Parameter

Almost all ROS programs require parameters. One of the advantages of using ROS is that the parameters can be managed by ROS, which means:

Listing (get) parameters registered to the nodes in real-time.
Loading parameters from a file without coding these things (opens in a new tab) on your own.
Dynamically setting parameters on node startup.

Examine Parameters of Running Nodes

List of Parameters

Before we start, let's run turtlesim_node by

 ros2 run turtlesim turtlesim_node

We will identify the associated parameters by:

Getting the list of parameters of /turtlesim

rosmarry@rosmarry:~$ ros2 param list /turtlesim # ros2 param list <node_name>
  background_b
  background_g
  background_r
  use_sim_time

We also run another executable turtle_teleop_key by

ros2 run turtlesim turtle_teleop_key

We now have two nodes /turtlesim and /teleop_turtle. This time, we will get the whole list of parameters of running nodes:

Getting whole list of parameters

rosmarry@rosmarry:~$ ros2 param list
/teleop_turtle:
  scale_angular
  scale_linear
  use_sim_time
/turtlesim:
  background_b
  background_g
  background_r
  use_sim_time

Getting Value of Parameters

Okay, we found total 6 parameters. Let us get the value of /turtlesim/background_r by:

Getting the value of a parameter

rosmarry@rosmarry:~$ ros2 param get /turtlesim background_r # ros2 param get <node_name> <parameter_name>
Integer value is: 69

We can get this value by calling the service /turtlesim/get_parameters as:

Getting the value of a parameter from service call

rosmarry@rosmarry:~$ ros2 service call /turtlesim/get_parameters rcl_interfaces/srv/GetParameters "{names: [background_r]}"
waiting for service to become available...
requester: making request: rcl_interfaces.srv.GetParameters_Request(names=['background_r'])
 
response:
rcl_interfaces.srv.GetParameters_Response(values=[rcl_interfaces.msg.ParameterValue(type=2, bool_value=False, integer_value=69, double_value=0.0, string_value='', byte_array_value=[], bool_array_value=[], integer_array_value=[], double_array_value=[], string_array_value=[])])

Loading Parameters

We identified what parameters the node /turtlesim has from ros2 param list /turtlesim, and their default values. The values of the parameters can be set when startup.

Setting Parameters by CLI

We can use --ros-args to set the parameters.

Setting parameter when ros2 run

ros2 run turtlesim turtlesim_node --ros-args -p background_r:=200

This will change the color of the GUI, as we changed the red color channel.

Setting Parameters from a File (yaml)

Assume that we have a bunch of parameters to be set. If then, having a file containing parameter values will be more convenient than manually writing all the parameters.

Let us write a file your_param.yaml setting the values:

Parameter description

turtlesim:
  ros__parameters: # parameters
    background_r: 200
    background_g: 200

Run the node by loading this file

Running node with parameter file

ros2 run turtlesim turtlesim_node --ros-args --params-file your_param.yaml

This would set a different background color (sky) for the GUI.

Namespace

We learned the namespace of a node in the previous chapter.

What if we run the node in a namespace my_namespace?

ros2 run turtlesim turtlesim_node --ros-args --params-file your_param.yaml -r __ns:=/my_namespace

This does not apply parameter values. The background color is the default. Putting the namespace nested even your parameters as ros2 param list shows:

rosmarry@rosmarry:~$ ros2 param list
/my_namespace/turtlesim:
  background_b
  background_g
  background_r
  use_sim_time

The contents of your_param.yaml should be wrapped inside the namespace:

Parameter file reflecting namespace

my_namespace:
  turtlesim:
    ros__parameters:
      background_r: 150
      background_g: 200

Now, running the below command will open a sky background:

ros2 run turtlesim turtlesim_node --ros-args --params-file your_param.yaml -r __ns:=/my_namespace

ℹ️

A homework. How should we write your_param.yaml if we run the turtlesim_node in a different name as ros2 run turtlesim turtlesim_node --ros-args -r __node:=crazy_turtle?

Dynamically Setting Parameters of Running Nodes

One of the most powerful features of ROS parameters is that we can change the parameter values on-the-fly. This is very useful in the case such as the tuning of control gains. Turn off everything and rerun turtlesim_node by

 ros2 run turtlesim turtlesim_node

This will open the blue-background GUI. We can change the red channel by

Changing parameter by param set

rosmarry@rosmarry:~$ ros2 param set /turtlesim background_r 200
Set parameter successful

Service for Parameter Changing

We can set the parameter by calling a service of the type []rcl_interfaces/srv/SetParameters](https://docs.ros2.org/foxy/api/rcl_interfaces/srv/SetParameters.html (opens in a new tab)). This approach has the following two advantages:

A node can change the parameters of another node by having a service client for SetParameters.
We can change multiple parameters simultaneously.

This can be done by the below long command:

Changing two parameters at the same time

ros2 service call /turtlesim/set_parameters rcl_interfaces/srv/SetParameters "{parameters: [{name: "background_r", value: {type: 2, integer_value: 200}}, {name: "background_g", value: {type: 2, integer_value: 200}}]}"

The result is as follows:

rosmarry@rosmarry:~$ ros2 service call /turtlesim/set_parameters rcl_interfaces/srv/SetParameters "{parameters: [{name: "background_r", value: {type: 2, integer_value: 200}}, {name: "background_g", value: {type: 2, integer_value: 200}}]}"
waiting for service to become available...
requester: making request: rcl_interfaces.srv.SetParameters_Request(parameters=[rcl_interfaces.msg.Parameter(name='background_r', value=rcl_interfaces.msg.ParameterValue(type=2, bool_value=False, integer_value=200, double_value=0.0, string_value='', byte_array_value=[], bool_array_value=[], integer_array_value=[], double_array_value=[], string_array_value=[]))])
 
response:
rcl_interfaces.srv.SetParameters_Response(results=[rcl_interfaces.msg.SetParametersResult(successful=True, reason='')])