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The Autonomous Underwater Vehicle (AUV) is a submersible with energy, autonomous navigation, and a wide range of sensors, which is an important means of marine observation. More and more attention has been paid to the long-term observation of the marine environment using AUVs. Due to the restriction of energy and operation mode, traditional autonomous underwater vehicle can work very short time. In order to meet the long-term observation requirements of marine environment, a new type of autonomous underwater vehicle named Long Term Station-keeping Profile Observing AUV with variable buoyancy system has been developed. By carrying the variable buoyancy system, the AUV can be adjusted to neutral before travelling which will help to reduce the resistance; When vertical profile observing, the AUV’s diving and surfacing are realized by VBS without continuous power consumption; During the observation interval, AUV is adjusted to neutral in water, and by closing the carrying equipment, the AUV gets into sleep state so as to save energy. Through these measures, it is expected to achieve long-term observation by AUV for one month. The key is the high precision variable buoyancy system with bidirectional regulation ability, which can adjust the residual buoyancy and pitch angle at any working depth. In order to realize the precise adjustment of buoyancy, a seawater piston cylinder is adopted. By measuring the position of the piston in the sea water cylinder accurately, the precise control of the amount of buoyancy adjustment can be realized. Through laboratory proofing and sea tests, the variable buoyancy system has attained its design targets expected. The AUV equipped with this system can realize underwater suspension at fixed depth, diving, surfacing and low energy sleep, which has carried out many marine applications in the Yellow Sea, the South China Sea and the East China Sea, and it proved to be stable and reliable. The results of laboratory and sea tests show that the variable buoyancy system with seawater cylinder features high accuracy and reliable operation.

The diving plane motion of autonomous underwater vehicles (AUVs) is a complex multivariable nonlinear system with pitch-heave coupling. Tornambe’s controllers (TCs) can online estimate the dynamic uncertainty regardless of the type of disturbance. The TCs for depth and pitch angle are adopted to achieve multivariable decoupling control by introducing virtual control inputs. The methodology can obtain fast and non-overshoot control of depth and pitch angle in the diving and floating process. Simulation results demonstrated the effectiveness of the proposed control scheme.