Nonlinear attributes modeling and analysis of harmonic drive manipulator joint

Harmonic drive transmission has been widely used in such accurate and reliable positioning and tracking fields as space mechanisms, robotics and high precision machines. However, the performance of harmonic drives is deteriorated by inherent nonlinear attributes including nonlinear stiffness, hysteresis, clearance, nonlinear friction, pure kinematic error and damping. Accurate modeling of the nonlinear attributes and analysis of their effects on system performance is crucial to improve the precision and reliability in positioning and tracking missions. In this work, a new model of harmonic transmission that comprehensively includes all the observed nonlinear attributes is proposed. In this model, nonlinear stiffness, hysteresis and joint clearance are modeled in an ordinary differential equation with coupled form, and its algebraic form under periodic input is put forward. The correctness and practicability are verified by using experiments data in previous studies. The comparison results show that the newly developed model has a 2.35% error. Then effects of these nonlinear attributes on system performance are studied. The main results show that the joint clearance effect leads to stiffness soften and deteriorates the velocity tracking performance. Furthermore, this analysis also reveals the velocity effects on position-tracking performance deterioration and `soft wind-up' behavior in harmonic drives.