As the name implies (JointControllerState
) this message is/may be used to publish the (internal) state of a joint (pid) controller.
As to the fields:
float64 set_point
the current set point (ie: desired state).
float64 process_value
the current value of the process (ie: latest sensor measurement on the controlled value).
float64 process_value_dot
first time-derivative of the process value.
float64 error
Essentially process_value - set_point
(for a regular PID implementation).
float64 time_step
time between two consecutive updates/execution of the control law (ie: delta-t).
float64 command
current output of the controller.
As an example: see the JointVelocityController in ros_control
.
Finally: it might be nice to contribute some comments to the message definition file in ros-controls/control_msgs if you now understand what the purpose of these fields is.
That would help future readers and users of the message avoid questions such as the one you posted.