Dynamics Modeling of Robot Joints with Asymmetric Load-Dependent Friction

This paper presents a strain wave gear model intended for industrial robots that are operated in both forward drive and backward drive under different load conditions. We show that a simple joint model captures the joint dynamics well at one load, but fails to capture the joint dynamics when the load is changed. Similarly, the simple model cannot capture the efficiency of the joint in both forward drive and backward drive. We propose a tooth model with sliding friction to resolve these issues. This model is simple to simulate and can thus be used for real-time control; the proposed model captures the load dependent friction and difference in efficiency in backward drive and forward drive. We validate the simulation performance of the tooth models on data from a Universal Robots UR10e robot with different payloads which shows an improvement compared to the standard rigid joint model.