Path Follower

Overview

The role of the path follower is to generate the correct airspeed, course, and altitude commands for the autopilot such that the aircraft follows the paths published by path_manager.

More information on the path_follower can be found in Small Unmanned Aircraft: Theory and Practice by Dr. Randal Beard and Dr. Tim McLain.

Path Follower Base

The path follower base contains all the ROS2 interfaces required for the path follower. A list of these interfaces is below.

The path_follower_base::follow method is a virtual method that should be implemented by a derived class. The path_follower_base publishes the controller commands calculated by the implementation of path_follower_base::follow.

List of ROS2 Interfaces

ROS2 Interface	Topic or Service	Explanation	Message or Service Type
vehicle_state_sub_	/estimated_state	Subscribes to the estimated aircraft state	rosplane_msgs::msg::State
current_path_sub_	/current_path	Subscribes to the current path messages published by path_manager	rosplane_msgs::msg::CurrentPath
controller_commands_pub_	/controller_commands	Publishes control commands to the autopilot	rosplane_msgs::msg::ControllerCommand
update_timer_	--	ROS2 timer that controls how frequently controller_commands_pub_ publishes	--

Interface with the Autopilot

The path_follower node interfaces with the MAV autopilot by publishing to the /controller_commands topic. The /controller_commands topic contains the following information calculated by the path_follower:

Member	Description	Type / units	Required
header	Standard message header that contains a valid timestamp	std_msgs::msg::Header	Yes
va_c	Commanded airspeed	double, m/s	Yes
h_c	Commanded altitude	double, m	Yes
chi_c	Commanded course	double, rad	Yes
phi_ff	Feedforward command (for orbits)	double, rad	No

Path Follower Example

The path_follower_example class contains the implementation of the path_follower_base::follow method. This method contains the control command computations for straight line and orbit-type paths as outlined in Small Unmanned Aircraft: Theory and Practice.

Parameters

See the Parameter Management page for more details on how parameter management works.

List of Parameters

Parameter	Explanation	Type / Units	Range
chi_infty	At infinity, the angle at which the aircraft will approach the desired line	double (rad)	$\frac{\pi}{2} \geq \chi^{\infty} > 0.0$
k_path	Constant that determines how quickly commanded course transitions from $\chi^{\infty}$ to 0 during line following	double	$> 0.0$
k_orbit	Constant that determines how quickly commanded course transitions from $\lambda \frac{\pi}{2}$ to 0 during an orbit	double	$> 0.0$

Since there are two different types of lines generated by the path_manager (straight lines and orbits), the path_follower uses different parameters to calculate the control output for each path type.

For line following, path_follower uses

• chi_infty and
• k_path to compute the control commands.

As described in Small Unmanned Aircraft: Theory and Practice, these two parameters define a vector field for the commanded course. The chi_infty parameter describes the angle at which the aircratft will approach a target line if the aircraft is infinitely far from the target line. For example, if chi_infty is set to $\frac{\pi}{2}$, then the aircraft will approach the target line at an angle perpindicular to the line when the aircraft is far away.

The k_path parameter defines how quickly the vectors at chi_infty "smooth" into the target line. When k_path is large, the transition will be abrupt, and when k_path is small, the transition will be smoother.

During orbit following, path_follower uses

• k_orbit to compute the control commands.

If the aircraft is very far away from the orbit, it will align itself so it flies directly at the center of the orbit. The k_orbit parameter is similar to the k_path parameter in that it determines how quickly the commanded angle of the aircraft will change from directly toward the center to the orbit direction.

Note

When tuning, make sure you are changing the correct gains for the type of line (straight or orbit) the aircraft is tracking. chi_infty and k_path will not affect the orbit performance!

Modifying the Path Follower

Changes to any of the ROS2 interface should be done in the path_follower_base.

Changes to how path_follower computes control commands to follow paths given by the path_manager should be done in the path_manager_example.