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Another type of solenoid valve used in robotic and other machine motion applications is a three-position valve. This valve is similar to the two-position valve where the solenoid is energized or de-energized. From the diagram in Ill. 1 notice that the valve has two solenoid coils, one on each end and a spring on each end. The actual coil symbol for this valve is more specific in that it shows a pilot valve is used to move the main spool. Hence the solenoid will energize a small valve, and when its spool moves, it will move fluid that will cause the main valve spool to move. The pilot valve in this application is like an amplifier.
It's easier to understand the operation of the valve if one disregards the pilot operation and explain the main spool movement. When the solenoid on the left side of the valve is energized, the spool (armature) in the valve will shift to the position shown in the diagram in the left box for the valve symbol. When the solenoid on the right side of the valve is energized, the valve will shift to the position shown in the diagram in the right box for the valve. When both solenoids are de-energized, spring pressure from the springs on each end will cause the valve spool to shift to a center position. In the diagram of this valve, notice that the center position causes the spool to block all ports. This is a safety feature used in robots and other motion control systems that allows the valve to move to the center position during an emergency stop condition and blocks the air to all ports of the robot's cylinder, causing all motion to stop.
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