Investigating PID Control for Station Keeping ROVs

For controlling Unmanned Underwater Vehicles (UUVs) in deep water, Proportional-Integral-Derivative (PID) control has previously been proposed. Disturbances due to waves are minimal at high depths, so PID provides an acceptable level of control for performing tasks such as station-keeping. In shallow water, disturbances from waves are considerably larger and thus station-keeping performance naturally degrades. By means of simulation, this letter details the performance of PID control when station keeping in a typical shallow-wave operating environment, such as that encountered during inspection of marine renewable energy devices. Using real wave data, a maximum positional error of 0.635m in the x-direction and 0.537m in the z-direction at a depth of 15 m is seen whilst subjected to a wave train with a significant wave height of 5.404m. Furthermore, estimates of likely displacements of a Remotely Operated Vehicle (ROV) are given for a variety of significant wave heights while operating at various depths. Our analysis provides a range of operational conditions within which hydrodynamic disturbances don’t preclude employment of UUVs and identify the conditions where PID-controlled station keeping becomes impractical and unsafe.