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Originally Posted by Dad1279
You need two valves. One for direction, which you hook up normally. Then you connect the vents from the first valve to a second, to stop the air from venting to the atmosphere. This stops the cylinder from moving until this second valve is opened.
Program the first for direction, the second for movement(on/off)
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This will work fine. Last year we found a better solution, called a "Three-position pressure center" valve. The spec sheet is
here. Look at valve type SY3520-6H, for example.
This kind of valve has three positions, and uses two solenoids: Position 1 is extend, Position 2 is hold, and Position 3 is retract. One solenoid is Extend, the other is Retract, and when neither is energized it means Hold. (This can and should be controlled with a single spike as explained by
coffeeism above).
We selected "Pressure Center" types, which apply full pressure to both sides of the cylinder when in the Hold position. You can also choose "Closed Center", which closes off all air to the cylinder. You don't want "Vent Center", which will allow the piston to move freely in the "Hold" (center) position.
One important thing we learned last year: Pistons work best in 'this end or that end' applications - they do not work well for non-endpoint positioning. The problem is that any variation in load causes the piston to move, because it's only air, and that will often cause a problem.
Good luck. I hope this post helps anyone considering midpoint positioning of pneumatic cylinders.
Don