Hydraulic control is controlled by pressure fluid as the control signal transmission mode. The control system composed of hydraulic technology is called hydraulic control system. Hydraulic control usually includes hydraulic open loop control and hydraulic closed loop control. The hydraulic closed loop control is also the hydraulic servo control. It consists of the hydraulic servo system, which usually includes the electrical hydraulic servo system (electro hydraulic servo system) and the mechanical hydraulic servo system (machine liquid servo system, or hydraulic servo mechanism).