Chapter-11.1-ROS2-Parameters-(C++).md

Chapter 11.1. ROS2 Parameters(C++)

Definition:

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.

Purpose/Use of Parameters

Let us once again take a look at the rpm_publisher.cpp source code - which we created in Project 1.1

rpm_publisher.cpp code:

// Including the rclcpp library - for ros2 c++ functionality.
#include "rclcpp/rclcpp.hpp"
// Next we are importing the interface of the messages we are going to publish through this node.
#include "std_msgs/msg/float64.hpp"

#include "chrono"
#include "functional"

using namespace std::chrono_literals;

const double RPM_VALUE = 100.0;  //<<--<<--<<--<<--

class RpmPubNode : public rclcpp::Node
{
private:
    rclcpp::Publisher<std_msgs::msg::Float64>::SharedPtr rpm_publisher_;
    rclcpp::TimerBase::SharedPtr timer_;
    void publish_rpm()
    {
        auto rpm_value = std_msgs::msg::Float64();
        rpm_value.data = RPM_VALUE;
        rpm_publisher_->publish(rpm_value);
    }

public:
    RpmPubNode() : Node("rpm_pub_node")
    {
        rpm_publisher_ = this->create_publisher<std_msgs::msg::Float64>("rpm", 10);
        timer_ = this->create_wall_timer(1s, std::bind(&RpmPubNode::publish_rpm, this));
        std::cout<<"RPM Publisher Node Is Running..."<<std::endl;
    }
};

int main(int argc, char *argv[])
{
    rclcpp::init(argc, argv);
    rclcpp::spin(std::make_shared<RpmPubNode>());
    rclcpp::shutdown();
    return 0;
}

Here, we have hard-coded the value of RPM using the const double RPM_VALUE = 100.0; line of code. Now, if we need to change the value of const double RPM_VALUE published by the rpm_publisher, we would usually think of directly editing the hard-coded value in rpm_publisher.cpp code i.e at const double RPM_VALUE = 100.0; . But by using the ROS Parameter Concept, we can change the value of the RPM that is being published by the rpm_publisher node — without directly editing the source code file rpm_publisher.cpp — from the terminal itself.

1. Open a new terminal in the workspace folder → Run the rpm_publisher node using the below command.

   source install/setup.bash
   ros2 run udemy_ros2_pkg rpm_publisher

2. Open a second terminal in the workspace folder → Run the rpm_subscriber node using the below command.

   source install/setup.bash
   ros2 run udemy_ros2_pkg rpm_subscriber

3. Open a third terminal in the workspace folder and run the below command.

   ros2 param list

   

   You will see a list of some default parameters for both the currently running nodes (rpm_publisher & rpm_subscriber).

4. To see the value of a parameter run the following command:

   #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 it uses the time of your local computer. 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.

5. If you want to see more information about a parameter, use the following command:

   #ros2 param describe <node_name> <param_name>
   ros2 param describe rpm_pub_node use_sim_time

   

Using ROS2 Parameters Concept in rpm_publisher.cpp Source Code

#include "rclcpp/rclcpp.hpp"
#include "std_msgs/msg/float64.hpp"

#include "chrono"
#include "functional"

using namespace std::chrono_literals;

const double RPM_DEFAULT_VALUE = 100.0; // Default constant RPM Value --- STEP 1

class RpmPubNode : public rclcpp::Node
{
private:
    rclcpp::Publisher<std_msgs::msg::Float64>::SharedPtr rpm_publisher_;
    rclcpp::TimerBase::SharedPtr timer_;

    double rpm_param_val_; // Creating a double variable rpm_param_val_ : for holding the value of 'rpm_val' parameter --- STEP 3

    void publish_rpm()
    {
        auto rpm_value = std_msgs::msg::Float64();

        // WAY 1:

        this->get_parameter("rpm_val", rpm_param_val_);
        // Using the 'Node::get_parameter()' method to summon the 'rpm_val' parameter and store its value (RPM_DEFAULT_VALUE) inside the 'rpm_param_val' variable. --- STEP 4

        rpm_value.data = rpm_param_val_; // --- STEP 5
        // We can change the value of rpm_value.data --> by changing the value of rpm_param_val_ variable --> by changing the value of 'rpm_val' parameter i.e associated with rpm_param_val_ variable ----> From the Terminal itself ---> Without having to change the actual source code (the value of RPM_DEFAULT_VALUE = 100.0 at line 9 of this file).

        // WAY 2:

        // rclcpp::Parameter rpm_param_val_object = this->get_parameter("rpm_val");
        // rpm_value.data = rpm_param_val_object.as_double();

        rpm_publisher_->publish(rpm_value);
    }

public:
    RpmPubNode() : Node("rpm_pub_node")
    {
        this->declare_parameter<double>("rpm_val", RPM_DEFAULT_VALUE); // Using the  'Node::declare_paramerter<ValueT>()' method to create a parameter 'rpm_val' for holding a 'double' type value and setting its default value to RPM_DEFAULT_VALUE   --- STEP 2
        rpm_publisher_ = this->create_publisher<std_msgs::msg::Float64>("rpm", 10);
        timer_ = this->create_wall_timer(1s, std::bind(&RpmPubNode::publish_rpm, this));
        std::cout << "RPM Publisher Node Is Running..." << std::endl;
    }
};

int main(int argc, char *argv[])
{
    rclcpp::init(argc, argv);
    rclcpp::spin(std::make_shared<RpmPubNode>());
    rclcpp::shutdown();
    return 0;
}

1. Before proceeding, make sure to build your code.

2. To see our new rpm_val parameter in the terminal, run the following commands:

   1st Terminal:

   source install/setup.bash
   ros2 run udemy_ros2_pkg rpm_publisher

   2nd Terminal:

   ros2 param list

   

3. To check the value of rpm_val parameter from the terminal, run the following command:

   2nd Terminal:

   ros2 param get rpm_pub_node rpm_val
   #ros2 param get <node_name> <param_name>
   #You will get the current value of the rpm_val parameter

4. To see the messages published by rpm_publisher node:

   3rd Terminal Parallel To 2nd Terminal:

   ros2 topic echo rpm

5. To change the value of RPM that is published by rpm_publisher node — from the terminal — without making any changes to the source code — run the following command:

   2nd Terminal:

   #ros2 param set <node_name> <parameter_name> <new_value>
   ros2 param set rpm_pub_node rpm_val 50.0