Pistons only halfway up

In 2008, with Overdrive, our team (2484) (2008 Las Vegas Regional) used a piston to move our arm up and down. Even though there are posts on CD now saying that it isn’t possible to have a piston go only part stroke, instead of all the way up or all the way down, we sure did it in 2008. The trouble is, we’re not sure how that was done. It was using the old control system, with the solenoids wired up to spike relays. Is that why we could go part way, instead of all the way? I know it can be done, so can anyone give me some insight as to why it worked back in 2008, but it won’t work now?

the control system should not make a difference. If the piston is large enough and has enough backpressure on it, you should be able to only open up the solenoid for a certain amount of time to not fill the piston up all the way. It will be difficult (impossible?) to do it with a small cylinder, but we easily did it with our 1.5" 1o inch stroke cylinders in 2008. Another option (havent tried it) is to lower your working pressure below 60psi but that may effect other cylinders on your bot

Meaning that if you have air going into both ends of the psiton, they should be able to counteract each other and stop the piston, right?

Use a second regulator to regulate the pressure down from 60 psi to maybe 30? it will lower the pushing force though, or use those fittings with valves which you can open and close as you test.

I’m not sure of the technical terms to use, but in 2010 we used a solenoid that normally kept both sides closed unless one side was activated. It was paired with a very large pneumatic cylinder to be used as a lifting arm. It raised and lowered fine, and you could even stop half way. We never got it fully functioning though.

Aren’t all solenoids closed unless you activate them, to open them?

Or you can look into using multi-positional sylinoids. They more or less control how much air goes in and out instead of traditional one where you get all or nothing out of the cylinder.

If you have extra cash you could purchase 3 position pistons from Bimba as well. Here is a link

You should also have some flow regulators that can slow the flow of air into smaller pistons.

Consider that air roughly follows the ideal gas laws of PV=nRT. P is the pressure, V is the volume and n is the number of moles of the gas. R is a constant and T, the temperature, is also roughly constant.

So as you put more moles (n) of gas into the cylinder, either the pressure or the volume must increase. Normally we extend a cylinder all the way… the pressure increases, forcing the piston outward, and allowing the volume to increase.

If, however, you put just a little bit of air into the cylinder, then as the volume increases the pressure will drop, until your piston position finds equilibrium with the applied load. (The pressure in your cylinder pushing on the piston will be balanced by the force applied to the piston.)

This sounds easy until you try to do it, and what works in your shop (at your altitude, temperature and humidity) might not work at your competition venue without some careful tweaking.

This is naturally quite a bit different from a hydraulic system as air is a compressible gas while hydraulic fluids are specifically chosen to be as incompressible as possible.

For precise, reliable positioning you may wish to consider some from of linear actuator… or perhaps using one of those lead screws that come in the KoP (longer ones are available) for something other than entertaining that team member who really, really, likes turning the lead screw up, and down… up and down…


Please can we call them cylinders :slight_smile:

It is very possible, I just wouldn’t recommend it.

Reducing the air will only reduce the amount of pressure and speed, the cylinder will still extend all the way.

A cylinder extends when air is applied on one end and exhausted out the other. To stop the piston inside the cylinder at a mid point you must stop the air from exiting.

That is how a flow control valve works, it is a one way valve that slows how much air comes out of the exhaust, slowing the movement.

So you must use a second valve (or a special valve) on the exhaust side to block the exit of air.

A easier and more reliable way to accomplish this is to put two cylinders back to back. Look at this pick from 1322. They are using three hooked together to get multiple positions.


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 :slight_smile:

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.

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

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.

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

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. *


*<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.

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.

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?

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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!