Quick guide

[Control a Servo] [Hardware]

To get your Arduino to control a plotter, following steps are needed:

Windows USB driver

Connect your Arduino with your PC, on windows 10 the needed USB driver will be installed automatically, if not: the cheap Arduino clones often uses a CH340 chip. Check here for more information https://www.google.de/search?q=arduino+ch340.

Grbl firmware

Download and unzip GRBL-Plotter. Start the program XLoader.exe. Select the desired firmware, correct device and the correct com port, then upload the firmware. With the shown settings, the upload took 18 seconds.

• grbl_v1.1h.20190825.hex is the latest regular grbl firmware for Arduino uno and nano, from https://github.com/gnea/grbl
• grbl_v1.1f_Servo.hex as above, but with Spindle-PWM adapted to RC-Servo specification (1ms - 2ms), from https://github.com/cprezzi/grbl-servo see also 'use of servo' below
• grbl_v1.1f_Servo_switch_dir_step.hex as above, but with Spindle-PWM adapted to RC-Servo specification (1ms - 2ms) and step/dir pins swapped for use with CNC Shield V4 for Arduino nano (see below), from https://github.com/cprezzi/grbl-servo
• grbl-Mega-5X-v1.1l.20190605.hex 5 axis version for  Arduino Mega2560 only, from https://github.com/fra589/grbl-Mega-5X

After successful upload, close XLoader and reset the controller.

1st connection

Install GRBL-Plotter by starting 'setup.exe' from the unzipped folder. Start GRBL-Plotter and select the correct COM-port for the controller. After successful connection, grbl sends a start-up message "Grbl 1.1h ['$' for help]".

Now the buttons inside the GUI are enabled and the stepper motors can be controlled.

Configuration of grbl

After pressing "$$", grbl lists the stepper motor settings, which must be adapted for each machine. Check the grbl-Wiki for more information. The screenshoot shows the initial settings after the installation of grbl.

The main settings are $3, $10x, $11x (x=0,1 or 2 depending on axis) to get the stepper motors running in the correct direction. The correct direction means:

• +X = to the right
• +Y = to the back
• +Z = upwards

Note: a too large number for $10x (steps/mm) and $11x (max. speed) will cause problems.
The product of $10x and $11x must stay below 1800000 to avoid pin frequencies above 30 kHz.
E.g. $100=250, $110 = 7200 is at the edge.

More information here: quick-guide-to-setting-up-your-machine-for-the-first-time

Spindle-PWM and use of a servo:

The pulse width modulation (PWM) for speed control of a motor or power control of a laser, is generated via a special pin function (hardware PWM) inside the ATmega328 (the microcontroller of the Arduino Uno / nano) on pin 11 (PB3, OC2A). This means the pin for the output of the PWM cannot be changed.

• By default, the PWM frequency is set to 1 kHz. The duty cycle can be controlled between 0% and 100% to get an control voltage of 0V to 5V for the spindle or laser.
• RC servos usually require a PWM signal of 50 Hz with a pulse width of 1 ms to 2 ms. The duty cycle is then just in the range of  5% to 10% - not suitable to control a spindle or laser.

Regular grbl: "By default, the spindle PWM frequency is 1kHz, which is the recommended PWM frequency for most current Grbl-compatible lasers system. If a different frequency is required, this may be altered by editing the cpu_map.h file."
Source: https://github.com/gnea/grbl/wiki/Grbl-v1.1-Laser-Mode#laser-mode-operation

Adapted grbl: "This is a special version with servo support (swichable in config.h) The PWM frequency is set to 61Hz (prescaler 1/1024). The pulse width range is 0.5 - 2.5ms." (Duty cycle it then from 3% to 15%).
Note: internally the PWM range goes from 7 (0.5 ms pulse width) to 38 (2.5 ms pulse width), which results to 32 possible different servo positions (probably less, because the regular range is from 1 ms to 2 ms). Source: https://github.com/cprezzi/grbl-servo
 

Common hardware

The following information refers to cheap (counterfeit?) boards that are available on ebay for 12€ to 15€ (including shield, arduino, A4988 drivers and USB cable).

The CNC shield V3 for Arduino Uno was designed for grbl 0.9. Because of a change in the pin-out in grbl version 1.1, the spindle-pwm signal can be found on the Z+ pin. Check schematic here.

The CNC shield V4 for Arduino nano shows a wrong layout for the micro-step jumpers. Check this link. Also step and dir-signals are swapped, which needs a adapted grbl version. I used this board at my foldable plotter. The adapted grbl firmware can be found in the GRBL plotter setup (grbl_v1.1f_Servo_switch_dir_step.hex).