Accurate altitude control of Autonomous Underwater Vehicle

Autonomous Underwater Vehicle (AUV) is a good instrument platform for carrying out various ocean and underwater explorations. One of the applications is to mount laser scanner on the platform to perform seafloor roughness measurement. The altitude of the AUV needs to be very close to the seafloor and very stable as well. In other words, to maintain very accurate and stable altitude, the vertical thrusters will operate around the zero point, and the direction of the propellers change frequently. Since the altitude needs to be very stable, the AUV will not experience abrupt changes. The command sent to the thrusters is minute such that the operation of the electric motor falls into so called "dead zone". Within the dead zone the actual output is null even though the commands is not. Once the command is large enough, getting away from the dead zone, the motor will suddenly rotates. This type of non-linearity is difficult to overcome just like backlash in a gear train. In this paper, we introduce the concept of virtual buoyancy to provide a condition in which the thrusters always operate out side of the dead zone. the idea is to configure two thrusters to work together as a pair. One thruster pushes downwards with a constant force while the other acts like a regular normal thruster. The former creates an adjustable buoyancy, the latter counter-acts the virtual buoyancy in addition to the commanding force requested by the controller. With this setting, both thrusters work out side of the dead zone, and accurate altitude control is achieved. The effectiveness of the design is both studied by simulation and experiments. The results prove that a small-weight AUV can maintain its altitude in 30 cm with errors less than 1 cm.