Pneumatic Position Control
Situation: Your rodless air cylinder needs to move to mid-position repeatably. The air cylinder hits a mid-point sensor and tries to stop. It continues to slowly move and eventually does stop. That will not work for this application. For this application on this new machine, I need the rodless cylinder to stop at mid-position +/- 0.0625 (1/16″) and repeat that move.
In order to stop the rodless air cylinder at a repeatable mid-position, you will need to select a 3-position air valve. These 3-position air valves come in three different configurations.
- • 3-positions closed center
- • 3-positions center exhaust
- • 3-positions pressure center
Which valve do you need to select?
I have seen many times a design engineer will select a closed center valve thinking that once the 3-position valve de-energized and centers the main spool, the valve will block both ports of the rodless air cylinder which will trap the compressed air on both sides of the air cylinder.
This will stop the rodless air cylinder, but it will not stop at your desired position. After tripping the mid-position sensor, the rodless air cylinder will try to stop, but it will continue to move past the desired stopping point very slowly and then stop.
Let’s breakdown step by step why the rodless air cylinder continues to move after tripping the sensor at mid-position.
Step 1: Before the assembly machine starts a new cycle, the rodless air cylinder is in the home position. The 3-position air valve “B” coil is energized which is holding the main spool in the shifted position. At this point, there is 80 PSI of compressed air on one side of the rodless air cylinder piston and 0 PSI on the other side of the piston. See the diagram below-
Step 2: The “B” coil of the solenoid valve gets de-energized and the “A” coil gets energized causing the main spool of the solenoid to be shifted. The rodless air cylinder is now moving towards the mid-position sensor.
Step 3: The rodless air cylinder has now tripped the mid-position sensor. The “A” coil of the solenoid de-energized which causes the main spool of the solenoid valve to move to the center position trapping the air between the air cylinder and solenoid valve.
Step 4: The rodless air cylinder has tripped the mid-position sensor but continues to slowly move and eventually does stops. This is happening because at Step 3 when the solenoid valve did get 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 rodless air cylinder is going to still move until the air pressure on both sides of the piston are equal.
The way to correct this is to select the 3-position valve that is a center pressure type.
All the above steps would be the same except Step 4. The below diagram shows how after the Rodless air cylinder does trip the mid-position sensor and the solenoid valve main spool is now in the center position, instead of trapping the air between the solenoid valve, the center pressure valve will send main air pressure to both the ports of the rodless air cylinder. This will stop the movement of the cylinder very quickly.
I hope this article was helpful to understand what type of 3-position valve you would need if you need to stop and repeat a mid-position on an air cylinder.
If you have questions regarding this article or have any questions regarding any new pneumatic application – Please e-mail me at [email protected]
Senior Pneumatic Technical Support Engineer Certified Pneumatic Specialist – CFPPS
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