cmd_vel mux

About

A ROS 2 package for selecting among a number of incoming geometry_msg/msg/Twist messages, choosing the highest priority one to republish on the output topic. It will automatically dislodge streams that are lower priority or that stop publishing for any reason. The stream currently in use is published on the "active" topic.

Published Topics

• /active (std_msgs/msg/String) - A latched topic. Publishes the "name" field of the currently active geometry_msgs/msg/Twist stream (see Parameters below), or "idle" if nothing is being a published.
• /cmd_vel (geometry_msgs/msg/Twist) - The current geometry_msgs/msg/Twist message. The values from the current highest-priority geometry_msgs/msg/Twist are republished here without change.

Subscribed Topics

• /any (geometry_msgs/msg/Twist) - For each named input (see Parameters below), a topic of type geometry_msgs/msg/Twist is subscribed to based on the topic parameter. The data from the highest-priority of these inputs is republished on the output /cmd_vel.

Parameters

The cmd_vel_mux node uses a dictionary to hold the subscribers, with name being the key of each element:

• name (string) - The "name" of each of the input geometry_msgs/msg/Twist topics. This is what will be published on the active topic when the currently active publisher changes.
• topic (string) - The "topic" corresponding to each of the input geometry_msgs/msg/Twist topics. This is what will be subscribed to.
• timeout (double) - The "timeout" of each of the input geometry_msgs/msg/Twist topics. If no data is received on the input topic for this amount of time, the input will be automatically disabled.
• priority (integer) - The "priority" of each of the input geometry_msgs/msg/Twist topics. The higher the number, the higher the priority. Higher priority topics will dislodge lower priority topics and start publishing to the output topic automatically.
• short_desc (string) - The "short description" of each of the input geometry_msgs/msg/Twist topics. This is informational only.

See config/cmd_vel_mux_params.yaml for an example of how to set these up.

Changing values

The dictionary translates to parameters in the form of subscribers. + name + topic/timeout/priority/short_desc. It can be changed on the fly using ros2 param set command, followed with the parameter in the form mentioned above and the value.

Adding subscriber

To add a new subscriber, call the service /cmd_vel_mux/set_parameters_atomically with a rcl_interfaces/srv/SetParametersAtomically message with a parameters value equal to an array of 4 rcl_interfaces/msg/Parameter messages, containing the new subscriber topic, timeout, priority and short_desc. All 4 need to be present for a new subscriber to be added.

Ex.1 for testing:

ros2 service call /cmd_vel_mux/set_parameters_atomically rcl_interfaces/srv/SetParametersAtomically "{parameters: [{name : subscribers.new_input.topic, value: {type: 4, string_value: /input/new_input}}, {name : subscribers.new_input.priority, value: {type: 2, integer_value: 20}}, {name : subscribers.new_input.timeout, value: {type: 3, double_value: 0.2}}, {name : subscribers.new_input.short_desc, value: {type: 4, string_value: 'test input'}}]}"

sets new_input subscriber listening at topic /input/new_input

Deleting subscriber

To delete a subscriber, call the service /cmd_vel_mux/set_parameters_atomically with a rcl_interfaces/srv/SetParametersAtomically message with a parameters value equal to an array of 4 rcl_interfaces/msg/Parameter messages, containing the new subscriber topic,timeout,priority and short_desc with the type key inside the rcl/parameter/ParameterValue being set to 0 (PARAMETER_NOT_SET).

Ex.2 for testing:

ros2 service call /cmd_vel_mux/set_parameters_atomically rcl_interfaces/srv/SetParametersAtomically "{parameters: [{name : subscribers.new_input.topic, value: {type: 0}}, {name : subscribers.new_input.priority, value: {type: 0}}, {name : subscribers.new_input.timeout, value: {type: 0}}, {name : subscribers.new_input.short_desc, value: {type: 0}}]}"

deletes new_input subscriber added at ex.1