Off robot air compressor

[Peter Matteson]

Not beat a dead horse but I want to see if others agree with me.

Wouldn't it be simpler to require the pressure relief valve (PSV) be mounted to the accumulator (ACC) or the vent valve (HV). This would make it very clear that the system was at a safe pressure and would eliminate the need to have many of these systems on the robot.

I have been a proponent of this for several safety reasons but it should also simplify inspection and allow for any charging source. If you over charge the ACC you will actually be at the lower reseating value of the PSV. Also having only the HV to vent does not adequetly prevent several potential over pressure issues particularly when a robot is shipped pressurized and exposed to extreme temperatures (in excess of 150F).

I have all season since reading the pnuematic rules related to an off board compressor been extremely disappointed with the wording. I would like to see FIRST take some of these items into account when writing next years rules.

[Tristan Lall]

Further to Ed's comments, I'd like to point out to anyone who's interested that the inspectors do discuss controversial topics like the necessity of the off-robot pneumatic requirements. And sometimes, we, like many of you, feel that certain rules (and their interpretations) don't always stand up to close scrutiny. If that's the case, the concerns are relayed to FIRST itself, to be considered for inclusion in a team update.

The trouble is, we've got the added burden of maintaining consistency and legitimacy throughout all of the events; this means that we deliberately restrict our flexibility in making rulings, because we wish to avoid situations where rules are applied unevenly, especially when we know that the issue is unresolved.

But, as Richard stated, even if the rules themselves are constant, the interpretations can change over the course of a season. When dealing with some contentious issues, what was properly ruled legal at a Regional will not necessarily be again ruled legal at the Championship, primarily because the inspectors at the Championship will settle upon standards that reflect the most up-to-date experiences of the entire FIRST community, while still adhering to the letter of the rules.

Basically, though my initial study of the off-board compressor rules lead me to agree with Dave's interpretation, in the later weeks of the season, FIRST has repeatedly clarified, both in the unofficial Q&A, and in an official update, that the rules should be interpreted differently. The Championship inspectors have agreed to interpret the robot rules as being applicable to off-robot compressors, and teams will therefore be expected to make the necessary modifications, even if their robot passed inspection elsewhere.

Like many people, I feel that the pneumatics rules could use a thorough re-write, but FIRST couldn't reasonably make that sort of change during the season, even if it wanted to. At the moment, the best solution is to express your views at the team forums, and to the appropriate FIRST personnel, all the while bearing in mind that those views won't necessarily sway an inspector into ruling in your favour at this event.

[Travis Covington]

Correct me if I am wrong, but not once did I mention that we weren't going to follow the rules. I agree that all teams should follow all of the rules. We have every intention of making our off board compressor legal. I was simply pointing out how frustrating these things can be. It is evident that the rule was a little unclear to almost everyone. As an inspector at SVR, I know of more than a few teams who had no idea of the complexity of the 'FIRST approved' off board compressor setup. Venting frustration is alot different than expecting to pass with an illegal setup.

[Ed Sparks]

Originally Posted by Travis Covington

Correct me if I am wrong, but not once did I mention that we weren't going to follow the rules. I agree that all teams should follow all of the rules..

Travis, I'm sure I could have worded my response better and I understand the frustration with this particular issue. Unfortunately when I read your last post, it reminded me of the dozen or so mentors that have stood toe to toe with me in the past quoting comments very simular to yours about rules they did not agree with. Believe me, inspectors can get a little frustrated too. I was trying to make a point but it looked like a personal attack and for that I appologize.

[SuperBK]

Hi. Looking at pneumatics for the first time. It seems that rule <R103> (2007 manual) is silly. It requires that the robot controller monitor the pressure switch and turn off the compressor when the limit is reached. Wouldn't it be much safer to put the pressure switch in series with the input signal to a spike and have the compressor safetly turn off when the pressure is reached without worring about the controller and software having to monitor it?According to <R103>, what the previous posted said would not be legal.

Brian

[Al Skierkiewicz]

Brian,
In order to keep in line with other parts of the manual (electrical) and in keeping with good electrical practice, the best solution is for the limit switch to be monitored by the RC and controlled through a spike. This switch does not handle the current reguired for compressor start and there can't be anything that controls a spike except the RC.

In addition, for all teams using the compressor for the first time, it is acceptable to replace the 20 amp fuse with a 20 amp circuit breaker on the spike that is used for the compressor. The start current for the compressor can exceed 20 amps for a very short time and the fuse will fail after a few starts.

<R65> ...
The fuse on the Spike relay for the air compressor may be replaced with a 20 Amp Snap-Action circuit breaker.

[DonRotolo]

Originally Posted by SuperBK

According to <R103>, what the previous posted said would not be legal.

...of course, that post was from a year ago, with different rules.

Don

[ChrisH]

Originally Posted by Don Rotolo

...of course, that post was from a year ago, with different rules.

Don

But unfortunately not substantially changed. The rules about off-board compressors are among the silliest in FIRST. From an engineering point of view I don't see any difference between air supplied by one compressor or another as long as they are at the same pressure. If there is a difference I wish somebody would provide us with a clear explanation as to what it is.

At the same time the rule is not that hard to comply with so as an Inspector I have to enforce it. I just don't have to like it.

[Al Skierkiewicz]

Originally Posted by ChrisH

If there is a difference I wish somebody would provide us with a clear explanation as to what it is.

Chris,
A team with the compressor on the robot must show that the compressor shuts off under RC control. By placing the compressor out of the robot, that rule must still be satisfied. Inspectors, as you know, have a hard enough time inspecting normal conditions, this rule should not cause a non-experienced inspector any trouble if there is a pressure check on the inspection list. Letting teams charge up the tanks with other compressor combinations may or may not give them added advantage but from a safety standpoint, I want to know that robot standing next to young field setup volunteer doesn't have more than 125 PSI onboard. If a team uses the pressure switch/RC/Spike combination then there are no worries.
As I interpret the rules, the compressor, pressure switch and spike may be mounted off the robot for weight savings. A four pole Anderson Power Pole connector and a hose will make this a very simple connection using the robot battery and satisfying the rules.

[Richard Wallace]

Originally Posted by Al Skierkiewicz

... As I interpret the rules, the compressor, pressure switch and spike may be mounted off the robot for weight savings. A four pole Anderson Power Pole connector and a hose will make this a very simple connection using the robot battery and satisfying the rules.

Al, I agree with your reasoning and believe that your interpretation (above) is consistent with the spirit and goals of the rules. However, the letter of the rules may be a different matter. <R101> is pretty explicit:

Originally Posted by <R101>

The only difference between an on- and off-board compressor is that the off-board compressor is physically removed from the ROBOT. Note: the intent of this rule is to permit teams to take advantage of the weight savings associated with keeping the compressor off-board. But using the compressor off-board of the ROBOT does NOT permit non-compliance with any other applicable rules.

Taken literally I think this says that everything except the compressor must be on the robot and configured to comply with all the rules, just as it would have been in the case of an on-board compressor. That would include the pressure switch and the Spike relay.

EDIT: It is worth posting a question on FIRST Q&A to get this clarified. To repeat: I think Al's interpretation makes sense.

[Tristan Lall]

Originally Posted by Al Skierkiewicz

A team with the compressor on the robot must show that the compressor shuts off under RC control. By placing the compressor out of the robot, that rule must still be satisfied. Inspectors, as you know, have a hard enough time inspecting normal conditions, this rule should not cause a non-experienced inspector any trouble if there is a pressure check on the inspection list. Letting teams charge up the tanks with other compressor combinations may or may not give them added advantage but from a safety standpoint, I want to know that robot standing next to young field setup volunteer doesn't have more than 125 PSI onboard. If a team uses the pressure switch/RC/Spike combination then there are no worries.

If we were dealing with a particularly volatile system, operating near its failure point, that would be an entirely reasonable precaution. However, the AVT-32-16 tanks, as supplied, have a rated pressure of 250 psi, and certainly a safety factor on top of that. The kit compressor can't physically sustain much more than 130 psi at 12 V without risking overheating (it may even have a thermal cutoff), and has an integral relief valve that restricts its upper limit anyway. There are pressure gauges to indicate the pressure in the system to anyone interested. Why must all of these safety features be additionally backed up by programmatic control which can be easily defeated in non-obvious ways—if anything, shouldn't the pressure switch be used to directly override the Spike's input signal from the RC*, rather than relying upon lines of code which can be removed by accident?

Basically, it seems like overkill, and it feels like wasted effort to protect a system which is already far more fail-safe than most of the robot.

*Don't do this. It's a violation of the rules. You must program the RC to accept a signal from the pressure switch over one of the digital inputs. Worse still, don't even think about wiring the pressure switch on the compressor's power lines.

[Peter Matteson]

I say this every year but I'll say it again. Put a pressure relief valve on the robot and I know that the team hasn't overpressureized with an on board compresser. You have one on the compressor and one on the accumulator to prevent someone from trying to charge into a closed valve and you're done. Do this and it becomes impossible to overpressurize any leg of the system. The additional parts of the pressure switch etc. could be on or off board and wired the same, but if teams actually had to have a pressure relief valve on board I would actually believe none of them tried to overpressurize.

Also most Thomas of this size have a thermal overload, so I would expect the kit one to as well.

[Richard Wallace]

Originally Posted by Tristan Lall

... The kit compressor can't physically sustain much more than 130 psi at 12 V without risking overheating (it may even have a thermal cutoff), ...

At the Thomas site (with some digging) you can find a .pdf datasheet for the 405ADC38/12 compressor. Its motor is rated for 12V, 10.5A continuous duty. It is thermally protected, which means that it will turn itself off before overheating. Rated current draw at stall is 25A, so presumably it will self-protect when stalled long enough to get hot. Rated pressure is 100 PSI continuously, and rated ambient temperature range is 50 deg F minimum to 104 deg F maximum.

All this supports what Tristan is arguing above -- the FRC rules for pneumatics provide multiple levels of safety. Possibly more levels than are really needed, but I'm comfortable leaving that to the GDC. The pneumatics rules as written are not hard to understand or police, except that I still think Al's suggestion above regarding configuration of components for use with an off-board compressor is very sensible. I would like to get a Q&A response on that.

[ChrisH]

Originally Posted by Al Skierkiewicz

Chris,
Letting teams charge up the tanks with other compressor combinations may or may not give them added advantage but from a safety standpoint, I want to know that robot standing next to young field setup volunteer doesn't have more than 125 PSI onboard. If a team uses the pressure switch/RC/Spike combination then there are no worries.

The same result may be achieved with a properly installed popoff valve which is already required, whether the compressor is mounted or not. I still don't see a valid engineering reason for requiring compressed air from one source over another.

I also do not see how it is enforcable as when the robot lines up in the que I have no idea just where the air on board came from. Yes, they had the required compressor set-up in the pit, but did they actually use it? or did they use something else? How would I tell the difference?

In the past this has been a minor issue. Only a couple of teams at an event would use an off-board compressor. But this year there are more applications where pnuematics are a good fit to the task AND we have twice the storage capacity. So teams like mine that in previous years figured that there wasn't enough storage capacity to ensure the shifters kept working the entire match will be taking another look at doing this.

If it was shown that the onboard system in the authorized configuration could not generate enough pressure to actually blow the popoff, that the popoff was actually there to protect against over pressures caused by mechanical action like forcing a cylinder in a direction it didn't want to go, then I would feel much more comfortable with this rule. Or FIRST could enact a rule prohibiting non-FIRST compressors or air storage devices in the competition area. Until then I will continue to dislike this particular rule, which is not to say I won't enforce it.

[sanddrag]

I still stand firm on my belief that compressed air is compressed air no matter how it is compressed, and that should the rules allow it, any team putting compressed air into their tanks would do so with a compressor that is manufactured by a professional company that does not put out more than 125 psi. I don't think anyone is going to bring their rusted out compressor, or engine driven compressor, or 5 HP compressor.

I think after the completion of a season, there should be a "petition for change of rules" forum/form submission/etc for the next year, done in a very formal and controlled manner by the mentor of each team who wishes to participate.

Anyhow, moving on.

On the topic of the pressure switch, as an inspector, I saw numerous teams without one at the regional. They didn't even know what it was or how to program it. No one had an extra. What were we supposed to do? Render their pneumatic system un-usable because it is missing the switch? Or (per the manual) not allow them to place their robot on the field because it was missing a (trivial) required part? They payed big bucks to field a robot (just like everyone else), so, that they did, and there were no problems. I guess what I'm saying is, we don't like the some of the rules, but lets get the word out anyhow so we can deal with them sooner rather than later.

Teams. Please install and program the pressure switch and wire and control the compressor by the rules regardless of whether it is on your robot or not.

And on my above note, I suppose we are expected to have a Spike relay on the robot, to control a compressor off the robot? I'd rather run out there with a pre-charged two gallon tank and go "pssh, pop" (about a 1.5 second process) than sit there and fiddle around with wires and whatnot waiting for it to finish going "brrmmmmmmmmmm" for a half mintue or so. Pneumatic systems leak. This is FRC, nothing is perfect. We'd like to be able to fill our tanks right on the field. But we don't want to hold up the event by waiting for the incredibly low CFM compressor to do it, especially now that we have 4 tanks.