MODEL-PREDICTIVE CONTROL OF A HYDRAULIC ACTIVE HEAVE COMPENSATION SYSTEM WITH HEAVE PREDICTION
This masters thesis presents the results of tracking ship heave motion with a heave prediction model-predictive controller (MPC) in the experimental actuation of a nonlinear, unloaded, full-scale active-heave compensation (AHC) hydraulic testbed. Implementing the MPC involves determining a system model for the AHC testbed and correcting for the nonlinear behavior of the AHC testbed. Multiple tuning parameters exist for MPC and so a single set of parameters was acceptably tuned and chosen to use for all experiments within this thesis work. The experimental heave tracking results collected are compared to an AHC testbed simulator developed in MATLAB Simulink. A load is applied within the simulator to determine the AHC testbed response to operating under load conditions. For the experimental unloaded case, as well as the simulator loaded and unloaded cases, the MPC results are compared to a tuned PID controller in tracking of sine waves as well as four heave motion test cases. The heave prediction MPC controller is found to track the test cases and sinusoidal references well while, additionally, outperforming the tuned PID controller in real-world experiments for all test cases and sine wave tracking. Two of the test cases introduced relatively high frequency components to the reference signal which the MPC is able to track, while the PID performance decreases dramatically with the addition of these high frequency components. Maintaining constant tuning parameters for each, the MPC is shown to be more robust for a range of operating conditions when compared to PID. Within the simulator the MPC controller performance is reduced compared to the experimental testbed performance while the PID controller is able to better track two of the four test cases. The loss in MPC performance is attributed to different implementations of MPC between the simulator and the experimental setup. Applying a load within the AHC testbed simulator shows two important results: first, that a counter-balance valve is necessary for the AHC testbed system under load conditions, and second, a parallel integral controller may be needed with the MPC controller to ensure motor leakage does not affect performance.