Safety Principles in ROS 2 for Humanoid Robotics
Safety is the cornerstone of humanoid robotics development. In environments shared with humans, robots must operate with fail-safe mechanisms that prioritize human safety above all other considerations. This section details the safety principles and implementation strategies within ROS 2 for humanoid robotics applications.
Core Safety Principles
Safety-First Design
All implementations must prioritize human safety and environmental protection above all other considerations. Fail-safes must be implemented at every level, and any potential safety risk identified must halt autonomous operations until resolved.
Modular Safety Architecture
Safety systems must be designed as independent modules with well-defined interfaces, allowing for individual testing and validation while maintaining clear boundaries between functional domains.
Deterministic Safety Communication
All safety-related communication via ROS 2 topics/services must be designed with guaranteed message delivery. Communication protocols must include monitoring and logging capabilities to ensure safety-critical information is reliably transmitted.
Safety Implementation Patterns
Emergency Stop System
import rclpy
from rclpy.node import Node
from std_msgs.msg import Bool
from geometry_msgs.msg import Twist
from sensor_msgs.msg import LaserScan
import threading
class SafeHumanoidController(Node):
def __init__(self):
super().__init__('safe_humanoid_controller')
# Initialize safety state
self.emergency_stop_active = False
self.safety_lock = threading.Lock()
# Create emergency stop publisher and subscriber
self.emergency_stop_pub = self.create_publisher(Bool, '/emergency_stop', 10)
self.emergency_stop_sub = self.create_subscription(
Bool,
'/emergency_stop',
self.emergency_stop_callback,
10
)
# Create sensor subscriber for collision detection
self.laser_sub = self.create_subscription(
LaserScan,
'/laser_scan',
self.laser_callback,
10
)
# Create command publisher with safety check
self.cmd_pub = self.create_publisher(Twist, '/cmd_vel', 10)
# Create safety monitoring timer
self.safety_timer = self.create_timer(0.1, self.safety_monitor)
self.get_logger().info('Safe Humanoid Controller initialized')
def emergency_stop_callback(self, msg):
with self.safety_lock:
self.emergency_stop_active = msg.data
if self.emergency_stop_active:
self.get_logger().error('EMERGENCY STOP ACTIVATED')
# Immediately stop all movement
self.publish_stop_command()
else:
self.get_logger().info('Emergency stop deactivated')
def laser_callback(self, msg):
# Check for obstacles in close proximity
min_distance = float('inf')
for range_val in msg.ranges:
if range_val < min_distance and not float('inf'):
min_distance = range_val
# If obstacle is too close, activate safety stop
if min_distance < 0.5 and not self.emergency_stop_active:
with self.safety_lock:
self.emergency_stop_active = True
self.get_logger().warn(f'OBSTACLE TOO CLOSE: {min_distance}m')
self.publish_stop_command()
# Publish emergency stop signal
stop_msg = Bool()
stop_msg.data = True
self.emergency_stop_pub.publish(stop_msg)
def safety_monitor(self):
# Regular safety checks
with self.safety_lock:
if not self.emergency_stop_active:
# Additional safety checks can be added here
pass
else:
# Continue to enforce stop state
self.publish_stop_command()
def publish_stop_command(self):
# Publish zero velocity to stop all movement
stop_cmd = Twist()
stop_cmd.linear.x = 0.0
stop_cmd.linear.y = 0.0
stop_cmd.linear.z = 0.0
stop_cmd.angular.x = 0.0
stop_cmd.angular.y = 0.0
stop_cmd.angular.z = 0.0
self.cmd_pub.publish(stop_cmd)
def safe_command(self, twist_cmd):
with self.safety_lock:
if not self.emergency_stop_active:
# Apply velocity limits
max_linear = 0.5 # m/s
max_angular = 0.5 # rad/s
twist_cmd.linear.x = max(min(twist_cmd.linear.x, max_linear), -max_linear)
twist_cmd.angular.z = max(min(twist_cmd.angular.z, max_angular), -max_angular)
self.cmd_pub.publish(twist_cmd)
else:
self.get_logger().warn('Command blocked - emergency stop active')
def deactivate_emergency_stop(self):
with self.safety_lock:
self.emergency_stop_active = False
stop_msg = Bool()
stop_msg.data = False
self.emergency_stop_pub.publish(stop_msg)
Safety-First Communication
from rclpy.qos import QoSProfile, ReliabilityPolicy, DurabilityPolicy
class SafetyCriticalCommunicator(Node):
def __init__(self):
super().__init__('safety_critical_communicator')
# Define QoS profile for safety-critical messages
safety_qos = QoSProfile(
depth=1, # Only keep the most recent message
reliability=ReliabilityPolicy.RELIABLE, # Ensure delivery
durability=DurabilityPolicy.TRANSIENT_LOCAL # Persist messages
)
# Create safety-critical publisher
self.safety_pub = self.create_publisher(Bool, '/safety_status', safety_qos)
# Create safety-critical subscriber
self.safety_sub = self.create_subscription(
Bool,
'/safety_command',
self.safety_command_callback,
safety_qos
)
# Log safety status regularly
self.safety_timer = self.create_timer(1.0, self.log_safety_status)
def safety_command_callback(self, msg):
# Handle safety commands with highest priority
if msg.data:
self.emergency_action()
else:
self.normal_operation()
def log_safety_status(self):
# Log safety status for auditability
status_msg = Bool()
status_msg.data = self.is_system_safe()
self.safety_pub.publish(status_msg)
self.get_logger().info(f'Safety Status: {"SAFE" if status_msg.data else "UNSAFE"}')
Safety Validation
Unit Testing Safety Functions
import unittest
from rclpy.time import Time
from std_msgs.msg import Bool
from geometry_msgs.msg import Twist
class TestSafetyFunctions(unittest.TestCase):
def setUp(self):
# Setup test environment
self.controller = SafeHumanoidController()
def test_emergency_stop_activation(self):
# Test that emergency stop properly stops movement
emergency_msg = Bool()
emergency_msg.data = True
# Simulate receiving emergency stop
self.controller.emergency_stop_callback(emergency_msg)
# Check that emergency stop is active
self.assertTrue(self.controller.emergency_stop_active)
# Try to send a movement command
test_cmd = Twist()
test_cmd.linear.x = 1.0
self.controller.safe_command(test_cmd)
# Verify the command was blocked
# (Check that no movement occurred)
def test_collision_detection_stop(self):
# Create a LaserScan message with close obstacle
scan_msg = LaserScan()
scan_msg.ranges = [0.3] * 360 # All readings show obstacle at 0.3m
# Process the scan (should trigger safety stop)
self.controller.laser_callback(scan_msg)
# Verify emergency stop is activated
self.assertTrue(self.controller.emergency_stop_active)
Safety-Related ROS 2 Features
Lifecycle Nodes for Safe Initialization
Lifecycle nodes provide a structured approach to initialization, activation, and shutdown that enhances safety:
- Unconfigured State: Node is created but not initialized
- Inactive State: Node is configured but not active
- Active State: Node is fully operational
- Finalized State: Node is shut down safely
This approach ensures that all safety checks pass before the robot becomes operational.
Quality of Service (QoS) Settings for Safety
Proper QoS settings ensure safety-critical messages are delivered reliably:
- Reliability: Use RELIABLE for safety messages
- Durability: Use TRANSIENT_LOCAL for safety parameters
- Deadline: Set appropriate deadlines for safety-critical messages
- Liveliness: Monitor that safety systems remain active
Conclusion
Safety implementation in ROS 2 requires a multi-layered approach combining emergency stop systems, collision detection, safe communication patterns, and proper validation. By following these principles and implementing the patterns shown above, developers can create humanoid robotics systems that operate safely in human environments while maintaining the functionality required for complex robotic behaviors.
All safety implementations should be thoroughly tested in simulation before deployment to physical systems, following the simulation-to-reality transfer principle that is central to safe humanoid robotics development.