This library allows to connect to low level hardware from a host that has I/O capabilities to interface with motors and control them. It contains all the level in between: communication interface, binary and PWM channels, encoders, motors and controllers. Each level has a clearly defined CPP interface that can easily be extended to support new implementation. On the communication interface level, currently, the BCM2835 (present on all Rapsberry Pis for example) and PCA9685 chips are implemented.
The package contains:
- The libray: containing all the levels and interfaces as well as some implementation that should allow to control DC and servo motors on a RaspberryPi
- ROS2 nodes: NOT IMPLEMENTED
- A Python wrapper for the library: usefull to plot the motor responses and tune the controllers. Note that this requires to build the library as SHARED which is less efficient.
In order to use this library, you need to have set up ROS2. The controllers implemented here work on hardware that have direct I/O to microcontrollers such as a RaspberryPi.
The lowest level of the library is the interface to the chips that are able to produce binary and or PWM signal. There are currently two chips implemented:
- PCA9685: uses i2c to communicate with the host (where the program runs)
- BCM2835 or BCM2711: is the chips present on RPis to controls the GPIOs. With this chip you can read and write on GPIOS as well as produce hardware or software PWM signals. This chip can be controlled with two libraries:
- The bcm2835 library (cannot produce software pwm signals)
- The pigpio library
The datasheet can be found @ https://www.nxp.com/docs/en/data-sheet/PCA9685.pdf
This controller is found on ready to use RaspberryPi hat to controller servo motors. This code is tested on RaspberryPi 4 with the "Adafruit Servo/PWM Pi HAT!".
sudo apt install i2c-tools libi2c-dev
This are the chips that equip the raspberry pi:
- for RaspberryPi 4 using BCM2711: https://elinux.org/RPi_BCM2711_GPIOs
- for RapsberryPi < 4 using BCM2835: https://elinux.org/RPi_BCM2835_GPIOs
Note that in both case, only a limited set of pins can produce a harware PWM signal. For the others, a software PWM signal must be used. Of course, a software one is costly on the CPU.
Both of these chips can be controller using either of the following libraries.
The following dependency is required:
sudo apt install libcap2 libcap-dev
If you already have installed the BCM2835 library on your PC, run the build with the option BUILD_3RDPARTY to OFF. Else CMake will download and install on the colcon install space the library.
BCM2835 edge detection feature requires to changes the BIOS configuration. See raspberrypi/linux#2550.
- On a RaspberryPi OS
/boot/config.txt - On a Ubuntu distribution (in my case Ubuntu20 on a RPi4)
/boot/firmware/usercfg.txt
Add the line
dtoverlay=gpio-no-irq
Note that this library does NOT allow to produce software PWM signals.
See http://abyz.me.uk/rpi/pigpio/cif.html#
Not implemented
Not implemented
The library comes with a serie of example programs that can you can use to build your own program.
The library is wrapped in python. To build it, install SWIG.
sudo apt install swig4.0
Because Python works with shared libraries, the project needs to be build with the BUILD_SHARED_LIBS flag. Use
colcon build --packages-select motor_controllers --cmake-args -DBUILD_PYTHON_WRAPPER=ON -DBUILD_SHARED_LIBS=ON
to build the project as SHARED libraries with the Python wrapper. Note that all the examples and nodes are availables but might be slighlty slower.
The wrapper could be implemented directly with pybind11 as the unique_ptr reaquired to write manual conversion files. However with it too, one cannot use wrap a function taking a unique pointer as input. There a bit of trickery has to be used and a better solution would be to provide a helper class for python that instanciate an Encoder directly from the pin numbers.