Custom position with pneumatics?

[TheHolyHades1]

Hi all.

Is there a way to make pneumatic cylinders extend to say ... 1/3 of the stroke length? Right now all I'm able to do with pneumatics is to fully extend or fully retract the cylinders. Is there a way to cutoff the air before the entire cylinder fills up so you have air on both sides (ie. custom length), or is there some other way of doing this that I'm missing?

Thanks.

[Walter Deitzler]

You can get 3 position cylinders ( http://www.bimba.com/Products-and-Cad/Actuators/Inch/Round-Line/Non-Repairable/Original-Line-Three-Position/ ).

Edit:: Apparently you can customize where it stops (read the later posts.) You learn something new every day.

[Racer26]

There exists three position valves with a center-stop position.

There also exists methods to achieve something similar by valving the exhaust side of the valve feeding your cylinder in a creative fashion.

1075 built an infinitely multiposition pneumatic arm this way in 2007.

[BBray_T1296]

Not effectively, or reliably, but:

If you filled the retracting side with ~40PSI and capped it shut, and applied 60PSI to the extending side, the cylinder would reach equilibrium at 1/3 extension.

That being said, as Delta P approaches zero, Force approaches zero too, so it would not be very powerful and thus not very useful

[jp1247]

We (1247) did this last year with a standard bimba two position cylinder for angling our shooter. Applying full working pressure on both sides simultaneously holds the cylinder at that position. Add magnetic reed switches and viola. We used a 1" cylinder I believe, very hard to move by hand, and worked very well. (only thing did on that bot :/)

[jimwick]

Yes, intermediate positioning is definitely possible. We built a fairly successful prototype this summer, although we didn't really have the necessary equipment to do the job right.

The basic idea is that you use a two position valve to control direction, and then use simple open/close valves on each exhaust to control motion. Nothing moves until ONE of the two exhaust valves is opened. Speed can be controlled by modulating air flow in the exhausts. We did not have solenoid operated open/close valves, so we just used manual ones from the hardware store. The system worked fairly well using the two manual valves.

I believe this can all be done using a three position valve. They are pricey so we haven't tried it yet.

[DampRobot]

We've also just tied a string to the end of the piston to limit how far it extends. Of course, you can't make it extend beyond that string somehow, without removing the string.

[MechEng83]

Another solution, depending on your space constraints could be to put 2 pistons in series and get a "4 position" setup. Say, using a 1 inch and 2 inch stroke piston, you could get 0, 1, 2, and 3 for lengths.

Those of you with any computer background might notice a binary system emerging...

[Bill_B]

We (1247) did this last year with a standard bimba two position cylinder for angling our shooter. Applying full working pressure on both sides simultaneously holds the cylinder at that position. Add magnetic reed switches and viola. We used a 1" cylinder I believe, very hard to move by hand, and worked very well. (only thing did on that bot :/)

The bimba cylinders are available in stainless steel and a magnetic plunger that can be detected by the reed switches mentioned above. Every order to bimba of mine spec'd the magnetic model but our team has yet to use any. Sorry to say I'm short of real-world experience on how hard it is to detect the transition point. Oh yeah, you get three free, so why not experiment a bit?