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Unread 15-10-2005, 17:40
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Oscillating a pnuematics valve?

I was wondering if quickly oscillating a double solenoid valve would damage it after repeated use. The reason would be to create a multi-position pnuematics system using only 1 valve switching back and forth at the correct time, instead of controlling the exaust. Are the double's even fast enough to do this?So the question is has anyone done this, or is it unfeasible?
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Last edited by Andrew Blair : 15-10-2005 at 18:11.
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Unread 15-10-2005, 17:50
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Re: Oscillating a pnuematics valve?

I have seen applications in the textile industry where pneumatic valves are actuated at speeds of over a hundred times a second all day long. (I was amazed to learn that pretty ordinary valves are capable of this). As long as you use a pretty solid valve, you should be ok. I don't know how fast any of the valves in the kit are but you don't need them to be all that fast. Keep in mind that that your setup will almost instantly drain your air supply if you are describing what i think you are.
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Unread 15-10-2005, 18:09
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Re: Oscillating a pnuematics valve?

Quote:
Originally Posted by Rickertsen2
I have seen applications in the textile industry where pneumatic valves are actuated at speeds of over a hundred times a second all day long. (I was amazed to learn that pretty ordinary valves are capable of this). As long as you use a pretty solid valve, you should be ok. I don't know how fast any of the valves in the kit are but you don't need them to be all that fast. Keep in mind that that your setup will almost instantly drain your air supply if you are describing what i think you are.
Darn, you're right. I didn't consider this. Perhaps if a flow control was used you could slow the air loss on the open side long enough to correct, then close it again. I'm considering an arm load where most of the time the load force is enough to allow control by opening only the lifting side for a moment. However, the arm itself wouldn't be heavy enough to lower the arm if it's not carrying something, so you would have to force it down using the other isde of the cylinder. Any ideas?
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Unread 15-10-2005, 18:25
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Re: Oscillating a pnuematics valve?

Quote:
Originally Posted by Andrew Blair
Darn, you're right. I didn't consider this. Perhaps if a flow control was used you could slow the air loss on the open side long enough to correct, then close it again. I'm considering an arm load where most of the time the load force is enough to allow control by opening only the lifting side for a moment. However, the arm itself wouldn't be heavy enough to lower the arm if it's not carrying something, so you would have to force it down using the other isde of the cylinder. Any ideas?
In the past we have created multi-position pneumatics by connecting a cylinder to a double solenoid valve and then plumbing the exhaust port of the valve to a single solenoid valve. The double solenoid valve is used to control the direction that you want the system to move and the single solenoid valve is then opened to allow movement, and closed to stop movement. It was totally manual and somewhat imprecise. With some feedback (switches or potentiometer) it should be better.
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Unread 15-10-2005, 19:34
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Re: Oscillating a pnuematics valve?

Quote:
Originally Posted by Andrew Blair
Darn, you're right. I didn't consider this. Perhaps if a flow control was used you could slow the air loss on the open side long enough to correct, then close it again. I'm considering an arm load where most of the time the load force is enough to allow control by opening only the lifting side for a moment. However, the arm itself wouldn't be heavy enough to lower the arm if it's not carrying something, so you would have to force it down using the other isde of the cylinder. Any ideas?
The best thing to do would be to use something like a gas spring to pull the thing down and use a special type of 3-way valve called a 3 position closed center double soleniod valve.

this valve would allow you to alternate one side of the cylinder between pressurized, sealed off and vented while only using a single valve. When neither soleniod is energized, the output is closed off. When one of the coils in energized, the output gets pressure. When the other is pressurized, the cylinder is vented. I did some tests with this sort of setup and it worked pretty well. The biasing spring also leads to increased stability when lifting light loads. You could also use a 4-way version of one of these if you want to control both sides of the cylinder.
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Last edited by Rickertsen2 : 15-10-2005 at 19:36.
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Unread 15-10-2005, 19:37
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Re: Oscillating a pnuematics valve?

I'm not real versed in pnuematic's rules, and I'm considering this for a FIRST robot. What are the rules concerning the purchase of solenoid valves and which ones you may use?
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Unread 15-10-2005, 21:30
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Re: Oscillating a pnuematics valve?

Quote:
Originally Posted by Andrew Blair
I'm not real versed in pnuematic's rules, and I'm considering this for a FIRST robot. What are the rules concerning the purchase of solenoid valves and which ones you may use?
The last revision of the rules allowed any unmodified off-the-shelf solenoid valve rated for 125 psi and consistent with the other generic parts restrictions (e.g. cost, safety, etc.). Rules change every year; verify that this is consistent with the 2006 rules when they appear.

Last edited by Tristan Lall : 15-10-2005 at 21:32.
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Unread 16-10-2005, 08:57
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Re: Oscillating a pnuematics valve?

The best way to do this is to use hydraulics, you can stop the cylinder at any place and it will be rock solid. If you want to do this for first, you need to get high presure on both sides of the piston or your set up will be spongy and not locate/hold location very good. You can make your cylinder way over sized to help make it stiffer. First's rules on pneumatics are very strick and really don't allow you to do much.
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Unread 16-10-2005, 14:09
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Re: Oscillating a pnuematics valve?

Quote:
Originally Posted by Eagle Master
The best way to do this is to use hydraulics, you can stop the cylinder at any place and it will be rock solid. If you want to do this for first, you need to get high presure on both sides of the piston or your set up will be spongy and not locate/hold location very good. You can make your cylinder way over sized to help make it stiffer. First's rules on pneumatics are very strick and really don't allow you to do much.
Haha true. Unfortunately we are not allowed to use hydraulics
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Unread 16-10-2005, 14:11
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Re: Oscillating a pnuematics valve?

Several comments here. First, the double solenoid valve spec sheet says the maximum operating frequency is 10 Hz on the solenoid valve. So take that into account in your multi-positioning thing.

Second, I just had a brain wave. From Eagle Master's post up there. It's hard to get high pressures on both sides of a FIRST piston and still move it and such. But you could use a piston to hold something in place if it was otherwise actuated. You connect the extend and retract ports of the piston together, except you put a solenoid valve between them. You close the valve when you want the piston to maintain position and the piston acts as an air spring. You open the valve to allow the piston to move freely. This has some interesting characteristics, however. The more you pre-pressurize this closed system, the stiffer the spring. The closer to the end of the stroke, the stiffer the spring becomes. At the middle of the stroke, the spring is reasonably linear over short displacements, but becomes less linear towards the ends of the stroke. The only real problem, I think, is that there would be a net extension force on the piston due to the shaft of the piston.

Similarly, you could make a shock absorber by putting a needle valve between the two ports.

Note, I'm doubting the FIRST legality of this. The pressures could get dangerously high. Especially if the system started at 60 psi.
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Unread 16-10-2005, 14:35
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Re: Oscillating a pnuematics valve?

Quote:
Originally Posted by Kevin Sevcik
Several comments here. First, the double solenoid valve spec sheet says the maximum operating frequency is 10 Hz on the
Figures. They are air piloted and very low wattage. You can faster acting ones that are FIRST legal.
Quote:
Originally Posted by Kevin Sevcik
Second, I just had a brain wave. From Eagle Master's post up there. It's hard to get high pressures on both sides of a FIRST piston and still move it and such. But you could use a piston to hold something in place if it was otherwise actuated.
This brings to mind the appropriate off the shelf part, which of couse the overly-prohibitive FIRST pneumatics rules prevent us from using.
http://www.bimba.com/break/news54.htm

Basically, it is an ordinary cylinder with two enhancements.
*A position feedback resistor
*a second single-acting cylinder which actuates an integrated device which locks the cylinder rod in place, preventing it from moving.
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Unread 16-10-2005, 16:36
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Re: Oscillating a pnuematics valve?

Or you could use 2 cylinders that are inline. Extending the first one gets you x distance and extending the 2nd gets you y distance. So actually if the two cylinders were different stroke you could get 4 positions.

Distance 1 = x and y retracted
Distance 2 = x and y extended
Distance 3 = x extended and y retracted
Distance 4 = x retracted and y extended
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Unread 16-10-2005, 17:29
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Re: Oscillating a pnuematics valve?

Quote:
Originally Posted by Eagle Master
Or you could use 2 cylinders that are inline. Extending the first one gets you x distance and extending the 2nd gets you y distance. So actually if the two cylinders were different stroke you could get 4 positions.

Distance 1 = x and y retracted
Distance 2 = x and y extended
Distance 3 = x extended and y retracted
Distance 4 = x retracted and y extended
Or, If the pneumatics rules weren't soo lame, you could buy off the shelf multiple position cylinders. These are more or less what you describe only integrated into a compact off the shelf part. The custom cylinder order form is the only first rule i disagree with. It has no rationale.
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Last edited by Rickertsen2 : 16-10-2005 at 17:42.
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Unread 17-10-2005, 17:12
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Re: Oscillating a pnuematics valve?

Well what would you be using a multi position cylinder for, because most likely you can use something else to do it that may be better or save weight so you dont have to have a pump and/or tanks and everything else.
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Unread 03-11-2005, 12:35
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Re: Oscillating a pnuematics valve?

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!

Last edited by KenWittlief : 03-11-2005 at 12:42.
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