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Situation: Your rod-less air cylinder needs to move to mid position repeatedly. The air cylinder hits a midpoint sensor and tries to stop; it continues to move slowly and eventually does stop. For this application on a new machine, the rod-less cylinder needs to stop at mid position +/- 0.0625 (1/16") and repeat that move.
To stop the rod-less air cylinder at a repeatable mid position, select a 3-position air valve. These 3-position air valves come in three different configurations:
- 3-positions closed center.
- 3-positoins center exhaust.
- 3-positions pressure center.
There have been times a design engineer will select a closed center valve thinking once the 3-position valve de-energized and centers the main spool, the valve will block both ports of the rod-less air cylinder which traps the compressed air on both sides of the cylinder.
This stops the rod-less air cylinder but won’t stop at the desired position. After tripping the mid position sensor, the rod-less air cylinder will try to stop but continues to move past the desired stopping point, eventually coming to a slow stop.
Here is a step-by-step breakdown of why the rod-less air cylinder continues to move after tripping the sensor at mid-position.
Step one
Before the assembly machine starts a new cycle, the rod-less air cylinder is in the home position. The 3-position air valve B coil is energized, which holds the main spool in the shifted position. At this point, there is 80 PSI of compressed air on one side of the rod-less air cylinder position and 0 PSI on the other side of the piston.
Step two
The B coil of the solenoid valve gets de-energized, and the A coil is energized, causing the main spool of the solenoid to be shifted. The rod-less air cylinder is now moving toward the mid-position sensor.
Step three
The rod-less air cylinder tripped the mid-position sensor. The A coil of the solenoid de-energized, causing the main spool of the solenoid valve to move to the center position trapping the air between the air cylinder and the solenoid valve.
Step four
The rod-less air cylinder has tripped the mid-position sensor but continues to move slowly and eventually stops. This happens because at step three, when the solenoid valve was de-energized, and the main spool went to the center position trapping the air between the solenoid valve and air cylinder, the air pressure is still higher on the one side of the cylinder piston than the other side. The rod-less air cylinder will still move until the air pressure on both sides of the piston are equal.
Correcting this is to select the 3-position valve that is a center pressure type. All the steps above would be the same except step four. The diagram below shows how after the rod-less air cylinder trip, the mid-position sensor and the solenoid valve main spool are now in the center position. Instead of trapping air between the solenoid valve, the center pressure valve will send the main air pressure to both ports of the rod-less air cylinder. This stops the movement of the cylinder quickly.
If you are still wondering which type of 3-position valve is right for you and your system, contact Motion Automation Intelligence today to speak to a rep.
