Nonlinear control of rigid-link flexible-joint robotic manipulators with harmonic drive transmissions using integrator backstepping

A harmonic drive is a special type of compact, in-line gearing with a high torque reduction ratio that has been shown to be virtually backlash free. As a result, it has become a very popular torque transmission device in robots. However, one of the most commonly known disadvantages of harmonic drive gearing is the existence of a large amount of nonlinear mechanical compliance in addition to kinematic error stemming from manufacturing inaccuracies. In this research, control of rigid-link flexible-joint (RLFJ) robotic manipulators with harmonic drive gearing is studied. First, effects of joint flexibility and kinematic error due to harmonic drive gearing in a robot manipulator are investigated using responses to step input torques and proportional and derivative (PD) control using motor and link feedback. It has been found that there exists mechanical resonance in the harmonic drive caused by kinematic error acting as a forcing function. As a result, PD motor and link control are limited in accomplishing high precision control of robot manipulators with harmonic drive gearing. Second, Integrator Backstepping design techniques are studied in systems with and without uncertainty. The presented examples include exact model knowledge, robust and adaptive backstepping based designs. It has been found that Integrator Backstepping is a very effective design methodology for handling unmatched system uncertainty while maintaining global stability results and eliminating the need for acceleration measurements. Consequently, controllers for N-Link RLFJ robot manipulators with harmonic drive gearing are designed using the Integrator Backstepping method. An adaptive/robust controller is simulated and found to verify theoretical results.