Category: Science

Having previously concluded running the CNC 3018-Pro steppers from 12 V would let the DRV8825 chips provide better current control in Fast Decay mode at reasonable speeds, I wondered what effect a 24 V supply would have at absurdly high speeds with the driver in 1:8 microstep mode to reduce the IRQ rate.

So, in what follows, the DRV8825 chip runs in 1:8 microstep mode with Fast Decay current control. You must apply some hardware hackage to the CAMTool V 3.3 board on the CNC 3018-Pro to use those modes.

In all the scope pix, horizontal sync comes from the DRV8825 Home pulse in the top trace, with the current in the two windings of the X axis motor in the lower traces at 1 A/div. Because only the X axis is moving, the actual axis speed matches the programmed feed rate.

Homework: figure out the equivalent two-axis-moving speed.

The 12 V motor supply works well at 140 mm/min, with Fast Decay mode producing clean microstep current levels and transitions:

The sine waves deteriorate into triangles around 1400 mm/min, suggesting this is about as fast as you’d want to go with a 12 V supply:

Although the axis can reach 3000 mm/min, it’s obviously running well beyond its limits:

The back EMF fights the 12 V supply to a standstill during most of the waveform, leaving only brief 500 mA peaks, so there’s no torque worth mentioning and terrible position control.

Increasing the supply to 24 V, still with 1:8 microstepping and Fast Decay …

At a nose-pickin’ slow 14 mm/min, Fast Decay mode looks rough, albeit with no missteps:

At 140 mm/min, things look about the same:

For completeness, a detailed look at the PWM current control waveforms at 140 mm/min:

The dead-flat microstep in the middle trace happens when the current should be zero, which is comforting.

At 1400 mm/min, where the 12 V waveforms look triangular, the 24 V supply has enough mojo to control the current, with increasing roughness and slight undershoots after the zero crossings:

At 2000 mm/min, the DRV8825 is obviously starting to have trouble regulating the current against the increasing back EMF:

At 2500 mm/min, the back EMF is taking control away from the DRV8825:

The waveforms take on a distinct triangularity at 2700 mm/min:

They’re fully triangular at 3000 mm/min:

In round numbers, you’d expect twice the voltage to give you twice the speed for a given amount of triangularity, because the current rate-of-change varies directly with the net voltage. I love it when stuff works out!

At that pace, the X axis carrier traverses the 300 mm gantry in 6 s, which is downright peppy compared to the default settings.

Bottom lines: the CNC 3018-Pro arrives with a 24 V supply that’s too high for the DRV8825 drivers in Mixed Decay mode and the CAMTool V3.3 board’s hardwired 1:32 microstep mode limits the maximum axis speed. Correcting those gives you 3000 mm/min rapids with good-looking current waveforms.

I’m reasonably sure engraving plastic and metal disks at 3000 mm/min is a Bad Idea™, but having some headroom seems desirable.

Herewith, a look at CNC 3018-Pro stepper motor current waveforms as a function of supply voltage, PWM decay mode, and motor speed.

The scope displays X and Y axis motor current at 1 A/div, with sensing through a pair of Tektronix Hall effect current probes:

The X axis driver is an unmodified DRV8825 PCB operating in default mixed-decay mode. The Y axis DRV8825 has its DECAY pin pulled high, thereby putting it in fast decay mode.

The scope timebase varies to match the programmed feed rate. Because the X and Y axes move simultaneously, each axis moves at 1/√2 the programmed speed:

G1 X10 Y10 F100 → 71 mm/min on X and Y

The motor generates minimal back EMF at slow speeds, so the winding sees nearly the full supply voltage. As described in the previous post, the basic problem arises when the current rises too fast during each PWM cycle:

V = L di/dt
di/dt = 24 V / 3 mH = 8 kA/s

The first 1:32 microstep away from 0 calls for 5% of max current = 50 mA at a 1 A peak. The DRV8825 datasheet says the PWM typically runs at 30 kHz = 33 µs/cycle, during which the current will change by 270 mA:

267 mA = 8 kA/s × 33.3 µs

Notice how the current slams to a nearly constant, much-too-high value just after the first microstep. The incorrect current level decreases with lower supply voltage, because the rate-of-change decreases and the commanded current level reaches the actual (incorrect) current sooner.

Varying the motor voltage at a constant 10 mm/min:

Note that reducing the supply voltage doesn’t change the motor winding current, because the DRV8825 controls the current during each microstep, at least to the best of its ability.

Also note that the current overshoots the target for those microsteps, even when the motor is stopped, because there’s no back EMF, so the power dissipation is too high even at rest.

Enough back EMF appears at 100 mm/min to begin tamping down the current overshoot at 24 V:

The current waveform looks good at 12 V:

The back EMF at 1000 mm/min nearly eliminates the overshoot at 24 V, with fast decay in the Y axis causing some PWM ripple:

Both decay modes look good at 12 V:

At 1500 mm/min, the highest reasonable speed for the thing, and a 24 V supply, both waveforms still look good:

However, the back EMF is now high enough to buck the 12 V supply, preventing the current from decreasing fast enough in mixed decay mode (top trace):

Tweaking the GRBL config to allow 2000 mm/min feeds shows the waveforms starting to become triangular, even at 24 V:

And a 12 V supply opposed by the back EMF simply can’t change the current fast enough to keep up with the DRV8825 microstep current levels:

Bottom line: a +12 V motor supply and DRV8825 drivers modified to run in fast decay mode look like the best setup for the 3018-Pro: good current control at low speeds with enough moxie to handle higher speeds.

I should hack the DRV8825 boards into 1:8 microstep mode to reduce the IRQ rate by a factor of four, then see what happens to the back EMF at absurd speeds.