Re: Pistons only halfway up
 

An easy way to do it is to control the exit of air out of the cylinder.
This option is more controllable if you use flow restrictors on the cylinder, to slow the movement of the air. If you don't, the cylinders move so quickly they can be hard to stop at a mid-point. You can keep the system at 60 psi, to give you the force you need - just apply the air more slowly.

Here is a sketch from 2004. You will need to be sure this is still legal with the one solenoid / cylinder rules.

Also look at 3 position cylinders from Bimba. We are using two this year. They are not much more expensive than the same length in a single. They require two solenoids to control them, because there are three ports - one solenoid would control one end and the second would control the extra extension. NOTE: I assume this set-up is legal with the current rules, because these are effectively 2 cylinders put together, but I am going to ask the Q+A to verify that we can use two solenoids to control this one "combo" cylinder.

http://www.chiefdelphi.com/media/papers/1476

Re: Pistons only halfway up
 

<R74> Each commanded motion of a pneumatic cylinder or rotary actuator must be accomplished via the flow of compressed air through only one approved pneumatic valve. Plumbing the outputs from multiple valves together into the same input on a pneumatic cylinder is prohibited.

Followup to above post. Based on the wording in this rule, I believe it is acceptable to use a set-up like in the sketch, and to use a 3 position (or more) cylinder. The key wording is that you cannot have multiple valves on the same input.

Re: Pistons only halfway up
 

The purpose of the magnetic switches on cylinders is to confirm that the piston has reached its destination (start and or finish). The reason that the switch can be placed anywhere along the travel is because people often design systems where the cylinder never reaches full stroke.

You should NEVER plan to try to stop a pneumatic cylinder partially through its stroke to a defined position with the use of air pressure / flow. There are precision valves which can accomplish this but there is no reason to do it that way. Hydraulic cylinders and valves do this all the time but oil is not as compressible as air!

As others have mentioned, use two position cylinders, mount cylinders in series (we are doing this), put intermediate hard stops or use motors and chain or belt.

Re: Pistons only halfway up
 

How would you connect the cyllinders together like that, tip to tail? Are they just the Bimba cyllinders ordered throught the KoP, or are they specialty cyllinders?

Re: Pistons only halfway up
 

I always connect them tip to tip or tail to tail.

We have three different length cylinders in our arm mechanism. The bottom one and the middle one are connected tip to tip and the middle and top ones are connected tail to tail.

This allows us to put our arm in 9 different positions.

If I could get into the shop I would post a pick but alas the weather will not allow that.

All three cylinders are from the Bimba donation.

I love those guys!

Re: Pistons only halfway up
 

I don't understand how the cyllinders in series would work. How can you put them together? When the bottom one actuates, does it physically move all of the cyllinders above it? Are they just stacked, or are they actually connected together somehow?

Re: Pistons only halfway up
 

1. We turn and tap joining rings on the lathe.
2. Yes
3. The are stacked and connected by the joining rings.

Sorry I don't have picks to share but we have been locked out of the shop for a week.

I'll get picks tomorrow and show you our method.

Re: Pistons only halfway up
 

You can join them 'rod to rod' by either joining the existing clevises with a bolt, or with a threaded coupler like these: http://www.mcmaster.com/#spacers-and-standoffs/=avwm3j

Re: Pistons only halfway up
 

I don't like tip to tip joining, but that may just be a preference. We are making a joining ring like IndySam suggested. One way is to weld the rear mount nuts together and then thread each cylinder onto the combined nut OR turn and thread a nut like IS suggested.

We have two long cylinders in series, joined butt to butt. Both rods are attached to brackets and the butts are supported on a slider mechanism. You have to very careful when joining esp. long cylinders to keep the load in column. If you do not, you will be buying more cylinders and changing your design.

Re: Pistons only halfway up
 

We used a 3 position Pressure held SMC valve ordered from Pneuaire.com. They are stocked and they match the approved V for the FIRST rules. We got them in 4 days. They are closed until you tell them to go one way or the other.

Our arm is pretty light so the bounce effect of regulated pressure is not significant. It is fully adjustable with needle valves on the inputs to slow it down.

If you do this you will need another relief valve for the end of the match to drain the trapped air in the system.

Re: Pistons only halfway up
 

Has anyone checked the legality of 3-position cylinders with the GDC yet? Are they legal?

Re: Pistons only halfway up
 

You may want to check on the legality of your design: http://forums.usfirst.org/showthread.php?t=16642

Re: Pistons only halfway up
 

So, how is the Sam Hill are you supposed to get a piston to stop in varying positions. We have done it for years using more than one valve is series and now we come to find that COTS 3 position valves are also illegal. I'm getting a bit frustrated here. These multi-rod solutions are nifty but impractical for what we already have designed. Is there any COTS solenoid that can do what what center blocked solenoid can do and is still legal?

Re: Pistons only halfway up
 

Don't shoot the messenger.

My team was also looking at a multi position cylinder solution but we caught this as soon as it was posted and went the more conventional motor route for moving our arm.

Good luck...

Re: Pistons only halfway up
 

You're good Mike. I just wish I understood the rationale of this ruling. It would be like having a motor driven elevator stuck at the top. That's ok, but having my arm at an angle at the end of the match is not.

sorry, for the sour grapes.

PS: thinking about it again, we can do it with 2 cylinders in line. This will give us 4 positions if I am thinking about this right. I didn't read the rules are exactly as I should have. The whole 1 solenoid output per piston rule let me think that our old series valve solution would be legal since the output would just be that one of one valve. Nope, that's illegal. Now the other option we had is illegal as well. I understand why it is now. I'm just mad at myself for not knowing about it earlier.