Velocity smoothing in ROS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the velocity smoothing node in ROS.

ROS

ros::init(argc, argv, [1]);

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

A"velocity_node"

B"velocity_smoothing_node"

C"smooth_velocity"

D"velocity_control"

Attempts:

3 left

💡 Hint

Common Mistakes

Using a generic node name that does not reflect velocity smoothing.

Forgetting to include quotes around the node name.

The node name must be "velocity_smoothing_node" to match the intended functionality.

2fill in blank

medium

Complete the code to declare a publisher for smoothed velocity commands.

ROS

ros::Publisher vel_pub = nh.advertise<geometry_msgs::Twist>([1], 10);

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

A"/velocity"

B"/cmd_vel_raw"

C"/cmd_vel_smooth"

D"/cmd_vel"

Attempts:

3 left

💡 Hint

Common Mistakes

Publishing on the raw velocity topic instead of the smoothed one.

Using a topic name that does not match the smoothing purpose.

The publisher should advertise on "/cmd_vel_smooth" to publish smoothed velocity commands.

3fill in blank

hard

Fix the error in the velocity smoothing function signature.

ROS

geometry_msgs::Twist [1](const geometry_msgs::Twist& input_vel) {

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

AsmoothVelocity

BvelocitySmooth

Csmooth_vel

Dvelocity_smooth

Attempts:

3 left

💡 Hint

Common Mistakes

Using underscores in function names instead of camelCase.

Using variable-like names instead of function names.

The function name should be 'smoothVelocity' following camelCase naming conventions.

4fill in blank

hard

Fill both blanks to complete the velocity smoothing condition and update.

ROS

if (std::abs(input_vel.linear.x - last_vel.linear.x) > [1]) {
    smoothed_vel.linear.x = last_vel.linear.x + [2] * (input_vel.linear.x - last_vel.linear.x);
}

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

Amax_acceleration

Bsmoothing_factor

Cmax_speed

Ddelta_time

Attempts:

3 left

💡 Hint

Common Mistakes

Using max_speed instead of max_acceleration for the threshold.

Using delta_time instead of smoothing_factor for the update.

The condition compares the difference to max_acceleration, and smoothing_factor controls the update step.

5fill in blank

hard

Fill all three blanks to complete the velocity smoothing loop with publishing.

ROS

ros::Rate loop_rate([1]);
while (ros::ok()) {
    ros::spinOnce();
    geometry_msgs::Twist smoothed = [2](current_vel);
    [3].publish(smoothed);
    loop_rate.sleep();
}

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

A10

BsmoothVelocity

Cvel_pub

D20

Attempts:

3 left

💡 Hint

Common Mistakes

Using too low or too high loop rate values.

Calling a non-existent smoothing function.

Publishing with the wrong publisher variable.

The loop rate is set to 20 Hz, smoothVelocity is called to smooth current_vel, and vel_pub publishes the result.

Practice

1. What is the main purpose of velocity smoothing in ROS robot control?

easy

A. To gradually change speed and avoid sudden jumps

B. To increase the maximum speed instantly

C. To stop the robot immediately

D. To ignore acceleration limits

Solution

1. Step 1: Understand velocity smoothing concept

   Velocity smoothing means changing speed gradually to avoid sudden jumps.

2. Step 2: Identify the purpose in robot control

   This gradual change improves safety and comfort by preventing abrupt movements.

3. Final Answer:

   To gradually change speed and avoid sudden jumps -> Option A

4. Quick Check:

   Velocity smoothing = gradual speed change [OK]

Hint: Velocity smoothing means smooth speed changes, not instant jumps [OK]

Common Mistakes:

• Thinking velocity smoothing increases speed instantly
• Confusing smoothing with emergency stop
• Ignoring acceleration limits in smoothing

2. Which of the following is the correct Python function signature for a velocity smoothing function in ROS?

easy

A. def smooth_velocity(current_vel: float, target_vel: float, max_accel: float):

B. def smooth_velocity(current_vel, target_vel, max_accel):

C. def smooth_velocity(current_vel: int, target_vel: int, max_accel: int, dt: int):

D. def smooth_velocity(current_vel: float, target_vel: float, max_accel: float, dt: float) -> float:

Solution

1. Step 1: Check function parameters for velocity smoothing

   The function needs current velocity, target velocity, max acceleration, and time delta (dt) to calculate smoothing.

2. Step 2: Verify correct typing and return type

   Using floats for velocities and acceleration is correct, and the function returns a float for new velocity.

3. Final Answer:

   def smooth_velocity(current_vel: float, target_vel: float, max_accel: float, dt: float) -> float: -> Option D

4. Quick Check:

   Correct parameters and types = def smooth_velocity(current_vel: float, target_vel: float, max_accel: float, dt: float) -> float: [OK]

Hint: Include all needed parameters with correct types and return value [OK]

Common Mistakes:

• Missing dt parameter for time step
• Using int instead of float for velocities
• No return type annotation

3. Given the following code snippet for velocity smoothing, what will be the output if current_vel = 1.0, target_vel = 3.0, max_accel = 1.0, and dt = 1.0?

def smooth_velocity(current_vel, target_vel, max_accel, dt):
    max_change = max_accel * dt
    delta = target_vel - current_vel
    if abs(delta) > max_change:
        delta = max_change if delta > 0 else -max_change
    return current_vel + delta

print(smooth_velocity(1.0, 3.0, 1.0, 1.0))

medium

A. 2.0

B. 3.0

C. 1.0

D. 4.0

Solution

1. Step 1: Calculate maximum allowed velocity change

   max_change = max_accel * dt = 1.0 * 1.0 = 1.0

2. Step 2: Calculate delta and limit it

   delta = target_vel - current_vel = 3.0 - 1.0 = 2.0, which is greater than max_change, so delta is limited to 1.0

3. Step 3: Calculate new velocity

   new velocity = current_vel + delta = 1.0 + 1.0 = 2.0

4. Final Answer:

   2.0 -> Option A

5. Quick Check:

   Velocity change limited by max_accel * dt = 2.0 [OK]

Hint: Limit velocity change by max_accel * dt before adding [OK]

Common Mistakes:

• Adding full delta without limiting by max_change
• Returning target_vel directly
• Ignoring sign of delta

4. Identify the bug in this velocity smoothing function and choose the correct fix:

def smooth_velocity(current_vel, target_vel, max_accel, dt):
    max_change = max_accel * dt
    delta = target_vel - current_vel
    if delta > max_change:
        delta = max_change
    elif delta < max_change:
        delta = -max_change
    return current_vel + delta

medium

A. Change max_change to max_accel / dt

B. Remove the if-else and always set delta = max_change

C. Change 'elif delta < max_change' to 'elif delta < -max_change'

D. Add abs() around delta in the if condition

Solution

1. Step 1: Analyze the conditions for limiting delta

   The function limits delta if it exceeds max_change positively or negatively.

2. Step 2: Identify incorrect condition

   The condition 'elif delta < max_change' is wrong because it triggers for any delta less than max_change, including values that don't need limiting. It should check if delta is less than negative max_change.

3. Step 3: Correct the condition

   Change 'elif delta < max_change' to 'elif delta < -max_change' to correctly limit negative large changes.

4. Final Answer:

   Change 'elif delta < max_change' to 'elif delta < -max_change' -> Option C

5. Quick Check:

   Negative delta limit needs correct comparison [OK]

Hint: Check negative limit uses -max_change, not max_change [OK]

Common Mistakes:

• Using max_change instead of -max_change for negative check
• Removing conditions and causing wrong velocity jumps
• Incorrect calculation of max_change

5. You want to implement velocity smoothing for a robot that receives a list of target velocities every second: [0, 2, 5, 3, 0]. The robot's max acceleration is 1.5 m/s² and the time step is 1 second. Which sequence of smoothed velocities will correctly apply velocity smoothing starting from 0 m/s?

hard

A. [0, 1.5, 3.0, 4.5, 3.0]

B. [0, 1.5, 3.0, 3.0, 1.5]

C. [0, 1.5, 2.5, 3.0, 1.5]

D. [0, 2, 5, 3, 0]

Solution

1. Step 1: Calculate smoothed velocities step-by-step

   Start at 0 m/s. Max change per step = 1.5 m/s. - Step 1: target 0 -> 0 (start) - Step 2: target 2, delta=2-0=2 >1.5, so velocity=0+1.5=1.5 - Step 3: target 5, delta=5-1.5=3.5 >1.5, velocity=1.5+1.5=3.0 - Step 4: target 3, delta=3-3=0 ≤1.5, velocity=3.0 - Step 5: target 0, delta=0-3=-3 < -1.5, velocity=3.0-1.5=1.5

2. Step 2: Compare with options

   The sequence [0, 1.5, 3.0, 3.0, 1.5] matches the calculated sequence.

3. Final Answer:

   [0, 1.5, 3.0, 3.0, 1.5] -> Option B

4. Quick Check:

   Apply max_accel limit each step = [0, 1.5, 3.0, 3.0, 1.5] [OK]

Hint: Apply max acceleration limit stepwise to each velocity [OK]

Common Mistakes:

• Using target velocities directly without smoothing
• Adding max_accel multiple times incorrectly
• Ignoring negative acceleration limits