ROS parameter server

Introduction

The ROS parameter server stores settings and data that nodes can share and use. It helps keep configuration in one place so many parts of a robot system can access it easily.

You want to set robot speed limits that many nodes need to know.

You need to share sensor calibration values between different parts of your robot software.

You want to change settings without restarting your robot program.

You want to store configuration data that can be read by multiple nodes at runtime.

You want to keep parameters persistent while your robot is running.

Syntax

ROS

rosparam set /parameter_name value
rosparam get /parameter_name
rosparam delete /parameter_name
rosparam list

Parameters are stored as key-value pairs on the parameter server.

Use '/' at the start of parameter names for global scope.

Examples

Sets a parameter named /robot_speed with the value 1.5.

ROS

rosparam set /robot_speed 1.5

Retrieves the value of the /robot_speed parameter.

ROS

rosparam get /robot_speed

Removes the /robot_speed parameter from the server.

ROS

rosparam delete /robot_speed

Shows all parameters currently stored on the server.

ROS

rosparam list

Sample Program

This Python ROS node sets a parameter called /greeting with a message. It then reads and prints it. After deleting the parameter, it tries to read it again and prints a default message because the parameter no longer exists.

ROS

#!/usr/bin/env python3
import rospy

rospy.init_node('param_example_node')

# Set a parameter
rospy.set_param('/greeting', 'Hello, ROS!')

# Get the parameter
message = rospy.get_param('/greeting', 'Default message')

print(message)

# Delete the parameter
rospy.delete_param('/greeting')

# Try to get the deleted parameter with default
message_after_delete = rospy.get_param('/greeting', 'Parameter not found')

print(message_after_delete)

Important Notes

Parameters can be simple types like numbers, strings, lists, or dictionaries.

Use rosparam command line tool or API functions to interact with parameters.

Parameters are useful for tuning robot behavior without changing code.

Summary

The ROS parameter server stores shared settings for nodes.

Use rosparam commands or API to set, get, and delete parameters.

Parameters help keep robot configuration organized and accessible.

Practice

1. What is the main purpose of the ROS parameter server?

easy

A. To compile ROS packages

B. To store and share configuration settings between ROS nodes

C. To execute robot movement commands

D. To visualize sensor data in real-time

Solution

1. Step 1: Understand the role of the parameter server

   The ROS parameter server is designed to hold parameters that nodes can access and share, such as configuration values.

2. Step 2: Compare options with this role

   Options A, B, and D describe other ROS functionalities unrelated to parameter storage.

3. Final Answer:

   To store and share configuration settings between ROS nodes -> Option B

4. Quick Check:

   Parameter server = shared settings [OK]

Hint: Parameter server holds shared settings, not commands or data [OK]

Common Mistakes:

• Confusing parameter server with command execution
• Thinking it compiles code
• Mixing it up with visualization tools

2. Which of the following is the correct command to set a parameter named robot_speed to 1.5 using rosparam?

easy

A. rosparam set robot_speed 1.5

B. rosparam get robot_speed 1.5

C. rosparam delete robot_speed 1.5

D. rosparam list robot_speed 1.5

Solution

1. Step 1: Identify the command to set a parameter

   The rosparam set command is used to assign a value to a parameter on the parameter server.

2. Step 2: Verify the syntax

   The syntax is rosparam set <param_name> <value>, so rosparam set robot_speed 1.5 is correct.

3. Final Answer:

   rosparam set robot_speed 1.5 -> Option A

4. Quick Check:

   Set parameter = rosparam set [OK]

Hint: Use 'rosparam set' to assign values to parameters [OK]

Common Mistakes:

• Using 'get' instead of 'set' to assign values
• Trying to delete or list with value arguments
• Incorrect command syntax

3. Given the following Python ROS node code snippet, what will be printed if the parameter max_speed is not set on the parameter server?

import rospy
rospy.init_node('speed_node')
max_speed = rospy.get_param('max_speed', 2.0)
print(max_speed)

medium

A. It will print 0

B. It will print None

C. It will print 2.0

D. It will raise a KeyError

Solution

1. Step 1: Understand the use of rospy.get_param with default

   The function rospy.get_param('max_speed', 2.0) tries to get the parameter 'max_speed'. If it does not exist, it returns the default value 2.0.

2. Step 2: Apply this to the scenario

   Since 'max_speed' is not set, the default 2.0 will be assigned and printed.

3. Final Answer:

   It will print 2.0 -> Option C

4. Quick Check:

   Missing param returns default = 2.0 [OK]

Hint: get_param returns default if param missing [OK]

Common Mistakes:

• Expecting an error if parameter missing
• Assuming None is returned by default
• Confusing default value usage

4. You wrote this code to get a parameter but it crashes with a KeyError. What is the problem?

import rospy
rospy.init_node('node')
value = rospy.get_param('unknown_param')
print(value)

medium

A. The parameter 'unknown_param' does not exist and no default was provided

B. The node was not initialized before getting the parameter

C. The print statement is incorrect syntax

D. The parameter server is not running

Solution

1. Step 1: Analyze the get_param call without default

   Calling rospy.get_param('unknown_param') without a default raises a KeyError if the parameter does not exist.

2. Step 2: Confirm the cause of the crash

   Since 'unknown_param' is missing and no default is given, the code crashes with KeyError.

3. Final Answer:

   The parameter 'unknown_param' does not exist and no default was provided -> Option A

4. Quick Check:

   Missing param + no default = KeyError [OK]

Hint: Always provide default or check param exists to avoid KeyError [OK]

Common Mistakes:

• Assuming node init causes KeyError
• Thinking print syntax is wrong
• Ignoring parameter server status

5. You want to set multiple parameters at once for a robot configuration using a YAML file named config.yaml. Which command correctly loads all parameters from this file onto the ROS parameter server under the namespace /robot?

hard

A. rosparam set config.yaml /robot

B. rosparam delete config.yaml /robot

C. rosparam get config.yaml /robot

D. rosparam load config.yaml /robot

Solution

1. Step 1: Identify the command to load parameters from a file

   The rosparam load command loads parameters from a YAML file onto the parameter server.

2. Step 2: Confirm the correct syntax for namespace

   Using rosparam load config.yaml /robot loads all parameters under the /robot namespace.

3. Final Answer:

   rosparam load config.yaml /robot -> Option D

4. Quick Check:

   Load YAML file with rosparam load [OK]

Hint: Use 'rosparam load' to import YAML parameters [OK]

Common Mistakes:

• Using 'set' instead of 'load' for files
• Trying to 'get' or 'delete' files
• Omitting namespace or using wrong syntax