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Manufacturing enterprise integration using hierarchial control and distributed database
Computers and Industrial Engineering Vol. 25, Nos 1-4, pp. 581-584, 1993 Printed in Great Britain. All fights reserved
0360-8352/9356.00+0.00 Copyright © 1993 Pergamon Press Ltd
MANUFACTURING ENTFJtPRISE INTEGRATION USING HIERARCHIAL CONTROL AND DISTRIBUTED DATABASE
t ALI K. KAMRANI, Ph.D, 2 KAMRAN K. KAMRANI, P.E, M.B.A.
I DEPARTMENT OF ENGINEERING MANAGEMENT UNIVERSITY OF MISSOURI.ROLLA ROLLA, MISSOURI 65401 2 INSTRUMENTATION AND CONTROL ENGINEER BURGESS AND NIPLE, LIMITED COLUMBUS, OHIO 437,20
ABSTRACT The advanced capabilities of computer systems and field sensory instruments have lead companies to consider the use of computers, smart sensors, and telecommunication networks. The application of a computerized system in a manufacturing and production environment includes data acquisition, supervisory control, status monitoring, operation optimization, and data management. A computer-automated and integrated manufacturing environment requires fast data sampling, high sensor capabilities, distributed control processing, reliable communication, easy-to-use software and operator interfaces. A computerized system should deliver the right information, to the right people, to the fight equipment, at the right time. This system mast use a distributed processing technique to achieve a comprehensive product and process quality monitoring, demand estimation and system monitoring and control while maintaining overall cost at minimum. This paper proposes a layout of a distributed computer control stxnctm'e for a computer integrated enterprise. It will also propose the supporting database associated with each level of this control hierarchy. It will also lists a set of activities and tasks associated with design and implementation of such automated system. INTRODUCTION A computer system network consists of several computers and computer networks which share critical process information through a common database and communicate via high speed media. The objective of a computer system is operation at optimum level with regard to prodact quality, production quantity, cost, and reliable service to customers. The latest technological trends in computer systems that can meet the operational objectives of a computer integrated enterprise are called distributive process control, open architecture, and universal networking. Distributive process control (DPC) is configured from field sensory instruments, computers, networks, and database software. DPC allows distribution of control actions and information throughout the entire system units. An open ~rchitecture approach consists of both hardwaxe and operating system which can be customized to meet specific application requirements. Universal networking, using protocol conversion and emulation, links unit stations to dissimilar unit computers on one universal network. DPC is a form of instrumentation that is used to monitor and control remotely located but centrally controlled stations. There is a continuous transmission of information between the two locations. The central control station is connected to devices to monitor process conditions and manipulate the control functions for the processes at the remote areas. In the DPC environment, upper level computers depend on lower level devices for process monitoring and data collection, and lower level systems depend on the higher level systems for sophisticated controlling functions for process optimization. One form of DPC environment is Sapervianry Control & Data Acquisition. SCADA is the science of sensing and measuring information at some remote location and transmitting the data to a convenient location for monitoring and control. SCADA includes computer-based systems that obtain data from field sensors and remote stations, transmit data from remote stations over communication links, present information to operators, and allow supervisory-type control. The features that distinguish SCADA from other types of computer-hased control systems are the communication-linkedsupervisory control, remote controllers that send commands and receive data from field sensors, and based on control strategy the remote controllers will cm.ry out these commands and report the information. The remote controllers collect data on process equipment operation and communicate these back to the master station for monitoring and control. Communication can be requested by the master and "Poll" basis or by the remote units on a "Contention" basis. Polling is a method of reporting alarm and/or status information in which the central unit requests update information from its attendant remote stations. Polling may be manually or automatically initiated. Contention is a method of alarm and/or status reporting in which remote stations report events "as soon as they happen". An imporlant feature of the SCADA strocture is its hierarchical arrangement. The hierarchy is arranged such that upper level computer(s) depend on lower level remote(s) and sensors for precess information and the lower level system, while performing local processing, depends on higher level system(s) for optimization and acknowledgement functions. The major functions of a typical SCADA system are: 1. 2. 3. 4.
Data Acquisition: The system collects and stores from the remote facilities, Monitoring the System: Displays alarm and status information for operator review and reports, Control.. The system provides command to the remote facilities to cause operations of the remote process sensors, Oporatlomff Data Reporting: The system provides analysis of the data collected from remote facilities and provides printed reports for management, operations, and maintenance staff review,
581
Proceedings of the 15th Annual Conference on Computers and Industrial Engineering
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5. 6.
Management Information: Supply status and variance information to management, Reliability Assurance: Perform serf-checks and diagnostic checks on itself.
Network topology is also an important factor. In order to provide supervisory control the network in a DPC structure should he able to download programs to lower levels with the required operational data at real-time in order to maintain efficient operation and upload data from production stations for analysis Of information such as material handling time, machine utniTnfion, and idle time. The network should also provide the database with reliable information such as raw material availability, finished part status, and tool information through communication network with all levels of the DPC structure. The networks can have multiple configurations. These configurations can hepoint-to-point, point-to-multiple points(star), multi-point-distributed(bus), and multi-point-fault-tolerant(ring). Reliable communication and standardization of protocol between DPC components of different manufacture is a necessity. The integration of DPC in a multi-vendor environment necessitates applications of open architecture software and protocol converters. This would result in connectivity among each of the network database, which this distributed format can be the source of the entire system data. There are a number of commonly data media types used in a DPC structure. Wire lines and optic fihera/lasers are some examples. Each of these has charactecistics which make them very desirable. However, when choosing a system, it is best to consider the limitations as a whole as well as the long-term operating costs. A commonly considered type of data path between the transmitter and the receiver is the use of wire lines. This alternative has proven to be unsatisfactory. Wire lines are usually buried in the same trench with the water lines and are subject to possible damage. A major disadvantage of wire fines is the high installation and maintenance expenses. Optic fibers are another form of communication media. Optical transmission of data requires the use of lasers. When lasers are used, they require an unobstructed line of light. Any break in the light beam may result in loss of data. Fiber optic cables tend to be fairly fragile. Damage to a fiber optical cable is not easily repairable and, in most cases, it may be patched. It should be noted that fiber optic cables have the advantage of carrying significantly more channels than their wire line counterparts. The information required for optimum operation of the system mast be stored in a database which can be easily accessed. The best apwoach for data distribution and management is to develop a number of different databases which reside within each level, and only contain the information which is required for the operation of the particular level. This will improve the real-time data accessibility and provides data security. The database in lower levels of a computer controlled hierarchy should also maintain critical information which is required, if the upper level computers are down. In order to perform data management effectively, communication model of Figure 1 is proposed. In this model there are two databases associated with each level, the permanent database and the working database. The working database is a temporary copy of a portion of permanent database, extracted for a selected task by the operator's or the appfication's program. The information within databases have two different data formats. The production system requires a set of information which allows for performance measurement and analysis. These data are part of the production specifications which are unchanged throughout the life cycle of the product and production. These sets of information are considered as static data. These sets commonly include manufactmed product specification, BOM for each woduct, routing and processing steps for product development, machine and resource location, machine specification and capabilities, vendor data for purchased material, customer data including historical information on sold product, cost and accounting information, and long-term product demand. Dynamic information is used to illustrate the state of the system during the operation. The information in this category may include status of a specific task, part, product and lot tracking, machine status and performance, operator performance tracking and status, and inventory level including work-in-process. Many researchers have proposed that the dynamic elements of the working database are to be maintained using object-oriented techniques. This is done by creating dynamic entries within the static database with static object characteristics. The state of the output can then he lagged and archived according to its occurrence at the real time and then updated within the working database. THE DISTRIBUTED PROCESS COMPUTER CONTROL STRUCTURE The structure of a computer network model is proposed in this section. This proposed structure is based on the model illustrated in Figure 2, can integrate and automate the overall activities of a manufacturing enterprise. Using this structure, computers in different levels can integram the production, scheduling and management functions, process control data, and maintain the overall operation at low cost. The model consists of four levels:
Corporate Level: The cca~rate computer coo~linales the operation of all factory computers, collects data and summarizes the operations and performance of the entire firm. The functions performed by the corporate computer are: customer information, customer billing, trouble call management, buy/sell management, report generation, strategic planning, human resource management, electronic mail service, and finance. The ,I-!~hase within this level operates as an Management Information System,/diS, and the communication is done through direct data entry, file transfer, inquiry response and remote job entry. An effective/dis will provide the required information at the right time, and in a format that is understandable by the user or any other devices which resides in the system hierarchy. At this level, the computer recommends the path of actions based on the information given to the computer, gathered and developed by operators. The final decision is made by the management group. The form of hardware in this level are mainframe computers. The traffic pattern within this level has long thwation. The infoemation is usually coordination and management which are considered to he not critical to real-time process control, but essential for short or long-term resource planning. Factory/,eve/." The overall production control and system optimization of each center is done at this level. This level decides the unit operation based on optimum combination of time, energy, material, and customer needs. The factory computer performs forecasting tasks to coordinate the processes at the manufacturing and production centers. The factory level computer should have the required knowledge and intelligence to make real-time operational decisions. Reports for each of these units are developed and archived in this level. The factory computer monitors and performs business-related functions such as data processing, document management, maintenance, operation management and material control. The role of the operator at this level can take one of the two forms. In the f'n~'t method, information on the controlled system is fed directly to the computer from the lower level. The data analysis and calculation of deciding factors is done by computer. It will recommend the action and the propca"controlling device to the human operator. The recommended action would be performed by the operator. In the second method, the computer will receive data directly from the lower level, and will recommend corrective actions to the operator, and also sends controlling commands to the lower level. The operator at the lower level updates the controlling algorithm required by these actions. Small mainframe computers are used in this level, since more compficated tasks and data analyses are performed. In this level, the data are transferred less frequently but for long durations. The information within this level provides scheduling and management instruction which are considered important. The database in this level maintains information such as operations data, operations procedure, quality control data, tool status, tool management procedures, process plans, machine maintenance, and expert system for material requirement planning. The Factory level communication unit within this level should transfer files, executive programs and supervisory instructions for operation optimization.
KAMRANI and KAMRANI: Manufacturing Enterprise Integration
583
Ceater Leve~. This level performs supervisory control. It coordinates cell level activities and optimize the operation at both cell and center levels, although it has limited authority. For more complicated commands this level relies on the information from upper level computers. This level computer has the responsibility of part scheduling, process priority setting, machine scheduling and control of materiai handling unit. The computer at this level should also responds to alarms and any other emergencies of the units under its control by shutting down the process, re-route the part to another process station, or with other interlncking safety actions. The computers at this level m'e capable of communicating to upper level computers for more optimization algorithms and scheduling infonnation. The operator is responsible for monitoring the process, initiating process start-up, and updating controlling parameters. For example, these pm.ameters provide the real-time decisions for material movement by coordinating the cell controller with the automated material handling controller. The hardware used at this level is minicomputers or personal computers. The data traffic in this level is less frequent, but they are in short or long duration. The respond time from this level is considered critical, since it provides sequencing and coordinating instructions. The database within this level includes machine status, process parameters, and cell status, operation status, tool status, quality status, process scheduling, and an expert system for resource planning and machine sequencing. Communication in this level is dane through simple queries, information transfer, and activity complete acknowledgment. Cell/eve/: This level control each operating unit which uses material and enexgy. The computers at this level monitor and control individual equipment using sensors. These operating units are controlled by supervisory functions from upper levels, to perform at optimum efficiency. The control within the level reacts to emergencies within the area of its control. The functional tasks of the first level are fixed and will not require human interaction. The main objective of the controller at this level is to translate the supervisory commands issued from upper level and convert into steps which maintains the goal of the production units. This level include equipments such as CNC machines, CMM and vision machines, robots, and automated material handling systems such as AGV's. The widely used system to establish monitoring and control in this level is the application of Programmable Logic Controllers (PLC). PLC systems were offered by major eleetrical suppliers beginning in the mid to late 1970's as one-for-one replacements for relays on the factory floor. They met with immediate and overwhelming success because they were relatively inexpensive, quite flexible and reliable, and because spares were readily available. Reasonably priced PLC products have matured and are now relied on by nearly all facilities with sizable logic requirements. The controllers' upplicatioas have varied widely as to equipment controlled, covering almost anything that is elec~cally operated or monitored. The main requirement for a PLC application has usually been the ability to represent its use on an eleetrical control diagram with timers, relays, switches, boolean logic, and electrical analog parameters. A PLC typically consists of a central processing unit (CPU), local or remote input and output boards for signal interfacing, and communication modules as required. The CPU, providing the "brainsn of the PLC, is capable of making a large number of logic control decisions rapidly and can also perform some regulatory analog control. The I/O modules serve as termination points for the field signals and as multiplexers, converting the field signals to a digital data stream understandable to the CPU. The controllers within the cell level are connected and communicate within a closed loop. Communication modules provide a data link from the CPU to any workstation or any other remote I/O board racks which are configured for the process application. The communication traffic patterns in this level is frequent and of short duration. Since the computer system is directly connected to the process and the equipment, the response time is considered to be very critical. Also, the database which resides in this level should include information such as diagnostic information, process procedures, and process control instructions. The information flow in the proposed hieran~hy possesses different priority. The real time requirements of information within the lower level is more critical and more frequent than of those in the higher level. Other requirements which all levels should include are fault tolerance operateability, error control mechanism, and priority scheduling of the message traffic. Some of the other chaxacteristics required by the communication units within all levels are: the data acquisition from devices, management of information, accumulation, reporting and analysis, and communication. The three major design challenges of a suitable communication unit are its flexibility (wide range of needs in the factory floor), connectivity (single and multi vendors platform), and reliable performance. CONCLUSION This paper illustrates a layout of a hierurchial computer control structure and describes the required supporting data fix each level. It will lists the critical activities, data characteristics, and tasks associated with these levels. Computer networks have contributed to further development and improvement of manufacturing systems and manufacturing operation performance. This evolution of knowledge and technology has introduce the Computer Integrated Manufacturing which is considered to be the manufacturing philosophy of 1990's. Related applications such as CAD, CAM, CANC, and CAPP demand a large amount of data and information. The control and management of these data within the hierarchy environment cannot be ignored, and its distribution while maintaining flexibility and performance must be considered as one of the important factors during the design and implementation. Critical activities considered dirdng the development and integration of computer networking and control of production systems arc: user needs, system architecture based on industry standards, development of unit control strategies, distributed process database, field sensors accuracies, selection of the proper communication medium and protocols, and finally, the operator interface graphics or panel layout. REFERENCES
l.
2. 3. 4.
5. 6. 7. 8.
K.K. Kamrani, A.K. Kamrani, and H.R. Parsani, "Distributive Computer Control Designfor Electric Power Utility," 5th International Power Conference, Tehran, Iran, 1990. A.K. Kamrani, K.K. Kamrani, H.R. Parsaei, and J.W. Wong, "A Computer Hierarchy Structure for Electric Power Utility," 13th Computers and Industrial Engineering Conference, Orlando, FL, 1991. K.K. Kamrani, and A,K. Kamrani, "The Technology and Economic Benefits of a Remote Controlled Distribution Substation," 7th International Power System Conference, Tehran, Iran, 1992. A.K. Kamrani, K.K. Kamrani, "ComputerNetworkingfor Operation Management and Optimization of Electric Power Utilities," 13th Annual National Conference of American Society for Engineering Management, Eatontown, NJ, 1992. "LAN : Local Area Networks," IBM Technical Report, International Business Machines Corporation, 1990. D.L. Spooner, M. Hardwick, and K.W. Liu, "Integrating the CIM Environment Using Object-Oriented Management Technology," International Conference on Computer Integrated Manufacturing, Troy, NY, 1988. M.G. Ketcham, J.M. Smith, and B.O. Nagi, "An Integrated Data Mode~for CIM Planning and Control," International Conference on Computer Integrated Manufacturing, Troy, NY, 1988. H. We,dekind, and G. Zoerntiein, ".4 Conceptual Basis for Database Application in Flexible Manufacturing Systems," IEEE International Conference on Robotics and Automation, Raleigh, NC, 1987.
584
Proceedingsofthe15thAnnualConferenceonComputersandIndustrialEngineering
f
Working Database
~
Per~,anent Database
r
i
Application Programs
User Interface
Figure i Proposed Database S t r u c t u r e
CorporateCornputer~
C
:I ~ ~
FaetoWjController CenterController ~
~
[
MaterialHandlingController
~.~.~_.~__ CellController
Figure 2 Overview of Proposed DPC System
0360-8352/9356.00+0.00 Copyright © 1993 Pergamon Press Ltd
MANUFACTURING ENTFJtPRISE INTEGRATION USING HIERARCHIAL CONTROL AND DISTRIBUTED DATABASE
t ALI K. KAMRANI, Ph.D, 2 KAMRAN K. KAMRANI, P.E, M.B.A.
I DEPARTMENT OF ENGINEERING MANAGEMENT UNIVERSITY OF MISSOURI.ROLLA ROLLA, MISSOURI 65401 2 INSTRUMENTATION AND CONTROL ENGINEER BURGESS AND NIPLE, LIMITED COLUMBUS, OHIO 437,20
ABSTRACT The advanced capabilities of computer systems and field sensory instruments have lead companies to consider the use of computers, smart sensors, and telecommunication networks. The application of a computerized system in a manufacturing and production environment includes data acquisition, supervisory control, status monitoring, operation optimization, and data management. A computer-automated and integrated manufacturing environment requires fast data sampling, high sensor capabilities, distributed control processing, reliable communication, easy-to-use software and operator interfaces. A computerized system should deliver the right information, to the right people, to the fight equipment, at the right time. This system mast use a distributed processing technique to achieve a comprehensive product and process quality monitoring, demand estimation and system monitoring and control while maintaining overall cost at minimum. This paper proposes a layout of a distributed computer control stxnctm'e for a computer integrated enterprise. It will also propose the supporting database associated with each level of this control hierarchy. It will also lists a set of activities and tasks associated with design and implementation of such automated system. INTRODUCTION A computer system network consists of several computers and computer networks which share critical process information through a common database and communicate via high speed media. The objective of a computer system is operation at optimum level with regard to prodact quality, production quantity, cost, and reliable service to customers. The latest technological trends in computer systems that can meet the operational objectives of a computer integrated enterprise are called distributive process control, open architecture, and universal networking. Distributive process control (DPC) is configured from field sensory instruments, computers, networks, and database software. DPC allows distribution of control actions and information throughout the entire system units. An open ~rchitecture approach consists of both hardwaxe and operating system which can be customized to meet specific application requirements. Universal networking, using protocol conversion and emulation, links unit stations to dissimilar unit computers on one universal network. DPC is a form of instrumentation that is used to monitor and control remotely located but centrally controlled stations. There is a continuous transmission of information between the two locations. The central control station is connected to devices to monitor process conditions and manipulate the control functions for the processes at the remote areas. In the DPC environment, upper level computers depend on lower level devices for process monitoring and data collection, and lower level systems depend on the higher level systems for sophisticated controlling functions for process optimization. One form of DPC environment is Sapervianry Control & Data Acquisition. SCADA is the science of sensing and measuring information at some remote location and transmitting the data to a convenient location for monitoring and control. SCADA includes computer-based systems that obtain data from field sensors and remote stations, transmit data from remote stations over communication links, present information to operators, and allow supervisory-type control. The features that distinguish SCADA from other types of computer-hased control systems are the communication-linkedsupervisory control, remote controllers that send commands and receive data from field sensors, and based on control strategy the remote controllers will cm.ry out these commands and report the information. The remote controllers collect data on process equipment operation and communicate these back to the master station for monitoring and control. Communication can be requested by the master and "Poll" basis or by the remote units on a "Contention" basis. Polling is a method of reporting alarm and/or status information in which the central unit requests update information from its attendant remote stations. Polling may be manually or automatically initiated. Contention is a method of alarm and/or status reporting in which remote stations report events "as soon as they happen". An imporlant feature of the SCADA strocture is its hierarchical arrangement. The hierarchy is arranged such that upper level computer(s) depend on lower level remote(s) and sensors for precess information and the lower level system, while performing local processing, depends on higher level system(s) for optimization and acknowledgement functions. The major functions of a typical SCADA system are: 1. 2. 3. 4.
Data Acquisition: The system collects and stores from the remote facilities, Monitoring the System: Displays alarm and status information for operator review and reports, Control.. The system provides command to the remote facilities to cause operations of the remote process sensors, Oporatlomff Data Reporting: The system provides analysis of the data collected from remote facilities and provides printed reports for management, operations, and maintenance staff review,
581
Proceedings of the 15th Annual Conference on Computers and Industrial Engineering
582
5. 6.
Management Information: Supply status and variance information to management, Reliability Assurance: Perform serf-checks and diagnostic checks on itself.
Network topology is also an important factor. In order to provide supervisory control the network in a DPC structure should he able to download programs to lower levels with the required operational data at real-time in order to maintain efficient operation and upload data from production stations for analysis Of information such as material handling time, machine utniTnfion, and idle time. The network should also provide the database with reliable information such as raw material availability, finished part status, and tool information through communication network with all levels of the DPC structure. The networks can have multiple configurations. These configurations can hepoint-to-point, point-to-multiple points(star), multi-point-distributed(bus), and multi-point-fault-tolerant(ring). Reliable communication and standardization of protocol between DPC components of different manufacture is a necessity. The integration of DPC in a multi-vendor environment necessitates applications of open architecture software and protocol converters. This would result in connectivity among each of the network database, which this distributed format can be the source of the entire system data. There are a number of commonly data media types used in a DPC structure. Wire lines and optic fihera/lasers are some examples. Each of these has charactecistics which make them very desirable. However, when choosing a system, it is best to consider the limitations as a whole as well as the long-term operating costs. A commonly considered type of data path between the transmitter and the receiver is the use of wire lines. This alternative has proven to be unsatisfactory. Wire lines are usually buried in the same trench with the water lines and are subject to possible damage. A major disadvantage of wire fines is the high installation and maintenance expenses. Optic fibers are another form of communication media. Optical transmission of data requires the use of lasers. When lasers are used, they require an unobstructed line of light. Any break in the light beam may result in loss of data. Fiber optic cables tend to be fairly fragile. Damage to a fiber optical cable is not easily repairable and, in most cases, it may be patched. It should be noted that fiber optic cables have the advantage of carrying significantly more channels than their wire line counterparts. The information required for optimum operation of the system mast be stored in a database which can be easily accessed. The best apwoach for data distribution and management is to develop a number of different databases which reside within each level, and only contain the information which is required for the operation of the particular level. This will improve the real-time data accessibility and provides data security. The database in lower levels of a computer controlled hierarchy should also maintain critical information which is required, if the upper level computers are down. In order to perform data management effectively, communication model of Figure 1 is proposed. In this model there are two databases associated with each level, the permanent database and the working database. The working database is a temporary copy of a portion of permanent database, extracted for a selected task by the operator's or the appfication's program. The information within databases have two different data formats. The production system requires a set of information which allows for performance measurement and analysis. These data are part of the production specifications which are unchanged throughout the life cycle of the product and production. These sets of information are considered as static data. These sets commonly include manufactmed product specification, BOM for each woduct, routing and processing steps for product development, machine and resource location, machine specification and capabilities, vendor data for purchased material, customer data including historical information on sold product, cost and accounting information, and long-term product demand. Dynamic information is used to illustrate the state of the system during the operation. The information in this category may include status of a specific task, part, product and lot tracking, machine status and performance, operator performance tracking and status, and inventory level including work-in-process. Many researchers have proposed that the dynamic elements of the working database are to be maintained using object-oriented techniques. This is done by creating dynamic entries within the static database with static object characteristics. The state of the output can then he lagged and archived according to its occurrence at the real time and then updated within the working database. THE DISTRIBUTED PROCESS COMPUTER CONTROL STRUCTURE The structure of a computer network model is proposed in this section. This proposed structure is based on the model illustrated in Figure 2, can integrate and automate the overall activities of a manufacturing enterprise. Using this structure, computers in different levels can integram the production, scheduling and management functions, process control data, and maintain the overall operation at low cost. The model consists of four levels:
Corporate Level: The cca~rate computer coo~linales the operation of all factory computers, collects data and summarizes the operations and performance of the entire firm. The functions performed by the corporate computer are: customer information, customer billing, trouble call management, buy/sell management, report generation, strategic planning, human resource management, electronic mail service, and finance. The ,I-!~hase within this level operates as an Management Information System,/diS, and the communication is done through direct data entry, file transfer, inquiry response and remote job entry. An effective/dis will provide the required information at the right time, and in a format that is understandable by the user or any other devices which resides in the system hierarchy. At this level, the computer recommends the path of actions based on the information given to the computer, gathered and developed by operators. The final decision is made by the management group. The form of hardware in this level are mainframe computers. The traffic pattern within this level has long thwation. The infoemation is usually coordination and management which are considered to he not critical to real-time process control, but essential for short or long-term resource planning. Factory/,eve/." The overall production control and system optimization of each center is done at this level. This level decides the unit operation based on optimum combination of time, energy, material, and customer needs. The factory computer performs forecasting tasks to coordinate the processes at the manufacturing and production centers. The factory level computer should have the required knowledge and intelligence to make real-time operational decisions. Reports for each of these units are developed and archived in this level. The factory computer monitors and performs business-related functions such as data processing, document management, maintenance, operation management and material control. The role of the operator at this level can take one of the two forms. In the f'n~'t method, information on the controlled system is fed directly to the computer from the lower level. The data analysis and calculation of deciding factors is done by computer. It will recommend the action and the propca"controlling device to the human operator. The recommended action would be performed by the operator. In the second method, the computer will receive data directly from the lower level, and will recommend corrective actions to the operator, and also sends controlling commands to the lower level. The operator at the lower level updates the controlling algorithm required by these actions. Small mainframe computers are used in this level, since more compficated tasks and data analyses are performed. In this level, the data are transferred less frequently but for long durations. The information within this level provides scheduling and management instruction which are considered important. The database in this level maintains information such as operations data, operations procedure, quality control data, tool status, tool management procedures, process plans, machine maintenance, and expert system for material requirement planning. The Factory level communication unit within this level should transfer files, executive programs and supervisory instructions for operation optimization.
KAMRANI and KAMRANI: Manufacturing Enterprise Integration
583
Ceater Leve~. This level performs supervisory control. It coordinates cell level activities and optimize the operation at both cell and center levels, although it has limited authority. For more complicated commands this level relies on the information from upper level computers. This level computer has the responsibility of part scheduling, process priority setting, machine scheduling and control of materiai handling unit. The computer at this level should also responds to alarms and any other emergencies of the units under its control by shutting down the process, re-route the part to another process station, or with other interlncking safety actions. The computers at this level m'e capable of communicating to upper level computers for more optimization algorithms and scheduling infonnation. The operator is responsible for monitoring the process, initiating process start-up, and updating controlling parameters. For example, these pm.ameters provide the real-time decisions for material movement by coordinating the cell controller with the automated material handling controller. The hardware used at this level is minicomputers or personal computers. The data traffic in this level is less frequent, but they are in short or long duration. The respond time from this level is considered critical, since it provides sequencing and coordinating instructions. The database within this level includes machine status, process parameters, and cell status, operation status, tool status, quality status, process scheduling, and an expert system for resource planning and machine sequencing. Communication in this level is dane through simple queries, information transfer, and activity complete acknowledgment. Cell/eve/: This level control each operating unit which uses material and enexgy. The computers at this level monitor and control individual equipment using sensors. These operating units are controlled by supervisory functions from upper levels, to perform at optimum efficiency. The control within the level reacts to emergencies within the area of its control. The functional tasks of the first level are fixed and will not require human interaction. The main objective of the controller at this level is to translate the supervisory commands issued from upper level and convert into steps which maintains the goal of the production units. This level include equipments such as CNC machines, CMM and vision machines, robots, and automated material handling systems such as AGV's. The widely used system to establish monitoring and control in this level is the application of Programmable Logic Controllers (PLC). PLC systems were offered by major eleetrical suppliers beginning in the mid to late 1970's as one-for-one replacements for relays on the factory floor. They met with immediate and overwhelming success because they were relatively inexpensive, quite flexible and reliable, and because spares were readily available. Reasonably priced PLC products have matured and are now relied on by nearly all facilities with sizable logic requirements. The controllers' upplicatioas have varied widely as to equipment controlled, covering almost anything that is elec~cally operated or monitored. The main requirement for a PLC application has usually been the ability to represent its use on an eleetrical control diagram with timers, relays, switches, boolean logic, and electrical analog parameters. A PLC typically consists of a central processing unit (CPU), local or remote input and output boards for signal interfacing, and communication modules as required. The CPU, providing the "brainsn of the PLC, is capable of making a large number of logic control decisions rapidly and can also perform some regulatory analog control. The I/O modules serve as termination points for the field signals and as multiplexers, converting the field signals to a digital data stream understandable to the CPU. The controllers within the cell level are connected and communicate within a closed loop. Communication modules provide a data link from the CPU to any workstation or any other remote I/O board racks which are configured for the process application. The communication traffic patterns in this level is frequent and of short duration. Since the computer system is directly connected to the process and the equipment, the response time is considered to be very critical. Also, the database which resides in this level should include information such as diagnostic information, process procedures, and process control instructions. The information flow in the proposed hieran~hy possesses different priority. The real time requirements of information within the lower level is more critical and more frequent than of those in the higher level. Other requirements which all levels should include are fault tolerance operateability, error control mechanism, and priority scheduling of the message traffic. Some of the other chaxacteristics required by the communication units within all levels are: the data acquisition from devices, management of information, accumulation, reporting and analysis, and communication. The three major design challenges of a suitable communication unit are its flexibility (wide range of needs in the factory floor), connectivity (single and multi vendors platform), and reliable performance. CONCLUSION This paper illustrates a layout of a hierurchial computer control structure and describes the required supporting data fix each level. It will lists the critical activities, data characteristics, and tasks associated with these levels. Computer networks have contributed to further development and improvement of manufacturing systems and manufacturing operation performance. This evolution of knowledge and technology has introduce the Computer Integrated Manufacturing which is considered to be the manufacturing philosophy of 1990's. Related applications such as CAD, CAM, CANC, and CAPP demand a large amount of data and information. The control and management of these data within the hierarchy environment cannot be ignored, and its distribution while maintaining flexibility and performance must be considered as one of the important factors during the design and implementation. Critical activities considered dirdng the development and integration of computer networking and control of production systems arc: user needs, system architecture based on industry standards, development of unit control strategies, distributed process database, field sensors accuracies, selection of the proper communication medium and protocols, and finally, the operator interface graphics or panel layout. REFERENCES
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K.K. Kamrani, A.K. Kamrani, and H.R. Parsani, "Distributive Computer Control Designfor Electric Power Utility," 5th International Power Conference, Tehran, Iran, 1990. A.K. Kamrani, K.K. Kamrani, H.R. Parsaei, and J.W. Wong, "A Computer Hierarchy Structure for Electric Power Utility," 13th Computers and Industrial Engineering Conference, Orlando, FL, 1991. K.K. Kamrani, and A,K. Kamrani, "The Technology and Economic Benefits of a Remote Controlled Distribution Substation," 7th International Power System Conference, Tehran, Iran, 1992. A.K. Kamrani, K.K. Kamrani, "ComputerNetworkingfor Operation Management and Optimization of Electric Power Utilities," 13th Annual National Conference of American Society for Engineering Management, Eatontown, NJ, 1992. "LAN : Local Area Networks," IBM Technical Report, International Business Machines Corporation, 1990. D.L. Spooner, M. Hardwick, and K.W. Liu, "Integrating the CIM Environment Using Object-Oriented Management Technology," International Conference on Computer Integrated Manufacturing, Troy, NY, 1988. M.G. Ketcham, J.M. Smith, and B.O. Nagi, "An Integrated Data Mode~for CIM Planning and Control," International Conference on Computer Integrated Manufacturing, Troy, NY, 1988. H. We,dekind, and G. Zoerntiein, ".4 Conceptual Basis for Database Application in Flexible Manufacturing Systems," IEEE International Conference on Robotics and Automation, Raleigh, NC, 1987.
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Proceedingsofthe15thAnnualConferenceonComputersandIndustrialEngineering
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Figure i Proposed Database S t r u c t u r e
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Figure 2 Overview of Proposed DPC System