A new control strategy for flexible-joint manipulators with joint friction is proposed. The proposed controller includes two main components: a friction compensating torque, and a composite controller torque to compensate for the friction in the joints and the flexibility of the joints, respectively. The novel approach that is used to compensate for the friction in the joints includes a nonlinear one-state LuGre model and a PD control. The second method of Lyapunov is utilized to find sufficient conditions for the global asymptotic stability of the friction compensating torque and it is shown that the tracking error is bounded as long as appropriate PD gains are chosen. The performance of the proposed controller is experimentally verified using a setup of a two-rigid-link flexible-joint manipulator. Experimental results are presented for different flexibilities of the joints and also for different desired trajectories. These results demonstrate the high performance of the controller under different situations while the tracking error remains very small.
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Salmasi et al. (2010) studied this question.
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