Design and Characteristic Analysis of Variable Buoyancy System for Long Range Autonomous Underwater Vehicle

Variable buoyancy system (VBS) is important for long-range autonomous underwater vehicles (AUVs) to adjust the buoyancy change caused by environmental density changes dynamically, and it is the core component of AUV to realize near-neutral buoyancy navigation and unpowered submerging and floating. Accuracy of AUV's pitching angle change model caused by buoyancy adjustment and the flow uniformity of hydraulic oil are the key factors to achieve efficient and high-precision buoyancy control and reduce the power consumption of buoyancy adjustment. Variable buoyancy system with the functions of quantitative oil exhaust, deep-sea passive oil entry and shallow-sea active oil entry was developed. The dynamic effects of buoyancy adjustment on AUV center of gravity, center of buoyancy and pitch angle were analyzed, and a correction model of moving amount of AUV center of gravity caused by buoyancy adjustment was established. With the help of designed pressure simulation system and hydraulic station, the oil exhaust flow rate and passive oil entry flow rate under different pressure were tested. Results shown that, when the oil from internal bladder to external bladder, the displacement flow rate decreases slightly but it is approximately uniform with the increase of external pressure (depth) and the flow rate in passive oil entry process is uniform and controllable. Finally, the depth-fixed navigation experiments were carried out in Thousand-island Lake, and the VBS performance is stable and reliable.