Measuring motor current (anyone tried this?)

[billbo911]

Originally Posted by Don Rotolo

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

You know, your Amprobe might do the trick. I know you are not measuring AC Amps, and 15KHz might not register, but it is pulsed DC. If all you are looking for is the out lier, any consistent measurement should work.

The song from Sesame Street keeps popping into my head... "One of these things is not like the others, one of these things just doesn't belong....."

[efoote868]

You could try measuring the deflection of a compass... just keep it far away from the motor

[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.

[Dale(294engr]]

Don..
I've been using a super convenient m/cheap & available ethod to measure robot (motor) currents since 1999..
(Harbor Freight DMM works fine, $1.99 w/9v batt on sale!)

Super easy & remarkably consistent considering CB "switch" action.

use any DMM on +_199.9mV scale

place it across the 120A main CB! (install permanent jacks for more ease)

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

(~ .96 millohm)

therefore 1A drawn per 1 mV measured for a resolution of .1A

you will be surprised at subtle mechanical misalignment/friction this method reveals

the caveate here is current is definately relative for single closed CB test, &
MOST handy to gauge fine "unloaded" motor current draw directly proportional to mechanical load.. commutator &two bearings baseline for ea motor as removed from KOP, then with added gear (test shift if used), again w/chains, wheels/bearings.

keeping log between competitions warns long before driver can sense change.
off floor: log current: full speed fwd, reverse, each motor or side..

Quiescent current is ~1A (power up all motors neutral "off" no drift etc)

compressor can be similarly characterized.

I've seen abused breakers (thousands cycles or tripped often) exhibit up to 2 milliohm but will also trip below 120A (or sporadically) so should be replaced )

Do some tests to assure yourself.. multi cycle the breaker - measure delta under constant load.. One time Calib test: use Lab 1 milliohm shunt in series as Gold Standard.. or use DMM 10 or 20A scale with a known load (10A = 1.2ohm 1% resistor, do measure exact V as battery health &state &charge &age determine its internal Vdrop with load)

email me at DaleScience@aol.com I'll reply with 50A constant load FIRST 18AH Batt curves demonstrating delta R with load time and workshop dwg of DMM across breaker for handy high current robot measurement

Many of you have heard this before in my workshops or as I RI at events (mini wkshops!)

in ATL I'll be RI on Newton and scootin around field on 3 wheels

[Racer26]

Or you could do that. Much simpler.

[Hugh Meyer]

Don,

If you have the option to use a Jaguar and read the data back I would highly recommend you do so. The data is more complete because of the time element included.

We have been looking at current draw during match play. Using the data from the Jaguars gives information that sheds interesting light on how two CIM motors mounted on one transmission behaves.

The attached excel file shows one Jaguar dropping out while the other stays running. The drivers were complaining about erratic driving response from time to time. After seeing this it was easy to understand their complaints.

For the most part the current draw from both motors is fairly close. What we don’t fully understand is why was one motor, or really the Jaguar, tripping out from time to time.

We suspect that the issue may be related to the “pumping” we see from our drivers. This causes very fast complete reversals which puts a very high load on the Jaguar. After we explained this to the drivers this problem was reduced.

We will likely add some form of rate control on next year’s system to limit the fast changes.

The data attached represents a reduced sample of what we get from one match. It is about 20 seconds of teleop run time. It is only part of the data from 4 jaguars. The fault lasted about 2.4 seconds which correlates to the approximate 3 second auto reset time inherent in the Jaguar.

[Ether]

Originally Posted by Hugh Meyer

We have been looking at current draw during match play. Using the data from the Jaguars gives information that sheds interesting light on how two CIM motors mounted on one transmission behaves.

http://www.chiefdelphi.com/forums/sh...ad.php?t=85117

See the above thread for discussion of various ways to collect runtime data during competition or practice.

~

[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

[Ether]

Originally Posted by dtengineering

allowed us to do PID control on the current to ensure that we did not exceed a maximum current draw.

Hi Jason,

Are you saying that your primary mode of control was current? Or were you controlling voltage (or perhaps speed), with a max current limiter?


~