pwmdriver - ROS2 SysFS PWM Driver

A simple PWM driver that uses the Linux Kernel sysfs interface to configure PWM devices. The driver also supports PPM style outputs (for servos and RC motor controllers).

Features

• Supports setting PWM duty or PPM compliant duty cycles.
• Set PWM devices using a normalised float value (0 to 1).
• PPM zero-points can be set to support reverse on RC motor controllers, allowing the duty to be set with a normalised float from -1.0 to 1.0.

Why

When researching how I was going to interface with a PCA9685 board for a DIY RoboCar project, I found that a driver for that hardware exists in the Linux kernel. This meant I was able to simply use the SysFS interface to configure that particular board, and any other PWM devices that kernel driver support exists for (of which there is a a lot). This also includes Single Board Computers such as Raspberry Pi and Rock Pi, where you can use the PWM port on the GPIO header.

A survey of the available ROS packages didn't show many ROS 2 PWM packages, and none of those used the SysFS interface. So here is my package to solve this.

Usage

1. Enable the kernel driver for the PWM hardware by configuring the Device Tree.
2. Set up the udev rule so the PWM device's SysFS interface can be written to from non-root users.
3. Install/compile this package into your ROS2 install or workspace.
4. Add the pwmdriver node to your launch scripts, or run the pwmdriver node, making sure to specify your configuration file.
5. Publish the normalised float to the /pwm/<output name> topic, setting the duty cycle.
6. Zoom around!

Messages

/pwm/<name>: std_msgs/msg/Float32 - Duty is set using a Float32 message. That's all there is to it.

Configuration

An example output motor is shown below:

pwm_driver:
  ros__params:
    outputs: ["esc", "servo"]
    esc:
      device_path: /sys/class/pwm/pwmchip1
      channel: 7
      period: 2040000
      type: ppm
      range: [1000000, 1500000, 2000000]
    servo:
      device_path: /sys/class/pwm/pwmchip2
      channel: 7
      period: 20000000
      type: ppm
      range: [1000000, 1500000, 2000000]

Parameter	Type	Description
device_path	string	Path to the PWM device. e.g. /sys/class/pwm/pwmchip1.
channel	int	The PWM channel that will be used for this output.
period	int	The period of the PWM signal in nanoseconds. For RC servos this is 20000000 (20 ms) and motor controllers this is 2040000 (2.04 ms).
type	string	Options are: ppm or pwm. The 'type' of PWM signal to output. RC servos and motor controllers needs this to be set to ppm.
range	int[]	Format is either [minimum, zero-point, maximum] or [minimum, maximum].
trim	float	Trim value for the scaled duty output. This value is added to the normalised duty value.

PPM Range

These are the signal durations in nanoseconds for minimum (0 or -1), zero-point (0) and maximum (1.0). If a zero-point is provided then the duty cycle input will accept normalised input from -1.0 to 1.0, otherwise 0.0 to 1.0. [1000000, 1500000, 2000000] this would cause the setting of the following duty cycles: -1 would be a duty cycle of 1 ms, 0 becomes 1.5 ms and 1 becomes 2 ms. This would be used for a motor controller that is capable of reverse.

Other Information

udev Rules

By default the SysFS interface for PWM devices is only writable by root. To allow non-root users to write to the interface, a udev rule must be used to change the permissions on PWM related events. Check to see the group and permissions on the PWM device. If your OS has a group that is allowed to write to the files then ensure your user is in that group. If not use the provided udev rule in the misc directory to change the permissions to allow users from the plugdev group to write to the PWM device.

TODO

• ros2_control plugin