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The goal of this paper is to examine the friction behaviour in a one-degree mechanical system designed for precise tracking. Friction as one of the main disturbances present in this system strongly influences its performance, which is most visible during the velocity reversals. Identification and compensation of the friction are crucial to achieve high tracking accuracy at very low velocities. In this paper the procedure for identification of static and dynamic frictional parameters of LuGre is presented. The experimental results show characteristic behaviours of friction present both in sliding and in presliding regime. Furthermore, it is experimentally proven in several control scenarios that dynamic friction model compensation causes significant decrease of trajectory tracking error.

In this paper, a problem of influence of an input-gain uncertainty on the tracking performance of a control structure designed according to the Active Disturbance Rejection Control (ARDC) paradigm is investigated. This problem is exemplified using the second-order plant. It is presented that conscious choice of the input-gain parameter different to that of the real plant may lead to a significant improvement of the control precision if the controller is designed in an error domain. Obtained results indicate that the closed-loop system remains stable, and the tracking errors decrease if the value of this parameter follows from multiplying the plant gain input by a positive factor smaller than a certain threshold value. The upper limit for this factor is investigated in the paper and outcomes are presented for various implementation variants. Results of numerical computations, simulations and experiments are presented to consider this bounding value in individual cases.

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