Subsumption architecture

The Subsumption architecture, designed by Rodney Brooks [8], was the first of the Behavior-based architectures. In this architecture each behavior is represented as a separate layer, having direct access to sensory information. Each layer has an individual goal, and they all work concurrently and asynchronously. A layer is constructed of a set of Augmented Finite State Machines (AFSM), connected by wires through which signals can be passed from one AFSM to another. These layers are organized hierarchically, and higher levels are allowed to subsume, hence the name, lower ones. This subsumption can take form of inhibition or suppression. Inhibition eliminates the signal coming out from an AFSM of the lower level, leaving it inactive. Suppression substitutes the signal of the AFSM by the signal given by the higher level. Higher level AFSMs can also send reset signals to lower ones. These mechanisms provide a competitive, priority-based coordination.

The hierarchical organization permits an incremental design of the system, as higher layers are added on top of an already working control system, with no need of modifying the lower levels. An example of such behavior layering is depicted in Figure 2.5.

**Figure 2.5:** Example of a control system using the subsumption architecture. Each box is an AFSM, and signals are passed through the arrows connecting the AFSMs. An encircled S is a suppression point, and an empty circle is a reset point
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The main strengths of this architecture are its incremental design methodology, which makes it easy and intuitive to build a system, its hardware retargetability (each of the layers can be implemented directly on logic circuitry), and the support for parallelism, since each layer can run independently and asynchronously.

However, this theoretic independence is not absolute, since higher layers can suppress, inhibit and also read the signals of lower layers. Moreover, these connections between layers are hard-wired, so they cannot be changed during execution, thus, not allowing on-the-fly adaptability of the system to changes in the environment. One final aspect against this architecture is that it forces the designer to prioritize behaviors, therefore, the case of behaviors with equal priority cannot be represented with the subsumption architecture.