Design and FEM Analysis of Miniature Torque Sensor for Finger Exoskeleton

Compliant torque sensors are needed in joints of hand and finger exoskeletons for enabling accurate and safe interaction through Series Elastic Actuation (SEA). SEAs normally 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.

I. Introduction

In United States about 795000 Stroke patients appear annually [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 rehabilitation 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.