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Introduction​

SLAM (simultaneous localization and mapping) is a technique for creating a map of environment and determining robot position at the same time. It is widely used in robotics.

While moving, current measurements and localization are changing, in order to create map it is necessary to merge measurements from previous positions.

For definition of SLAM problem we use given values (1,2) and expected values (3,4):

1. Robot control

u{1:t} = { u1, u2, u3, ..., ut}(1)

2. Observations

z{1:t} = { z1, z2, z3, ..., zt}(2)

3. Environment map

m(3)

4. Robot trajectory

x{1:t} = { x1, x2, x3, ..., xt}(4)

6. Robot trajectory estimates are determined with:

p(x0:t,m|z1:t, u1:t)(5)
## Coordinate frames tracking ##

ROS can help you with keeping track of coordinate frames over time. Package for it is tf2 - the transform library, it comes with a specific message type: tf/Transform and it is always bound to one topic: /tf. Message tf/Transform consist of transformation (translation and rotation) between two coordinate frames, names of both frames and timestamp. Publishing to /tf is done in different way than to any other topic, we will write tf publisher in the example.

Publishing transformation​

We will make a node that subscribe to /pose topic with message type geometry_msgs/PoseStamped and publish transformation between objects mentioned in /pose. This node is required only on ROSbot, Gazebo is publishing necessary tf frames itself.

Begin with headers:

#include <ros/ros.h>
#include <geometry_msgs/PoseStamped.h>
#include <tf/transform_broadcaster.h>

Publisher for transformation:

static tf::TransformBroadcaster br;

Transformation message:

tf::Transform transform;

Quaternion for storing rotation data:

tf::Quaternion q;

Function for handling incoming /PoseStamped messages, extract quaternion parameters from message and put it to quaternion structure, put translation parameters into transform message, put quaternion structure into transform message, publish transform:

void pose_callback(const geometry_msgs::PoseStampedPtr &pose)
{
q.setX(pose->pose.orientation.x);
q.setY(pose->pose.orientation.y);
q.setZ(pose->pose.orientation.z);
q.setW(pose->pose.orientation.w);

transform.setOrigin(tf::Vector3(pose->pose.position.x, pose->pose.position.y, 0.0));
transform.setRotation(q);

br.sendTransform(tf::StampedTransform(transform, ros::Time::now(), "odom", "base_link"));
}

Publishing of transform is done with sendTransform function which parameter is StampedTransform object. This object parameters are:

  • transform - transformation data

  • ros::Time::now() - timestamp for current transformation

  • odom - transformation parent frame - the one that is static

  • base_link - transformation child frame - the one that is transformed

In main function just initialize node and subscribe to /pose topic:

int main(int argc, char **argv)
{
ros::init(argc, argv, "drive_controller");
ros::NodeHandle n("~");
ros::Subscriber pose_sub = n.subscribe("/pose", 1, pose_callback);
ros::Rate loop_rate(100);
while (ros::ok())
{
ros::spinOnce();
loop_rate.sleep();
}
}

Your final file should look like this:

#include <ros/ros.h>
#include <geometry_msgs/PoseStamped.h>
#include <tf/transform_broadcaster.h>

tf::Transform transform;
tf::Quaternion q;

void pose_callback(const geometry_msgs::PoseStampedPtr &pose)
{
static tf::TransformBroadcaster br;
q.setX(pose->pose.orientation.x);
q.setY(pose->pose.orientation.y);
q.setZ(pose->pose.orientation.z);
q.setW(pose->pose.orientation.w);

transform.setOrigin(tf::Vector3(pose->pose.position.x, pose->pose.position.y, 0.0));
transform.setRotation(q);

br.sendTransform(tf::StampedTransform(transform, ros::Time::now(), "odom", "base_link"));
}

int main(int argc, char **argv)
{
ros::init(argc, argv, "drive_controller");
ros::NodeHandle n("~");
ros::Subscriber pose_sub = n.subscribe("/pose", 1, pose_callback);
ros::Rate loop_rate(100);
while (ros::ok())
{
ros::spinOnce();
loop_rate.sleep();
}
}

Save it as drive_controller.cpp.

Now in CMakeLists.txt file find line:

    find_package(catkin REQUIRED COMPONENTS
roscpp
)

and change it to:

    find_package(catkin REQUIRED COMPONENTS
roscpp tf
)

then declare executable:

add_executable(drive_controller_node src/drive_controller.cpp)

And specify libraries to link:

target_link_libraries(drive_controller_node
${catkin_LIBRARIES}
)

Publisher for static transformations​

If we want to publish transformation which is not changing in time, we can use static_transform_publisher from tf package. We will use it for publishing relation between robot base and laser scanner.

We can run it from command line:

$ rosrun tf static_transform_publisher 0 0 0 3.14 0 0 base_link laser 100

Arguments are consecutively:

  • 0 0 0 - x y z axes of translation, values are in meters

  • 3.14 0 0 - z y x axes of rotation, values are in radians

  • base_link - transformation parent frame - the one that is static

  • laser - transformation child frame - the one that is transformed

  • 100 - delay between consecutive messages

You can use it to adjust position of your laser scanner relative to robot. The best would be, if you place scanner in such a way that its rotation axis is coaxial with robot rotation axis and front of laser scanner base should face the same direction as robot front. Most probably your laser scanner will be attached above robot base. To set scanner 10 centimeters above robot you should use:

rosrun tf static_transform_publisher 0 0 0.1 0 0 0 base_link laser 100

And if your scanner is also rotated by 180ΒΊ around z-axis it should be:

rosrun tf static_transform_publisher 0 0 0.1 3.14 0 0 base_link laser 100

Remember that if you have improperly mounted scanner or its position is not set correctly, your map will be generated with errors or it will be not generated at all.

Visualizing transformation with rviz​

You can visualize all transformations that are published in the system. To test it run:

  • CORE2 bridge node - roslaunch rosbot_ekf rosserial_bridge.launch

    Rosbot PRO - roslaunch rosbot_ekf rosserial_bridge.launch serial_port:=/dev/ttyS4 serial_baudrate:=460800

  • drive_controller_node - publisher that you just created

Or instead ot these two start Gazebo:

  • roslaunch rosbot_gazebo maze_world.launch

You will also need rviz and keyboard controller:

  • teleop_twist_keyboard - keyboard controller

  • rviz - visualization tool

You may also add some static_transform_publisher nodes.

Now click Add from object manipulation buttons, in new window select By display type and from the list select Tf. You can also add Pose visualization. You can observe as robot_base frame moves accordingly to robot movement.

image

SLAM implementation in ROS​

To perform accurate and precise SLAM, the best is to use laser scanner and odometry system with high resolution encoders.

Running the laser scanner​

In this example we will use rpLidar laser scanner. Place it on your robot, main rotation axis should pass the centre of robot. Front of the rpLidar should face the same direction as front of the robot.

As a driver for the rpLidar we will use rplidarNode from rplidar_ros package. This node communicate with device and publish its scans to /scan topic with message type sensor_msgs/LaserScan. We do not need more configuration for it now.

To test it you can run only this one node:

rosrun rplidar_ros rplidarNode

For Gazebo you do not need any additional nodes, just start simulator and laser scans will be already published to appropriate topic.

In case there are no scans showing, there may be a problem with laser scanner plugin for Gazebo. Some GPUs, mainly the integrated ones have problems with proper rendering of laser scanner. To solve it, you will have to change the used plugin to CPU based. Go to file rosbot.gazebo located in rosbot_description/src/rosbot_description/urdf, comment out section succeeding <!-- RpLidar using GPU --> line with <!-- to begin comment and --> to end comment and uncomment section succeeding <!-- RpLidar using CPU --> line by deleting <!-- and -->.

You should have /scan topic in your system. You can examine it with rostopic info but better do not try to echo it, it is possible but you will get lots of output that is hard to read. Better method for checking the /scan topic is to use rviz. Run rviz and click Add from object manipulation buttons, in new window select By topic and from the list select /scan. In visualized items list find position Fixed Frame and change it to laser_frame. To improve visibility of scanned shape, you may need to adjust one of visualized object options, set value of Style to Points. You should see many points which resemble shape of obstacles surrounding your robot.

image

Shut down rplidarNode and run it again, but with some other nodes:

  • CORE2 bridge node - roslaunch rosbot_ekf rosserial_bridge.launch

    Rosbot PRO - roslaunch rosbot_ekf rosserial_bridge.launch serial_port:=/dev/ttyS4 serial_baudrate:=460800

  • rplidarNode - driver for rpLidar laser scanner

  • drive_controller_node - publisher that you just created

Or instead of these three, start Gazebo:

  • roslaunch rosbot_gazebo maze_world.launch

You will also need:

  • static_transform_publisher - tf publisher for transformation of laser scanner relative to robot

  • teleop_twist_keyboard - keyboard controller

  • rviz - visualization tool

You can use below launch file:

<launch>

<arg name="rosbot_pro" default="false" />
<arg name="use_gazebo" default="false" />

<!-- Gazebo -->
<group if="$(arg use_gazebo)">
<include file="$(find rosbot_gazebo)/launch/maze_world.launch" />
<include file="$(find rosbot_description)/launch/rosbot_gazebo.launch"/>
<param name="use_sim_time" value="true" />
</group>

<!-- ROSbot 2.0 -->
<group unless="$(arg use_gazebo)">
<include file="$(find rosbot_ekf)/launch/all.launch">
<arg name="rosbot_pro" value="$(arg rosbot_pro)" />
</include>

<include if="$(arg rosbot_pro)" file="$(find rplidar_ros)/launch/rplidar_a3.launch" />
<include unless="$(arg rosbot_pro)" file="$(find rplidar_ros)/launch/rplidar.launch" />
</group>

<node unless="$(arg use_gazebo)" pkg="tf" type="static_transform_publisher" name="laser_broadcaster" args="0 0 0 3.14 0 0 base_link laser 100" />

<node pkg="teleop_twist_keyboard" type="teleop_twist_keyboard.py" name="teleop_twist_keyboard" output="screen"/>

<node pkg="rviz" type="rviz" name="rviz"/>

</launch>

In rviz add Tf and /scan. This time set Fixed Frame to odom.

Try to move around your robot, you should see as laser scans change its shape accordingly to obstacles passed by robot.

image

For building a map and localizing robot relative to it, we will use slam_gmapping node from gmapping package.

This node subscribes /tf topic to obtain robot pose relative to starting point and laser scanner pose relative to robot and also subscribe /scan topic to obtain laser scanner messages. Node publishes to /map topic with message type nav_msgs/OccupancyGrid, this contain the actual map data.

We need to set few parameters:

  • base_frame - name of frame related with robot, in our case it will be base_link

  • odom_frame - name of frame related with starting point, in our case it will be odom

  • delta - map resolution expressed as size of every pixel in meters

You can use below launch file:

<launch>

<arg name="rosbot_pro" default="false" />
<arg name="use_gazebo" default="false" />

<!-- Gazebo -->
<group if="$(arg use_gazebo)">
<include file="$(find rosbot_gazebo)/launch/maze_world.launch" />
<include file="$(find rosbot_description)/launch/rosbot_gazebo.launch"/>
<param name="use_sim_time" value="true" />
</group>

<!-- ROSbot 2.0 -->
<group unless="$(arg use_gazebo)">
<include file="$(find rosbot_ekf)/launch/all.launch">
<arg name="rosbot_pro" value="$(arg rosbot_pro)" />
</include>

<include if="$(arg rosbot_pro)" file="$(find rplidar_ros)/launch/rplidar_a3.launch" />
<include unless="$(arg rosbot_pro)" file="$(find rplidar_ros)/launch/rplidar.launch" />
</group>

<node unless="$(arg use_gazebo)" pkg="tf" type="static_transform_publisher" name="laser_broadcaster" args="0 0 0 3.14 0 0 base_link laser 100" />

<node pkg="teleop_twist_keyboard" type="teleop_twist_keyboard.py" name="teleop_twist_keyboard" output="screen"/>

<node pkg="rviz" type="rviz" name="rviz"/>

<node pkg="gmapping" type="slam_gmapping" name="gmapping">
<param name="base_frame" value="base_link"/>
<param name="odom_frame" value="odom" />
<param name="delta" value="0.1" />
</node>

</launch>

In rviz add Tf and /scan, again open object adding window, select By topic and from the list select /map.

At the beginning there could be no map, you may need to wait few second until it is generated. Starting state should be similar to the one on picture:

image

Now drive your robot around and observe as new parts of map are added, continue until all places are explored. Final map should look like below:

image

Summary​

After completing this tutorial you should be able to publish transformations between various frames, connect laser scanner to the system, set up slam_gmapping node to perform SLAM task and visualize map, robot position and all related frames.


by Łukasz Mitka, Husarion

Do you need any support with completing this tutorial or have any difficulties with software or hardware? Feel free to describe your thoughts on our community forum: https://community.husarion.com/ or to contact with our support: support@husarion.com