🎮 Nav2 Controllers Extensions#

The pal_nav2_controllers package extends the nav2_controllers package, providing implementations of controller modules which act as plugins of the nav2 base class nav2_core::Controller, also known as Local Planners.

These controller plugins are loaded by the Nav2 Controller Server and are used when calling the /follow_path.

PID Controller#

The PID Controller implements a motion planning algorithm that can be used for path following or for pose tracking.
It implements a simple PID Control
strategy to minimize the distance of the robot from any given path or any give goal.

Apart from being used as a Nav2 Controller Plugin, it can also be used within the Target Navigation framework as a Target Tracker.

It accepts a nav_msgs::msg::Path with just one Pose and reaches it by minimizing at the same time the x, y, and theta error from the given Pose.

Alternatively, this functionality can also be triggered by sending an action goal to the /follow_path.

Send a goal from the command-line#

  ros2 action send_goal /follow_path nav2_msgs/action/FollowPath "path:
     header:
        stamp:
           sec: 0
           nanosec: 0
        frame_id: 'base_footprint'
     poses:
       - header:
           stamp:
             sec: 0
             nanosec: 0
           frame_id: 'base_footprint'
         pose:
           position:
             x: 0.0
             y: 0.0
             z: 0.0
           orientation:
             x: 0.0
             y: 0.0
             z: 0.0
             w: 1.0
     controller_id: 'TrackPose'
     goal_checker_id: ''"

You can set the usage of the PID Controller by setting the controller_id field to 'TrackPose' and change the pose.position and the pose.orientation values to the desired pose. Furthermore, by setting the frame_id to the base_footprint, this works as a Cartesian Controller that moves the robot to a goal expressed relative to its base frame. In this configuration, it can also be used for Instruction-based Navigation (e.g. move three meters forward, then turn right).

Obstacle Avoidance#

This controller implements a basic collision detection algorithm that before sending a velocity commands to the robot, simulates its consequences (e.g. where will the robot be after this command). If a collision is detected, the robot is stopped.

Configuration#

Parameter	Description
`simulate_ahead_time`	The time (in seconds) to project the velocity when looking for collisions.
`max_linear_vel`	Maximum linear velocity (m/s) allowed by the controller.
`max_angular_vel`	Maximum angular velocity (rad/s) allowed by the controller.
`kp_gain`	Proportional Gain of the PID controller.
`ki_gain`	Integral Gain of the PID controller.
`kd_gain`	Derivative Gain of the PID controller.
`antiwindup`	Whether to enable the antiwindup.
`integration_clamp`	Max Integral error allowed.
`step_size`	Sampling size of the path. Lower values correspond to stricter path following.
`motion_model`	Desired motion model used for motion planning. Options are: `'DiffDrive'`, `'OmniDrive'`.
`transform_tolerance`	Time tolerance for data transformations with TF (in seconds).

All the parameters above are dynamic, and can be changed at runtime producing a change in the behavior of the controller.