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A PAPER ON FLEXIBLE MANUFACTURING SYSTEMS
Submitted by: Jayaditya Sarma K PGDM, Institute of Public Enterprise OU Campus, Hyderabad
ABSTRACT This paper defines the flexible manufacturing system and explains the growth of the systems over the years and their deployment for various manufacturing processes. It gives an insight into the development of the systems with the advent of Computer Numerically Controlled (CNC) machine tools. The key idea in FMS is that the coordination of the flow of work is carried out by a central control computer. This computer performs functions such as: Scheduling jobs onto the machine tools Downloading part-programs part to the machines. Sending instructions to the automated vehicle system for transportation The paper also focuses on the pros and cons of these systems and delves into the productivity and investment facets. The role of FMS communication protocol and the significance of FMS data traffic are expounded. The real strength of these FMS lay in the fact that they brought tremendous benefits in inventory reduction (often 85%), quality improvement and lead time. In many installations, the inventory reduction alone was sufficient to justify the investment in hardware, software and system design effort. With the advent of massive industrialization and globalization across the world manufacturers are being compelled to respond flexibly and rapidly to the changing market conditions. Success in manufacturing requires the adoption of methods in customer-acquisition and order-fulfillment processes that can manage anticipated change with precision while providing a fast and flexible response to unanticipated changes. At this juncture the paper throws some light on the solution i.e., mass customization manufacturing.
CONTENTS
1. Introduction : Flexible Manufacturing System 2. Numerically Controlled Machines 3. The Internal Working Of FMS 4. Advantages 5. Disadvantages 6. Mass Customization Manufacturing
1.0 FLEXIBLE MANUFACTURING SYSTEM: Traditional FMS: A flexible manufacturing system (FMS) is an arrangement of machines.... interconnected by a transport system. The transporter carries work to the machines on pallets or other interface units so that work-machine registration is accurate, rapid and automatic. A central computer controls both machines and transport system. The key idea in FMS is that the co-ordination of the flow of work is carried out by a central control computer. This computer performs functions such as: Scheduling jobs onto the machine tools Downloading part-programs (giving detailed instructions on how to produce a part) to the machines. Sending instructions to the automated vehicle system for transportation. Products to be produced are manually loaded onto pallets at a load station, and the computer system takes over, moving the product to the various processing stations using automatic vehicles, which may be rail-guided, guided by wires embedded in the floor or free-roving. After having visited all necessary stations, usually only two or three, the job is taken back to the load station, where it is removed from the pallet and passed to the next process. Flexibility: A modern Flexible manufacturing system (FMS) is essentially an automated manufacturing cell- a group of interconnected, numerically controlled machines with automated material-handling capabilities and a shared control system. The automated material-handling system must be capable of loading and unloading materials on the Numerically Controlled (NC) machines, as well as transporting parts between them. An FMS then is capable of making a wide variety of parts, even in small quantities, without
human intervention. Although flexible manufacturing systems are very expensive, they can frequently be justified in the context of group technology. Without technology an FMS is likely to be underutilized and eventually to be removed. In a flexible manufacturing system (FMS) there is some amount of flexibility that allows the system to react in the case of changes, whether predicted or unpredicted. Flexibility is one of the benefits of small-batch manufacturing. It is the ability of a manufacturing system to respond at a reasonable cost and at an appropriate speed, to planned and unanticipated changes in external and internal environments. While a small-batch shop may produce lower unit output than a shop dedicated to one or two lines, its strength is that it can make a variety of different products in small volumes. The complexity of coordinating manual small-batch production had, until the early 1980s, confined automation of the manufacturing system as a whole to industries producing in large-batches, with a small, slowly-changing range of products. Small-batch production relied on stand-alone processing machines, which were coordinated by human operators and schedulers. The complex nature of producing a wide-range of products brought what were seen as necessary evils accommodated in the name of flexible manufacturing. The flexibility is generally considered to fall into two categories, which both contain numerous Sub-categories. The first category, machine flexibility, covers the system's ability to be changed to produce new product types, and ability to change the order of operations executed on a part. The second category is called routing flexibility, which consists of the ability to use multiple machines to perform the same operation on a part, as well as the system's ability to absorb large-scale changes, such as in volume, capacity, or capability. 2.0 NUMERICALLY CONTROLLED (NC) MACHINES: When the positions or paths of cutter tools are under the control of a digital computer, we have numerical control .The feedback control paths emanate from the basic positioning controls of the tools or work tables that determine the position of the cutters relative to the work. These feedback control loops continually compare the actual position with the programmed position and apply correction when necessary. group
When two dimensions are controlled, we have position control, illustrated by the drilling holes that must be positioned accurately. The drill tool can be moved in two dimensions to achieve the desired position, after which the tool does the work to produce the hole. Such a system can be programmed to drill a series of accurately positioned holes. When position control is carried one step further, by controlling three dimensions , we have contour control, controlling the actual path of the cutter. Contour control involves a much more complex programming problem because curves and surfaces must be specified. Contour control systems have great flexibility in terms of the part shapes that can be produced as well as in the change of shapes from job to job. Instead of the part being processed through a sequence of machines or machine-centers, it is often possible to perform all the required operations with a single set-up because the cutter can be programmed to make cuts along any path needed to produce the required configuration. Very complex parts can be produced with a single set-up. Advantages of NC Machines: One of the great advantages of numerically controlled systems is that the machine tool is not tied up for long periods during set-up because practically all the preparation time is in programming, which does not involve the actual machine tool. In addition, repeat orders require virtually no set-up time. Thus, the field of applicability includes parts that are produced in low volumes. Therefore, through numerically controlled processes, automation is having an important impact on process technology for both high- volume, standardized types of products and low- volume products(even custom designs). Computer Numerically Controlled Machines: The abbreviation CNC stands for computer numerical control, and refers specifically to a computer "controller" that reads G-code instructions and drives a machine tool, a powered mechanical device typically used to fabricate components by the selective removal of material. CNC does numerically directed interpolation of a cutting tool in the
work envelope of a machine. The operating parameters of the CNC can be altered via a software load program. CNC was preceded by NC (Numerically Controlled) machines, which were hard wired and their operating parameters could not be changed. NC was developed in the late 1940s and early 1950s by John T. Parsons in collaboration with the MIT Servomechanisms Laboratory. The first CNC systems used NC style hardware, and the computer was used for the tool compensation calculations and sometimes for editing. 3.0 THE INTERNAL WORKING OF FMS. An Industrial Flexible Manufacturing System (FMS) consists of robots, Computercontrolled Machines, Numerical controlled machines (CNC), instrumentation devices, computers, sensors, and other stand alone systems such as inspection machines. The use of robots in the production segment of manufacturing industries promises a variety of benefits ranging from high utilization to high volume of productivity. Each Robotic cell or node will be located along a material handling system such as a conveyor or automatic guided vehicle. The production of each part or work-piece will require a different combination of manufacturing nodes. The movement of parts from one node to another is done through the material handling system. At the end of part processing, the finished parts will be routed to an automatic inspection node, and subsequently unloaded from the Flexible Manufacturing System. The FMS data traffic consists of large files and short messages, and mostly come from nodes, devices and instruments. The message size ranges between a few bytes to several hundreds of bytes. Executive software and other data, for example, are files with a large size, while messages for machining data, instrument to instrument communications, status monitoring, and data reporting are transmitted in small size. There is also some variation on response time. Large program files from a main computer usually take about 60 seconds to be down loaded into each instrument or node at the beginning of FMS operation. Messages for instrument data need to be sent in a periodic time with deterministic time delay. Other type of messages used for emergency reporting
is quite short in size and must be transmitted and received with almost instantaneous response. The demands for reliable FMS protocol that support all the FMS data characteristics are now urgent. The existing IEEE standard protocols do not fully satisfy the real time communication requirements in this environment. The delay of CSMA/CD is unbounded as the number of nodes increases due to the message collisions. Token Bus has a deterministic message delay, but it does not support prioritized access scheme which is needed in FMS communications. Token Ring provides prioritized access and has a low message delay; however, its data transmission is unreliable. A single node failure which may occur quite often in FMS causes transmission errors of passing message in that node. In addition, the topology of Token Ring results in high wiring installation and cost. A design of FMS communication protocol that supports a real time communication with bounded message delay and reacts promptly to any emergency signal is needed. Because of machine failure and malfunction due to heat, dust, and electromagnetic interference is common, a prioritized mechanism and immediate transmission of emergency messages are needed so that a suitable recovery procedure can be applied. A modification of standard Token Bus to implement a prioritized access scheme was proposed to allow transmission of short and periodic messages with a low delay compared to the one for long messages. 4.0 ADVANTAGES:
• • • •
Productivity increment due to automation Preparation time for new products is shorter due to flexibility Saved labor cost, due to automation Improved production quality, due to automation The real strength of these FMS lay in the fact that they brought tremendous
benefits in inventory reduction (often 85%), quality improvement and lead time. In many installations, the inventory reduction alone was sufficient to justify the investment in hardware, software and system design effort.
5.0 DISADVANTAGES: 1) Narrow Process Focus: The types of manufacturing processes suitable for integration into traditional FMS remain limited: turning, milling and sheet metal work dominate FMS processes while many other, less well automated, processes remain unintegrated. This is mainly because they are not computerized at the machine level and are hence not yet ready for computer integration at the system level. Nevertheless, even in metal cutting, with much wider application of Computer Numerical Control, comparatively little output is due to FMS. 2) Technological uncertainty: When FMS were first introduced, the novelty of the integration technology naturally made many firms "wait-and-see" until the technology had settled. This was particularly true in the smaller companies. The technology of FMS has, at least in the West, not become mature and well understood and many companies would still consider FMS All-or-Nothing The monolithic all-or-nothing nature of FMS increases the risk of projects, causing companies to shy away. This is particularly true of those companies whose products are a little different from those for which FMS has already proven itself --- the scale of the effort required, in conjunction with their less standard processes is sufficient to dissuade them from undertaking the project.
3) Productivity: In many applications, the productivity of the prospective system --- in terms of its output with respect to its capital input --- is insufficient. Practical experience has also shown that the utilization of the systems may be much lower than predicted when they were designed, further reducing productivity. While productivity may not be the manufacturing performance criterion most closely associated with the competitive focus of the system, there are bare minima to be exceed in any industry. Without a reasonable level of practical productivity (and hence return) from capital, the project will founder, perhaps rightly, in the capital investment procedures of the firm. 4) Shallow learning curve: It takes a long time for an organization to learn about FMS technology. Much of the technology is embodied in software integration, and software engineering is not a skill which many manufacturing companies acquire quickly. Second, the highly interdependent and specialized nature of the technology means that integration is best handled by a very tight nucleus of people . While this might get the job done at the outset (once these scarce people have been found), it often means that just a few people hold the key competencies. This concentration of knowledge inhibits learning in the organization as a whole. The nature of the skills required means that these skilled people have often been imported from outside the firm and owe it only fleeting allegiance. When they leave, they take their skills with them, which further flatten the learning curve of the company. 5) Level of Investment: The investment in FMS (as characterized by Ingersoll Engineers is often in the range of $10 to 15 million). The amount of money needed to finance an FMS is thus a significant barrier to its introduction, particularly in smaller companies. Smaller firms currently perform most of the small batch work, so it is here
where FMS would be most appropriate. However, for most small firms, an investment in FMS would mean "betting the farm". Quite reasonably, given the plethora of other difficulties, they choose not to. 6) Inflexibility: The main disadvantage with FMS technology lies, paradoxically, in its inflexibility. FMS are flexible in that they can, in the short-term, produce a range of known products. However, the complexity necessary to automatically achieve short-term flexibility makes it difficult to introduce new families of products into the system, and certainly much more difficult than in a manual shop. Similarly, when new machines are to be added (or old ones updated) it can be very costly. Changes in system configuration require timeconsuming, expensive alteration to software particularly in complex, Western systems. These six reasons, in concert, marshal against the diffusion of current FMS technology. This is not to say however, that these are sound reasons why FMS should not be embraced. Many argue that the difficulties described above are the price one has to pay, and that technologies such as FMS must be seen as a strategic investment --- the shortterm hurdles must be compared against the long-term strategic and intangible cost of being ignorant of the technology. If this argument were truly compelling, one might expect many more of the forward-thinking companies, whose competitiveness is tightly linked to their small-batch effectiveness to have grasped the nettle, and to have adopted FMS technology as a stepping-stone towards the future factory and as a strategic investment in the flexible technology of the 21st-century plant 6.0 MASS CUSTOMIZATION MANUFACTURING Competition in the manufacturing industry over the next decade will be focused on the ability to flexibly and rapidly respond to changing market conditions. With significantly shortened product life cycles, manufacturers have found that they can no longer capture market share and gain higher profits by producing large volumes of a standard product for a mass market. Success in manufacturing requires the adoption of methods in customer-acquisition and order-fulfillment processes that can manage anticipated change
with precision while providing a fast and flexible response to unanticipated changes Many companies are confronted with the challenge of changing their strategic orientations to meet demands of the current market place. Mass customization manufacturing (MCM) is a solution to this challenge. The design: The design of an MCM system is an extension of the customer-centered concept in fabrication. The design goal is to achieve a balance between product standardization and manufacturing flexibility. Success in mass customization manufacturing is achieved by swiftly reconfiguring operations, processes, and business relationships with respect to customers’ individual needs and dynamic manufacturing requirements. It is thus critical to develop a manufacturing system that will achieve this goal. A competitive manufacturing system is expected to be flexible enough to respond to small batches of customer demand .Because the construction of any new production line is a large investment, current production lines must be able to be reconfigured to keep up with increased frequency of new product designs. In MCM, each unpredictable feature demanded by customers is considered an opportunity, whereas current system capabilities may not be able to support new customer requirements. The key to adjusting the manufacturing capability successfully is to reconfigure the system, developing and integrating new functions when necessary. Challenges: The revolutionary MCM system is characterized by four challenging characteristics: degrees of flexibility, production capability adjustments, modularization methods, and dynamic network-control system structure. 1) Degrees of flexibility The traditional flexible manufacturing system (FMS) is based on numerically controlled machines in addition to other value-added, automatic, material handling facilities. A degree of flexibility within FMS serves to satisfy demands for a relatively diverse range of products with a small to medium batch size production. Compared with FMS, more part varieties are produced in a mass-customized production environment, and manufacturing requirements are often dynamically changed. In addition, customer orders
come through more randomly with different delivery dates. Thus, an MCM system must possess sufficient flexibility and rapid response capability to deal with complex manufacturing situations. 2) Production capability adjustments The expandability of production capability for traditional FMS is limited by the scope of product families during design stages. It is usually a difficult task to renovate a FMS to accommodate new features demanded by market changes. MCM requires rapid adjustment of production capability based on customer demands. To accommodate everchanging manufacturing requirements, an MCM system needs to be equipped with rapid, production-plan-configuration and resource-allocation capabilities. Since one of the MCM philosophies is to face a certain level of unknown customized demands, a key objective for the development of an MCM system is continuous satisfaction of customer demand. 3) Modularization methods Modularization methods in traditional manufacturing systems are often product-oriented, where modules are grouped in teams with intercross functions. It is difficult for such a system to change structures when products need to be changed and production capability needs to be adjusted. In addition, the old modularization method is likely to cause inner frictions when adjustments are performed. In an MCM system, it is more desirable to categorize modules based on their functionalities: the greater the diversity of module classifications, the better the system’s potential to satisfy different customized demands. 4) Dynamic-network-control system structure Control system structures in FMS are often constructed in a hierarchical mode. Modules assigned at various closely interactive layers result in the limitation of the capability for system reconfiguration, reliability, and system expandability. Moreover, the complexity of this type of system structure will increase as the scope of the system increases. Standalone technologies may not be sufficient to satisfy the operation of a highly complex MCM system. Dynamic network control is needed to maximize the optimal potential benefit. Because of the complexity in ever-changing manufacturing requirements and flexible process routing, fixed and centralized control is almost impossible in a MCM system. Dynamic and flexible network utilizations in MCM functional modules can
maximize the strength of each empowered resource, and hence, the overall risk and costs are reduced.