Discrete Event Process Modeling of Manufacturing Systems Using Sensor Graphs
Tord Alenljung · Chalmers Publication Library (Chalmers University of Technology) · 2009
The design of control programs for manufacturing systemsis becoming more and more complex as the demands for flexibility,efficiency and reconfigurability increase, due tochanging consumer demands and fierce competition.There are methods stemming from academia that can assist the control programmersin facing these challenges. Some of those methods require a formal modelof the system to be controlled.This thesis presents the language Sensor Graphs for modeling systemsthat are to be controlled by a PLC.The system under control, the process, is typically mechanical,containing equipment such as lifts, fixtures and robots, aswell as mobile objects like pallets and manufacturing parts.The controller observes the state of the processthrough binary position sensors and influences it through binary control signals.The Sensor Graph model describes, as a discrete event system,how the physical objects activateand deactivate the sensors under the influence of control signals.The purpose of the suggested language is to make modeling easier and less fallible for the kind of systems described above, compared to existing general purpose discrete event languages.The language has a graphical syntax that often makes the models more compact and concise than models expressed using classical state machine-or Petri net-based languages.It does also support hierarchical and component-based modeling.It is shown how a closed-loop system can be formally verified basedon a Sensor Graph process model and a discrete state equation representingthe PLC program.It is also shown how the Sensor Graph process model can be translatedinto an observer that provides the controller with an estimate of the process' state. This state estimate is used for state feedback control and automated fault monitoring.The semantics of Sensor Graphs takes care of the details concerning theinteraction between the discrete event process and the discrete-time controller,for instance the computation delay between thecontroller's input sampling and its output response, and the timing of theprocess in relation to the controller.