Generalized front-end controller design

Abstract

Many manufacturing systems can be modeled as a hierarchical, distributed control structure in which each level of the hierarchy represents a different level of information abstraction. Tasks passed to each level from the previous levels are decomposed into a set of simpler tasks. If these simpler tasks are within the conceptual domain of the current level of the hierarchy, they are acted upon. If not, they are decomposed further to be passed onto the next lower level of the hierarchy. The decomposition of tasks frees the upper levels of the hierarchy to make global decisions about the manufacturing process. With the advent of the personal computer, microprocessor-based hierarchical distribution has become a reality upon the factory floor. Today personal computers, PCs, account for the vast majority of cell controller installations. Among the reasons for this dominance are low cost, ease of programming, and the ability to combine the capabilities of third party products. Adding these capabilities to the manufacturing system increases flexibility creating an intelligent, closed loop environment. Within this environment, the PC takes on many roles. One of these roles is as a front-end controller. The front-end controller provides two basic functions for the manufacturing environment. First, the front-end controller provides a “plug” for systems integration. Equipment and software manufacturers have not yet agreed upon standards for system interfaces. That is, they cannot be plugged together as home stereo components can be; they are not “plug” compatible. The front-end controller provides this plug by creating an interface that allows previously incompatible systems to work together. Secondly, the front-end controller provides a local processing node within a level of the distributed manufacturing environment. The manufacturing environment needs this because much of today's equipment and software are underdeveloped. They lack the processing power to formulate the kinds of environment-based decisions necessary to achieve the implementation of the unmanned machine cell. The front-end controller bridges this gap enhancing the existing capabilities of equipment and software while adding new capabilities. Some of the new capabilities may be low level error recovery, a local database for equipment or tool histories, a local user interface, and data acquisition, assimilation, and management. In a program sponsored by the National Institute of Standards and Testing Automated Manufacturing Research Facility (NIST AMRF), the University of Kansas (KU) has been developing work station level rules for a vertical machining work station. The vertical machining work station at KU consists of an American Robot, a Hurco KMC-3P Three Axis Vertical Milling Machine, an automated fixture, and a Sun Microsystems 4/260 work station controller. The architecture of the work station is based on the five level control system under implementation at the AMRF. Currently, the two lowest levels of the hierarchy, work station and equipment, are implemented at the University of Kansas Computer Integrated Manufacturing Lab. During system integration of the equipment, it was necessary to design and implement several front-end controllers. Following the hierarchical abstraction of the rest of the manufacturing control hierarchy, a general model for the front-end controller was developed consisting of a common frame of reference, structure, and command set for the work station. The architecture of the front-end controller is modular and expandable allowing it to act as an open system in a complex environment. This technique allows the front-end controller to be ported from one piece of equipment to the next with little modification for the equipment's full integration within the work station, thus providing low integration and development time.