Eks loaned me a Tek AM503 Current Probe Amplifier, one of those gorgeous instruments that Just Works: a clamp-on DC to 50 MHz Hall Effect current meter. Because it’s electrically isolated from all the hideous electrical hash that surrounds any stepper motor driver circuit, it doesn’t see much of the garbage that pollutes any current sensor depending on a series resistance and a differential amplifier.
Which lets you take pix like this:
From top to bottom:
- Generated 1/rev sync pulse
- Winding current at 200 mA/div (1 A peak current)
- Step drive pulses at about 3 rev/s
The initial ramp occupying the first third of each step comes from the motor’s L/R time constant coupled with the 9 V supply I was using. Back of the envelope: 2 mH / 2 Ω = 1 ms. With 8 V (9 V less MOSFET drops &c) applied, the initial slope = 8 V / 2 mH = 2500 A/s, so in 75 ms it rises 187 mA: close enough.
The small ripples show the A4988 chopping the current to maintain the proper value for each microstep.
Looks just like the pretty pictures in the datasheet, doesn’t it?