Measuring Amperage

[fovea1959]

we may want to measure amperage for our shooter. I'm looking at using a Polulu Hall Effect current sensor (http://www.pololu.com/catalog/product/2199), and am reading the rules:

R49

Branch circuits may include intermediate elements such as COTS connectors, splices, COTS flexible/rolling/sliding contacts, and COTS slip rings, as long as the entire electrical pathway is via appropriately gauged/rated elements.


R54

Custom circuits shall not directly alter the power pathways between the ROBOT battery, PD Board, motor controllers, relays, motors, or other elements of the ROBOT control system (including the power pathways to other sensors or circuits). Custom high impedance voltage monitoring or low impedance current monitoring circuitry connected to the ROBOT’S electrical system is acceptable, if the effect on the ROBOT outputs is inconsequential.


Would putting one of these in the controller to motor pathway (not a CIM motor, a 30A branch circuit) be "inconsequential"? Everyone on our team that's looked at this so far thinks we're legal, but getting another set of eyes on it seems prudent.

[Joe Ross]

In my mind, 1.1 mOhm is low impedance.

[Jon Stratis]

That would seem to be exactly what R54 had in mind when it says "Custom high impedance voltage monitoring or low impedance current monitoring circuitry connected to the ROBOT’S electrical system is acceptable".

The key here is that your additional circuitry isn't doing and direct control of the motor - without it present, the motor would act exactly the same.

[Hugh Meyer]

Be sure to purchase some extras and expect to let out some smoke. We used a similar device a few years ago. It worked great as long as the motor didn't stall.

You might want to search on here for "stang sense". You can use a 12" piece of # 10 Ga wire and make a 0.001 ohm resistor. With a voltmeter that will read milli volts it makes a great ammeter. A simple op amp circuit gets you a signal to read with the cRIO.

-Hugh

[Ether]

What motor controller and motor combination, and at what motor speed and current are you trying to measure?

Answers to those questions could affect the method required to get a useful current reading.

FWIW, using Jag with CAN you just ask the Jag to tell you what current the motor is getting.

[Saberbot]

We've been using a CAN Jaguar with BDC-Com for all of our shooter testing this year. It's great because it tells us current draw and an exact voltage.

[dellagd]

Yep. Drive your Jaguars off of CAN and you can get the current amperage draw of the motors. Helped us alot last year.

[Al Skierkiewicz]

Doug,
The answer depends on how you implement the introduction of this device on your robot. You are more likely to get accurate readings if you monitor the input current to the speed controller. While it does exhibit some of the PWM current spikes, it has less of the spikes, overshoots and noise that appear at the output of a controller or motor wiring. Without getting into the detail of how to use this device, it would be more appropriate to insert it in the common lead of the speed controller input wiring, if possible. The controller input wiring does not change polarity (although it appears that this device may be designed to look at varying polarity signals). That puts the board at a lower potential and if insulation should fail, less likely to cause control issues in other circuitry. The circuit itself will be inspected as a custom circuit under the rules quoted above for each event you attend. It will also be required to be correctly insulated and isolated from the frame while meeting all other robot rules.

[jmiller18]

You could always use a current clamp, you just clip it around the wire and it will measure the current without actually having to be part of the circuit between the motor and jag

http://en.wikipedia.org/wiki/Current_clamp

Also, amps are a unit that measures current not amperage.

[Jon Stratis]

Also, amps are a unit that measures current not amperage.

http://www.thefreedictionary.com/amperage

amperage: The strength of an electric current expressed in amperes

Current and amperage are generally treated as synonymous.

[jmiller18]

http://physics.nist.gov/Pubs/SP330/sp330.pdf
bottom of page 19

this is the United States's National Institute of Technology (NIST) reproduction of the International System of Units (SI) standards. It clearly says that amps measure current. Amperage isn't technically a word. It is like guesstimate, it is only defined in the dictionary because it is used so often.

[EricVanWyk]

http://physics.nist.gov/Pubs/SP330/sp330.pdf
bottom of page 19

this is the United States's National Institute of Technology (NIST) reproduction of the International System of Units (SI) standards. It clearly says that amps measure current. Amperage isn't technically a word. It is like guesstimate, it is only defined in the dictionary because it is used so often.

Amperage is most commonly measured in amps. What you have said is essentially "Length isn't a real word, meters is".

[Ether]

What you have said is essentially "Length isn't a real word, meters is".

Actually, what he is saying is "Meterage isn't a real word, length is".

[Al Skierkiewicz]

While I will understand what you are saying, I am likely to correct you when you use "amperage" instead of "current". For me it is like putting ketchup on cake. It is possible but it makes me cringe.

[Ether]

For me it is like putting ketchup on cake. It is possible but it makes me cringe.

Few posts make me actually "laugh out loud" but this sure did.

Thanks for brightening my day :)

[Al Skierkiewicz]

Thanks for brightening my day :)




That's why I get paid the big bucks.

[EricVanWyk]

I must be a ketchup type of guy. I commonly use the words amperage and ampacity when describing circuits.

[fovea1959]

I'm probably going to sic a student on getting CAN set up. The team used CAN last year, and the programmer's didn't like the reliability (I spent a good chunk of yesterday peeling solidified hot-glue off old Jags), but if we just use them for the shooting, I think we will be ok. (I was elsewhere last year, don't have details)

May end up using them everywhere; I understand we can't get Talons. Too bad, Jags are *bulky*.

...and my Websters agrees with freedictionary.com. "Amperage" is indeed a word, though one that Al doesn't like :)

[JDL]

Anytime you can get motor stats like current from a motor drive to your controller via some form of communication like CAN it's usally the best and easiest way to go. Adding a Hall effect current sensor like that equals more connections between the drive and the motor and more places for failure and more work. It can also be the most accurate way to go too.

[fovea1959]

JDL: agreed. I've thought it over, and if we are going to measure current, we'll just use CAN.