Friction Compensation in Servo-Drives
Friction is detrimental to the performance of a number of systems, specifically those demanding ultra-accurate positioning. Some debilitating effects of friction include non-zero steady-state errors in position and discontinuity in the velocity leading to performance degradation, limit cycles, or instability. Most existing techniques for friction compensation require direct or indirect friction parameter estimation for compensation purposes [43, 45, 56, 57, 126] . This approach, though conceptually simple, suffers from being heavily model-dependent and thus friction compensation is achieved up to the level of “goodness” of the model. Also, when a direct estimation of friction parameters (static or dynamic) is done, the following undesirable consequences may occur: (i) the use of approximate linearly parameterized models (in many cases, this leads to over/under compensation), (ii) large order dynamic compensators (even for a one degree-offreedom system), (iii) the necessity to measure acceleration, and (iv) the lack of robustness with respect to all friction and plant parameters. Some of the aforementioned methods that employ friction parameter estimation for compensation are briefly outlined next.
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