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?

[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?

[DonRotolo]

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 :ahh:

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

[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]

Using a fluke with a clampon dc ampmeter.
Um, well, yeah, but I don't have one :rolleyes:

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]

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/roboticsprograms/frc/content.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]

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]

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 :yikes:, 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]

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.

[billbo911]

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.....":rolleyes:

[efoote868]

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

[Racer26]

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]

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/showthread.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]

Now, if the breaker had a few hundred ohms of resistance......I'll take 0.1 Ohm!
and it took me 3 years to do the first one.Oh, so you're the guilty party? Nice job with it, BTW
;953253']120A CB has tightly controlled resistance of almost exactly 1 milliohm
Exactly the information I was looking for! Perfect, and thanks.
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?ViewItem&item=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/Products/Design/current_sensors/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]

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?


~

[chuckmerja]

Any progress on this? We built a rough terrain bot with CIMS and we know we are at or beyond the design limits for our CIMS and Victors, but that is what we had, and so that is where we started. We have 4 motors being driven by 4 Victors and a 55Ahr deep cycle battery. We ran a few hundred yards and measured the temps on the motors and discovered 3 of them at roughly 105 deg F (on 82 ish degree day) but one motor was 160 to 180 deg F! That's definitely an outlier, and we need to look into this further. I really think we need to monitor and record amps on each of the 4 motors - simultaneously. I think that hall effect current sensors is where we are headed http://www.allegromicro.com/en/Products/Design/current_sensors/index.asp any help would be great.

Chuck

[kaliken]

In 2010/2011 we used the CAN interface with jaguars to get a simple current draw measurement. We actually tied the current measurement to a simple cut off for our ball handler.

Perhaps it may not be exactly what you want but it should be a zero cost, quick test approach

[ratdude747]

I had an Idea for future use.

I wonder if you could use an infrared thermometer to measure temperature? the extra amps being drawn equates to extra energy being used, and the energy has to be going somewhere (conservation of energy), which if the kinetic output is the same, the only other place would be heat. if it is bad enough, you might get burned by the motor heat (I did by accident a long time ago).

It doesn't give you amps but it would tell you which motor is drawing more than it should.

[Ether]

I had an Idea for future use.

I wonder if you could use an infrared thermometer to measure temperature?


Just tape or glue a thermistor to the motor case and run the signal to an analog input.

[philso]

At work,we use larger versions of the LEM Hall-effect current sensors shown in the link below in the 3-phase motor controllers we manufacture. They will work down to DC and will have sufficient bandwidth to detect the PWM current into your motor. The one shown needs a +-15V supply. You may have to add a simple R-C filter on its output before you connect to your DVM. Many DVMs, especially the less expensive ones, will give wildly inaccurate readings if connected to a PWM signal.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=398-1049-ND


I have seen clamp-on type DVM's by Kein Tools at Home Depot that claim to work down to DC but I have never tried them.

Phil

[roystur44]

Take a look at this product.

http://www.batteryspace.com/compactdigitalbatteryanalyzerforany4-60vrechargeablebatterypackupto50amp.aspx

We used this to tune our minibot motors and it worked out great. Reading the specs I think the analyzer is within your test range. We used this with a variable DC power supply.

Here are some uses:
Set speed control cutoff voltages and currents
Tune performance by comparing different setups of propeller, motor, gearing, speed control and battery
Measure a full battery charge and compare to spec when charger claims done
Check peak currents are safe for battery, speed control, motor, wiring and connectors
Predict airplane flight time based on ACTUAL conditions in your model
Check battery capacity and health and whether battery should be retired
Verify that the minimum voltage under load is within specifications
Check for wiring and connector power losses.



Roy

[Dale(294engr]]

Don hope I'm not too late with this for you to use else good for future ref.!

Contrary to what many here may have reasoned
120A CB is a great poorman's 1 milliohm shunt!! (actually ~.96milliohm)

consider if Rcb was not tightly controlled trip, current would vary likewise!

(caveate: defective breakers internal R varies significantly =poor candidate)

I've been using 120A CB shunt yrs, include it in my FIRST wkshops & demo's
(previously 60A =bit over a milliohm so calibration factor req'd incr absolute accuracy - but relative current stability is quite good enough, in practice)

Using any DMM on 200mV scale place it across the 120A CB terminals

(best = install jacks to accept DMM pointed probes for competition ease use)

(Harbor Freight DMM works fine w/ 1Meg Rint on 200mV scale)

(when CB is open DMM reads overrange but csuses no harm [i sprotected])

when closed read the DMM +_199.9 A / mV full scale
polarity useful for drain/ batt charge current monitor if batt charged in robot

resolution is .1A!!

convenient to keep track of Robot quiescent current draw ~2A

useful to run each of 4 CIMs noload off floor as gauge of overall drive friction
.. motor, motor bearings& alignment, brush/commutator, gearing, chains, wheel bearing alignment, keeping in mind at zero friction current draw is Zero!

.. so keep log ea motor out of kit I-noload 12.6v reg pwr supply best cntrl
burn in to stable lowest full speed current draw. Future comparison will indicate: worn brush/commutator, bearings and shorted/open rotor windings

In competition = good quick test of left Vs right tank drive (or ea of 4 if 4WD)
do one at a time, subtracting quiescent draw, log result

It is remarkable just how sensitive this test is!!

NL current increase is measurable predicts a problems developing well before driver becomes aware of it!

and all Robot loads.. simply run each alone and subtract quiescent constant

using on-board A/D, a dynamic history may be recorded!!

with a little ingenuity ext. variable mechanical load constitutes handy dyno!

or with locked shaft and careful pre-set control of Victor / Jaguar
can test at any desired constant current up to 40A CB cycling which occurs at ~50A for each motor (for 3 motors tank cfg ~150A max ea side (~300A total) may be attained for a few seconds! - wise to monitor motor temp in this mode!)

.. great for finding resistive connections in wire, connectors, CB's, aging SLA's

also as dynamic 18AH SLA constant load battery characterization test:
use 50A, expect 9 minutes recording Vbatt every 10 sec
use Excel to produce Vbatt-at-terminals Vs Time for true batt discharge characteristic enlightenment!

[DonRotolo]

Any progress on this?See my Post #28 for what we actually did.
;1069257']Don hope I'm not too late with this for you to use else good for future ref.!
Well, yes it is far too late, but that IS excellent info for future reference, I think we'll incorporate dedicated test sockets across the 120A CB in future designs. Thanks!

[Al Skierkiewicz]

As a caveat, the resistance across the 120 amp breaker varies with temperature and manufacture. A one foot length of #10 wire is very close to 1 milliohm. Placed in the power return at the input of a speed controller allows one to measure input current, of constant polarity, at 1mV/A. Sample at 500 Hz or higher and throw away measurements that are less than 20mV and you will get a pretty good idea what the current is.

[Mike AA]

As a caveat, the resistance across the 120 amp breaker varies with temperature and manufacture. A one foot length of #10 wire is very close to 1 milliohm. Placed in the power return at the input of a speed controller allows one to measure input current, of constant polarity, at 1mV/A. Sample at 500 Hz or higher and throw away measurements that are less than 20mV and you will get a pretty good idea what the current is.

Al,

I have seen you say this a few times and I am now wondering, do I just leave the 1 foot of wire straight, should I wrap it around something like a screwdriver handle to take up space? I am thinking about setting up something for 2012 where we can monitor our motors a little better as I have gotten better with programming.

-Mike

[Al Skierkiewicz]

Mike,
A sense wire from each end of the #10 wire is all you need. If you search StangSense here on CD you will find a solution using a Maxim battery charging chip that we used many years ago. We used the same one foot piece for each channel.
Al

[WizenedEE]

Why not just get a 40-60 amp ammeter?

[Al Skierkiewicz]

Why not just get a 40-60 amp ammeter?

CIM stall is 129 amps and it is hard to chase the robot with an ammeter in your hand. I did it once (long ago) so I am speaking from experience.

[DonRotolo]

Why not just get a 40-60 amp ammeter?
...at a cost of...

"Inexpensive" and "40-60 A ammeter" are polar opposites. Sure, I'd do that at work, or mount a 200A shunt... but in the original post, I was off to St Louis with carry-on only.