Chief Delphi - Some basic Pneumatic Design Questions

Quote:

Originally Posted by Jeffy (Post 1289090)

1)It is a smart choice not to carry a compressor,
If you keep the pressure above 60 psi, the answer is simple and as you have explained. Once the pressure in the storage tanks goes below the regulator pressure, the relationship is no longer linear. I will let someone else get into it because I am not sure how to calculate it.

Until you run out of pressure, or forget to recharge between matches, or don't have time to recharge during matches (think eliminations).

The new compressor is light and small and only needs a spike to control it. You will be investing in all of these things anyway since the robot needs to control the compressor to charge it's tanks per 2013 rules.

Quote:

Originally Posted by Jeffy (Post 1289090)

2) There are two kinds of motions you can get from pneumatics; linear or some curve using a linkage.

There are legal rotary actuators available per 2013 rules.

Quote:

Originally Posted by Jeffy (Post 1289090)

When designing, I like to make the best estimate I can as to how long a cylinder I can use. Then, I will often use one that is a bit longer.

This is very conservative, and unnecessary in my experience. It will cost extra air even if you restrict throw on the cylinder somehow, it adds extra weight, and cannot increase your force capability without re-mounting the cylinder.

If you want to be conservative I would suggest getting the right stroke air cylinder, but in a diameter slightly larger than what you need. To conserve air: regulate your operating pressure down below 60psi. This gives you some headroom to adjust pressure (and thus force) up later if need be. It does add some weight too, of course, but is more efficient with compressed air than restricting stroke.

McMaster and Bimba have very good CAD models of their air cylinders available in a large number of CAD formats (Solidworks Native IIRC). The model includes a deployed and retracted position, so it's very easy to figure out what length cylinder you need and where you need to mount it.

Quote:

Originally Posted by Jeffy (Post 1289090)

When designing linearly remember that you can adjust how far the cylinder retracts easily by adding a spacer to the piston. Adjusting how far the cylinder extends takes a new mounting system.

For linkages:
Design the system so that you can mount the cylinder in many different places. A hole pattern spaced ~ .375 inches can do this nicely. And remember that you can restrict retraction with a spacer too.

Note that basically every air cylinder I've ever worked with has a threaded end. This allows for a large amount of adjustment forward and back of the rod end on the cylinder. These adjustments can be made with the cylinder in place too. This is a great way to fine-tune the overall length of an air cylinder. It would be excessive to have a hole pattern spaced smaller than the amount of adjustment the threaded end gives you, but this varies per cylinder. A decent rule of thumb is that you only need 3 complete turns of thread to engage the full strength of the rod-end, so there is a lot of adjustment to be had!

I don't mean to sound too critical of you Jeffy :) I've been designing FRC pneumatic systems since I was a freshman in 2001. I used to design things the same way you're suggesting for a while. It works, but it's not the best way. I have found what I think are better design practices in the intervening 12 or so years.