Dynamic Composition and Execution of Behaviors in a Hierarchical Control System

Abstract

The behavior of a system can be defined as its trajectory in state space. A system's behavior comes about through the interaction of its control law with the plant dynamics. In systems where the plant dynamics vary considerably a constant control law can lead to varying behaviors. A more consistent operation of a system can be attained by specifying the desired behavior and then, based on the current plant dynamics, selecting the appropriate control law. Assuming that only the lowest level of a hierarchical controller is responsible for selecting the appropriate control law, the rest of the hierarchy needs only to represent and operate on abstracted state spaces and behaviors. Here a hierarchical control system is presented that operates as a hierarchy of behaviors, rather than control laws, to achieve adaptive real-time control performance. A planning system at each level of the hierarchy composes sequences of behaviors that will implement a more abstract behavior specified by the next higher level. The first intermediate goal state generated by a plan is passed to the next lower level of the hierarchy as the goal state, and the planning sequence repeated. A set of relations is used to translate between the state abstractions at different levels. At the bottom level the behavior is mapped into a control law that will achieve the behavior when applied to the current plant dynamics. The system has been prototyped and demonstrated on a wheeled mobile robot platform with sonar range sensing.