Hybrid Control Systems
Subsumption architectures are inherently reactive. They intend to enable real-time control, robust performance, modular development, learning through incremental growth, bottom-up design and a tight coupling between action and incoming sensory data. For some tasks, however, there is an inescapable need for “willed” rather than automatic control. Although subsumption architectures seek to avoid representation, it can be very useful to link symbolic meaning to action. Representation allows the designer to explicitly evoke intentionality from the robot. Deliberative systems can readily integrate world knowledge into a system, whereas purely reactive systems cannot.
Ideally, there should be some way to combine the benefits of a reactive strategy with a mechanism that can be aware of its environment and able to communicate about it. Hybrid approaches have attempted to make planning subordinate to reactivity and yet still use it to guide reactivity at a high level. Arkin believes that reactive and deliberative control can be complementary: “The false dichotomy that exists between hierarchical control and reactive systems should be dropped.” (Arkin 1989) In fact, planning can be viewed as a configuration for selecting behaviors. Arkin designed an autonomous robot architecture (AURA) that used representation-based, hierarchical components including a mission planner, spatial planner and plan sequencer to advise a reactive component. (Arkin 1986) NASA rovers such as Robby use a hybrid architecture that could acknowledge failure and adapt the reactive controller accordingly. (Gat 1992) Lyons used a planner to produce continuous modification of a reactive system according to some high-level goal. (Lyons 1992) In the same year, Connell developed a system with a servo layer, subsumption layer and symbolic layer. At its best, planning becomes cognition — a means to adapt online to new events as they occur. The symbolic layer should decide where to go next while the subsumption layer decides where to go now.