ROS2 parameters are variables used to store values that can be changed during runtime - from the terminal. ROS2 parameters are stored in a centralized parameter server, which allows nodes to access and modify them dynamically. ROS2 parameters can be of various types, including integers, floating-point numbers, strings, booleans, arrays, and more.
Let us once again take a look at the rpm_subscriber.py source code - which we created in Project 1.2
#! /usr/bin/env python3
import rclpy
from std_msgs.msg import Float32
from rclpy.node import Node
DIAMETER = 10.0 # meters # Float
class RPMSubscriber(Node):
def __init__(self):
super().__init__("rpm_sub_node")
self.sub = self.create_subscription(Float32, 'rpm', self.subscriber_callback_func, 10)
self.pub = self.create_publisher(Float32, 'speed', 10)
def subscriber_callback_func(self, msg):
SPEED = (msg.data * 3.14 * DIAMETER)/60 # SPEED = DISTANCE/TIME
# DISTANCE = WHEEL CIRCUMFERENCE * RPM
# TIME = 60 secs (1 minute)
msg = Float32()
msg.data = float(SPEED)
self.pub.publish(msg)
def main(args=None):
rclpy.init()
rpm_sub_node = RPMSubscriber()
print("Waiting for rpm data to be published...")
try:
rclpy.spin(rpm_sub_node)
except KeyboardInterrupt:
print("Terminating subscriber...")
rpm_sub_node.destroy_node()
if __name__ == '__main__':
main()Here, we have hard-coded the value of DIAMETER using the DIAMETER = 10.0 line of code. Now, if we need to change the value of DIAMETER (lets say we want to use this code for some different robot with wheels of different diameter), we would usually think of directly editing the hard-coded value of DIAMETER variable in rpm_subscriber.py code i.e at DIAMETER = 10.0 . But by using the ROS Parameter Concept, we can change the value of the DIAMETER that is being used by the rpm_subscriber.py code — without directly editing the source code file rpm_subscriber.py — from the terminal itself.
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Open a new terminal in the workspace folder → Run the rpm_publisher.py code using the below command.
source install/setup.bash ros2 run udemy_ros2_pkg rpm_publisher.py
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Open a new terminal in the workspace folder → Run the rpm_subscriber.py code using the below command.
source install/setup.bash ros2 run udemy_ros2_pkg rpm_subscriber.py
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Open a new terminal and run the below command.
ros2 param list
You will see a list of some default parameters for both the currently running nodes (/rpm_pub_node & /rpm_sub_node).
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To see the value of a parameter run the following command from the same terminal:
#ros2 param get <node_name> <param_name> ros2 param get /rpm_pub_node use_sim_time # Checking the value of use_sim_time - a default parameter of rpm_pub_node.
You will see that use_sim_time has a Boolean type value which is currently set to False — by default — because this node is currently using the time of your local computer and not that of a simulation environment . You set the value of use_sim_time parameter to true if you want to use the time being generated by a simulation which you may have been running.
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If you want to see more information about a parameter, use the following command from the same terminal:
#ros2 param describe <node_name> <param_name> ros2 param describe /rpm_pub_node use_sim_time
#! /usr/bin/env python3
import rclpy
from std_msgs.msg import Float32
from rclpy.node import Node
DEFAULT_DIAMETER = 10.0 # meters
class RPMSubscriber(Node):
def __init__(self):
super().__init__("rpm_sub_node")
self.declare_parameter('wheel_diameter', DEFAULT_DIAMETER)
# Creating a parameter object named 'wheel_radius'and setting its default value equal to DEFAULT_DIAMETER variable.
self.sub = self.create_subscription(Float32, 'rpm', self.subscriber_callback_func, 10)
self.pub = self.create_publisher(Float32, 'speed', 10)
# print(self.get_parameter('wheel_diameter').get_parameter_value())
# printing the value of the 'wheel_diameter' parameter object
# Output:
#*********
# rcl_interfaces.msg.ParameterValue(type=3, bool_value=False, integer_value=0, double_value=10.0, string_value='', byte_array_value=[], bool_array_value=[], integer_array_value=[], double_array_value=[], string_array_value=[])
def subscriber_callback_func(self, msg):
wheel_diameter_param = self.get_parameter('wheel_diameter').get_parameter_value().double_value
# storing the 'double_value' attribute of the 'wheel_diameter' parameter object, inside the 'wheel_diameter_param' variable.
# 'double_value' is the attribute of the 'wheel_diameter' parameter object which stores the default 'double' type value of DEFAULT_DIAMETER (= 10.0) variable.
SPEED = (msg.data * 3.14 * wheel_diameter_param)/60
# SPEED = DISTANCE/TIME
# DISTANCE = WHEEL CIRCUMFERENCE * RPM
# TIME = 60 secs (1 minute)
msg = Float32()
msg.data = float(SPEED)
self.pub.publish(msg)
def main(args=None):
rclpy.init()
rpm_sub_node = RPMSubscriber()
print("Waiting for rpm data to be published...")
try:
rclpy.spin(rpm_sub_node)
except KeyboardInterrupt:
print("Terminating subscriber...")
rpm_sub_node.destroy_node()
if __name__ == '__main__':
main()-
Open a new terminal in the workspace folder → Run the rpm_publisher.py code using the below command.
source install/setup.bash ros2 run udemy_ros2_pkg rpm_publisher.py -
Open a new terminal in the workspace folder → Run the rpm_subscriber.py code using the below command.
source install/setup.bash ros2 run udemy_ros2_pkg rpm_subscriber.py -
Open a new terminal in the workspace folder and run the following commands:
ros2 topic list # Gives the list of currently running ros2 topics ros2 topic echo /speed # /speed is the topic to which our rpm_sub_node # is publishing its calculated speed values.
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Open a new terminal and run the following commands:
ros2 param list # To get the current list of active ros2 parameters in the # central parameters pool ros2 param get /rpm_sub_node wheel_diameter # To get the current value of parameter wheel_diameter ros2 param set /rpm_sub_node wheel_diameter 15.23 # To set a new value for parameter wheel_diameter # The changes will reflect in the runtime itself. # Changing the wheel diameter is going to change the # calculated speed of the robot. # Check the new values of speed echoed by the /speed topic # in terminal 3.
