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Archive for February 2nd, 2018

MPCNC: Stepper Drive vs. Back EMF at the Edge of Madness

This is what the stepper current looks like when you run an MPCNC at 12000 mm/min = 200 mm/s from a 24 V supply:

G0 X 25.6 - A drive - 200 mm-s - low I - 24V 200mA-div

G0 X 25.6 – A drive – 200 mm-s – low I – 24V 200mA-div

The vertical scale for the winding current in the top trace is 200 mA/div, so those flat sections run about 150 mA, well under the 600 mA peak found at lower speeds. This G0 move ramps up to 12000 mm/min and shows the current falling off and the sine wave deteriorating:

G0 X 200 mm-s - 24V 200mA-div

G0 X 200 mm-s – 24V 200mA-div

The motor turns at 375 RPM, which means each stepper generates 12 Vpk back EMF, so the A4988 driver has absolutely no control authority near what should be the zero crossings of the current waveform.

The taller waveform along the bottom of the first scope shot comes from the H bridge output tending to increase the current waveform, so it’s applying a solid +24  during the flat top section and not only gets no traction, but actually loses ground just after the middle of the screen. Eventually the back EMF drops and the current begins increasing, but long after what should be the 600 mA peak current where the driver flips the H bridge and begins trying to decrease the current.

Here’s a detailed view of the section just after the left cursor:

G0 X 25.6 - A drive - low I - detail - 24V 200mA-div

G0 X 25.6 – A drive – low I – detail – 24V 200mA-div

The step pulses tick along at 50 μs intervals, with only two driver PWM pulses for each one.

A 2-flute carbide cutter spinning at 20000 RPM with a 0.25 mm chip load calls for a speed of 10000 mm/min. Alas, the stock 12 V supply stalls at 8000 mm/min:

MPCNC G1X20F8000 - 500 mA-div 50 ms-div

MPCNC G1X20F8000 – 500 mA-div 50 ms-div

It’s facing something over 9 V of back EMF from the motors at that speed. The chaotic spikes near the middle of the current waveform show where the rotor dropped out of lock with the driver and begins shoving the current around.

So that’s why you really can’t use a 12 V power supply with series stepper motors, at least if you want to run ’em at more than moderate speeds. It’s not at all clear the 24 V supply will provide enough motor torque to run at 10 k mm/min, so I gotta wrap an enclosure around the electronics and start making some chips to see how it behaves …

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