A Simple Hall Effect Current Monitor
 

I wrote a detailed description of a DC Current to Analog Input circuit we constructed based around a Hall Effect sensor available from Digi-Key. Its pretty simple to build and provides current readings to an Analog Input using a single electronic component.

The paper is too large to post as an attachement (629KB) , but here is a link to where you can pick up a copy.

Robot Current Monitor Paper

cost?
 

Hey Steve,


I read through your paper. Great study! Thanks for sharing it. I looked up the transducer in digikey.... it was $35 a unit! Pricey little device, but with the information you provided, I'll bet there's a really clever application for this circuit!


Thanks!



Steve Alaniz


"What good is technology if you can't abuse it?" - Ted Forth

Price of the Hall Effect Transducer
 

If you check the price of the Hall Effect transducer on-line, it has dropped quite a bit to $19.20 in single quantities.

Re: Re: Display Software?
 

What did you find out?

Re: A Simple Hall Effect Current Monitor
 

Thanks. I bought 4 current sensors from DigiKey, and they appear to work nicely.

To see how they worked, we took two of them and put one on each of the drive motors in last years robot, and used GreenDice's Dashboard Monitor 1.3 to view and log the output.

I haven't had a chance to plot the logged output yet, but will do that soon.

Hopefully, next year, FIRST will allow us to mount current sensors without having to put them inside a project box. On this year's robot, it would be difficult to route the wires for our four drive motors through a project box.

I'd also like to mount a drivetrain speed sensor on each side of our robot. Any suggestions on how to measure relative speed?

The two ideas I had were: 1. use a small motor to generate a voltage relative to speed, 2. use an optical sensor and a rotating disk with an alternating black and white pattern and convert the pulse rate into an analog voltage.

I like idea #2, but it requires circuitry to convert the pulses rate to an analog voltage.

Hey guys,
I just posted a writeup of our current sensing circuit in the Technical Discussion forum. We avoided running the motor power lines through the sensing box by using 1 foot of the #10 wire as a resistor. Then we just used some sense leads from the #10 to the project box.

The cool thing about the hall-effect current sensors from DigiKey is that there isn't any additional circuitry. They connect directly to the Analog inputs.

The MAX4172s that we use could also go right to the robot controller if we wanted. We just wanted the ability to do some real processing at the robot end.

Current measurement results
 

Here are our results using the http://www.lemusa.com/ current sensors from Digikey on our old robot, which has 2 drive motors. (The current sensor is pictures in the center of their main page, standing on the triangle.)

I had to average each reading with it's nearest 4 neighbors to smooth out the graph.

The values for amps are only an estimate since we haven't calibrated the readings yet.

Re: Current measurement results
 

Fantastic results! One question....what is the X-Axis time scale for those readings? (Seconds, milliseconds, etc...?) Is the total time shown over a two minute round, or are you saying the current varies that much over a short time?

Do you see the chopping (PWM) variations, and can you perhaps just filter them out with an R/C filter?

We filtered these out/down on our current measuring circuit.

Just a thought/question,
-Quentin

Re: Re: Current measurement results
 

The timeframe was somewhere around two minutes, but it was just random driving around. I didn't time it, since my only goal was to get some data to plot. We'll run some more controlled tests later.

Each tick represents one log entry on the Dashboard 1.3 program. I don't know how much time that is per tick.

Yes, an RC filter would be good to smooth out the PWM pulses. Any suggestions for RC circuit values between the current sensors and the analog inputs?

I suppose if the output impedence of the sensor and the input impedence of the analog inputs were appropriate, I would only need a capacitor.

ECarlson,
Dave Flowerday and I have already written on our use of the Stangsense circuitry in the electrical forum. One of the big advantages we have in A/D in the circuit is the ability to integrate the results and multiplex up to eight motors into one stream. Additionally, the use of current monitor on the input of the speed controller is unipolar (only flowing in one direction) and measures the exact load presented to the circuit breaker.
In either case you can see from your data (even after averaging) that instantaneous currents are pretty high. We can easily see from the graph you posted when you turned obviously using tank drive (one motor turning in the opposite direction of the other motor.) and more interesting is the higher current being drawn by the "blue" motor. If you were driving straight I would look to a tight transmission on that side or a misaligned wheel. Have you tried plotting the battery voltage against the currents yet?
Good Luck,

Thanks for the reply. I have the battery readings, but I have not yet plotted them.

We also need to do some test under more controlled conditions to really see what is going on with our old robot.

Re: Here are our Motor Monitor Schematics
 

Nice job, Quentin! A real KISS solution.

A couple of questions for you:

1) Your equation on the first page of the PDF says: [Vout = (V1-V2)*R1/R2]. I think that R1 and R2 are backwards in that equation. Shouldn't that be: [Vout = (V1-V2)*R2/R1] ??? (100K / 12.1K ~= 8.26)

2) Page 3 shows you are taking MxV+, and the power for the CC off of the same breaker CB1. I know you have bypass caps (C3, C4, C7, C8, C11, C12, & C14), and the CC doesn't use much power, but there are still common mode transient loads from line driving the wiring between the CC and the RC.

Assuming you have the spare breakers, wouldn't it be cleaner to simply use 2 breakers, one for "+12v_FUSED" to power the CC, and a second one as the MxV+ "common high side sniffing probe", or did you find the difference too small to worry about?

BTW... Everyone: make sure you don't add any other loads to CB1, especially motors! They'll mess up your data.

- Keith