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Section 2: Working with Parameters

Estimated Reading Time: 8 minutes

What are Parameters?​

Parameters are configuration values stored on the ROS 2 parameter server. They allow you to:

  • Configure node behavior at runtime (no recompile needed)
  • Change settings without restarting the node
  • Share configuration across multiple nodes

Common parameters:

  • max_velocity: Speed limits for safety
  • control_frequency: How often to update (Hz)
  • enable_logging: Debug output on/off
  • robot_name: Which robot to control

Declaring Parameters​

Before using a parameter, declare it with a default value:

self.declare_parameter('max_velocity', 1.0)
self.declare_parameter('control_frequency', 10.0)
self.declare_parameter('enable_logging', True)
self.declare_parameter('robot_name', 'default_robot')

Reading Parameters​

Read parameter values in your code:

# Read a double parameter
max_vel = self.get_parameter('max_velocity').get_parameter_value().double_value

# Read a boolean parameter
enable_log = self.get_parameter('enable_logging').get_parameter_value().bool_value

# Read a string parameter
robot_name = self.get_parameter('robot_name').get_parameter_value().string_value

Using Parameters in Code​

Use parameter values to control behavior:

# Clamp desired velocity to max
desired_vel = 2.0
max_velocity = self.get_parameter('max_velocity').get_parameter_value().double_value
actual_vel = min(desired_vel, max_velocity)

msg = Twist()
msg.linear.x = actual_vel
self.publisher_.publish(msg)

Reacting to Parameter Changes​

Register a callback to respond when parameters change:

self.add_on_set_parameters_callback(self.parameter_callback)

def parameter_callback(self, params):
    for param in params:
        if param.name == 'max_velocity':
            self.get_logger().info(f'Max velocity changed to {param.value}')
    return SetParametersResult(successful=True)

Using Parameters from Command Line​

View and modify parameters without restarting:

# List all parameters
ros2 param list

# Get a parameter value
ros2 param get /node_name max_velocity

# Set a parameter
ros2 param set /node_name max_velocity 2.5

Example: Parameter-Based Controller​

See param_node.py in code-examples for a complete example that:

  1. Declares parameters with defaults
  2. Reads parameters in callbacks
  3. Responds to parameter changes
  4. Uses parameters to limit behavior

Benefits of Parameters​

  • Flexibility: Change behavior at runtime
  • Reusability: Same code works for different robots
  • Tuning: Adjust PID gains, speeds, without recompiling
  • Safety: Limit maximum speeds without code changes

Key Takeaways​

  • Parameters are configuration values on the parameter server
  • Declare parameters with default values
  • Read parameters using get_parameter()
  • React to changes with callbacks
  • Change at runtime using ros2 param set
  • Use parameters to make code flexible and reusable

Next: Section 3: Actions