Using generic database access technology to expand OPC
By Randy Kondor
At the foundation of most automation applications are databases. They are the windows to our innermost operations as well to our connections with the outside world when used as a gateway to a process optimization application, computer maintenance management system (CMMS), composition analyzer, or a custom, in-house application.
As our dependence on databases has increased, so too has our reliance on OLE for Process Control (OPC)—the open connectivity standard for industrial automation and enterprise systems. OPC standardizes the way that plant floor devices such as PLCs, DCSs, and analyzers share data with software applications such as HMIs, process historians, and trending applications. Regardless of the type of device, data is always shared in the same standardized format using OPC. As a result, it has become the standard used by applications to poll devices for real-time data.
The widespread industry acceptance of OPC has led many companies to pursue conversion of archived, relational data to real-time data—and vice versa—so that different databases can communicate more effectively, ultimately enabling better decision-making at all levels of the manufacturing organization.
A number of access methods are capable of handling data conversion, such as ODBC; however most of these methods are intended for use with relational data, not real-time transfers. Further complicating the data conversion issue is the fact that products from Sybase, Oracle, and Microsoft all have their own application programming interfaces (APIs) used to optimize operations.
OPC can handle the issues of data transfers between real-time and relational databases as well as multiple API use with the application of generic database access (GDA) technology. GDA helps OPC take advantage of real-time values from a relational database. For a frame of reference, consider that Microsoft ActiveX Data Objects are examples of GDA technology. ActiveX Data Objects are COM (component object model) objects used to gain access to a database through active server page (ASP) technology.
OPC generic database access
OPC GDA technology deals with the translation of data from databases to standardized OPC values and back again. GDA can connect an application to any database, regardless of the table structure—even a poorly configured database. GDA is built with an OPC front end, and can therefore be effectively used to provide a host of database connectivity solutions. For example, OPC applications such as human/machine interfaces (HMI), expert systems, and historians can use existing OPC interfaces to integrate a database as a real-time data source. In addition, since OPC GDA technology also enables a database to be used to store new information, GDA can be used to collect data from an OPC source and archive it in the database, as is done in CMMS.
Because OPC for GDA standardizes the data access method, every point is guaranteed to have three attributes: a value, quality, and timestamp, just like data from any other OPC source. It is irrelevant to OPC whether the application connects to a DCS, PLC, database, or other device, because the interface is always the same.
Randy Kondor is OPC product manager for Matrikon, a provider of software solutions for the process industries; www.matrikon.com . For more information about OPC, visit www.opcfoundation.org .