Collision Checker Module User Guide

This guide covers the ManipulaPy.potential_field.CollisionChecker class, which performs geometry-based self-collision detection for serial manipulators using URDF meshes.

Note

Examples target Python 3.10+, SciPy 1.10+, and the urchin.urdf parser for loading URDF meshes and building convex hulls for collision tests.

Introduction

Collision checking is essential for safe motion planning. The CollisionChecker builds convex-hull approximations of each link’s visual mesh and tests for pairwise intersection at a given robot configuration.

Key features

  • Convex hull construction from URDF mesh vertices

  • Fast pairwise collision tests using convex polytope intersection checks

  • Pose transforms of hulls via forward kinematics

  • Self-collision detection between any two non-adjacent links

Mathematical Background

Given a robot configuration \(\mathbf q\) (joint angles), the forward-kinematics function \(T_i(\mathbf q)\in SE(3)\) returns the homogeneous transform of link i. Each link’s mesh is approximated by its convex hull \(H_i\). Under transform \(T_i\), hull vertices map to

\[\{\,T_i(\mathbf q)\,p \mid p\in H_i\,\}.\]

Two convex polyhedra intersect iff their convex hulls intersect. In practice, the check can be implemented by half-space evaluations or separating-axis tests on the transformed hulls.

Workflow

  1. Load URDF β†’ extract mesh vertices.

  2. Build hulls \(\{H_i\}\) offline.

  3. For each configuration \(\mathbf q\): - Compute \(T_i(\mathbf q)\) for each link. - Transform \(H_i \rightarrow T_i(H_i)\). - Test all pairs (i, j) with j > i + 1 (skip adjacent links) for intersection. - If any pair intersects β†’ collision.

API Reference

class ManipulaPy.potential_field.CollisionChecker(urdf_path, backend='builtin', load_meshes=True)[source]

Collision checker using URDF visual/collision geometry and convex hulls.

Supports multiple URDF parser backends:

  • β€œbuiltin”: Native ManipulaPy parser (NumPy 2.0 compatible, default)

  • β€œurchin”: Legacy urchin parser (requires urchin, not NumPy 2.0 compatible)

  • β€œpybullet”: PyBullet-based parser (requires pybullet)

Parameters:

  • backend (str) –

  • load_meshes (bool) –

check_collision(thetalist)[source]

Check for self-collision at a given joint configuration.

Parameters:

thetalist – Joint configuration (array or dict)

Returns:

True if collision detected, False otherwise

Return type:

bool

Installation

Ensure dependencies are installed:

pip install manipulapy[core] scipy urchin

Usage Examples

Basic collision check

from ManipulaPy.potential_field import CollisionChecker

# Initialize with your robot URDF
cc = CollisionChecker("robot.urdf")

# Test a single joint configuration
q = [0.0, -0.5, 0.3, 0.0, 0.2, -0.1]
if cc.check_collision(q):
    print("In collision!")
else:
    print("Collision-free.")

Batch checking

from ManipulaPy.potential_field import CollisionChecker
import numpy as np

cc    = CollisionChecker("robot.urdf")
poses = np.random.uniform(-0.5, 0.5, size=(100, 6))

collisions = [cc.check_collision(q) for q in poses]
print(f"{sum(collisions)} / {len(poses)} configurations in collision")

Integration with trajectory planning

from ManipulaPy.path_planning import TrajectoryPlanning
from ManipulaPy.potential_field import CollisionChecker

planner = TrajectoryPlanning(robot, "robot.urdf", dynamics, joint_limits)
cc      = CollisionChecker("robot.urdf")

traj = planner.joint_trajectory(q_start, q_goal, Tf=2.0, N=200, method=3)
safe = []
for q in traj["positions"]:
    if not cc.check_collision(q):
        safe.append(q)
print(f"{len(safe)} collision-free waypoints")

Advanced Topics

Convex hull caching

Hulls are built once at initialization. For dynamic meshes, you can rebuild via cc._create_convex_hulls() (may be expensive; prefer offline caching).

Custom mesh precision

Simplify (decimate) meshes before hull construction to reduce hull size and speed up intersection tests.

Troubleshooting

Mesh loading errors

Ensure your URDF’s <mesh> elements point to files with valid vertex arrays.

False negatives/positives

Convex hulls approximate concave geometry. For high precision, refine meshes or augment with additional proxy geometry.

Performance bottlenecks

  • Precompute and cache hulls.

  • Use fewer sample configurations in look-ahead checks.

  • Parallelize check_collision calls with multiprocessing.

See Also

References