Nonlinear modeling and parameter identification of harmonic drive robotic transmissions

Harmonic drive gears, of extensive use in robotics, exhibit a non-smooth torque transmission characteristic that significantly influences the dynamic behavior of the driving and driven elements. In this paper, the authors propose a mechanical model for the joint transmission unit that takes into account compliance, dynamic friction, static friction (startup torque) and hysteresis, the last two items also depend on the load history of the harmonic drive. Based on a series of quasistatic and stationary dynamic experiments, the authors describe a scheme to identify all necessary parameters. Finally, the quality of the model and parameters is verified experimentally with respect to its dynamic behavior by driving the actual system and the model with the same input and comparing the output signals.