Joint Torque Sensory in Robotics

Abstract

Joint Torque sensory Feedback (JTF) can substantially improve the performance of a robotic system. JTF makes it possible to achieve dynamic control of a robotic manipulator without the need for modeling its link dynamics. Moreover, it has been proved that JTF can achieve a precise torque tracking in a manipulator joint by compensating the e ect of joint friction and actuator nonlinearities. Despite these advantages, accurate joint torque measurement encounters several practical challenges. Since much of the torque/force reaction of the link load on the joints appears in the form of nontorsional components, e.g. overhung force and bending moment, the torque sensing device has to not only bear but remain insensitive to these force/moment components. In addition, it is desirable to design the structure of the sensor such that it generates a large strain for a given load torque and therefore has a high sensitivity. However, this is in conflict with the high-sti ness requirement for minimizing the joint angle error introduced by the sensor. The main objectives of this chapter are twofold: Firstly, in Sections 2 and 3, we describe the technical challenges and practical solutions to accurate measurement of joint torques in the presence of the non-torsional components in a robot joint. For a torque sensing device, di erent mechanical structures will be examined and their properties, such as sensitivity and sti ness in di erent directions and decoupling, will be explored. This allows a systematic design of a sensor structure which is suitable for torque measurement in a robot joint. Finally, the state-of-the-art and design of a torque sensor for robots will be presented in Section 4. The design achieves the conflicting requirements of high sti ness for all six force and torque components, high sensitivity for the one driving torque of interest, yet very low sensitivity for the other five force/torque components. These properties, combined with its donut shape and small size make this sensor an ideal choice for direct drive robotic applications. Experimental data validates the basic design ideas underlying the sensor’s geometry, the finite element model utilized in its optimization, and the advertised performance. The second objective is to describe the application of joint torque sensory feedback (JTF)in robot control. The main advantages claimed for JTF are (i) it can simplify the complexity of the system dynamics by eliminating its link dynamics; and (ii) the control system is inherently robust with respect to both external force disturbance and parameter variation. First, assuming both actuator and torque sensor are ideal, we describe robot control with JTF in Section 5. Then, development of an adaptive JTF is presented in Section 6that requires only the incorporation of uncalibrated joint torque signals, i.e., the gains and o sets of multiple sensors are unknown. Also, all physical parameters of the joints including inertia of the rotors, link twist angles, and friction parameters are assumed unknown to the