PyCNC is a free open-source high-performance G-code interpreter and CNC/3D-printer controller. It can run on a variety of Linux-powered ARM-based boards, such as Raspberry Pi, Odroid, Beaglebone and others. This gives you a flexibility to pick a board you are most familiar with, and use everything Linux has to offer, while keeping all your G-code runtime on the same board without a need to have a separate microcontroller for real-time operation. Our choice of Python as main programming language significantly reduces code base compared to C/C++ projects, reduces boilerplate and microcontroller-specific code, and makes the project accessible to a broader audience to tinker with.

Realtime Motor Control in Linux?

Typically there is no way to control stepper motors from Linux runtime environment due to the lack of real time GPIO control. Even kernel based modules can not guarantee precise control of pulses for steppers. However, we can use a separate hardware module, DMA (Direct Memory Access) which provides high precision for GPIO outputs. This module can copy bytes which represent GPIO states from RAM buffer directly to GPIO with some clock based on main chip internal oscillator without using CPU's cores. Using such approach this project generates impulses for moving stepper motors and that is very precise way regardless CPU load and OS time jitter.
This approach also allows to use Python language for this project. Typically, Python is not good choice for real time application, but since project just needs to set up DMA buffers and hardware will do the rest, Python become the perfect choice for easy development of this project.

Video demo - YouTube video
And the original video when PyCNC was just a prototype YouTube video

Current gcode and features support

• Commands G0, G1, G2, G3, G4, G17, G18, G19, G20, G21, G28, G53, G90, G91, G92, M2, M3, M5, M30, M84, M104, M105, M106, M107, M109, M114, M140, M190 are supported. Commands can be easily added, see gmachine.py file.
• Four axis are supported - X, Y, Z, E.
• Circular interpolation for XY, ZX, YZ planes is supported.
• Spindle with rpm control is supported.
• Extruder and bed heaters are supported.
• Hardware watchdog.

Watchdog

PyCNC uses one of DMA channels as hardware watchdog for safety purpose. If board, OS or PyCNC hangs this watchdog should disable all GPIO pins(by switching them into input state, for RPi this would be GPIO0-29) in 15 seconds. Since there is a high current and dangerous devices like heated bed, extruder heater, this feature should prevent uncontrollable overheating. But don't count on such software features too much, they can hang too or output MOSFET become shorted, use hardware protection like thermal cutoff switches in your machines.

Hardware

Currently, this project supports Raspberry Pi 1-3. Developed and tested with RPI3. And there is a way to add new boards. See hal.py file.
Note: Current Raspberry Pi implementation uses the same resources as on board 3.5 mm jack(PWM module), so do not use it. HDMI audio works.

Config

All configs are stored in config.py and contain hardware properties, limitations and pin names for hardware control.
Raspberry Pi implementation should be connected to A4988, DRV8825 or any other stepper motor drivers with DIR and STEP pin inputs. Default config is created for Raspberry Pi 2-3 and this wiring config:

Circuit name	RPi pin name	RAMPSv1.4 board pin name	Note
X step	GPIO21	A0
X dir	GPIO20	A1
steppers enable	GPIO26	A2, A8, D24, D30, D38	all steppers
Y step	GPIO16	A6
Y dir	GPIO19	A7
Z dir	GPIO13	D48
Z step	GPIO12	D46
E1 step	GPIO6	D36	reserve
E1 dir	GPIO5	D34	reserve
E0 dir	GPIO7	D28
E0 step	GPIO8	D26
Z max	GPIO11	D19
Z min	GPIO25	D18
Y max	GPIO9	D15
Y min	GPIO10	D14
X max	GPIO24	D2
X min	GPIO23	D3
heater bed	GPIO22	D8
heater 2	GPIO27	D9	uses for fan
heater 1	GPIO18	D10
ser 1	GPIO17	D11	reserve
ser 2	GPIO15	D6	reserve
ser 3	GPIO4	D5	reserve
ser 4	GPIO14	D4	reserve
I2C SCL	GPIO3	-	to ads111x
I2C SDA	GPIO2	-	to ads111x
ads1115 ch0	-	A15	heater 2 - nc
ads1115 ch1	-	A14	bed sensor
ads1115 ch2	-	A13	extruder sensor
ads1115 ch3	-	-	not connected

So having Raspberry Pi connected this way, there is no need to configure pin map for project. RAMPS v1.4 board can be used for this purpose. Full reference circuit diagram and photos of assembled controller(click to enlarge):

Usage

Just clone this repo and run ./pycnc from repo root. It will start in interactive terminal mode where gcode commands can be entered manually.
To run file with gcode commands, just run ./pycnc filename.
Optionally, pycnc can be installed. Run

sudo pip install .

in repo root directory to install it. After than, pycnc command will be added to system path. To remove installation, just run:

sudo pip remove pycnc

Performance notice

Pure Python interpreter would not provide great performance for high speed machines. Overspeeding setting causes motors mispulses and probably lose of trajectory. According to my tests, Raspberry Pi 2 can handle axises with 400 pulses on mm with top velocity ~800 mm per min. There is always way out! :) Use JIT Python implementation like PyPy. RPi2 can handle up to 18000 mm per minute on the machine with 80 steps per millimeter motors with PyPy.
Note: Raspbian has outdated PyPy version in repositories(v4.0). Moreover v4.0 has issue with mmap module implementation. Use PyPy v5.0+, download it for your OS from here.
PyPy installation:

wget wget https://bitbucket.org/pypy/pypy/downloads/pypy2-v5.7.1-linux-armhf-raspbian.tar.bz2
sudo mkdir /opt/pypy
sudo tar xvf pypy2-v5.7.1-linux-armhf-raspbian.tar.bz2 --directory /opt/pypy/ --strip-components=1
sudo ln -s /opt/pypy/bin/pypy /usr/local/bin/pypy

Project architecture

Dependencies

Nothing for runtime. Just pure Python code. For uploading to PyPi there is a need in pandoc:

sudo dnf install pandoc
sudo pip install pypandoc

GCode simulation

Just a link, mostly for myself :), to a nice web software for gcode files emulation (very helpful for manual creating of gcode files): https://nraynaud.github.io/webgcode/

License

see LICENSE file.

Author

Nikolay Khabarov