GDC cylinder answer on Q&A

FIRST General Forum
1

I read this answer with confusion, and when this answer came out I was truly baffled.

For clarification, I am not affiliated with the team asking the questions, and don’t plan to implement this on our robot.

I don’t recall the GDC ruling that something is not permitted without backing that up with a rule that is violated. I don’t see any rule that requires a cylinder to connected to a valve at all. If such a rule existed, a team could connect the cylinder to a valve and just not activate that valve.

And what about single acting cylinders? if one wanted to use a spring to retract a cylinder, one would not need to connect the rod end to a valve. Is that also illegal?

Does anyone see the logic here? Or am I missing something?

2

My guess would be there exists a possibility for the pneumatic cylinder acting as a shock absorber to exceed the 60psi working pressure allowed on the robot. While the cylinders may be able to handle considerably more than that, it appears FIRST wants to stay away from any potential failure.

Thats the only logic I can see.

-Brando

3

Brandon’s guess is consistent with past years’ Q&A explanations of why the proposed usage is not permitted.

4

I think the laws of nature (or perhaps our definitions of mechanical things) would prevent using a cylinder as a shock absorber. I’m assuming the intent is to seal one end of the cylinder, and use it as a gas spring.

A shock absorber (in the normal automotive use of the term) is a damper, not a spring.

So you can’t use a cylinder as a shock absorber unless you fill it with a liquid and have that liquid go through an orifice from one side of the piston, to the other.

I like Brandon’s answer better though :smiley: :smiley:

5

I don’t see the question that way. They specifically said that the cylinder would not be hooked up, I take that to mean that the ports are open, or have adjustable flow fittings (which are legal) that are themselves open to the atmosphere. Therefore, you cannot trap air in the cylinder, and no rule prohibits it. And it would definitely provide damping because it IS filled with a fluid (air). Damping is defined as a resistance to movement that is proportional to the speed of that movement.

6

Wouldn’t the cylinder be considered a pneumatic component? Because it is a pneumatic component, it must be hooked up to a full pneumatics system and cannot be isolated.

Gas shocks are the exception to the rule.

7

I expect the GDC does not make that assumption, and I think they’re right to not make that assumption.

If there is an orifice on one of the openings on the cylinder, then it could act as a spring, and exceed the rated pressure.

8

But flow control fittings are legal, and have been included in the KOP in the past (see 2008 SMC parts, flow control fittings NAS2201F-N01_07S) Again, if the team in question were to hook up the blind end of the cylinder to the normally open side of a legal pneumatic valve through a legal flow control fitting, they would achieve the same result. If they choose not to activate that valve, there is no rule to prohibit this use.

If the GDC had explained the ruling by citing the rule that prohibits the design as they see it, the team could clarify, or modify their design to be legal. My biggest complaint is the lack of clarity and completeness in the answer.

9

That is a valid complaint.

10

What if the cylinder was only connected at one end and allowed to vent to atmosphere on the other end? Would this be illegal? I hope not else our last year’s robot was illegal and we didn’t know it. Our main kicker cylinder was only presurized on one side.

This really doesn’t make sense to me either. I was actually thinking about using an unconnected cylinder with an adjustable flow control valve to dampen our minibot deployment mechanism so it doesn’t slam into the pole so hard if needed. I guess this Q&A ruling makes that illegal. Back to the drawing board…

11

Martin,
As you pointed, it would have been nice if the GDC would stated which rule applied. The user had specifically asked if a cylinder (they used the term piston) could be used as a shock absorber without being

R66J defines a cylinder as a pneumatic component.
R70A defines that all working pneumatic components must be connected to the working pressure side of the pneumatic system.

I was wondering why someone would want to do this. I believe Chuck answered that question for me in the previous post. I do believe using an independent cylinder with restricters would violate R70A.

I guess the GDC left us guessing on this one.

12

<R65>, in the wording of the first paragraph, stipulates that all pneumatic parts must be COTS and unmodified except for assembly. Combined with <R70>, you get the working psi of all COTS pneumatics shall not exceed 60psi. It’s a bit of a stretch, but <R74> also says something about “Commanded Motions” going through one valve.

Without further details providing evidence that the cyllinder would not absorb more than 60psi worth of force AND that the cyllinder would not intentionally absorb shock due to another mechanism’s actuation, the three rules combine to make a COTS cyllinder-based shock absorber illegal.

I agree, that answer is somewhat unclear, but perhaps these three rules are the basis of the ruling? I do not necessarily agree with the GDC’s justification of their ruling since it’s a quite a stretch as it’s worded. The underlying issue from their perspective is probably safety and without many more details on the design they probably wanted to error on the side of safety, thus finding some nuance words to justify it. I personally would not want a 125-psi rated shock absorber absorbing the impact of some poorly-designed robotics subsystem anywhere near someone who wasn’t behind a safety wall.