HCSM

This paper presents HCSM, a framework for behavior and scenario control based on communicating hierarchical, concurrent state machines. We specify the structure and an operational execution model of HCSM's state machines. Without providing formal semantics, we provide enough detail to implement the state machines and an execution engine to run them. HCSM explicitly marries the reactive (or logical) portion of system behavior with the control activities that produce the behavior. HCSM state machines contain activity functions that produce outputs each time a machine is executed. An activity function's output value is computed as a function of accessible external data and the outputs of lower-level state machines. We show how this enables HCSM to model behaviors that involve attending to multiple concurrent concerns and arbitrating between conflicting demands for limited resources. The execution algorithm is free of order dependencies that cause robustness and stability problems in behavior modeling. In addition, we examine the problems of populating virtual environments with autonomous agents exhibiting interesting behavior and of authoring scenarios involving such agents. We argue that HCSM is well suited for modeling the reactive behavior of autonomous agents and for directing such agents to produce desired situations. We demonstrate use of HCSM for modeling vehicle behavior and orchestrating scenarios in the Iowa Driving Simulator, an immersive real-time virtual driving environment.