Archive for November 8th, 2017

Mostly Printed CNC: Stepper Drive

The MPCNC kit includes five Automation Technology KL17H248-15-4A stepper motors:

MPCNC Stepper label - KL17H248-15-4A

MPCNC Stepper label – KL17H248-15-4A

If link rot should set in, a direct rip from the website:

NEMA 17 BIPOLAR STEPPER MOTOR, KL17H248-15-4A, 76 oz-in

Shaft: 5mm diameter with flat
Current Per Phase: 1.5A
Holding Torque: (76 oz-in)
Rated Voltage: 4.2V
NO.of Phase: 2
Step Angle: 1.8° ± 5%
Resistance Per Phase: 2.8Ω± 10%
Inductance Per Phase: 4.8mH± 20%
Insulation Class: Class B
Dielectric Strength: 100Mohm
Operation Temp Range: -20 ~ +40° C
Lead Wire: 22AWG / 750mm with connector to stepper motor driver

Red- 1A
Green- 1B
Yellow- 2A
Blue- 2B

A nice torque curve:

KL17H248-15-4A - Torque curve

KL17H248-15-4A – Torque curve

The present MPCNC design wires the motors on each end of X / Y axes in series. Each motor has 2.8 Ω of DC resistance = 5.6 Ω total and, given the small wire gauge (allegedly 22 AWG on the motors and unspecified for any eBay cables) and six (!) teeny header pins in series along the wires for each winding, a total series resistance of 6 Ω seems reasonable and is, in fact, what I measure with an ohmmeter.

The stepper drivers arrived preset for 1 A peak:

MPCNC Stepper Drive - as delivered - 500 mA div

MPCNC Stepper Drive – as delivered – 500 mA div

The vertical scale is 500 mA/div. The waveform comes from a 10 mm move at 5000 mm/min = 83 mm/s, which is absurdly fast for such a machine, particularly seeing as how the default firmware limits it to 190 mm/min = 3 mm/s. Cutting speeds will be much lower than either of those.

The default DRV8825 current-setting pot setting was 600 mV, for a nominal current motor current of 1.2 A peak. That’s reasonably close to the measurement, all things considered.

However, because the motors run from a 12 V supply at 1 A, the winding and wiring losses mean they operate at a bit over 8 V: much much less than the nominal 24 and 32 V plotted in the torque curve. More voltage = faster response to microstep current changes = higher top speed. At sensible speeds, this surely does not matter.

The default DRV8825 stepper driver module jumpers select 32 microsteps = 6400 step/rev, a factor of four higher than the chart.

Part of the tweakage will be to sort that out; a 24 V supply may be in order. Driving each motor separately (as required for automatic de-racking homing) at 1.5 A/phase would require 3  1.5×√2 A/motor × 5 motors = 15 10.5 A, which seems excessive even to me, particularly in light of sending it across a RAMPS board. At 1 A/phase, you need 10 7 A, which falls within the realm of reason and would be kinder to the PLA motor mounts. It’s not clear boosting the motor voltage will produce any real benefit, although giving the drivers more headroom seems reasonable.

The GT2 drive belts have 2 mm pitch, so the 16 tooth drive pulleys move 32 mm/rev and require 200 step/mm, which seems high to me. At a nice round 100 mm/s, the steppers must tick along at 20 k step/s, half of Marlin’s top speed, which may explain some of the roughness around 80 mm/s.

The torque curve suggests the motors want to run under 200 RPM = 3.3 rev/s = 100 mm/s with the stock 16 tooth pulley. No problem with those numbers!

Using 16:1 microstepping would produce 3200 step/rev, 100 step/mm, thus half the step rate at any speed. Reducing the driver step frequency can’t possibly be a Bad Thing for Marlin.