Minimum energy based fine motion control of underwater robots in the presence of thruster nonlinearity

This paper considers thruster dead zones and saturation limits in fine motion control of underwater robots. Thruster dead zones and saturation limits are major nonlinear elements that make fine motion control of underwater robots difficult. If the vehicle is designed with a redundant thruster configuration, the dead zone and the nonlinear region around it can be completely avoided by implementing a null motion solution for the command input of the vehicle. This solution is derived primarily from the hardware geometry, and is actually realized before the application of the motion control algorithm. The result is an improvement in system performance regardless of the implemented controller type. This paper describes the mathematical derivation of a null motion solution, the minimum energy solution, and a hybrid of the two. The approach is then illustrated through simulation, and followed by empirical application to an underwater vehicle that was developed at the Autonomous Systems Laboratory of the Mechanical Engineering Department of the University of Hawai'i at Manoa.