COMPUTER INTEGRATED MANUFACTURING:CIMS STRUCTURE AND FUNCTIONS

CIMS STRUCTURE AND FUNCTIONS

CIMS Structure

The components of CIMS include both hardware and software. The hardware includes computer hardware, network, manufacturing devices, and peripherals. The software includes operating systems, communication software, database management systems, manufacturing planning and control soft- ware, management information software, design software, office automation software, and decision support software. These different hardware and software systems have different functions and work together to fulfill the company’s business goals. To make it easier to understand, CIMS is normally decomposed into a number of subsystems interacting with each other. Unfortunately, no unique and standard decomposition method exists. Every company can define a method according to its specific situation and requirements. One decomposition method is shown in Figure 5.

From Figure 5, it can be seen that CIMS consists of four functional subsystems and two support subsystems. The four functional subsystems are management information, CAD / CAPP / CAM, man- ufacturing automation, and computer-aided quality management. These functional subsystems cover the business processes of a company. The two support subsystems are computer network and database management. They are the basis that allows the functional subsystems to fulfill their tasks. The arcs denote the interfaces between different subsystems. Through these interfaces, shared data are ex- changed between different subsystems.

Components of CIMS

This section briefly describes the components of CIMS.

Management Information System

Management information system (MIS) plays an important role in the company’s information system. It manages business processes and information based on market strategy, sales predictions, business decisions, order processing, material supply, finance management, inventory management, human resource management, company production plan, and so on. The aims of MIS are to shorten delivery time, reduce cost, and help the company to make rapid decision to react to market change.

Currently, Enterprise Resource Planning (ERP) software is normally used as the key application software in MIS. Many commercial ERP software products are on the market, such as SAP R / 3, developed by SAP, and BaanERP, developed by Baan.

Basic Concept of ERP In balancing manufacturing, distribution, financial, and other business functions to optimize company productivity, ERP systems are considered to be the backbone of corporate infrastructure. The ERP concept is derived from and extends the functions of MRPII (Manufacturing Resources Planning) system (Wright 1992). Besides the traditional functions of MRPII in manufacturing management, material supply management, production planning, finance, and sales management, ERP introduces new functions, such as transportation management, supply chain management, corporate strategy planning, workflow management, and electronic data exchange, into the system. The ERP system thus provides more flexibility and ability to the company in business process reengineering, integration with customers, and integration with material suppliers as well as product dispatchers.

Manufacturing Resource Planning The basis of MRPII is MRP (material requirements planning), which dates back to the 1940s. MRPII uses computer-enhanced materials ordering and inventory control methods. It enhances speed and accuracy in issuing raw materials to factory work- stations. It is clear that linking materials with production demand schedules could optimize the flow of the product as it is being constructed in the factory. This could be done in such a manner that material queue times could be minimized (e.g., have the material show up only when needed), and the amount of material needed throughout the factory at any one time could be reduced ultimately. This is an optimization technique that allocates identified sets of materials (sometimes called kits) to specific jobs as they go through the manufacturing process.

Because it is possible for a computer to keep track of large numbers of kits, it is reserves or mortgages materials for specific jobs in time-order sequences. Linking these sequences with a pro- duction plan based on customer need dates allows management to release and track orders through the shop accurately. Prior to releasing orders by means of the kitting process based on the production schedule, it was necessary to obtain supplies. The supplies are based on a gross basis depending on the number of orders expected to be shipped to customers over the selected time period and by having the gross amount of inventory on hand at the start of the period to support production. Obviously, the kit will result in fewer extra materials on hand at any point in the production period. This results in large reductions in raw material and work in process and hence in lower operation costs.

Figure 6 gives a flow diagram of an MRPII system (Waldner 1992).

Just-in-Time Another method that has received much attention for production planning and control is just-in-time theory. In contrast to MRPII, which is ‘‘push’’ oriented, the JIT philosophy of management is ‘‘pull’’ oriented—that is, it calls for something to be manufactured only when there is a firm order for it. JIT is a productivity enhancer based on the simple proposition that all waste in the manufacturing process must be eliminated.

JIT theory states that wastes can only begin to be eliminated if the push production control system is replaced with a pull production control system. Inventory levels contain a very large volume of waste. Therefore, a way must be found to minimize inventory levels. If this is done without the analytical capability of the computer, it would be logical to conceive a system that would not let material move or be used until it is necessary. This is what Toyota did. They instituted a backward scheduling technique that started with the desired ship date. They had to know when the product needed to be at final assembly and before that when it needed to be at the subassembly levels and so forth, back through component part manufacturing. Ultimately, this means determining precisely when the raw materials should show up at the receiving dock. This in itself is not unusual or unique.

Although JIT proposes ways to reduce a great deal of waste, it cannot be implemented without the help of CIM and MRPII systems. For example, the means for producing products only at the rate at which the customer wants them can best be realized using the feedback control system production schedule of MRPII. By using the MRPII system, we can monitor the progress of all workstations carrying out the dictates of the strategic plan and thus speed up or slow down the preceding operation to optimize the usage of materials and labor. Koenig (1990) explains in detail the relationship of JIT with the MRPII and CIM systems.

Because JIT and MRPII have their advantages as well as limitations in applications, the combi- nation of JIT and MRPII systems in the common framework of CIM may produce excellent results in production scheduling and control.

CAD / CAPP / CAM System

CAD / CAPP / CAM stands for computer-aided design / computer-aided process planning / computer- aided manufacturing. The system is sometimes called the design automation system, meaning that CAD / CAPP / CAM is used to promote the design automation standard and provide the means to design high-quality products faster.

Computer-Aided Design CAD is a process that uses computers to assist in the creation, modification, analysis, or optimization of a product design. It involves the integration of computers into design activities by providing a close coupling between the designer and the computer. Typical design activities involving a CAD system are preliminary design, drafting, modeling, and simulation. Such activities may be viewed as CAD application modules interfaced into a controlled network operation under the supervision of a computer.

A CAD system consists of three basic components: hardware, which includes computer and input– output devices, application software, and the operating system software (Figure 7). The operating system software acts as the interface between the hardware and the application software system.

The CAD system function can be grouped into three categories: geometric modeling, engineering analysis, and automated drafting.

Geometric modeling constructs the graphic images of a part using basic geometric elements such as points, lines, and circles under the support of CAD software. Wire frame is one of the first geometric modeling methods. It uses points, curves, and other basic elements to define objects. Then the surface modeling, solid modeling, and parametric modeling methods are presented in the area of geometric modeling area. Saxena and Irani (1994) present a detailed discussion of the development of geometric modeling methods.

Engineering design completes the analysis and evaluation of product design. A number of com- puter-based techniques are used to calculate the product’s operational, functional, and manufacturing parameters, including finite-element analysis, heat-transfer analysis, static and dynamic analysis, mo- tion analysis, and tolerance analysis. Finite-element analysis is the most important method. It divides an object into a number of small building blocks, called finite elements. Finite-element analysis will fulfill the task of carrying out the functional performance analysis of an object. Various methods and packages have been developed to analyze static and dynamic performance of the product design. The objectives and methods can be found in any comprehensive book discussion of CAD techniques. After the analysis, the product design will be optimized according to the analysis results.

The last function of the CAD system is automated drafting. The automated drafting function includes 2D and 3D product design drafting, converting a 3D entity model into a 2D representation.

Computer-Aided Process Planning CAPP is responsible for detailed plans for the pro- duction of a part or an assembly. It acts as a bridge between design and manufacturing by translating design specifications into manufacturing process details. This operation includes a sequence of steps to be executed according to the instructions in each step and is consistent with the controls indicated in the instructions. Closely related to the process-planning function are the functions that determine the cutting conditions and set the time standards. The foundation of CAPP is group technology (GT),

which is the means of coding parts on the basis of similarities in their design and manufacturing attributes. A well-developed CAPP system can reduce clerical work in manufacturing engineering and provide assistance in production.

One of the first tasks of the CAPP system is to complete the selection of the raw workpiece. According to the functional requirements of the designed part, it determines the attributes of the raw workpiece, such as shape, size (dimension and weight), and materials. Other jobs for the CAPP system are determining manufacturing operations and their sequences, selecting machine tools, and selecting tools, fixtures, and inspection equipment. Determination of manufacturing conditions and manufacturing times are also part of the work of CAPP. These conditions will be used in optimizing manufacturing cost.

The CAPP system consists of computer programs that allow planning personnel interactively to create, store, edit, and print fabrication and assembly planning instructions. Such a system offers the potential for reducing the routine clerical work of manufacturing engineers. Figure 8 presents the classification of various CAPP systems.

Computer-Aided Manufacturing In this section, computer-aided manufacturing (CAM) refers to a very restricted area that does not include general production control functions. The pro- duction control functions will be introduced in the manufacturing automation subsystem (MAS) section. Here, CAM includes preparing data for MAS, including producing NC code for NC ma- chines, generating tool position, planning tool motion route, and simulating tool movement. Auto- matic NC code generation is very important for increasing work efficiency. Before the NC code for NC machine centers can be generated, a number of parameters regarding machine tool specification, performance, computer numerical control system behavior, and coding format should be determined first. The manufacturing method and operations will be selected according to these parameters, ge- ometric dimensions, solid forms, and designed part specifications. The CAM system will calculate the tool position data. Then the data regarding the part dimension, the tool motion track, cutting parameters, and numerical control instructions are generated in a program file. This file, called the NC program, is used by the machine tool to process part automatically.

CAD / CAPP / CAM Integration Besides the utilization of CAD, CAPP, and CAM tech- nology alone, the integration of CAD, CAPP, and CAM is an important way to enhance the company’s product design standards. Three methods can be used in the integration of CAD / CAPP / CAM: exchange product data through specific defined data format; exchange product data through standard data format, such as STEP, IGES, and DXF; and define a unified product data model to exchange product information.

Figure 9 is a STEP-based CAD / CAPP / CAM integration system developed at the State CIMS Engineering Research Center of China (located at Tsinghua University, Beijing). It was developed as a part of the CIMS application integration platform (Fan and Wu 1997) for manufacturing enterprises. This system focuses on part-level CAD / CAPP / CAM integration. XPRESS language and the STEP development tool ST-developer are used to define and develop the integration interfaces. Different kinds of CAD, CAPP, and CAM systems can be integrated using the interfaces provided.

Manufacturing Automation System

Manufacturing automation system is a value-added system. The material flow and information flow come together in MAS. For a discrete manufacturing company, MAS consists of a number of man- ufacturing machines, transportation systems, high-bay stores, control devices, and computers, as well as MAS software. The whole system is controlled and monitored by the MAS software system. For the process industry, MAS consists of a number of devices controlled by DCS, the monitor system, and the control software system. The objectives of MAS are to increase productivity, reduce cost, reduce work-in-progress, improve product quality, and reduce production time.

MAS can be described from three different aspects: structural description, function description, and process description. Structural description defines the hardware and the software system asso- ciated with the production processes. Function description defines the MAS using a number of functions that combine to finish the task of transforming raw material into products. The input–output mapping presented by every function is associated with a production activity of the MAS. Process description defines the MAS using a series of processes covering every activity in the manufacturing process.

In the research field of MAS, a very important topic is the study of control methods for manu- facturing devices, from NC machines to automatic guided vehicles. But the focus of this chapter is on studying MAS from the CIM system point of view. We will describe the shop-floor control and management system functions and components below.

The shop-floor control and management system is a computer software system that is used to manage and control the operations of MAS. It is generally composed of several modules as shown in Figure 10. It receives a production plan from the MRPII (ERP) system weekly. It optimizes the sequence of jobs using production planning and scheduling algorithms, assigns jobs to specific de- vices and manufacturing groups, controls the operation of the material-handling system, and monitors the operations of the manufacturing process.

Task planning decomposes the order plan from MRPII system into daily tasks. It assigns job to specific work groups and a set of machines according to needed operations. Group technology and optimization technology are used to smooth the production process, better utilize the resources, reduce production setup time, and balance the load for manufacturing devices. Hence, good task planning is the basis for improving productivity and reducing cost of production.

Job scheduling is used to determine the entry time and sequence for different production jobs. It consists of three main functions: static scheduling, dynamic scheduling, and real-time resource sched- uling. Material-flow control is one of the tasks for real-time resource scheduling. Static scheduling is a off-line scheduling method. It determines operation sequences before the production starts. The aim of static scheduling is to reduce the makespan (the time duration between when the first task enters the system and when the last task leaves the system). Operations research is an important method for generating static scheduling. Because errors and uncertainties may be caused by machine breakdown, task priorities change and dynamic scheduling is needed for rescheduling the operation sequences and production routes. It is the best method for increasing the flexibility of production system. Heuristic rules are normally used in generating dynamic scheduling. Job scheduling aims to optimize the operation of production system and increase the system flexibility.

Production activity control is used to control the operations of tasks, material flow, and manufac- turing resources. Real-time data collecting, processing, and decision making are important tasks of production activity control, which aims to regulate and smooth the production processes even when errors and disturbances occur.

Tool management is also a very important task for the shop-floor control and management system. In a manufacturing system, a large number of tools are needed and the supply of necessary tools on time is vital for improving productivity. Tool quality is important to product quality. The parameters of every tool should be maintained in a correct and real-time fashion because these parameters will be used by machine centers in controlling manufacturing processes.

Quality control, production monitoring, fault diagnosis, and production statistics are important supplementary functions for the shop-floor control and management system to be operated efficiently and effectively.

Computer-Aided Quality-Management System

Since the 1970s, quality has become an extremely important factor for a company to win market competition. Customers always want higher product quality for their investment. The computer-aided quality-management system of CIMS is a system used to guarantee the product quality. It covers a wide range, from product design to material supply to production quality control. The International Standards Organization (ISO) has established a series of quality assurance standards, such as ISOs 9000, 9001, 9002, 9003, and 9004. The computer-aided quality-management system has also been called the integrated quality system.

The computer-aided quality system consists of four components: quality planning, inspection and quality data collection, quality assessment and control, and integrated quality management.

The quality-planning system consists of two kinds of functions: computer-aided product-quality planning and inspection-plan generating. According to the historical quality situation and production- technology status, computer-aided product-quality planning first determines the quality aims and assigns responsibility and resources to every step. Then it determines the associated procedure, method, instruction file, and quality-inspection method and generates a quality handbook. Computer- aided inspection planning determines inspection procedures and standards according to the quality aims, product model, and inspection devices. It also generates automatic inspection programs for automatic inspection devices, such as a 3D measuring machine.

Guided by the quality plan, the computer-aided quality inspection and quality data collection receive quality data during different phases. The phases include purchased-material and part-quality inspection, part-production-quality data collection, and final-assembly quality inspection. The meth- ods and techniques used in quality inspection and data collection are discussed in special books on quality control (Taguchi et al. 1990).

Quality assessment and control fulfills the tasks of manufacturing process quality assessment and control and supply part and supplier quality assessment and control. Integrated quality management includes the functions of quality cost analysis and control, inspection device management, quality index statistics and analysis, quality decision making, tool and fixture management, quality personnel management, and feedback information storage on quality problems, and quality problems backtrack into manufacturing steps.

Quality cost plays an important role in a company’s operation. The quality cost analysis needs to determine the cost bearer and the most important cost constituent part to generate a quality cost plan and calculate real cost. It also optimizes the cost in the effort to solve quality problems. Figure 11 presents a quality cost analysis flowchart.

Computer Network and Database Management Systems

Computer network and database management systems are supporting systems for CIMS. The com- puter network consists of a number of computers (called nodes in the network) and network devices, as well as network software. It is used to connect different computers together to enable the com- munication of data between different computers. The computer network can be classified as a local area network (LAN) or a wide area network (WAN). LAN normally means a restricted area network, such as in a building, factory, or campus. WAN means a much wider area network, across a city or internationally. Network technology is developing rapidly. The Internet concept has changed manu-

facturing companies’ operation method greatly. Global manufacturing, agile manufacturing, and net- work-based manufacturing paradigms have seen rapid development. A computer network is the infrastructure for these new manufacturing paradigms to be realized in a cost-effective way.

The database management system provides basic support for the data storage and information sharing of manufacturing companies. Currently, relational database management systems are the principal databases used. Information integration of a company is concerned with integration data sources in different locations and with different kinds of database management systems. The heter- ogeneous properties of computer operating systems and database management systems are the major difficulties in information integration. Advanced software techniques have been developed to cope with the heterogeneity problem. Techniques include CORBA, as well as OLE / DCOM, developed by Microsoft, and the Java language, developed by Sun.

Hundreds of books discussing computer network and database techniques can be found in almost any bookstore.