Get closer to history with OPC

Offline analysis of historical process data is an integral part of process optimisation and production tracking. OPC helps maximise the return on process history investments by providing a standardised mechanism for collection of real-time process data and by enabling scalable multivendor and distributed historical analysis solutions.

To optimise 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, and 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 optimised for storing and accessing large amounts of time-based data. Unlike a relational database, which is optimised for finding relationships between stored non-temporal data, a process historian is optimised for retrieving and storing series process data. Organisations that are serious about process optimisation have a dedicated process historian as the foundation of an optimisation strategy.

Enterprise versus desktop historians

Process historians come in different shapes and sizes. For large organisations there are enterprise-class process historians. The enterprise process historian provides a robust solution for long-term storage of process data. These applications are optimised 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, and 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 localised data history 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, and the focus of a desktop historian is on high-performance real-time data acquisition and storage. A good example of a desktop process historian is Matrikon’s OPC Desktop Historian.

OPC enables history collection

OPC is an open standard that permits a consistent method of accessing field data from plant floor devices and, as such, 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 so that the method of application communication using OPC standards remains the same regardless of the type and source of data. End users are, therefore, 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, which provides a standard API that is optimised for inter-application transfer of real-time data. It 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, multivendor 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.

Figure 1: OPC DA and OPC HDA history solution.

OPC HDA standardises 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 visualisation, interpretation and report generation can access data from any OPC HDA-enabled process historian. Using OPC HDA, organisations that are interested in reaping the benefits of process optimisation through offline data analysis are free to choose the best-of-breed historian and the best-of-breed analysis packages.

OPC HDA is a mature specification that has widespread use today, and every major brand of process historian has OPC HDA connectivity available. If the vendor of the historian does not provide OPC HDA connectivity, third-party vendors like Matrikon provide standardised access to the process historian. Many applications exist that leverage the OPC HDA specification, and OPC HDA-enabled applications that perform process optimisation based upon historical data calculations can easily integrate with any existing OPC HDA-enabled process historian.

Several vendors provide OPC HDA-enabled trending packages as well. OPC HDA is indeed a standard that has wide adoption. Although most organisations only have a single enterprise process historian, many, either through asset acquisition or multisite integration, are faced with multiple enterprise process historians. Before OPC HDA, an organisation faced with multiple vendor historians was forced to make a choice between two vendors’ historians. Integration costs, the costs associated with data-migration services, and the costs of training personnel who were using the about-to-be-displaced historian combined to make this a very expensive choice. OPC HDA enables multivendor historians to provide a distributed architecture that is not only cost effective to implement, but leverages existing multivendor history assets. OPC HDA-enabled products exist that enable data stored in a historian developed by vendor X to be transferred or shared with a historian from vendor Y. MatrikonOPC HistoryLink is an example. OPC HDA has effectively turned a problem of the past into an advantage for the future.

OPC DA and HDA: a powerful combination

One can easily see how OPC DA has provided a process historian with a standardised mechanism for real-time data collection. OPC HDA can be leveraged to provide a standardised interface to the stored process history data.

The combination is very useful for solving multivendor, best-of-breed architectures. OPC DA and OPC HDA can also be combined to solve the store-and-forward problem. Store-and-forward is a concept that is used by process historians to enable remote data collection for a process historian. During a connectivity break between the collector and the process historian, data is buffered at the remote location and forwarded to the process historian when the collector-historian link is restored.

The difficulty with the proprietary collector problem is the lack of access to the buffered history data. Although data is constantly buffered at the remote location, the collectors provided by the historian vendors do not typically provide historian tool access to this buffer.

Using an OPC HDA desktop-class historian, data can not only be collected remotely using the OPC DA specification, the data can be transferred to the enterprise process historian using OPC HDA. This can either be done on a scheduled basis or only in the event of communication failure between the remote site and the enterprise process historian. Both of these solutions provide full time access to the historical data at the remote site, even if communication to the enterprise historian is not possible. Solutions like those mentioned leverage a powerful combination of OPC DA and OPC HDA connectivity.

Figure 2: Distributed OPC history architecture.

Usage examples

The following are some typical situations that would require a historical data transfer tool.

• Automated transfer of data for SCADA environments where data is collected remotely and needs to be transferred to the enterprise level for analysis or long-term storage - OPC HDA can be used to schedule regular and automatic transfers of historical data generated by RTUs, analysers or other devices to a centralised process historian or CSV file.
• Multisite data migration and synchronisation - OPC HDA facilitates the transfer and synchronisation of historical data from archived sources of data to one or more databases or CSV files.
• Trip capture - OPC HDA technology can be used to collect sub-second data surrounding a trip event (or any significant event) and deliver the data to the operation’s existing process historian; analysis of the sub-second process data can be done using the analysis tools already deployed.
• Batch capture so that only relevant the data is archived - Instead of collecting data that is not relevant to specific phases of a process, OPC HDA can be used to deliver data defined by a specific phase of a process to the process historian, thus eliminating the need to sort through irrelevant data.
• OPC store-and-forward if communication is lost - To ensure all remote real-time data is delivered reliably to the process historian, OPC HDA Recognize and Retrieve can be used.

Conclusion

Offline process history analysis using a process historian and a set of analysis tools has, indeed, proven to reduce costs and provide vital production tracking. OPC has provided a standardised solution to real-time data collection. Real-time data sources are accessed using the OPC standard interface. Historian vendors no longer need to focus on data collection or data visualisation and presentation due to the widely adopted OPC HDA specification. Using OPC, organisations are free to choose not only what vendor supplies the data analysis and presentation tools, but also to choose the vendor or vendors that supply the process historians for the organisation.

By Sean Leonard, BSc Eng CompE, Msc (CompE), MBA (Technology Commercialization), Product Manager for MatrikonOPC. Sean also works with the OPC Foundation’s technical working groups to help set the future direction of OPC standards.