Safety velocity limits in ROS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set the maximum linear velocity limit in the ROS node.

ROS

max_linear_velocity = [1]  # meters per second

Drag options to blanks, or click blank then click option'

A5.0

B0.5

C10.0

D50.0

Attempts:

3 left

💡 Hint

Common Mistakes

Setting velocity too high causing unsafe robot speed.

Using zero which stops the robot completely.

The maximum linear velocity is set to 0.5 m/s to ensure safe robot movement.

2fill in blank

medium

Complete the code to limit the angular velocity in the ROS velocity command message.

ROS

cmd_vel.angular.z = [1]  # radians per second

Drag options to blanks, or click blank then click option'

A6.28

B3.14

C1.0

D10.0

Attempts:

3 left

💡 Hint

Common Mistakes

Using too high angular velocity causing unstable turns.

Using zero which prevents rotation.

Angular velocity is limited to 1 radian per second for safe turning speed.

3fill in blank

hard

Fix the error in the velocity limiting function to clamp the linear velocity correctly.

ROS

def limit_velocity(velocity, max_velocity):
    return min(velocity, [1])

Drag options to blanks, or click blank then click option'

Amax_velocity

Bvelocity

C0

Dmax_velocity * 2

Attempts:

3 left

💡 Hint

Common Mistakes

Returning velocity without limiting it.

Using zero which always returns zero.

The function should return the smaller of the current velocity and the maximum allowed velocity.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps each velocity component to its limited value.

ROS

limited_velocities = {axis: min(value, [1]) for axis, value in velocities.items() if value [2] 0}

Drag options to blanks, or click blank then click option'

Amax_velocity

B>

C<

Dvelocity_limit

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong comparison operator causing wrong filtering.

Using incorrect variable names.

The comprehension limits each velocity to max_velocity only if the velocity is positive.

5fill in blank

hard

Fill all three blanks to create a ROS subscriber callback that limits and publishes safe velocities.

ROS

def velocity_callback(msg):
    safe_linear = min(msg.linear.x, [1])
    safe_angular = min(msg.angular.z, [2])
    safe_msg = Twist()
    safe_msg.linear.x = safe_linear
    safe_msg.angular.z = safe_angular
    [3].publish(safe_msg)

Drag options to blanks, or click blank then click option'

Amax_linear_velocity

Bmax_angular_velocity

Cvelocity_publisher

Dvelocity_subscriber

Attempts:

3 left

💡 Hint

Common Mistakes

Publishing with the subscriber instead of the publisher.

Not limiting velocities before publishing.

The callback limits linear and angular velocities using max limits and publishes the safe message.

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