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-   -   Oscillating a pnuematics valve? (http://www.chiefdelphi.com/forums/showthread.php?t=40040)

Squirrelrock 03-11-2005 13:10
Re: Oscillating a pnuematics valve?
 
I like this - I have attempted to get a cylinder to run from two single solinoid valves as a multi-positioner, and it is hard to do, until you figure out how to do it. I no longer have the setup, though, but I believe that the exhaust of the first valve had to be routed through the input of the second, and the second had to have one output blocked and be set to it as default. Then, the system backpressures itself (probably above 60 psi, so this probably is not competition legal) until the arm stops whereever it is. The way you described it would seem to be a much easier (wiring and tubing wise) way to get this done, and it has a much higher likelihood of being competition legal.

Quote:
Originally Posted by KenWittlief
The idea of having an arm that is moved by pneumatics, and locked in place anywhere within its range of motion - thats very clever! I dont think I have ever seen it done on a FIRST robot

you dont need to get fancy with the valves, simply use the flow restrictors to limit the speed at which the arm will move

and to get precise control, use feedback. Instead of having the operator release the clutch and move the arm up or down, and then engage the clutch at the right position, have something like a potentiometer on the arm reporting position. the operator would move the joystick up or down, and the system will sense the arm is not where the joystick says it should be, unlocks the clutch, pressurizes the cylinder in the right direction, and engages the clutch when the feedback sensor = the joystick setting.

I really like this idea. You could have a very light, very simple, multiple axis or multiple elbow articulated arm, with very precise control of its motion - using nothing but pnuematics!

Nitroxextreme 07-11-2005 22:24
Re: Oscillating a pnuematics valve?
 
last year we used multi positioning on 2 pneumatic cylinders and needed a total of 2 double solenoids and 2 single solenoids

http://www.chiefdelphi.com/forums/sh...ad.php?t=33258
has a good description as to what needs to be done for it to work

KenWittlief 07-11-2005 23:22
Re: Oscillating a pnuematics valve?
 
the problem with using two valves to create a multiposition system is that the arm is balanced by the pressure in the cylinders. If you pulled or pushed on the arm at any given position, the arm would move (for example, if you dropped a tetra, the arm would rise up on its own).

using a locking mechanism, or clutch, would make the arm stiff in all positions.

Cuog 14-11-2005 15:00
Re: Oscillating a pnuematics valve?
 
yeah thats what the potentiometer is for you make pregramming increase or decrease the pressure until the arm is within like 1 or 2 degrees of where the joystick is its very logical and a really great idea

What would happen is the arm would move due to the pressure change of shifting wieght but would automatically readjust itself to wherever it is supposed to be.

DonRotolo 20-11-2005 09:47
Re: Oscillating a pnuematics valve?
 
Team 1676 tried that last year, but we had mixed results. The feedback loop (including the valves, movement of the air, etc.) wasn't fast enough to hold a position pneumatically when faced with variable loading. The problem is that air compresses, and so changes in load made the arm move: We hung a tetra on the arm and it drooped down. Also, the operator cannot expect instant response - we had something like a 1/2 second delay, and that was hard on the operator. We used a double-solenoid valve (3-way), no fooling with modulating exhausts and whatnot.

In short: Pneumatics are excellent for endpoint positioning, but awful for precision or intermediate positioning. Not that it can't be done - we did it - just that motors are a FAR better choice.

My advice would be to abandon ideas of variable mid-point positioning using pneumatics.

On the other hand, pneumatics were excellent for our gripping claw - open/closed.

Don



Quote:
Originally Posted by KenWittlief
The idea of having an arm that is moved by pneumatics, and locked in place anywhere within its range of motion - thats very clever! I dont think I have ever seen it done on a FIRST robot

you dont need to get fancy with the valves, simply use the flow restrictors to limit the speed at which the arm will move

and to get precise control, use feedback. Instead of having the operator release the clutch and move the arm up or down, and then engage the clutch at the right position, have something like a potentiometer on the arm reporting position. the operator would move the joystick up or down, and the system will sense the arm is not where the joystick says it should be, unlocks the clutch, pressurizes the cylinder in the right direction, and engages the clutch when the feedback sensor = the joystick setting.

I really like this idea. You could have a very light, very simple, multiple axis or multiple elbow articulated arm, with very precise control of its motion - using nothing but pnuematics!

KenWittlief 20-11-2005 10:16
Re: Oscillating a pnuematics valve?
 
Quote:
Originally Posted by Don Rotolo
Team 1676 tried that last year, but we had mixed results. The feedback loop (including the valves, movement of the air, etc.) wasn't fast enough to hold a position pneumatically when faced with variable loading. The problem is that air compresses, and so changes in load made the arm move: We hung a tetra on the arm and it drooped down.
I think you missed part of the description.

you include a clutch or locking mechanism on the joint, so once the arm is in the proper location, the clutch locks, holding the arm rigid. Unless you add enough weight to the end of the arm to overpower the clutch, its not going to move

and with this approach you dont try to control the pressure in the cylinder, or to apply pressure to both ends - you use the cylinder in the normal configuration, with the little flow restrictors to slow down their movement.

The clutch is in the feedback loop, not the valves. Once the arm has moved to the right position (up or down) the clutch locks and holds it against the pnuematic pressure, and against any added weight to the end of the arm.

Rickertsen2 20-11-2005 13:18
Re: Oscillating a pnuematics valve?
 
Quote:
Originally Posted by KenWittlief
I think you missed part of the description.

you include a clutch or locking mechanism on the joint, so once the arm is in the proper location, the clutch locks, holding the arm rigid. Unless you add enough weight to the end of the arm to overpower the clutch, its not going to move

and with this approach you dont try to control the pressure in the cylinder, or to apply pressure to both ends - you use the cylinder in the normal configuration, with the little flow restrictors to slow down their movement.

The clutch is in the feedback loop, not the valves. Once the arm has moved to the right position (up or down) the clutch locks and holds it against the pnuematic pressure, and against any added weight to the end of the arm.
I have tried this and it works! There are nifty off the shelf cylinders with pneumatic cluthes built in for exactly this application but they are of course not FIRST legal.



It might still be nifty to use a closed center valve in addition to a locking mechanism. By pulsing it on and off at different duty cycles, you could move at different speeds. Another strategy is to use more than one valve to control a cylinder. Each valve has it's flow restricted to a different extent.


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