How to calculate actuator time?

[DonRotolo]

I suppose it is possible to calculate, but most of us just set it up and measure.

Let's just say you want to use pneumatics to lift your robot in the air, and it weighs 150 pounds. If you have 60 PSI to work with, you know anything less than a piston area of 2.5 in^2 won't do it (2.5 * 60 = 150). Of course, that ignores friction (which cannot be ignored).

OK now that you know the VOLUME of the pneumatic cylinders you need (cross-section times length), you can now start to calculate time, based on volume flow through the smallest element in the path (likely the pneumatic solenoid valve). How long will it take for xx cubic inches of air to flow at a pressure difference between source and chamber starting at 60 PSI and ending at zero (as the cylinder pressurizes, the pressure difference eventually goes to zero), of course while simultaneously calculating the volume (which goes from about zero eventually to maximum...

It gets complicated.

It also depends on how much air you have stored, and how much your compressor can supply.

Take two hypothetical 2" cylinders, each 30" long, using 60 PSI air and a solenoid valve for each. You have 8 large plastic storage tanks, fully charged. I am guessing it will take 15-40 seconds to fully retract those cylinders. Assuming you didn't run out all the air extending them....

[mikegrundvig]

Ha, I spend all that time writing it out and someone beat me to it and more succinctly at that! Serves me right for being so wordy.

Originally Posted by DonRotolo

Take two hypothetical 2" cylinders, each 30" long, using 60 PSI air and a solenoid valve for each. You have 8 large plastic storage tanks, fully charged. I am guessing it will take 15-40 seconds to fully retract those cylinders. Assuming you didn't run out all the air extending them....

I just tossed this into our spreadsheet calculator and it's lots faster than 15 seconds in practice. Even at a very slow 3 GPM, it's going to take ~8 seconds or so to travel either direction. Flow rates of air are pretty dang high:
http://rapidairproducts.com/flowrate.asp

The solenoid is absolutely the slowest part but I think what limits most teams pneumatic speed is that they under tank the system. The pressure differential is a big part of how fast air will travel from the high to low side. When you have a lot of volume to work with in tanks and the differential stays high consistently, you get very speedy air flow and movement. It's when the differential gets low that things start slowing down very quickly. To make it worse, the compressors we use in FRC are very small and slow so assuming they can keep up with large cylinders is a huge mistake.

-Mike

[matthewdenny]

So I took everyones words of wisdom, went and did some research, and found some additional info, matched it to a couple charts to make sure it was right and made a spreadsheet. I'd love to share it here, but I'm not sure how to post it to CD. Anyone know how?

[mikegrundvig]

Use Google docs and post the link

[Ether]

Originally Posted by matthewdenny

I'd love to share it here, but I'm not sure how to post it to CD. Anyone know how?

Yes. Click on the "Manage Attachments" button. (see attachment to this post).

[matthewdenny]

Okay, got it. Let me hear suggestions/critiques/etc.

[Ether]

Originally Posted by matthewdenny

Okay, got it. Let me hear suggestions/critiques/etc.

Are you assuming a strict isothermal process for all your computations?

[matthewdenny]

Originally Posted by Ether

Are you assuming a strict isothermal process for all your computations?

Yes. I also assumed a min operating pressure of ~30psi. Theoretically if someone is at 25psi (<2x ambient pressure) I should switch over to Low pressure drop flow equations, but I stuck with High pressure drop flow. I figured that it was close enough to the transition that it would only have 5-10% effect on time measurements.

[Ether]

Originally Posted by matthewdenny

Yes. I also assumed a min operating pressure of ~30psi. Theoretically if someone is at 25psi (<2x ambient pressure) I should switch over to Low pressure drop flow equations, but I stuck with High pressure drop flow. I figured that it was close enough to the transition that it would only have 5-10% effect on time measurements.

It would make a good science project for an interested student to validate your model. For fast-acting pistons (as in previous years) I wonder if adiabatic behavior would be a better model.

[Kevin Sevcik]

Matthew, Excel is telling me your file is corrupt, so I can't really look at it.

I'm not an expert at pneumatics, but I'm pretty sure the pressure differential you should be using is difference between your supply pressure and the pressure required to move the cylinder. Yes, at the instant you switch the valve, you have a full 60psi differential, but that assumes 0 psi in your cylinder which means you're not moving. Once you hit the pressure you need to move the cylinder, the cylinder moves (until the pressure drops enough to stop it). So I think a reasonable steady-state case for a long stroke is to assume that your pressure differential is supply pressure minus moving pressure.

That means sizing a cylinder exactly as big as necessary to move your robot is a bad idea, since you'd have zero pressure differential and thus no flow. Which explains why all the speed ratings I'm seeing in SMC's data sheet assume a cylinder sized with a 50% load rate. That is, the cylinder is twice as big as strictly necessary to move the load.

[DonRotolo]

0.02 seconds to move 12" is about right, with no load. It gets a lot slower under load.

Data point: in our Rack & Roll bot, we had 2 12" x 2" cylinders that raised a robot on our ramp up to 12" off the ground, it took about 2 seconds.

[matthewdenny]

So I have been thinking about it, and the times to expand and contract are the same regardless of the load they are working against. In theory we should have to account for F=ma, shouldn't we? For example if a have a cylinder that can exert 180 lbs of force lifting 5lbs, then the limiting factor to speed should likely be the Cv of the solenoid. If it is lifting say 178 lbs then we only have 2lbs of net force lifting a 178lb object. In this situation the delay to very slow acceleration of the cylinder/object seems like it would be a limiting factor. What do you all think?

[Taylor]

Thank you very much for sharing your thought processes and spreadsheet. For teams like us with minimal expert help, ChiefDelphi is one of the best mentors out there!