| The machining systems considered in this dissertation are high volume transfer lines which are widely used in automotive manufacturing. In these machining systems, several machines are linked together to provide complete processing of a machined part. The logic controller for a machining system is a discrete event supervisory system. Generally, it has three control modes: auto, hand, and manual. This dissertation addresses a formal representation of logic controllers with three control modes in modularized structures using Petri net formalism. By considering the fault recovery processes of operations, two operation modules are developed. The structure of the modular logic controller is introduced and its properties are verified using well-known theorems for state machines. Each station control module can be represented by connecting operation modules; their connection algorithms are provided. If the connection algorithms are followed, live, safe, and reversible Petri nets can be obtained. A condition for a modular logic controller to generate correct control logic is presented. A modular logic controller for the normal operation cycle is introduced and formalized for performance analysis; its properties are verified using well-known theorems for marked graphs. The performance analysis using the modular logic controller for the normal operation cycle is introduced. The cycle time during normal operation is used as the performance metric of a transfer line. By adding time specifications to the modular logic controller for the normal operation cycle, a timed modular logic controller is generated. The time-based cyclic behavior of high volume transfer lines is thus characterized. Two efficient algorithms to compute the cycle time and critical operations are developed. It is also shown that this modular structure can be used in control logic reconfiguration and automatic code generation. |
