Backlash, also known as mechanical play, is a piecewise differentiable nonlinearity which exists in several actuated systems, comprising, e.g., rack-and-pinion drives, shaft couplings, toothed gears, and other machine elements. Generally, the backlash is nested between the moving parts of a complex dynamic system, which handicaps its proper detection and identification. A classical example is the two-mass system which can approximate numerous mechanisms connected by a shaft (or link) with relatively high stiffness and backlash in series. Information about the presence and extent of the backlash is seldom exactly known and is rather conditional upon factors such as wear, fatigue and incipient failures in the components. This paper proposes a novel backlash identification method using one-side sensing of a two-mass system. The method is based on the delayed relay operator in feedback that allows stable and controllable limit cycles to be induced and operated within the (unknown) backlash gap. The system model, with structural transformations required for the one-side backlash measurements, is given, along with the analysis of the delayed relay in velocity feedback. Experimental evaluations are shown for a twoinertia motor bench that has coupling with backlash gap of about one degree.

Force/torque control is attracting considerable attention for the realization of compliant motion in mechatronic systems such as industrial robots and human-support robots. They have transmission mechanisms such as gear reducers, which introduce resonance in the low frequency range and nonlinearity by backlash. To achieve high precision even with transmission mechanisms, the reduced cost of high-resolution encoders has increased the number of devices with load-side (gear-output-side) encoders in industry. Therefore, this study proposes a precise joint torque control method with backlash compensation by using load-side encoder information. The effective use of the load-side encoder information enables the proposed method to compensate the fast backlash effect in a feed forward manner. Moreover, based on the experimental analyses, the novel backlash compensation model is proposed to solve the problems caused by the conventional backlash compensation model. Simulation and experimental results demonstrate the advantages of the proposed method.

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Backlash degrades positioning accuracy and can induce mechanical wear and breakage by collisions. Therefore, a lot of studies have been conducted on backlash compensation. The simplest control method for the impact attenuation is torsional damping addition by feedback of torsional velocity. This paper reveals the advantages and disadvantages of the torsional damping addition. Based on the analyses, a novel switched damping control is proposed to realize the responses with smaller overshoot while attenuating the impact. The mechanical system and the proposed controller are described as piecewise affine systems for analyses. The performance of the proposed method is compared with a linear damping control method in simulations and experiments.