Reactivity and statefulness: Action-based sensors, plans, and necessary state

Dylan Shell, Grace McFassel

Abstract:

Typically to a roboticist, a plan is the outcome of other work, a synthesized object that realizes ends defined by some
problem; plans qua plans are seldom treated as first-class objects of study. Plans designate functionality: a plan can
be viewed as defining a robot’s behavior throughout its execution. This informs and reveals many other aspects of
the robot’s design, including: necessary sensors and action choices, history, state, task structure and how to define
progress. Interrogating sets of plans helps in comprehending the ways in which differing executions influence the
interrelationships between these various aspects. Revisiting Erdmann’s theory of action-based sensors, a classical
approach for characterizing fundamental information requirements, we show how plans (in their role of designating
behavior) influence sensing requirements. Using an algorithm for enumerating plans, we examine how some plans for
which no action-based sensor exists can be transformed into sets of sensors through the identification and handling of
features that preclude the existence of action-based sensors. We are not aware of those obstructing features having
been previously identified. Action-based sensors may be treated as standalone reactive plans; we relate them to the set
of all possible plans through a lattice structure. This lattice reveals a boundary between plans with action-based sensors
and those without. Some plans, specifically those that are not reactive plans and require some notion of internal state,
can never have associated action-based sensors. Nevertheless, action-based sensors can serve as a framework to
explore and interpret how such plans make use of state.