Table of Contents

Editor: Controlling Simulations with MQTT

Description

Via the use of MQTT, this guide will walk Nominal Users through the steps required to create a Simulation that has its tick controlled via a remote source. The MQTT Tick Control Nominal Editor demo is referenced in this guide and is a provided example of remote control ticking with MQTT. This guide also assumes a basic knowledge of the Message Queuing Telemetry Transport (MQTT) protocol.

Configuring the Simulation

During the Event Begin Play of the simulation level, an MQTT Client needs to be created and connected. For the MQTT Tick Control demo, a public broker is used to highlight the remote aspect. However, local hosts or private brokers can be used instead.

Note

The Connect function of the MQTT client will need to be updated for the selected external broker if used. The following broker is a free and public broker that can be used for testing purposes and is used in many of the provided Nominal MQTT demo levels.

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Once the Client is connected, it must Subscribe to the Tick Topic. As the Demo is using a public broker, users are advised to add a Topic Prefix to prevent others from controlling the tick of your simulations.

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Ticking the Simulation

The simulation level will only start processing Tick commands once the simulation has been initialized. When a new Tick command has been received, the MQTT Tick Control Demo extracts the command type for the JSON-formatted MQTT message payload.

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An example of the MQTT message payload is shown below, where the command is to start ticking the simulation continuously with a step size of 0.1 and 10 iterations/steps per Unreal frame.

data = {
	"command": "start",
	"continuous": True,
	"step": 0.1,
	"iterations": 10
}
Note

In this example, an external Python script is used to send the JSON data over MQTT to be processed by the level. This is not covered in the guide.

Based on the received command, the MQTT Tick Control demo then extracts the rest of the information in the MQTT JSON Payload and actions the command. The referenced demo is capable of starting, stopping, ticking and resetting the simulation.

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Example Result

A Python script is also provided with the MQTT Tick Control demo that is preconfigured to cycle through several commands. The MQTT Tick Control demo itself is a simulation of a spacecraft in orbit around Jupiter in a sun-pointing mode.

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