Gazebo physics and collision in ROS - Step-by-Step Execution

Concept Flow - Gazebo physics and collision

Start Simulation

↓

Load World & Models

↓

Initialize Physics Engine

↓

For each simulation step

↓

Update Object States

↓

Detect Collisions

↓

Calculate Collision Responses

↓

Apply Forces & Update Positions

↓

Render Scene

↓

Repeat Next Step

This flow shows how Gazebo runs physics and collision detection step-by-step during simulation.

Execution Sample

ROS

# Pseudocode for Gazebo physics step
world->Step()
// Updates physics and collisions

// Inside Step:
physicsEngine->Update()
collisionManager->Detect()
collisionManager->Resolve()
world->UpdateModels()

This code runs one simulation step updating physics and handling collisions.

Execution Table

Step	Action	Physics State	Collision Detection	Collision Response	Model Positions
1	Initialize physics engine	Physics engine ready	No collisions yet	No response	Models at start positions
2	Update physics	Velocities and forces calculated	Check collisions between models	No collisions detected	Models moved by physics
3	Detect collisions	Physics state updated	Collision detected between Model A and Model B	Calculate contact forces	Models positions adjusted
4	Resolve collisions	Contact forces applied	Collision resolved	Apply impulses to models	Models separated to avoid overlap
5	Update models	Physics stable	No new collisions	No response needed	Models at new positions
6	Render scene	Final physics state for step	Collision info used for visuals	No changes	Scene displayed
7	Repeat next step	Prepare for next update	Reset collision flags	Clear forces	Ready for next physics update

💡 Simulation runs continuously; this table shows one step cycle.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
Model A Position	(0,0,0)	(0.1,0,0)	(0.15,0,0)	(0.14,0,0)	(0.14,0,0)	(0.14,0,0)
Model B Position	(1,0,0)	(0.9,0,0)	(0.85,0,0)	(0.86,0,0)	(0.86,0,0)	(0.86,0,0)
Collision Flag	False	False	True	True	False	False
Contact Force	0	0	5N	5N	0	0

Key Moments - 3 Insights

Why does the model position slightly move back after collision detection?

After collision detection (Step 3), Gazebo calculates contact forces and applies impulses (Step 4) to prevent models from overlapping, causing slight position adjustments as shown in execution_table rows 3 and 4.

What happens if collision detection is skipped in a physics step?

If collision detection is skipped, models may pass through each other unrealistically because no contact forces are calculated or applied, breaking the simulation's physical accuracy. This is why collision detection is essential every step (see Step 3).

How does Gazebo know when to stop applying collision responses?

Gazebo applies collision responses until models no longer overlap and contact forces stabilize, indicated by collision flags turning false and no new collisions detected in Step 5 and 6.

Visual Quiz - 3 Questions

Look at the execution table, what is the collision flag value after Step 3?

A False

B True

C Undefined

D Zero

💡 Hint

Check the 'Collision Flag' variable in variable_tracker after Step 3.

At which step does Gazebo apply contact forces to models?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look at the 'Collision Response' column in execution_table where impulses are applied.

If the collision flag remains true after Step 5, what would likely happen?

A Models separate automatically without forces

B Simulation ends immediately

C Models remain overlapping causing unrealistic simulation

D No change in model positions

💡 Hint

Refer to key_moments about collision response stopping conditions.

Concept Snapshot

Gazebo physics runs in steps:
1. Update physics states (forces, velocities)
2. Detect collisions between models
3. Calculate and apply collision responses
4. Adjust model positions to avoid overlap
5. Render updated scene
Collision detection and response keep simulation realistic and prevent models from passing through each other.

Full Transcript

Gazebo simulation runs in repeated steps. Each step starts by updating physics states like forces and velocities on models. Then, Gazebo checks if any models collide. If collisions are found, it calculates contact forces and applies impulses to models to prevent overlap. Models' positions are adjusted accordingly. Finally, the scene is rendered with updated positions. This cycle repeats continuously to simulate realistic physics and collisions. Variables like model positions and collision flags change each step to reflect the current simulation state. Understanding this flow helps grasp how Gazebo keeps simulations physically accurate.

Practice

1. What is the main purpose of physics in Gazebo simulations?

easy

A. To define the color and texture of objects

B. To control how objects move and react to forces like gravity

C. To create user interfaces for robot control

D. To store sensor data from the robot

Solution

1. Step 1: Understand the role of physics in Gazebo

   Physics in Gazebo simulates real-world forces like gravity and friction affecting objects.

2. Step 2: Identify the correct purpose

   Physics controls object movement and reactions, not appearance or data storage.

3. Final Answer:

   To control how objects move and react to forces like gravity -> Option B

4. Quick Check:

   Physics = object movement and forces [OK]

Hint: Physics = movement and forces, not visuals or data [OK]

Common Mistakes:

• Confusing physics with visual appearance
• Thinking physics stores sensor data
• Assuming physics creates user interfaces

2. Which of the following is the correct way to define a collision shape in a Gazebo SDF file?

easy

A. 1 1 1

B. red

C. camera

D. metal

Solution

1. Step 1: Review SDF collision shape syntax

   Collision shapes use <geometry> tags with shape types like <box>, <sphere>, or <cylinder>.

2. Step 2: Identify the valid collision definition

   <collision><geometry><box><size>1 1 1</size></box></geometry></collision> correctly defines a box size inside geometry within collision tags.

3. Final Answer:

   <collision><geometry><box><size>1 1 1</size></box></geometry></collision> -> Option A

4. Quick Check:

   Collision shape = geometry + shape tags [OK]

Hint: Collision needs geometry and shape tags, not color or sensor [OK]

Common Mistakes:

• Using color or material tags inside collision
• Confusing sensors with collision shapes
• Omitting geometry tag inside collision

3. Given this SDF snippet, what will happen when the robot collides with the box?

<collision name="box_collision">
  <geometry>
    <box><size>1 1 1</size></box>
  </geometry>
</collision>

medium

A. The box will change color on collision

B. The robot will pass through the box without any effect

C. The simulation will crash due to missing physics

D. The robot will detect collision and stop or react physically

Solution

1. Step 1: Understand collision shape effect

   The collision shape defines where objects physically interact and block each other.

2. Step 2: Predict behavior on collision

   When the robot hits the box collision shape, physics will cause it to stop or react realistically.

3. Final Answer:

   The robot will detect collision and stop or react physically -> Option D

4. Quick Check:

   Collision shape causes physical interaction [OK]

Hint: Collision shapes cause physical blocking, not visual changes [OK]

Common Mistakes:

• Thinking collision changes color
• Assuming robot passes through objects with collision
• Believing simulation crashes without physics tag

4. You wrote this collision tag in your Gazebo model but the robot passes through the object. What is the likely error?

<collision>
  <geometry>
    <sphere><radius>0.5</radius></sphere>
  </geometry>
</collision>

medium

A. The physics engine is not enabled or configured properly

B. The collision tag is missing a name attribute

C. The sphere radius is too small to detect collision

D. The geometry tag should be outside the collision tag

Solution

1. Step 1: Check collision tag correctness

   The collision tag syntax is correct and includes geometry with a sphere shape.

2. Step 2: Identify common cause of passing through objects

   If physics is disabled or misconfigured, collisions won't be processed, causing objects to pass through.

3. Final Answer:

   The physics engine is not enabled or configured properly -> Option A

4. Quick Check:

   Physics engine must be active for collisions [OK]

Hint: Collision needs physics engine enabled to work [OK]

Common Mistakes:

• Thinking missing name attribute breaks collision
• Assuming small radius disables collision
• Placing geometry outside collision tag

5. You want to simulate a robot pushing a box realistically in Gazebo. Which combination of settings is best to achieve this?

hard

A. Only define visual shapes and manually move the box in code

B. Disable physics engine, use only visual shapes for robot and box

C. Enable physics engine with friction, define collision shapes for both robot and box

D. Use collision shapes but set friction to zero in physics settings

Solution

1. Step 1: Understand realistic pushing requires physics and collision

   Physics engine simulates forces like friction and collision shapes define contact areas.

2. Step 2: Evaluate options for realistic interaction

   Enable physics engine with friction, define collision shapes for both robot and box enables physics with friction and collision shapes, allowing realistic pushing behavior.

3. Step 3: Reject options disabling physics or friction

   Disabling physics or friction or using only visuals prevents realistic physical interaction.

4. Final Answer:

   Enable physics engine with friction, define collision shapes for both robot and box -> Option C

5. Quick Check:

   Physics + friction + collision = realistic pushing [OK]

Hint: Physics + friction + collision shapes = realistic object interaction [OK]

Common Mistakes:

• Disabling physics engine expecting realistic forces
• Setting friction to zero for pushing
• Using only visual shapes without collision