The hydraulic propulsion system is widely used in underwater vehicles due to its high power density and good control performance. In this paper, a torque-controlled hydraulic propulsion system which consists of a pump, a couple of proportional pressure reducing valves and a thruster driven by a hydraulic motor is presented. It has a much lower cost and a higher efficiency compared with servo valve control systems. And its response is still quick enough to control the underwater vehicles’ attitude compared with secondary regulation systems. However, while the propeller is running at a low shaft speed, its load characteristics may lead to instability due to its low load stiffness at low speed, low friction and small internal leakage inside the hydraulic motor.
In this paper, all these hydraulic parameters, even the fluid viscosity and the effective bulk modulus, which may influence its stability, are all analyzed and simulated. Based on this, a drive-pressure and back-pressure independent regulation control method is proposed. Pool experiments shows that this double pressure regulated hydraulic propulsion system is quite stable and reliable at low shaft speed.