The reason for this is that many Electro Hydraulic Servo Valve rely on a feedback wire or spring to perform spool position proportioning.
The employment of an LVDT spool position sensor with electronic feedback is common in proportional valves, resulting in a "tighter" spool position control loop and lower hysteresis.
What is a hydraulic sequence valve?
Sequence Electro Hydraulic Servo Valve regulates the operation of two branches in a circuit, allowing one unit to automatically start the operation of the other. This is accomplished by a sequence valve put in each landing gear actuation line.
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