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Estimating Gripping Forces During Robot- Assisted Surgery Based on Motor Current

Estimating Gripping Forces During Robot- Assisted Surgery Based on Motor Current DE GRUYTER Current Directions in Biomedical Engineering 2022;8(1): 105-108 Max B. Schäfer*, Johannes G. Meiringer, Julia Nawratil, Lukas Worbs, Giuliano A. Giacoppo and Peter P. Pott Estimating Gripping Forces During Robot- Assisted Surgery Based on Motor Current https://doi.org/10.1515/cdbme-2022-0027 implemented in the surgical tools, thus might add considerable cost to the setup [3]. The use of existing surgical instruments Abstract: Accurate measurement of interaction forces during for RAS would be beneficial regarding economical and robot-assisted surgery requires compact force sensing infrastructural aspects, such as facilitating the adoption into modalities in the surgical tools, thus might add considerable clinical workflows, to enable widespread usage, and to enable cost to the setup. Measuring the motor current to estimate a versatile application of various instruments in different gripping forces, is an advantageous approach since no surgical scenarios. There are different methods to enhance expensive force sensor is needed. In this paper, a mechanical instruments with a gripping force measurement, such as interface is presented, which allows actuating conventional obtaining forces or torques in the drive train of the instrument articulated instruments for robot-assisted surgery. The or the motor current [4 - 6]. In contrast to the development of interface features the estimation of static gripping forces at the instruments with integrated force sensing capabilities, such as instrument’s tip based on the motor current. The evaluation by KIM et al. [7] or THOLEY et al. [8], the enhancement of shows reproducible results, and the current-based approach existing instruments with a sensing approach at the proximal seems to be a cost-efficient way to estimate gripping forces. part of the instrument seems to be a resource-efficient alternative. Especially the estimation of gripping forces based Keywords: robot-assisted surgery, articulated instruments, on the motor current is advantageous, since no expensive force gripping forces, haptic feedback, telemanipulation sensors are needed, and presumably, a rough estimation of the acting gripping force is sufficient for the user to achieve a more delicate interaction with the tissue. 1 Introduction In this paper, a mechanical interface is presented, which allows to robotically guide and actuate instruments with wrist During minimally invasive surgery (MIS), the limited degrees articulation based on rigid kinematics instead of the frequently of freedom (DOF) of the instrument reduces the dexterity for used cable-based actuation. The mechanical interface inherits the surgeon. Instruments for manual use with additional DOF, current sensing, enabling the estimation of gripping forces. In such as wrist-like articulation of the tip, address this issue [1], the following, the interface is briefly described, and the however, finding an intuitive user interface remains gripping force estimation is presented and assessed. challenging. In contrast, using such instruments in robot- assisted surgery (RAS) is state-of-the-art due to the greater design freedom of the user interface. To preserve a haptic 2 System Design impression for the surgeon, not only the translational forces between instrument and tissue, but also gripping forces are of relevance [2] and must be obtained and displayed to the user. 2.1 Telemanipulation System Accurate measurement of interaction and gripping forces during RAS requires compact force sensing modalities For research purposes, a telemanipulation system is used that consists of standard or commercially available components and can be easily reconfigured. As the manipulator of the ______ *Corresponding author: Max B. Schäfer: Institute of Medical telemanipulation system, a seven-DOF articulated robotic Device Technology, University of Stuttgart, Pfaffenwaldring 9, arm (Panda, Franka Emika GmbH, Munich, DE) is Stuttgart, Germany, max.schaefer@imt.uni-stuttgart.de used (Fig. 1). To guide and actuate surgical instruments, a Johannes G. Meiringer, Julia Nawratil, Lukas Worbs, Giuliano mechanical interface is mounted to the tool center point (TCP) A. Giacoppo, Peter P. Pott: Institute of Medical Device of the manipulator by a passive universal joint. As a result, Technology, University of Stuttgart, Germany Open Access. © 2022 The Author(s), published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 International License. 105 when forces are applied to the tissue by the instrument’s tip, maximum opening angle of the respective endeffector, which lateral forces onto the abdominal wall in the trocar point are is 60° for the atraumatic grasper and dissector and 45° for the accepted. With the position of the manipulator’s TCP and an needle holder. Further, the wrist joint enables a 90° articulation inertial measurement unit to determine the orientation of the angle in one direction, the endeffector rotation is infinite, and mechanical interface, the pose of the instrument’s tip can be the shaft rotation is limited due to cable guidance to 360°. obtained. The manipulator and the mechanical interface can be Additionally, to obtain the pose of the instrument’s controlled with a haptic input device that allows the user to endeffector, an inertial measurement unit (IMU) with a command poses and gripping movements to the instrument. BNO055 chip (Bosch Sensortec GmbH, Reutlingen, DE) is integrated. The actuation unit of the mechanical interface has an overall length of 246 mm and a diameter of 70 mm. 2.3 Control of the Mechanical Interface In the control unit, the control algorithms for the actuators are executed and the current sensor is read out to estimate a gripping force based on the motor current. All motors are supplied with a nominal voltage of 12 V and pulse width modulation. The current sensor is placed between power supply and motor driver board and is read out by the Arduino via I C. Averaging over four measurement values, to reduce Figure 1: Manipulator with the mechanical interface (a) and close- sensor noise, results in an overall sampling rate of 300 Hz. up view of the instruments endeffector (b). Gripping movement, as well as endeffector and shaft rotation, are controlled by a proportional controller. Due to the high friction, for the wrist articulation proportional-integral- 2.2 Mechanical Interface derivative control is applied. Since incremental encoders are The mechanical interface is designed for the r2 instruments by used and thus, no absolute position feedback is available, Tuebingen Scientific Medical (Tuebingen Scientific Medical every axis must perform a homing movement. Therefore, GmbH, Tübingen, DE). The instruments allow articulating gripping and wrist articulation move in one direction until the their endeffector around a wrist joint, rotating the endeffector motor stalls, which is detected by an increase in the motor around its axis, rotating the instrument around its shaft axis, current. Both, endeffector and shaft rotation feature reflective and a gripping movement. To access the actuation part of the optical switches (ITR 20001/T, Everlight Electronics Co. Ltd., instrument, the handle for the manual operation was removed. TW) to enable the homing procedure. The mechanical interface consists of a control unit, a universal joint, and the actuation unit with an integrated sensor unit (Fig. 1). The control unit comprises the electronic 3 Methods components necessary for the control of the actuation unit, such as an Arduino Nano (Arduino S.r.l., Monza, IT), the To evaluate the forces, which can be applied to an object with motor controllers (DRV8801, Texas Instruments Inc., US), the the instrument’s endeffector, and to validate the relation current sensor (INA219, Texas Instruments Inc., US) and between motor current and gripping force, a test rig was set communication interfaces for the integration into the up (see Fig. 2). Motor current and gripping force are expected telemanipulation system’s control architecture. The sensor to follow a linear relationship, mainly influenced by the gear unit contains the HEX 21 6-axis force and torque sensor reduction ratio and the instrument mechanics. The test rig (Wittenstein SE, Igersheim, DE), to determine translational offers the possibility to measure forces between two small interaction forces at the instrument’s tip. The actuation unit gripper branches without the need of placing a small force contains the electromechanical components for the actuation sensor between those branches. Therefore, a lever kinematic of the DOF of the instrument, such as four DC gear motors allows equiping the test rig with conventional S-shaped force with incremental encoders (Dr. Fritz Faulhaber GmbH & Co. sensors. Due to the lever kinematic, all measured values must KG, Schönaich, DE). The gripping movement is actuated by a be divided by a factor of two. A type KD40s force sensor (ME- type 1512 DC gear motor with 112:1 gear ratio and a rack and Meßsysteme GmbH, Hennigsdorf, DE) with a measurement pinion gear. The gripping movement is limited to the 106 range of ± 50 N and a sampling rate of 200 Hz was used. The and then increased by a multiple of the step width. This gripping width can be adapted to the instruments endeffector procedure was repeated until a local maximum of the type by adjusting the distance between the gripping brackets. derivative of the force signal was detected. The step in the duty cycle and thus in the motor current, which causes a change in the force signal, can be understood as the smallest differential force, which can be obtained during operation. This resolution is not limited by the current sensor but rather caused by friction of the actuation path with a high break-away force. The measurement was carried out ten times with the atraumatic gripper in the medial position. Since the test rig does not allow endeffector movements, and thus hinders the actuation from overcoming initial break-away loads caused by static friction, a 5 mm layer of polyurethane foam was placed between sensor Figure 2: Test rig for the measurement of gripping forces. and force stamp. This ensures a more realistic behavior but still The maximum gripping forces of the endeffector type allows the assumption of a constant opening angle. dissector, atraumatic grasper, and needle holder were With the suggested method of estimating gripping forces evaluated by grasping the gripping brackets with a duty cycle based on the motor current, it is not possible to determine of 100 % leading to the highest possible motor torque. The absolute gripping forces, since the position of a grasped object maximum gripping forces were measured at three between the gripping branches as well as the opening angle of measurement points on dissector and atraumatic grasper and the gripping branches changes the transmission ratio of the on one measurement position on the needle holder (Fig. 3). For instrument’s gripping mechanism. It is instead possible to all endeffector types, a wrist articulation of 0°, corresponding determine what percentage of the available actuation force is to a straight instrument, and an opening angle of 50° were provided from the actuator. chosen for the isometric measurement. For every measurement point, twelve measurements were carried out to obtain a mean maximum gripping force. On every measurement point, the 4 Results force was held for 0.5 s to reduce dynamic effects and to enable averaging to reduce noise of both, current and force The mean maximal gripping forces of the three different sensor. endeffector types are listed in Table 1 with respect to the measurement position. Table 1: Mean maximal gripping forces and standard deviation. Figure 3: Atraumatic grasper (a) and dissector (b) with proximal, Proximal Medial Distal medial, and distal measurement positions, and needle holder (c) with only the medial measurement position. Atraumatic Grasper 8.37 ± 0.28 N 4.62 ± 0.24 N 3.21 ± 0.13 N Dissector 9.02 ± 0.05 N 4.41 ± 0.03 N 3.12 ± 0.06 N To characterize the relation between motor current and Needle Holder - 16.44 ± 0.22 N - gripping force, the duty cycle was increased in steps of 4 % The relation between the measured motor current and the from 28 % to 100 % and the resulting motor currents and measured gripping force is shown in Fig. 4. gripping forces were measured. Between every step, the gripper was completely opened to reduce the influence of static friction. This resembles a scenario where the endeffector is completely opened and a user starts grasping tissue. Analogous to the maximum gripping force measurement, different measuring points were used (Fig. 3) and for every measurement point, twelve measurements were carried out. A second measurement covers the scenario, in which a user has already grasped an object and then wants to apply a higher gripping force. Therefore, a base duty cycle of 28 % was applied to the actuator and was then increased by a Figure 4: Relation between gripping force and motor current with differential step of 0.8 %. Afterwards, the gripper was respect to the different measurement positions. completely opened, again actuated with the base duty cycle, 107 The smallest differential change in the force signal, which presented setup. However, due to the user being part of the could be measured at the medial measurement position of the control loop, and thus perceiving a visual impression of the atraumatic gripper is a mean step of 0.07 ± 0.05 N, equivalent deformation of a grasped object, the relative force to 1.5 % of the mean maximal force. To overcome the break- measurement is expected to be sufficient. away load, this step is related to a mean change in the motor The use of available articulated instruments and the current of 3.86 ± 0.84 mA. The mean maximal noise estimation of gripping forces by measuring the motor current amplitude is 0.14 ± 0.06 mA. showed promising results. Additional evaluation needs to be carried out in a realistic setting with tissue-like objects instead of using the isometric measurement setup. Further, the gripping force must be evaluated under different wrist 5 Discussion articulation angles since higher friction is expected to occur. In this paper, a mechanical interface for the actuation of Acknowledgement conventional articulated instruments in a telemanipulation The authors thank the company Dr. Fritz Faulhaber GmbH & system for research purposes is presented and assessed Co. KG (Schönaich, DE) for supporting this project by regarding its ability to estimate gripping forces based on motor providing DC motors, the company Tuebingen Scientific current. Despite the high static friction and the resulting break- Medical GmbH (Tübingen, DE) for the support with surgical away load, a clear and reproducible linear relation between instruments, and the company Wittenstein SE (Igersheim, DE) motor current and force can be seen, which benefits the for the support with a force/torque sensor. estimation of interaction forces during operation. The high standard deviation of the differential force measurement is Author Statement presumably due to the high friction that leads to varying Research funding: The author state no funding involved. currents necessary to achieve a breakaway of the mechanics. Conflict of interest: Authors state no conflict of interest. A strong influence of static friction could be observed, resulting in a high break-away force necessary for initiating a movement of the gripping branches. Especially during the References isometric measurements, this hindered the setup from applying [1] Anderson PL, Lathrop RA, Webster RJ III. Robot-like dexterity forces to the test rig. Since such a stiff object is not very likely without computers and motors: a review of hand-held to appear in a realistic setting, a flexible element was placed in laparoscopic instruments with wrist-like tip articulation. Expert the measurement setup to allow for a slight displacement and Rev Med Devices, vol. 13,7: 661-72, 2016. thus avoiding a complete blocking of the instrument’s [2] Gibo TL, Bastian AJ, Okamura AM. Grip Force Control during Virtual Object Interaction: Effect of Force Feedback, Accuracy mechanism. In the case of static measurements, the flexible Demands, and Training. IEEE Transactions on Haptics, vol. 7, element is assumed to not have any influence on the absolute no. 1, pp. 37-47, Jan.-March 2014. force measured. To improve the mechanical setup of the [3] Okamura AM, Verner LN, Yamamoto T, Gwilliam JC, Griffiths interface in the future, the friction can be reduced by using PG. Force Feedback and Sensory Substitution for Robot- Assisted Surgery. Rosen, J., Hannaford, B., Satava, R. (eds) better tribological pairing and lubricant or by reducing the Surgical Robotics. Springer, Boston, MA. 2011. reduction ratio of the actuator and thus achieving a back- [4] Li Y, Miyasaka M, Haghighipanah M, Cheng L, Hannaford B. drivable system. While the test rig is well suited for static Dynamic modeling of cable driven elongated surgical measurements, due to its high mass, dynamic measurements instruments for sensorless grip force estimation. IEEE Int. Conf. on Robotics and Automation (ICRA), 2016. of fast changing gripping forces are only possible in limited [5] Lee DH, Kim U, Gulrez T, Yoon WJ, Hannaford B, Choi HR. A extent. Despite the strong influence of the friction, the Laparoscopic Grasping Tool With Force Sensing Capability. measurements show reproducible results for all endeffector IEEE/ASME Transactions on Mechatronics, vol. 21, no. 1, pp. types and measurement positions, which benefits the goal to 130-141, 2016. provide haptic feedback to the user concerning the gripping [6] Zhao B, Nelson CA. Estimating Tool-Tissue Forces Using a 3-Degree-of-Freedom Robotic Surgical Tool. J Mech Robot. force. The presented approach seems to be a cost-efficient and easy-to-implement way to estimate gripping forces and enable [7] Kim U, Kim YB, So J, Seok D-Y, Choi HR. Sensorized Surgical to distinguish if an object is grasped too tight or too weak. Forceps for Robotic-Assisted Minimally Invasive Surgery. IEEE To provide feedback with absolute forces, the position of Transactions on Industrial Electronics, 2018. [8] Tholey G, Pillarisetti A, Green W, Desai JP. Design, a grasped object within the gripper branches, as well as the Development, and Testing of an Automated Laparoscopic opening angle of the gripper branches, must be known, thus, Grasper with 3-D Force Measurement Capability. International only relative force feedback can be provided with the Symposium on Medical Simulation, 2004. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Current Directions in Biomedical Engineering de Gruyter

Estimating Gripping Forces During Robot- Assisted Surgery Based on Motor Current

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© 2022 by Walter de Gruyter Berlin/Boston
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Abstract

DE GRUYTER Current Directions in Biomedical Engineering 2022;8(1): 105-108 Max B. Schäfer*, Johannes G. Meiringer, Julia Nawratil, Lukas Worbs, Giuliano A. Giacoppo and Peter P. Pott Estimating Gripping Forces During Robot- Assisted Surgery Based on Motor Current https://doi.org/10.1515/cdbme-2022-0027 implemented in the surgical tools, thus might add considerable cost to the setup [3]. The use of existing surgical instruments Abstract: Accurate measurement of interaction forces during for RAS would be beneficial regarding economical and robot-assisted surgery requires compact force sensing infrastructural aspects, such as facilitating the adoption into modalities in the surgical tools, thus might add considerable clinical workflows, to enable widespread usage, and to enable cost to the setup. Measuring the motor current to estimate a versatile application of various instruments in different gripping forces, is an advantageous approach since no surgical scenarios. There are different methods to enhance expensive force sensor is needed. In this paper, a mechanical instruments with a gripping force measurement, such as interface is presented, which allows actuating conventional obtaining forces or torques in the drive train of the instrument articulated instruments for robot-assisted surgery. The or the motor current [4 - 6]. In contrast to the development of interface features the estimation of static gripping forces at the instruments with integrated force sensing capabilities, such as instrument’s tip based on the motor current. The evaluation by KIM et al. [7] or THOLEY et al. [8], the enhancement of shows reproducible results, and the current-based approach existing instruments with a sensing approach at the proximal seems to be a cost-efficient way to estimate gripping forces. part of the instrument seems to be a resource-efficient alternative. Especially the estimation of gripping forces based Keywords: robot-assisted surgery, articulated instruments, on the motor current is advantageous, since no expensive force gripping forces, haptic feedback, telemanipulation sensors are needed, and presumably, a rough estimation of the acting gripping force is sufficient for the user to achieve a more delicate interaction with the tissue. 1 Introduction In this paper, a mechanical interface is presented, which allows to robotically guide and actuate instruments with wrist During minimally invasive surgery (MIS), the limited degrees articulation based on rigid kinematics instead of the frequently of freedom (DOF) of the instrument reduces the dexterity for used cable-based actuation. The mechanical interface inherits the surgeon. Instruments for manual use with additional DOF, current sensing, enabling the estimation of gripping forces. In such as wrist-like articulation of the tip, address this issue [1], the following, the interface is briefly described, and the however, finding an intuitive user interface remains gripping force estimation is presented and assessed. challenging. In contrast, using such instruments in robot- assisted surgery (RAS) is state-of-the-art due to the greater design freedom of the user interface. To preserve a haptic 2 System Design impression for the surgeon, not only the translational forces between instrument and tissue, but also gripping forces are of relevance [2] and must be obtained and displayed to the user. 2.1 Telemanipulation System Accurate measurement of interaction and gripping forces during RAS requires compact force sensing modalities For research purposes, a telemanipulation system is used that consists of standard or commercially available components and can be easily reconfigured. As the manipulator of the ______ *Corresponding author: Max B. Schäfer: Institute of Medical telemanipulation system, a seven-DOF articulated robotic Device Technology, University of Stuttgart, Pfaffenwaldring 9, arm (Panda, Franka Emika GmbH, Munich, DE) is Stuttgart, Germany, max.schaefer@imt.uni-stuttgart.de used (Fig. 1). To guide and actuate surgical instruments, a Johannes G. Meiringer, Julia Nawratil, Lukas Worbs, Giuliano mechanical interface is mounted to the tool center point (TCP) A. Giacoppo, Peter P. Pott: Institute of Medical Device of the manipulator by a passive universal joint. As a result, Technology, University of Stuttgart, Germany Open Access. © 2022 The Author(s), published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 International License. 105 when forces are applied to the tissue by the instrument’s tip, maximum opening angle of the respective endeffector, which lateral forces onto the abdominal wall in the trocar point are is 60° for the atraumatic grasper and dissector and 45° for the accepted. With the position of the manipulator’s TCP and an needle holder. Further, the wrist joint enables a 90° articulation inertial measurement unit to determine the orientation of the angle in one direction, the endeffector rotation is infinite, and mechanical interface, the pose of the instrument’s tip can be the shaft rotation is limited due to cable guidance to 360°. obtained. The manipulator and the mechanical interface can be Additionally, to obtain the pose of the instrument’s controlled with a haptic input device that allows the user to endeffector, an inertial measurement unit (IMU) with a command poses and gripping movements to the instrument. BNO055 chip (Bosch Sensortec GmbH, Reutlingen, DE) is integrated. The actuation unit of the mechanical interface has an overall length of 246 mm and a diameter of 70 mm. 2.3 Control of the Mechanical Interface In the control unit, the control algorithms for the actuators are executed and the current sensor is read out to estimate a gripping force based on the motor current. All motors are supplied with a nominal voltage of 12 V and pulse width modulation. The current sensor is placed between power supply and motor driver board and is read out by the Arduino via I C. Averaging over four measurement values, to reduce Figure 1: Manipulator with the mechanical interface (a) and close- sensor noise, results in an overall sampling rate of 300 Hz. up view of the instruments endeffector (b). Gripping movement, as well as endeffector and shaft rotation, are controlled by a proportional controller. Due to the high friction, for the wrist articulation proportional-integral- 2.2 Mechanical Interface derivative control is applied. Since incremental encoders are The mechanical interface is designed for the r2 instruments by used and thus, no absolute position feedback is available, Tuebingen Scientific Medical (Tuebingen Scientific Medical every axis must perform a homing movement. Therefore, GmbH, Tübingen, DE). The instruments allow articulating gripping and wrist articulation move in one direction until the their endeffector around a wrist joint, rotating the endeffector motor stalls, which is detected by an increase in the motor around its axis, rotating the instrument around its shaft axis, current. Both, endeffector and shaft rotation feature reflective and a gripping movement. To access the actuation part of the optical switches (ITR 20001/T, Everlight Electronics Co. Ltd., instrument, the handle for the manual operation was removed. TW) to enable the homing procedure. The mechanical interface consists of a control unit, a universal joint, and the actuation unit with an integrated sensor unit (Fig. 1). The control unit comprises the electronic 3 Methods components necessary for the control of the actuation unit, such as an Arduino Nano (Arduino S.r.l., Monza, IT), the To evaluate the forces, which can be applied to an object with motor controllers (DRV8801, Texas Instruments Inc., US), the the instrument’s endeffector, and to validate the relation current sensor (INA219, Texas Instruments Inc., US) and between motor current and gripping force, a test rig was set communication interfaces for the integration into the up (see Fig. 2). Motor current and gripping force are expected telemanipulation system’s control architecture. The sensor to follow a linear relationship, mainly influenced by the gear unit contains the HEX 21 6-axis force and torque sensor reduction ratio and the instrument mechanics. The test rig (Wittenstein SE, Igersheim, DE), to determine translational offers the possibility to measure forces between two small interaction forces at the instrument’s tip. The actuation unit gripper branches without the need of placing a small force contains the electromechanical components for the actuation sensor between those branches. Therefore, a lever kinematic of the DOF of the instrument, such as four DC gear motors allows equiping the test rig with conventional S-shaped force with incremental encoders (Dr. Fritz Faulhaber GmbH & Co. sensors. Due to the lever kinematic, all measured values must KG, Schönaich, DE). The gripping movement is actuated by a be divided by a factor of two. A type KD40s force sensor (ME- type 1512 DC gear motor with 112:1 gear ratio and a rack and Meßsysteme GmbH, Hennigsdorf, DE) with a measurement pinion gear. The gripping movement is limited to the 106 range of ± 50 N and a sampling rate of 200 Hz was used. The and then increased by a multiple of the step width. This gripping width can be adapted to the instruments endeffector procedure was repeated until a local maximum of the type by adjusting the distance between the gripping brackets. derivative of the force signal was detected. The step in the duty cycle and thus in the motor current, which causes a change in the force signal, can be understood as the smallest differential force, which can be obtained during operation. This resolution is not limited by the current sensor but rather caused by friction of the actuation path with a high break-away force. The measurement was carried out ten times with the atraumatic gripper in the medial position. Since the test rig does not allow endeffector movements, and thus hinders the actuation from overcoming initial break-away loads caused by static friction, a 5 mm layer of polyurethane foam was placed between sensor Figure 2: Test rig for the measurement of gripping forces. and force stamp. This ensures a more realistic behavior but still The maximum gripping forces of the endeffector type allows the assumption of a constant opening angle. dissector, atraumatic grasper, and needle holder were With the suggested method of estimating gripping forces evaluated by grasping the gripping brackets with a duty cycle based on the motor current, it is not possible to determine of 100 % leading to the highest possible motor torque. The absolute gripping forces, since the position of a grasped object maximum gripping forces were measured at three between the gripping branches as well as the opening angle of measurement points on dissector and atraumatic grasper and the gripping branches changes the transmission ratio of the on one measurement position on the needle holder (Fig. 3). For instrument’s gripping mechanism. It is instead possible to all endeffector types, a wrist articulation of 0°, corresponding determine what percentage of the available actuation force is to a straight instrument, and an opening angle of 50° were provided from the actuator. chosen for the isometric measurement. For every measurement point, twelve measurements were carried out to obtain a mean maximum gripping force. On every measurement point, the 4 Results force was held for 0.5 s to reduce dynamic effects and to enable averaging to reduce noise of both, current and force The mean maximal gripping forces of the three different sensor. endeffector types are listed in Table 1 with respect to the measurement position. Table 1: Mean maximal gripping forces and standard deviation. Figure 3: Atraumatic grasper (a) and dissector (b) with proximal, Proximal Medial Distal medial, and distal measurement positions, and needle holder (c) with only the medial measurement position. Atraumatic Grasper 8.37 ± 0.28 N 4.62 ± 0.24 N 3.21 ± 0.13 N Dissector 9.02 ± 0.05 N 4.41 ± 0.03 N 3.12 ± 0.06 N To characterize the relation between motor current and Needle Holder - 16.44 ± 0.22 N - gripping force, the duty cycle was increased in steps of 4 % The relation between the measured motor current and the from 28 % to 100 % and the resulting motor currents and measured gripping force is shown in Fig. 4. gripping forces were measured. Between every step, the gripper was completely opened to reduce the influence of static friction. This resembles a scenario where the endeffector is completely opened and a user starts grasping tissue. Analogous to the maximum gripping force measurement, different measuring points were used (Fig. 3) and for every measurement point, twelve measurements were carried out. A second measurement covers the scenario, in which a user has already grasped an object and then wants to apply a higher gripping force. Therefore, a base duty cycle of 28 % was applied to the actuator and was then increased by a Figure 4: Relation between gripping force and motor current with differential step of 0.8 %. Afterwards, the gripper was respect to the different measurement positions. completely opened, again actuated with the base duty cycle, 107 The smallest differential change in the force signal, which presented setup. However, due to the user being part of the could be measured at the medial measurement position of the control loop, and thus perceiving a visual impression of the atraumatic gripper is a mean step of 0.07 ± 0.05 N, equivalent deformation of a grasped object, the relative force to 1.5 % of the mean maximal force. To overcome the break- measurement is expected to be sufficient. away load, this step is related to a mean change in the motor The use of available articulated instruments and the current of 3.86 ± 0.84 mA. The mean maximal noise estimation of gripping forces by measuring the motor current amplitude is 0.14 ± 0.06 mA. showed promising results. Additional evaluation needs to be carried out in a realistic setting with tissue-like objects instead of using the isometric measurement setup. Further, the gripping force must be evaluated under different wrist 5 Discussion articulation angles since higher friction is expected to occur. In this paper, a mechanical interface for the actuation of Acknowledgement conventional articulated instruments in a telemanipulation The authors thank the company Dr. Fritz Faulhaber GmbH & system for research purposes is presented and assessed Co. KG (Schönaich, DE) for supporting this project by regarding its ability to estimate gripping forces based on motor providing DC motors, the company Tuebingen Scientific current. Despite the high static friction and the resulting break- Medical GmbH (Tübingen, DE) for the support with surgical away load, a clear and reproducible linear relation between instruments, and the company Wittenstein SE (Igersheim, DE) motor current and force can be seen, which benefits the for the support with a force/torque sensor. estimation of interaction forces during operation. The high standard deviation of the differential force measurement is Author Statement presumably due to the high friction that leads to varying Research funding: The author state no funding involved. currents necessary to achieve a breakaway of the mechanics. Conflict of interest: Authors state no conflict of interest. A strong influence of static friction could be observed, resulting in a high break-away force necessary for initiating a movement of the gripping branches. Especially during the References isometric measurements, this hindered the setup from applying [1] Anderson PL, Lathrop RA, Webster RJ III. Robot-like dexterity forces to the test rig. Since such a stiff object is not very likely without computers and motors: a review of hand-held to appear in a realistic setting, a flexible element was placed in laparoscopic instruments with wrist-like tip articulation. Expert the measurement setup to allow for a slight displacement and Rev Med Devices, vol. 13,7: 661-72, 2016. thus avoiding a complete blocking of the instrument’s [2] Gibo TL, Bastian AJ, Okamura AM. Grip Force Control during Virtual Object Interaction: Effect of Force Feedback, Accuracy mechanism. In the case of static measurements, the flexible Demands, and Training. IEEE Transactions on Haptics, vol. 7, element is assumed to not have any influence on the absolute no. 1, pp. 37-47, Jan.-March 2014. force measured. To improve the mechanical setup of the [3] Okamura AM, Verner LN, Yamamoto T, Gwilliam JC, Griffiths interface in the future, the friction can be reduced by using PG. Force Feedback and Sensory Substitution for Robot- Assisted Surgery. Rosen, J., Hannaford, B., Satava, R. (eds) better tribological pairing and lubricant or by reducing the Surgical Robotics. Springer, Boston, MA. 2011. reduction ratio of the actuator and thus achieving a back- [4] Li Y, Miyasaka M, Haghighipanah M, Cheng L, Hannaford B. drivable system. While the test rig is well suited for static Dynamic modeling of cable driven elongated surgical measurements, due to its high mass, dynamic measurements instruments for sensorless grip force estimation. IEEE Int. Conf. on Robotics and Automation (ICRA), 2016. of fast changing gripping forces are only possible in limited [5] Lee DH, Kim U, Gulrez T, Yoon WJ, Hannaford B, Choi HR. A extent. Despite the strong influence of the friction, the Laparoscopic Grasping Tool With Force Sensing Capability. measurements show reproducible results for all endeffector IEEE/ASME Transactions on Mechatronics, vol. 21, no. 1, pp. types and measurement positions, which benefits the goal to 130-141, 2016. provide haptic feedback to the user concerning the gripping [6] Zhao B, Nelson CA. Estimating Tool-Tissue Forces Using a 3-Degree-of-Freedom Robotic Surgical Tool. J Mech Robot. force. The presented approach seems to be a cost-efficient and easy-to-implement way to estimate gripping forces and enable [7] Kim U, Kim YB, So J, Seok D-Y, Choi HR. Sensorized Surgical to distinguish if an object is grasped too tight or too weak. Forceps for Robotic-Assisted Minimally Invasive Surgery. IEEE To provide feedback with absolute forces, the position of Transactions on Industrial Electronics, 2018. [8] Tholey G, Pillarisetti A, Green W, Desai JP. Design, a grasped object within the gripper branches, as well as the Development, and Testing of an Automated Laparoscopic opening angle of the gripper branches, must be known, thus, Grasper with 3-D Force Measurement Capability. International only relative force feedback can be provided with the Symposium on Medical Simulation, 2004.

Journal

Current Directions in Biomedical Engineering – de Gruyter

Published: Jul 1, 2022

Keywords: robot-assisted surgery; articulated instruments; gripping forces; haptic feedback; telemanipulation

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