Design and FEM Analysis of Miniature Torque Sensor for Finger Exoskeleton Faryal Gula Department of Mechatronics Engineering National University of Sciences and Technology Islamabad, Pakistan fgula.mts17ceme@student.nust.edu.pk Department of Avionics Engineering National University of Sciences and Technology Islamabad, Pakistan h.munawar@cae.nust.edu.pk Department of Mechatronics Engineering National University of Sciences and Technology Islamabad, Pakistan a.hamza@ceme.nust.edu.pk Abstract—Compliant torque sensors are needed in joints of hand and finger exoskeletons for enabling accurate and safe interaction through Series Elastic Actuation (SEA). SEAs nor- mally use springs in coordination with displacement sensors such as photo interrupters, magnetic encoders and optical encoders. Using these sensors makes the compliant torque sensor complex and relatively large. In this paper, a novel compliant element is presented whose torque sensing uses the principle of strain measurement. Combined together the torque sensor is simpler, more reliable and smaller in size. The design consists of a single spiral link as a medium between the fixed outer circumference and a rotating center. Poly-lactic acid is selected as the material for the monolithic design using additive manufacturing technique for fabrication. FEM analysis shows that desired results have been achieved. Index Terms—Sensor, Torque, Exoskeleton, Rehabilitation. I. I NTRODUCTION In United States about 795000 Stroke patients appear annu- ally [1]. In Pakistan although the data is not available but the situation is likely to be even worse. For rehabilitation of stroke patients, wearable robots are used. In the design of these robots, sensors are used to measure torque, force, position and trajectory of joints. Accurate torque sensing is the basic requirement of all physical human robot interactions. There are techniques to mitigate the errors and inaccuracies but to accomplish that task there is a need to measure these errors within a system. For this purpose sensors are installed at locations vulnerable to errors. Such is the case with rehabil- itation exoskeletons where a machine is physically interacting with a human bodypart. The force, torque and trajectory that is being generated by exoskeleton the human should be very accurate as threshold of applied force for human beings should not be breached. In exoskeleton design for rehabilitation of human finger, torque sensor is required that can measure the single axis force applied. For human machine interaction in rehabilitation there is a dire need of low stiffness materials and soft robotics design. Many researchers have studied torque sensors employed in exoskeletons. These sensors need a transducer for measure- ment which can be encoders, photo interrupters, strain gauges etc. Some researchers have designed customized transducers while others use off the shelf transducers. Norman et al [2] used photointerrupters in torque sensor designs ranging from a simple customized 1-DOF force sensor to fully decoupled six-axis F/T sensors of different shapes. The author did not present the theoretical model of his design as the sensor was manufactured using FFF 3D printing technique, which results in an anisotropic material. A human finger can exert a torque at maximum of 0.3Nm [3]. In another study, a compliant frame of ABS having 3 links ensemble LED and photo-detector for measurement of applied torque is implemented [4]. A linkage structure was designed for hand rehabilitation has springs that are attached to the bend link to provide force when finger deviated from the desired posture [5]. The guiding force is calculated through a potentiometer as shown in figure 1 that measures the deflection of spring. Fig. 1: Potentiometer based torque sensor in finger exoskeleton Miniature Bowden Cable Series Elastic Actuation is used in Finger exoskeleton design for accurate torque control, high backdrivability, low reflected inertia, and comfortable and safe interaction with the device [6]. Measurements of joint displacement are obtained using the magnetic angle sensor mounted at the joint and the motor encoder. Hence the torque control problem is converted to a position control problem. Andrea et al [7] proposed two sensor designs that are single- axis torque-sensitive miniaturized compliant elastic elements. The first is a cam-based design with 28 mm length and 17 mm diameter and the second is based on tangential springs of 30 mm length and 16 mm diameter . The estimated stiffness of second design is 1.985 Nm/rad resulting in precise torque and angle measurements. The limitation of this design is geometric Hammad Munawar Amir Hamza 2021 International Conference on Robotics and Automation in Industry (ICRAI) | 978-1-6654-2343-4/21/$31.00 ©2021 IEEE | DOI: 10.1109/ICRAI54018.2021.9651427 978-1-6654-2343-4/21/$31.00 © 2021 IEEE Authorized licensed use limited to: NUST School of Electrical Engineering and Computer Science (SEECS). Downloaded on January 07,2022 at 13:26:31 UTC from IEEE Xplore. Restrictions apply.