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Semi-active suspension systems from research to mass-market – A review:

Semi-active suspension systems from research to mass-market – A review: It is well documented that active suspension systems offer substantial benefits in ride comfort, handling control over traditional passive systems. However, restrictive features such as the power required and costs make an active system impractical. To solve those problems, semi-active suspension systems have been developed. This paper aims at providing a review of the present state-of-the-art in the semi-active suspension control field in terms of vehicle ride comfort and road-holding performance evaluation. Strengths and weaknesses of the semi-active suspension systems are identified and their relative performance capabilities and equipment requirements are discussed. Furthermore, examples of the current mass market implementation for semi-active suspension systems for road vehicle are discussed. Keywords Semi-active vehicle suspension, skyhook control, fuzzy logic control, adaptive control, preview control Introduction The goal of the vehicle suspension system is to isolate the vehicle from the road irregularities while improving the road-holding characteristics. The limitations of passive suspensions to improve the ride comfort and road holding together have motivated the investigation of controlled suspension systems, both active and semi-active. The foundation of controlled suspensions for the car mass-market can probably be dated back to the early 1960s, when Citroen introduced hydro-pneumatic active suspension in its luxury cars; at that time those suspen- sions were still untouched by electronics. Given this tribute to Citroen, the 1980s was the real start of electronic suspension; analog electronics were already developed, and the magic of the fully active suspensions attracted both the Formula 1 competition and the car manufacturers. As shown in Figure 1, during this decade the exceptional potential of replacing the conventional spring-damper system with a fully electronically controllable fast-reacting hydraulic actuator was demonstrated. While the high costs, significant power absorption, bulky and unreliable hydraulic systems, and uncertain management of the safety issues made it difficult to have significant impact on the mass-market of vehicles. Since the 1990s, a new trend emerged: it became increasingly clear that the best compromise of cost (component cost, weight, electronics and sensors, also power consumption, etc.) and performance (comfort, handling, and safety) was to be found in another technology of electronically controllable suspensions: the semi-active suspen- sion or the continuous/variable damping suspension. For example, car manufacturers as Audi, VW, Mercedes, Ford, Volvo, and GM cooperated together with controlled damper suppliers like Sachs, Tenneco, Bilstein, and Monroe to design, develop, and manufacture different types of the semi-active suspension systems, which are introduced in the mass-market of cars. Faculty of Engineering-Benha, Benha University, Egypt Automotive & Tractors Engineering Department, Faculty of Engineering, Minia University, Egypt Corresponding author: MMS Kaldas, Automotive & Tractors Engineering Department, Faculty of Engineering, Minia University, Egypt. Email: m.kaldas@web.de Creative Commons CC BY: This article is distributed under the terms of the Creative Commons Attribution 4.0 License (http://www. creativecommons.org/licenses/by/4.0/) which permits any use, reproduction and distribution of the work without further permission provided the original work is attributed as specified on the SAGE and Open Access pages (https://us.sagepub.com/en-us/nam/open-access-at-sage). 1006 Journal of Low Frequency Noise, Vibration and Active Control 40(2) 2 Journal of Low Frequency Noise, Vibration and Active Control 0(0) Semi-active suspension system The main problem in the design of the vehicle suspension is that designers have to deal with conflicting demand, 1,2 namely good comfort of the vehicle’s occupants, low suspension working space, and good handling properties. In order to provide good ride quality, the vehicle body should be isolated from the road disturbances. The ride quality in general can be quantified by the vertical acceleration of the passenger locations. The presence of a well- designed suspension provides isolation by reducing the vibratory forces transmitted from the axle to the vehicle body; this in turns reduces vehicle body acceleration. On the other hand, to keep a good road-holding perfor- mance that can be characterized in terms of vehicle’s cornering, braking, and traction abilities, the normal tyre loads should be minimized. This is because the lateral and longitudinal forces generated by tyres depend directly on the normal tyre loads. Since the tyres roughly behave like springs in response to vertical forces, variations in normal tyre loads can be directly related to vertical tyre deflections. Furthermore, the optimal ride comfort and road-holding performance should be obtained considering the allowable suspension working space of the vehicle. The conflicting demand in the design of vehicle suspension is shown in Figure 2. The x-axis represents the root mean square (RMS) value of the dynamic tyre load with respect to the static wheel load. The y-axis represents the Figure 1. Number of the car manufactures for controlled suspension. Figure 2. The ride comfort and road holding conflict. Soliman and Kaldas 1007 Soliman and Kaldas 3 RMS value of the weighted body vertical acceleration. The “ride comfort value” consists of a weighted sum made up of the accelerations of the driver’s seat, hands, and feet. The ride comfort value reflects the human ability to perceive the strength of various oscillatory excitations. As a result of the hooked shapes of the constant damping curves, a continued increase in the overall suspension stiffness leads not only to a decrease in the passenger comfort, but also (beyond a certain point) an increase in the dynamic tyre load. In the same manner, a continued increase in the overall suspension damping eventually leads to a decrease in the occupant comfort. The selection of the optimal damping requires a compromise between harder “safer dampers” and softer “more comfortable” dampers. This selection is made more difficult by the fact that the optimal damper settings are dependent on the road surface as well as driving maneuver dictated by the driver. Therefore, controlled dampers are developed to resolve this conflict. In the visual representation of the conflict shown in Figure 2, a conventional spring-damper combination represents the intersection between a constant damping curve and a constant spring stiffness curve. The controlled damper is capable of representing any point on a constant spring stiffness curve and 1,2 can be adjusted to optimize either safety or comfort. For driving situations where comfort is especially relevant, the amplitude of the vehicle body accelerations can be reduced in the region between resonant frequencies by reducing the damping. In critical driving situations, the dynamic wheel loads can be reduced in regions near the natural frequencies by increasing the damping. Nowadays, the semi-active suspension systems are used to offer solutions for overcoming the ride performance limitations of passive suspension systems on the one hand and on the other hand for reducing the high system cost and energy consumption of fully active suspension systems. The hardware and the control equipment of an active suspension system are also required for a semi-active system. However the actuator, hydraulic pumps, and accumulators are replaced with a controllable damper. The semi-active systems can clearly offer cheaper suspen- sion control solutions compared to the active systems. The main components of the semi-active suspension system are the semi-active damper, acceleration sensors, ride height sensors, and controller. In this paper, the focus will be on the semi-active dampers and the control algorithms. Semi-active dampers Semi-active dampers are hydro-mechanical control devices with the capability to vary the amount of energy they dissipate using a small source of power. For example, the hydraulic dampers are designed in such a way that controllable force is proportional to the velocity. Such systems extend the possible range of damping character- istics obtainable from a regular (passive) damper. For the purpose of semi-active damping control, various energy-dissipating devices as following have been used to obtain the desired damping. • Servo/Solenoid valve dampers • Magnetorheological (MR)/electrorheological (ER) dampers • Electromagnetic dampers Some of these technologies have already found their way to the vehicles market. Currently available semi-active dampers use the solenoid valves or MR fluids, given their technological advantages and cost efficiency. Solenoid/Servo valve dampers. In general, the controllable valve requires certain characteristics in order to be useful in semi-active damper applications. The valve must be able to handle at maximum velocity a flow rate corre- sponding to the volume of displaced fluid in the damper. The maximum operating pressure of the valve must also correspond to the maximum pressure found in a passive damper, because of the suspension loading. In addition, the controllable valve must have a fast-enough response to cause variations in damping characteristics during operation. A damper consisting of a hydraulic actuator in conjunction with an electro-hydraulic servo-valve 7 8 modulating the controlling orifice area was used by Krasnicki and Patten et al. Practical applications of the servo-valve-skyhook dampers, namely extreme isolation for delicate manufacturing operations against seismic 9 10 input and the automotive suspensions were discussed by Karnopp and Soliman et al. Solenoid valves provide an alternative to servo-valves. Solenoid valves do not have as fast or accurate responses as servo-valves; however, a servo-valve is much more expensive than a solenoid valve. A solenoid valve is much simpler in design than a servo-valve, and it could possibly be manufactured in-house, bringing the added benefit of being able to optimize the valve design for the particular application including force, stroke, 11,12 and general packaging. Solenoid valves have also been used in two-or three-state dampers that can change characteristics between hard damping and soft damping by opening or closing a bypass valve. These dampers have 1008 Journal of Low Frequency Noise, Vibration and Active Control 40(2) 4 Journal of Low Frequency Noise, Vibration and Active Control 0(0) Figure 3. The DampTronic-Select damper from Bilstein. resulted in improvements in ride quality when appropriate control schemes are used. Although these dampers do not have the capability of being continuously variable, continuously variable solenoid valve dampers are also available. Solenoid valves of this type have been tested, with promising results by Kitching et al. In the automobile market, many suppliers of controlled dampers developed different types of the solenoid valve dampers. For example, ThyssenKrupp Bilstein developed the DampTronic-Select damper shown in Figure 3, a two-stage selectable damper closing the gap between conventional, passive shock absorbers and elaborate elec- tronic continuously variable dampers. The DampTronic-Select valve, which is installed inside the shock absorb- er piston provides two generally independently adaptable damping-force characteristics within a very compact installation space. The aim of the DampTronic-Select damper is to enable the driver to decide whether he would like a standard” Soft” or a sport “Hard” suspension system, and can simply alternate between the two suspension configurations by the touch of a button on the dashboard. Similar to the DampTronic-Select dampers, Tenneco Automotive’s Monroe developed a Dual Mode Damper, which provides an enhanced ride experience at a minimal system cost for the small and midsized vehicle segment. As shown in Figure 4, typically one characteristic is a higher level of comfort than a passive damper. On the other hand, when the driver wishes to get the most out of his vehicle, he can select the sport characteristic. The dampers are then set to a firmer damping level to deliver the needed road holding. This is achieved via two valve setting, the first one is NC “Normally Closed”, which offers firm damping without energizing the valve “0.0 A”, soft damping is achieved at approximately “0.5 A” holding current, while the second one is NO “Normally Open” offers soft damping without energizing the valve “0.0 A”, firm damping is achieved at approximately “0.5 A” hold- ing current. In order to provide different choices for the costumers, the adaptive damping system (ADS) has been developed by ThyssenKrupp Bilstein. The ADS is a four-stage damping system designed by combining a soft and a hard characteristic for the extension and compression stages in the compression characteristics. The four different damping characteristics are obtained through two pistons in an external adjustment module connected individ- ually in parallel to the conventional working piston. The four damping characteristics in extension or compres- sion are soft/soft; soft/hard; hard/soft; hard/hard. As an actual mass-market semi-active damper, the DampTronic-Sky damper shown in Figure 5 is developed by ThyssenKrupp Bilstein. The DampTronic-Sky damper is a further milestone in resolving the conflict between driving comfort on the one hand and the driving safety and agility of the car on the other hand. In this damper, two continuously variable valves adjust the damping force in the damper: one valve controls the extension phase i.e. the force that ensues when the wheel rebounds, and the other the compression phase. As a competition between the controlled damper manufacturers, Tenneco Automotives Monroe developed a semi-active damper” Continuously Controlled Damper” shown in Figure 6, is a semi-active suspension damper that continuously adjusts damping levels according to road conditions and vehicle dynamics, such as speed, turning and cornering, delivering comfort without sacrificing the safety of sure handling. The semi-active damper adjusts damping levels through a bypass solenoid valve attached to the outer tube of the damper. The softer damping setting is achieved at approximately 0.3 A, while the hard damping setting is obtained at approximately 1.6 A. In the case of the failsafe, the semi-active damper provides approximately the characteristics of the passive damper without energizing the valve (0.0 A). Soliman and Kaldas 1009 Soliman and Kaldas 5 Figure 4. The damping characteristics of the dual-mode damper from Tenneco. Figure 5. The DampTronic-Sky damper from Bilstein. Figure 6. (a) The continuously controlled damper from Tenneco. (b) Schematic drawing for the continuously controlled damper characteristics. MR and ER dampers. MR or ER damper is hydraulic damper consist of a hydraulic cylinder containing micron- sized polarizable particles in a MR or ER fluid (usually oil). MR and ER fluids are non-Newtonian fluids that change its properties in the presence of a magnetic or electric field. Micron-size iron particles suspended in a carrier fluid (water, petroleum-based oil, or silicon-based oil) align in chain-like structures along the flux lines of a magnetic or electrical field, changing the rheological properties of the fluid. Both the MR and ER materials have the ability to change from free flowing viscous fluids to a semi-solid state in a matter of milliseconds when exposed to a magnetic or an electric field. MR and ER dampers are mechanically reliable, since they do not contain any moving parts. Figure 7 shows the working principal for MR dampers. 1010 Journal of Low Frequency Noise, Vibration and Active Control 40(2) 6 Journal of Low Frequency Noise, Vibration and Active Control 0(0) Figure 7. The MR damper working principal. There are numerous published references on vibration control using MR and ER dampers. For example, Wu and Griffin used an ER damper to reduce the severity of shocks caused by suspension seat end-stop impacts or high magnitude vibration. The ER damper was used to realize the required two-state damping. The vibration isolation performance of a MR damper under the control of the on–off skyhook control strategy is studied by Jeon et al. The damping constant and response time of the damper were measured. The time delay in the response of the MR damper was measured and incorporated into the control under harmonic disturbances. Experimental results showed that on–off skyhook control strategy, which includes the damper time delay, per- forms less effectively than the one without the consideration of time delay. Since the 2000s, Delphi Automotive Systems has developed the MagneRide system shown in Figure 8, which is a semi-active suspension system based on MR dampers. The MagneRide system employs a unique “MR” fluid in its monotube dampers. In the beginning of 2003, the MagneRide has appeared for some luxury vehicles in the European and American market. Initially, it was produced for the Cadillac Seville STS and Chevrolet Corvette, and the latest iteration of the system is fitted to the Ferrari 599 GTB Fiorano, Audi TT, and Audi R8. Recently, MagneRide suspension is selected for the Range Rover Evoque. The MagneRide suspension for Range Rover is supplied by BWI Group’s, which developed the third generation of the MagneRide. Figure 8 shows the MagneRide damper and a comparison between the damping characteristics of the MagneRide damper with a typical variable valve damper and a passive damper. Electromagnetic dampers. Electromagnetic dampers use the interaction between the movement of a coil and the 23,24 magnetic field of a permanent magnet or electromagnet to provide a damping effect. When an electromagnetic damper coil is shorted or connected to an external resistor, the device becomes a linear mechanical damper. The Soliman and Kaldas 1011 Soliman and Kaldas 7 Figure 8. The MagneRide damper and its damping characteristics from Delphi. damping level can be varied by changing the external resistance or the strength of the magnetic field. If the external resistance is varied, the damping coefficient is varied. In the open circuit state the damping coefficient vanishes, while it reaches a maximum value if the coil is shorted. Since effective resistance can be rapidly varied electronically, an electrical actuator can function as a semi-active damper in vehicle or vibration isolation sus- pension systems. For example, in Karnopp the possibility of using permanent magnet linear motors as variable mechanical dampers for vehicle suspensions is studied. Two basic electromagnetic designs are analyzed, namely the moving coil and the moving magnet approach. The electromagnetic damper use consists of a tubular coil of wire situated within a radially oriented constant magnetic field produced by a permanent magnet. The damping coefficient is varied by changing the external resistance. The high cost of the electromagnetic dampers made it difficult to appear in the vehicle market, but the compact size and weight motivate the shock absorbers suppliers and the automobile companies to consider it in their research studies. Control strategies 25,26 Semi-active suspensions were firstly introduced in literature as an alternative to the high cost, highly compli- cated, and power density active systems. A vast amount of work on controlled suspension systems is presented in the technical and scientific literature. One of the reviews of the state-of-the-art of controlled suspensions was carried out by Sharp and Crolla in the semi-active suspension and other active suspensions. The most attractive feature of that work was that control strategies were based only upon the measurement of the relative displace- ment and velocity of the suspension system. Semi-active suspension systems provide not only vehicle ride comfort and control, but also better road holding. These systems are able to tune the amount of damping in response to measured vehicle-ride and handling indicators. Research and development projects in semi-active suspension have been carried out in order to 28–32 improve the stability and ride handling performance of the vehicle. However, these approaches need to solve some practical applications. The neural network-based robust control system gives better ride comfort compared to the standard proportional–integral–derivative (PID) controller. Preview control for a semi- active suspension system was investigated by Soliman and Crolla. Furthermore, the influence of the preview control of the active suspension on the vehicle ride and braking performance was investigated by Kaldas and Soliman. Control approaches such as skyhook control, adaptive control, robust control, fuzzy logic, neural network methods, linear quadratic regulator (LQR), linear optimal control, and preview control have been used in the field of semi-active suspensions. Practical semi-active suspensions have only recently become possible with the advent of powerful but relatively inexpensive signal processors. Classical control strategies for semi-active suspension system. Several semi-active control strategies have been proposed and investigated since Crosby and Karnopp developed the skyhook control strategy. The common goal of these initiatives is to achieve a higher level of vibration isolation or to find practical and easy implementation methods, or both. In this section, the most classical methods are briefly introduced. 1012 Journal of Low Frequency Noise, Vibration and Active Control 40(2) 8 Journal of Low Frequency Noise, Vibration and Active Control 0(0) Figure 9. Skyhook and groundhook models. Skyhook control. The skyhook strategy is used to minimize disturbance effects that can cause the vehicle body to oscillate. Depending on the direction of motion of the vehicle body, the system compression and rebound damping are either increased or reduced. The main goal of the skyhook strategy is to decouple the vehicle body from disturbances on the road surface. This strategy attempts to simulate the responses that would occur if a vibration damper were mounted between the vehicle body and a traveling hook attached to a fixed inertial system (the sky) rather than between the body and the wheels. Historically, the skyhook control strategy was developed by DC Karnopp. Krosby and Karnopp and Karnopp et al. reviewed the historical development of theoretical concepts necessary for the design of isolation systems and indicates how control theory began to influence the vibration isolation in the last half of this period. He focused on a relatively simple idea namely the “skyhook” damper. This idea came through purely theoretical studies but is now used in combination with other concepts in production suspension systems. Since it is not practically possible to develop the ideal skyhook system presented in Figure 9(a), it is realized by using the damper force C Z that allows reproducing the skyhook behavior for the sprung mass. This desired behavior is then sky b modeled as € _ M Z ¼� K ðÞ Z � Z � C Z (1) b b s b w sky b � � _ _ _ C Z ¼ C Z � Z (2) sky b s b w � � C ¼ C (3) s sky _ _ Z � Z b w M Z ¼� K ðÞ Z � Z þ K ðÞ Z � Z (4) w w t w o s b w where C is the damping coefficient of the skyhook damper. sky In order to realize the skyhook damper to semi-active damper, an on/off strategy that switches between the low damping and the skyhook “high” damping is used. This control law consists in changing the damping coefficient � � _ _ _ C of the semi-active damper according to the chassis velocity Z and the suspension deflection velocity Z � Z s b b w by using the following logical rule �� _ _ _ C if Z Z � Z � 0 min b b w �� C ¼ (5) _ _ _ C if Z Z � Z > 0 sky b b w where C and C are the minimal and skyhook “maximal” damping coefficients achievable by the considered min sky semi-active damper, respectively (and usually C ¼ C ). Basically, this control law consists in switching sky max Soliman and Kaldas 1013 Soliman and Kaldas 9 Figure 10. Damping settings of the skyhook control. controller which deactivates the controlled damper when the body velocity and relative velocity between vehicle body and wheel (damper velocity) have opposite signs as shown in Figure 10. Many studies have been carried out on the skyhook control strategy since it represents a simple way to achieve 37 38 a good comfort requirement, as for instance the no-jerk version. Some extended versions of the skyhook control have been also developed, such as the adaptive-skyhook in Song et al. to MR dampers or the gain- scheduled-skyhook in Hong et al. This control strategy presents the advantage of being simple, but it requires at least two sensors, filters, and state estimation algorithm. Yi and Song presented an adaptive skyhook control for semi-active suspension, which consists of a skyhook control law with road adaptive gains and a road detection algorithm. The new skyhook control law is a combi- nation of the sprung mass and unsprung mass velocity feedbacks with time varying gains, and the unsprung mass velocity feedback is seen to be an important factor in increasing damping at the wheel hop frequency. The road detection algorithm is designed to adapt the optimal gains on the basis of the frequency content of the road inputs. The results showed that the proposed control law provides adequate damping for the wheel hop frequency and improved performance compared with that of skyhook control law. Consequently, the proposed control law obtains a performance enhancement in ride comfort and road holding. Groundhook control. The groundhook approach, which is in some sense the dual of the skyhook, is described below. It consists of increasing the damping, then reducing the deflection, to reduce the road–tyre forces, as illustrated in literature. As in the skyhook, the ideal groundhook cannot be achieved and needs to be approx- imated. The groundhook control method is similar to the on–off skyhook control method, except that control is based on the unsprung mass damping control, as illustrated in Figure 9(b). The on–off groundhook semi-active policy emulates the ideal tyre displacement control configuration of a passive damper “hooked” between the tyre and the “ground”. The semi-active on-off control law can be considered as the following �� _ _ _ C if � Z Z � Z � 0 min w b w �� C ¼ (6) in _ _ _ C if � Z Z � Z > 0 ground w b w where C and C are the minimal and groundhook “maximal” damping coefficients achievable by the min ground considered semi-active damper, respectively (and usually C ¼ C ). Some extended cases, such as displace- ground max 43,44 ment and velocity-based groundhook control policies, are presented and tested experimentally in literature. Vala´ �sek et al. applied the groundhook concept for semi-active suspension for heavy trucks with the ultimate objective to minimize dynamic tyre–road force and thus the road damages. The basic groundhook concept is extended to the several variants, which enables a decrease in road damage whilst maintaining driver comfort over a broad range of road unevenness. The influence and interaction of the damping rate limits and time constants of 1014 Journal of Low Frequency Noise, Vibration and Active Control 40(2) 10 Journal of Low Frequency Noise, Vibration and Active Control 0(0) variable shock absorbers are also taken into account. It was also concluded that the concept could be used to increase the truck payload. Hybrid skyhook–groundhook control. An alternative semi-active control policy known as hybrid control has been shown to take advantage of the benefits of both skyhook and groundhook controls. With hybrid control, the user has the ability to specify how closely the controller emulates skyhook or groundhook. In other words, hybrid control can divert the damping energy to the bodies in a manner that eliminates the compromise that is inherent in passive dampers. This logic is aimed to reducing both body acceleration and dynamic tyre force by combining the skyhook and groundhook �� C ¼ lC þ ðÞ 1 � l C (7) hybrid sky ground With the following four-state switching condition � � _ _ _ C if Z Z � Z � 0 > min b b w � � _ _ _ C if Z Z � Z > 0 hybrid b b w C ¼ �� (8) in > _ _ _ > C if � Z Z � Z � 0 min w b w �� _ _ _ C if � Z Z � Z > 0 hybrid w b w The variable l is the relative ratio between the skyhook and groundhook control. If l is set to 1, the hybrid control policy is switched to pure skyhook and, if l is set to 0, the hybrid control is switched to pure groundhook. Ahmadian studied the performance of five different skyhook control methods experimentally, using a quarter-car rig. The control methods that are analyzed include skyhook control, groundhook control, hybrid control, displacement skyhook, and relative displacement skyhook. The results indicated that hybrid control, however, comes close to providing the best compromise between different dynamic demands on a primary sus- pension, also hybrid control can offer benefits to both the sprung and unsprung mass with control gain settings that provide equal contributions from skyhook control and groundhook control. Acceleration driven damper control. The acceleration-driven damper (ADD) control is a semi-active control law 48,49 described in Savaresi et al., which consists of changing the damping factor C as in � � € _ _ C if Z Z � Z � 0 min b b w � � C ¼ (9) in € _ _ C if Z Z � Z > 0 max b b w where C and C are the minimal and maximal damping factors achievable by the controlled damper. This min max strategy is shown to be optimal in the sense that it minimizes the vertical body acceleration when no road information is available (therefore, this control law is a comfort-oriented one). This control strategy presents the advantage of being simple, but it requires at least two sensors as the skyhook and groundhook control law, in addition to the state estimation algorithm and filters. It should be noted that the control law is very similar to the skyhook algorithm, with the difference that the switching law depends on body acceleration Z , instead of body velocity (which is easier to measure on real vehicles). It is worth noting that the ADD design is well adapted to comfort improvement but not to road-holding. Moreover, the “switching dynamic” may influence the closed- loop performances. Modern and intelligent controllers for semi-active suspension system Optimal control. The linear quadratic (state feedback) control is one of the first methodologies that have been applied to suspension control, mainly in the active suspension systems. Some applications of semi-active sus- 50–52 53 pensions have been proposed using different control strategies based on optimal control. Recently, Soliman compared the performance of three control laws for a semi-active suspension system, based on simulated results obtained from a quarter car model. The semi-active damper is modeled as a continuously variable device with maximum and minimum limits and a time delay between the demand and actual forces. The three control laws are Soliman and Kaldas 1015 Soliman and Kaldas 11 based on (a) full state feedback, (b) limited state feedback, and (c) skyhook damping. The results showed that semi-active suspension system with full state feedback control force gives a better improvement in terms of ride comfort compared to a semi-active with limited state feedback and skyhook control force. The mean power dissipation in the semi-active with the three control laws and passive suspension systems relative to the rolling resistance power losses are ranged between 14% and 19%. An adaptation algorithm to maintain optimal performance over the wide range of input conditions typically encountered by a vehicle is proposed in literature. The adaptive control loop is based on a gain scheduling approach and two strategies are examined both theoretically and experimentally using a quarter vehicle test rig. For the first strategy, the gains are selected on the basis of RMS wheel acceleration measurements whereas in the second approach the RMS value of suspension working space is used. A composite input is used consisting of sections of a road input disturbance of differing levels of magnitude in order to test the control systems abilities to identify and adapt efficiently as the severity of the road input changes. Both simulated and measured results are used to investigate design decisions regarding the pre-calculated gains, averaging of the monitored variable on which the adaptation is based and the detailed way in which the gains are altered when a change is required. The obtained results showed that, vehicle ride comfort performance is improved with both adaptation control strategies in comparison with the same system without the adaptation algorithm. An optimal self-tuning control algorithm for active and semi-active suspension systems incorporating a weight- ing controller, state observer, and parameter estimator, based on LQG and LQR theories is suggested by Yu and 55 56 Crolla and Soliman and Crolla, respectively. The controller is based on the updated estimates of vehicle parameters and states, and the adapted weighting parameters. The LQG/LQR controllers provide the optimal set of gains over different operating conditions. The feasibility and effectiveness of the self-tuning system was investigated and proved by simulation studies. The obtained results showed that the self-tuning control algorithm improved the vehicle ride performance in comparison with the LQR/LQG control without the self-tuning algorithm. However, the performance improvement depends mainly on the accuracy of the state observer and parameter estimator. Since the semi-active suspensions are feedback-controlled systems in which the actuator is limited to providing energy dissipation, the power required to perform modulation of the continuously variable damper is insignificant in comparison with the power needed for other active suspension systems. However, the performance benefits available with a semi-active suspension system are, not surprisingly, less than those for an active system. Therefore, some researchers used a preview strategy to improve the behavior of semi-active systems. Two forms of preview control are distinguishable; the look-ahead preview and wheel base preview. As an application of LQR including a look-ahead preview algorithm for semi-active suspension system is suggested by literature. The control strategy has been applied to a quarter vehicle model. The performance of a semi-active suspension system with preview control is compared with an active system with and without preview control, and with a passive suspension system. The simulation results presented that the performance of a semi-active suspension can be improved significantly if preview information about the road input, ahead of the wheel is available. One of the most important findings in the study is that the performance of the semi-active system with preview is as good as the performance of the active system without preview. Model predictive control. As a result of the development and improvements in the optimization algorithms, the model predictive control (MPC) is being increasingly used in automotive applications. An MPC semi-active suspension controller, which provides better performance compared to those of the skyhook and LQR approaches is introduced by Giua et al. The obtained results are interesting and deserve a closer view, but the main draw- backs are the requirement of an online “fast” optimization procedure in the control loop, optimal control, full state measurement, and a good knowledge of the model parameters. The online computation difficulties related to the predictive control law for a semi-active suspension system are overcome in Canale et al. by means of a “fast” implementation of the MPC algorithm (FMPC). The estimation of the system state variable needed for control law computation is provided by a suitable “robust” observer, whose accuracy is not affected by variation of the system parameters (i.e. masses, damping and stiffness coef- ficients, etc.). The computed control law aims to optimize the suspension performance, by minimizing a quadratic cost function while ensuring that the magnitude of the forces generated by the control law satisfies the physical constraints of passive damping. A performance comparison with the well-established semi-active skyhook strategy is presented. The achievable performance improvements of the proposed design procedure over skyhook control law are showed by means of simulation results. 1016 Journal of Low Frequency Noise, Vibration and Active Control 40(2) 12 Journal of Low Frequency Noise, Vibration and Active Control 0(0) A MPC hybrid control tool to derive an optimal control law for semi-active suspension system is designed by Giorgetti et al. A quarter-car semi-active suspension model has been modeled as a linear system under state- dependent linear and nonlinear constraints. The constrained quarter-car semi-active suspension is modeled as a switching affine system, where the switching is determined by the activation of passivity constraints, force satu- ration, and maximum power dissipation limits. The performance of different finite-horizon hybrid MPC controllers is tested in simulation using mixed-integer quadratic programming. A comparison between different semi-active control laws and the proposed MPC hybrid control law is performed as a way for obtaining control laws with different degrees of optimality, depending on the chosen control horizon. The obtained results showed that when the horizon is equal to one the hybrid MPC law corresponds to the “clipped-optimal” control law, obtained by computing the active LQR control law and clipping it to enforce the given constraints. Furthermore, with increased control horizon, the results showed that significant deviation of optimal from the “clipped optimal” can occur. H1 clipped control. Many studies deals with on the application of the H control approach (sometimes in the framework of LPV systems) for the suspension system. However, most of the studies applied for active suspen- 60 61 62 sions, as in Zin et al., Karnopp, and Margolis suggested a semi-active controller by passing the optimal active controller through a limiter (saturation). In the control step, the force applied by the semi-active damper is then chosen to be as close to the force required by the controller for a given suspension deflection speed and for the possible range of forces the damper can deliver. This simple strategy has been applied in many cases, whatever 63 64 65 65 the control method is: optimal, robust, or state-feedback. For example, in Du et al. a static output feed- back (clipped) H controller is designed for a suspension equipped with an MR damper. This strategy uses two measured states: the suspension deflection and the velocity of the sprung mass. The results showed that the proposed strategy allows to improve comfort and road-holding as well, compared to a passive suspension. Rossi and Lucente studied the control of semi-active suspensions using H state-space optimization techni- ques. Two standard models of the system, quarter car and half-car model with three H controllers are developed in the fully active case. Then, they adapted to the nonlinear real system using a sort of clipped control. The first controller is focused on ride comfort optimization, the second on both comfort and handling improvement, the third is a global controller derived from half car model. Performance indices related to ride comfort and drive safety are introduced to evaluate proposed controllers. The obtained results indicated that controllers had better ride comfort than a passive setting. Drive safety performances can be improved including variables related to unsprung mass dynamics in the error signals vector. Simulations were performed to show that semi-active controlled suspensions succeed in achieving higher levels of ride comfort and drive safety with respect to a passive setting. Fuzzy-logic control. Although Zadeh developed fuzzy-logic in 1965, it has been applied to vehicle suspension control only in recent years. The fuzzy-logic is one of the most intelligent control methods, offers several unique features that make it a particularly good choice for many control problems. It can be used to control nonlinear systems that are difficult or impossible to model mathematically. In addition to this, it does not require precise, noise-free inputs, and can be programmed to control the system safely if a feedback sensor fails. The control output is a smooth function despite a wide range of input variations. Therefore, any sensor data that provides 67–69 some indication of the system actions and reactions is sufficient. Fuzzy-logic is one of the model-free and robust intelligent controllers introduced to solve the problems of uncertainty effects. The fuzzy logic controller (FLC) is an adequate methodology for designing robust controllers that are capable of delivering a satisfactory performance in the face of uncertainty and imprecision. As a result, the FLC has become a popular approach to nonlinear and uncertain system control in recent years. There are different ways to design FLCs for vehicle suspension control, with the most common method to design FLCs being by eliciting the fuzzy rules and 68,69 its membership functions based on experts’ knowledge or experience. The semi-active suspension control problem is a well-suited problem for taking advantage of the fuzzy logic features: it is highly nonlinear, it is difficult to simulate adequately, the control improvements are quite subjective, and controllers should deal with a compromise between opposite properties (ride safety and passenger comfort). Among the first application attempts, Al-Holou et al. adopted fuzzy logic to control the vibration of semi-active suspension systems. Although results obtained by fuzzy logic have shown improvement over passive suspension, not much work has been done to compare the results from FLCs with those from other conventional methods introduced above. A semi-active vehicle suspension system using an adjustable shock absorber applied to a quarter vehicle model is presented by Wu et al. Two control techniques are developed for assessing both ride Soliman and Kaldas 1017 Soliman and Kaldas 13 comfort and road handling. A part from the PID controller, a controller using fuzzy logic is developed and its performance is experimentally tested. The simulation results showed that the semi-active control provides lower RMS values and reduced settling time compared to the passive suspension system. It was demonstrated that the proposed FLC achieves much better performance compared to the semi-active suspension with PID controller and the passive suspension under various road conditions. Nicola´ s et al. described the practical application of fuzzy logic to the design of semi-active suspension systems control strategies. The intelligent suspension systems are based on a continuously adjustable shock absorber. Two different fuzzy approaches are described. The first strategy is based on the actions of the driver and the second one is based on the vehicle dynamics. Both strategies have been tested theoretically in simulation models and exper- imentally on-road. The system was tested in a number of vehicles and the results are compared with those obtained with other conventional control strategies based on selection amongst several discrete damping settings. The main advantage of the proposed fuzzy strategies is the small number of required sensors, achieving similar performances than other control algorithms with less cost. The obtained results showed that the fuzzy controller based on the driver actions shows excellent performances with a low cost sensor system. The behavior of the controller is close to the theoretical optimum for a semi-active suspension system. An adaptive FLC for an active suspension system is developed by Huang and Chao. The inputs of the FLC are the vertical position error of the vehicle sprung mass and the rate of change of this error, while the output is the damper control voltage. The antecedents membership functions consists of 11 equal triangular type functions. The voltage increment membership function is a set of 15 equal triangular type functions. The self-tuning property is implemented by adjusted scaling factors for controller inputs and output. A total of 121 fuzzy rules are employed to suppress the sprung mass vibration amplitude due to road inputs. In order to evaluate the fuzzy control system, a two-degrees-of-freedom (DOF) quarter-vehicle suspension model was used. The dynamic response of the semi-active suspension system was presented for vehicle ride performance on a rough concave- convex road with 25 mm obstacles but not compared with the known control strategies. It was noted that the control signal was very smooth and easy to employ in the practical vehicle. The adjusted scaling factors were chosen by experiments and many simulations, which limit the flexible and adaptive abilities of the adaptive FLC. On the other hand, Kaldas et al. developed a semi-active suspension system controller using adaptive fuzzy logic control theories together with a Kalman filter for the state estimation. A quarter vehicle ride dynamics model is constructed and validated through laboratory tests performed on a hydraulic four-poster shaker. A Kalman filter algorithm is constructed for bounce velocity estimation, and its accuracy is verified through measurements performed with external displacement sensors. The benefit of using adaptive control with fuzzy logic to maintain the optimum performance over a wide range of road inputs is enhanced by the accuracy of the Kalman filter in estimating the controller inputs. A gradient-based optimization algorithm is applied for improving the FLC parameters. The vehicle model is simulated with the developed semi-active suspension controller on different road profiles. A comparison is performed between the adaptive fuzzy semi-active controller, the optimum LQR semi-active controller, and the optimum passive suspension system in terms of ride comfort and road holding. The results showed that the proposed semi-active suspension system controller provides significant improvements in both ride comfort and road holding of the vehicle on different road profiles. Application of the FLC to the MR damper was presented by Kurczyk and Pawel. In this study, a semi-active control of the suspension of an all-terrain vehicle is developed. A 7-DOF suspension model is developed and a fuzzy approach for controller synthesis is proposed. The fuzzy system output is the damping coefficient. In contrast to many other control algorithms, the presented fuzzy algorithm does not require inverse modeling of the MR damper. Instead, some scaling parameters are set that can be chosen experimentally, or through a bio- inspired strategy. The FLC is compared with the skyhook control in terms of road holding and driving comfort indicators. The simulation results showed that the FLC used for vehicle suspension vibration damping can per- form, in some cases, as well as the skyhook algorithm, but without necessity of using an inverse damper model. The use of the fuzzy algorithm is more computationally complex than the skyhook algorithm. However, consid- ering cutting edge technology it can be successfully implemented, although with a higher cost controller. There are FLC operations that can be performed once in order to reduce the system complexity. Moreover, hardware computing can be used, e.g. the Field Programmable Gate Array technology. In Nabaglo, the FLC for MR dampers is optimized by the genetic algorithms (GAs) and the optimization is performed theoretically using ADAMS/Car and MATLAB simulations. The objective of the control system is to reduce the vertical acceleration and the counteraction against moments of forces acting around the longitudinal and transverse axes of the vehicle in varied ride conditions. The obtained results showed that the FLC is effective, reliable, safe, and superior in ride comfort improvement as well as vehicle safety. 1018 Journal of Low Frequency Noise, Vibration and Active Control 40(2) 14 Journal of Low Frequency Noise, Vibration and Active Control 0(0) In a more advanced optimization method, Kaldas et al. developed an advanced optimization process to obtain the optimum fuzzy logic membership functions and the optimum rule-base of the FLC applied in the semi- active suspension. Discrete optimization has been performed with a GA to find the global optima of the cost function which considers the ride comfort and road-holding performance of the full vehicle. The proposed rule- optimized fuzzy logic semi-active controller is compared to the optimum linear quadratic regulator (LQR) semi- active controller and the optimum passive suspension system in terms of ride comfort and road holding. The results showed that the proposed semi-active suspension system controller provides significant improvements in both ride comfort and road-holding performance of the vehicle. Furthermore, in C¸ alıs¸ kan et al. the rule- optimized fuzzy logic semi-active controller is enhanced through preview capability. The preview-enhanced FLC uses the feedforward road input information and the feedback vehicle state information as the controller inputs. An 11-DOF full vehicle model, which is validated through laboratory tests performed on a hydraulic four- poster shaker, is used for the controller synthesis. The preview distance is also considered as a design parameter during the optimization process. The performance of the preview-enhanced rule-optimized FLC is evaluated by using a measured stochastic road profile as vehicle model input. The results demonstrate the potential of the preview-enhanced controller in improving all aspects of system performance compared to the rule-optimized FLC without preview. With the increase of the customer demand for adjustable chassis control features, which enable adaption of the vehicle comfort and driving characteristics to the customer requirements, Kaldas et al. improved the rule- optimized FLC for semi-active suspension systems, which can continuously adjust itself not only according to the road conditions, but also according to the driver requirements. The proposed controller offers three different control modes (comfort, normal, and sport), which can be switched by the driver during driving. The comfort mode minimizes the accelerations imposed on the driver and passengers by using a softer damping. On the other hand, the increased damping in the sport mode provides better road-holding capability, which is critical for sporty handling. The normal mode is adjusted to provide an overall balance between the vehicle ride comfort and road holding. A comparison between the three control modes in terms of ride comfort and road holding is performed. The results show that the proposed control modes provide three different vehicle characteristics to the driver. In addition to this, all three control modes are superior to the optimal passive suspension in terms of both ride comfort and road holding. Semi-active suspension system in vehicles mass-market In recent years, the automobile companies have commercially developed several vehicle models with semi-active suspension systems. The control algorithms have been continuously developed and new control features have been added, which make the customers satisfied with these vehicles. For example, Tenneco in conjunction with Ohlins Racing developed the continuously controlled electronic suspension “CES”. The CES is a semi-active suspension system that continuously adjusts damping levels according to road conditions and vehicle dynamics, such as speed, turning and cornering, delivering comfort without sacrificing safety and handling. The CES controller is designed to exploit the full potential of the electro-hydraulic valving system by processing input data sent by the acceleration sensors placed on the vehicle body and the suspension position sensors, installed between the vehicle body and the lower control arms of the suspension system. The position and acceleration sensors monitoring the movements of the wheels and body, and an electronic control unit realizing the control functionality and man- aging the necessary communication between the shock absorbers, the sensors and other control systems in the vehicle. A typical CES system architecture is shown in Figure 11. The CES utilizes control software that processes the sensor information regarding steering wheel angle, vehicle speed, brake pressure, and other chassis control information and sends signals that independently adjust the damping level of each shock absorber valve—up to 100 times a second. CES dampers allow a large separation between maximum and minimum damping levels and adjust instantaneously to ensure the optimum in ride comfort and road holding. The CES is in production in 37 different vehicle models. For example, CES has been supplied to Volvo for the S60R and V70R and General Motors Corp.’s for Cadillac Seville. Currently it has been supplied to the latest generation of Renault Espace. In 2004, continuous damping control (CDC) was developed by ZF Sachs AG. The CDC system uses six strategically placed accelerometers in the vehicle to detect wheel and body movement. An electronic control unit processes that information and interacts with the antilock brakes, steering inputs, engine speed, and elec- tronic stability control unit in deciding within milliseconds the necessary damping force for optimum handling, regardless of road conditions. In recent years, ZF Sachs has assembled an impressive list of vehicles using the Soliman and Kaldas 1019 Soliman and Kaldas 15 Figure 11. CES system architecture from Tenneco – Ohlins Racing. electronic chassis system, including the Audi A8, BMW 7-Series, Maserati 3200 Coupe, Volkswagen Phaeton, Ferraris, Porsche Cayenne, and Volkswagen Touareg. Recently, ZF Sachs has been selected by Adam Opel AG’s to supplie the CDC suspension system to Opel Astra. Also, ThyssenKrupp Bilstein developed the DampTronic-Sky system, which is a complete semi-active suspen- sion system. In the system, two continuously variable valves adjust the damping force in each damper: one valve controls the extension phase, i.e. the force that ensues when the wheel rebounds and the other the compression phase. Using the data it receives from the acceleration and suspension travel sensors, the control module of the suspension system can individually adapt the damping forces for each wheel within just a few milliseconds to eliminate the effects of rough road-ride conditions that may detract from the driving comfort for the passengers, at the same time controlling the dampers in such a way that the chassis is stabilized to the best possible degree. The damper is also able to ensure that the damping force can be adjusted according to the skyhook principle even in the high-frequency spectrum of the wheel vibrations. Currently, the semi-active suspension system DampTronic- Sky has appeared in the new Nissan GT-R Nismo. Other car manufacturers decided to design and develop the software of the semi-active suspension system controller in-house, while the hardware components like dampers, sensors, and ECU hardware are supplied by different suppliers. For example, Volkswagen AG developed the dynamic chassis control (DCC) system, which is based on semi-active dampers. The DDC system uses three accelerometers placed on the vehicle body and three suspension travel sensors to detect body and suspension movement. In this system, the control algorithm of the ECU is developed by Volkswagen engineers. The semi-active valve is mounted externally on the side of the shock absorber so that oil from the shock absorber ring channel flows to the valve. The valve is adjusted by applying a current to the coil of the valve (0.24 A to max. 2.0 A) and the resulting changes inside the adjustment valve. The DDC suspension constantly adjusts itself to the road conditions, the driving situation and the driver’s require- ments. The driver can switch the vehicle driving characteristics from the normal mode to the comfort or sport modes through a button in the dashboard. As well, Jaguar Land Rover designed a semi-active suspension system which is commercially called the con- tinuously variable damping (CVD) system. The controller software is designed in-house by Jaguar Land Rover. The same system is developed for Jaguar and Land Rover cars, while tuning parameters for the control algorithm are considered to optimize the vehicle performance for both. The CVD uses semi-active dampers which employ continuously variable valves inside the damper and placed in the damper piston. As lead in the automotive industry, Ford Motor Company developed a semi-active suspension system that commercially called the contin- uously controlled damping (CCD). The CCD system uses four height sensors to measure the relative displace- ment between the vehicle body and wheels, in addition to the four solenoid valve dampers and the ECU. The CCD control algorithm is developed by suspension control engineers and tuned by vehicle dynamics engineers of Ford. 1020 Journal of Low Frequency Noise, Vibration and Active Control 40(2) 16 Journal of Low Frequency Noise, Vibration and Active Control 0(0) Recently, the CCD suspension system is implemented in most of the Ford vehicles, in addition to the luxury vehicles Lincoln. Conclusion After a survey of the previous research studies on the semi-active suspension system and with the focus on the controlled suspension systems in the automobiles market, it can be concluded that the solenoid valve dampers as semi-active dampers are developed and made commercially available, and have already found their way to the vehicles market in comparison with the other types of the controlled dampers. On the other hand, semi-active suspension control is studied and it is still a scoop of studies for the classical control as skyhook and groundhook and for the modern control like optimal control and fuzzy Logic. The semi- active suspension control problem is highly nonlinear, it is difficult to simulate adequately, the control improve- ments are quite subjective, and controllers should deal with a compromise between opposite properties (ride safety and passenger comfort). Semi-active control algorithms have been developed by suppliers or OEMs to achieve the correct balance of ride comfort and road-holding control and usually based on the skyhook control strategy. Declaration of conflicting interests The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. Funding The author(s) received no financial support for the research, authorship, and/or publication of this article. 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Abstract

It is well documented that active suspension systems offer substantial benefits in ride comfort, handling control over traditional passive systems. However, restrictive features such as the power required and costs make an active system impractical. To solve those problems, semi-active suspension systems have been developed. This paper aims at providing a review of the present state-of-the-art in the semi-active suspension control field in terms of vehicle ride comfort and road-holding performance evaluation. Strengths and weaknesses of the semi-active suspension systems are identified and their relative performance capabilities and equipment requirements are discussed. Furthermore, examples of the current mass market implementation for semi-active suspension systems for road vehicle are discussed. Keywords Semi-active vehicle suspension, skyhook control, fuzzy logic control, adaptive control, preview control Introduction The goal of the vehicle suspension system is to isolate the vehicle from the road irregularities while improving the road-holding characteristics. The limitations of passive suspensions to improve the ride comfort and road holding together have motivated the investigation of controlled suspension systems, both active and semi-active. The foundation of controlled suspensions for the car mass-market can probably be dated back to the early 1960s, when Citroen introduced hydro-pneumatic active suspension in its luxury cars; at that time those suspen- sions were still untouched by electronics. Given this tribute to Citroen, the 1980s was the real start of electronic suspension; analog electronics were already developed, and the magic of the fully active suspensions attracted both the Formula 1 competition and the car manufacturers. As shown in Figure 1, during this decade the exceptional potential of replacing the conventional spring-damper system with a fully electronically controllable fast-reacting hydraulic actuator was demonstrated. While the high costs, significant power absorption, bulky and unreliable hydraulic systems, and uncertain management of the safety issues made it difficult to have significant impact on the mass-market of vehicles. Since the 1990s, a new trend emerged: it became increasingly clear that the best compromise of cost (component cost, weight, electronics and sensors, also power consumption, etc.) and performance (comfort, handling, and safety) was to be found in another technology of electronically controllable suspensions: the semi-active suspen- sion or the continuous/variable damping suspension. For example, car manufacturers as Audi, VW, Mercedes, Ford, Volvo, and GM cooperated together with controlled damper suppliers like Sachs, Tenneco, Bilstein, and Monroe to design, develop, and manufacture different types of the semi-active suspension systems, which are introduced in the mass-market of cars. Faculty of Engineering-Benha, Benha University, Egypt Automotive & Tractors Engineering Department, Faculty of Engineering, Minia University, Egypt Corresponding author: MMS Kaldas, Automotive & Tractors Engineering Department, Faculty of Engineering, Minia University, Egypt. Email: m.kaldas@web.de Creative Commons CC BY: This article is distributed under the terms of the Creative Commons Attribution 4.0 License (http://www. creativecommons.org/licenses/by/4.0/) which permits any use, reproduction and distribution of the work without further permission provided the original work is attributed as specified on the SAGE and Open Access pages (https://us.sagepub.com/en-us/nam/open-access-at-sage). 1006 Journal of Low Frequency Noise, Vibration and Active Control 40(2) 2 Journal of Low Frequency Noise, Vibration and Active Control 0(0) Semi-active suspension system The main problem in the design of the vehicle suspension is that designers have to deal with conflicting demand, 1,2 namely good comfort of the vehicle’s occupants, low suspension working space, and good handling properties. In order to provide good ride quality, the vehicle body should be isolated from the road disturbances. The ride quality in general can be quantified by the vertical acceleration of the passenger locations. The presence of a well- designed suspension provides isolation by reducing the vibratory forces transmitted from the axle to the vehicle body; this in turns reduces vehicle body acceleration. On the other hand, to keep a good road-holding perfor- mance that can be characterized in terms of vehicle’s cornering, braking, and traction abilities, the normal tyre loads should be minimized. This is because the lateral and longitudinal forces generated by tyres depend directly on the normal tyre loads. Since the tyres roughly behave like springs in response to vertical forces, variations in normal tyre loads can be directly related to vertical tyre deflections. Furthermore, the optimal ride comfort and road-holding performance should be obtained considering the allowable suspension working space of the vehicle. The conflicting demand in the design of vehicle suspension is shown in Figure 2. The x-axis represents the root mean square (RMS) value of the dynamic tyre load with respect to the static wheel load. The y-axis represents the Figure 1. Number of the car manufactures for controlled suspension. Figure 2. The ride comfort and road holding conflict. Soliman and Kaldas 1007 Soliman and Kaldas 3 RMS value of the weighted body vertical acceleration. The “ride comfort value” consists of a weighted sum made up of the accelerations of the driver’s seat, hands, and feet. The ride comfort value reflects the human ability to perceive the strength of various oscillatory excitations. As a result of the hooked shapes of the constant damping curves, a continued increase in the overall suspension stiffness leads not only to a decrease in the passenger comfort, but also (beyond a certain point) an increase in the dynamic tyre load. In the same manner, a continued increase in the overall suspension damping eventually leads to a decrease in the occupant comfort. The selection of the optimal damping requires a compromise between harder “safer dampers” and softer “more comfortable” dampers. This selection is made more difficult by the fact that the optimal damper settings are dependent on the road surface as well as driving maneuver dictated by the driver. Therefore, controlled dampers are developed to resolve this conflict. In the visual representation of the conflict shown in Figure 2, a conventional spring-damper combination represents the intersection between a constant damping curve and a constant spring stiffness curve. The controlled damper is capable of representing any point on a constant spring stiffness curve and 1,2 can be adjusted to optimize either safety or comfort. For driving situations where comfort is especially relevant, the amplitude of the vehicle body accelerations can be reduced in the region between resonant frequencies by reducing the damping. In critical driving situations, the dynamic wheel loads can be reduced in regions near the natural frequencies by increasing the damping. Nowadays, the semi-active suspension systems are used to offer solutions for overcoming the ride performance limitations of passive suspension systems on the one hand and on the other hand for reducing the high system cost and energy consumption of fully active suspension systems. The hardware and the control equipment of an active suspension system are also required for a semi-active system. However the actuator, hydraulic pumps, and accumulators are replaced with a controllable damper. The semi-active systems can clearly offer cheaper suspen- sion control solutions compared to the active systems. The main components of the semi-active suspension system are the semi-active damper, acceleration sensors, ride height sensors, and controller. In this paper, the focus will be on the semi-active dampers and the control algorithms. Semi-active dampers Semi-active dampers are hydro-mechanical control devices with the capability to vary the amount of energy they dissipate using a small source of power. For example, the hydraulic dampers are designed in such a way that controllable force is proportional to the velocity. Such systems extend the possible range of damping character- istics obtainable from a regular (passive) damper. For the purpose of semi-active damping control, various energy-dissipating devices as following have been used to obtain the desired damping. • Servo/Solenoid valve dampers • Magnetorheological (MR)/electrorheological (ER) dampers • Electromagnetic dampers Some of these technologies have already found their way to the vehicles market. Currently available semi-active dampers use the solenoid valves or MR fluids, given their technological advantages and cost efficiency. Solenoid/Servo valve dampers. In general, the controllable valve requires certain characteristics in order to be useful in semi-active damper applications. The valve must be able to handle at maximum velocity a flow rate corre- sponding to the volume of displaced fluid in the damper. The maximum operating pressure of the valve must also correspond to the maximum pressure found in a passive damper, because of the suspension loading. In addition, the controllable valve must have a fast-enough response to cause variations in damping characteristics during operation. A damper consisting of a hydraulic actuator in conjunction with an electro-hydraulic servo-valve 7 8 modulating the controlling orifice area was used by Krasnicki and Patten et al. Practical applications of the servo-valve-skyhook dampers, namely extreme isolation for delicate manufacturing operations against seismic 9 10 input and the automotive suspensions were discussed by Karnopp and Soliman et al. Solenoid valves provide an alternative to servo-valves. Solenoid valves do not have as fast or accurate responses as servo-valves; however, a servo-valve is much more expensive than a solenoid valve. A solenoid valve is much simpler in design than a servo-valve, and it could possibly be manufactured in-house, bringing the added benefit of being able to optimize the valve design for the particular application including force, stroke, 11,12 and general packaging. Solenoid valves have also been used in two-or three-state dampers that can change characteristics between hard damping and soft damping by opening or closing a bypass valve. These dampers have 1008 Journal of Low Frequency Noise, Vibration and Active Control 40(2) 4 Journal of Low Frequency Noise, Vibration and Active Control 0(0) Figure 3. The DampTronic-Select damper from Bilstein. resulted in improvements in ride quality when appropriate control schemes are used. Although these dampers do not have the capability of being continuously variable, continuously variable solenoid valve dampers are also available. Solenoid valves of this type have been tested, with promising results by Kitching et al. In the automobile market, many suppliers of controlled dampers developed different types of the solenoid valve dampers. For example, ThyssenKrupp Bilstein developed the DampTronic-Select damper shown in Figure 3, a two-stage selectable damper closing the gap between conventional, passive shock absorbers and elaborate elec- tronic continuously variable dampers. The DampTronic-Select valve, which is installed inside the shock absorb- er piston provides two generally independently adaptable damping-force characteristics within a very compact installation space. The aim of the DampTronic-Select damper is to enable the driver to decide whether he would like a standard” Soft” or a sport “Hard” suspension system, and can simply alternate between the two suspension configurations by the touch of a button on the dashboard. Similar to the DampTronic-Select dampers, Tenneco Automotive’s Monroe developed a Dual Mode Damper, which provides an enhanced ride experience at a minimal system cost for the small and midsized vehicle segment. As shown in Figure 4, typically one characteristic is a higher level of comfort than a passive damper. On the other hand, when the driver wishes to get the most out of his vehicle, he can select the sport characteristic. The dampers are then set to a firmer damping level to deliver the needed road holding. This is achieved via two valve setting, the first one is NC “Normally Closed”, which offers firm damping without energizing the valve “0.0 A”, soft damping is achieved at approximately “0.5 A” holding current, while the second one is NO “Normally Open” offers soft damping without energizing the valve “0.0 A”, firm damping is achieved at approximately “0.5 A” hold- ing current. In order to provide different choices for the costumers, the adaptive damping system (ADS) has been developed by ThyssenKrupp Bilstein. The ADS is a four-stage damping system designed by combining a soft and a hard characteristic for the extension and compression stages in the compression characteristics. The four different damping characteristics are obtained through two pistons in an external adjustment module connected individ- ually in parallel to the conventional working piston. The four damping characteristics in extension or compres- sion are soft/soft; soft/hard; hard/soft; hard/hard. As an actual mass-market semi-active damper, the DampTronic-Sky damper shown in Figure 5 is developed by ThyssenKrupp Bilstein. The DampTronic-Sky damper is a further milestone in resolving the conflict between driving comfort on the one hand and the driving safety and agility of the car on the other hand. In this damper, two continuously variable valves adjust the damping force in the damper: one valve controls the extension phase i.e. the force that ensues when the wheel rebounds, and the other the compression phase. As a competition between the controlled damper manufacturers, Tenneco Automotives Monroe developed a semi-active damper” Continuously Controlled Damper” shown in Figure 6, is a semi-active suspension damper that continuously adjusts damping levels according to road conditions and vehicle dynamics, such as speed, turning and cornering, delivering comfort without sacrificing the safety of sure handling. The semi-active damper adjusts damping levels through a bypass solenoid valve attached to the outer tube of the damper. The softer damping setting is achieved at approximately 0.3 A, while the hard damping setting is obtained at approximately 1.6 A. In the case of the failsafe, the semi-active damper provides approximately the characteristics of the passive damper without energizing the valve (0.0 A). Soliman and Kaldas 1009 Soliman and Kaldas 5 Figure 4. The damping characteristics of the dual-mode damper from Tenneco. Figure 5. The DampTronic-Sky damper from Bilstein. Figure 6. (a) The continuously controlled damper from Tenneco. (b) Schematic drawing for the continuously controlled damper characteristics. MR and ER dampers. MR or ER damper is hydraulic damper consist of a hydraulic cylinder containing micron- sized polarizable particles in a MR or ER fluid (usually oil). MR and ER fluids are non-Newtonian fluids that change its properties in the presence of a magnetic or electric field. Micron-size iron particles suspended in a carrier fluid (water, petroleum-based oil, or silicon-based oil) align in chain-like structures along the flux lines of a magnetic or electrical field, changing the rheological properties of the fluid. Both the MR and ER materials have the ability to change from free flowing viscous fluids to a semi-solid state in a matter of milliseconds when exposed to a magnetic or an electric field. MR and ER dampers are mechanically reliable, since they do not contain any moving parts. Figure 7 shows the working principal for MR dampers. 1010 Journal of Low Frequency Noise, Vibration and Active Control 40(2) 6 Journal of Low Frequency Noise, Vibration and Active Control 0(0) Figure 7. The MR damper working principal. There are numerous published references on vibration control using MR and ER dampers. For example, Wu and Griffin used an ER damper to reduce the severity of shocks caused by suspension seat end-stop impacts or high magnitude vibration. The ER damper was used to realize the required two-state damping. The vibration isolation performance of a MR damper under the control of the on–off skyhook control strategy is studied by Jeon et al. The damping constant and response time of the damper were measured. The time delay in the response of the MR damper was measured and incorporated into the control under harmonic disturbances. Experimental results showed that on–off skyhook control strategy, which includes the damper time delay, per- forms less effectively than the one without the consideration of time delay. Since the 2000s, Delphi Automotive Systems has developed the MagneRide system shown in Figure 8, which is a semi-active suspension system based on MR dampers. The MagneRide system employs a unique “MR” fluid in its monotube dampers. In the beginning of 2003, the MagneRide has appeared for some luxury vehicles in the European and American market. Initially, it was produced for the Cadillac Seville STS and Chevrolet Corvette, and the latest iteration of the system is fitted to the Ferrari 599 GTB Fiorano, Audi TT, and Audi R8. Recently, MagneRide suspension is selected for the Range Rover Evoque. The MagneRide suspension for Range Rover is supplied by BWI Group’s, which developed the third generation of the MagneRide. Figure 8 shows the MagneRide damper and a comparison between the damping characteristics of the MagneRide damper with a typical variable valve damper and a passive damper. Electromagnetic dampers. Electromagnetic dampers use the interaction between the movement of a coil and the 23,24 magnetic field of a permanent magnet or electromagnet to provide a damping effect. When an electromagnetic damper coil is shorted or connected to an external resistor, the device becomes a linear mechanical damper. The Soliman and Kaldas 1011 Soliman and Kaldas 7 Figure 8. The MagneRide damper and its damping characteristics from Delphi. damping level can be varied by changing the external resistance or the strength of the magnetic field. If the external resistance is varied, the damping coefficient is varied. In the open circuit state the damping coefficient vanishes, while it reaches a maximum value if the coil is shorted. Since effective resistance can be rapidly varied electronically, an electrical actuator can function as a semi-active damper in vehicle or vibration isolation sus- pension systems. For example, in Karnopp the possibility of using permanent magnet linear motors as variable mechanical dampers for vehicle suspensions is studied. Two basic electromagnetic designs are analyzed, namely the moving coil and the moving magnet approach. The electromagnetic damper use consists of a tubular coil of wire situated within a radially oriented constant magnetic field produced by a permanent magnet. The damping coefficient is varied by changing the external resistance. The high cost of the electromagnetic dampers made it difficult to appear in the vehicle market, but the compact size and weight motivate the shock absorbers suppliers and the automobile companies to consider it in their research studies. Control strategies 25,26 Semi-active suspensions were firstly introduced in literature as an alternative to the high cost, highly compli- cated, and power density active systems. A vast amount of work on controlled suspension systems is presented in the technical and scientific literature. One of the reviews of the state-of-the-art of controlled suspensions was carried out by Sharp and Crolla in the semi-active suspension and other active suspensions. The most attractive feature of that work was that control strategies were based only upon the measurement of the relative displace- ment and velocity of the suspension system. Semi-active suspension systems provide not only vehicle ride comfort and control, but also better road holding. These systems are able to tune the amount of damping in response to measured vehicle-ride and handling indicators. Research and development projects in semi-active suspension have been carried out in order to 28–32 improve the stability and ride handling performance of the vehicle. However, these approaches need to solve some practical applications. The neural network-based robust control system gives better ride comfort compared to the standard proportional–integral–derivative (PID) controller. Preview control for a semi- active suspension system was investigated by Soliman and Crolla. Furthermore, the influence of the preview control of the active suspension on the vehicle ride and braking performance was investigated by Kaldas and Soliman. Control approaches such as skyhook control, adaptive control, robust control, fuzzy logic, neural network methods, linear quadratic regulator (LQR), linear optimal control, and preview control have been used in the field of semi-active suspensions. Practical semi-active suspensions have only recently become possible with the advent of powerful but relatively inexpensive signal processors. Classical control strategies for semi-active suspension system. Several semi-active control strategies have been proposed and investigated since Crosby and Karnopp developed the skyhook control strategy. The common goal of these initiatives is to achieve a higher level of vibration isolation or to find practical and easy implementation methods, or both. In this section, the most classical methods are briefly introduced. 1012 Journal of Low Frequency Noise, Vibration and Active Control 40(2) 8 Journal of Low Frequency Noise, Vibration and Active Control 0(0) Figure 9. Skyhook and groundhook models. Skyhook control. The skyhook strategy is used to minimize disturbance effects that can cause the vehicle body to oscillate. Depending on the direction of motion of the vehicle body, the system compression and rebound damping are either increased or reduced. The main goal of the skyhook strategy is to decouple the vehicle body from disturbances on the road surface. This strategy attempts to simulate the responses that would occur if a vibration damper were mounted between the vehicle body and a traveling hook attached to a fixed inertial system (the sky) rather than between the body and the wheels. Historically, the skyhook control strategy was developed by DC Karnopp. Krosby and Karnopp and Karnopp et al. reviewed the historical development of theoretical concepts necessary for the design of isolation systems and indicates how control theory began to influence the vibration isolation in the last half of this period. He focused on a relatively simple idea namely the “skyhook” damper. This idea came through purely theoretical studies but is now used in combination with other concepts in production suspension systems. Since it is not practically possible to develop the ideal skyhook system presented in Figure 9(a), it is realized by using the damper force C Z that allows reproducing the skyhook behavior for the sprung mass. This desired behavior is then sky b modeled as € _ M Z ¼� K ðÞ Z � Z � C Z (1) b b s b w sky b � � _ _ _ C Z ¼ C Z � Z (2) sky b s b w � � C ¼ C (3) s sky _ _ Z � Z b w M Z ¼� K ðÞ Z � Z þ K ðÞ Z � Z (4) w w t w o s b w where C is the damping coefficient of the skyhook damper. sky In order to realize the skyhook damper to semi-active damper, an on/off strategy that switches between the low damping and the skyhook “high” damping is used. This control law consists in changing the damping coefficient � � _ _ _ C of the semi-active damper according to the chassis velocity Z and the suspension deflection velocity Z � Z s b b w by using the following logical rule �� _ _ _ C if Z Z � Z � 0 min b b w �� C ¼ (5) _ _ _ C if Z Z � Z > 0 sky b b w where C and C are the minimal and skyhook “maximal” damping coefficients achievable by the considered min sky semi-active damper, respectively (and usually C ¼ C ). Basically, this control law consists in switching sky max Soliman and Kaldas 1013 Soliman and Kaldas 9 Figure 10. Damping settings of the skyhook control. controller which deactivates the controlled damper when the body velocity and relative velocity between vehicle body and wheel (damper velocity) have opposite signs as shown in Figure 10. Many studies have been carried out on the skyhook control strategy since it represents a simple way to achieve 37 38 a good comfort requirement, as for instance the no-jerk version. Some extended versions of the skyhook control have been also developed, such as the adaptive-skyhook in Song et al. to MR dampers or the gain- scheduled-skyhook in Hong et al. This control strategy presents the advantage of being simple, but it requires at least two sensors, filters, and state estimation algorithm. Yi and Song presented an adaptive skyhook control for semi-active suspension, which consists of a skyhook control law with road adaptive gains and a road detection algorithm. The new skyhook control law is a combi- nation of the sprung mass and unsprung mass velocity feedbacks with time varying gains, and the unsprung mass velocity feedback is seen to be an important factor in increasing damping at the wheel hop frequency. The road detection algorithm is designed to adapt the optimal gains on the basis of the frequency content of the road inputs. The results showed that the proposed control law provides adequate damping for the wheel hop frequency and improved performance compared with that of skyhook control law. Consequently, the proposed control law obtains a performance enhancement in ride comfort and road holding. Groundhook control. The groundhook approach, which is in some sense the dual of the skyhook, is described below. It consists of increasing the damping, then reducing the deflection, to reduce the road–tyre forces, as illustrated in literature. As in the skyhook, the ideal groundhook cannot be achieved and needs to be approx- imated. The groundhook control method is similar to the on–off skyhook control method, except that control is based on the unsprung mass damping control, as illustrated in Figure 9(b). The on–off groundhook semi-active policy emulates the ideal tyre displacement control configuration of a passive damper “hooked” between the tyre and the “ground”. The semi-active on-off control law can be considered as the following �� _ _ _ C if � Z Z � Z � 0 min w b w �� C ¼ (6) in _ _ _ C if � Z Z � Z > 0 ground w b w where C and C are the minimal and groundhook “maximal” damping coefficients achievable by the min ground considered semi-active damper, respectively (and usually C ¼ C ). Some extended cases, such as displace- ground max 43,44 ment and velocity-based groundhook control policies, are presented and tested experimentally in literature. Vala´ �sek et al. applied the groundhook concept for semi-active suspension for heavy trucks with the ultimate objective to minimize dynamic tyre–road force and thus the road damages. The basic groundhook concept is extended to the several variants, which enables a decrease in road damage whilst maintaining driver comfort over a broad range of road unevenness. The influence and interaction of the damping rate limits and time constants of 1014 Journal of Low Frequency Noise, Vibration and Active Control 40(2) 10 Journal of Low Frequency Noise, Vibration and Active Control 0(0) variable shock absorbers are also taken into account. It was also concluded that the concept could be used to increase the truck payload. Hybrid skyhook–groundhook control. An alternative semi-active control policy known as hybrid control has been shown to take advantage of the benefits of both skyhook and groundhook controls. With hybrid control, the user has the ability to specify how closely the controller emulates skyhook or groundhook. In other words, hybrid control can divert the damping energy to the bodies in a manner that eliminates the compromise that is inherent in passive dampers. This logic is aimed to reducing both body acceleration and dynamic tyre force by combining the skyhook and groundhook �� C ¼ lC þ ðÞ 1 � l C (7) hybrid sky ground With the following four-state switching condition � � _ _ _ C if Z Z � Z � 0 > min b b w � � _ _ _ C if Z Z � Z > 0 hybrid b b w C ¼ �� (8) in > _ _ _ > C if � Z Z � Z � 0 min w b w �� _ _ _ C if � Z Z � Z > 0 hybrid w b w The variable l is the relative ratio between the skyhook and groundhook control. If l is set to 1, the hybrid control policy is switched to pure skyhook and, if l is set to 0, the hybrid control is switched to pure groundhook. Ahmadian studied the performance of five different skyhook control methods experimentally, using a quarter-car rig. The control methods that are analyzed include skyhook control, groundhook control, hybrid control, displacement skyhook, and relative displacement skyhook. The results indicated that hybrid control, however, comes close to providing the best compromise between different dynamic demands on a primary sus- pension, also hybrid control can offer benefits to both the sprung and unsprung mass with control gain settings that provide equal contributions from skyhook control and groundhook control. Acceleration driven damper control. The acceleration-driven damper (ADD) control is a semi-active control law 48,49 described in Savaresi et al., which consists of changing the damping factor C as in � � € _ _ C if Z Z � Z � 0 min b b w � � C ¼ (9) in € _ _ C if Z Z � Z > 0 max b b w where C and C are the minimal and maximal damping factors achievable by the controlled damper. This min max strategy is shown to be optimal in the sense that it minimizes the vertical body acceleration when no road information is available (therefore, this control law is a comfort-oriented one). This control strategy presents the advantage of being simple, but it requires at least two sensors as the skyhook and groundhook control law, in addition to the state estimation algorithm and filters. It should be noted that the control law is very similar to the skyhook algorithm, with the difference that the switching law depends on body acceleration Z , instead of body velocity (which is easier to measure on real vehicles). It is worth noting that the ADD design is well adapted to comfort improvement but not to road-holding. Moreover, the “switching dynamic” may influence the closed- loop performances. Modern and intelligent controllers for semi-active suspension system Optimal control. The linear quadratic (state feedback) control is one of the first methodologies that have been applied to suspension control, mainly in the active suspension systems. Some applications of semi-active sus- 50–52 53 pensions have been proposed using different control strategies based on optimal control. Recently, Soliman compared the performance of three control laws for a semi-active suspension system, based on simulated results obtained from a quarter car model. The semi-active damper is modeled as a continuously variable device with maximum and minimum limits and a time delay between the demand and actual forces. The three control laws are Soliman and Kaldas 1015 Soliman and Kaldas 11 based on (a) full state feedback, (b) limited state feedback, and (c) skyhook damping. The results showed that semi-active suspension system with full state feedback control force gives a better improvement in terms of ride comfort compared to a semi-active with limited state feedback and skyhook control force. The mean power dissipation in the semi-active with the three control laws and passive suspension systems relative to the rolling resistance power losses are ranged between 14% and 19%. An adaptation algorithm to maintain optimal performance over the wide range of input conditions typically encountered by a vehicle is proposed in literature. The adaptive control loop is based on a gain scheduling approach and two strategies are examined both theoretically and experimentally using a quarter vehicle test rig. For the first strategy, the gains are selected on the basis of RMS wheel acceleration measurements whereas in the second approach the RMS value of suspension working space is used. A composite input is used consisting of sections of a road input disturbance of differing levels of magnitude in order to test the control systems abilities to identify and adapt efficiently as the severity of the road input changes. Both simulated and measured results are used to investigate design decisions regarding the pre-calculated gains, averaging of the monitored variable on which the adaptation is based and the detailed way in which the gains are altered when a change is required. The obtained results showed that, vehicle ride comfort performance is improved with both adaptation control strategies in comparison with the same system without the adaptation algorithm. An optimal self-tuning control algorithm for active and semi-active suspension systems incorporating a weight- ing controller, state observer, and parameter estimator, based on LQG and LQR theories is suggested by Yu and 55 56 Crolla and Soliman and Crolla, respectively. The controller is based on the updated estimates of vehicle parameters and states, and the adapted weighting parameters. The LQG/LQR controllers provide the optimal set of gains over different operating conditions. The feasibility and effectiveness of the self-tuning system was investigated and proved by simulation studies. The obtained results showed that the self-tuning control algorithm improved the vehicle ride performance in comparison with the LQR/LQG control without the self-tuning algorithm. However, the performance improvement depends mainly on the accuracy of the state observer and parameter estimator. Since the semi-active suspensions are feedback-controlled systems in which the actuator is limited to providing energy dissipation, the power required to perform modulation of the continuously variable damper is insignificant in comparison with the power needed for other active suspension systems. However, the performance benefits available with a semi-active suspension system are, not surprisingly, less than those for an active system. Therefore, some researchers used a preview strategy to improve the behavior of semi-active systems. Two forms of preview control are distinguishable; the look-ahead preview and wheel base preview. As an application of LQR including a look-ahead preview algorithm for semi-active suspension system is suggested by literature. The control strategy has been applied to a quarter vehicle model. The performance of a semi-active suspension system with preview control is compared with an active system with and without preview control, and with a passive suspension system. The simulation results presented that the performance of a semi-active suspension can be improved significantly if preview information about the road input, ahead of the wheel is available. One of the most important findings in the study is that the performance of the semi-active system with preview is as good as the performance of the active system without preview. Model predictive control. As a result of the development and improvements in the optimization algorithms, the model predictive control (MPC) is being increasingly used in automotive applications. An MPC semi-active suspension controller, which provides better performance compared to those of the skyhook and LQR approaches is introduced by Giua et al. The obtained results are interesting and deserve a closer view, but the main draw- backs are the requirement of an online “fast” optimization procedure in the control loop, optimal control, full state measurement, and a good knowledge of the model parameters. The online computation difficulties related to the predictive control law for a semi-active suspension system are overcome in Canale et al. by means of a “fast” implementation of the MPC algorithm (FMPC). The estimation of the system state variable needed for control law computation is provided by a suitable “robust” observer, whose accuracy is not affected by variation of the system parameters (i.e. masses, damping and stiffness coef- ficients, etc.). The computed control law aims to optimize the suspension performance, by minimizing a quadratic cost function while ensuring that the magnitude of the forces generated by the control law satisfies the physical constraints of passive damping. A performance comparison with the well-established semi-active skyhook strategy is presented. The achievable performance improvements of the proposed design procedure over skyhook control law are showed by means of simulation results. 1016 Journal of Low Frequency Noise, Vibration and Active Control 40(2) 12 Journal of Low Frequency Noise, Vibration and Active Control 0(0) A MPC hybrid control tool to derive an optimal control law for semi-active suspension system is designed by Giorgetti et al. A quarter-car semi-active suspension model has been modeled as a linear system under state- dependent linear and nonlinear constraints. The constrained quarter-car semi-active suspension is modeled as a switching affine system, where the switching is determined by the activation of passivity constraints, force satu- ration, and maximum power dissipation limits. The performance of different finite-horizon hybrid MPC controllers is tested in simulation using mixed-integer quadratic programming. A comparison between different semi-active control laws and the proposed MPC hybrid control law is performed as a way for obtaining control laws with different degrees of optimality, depending on the chosen control horizon. The obtained results showed that when the horizon is equal to one the hybrid MPC law corresponds to the “clipped-optimal” control law, obtained by computing the active LQR control law and clipping it to enforce the given constraints. Furthermore, with increased control horizon, the results showed that significant deviation of optimal from the “clipped optimal” can occur. H1 clipped control. Many studies deals with on the application of the H control approach (sometimes in the framework of LPV systems) for the suspension system. However, most of the studies applied for active suspen- 60 61 62 sions, as in Zin et al., Karnopp, and Margolis suggested a semi-active controller by passing the optimal active controller through a limiter (saturation). In the control step, the force applied by the semi-active damper is then chosen to be as close to the force required by the controller for a given suspension deflection speed and for the possible range of forces the damper can deliver. This simple strategy has been applied in many cases, whatever 63 64 65 65 the control method is: optimal, robust, or state-feedback. For example, in Du et al. a static output feed- back (clipped) H controller is designed for a suspension equipped with an MR damper. This strategy uses two measured states: the suspension deflection and the velocity of the sprung mass. The results showed that the proposed strategy allows to improve comfort and road-holding as well, compared to a passive suspension. Rossi and Lucente studied the control of semi-active suspensions using H state-space optimization techni- ques. Two standard models of the system, quarter car and half-car model with three H controllers are developed in the fully active case. Then, they adapted to the nonlinear real system using a sort of clipped control. The first controller is focused on ride comfort optimization, the second on both comfort and handling improvement, the third is a global controller derived from half car model. Performance indices related to ride comfort and drive safety are introduced to evaluate proposed controllers. The obtained results indicated that controllers had better ride comfort than a passive setting. Drive safety performances can be improved including variables related to unsprung mass dynamics in the error signals vector. Simulations were performed to show that semi-active controlled suspensions succeed in achieving higher levels of ride comfort and drive safety with respect to a passive setting. Fuzzy-logic control. Although Zadeh developed fuzzy-logic in 1965, it has been applied to vehicle suspension control only in recent years. The fuzzy-logic is one of the most intelligent control methods, offers several unique features that make it a particularly good choice for many control problems. It can be used to control nonlinear systems that are difficult or impossible to model mathematically. In addition to this, it does not require precise, noise-free inputs, and can be programmed to control the system safely if a feedback sensor fails. The control output is a smooth function despite a wide range of input variations. Therefore, any sensor data that provides 67–69 some indication of the system actions and reactions is sufficient. Fuzzy-logic is one of the model-free and robust intelligent controllers introduced to solve the problems of uncertainty effects. The fuzzy logic controller (FLC) is an adequate methodology for designing robust controllers that are capable of delivering a satisfactory performance in the face of uncertainty and imprecision. As a result, the FLC has become a popular approach to nonlinear and uncertain system control in recent years. There are different ways to design FLCs for vehicle suspension control, with the most common method to design FLCs being by eliciting the fuzzy rules and 68,69 its membership functions based on experts’ knowledge or experience. The semi-active suspension control problem is a well-suited problem for taking advantage of the fuzzy logic features: it is highly nonlinear, it is difficult to simulate adequately, the control improvements are quite subjective, and controllers should deal with a compromise between opposite properties (ride safety and passenger comfort). Among the first application attempts, Al-Holou et al. adopted fuzzy logic to control the vibration of semi-active suspension systems. Although results obtained by fuzzy logic have shown improvement over passive suspension, not much work has been done to compare the results from FLCs with those from other conventional methods introduced above. A semi-active vehicle suspension system using an adjustable shock absorber applied to a quarter vehicle model is presented by Wu et al. Two control techniques are developed for assessing both ride Soliman and Kaldas 1017 Soliman and Kaldas 13 comfort and road handling. A part from the PID controller, a controller using fuzzy logic is developed and its performance is experimentally tested. The simulation results showed that the semi-active control provides lower RMS values and reduced settling time compared to the passive suspension system. It was demonstrated that the proposed FLC achieves much better performance compared to the semi-active suspension with PID controller and the passive suspension under various road conditions. Nicola´ s et al. described the practical application of fuzzy logic to the design of semi-active suspension systems control strategies. The intelligent suspension systems are based on a continuously adjustable shock absorber. Two different fuzzy approaches are described. The first strategy is based on the actions of the driver and the second one is based on the vehicle dynamics. Both strategies have been tested theoretically in simulation models and exper- imentally on-road. The system was tested in a number of vehicles and the results are compared with those obtained with other conventional control strategies based on selection amongst several discrete damping settings. The main advantage of the proposed fuzzy strategies is the small number of required sensors, achieving similar performances than other control algorithms with less cost. The obtained results showed that the fuzzy controller based on the driver actions shows excellent performances with a low cost sensor system. The behavior of the controller is close to the theoretical optimum for a semi-active suspension system. An adaptive FLC for an active suspension system is developed by Huang and Chao. The inputs of the FLC are the vertical position error of the vehicle sprung mass and the rate of change of this error, while the output is the damper control voltage. The antecedents membership functions consists of 11 equal triangular type functions. The voltage increment membership function is a set of 15 equal triangular type functions. The self-tuning property is implemented by adjusted scaling factors for controller inputs and output. A total of 121 fuzzy rules are employed to suppress the sprung mass vibration amplitude due to road inputs. In order to evaluate the fuzzy control system, a two-degrees-of-freedom (DOF) quarter-vehicle suspension model was used. The dynamic response of the semi-active suspension system was presented for vehicle ride performance on a rough concave- convex road with 25 mm obstacles but not compared with the known control strategies. It was noted that the control signal was very smooth and easy to employ in the practical vehicle. The adjusted scaling factors were chosen by experiments and many simulations, which limit the flexible and adaptive abilities of the adaptive FLC. On the other hand, Kaldas et al. developed a semi-active suspension system controller using adaptive fuzzy logic control theories together with a Kalman filter for the state estimation. A quarter vehicle ride dynamics model is constructed and validated through laboratory tests performed on a hydraulic four-poster shaker. A Kalman filter algorithm is constructed for bounce velocity estimation, and its accuracy is verified through measurements performed with external displacement sensors. The benefit of using adaptive control with fuzzy logic to maintain the optimum performance over a wide range of road inputs is enhanced by the accuracy of the Kalman filter in estimating the controller inputs. A gradient-based optimization algorithm is applied for improving the FLC parameters. The vehicle model is simulated with the developed semi-active suspension controller on different road profiles. A comparison is performed between the adaptive fuzzy semi-active controller, the optimum LQR semi-active controller, and the optimum passive suspension system in terms of ride comfort and road holding. The results showed that the proposed semi-active suspension system controller provides significant improvements in both ride comfort and road holding of the vehicle on different road profiles. Application of the FLC to the MR damper was presented by Kurczyk and Pawel. In this study, a semi-active control of the suspension of an all-terrain vehicle is developed. A 7-DOF suspension model is developed and a fuzzy approach for controller synthesis is proposed. The fuzzy system output is the damping coefficient. In contrast to many other control algorithms, the presented fuzzy algorithm does not require inverse modeling of the MR damper. Instead, some scaling parameters are set that can be chosen experimentally, or through a bio- inspired strategy. The FLC is compared with the skyhook control in terms of road holding and driving comfort indicators. The simulation results showed that the FLC used for vehicle suspension vibration damping can per- form, in some cases, as well as the skyhook algorithm, but without necessity of using an inverse damper model. The use of the fuzzy algorithm is more computationally complex than the skyhook algorithm. However, consid- ering cutting edge technology it can be successfully implemented, although with a higher cost controller. There are FLC operations that can be performed once in order to reduce the system complexity. Moreover, hardware computing can be used, e.g. the Field Programmable Gate Array technology. In Nabaglo, the FLC for MR dampers is optimized by the genetic algorithms (GAs) and the optimization is performed theoretically using ADAMS/Car and MATLAB simulations. The objective of the control system is to reduce the vertical acceleration and the counteraction against moments of forces acting around the longitudinal and transverse axes of the vehicle in varied ride conditions. The obtained results showed that the FLC is effective, reliable, safe, and superior in ride comfort improvement as well as vehicle safety. 1018 Journal of Low Frequency Noise, Vibration and Active Control 40(2) 14 Journal of Low Frequency Noise, Vibration and Active Control 0(0) In a more advanced optimization method, Kaldas et al. developed an advanced optimization process to obtain the optimum fuzzy logic membership functions and the optimum rule-base of the FLC applied in the semi- active suspension. Discrete optimization has been performed with a GA to find the global optima of the cost function which considers the ride comfort and road-holding performance of the full vehicle. The proposed rule- optimized fuzzy logic semi-active controller is compared to the optimum linear quadratic regulator (LQR) semi- active controller and the optimum passive suspension system in terms of ride comfort and road holding. The results showed that the proposed semi-active suspension system controller provides significant improvements in both ride comfort and road-holding performance of the vehicle. Furthermore, in C¸ alıs¸ kan et al. the rule- optimized fuzzy logic semi-active controller is enhanced through preview capability. The preview-enhanced FLC uses the feedforward road input information and the feedback vehicle state information as the controller inputs. An 11-DOF full vehicle model, which is validated through laboratory tests performed on a hydraulic four- poster shaker, is used for the controller synthesis. The preview distance is also considered as a design parameter during the optimization process. The performance of the preview-enhanced rule-optimized FLC is evaluated by using a measured stochastic road profile as vehicle model input. The results demonstrate the potential of the preview-enhanced controller in improving all aspects of system performance compared to the rule-optimized FLC without preview. With the increase of the customer demand for adjustable chassis control features, which enable adaption of the vehicle comfort and driving characteristics to the customer requirements, Kaldas et al. improved the rule- optimized FLC for semi-active suspension systems, which can continuously adjust itself not only according to the road conditions, but also according to the driver requirements. The proposed controller offers three different control modes (comfort, normal, and sport), which can be switched by the driver during driving. The comfort mode minimizes the accelerations imposed on the driver and passengers by using a softer damping. On the other hand, the increased damping in the sport mode provides better road-holding capability, which is critical for sporty handling. The normal mode is adjusted to provide an overall balance between the vehicle ride comfort and road holding. A comparison between the three control modes in terms of ride comfort and road holding is performed. The results show that the proposed control modes provide three different vehicle characteristics to the driver. In addition to this, all three control modes are superior to the optimal passive suspension in terms of both ride comfort and road holding. Semi-active suspension system in vehicles mass-market In recent years, the automobile companies have commercially developed several vehicle models with semi-active suspension systems. The control algorithms have been continuously developed and new control features have been added, which make the customers satisfied with these vehicles. For example, Tenneco in conjunction with Ohlins Racing developed the continuously controlled electronic suspension “CES”. The CES is a semi-active suspension system that continuously adjusts damping levels according to road conditions and vehicle dynamics, such as speed, turning and cornering, delivering comfort without sacrificing safety and handling. The CES controller is designed to exploit the full potential of the electro-hydraulic valving system by processing input data sent by the acceleration sensors placed on the vehicle body and the suspension position sensors, installed between the vehicle body and the lower control arms of the suspension system. The position and acceleration sensors monitoring the movements of the wheels and body, and an electronic control unit realizing the control functionality and man- aging the necessary communication between the shock absorbers, the sensors and other control systems in the vehicle. A typical CES system architecture is shown in Figure 11. The CES utilizes control software that processes the sensor information regarding steering wheel angle, vehicle speed, brake pressure, and other chassis control information and sends signals that independently adjust the damping level of each shock absorber valve—up to 100 times a second. CES dampers allow a large separation between maximum and minimum damping levels and adjust instantaneously to ensure the optimum in ride comfort and road holding. The CES is in production in 37 different vehicle models. For example, CES has been supplied to Volvo for the S60R and V70R and General Motors Corp.’s for Cadillac Seville. Currently it has been supplied to the latest generation of Renault Espace. In 2004, continuous damping control (CDC) was developed by ZF Sachs AG. The CDC system uses six strategically placed accelerometers in the vehicle to detect wheel and body movement. An electronic control unit processes that information and interacts with the antilock brakes, steering inputs, engine speed, and elec- tronic stability control unit in deciding within milliseconds the necessary damping force for optimum handling, regardless of road conditions. In recent years, ZF Sachs has assembled an impressive list of vehicles using the Soliman and Kaldas 1019 Soliman and Kaldas 15 Figure 11. CES system architecture from Tenneco – Ohlins Racing. electronic chassis system, including the Audi A8, BMW 7-Series, Maserati 3200 Coupe, Volkswagen Phaeton, Ferraris, Porsche Cayenne, and Volkswagen Touareg. Recently, ZF Sachs has been selected by Adam Opel AG’s to supplie the CDC suspension system to Opel Astra. Also, ThyssenKrupp Bilstein developed the DampTronic-Sky system, which is a complete semi-active suspen- sion system. In the system, two continuously variable valves adjust the damping force in each damper: one valve controls the extension phase, i.e. the force that ensues when the wheel rebounds and the other the compression phase. Using the data it receives from the acceleration and suspension travel sensors, the control module of the suspension system can individually adapt the damping forces for each wheel within just a few milliseconds to eliminate the effects of rough road-ride conditions that may detract from the driving comfort for the passengers, at the same time controlling the dampers in such a way that the chassis is stabilized to the best possible degree. The damper is also able to ensure that the damping force can be adjusted according to the skyhook principle even in the high-frequency spectrum of the wheel vibrations. Currently, the semi-active suspension system DampTronic- Sky has appeared in the new Nissan GT-R Nismo. Other car manufacturers decided to design and develop the software of the semi-active suspension system controller in-house, while the hardware components like dampers, sensors, and ECU hardware are supplied by different suppliers. For example, Volkswagen AG developed the dynamic chassis control (DCC) system, which is based on semi-active dampers. The DDC system uses three accelerometers placed on the vehicle body and three suspension travel sensors to detect body and suspension movement. In this system, the control algorithm of the ECU is developed by Volkswagen engineers. The semi-active valve is mounted externally on the side of the shock absorber so that oil from the shock absorber ring channel flows to the valve. The valve is adjusted by applying a current to the coil of the valve (0.24 A to max. 2.0 A) and the resulting changes inside the adjustment valve. The DDC suspension constantly adjusts itself to the road conditions, the driving situation and the driver’s require- ments. The driver can switch the vehicle driving characteristics from the normal mode to the comfort or sport modes through a button in the dashboard. As well, Jaguar Land Rover designed a semi-active suspension system which is commercially called the con- tinuously variable damping (CVD) system. The controller software is designed in-house by Jaguar Land Rover. The same system is developed for Jaguar and Land Rover cars, while tuning parameters for the control algorithm are considered to optimize the vehicle performance for both. The CVD uses semi-active dampers which employ continuously variable valves inside the damper and placed in the damper piston. As lead in the automotive industry, Ford Motor Company developed a semi-active suspension system that commercially called the contin- uously controlled damping (CCD). The CCD system uses four height sensors to measure the relative displace- ment between the vehicle body and wheels, in addition to the four solenoid valve dampers and the ECU. The CCD control algorithm is developed by suspension control engineers and tuned by vehicle dynamics engineers of Ford. 1020 Journal of Low Frequency Noise, Vibration and Active Control 40(2) 16 Journal of Low Frequency Noise, Vibration and Active Control 0(0) Recently, the CCD suspension system is implemented in most of the Ford vehicles, in addition to the luxury vehicles Lincoln. Conclusion After a survey of the previous research studies on the semi-active suspension system and with the focus on the controlled suspension systems in the automobiles market, it can be concluded that the solenoid valve dampers as semi-active dampers are developed and made commercially available, and have already found their way to the vehicles market in comparison with the other types of the controlled dampers. On the other hand, semi-active suspension control is studied and it is still a scoop of studies for the classical control as skyhook and groundhook and for the modern control like optimal control and fuzzy Logic. The semi- active suspension control problem is highly nonlinear, it is difficult to simulate adequately, the control improve- ments are quite subjective, and controllers should deal with a compromise between opposite properties (ride safety and passenger comfort). Semi-active control algorithms have been developed by suppliers or OEMs to achieve the correct balance of ride comfort and road-holding control and usually based on the skyhook control strategy. Declaration of conflicting interests The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. Funding The author(s) received no financial support for the research, authorship, and/or publication of this article. 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Journal

"Journal of Low Frequency Noise, Vibration and Active Control" – SAGE

Published: Oct 22, 2019

Keywords: Semi-active vehicle suspension; skyhook control; fuzzy logic control; adaptive control; preview control

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