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4.2. ANALYSIS OF REQUIREMENTS 23
To capture a 3D point cloud the arm has to pan the laser ranger over the shelf. This can be
done in two different ways. The first one is to move the arm to a specified position, stop there, take
the laser scan and then move on to the next position. This is the simpler approach since the whole
transformation chain is fixed at the point in time the laser scan is taken. Another approach is to
continuously take laser scans while the arm is moving. This is of course more complex, because the
transformation chain changes over time. Further the same effect as described in Section 4.1.2 appears
which makes it even more complex to accurately capture the whole 3D point cloud.
Main challenges in this use-case:
• Synchronize long transformation chains: Long chains of more than just one or two joints
have to be synchronized. This is especially challenging it the TF Data is generated by
components running at different rates.
• Consider fixed and moving state: The two different states at which a laser scan can be taken
have to be considered, because this influences the accuracy of the pose where the scan
was taken.
4.2 Analysis of Requirements
After the analysis of the use-cases described above different requirements evolve for a tf system.
Further the use of a component-based system introduces some requirements which are not directly
related to the transformation problem. Nevertheless only the use of a component-based system makes
it possible to handle the overall system complexity of an advanced robotic systems.
I. Publish Static Transformation Once
Static Transformations have to be published only once to all interested components.
The values for a Static Transformation must only be sent once to all interested components, because
they do not change over time. The interested components are the ones, where this transformation
is a node in the tf chain, the component is interested in. However it is also possible to publish the transformation
periodically, but with the drawback of wasted resources. If the TF Data is exchanged via
publish/subscribe, the framework must ensure, that components which later subscribe to the message
do get it.
II.
Publish Dynamic Transformation Continuously
Dynamic Transformations have to be published continuously when they are in moving state.
If a Dynamic Transformation is changing, because the actuator is moving, the transformation has
to be published with a previously defined frequency. This rate has to be chosen in a way, that the
needed quality is provided. A higher frequency increases the quality of the transformation as it will
be shown in Section 4.4.4.
III.
Consider Fixed and Moving State
Fixed and Moving State of a joint have to be distinguished to achieve optimal resource usage.