Reactive Interaction Through Body Motion and the Phase-State-Machine

Abstract

Between humans, body motion is an intuitive and ubiquitous means to coordinate interactions, but so far collaborative robots have not embraced this mode of communication. One reason for this is that conventional behavior generation systems use finite state machines which make it exceptionally difficult to ingest and produce the inherently continuous and concurrent information flow that body motion provides. We propose a new, reactive motion generator based on a dynamical system instead. It mimics a conventional state machine except that transitions are not instantaneous but time-extended, reversible, and have phases. Moreover, consecutive states and transitions may overlap during execution. Most notably, more than one transition can be active at the same time if they share predecessor state. Together, these unique features enable instantaneous and gradual responses to novel information and continuous decision processes without changing the state graph itself. We demonstrate the system's capabilities in an object handover task coordinated by body motion.