Hard wiring?

[Dakota]

I am trying to minimize processing required by the cRIO by not hooking the compressor up to it. I simply hard-wired the pressure switch directly to the power wires of the compressor. While this works absolutely flawlessly, is it legal? I don't want problems with inspectors over something that simple.
Thanks

[eugenebrooks]

This is not legal and will be rejected by the inspectors.

Your switch must be detected by the cRIO which then
controls the compressor through a spike. Check the
rules carefully, as they are very specific about this.

Eugene

[EricVanWyk]

No, it is not legal.

The saved processing time is negligible and unnecessary. Honestly, I do not believe you would be able to measure the difference.

[Dakota]

D'oh! Well, it was good for practice anyways. Thanks.

[laxcompute]

Does anyone know why that is?

It almost seems like having the pressure switch hooked directly to the compressor and the power source would be much more elegant and less likely to go wrong than the more complicated system using a relay and the cRIO.

[Dakota]

Originally Posted by laxcompute

Does anyone know why that is?

It almost seems like having the pressure switch hooked directly to the compressor and the power source would be much more elegant and less likely to go wrong than the more complicated system using a relay and the cRIO.

Exactly my thoughts. That's how a real air compressor works too.

[kevin.li.rit]

The Pressure switch was not designed to handle the current of a running compressor. It could fail as you use it for extended periods of time.

[EricVanWyk]

Originally Posted by laxcompute

Does anyone know why that is?

It almost seems like having the pressure switch hooked directly to the compressor and the power source would be much more elegant and less likely to go wrong than the more complicated system using a relay and the cRIO.

I have two reasons, I'm not sure if either are correct:

1) Current. The compressor draws over 20 amps, which is a good chunk of current. I don't think the pressure switch is rated for that.

2) Control-ability. There are a few rules to the effect that all control signals must come from the cRIO. This wiring would run counter to that.

[Tom Line]

I suspect it comes from #2 eric. The field software makes us safe by killing all the outputs on the robot. If it were not able to do that, the compressor could choose to fill at any time it likes, with unexpected results.

For instance, say you have a pneumatic firing mechanism, and a spike shorts or malfunctions so that robot keeps shooting. Without being able to kill the compressor with the E-stop, the firing mechanism will just keep firing while the compressor works happily to refill it.

[MrForbes]

I think both of Eric's answers are right. Look at the pressure switch on a "big" air compressor (such as in a shop), it's a lot larger than the little Nason one we get, mainly to have room for contacts and springs and a pressure diaphram big enough to handle the current load. Since ours runs on 12v instead of 120/240, the current load is similar. And the safety thing....the field personnel need to be able to turn off all robot operations, they can't turn off the compressor if it's not under computer control.

[Al Skierkiewicz]

The Nasson pressure switch is not able to handle the current of a compressor. As Erik pointed out, the start current is 25 amps and run current is between 10 and 12 amps. More importantly, however, is this...

<R78> The Nason pressure switch must be connected to the high-pressure side of the pneumatic circuit (i.e. prior to the pressure regulator) to sense the “stored” pressure of the circuit. The two wires from the pressure switch must be connected directly to a digital input and ground port on the Digital Sidecar, and the cRIO-FRC must be programmed to sense the state of the switch and operate the relay module that powers the compressor to prevent over-pressuring the system.

[ebarker]

Originally Posted by Dakota

I am trying to minimize processing required by the cRIO by not hooking the compressor up to it.

The cRio is a 400MHz power pc.

There is no real load on the cRio. The real load is on the student.

[Mark McLeod]

Here are the specs on the pressure switch from the Nason catalog.
The switch is rated at 5A.