Structurally integrated joint torque sensors in an assistive upper limb exoskeleton

This paper describes the design process of a torque sensor, which is structurally integrated into the kinematic chain of an assistive upper limb exoskeleton to measure user interaction. Two different approaches of torque sensor integration are discussed regarding the nominal torque loads on each joint and applied transverse stresses. The final sensor concept consists of an aluminum spoke wheel deformation element with four strain gauges connected to a Wheatstone bridge. It is mounted between the motor casing and gear drive of each joint to measure the torque of the actuator and user. A sensor electronics board including signal processing and an EtherCAT interface is integrated in each joint. The torque sensor shows linear transmission characteristics for 0.4 Nm nominal rated torque with a linear error of 0.51 % and a hysteresis error of 0.53 %.