Sliding mode based rejection of load and torque ripple in a direct-drive 2 DOF robot arm

This study aims to demonstrate the importance of torque ripple and its elimination for both position control and trajectory tracking applications in multi-degree of freedom (DOF) direct-drive (DD) systems. The problem of torque ripple has been studied intensively for 1 DOF systems, but to the best knowledge of the authors, this is the first study addressing this issue for a 2 DOF DD system under gravity effects. For this purpose the system performance is evaluated with realistic simulation models using a chattering-free sliding mode controller (SMC) for a variety of reference trajectories for both links, also taking ripple torque and cogging torque into account for each link. The comparison of the results obtained with a well-turned PID controller demonstrates significant rejection of both torque ripple and load uncertainties achieved with the SMC and the resulting increase in tracking accuracy of the overall system.