Safety velocity limits in ROS - Mini Project: Build & Apply

Safety velocity limits

📖 Scenario: You are programming a robot that moves around. To keep it safe, you need to make sure it never goes faster than certain speed limits.

🎯 Goal: Build a ROS node that reads velocity commands, checks them against safety limits, and publishes safe velocity commands.

📋 What You'll Learn

Create a dictionary with exact velocity limits for linear and angular speeds

Add a variable for the safety margin factor

Write a function that limits the velocity commands based on the limits and margin

Publish the safe velocity commands in a ROS publisher

💡 Why This Matters

🌍 Real World

Robots must move safely without exceeding speed limits to avoid accidents or damage.

💼 Career

Understanding how to enforce safety constraints in robot control software is essential for robotics engineers and developers.

1

Create velocity limits dictionary

Create a dictionary called velocity_limits with these exact entries: 'linear_max': 1.0 and 'angular_max': 0.5.

ROS

# Create the velocity_limits dictionary with linear_max and angular_max
# Your code here

Hint

Use curly braces to create a dictionary with keys 'linear_max' and 'angular_max'.

2

Add safety margin factor

Add a variable called safety_margin and set it to 0.9 to reduce the velocity limits by 10%.

ROS

velocity_limits = {'linear_max': 1.0, 'angular_max': 0.5}
# Add safety_margin variable here
# Your code here

Hint

Set safety_margin to 0.9 to keep speeds below 90% of max.

3

Write velocity limiting function

Write a function called limit_velocity that takes linear_vel and angular_vel as inputs and returns limited velocities. Use velocity_limits and safety_margin to cap the velocities so that linear velocity does not exceed velocity_limits['linear_max'] * safety_margin and angular velocity does not exceed velocity_limits['angular_max'] * safety_margin. Use min and max to handle negative velocities correctly.

ROS

velocity_limits = {'linear_max': 1.0, 'angular_max': 0.5}
safety_margin = 0.9

# Define limit_velocity function here
# Your code here

Hint

Use min and max to clamp the velocities between negative and positive limits.

4

Publish safe velocity commands

Complete the ROS node by importing rclpy and Twist from geometry_msgs.msg. Create a publisher called cmd_vel_pub that publishes Twist messages on the topic '/cmd_vel_safe'. Write a callback function called cmd_vel_callback that receives a Twist message, uses limit_velocity to limit the linear.x and angular.z velocities, creates a new Twist message with the limited velocities, and publishes it. Initialize the ROS node with the name 'safety_velocity_node' and spin it.

ROS

velocity_limits = {'linear_max': 1.0, 'angular_max': 0.5}
safety_margin = 0.9

def limit_velocity(linear_vel, angular_vel):
    max_linear = velocity_limits['linear_max'] * safety_margin
    max_angular = velocity_limits['angular_max'] * safety_margin
    limited_linear = max(-max_linear, min(linear_vel, max_linear))
    limited_angular = max(-max_angular, min(angular_vel, max_angular))
    return limited_linear, limited_angular

# Import rclpy and Twist
# Create publisher cmd_vel_pub
# Define cmd_vel_callback to limit and publish safe velocities
# Initialize and spin node
# Your code here

Hint

Use ROS 2 Python API to create a node, publisher, and subscriber. Limit velocities in the callback and publish safe commands.

Practice

1. What is the main purpose of safety velocity limits in ROS?

easy

A. To keep robot speeds within safe ranges

B. To increase the robot's maximum speed

C. To disable robot movement completely

D. To control the robot's battery usage

Solution

1. Step 1: Understand the role of safety velocity limits

   Safety velocity limits are designed to prevent the robot from moving too fast, ensuring safety.

2. Step 2: Identify the correct purpose

   Among the options, only keeping speeds safe matches the purpose of safety velocity limits.

3. Final Answer:

   To keep robot speeds within safe ranges -> Option A

4. Quick Check:

   Safety velocity limits = keep speeds safe [OK]

Hint: Safety limits control max speed, not disable or increase it [OK]

Common Mistakes:

• Thinking safety limits increase speed
• Confusing safety limits with power control
• Assuming safety limits stop all movement

2. Which ROS parameter syntax correctly sets a maximum linear velocity limit to 0.5 m/s?

easy

A. max_linear_velocity 0.5

B. max_linear_velocity = 0.5

C. max_linear_velocity->0.5

D. max_linear_velocity: 0.5

Solution

1. Step 1: Recall ROS parameter YAML syntax

   ROS parameters in YAML use colon and space, like param_name: value.

2. Step 2: Match syntax to options

   Only max_linear_velocity: 0.5 uses correct YAML syntax for setting parameters.

3. Final Answer:

   max_linear_velocity: 0.5 -> Option D

4. Quick Check:

   ROS YAML param = key: value [OK]

Hint: ROS params use colon and space, not equals or arrows [OK]

Common Mistakes:

• Using equals sign instead of colon
• Omitting colon and space
• Using arrow notation which is invalid

3. Given this ROS node snippet setting velocity limits:

velocity_limits:
  max_linear: 1.0
  max_angular: 0.5

robot_velocity:
  linear: 1.2
  angular: 0.4

What will be the effective linear velocity after applying safety limits?

medium

A. 1.0 m/s

B. 1.2 m/s

C. 0.5 m/s

D. 0.4 m/s

Solution

1. Step 1: Compare robot velocity to max limits

   The robot's linear velocity is 1.2 m/s, which exceeds the max_linear limit of 1.0 m/s.

2. Step 2: Apply safety velocity limit

   The effective linear velocity must be capped at the max_linear limit, 1.0 m/s.

3. Final Answer:

   1.0 m/s -> Option A

4. Quick Check:

   Velocity capped at max_linear = 1.0 [OK]

Hint: If velocity > max limit, use max limit value [OK]

Common Mistakes:

• Using original velocity without capping
• Confusing angular and linear limits
• Choosing the lower angular limit for linear velocity

4. You wrote this YAML for velocity limits but the robot ignores the limits:

velocity_limits:
  max_linear = 0.8
  max_angular: 0.4

What is the likely error?

medium

A. Incorrect indentation for max_angular

B. Missing quotes around numbers

C. Using '=' instead of ':' for max_linear

D. Parameter names must be uppercase

Solution

1. Step 1: Check YAML syntax for parameters

   YAML requires colon and space to assign values, not equals sign.

2. Step 2: Identify the error in max_linear line

   Using '=' instead of ':' causes the parameter to be ignored or cause parsing errors.

3. Final Answer:

   Using '=' instead of ':' for max_linear -> Option C

4. Quick Check:

   YAML param syntax = colon, not equals [OK]

Hint: YAML uses colon, not equals, to assign values [OK]

Common Mistakes:

• Using equals sign in YAML
• Forgetting indentation rules
• Thinking quotes are mandatory for numbers

5. You want to set different safety velocity limits for two robot modes: normal and cautious. Which YAML structure correctly defines max linear velocities for both modes?

hard

A. velocity_limits:
normal:
max_linear: 1.0
cautious:
max_linear: 0.5

B. velocity_limits:
max_linear:
normal: 1.0
cautious: 0.5

C. velocity_limits:
normal_max_linear: 1.0
cautious_max_linear: 0.5

D. velocity_limits:
max_linear_normal: 1.0
max_linear_cautious: 0.5

Solution

1. Step 1: Understand hierarchical YAML for modes

   Grouping limits under max_linear with mode keys is clear and scalable.

2. Step 2: Compare options for clarity and structure

   velocity_limits:
     max_linear:
       normal: 1.0
       cautious: 0.5

   nests modes under max_linear, which is a common pattern for related parameters.

3. Final Answer:

   velocity_limits: max_linear: normal: 1.0 cautious: 0.5 -> Option B

4. Quick Check:

   Nested keys for modes under max_linear =

   velocity_limits:
     max_linear:
       normal: 1.0
       cautious: 0.5

   [OK]

Hint: Nest modes under max_linear for clear grouped limits [OK]

Common Mistakes:

• Using flat keys with mode suffixes
• Not grouping related parameters
• Confusing key names and structure