Measuring motor current (anyone tried this?)

[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.

[roystur44]

Take a look at this product.

http://www.batteryspace.com/compactd...upto50amp.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

[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

[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.

~

[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]

Originally Posted by chuckmerja

Any progress on this?

See my Post #28 for what we actually did.

Originally Posted by Dale(294engr]

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]

Originally Posted by 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.

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