Measuring motor current (anyone tried this?)

[DonRotolo]

I have a need to measure the current draw under load of each of our four CIM motors, to diagnose which of the four is drawing more than it should. (This isn't meant as a long-term measurement, more of a quick diagnostic)

Looking at the 40A snap-action fuse, I see the copper connection feet pass all the way through the body and have a small part that comes up through the top of the breaker.

My thought is to measure the voltage drop across the breaker, and use that to calculate the current draw. My multimeter cannot measure above 10 Amps directly, under load the motor draws more than that.

Has anyone ever done this? Is the voltage drop reasonably measurable? Can I measure to within 1/2 Amp this way?

[MrForbes]

The resistance of each breaker will probably be different. How about replacing one wire from per motor from each speed controlller to fuse panel with a longer one, same size/length for all four, and measure the voltage drop on the wires?

[DonRotolo]

Originally Posted by squirrel

The resistance of each breaker will probably be different. How about replacing one wire from per motor from each speed controlller to fuse panel with a longer one, same size/length for all four, and measure the voltage drop on the wires?

I was thinking of that, a 3 or 4 foot length of #18 with 1/4" male quick connects on the ends to fit into the PDB - I'd surely get the voltage drop I need there.

And, the wire can act as a fuse

I'm not sure I'll be able to see a speed difference between motors; they are installed two-to-a-gearbox, which slightly complicates it.

Oh yes, and I have only a few minutes to get it done.

[MrForbes]

YOu don't need to make the wire that small, you can use normal size wire, the important thing is to control the gage/length of all four, so you get useful results. The longer the wire, the more voltage drop. Try a relatively short length and see if you can measure it accurately

[45Auto]

I would hope that there is an EXTREMELY small voltage drop across the breaker. Probably not enough to give you any degree of confidence unless the motor is dragging very badly.

Have you thought about using one of your Victors or Jaguars at a fixed setting (maybe 25%), then swap the connection between motors and see if one is noticeably slower?

[dcherba]

Using a fluke with a clampon dc ampmeter and the peak record function would be the easy way. I have a 50 amp shunt that gives 50mv full scale that I use to make these type of precision measurements. The clampon dc ampmeter for the fluke is fairly expensive. I have also put a .01 ohm 20 watt resistor in series and done the same thing.

[EricVanWyk]

If you have CAN set up, you can ask for the currents in each motor over the bus.

You'll probably need a 4 wire multimeter to get any appreciable voltage. Its on the order of a dozen milliohms.

I'd pull the breakers individually and measure resistances. They'll change a bit under load, but such is life.

[DonRotolo]

Originally Posted by dcherba

Using a fluke with a clampon dc ampmeter.

Um, well, yeah, but I don't have one

I do have a genuine Amprobe, but it measures only AC Amps.

Milli-ohms is not enough for me Eric, and we're running PWM, so a few pieces of wire it is. Maybe just one, allowing the others to run through their breakers. After all, I only need a comparative reading - if they are all the same, they're all good (or all bad!), but if one is appreciably different, the mechanical team has some work to do...

[billbo911]

This is just a thought, without all the details to KNOW if this will work, I can only throw it out as a suggestion.

The power distribution board has led indicators on it to show when a CB is open or missing. The only way these can work is if there is some sort of sensing circuitry across the contacts of the breaker connections. If you are willing to open the PDB, which I am not suggesting you do, you might be able to gain access to these components. Measuring the drop across the sense circuit "could" give you a reading relative to the current flow. Again, without a schematic of the board, I can not say if this will work or not. It is just food for thought.

[Joe Ross]

Originally Posted by billbo911

This is just a thought, without all the details to KNOW if this will work, I can only throw it out as a suggestion.

The power distribution board has led indicators on it to show when a CB is open or missing. The only way these can work is if there is some sort of sensing circuitry across the contacts of the breaker connections. If you are willing to open the PDB, which I am not suggesting you do, you might be able to gain access to these components. Measuring the drop across the sense circuit "could" give you a reading relative to the current flow. Again, without a schematic of the board, I can not say if this will work or not. It is just food for thought.

The schematic is available here: http://www.usfirst.org/roboticsprogr....aspx?id=16337

[reversed_rocker]

i'm not sure if i'm remembering this correctly, but we were playing around with connecting a wire to the negative portion of the main breaker lead and measuring the voltage drop when current draw increases using an analog in on the CRIO. The voltage drop has a linear relationship with the current draw, so if you apply a couple known loads and measure the voltage drop you can get a constant to multiply your voltage drop by to give you your current draw. If you have your controller on then run all 4 CIMS at full power you should be able to calculate how much current they draw. If you work out the kinks this can be a great way to monitor current during robot operation and put some safeguards in the code to stop you from drawing too much.

Our mentor showed up with a hall effect sensor rated up to 180 amps that he said he got for 8 bucks. We were going to put this on our robot to see if we were drawing to much current during the comp, but when the battery voltage drops below 9v it sends back angry readings.

[EricVanWyk]

Originally Posted by billbo911

This is just a thought, without all the details to KNOW if this will work, I can only throw it out as a suggestion.

The power distribution board has led indicators on it to show when a CB is open or missing. The only way these can work is if there is some sort of sensing circuitry across the contacts of the breaker connections. If you are willing to open the PDB, which I am not suggesting you do, you might be able to gain access to these components. Measuring the drop across the sense circuit "could" give you a reading relative to the current flow. Again, without a schematic of the board, I can not say if this will work or not. It is just food for thought.

Cool idea, but not feasible with the current PD hardware (ask again in ~3 years ). The sensing circuit you refer to is super cheap and doesn't actually measure the current. You can check it out, per Joe's suggestion, if you'd like. Its just a FET being used as a comparator, plus some protection.

[billbo911]

Originally Posted by EricVanWyk

Cool idea, but not feasible with the current PD hardware (ask again in ~3 years ). The sensing circuit you refer to is super cheap and doesn't actually measure the current. You can check it out, per Joe's suggestion, if you'd like. Its just a FET being used as a comparator, plus some protection.

Yep, that is what I was afraid of. Now that I've seen the schematics, I agree 100%, that circuit will not allow a measurement with any meaning.
Now, if the breaker had a few hundred ohms of resistance...... it would not be a breaker , but you could then measure a nice proportional voltage. But then again, you knew that.
Well, like I said, it was just a thought.


BTW Eric, are you saying there is a new PDB in the works???

[EricVanWyk]

Originally Posted by billbo911

BTW Eric, are you saying there is a new PDB in the works???

No, I'm just saying that I'd love the opportunity to make it prettier and it took me 3 years to do the first one.

[jonboy]

What about fabricating a cable out of the Anderson power connectors pig tails the goes between the battery and the robot power connector? The special cable can have either a 50mv shunt or an appropriate length and sized wire to provide a voltage drop when the CIMs are running. Now remove all ckt bkrs but the one CIM under test and measure the voltage drop/current. This cable can be fabricated without interfering with the bot and quickly installed and removed.