How Unreal Engine Simulation for Vehicle Dynamics Blockset Works

The vehicle dynamics models run programmable maneuvers in a photorealistic 3D visualization environment. Vehicle Dynamics Blockset™ integrates the 3D simulation environment with Simulink^® so that you can query the world around the vehicle for virtually testing perception, control, and planning algorithms. The Vehicle Dynamics Blockset visualization environment uses the Unreal Engine^® by Epic Games^®.

Understanding how this simulation environment works can help you troubleshoot issues and customize your models.

Note

Simulating models in the 3D visualization environment requires Simulink 3D Animation™.

Communication with 3D Simulation Engine

When you run your algorithms using Simulink 3D Animation, MATLAB^® and Simulink co-simulate the algorithms with the 3D simulation engine through, lock-step mechanism. The lock-step mechanism is a synchronization approach where the simulation progresses in fixed time steps, and the two simulation engines, either MATLAB or Simulink and the 3D simulation engine, run sequentially. One simulation engine waits while the other simulation engine proceeds, so each simulation produces consistent results.

During each simulation step:

MATLAB or Simulink configures the initial settings of the 3D environment, including actor transformations and properties and the custom scene viewpoint. MATLAB or Simulink also determines the configuration parameters of the 3D environment and the actors using the 3D simulation engine feedback.
The co-simulation framework sends data from MATLAB or Simulink to the 3D simulation engine.
The co-simulation framework sends a command signal to the 3D simulation engine indicating the end of data transfer.
The 3D simulation engine executes the simulation. The 3D simulation engine configures the 3D environment and the actors based on the data from MATLAB or Simulink, then provides updated information about the 3D environment and the actors.
The co-simulation framework sends data from the 3D simulation engine to MATLAB or Simulink.
The co-simulation framework sends an acknowledge signal to MATLAB or Simulink, indicating the end of data transfer from the 3D simulation engine.

Limitations

Simulink Online™ does not support co-simulation between Simulink and the Unreal Engine.

Block Execution Order

During simulation, the 3D simulation blocks follow a specific execution order:

The vehicle blocks initialize the vehicles and send their X, Y, and Yaw signal data to the Simulation 3D Scene Configuration block.
The Simulation 3D Scene Configuration block receives the vehicle data and sends it to the sensor blocks.
The sensor blocks receive the vehicle data and use it to accurately locate and visualize the vehicles.

The Priority property of the blocks controls this execution order. To access this property for any block, right-click the block, select Properties, and click the General tab. By default, Simulation 3D Vehicle with Ground Following blocks have a priority of -1, Simulation 3D Scene Configuration blocks have a priority of 0, and sensor blocks have a priority of 1.

If your sensors are not detecting vehicles in the scene, it is possible that the 3D simulation blocks are executing out of order. Try updating the execution order and simulating again. For more details on execution order, see Control and Display Execution Order.

Also be sure that all 3D simulation blocks are located in the same subsystem. Even if the blocks have the correct Priority settings, if they are located in different subsystems, they still might execute out of order.

Simulation Speed

Simulation speed depends on the stop time and the sample time of the simulation. The stop time is the duration of the simulation. The sample time is the time interval at which the output is updated during the simulation. The simulation step is the ratio of stop time to sample time. A smaller step enables more detailed and accurate simulation but can increase computational load and simulation time. It determines how often the state of the simulation is updated, including the positions, velocities, and other properties of objects within the simulation. If you decrease the sample time, the software takes more samples per second, so simulation speed decreases. To set the sample time in Simulink, use the Sample time parameter in the Simulation 3D Scene Configuration block.

If you want to slow down a 3D simulation to investigate system behavior, you can use simulation pacing. The pacing rate determines how fast the simulation runs relative to real time. The pacing rate adjusts the speed at which the simulation progresses. A pacing rate of 1 means the simulation runs in real time, a rate greater than 1 increases the simulation beyond real-time speed, and a rate less than 1 slows down the simulation. You can configure simulation pacing in Simulink using the Simulation Pacing Options.

Simulation speed also depends on model complexity, computational speed, and the speed at which the system can render a scene, which depends on the system configuration. You can use the profile function in MATLAB or the Simulink Profiler in Simulink to analyze the distribution of simulation execution time among the simulation components and see the computational time of each component in the simulation. For more information on profiling, see Profile Your Code to Improve Performance.