Parallel loop recovery with quiescent compensation for high performance feedback control of systems with imperfect actuators

Actuator limits and imperfections introduce multiple nonlinearities in a feedback loop. While standard mild-feedback systems (e.g., PID) are intrinsically robust to such nonlinearities, the stability of large feedback systems is threatened. This eliminates from consideration the usage of imperfect actuators in large feedback applications and available disturbance rejection as a result is not realized. The implementation of multiple-path nonlinear dynamic compensation (NDC), however, allows the combination of large feedback and imperfect actuation and is the focus of this work. As nonlinearities due to actuator imperfection are often difficult to model, the stability of the NDC systems is assessed using multivariable absolute stability analysis. The features of a parallel loop recovery system with loop transmission modulus reduction compensation in the quiescent condition are compared to an alternative approach of strict modulus reduction via nonlinear compensation for stability retention. Efficacy of the loop recovery approach is illustrated using experimental data from a voice-coil actuated disturbance rejection system.