BackUp Action

Package: nav2_msgs
Category: Behaviors

Move robot backwards a specified distance with obstacle detection

Message Definitions

Goal Message

Field Type Description
target geometry_msgs/Point Target position or velocity vector
speed float32 Movement speed in meters per second for the specified motion
time_allowance builtin_interfaces/Duration Maximum time limit for completing the action before timing out

Result Message

Field Type Description
NONE uint16 Success status code indicating the action completed without errors
UNKNOWN uint16 Generic error code for unexpected or unclassified failures
TIMEOUT uint16 Error code indicating the action exceeded its maximum allowed time
TF_ERROR uint16 Error code indicating a transform/localization failure
INVALID_INPUT uint16 Error code indicating invalid parameters were provided
COLLISION_AHEAD uint16 Error code indicating an obstacle was detected blocking the path
total_elapsed_time builtin_interfaces/Duration Total time taken to complete the action
error_code uint16 Numeric error code indicating specific failure reason (0=success, various codes for different failure types)
error_msg string Human-readable error message describing what went wrong during action execution

Feedback Message

Field Type Description
distance_traveled float32 Real-time feedback of distance moved from starting position

Usage Examples

Python

import rclpy
from rclpy.node import Node
from rclpy.action import ActionClient
from nav2_msgs.action import BackUp

class Nav2ActionClient(Node):
    def __init__(self):
        super().__init__('nav2_action_client')
        self.action_client = ActionClient(self, BackUp, 'back_up')
        
    def send_goal(self):
        goal_msg = BackUp.Goal()
        goal_msg.target.x = -0.5
        goal_msg.target.y = 0.0
        goal_msg.target.z = 0.0
        goal_msg.speed = 0.25
        goal_msg.time_allowance = Duration(seconds=10.0)
        
        self.action_client.wait_for_server()
        future = self.action_client.send_goal_async(
            goal_msg, feedback_callback=self.feedback_callback)
        return future
        
    def feedback_callback(self, feedback_msg):
        self.get_logger().info(f'Received feedback: {feedback_msg.feedback}')

C++

#include "rclcpp/rclcpp.hpp"
#include "rclcpp_action/rclcpp_action.hpp"
#include "nav2_msgs/action/back_up.hpp"

class Nav2ActionClient : public rclcpp::Node
{
public:
    using BackUpAction = nav2_msgs::action::BackUp;
    using GoalHandle = rclcpp_action::ClientGoalHandle<BackUpAction>;

    Nav2ActionClient() : Node("nav2_action_client")
    {
        action_client_ = rclcpp_action::create_client<BackUpAction>(
            this, "back_up");
    }

    void send_goal()
    {
        auto goal_msg = BackUpAction::Goal();
        goal_msg.target.x = -0.5;
        goal_msg.target.y = 0.0;
        goal_msg.target.z = 0.0;
        goal_msg.speed = 0.25;
        goal_msg.time_allowance = rclcpp::Duration::from_seconds(10.0);
        
        action_client_->wait_for_action_server();
        
        auto send_goal_options = rclcpp_action::Client<BackUpAction>::SendGoalOptions();
        send_goal_options.feedback_callback = 
            std::bind(&Nav2ActionClient::feedback_callback, this, 
                     std::placeholders::_1, std::placeholders::_2);
        
        action_client_->async_send_goal(goal_msg, send_goal_options);
    }

private:
    rclcpp_action::Client<BackUpAction>::SharedPtr action_client_;
    
    void feedback_callback(GoalHandle::SharedPtr, 
                          const std::shared_ptr<const BackUpAction::Feedback> feedback)
    {
        RCLCPP_INFO(this->get_logger(), "Received feedback");
    }
};