Motion Control Of Underwater Flight Vehicle At Low Speed And Depth

The operational quality of underwater flight vehicles is limited by the performance of the onboard motion control systems,the design of which is subject to high degree of model uncertainty,non-linearity of dynamics and environmental disturbances e.g.underwater currents.Motion control of underwater flight vehicles at low speed and depth is studied in this paper for underwater flight vehicles to extend their operational range towards both low speed and low depth,within which some onboard payloads can be deployed near surface from stable underwater platforms.Due to low effectiveness of both bow and stern planes at low speeds,the near-surface suction force experienced by an underwater flight vehicle can hardly be counteracted by lifting forces generated by the control surfaces.To improve the performance of depth control system,therefore,variable water ballast mechanism is introduced and thus leads to overactuating control system.A composite control scheme is proposed for such an over-actuated control system.The input of the feedforward controller is the disturbance force,which is estimated by the disturbance observer,and the output of the feedforward controller is the mass of ballast water.Besides,the input of the feedback controller is the depth error and the output of the controller is the deflection angle of control surfaces.In addition to composite depth control system design,both PID-based speed control system and ADRC-based heading control systems are designed.The proposed motion control system,which comprises speed,heading and depth control subsystems,is then ported to the embedded control computer on board an underwater flight vehicle and verified via tank tests under both still water and wave conditions.