Quote:
Originally Posted by grosh
Can we control how far the piston on a pneumatic cylinder extends and contracts? We are using the double solenoid with a dual actuated cylinder. When we turn the relay on the piston extends to its FULL length. We tried turning the relay off before the piston was in it's fully extended position, but it didn't stop. Can you control the length the piston extends with programming?
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Again, not with the kit valves UNLESS you use ONE valve to plug the exhaust ports of your cylinder's directional control valve when commanded.
However, there are two things you need to do, to make this work cleanly: Get the correct TYPE of valve, and add feedback sensing.
1) Valve change - If you go to the SMC website, you can order a Center Off version of the same series of SMC dual coil valve we use, and substitute it. In this style, turning coil 1 ON (and coil 2 OFF) moves it one way, and turning coil 2 ON (and coil 1 OFF) moves it the other way. When both coils are OFF, it plugs both ends of the cylinder, and it "stops". Buy the valve itself, and use the 12V coils and the manifold from one of the kit valves.
This however is only half of the problem, as others have mentioned air is compressable. Change the load (or the direction of gravity), and the cylinder rod may move on you because after all, you are only balancing a PAIR of forces against the load (the air charge on either side of the disc inside).
2) Next, "close the loop" by adding feedback to directly (or INDIRECTLY) detect the position of the piston, and DO something about it if it is in the wrong place.
The simplest form with is to add sensing to the thing you are operating. If the cylinder is twisting a joint, add a potentiometer to that joint. It's value is the important thing anyway, and will have some direct functional relationship to the piston's position.
If you order the MAGNETIC version of the Bimba cylinders (which we ALWAYS do!), you can sense key positions with the supplied strap on reed switches. It's also theoretically possible to whip up a sensor on the outside of the cylinder and have the magnet act like a LVDT core, but I've yet to see anyone desparate enough to directly measure the cylinder to implement one. (Again, it's easier to measure the thing you are operating!)
You can also add a String Potentiometer, or a String Encoder alongside the cylinder's back end, and attach the string to the Clevis. This gives you a direct readout of the cylinder's position, but this is the EXPENSIVE way to go, if you try to buy a commercial device vs making one. (Yet again, you're often MUCH better off watching the device the cylinder is operating!)
In all cases, if your sensor shows that the cylinder/device setup has moved too far away from where you want it to be, simply pulse the appropriate valve to move it back. This kind of closed loop control automatically compensates for load variances, such as grabbing or releasing the scoring object for this year's game from a gripper.
Does this help?
- Keith
As I told my friend Joe Johnson, just because you aren't around as much doesn't mean that you can just close the book on this chapter, instead - just keep a book marker in this place, so you can remember how to find your way back home - when the time is right.
- meaubry [more] 
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10-10-2008, 22:54
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