Depth Control of Underwater Robot with Metal Bellows Mechanism for Buoyancy Control Device Utilizing Phase Transition

The ultimate goal of this study is to develop a buoyancy control device that utilizes volume change due to phase transition of material between solid and liquid states. This paper describes the depth control method for an underwater robot fitted with the metal bellows buoyancy control devices that we have developed in this study. Four metal bellows buoyancy control devices are installed on an underwater robot. We first measured underwater robot buoyancy change and found that it agreed roughly with theoretical values. We then checked whether the robot could change buoyancy successively so that the robot rises or sinks as commanded. We then conducted a series of experiments on robot depth control in which if the robot depth is more than a certain distance different from the target depth, control devices are either heated or cooled at maximum output. If such a difference is within the threshold, proportional control is applied to develop output in proportion to the distance to the target depth. Experimental results showed that the underwater robot followed varied target depth with a steady-state deviation of a few cmor so, except in some cases of failure.