Three-axis motion with GRBL

For an upcoming project I want to get three axis motion working with stepper motors and linear stages which use a trapezoidal thread to move in a very accurate way.

Stepper Stages

I used these stages which have a NEMA11 motor and a Tr6-2 thread (1mm pitch, double start), resulting in 100 steps per mm (not 200 like the listing said). As shipped the end plates were a little misaligned, and needed to be properly set up¹.

The actual length of travel is about 103mm. I’ll post some measurements later.

GRBL board

I combined them with this GRBL driver board from Ebay. In the photos it is identified as LY-GRBL3-10086-V11 but the board as shipped is identified as LY-3Axis-4.0-V2.0. It is otherwise the same.

It shows up as a USB device with identifiers 0483:5740 STMicroelectronics Virtual COM Port and device strings Product: LUNYEE_4axis_Control and Manufacturer: tomeko net. In Linux it appears as /dev/ttyACM0 (etc).

The $I command reveals it is running a custom build of GRBL 1.1f:

Monport
[VER:1.1f.20230316:������������������������]
[OPT:VMZHL,35,254]

some � omitted for clarity

This particular board uses a Gigadevice ARM Cortex CPU, probably a GD32F303VCT6 but I’m damned if I can read the inscription.

I probably should have found an ESP32 based board but this one was cheap and ticked all the boxes.

Stepper Drivers

The board comes pre-populated with three A4988 drivers. They have little heatsinks and current limit trimpots which need to be set up to suit your stepper motors.

As shipped the board has all three stepper driver jumpers populated, meaning each step is 1/16th of a real step. So we get 1600 microsteps per mm.

GRBL Settings

GRBL keeps a bunch of settings in NVRAM and these are the ones I changed:

By keeping the limit switch activation point about 1mm from the end and reducing homing pull-off to 1mm the stage limits are a very neat 0 - 100 mm.

For some reason the Z axis is inverted in hardware, I checked my stepper wiring many times and it just is. So I invert X and Y to match, so that 0 is with the stage at the stepper end and 100 is with the stage at the far end. Homing direction is also inverted, so the homing cycle travels to the negative X, Y and Z direction.

Axis Connectors

Each axis has it’s own JST XH2.5 with the following pinout.

Except Y2 which has the pins opposite for some reason, but I’m not using that one.

The steppers had bare wires and I had some spare pre-wired headers so I just made up some frankencables. Using motors with pre-installed connectors would save a lot of messing about with solder.

Limit / Home switches

When the board first wakes up, it doesn’t know where the stepper stages are positioned. So it has to perform a homing cycle, and to do that it needs some switches to tell it when each axis is ‘home’.

These wire to two-pin “dupont” connectors on the board. For now I’ve just used microswitches in a 3d printed plastic carrier which clips to the stepper.

Board Power

The stepper drivers are powered from the DC connector, 5.5mm OD / 2.5mm center positive, 7 - 36V.
I found a 19V 4A supply to suit. Don’t forget to turn on the switch.

The logic is, or at least can be, powered from the USB-C port while the motor power is off. There are a number of other power connectors on the board whose purpose is not entirely clear but is presumably for spindles, lasers, etc.

There are three red power LEDs on the board which might be helpful to know about:

• A: Motor Power labelled PWR
• B: USB-C Power (no label)
• C: Either Motor or USB-C Power labelled 3V3

Does it work?

Yes! The three axis can be controlled independently or together using G-Code commands. Note that GRBL starts a homing cycle with $H instead of the more common G28.

Next Steps

• printing a case for the board and some parts to hold the stages together and more elegant end stop switch holders.
• implementing smoother kinematics by sending way too much Gcode.