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Feedback Control of a ROS-Enabled Robot Over ROS 2

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This example shows you how to use Simulink® to control a simulated robot running in a Gazebo® robot simulator over ROS 2 network.

Introduction

In this example, you will run a model that implements a simple closed-loop proportional controller. The controller receives location information from a simulated robot and sends velocity commands to drive the robot to a specified location. You will adjust some parameters while the model is running and observe the effect on the simulated robot.

The following diagram summarizes the interaction between Simulink and the robot simulator (the arrows in the diagram indicate ROS 2 message transmission). The/odom主题表达位置信息,/cmd_veltopic conveys velocity commands.

Start a Robot Simulator and Configure Simulink

In this task, you will start a ROS-based simulator for a differential-drive robot, start the ROS bridge configure MATLAB® connection with the robot simulator.

  • In the Ubuntu desktop, click theGazebo Emptyicon to start the empty Gazebo world.

  • Click theROS Bridgeicon to start the ROS bridge to relay messages between Simulink ROS 2 node and Turtlebot3 ROS-enabled robot.

  • In MATLAB on your host machine, set the proper domain ID for the ROS 2 network using the'ROS_DOMAIN_ID'environment variable to25to match the robot simulator ROS bridge settings and runros2 topic listto verify that the topics from the robot simulator are visible in MATLAB.

setenv('ROS_DOMAIN_ID','25') ros2topiclist
/clock /gazebo/link_states /gazebo/model_states /gazebo/performance_metrics /imu /joint_states /odom /parameter_events /rosout /rosout_agg /scan /tf

Open Existing Model

After connecting to the ROS 2 network, open the example model.

open_system('robotROS2FeedbackControlExample.slx');

The model implements a proportional controller for a differential-drive mobile robot. On each time step, the algorithm orients the robot toward the desired location and drives it forward. Once the desired location is reached, the algorithm stops the robot.

open_system('robotROS2FeedbackControlExample/Proportional Controller');

Note that there are four tunable parameters in the model (indicated by colored blocks).

  • Desired Position(at top level of model): The desired location in (X,Y) coordinates

  • Distance Threshold: The robot is stopped if it is closer than this distance from the desired location

  • Linear Velocity: The forward linear velocity of the robot

  • Gain: The proportional gain when correcting the robot orientation

The model also has aSimulation Rate Controlblock (at top level of model). This block ensures that the simulation update intervals follow wall-clock elapsed time.

Configure Simulink and Run the Model

In this task, you will configure Simulink to communicate with ROS-enabled robot simulator over ROS 2, run the model and observe the behavior of the robot in the robot simulator.

To configure the network settings for ROS 2.

  • Under theSimulationtab, inPREPARE, selectROS Toolbox > ROS Network.

  • InConfigure ROS Network Addresses, set the ROS 2 Domain ID value to25and set the ROS 2 RMW Implementation asrmw_fastrtps_cpp.

  • ClickOKto apply changes and close the dialog.

To run the model.

  • Position windows on your screen so that you can observe both the Simulink model and the robot simulator.

  • Click the Play button in Simulink to start simulation.

  • While the simulation is running, double-click on theDesired Positionblock and change the Constant value to[2 3]. Observe that the robot changes its heading.

  • While the simulation is running, open theProportional Controllersubsystem and double-click on theLinear Velocity (slider)块。Move the slider to 2. Observe the increase in robot velocity.

  • Click the Stop button in Simulink to stop the simulation.

Observe Rate of Incoming Messages

In this task, you will observe the timing and rate of incoming messages.

  • Click the Play button in Simulink to start simulation.

  • Open the Scope block. Observe that theIsNewoutput of theSubscribeblock is always zero, indicating that no messages are being received for the /odom topic. The horizontal axis of the plot indicates simulation time.

  • Start Gazebo Simulator in ROS network and start ROS Bridge in ROS 2, so that ROS 2 network able receive messages published by Gazebo Simulator.

  • In the Scope display, observe that theIsNewoutput has the value 1 at an approximate rate of 20 times per second, in elapsed wall-clock time.

The synchronization with wall-clock time is due to theSimulation Rate Control块。通常,一个仿真软件模拟执行金宝appn a free-running loop whose speed depends on complexity of the model and computer speed (seeSimulation Loop Phase(Simulink)). TheSimulation Rate Controlblock attempts to regulate Simulink execution so that each update takes 0.02 seconds in wall-clock time when possible (this is equal to the fundamental sample time of the model). See the comments inside the block for more information.

In addition, the Enabled subsystems for the Proportional Controller and the Command Velocity Publisher ensure that the model only reacts to genuinely new messages. If enabled subsystems were not used, the model would repeatedly process the same (most-recently received) message over and over, leading to wasteful processing and redundant publishing of command messages.

Summary

This example showed you how to use Simulink for simple closed-loop control of a simulated robot. It also showed how to use Enabled subsystems to reduce overhead in the ROS 2 network.