Local Control Methods Global path planning

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05/06/23 CS225B Kurt Konolige

Local Control Methods

Global path planning•Expensive to plan in a dynamic world•Dealing with dynamics of the robot is difficult

=> Global planner + local control method

Local Control Methods

• Potential fields • Vector Field Histogram [Borenstein and Koren]• Elastic Bands [Quinlan and Khatib]• Elastic Strips [Brock and Khatib]

• Trajectory sampling• Dynamic Window [Fox et al.]• Trajectory Rollout [Gerkey et al.]

• Local gradient + lookahead control

Vector Field Histogram [Borenstein and Koren]

Potential field method

• Workspace obstacles• Obstacle probabilities from Cartesian histogram• Polar histogram of good directions

Potential field method

• Workspace obstacles• Obstacle probabilities from Cartesian histogram• Polar histogram of good directions

Issues

• Width of robot, safety margin• Cost function for handling tradeoffs: safety, progress, etc. • Trajectory and dynamics• Oscillation

Elastic Band [Quinlan and Khatib 1996]

Manhattanglobal path

Smoothing byexternal andInternal forces

Obstacle Obstacle

Bubbles along a pathInternal forces minimizing length

Exernal forces minimizing contactdistance

Bubbles along a path

Implementation on Care-O-Bot and PR2 by Christian Connette

Dynamic Window Method [Fox et al.]

Evaluating constant curvature path in configuration spaceWindow of values based on one-step accelerationWhen will the robot crash?

dtadtadtavvdtav

Admissible trajectories: braking before collision

Heading: achieve the goalDistance: avoid obstaclesVelocity: do it fast

Trajectory Rollout [Konolige et al.]

DWA Issues

• Computation• Evaluation function tuning: small openings• Longer paths / lower acceleration• Using the global path

Desired velocities:

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