Measuring motor current (anyone tried this?)

[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.

[Racer26]

Originally Posted by 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.

Seems like this would be easy enough to do. Take two APP connectors, and just connect them such that you can access known test points (with long enough wire between to generate your voltage drop), and then plug the robot and battery into this current sense cable, then only run one motor at a time, check the current, if you get an outlier, you know where your problem is.

[Al Skierkiewicz]

Don,
Years ago we set up StangSense to measure current in motors. It used a one foot length of #10 wire in the negative lead of the speed controller input. Using a Maxim battery charge monitor IC we could scale the voltage dropped across the wire into a current through the wire. 100 amps in a one foot piece of #10 generates 0.1 volts. You will have to search here but I am pretty sure we uploaded the schematics. We used StangSense to alert our drivers when they were pushing too hard and to analyze other teams robots. Our software plotted the battery voltage, and motor current against time ticks provided in the IFI controller and then displayed them on a Palm. We then were able to download the Palm data and generate a spreadsheet and plot. In odd situations, we merely played back video of the match and timed out to our high current demands to plan on software changes to prevent the IFI controller at that time from reboot. The old IFI controller which did not have a backup battery, dropped out below 8 volts. We regularly saw small drops into the 4 volt range as motors turned on in drive or pushing. If you want more range, move to #12 and the voltage drops goes up to 0.160 volts per hundred amps. It is a little harder to scale but it is possible.

[DonRotolo]

Originally Posted by billbo911

Now, if the breaker had a few hundred ohms of resistance......

I'll take 0.1 Ohm!

Originally Posted by EricVanWyk

and it took me 3 years to do the first one.

Oh, so you're the guilty party? Nice job with it, BTW

Originally Posted by Dale(294engr]

120A CB has tightly controlled resistance of almost exactly 1 milliohm

Exactly the information I was looking for! Perfect, and thanks.

Originally Posted by Al Skierkiewicz

a one foot length of #10 wire in the negative lead of the speed controller input.

Thanks Al, I was headed in that direction, until Dale's comments. This is for a one-time measurement, not a monitor, so less is better.

We're on our way tomorrow, see y'all Wednesday.

[Al Skierkiewicz]

six inches of #6 = twelve inches of #10 or 12 inches of #6 at 100 amps gets you 0.2 volts.

[MrForbes]

I was thinking something like that would be true....thanks for the info Al. If you can measure to a couple decimal places with a dvm then measuring the voltage drop across the same length of wire on each motor circuit should do the job.

KISS?

[vamfun]

Here was our simple solution.... if you can't afford a fluke

http://cgi.ebay.com/ws/eBayISAPI.dll...m=300417617202

Heck of a deal 40/400 DC amp scales w/clamp on

[DonRotolo]

OK, so here was the solution we went with:

Measured the voltage drop from the battery + cable end to the + terminal on the power distribution board. I am estimating we had about 5 mV per amp, but all we needed were relative readings, so that was more than sensitive enough (our DVM could display 0.1 mV).

The problem we needed to solve - one breaker was popping - turned out to be a sensitive breaker. Many thanks for team 1732's advice and assistance on Wednesday evening as we worked the checklist.

For what its worth: The right side measured "43" units, the left side measured "38" units, both at full speed off the ground. We attributed the difference to a tight chain; we didn't change it, added some lube, and went to the field like that. We did see some breakers popping on occasion, but it wasn't a problem.

[dtengineering]

Last year we built a couple of Allegro Current sensors in to our robot. If you'll recall we actually got some current sensors in the KoP, along with some nifty little circuit boards to mount them on... oh... about five years ago, I think.

http://www.allegromicro.com/en/Produ...sors/index.asp

This gave us real time feedback to an analog input on the control system, and allowed us to do PID control on the current to ensure that we did not exceed a maximum current draw.

I guess you can do this directly with the new Jags, but getting a few current sensors from Digikey is a cheaper way to go about it.

Jason