Excerpted from a knowledge paper from Process Automation Systems, Siemens Energy & Automation Inc.
In the early 1990s, manufacturing execution systems (MES) were touted as the panacea for shop floor management problems, promising to do away with “islands of automation” and to seamlessly connect the plant floor with enterprise systems. Unfortunately, the early MESs were too vertical in scope. There was an MES for silicon wafer assembly, one for chemicals, another for packaging, and so on, with dozens of companies specializing in finely honed products for vertical applications. This contributed little toward ending the islands of automation, and the seamless connections often required skilled coding to connect to enterprise resource planning (ERP) and process control systems.
In some respects, the concept of MES was ahead of its time. Networking systems and operating systems weren’t quite ready to handle the degree of communication necessary between plant floor and enterprise systems. A potential solution to the communication problem—object-oriented communication—was developing both the PC-based operating systems (OS) often used on the plant floor and the UNIX systems typically used for enterprise systems, but definitions seemed to change with each new release of an OS. Distributed Component Object Model (DCOM) in Windows and Common Object Requester Broker Architecture (CORBA) in UNIX never quite seemed finished and were in a constant state of flux. In addition, the inability of these object platforms to communicate with one another impeded progress.
Bridge the Gap
Today, the availability of Web-based Extensible Markup Language (XML) communications successfully bridges the gaps between these systems, allowing object-based communications. Built on XML, the business-to-manufacturing markup language (B2MML) standard specifies accepted definitions and data formats for information exchange between systems and facilitates information flow and updates between systems.
An MES brings together many functions that fall outside the realm of the plant floor control system. Some of these include plant maintenance, laboratory information management, standard operating procedures, document control, training documentation, raw materials management, and corporate information flow. The MES pulls information from the ERP system concerning orders, bills of materials, work instructions, standard operating procedures, assets, assembly steps, and so on. It also creates a practical manufacturing plan based on prevailing factory floor conditions.
Recent changes make the MES an ideal partner with both plant floor systems and the corporate ERP system. When coupled effectively with an ERP system, today’s MES tightly integrates plant floor events with business events, empowers plant personnel with real-time/near-real-time vision into relevant events and metrics, provides accurate manufacturing costs based on productivity, and shows how much value was added on the plant floor on a shift, daily, or weekly basis. Because past MES/ERP integration problems often occurred due to a lack of definition as to the boundaries of each system, it’s important to define the role of MES functionality in a specific application before deciding how an ERP system might complement it.
Redefining the MES
While the MES has always promised to improve manufacturing agility—making it easier to introduce more new products, create smaller and specialized batches, and shorten lead times—it also satisfied requirements for electronic record keeping and traceability. But in today’s global markets, other important considerations include redefining MES implementation. These include global markets and production facilities, product quality and safety, production efficiency, and product genealogy.
Also helping to redefine the role of the MES is the four-level Instrumentation, Systems, and Automation Society’s (ISA) SP-95 Enterprise-Control System Integration specification, an international standard agreed upon by a consortium of manufacturers, system suppliers, and opinion leaders. This model breaks down business-to-plant floor operations into four levels.
Levels 1 and 2 include batch, continuous, and discrete control. (See Figure 1, p.35.) Level 3 (MES layer) includes manufacturing, operations management, dispatching production, detailed production scheduling, reliability assurance, and so on. Level 4 (ERP) defines business planning and logistics, plant production scheduling, operational management, and related areas.
An ISA SP-95-Compliant MES
With an ERP system at the enterprise level (top) and the control layers at the plant floor (bottom), an ISA-compliant MES will need to communicate data in both directions to help managers decide how to produce, what can be produced, when and what to produce, and when and what was produced. (See Figure 2, right.) In the ISA model, a B2MML layer provides connectivity between the ERP system and the MES. At the bottom, OLE (Object Linking and Embedding) for Process Control (OPC), which began as part of Microsoft’s COM/DCOM object model, connects data from the control system to the MES. The B2MML layer provides protocols for product definition, production capability, production schedule, and production performance. Product definition protocols bring data from the MES Product Definition Management module to the ERP.