README

Graceful Motion Controller

The graceful motion controller implements a controller based on the works of Jong Jin Park in “A Smooth Control Law for Graceful Motion of Differential Wheeled Mobile Robots in 2D Environment”. In this implementation, a motion_target is set at a distance away from the robot that is exponentially stable to generate a smooth trajectory for the robot to follow.

See its Configuration Guide Page for additional parameter descriptions.

Smooth control law

The smooth control law is a pose-following kinematic control law that generates a smooth and confortable trajectory for the robot to follow. It is Lyapunov-based feedback control law made of three components:

• The egocentric polar coordinates of the motion target (r, phi, delta) with respect to the robot frame.

• A slow subsystem which describes the position of the robot.

• A fast subsystem which describes the steering angle of the robot.

Parameters

Parameter	Description
transform_tolerance	The TF transform tolerance.
motion_target_dist	The lookahead distance to use to find the motion_target point. This distance should be a value around 1.0m but not much farther away. Greater values will cause the robot to generate smoother paths but not necessarily follow the path as closely.
max_robot_pose_search_dist	Maximum integrated distance along the path to bound the search for the closest pose to the robot. This is set by default to the maximum costmap extent, so it shouldn’t be set manually unless there are loops within the local costmap.
k_phi	Ratio of the rate of change in phi to the rate of change in r. Controls the convergence of the slow subsystem. If this value is equal to zero, the controller will behave as a pure waypoint follower. A high value offers extreme scenario of pose-following where theta is reduced much faster than r. Note: This variable is called k1 in earlier versions of the paper.
k_delta	Constant factor applied to the heading error feedback. Controls the convergence of the fast subsystem. The bigger the value, the robot converge faster to the reference heading. Note: This variable is called k2 in earlier versions of the paper.
beta	Constant factor applied to the path curvature. This value must be positive. Determines how fast the velocity drops when the curvature increases.
lambda	Constant factor applied to the path curvature. This value must be greater or equal to 1. Determines the sharpness of the curve: higher lambda implies sharper curves.
v_linear_min	Minimum linear velocity. Units: meters/sec.
v_linear_max	Maximum linear velocity. Units: meters/sec.
v_angular_max	Maximum angular velocity produced by the control law. Units: radians/sec.
slowdown_radius	Radius around the goal pose in which the robot will start to slow down. Units: meters.
initial_rotation	Enable a rotation in place to the goal before starting the path. The control law may generate large sweeping arcs to the goal pose, depending on the initial robot orientation and k_phi, k_delta.
initial_rotation_min_angle	The difference in the path orientation and the starting robot orientation to trigger a rotate in place, if initial_rotation is enabled.
final_rotation	Similar to initial_rotation, the control law can generate large arcs when the goal orientation is not aligned with the path. If this is enabled, the final pose will be ignored and the robot will follow the orientation of he path and will make a final rotation in place to the goal orientation.
rotation_scaling_factor	The scaling factor applied to the rotation in place velocity.
allow_backward	Whether to allow the robot to move backward.

Topics

Topic	Type	Description
transformed_global_plan	nav_msgs/Path	The global plan transformed into the robot frame.
local_plan	nav_msgs/Path	The local plan generated by appliyng iteratively the control law upon a set of motion targets along the global plan.
motion_target	geometry_msgs/PointStamped	The current motion target used by the controller to compute the velocity commands.
slowdown	visualization_msgs/Marker	A flat circle marker of radius slowdown_radius around the motion target.