ros2 rpi-ppm-input

This node reads a PPM signal from a Raspberry Pi GPIO pin using the RPi.GPIO library. PPM signals are often output by RC type radio receivers. I have a Turnigy 9x radio with a PCM to PPM converter and also a Radiolink R8FM. The R8FM makes a nice simple controller for robots.

• PCM signals have a wire for each channel and encode the value as a pulse between 1000 and 2000 microseconds. These mode of signaling is NOT supported by this node.
• PPM signals are similar to PCM, but the 8-12 channels are encoded on one wire where each channel is output serially one after the other with a missing pulse indicating the start/end of the channel grouping. This single-wire type of RC signaling is what this node reads.

Raw PPM mode

By default this node will publish Int16MultiArray messages (array of 16bit ints) to the /input/ppm topic. Each frame of PPM data will publish a new message and the size of the message int array will match the number of channels read in the last PPM frame. This means the number of elements in the array may change.

Joystick mode

If 'mode' parameter is set to 'joy' then this node will publish as sensor_msgs/Joy. This requires PPM channels are mapped to axes and buttons.

Axes

The axis will be map PPM values of 1000 to 2000 to float ranges -1.0 to +1.0 but how these are mapped exactly depends on the axis type. You can specify per-axis parameters using the parameter namespace axis-# with sub-parameters of type, deadband, center, zero_point and min_throttle. All axis numeric parameters are in PPM raw units (i.e. between 1000-2000)

type=midstick

This is your typical axis that snaps to center and will map to -1.0 to 1.0. You can optionally provide a deadband (in PPM int counts, such as 30)

type=throttle

This mode acts like a throttle where full down is 0.0, there is some safety margin (zero_point=1200) before throttle is activated (outputs non-zero) and the first throttle value starts at a minimum amount of power (min_throttle=0).

type=trim

This axis assumes a trim pot and has a linear output of 0.0 to 1.0.

Buttons

Buttons are channels mapped to simple integer switch positions 0, 1, 2, ... N where N is the number of poles. Most switches are either 2 or 3 poles (2 or 3 way). By default switches are 2-way but you can easily specify which buttons are 3-way by listing them in an array '3-way'.

Default Joystick Configurations

Look in the config directory to see if your transmitter already has a configuration. If not you can at least base your configuration on one of the existing params file there. Using the standard configs is very easy in a launch file. Just specify the config name (ex. r8fm) as the one and only argument.

    joystick = Node(
        package="rpi_ppm_input",
        executable="ppm_input",
        arguments=["r8fm"],
        output="screen")

Sample Params File

This example params.yml is for the R8FM tramsmitter. Using standard configurations is recommended but if you create your own for your transmitter please consider including it by submitting a PR or create an Issue with the config pasted in the body.

# Default channel settings for R8FM
# 0 - Roll (right)
# 1 - Pitch (right)
# 2 - Throttle (left)
# 3 - Yaw (left)
# 4 - Right 3-way
# 5 - Right button
# 6 - Left 3-way
# 7 - Left trim POT

ppm_input:
    ros__parameters:
        mode: joy
        # frame_id: preview
        # pin: <rpi gpio pin number, defaults to 7>
        joystick:
            # indexes channel IDs in the received PPM frame
            axis: [0,1,2,3,7]
            buttons: [4,5,6]
            # indicate some buttons are 3-way (have 3 poles)
            3-way: [4,6]
        # per-axis configuration
        axis-0:
            type: midstick
            deadband: 30
        axis-1:
            type: midstick
            deadband: 30
        axis-2:
            type: throttle
        axis-3:
            type: midstick
            deadband: 30
        axis-4:
            type: trim

Raspberry PI 3.3v GPIO

The RPi pins are NOT 5v tolerant! Most RC transmitters will output 5v TTL level. So be sure to protect your RPi inputs with a 5v to 3.3v level converter. There are many options available.

I used this simple resistor and diode circuit to protect my RPi and it is working but I make no warranties about it. I got this advice from tansi.org who got it off the from someone on the RPi community. This circuit reduces the 4v down to 4.2 volt which is apparently within the RPi pin input limits.

Joystick Remapper (Related software)

Although this node already supports remapping of axis and buttons you may also want to look at the joystick remapper node which has similar features.