Bounded-saturation-based disturbance observer for precision position control of an indirect transmission system

Indirect transmission systems achieve higher output torque compared to direct transmission systems under constrained conditions and limited space. However, the prevalent undesired backlash dynamics in indirect transmission systems can lead to an increase positional error and unstable limit cycle oscillations. In order to suppress the adverse effects of backlash dynamics using a simple and practical approach, we propose a bounded-saturation-based disturbance observer (DOB) to enhance the precision positioning performance of transmission systems. The core of this approach lies in the incorporation of a nonlinear saturation function into the traditional DOB, limiting the amplitude output of the compensation signal, thereby altering the trajectory of closed-loop poles, resulting in a backlash-suppression structure with higher bandwidth and improved stability performance compared to conventional observers. In the novel full-closed loop feedback structure, we also analyze the suppression effect of the introduced saturation function on limit cycle oscillations by the extended Nyquist stability criterion. Experimental validation is conducted on a positioning turntable platform driven by the flexible timing-belt. The results demonstrate an effective suppression on the severe reverse backlash dynamics induced during velocity reversals, with a reduction of over 30% in tracking positioning error. The method proposed in this paper does not involve intricate mathematical derivations, exhibits strong real-time suppression capability for backlash dynamics, and can be extended to precision positioning systems in practical industrial applications featuring backlash dynamics.