Running Simulations

This guide explains how to start, control, and monitor Webots-Simulink simulations.

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Simulation Control Interface

The simulation is controlled through both Webots and Simulink interfaces. Webots handles physics simulation while Simulink manages the control algorithms.

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Starting a Simulation

Step 1: Launch Webots

1. Open your Webots world file (.wbt)
2. Verify the robot controller is set to <extern>
3. Keep Webots in pause mode initially

Step 2: Run MATLAB Controller

Execute the main controller script in MATLAB:

% Initialize the robot
wb_robot_init();

% Load Simulink model
model_name = 'controller_model';
load_system(model_name);
open_system(model_name);

% Start simulation
set_param(model_name, 'SimulationCommand', 'start');

Step 3: Start Webots Simulation

Press the Play button in Webots or use the keyboard shortcut.

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Simulation Controls

Action	Webots Interface	MATLAB Command
Start	Click Play button	set_param(model, 'SimulationCommand', 'start')
Pause	Click Pause button	set_param(model, 'SimulationCommand', 'pause')
Stop	Click Stop button	set_param(model, 'SimulationCommand', 'stop')
Step	Click Step button	set_param(model, 'SimulationCommand', 'step')
Reset	Click Reset button	wb_supervisor_simulation_reset()

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Step-by-Step Simulation

For debugging purposes, you can advance the simulation one step at a time:

% Single step execution
while wb_robot_step(TIME_STEP) ~= -1
    % Read sensors
    position = wb_gps_get_values(gps);
    orientation = wb_inertial_unit_get_roll_pitch_yaw(imu);

    % Process control algorithm
    control_output = controller(position, orientation);

    % Send commands
    wb_motor_set_velocity(motor, control_output);

    % Optional: pause for visualization
    pause(0.01);
end

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Real-Time Monitoring

Using Simulink Scopes

Add Scope blocks to visualize signals in real-time:

1. Drag a Scope block from the Simulink library
2. Connect it to the signal you want to monitor
3. Double-click to open the scope window during simulation

Using MATLAB Command Window

Print values to the command window:

% Inside simulation loop
fprintf('Position: [%.3f, %.3f, %.3f]\n', position(1), position(2), position(3));
fprintf('Orientation: [%.3f, %.3f, %.3f]\n', orientation(1), orientation(2), orientation(3));

Data Logging

Log simulation data to workspace variables:

% Pre-allocate arrays
max_steps = 10000;
position_log = zeros(max_steps, 3);
time_log = zeros(max_steps, 1);

step = 1;
while wb_robot_step(TIME_STEP) ~= -1 && step <= max_steps
    position_log(step, :) = wb_gps_get_values(gps);
    time_log(step) = step * TIME_STEP / 1000;
    step = step + 1;
end

% Plot results
figure;
plot(time_log(1:step-1), position_log(1:step-1, :));
xlabel('Time (s)');
ylabel('Position (m)');
legend('X', 'Y', 'Z');

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Simulation Video

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Simulation Parameters

Time Step Configuration

Parameter	Typical Value	Description
TIME_STEP	16 ms	Webots simulation step
FixedStep	0.016 s	Simulink solver step
StopTime	inf	Continuous operation

Ensure these values match between Webots and Simulink for proper synchronization.

Performance Settings

% Simulink model configuration
set_param(model_name, 'Solver', 'ode4');
set_param(model_name, 'FixedStep', '0.016');
set_param(model_name, 'SimulationMode', 'normal');

% For faster-than-realtime simulation
set_param(model_name, 'SimulationMode', 'accelerator');

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Stopping a Simulation

Graceful Shutdown

% Stop Simulink
set_param(model_name, 'SimulationCommand', 'stop');

% Cleanup Webots connection
wb_robot_cleanup();

% Close model (optional)
close_system(model_name, 0);

Emergency Stop

If the simulation becomes unresponsive:

1. Press Ctrl+C in MATLAB command window
2. Click Stop in Webots
3. If necessary, close Webots from Task Manager

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Common Issues

Issue	Cause	Solution
Simulation doesn't start	Controller not set to <extern>	Check Webots robot settings
Simulation runs slowly	Time step mismatch	Verify TIME_STEP values match
Data not updating	Missing wb_robot_step() call	Ensure step function is called each iteration
Motors not moving	Actuators not enabled	Call wb_motor_set_position() with INFINITY for velocity control

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Next Steps

• Customization: Learn how to modify the bridge for different robots
• Debugging: Troubleshoot simulation issues