MicroControllers at competitions

[yash101]

I know it is a tad early for me to post about this, but I think that when I go to competition, I should take a Propeller Chip, MCP3204, a 120Amp shunt resistor, and some other various things. The reason why is because last year, we experienced the problem that our shooter motors kept dying and we had no way to measure the motor's current draw (because we didn't want to damage our fluke by a current spike). What do you guys think of this idea to aid in competition debugging?

[Al Skierkiewicz]

Dev,
You are asking for a 0.1 ohm/1500 watt resistor. Do you have those laying around?
http://store.mwands.com/diversion-du...12-24-48-volt/

[RyanCahoon]

Originally Posted by Al Skierkiewicz

Dev,
You are asking for a 0.1 ohm/1500 watt resistor. Do you have those laying around?
http://store.mwands.com/diversion-du...12-24-48-volt/

Al,
What's the difference between the device you linked and this one: http://www.amazon.com/Amico-current-.../dp/B008SOXFZC ?

[Ether]

Originally Posted by RyanCahoon

What's the difference between the device you linked and this one: http://www.amazon.com/Amico-current-.../dp/B008SOXFZC ?

Yours is 200 amps at 0.075 volts (shunt)

Al's is 125 amps at 12 volts (load resistor)

[MikeE]

Originally Posted by RyanCahoon

Al,
What's the difference between the device you linked and this one: http://www.amazon.com/Amico-current-.../dp/B008SOXFZC ?

Off topic but I can't help but chuckle at Amazon's shipping info tag "Ordering for Christmas?"

I can just imagine the little 'uns eagerly tearing open the wrapping paper under the tree, hoping after hope that they were on Santa's nice list...

[Ether]

Originally Posted by Al Skierkiewicz

You are asking for a 0.1 ohm/1500 watt resistor.

His post was admittedly somewhat ambiguous, but that's not the way I interpreted it.

I think he wants to insert the shunt in series in one of the motor leads and measure the voltage drop across it to get current.

Aside from questions of FRC legality, the shunt Ryan linked would would work: it would dissipate roughly 7 watts at 133amps:

0.075V/200A = 375e-6 Ohms. 133amps * 375e-6 ohms = ~7watts.

As you have previously pointed out on many occasions Al, instead of a shunt the voltage drop across a length of existing wiring would suffice to get a rough measure of current (for large currents).

[RyanCahoon]

Out of curiosity, what motors were you using?

Are you thinking for in-match debugging, or in the pits? Be aware that in-line current monitoring has been illegalrestricted by past years' rules.

Seems like this would be a good thing to have tested enough in your shop to know that the motors wouldn't die. That way you have time if you need to adjust gear ratios, etc.

Speaking of Fluke, what about an AC/DC clamp-on current meter?

[Ether]

Originally Posted by RyanCahoon

Be aware that in-line current monitoring has been illegal by past years' rules

Depends on how you do it I would think. What rule did you have in mind?

[RyanCahoon]

Originally Posted by Ether

Depends on how you do it I would think. What rule did you have in mind?

Thanks for catching my error (again ). I've edited my original post. Contrary to the wording of my original post, I was only thinking of the general "custom circuits can't affect power pathways" rule and forgot the exception for low-impedance current monitoring. Presumably (0.075 volts) / (200 amperes) = 0.000375 ohms is low impedance...

Originally Posted by Ether

Yours is 200 amps at 0.075 volts (shunt)
Al's is 125 amps at 12 volts (load resistor)

Oh that makes sense. I was reading the 12V as a safety rating or something along those lines.

Originally Posted by magnets

Your multimeter (should) have a fuse to protect itself from current, so you should be all set.

Well it would be good to check the rating on the multimeter first. Not worth blowing unnecessary fuses. Also make sure that the probe leads are large enough to handle the current as well.

[magnets]

Originally Posted by yash101

I know it is a tad early for me to post about this, but I think that when I go to competition, I should take a Propeller Chip, MCP3204, a 120Amp shunt resistor, and some other various things. The reason why is because last year, we experienced the problem that our shooter motors kept dying and we had no way to measure the motor's current draw (because we didn't want to damage our fluke by a current spike). What do you guys think of this idea to aid in competition debugging?

Your multimeter (should) have a fuse to protect itself from current, so you should be all set.

What does the current draw tell you? Our team has attended 42 robotics competitions, countless offseason, and have played close to 1000 matches. We have never needed to measure the current draw of a motor at competition, nor have we heard of anybody needing to.

[Andrew Schreiber]

Originally Posted by magnets

Your multimeter (should) have a fuse to protect itself from current, so you should be all set.

What does the current draw tell you? Our team has attended 42 robotics competitions, countless offseason, and have played close to 1000 matches. We have never needed to measure the current draw of a motor at competition, nor have we heard of anybody needing to.

I've used the current monitoring in Jags to diagnose binding in the drivetrain (old robot had picked up some gunk and locked up one side of the drive) as well as to identify that we were initially over geared this year. It's useful data to have for debugging.

[magnets]

Originally Posted by Andrew Schreiber

I've used the current monitoring in Jags to diagnose binding in the drivetrain (old robot had picked up some gunk and locked up one side of the drive) as well as to identify that we were initially over geared this year. It's useful data to have for debugging.

Was there anything you couldn't measure with a multimeter? A clamp on ammeter costs $60. The shunt resistor costs over $100.

[Andrew Schreiber]

Originally Posted by magnets

Was there anything you couldn't measure with a multimeter? A clamp on ammeter costs $60. The shunt resistor costs over $100.

Dunno, didn't need to. We had Jags on a CAN bus so it was baked in for us. But my gut feeling is no, a clamp on ammeter woulda been fine.

[FrankJ]

Beware that most current clamps measure AC only. The DC capable ones tend to be expensive.

[magnets]

Originally Posted by FrankJ

Beware that most current clamps measure AC only. The DC capable ones tend to be expensive.

Here's a 400 amp dc one for $50.

http://www.sears.com/craftsman-digit...p-03482369000P