Hybrid actuation with complementary allocation for depth control of a Lagrangian sea-floor imaging platform

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A hybrid depth control approach, which utilizes both active buoyancy control and a bi-directional vertical thruster, is presented for the Shallow Water Lagrangian Imaging Float. The approach is shown to reduce propulsion power and increase tracking accuracy compared to a single actuator system. Results are presented from field trials at Cordell Bank Marine Sanctuary and Scott Reef Nature Reserve at depths up to 60 m. Using heuristically tuned controllers, the hybrid actuation approach reduces average propulsion power by 65% with respect to a thruster-only system, and by 73% with respect to a piston-only system. A further 58% reduction in power consumption, and a 26% reduction in mean tracking error, is realized by combining state feedback control with a pair of complementary filters. The complementary filters are used to allocate vertical force command between the two actuation modes. Simulations are used to demonstrate that the power savings compared to a thruster-only system result from elimination of steady state trim errors that are otherwise inevitable in field deployments. The control architecture described allows the user to directly tune (a) the time constant of the desired first-order system dynamics and (b) the frequency cutoff between control using the “slow” active buoyancy system and control using the “fast” bi-directional thruster. The results indicate that hybrid actuation and complementary filtering for control allocation are enabling technologies for energy-constrained deep water platforms.

Publication Title

Journal of Field Robotics