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By Eric Murphy, ControlGlobal.com Columnist
There are many sources describing the differences between discrete and process type manufacturing. One of the simplest definitions defines discrete systems as dealing with ‘hard’ things like car parts, widgets and computers. Process systems handle ‘gooey’ things like gasoline, corn syrup and waste water. Since OPC originally stood for ‘OLE for Process Control’, people tend to think it only works for ‘gooey’ applications. However, OPC has moved beyond just process control, and is a reliable and versatile standard that deals with the ‘hard’ systems too.
Several factors other than product density define the unique characteristics between discrete and process manufacturers. Process manufacturing usually deals with continuous stream production of amorphous materials that you measure, as opposed to things you count. Process manufacturers organize their production facilities into lines or departments, and typically make significantly high capital investments in specialized equipment. These systems are relatively fixed, and manufacture high volumes of product on a scheduled basis. In contrast, Discrete manufacturing produces individual, countable products typically created from other individual inputs. Discrete manufacturing tends to be organized in smaller, more flexible and specialized cells of production. The more dynamic nature of batch type or work-order driven business means data collection requirements are geared towards reliable counting, inventory control and asset utilization.
Fortunately, OPC provides standardized data access that meets the characteristics of both process and discrete manufacturing. OPC efficiently collects large amounts of data, derived from multiple sources and delivers it simultaneously to many destinations. OPC effectively handles data collection whether represented by many static items changing rapidly on an exception basis, or multiple sets of items that update on a batch or transition phase manner. In addition OPC offers separate specifications to address different data semantics, including real-time data, historical data, alarm and event information and batch data.
Another distinguishing characteristic between process and discrete manufacturing is the type of control equipment typically employed. Process control systems are often controlled by a distributed control system (DCS), where programmable logic controllers (PLCs) are more common to discrete manufacturing. DCS systems can be thought of multiple, interconnected sub-systems that control a larger, complex process system. PLCs are designed for more modular and localized automation control. Although this distinction is beginning to disappear due to advancements in computer hardware, communication media and networking, for many installations this general separation still exists.
The invention of the PLC was driven by the needs of the American automotive industry, therefore it’s not surprising that auto manufacturing and other discrete type processes are still large PLC users. A handful of vendors hold the majority of the PLC market share, with Siemens, Rockwell Automation and Schneider Electric as some of the more prevalent brands. Other well known names in the industry include ABB, General Electric, Honeywell, Koyo, Mitsubishi and Omron.
The lifetime of most PLC brands span many years of development, device types and network communication options. A typical discrete manufacturer installation may have a mixture of different PLC brands or several product lines from a single vendor. As product lines adopt new standards, it’s also common to have different communication options such as serial, Ethernet or proprietary connections. Discrete manufactures need standardized, reliable access to these devices, and other control equipment. OPC provides this standardization. There are OPC Servers available for all major PLC brands, including Siemens, Rockwell Allen-Bradley, Mitsubishi and Schneider Electric.
Discrete manufacturer’s success depends on responding to a wide variety of dynamic product demands in order to achieve on-time delivery and maintain competitive pricing. Therefore discrete companies need flexible productions facilities that allow manufacturers to deal with the wide range of customer demands. Well designed OPC servers will also support features that save configuration time, recognize dynamic changes in the system and provide high reliability. OPC Servers suited to meet the needs of discrete manufacturing would support additional options such as:
Whether manufacturers produce hard things that ‘thunk’ or gooey stuff that splats, they require software that offers fast, flexible and reliable access to data. Companies are continually facing environments that are evolving in diversity and complexity. Standard, industrial strength OPC products ensure that regardless of their business, production style or control equipment, their choice for data access is not a hard one to make.