Safety velocity limits in ROS

Introduction

Safety velocity limits help robots move safely by setting maximum speeds. This stops robots from moving too fast and causing accidents.

When a robot moves near people or fragile objects.

When a robot operates in a small or crowded space.

When testing new robot movements to avoid damage.

When the robot's environment changes and needs safer speed settings.

Syntax

ROS

safety_limits:
  max_vel_x: float  # maximum forward speed in meters per second
  max_vel_y: float  # maximum sideways speed in meters per second
  max_vel_theta: float  # maximum rotational speed in radians per second

The values are usually set in a YAML file or parameter server in ROS.

These limits apply to the robot's velocity commands to keep movement safe.

Examples

This sets the robot to move forward at max 0.5 m/s, no sideways movement, and rotate at max 1 rad/s.

ROS

safety_limits:
  max_vel_x: 0.5
  max_vel_y: 0.0
  max_vel_theta: 1.0

This allows slower movement forward and sideways, with slower rotation for extra safety.

ROS

safety_limits:
  max_vel_x: 0.2
  max_vel_y: 0.2
  max_vel_theta: 0.5

Sample Program

This ROS 2 Python node listens to raw velocity commands on 'cmd_vel_raw'. It limits the speeds to safe maximums set by parameters. Then it publishes the safe commands on 'cmd_vel_safe'.

This helps keep the robot from moving too fast and causing accidents.

ROS

#!/usr/bin/env python3
import rclpy
from rclpy.node import Node
from geometry_msgs.msg import Twist

class SafetyVelocityNode(Node):
    def __init__(self):
        super().__init__('safety_velocity_node')
        self.declare_parameter('max_vel_x', 0.5)
        self.declare_parameter('max_vel_y', 0.0)
        self.declare_parameter('max_vel_theta', 1.0)

        self.max_vel_x = self.get_parameter('max_vel_x').value
        self.max_vel_y = self.get_parameter('max_vel_y').value
        self.max_vel_theta = self.get_parameter('max_vel_theta').value

        self.subscriber = self.create_subscription(
            Twist,
            'cmd_vel_raw',
            self.cmd_vel_callback,
            10)
        self.publisher = self.create_publisher(Twist, 'cmd_vel_safe', 10)

    def cmd_vel_callback(self, msg):
        safe_msg = Twist()
        safe_msg.linear.x = max(min(msg.linear.x, self.max_vel_x), -self.max_vel_x)
        safe_msg.linear.y = max(min(msg.linear.y, self.max_vel_y), -self.max_vel_y)
        safe_msg.angular.z = max(min(msg.angular.z, self.max_vel_theta), -self.max_vel_theta)

        self.publisher.publish(safe_msg)
        self.get_logger().info(f"Published safe velocities: linear.x={safe_msg.linear.x}, linear.y={safe_msg.linear.y}, angular.z={safe_msg.angular.z}")


def main(args=None):
    rclpy.init(args=args)
    node = SafetyVelocityNode()
    rclpy.spin(node)
    node.destroy_node()
    rclpy.shutdown()

if __name__ == '__main__':
    main()

Important Notes

Always test safety limits in a controlled environment before using on real robots.

Adjust limits based on robot size, weight, and environment for best safety.

Summary

Safety velocity limits keep robot speeds within safe ranges.

They prevent accidents by stopping the robot from moving too fast.

Use parameters to set and adjust these limits easily in ROS.

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