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By Sean Leonard, MBA Eng.
OFF-LINE ANALYSIS of historical process data is an integral part of process optimization and production tracking. OPC helps maximize return on process history investments. OPC not only provides a standardized mechanism for collection of real time process data, it also enables multi-vendor historical analysis solutions. OPC facilitates highly scalable multi-vendor distributed historian architectures. OPC Data Access (DA) and OPC Historical Data Access (HDA) are two specifications that can be combined to solve many of the process history problems.
Process Historian: The Heart of Process Optimization
Off-line analysis of historical process data is an integral part of process optimization and production tracking. To optimize processes, one needs to understand how the process is doing over time, not just what the value of the process variables are at a specific instant in time. Trending, process tuning, reporting, and environmental auditing all require access to historical process data. Industries use an application known as a process historian to store and retrieve historical process data. A process historian is a database-like application that is optimized for storing and accessing large amounts of time based data. Unlike a relational database, which is optimized for finding relationships between stored non-temporal data, a process historian is optimized for retrieving and storing series process data. Organizations that are serious about process optimization have a dedicated process historian as the foundation of an optimization strategy.
Enterprise vs. Desktop Historians
Process historians come in different shapes and sizes. For large organizations there are enterprise class process historians. The enterprise process historian provides a robust solution for long-term storage of process data. These applications are optimized for multiple concurrent users accessing history, as well as for collecting hundreds of thousands of process variables. Typically the enterprise process historian stores interpreted (compressed) data. There are several common algorithms for deciding, in real time, what data is to be ignored during collection, and what data is to be stored. The typical enterprise process historian has a set of shared data analysis tools that are deployed via a web interface or installed on engineering workstations.
Another form of process historian is the desktop class process historian. A desktop process historian provides localized data historization at the operation level. For example, seasonal unit performance metrics may be useful closer to the plant floor than the enterprise historian deployment. A desktop historian typically provides targeted history analysis (both long term and short term) on high-resolution uncompressed data. The focus of a desktop historian is on high-performance real-time data acquisition and storage.
OPC Enables History Collection
OPC is an open standard that permits a consistent method of accessing field data from plant floor devices. OPC provides a set of specifications that are created by representatives from many vendors from around the world. This set of specifications enables inter-application communication for the purposes of process data transfer. The method of application communication using OPC standards remains the same regardless of the type and source of data. Therefore, end users are free to choose the software and hardware that meets their primary production needs, without having to consider the availability of proprietary drivers.
The most popular specification is the OPC Data Access (DA) specification. The OPC DA specification provides a standard API (Application Program Interface) that is optimized for inter-application transfer of real time data. The OPC DA specification enables real-time data produced by hardware or software from vendor X to be easily transferred to hardware or software from vendor Y, thus enabling a true best of breed solution. Using OPC DA, multi-vendor distributed architecture solutions are deployed with ease.
The OPC DA specification has simplified the historical data collection problem. Before OPC, historian vendors were required to build custom software for every type of data source. A historian is completely useless if it has no data to store, thus historian vendors spent a great deal of time building custom drivers to get data into their product. With the advent of OPC DA, historian vendors need only build an OPC DA connection, and instantly the historian can collect data from any of the hundreds of OPC DA enabled products. To an OPC DA enabled process historian, an OPC DA enabled product is just a generic source of data; the historian need not know any of the details regarding the proprietary data source. Historian vendors who provide OPC DA connectivity are now able to focus their development efforts on providing a better historian solution, instead of focusing on getting data into an existing technology.
OPC HDA Standardizes Historical Data Access
The OPC Historical Data Access (HDA) specification provides a standard method for access to historical data. Similar to OPC DA, OPC HDA provides a standard mechanism for inter-application transfer of data. The fundamental difference between OPC HDA and OPC DA is that OPC HDA enabled applications share process data that is historical in nature. OPC HDA enabled applications that provide visualization, interpretation, and report generation can access data from any OPC HDA enabled process historian. Using OPC HDA, organizations that are interested in reaping the benefits of process optimization through off-line data analysis are free to choose the best of breed historian and the best of breed analysis packages.
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