A servo-valve of this type and known from DE 41 35 822 A1 contains up to four individual piezo-valves in a first stage, all four piezo-valves, or in the case of a lower number together with fixed resistances, being arranged in a bridge circuit. Each piezo-valve contains its own actuatable piezo-element, which, together with a deflector, forms a controllable flow resistance. The bridge circuit serves to generate a differential pressure of variable size and sign, with which, in a further stage of the servo-valve, a control piston can be adjusted and a three-way valve can be adjusted by way of said control piston, said three-way valve serving to adjust a hydraulic cylinder.
Alternative designs of piezo-valves are known from EP 0 943 812 A1 or the subject matter of the former German patent application with the official file reference 10 2004 035 844.3.
Instead of piezo-valves, electromagnetic or any other electrically actuatable valves can also be used in the bridge circuit.
DE 42 01 442 A1 shows an alternative embodiment of the three-way valve, which, in a known manner, can also be configured as a slide valve, rotary valve or can consist of individual valves such as for instance check valves.
Servo-valves are used in many areas of technology, thus also with electro-pneumatic positioners, which serve to control the drive of process valves. In the event of a power failure, a request is often made for the drive actuated by the positioner to automatically bring the process valve into a secure position, generally “open” or “close”.