ROS parameter server - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the ROS parameter server?

The ROS parameter server is a shared, centralized storage for parameters that nodes can read from and write to during runtime. It helps nodes share configuration data easily.

beginner

How do you set a parameter on the ROS parameter server using the command line?

You use the command rosparam set <parameter_name> <value> to store a parameter on the server.

beginner

What is the difference between rosparam get and rosparam set?

rosparam set stores a parameter on the server, while rosparam get retrieves the value of a parameter from the server.

beginner

Why is the ROS parameter server useful in a robot system?

It allows different nodes to share configuration settings like robot speed or sensor thresholds without hardcoding values, making the system flexible and easier to update.

intermediate

How can a ROS node access parameters from the parameter server in code?

A node can use the ROS client library functions like getParam() to read parameters and setParam() to write parameters during runtime.

What does the ROS parameter server store?

A Robot sensor data streams

B Configuration parameters shared by nodes

C Node executable files

D Robot movement commands

The parameter server stores configuration parameters that nodes can share and access.

Which command sets a parameter on the ROS parameter server?

A rosparam get

B rosnode kill

C rosparam set

D roslaunch start

rosparam set is used to store parameters on the server.

How do nodes typically access parameters in their code?

A Using rosrun command

B By reading files directly

C By subscribing to topics

D Using getParam() and setParam() functions

Nodes use getParam() and setParam() to interact with the parameter server.

What happens if a node requests a parameter that does not exist on the server?

A The node receives a default or empty value

B The robot stops moving

C The parameter server creates it automatically

D The node crashes immediately

If a parameter is missing, nodes usually get a default or empty value, depending on the code.

Why is using the parameter server better than hardcoding values in nodes?

A It allows changing parameters without recompiling code

B It makes the robot run faster

C It stores sensor data permanently

D It replaces the need for topics

Using the parameter server lets you update settings without changing the node code.

Explain what the ROS parameter server is and why it is useful in robot systems.

Think about how different parts of a robot can share settings easily.

Describe how you would set and get a parameter on the ROS parameter server using command line tools.

Focus on the rosparam commands and their purpose.

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