In the ETF Q library.
The reference frames that make up our robot or any application can be mainly of two types static or
dynamic.
Let's go back to the RDF model of the robot and the links and the joints.
We used to create the reference frames and connect them to each other using transformation matrices.
Some joints are defined in the RDF model of type fixed and therefore the transformation matrices that
connect the two reference frames are static, so they do not change over time.
Disconnections between static reference frames often refer to actual mechanical connection between components.
For example, when they are bolted together, such as when mounting a sensor like a camera on the robot
structure.
In this case, the camera link is fixed, so is static and does not move with respect to the robot's
base footprint.
Regarding the transformation matrices that connects static reference frames which are only connected
by a fixed joints.
Since the transformation matrix is static, its value do not change over time, and so it is published
only once within the two topic called TF Static.
As for movable joints such as those that actuate the robot's arm, the transformation matrix that express
how the links of the robot rotate can vary over time depending on how the motors are actuated and thus
rotated.
In this case, this transformation is repeatedly published with a certain frequency within the TF topic.
At each time step, each motor can be in a different position relative to the robot's reference frame,
and thus the transformation matrix that connects the motor link to the robot's base is published in
the TF topic for that specific time step.
In our subsequent time step, A new transformation matrix that connects these two frames will be again
published in the TF topic, representing the new position of the joints of the robot.
All this information that is published within the TF and TF static topics can then be used, can then
be read by other nodes such as RVs, for example, to visualize the static model of the robot in real
time and observe the movement of its dynamic parts.