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英文原文
COMPUTER AIDED MANUFACTURING
The term Computer Aided Manufacturing (CAM) covers many areas from information processing and decision making to manufacturing and machining, which makes giving a single definition for CAM extremely difficult. D. Kochan gave fitting definition for CAM, with its diversity and wide range of use, in his book, “CAM can be defined as computer-aided preparation of manufacturing including decision-making, process and operational planning, software design techniques, and artificial intelligence, and manufacturing with different types of automation (NC machine, NC machine centers, NC machining cells, NC flexible manufacturing systems), and different types of realization (CNC single unit technology, DNC group technology).”
Since CAM has such a wide range of use, a better way too look at CAM is through CAM technologies. The CAM technologies covered are group technology, manufacturing database, automated and tolerancing. Fig.22-1 illustrates the general scope of CAM.
The essential role of the computer in the production function is to capture and process the data relating to a large number of transaction which continuously take place in different departments of the company. The initial research activity for CAM was Numerical Control (NC) for machine tools at the Massachusetts Institute of Technology (MIT) in 1953. The first programming language was Automatically Programming Tools (APT) created at MIT, and it was the pattern for many further developments. Currently, many manufacturing functions have been addressed by CAM including the following:
Fig.1 The general scope of CAM
·Numerical Control (NC)
·Computer Numerical Control (CNC)
·Direct Numerical Control (DNC)
·Computer controlled conveyor systems
·Computer controlled machining process
·Computer aided process monitoring
·Computer aided fixturing design
·Computer aided tooling design
·Computer aided tolerancing design
·Computer aided cost estimating
·Material Requirement Planning (MRP)
·Computer aided Process Planning (CAPP)
·Computerized machinability data system
·Manufacturing Resources Planning (MRPⅡ)
Computer Numerical Control
Numerical control (NC) is a form of programmable automation in which the processing equipment is controlled by means of numbers, letters, and other symbols. The number, letters, and symbols are coded in an appropriate format to define a program of instructions for the particular work piece changes. The capability to change the program is what makes NC suitable for low-volume and medium-volume production, and it is much easier to write new programs than to make major alteration to the processing equipment.
The principle of numerical control was first applied to the milling process, and then later to turning process, flame cutting , drilling, and grinding. NC technology is now used more and more for other manufacturing processes, such as forming (fine forging, rolling, etc.), engraving, and laser cutting.
The current NC equipment is relatively more mature. Many machines posses multiple processing function, such as milling centers which can perform vertical and horizontal milling, drilling, boring, reaming, slotting, shaping, and turning processes. Of course, with a high capacity automated tooling library, CNC machines’ functions can be considerably more abundant.
Programmable Logic Controller
Programmable logic controller are widely used in computer aided manufacturing. Actually, PLCs are used in virtually every segment of industry where automation is required. PLCs represent one of the faster growing segments of the electronics industry. Since their inception, PLCs have proved to be the salvation of many manufacturing plans which previously relied on electro-mechanical control system. A PLC is a solid-state device designed to perform logic functions previously accomplished by electro-mechanical relays. The design of most PLC is similar to that of a computer. Basically, the PLC is an assembly of solid-state digital logic elements designed to make logical decisions and provide outputs, programmable logic controllers are used for the control and operation of manufacturing process equipment and machinery.
Computer Aided Material Handing
Material handling (MH) is a very important factor in how efficiently a workshop or company can be operated. An efficient MH system will help reduce waiting time, and it may even help increase safety or the effectiveness of the entire manufacturing process.
Cabbert and Brown indicated that as much as 60% of the total production cost many be accounted for by material handing. It is also evidenced that most discrete manufacturing products spend 90% of their manufacturing lead time on the duration of material handing and storage.with MH accounting for such a large amount of the total production cost,it is obvious that reducing the amount of time a produce is handled will dramatically reduce production cost.One way of helping reduce these costs is by using computers to do some material handing.
There is a great variety of material handing equipment available commercially and there are many types of MH approaches used today. One of these approaches is to used a computer database to store listing of MH equipment and the user’s input of factor values. The computer takes the user’s required level of, and preferred importance for, each criterion, and the feasible MH equipment for the task at hand, and produces a category of equipment from which the user can choose the proper type or piece of MH equipment.
Computer Monitoring and Diagnostics for Manufacturing Processes
In a computer monitoring and diagnostic system, the aim of monitoring is to detect failure, while the aim of diagnostics includes fault localization and identification. Both monitoring and diagnostics should appear at all levels of the control-monitoring hierarchy.
There are some essential requirements that almost every monitoring and diagnostics system should posses. Some of the requirement for a monitoring system are : (1) the ability to measure and process relatively numerous analogue and digital signals; (2) the capability of profound preprocessing of measured signals, including statistical and frequency based analysis; (3) the ability for complex, multi-parameter decisions; (4) modular, extendable, reconfigurable structure; (5) programmability in all functions; and (6) standardized bi-directional software/hardware interfaces to the CNC/DNC controllers. Some of the requirements for a diagnostic system are : (1) the system should easily provide knowledge about the causal interrelationship when faults arise, to enable even worker who are not well acquainted with the process to localize faults ; (2) the consequences of faults should be readily available in the system so that the severity of a given fault for the further production process can be estimated ; (3) the user should have the possilibity of repairing the fault alone, I . e . repair instructions should be available to the user in a suitable form ; (4) the operation of the expert system should be possible by employees who have no previous experience with computer ; and (5) after a short training period, the system should be maintained by the employees running the facility so that the presence of expert engineers is no longer necessary.
There are three major types of M/D systems that can be classified by their place and function in the manufacturing system. These M/D systems are : (1) autonomous subsystem monitoring, which gets only messages containing environment or condition descriptions from upper levels of control, and supplies all of the elements of the monitoring process with instruction, parameters, or setting needed for measuring, processing, classification, and intervention ; (2) complementary subsystem monitoring, which undertakes only the task of measuring and processing and passes classification and intervention to system level; and (3) semi-autonomous monitoring, which performs only simple, quick monitoring functions autonomously on its own level, and turn to upper levels in case of sophisticated classification and intervention tasks.
The ideal computer monitoring and diagnostic system can be summed up as being a system that can be used during the absence of the human expert, for example, when the expert is on vacation, during breaks, or if a company wants to have three shift with few people on the third shift.
(A) COMPUTER AIDED DESIGN
Computer Aided Design(CAD) can be defined as using computer to aid engineering design process by means of effectively creating modifying, or documenting the part’s geometrical modeling. CAD is most commonly associated with the use of an interactive computer graphics system..The object of the engineering design is stored and represented in the from of geometric model. Geometric modeling is concerned with the use of a CAD system to develop a mathematical description of the geometry of an object. The mathematical description is called a model. There are three types of models (wire-frame,surface model, and solid models), that are commonly used to represent a physical object. Wire –frame model ,also called edage-vertex or stick-figure models,are the simplest method of modeling and ate most commonly used to define computer models of parts. Surface models may be constructed using a large variety of surface features. Solid models are recorded in the computer mathematically as volumes bounded by surfaces rather that as stick-figure structures. As a result, it is possible to calculate mass properties of the parts, which is often required for engineering analysis such as finite element methods, kinematic or dynamic studies studies, and mass or heat transfer for interference checking .
Models in CAD also be classified as being two-dimensional (2D) models, two-and-half-dimensional models , or three-dimensional (3D) models . A 2Dmodel represents a flat part and a 3D model provides representation of a generalized part shape . a 2.5D model can be used to respresent a part of constant section with no side-wall details . the major advantage of a 2.5D model is that it give a certain amount of 3D information about a part without the need to create the database of a full 3D model .
After a particular design alternative has been developed, some from of engineering analysis must often be performed as a part of the design process .The analysis may take the form of stress-strain calculations, heat transfer analysis, dynamic simulation etc. some examples of the software typically offered on CAD systems are properties and Finite Element Method analysis .Mass properties analysis involves the computation of such features of a solid object as its Volume、surface area、weight、and center of gravity. FEM analysis is available on most cad systems to aid in heat transfer, stress-strain analysis, dynamic characteristics, and other engineering computations. Presently, many CAD systems can be automatically generate the 2D or 3D FEM meshes which are essential to FEM analysis.
As a matter of fact , development of CAD systems is now quite mature . however, considering the interface between CAD and CAPP , many problems still remain .the main problem is transformation of geometrical mode or ,more strictly , geometrical model representations, from CAD to CAPP . for instance ,in the simplest 2D form , Initial Graphics Exchange Specification (IGES) can represent an engineering drawing ,but items such as dimensions can be represented in different ways . Also different drawing systems use different technologies to group lines into profiles. As a result, there appear to be major problems in using IGES to transfer data between different systems . In 3D, The problems are worse because many ways of sorting surface and space curvese are incompatible. Some other attempt, such as the approach of Boundary Representation (B-Rep) and the approach of Constructive solid Geometry (CSC) tress in which the cavities are recognized from the special relationships between the primitive volumes, do not provide any semantic which could be associated with the machined volumes and are based on local information . Nevertheless, great efforts have been made in this area , and many approaches have been provided to interface CAPP with CAD .
(B) COMPUTER AIDED PROCEDD PLANNING
Computer Aided Process Planning (CAPP) can be defined as the functions which use computers to assist the work of process planners. The levels of assistance depend on the different strategies employed to implement the system. Lower level strategies only use computers for storage and retrieval of the data for the process plans which will be constructed manually by process planners, as well as for supplying the data which will be used in the planner’s new work. In comparison with lower level strategies, higher level strategies use computers to automatically generate process plans for some workpieces of simple geometrical shapes. Sometimes a process planner is required to input the data needed or to modify plans which do not fit specific production requirement well. The highest level strategy, which is the ultimate goal of CAPP, generates process plans by computer, which may replace process planners, when the knowledge and expertise of process planning and working experience have been incorporated into the computer programs. The database in a CAPP system based on the highest level strategy will be directly integrated with conjunctive system , e . g. CAD and CAM . CAPP has been recognized as playing a key role in CIM.
More than 20 years have elapsed sine the use of computers to assist process planning tasks was first proposed. Tremendous efforts have made in the development of CAPP system. For the time being, the research interests for development of CAPP system are focused on intelligent and integrated process planning systems. For increasing the intelligence of CAPP systems, some new concepts, such as neural networks, fuzzy logic, and machine learning have been explored for the new generation of CAPP system. For increasing the integrability of CAPP system, feature based design, the roles of features, integrating process planning with scheduling, and integrating process planning with manufacturing resources planning have been focused on . this phenomenon is entitled concurrent or simultaneous engineering.
Why computer aided process planning? It is obvious that CAPP development has been addressed by many universities, institutions, research organization and corporate development departments. A great effort has been made on the subject. However, the question of why CAPP is so important for the current production environment still needs to be answered. In this section the issues will be addressed. In general, there are three main arguments that are involved in the subject.
Since a process plan determines the methods, machines, sequences, fixturing, and tools required in the fabrication and assembly of components, it is easy to see that process planning is one of the basic tasks to be performed in manufacturing systems. The task of carrying out the difficult and detailed process plans has traditionally been done by workers with a vast knowledge and understanding of the manufacturing process. Many of these skilled workers, now considered process planners, are either retired or close to retirement, with no qualified young process planners to take their place. An increasing shortage of process planners has been created. With the high pressure of serious competition in the world market, integrated production has been pursued as way for companies to survive and succeed. Automated process planning systems have been recognized as playing a key role in CIM. It is for reasons such as these that many companies look for computer aided process planning systems.
Computer aided process planning is the way in which most companies are solving the problem of automating process planning and overcoming the shortage of skilled process planners. As the American Machinist and Automated Manufacturing Society has reported in the paper Process Planning Software Enhances Accuracy and Consistency, a computerized process planning system has essentially four goals:
(1) reduces the clerical load of plan preparation on the manufacturing engineers and skilled process planners, who are in short supply;
(2) optimize existing plans using the best available information on machines, tools, speeds, etc. ;
(3) standardize what are known to be the ‘best’ process plans for families of components within a company, thereby capturing the knowledge of the skilled planners;
(4) standardize production time/costs for particular families of components.