Bio-inspired backlash reduction of a low-cost robotic joint using closed-loop-commutated stepper motors

Abstract

The majority of current robotic joints are primarily actuated by rotational mechanisms. These electrical drives have substantially different features than the features found in human muscular systems. This paper presents a cost-effective solution to the backlash of a phenomenon known to cause positioning errors and other undesirable dynamic effects in drives. These errors are particularly pronounced when relatively major changes appear in the pre-load conditions of the motor such as in the case of a robotic leg or arm with a high degree of freedom. Current solutions require an accurate time-varying model of drives that is not available in the majority of practical cases. Therefore, in this paper a digitally controlled mechanical solution is proposed which is inspired by the human flexor-extensor mechanism. The idea is to construct an antagonistic actuator pair analogous to the flexor and extensor muscles. In order to obtain good control performance even in the low-speed range, permanent magnet stepper motors were chosen as actuators that are commutated in a digitally closed-loop fashion. The operation of the controlled structure has been verified in a real experimental environment where measurements showed good results and match with previous simulations. Nomenclatureb Measure of a backlash as a distance [m] β Measure of a backlash as an angle [rad] r Radius of a gear [m] τ Torque [Nm] i Current [A] I Nominal current [A] θ Angle [rad] J Inertia [kgm 2 ]