Sensors#
Note: This tutorial is a continuation from the Moving the robot tutorial.
In this tutorial we will learn how to add sensors to our robot and
to other models in our world. We will use three different sensors:
an IMU sensor, a Contact sensor and a Lidar sensor. We will also
learn how to launch multiple tasks with just one file using gz launch.
You can find the final world of this tutorial showing all these plugins in use here.
You may also find an extensive set of world examples with many possible sensors and actuation capabilities in gz-sim/examples/worlds for individual examples.
The full set of sensors can be found in the gz-sensors library.
If using ROS, you can see demo launches and bridging configuration for these examples here.
Preliminaries#
When adding a plugin to an SDF file which does not currently contain one, the default plugins are not loaded. Before adding a sensor, make sure to add in a couple of logical defaults to your world so that it is possible to continue to use the GZ GUI:
<sdf version='1.9'>
<world name='demo'>
<plugin
filename="gz-sim-physics-system"
name="gz::sim::systems::Physics">
</plugin>
<plugin
filename="gz-sim-scene-broadcaster-system"
name="gz::sim::systems::SceneBroadcaster">
</plugin>
<!-- ... -->
IMU sensor#
The inertial measurement unit (IMU) outputs the orientation of our
robot in quaternions, the angular_velocity in the three axes (X, Y, Z),
and the linear_acceleration in the three axes. Let’s use our
moving_robot.sdf world and modify it. Create a new file
sensor_tutorial.sdf and add the code from moving_robot.sdf to it.
To define the IMU sensor add this code under the <world> tag:
<plugin filename="gz-sim-imu-system"
name="gz::sim::systems::Imu">
</plugin>
This code defines the IMU sensor plugin to be used in our world.
Now we can add the IMU sensor to our robot as follows:
<sensor name="imu_sensor" type="imu">
<always_on>1</always_on>
<update_rate>1</update_rate>
<visualize>true</visualize>
<topic>imu</topic>
</sensor>
The sensor is usually added to one of the links of our model; we added
it under the chassis link.
Let’s describe the tags:
<always_on>if true the sensor will always be updated according to the update rate.<update_rate>the frequency at which the sensor data is generated.<visualize>if true the sensor is visualized in the GUI.<topic>name of the topic on which data is published.
Note: Not all the tags are supported for all sensors yet.
Read data from IMU#
To read the data from the IMU, run the world in one terminal and press the play button:
gz sim sensor_tutorial.sdf
In another terminal, run:
gz topic -e -t /imu
The last command listens to the messages sent over the /imu topic. The IMU data are orientation, angular_velocity and linear_acceleration as described above. It should look like this:
Move your robot forward using the keyboard up key. You should see the sensor values changing.
Contact sensor#
Let’s introduce a different type of sensor, the contact sensor.
You can guess from the name that this sensor gives indication when
it touches (contacts) something else. We will build an obstacle (wall)
and add the contact sensor to it. If the robot hits the obstacle it will
stop, preventing the robot from damaging itself. Let’s first build the
obstacle as follows:
<model name='wall'>
<static>true</static>
<pose>5 0 0 0 0 0</pose><!--pose relative to the world-->
<link name='box'>
<visual name='visual'>
<geometry>
<box>
<size>0.5 10.0 2.0</size>
</box>
</geometry>
<!--let's add color to our link-->
<material>
<ambient>0.0 0.0 1.0 1</ambient>
<diffuse>0.0 0.0 1.0 1</diffuse>
<specular>0.0 0.0 1.0 1</specular>
</material>
</visual>
<collision name='collision'>
<geometry>
<box>
<size>0.5 10.0 2.0</size>
</box>
</geometry>
</collision>
</link>
</model>
It is just a simple model with one link of box shape. You can check the Build your own robot tutorial to learn how to build models.
Now run the world and make sure that the wall appears in the simulation like this:
Let’s add the contact sensor to the wall. As with the IMU sensor, we should first define the Contact sensor by adding the following code:
<plugin filename="gz-sim-contact-system"
name="gz::sim::systems::Contact">
</plugin>
Now we can add the contact sensor to the box link of the wall model:
<sensor name='sensor_contact' type='contact'>
<contact>
<collision>collision</collision>
</contact>
</sensor>
The definition of the <sensor> is straight forward, we just define the name and the type of the sensor. And inside the collision we define the box link collision name which is collision.
We need also to add the TouchPlugin under the wall model as follows:
<plugin filename="gz-sim-touchplugin-system"
name="gz::sim::systems::TouchPlugin">
<target>vehicle_blue</target>
<namespace>wall</namespace>
<time>0.001</time>
<enabled>true</enabled>
</plugin>
The TouchPlugin will publish (send) a message when the wall
has been touched. The tags of the plugin are as follows:
<target>which will be in contact with our wall, in our casevehicle_blue.<namespace>takes the namespace of our topic, so when our robot hits the wall it will send a message to/wall/touchedtopic.
Run the world in one terminal:
gz sim sensor_tutorial.sdf
In another terminal, listen to the /wall/touched topic:
gz topic -e -t /wall/touched
Drive your robot forward to the wall using the keyboard arrow keys. Make sure to start the simulation by hitting the play button, and enable the Key Publisher plugin as well by clicking on the plugins dropdown list (vertical ellipsis), then selecting “Key Publisher”.
When you hit the bump you should see a message data: true on the terminal where you ran the gz topic -e -t /wall/touched.
Now we can use the TriggeredPublisher plugin to make our robot stop when hits the wall as follows:
<plugin filename="gz-sim-triggered-publisher-system"
name="gz::sim::systems::TriggeredPublisher">
<input type="gz.msgs.Boolean" topic="/wall/touched">
<match>data: true</match>
</input>
<output type="gz.msgs.Twist" topic="/cmd_vel">
linear: {x: 0.0}, angular: {z: 0.0}
</output>
</plugin>
As explained in the Moving robot tutorial,
we can publish an output depending on a received input. So when we receive
data: true on the /wall/touched topic we publish
linear: {x: 0.0}, angular: {z: 0.0} to make our robot stop.
Lidar sensor#
We don’t want our robot to touch the wall at all because this may cause some damage, so instead of the contact sensor we can use the Lidar. Lidar is an acronym for “light detection and ranging”. This sensor can help us detect obstacles around the robot. We will use it to measure the distance between our robot and the wall.
First let’s create a frame to fix our lidar to. This should be added inside of the vehicle_blue <model> tag, since the lidar frame is attached to the robot’s chassis:
<frame name="lidar_frame" attached_to='chassis'>
<pose>0.8 0 0.5 0 0 0</pose>
</frame>
Then add this plugin under the <world> tag, to be able to use the lidar sensor:
<plugin
filename="gz-sim-sensors-system"
name="gz::sim::systems::Sensors">
<render_engine>ogre2</render_engine>
</plugin>
Under the chassis link we can add the lidar sensor as follows:
<sensor name='gpu_lidar' type='gpu_lidar'>"
<pose relative_to='lidar_frame'>0 0 0 0 0 0</pose>
<topic>lidar</topic>
<update_rate>10</update_rate>
<ray>
<scan>
<horizontal>
<samples>640</samples>
<resolution>1</resolution>
<min_angle>-1.396263</min_angle>
<max_angle>1.396263</max_angle>
</horizontal>
<vertical>
<samples>1</samples>
<resolution>0.01</resolution>
<min_angle>0</min_angle>
<max_angle>0</max_angle>
</vertical>
</scan>
<range>
<min>0.08</min>
<max>10.0</max>
<resolution>0.01</resolution>
</range>
</ray>
<always_on>1</always_on>
<visualize>true</visualize>
</sensor>
First we defined the
nameandtypeof our sensor, then we defined its<pose>relative to thelidar_frame.In the
<topic>we define the topic on which the lidar data will be published.<update_rate>is the frequency at which thelidardata is generated, in our case10 Hzwhich is equal to0.1 sec.Under the
<horizontal>and<vertical>tags we define the properties of the horizontal and vertical laser rays.<samples>is the number of simulated lidar rays to generate per complete laser sweep cycle.<resolution>: this number is multiplied by samples to determine the number range data points.The
<min_angle>and<max_angle>are the angle range of the generated rays.Under the
<range>we define range properties of each simulated ray<min>and<max>define the minimum and maximum distance for each lidar ray.The
<resolution>tag here defines the linear resolution of each lidar ray.
<always_on>: if true the sensor will always be updated according to the<update_rate>.<visualize>: if true the sensor is visualized in the GUI.
Now run the world and press the play button in the bottom-left corner:
gz sim sensor_tutorial.sdf
Look at the lidar messages on the /lidar topic, specifically the ranges data:
gz topic -e -t /lidar
The lidar message has the following attributes:
message LaserScan
{
Header header = 1;
string frame = 2;
Pose world_pose = 3;
double angle_min = 4;
double angle_max = 5;
double angle_step = 6;
double range_min = 7;
double range_max = 8;
uint32 count = 9;
double vertical_angle_min = 10;
double vertical_angle_max = 11;
double vertical_angle_step = 12;
uint32 vertical_count = 13;
repeated double ranges = 14;
repeated double intensities = 15;
}
Avoid the wall#
Now as we have the lidar on our robot, we can use the ranges data
and make our robot avoid hitting the wall.
To do that, we’ll write a short C++ program that listens to
the sensor data and sends velocity commands to the robot.
This program is called a node. We will build a node that subscribes
to the /lidar topic and reads its data.
Have a look at this tutorial
to learn how to build a publisher and a subscriber node.
You can download the finished node for this demo from here.
The lidar_node#
std::string topic_pub = "/cmd_vel";
gz::transport::Node node;
auto pub = node.Advertise<gz::msgs::Twist>(topic_pub);
We declare a node which will publish to cmd_vel topic. Then advertise our node.
void cb(const gz::msgs::LaserScan &_msg)
{
gz::msgs::Twist data;
bool allMore = true;
for (int i = 0; i < _msg.ranges_size(); i++)
{
if (_msg.ranges(i) < 1.0)
{
allMore = false;
break;
}
}
if (allMore) //if all bigger than one
{
data.mutable_linear()->set_x(0.5);
data.mutable_angular()->set_z(0.0);
}
else
{
data.mutable_linear()->set_x(0.0);
data.mutable_angular()->set_z(0.5);
}
pub.Publish(data);
}
Inside the callback function we check if the range of all rays are bigger than 1.0.
If so we publish a message to our car to move forward. Otherwise we make the robot rotate.
int main(int argc, char **argv)
{
std::string topic_sub = "/lidar";
// Subscribe to a topic by registering a callback.
if (!node.Subscribe(topic_sub, cb))
{
std::cerr << "Error subscribing to topic [" << topic_sub << "]" << std::endl;
return -1;
}
// Zzzzzz.
gz::transport::waitForShutdown();
return 0;
}
Inside the main we subscribe to the lidar topic, and wait until the node is shut down.
Build the node#
Download the CMakeLists.txt, and in the same folder of lidar_node create build/ directory:
mkdir build
cd build
Run cmake and build the code:
cmake ..
make lidar_node
Run the node#
Run the node from terminal 1:
./build/lidar_node
Run the world from terminal 2:
gz sim sensor_tutorial.sdf
Now you can see the robot move forward and as it approaches the wall it starts to turn left until it’s clear and moves forward again (be sure to press the play button in the bottom-left corner to make the robot start moving).
Gazebo launch#
Instead of running two different tasks from two different terminals we can make a launch file which will run the sensor_world and the lidar_node at the same time. Open your text editor and add the following code.
<?xml version='1.0'?>
<gz version='1.0'>
<executable name='sensor-world'>
<command>gz sim sensor_tutorial.sdf</command>
</executable>
<executable name='lidar_node'>
<command>./build/lidar_node</command>
</executable>
</gz>
The launch file is an XML file. We simply define what commands will run under the <executable> tag.
The first command is gz sim sensor_tutorial.sdf which launches the world.
And the second command is ./build/lidar_node which runs the lidar_node.
Save the file as sensor_launch.gzlaunch, and then run it using the following command:
gz launch sensor_launch.gzlaunch
Press the play button to start the simulation. Hurray! Our robot is now moving and avoiding the wall.
To add even more complexity to your simulation, learn how to add actors to a world in the next tutorial.
Video walk-through#
A video walk-through of this tutorial is available from our YouTube channel: Gazebo tutorials: Sensors