Mastering ROS for Robotics Programming（Second Edition）

上QQ阅读APP看书，第一时间看更新

Using the ROS parameter

When programming a robot, we might have to define robot parameters, such as robot controller gains P, I, and D. When the number of parameters increases, we might need to store them as files. In some situations, these parameters have to share between two or more programs too. In this case, ROS provides a parameter server, which is a shared server in which all ROS nodes can access parameters from this server. A node can read, write, modify, and delete parameter values from the parameter server.

We can store these parameters in a file and load them into the server. The server can store a wide variety of data types and can even store dictionaries. The programmer can also set the scope of the parameter, that is, whether it can be accessed by only this node or all the nodes.

The parameter server supports the following XMLRPC datatypes:

32-bit integers
Booleans
Strings
Doubles
ISO8601 dates
Lists
Base64-encoded binary data

We can also store dictionaries on the parameter server. If the number of parameters is high, we can use a YAML file to save them. Here is an example of the YAML file parameter definitions:

/camera/name : 'nikon'  #string type 
/camera/fps : 30     #integer   
/camera/exposure : 1.2  #float  
/camera/active : true  #boolean

The rosparam tool is used to get and set the ROS parameter from the command line. The following are the commands to work with ROS parameters:

$ rosparam set [parameter_name] [value]: This command will set a value in the given parameter
$ rosparam get [parameter_name]: This command will retrieve a value from the given parameter
$ rosparam load [YAML file]: The ROS parameters can be saved into a YAML file and it can load to the parameter server using this command
$ rosparam dump [YAML file]: This command will dump the existing ROS parameters to a YAML file
$ rosparam delete [parameter_name]: This command will delete the given parameter
$ rosparam list: This command will list existing parameter names

The parameters can be changed dynamically during the execution of the node that uses these parameters, using the dyamic_reconfigure package (http://wiki.ros.org/dynamic_reconfigure).