Nonlinear modeling and robust H∞-based control of flexible joint robots with harmonic drives

Motion control of manipulators relies on the ability of the actuation system to provide desired joint torques. For robotic manipulators, particularly those equipped with harmonic drives (HD), this ability is considerably restricted by the inherent nonlinearity, friction and flexibility in the actuator-transmission systems. In this work, a joint torque feedback scheme is used for torque control. The design of this method requires that the actuation system is modeled accurately. The motion control design consists of two parts. In the first part, the error dynamic model of the rigid robot arm is derived, followed by obtaining a computed torque control law based on H_∞. The control torque must be provided accurately. The second part of the control design addresses this issue. It has to perform robustly in the presence of hysteresis, nonlinearity and friction. Describing function and conic sector bounded nonlinearity methods are proposed to model the effect of hysteresis, friction and nonlinear stiffness in the control design