MES (manufacturing execution system) architecture

INT. J. COMPUTER INTEGRATED MANUFACTURING, 2002, VOL. 15, NO. 3, 274–284

MES (manufacturing execution system) architecture for FMS compatible to ERP (enterprise planning system) BYOUNG K. CHOI and BYUNG H. KIM

Abstract. Presented in the paper is MES (manufacturing execution system) architecture, which is suitable for managing FMS (flexible manufacturing system) lines under an ERP (enterprise planning system) environment. An IDEF0-model of an ‘order handling’ shop floor having an FMS line is developed to identify functional requirements of MES, and then a two-tier MES architecture satisfying the functional requirements is proposed. The proposed MES is composed of a Main-MES (for the main shop floor) and an FMS-MES (for the FMS line). A BOP (bill of processes) is used as a means to represent process plans, and a LS-Net (loading schedule network) is used as a mechanism for representing and manipulating loading schedules. Object models of BOP and LS-Net are presented, and the effectiveness of the proposed MES is demonstrated by applying it to two FMS lines, a stamping-die machining line and a mechanical part machining line.

1. Introduction In this paper, the term FMS (flexible manufacturing system ) is used to mean a computer-integrated machining system consisting of machining centres, automated handling systems for jobs and tools, AS/RS (automated storage/retrieval systems), auxiliary processing facilities, and set-up stations. If properly managed, a FMS has a high potential for increased productivity with enhanced flexibility (Basnet and Mize 1994, Pyoun and Choi 1994). An ERP (enterprise resource planning) system is an integrated information processing system supporting various business processes such as finance, distribution, human resources and manufacturing. Since the 1990s, Authors: B. K. Choi (e-mail: [email protected]) and B. H. Kim, Department of Industrial Engineering, Korea Advanced Institute of Science and Technology (KAIST), 373-1, Kusong-dong, Yusong-gu, Taejon, 305-701, Republic of Korea.

ERP systems have gained an explosive popularity among manufacturing enterprises worldwide (Hicks and Steche 1995, Welti 1999). An ERP system by nature is not suitable for controlling day to day shop floor operations, and for this purpose, a new type of industrial software called MES (manufacturing execution system) emerged during the 1990s. An MES aims to provide an interface between an ERP system and shopfloor controllers by supporting various ‘execution’ activities such as scheduling, order release, quality control, and data acquisition (MESA 1997). In particular, this role of MES has been well established in the semiconductor industry (SEMATECH 1997). For a manufacturing company employing traditional manufacturing facilities and conventional MIS (management information systems), a FMS and ERP system may be the two major areas of investment if the company wants to be more competitive. The amount of investment for a typical FMS is about US$5 – 10 million (Kim 1996), and that for a typical ERP system is about US$3 – 5 million (Welti 1999). Thus, for a company that has invested on both FMS and ERP, it is essential to have a suitable MES that can provide an adequate interface between the two. However, to the best of our knowledge, there are no known MES models for this purpose. Existing works on MES include a Gantt Chart based MES for die and mould manufacturing (Choi et al. 1995), an integrated MES framework (Scott 1996), a CIM framework for semiconductor manufacturing (SEMATECH 1997), a distributed and object-oriented MES framework (Cheng et al. 1999), and MES development cases for semiconductor manufacturing (Pickett and Zuniga 1997, Cheng et al. 1998, Westphal and Gramlich 1998). Quite a few commercial MES software systems are also available, but unlike ERP systems

International Journal of Computer Integrated Manufacturing ISSN 0951-192X print/ISSN 1362-3052 online # 2002 Taylor & Francis Ltd http://www.tandf.co.uk/journals DOI: 10.1080/09511920110059106

MES architecture for FMS compatible to ERP there are no reference MES models that can be used for general manufacturing environments (MESA 1997). Proposed in the paper is a two-tier MES architecture suitable for bridging the gap between an FMS controller and an ERP system. Since an FMS line is a portion of a ‘main’ shop floor, the two-tier MES consists of a main-MES in charge of main shop-floor operations and an FMS-MES in charge of FMS operations. The main-MES is connected to the ERP system, and the FMS-MES is connected to the FMS controller. The twotier MES architecture has been implemented for the two FMS lines shown in figure 1. The FMS line in figure 1(a) is an automobile stamping-die machining line having three high-speed machining centres and a CMM, and the FMS line in figure 1(b) is a mechanical parts machining line having four machining centres and a washing machine. The paper is organized as follows. In section 2, a structured model of FMS operations is presented together with a set of functional requirements for MES. A two-tier MES architecture satisfying the functional requirements is presented in section 3, and some details of the internal structure of the proposed MES architecture is described in section 4. A brief description of implementation experience is given in the section that follows, and concluding remarks and discussions are given in the final section.

set-up stations. The devices in the FMS line are controlled by an FMS controller that is responsible for releasing orders (for machining, set-up, etc) and for receiving status reports and messages from the devices (SNK 1996, Kim 1996). 2.1. Overall structure of shop floor operations In general, the ‘main’ shop floor containing an FMS line is a part of an Order Handling Manufacturing System (Wu 1994). Depicted in figure 2 is an IDEF0 model of such shop-floor operations. The IDEF0 model was constructed based on a detailed analysis of the two shop floors having the FMS lines given in figure 1. The shop floor operations may be grouped into the following categories (A1, A2, . . . represent Activity boxes of the IDEF0 model in figure 2):

(1) Customer Inquiry Handling: Inquiry receiving (A1)ïTemporary process planning (A3)ïLoading scheduling simulation (A5)ïConfirm due dates. (2) Received Order Handling: Order receiving (A1)ïShop-level process planning (A2)ïLoading schedule generation (A5)ïDispatch jobs to FMS or main shop. (3) Load/Progress Control: Job operations (A6: FMS; A7: main shop)ïMonitoring and data acquisition (A8)ïLoading schedule re-generation (A5) ïDispatch jobs. 4 ( ) ECO (engineering change orders) Handling: Planning for ECO/repair (A4)ïLoading schedule generation (A5)ïDispatch jobs.

2. Functional modelling of FMS operation As mentioned earlier, a typical FMS line consists of such ‘devices’ as machining centres, automated handling systems, AS/RS, auxiliary processing facilities, and a

Figure 1.

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Two FMS lines for which the two-tier MES architecture was applied: (a) stamping-die machining, (b) mechanical parts machining.

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Figure 2. IDEF0-model for the main shop floor having an FMS line.

In figure 2, BOP (bill of processes) represents a network structure in which BOM (bill of materials) and the routing information for a product are stored (Choi et al. 1995). Contained in the Master Data are all the technical data related to production resources, production processes, components of the products, and working hours and maintenance schedules. 2.2. Overall structure of FMS operations Details of Activity box A6 (FMS operations) of figure 2 are described in figure 3 as a level-2 IDEF0 model. Once FMS production orders are dispatched, FMS operations for ‘repeating’ jobs whose NC-programmes and tooling information are already available are carried out as follows.

(1) Retrieve detailed machining-process plans for the jobs (A61). (2) Generate FMS machining schedules and send out NC-file transfer instructions and tool preparation instructions (A62).

(3) Download NC files and send out set-up instructions (A63). (4) Tool presetting and installation (A64). (5) Perform machining operations (A65) while collecting data (A66). For a ‘new’ job whose NC-programmes and tooling information are not available, the FMS operations are carried out in two phases (planning and execution phases ). During the planning phase, a tentative process plan for FMS machining is generated in A61, and FMS loading schedules together with NC programming requests are generated in A62. Then, NC files together with a UMO (unit machining operation) list for the job are prepared in A63. The FMS operations for ‘new’ jobs at the execution phase are similar to those for repeating jobs. Once the NC files are downloaded and the tools are installed, the FMS line may operate in an ‘unmanned mode’ as long as the jobs to be machined are available. The length of the unmanned operation is an important design parameter when planning for an FMS line. For example, to run an FMS line in an unmanned mode

MES architecture for FMS compatible to ERP

Figure 3.

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IDEF0-model for FMS operations.

‘over the weekend’ (18:00 Friday–8:00 Monday), the required length of unmanned operation is 62 hours. 2.3. Functional requirements of MES There are a number of distinctive features of a shop floor having an FMS line. First, as can be seen from the shop floor model of figure 2, an FMS line is a component of the main shop. Thus, FMS operations are tightly coupled with the rest of the shop floor operations. Namely, a job can be dispatched either to the FMS line or to non-FMS machines, and an FMS job may need pre- and/or post-processing operations performed on non-FMS machines. Existing research works (Basnet and Mize 1994, Sabuncuoglu and Karabuk 1999) focus on finding ‘optimal schedules’ only for FMS operations and, as a result, there is a gap between those research results and industrial practices (Sauvaire et al. 1998). Second, it is obvious from the FMS IDEF0-model (figure 3) that timely delivery of reliable NC files and cutting tools is critical to a successful operation of FMS. Thus, it is essential to have an interface mechanism for

coordinating the activities of the NC programming room, the tooling room, and the FMS operators. Third, it is required to allocate jobs having a longer processing time to the ‘over the weekend’ unmanned periods (to prevent starving). A commercial FMS controller that is designed as a ‘standalone’ controller does not easily accommodate these requirements (Lin and Lee 1997, Maione and Piscitelli 1999). The above ‘distinctive features’ have to be handled by an MES system. Thus, in summary, major functional requirements of MES are:

(1) De-coupling the FMS operations from the rest of the shop floor operations. (2) Interfacing and coordinating the activities of various sub-systems (programming room, tooling room, FMS control room, etc). (3) Allocating ‘longer processing’ jobs to over-theweekend unmanned periods In addition, the MES is required to communicate with the corporate ERP system as well as with the POP/ DAS (point of production/data acquisition systems ) of the main shop floor.

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A shop floor having an FMS line may contain a number of sub-systems, including POP/DAS, NC programming room, tooling room, and FMS controller. The shop floor interacts with the outside world through an ERP system. 3.1. Two-tier MES architecture As discussed in the previous section, FMS operations need to be de-coupled from the main shop floor operations by introducing an MES. To meet this requirement, we propose a two-tier architecture composed of a Main-MES and an FMS-MES. (We could have two FMS-MES systems if the shop floor has two FMS lines). The Main-MES is directly linked to the ERP system and to the POP/DAS system of the shop floor, and the FMS-MES is connected to NC Programming and Tooling rooms. In fact, the two-tier architecture is similar to the ‘proper hierarchical form’ of advanced manufacturing system control (Dilts et al. 1991). Hierarchical architectures are widely employed in scheduling systems for a general job shop (Tagawa 1996) and for an autonomous and distributed shopfloor environment (Tharumarajah and Bemelman 1997). Referring to the shop floor model of figure 2, orderreceiving (A1) is handled by the corporate ERP system, and the main-MES is responsible for shop-level process planning (A2, A3, A4) and loading schedule generation (A5) for the entire shop floor including FMS operations. As shown in figure 4, the schedule generated by the main-MES is sent to the FMS-MES in the form of ‘FMS order’ to be used as constraints when the FMSMES is generating its ‘FMS schedule’, which is then

Figure 4.

Two-tier MES architecture.

sent back to the main-MES. When there is a conflict in the FMS schedule, the main-MES will generate a revised schedule, and so on (until the conflict is resolved). 3.2. Architecture of FMS-MES Referring to the FMS model of figure 3, the FMSMES is responsible for FMS-level process planning (A61) and FMS-schedule generation (A62). Figure 5 shows an overview of the FMS-MES architecture containing three ‘internal modules’ – FMS-level process planning, schedule generation, and schedule editing. For FMS-level process planning, it receives an FMS order (shop-level process plan) from the main-MES and a UMO (unit machining operation) list from the NC P/G room. Process planning is carried out interactively by ‘editing’ the standard BOP stored in a database, and the resulting BOP is stored in the actual BOP database. For schedule generation, a ‘loading simulation’ is performed by assigning available resources in the resource DB to the individual processes in each of the actual BOPs while taking into account the existing loading status. It is a ‘finite capacity scheduling’ method that is widely used (Ying and Clark 1994, Kim 1995, Yeh 1997, Baker 1998). The resulting schedules are stored in a LS-Net (loading schedule network). In practice, the schedules obtained from loading simulation may have to be ‘edited’ manually (e.g. by outsourcing some of the over-loaded processes and/or adding overtime schedules), which is called schedule

Figure 5. Architecture of FMS-MES.

MES architecture for FMS compatible to ERP editing. Every time a new schedule is generated for a job (or the existing schedule is updated), the FMS-MES takes the following actions:

(1) A ‘schedule report’ of the FMS schedule is sent to the main-MES; (2) Instructions for set-up and machining are sent to the FMS controller; (3) Instructions for NC-data preparation are sent to the NC programming room; (4) Instructions for tool preparation are sent to the tooling room. The existing schedule is updated every time execution reports are received from the FMS controller. 4. Internal structure of FMS-MES Described in this section are object model structures for BOP (bill of processes) and LS-Net (loading schedule network) as they play a key role in FMS-MES (figure 5). 4.1. BOP structure As explained earlier, BOP is a network structure in which BOM (bill of materials) and routing information for a product is stored. To support the FMS-MES requirements, the basic BOP structure (Choi et al. 1995) has to be extended in a hierarchical manner. Shown in figure 6 is an example of hierarchical BOP structure for

Figure 6.

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a machine component to be machined in an FMS (figure 1(b)). In the shop floor level, a process in the BOP represents a group of operations of a given type such as NC milling (NM) and drilling (DR). The shop-level BOP is utilized by the main-MES, and associated with each shop floor level process are earliest start time (EST) and latest finish time (LFT). A shop-level process is decomposed into machine-level processes. For example, NM01 is decomposed into FX (fixturing), SM01 (surface milling 01), etc. The machine level process SM01 is again decomposed into a number of tool-level processes called unit-machining operations (UMO). Processes (and parts) at the machine and tool level are stored in an FMS-BOP. Machine level processes in an FMS-BOP may be grouped into: (1) fix-type processes (fixturing, refixturing) in which such data as pallet type, number of parts on a pallet, and processing times are stored; (2) NC-type processes (machining, measuring) in which a UMO-list is stored; and (3) auxiliary-type processes (washing, unfixturing). A portion of an FMS-BOP is shown in figure 7. In an FMS-BOP network, a node (BOP-node) represents either a process (circle) or a part (rectangle). Stored in a process-node are such items as eligible resources (i.e. machines that can handle the process) and expected processing times, while stored in a part-node are production volume, size, material type, order number, etc. Shown in figure 8 is an object model of FMS-BOP in which detailed relationships among the object classes (BOP-node, part, processes, and BOP) are specified.

Hierarchical BOP structure.

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An FMS-BOP is constructed every time a job is dispatched to the FMS line, and it is updated when an ECO (engineering change order) is received or when the process plan is changed (due to defective processing and/or machine breakdown, etc). 4.2. LS-Net structure Figure 7.

Figure 8.

FMS-BOP structure.

Object model of FMS-BOP.

Figure 9.

Once a resource is scheduled to every process of each BOP, we obtain a linked-list structure, called a LSNet (loading schedule network), in which the entire schedule for all jobs is stored (Choi et al. 1995). However, for FMS operations, more than one part may be set up on a pallet, and multiple parts on a pallet are treated as a single job. Thus, this ‘palletizing effect’ must be explicitly represented in the LS-Net as shown in figure 9. In the LS-Net object model of figure 9, a ‘Palletized Part’ represents multiple parts on a single pallet and a ‘Palletized Process’ represents the processing operation for a palletized part, and the BOP model (the shaded rectangle) is called a Palletised BOP. The palletizing effect can be modelled as follows. When the number of parts to be machined is n and the pallet size (i.e. number of parts on a pallet) is s, n/s

Object model of LS-Net for FMS-MES.

MES architecture for FMS compatible to ERP palletized parts are created. In addition, a palletized process is created for each palletized part in the palletized BOP of the LS-Net. The LS-Net may conveniently be displayed using a Gantt chart: a Gantt chart for a job (or a group of parts) is called a ‘progress chart’, and a Gantt chart for a group of resources is called a ‘load chart’. 5. Implementation of the MES architecture The two-tier MES architecture proposed in the paper has been implemented for a passenger car stamping-die machining shop having the FMS line of figure 1(a) and for a mechanical parts machining shop having the FMS line of figure 1(b). 5.1. FMS-MES for stamping-die machining An overall layout of a stamping-die shop together with a detailed layout of the FMS line shown in figure 1(a) is depicted in figure 10. In the FMS line are three M/Cs (machining centres) equipped with ‘highspeed’ attachments, one CMM (coordinate measuring machine), four pallet stockers, one set-up station, and seven pallets that are transported by a RGV (rail-guided vehicle). Mainly large-sized dies are dispatched to the FMS line (the average machining time on a M/C is about 20 hours). A FANUC FD-Mate controller controls the FMS line. Its main functions are to (1) download NC files to the M/Cs and CMM; (2) issue machining/measuring commands to them; (3) issue set-up orders to the setup station; (4) control the RGV operation; and (5) receive messages from the devices (SNK 1996). The main objectives for developing an FMS-MES (on top of the main-MES that had been in use when the FMS line was installed in 1997) were:

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(1) Scheduling of FMS operations to meet the due dates of stamping-dies. (2) Scheduling of FMS operations to cope with disturbances of the main shop floor in preparing ‘raw’ dies to be machined in the FMS line. (3) Elimination of machine idle periods due to delays in NC-file and tool preparation. (4) Scheduling of FMS operations to provide enough jobs for the over-the-weekend unmanned period of 44 hours (Saturday 13:00 – Monday 8:00). An FMS-MES software system was developed during 1997 using C++ and Motif, and was deployed in the FMS line in early 1998. The programme runs on a Workstation under an X-Window environment. Establishing an interface with the FMS controller was the most difficult part of the development project (the commercial controller was not friendly for an ‘open’ interface). Shown in figure 11 is the main screen of the FMS-MES software. Shown at the left of the screen is a list of stamping-dies dispatched to the FMS line. The status of the FMS line is displayed at the upper-right portion, and a list of ‘current’ and ‘waiting’ operations is shown at the lower-right portion. It is not easy to quantify the effectiveness of the FMSMES software system, but it is regarded as a successful project since the main objectives of the project have been met to some extent. Currently the FMS line is operated at a utilization rate of 90%. Along the line, however, the FMS line went through a number of ‘trialand-error’ processes in setting up a stabilized management procedure. 5.2. FMS-MES for mechanical part machining Sketched in figure 12 is an overall layout of a mechanical part machining shop together with a

Figure 10. Shop floor and FMS line layout for stamping-die machining.

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Figure 11. Main screen of FMS-MES for stamping-die machining.

Figure 12.

Shop floor and FMS line layout for mechanical part machining.

detailed layout of the FMS line shown in figure 1(b). In the FMS line are four M/Cs (three 3-axis M/Cs and a 5-axis M/C), one washing machine, one stacker crane, three set-up stations, and a pallet stocker system of 96 cells. Medium-sized ‘prismatic’ parts are machined (milling, boring, drilling, tapping) at the FMS line (with an average machining time on a M/C of about 5 hours). Unlike stamping dies, multiple parts are set up on a pallet. A proprietary controller, TONGIL-Mastrol shown in figure 13, controls the FMS-line (Kim 1996). The main

objectives for developing an FMS-MES were similar to those for the stamping-die shop case. FMS-MES and main-MES were developed simultaneously using Visual C++ and Oracle DBMS. The software runs on a PC under a Windows 9x/NT environment. The establishing interface with the FMS controller posed no difficulty because the company makes the controller. However, difficulties encountered during the MES development came from the ‘DB interface’ because different database systems were used for different applications: IBM’s DB2 for the ERP system (BPCS);

MES architecture for FMS compatible to ERP Oracle DB for the POP system; MS Access for the FMS controller. The Main-MES is in use currently, but the FMS-MES is in its final tuning stage. Shown in figure 14 are a progress chart for a group of parts and a load chart for the major resources in the FMS line. It is too early to assess the benefit of the FMS-MES, but the MES system is expected to serve its purpose.

applying it to two FMS lines in Korea. Key characteristics of the proposed MES architecture are:

(1) It has a hierarchical structure consisting of a MainMES and an FMS-MES. The former controls the latter, but the two have the same basic structure. (2) The Main-MES is in charge of the entire shop floor operations (including FMS) and is directly connected to ERP System and POP/DAS System. (3) The FMS-MES is in charge of the FMS operations and is interfaced with the FMS controller, NC programming room, and tooling room. (4) The concept of BOP (bill of processes) is utilized as a vehicle for representing, planning, storing and displaying process plans. (5) The concept of LS-Net (loading schedule network) is utilized as a mechanism for representing, generating, storing, and displaying schedules.

6. Conclusions and discussions Proposed in the paper is a two-tier MES architecture suitable for managing FMS operations under an ERPsystem environment. The proposed MES architecture was developed based on a set of functional requirements observed in ‘real’ FMS, and the effectiveness of the two-tier architecture has been demonstrated by

Figure 13.

The proposed MES architecture is derived from the operating characteristics of two ‘linear type’ FMS lines that may not necessarily possess the general characteristics of a generic FMS line. Thus, it is necessary to validate the effectiveness of the proposed MES architecture for different types of FMS. An architecture represents a set of rules that defines (1) a unified structure consisting of constituent parts and (2) the connections that establish how those parts fit and work together (SEMATECH 1997). In this respect, it is necessary to develop object-oriented models for the semantic/syntactic behaviour of the MES components (the paper focused on the first aspect of the MES architecture ).

Structure of the FMS controller for mechanical part machining.

a

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Figure 14.

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Gantt Chart display of FMS schedule (mechanical part machining): (a) progress chart, (b) load chart.

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Acknowledgements The Ministry of Science & Technology of Korea supported the research through a ‘National Research Lab’ grant to the first author of the paper. The research was also supported by the Korean government through a ‘G7 Project’ grant.

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