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Using an oscilloscope to view current waveform through winding


#1

Hello group,

I wish to analyze the current waveform through my stepper windings using a Rigol oscilloscope which I will be receiving shortly. :rofl: I have no experience with an oscilloscope, and am looking for guidance.

The stepper is driven by a Pololu A4988 motor driver with voltage regulators, configured at 16 uSteps/full step;

The stepper’s specs:

  • Stepper rated voltage: 12V
  • Stepper rated current: 0.33A
  • Supply voltage: 18VDC
  • L=48mH
  • R=32.6ohms
  • f= 10Hz (ie 10 full cycles of 16uSteps per full step per second)

I do not have a current probe, so plan to use a shunt resistor in series.
For shunt resistor value, my calcs so far if I am correct point to a 1 ohm resistance.

The only technique I have found on the internet involves the use the A -B differential method described here to measure voltage difference between two probes.

Is there another way to see the current waveform using this scope?

Thanks in advance for your responses!

Ron
Montreal


#2

Hello, Ron.

A differential measurement using a shunt resistor is a reasonable approach. Adding resistance in series with the coils will change the behavior of the driver a little bit (the driver will have to run a slightly higher duty cycle to product the target currents) but a 1 ohm resistor probably will not affect a 32.6 ohm coil much. Lower resistance shunt resistors will have less of an effect, but also might affect the resolution of your measurements.

It is worth noting that there are current sensing resistors on the A4988 board, however those are connected to the coils through the H-bridge, so they will be frequently disconnected from and connected in reverse to the coil, which makes measurements there much more difficult to interpret.

-Nathan


#3

Thank you Nathan for your response.
I’m glad to see that I’m on the right track with this project.
Thanks once again!


#4

Ron,

Back in the old days of analog oscilloscopes we had current probes that clipped over the wire. Tektronix made the scopes and probes. Dont know if these would work with your new scope though.

Best,

Mike


#5

Thank you Mike.
I’m aware of current probes, I’ve looked them up, and found them a bit pricey.
Being a DIYer on a budget, and since I can get an indirect measurement using the scope and it’s probes, I’ll give that a shot and see.
Ciao!

Ron


#6

Ron,

If you are a DIYer and familiar with analog electronics you could make a simple current probe with a small toroid core and a few turns of wire. Wind a few turns of small wire around the toroid and slip the toroid core (motor wire goes thru the toroid center) over one of the motor wires. Place a small resistor across the wire ends of the wire you wrapped around the toroid, probably 1000 ohms for starters. Connect your scope probe across the resistor and you should be able to see the motor current waveform. Add more turns of wire for more sensitivity. You can calibrate your probe by passing a pulsed current (square wave for example) of known amplitude thur the probe. Most simple function generators have a 50 ohm output impedance, so placing a 5 volt open circuit square wave pulse at the frequency of your stepper driver on the function generator output then simply short the output with a short wire going thru your probe toroid center. The current going thru the probe should be 5v/50 ohms or ~100ma.

A small iron (magnetic) nut or washer might work for the toroid.

Anyway just a thought,

Best,

Mike