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Chapter: Automation, Production Systems, and Computer Integrated Manufacturing - Flexible Manufacturing Systems

Flexible Manufacturing Systems(FMS) Components

FMS Components: a. Workstations b. Material Handlingand Storage System c. ComputerControl System d. Human Resources


        FMS COMPONENTS

 

As indicated in Out definition. there are several basic components of an FMS: (1) workstations, (2) material handling and storage system, and (3) computer control system. In addition, even though an FMS is highly automated, (4) people are required to manage and operate the system. We discuss these four FMS components in this section.


 

           Workstations

 

The processing or assembly equipment used in an FMS depends on the type of work accomplished by the system. In a system designed for machining operations, the principle types of processing station are CNC machine tools. However, the FMS concept is also applicable to various other processes as well. Following are the types of workstations typically found in an FMS.

 

Load/Unload Stations. The load/unload station is the physical interface between the FMS and the rest of the factory. Raw work-parts enter the system at this point, and finished parts exit the system from here. Loading and unloading can be accomplished either manually or by automated handling systems. Manual loading and unloading is prevalent in most FMSs today. The load/unload station should be ergonomically designed to permit convenient and safe movement of work parts. For parts that are too heavy to lift by the operator, mechanized cranes and other handling devices are installed to assist the operator.

 

A certain  level of cleanliness must  be maintained at the workplace. and air hoses  or other washing facilities are often required to flush away chips and ensure clean mounting and locating points. The station is often raised slightly above floor level using an open-grid platform to permit chips and cutting fluid to drop through the openings for subsequent recycling or disposal

 

The load/unload station should include a data entry unit and monitor for communication between the operator and the computer system. Instructions must be given to the operator regarding which part to load onto the next pallet to adhere to the production schedule. In cases when different pallets are required for different parts, the correct pallet must be supplied to the station. In cases where modular fixturing is used, the correct fixture must be specified. and the required components and tools must be available at the workstation to build it. 'When the part loading procedure has been completed. the handling system must proceed to launch the pallet into the system; however, the handling system must be prevented from moving the pallet while the operator is still working. All of these circumstances require communication between the computer system and the operator at the load/unload station

 

Machining Stations. The most common applications of FMSs arc machining operations, The workstations used in these systems are therefore predominantly CNC machine tools. Most common is the CNC machining center (Section 14.3.3): in particular. the horizontal rnachining center. CNC machining centers possess features that make them compatible with the FMS, including automatic tool changing and tool storage, use of palletized work-parts, CNC, and capacity for distributed numerical control (DNC) (Section 6.3). Machining centers can be ordered with automatic pallet changers that can be readily interfaced with the FMS part handling system. Machining centers are generally used for non-rotational parts. For rotational parts, turning centers are used; and for parts that are mostly rotational hut require multi-tooth rotational cutters (milling and drilling), mi11·turn centers can be used.

 

In some machining systems, the types of operations performed are concentrated in a certain category, such as milling or turning. For milling, special milling machine modules can be used to achieve higher production levels than a machining center is capable of. The milling module can be vertical spindle, horizontal spindle, or multiple spindle. For turning operations. Special turning modules can be designed for the FMS, In conventional turning, the work-piece IS rotated against a tool that is held in the machine and fed in a direction parallel to the axis of work rotation. Parts made on most FMSs are usually non-rotational: however, they may require some turning in their process sequence. For these cases, the parts are held in a pallet fixture throughout processing on the FMS, and a turning module is designed to rotate the single point tool around the work.

 

Other Processing Stations. The FMS concept has been applied to other processing operations in addition to machining. One such application is sheet metal fabrication processes. The processing workstations consist of press-working operations, such as punching, shearing, and certain bending and forming processes. Also, flexible systems are being developed to automate the forging process. Forging is traditionally a very labor-intensive operation. The workstations in the system consist principally of a heating furnace, a forging press. and a trimming station.

 

Assembly. Some FMSs are designed to perform assembly operations. Flexible automated assembly systems are being developed to replace manual labor in the assembly of products typically made in batches. Industrial robots are often used as the automated workstations in these flexible assembly systems. They can be programmed to perform tasks with variations in sequence and motion pattern to accommodate the different product styles assembled in the system. Other examples of flexible assembly workstations are the programmable component placement machines widely used in electronics assembly.

 

Other Stations and Equipment. Inspection can be incorporated into an FMS, cither by including, an inspection operation at a processing workstation or by including a station specifically designed for inspection. Coordinate measuring machines (Section 23.4), special inspection probes that can be used in a machine tool spindle (Section 23.4.b), and machine vision (Section 23.0) are three possible technologies for performing inspection on an FMS. Inspection has been found to be particularly important in flexible assembly systems to ensure that components have been properly added at the workstations. We examine  the topic of automated inspection in more detail in Chapter 22 (Section 22.3).

In addition to the above, other operations and functions are often accomplished on an FMS. These include stations for cleaning parts and/or pallet fixtures. central coolant de

 

livery systems for the entire FMS, and centralized chip removal systems often installed below floor level

 

           Material  Handling and Storage  System

 

The second major component of an FMS is its material handling and storage system. In this subsection, we discuss the functions of the handling system, material handling equipment typically used in an FMS, and types of FMS layout.

 

Functions of the Handling System. The material handling and storage system in an FMS performs the fol1owing functions:

 

   Random, independent movement of work-parts between stations. This means that parts must be capable of moving from anyone machine in the system to any other machine. to provide various routing alternatives for the different parts and to make machine substitutions when certain stations are busy.

 

   Handle a variety of work-part configurations. For prismatic parts, this is usually accomplished by using modular pallet fixtures in the handling system. The fixture is located on the top face of the pallet and is designed to accommodate different part configurations by means of common components, quick change features, and other devices that permit a rapid buildup of the fixture for a given part. The base of the pallet is designed for the material handling system. For rotational parts, industrial robots are often used to load and unload the turning machines and to move parts between stations.

 

   Temporary storage. The number of parts in the FMS will typically exceed the number of parts actually being processed at any moment. Thus, each station has a small queue of parts waiting to be processed. which helps to increase machine utilization.

 

   Convenient access for loading and unloading work-parts. The handling system must include locations for load/unload stations.

 

Compatible with computer control. The handling system must be capable of being controlled directly by the computer system to direct it to the various workstations, load/unload stations, and storage areas

Material Handling Equipment. The types of material handling systems used to transfer parts between stations in an F.\1S include a variety of conventional material transport equipment (Chapter 10), inline transfer mechanisms (Section 18.1.2),and industrial robots (Charter 7) The material handling function in an FMS is often shared between two systems: (1) a primary handling system and (2) a secondary handling system. The primary handling system establishes the basic layout of the FMS and is responsible for moving work-parts between stations in the system. The types of material handling equipment typically utilized for FMS layouts are summarized in Table 16.5

 

The secondary handling system consists of transfer devices, automatic pallet changers. and similar mechanisms located at the workstations in the FMS. The function of the secondary handling system is to transfer work from the primary system to the machine tool or other processing station and to position the parts with sufficient accuracy and repeatability to perform the processing or assembly operation. Other purposes served by the secondary handling system include: (1) reorientation of the work-part if necessary to present the surface that is to be processed and (2) buffer storage of parts to minimize work change time and maximize station utilization. In some FMS installations, the positioning and requirements at the individual workstations are satisfied by the primary work handling system. In these cases, the secondary handling system is not included,

 

The primary handling system is sometimes supported by an automated storage system (Section: 1.4). An example of storage in an FMS is illustrated in Figure 16.6. The FMS is integrated with an automated storage/retrieval system (AS/RS), and the S/R machine serves the work handling function for the workstations as well as delivering parts to and from the storage racks,

 

FMS Layout Configurations. The material handling system establishes the FMS layout. Most layout configurations found in today's FMSs can he divided into five categories: (1) inline layout, (2) loop hl)'OU1, (3) ladder layout. (4) open field layout, and (5) robot-centered cell.

 

In the inline layout, the machines and handling system are arranged in a straight line, as illustrated in Figure, 16.6 and 16.7. In its simplest form. the parts progress from one workstation to the next in a well defined sequence, with work always moving in one direction and no back flow, as in Figure 16.7(a). The operation of this type of system is similar to a transfer lin., (Chapter 18). except that a variety of work-parts are processed in the

 

TABLE 16.5 Material Handling Equipment Typically Used as the Primary Handling System for the Five FMS Layouts (Chapter or Section Identified in




system. Since all work units follow the same routing sequence, even though the processing varies at each station, this system is classified as type III A in our manufacturing systems classification system. For inline systems requiring greater routing flexibility, a linear transfer system that permits movement in two directions can be installed. One possible arrangement for doing this is shown in Figure 16.7(b), in which a secondary work handling system is provided at each workstation to separate most of the parts from the primary line. Because of the variations in routings, this is II  type  II A manufacturing system.

 

In the loop layout, the workstations are organized in a loop that is served by II part handling system in the same shape, as shown in Figure 16.8(a). Parts usually flow in one direction around the loop, with the capability to stop and be transferred to any station. A secondary


 handling system is shown at each workstation to permit parts to move without obstruction around the loop. The load/unload station(s) arc typically located at one end of the loop. An alternative form of loop layout is the rectangular layout. As shown in Figure 16.8(b), this arrangement might be used to return pallets to the starting position in a straight line machine arrangement.

 

The ladder layout consists of a loop with rungs between the straight sections of the loop, on which workstations are located, as shown in Figure 16.9.The rungs increase the possible ways of getting from one machine to the next, and obviate the need for a secondary handling system. This reduces average travel distance and minimizes congestion in the handling system, thereby reducing transport time between workstations.

The open field layout consists of multiple loops and ladders and may include sidings as well. as illustrated in Figure 16.m This layout type is generally appropriate for processing a large family of parts. The number of different machine types may be limited, and parts are routed to different workstations depending on which one becomes available first.

 

The robot-centered cell (Figure 16.1) uses one or more robots as the material handling system. Industrial robots can be equipped with grippers that make them well suited for the handling of rotational parts, and robot centered FMS layouts are often used to process cylindrical or disk shaped parts


 

Computer   Control  System

 

The FMS includes a distributed computer system that is interfaced to the workstations, material handling system, and other hardware components. A typical FMS computer system consists of a central computer and microcomputers controlling the individual machines and other components. The central computer coordinates the activities of the components to achieve smooth overall operation of the system. Functions performed by the FMS computer control system can be grouped into the following categories:

 

    Workstation control. In a fully automated FMS, the individual processing or assembly stations generally operate under some form of computer control. For a machining system, CNC is used to control the individual machine tools.

Distribution of control instructions to workstations. Some form of central intelligence is also required to coordinate the processing at individual stations. In a machining FMS, part programs must be downloaded to machines, and DNC is used for this purpose, The DNC system stores the programs, allows submission of new programs and editing of existing programs as needed, and performs other DNC functions (Section 6.3).



 

    Production control. The part mix and rate at which the various parts are launched into the system must be managed. Input data required for production control includes desired daily production rates per part. numbers of raw work-parts available, and number of applicable pallets.' The production control function is accomplished by routing an applicable pallet 10 the load/unload area and providing instructions to the operator for loading the desired work-part.

 

    Traffic control. This refers to the management of the primary material handling system that moves workparts between stations. Traffic control is accomplished by actuating switches at branches and merging points. stopping parts at machine tool transfer locations, and moving pallets to load/unload stations.

 

    Shuttle control. This control function is concerned with the operation and control of the secondary handling system at each workstation. Each shuttle must be coordinated with the primary handling system and synchronized with the operation of the machine tool it serves,


    Work-piece  monitoring. The computer must monitor the status of each cart and/or pallet in the primary and secondary handling systems as well as the status of each of the various workpiece types.

    Tool control. In a machining system, cutting tools are required. Tool control is concerned with managing two aspects of the cutting tools:

Tool location. This involves keeping track of the cutting tools at each workstation, If one or mere tools required to process a particular workpiece is not present at the station that is specified in the part's routing, the tool control subsystem takes one or both of the following actions: (a) determines whether an alternative workstation that has the required tool is available and/or (b) notifies the opera tor responsible for tooling in the system that the tool storage unit at the station must be loaded with the required cutter(s).

Tool life monitoring. 1.nthis aspect of tool control, a tool life isspecif.ied to the computer for each cutting tool in the FMS. A record of the machining time usage i••maintained for each of the tools, and when the cumulative machining time reaches the specified life of the tool, the operator is notified that a tool replacement is needed.

 

K Performance monitoring and reporting. The computer control system is programmed to collect data on the operation and performance of the FMS. This data is periodically summarized, and reports are prepared for management on system performance. Some of the important reports that indicate FMS performance are listed in Table 16.6

 

    Diagnostics. This function is available to a greater or lesser degree on many manufacturing systems to indicate the probable source of the problem when a malfunction occurs. It can also be used to plan preventive maintenance iu the system and to identify Impending failures. The purpose of the diagnostics function is to reduce breakdowns and downtime and increase availability of the system.

 

The modular structure of the FMS application software for system control is illustrated in Figure 16.11. It should be noted that an FMS possesses the characteristic architecture or a DNC system. As in other DNC systems. Two-way communication is used. Data and commands an: sent from the central computer to the individual machines and other hardware components, and data on execution and performance are transmitted from the components hack up to the central computer. In addition, an uplink from the FMS to the corporate host computer is provided

 

           Human Resources

 

One additional component in the FMS is human labor. Humans are needed to manage the operations of the FMS. Functions typically performed by humans include: (1) loading raw workparts into the system, (2) unloading finished parts (or assemblies) from the system.

(3) changing and setting tools. (4) equipment maintenance and repair, (5) NC part programming in a machining system, (6) programming and operating the computer system, and (7) overall management of the system



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