What is a servo-controlled solenoid valve?

A (direct acting) plunger-type solenoid is used as a pilot valve in servo-controlled solenoid valves. The main valve seat is opened and closed by a diaphragm. The closing function is enabled through the force of the pressure spring as well as through the medium pressure.

When the pilot bore is opened, the pressure in the fluid chamber above the membrane is relieved. The medium pressure below the membrane increases it and opens the valve.

If the pilot valve is closed, the medium pressure above the membrane builds up again through a servo bore in the membrane or through a bypass channel in the valve body. If the medium pressure above and below the membrane is the same, the relatively weak spring closes the valve. Pressure is always required to actuate the valve, since the solenoid valve is not actuated at 0 bar.

Solenoid valve closed:	Solenoid valve opens:	Solenoid valve open:	Advantages:
			-    High pressure ranges possible -    Low magnetic force -    Large diameters -    Inexpensive

Appropriate servo-controlled solenoid valves are available here from NieRuf.