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

[Racer26]

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.

[Tds123]

Originally Posted by 1075guy

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.

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

[MrForbes]

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.

[Kirk]

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.

[Racer26]

Originally Posted by Tds123

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

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.

Originally Posted by squirrel

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.

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.

Originally Posted by Kirk

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.

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.

[MrForbes]

Originally Posted by 1075guy

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.

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)

[Kirk]

Originally Posted by 1075guy

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.

<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.

[XXShadowXX]

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.

[Racer26]

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|.....................\___/

[Guarana]

Originally Posted by 1075guy

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|.....................\___/

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.

[Racer26]

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

[CyberWolf_22]

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

[ngreen]

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.

[jdejoannis]

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

[Daniel_LaFleur]

Originally Posted by squirrel

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.

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.