How to calculate actuator time?

[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!