Using the simulator

The Clover simulation environment allows the user to test their code without any risk of equipment damage. Additionally, the virtual machine-based environment has additional (non-ROS) services that are present on a real drone, like Monkey web server.

Running the simulation

After setting up the simulation packages or importing and running the VM, you can use roslaunch to start Gazebo simulation:

# Be sure to activate your workspace first
source ~/catkin_ws/devel/setup.bash
roslaunch clover_simulation simulator.launch

Alternatively, if you are using the VM, just double-click on the Gazebo PX4 icon on the desktop.

This will launch Gazebo server and client, the PX4 SITL binary and Clover nodes. The terminal in which the command was run will display diagnostic messages from the nodes and PX4, and will accept input for the PX4 command interpreter:

Gazebo simulation screenshot

You can use QGroundControl to configure the simulated drone parameters, plan missions (if GPS is simulated) and control the drone using a joystick:

Gazebo and QGC

You can also use our simplified OFFBOARD control to control the drone, and traditional ROS GUI utilities like rviz and rqt to analyze the drone state:

Gazebo and RQT

Configuring the simulation

The simulation can be configured by passing additional arguments to the roslaunch command or by changing the ~/catkin_ws/src/clover/clover_simulation/launch/simulator.launch file. Nodes that provide ArUco detection, optical flow calculation and other services can be configured by changing their respective .launch files, just like on a real drone.

Changing the drone parameters

vscode with simulator.launch open

You can enable or disable some of the drone sensors by changing parameters in the simulator.launch file. For example, in order to enable GPS, set the gps argument to true:

    <arg name="gps" value="true"/>

Note that this will simply enable the sensor, you will have to change the PX4 estimator parameters to enable GPS fusion.

If you wish to add additional sensors or change their placement, you will have to change the drone description. The description file is located in ~/catkin_ws/src/clover/clover_description/urdf/clover/clover4.xacro, and uses the xacro format to build URDF description.

Changing the default world

Gazebo plugins for the drone currently require the real_time_update_rate world parameter to be 250, and max_step_size to be 0.004. Using other values will not work. Consider using ~/catkin_ws/src/clover/clover_simulation/resources/worlds/clover.world as a base.

Performance suggestions

Gazebo simulation environment is resource-intensive, and requires a fast CPU and a decent GPU for real-time execution. However, the simulation may still work on less powerful systems at slower-than-realtime rate. Below are some suggestions for running Gazebo on hardware that does not allow realtime execution.

Use throttling_camera plugin

By default, Gazebo does not slow simulation down for visual sensors. This can be remedied by using the throttling_camera plugin from clover_simulation.

You can enable it for the drone by changing the maintain_camera_rate argument to true in clover_description/launch/spawn_drone.launch:

    <!-- Slow simulation down to maintain camera rate -->
    <arg name="maintain_camera_rate" default="true"/>

The plugin will collect publishing rate statistics and slow the simulation down so that the camera publishing rate is no less than requested. However, large slowdowns may negatively affect non-ROS software that connects to PX4. If your simulation is being slowed down to values lower than 0.5 of realtime, consider using the next suggestion.

Set simulation speed

Since v1.9 the PX4 SITL setup supports setting the simulation speed by setting the PX4_SIM_SPEED_FACTOR environment variable. Its value is picked up by PX4 startup scripts, which in turn reconfigure it to expect a certain speedup/slowdown.

You should set its value to the actual real time factor that you get with throttling_camera. The real time factor may be found in the Gazebo GUI window at the bottom:

Real Time Factor

In this example you should set PX4_SIM_SPEED_FACTOR to 0.42 when launching the simulation:

PX4_SIM_SPEED_FACTOR=0.42 roslaunch clover_simulation simulator.launch

If you are using the VM, it may be convenient to put the value in the Gazebo desktop shortcut. Right-click on the Gazebo icon, select "Properties..." and add PX4_SIM_SPEED_FACTOR=0.42 to the Command field as follows:
Gazebo PX4 launcher command

Allocate more resources to the VM

The virtual machine may benefit from several CPU cores, especially if the cores are not very performant. In our tests, a four-core machine with only a single core allocated to the VM was unable to run the simulation, with constant interface freezes and dropped ROS messages. The same machine with all four cores available to the VM was able to run the simulation at 0.25 real-time speed.

Do note that you should not allocate more resources than you have on your host hardware.

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