Using an oscilloscope to view current waveform through a stepper winding

[presse55]

Hello group,

I wish to analyze the current waveform through the my stepper's windings.

Reason: uneven steps - .  As you can see, the application runs fine, the stepper steps, forwards, reverse, accelerates, etc; just the steps are uneven at low speed.

I will be receiving a Rigol DS1102E shortly. I have no experience with an oscilloscope, and am looking for guidance on viewing the current waveform on this stepper.

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

The stepper’s characteristics:

• Stepper rated voltage: 12V
• Stepper rated current: 0.33A
• Supply voltage: 18VDC, Li Ion battery, floating, ie not ground referenced
• L=48mH
• R=32.6ohms
• f= 10Hz - 100Hz (ie 10 to 100 full cycles of 16uSteps per full step per second)

My questions:

• I do not have a current probe, so plan to use a shunt resistor in series.  The only technique I have found on the internet involves the use the A-B differential method illustrated here to measure voltage difference between two probes.
  Is there another more appropriate or direct way to plot the current waveform using this scope?
  
• For shunt resistor value, my calcs so far if I am correct point to a 1 ohm resistance.
  Does this look right?
  

Feel free to redirect me to any existing post dealing with this topic.

Thanks in advance for your responses!

Ron
Montreal

PS: I'm a bit of a noob with electronics, but I've been through the phasor representation for this setup.

[MrW0lf]

This best you can do with 2 ch scope, shunt and floating DUT to see current and voltage. Main thing: Ground clips must be connected  together to same place. 1ohm is fairly large for shunt. I use special 0.1ohm non-inductive max. But in this case might not matter much since this is pretty civilized motor not near-short like brushless outrunners.

[presse55]

Cool, thanks for the reply MrW0lf!
I like your suggestion cause that way I'll get to see both voltage and current!
Thanx!

[threephase]

Good advice MrW0lf

[korlatos]

You can easily build a low-cost current probe that covers DC-200kHz range. Please look at the attached application note for details.