Dual-Stage Adaptive Friction Compensation for Precise Load Side Position Tracking of Indirect Drive Mechanisms

This paper investigates the friction compensation of an indirect drive mechanism in the absence of precise end-effector position measurement. Unlike direct drive mechanisms, a typical indirect drive mechanism in the mechanical systems is composed of an actuator (motor side) and an inertia load (load side/end-effector), separated by the gear transmission. Friction in the gear transmission makes the dynamics of the indirect drive mechanism complicated and the precise position control of the end-effector challenging. More specifically, when joint compliance exists in the gear transmission, it is not simple to observe and compensate for the friction. In this paper, a mathematical model and its parameter adaptation method are proposed for the estimation and compensation of the friction in indirect drive mechanisms. For the ultimate objective of tracking the predefined load side reference trajectory, we feed back the estimated friction into the control system by refining the motor side reference trajectory, as well as by adding a feedforward signal to the control input of the motor. Both methods (i.e., the torque compensation and the motor side reference modification) are combined to effectively reject the friction effects in indirect drive mechanisms. The success of the two friction compensation methods depends on their effective integration. For this purpose, a hybrid decision making is adopted to engage or disengage the load side compensation algorithm when the task is repetitive. The proposed method is verified by experimental results on a single-joint indirect drive testbed.