Pistons only halfway up

[akoscielski3]

two different cylinders and connecting them to eachother. extend one to get half way up, extend both for all the way up, and none for all the way down

[tonystark_23]

Hi, I am not an expert on the situation, but i have mentored a team that did multi position pneumatic cylinders. (pistons). Bimba sells pistons with magnetic switches attached to the outside. The way this works is that the "plunger" in the cylinder has a magnetic tip. The magnetic switches on the outside of the cylinder detect when the plunger passes and return a value 0 or 1. You can use this value to write code that turns on the solenoid and then turns it off when it reaches the appropriate switch position.

[cmass]

Originally Posted by tonystark_23

Hi, I am not an expert on the situation, but i have mentored a team that did multi position pneumatic cylinders. (pistons). Bimba sells pistons with magnetic switches attached to the outside. The way this works is that the "plunger" in the cylinder has a magnetic tip. The magnetic switches on the outside of the cylinder detect when the plunger passes and return a value 0 or 1. You can use this value to write code that turns on the solenoid and then turns it off when it reaches the appropriate switch position.

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.

[Matt C]

Originally Posted by akoscielski3

two different cylinders and connecting them to eachother. extend one to get half way up, extend both for all the way up, and none for all the way down

Or you can use two different length pistons attached end to end, and have 4 positions!

[MrForbes]

Air is a spring, so it's difficult to position the piston in a pneumatic cylinder accurately unless it's at one end of it's travel, or the other.

Good design with pneumatics is to have the pistons travel all the way to the end. Matt C made an excellent suggestion for how to do this.

[Dale]

Also, you'd need to be careful to design a system that didn't trap compressed air when the robot is deactivated. There's a Q&A about that at http://forums.usfirst.org/showthread.php?t=16642

[Chris Fultz]

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

[Chris Fultz]

<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.

[implosionprez]

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?

[IndySam]

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!

[implosionprez]

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?

[IndySam]

Originally Posted by implosionprez

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?

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.

[Dad1279]

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

[cmass]

Originally Posted by implosionprez

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?

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.

[Team2339]

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.