You might think you know programming languages, from VHDL to Scheme to Java, however most of the times, knowledge of the language itself is just the beginning. This week I spent three days creating a subsumption architecture using the Boost Graph Library. The latter is part of the Boost libraries, that are such an entire source of additional knowledge on top of ordinary C++ know-how.

Over time several robot architectures have been implemented. I will leave out very interesting ones (see also the internship description and the Replicator deliverables, if you are interested in those). A comparison between three architectures is given by Georgas and Taylor. They compare the subsumption architecture with the three-layer 3L approach and the reactive-concentric architecture in An Architectural Style Perspective on Dynamic Robot Architectures. Most conspicuous in this article is by the way the ease with which layers are accepted as the way to organize a robot brain. It is without doubt that the brain is built up in a modular fashion. However, how this modularity is actually materialized needs to be handled very carefully.

Notwithstanding those concerns, Brooks' subsumption architecture is the prime example of a robot architecture. The movie at the top is an appetizer of Brooks' film "Rodney's Robot Revolution" in which he wants to create a robot physically playing the board game Go. But for now we will only focus on the subsumption architecture itself. This is in incredibly simple structure. Namely of tasks or controllers, in a layered structure. Each controller might give commands. For example there is a collision avoidance controller reading infrared sensors and sending commands to the wheels to avoid collisions. However, in certain circumstances, for example when a robot needs to dock to another robot, this controller needs to be overruled. In a subsumption architecture the output of the collision avoidance controller is subsumed by the output of the docking controller. In this way an entire hierarchy of controllers can be built.

The Boost Graph Library contains a lot of predefined classes. Because of the use of generics the vertex and edge entities (or descriptors) can be easily replaced by custom made versions. This was not needed for the purpose of presenting a subsumption hierarchy by a graph though. At our SVN server you can see (check also this corresponding test unit) how this becomes incredibly easy. For example, output to GraphViz in the form of a .dot file (a kind of XML for graphs, although there are more XML-like versions) is just one line of code! Like always, feel free to use the code, it is open-source.