1. Obtaining RPLidar Data

1.1. Install and compile the RPLidar ROS package

The installation process is described here: Install the Slamtec RPLidar ROS package

This process clones the RPLidar ROS package git repository into the catkin_ws/src/ of your git repository. Hence the nodes of the RPLidar ROS package are available after a catkin_make, i.e.,:

cd ~/asclinic-system/catkin_ws
catkin_make

1.2. Launch the RPLidar node

The RPLidar ROS package provide a node named rplidarNode that reads data from the RPLidar device and publishes it to the topic /scan. As the RPLidar ROS package is part of your catkin workspace, you can launch an rplidarNode in the same way you would launch any other node. The following is a complete launch file for launching the rplidarNode into the group namespace /asc:

<launch>

  <group ns="asc">
    <!-- LAUNCH A RPLIDAR NODE -->
    <!-- Note: for model A1/A2 use baudrate 115200 -->
    <!--       for model A3    use baudrate 256000 -->
    <node
      pkg    = "rplidar_ros"
      name   = "rplidarNode"
      output = "screen"
      type   = "rplidarNode"
      >
      <param name="serial_port" type="string" value="/dev/rplidar"/>
      <param name="serial_baudrate" type="int" value="115200"/>
      <param name="frame_id" type="string" value="laser"/>
      <param name="inverted" type="bool" value="false"/>
      <param name="angle_compensate" type="bool" value="true"/>
    </node>
  </group>

</launch>

For example, you can place this launch file at the location:

~/asclinic-system/catkin_ws/src/asclinic_pkg/launch/rplidar.launch

And then launch it with the following command:

roslaunch asclinic_pkg rplidar.launch

The following are some important details about launching an rplidarNode in this way:

  • As launch file uses the group namespace /asc, the data published to by the rplidarNode is available on the topic /asc/scan

  • You can add the <node>...</node> part of the launch file to any other launch file you have.

  • If you need to know some detail about what the rplidarNode is doing, then simply look at the source code located at:
    catkin_ws/src/rplidar_ros/src/node.cpp
    This one file is the whole not, it is a reasonable length, and it is reasonably easy to parse. You can also view the rplidarNode code on the RPLidar git hub.

Note

For more information about ROS launch file, see the Run, launch, and interrogate nodes page.

1.3. Format of the RPLidar scan data

The rplidarNode publishes one message of type sensor_msgs::LaserScan for every complete rotation of the Lidar device. The definition of the LaserScan message type shows that it contains the following properties:

  • angle_min - this is the angle at the start of the scan, i.e., the angle of the first measurement in the ranges property. In units of [radians].

  • angle_max - this is the angle at the end of the scan, i.e., the angle of the last measurement in the ranges property. In units of [radians].

  • angle_increment - this is the angular distance between measurements. In units of [radians].

  • time_increment - this is the time between measurements. In units of [seconds].

  • scan_time - this is the time between scans, i.e., the time between the first measurement of this scan and the first measurement of the previous scan. In units of [seconds].

  • range_min - this is the minimum range value. In units of [meters].

  • range_max - this is the maximum range value. In units of [meters].

  • ranges - this is an array property that contains the range data of each measurement. The length of this array should be number of time that angle_increment fits in between angle_min and angle_max, i.e., the measurement ranges[0] was taken at angle_min, and the measurements ranges[i] were taken at (angle_min + i*angle_increment)

Note

Values in the ranges array should be discarded if they are less than the range_min or greater than the range_max property.

1.4. Subscribe to the RPLidar scan data

Step 1. include the header that defines the message type.

For C++: the LaserScan include is:

#include "sensor_msgs/LaserScan.h"

For Python: the LaserScan import is:

from sensor_msgs.msg import LaserScan

Step 2. Add a ROS subscriber to the /scan topic.

For C++: the topic subscriber is:

// Initialise a node handle to the group namespace
ros::NodeHandle nh_for_asc_group("/asc);
// Initialise a subscriber to the RPLidar scan
ros::Subscriber rplidar_scan_subscriber = nh_for_asc_group.subscribe("scan", 10, laserScanSubscriberCallback);

For Python: the topic subscriber is:

# Initialise a subscriber to the RPLidar scan
rospy.Subscriber("/asc"+"/scan", LaserScan, self.laserScanSubscriberCallback)

Step 3. Implement the subscriber callback to process the scan data.

For C++: the subscriber callback is:

// Respond to subscriber receiving a message
void laserScanSubscriberCallback(const sensor_msgs::LaserScan& msg)
{
  ROS_INFO_STREAM("Message received with angle_min = " << msg.angle_min << " [rad], angle_max = " << msg.angle_max << " [rad], range_min = " << msg.range_min << " [m], range_max = " << msg.range_max << " [m]");

  // Now process the msg.ranges data to
  // interpret the robot's surroundings
}

For Python: the subscriber callback is:

# Respond to subscriber receiving a message
def laserScanSubscriberCallback(self, msg):
    rospy.loginfo("Message receieved with angle_min = " + str(msg.angle_min) + " [rad], angle_max = " + str(msg.angle_max) + " [rad], range_min = " + str(msg.range_min) + " [m], range_max = " + str(msg.range_max) + " [m]")

    # Now process the msg.ranges data to
    # interpret the robot's surroundings

Step 4. Ensure that the sensor_msgs are made available via the pacakge.xml and CMakeLists.txt at the path:

~/asclinic-system/catkin_ws/src/asclinic_pkg/

Simply copy the syntax and directives used for adding the std_msgs to these two files.

For the pacakge.xml, add the following lines:

<build_depend>sensor_msgs</build_depend>
<build_export_depend>sensor_msgs</build_export_depend>
<exec_depend>sensor_msgs</exec_depend>

For the CMakeLists.txt, add the lines that are highlighted in the following:

find_package(catkin REQUIRED COMPONENTS
  message_generation
  roscpp
  rospy
  std_msgs
  geometry_msgs
  sensor_msgs
  genmsg
  roslib
)

## Generate added messages and services with any dependencies listed here
generate_messages(
  DEPENDENCIES
  std_msgs
  geometry_msgs
  sensor_msgs
)