twist_mux

Intro

twist_mux is a ROS package that handles multiplexing multiple geometry_msgs/Twist messages into a single output stream. This is particularly useful in robotic systems where multiple nodes or sources may try to control a robot’s movement simultaneously, such as:

1. Autonomous Navigation Nodes: Commands from an autonomous navigation stack.
2. Teleoperation Nodes: Commands from a joystick or remote control interface.
3. Safety Controllers: Commands to stop or override movement in critical situations.

The twist_mux package allows these multiple command sources to be prioritized, ensuring that only the highest-priority commands are sent to the robot’s actuators.

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Core Features:

1. Input Prioritization: Defines a priority system for input topics so that higher-priority commands override lower-priority ones.
2. Topic Subscription: Listens to multiple cmd_vel (velocity command) topics as inputs.
3. Output Publishing: Publishes the selected cmd_vel message to the robot’s motor controller.

• install tmux-twist package

sudo apt install ros-humble-twist-mux

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Example Use Case:

Suppose a robot is controlled both manually using a joystick (cmd_vel_teleop) and autonomously through a navigation stack (cmd_vel_nav). To ensure smooth operation:

• If the joystick is in use, it overrides the navigation commands.
• If no joystick input is detected, the robot switches back to the navigation stack.

With twist_mux, you can configure this behavior using a YAML configuration file specifying priorities and timeouts.

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Typical Configuration:

A YAML file might look like this:

subscribers:
  - name: teleop
    topic: cmd_vel_teleop
    timeout: 0.5
    priority: 2
  - name: navigation
    topic: cmd_vel_nav
    timeout: 1.0
    priority: 1
publisher:
  topic: cmd_vel

Here:

• teleop has a higher priority (2) and a timeout of 0.5 seconds.
• navigation has a lower priority (1) and a timeout of 1 second.
• The selected command is published on the cmd_vel topic.

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Why is it Needed?

Without twist_mux, multiple nodes publishing to the same cmd_vel topic could create conflicts, causing erratic robot behavior. By multiplexing, twist_mux ensures orderly control transitions and prevents command conflicts.