Table of Contents
Firmware
There is a nice fork of the original GRBL that works with a servo instead of a spindle: https://github.com/cprezzi/grbl-servo
Configuration
I had to make just a few adjustments to defaults.h in order to set the machine properties, such as axis lengths, accelerations and steps per unit.
defaults.h
// […]
#define DEFAULT_X_STEPS_PER_MM 53.555
#define DEFAULT_Y_STEPS_PER_MM 53.555
// […]
#define DEFAULT_X_MAX_RATE 6400.0 // mm/min
#define DEFAULT_Y_MAX_RATE 6400.0 // mm/min
// […]
#define DEFAULT_X_ACCELERATION (80.0*60*60) // 80*60*60 mm/min^2 = 80 mm/sec^2
#define DEFAULT_Y_ACCELERATION (80.0*60*60) // 80*60*60 mm/min^2 = 80 mm/sec^2
// […]
#define DEFAULT_X_MAX_TRAVEL 300.0 // mm NOTE: Must be a positive value.
#define DEFAULT_Y_MAX_TRAVEL 300.0 // mm NOTE: Must be a positive value.
// […]
#define DEFAULT_DIRECTION_INVERT_MASK 1 // invert X axis
// […]
Compile & Flash
After changing values in the code, it needs to be recompiled and flashed (requires avr-gcc and avrdude):
make
make flash
In order to restore settings in the EEPROM to their firmware defaults, one can issue the following command via serial terminal:
$RST=*
How to connect the servo to the GRBL board?
Everything worked fine except for the servo control. According to the readme, one should be able to control the servo using M3 S0 … M3 S255 commands. I assumed that the servo had to be connected to the “Spindle Enable” (D12) pin, which vanilla GRBL can natively drive using PWM. Took me a while before i found the following in grbl’s config.h:
When enabled, the Z-limit pin D11 and spindle enable pin D12 switch! The hardware PWM output on pin D11 is required for variable spindle output voltages.
Hooked the servo up to D11 instead, and suddely it worked! Magic! Only thing left to do was to adjust minimum and maximum pulse values in order to restrict servo movement to the absolutely necessary:
cpu_map.h:127-132
#ifdef SPINDLE_IS_SERVO
#define SPINDLE_PWM_MAX_VALUE 60
#ifndef SPINDLE_PWM_MIN_VALUE
#define SPINDLE_PWM_MIN_VALUE 20
#endif
#else
Control Software
GCode generation
Inkscape Plugin: https://github.com/arpruss/gcodeplot (there you can set the gcode to lift and lower the pen using the servo as “before path” and “after path” actions)
Machine control & GCODE sender
http://chilipeppr.com/jpadie (the jpadie workspace is compatible with latest GRBL)