What Are Manufacturing Execution Systems?

Originally published - November 29th, 2005
Revised and updated March 5th, 2008

Plant execution software systems have many different scopes, forms, and formats, and they mean different things to different folks. Although plant execution software is used widely in a number of industries, it is rarely described similarly, and its functions are hardly ever identical.

An execution system used at an electronics discrete manufacturing facility is similar only in concept to one used at a food processing plant, and these differ substantially from that used by a pharmaceutical or chemical manufacturer. Time and experience have led the most successful vendors to pursue a “narrow-and-deep” strategy, and to devote their software development to the industries they know best. Even still, the names given to the various components of the execution systems vary greatly among industries and even among companies within an industry—if not between plants within a company.

To add further confusion, official definitions of a manufacturing execution system (MES) differ as well. APICS Dictionary (11th edition) defines it as

[p]rograms and systems that participate in shop floor control, including programmed logic controllers and process control computers for direct and supervisory control of manufacturing equipment; process information systems that gather historical performance information, then generate reports; graphical displays; and alarms that inform operations personnel what is going on in the plant currently and a very short history into the past. Quality control information is also gathered and a laboratory information management system [LIMS—applications used to manage the collection of samples, collection and formatting of test results, and the reporting of results by sample or product category, whereas these applications may be environmental-, medical- or research-focused] may be part of this configuration to tie process conditions to the quality data that are generated. Thereby, cause-and-effect relationships can be determined. The quality data at times affect the control parameters that are used to meet product specifications either dynamically or off line. [italics added]

Gartner’s IT Glossary defines MES as a

computerized system that formalizes production methods and procedures within the manufacturing environment, providing online tools to execute work orders. The term is generally used to encompass any manufacturing system not already classified in the enterprise resource planning (ERP) or open control system [OCS—a manufacturing system that is based on a set of commercially available, standards-based technologies, and that permits the open exchange of process data with plant systems and business systems throughout a manufacturing enterprise, whereas "control" refers to process control for discrete, batch, and continuous-process manufacturing, as well as computer numerical control and other motion controls] categories. In the broadest definition, MESs include computerized maintenance management systems (CMMSs), LIMSs, shop floor controls (SFC—a system of computers and controllers used to schedule, dispatch and track the progress of work orders through manufacturing based on defined routings), statistical process control [SPC] systems, quality control systems, and specialized applications such as batch reporting and control. [italics added]

What these lengthy definitions illustrate is that it can be difficult to easily identify or define the full range of applications used on the plant floor, let alone determine what falls exclusively under MES. Moreover, vendors never hesitate to add to the confusion by using creative labeling to suggest difference.

To put MES into perspective, it can be defined both broadly and specifically. Broadly speaking, MES can be regarded as a collection of business processes that provide event-by-event, real-time execution of planned production requirements. For example, it can calculate what and how much to produce, based on information from the enterprise planning level. From electronic production management systems to shop floor data capture, MES functions manage operations from point of order release to manufacturing, to point of product delivery to finished goods.

A narrow definition of MES is that it serves as a work order–driven, work-in-process (WIP) tracking system that manages and monitors production events and reporting activities. It captures “live” information about setups, run times, throughput, yields, etc., allowing managers to better measure constraints, identify bottlenecks, and get a better understanding of capacity. It closes the loop for production management and helps ensure production is followed as planned.

MES Today

Seen as a bridge from the plant floor to the rest of the enterprise, MES has become the principal means of delivering real-time order status to the supply chain, for available-to-promise (ATP) processing, and for “closing the loop” with sophisticated enterprise and supply chain planning systems.

As a result, and despite the disparities surrounding MES systems, some similarities exist regarding its general functional scope. The functions and information collected in these systems can be categorized similarly. Overall, MESs try to bring pervasive computerization to plant floors in a systematic way by placing diverse functions on a single platform, including quality management, document management, and plant-floor dispatching. The components of these systems can, in principle, be divided into two categories: 1) core functions, which are directly associated with managing the production process and are included in most vendor packages; and 2) support functions, which are somewhat peripheral to the central order management process, and are only provided as options.

An MES tracks WIP through detailed product routing and tracking, labor reporting, resource and rework management, production measurement, and automated data collection (ADC). In other words, it acts as a collection point, clearinghouse, and translator for data that is needed on or is generated by the plant floor (see The Why of Data Collection for more information). It also offers exception management, which provides the ability to respond to unanticipated events that affect the production plan, such as a bill of materials (BOM) item shortage for a work order in process. Most systems include the ability to react to exceptions, following rules that are typically plant-centric, and exception management generally requires some level of configuration or customization in order to meet local requirements.

These core functions are fulfilled through modules like Order Management, which can accumulate and manage work orders that have been received from the planning system, often through some planning system interface that defines what and how information is exchanged. It performs common tasks such as quantity changes to orders; combining or splitting orders; running short-term what-if analyses to determine best current resource use; and prioritizing, dispatching, and scheduling.

A Workstation/Work Centers Management module can implement a work order production plan, and assign workstation scheduling. It is also responsible for the logical configuration of each workstation. The availability of current resources and current scheduling requirements, by operation, are normally maintained here.

Additionally, the Inventory Tracking and Management function develops, stores, and maintains the details of each batch, lot, or unit of inventory of the WIP. The Material Movement Management module schedules and manages the movement of material, either manually or automatically. Through such modules, an MES application can deliver a proven, reasonably justified, closed-loop system for highly complex manufacturing environments that have high product mix; real-time, event-driven conditional workflows; and heavy ADC requirements (such as for lot/serial tracking).

Peripheral MES Functions

What may be a critical flaw of the MES vendor community is its failure to define clearly and consistently the functionality of MES, adding to the confusion of buyers. When a provider declares itself to be an MES vendor, often all it is really saying is that it is not an ERP, enterprise asset management (EAM), or open control software (OCS) vendor, which leaves the user to guess what functionality scope the vendor really provides. MESs come in all shapes and sizes and can have one or more of the components outlined above, depending on the industry and user company.

For instance, a vendor might call a single module—such as a SPC or a physical infrastructure management system (PIMS) package—an MES system. Others may offer a wide assortment of systems and collectively referred to them as an MES, but have no tie between the packages. Also, in some instances, core functions will generally be well integrated, but most of the support functions will not. For example, while more modern applications pay more attention to data integration issues, most current plant-level execution systems still consist of disparate components.

Following is a list of key MES functionality, which also illustrates the overlap that can occur between enterprise applications and plant-level systems:

  • Computerized maintenance management system (CMMS) or EAM system
    These systems manage production equipment maintenance and repair-related issues, including predictive and preventative maintenance; work order and labor scheduling; procurement and storage of the repair parts inventory; and equipment-record maintenance. For more information, see EAM Versus CMMS: What's Right for Your Company?

  • Time and attendance/clock system
    These systems usually include clock-in/clock-out information and labor and employee skills data. Such systems minimize the time required for labor data input. The product generates payroll timecards or an output file for external payroll systems. It also generates more accurate job costing information by tracking labor to specific tasks.

  • Warehouse management system (WMS)
    These systems are primarily used for monitoring and managing outbound inventory activities. Supply chain execution (SCE) tasks, for example, can also include logistics and other transportation management data, and some systems are also capable of inbound raw or purchased material management. Product location information and order fulfillment instructions are two of many online functions.

  • Statistical process control (SPC)
    This is a quality control method that focuses on continuous process monitoring rather than the inspection of finished individual products—it has the ability to do capability calculations based on the data that users capture from the shop floor.

  • Quality management system (QMS)
    These systems may or may not be tied together with SPC or ISO 9000 quality standards, but whether stand-alone or combined, these packages are frequent components of the production process. The same holds for LIMS and environmental safety and health (ES&H) systems, which is a category of software applications that deals with regulatory compliance, such as the US Environmental Protection Agency (EPA) or Occupational Safety and Health Administration (OSHA) requirements.

  • Process data/performance analysis or process information management system (PIMS)
    Such systems that involve process data collection and management can be a standard package developed for specific applications, such as time/cost variance information or manufacturing process records.

  • Document management (DM) or product data management (PDM) system
    These systems can handle unstructured data, which can be used to create product drawings and process information, and supply data for plant-floor use. For more information, see Mainstream Enterprise Vendors Begin to Grasp Content Management.

User Recommendations

Ultimately, all major plant automation systems vendors have devised some form of software architecture to support not only open integration, but a common approach to plant data and common services like alarming and trending. Additionally, traditional MES technology providers have been extending their systems to include improved support for a wider variety of functions, including quality assurance (QA), product lifecycle management (PLM), and supply chain collaboration.

In any case, prospective manufacturing system users should not be too concerned about buzzwords and acronyms, but rather define their requirements around the key manufacturing processes, such as “get-ready to” or “plan and prepare” production, production execution, information processing, information analysis, and action coordination. Enterprises should, to that end, thoroughly analyze their typical manufacturing scenarios, identify their information collection requirements, both on the production side and on the planning side, and evaluate whether these are things their existing enterprise system can handle.

Depending on the enterprise’s style of manufacturing, deploying an MES or other plant-level systems might not be particularly beneficial. Namely, a predominantly manual-job shop with low product volume might manage with an ERP system that can deliver work instructions and provide a manual or "paper-on-glass" interface for shop floor personnel to enter job completion and quality status information.

But, traditional ERP systems cannot effectively handle high-speed, high-volume, and high-mix environments where data must be collected and coordinated with plant automation equipment. Enterprises that fall into this category, or enterprises that are in regulated industries where there is a greater need for tracking and tracing capabilities, or plants struggling with excessive scrap levels, poor machine/personnel utilization, or quality issues, might be good MES candidates, since an MES will give users better visibility into plant operations.

While MES systems typically involve more customization than their ERP counterparts, MES vendors should offer templates containing requirements of the prospect’s industry, so that no one has to start from scratch to determine what type of functionality falls within the vendor’s definition of MES. This should keep cost and risks down. Also, for enterprises that will likely grow and expand into mixed manufacturing environments, scalability remains an important issue, because it will help “future-proof” their MES investment. Such enterprises should select a product that covers a variety of manufacturing operations, from batch processing via repetitive discrete manufacturing to assembly-to-order (ATO).

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