Using the Simulator


Within the Webots IDE, there are a few different panels:

  • In the centre of your screen is the 3D simulated view of the arena. It can be useful to change your view into the world using the camera controls.
  • On the left is a tree hierarchy of all elements in this “world”.
  • On the right is a text editor which you can use to develop your code, however we recommend using a dedicated code editor if you can.
  • At the bottom is the console, where output from your robot code will be displayed.
  • At the top are your general controls which include the time controls. Press the centre play button to run the simulation at normal speed.

Webots overview screenshot


In the simulated environment, time advances only at the pace that the simulator is run. The relation between this time and the real passage of time depends on a couple of factors: the speed the simulation is configured to run at and the ability of the computer running the simulation to process it fast enough.

You can configure and observe the speed the simulator is running at from the toolbar in webots:

Here the simulation has run for 13.28 seconds, but is currently paused (the speed multiplier shows 0.00×). You could choose to step a single time increment, run the simulator at real speed (▶), or run the simulator at various faster speeds (▶▶ and ▶▶▶).

Programming for Simulated Time

As time in the simulator is part of the simulation itself, your code must be careful not to block for too long.

For example, if you have used Python before and have used time.time to determine the current time or time.sleep to wait for some duration, you will find these to be unreliable in the simulator.

Instead you must use robot.time and robot.sleep, which are provided as direct replacements of time.time and time.sleep respectively. These methods are also available on the physical kits.

Since the simulator does not simulate the time taken to execute your code, any loop or decision which needs an event to occur must be accompanied by a robot.sleep even if with a small value. If in doubt add a robot.sleep. If you find that the simulator freezes then this indicates that your code is reaching a loop which does not contain any robot.sleep and is expecting time to advance.

Developing your code

You should develop your code outside the folder which contains the world simulation, in a file called This should be within the parent directory you created when setting up the world simulation.

├── competition-simulator-<version>
│   ├── ...
│   └── worlds
│       └── Arena.wbt

If you run the simulation without a file in place, the simulation will copy an example there and run it.

Making changes

In order for your simulated robot to pick up changes to your code you need to save the files you are working on and then reload the simulation. This will also reset the state of the simulated world, allowing your robot to start afresh.

Only your controller code will be present in the competition environment.

Running multiple robots

To test how your robot behaves in each starting zone of the arena, you can copy your robot’s code to run in each corner. Rather than having a single, code can be placed in a number of zone-<zone> folders to run in starting zone <zone>:

├── competition-simulator-<version>
│   ├── ...
│   └── worlds
│       └── Arena.wbt
├── zone-0
│   └──
└── zone-1

This will run two robots in the arena, each with different files. You can run as many or as few zones as you like, in any combination.

Robot logs

When your program runs in the simulator, the output of print(...) statements and any errors which occur are written to a log file next to your code as log-zone-<zone>-<date-time>.txt.

These logs are also available to view live via the console at bottom of the simulator.

In the simulator, all logs are prefixed with the zone that the robot code is running in. For example, a robot running in zone 2 might have logs like:

2| Using /home/srobo/my-robot/ for Zone 2
2| Robot Initialized. Zone: 2, Mode: RobotMode.DEV.
2| Waiting for start signal.
2| Starting
2| I saw a marker!
2| I turned left!

This makes it possible to distinguish which logs are from which robot when they are viewed in Webots console panel.

Simulated robot inputs

By default Webots will show an overlay of what the robot’s camera can see in the top right of the simulation window. This can be useful to understand how the robot sees its world. Double-clicking this overlay will move the overlay to its own window.

Note that the view provided by Webots may include highlights of simulated markers which a real camera would not be able to see. These are filtered out for you by the simulated SR API.

The robot also features distance sensors. To better understand how these sensors perceive the world it is possible to turn on a display of their interactions with the simulated world. Go to the menu ViewOptional Rendering and select the option Show DistanceSensor Rays.