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RPLidar C1 Connection Testing

This tutorial covers testing the connection and functionality of a C1 RPLidar sensor with Perseus. The RPLidar provides 2D laser scan data essential for navigation and mapping.

Prerequisites

The following conditions must be met before beginning:

  • The laptop runs a Linux distribution with the latest code from the perseus-v2 Github repository: 
    git checkout main
git pull
  • RPLidar C1 is physically connected via USB
  • Required fonts are installed for rendering the terminal with the powerlevel10k theme (available in /perseus-v2/fonts)

Summary

  1. Verify hardware connection
  2. Check device permissions
  3. Launch RPLidar driver
  4. Verify scan data
  5. Test with visualization

Hardware Connection Verification

Before launching the RPLidar software, verify the hardware connection:

Check USB Devices

lsusb

Look for the RPLidar device in the output. You should see a USB-to-serial adapter.

Check Serial Devices

ls -la /dev/ttyUSB*
ls -la /dev/ttyACM*

The RPLidar typically appears as /dev/ttyUSB0 or similar.

Check Device Permissions

ls -la /dev/ttyUSB0

If you don't have read/write permissions, you can temporarily fix them:

sudo chmod 666 /dev/ttyUSB0

For a permanent solution, add your user to the dialout group:

sudo usermod -a -G dialout $USER

Then log out and log back in for the changes to take effect. To apply the group change immediately without logging out, use:

newgrp dialout

Alternatively, create a udev rule for persistent permissions:

echo 'KERNEL=="ttyUSB*", ATTRS{idVendor}=="10c4", ATTRS{idProduct}=="ea60", MODE="0666"' | sudo tee /etc/udev/rules.d/99-rplidar.rules
sudo udevadm control --reload-rules && sudo udevadm trigger

:::{warning} MODE="0666" grants read/write permissions to all users on the system (owner, group, and others). This is convenient but may be a security concern in multi-user environments. For better security, use MODE="0660", GROUP="dialout" instead and ensure your user is in the dialout group. :::

RPLidar Driver Testing

Basic Driver Launch

Launch the RPLidar driver with default settings:

nix develop --command ros2 launch rplidar_ros rplidar_c1_launch.py

This starts the RPLidar node with these default parameters:

  • Serial port: /dev/ttyUSB0
  • Baudrate: 460800
  • Frame ID: laser
  • Scan mode: Standard

Custom Parameters

If your RPLidar uses a different port or requires custom settings:

nix develop --command ros2 launch rplidar_ros rplidar_c1_launch.py serial_port:=/dev/ttyUSB1 serial_baudrate:=460800

Data Verification

After launching the driver, verify that scan data is being published:

Check Active Nodes

ros2 node list

You should see /rplidar_node in the output.

Monitor Scan Data

ros2 topic echo /scan

This displays real-time laser scan data. Press Ctrl+C to stop.

Check Topic Information

ros2 topic info /scan
ros2 topic hz /scan

The scan frequency should be around 10-15 Hz for normal operation.

Visualization Testing

For visual confirmation of scan data, launch with RViz:

nix develop --command ros2 launch rplidar_ros view_rplidar_c1_launch.py

This launches both the RPLidar driver and RViz. In RViz:

  1. You should see a circular pattern of laser points around the origin
  2. The laser data updates in real-time as objects move around the sensor
  3. The coordinate frame should be labeled as laser

Motor Control

The RPLidar driver provides services to control the sensor's motor:

Stop Motor

ros2 service call /stop_motor std_srvs/srv/Empty

Start Motor

ros2 service call /start_motor std_srvs/srv/Empty

Troubleshooting

No Scan Data

  1. Verify USB connection and device permissions
  2. Check that the motor is spinning (you should hear it)
  3. Ensure no other processes are using the serial port
  4. Try unplugging and reconnecting the USB cable

Permission Denied Errors

sudo usermod -a -G dialout $USER

Then log out and log back in for the changes to take effect.

Wrong Serial Port

If /dev/ttyUSB0 doesn't exist, check for other devices:

dmesg | grep tty

This shows recent serial device connections.

Integration with Perseus

The RPLidar data integrates with Perseus navigation through:

  • Topic remapping: /scan data is used by navigation algorithms
  • Frame transforms: The laser frame is linked to the robot's base frame
  • Sensor fusion: Combined with other sensors for autonomous navigation