A Mechanical Model for Robotic Joints with Harmonic Drives

Harmonic drive gears, of extensive use in robotics, exhibit a non-smooth torque transmission characteristic that signiicantly innuences the dynamic behaviour of the driving and driven elements. For the design of robust motion-and force-control schemes, it is therefore necessary to model accurately the indirect drives involved. In this report, we analyze nonlinear harmonic drive properties, that have been observed in experiments, and propose a mechanical model for the joint transmission unit. The main sources of nonlinearity, that have also been taken into account in the modelling, are identiied as compliance, dynamic friction, static friction (startup torque) and hysteresis, the last two items also depending on the load history of the harmonic drive due to frictional memory. Based on a series of quasistatic and stationary dynamic experiments, we then describe a scheme to identify all necessary parameters. Finally, the quality of the model and parameters is veriied experimentally with respect to its dynamic behaviour by driving the actual system and the model with the same input and comparing the output signals. Numerical and experimental results are given for two harmonic drives and illustrate that the nonlinear system dynamics can be accurately predicted. Humboldt au premier auteur.