PRESSURE SWITCH

Quoted from the FIRST Pneumatics Manual:

We have included a pressure switch manufactured by The Nason Company. These switches are normally closed. The switches will open at approximately 115psi and will not close again until the pressure drops to approximately 95psi. This will allow you to turn off the compressor once you are up to 115psi, saving power in the battery. It should be wired directly to the robot controller digital input bank with PWM type cable. No specific Digital Input Port is designated for the pressure switch. The Robot Controller must be programmed to react to the Input Port that is connected to the pressure switch The Robot Controller will activate the designated Spike Relay to turn the Compressor “on” and “off”. There is no default program in the Robot Controller to control the Compressor power. Do not put the pressure switch in series with the power supply to the compressor.

Maybe I am missing something, but how is this of any use to us. The highest we can run our pneumatic system is at 60psi, so how is a sensor that measures 95/115 psi be of any use?

Am I just way off track?

The highest you can run the pneumatic system is 60psi (to solenoids, pistons, etc) but you can store up to 115 psi in the two pneumatic tanks that are connected before the main 60 psi regulator. I guess this is so you can charge up the tanks to 115 psi and run the pistons numerous times before charging the tank up again (since charging the air tanks takes longer than using the air in them).

Thanks for the reply, is there a rule number for this? I couldn’t find anything regarding 115psi in pneumatic part of section 5 in the manual.

I thought it was 120 psi in the system not 115?

It is 120…

Rule R02, finally found it. Thanks

120 psi and 115psi are both correct…in a way:

The maximum allowed by the rules is 120psi. Your robot will be limited to 115 psi because the Nason Pressure Switch shuts off at 115psi.

“The switches will open at approximately 115psi and will not close again until the pressure drops to approximately 95psi.”

from page 3 of : http://www2.usfirst.org/2005comp/Manuals/2005PneumaticsManual.pdf

When your robot is inspected, you will need to demonstrate that the compressor turns of via software at a high side pressure of no more than 120 PSI. For that reason it is recommended you follow the system diagram at the bottom of page 20 of the robot manual and include a presuure guage at both the high side (120 PSI) and working pressure (60 PSI) side of the regulator.

“<R88> The Nason Co. pressure switch must be connected to the output end of one of the Clippard tanks to sense the tank’s pressure. The two wires from the pressure switch must be connected directly to a digital input and ground terminals on the Robot Controller, and the controller must be programmed to sense the state of the switch to operate the relay that powers the compressor.”

FIRST has indicated that it is legal to run the pump all the time and ignore the pressure switch:

Good info on this thread!

To follow-up:

I’m sure teams are going to want to push the limit and get exactly 120 psi into their Clippards. 115 wont cut it - whether it be for practical reasons, or just out of principle =).

I see two options here:

  1. use only the Nason pressure switch, and code a timer that causes the compressor to run just long enough after the 115 psi cut-off to get roughly 120 psi.

  2. connect both the Nason pressure switch AND the analog pressure sensor. Ignore the output from the pressure SWITCH, and use only the reading from the pressure sensor to dictate compressor shutoff at exactly 120 psi.

Seems a littleweird to me, but are there other options here to get 120psi legally?

-SlimBoJones…

Unfortunately this year there is also a rule (or in Q&A) that states that you cannot use the pressure sensor to shut off the compressor .

Can we use the Pressure Sensor to turn ON the compressor?
We don’t like the idea that we need to run the compressor at 95psi when only 60 is passed beyond the next gauge.
If we code the "Start compressor with the Analog, we can run down the pressure to 60-65 and use the battery less… I think :slight_smile:

Running the high side at about 115 psi gives your system more air volume to work with. If you run the high side at 65 psi your compressor may not be able to keep up with the cycles or speed that are need during competition.

Looking toward using the Nason as the high side (115psi) and the Analog as the low side so that we can detect when the pressure drops to say 60-65 psi.

Remember, you must connect the pressure relief valve when using the pump. This valve relieves pressure at around 120psi, no matter what! This is why it is legal to run your pump continuously.

Why pump up to 120 psi when you only use 60psi? That is a matter of “pressure volume”. Let’s say I have a cylinder that uses the exact volume of air that is in the tanks. If the pressure of the tank is only 60psi, then when I fire the cylinder once I expel all of the air in the tanks. If the pressure in the tanks is 120psi, then I have double the available air to use at 60psi. Air is compressible, which means as you increase the pressure the volume the same mass of air takes up gets smaller. Since the air tanks are a constant volume, when you increase the pressure then the amount of usable air increases. For this reason, you should try to maximize the pressure you have in the tanks.

That is also a reason why not to have to large an accumulator (not legal anyways)
but last year after our season, i wasnt at the meeting the night before an exibitoin we were going to, so the mechanical fools deicided that since our robot was pneumatics heavy (used a lot) that a larger accumulator would be a great idea. So they installed a 10lb 4ga Air tank on our bot. They experimented with it, and since the compressor had to work a lot more, is started to overheat, instead of realizing that they messed something up, they instead put 2 120mm fans near the compressor to cool it down.

When i got back, i was astonished at what the gigantic growth was, and asked about it, and got the answer.

Unfotunatley they didnt understand that 1unit of air at 60lbs provides the smae force as 4ga or air at 60lbs and that the compressor had to work x times harder to fill the large tank, draining our batter (exibition, not 2min mathces)

sigh

But its all better now, i tok the monstrosity out.

[quote=Mr. Steve]Quoted from the FIRST Pneumatics Manual:

Hello everyone,

I’m doing some research on the internet and happening to into this msg forum. I see you guys are discussing something about pressure switch & control system. Maybe you can help me out. I’m looking at someone’s patent and one of the independent claim that person makes is that he uses a pressure switch to turn on a PWM circuit/controller when the liquid pressure is low enough to trigger the switch. The exact claim is " Pressure switch can sense the output pressure, said pressure switch activating said pulse width modulating circuit when output pressure is less than a threshold value". I believe the claim is too broad. I meant people have been using pressure switch and pwm for decades. I’m looking at some proof to invalidate that claim. If you guys happen to know any product or publication out there prior to 2001, please let me know.

Thanks,
Kevin[/quote]

Kevin,

First of all, this forum is for high school students and their mentors who build robots as part of an educational endeavour. As such, your query is not quite appropriate.

Second, I suggest that you contact a competent patent attorney regarding patents. Anything can be patented if it is used in a unique or novel manner or for a unique or novel purpose. I have seen patents awarded for off the shelf bevel washers simply because of where they were used.

Patents are an issue of law which, alas, has little to do with engineering or common sense.

Regards,

Mike

If you fire a large cylinder a few times rapidl, you will notice how quickly the system pressure (at the tanks) drops below 60 PSI, even when you started at 95 - 115 psi. It takes a long time for the compressor to recover and your cylinders may move slower while the compressor is trying to catch up.

If you limited your reservior pressure to 60 psi, the system might never recover during a match if you were trying to operate pneumatics.

[quote=Paul Copioli]If the pressure of the tank is only 60psi, then when I fire the cylinder once I expel all of the air in the tanks. If the pressure in the tanks is 120psi, then I have double the available air to use at 60psi./QUOTE]

If we assume the volume used by the cylinder equals that of the tank, then the amount of air you can get from the tank is one half of what is there - the system will equalize at 30 PSI. That is why you use 120 PSI, so your cylinder can be sure to see the full 60 PSI, just as you stated.

You see, while the compressor can keep up pressure, it is limited in volume (Cubic Feet per Minute). The tanks act like ‘pressure inductors’, allowing brief periods of high flow volume despite a lower volume compressor.

Besides, isn’t more better? :wink:

Don[/quote]