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 Chief Delphi Pneumatic Actuator Speed?
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#1
01-13-2010, 07:47 PM
 WorkingFromHome Full of Awesomeness AKA: That One Guy Who Does Stuff FRC #2587 (Discobots) Join Date: Jan 2010 Rookie Year: 2008 Location: Earth. Where else? Posts: 1
Pneumatic Actuator Speed?

Does anyone know a good formula for figuring out the speed of a pneumatic actuator? I've looked all over the place, but all I can find are ways to get the force.

Thanks for the help,
WorkingFromHome
#2
01-13-2010, 10:01 PM
 kirtar Alumnus FRC #0461 (Westside Boiler Invasion) Team Role: Alumni Join Date: Jan 2008 Rookie Year: 2007 Location: West Lafayette, IN Posts: 169
Re: Pneumatic Actuator Speed?

One thing you can do is take a scale (that is to say a reference such as a meter stick) and use a video camera. This may require a high speed camera that you don't have though depending on how fast this is. What you can do is tape it with the scale and go frame by frame (remember, frames have a constant rate).

If you're using a piston and not a rotary pneumatic, you can actually just focus on the piston and put the scale there so you won't reallly need a high speed camera. Basically, start at the first frame in which it starts to move (record measurement), go to the frame in which it is fully extended and then divide the distance traveled by (Numer of frames/Frame rate), and then carry through the calculation through any linkage you have it hooked up to.

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#3
01-15-2010, 02:41 PM
 StevenB is having FRC withdrawal symptoms. AKA: Steven Bell no team Team Role: College Student Join Date: May 2005 Rookie Year: 2005 Location: Stanford, CA Posts: 374
Re: Pneumatic Actuator Speed?

Quote:
 Originally Posted by WorkingFromHome Does anyone know a good formula for figuring out the speed of a pneumatic actuator? I've looked all over the place, but all I can find are ways to get the force.
I'm looking for the same thing. All I know is that it has to do with the rate of airflow through the tubes.
This paper has some useful information, but I don't really understand the equations they present.
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#4
01-20-2010, 12:09 AM
 vamfun Mentor :Contol System Engineer AKA: Chris FRC #0599 (Robodox) Team Role: Engineer Join Date: Jan 2009 Rookie Year: 2003 Location: Van Nuys, California Posts: 178
Re: Pneumatic Actuator Speed?

I couldn't find any either so I wrote an excel program that integrates the forces to get the piston step response. The response is dictated by the valve coefficent of flow Cv or effective area. The effective area of the SMC valve is 4.86mm^2. A valve is either operating in subsonic or sonic conditions. When it is in sonic conditions, the flow rate is limited by the speed of sound. Otherwise , it varies as the square root of the pressure drop across it. It gets a little more complicated because every component in the pneumatic flow chain has its own Cv (area) and the squared Cv's add like resisters in parallel.

You can take a wag at the maximum possible piston speed assuming it is restricted by the speed of sound.

speed_ips = 13500*factor*A_valve/A_piston

where A_valve = 4.86mm^2, A_piston is the area of the bore and factor is a degradation due to the system fittings and piping. factor reduces the effective A_valve. I would use factor of .3 to .5.

So rewriting this in terms of cylinder bore_in we get

speed_ips = 84.76*factor/bore_in^2

Assuming a factor of .5 and a bore of 1 in then the max speed would be about 42 ips. A 2 in bore would be 1/4 that.

The above assumes standard day sea level conditions.

My model shows that the SMC valve is sized well to keep the .75 in bore cylinder in sub sonic flow conditions most of the time. The 1 and 2 inch bores are flow limited unless driving very large loads. So if you want speed, use the smallest bore. If you want kinetic energy, load the piston as much as you can with a weight. This slows the piston down and allows the piston to be work limited (piston force*stroke) rather than speed limited. When work limited, you can typically get an efficiency of about 35% of what the piston could deliver if moving really slow against a heavy weight than when it is pushing a lighter load.

For example, when pushing a 10 lb load at 60psi with a .75 in bore , a 12 in stroke can deliver about 8.6 ft lbs of kinetic energy. The max would be =Bore*Force = 1ft*24lbs=24 ft lbs, so 35% efficiency. A 10 lb load would be typical of a piston pushing a 2lb weight with a 5 to 1 lever arm.

These results are yet to be verified with FRC testing... I have however done some verification with the vex pneumatics while working with a vex catapult. Here are some forum posts if you are interested:
Also a little more in my vex blog: http://vamfun.wordpress.com/

Last edited by vamfun : 01-20-2010 at 01:39 AM.

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