| 摘要 |
A two-stage servo-valve has a first stage in which four flow resistances are arranged in a bridge circuit. At least one of the flow resistances is variable, and two opposite switching points of the bridge circuit are connected to an air inlet and to an air outlet respectively. A differential pressure of variable size and sign is provided between the two other switching points. The servo-valve also has a second stage in which a control piston which can be actuated by the differential pressure is arranged and controls a three-way valve. In order to enable the current position of a process valve driven by the servo-valve to be maintained by the most simple means in the event of a power failure, the switching points which provide the differential pressure are interconnected by a magnetic valve which is closed in the driven state and open in the unconnected state, thus pneumatically interconnecting the switching points.
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