Complementary Control of the Depth of an Underwater Robot

The depth control of an Autonomous Underwater Vehicle (AUV) is addressed. The vehicle is equipped with two separate actuation systems for the heave axis: a ballast tank system and a set of four jet motors generating vertical forces in the bow and stern of the vehicle. These two actuation systems have different time constants: the ballast tank system is rather slow, while the jet actuators have a much faster dynamics. The proposed control systems is inspired by the theory of complementary filtering in estimation where the output of slow sensors (low pass filters) is combined with the output of faster sensors (or the very plant model, i.e. high pass filters) to generate an estimate. In the given setting a heave controller is designed generating a desired force command. This signal is partitioned in a lower and higher frequency component: the former is sent to the ballast tank system and the latter to the jet thruster actuators. As a result, the depth force command is seemingly executed concurrently by the two actuation systems each working in its most natural frequency domain. The control design methodology is outlined and simulation results are reported illustrating the overall performance.