Model development and controller design for an electrohydraulic servo actuator

This thesis presents an analysis of an electrohydraulic servo actuator. A fully nonlinear model of an electrohydraulic actuator has been created in Simulink to fully model the dynamics of the entire system. The entire system was modeled and variables were estimated, due to the inability to accurately measure the actual values. Validation steps including step response, frequency response, and literature verification were performed on the model to verify the accuracy of the model. The model provides the unique advantage of being able to capture the nonlinearity of an electrohydraulic system including the servovalve whose dynamics usually are ignored or approximated. This system is dependent on the values chosen and exhibits typical nonlinearities, which allows this model to be used for other electrohydraulic actuators after tuning the variables to match the response of a system. The full nonlinearity and modeling approach allows the response of any aspect of the system to be analyzed, increasing the value this model has over other model examined in the literature review. Three controllers were analyzed; a PID with anti-windup, full state feedback with state estimators, and a hybrid PI full state feedback, with regards to the nonlinear system. Due to the large order of magnitudes found in the transfer function of the plant, additional procedures had to be developed to analyze and design controllers with the state space representation. Equations were developed to create state feedback and estimator gains based off of scaled step responses. These equations can be used when numerical issues arise from traditional pole placement techniques. In addition to improving the response time by looking at the settling time, the three controllers that were also analyzed based on the resulting input signal to the plant. Lastly the three controllers are also based on their ability to reject a pulse disturbance added after the controller.