The MOOG servo valve is mainly composed of permanent magnet torque motor, nozzle baffle valve, valve core valve sleeve and other components [2] [6] [8]. A typical servo valve consists of permanent magnet torque motor, nozzle, baffle, valve core, valve sleeve and control chamber [13-14]. The input current drives the torque motor, causing the baffle to move and change the pressure on both sides of the nozzle, thereby pushing the valve core to move and regulating the flow and pressure of the hydraulic oil [2] [8]. When the input coil is energized, the baffle moves to change the throttling effect of the nozzle, resulting in an imbalance of forces at both ends of the valve core and causing it to move, thereby controlling the flow direction of the oil; the displacement of the valve core is proportional to the input current of the torque motor [13-14].
According to the number of amplification stages, MOOG servo valves can be classified as single-stage servo valves, two-stage servo valves and three-stage servo valves. The electro-hydraulic servo valve converts weak electrical signals into large power hydraulic energy (flow and pressure) output, and is the core of the electro-hydraulic servo system [11] [20].
New servo valves are attempting to overcome the disadvantages of complex structure and high requirements for oil in traditional servo valves, such as using super magnetostrictive materials (GMM) or direct-acting servo valves driven by stepper motors. The structure is more simplified and has advantages such as high frequency response and high precision. Electronicization, digitization and intelligence are also important development directions of servo valve technology [14].