optical_flow_ros

ROS 2 package for the PMW3901 optical flow sensor and it's short-range variant PAA5100.

Note: This implementation is a bit over-engineered, as I have been experimenting with ROS 2 managed/lifecycle nodes using Python.

Implementation details

• optical_flow_publisher: This executable uses the pmw3901-python library to access sensor data over SPI. The delta X and Y measurements from the sensor are converted to 2D odometry data, which is published periodically using a timer as an Odometry message to the /odom topic and as a transform broadcast to /tf. The odometry publisher uses the Sensor Data QoS profile as default. This implementation is designed as a lifecycle component and can be run individually as well.

• optical_flow_launch.py: This is the launch file that launches optical_flow_publisher as a lifecycle node, loads its parameters, and then configures and activates it. The lifecycle node is first initialized, then set to 'configure' from the launch file. When the 'inactive' state is reached, the registered event handler activates the node.

Parameters

• timer_period: Timer period in seconds (Default: 0.01)
• sensor_timeout: Sensor timeout in seconds in case of no movement, or sensor failure (Default: 1.0)
• parent_frame: Parent frame for the Odometry message and Transform (Default: odom)
• child_frame: Child frame for the Odometry message and transform (Default: base_link)
• x_init: Initial position in the X axis, in meters (Default: 0.0)
• y_init: Initial position in the Y axis, in meters (Default: 0.0)
• z_height: Height of the sensor from the ground, in meters (Default: 0.025)
• board: Sensor type - pmw3901 or paa5100 (Default: paa5100)
• scaler: Scaling factor, i.e. the sensor value returned for 1 pixel move (Default: 5)
• spi_nr: SPI port number (Default: 0)
• spi_slot: SPI CS pin - front (BCM pin 7 on RPi) or back (BCM pin 8 on RPi) (Default: front)
• rotation: Rotation of the sensor in 90 degree increments - 0, 90, 180, 270 (Default: 270)
• publish_tf: Boolean value to turn transform publisher on/off (Default: true)

How to use

• Connect the sensor breakout (PMW3901 or PAA5100) to the SPI GPIO pins of a Raspberry Pi device:
  • 3-5V to a 3.3V pin
  • CS to BCM 7
  • SCK to BCM 11
  • MOSI to BCM 10
  • MISO to BCM 9
  • INT to BCM 19
  • GND to any ground pin
• Install the pmw3901-python library: sudo pip install pmw3901
• Install the RPi.GPIO library: sudo pip install RPi.GPIO
• Clone this repository in a ROS 2 workspace. Check the sensor_params.yaml file in the config directory, and make any necessary changes.
• Build the package and run the launch file: ros2 launch optical_flow_ros optical_flow_launch.py

Results

This package was tested using a PAA5100JE Near Optical Flow sensor from Pimoroni and ROS 2 Humble on two devices:

• A Raspberry Pi 4 (4GB) running Ubuntu 22.04 with a real-time kernel
• A Raspberry Pi Zero 2 W running Ubuntu 22.04 without any kernel modifications In both cases, the output frequency of 100Hz was achieved.

Challenges

Both these challenges are common for both PMW3901 and PAA5100 boards:

• Secret sauce: the library has a lot of proprietary information which cannot be explained. The manufacturer is apparently very guarded about this information, and the only people who seem to have this information have had to sign an NDA. This includes stuff like the scaling factor to convert from pixel values to distances as well as functionality such as turning the LEDs on and off.

• Off-centered: The sensor breakout from Pimoroni is not centered on the breakout board. Even if this offset is corrected, the aperture of the sensor itself is not centered on the sensor package. Fortunately, Pimoroni has provided a mechanical drawing of the sensor breakout.