How do you control the length a piston will extention/retraction?
 

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?

Re: How do you control the length a piston will extention/retraction?
 

There are already a whole bunch of threads about this topic, but here's one explanation:

http://www.chiefdelphi.com/forums/sh...4&postcount=14

Re: How do you control the length a piston will extention/retraction?
 

Yes, many threads already, but I'll answer it again.

The short answer is No, you can't stop a piston in mid-travel.

The only *easy* way to do it is to mechanically block the piston from traveling any further than you want it to.

The long answer is Yes, but its a bit complicated.

In the past (Read: 2004 Off-Season, and 2007 Regular/Off-Season) 1075 has built arms that stop in mid-travel with pneumatics.

In 2004 we used 2 valves. One to switch the direction of travel, and then a second to switch the exhaust of the first. By not letting the exhaust out of the system, the piston doesn't move.

That system was plagued with large amounts of drift, but it served its purpose.

In 2007 where accuracy was paramount, we used a 3rd valve, as well as a mechanical brake. This allowed us to switch the feed side as well for added control. This reduced the drifting problem. Additionally, we used potentiometers, and some fancy code to make it all work nicely together.

Searching CD should yield several drawings of how to plumb such a system.

Re: How do you control the length a piston will extention/retraction?
 

THis is not true actually u can , u connect a single solonoid to a double solonoid to the piston , if u hooped up the single correctly, it shoudl work just turn that little blue thingy while pressing the double solonoid buttons, the piston wont extend/retract but if u release the blue thing a little the piston will extend/retract the more u release the fast and more it will extend/retract

Re: How do you control the length a piston will extention/retraction?
 

You cannot accurately control the length a piston will extend, if there is a load on it, because the air in the cylinder acts like a spring. You're better off using a motor instead of pneumatics if you need to have good control over how far something moves.

The neat thing about this year's game is that for the most part there is no need to have good control over how far something moves...I mean, the game piece is 40" across, and it just has to go over or under the overpass, it doesn't have to be any specific height. We're using pneumatics that are either up or down for our 3 ball manipulator mechanisms.

Re: How do you control the length a piston will extention/retraction?
 

If you are trying to control the length of a cylinder you want a "center off or center closed" solenoid. Instead of the spool inside moving and staying in position to supply air to port A or port B, when it gets power only momentary to allow air into port A or B allowing for small adjustments of a cylinder. As stated above this can be tricky since the air in the cylinder isn't the full regulated pressure but an average and can act like a spring. If you are interested in these contact SMC and they can help you select the proper one for your situation.

Re: How do you control the length a piston will extention/retraction?
 

I'm not really sure how what I'm saying is "not true" since you basically described a similar method to what I did... I have personally built 3 of these type of systems now, with considerable success.

While the trapped air in the cylinder can effectively turn it into a spring, I would argue that it CAN be controlled to a reasonable level of accuracy, as evidenced by 1075's 2007 arm. We did however, use, in addition to the ram driving the arm, another ram which clamped down on a piece of Al angle attached to the arm that slid through a piece of Al square tube. This basically functioned as a physical brake, reducing the "air spring" effect.

I forgot about these "center-off" valves. My ongoing question about these is "Are they allowed?". Besides, the method I feel I was one of the pioneers of, using a plurality of valves to control one ram fits more within the FIRST mantra of using creativity to solve problems, rather than store-bought parts.

Re: How do you control the length a piston will extention/retraction?
 

Adding a mechanism to limit travel can be a big help, you could also have a movable physical stop (perhaps a small servo could actuate a pin or something, as long as the load is removed before you try to move it)

Re: How do you control the length a piston will extention/retraction?
 

With the current KoP parts, this is correct, However, Both Bimba and Parker have cylinders with potentiometers built into the cylinder that give positional feedback. Either of these, used with a pressure controller, could be used to control the stroke of a cylinder even under load.

And this is why I don't like the pneumatics setup that FIRST requires us to use, as it really limits creative pneumatics.

Re: How do you control the length a piston will extention/retraction?
 

I look at it the other way...it forces you to be more creative in your use of pneumatics!

And the limited compressor/storage capacity makes you have to think even more :)

.

Re: How do you control the length a piston will extention/retraction?
 

<R88> states that there is no limit to the number of soleniod valves as long as they are off the shelf. We used center off valves a few years ago. They are also alot easier than multiple soleniods.

Re: How do you control the length a piston will extention/retraction?
 

short answer yes, we do so yes.


you need a double solinoid, double solinoid. or 2 double solinoids.


I don't know the programming for this and this may be to late, but to hook it all up.

solinoid 1
You need the pressure goes in the p hole
top of clynider to the a hole
bottom of the clynider to the b hole
then you also fill the ea, and ab holes but ill get to that later.


solinoid 2.

connect tubing to the a, and b holes
plug p
leave ea, eb empty.

connecting the two

connect ea on solinoid 1 to b on solinoid 2
connect eb on solinoid 1 to a on solinoid 2

in short
connect ea on solinoid 1 to b on solinoid 2
connect eb on solinoid 1 to a on solinoid 2
connect solinoid 1 a to clynider
connect solinoid 1 b to clynider
connect solinoid 1 p to pressure source
plug solinoid 2 p
leave solinoid 2 ea empty
leave solinoid 2 eb empty


im sry this is late, i got to go back to work on our robot sry i can't post code for this, you can't reverse and you need 2 spikes good luck.

Re: How do you control the length a piston will extention/retraction?
 

This is incorrect. The 2nd valve will not switch without pressure in the P hole. I believe the SMC valves require about 20psi to switch. A method that will work, is looping the A and B ports together on the 2nd solenoid, feeding EA, EB, or both from solenoid 1 to the P port (for one direction, the other, or both) on the 2nd solenoid. plugging EA, and leaving EB open to atmosphere. I would recommend using a Single Solenoid for the 2nd one.

Basically the system looks like this:

Pressure in-->|Solenoid 1|-EA------P-|Solenoid 2|--EA--|
...................|________|-EB-/..........|________|--EB------
......................A|....B|.................... A|....B|
................----|cylinder|.....................\___/

Re: How do you control the length a piston will extention/retraction?
 

LOL.

Oh, we can be as creative as the next guy ... No doubt about that. It's just tough because i do work for Parker and I know what is commonly (an uncommonly) available, and I see the restrictions on pneumatics that the students dont even know exist.

I guess my point is First should only limit the amout of power available to pneumatics (limit the working and storage pressures, and the overall maximum volume). That way we could see the real power of pneumatics without having to re-invent the wheel each time.

JM(NS)HO :D

Re: How do you control the length a piston will extention/retraction?
 

with this, the pneumatic will stop when the sol 2 switches to A. when its on B it will move freely. is this correct? trying to figure out programming real quick.

Re: How do you control the length a piston will extention/retraction?
 

actually, i think its the opposite of what you'd said... its easy enough to flip the one variable if its wrong.

Re: How do you control the length a piston will extention/retraction?
 

For those teams that have used center off solenoids in the past, what brand and model solenoids did you use?

Re: How do you control the length a piston will extention/retraction?
 

Just to reiterate, coarse control of piston length has been around for a while. Here is a post about the setup on 1108's 2003 robot with picture and diagram. http://www.chiefdelphi.com/forums/sh...2&postcount=11

The two solenoid system work for this. I've always thought that the center stop solenoid would be a more elegant solution but never got my hands on one and the rules have changed around some as far as outside part go (I don't know what is allowed this year).

The interesting challenge would be to do fine controls, especially under load, which I haven't done much exploring in.

Re: How do you control the length a piston will extention/retraction?
 

could you use this

http://www.alibaba.com/product-gs/20..._Five_Way.html

and could anyone give more info on how to stop a ram using a three way solenoid

Re: How do you control the length a piston will extention/retraction?
 

In previous years (including last year) valves not supplied in the KoP were not legal to use, so these would not be legal (although this could change in the future)

I'm assuming that when you say ram you mean actuator (cylinder).

You would need 2 3-way valves on the primary side of the cylinder and seal the secondary side.
The first valve is used to add pressure or seal the primary side.
The second valve is used to release pressure or seal the primary side.

When you actuate the first valve pressure will build on the primary side of the cylinder and start moving the rod. as the rod moves the gas on the secondary side will be compressed and pressure will build. When you seal the first valve the rod will continue to move until the pressure on both the primary side and secondary side of the cylinder are near equal (difference being friction).

To retract the cylinder you'll just need to activate the second valve.

*Note* the pressure on the sealed secondary side needs to be above atmosphere when the cylinder is fully retracted or the cylinder will only partially retract when the second valve is actuated unless you assist the cylinder with some external spring source like surgical tubing

*Note2* you could add 2 more valves on the secondary side and reverse the actuation from the first side to get quicker (while less efficient) response.

Re: How do you control the length a piston will extention/retraction?
 

I have always been a little cautious about the legality of magnets on robots, but there are multiple magnetic position sensors out there that with a 2-way solenoid would give fairly accurate position. it was one of the ideas we talked about using for this year that we decided we didn't need.


http://www.bimba.com/Products/PositionSensingSolutions/

Re: How do you control the length a piston will extention/retraction?
 

<R88> allowed other pneumatic valves last year. <R87> muddies the waters a little, but it was clarified in this Q/A post. http://forums.usfirst.org/showthread.php?t=8292

Re: How do you control the length a piston will extention/retraction?
 

I wish I saw that during the build season :(



Or you could try a String encoder

Re: How do you control the length a piston will extention/retraction?
 

If you simply want to shorten its travel so that only goes to a single location - You use something mechanical to stop it when it has gone that far (you put something sturdy enough in front of it...;) ).

I'll see if I can dig up a URL to an article I recently read in Machine deisn about the math behind doing this with high precision.

Blake

Re: How do you control the length a piston will extention/retraction?
 

Hi,

I would love to see sketches or photos of any of the multi-valve solutions. It would be fun to try them out in the lab to learn more about pneumatics fundamentals.

Here is a thought on the KoP. It seems to me that FIRST tries to use equipment that is relatively simple and general rather than highly specialized items. The basic elements of robotics - you might say. I guess it also depends on the cost of the items in question. Does anyone have a sense of the comparative costs of regular versus specialized pneumatic components?

-Jason

Re: How do you control the length a piston will extention/retraction?
 

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