Pure Pursuit

Linear and angular velocity control commands

• Library:
• Robotics System Toolbox / Mobile Robot Algorithms

• Description

The Pure Pursuit block computes linear and angular velocity commands for following a path using a set of waypoints and the current pose of a differential drive vehicle. The block takes updated poses to update velocity commands for the vehicle to follow a path along a desired set of waypoints. Use the Max angular velocity and Desired linear velocity parameters to update the velocities based on the performance of the vehicle.

The Lookahead distance parameter computes a look-ahead point on the path, which is an instantaneous local goal for the vehicle. The angular velocity command is computed based on this point. Changing Lookahead distance has a significant impact on the performance of the algorithm. A higher look-ahead distance results in a smoother trajectory for the vehicle, but can cause the vehicle to cut corners along the path. Too low of a look-ahead distance can result in oscillations in tracking the path, causing unstable behavior. For more information on the pure pursuit algorithm, see Pure Pursuit Controller.

Ports

Input

expand all

Current vehicle pose, specified as an [x y theta] vector, which corresponds to the x-y position and orientation angle, theta. Positive angles are measured counterclockwise from the positive x-axis.

Waypoints, specified as an n-by-2 array of [x y] pairs, where n is the number of waypoints. You can generate the waypoints using path planners like mobileRobotPRM (Robotics System Toolbox) or specify them as an array in Simulink®.

Output

expand all

Linear velocity, specified as a scalar in meters per second.

Data Types: double

Angular velocity, specified as a scalar in radians per second.

Data Types: double

Target direction for the vehicle, specified as a scalar in radians. The forward direction of the vehicle is considered zero radians, with positive angles measured counterclockwise. This output can be used as the input to the TargetDir port for the Vector Field Histogram block.

Dependencies

To enable this port, select the Show TargetDir output port parameter.

Parameters

expand all

Desired linear velocity, specified as a scalar in meters per second. The controller assumes that the vehicle drives at a constant linear velocity and that the computed angular velocity is independent of the linear velocity.

Maximum angular velocity, specified as a scalar in radians per second. The controller saturates the absolute angular velocity output at the given value.

Look-ahead distance, specified as a scalar in meters. The look-ahead distance changes the response of the controller. A vehicle with a higher look-ahead distance produces smooth paths but takes larger turns at corners. A vehicle with a smaller look-ahead distance follows the path closely and takes sharp turns, but oscillate along the path. For more information on the effects of look-ahead distance, see Pure Pursuit Controller.

Select this parameter to enable the TargetDir out port. This port gives the target direction as an angle in radians from the forward position, with positive angles measured counterclockwise.