A small pneumatics spreadsheet

[WorldShaper]

I recently graduated from college with a physics degree and so now I don't have much to do. At least, until January 9th rolls around. This spreadsheet is some of the fruits of my boredom.

The basic jist of the thing is an ideal gas calculation of a pneumatic system. Three layers of varying complexity are included.

1. The simplest layer is has no onboard compression capability and only a single cylinder. This sheet will calculate the number of actuations that cylinder may undergo while stored pressure exceeds working pressure; it calculates the number of actuations before the system no longer works with full pressure.

This sheet might be nice to get a feel for how much firing a large cylinder might effect your system.

2. The middle layer can provide a decent first order approximation of when the stored pressure of a robot with up to 8 cylinders will fall below the working pressure of those cylinders. The time it calculates is based simply off of how much air, on average, several actuators will use per second (given each actuator's distinct pressures, volumes, and actuation rates).

This sheet is a reasonable approximation of if your robot can make it through a match and still have air.

3. The last layer provides a fairly simple simulative analysis of a robot's pressure during a match. Each actuator has several parameters: extension and retraction volume, working pressure, and a default state. One may enter the times (with 1/2 second resolution) at which each actuator fires. A graph of stored pressure vs. time is generated.

This sheet is only really applicable if you have a pretty good understanding of what your robot looks like and how a match will unfold. Speaking as a former operator, you don't know how the match will unfold. But you could at least enter in how it is supposed to.

My apologies for the wall of text, but if you give me a pulpit I tend to preach.

I hope you enjoy, and have a boisterous build season!

[GeeTwo]

Neat stuff! A few suggestions that pop out at me:

• The air volumes are huge! The first sheet has a 12 cubic foot tank - that's almost 90 gallons, or bigger than some whole FRC robots. Typical cylinders used in FRC range from about .0005 cubic feet (3/4" diameter, 2 inch stroke to 0.065 cubic feet (2" diameter, 36" stroke).
• It would be great if the cylinder volumes were calculated from diameter and stroke (and possibly diameter of the shaft for the return stroke). Whenever feasible, spreadsheet or other UI inputs should be numbers available on the manufacturer's data, without requiring a lot of user pre-calculation.
• For small cylinders and long tubing, the length of tube from the solenoid valve to the cylinder should enter into this. It doesn't do any work, but it does use up air.
• For the versions with the compressor, are you assuming that the compressor will run continuously through the match? This is usually not a good idea. I also need to go back to see if your spreadsheet "turns off the compressor" when at full pressure. If you incorporate this, you should add the pressure at which the switch goes on as well as off.

[WorldShaper]

Thanks for the feedback GeeTwo!

The air volumes in the original copy were just there from me playing around with it to see what would happen. Integers were just a lot easier to type than small fractions and everything is nearly proportional, so they were nice test numbers for me.

It would be great if the cylinder volumes were calculated from diameter and stroke

Well now that you say that, its totally obvious. I've edited the original post to contain a spreadsheet that does just that, using Power Factor (Bimba's name for surface area, who knows why) and stroke length.

While the old sheet assumed that you had taken the volume of the tubing into account (or were at least comfortable with ignoring it), the new sheet has the option to include a specific quantity for it, based on the tubing's inner diameter and the length of tubing.

The middle complexity sheet only assess air balance, and so never shuts off the compressor. The assumption therein is that t=0 is the first actuation and that the pressure never returns to the maximum storage pressure, which at least in my team's history, is par for a match. That said, the more complex sheet does shut off the compressor when the maximum storage pressure is attained.

Cheers!

[GeeTwo]

Originally Posted by WorldShaper

Thanks for the feedback GeeTwo!

Always a pleasure to help someone trying to help! Especially someone applying physics knowledge! (My BS and MS are in physics).

Originally Posted by WorldShaper

Well now that you say that, its totally obvious.

So's the wheel, but no one seemed to use it much until a very few thousand years ago. I didn't invent this rule of user interface, just passing it along.

Originally Posted by WorldShaper

I've edited the original post to contain a spreadsheet that does just that, using Power Factor (Bimba's name for surface area, who knows why) and stroke length.

As I'm sure you know, the force produced by a piston extending is (ignoring friction) F=Pa, where P is pressure and a is the area. Most typically in the US F is in lbf and P in psi, so the natural unit for a is square inches. Force Factor would be a more accurate name for a, but I'm speculating that the marketing folks at Bimba thought Power Factor sounds more impressive (and inaccuracy aside, I concur). The bottom line is that publishing the "Power Factor" and even making it a central element of the part number makes it easy for engineers (including amateurs, hobbyists, and FRC teams) to figure out what diameter piston their actuator should have.

[ScandallB225]

Great information.

Originally Posted by WorldShaper

While the old sheet assumed that you had taken the volume of the tubing into account (or were at least comfortable with ignoring it), the new sheet has the option to include a specific quantity for it, based on the tubing's inner diameter and the length of tubing.

From my limited experience, the shaft volume is not insignificant for most FRC applications. We have found that the smaller the cylinder the more significant it becomes.

Originally Posted by WorldShaper

The middle complexity sheet only assess air balance, and so never shuts off the compressor. The assumption therein is that t=0 is the first actuation and that the pressure never returns to the maximum storage pressure, which at least in my team's history, is par for a match. That said, the more complex sheet does shut off the compressor when the maximum storage pressure is attained.

Again from limited experience, over the past two years we have trimmed weight to maximize onboard storage and still run below 60 psi with the compressor running full time.
The two years prior the compressor was on for short bursts at a time. So, both situations are good to have accounted for.

Did I mention, thank you for tabulating this for us?

[GeeTwo]

Originally Posted by ScandallB225

From my limited experience, the shaft volume is not insignificant for most FRC applications. We have found that the smaller the cylinder the more significant it becomes.

The quantity referenced was not the shaft volume, but the tubing between the solenoid valve and the cylinder.

Originally Posted by ScandallB225

Again from limited experience, over the past two years we have trimmed weight to maximize onboard storage and still run below 60 psi with the compressor running full time.
The two years prior the compressor was on for short bursts at a time. So, both situations are good to have accounted for.

In 2013 Ultimate Ascent, we used air during the main part of autonomous only to trigger our launcher (less than 10 cu in of 60 psi air per cycle); our climb was about 60% pneumatics and 40% electric powered. If we started with full tanks (7 clippards at approx 120psi), in match-duration drills we usually wound up starting the compressor as we lifted the robot half-off the floor. I suspect we used even less air during the matches due to defenses slowing us down.

Originally Posted by ScandallB225

Did I mention, thank you for tabulating this for us?

Hear, hear!