An experimental study of nonlinear stiffness, hysteresis, and friction effects in robot joints with harmonic drives and torque sensors

Abstract

Despite widespread industrial applications of harmonic drives, the source of some elastokinetic phenomena causing internal instability has not been fully addressed thus far. This paper describes a new phenomenon named "torque-transmission paradox" related to the inability of transmitting the input motor torque to the output link. Also, we describe experiments and mathematical models related to other physical phenomena, such as nonlinear stiffness, hysteresis and soft-windup. The goal of our modeling strategy was not in developing very precise and possibly complicated model, but to distill an appropriate model that can be easily used by control engineers to improve joint behavior. To visualize the developed model, equivalent mechanical and electrical schemes of the joint are introduced. Finally, a simple and reliable estimation procedure has been established not only for obtaining the parameters, but also for justifying the integrity of the proposed model.