For use in feedback controls this is a group of continuously adjustable valves, in which a constant input signal brings about a proportionaloutput signal. A distinction is made between mechanical-hydraulic servo valves (copying valve) and electro-hydraulic servo valves: Valve produced with a high degree of precision, which has an electro-mechanical converter (e.g. torque motor) as an input stage, which converts a very weak control current into a path, which is taken over by the hydraulic control unit. A distinction is made between 1-stage and 2-stage or 3-stage servo valves.
- In one-stage servo valves, the electrical input stage (torque motor, moving coil) acts directly on the hydraulic valve control spool (Figure S 49). Both work in an open loop control system,i.e. without feedback so that interferences are not taken into consideration. They are used for up to approx. 20 lpm flow rate.
- The most commonly used two-stage servo valves have a piloting stage, consisting of the electro-mechanical converter (normally a torque motor) and a hydraulic amplifier stage (normally a flapper-nozzle system), which adjusts the piston of the primary control stage in proportion to the input signal (Figure S 50). A feedback spring is used as standard for the position feedback.
- For flows greater than 150 to 200 lpm, three-stage servo valves are used, in which the third (main control) stage is pilot-controlled by a two-stage servo valve (Figure S 51). This means that overall gains of up to a factor of 108 can be achieved.
The servo valve and its qualities largely determine the behaviour of the drive. That is particularly true in terms of its dynamic properties. Therefore, frequencies over 150 Hz can be expected from a servo valve today at a damping of 0.6. However, that can only be achieved with extremely short control distances of a few tenths of millimetres. Therefore, a high pressure difference at the control edges is required to force the desired flow rate through this gap.
Servo valves operate according to the principle of control with throttle. This control principle sees the maximum hydraulic effective power passed on from the valve to the consumer if approximately 1/3 of the available system pressure p max reaches the pressure difference on the valve and approximately 2/3 is available as load pressure on the consumer. The servo valves are dimensioned amongst other things for this point of the operation (Figure S 52).
Therefore, relatively high losses are the price paid for good dynamic performance.
Figure S 49: One-stage servo valve
Figure S 50: Two stage servo valve
Figure S 51: Three stage servo valve
Figure S 52: Effective power and flow rate contingent on Δp in the case of servo valves