Skip to main content
Module 1: ROS 2Module 2: The Digital Twin Module 3: AI-Robot BrainModule 4: Vision-Language-Actions

Collision Detection and Response in Gazebo

Collision detection and response are critical components of physics simulation that determine how objects interact when they come into contact with each other. In Gazebo, these systems ensure realistic physical interactions between robots, obstacles, and the environment.

Understanding Collision Detection

Collision detection is a two-phase process:

  • Broad Phase: Quickly eliminates pairs of objects that are far apart
  • Narrow Phase: Performs precise collision detection between nearby objects

Collision Detection Methods

Gazebo supports several collision detection algorithms:

  • FCL (Flexible Collision Library): Default for geometric shapes
  • Bullet Collision: Alternative with different performance characteristics
  • ODE Collision: Legacy support for backward compatibility

Types of Collisions

  • Discrete Collision Detection: Checks for collisions at fixed intervals
  • Continuous Collision Detection: Predicts collisions between time steps
  • Ray Casting: Detects collisions along a ray (for sensors)

Collision Geometry

Collision Meshes

Objects require collision geometry to interact properly:

<link name="link_name">
  <collision name="collision_name">
    <geometry>
      <!-- Box shape -->
      <box>
        <size>0.1 0.1 0.1</size>
      </box>

      <!-- Sphere shape -->
      <!-- <sphere><radius>0.05</radius></sphere> -->

      <!-- Cylinder shape -->
      <!-- <cylinder><radius>0.05</radius><length>0.1</length></cylinder> -->

      <!-- Mesh shape -->
      <!-- <mesh><uri>model://my_model/meshes/link_shape.stl</uri></mesh> -->
    </geometry>

    <surface>
      <contact>
        <ode>
          <max_vel>100.0</max_vel>
          <min_depth>0.001</min_depth>
        </ode>
      </contact>
    </surface>
  </collision>
</link>

Visual vs. Collision Geometry

  • Visual Geometry: Determines appearance (rendered in GUI)
  • Collision Geometry: Determines physical interactions (computational)
  • Best Practice: Use simplified collision geometry for performance

Contact Properties

Material Properties

Contacts have various properties that affect interaction:

  • Restitution (Bounciness): How much kinetic energy is preserved
  • Friction: Resistance to sliding motion
  • Stiffness/Damping: How contact forces are computed

Contact Parameters

<collision name="collision_name">
  <surface>
    <contact>
      <ode>
        <soft_cfm>0</soft_cfm>
        <soft_erp>0.2</soft_erp>
        <kp>1e+13</kp>
        <kd>1.0</kd>
        <max_vel>100.0</max_vel>
        <min_depth>0.001</min_depth>
      </ode>
    </contact>
    <friction>
      <ode>
        <mu>1.0</mu>
        <mu2>1.0</mu2>
        <fdir1>0 0 0</fdir1>
      </ode>
    </friction>
  </surface>
</collision>

Collision Response

Contact Force Computation

When collisions occur, Gazebo computes forces using:

  • Penalty Method: Applies spring-damper forces to separate objects
  • Impulse-Based Method: Instantaneous momentum transfer
  • Constraint-Based Method: Formulates as constrained optimization

Contact Information

During simulation, contact information includes:

  • Contact Points: Locations where objects touch
  • Normal Vectors: Direction of contact forces
  • Depth Penetration: Amount objects overlap
  • Contact Forces: Magnitudes of interaction forces

Sensor Integration

Contact Sensors

Gazebo provides contact sensors for detecting collisions:

<sensor name="contact_sensor" type="contact">
  <always_on>1</always_on>
  <update_rate>30</update_rate>
  <contact>
    <collision>link_name_collision</collision>
  </contact>
</sensor>

Force/Torque Sensors

Monitor forces at joints during collisions:

  • Joint-Level: Forces and torques at specific joints
  • Link-Level: Forces acting on specific robot links
  • Wrench Output: 6DOF force/torque measurements

Performance Considerations

Optimization Strategies

  • Simple Geometries: Use primitive shapes when possible
  • Hierarchical Structures: Organize collision detection efficiently
  • Contact Filtering: Exclude unnecessary collision pairs
  • Adaptive Timestepping: Adjust based on collision activity

Collision Mesh Simplification

  • Decimate Complex Models: Reduce polygon count for collision geometry
  • Convex Decomposition: Break concave objects into convex parts
  • Bounding Volumes: Use spheres, boxes, or capsules for coarse detection

Common Collision Challenges

Jittering

Problem: Objects vibrate or oscillate during contact Solutions:

  • Increase damping parameters
  • Adjust ERP/CFM values
  • Use softer contact parameters
  • Reduce simulation timestep

Tunneling

Problem: Fast-moving objects pass through each other Solutions:

  • Enable continuous collision detection
  • Reduce timestep
  • Increase collision mesh resolution
  • Use swept-volume detection

Stacking Instability

Problem: Stacked objects become unstable or collapse Solutions:

  • Increase solver iterations
  • Adjust ERP and CFM parameters
  • Use damping forces
  • Verify mass and inertia properties

Robot-Specific Considerations

Manipulation Tasks

For robots manipulating objects:

  • Grasp Stability: Proper friction values for secure grasps
  • Object Properties: Accurate mass and inertia for manipulation
  • Sensor Feedback: Contact sensors for grasp detection
  • Control Integration: Force control during contact transitions

Navigation Challenges

For mobile robots navigating:

  • Terrain Adaptation: Different surface properties for various terrains
  • Obstacle Avoidance: Reliable collision detection for safety
  • Slip Modeling: Friction effects on different surfaces
  • Dynamic Obstacles: Moving object collision handling

Debugging Collisions

Visualization Tools

  • Contact Visualization: Show contact points and normals in GUI
  • Force Display: Visualize contact forces during simulation
  • Collision Mesh Overlay: Highlight collision geometries

Logging and Monitoring

  • Contact Events: Log collision occurrence and properties
  • Force Measurements: Monitor contact forces over time
  • Performance Metrics: Track collision detection computation time

Best Practices

  • Start Conservative: Begin with stable contact parameters
  • Incremental Complexity: Add detailed collision geometry gradually
  • Validation: Compare simulated collisions to real-world behavior
  • Documentation: Record successful parameter sets for reuse
  • Testing: Validate collision behavior across various scenarios
  • Performance Monitoring: Watch for computational bottlenecks in collision detection