OLE for Process Control (OPC)

Information Architecture and Communication

The information architecture for the Process Industry involves the following levels:

• Field Management - provides data on the field devices, device configuration parameters, materials of construction, etc.
• Process Management - installs Distributed Control Systems (DCS) and SCADA systems to monitor and control manufacturing processes.
• Business Management - integrates the collected information into the business systems.

To do these things effectively, manufacturers need to access data from the plant floor and integrate it into their existing business systems. Manufacturers must be able to utilize off the shelf tools (SCADA Packages, Databases, spreadsheets, etc.) to assemble a system to meet their needs. The key is an open and effective communication architecture concentrating on data access, and not the types of data. The most common ways to implement communications are OPC, ActiveX, DLLs, and now .NET objects.

The OPC

The OPC (OLE for Process Control) Specification is a non-proprietary technical specification that defines a set of standard interfaces based upon Microsoft's OLE (now Active X), COM (component object model) and DCOM (distributed component object model) technologies. The application of the OPC standard interface makes possible interoperability between automation/control applications, field systems/devices and business/office applications. COM is a software architecture that allows one program to use the interface of another program to get information from it (as long as it is programmed as a COM component). DCOM is the network-aware version of COM technology.

Traditionally, each software or application developer was required to write a custom interface, or server/driver, to exchange data with hardware field devices. OPC eliminates this requirement by defining a common, high performance interface that permits this work to be done once, and then easily reused by HMI, SCADA, Control and custom applications.

OPC is created with the collaboration of a number a leading worldwide automation and hardware software suppliers working in cooperation with Microsoft. The organization that manages this standard is the OPC Foundation (www.opcfoundation.org).

Benefit of OPC

OLE for Process Control (OPC™) is designed to allow client applications access to plant floor data in a consistent manner. With the OPC standard, every piece of hardware in the control system is just like a plug-and-play component - it has one and only one OPC compliant interface. This benefits both to the hardware and software vendors, and the end users.

• Hardware manufacturers only have to make one set of software components for customers to utilize in their applications.
• Software developers will not have to rewrite drivers because of feature changes or additions in a new hardware release.
• Customers will have more choices with which to develop World Class integrated manufacturing systems.

With OPC, system integration in a heterogeneous computing environment will become simple. Leveraging OLE/COM the environment shown in Figure 0-3 becomes possible.

However ...

The OPC is facing a strong competition from other communication objects, though the theories behind OPC are great ideas. Some people believe that ActiveX controls are the easiest to use but DLLs offer higher performance than ActiveX.

OPC is founded on the Microsoft COM model (which is a little bit too old today). VB (or C#) users expect that the supplier provides an OPC client (like an ActiveX control or DLL) specifically designed for VB (or C#). However, nobody wants to provide such an OPC client. Even if some suppliers provides such an OPC client, the cost is very high, since the user needs to spend thousands of dollars for each OPC server and thousands of dollars for each client. OPC servers are typically licensed for one computer, while ActiveX and DLLs are typically licensed for each developer and can be used on many computers.

Even though there is still good reasons to believe that OPC have a good opportunity to take the lead in automation communications. OPC foundation is trying hard to solve the problems, hopefully within a few years. Microsoft .Net also provides a perfect opportunity for OPC.

For more information, visit the site of The OPC Foundation.