New control design and analysis techniques for plants with actuator nonlinearities

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Abstract

Actuator saturation is ubiquitous in physical plants. In closed-loop systems limitsimposed on the actuators may result in degraded performance of the control law and,ultimately, instability of the system. When other nonlinearities, such as deadzone,backlash or stiction, are also present in a system’s input, the analysis and designprocedures become more involved.The core of this thesis is a new structure based on the right inverse approach fordeadzone and backlash, which is extended to linear plants that exhibit a combinationof saturation and either deadzone, backlash or stiction, in the actuator. It is shownthat, for this type of systems, the inclusion of the right inverse nonlinearity resultsin the linear plant being subject to a new input saturation. Then, one can designstandard controllers such as anti-windup or input constrained MPC around this saturation.This simplifies the analysis and design processes, in spite of the presence ofcomplex nonlinearities.The results for deadzone and backlash are extended to stiction by proposing anapproximate stiction nonlinearity, and then introducing a right inverse to this approximation.It is demonstrated that the systems studied can be compensated by a standardinput constrained MPC which can be solved by a convex quadratic program. Additionally,a simple anti-windup structure is used to demonstrate the applicability ofthe proposed structure using existing control strategies.

Keyword(s)

Anti-windup; Backlash; Deadzone; Model Predictive Control; Nonlinear control; Stiction

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