Raspberry Pi Sensor Synchronization Tool

This is a simple tool for synchronization of LiDAR and camera sensors using a 1PPS signal, either generated internally or from an attached GNSS unit.

Use case scenarios

• Synchronize LiDAR and multiple cameras using a generated 1PPS and synchronized, phase and frequency adjustable camera trigger waveforms. The LiDAR can also be sent spoofed NMEA sentences.
• Synchronize output waveforms to an input 1PPS from a GNSS receiver.

Getting started

Prerequisites

You will require the following hardware:

• Raspberry Pi (tested on Raspberry Pi 4 Model B)
• Pre-installed Rasbian OS (tested on Raspbian Buster)
• LiDAR sensor that synchronizes to a 1PPS (pulse per second) signal (tested with Velodyne Alpha Prime)
• One or more sensors (i.e. cameras) that are triggered to capture with a TTL rising edge (tested with FLIR Grasshopper)

Installing

This tool is python-based and makes use of the pigpio library.

Start by downloading and installing pigpio on the Raspberry Pi:

wget https://github.com/joan2937/pigpio/archive/master.zip
unzip master.zip
cd pigpio-master
make
sudo make install

Then start the daemon (this must be done after every boot):

sudo pigpiod

Running synchronization

Now you can connect the sensor triggers/PPS to the desired Raspberry Pi GPIO and run the synchronization script found in this repository.

https://github.com/drwnz/rpi_sensor_sync

Configure as required in sync_config.py (see details below) and run:

python run_sync.py

Configuration

Case 1: No GNSS available, synchronization of LiDAR(s) and camera(s)

In this case, the Raspberry Pi will generate the 1PPS for the LiDAR and synchronized camera trigger pulses. Configuration is in sync_config.py. Configure the LiDAR sensor:

• PPS_INPUT_GPIO = -1 since there is no input PPS
• PPS_OUTPUT_GPIO = 2 if the output 1PPS (connected to LiDAR) is connected to GPIO2 (edit to desired GPIO pin)
• SEND_DUMMY_NMEA = True to send a spoofed NMEA message to the LiDAR - time is Raspberry Pi software clock time and GNSS position is a placeholder
• NMEA_DESTINATION_PORT = 10110 is the destination port (10110 is the Velodyne default)
• NMEA_DESTINATION_HOST = '192.168.1.201' is the IP address of the LiDAR sensor

Configure the camera(s) or other triggered sensors:

• TRIGGER1_GPIO = 3 if the camera 1 trigger is connected to GPIO3 (edit to desired GPIO pin)
• TRIGGER1_FREQUENCY = 10 the camera 1 trigger frequency in Hz
• TRIGGER1_PHASE = 0 the camera 1 trigger phase in degrees clockwise (see notes about phase below)

The other camera(s) are configured in the same fashion.

Case 2: GNSS available, synchronization of LiDAR(s) and camera(s)

In this case, the Raspberry Pi will synchronize to the 1PPS of the GNSS, which must be connected on a GPIO pin. The GNSS 1PPS can be connected directly to the LiDAR as well, or the PPS_OUTPUT_GPIO can be used if connected to the Raspberry Pi. Configuration is in sync_config.py. Configure the LiDAR sensor:

• PPS_INPUT_GPIO = 10 if the input 1PPS (from GNSS) is connected to GPIO10
• PPS_OUTPUT_GPIO = 2 if the output 1PPS (connected to LiDAR) is connected to GPIO2 (edit to desired GPIO pin)
• SEND_DUMMY_NMEA = False if the GNSS is connected directly to the LiDAR

Configure the camera(s) or other triggered sensors:

• TRIGGER1_GPIO = 3 if the camera 1 trigger is connected to GPIO3 (edit to desired GPIO pin)
• TRIGGER1_FREQUENCY = 10 the camera 1 trigger frequency in Hz
• TRIGGER1_PHASE = 0 the camera 1 trigger phase in degrees clockwise (see notes about phase below)

The other camera(s) are configured in the same fashion.

Notes on trigger phase

The standard Velodyne phase configuration is alignment of the laser firing along the Velodyne Y-axis (the direction pointing away from the cable entry point) with the 1PPS rising edge. This can be configured in the web UI. If the trigger phase of a camera is set to 0, it will fire exactly as the LiDAR lasers reach the Y-axis direction. The phase of each camera should be set such that the capture time corresponds to when the LiDAR 'sweeps' the direction the camera is facing. For example, for a forward facing camera with a narrow FoV, a phase of 0 is probably about right, as the camera exposure time is typically much shorter than the laser sweep time over the FoV. For a rearward facing camera, the phase should be set to about 180 - again precise phase timing depends on the exposure time and FoV.