Upstairs, Downstairs

Data Going Up, Data Going Down? Engineers and IT Shake Hands in the Middle as OPC Gives Way to XML, Web Services

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ERP may “own” cost accounting, and control systems may “own” loop parameters, but there’s lots of functional overlap, in particular, at ISA5 Level 3. So while DCS and ERP vendors offer interfaces between their systems, usually the connection is through the DCS vendor’s historian, which reaches up to Level 3, which serves as a demilitarized zone to negotiate the architectural conflicts between real-time control processors and transactional business systems. It provides that middle zone where key enabling technologies meet.

For Zywicki’s ERP-to-DCS integration project at Dow Corning, RLINK, part of OSI’s Pi System, provides the data map from above and below.

Ask a Historian

Historians and application databases have been relatively easy to access from above or below using SQL Server-type databases. Since the early 1990s, Open Database Connectivity or ODBC has provide a standard way to map data paths between business and plant systems’ relational databases.

In the mid-1990s, OLE for Process Control (OPC) advanced this mapping. The most familiar OPC protocol, DA (Data Access) provided a format to map data in real time and made custom drivers, such as the drivers that were once the basis for many a HMI/SCADA software vendor, obsolete. Another OPC protocol, AE (Alarm and Events), offers transactional, on-demand transfers; a host of others protocols specifically address historical data, batch data, server networks and a platform-neutral XML documents. The latter illustrates the convergence of IT and Web standards in the plant, which is no longer the exclusive domain of engineers.

The most ambitious OPC standard further reflects this. The first draft of OPC UA (Unified Architecture) was released in February 2008, with a final approved specification planned by year’s end. “UA” combines all OPC protocols into one, while shedding its Microsoft underpinnings for one based on a service-oriented architecture (SOA) that follows the World Wide Web Consortium’s Internet standards. But UA will be years in coming.

“The link between the DCS and the plant data system has become easier with the maturing of OPC,” says Pat Kennedy, CEO of OSIsoft. “The real issue is configuration and maintenance, not installation. Also the systems are getting so big that some sort of meta data is useful to understand the information, and this has to be configured too.”

Integrator Timothy Alosi, vice president of engineering services with New England Controls, Mansfield, Mass., knows it. To connect two computers with OPC servers via an OPC mirror in an Emerson Delta V DCS, he first “sat down at one computer and pinged the other one by IP address. Then I pinged it by name; then I created a shared directory on each side, and logged-in with the user I thought we’d use for this computer. Then I checked to see if I could browse for the user and if I could copy files back and forth. Also in my Delta V, I had to set user priveliges for the right process areas... and I hadn’t even gotten to OPC yet!”

The two computers were sitting next to each other with nothing but a cable between them. Alosi says connections become exponentially more complicated by network swtiches, and “once you want to connect to another site, it’s essentially impossible. I’ve seen engineers pull their hair out and do a lot of yelling and cursing for two weeks to make an OPC connection work.”

Markup of Maturity

In the control arena, all DCS vendors either offer a historian or tie to others, and they are typically installed with a firewall above. This presents problems for OPC, but not for SOA-generation tools such as Web services and XML that, by design, can traverse firewalls with the ease of Web browsing. And both have their purpose for plants trying to bring data above and below the firewall.

“OPC is well known and understood by control engineers, and Web services are well known and understood by the IT guys,” says Yokogawa’s Emerson, whose latest DCS architecture offers Web services. A few, or possibly a single engineer at a site will want to learn enough XML to convert ERP-level data into operational or control systems, so “the guys doing the control applications and function-block work won’t have to worry about it,” Emerson says.

This will make life easier in the real world for empathetic IT leaders who seek to keep engineers “as far from becoming IT specialists as possible, so they can focus on engineering their products and processes,” says Zywicki, who is co-leader of Dow Corning’s global SOA initiative.

The WBF organization’s XML committee, which Dave Emerson chairs, is in its fourth release of Business To Manufacturing Markup Language (B2MML), a set of XML schemas based on ISA95 data models. Co-authored by Brandl, B2MML provides a royalty-free, common data roadmap. At present, the data mapping is still a manual affair, but broad adoption could lead to automation analagous to OPC’s elimination of custom software drivers more than a decade ago.

At minimum today, SOA, Web services and XML serve as the state of the integration art and a utilitarian software linga franca.

Bob Sperber is a contributing editor to Control.

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