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actuators Article Buttons on Demand Sliding Mechanism Driven by Smart Materials and Mechanical Design 1 1 2 3 Christianto Renata , Manivannan Sivaperuman Kalairaj , Hong Mei Chen , Gih Keong Lau 1 , and Wei Min Huang * School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore; chrisrenata19@gmail.com (C.R.); manivann003@ntu.edu.sg (M.S.K.) College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610066, China; chenhongmei@sicnu.edu.cn Department of Mechanical Engineering, National Yang Ming Chiao Tung University, Hsinchu 30010, Taiwan; mgklau@nctu.edu.tw * Correspondence: mwmhuang@ntu.edu.sg Abstract: In this paper, we describe a novel human interaction platform in a car, called buttons on demand, that will serve as buttons inside the interior of a car, which can be called upon and activated when required but remain concealed and inactive when not required. The mechanism to obtain such interaction is driven by a combination of smart materials and mechanical design. The elaboration of smart materials and mechanical design employed to achieve this mechanism is discussed. A demonstration of how the buttons on demand mechanism described in this paper can potentially substitute or minimize the use of bulkier physical buttons in cars and provide the user with haptic and tactile feedback with low power consumption and fast response time is also presented. Keywords: buttons on demand; smart material; shape memory material; mechanical design; Citation: Renata, C.; Kalairaj, M.S.; mechanism; buttons interaction Chen, H.M.; Lau, G.K.; Huang, W.M. Buttons on Demand Sliding Mechanism Driven by Smart Materials and Mechanical Design. 1. Introduction Actuators 2021, 10, 251. https:// Automobiles have become an integral part of people’s everyday lives as they help doi.org/10.3390/act10100251 people commute from one place to another. Based on the study conducted by Ward’s Auto in 2010, there are 1.015 billion cars globally, or 1 car per 6.75 people in the world, Academic Editor: Manfred Kohl and this number is expected to increase to 2.5 billion cars by 2050 [1,2]. With the huge and continuously increasing number of cars being owned by people today, buttons that Received: 14 August 2021 are installed in cars play an increasingly important role in people’s lives as they help to Accepted: 24 September 2021 perform various functions, such as turning the engine on and off, helping park the car, or Published: 29 September 2021 even functions that are related to autonomous driving. However, most buttons that are installed in the majority of automobiles today are Publisher’s Note: MDPI stays neutral physical buttons that require inputs from human touch, even if the touch screen feature has with regard to jurisdictional claims in been integrated into the automobiles for the user to interact and input their commands to published maps and institutional affil- the cars. Some of the drawbacks from the use of physical buttons for human–automobile iations. interaction include the lack of haptic feedback to the user, the bulky size of the buttons, and these buttons will always remain even when it is not required or when it is inactive. Haptic feedback can be defined as feedbacks that can be received by the user ’s sense receptors, so that the user will obtain feedback information when certain tasks or com- Copyright: © 2021 by the authors. mands have been performed [3,4]. The inability of the physical buttons to provide haptic Licensee MDPI, Basel, Switzerland. feedback to the user implies that there is no reciprocal information being transmitted by This article is an open access article the automobile to the user after inputting the command. This situation may lead to some distributed under the terms and safety issues as the driver may be distracted and thus, their attention on the road will conditions of the Creative Commons be reduced [5]. Attribution (CC BY) license (https:// Buttons on demand can be described as a controller whereby buttons will arise creativecommons.org/licenses/by/ and become active when they are required to perform certain functions and disappear, 4.0/). Actuators 2021, 10, 251. https://doi.org/10.3390/act10100251 https://www.mdpi.com/journal/actuators Actuators 2021, 10, 251 2 of 13 i.e., become flat and inactive when their particular functions are no longer needed. The installation of buttons on demand in automobiles will potentially enable users to have better interaction with their automobiles and increase the safety of driving as the risks of drivers choosing the wrong buttons or inputting the wrong commands will be minimized. Several works on the development of buttons that can provide haptic feedbacks and appear or disappear depending on their state have been undertaken by several groups (including Continental Automotive GmbH), most notably mechanical buttons on demand that are driven using the electric motor [6], Tactus technology whereby the protrusion of the buttons are controlled by the movement of compressed fluid through narrow channels [7], and buttons whose actuation are controlled by an electro-magnetic mechanism [8]. How- ever, these works have their respective limitations, such as the bulky size of the mechanical motor, the possible leakage of the fluid and friction between the fluid and the channels, and the relatively high voltage required to drive the electro-magnetic mechanism. In this paper, we present a novel approach for the buttons on demand mechanism that combines mechanical design with smart materials to obtain the activation and inactivation of the buttons when required. The mechanism described in this paper makes use of a sliding mechanism whose movement is controlled by a shape memory alloy (SMA) and whose patterns and positions are controlled by the locking mechanism achieved by the design profiling of the buttons. Demonstration of the buttons on demand prototype that has been connected with the user interface software and efforts on visual enhancement of the buttons on demand are also described in this paper. 2. Materials 2.1. Shape Memory Alloy (SMA) The shape memory alloy (SMA) has gained significant attention in recent years due to its capability to be deformed into a temporary shape and recovered to its original shape through the application of appropriate stimulus. The unique properties of SMA have resulted in them gaining popularity in the field of robotics and mechatronics, automo- tive [9–11], and biomedical [12–14]. Although there are two categories of SMA, namely thermo-responsive and magneto-responsive SMAs, thermo-responsive SMA is more pop- ular as its transition can be easily achieved through the application of heat above its transition temperature. The stress–strain relationship of the SMAs also varies based on their temperature [15]. Shape memory polymers (SMP) also exhibit similar properties but due to their slow response time and small force exertion, they are not suitable for this application [16]. The SMA used to drive the movement of the sliding mechanism used in this paper is a Ni–Ti one-way SMA spring with a spring diameter of 6.2 mm and a wire diameter of 0.8 mm with a transition temperature of 65 C, which was purchased from Grand Illusion. The SMA spring comes in a coiled form and can be easily deformed and manually stretched. The SMA spring is a thermally responsive smart material. As such, the application of heat to a temperature above its transition temperature will cause the deformed spring to return to its original shape and conformation. Figure 1 shows the original shape of the SMA spring (Figure 1A) and the shape of the spring after it was deformed (Figure 1B). Refer to Figure S4 in [15] for its stress–strain curves in uniaxial tension at different temperatures. In our prototype, the activation of the SMA spring by heating to a temperature above its transition temperature was achieved by Joule heating by connecting the SMA spring to an electrical power source, as illustrated in Figure 2A. A simple 9 V battery was found to be sufficient to return the deformed SMA to its original shape. The illustration of the simple electronic circuit that can generate Joule heating to a different portion of the spring is shown in Figure 2A,B. In the illustration, the red portion of the spring indicates that the current is passing through that area and therefore, that portion of the spring is subjected to Joule heating. Actuators 2021, 10, x FOR PEER REVIEW 3 of 14 Actuators 2021, 10, 251 3 of 13 Actuators 2021, 10, x FOR PEER REVIEW 3 of 14 Figure 1. (A) Original shape of SMA spring. (B) Shape of SMA spring after deformation. In our prototype, the activation of the SMA spring by heating to a temperature above its transition temperature was achieved by Joule heating by connecting the SMA spring to an electrical power source, as illustrated in Figure 2A. A simple 9 V battery was found to be sufficient to return the deformed SMA to its original shape. The illustration of the simple electronic circuit that can generate Joule heating to a different portion of the spring is shown in Figure 2A,B. In the illustration, the red portion of the spring indicates that the current is passing through that area and therefore, that portion of the spring is subjected Figure 1. (A) Original shape of SMA spring. (B) Shape of SMA spring after deformation. Figure 1. to Joule he (A)ating. Original shape of SMA spring. (B) Shape of SMA spring after deformation. In our prototype, the activation of the SMA spring by heating to a temperature above its transition temperature was achieved by Joule heating by connecting the SMA spring to an electrical power source, as illustrated in Figure 2A. A simple 9 V battery was found to be sufficient to return the deformed SMA to its original shape. The illustration of the simple electronic circuit that can generate Joule heating to a different portion of the spring is shown in Figure 2A,B. In the illustration, the red portion of the spring indicates that the current is passing through that area and therefore, that portion of the spring is subjected to Joule heating. Figure Figure 2. 2. (( A A) Electronic cir ) Electronic cir ccuits uits to trigger the to trigger the contract contraction ion of of the specif the specific ic portion of SM portion of SMA A spring spri . ( ng. B) Connection of electric wires to different parts of SMA spring. (B) Connection of electric wires to different parts of SMA spring. 2.2. Device Prototypes 2.2. Device Prototypes Three types of device prototypes are fabricated in our work, namely the square 9- Three types of device prototypes are fabricated in our work, namely the square 9- button set (Button set A, Figure 3A), the elongated 4-button set with switches (Button set B, button set (Button set A, Figure 3A), the elongated 4-button set with switches (Button set Figure 3B), and rectangular 4-button set (Button set C, Figure 3C). Figure 3A–C show the B, Figure 3B), and rectangular 4-button set (Button set C, Figure 3C). Figure 3A–C show Actuators 2021, 10, x FOR PEER REVIEW 4 of 14 three device prototypes that are fabricated and assembled. Each of these three device the three device prototypes that are fabricated and assembled. Each of these three device prototypes has a different design, although their working mechanisms are similar. prototypes has a different design, although their working mechanisms are similar. Figure 2. (A) Electronic circuits to trigger the contraction of the specific portion of SMA spring. (B) Connection of electric wires to different parts of SMA spring. 2.2. Device Prototypes Three types of device prototypes are fabricated in our work, namely the square 9- button set (Button set A, Figure 3A), the elongated 4-button set with switches (Button set B, Figure 3B), and rectangular 4-button set (Button set C, Figure 3C). Figure 3A–C show the three device prototypes that are fabricated and assembled. Each of these three device prototypes has a different design, although their working mechanisms are similar. Figure 3. Prototypes of (A) button set A (top surface: 10 cm × 10 cm); (B) buttons set B (top surface: Figure 3. Prototypes of (A) button set A (top surface: 10 cm 10 cm); (B) buttons set B (top ≈ 20 cm × 3 cm); and (C) button set C (top surface: 14 cm × 3 cm). surface: 20 cm 3 cm); and (C) button set C (top surface: 14 cm 3 cm). The top cover, buttons, and sliders used in our prototypes are made from aluminum alloy whose surface has been oxidized. These parts were fabricated by CNC machining to ensure the precision of the parts. The designs of the top cover, buttons, and sliders were made and simulated using SolidWorks (Dassault Systèmes, Vélizy-Villacoublay, France). The top cover is the topmost element of the prototype that is cut out for the buttons to slot in. As the design of the three device prototypes used in our work varies, the design and the shape of their top cover are different from one another. The top cover for button set A has dimensions of 100 mm long, 100 mm wide, and 5 mm thick. It has 9 circular holes with a dimension of 20 mm in diameter. The top cover for button set B has a dimen- sion of 200 mm long, a largest width of 40 mm, and 5 mm thick. In addition to the 4 square holes to slot in the buttons, two additional circular holes to fit in push button switches are also present in the top cover for button set B. Lastly, the top cover for button set C has a dimension of 140 mm long, 30 mm wide, and 5 mm thick. Similar to button set B, button set C has four holes although in this case the hole is circular in shape. As mentioned above, the three button sets use the different shape and size of the buttons as button set A and C use circular buttons, whereas button set B uses square but- tons. Both the circular and square buttons are designed to have a flat top surface, but have protrusions and profiles on the bottom to accommodate the slider’s movement and the rising and falling action of buttons. Figure 4A shows the top and bottom view of the cir- cular button. The square button has a similar profile to the circular button as they only differ in size and shape. The slider is the movable part that controls the rise and fall of the buttons. All three button sets have different slider designs as the three button sets have different button patterns. Even though the design of the sliders for the button set varies, one of the surfaces of all the sliders have protrusions and profiles that allow the buttons to rise and fall de- pending on the button patterns of the set. The other side of the slider is a flat surface on which a small elliptical piece is attached to control the sliding orientation and the direction of the slider, as shown in Figure 4B; whereas Figure 4C shows the side view of the overall assembly of the top cover, buttons, and the sliders. The design profile and protrusions on the buttons and the sliders that facilitate the locking mechanism of the patterns and posi- tions of the buttons can be clearly seen in Figure 4C. Actuators 2021, 10, 251 4 of 13 The top cover, buttons, and sliders used in our prototypes are made from aluminum alloy whose surface has been oxidized. These parts were fabricated by CNC machining to ensure the precision of the parts. The designs of the top cover, buttons, and sliders were made and simulated using SolidWorks (Dassault Systèmes, Vélizy-Villacoublay, France). The top cover is the topmost element of the prototype that is cut out for the buttons to slot in. As the design of the three device prototypes used in our work varies, the design and the shape of their top cover are different from one another. The top cover for button set A has dimensions of 100 mm long, 100 mm wide, and 5 mm thick. It has 9 circular holes with a dimension of 20 mm in diameter. The top cover for button set B has a dimension of 200 mm long, a largest width of 40 mm, and 5 mm thick. In addition to the 4 square holes to slot in the buttons, two additional circular holes to fit in push button switches are also present in the top cover for button set B. Lastly, the top cover for button set C has a dimension of 140 mm long, 30 mm wide, and 5 mm thick. Similar to button set B, button set C has four holes although in this case the hole is circular in shape. As mentioned above, the three button sets use the different shape and size of the buttons as button set A and C use circular buttons, whereas button set B uses square buttons. Both the circular and square buttons are designed to have a flat top surface, but have protrusions and profiles on the bottom to accommodate the slider ’s movement and the rising and falling action of buttons. Figure 4A shows the top and bottom view of the Actuators 2021, 10, x FOR PEER REVIEW 5 of 14 circular button. The square button has a similar profile to the circular button as they only differ in size and shape. Figure 4. (A) Top and bottom view of the circular button. (B) Top and bottom of the slider. (C) Side Figure 4. (A) Top and bottom view of the circular button. (B) Top and bottom of the slider. (C) Side view of view of assembled prototype. assembled prototype. The slider is the movable part that controls the rise and fall of the buttons. All three Similarly, the bottom covers of the three button sets vary in size and shape, but all of button sets have different slider designs as the three button sets have different button them are made from the laser cutting of PMMA. The bottom covers have a varying num- patterns. Even though the design of the sliders for the button set varies, one of the surfaces ber of holes along the edges that are used to slot in the screws and nuts to hold and tighten of all the sliders have protrusions and profiles that allow the buttons to rise and fall all the components together. In the middle of the bottom cover, there are cut out spaces to depending on the button patterns of the set. The other side of the slider is a flat surface on fit in the small elliptical piece protruding from one of the surface of the slider. This cut out which a small elliptical piece is attached to control the sliding orientation and the direction space is used to control the sliding orientation and alignment and also limit the sliding of the slider, as shown in Figure 4B; whereas Figure 4C shows the side view of the overall distance. After the top cover, buttons, sliders, and bottom cover have been put together, assembly of the top cover, buttons, and the sliders. The design profile and protrusions the prototype is slot into a plastic case that has been cut out to slot in the device prototypes, on the buttons and the sliders that facilitate the locking mechanism of the patterns and as shown in Figure 5A,B. This plastic case is an original part of one of vehicle that will be positions of the buttons can be clearly seen in Figure 4C. used for demonstration purposes. Similarly, the bottom covers of the three button sets vary in size and shape, but all of Prototype devices with other shapes and button configurations have also been fabri- them are made from the laser cutting of PMMA. The bottom covers have a varying number cated and tested, such as the device prototype with 9 circular buttons and 3 button acti- of holes along the edges that are used to slot in the screws and nuts to hold and tighten vation patterns and a device prototype with a similar size and shape to the device de- all the components together. In the middle of the bottom cover, there are cut out spaces scribed above but with rectangular buttons. However, we focused on using the above de- to fit in the small elliptical piece protruding from one of the surface of the slider. This cut vice set for the demonstration with the user interface. Figure 5. Final appearances of the device prototype: (A) top view and (B) bottom view. The embed- ded is Button set C as shown in Figure 3C. 2.3. Color Change Visual Enhancement Materials Various color change materials can be laid on top of the top surface of the device prototype to enhance the visual appearance and distinction of the active buttons from the inactive ones. These color change materials belong to a class of mechanochromic materi- als, which are materials that undergo color change when the mechanical load is applied on it [17, 18]. The rise of the buttons when they are activated exerts a localized mechanical load on the color change materials and therefore, causing some or all parts of the material directly on top of the active buttons to change its color or form a distinguishing contour of the active buttons. Consequently, the active and inactive buttons can also be visually distinguished as they have different colors. Actuators 2021, 10, x FOR PEER REVIEW 5 of 14 Figure 4. (A) Top and bottom view of the circular button. (B) Top and bottom of the slider. (C) Side view of assembled prototype. Similarly, the bottom covers of the three button sets vary in size and shape, but all of them are made from the laser cutting of PMMA. The bottom covers have a varying num- ber of holes along the edges that are used to slot in the screws and nuts to hold and tighten all the components together. In the middle of the bottom cover, there are cut out spaces to fit in the small elliptical piece protruding from one of the surface of the slider. This cut out space is used to control the sliding orientation and alignment and also limit the sliding distance. After the top cover, buttons, sliders, and bottom cover have been put together, Actuators 2021, 10, 251 5 of 13 the prototype is slot into a plastic case that has been cut out to slot in the device prototypes, as shown in Figure 5A,B. This plastic case is an original part of one of vehicle that will be used for demonstration purposes. Prototype devices with other shapes and button configurations have also been fabri- out space is used to control the sliding orientation and alignment and also limit the sliding cated and tested, such as the device prototype with 9 circular buttons and 3 button acti- distance. After the top cover, buttons, sliders, and bottom cover have been put together, vation patterns and a device prototype with a similar size and shape to the device de- the prototype is slot into a plastic case that has been cut out to slot in the device prototypes, scribed above but with rectangular buttons. However, we focused on using the above de- as shown in Figure 5A,B. This plastic case is an original part of one of vehicle that will be vice set for the demonstration with the user interface. used for demonstration purposes. Figure 5. Final appearances of the device prototype: (A) top view and (B) bottom view. The embed- Figure 5. Final appearances of the device prototype: (A) top view and (B) bottom view. The ded is Button set C as shown in Figure 3C. embedded is Button set C as shown in Figure 3C. 2.3. Color C Prototype hange devices Visual E with nhance other menshapes t Materials and button configurations have also been fab- ricated and tested, such as the device prototype with 9 circular buttons and 3 button Various color change materials can be laid on top of the top surface of the device activation patterns and a device prototype with a similar size and shape to the device prototype to enhance the visual appearance and distinction of the active buttons from the described above but with rectangular buttons. However, we focused on using the above inactive ones. These color change materials belong to a class of mechanochromic materi- device set for the demonstration with the user interface. als, which are materials that undergo color change when the mechanical load is applied on it [17, 18]. The rise of the buttons when they are activated exerts a localized mechanical 2.3. Color Change Visual Enhancement Materials load on the color change materials and therefore, causing some or all parts of the material Various color change materials can be laid on top of the top surface of the device directly on top of the active buttons to change its color or form a distinguishing contour prototype to enhance the visual appearance and distinction of the active buttons from the of the active buttons. Consequently, the active and inactive buttons can also be visually inactive ones. These color change materials belong to a class of mechanochromic materials, distinguished as they have different colors. which are materials that undergo color change when the mechanical load is applied on it [17,18]. The rise of the buttons when they are activated exerts a localized mechanical load on the color change materials and therefore, causing some or all parts of the material Actuators 2021, 10, x FOR PEER REVIEW 6 of 14 directly on top of the active buttons to change its color or form a distinguishing contour of the active buttons. Consequently, the active and inactive buttons can also be visually distinguished as they have different colors. Sever Several al co color lor change change mater materials ials have be have enbeen explored explor and tested ed and tested in this wor in this k, suc work, h as such as the standard elastic cloth, pristine and modified holographic film (Figure 6A), sublimation the standard elastic cloth, pristine and modified holographic film (Figure 6A), sublimation printed elastic cloth (Figure 6B,C), and elastomer-based transparency film (Figure 6D). printed elastic cloth (Figure 6B,C), and elastomer-based transparency film (Figure 6D). Apart from the elastomer-based transparency film, the remaining color change materials Apart from the elastomer-based transparency film, the remaining color change materials are commercially purchased from various sources. are commercially purchased from various sources. Figure 6. (A) Modified holographic film (button set A; top surface: 10 cm × 10 cm); (B) (button set Figure 6. (A) Modified holographic film (button set A; top surface: 10 cm 10 cm); (B) (button set A; top surface: 10 cm × 10 cm); and (C) sublimation printed elastic cloth (button set B; top surface: ≈ A; top surface: 10 cm 10 cm); and (C) sublimation printed elastic cloth (button set B; top surface: 20 cm × 3 cm); (D) elastomer-based transparency film (button set A; top surface: 10 cm × 10 cm). 20 cm 3 cm); (D) elastomer-based transparency film (button set A; top surface: 10 cm 10 cm). 3. Working Mechanism of Buttons on Demand The working mechanism of the buttons on demand device discussed in this paper is through the combination of smart mechanical design and smart materials. The smart me- chanical design defines the patterns of the buttons when activated and also locks the po- sition of the buttons according to the defined pattern. On the other hand, the smart mate- rial used in the device triggers the activation and deactivation of the buttons by controlling the movement of the sliders. The movement of the slider and its corresponding activation and locking of the buttons’ position is illustrated in Figure 7A,B. Figure 7. Activation (protrusion, 1.5 mm) of the buttons achieved through the movement of the slider driven by the smart material. Final appearances of the device prototype: (A) top view and (B) bottom view. 3.1. Mechanism of Two-Button and Three-Button Patterns of Buttons on Demand Device As mentioned earlier, the components that make up the device prototype are the top cover, slider, buttons, SMA, wires, clamps, and power source to make up the electrical circuit. For both two-button and three-button patterns devices, the SMA will be clamped at three separate locations with electrical wires being attached to the three terminals, as shown in Figure 8A–C. Depending on which part of the electrical circuits are being closed, Actuators 2021, 10, x FOR PEER REVIEW 6 of 14 Several color change materials have been explored and tested in this work, such as the standard elastic cloth, pristine and modified holographic film (Figure 6A), sublimation printed elastic cloth (Figure 6B,C), and elastomer-based transparency film (Figure 6D). Apart from the elastomer-based transparency film, the remaining color change materials are commercially purchased from various sources. Figure 6. (A) Modified holographic film (button set A; top surface: 10 cm × 10 cm); (B) (button set A; top surface: 10 cm × 10 cm); and (C) sublimation printed elastic cloth (button set B; top surface: ≈ Actuators 2021, 10, 251 6 of 13 20 cm × 3 cm); (D) elastomer-based transparency film (button set A; top surface: 10 cm × 10 cm). 3. Working Mechanism of Buttons on Demand 3. Working Mechanism of Buttons on Demand The working mechanism of the buttons on demand device discussed in this paper is The working mechanism of the buttons on demand device discussed in this paper through the combination of smart mechanical design and smart materials. The smart me- is through the combination of smart mechanical design and smart materials. The smart chanical design defines the patterns of the buttons when activated and also locks the po- mechanical design defines the patterns of the buttons when activated and also locks the sition of the buttons according to the defined pattern. On the other hand, the smart mate- position of the buttons according to the defined pattern. On the other hand, the smart rial used in the device triggers the activation and deactivation of the buttons by controlling material used in the device triggers the activation and deactivation of the buttons by the movement of the sliders. The movement of the slider and its corresponding activation controlling the movement of the sliders. The movement of the slider and its corresponding and locking of the buttons’ position is illustrated in Figure 7A,B. activation and locking of the buttons’ position is illustrated in Figure 7A,B. Figure 7. Activation (protrusion, 1.5 mm) of the buttons achieved through the movement of the slider driven by the smart Figure 7. Activation (protrusion, 1.5 mm) of the buttons achieved through the movement of the slider driven by the smart material. Final appearances of the device prototype: (A) top view and (B) bottom view. material. Final appearances of the device prototype: (A) top view and (B) bottom view. 3.1. Mechanism of Two-Button and Three-Button Patterns of Buttons on Demand Device 3.1. Mechanism of Two-Button and Three-Button Patterns of Buttons on Demand Device As mentioned earlier, the components that make up the device prototype are the top As mentioned earlier, the components that make up the device prototype are the top cover, slider, buttons, SMA, wires, clamps, and power source to make up the electrical Actuators 2021, 10, x FOR PEER REVIEW 7 of 14 cover, slider, buttons, SMA, wires, clamps, and power source to make up the electrical circuit. For both two-button and three-button patterns devices, the SMA will be clamped at circuit. For both two-button and three-button patterns devices, the SMA will be clamped three separate locations with electrical wires being attached to the three terminals, as shown at three separate locations with electrical wires being attached to the three terminals, as in Figure 8A–C. Depending on which part of the electrical circuits are being closed, the shown in Figure 8A–C. Depending on which part of the electrical circuits are being closed, the current and therefore the corresponding Joule heating will be applied to the corre- current and therefore the corresponding Joule heating will be applied to the corresponding sponding section of the SMA and bring about the contraction of the left or right portion section of the SMA and bring about the contraction of the left or right portion of the SMA. of the SMA. Using Figure 8A–C as an example, closing the circuit created by the black and Using Figure 8A–C as an example, closing the circuit created by the black and yellow wires yellow wires will cause contraction of the left portion of the SMA and therefore cause the will cause contraction of the left portion of the SMA and therefore cause the slider to move slider to move to the left. to the left. Figure 8. Movement of the slider due to the contraction of a portion of the SMA (button set A; top Figure 8. Movement of the slider due to the contraction of a portion of the SMA (button set A; top surface: 10 cm × 10 cm); (A) slider at the left-most position; and (B) slider in the middle, and (C) surface: 10 cm 10 cm); (A) slider at the left-most position; and (B) slider in the middle, and (C) slider at the right-most position. slider at the right-most position. In a two-button pattern device, the position of the buttons, and therefore the activa- In a two-button pattern device, the position of the buttons, and therefore the activation tion and deactivation of the buttons, is governed by whether the slider is located at the and deactivation of the buttons, is governed by whether the slider is located at the left-most left-most or right-most position. When it is inactive, all of the buttons will be flat, whereas or right-most position. When it is inactive, all of the buttons will be flat, whereas when it when it is active, all of the buttons will protrude upwards. On the other hand, in a three- is active, all of the buttons will protrude upwards. On the other hand, in a three-button pattern button pa device, ttern devi an additional ce, an addi active tiona buttons l active bu pattern ttons pattern ca can be generated n be genera as theted as the mi middle position ddle position of the slider and can be used to define an additional buttons pattern. Similarly, of the slider and can be used to define an additional buttons pattern. Similarly, the left- the left-most and right-most position will cause all of the buttons to be flat or protrude most and right-most position will cause all of the buttons to be flat or protrude upwards. Illustration upwards. Ilof lustra theti movement on of the movement of of the slider the to generate slider to genera three patterns te three pa oftterns of the buttons the but- on demand tons on dem device and device is illustrated is illustr in Figur ated ein F 9A–C. igure 9A–C. Figure 9. Illustration of the generation of 3 patterns on the buttons on demand set: (A) pattern 1; (B) pattern 2; and (C) pattern 3. 3.2. Mechanism of Button Set C Figure 10A,B show the top and bottom appearances of the buttons on demand device prototype after it was slotted into the plastic case with the electrical wires connected at various points along the spring. In Figure 10A, the buttons are marked from top to bottom as 1, 2, 3, and 4, whereas in Figure 10B, the wires are denoted with the letters A, B, G, C, and D. There are two sliders in the above prototype: one controlling buttons 1 and 2, and the other controlling buttons 3 and 4. The two sliders and therefore, the button pairs 1–2 and 3–4, can be independently controlled by connecting the wires into separate power sources. Actuators 2021, 10, x FOR PEER REVIEW 7 of 14 the current and therefore the corresponding Joule heating will be applied to the corre- sponding section of the SMA and bring about the contraction of the left or right portion of the SMA. Using Figure 8A–C as an example, closing the circuit created by the black and yellow wires will cause contraction of the left portion of the SMA and therefore cause the slider to move to the left. Figure 8. Movement of the slider due to the contraction of a portion of the SMA (button set A; top surface: 10 cm × 10 cm); (A) slider at the left-most position; and (B) slider in the middle, and (C) slider at the right-most position. In a two-button pattern device, the position of the buttons, and therefore the activa- tion and deactivation of the buttons, is governed by whether the slider is located at the left-most or right-most position. When it is inactive, all of the buttons will be flat, whereas when it is active, all of the buttons will protrude upwards. On the other hand, in a three- button pattern device, an additional active buttons pattern can be generated as the middle position of the slider and can be used to define an additional buttons pattern. Similarly, the left-most and right-most position will cause all of the buttons to be flat or protrude Actuators 2021, 10, 251 7 of 13 upwards. Illustration of the movement of the slider to generate three patterns of the but- tons on demand device is illustrated in Figure 9A–C. Figure 9. Figure 9.Illust Illustration ration of the generation of 3 pa of the generation of 3 patterns tterns on the buttons on de on the buttons on mand set: ( demand set: A) (pattern 1; ( A) patternB 1; ) pattern 2; and ( (B) pattern 2; and C) pattern 3. (C) pattern 3. 3.2. Mechanism of Button Set C 3.2. Mechanism of Button Set C Figure 10A,B show the top and bottom appearances of the buttons on demand device Figure 10A,B show the top and bottom appearances of the buttons on demand device prototype after it was slotted into the plastic case with the electrical wires connected at prototype after it was slotted into the plastic case with the electrical wires connected at various points along the spring. In Figure 10A, the buttons are marked from top to bottom various points along the spring. In Figure 10A, the buttons are marked from top to bottom as 1, 2, 3, and 4, whereas in Figure 10B, the wires are denoted with the letters A, B, G, C, and as 1, 2, 3, and 4, whereas in Figure 10B, the wires are denoted with the letters A, B, G, C, D. There are two sliders in the above prototype: one controlling buttons 1 and 2, and the and D. There are two sliders in the above prototype: one controlling buttons 1 and 2, and Actuators 2021, 10, x FOR PEER REVIEW 8 of 14 other controlling buttons 3 and 4. The two sliders and therefore, the button pairs 1–2 and the other controlling buttons 3 and 4. The two sliders and therefore, the button pairs 1–2 3–4, can be independently controlled by connecting the wires into separate power sources. and 3–4, can be independently controlled by connecting the wires into separate power sources. Figure 10. Figure 10. ((A A)) Top and ( Top and (B B)) bo bottom ttom appearances of appearances of the butto the buttons ns on on demand demand set C set C (refer to (refer to F Figur igure 3C) e 3C) with wire connections. with wire connections. By re By referring ferring to to Fig Figur ure 10A e 10A,B, ,B, the work the working ing mechan mechanism ism of the buttons on deman of the buttons on demand d pro- prototype will be explained in greater detail. As mentioned above, there are two sliders totype will be explained in greater detail. As mentioned above, there are two sliders in- installed in the above prototypes and therefore the pair of buttons can be independently stalled in the above prototypes and therefore the pair of buttons can be independently controlled. Wires A, B, and G control the travel of buttons 1 and 2, whereas wires C, D, and controlled. Wires A, B, and G control the travel of buttons 1 and 2, whereas wires C, D, G controls the travel of buttons 3 and 4. and G controls the travel of buttons 3 and 4. When wires A and B are connected to the power source, electrical current will pass When wires A and B are connected to the power source, electrical current will pass through the section of the SMA spring between wires A and B and therefore cause the through the section of the SMA spring between wires A and B and therefore cause the Joule heating of that portion of spring. As the entire spring is in a stretched and deformed Joule heating of that portion of spring. As the entire spring is in a stretched and deformed state, the Joule heating of the portion of the spring between wires A and B will cause it state, the Joule heating of the portion of the spring between wires A and B will cause it to to shrink and therefore, pull the slider towards point A. Movement of the slider towards shrink and therefore, pull the slider towards point A. Movement of the slider towards point A will cause buttons 1 and 2 to rise and go into active mode. The same principle and point A will cause buttons 1 and 2 to rise and go into active mode. The same principle and buttons activation can be said for buttons 3 and 4 when wires C and D are connected to buttons activation can be said for buttons 3 and 4 when wires C and D are connected to the power source. The Joule heating of the portion of the SMA spring between C and D the power source. The Joule heating of the portion of the SMA spring between C and D will cause the spring to contract and therefore pull the slider towards point D. Once the will cause the spring to contract and therefore pull the slider towards point D. Once the buttons have risen and are in active mode, the application of the electrical current is no longer necessary as the design profile on the bottom surface of the buttons allows their positions to be locked. On the other hand, connecting wires B and G and C and G to the power source can cause button pairs 1–2 and 3–4, respectively, to fall, and therefore, go into inactive mode. Here, the respective slider will be pulled by the contraction of the spring towards point G. Switches, such as the simple push button switches or proximity sensor switches can be used to help control the passing of the electrical current through the various portions of the springs, and therefore, help control the activation and deactivation of the correspond- ing buttons. 4. Simulation and Demonstration of Buttons on Demand Device 4.1. Electrical Voltage Requirement for Activation As mentioned above, the shape memory effect of the SMA is achieved through ther- mal stimulation via Joule heating. As such, a power source that is able to drive current through the SMA is required. Various kinds of power source can be used in order to trig- ger the shape memory effect of the SMA and therefore, cause portions of the SMA spring to contract. Upon connecting the SMA spring integrated prototype with a power source and a multimeter, we noticed that an electrical current of approximately 2.5 A is required to trigger the activation of the shape memory effect of the SMA. One of the power sources that were used in our work was the commercial Alkaline 9 V battery. From our test and simulations, we observed that this battery was able to drive Actuators 2021, 10, 251 8 of 13 buttons have risen and are in active mode, the application of the electrical current is no longer necessary as the design profile on the bottom surface of the buttons allows their positions to be locked. On the other hand, connecting wires B and G and C and G to the power source can cause button pairs 1–2 and 3–4, respectively, to fall, and therefore, go into inactive mode. Here, the respective slider will be pulled by the contraction of the spring towards point G. Switches, such as the simple push button switches or proximity sensor switches can be used to help control the passing of the electrical current through the various por- tions of the springs, and therefore, help control the activation and deactivation of the corresponding buttons. 4. Simulation and Demonstration of Buttons on Demand Device 4.1. Electrical Voltage Requirement for Activation As mentioned above, the shape memory effect of the SMA is achieved through thermal stimulation via Joule heating. As such, a power source that is able to drive current through the SMA is required. Various kinds of power source can be used in order to trigger the shape memory effect of the SMA and therefore, cause portions of the SMA spring to contract. Upon connecting the SMA spring integrated prototype with a power source and a multimeter, we noticed that an electrical current of approximately 2.5 A is required to trigger the activation of the shape memory effect of the SMA. One of the power sources that were used in our work was the commercial Alkaline 9 V battery. From our test and simulations, we observed that this battery was able to drive sufficient current into the SMA spring to generate the heat necessary for the activation of the SMA spring, which has a shape memory activation temperature of 60–65 C. The advantage of using this battery is that it is commercially available, cheap, and easy to use. However, one of the drawbacks of using this particular battery is the relatively longer time taken to heat up the SMA spring to its activation temperature. When the Alkaline 9 V battery is used as the power source, the contraction of the SMA spring is achieved after approximately 10 s. For vehicle buttons’ application, the activation period of 10 s may be too long. To accelerate the activation of the SMA spring, other types of battery can be used, such as the lead-acid rechargeable battery. The lead-acid battery that we tested was 12 V, 2000 mAh battery. When this battery was used as the power source, the activation of the SMA spring can be achieved instantaneously, i.e., within 1–2 s. However, the drawback of using this battery is that as it is a more powerful battery than the Alkaline 9 V battery, the current discharge may be excessively large. As a consequence of the large current discharge, the battery may heat up the SMA spring to a very high temperature, which causes the deformed and elongated shape of the SMA spring to be permanent. Once the deformation of the SMA spring has become permanent, connecting that particular portion of the SMA spring to a power source may not be able to bring about the contraction and recovery of the spring. In addition, the very high temperature of the spring upon connecting the SMA spring to the power source may also burn other components of the prototypes. From the various types of battery that have been used in our test, the best battery that is able to deliver the relatively quick activation of the SMA, safe to use without potentially causing any burning and permanent deformation of the spring, compact, and easy to use is the Lithium rechargeable 18,650 battery. The 18,650 Lithium battery that was used in our test has an electrical voltage of 3.7 V and an electrical discharge of 2200 mAh. When the 18,650 lithium battery was used as the power source for the buttons on demand prototype, the activation of the SMA spring can be achieved within 3–4 s, which is acceptable for vehicle buttons’ application. In addition, this battery is not capable to heat the SMA spring to an excessively high temperature that may cause the burning of the components surrounding the spring and its permanent deformation. It is important to note, however, that at this current stage, regardless of the type of battery that is used as the power source to trigger the Joule heating of the SMA spring, the Actuators 2021, 10, 251 9 of 13 cooling down period of approximately 10–15 s is necessary after every activation cycle of the SMA spring. In order to obtain an optimum contraction of different portions of the SMA spring, it is best to allow the SMA spring to cool down to approximately 35 C before the next activation cycle is triggered. In the future, we hope to shorten the cooling down period by means of integrating a cooling system into the prototype. 4.2. In-Car Demonstration with User Interface After the assembly of the buttons on demand prototype set C and building the electronic circuit, the device prototype was mounted and fixed onto a plastic frame case that was cut out in the middle. As the plastic frame was an original part of the vehicle, the mounting and dismounting of the case to and from the vehicle was relatively easy. A force sensor was fixed on top of each circular button so that the touching of each button will allow input command to be independently delivered into the user interface. A user interface simulation that resembles the user interface used in current vehicle was used for the in-car demonstration. For the demonstration, the user interface simulation was installed in a tablet PC and it was connected to the device prototype using an Arduino USB Actuators 2021, 10, x FOR PEER REVIEW 10 of 14 driver. Figure 11 shows the appearance of the interior of the car after the installation of the buttons on demand device set together with placing the tablet PC to control the simulation. Figure Figure 11. 11. Appearance Appearance of a of a car car’s ’s interior with interior with the the buttons buttons on on demand demand d device evice(r (refer to Fi efer to Figur gure 5) bei e 5) being ng installed inside the car. installed inside the car. The user interface simulation was programmed to simulate a driving scenario whereby The user interface simulation was programmed to simulate a driving scenario the driver can choose interchangeably between parking mode at 0 mph, city driving mode whereby the driver can choose interchangeably between parking mode at 0 mph, city at 50 mph, and highway driving mode at 110 mph, by means of swiping left and right driving mode at 50 mph, and highway driving mode at 110 mph, by means of swiping left at the screen of the tablet. In addition, different functions and features corresponding to and right at the screen of the tablet. In addition, different functions and features corre- different buttons’ activation have also been programmed in the simulation depending sponding to different buttons’ activation have also been programmed in the simulation on which mode the simulation was currently in. Figure 12A–D show the snapshots of depending on which mode the simulation was currently in. Figure 12A–D show the snap- the user interface simulation in the engine’s off state, parking mode, city driving mode, shots of the user interface simulation in the engine’s off state, parking mode, city driving and highway driving mode, while Figure 13A,B shows the appearance of the buttons on mode, and highway driving mode, while Figure 13A,B shows the appearance of the but- demand device set when only two buttons and all four buttons are active. tons on demand device set when only two buttons and all four buttons are active. Figure 12. Snapshots of the user interface simulation at the (A) off state; (B) parking mode; (C) city driving mode; and (D) highway driving mode. Actuators 2021, 10, x FOR PEER REVIEW 10 of 14 Figure 11. Appearance of a car’s interior with the buttons on demand device (refer to Figure 5) being installed inside the car. The user interface simulation was programmed to simulate a driving scenario whereby the driver can choose interchangeably between parking mode at 0 mph, city driving mode at 50 mph, and highway driving mode at 110 mph, by means of swiping left and right at the screen of the tablet. In addition, different functions and features corre- sponding to different buttons’ activation have also been programmed in the simulation depending on which mode the simulation was currently in. Figure 12A–D show the snap- shots of the user interface simulation in the engine’s off state, parking mode, city driving Actuators 2021, 10, 251 10 of 13 mode, and highway driving mode, while Figure 13A,B shows the appearance of the but- tons on demand device set when only two buttons and all four buttons are active. Actuators 2021, 10, x FOR PEER REVIEW 11 of 14 Figure 12. Snapshots of the user interface simulation at the (A) off state; (B) parking mode; (C) city Figure 12. Snapshots of the user interface simulation at the (A) off state; (B) parking mode; (C) city driving mode; and (D) highway driving mode. driving mode; and (D) highway driving mode. Figure 13. Appearances of the buttons on demand device set (refer to Figure 5) when (A) only 2 Figure 13. Appearances of the buttons on demand device set (refer to Figure 5) when (A) only buttons are active and (B) all 4 buttons are active. 2 buttons are active and (B) all 4 buttons are active. Fr From Figure om Figure 12 12B–D B–D,, it c it can an be observed be observed that that different different options are options are available available in in e each ach mode. mode. For ex For example, ample, in par in parking king mode mode, , wwher hereby th eby the e cacar r is in is in stat stationary ionary mode mode (0 mph), t (0 mph), he the top two selections will appear and become active while the bottom two selections top two selections will appear and become active while the bottom two selections will will rema remain in idleidle and i and nac inactive. tive. Correspondingl Correspondingly y, the buttons on dem , the buttons on demand and devi device ce will will ada adapt pt to to the driving mode that the car is in and make adjustments in terms of the activation the driving mode that the car is in and make adjustments in terms of the activation and and inactivation of the buttons. As such, when the car is in parking mode, the top two inactivation of the buttons. As such, when the car is in parking mode, the top two buttons buttons will rise and become active while the bottom two buttons will remain flat and will rise and become active while the bottom two buttons will remain flat and inactive. As inactive. As soon as the corresponding buttons become active, pressing the active buttons soon as the corresponding buttons become active, pressing the active buttons on the but- on the buttons on demand device will allow the driver to input a command into the car tons on demand device will allow the driver to input a command into the car and therefore and therefore perform functions that are desired by the driver. Figure 14A–C gives some perform functions that are desired by the driver. Figure 14A–C gives some examples of examples of pressing active buttons on the device and delivering a command to the car pressing active buttons on the device and delivering a command to the car in different in different modes. Similarly, when the car is in city driving mode, only the bottom two modes. Similarly, when the car is in city driving mode, only the bottom two buttons will buttons will become active and when the car is in highway driving mode, all four buttons become active and when the car is in highway driving mode, all four buttons will become will become active. It is also important to note that when the car is in highway driving active. It is also important to note that when the car is in highway driving mode, the top mode, the top high buttons will no longer perform the same function as when they are in high buttons will no longer perform the same function as when they are in the parking the parking mode. mode. Figure 14. Pressing activated buttons on the buttons on demand device (refer to Figure 5) to perform functions in (A) parking mode; (B) city driving mode; and (C) highway driving mode. 4.3. Visual Enhancement with Mechanochromic Material Various mechanochromic and color change materials were laid on top of the buttons on demand sets in order to test their capabilities in terms of enhancing the visual appear- ance of the set and at the same time, further distinguish the active buttons from the inac- tive ones. As mentioned in Section 2.3 and Figure 6, some of the materials that were tested were elastic cloth, modified holographic film, sublimation printed elastic cloth, and elas- tomer-based transparency film. In general, all the materials tested in our work showed promising abilities in enhanc- ing the visual appearance of the active buttons. The most promising result was provided by the sublimation printed elastic cloth. This particular cloth was relatively thin, highly stretchable, and easy to handle. Figure 15 shows the appearance of the cloth covered but- ton set when the buttons are inactive (Figure 15A) and when the buttons are active (Figure 15B). From Figure 15A, it can be observed that when the buttons are inactive, the cloth Actuators 2021, 10, x FOR PEER REVIEW 11 of 14 Figure 13. Appearances of the buttons on demand device set (refer to Figure 5) when (A) only 2 buttons are active and (B) all 4 buttons are active. From Figure 12B–D, it can be observed that different options are available in each mode. For example, in parking mode, whereby the car is in stationary mode (0 mph), the top two selections will appear and become active while the bottom two selections will remain idle and inactive. Correspondingly, the buttons on demand device will adapt to the driving mode that the car is in and make adjustments in terms of the activation and inactivation of the buttons. As such, when the car is in parking mode, the top two buttons will rise and become active while the bottom two buttons will remain flat and inactive. As soon as the corresponding buttons become active, pressing the active buttons on the but- tons on demand device will allow the driver to input a command into the car and therefore perform functions that are desired by the driver. Figure 14A–C gives some examples of pressing active buttons on the device and delivering a command to the car in different modes. Similarly, when the car is in city driving mode, only the bottom two buttons will become active and when the car is in highway driving mode, all four buttons will become active. It is also important to note that when the car is in highway driving mode, the top Actuators 2021, 10, 251 11 of 13 high buttons will no longer perform the same function as when they are in the parking mode. Figure 14. Pressing activated buttons on the buttons on demand device (refer to Figure 5) to perform Figure 14. Pressing activated buttons on the buttons on demand device (refer to Figure 5) to perform functions in (A) parking mode; (B) city driving mode; and (C) highway driving mode. functions in (A) parking mode; (B) city driving mode; and (C) highway driving mode. 4.3. 4.3. Vi Visual sual Enhancement Enhancement wi with thMechanochr Mechanochromic omic Material Material Various mechanochromic and color change materials were laid on top of the buttons on Various mechanochromic and color change materials were laid on top of the buttons demand sets in order to test their capabilities in terms of enhancing the visual appearance on demand sets in order to test their capabilities in terms of enhancing the visual appear- of the set and at the same time, further distinguish the active buttons from the inactive ones. ance of the set and at the same time, further distinguish the active buttons from the inac- As mentioned in Section 2.3 and Figure 6, some of the materials that were tested were elastic tive ones. As mentioned in Section 2.3 and Figure 6, some of the materials that were tested cloth, modified holographic film, sublimation printed elastic cloth, and elastomer-based were elastic cloth, modified holographic film, sublimation printed elastic cloth, and elas- transparency film. tomer-based transparency film. In general, all the materials tested in our work showed promising abilities in enhancing In general, all the materials tested in our work showed promising abilities in enhanc- the visual appearance of the active buttons. The most promising result was provided by ing the visual appearance of the active buttons. The most promising result was provided the sublimation printed elastic cloth. This particular cloth was relatively thin, highly by the sublimation printed elastic cloth. This particular cloth was relatively thin, highly Actuators 2021, 10, x FOR PEER REVIEW 12 of 14 stretchable, and easy to handle. Figure 15 shows the appearance of the cloth covered button stretchable, and easy to handle. Figure 15 shows the appearance of the cloth covered but- set when the buttons are inactive (Figure 15A) and when the buttons are active (Figure 15B). ton set when the buttons are inactive (Figure 15A) and when the buttons are active (Figure From Figure 15A, it can be observed that when the buttons are inactive, the cloth will fully 15B). From Figure 15A, it can be observed that when the buttons are inactive, the cloth will fully conform to the flat surface of the buttons. On the other hand, when the buttons conform to the flat surface of the buttons. On the other hand, when the buttons are active are active (Figure 15B), due to the 1.5 mm rise of the buttons, the portion of the cloth that (Figure 15B), due to the 1.5 mm rise of the buttons, the portion of the cloth that is directly is directly on top of the buttons will be stretched and therefore, become lighter in color— on top of the buttons will be stretched and therefore, become lighter in color—especially especially along the border of the buttons. This is because the elastic cloth adheres to the along the border of the buttons. This is because the elastic cloth adheres to the top cover of top cover of the button set, the button set, but it does but i not t does not a adhere todthe here to the buttons. In buttons. In our test, our test, thi this particular s particloth cular performs cloth perform better s better than other elastic c than other elastic clothslbecause oths because onceonce the protrusions of the buttons the protrusions of the buttons are r aemoved re removed when when the but the buttons tons return to i return to inactive nactistate, ve stathe te, the cl clothoth instantaneously instantaneousl returns y returns to being to being flat, flat, i.e., i.e., no no re residual sidustr al stretchi etching ng observed observed along along the border of the border of the the b buttons. uttons. Figure Figure 15. 15. Sublimation Sublimation printed ela printed elastic stic cloth cloth cover covered button set, ( ed button set, (A A)) when when buttons buttons are ina are inactive ctive and and (B) when buttons are active. (B) when buttons are active. Other color change materials such as the modified holographic film and elastomer- Other color change materials such as the modified holographic film and elastomer- based transparency film also showed promising abilities; however, both of them possess based transparency film also showed promising abilities; however, both of them possess some limitations that need to be addressed before they can be mounted on top of the some limitations that need to be addressed before they can be mounted on top of the but- buttons on demand set. In the case of the modified holographic film, from our test, we tons on demand set. In the case of the modified holographic film, from our test, we noticed that this particular film is relatively rigid. As such, the protrusion of buttons larger than 1 mm may cause the film to break, especially along the border of the buttons. On the other hand, for the elastomer-based transparency film, which has nanometer-sized metal parti- cles being sputtered on top of the elastomer film, some design parameters need to be ad- dressed so that the biaxial strain applied by the protrusion of the buttons could generate change in the transparency of the film, as illustrated in Figure 16. Figure 16. Change in transparency of elastomer-based transparency film resulting in visual clarity of symbol underneath. 5. Future Works Although the work that we performed successfully demonstrated the working con- cept of buttons on demand, including the successful installation and simulation of the device in the car, there are some aspects that can be addressed and improved before the Actuators 2021, 10, x FOR PEER REVIEW 12 of 14 will fully conform to the flat surface of the buttons. On the other hand, when the buttons are active (Figure 15B), due to the 1.5 mm rise of the buttons, the portion of the cloth that is directly on top of the buttons will be stretched and therefore, become lighter in color— especially along the border of the buttons. This is because the elastic cloth adheres to the top cover of the button set, but it does not adhere to the buttons. In our test, this particular cloth performs better than other elastic cloths because once the protrusions of the buttons are removed when the buttons return to inactive state, the cloth instantaneously returns to being flat, i.e., no residual stretching observed along the border of the buttons. Figure 15. Sublimation printed elastic cloth covered button set, (A) when buttons are inactive and (B) when buttons are active. Other color change materials such as the modified holographic film and elastomer- Actuators 2021, 10, 251 12 of 13 based transparency film also showed promising abilities; however, both of them possess some limitations that need to be addressed before they can be mounted on top of the but- tons on demand set. In the case of the modified holographic film, from our test, we noticed that this particular film is relatively rigid. As such, the protrusion of buttons larger than 1 noticed that this particular film is relatively rigid. As such, the protrusion of buttons larger mm may cause the film to break, especially along the border of the buttons. On the other than 1 mm may cause the film to break, especially along the border of the buttons. On the hand, for the elastomer-based transparency film, which has nanometer-sized metal parti- other hand, for the elastomer-based transparency film, which has nanometer-sized metal cles being sputtered on top of the elastomer film, some design parameters need to be ad- particles being sputtered on top of the elastomer film, some design parameters need to be dressed so that the biaxial strain applied by the protrusion of the buttons could generate addressed so that the biaxial strain applied by the protrusion of the buttons could generate change in the transparency of the film, as illustrated in Figure 16. change in the transparency of the film, as illustrated in Figure 16. Figure 16. Change in transparency of elastomer-based transparency film resulting in visual clarity Figure 16. Change in transparency of elastomer-based transparency film resulting in visual clarity of of symbol underneath. symbol underneath. 5. Future Works 5. Future Works Although the work that we performed successfully demonstrated the working concept Although the work that we performed successfully demonstrated the working con- of buttons on demand, including the successful installation and simulation of the device cept of buttons on demand, including the successful installation and simulation of the in the car, there are some aspects that can be addressed and improved before the device device in the car, there are some aspects that can be addressed and improved before the starts to be used commercially. One area that can be further improved is the insulation of the heat generated within the SMA spring so as to eliminate any safety risks of burns and fire as well as cool down the SMA spring after its activation so that the SMA spring can be repeatedly activated at ease. Furthermore, similarly to the smart surface [19], the test on the long-term lifecycle of the buttons on demand set at various ambient temperatures also needs to be conducted before this particular device can be used commercially. In addition, other SMA materials that are relatively flat can be explored in order to reduce the overall thickness of the device set. Although the current SMA spring was working fine in our test, one of the issues that we noticed was the relatively bulky size of the SMA spring as it comes in a solenoid form. Other SMA materials that are flat and relatively pliable can be heat-treated into a flat coil shape and this can potentially significantly reduce the thickness. Early work on flat SMA coil has been performed; however, further tests, especially on its lifecycle, will be performed in the future. 6. Conclusions In this paper, we presented a novel buttons on demand concept driven by smart materials and smart mechanical design. Here, the buttons will protrude and appear when activated and remain flat and concealed when inactive. As a result, the possibilities of wrong input by the driver can be minimized, enabling the driver to maintain their attention on the road while driving. In our work, we made several device prototypes using relatively inexpensive materials, such as oxidized aluminum, PMMA cover, and Ni–Ti shape memory alloy, and successfully integrated and simulated its working principle together with the user interface simulation in the car. Since the smart mechanical design enables the patterns and positions of the buttons to be locked, only a short application of electrical current is required as the power source from the off-the-shelf batteries. We also successfully covered the button set with elastic cloth, which improves the visual appearance of the set in addition to distinguishing the active buttons from the inactive ones. Actuators 2021, 10, 251 13 of 13 Author Contributions: Conceptualization, M.S.K., G.K.L. and W.M.H.; data curation, C.R.; formal analysis, H.M.C.; funding acquisition, W.M.H.; investigation, C.R., M.S.K., H.M.C. and W.M.H.; methodology, M.S.K. and G.K.L.; project administration, W.M.H.; supervision, W.M.H.; visualization, M.S.K.; writing—original draft, C.R.; writing—review and editing, M.S.K. and W.M.H. All authors have read and agreed to the published version of the manuscript. Funding: This project was supported by BMW-NTU Joint R&D program and AcRF Tier 1 (RG172/15), Singapore. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The authors exclude this statement. Acknowledgments: We thank Michael Haller and Christian Rendl from the Media Interaction Lab of University of Applied Sciences Upper Austria (Hagenberg, Austria) for constructive discussions and help. 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Actuators – Multidisciplinary Digital Publishing Institute
Published: Sep 29, 2021
Keywords: buttons on demand; smart material; shape memory material; mechanical design; mechanism; buttons interaction
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