Active nonlinear control of a stroke limited inertial actuator: Theory and experiment

This paper presents a theoretical and experimental study of a stroke limited inertial actuator when used for active vibration control. The active control system under investigation consists of the inertial actuator attached to a flexible structure, a collocated vibration sensor and a velocity feedback controller (VFC). Controlling low frequency motions or large amplitude vibrations requires a very long stroke for the proof mass. However, a physical limitation of inertial actuators is that the stroke length is finite. Stroke saturation results in impulse-like excitation, which is transmitted to the structure and may result in damage. Additionally, these impacts between the proof mass and the end-stops can be in phase with the velocity of the structure, reducing the overall damping of the system, which leads to instability and limit cycle oscillations. This paper examines the implementation of a nonlinear feedback controller (NLFC) to avoid collisions of the proof mass with the actuator's end-stops, thus preventing this instability. The nonlinear control strategy actively increases the internal damping of the actuator when the proof mass approaches the end-stops. The experimental implementation of the NLFC is investigated for the control of the first mode of a cantilever beam, and it is shown that the robustness of the VFC system to external perturbations is much improved with the NLFC. It is shown experimentally that larger velocity feedback gains can be used without the system becoming unstable when the NLFC is adopted and the theoretical reasons for this increase in stability margin are explored.