Development of a hydraulic buoyance adjustment module for underwater robots

Underwater robots such as AUVs may work in different depth. In many cases, they need to maintain certain depth, which can be achieved through cooperation of rudders and propellers. However, this is quite energy-consuming process because the propellers and rudders have to keep working. In this article, a novel hydraulic buoyancy adjustment module(BAM) for underwater robot is proposed. The mechanical design, the dynamics modelling and a optimal depth control scheme are discussed in the paper. This mechanical design offers an easy way to control the buoyancy at shallow depth, and offers simpler security solution at deep depth by adopting accumulator as a passive pressure source. And the model of this BAM can be described by three-order nonlinear hybrid system with state constraints. Considering the uniqueness, it is proven that the limited switching times condition is fulfilled and the switching sequence is derived. On the basis of switching sequence, a improved time optimal control scheme is developed.