Introduction to Electro-hydraulic servo valve technology Electro-hydraulic servo valves are no longer the preserve of the Aerospace and military markets, they are common place in the industrial markets, in machine tool applications. The principle of operation being the hydraulic flow output being directly proportional to the electrical input current. Valve construction commonly consists of a main spool piloted by a double nozzle flapper powered by a torque motor. The input current is applied to the control coils, the electro-magnetic force is applied to either side of the flapper assembly via a flexure tube. As the flapper moves the nozzle opening is varied. Pilot pressure is applied to each nozzle and the pressure difference through these variable nozzles produce is applied to the ends of the main control spool. As the main four-way spool moves it varies hydraulic flow to the control ports in a proportional amount dependent upon the spool movement. The flapper assembly is connected to the main spool via a feedback wire. The torque the movement produces is a produces a movement in the flapper, this torque becomes balanced with the electromagnetic torque produced from the input current. This internal feedback loop ensures the main spool can be designed to give a highly proportional output characteristic. Using this proportionality Electro-hydraulic servo valves are effective in applications such as steel mills, cargo cranes, flight simulation, construction equipment and mobile vehicles. With the addition of a sensor such as load cell or potentiometer the actuator performance is feedback into a controller. PID or fuzzy logic closed loop control philosophies can be used to give exceptional control of this equipment and processes.