OPC maximizes object-oriented systems

THE OPC COMPLEX Data specification, released in December 2003, describes a new way to pass data from one OPC DA server to another, making it easier for vendors, OEMs, machine builders and end users to connect new, smarter devices together. The current DA specification requires data items to be simple types or arrays of simple types, so OPC DA servers represent data as a sequence of bytes. Under the current specs, there is no way describe the structure of these bytes. Clients receiving structured  data are unable to interpret it unless the OPC server provides the data as items or arrays of simple data types.

As OPC field devices get more complex, they are creating much more information, and it is becoming much more complicated and harder to deal with them. Some PLCs, for example, are now using structured variables. Field devices are getting much smarter, and have more data to transmit than just process variables. The Complex Data specification will solve the problem.

“OPC Complex Data is all about providing a way to pass data that doesn’t fit into the 'word, bit, integer, float’ tag format of OPC DA,” explains John Weber, president of Software Toolbox. “For example, a variable named 'My Structure’ might contain five floating-point variables, two Boolean values, and three integers. A common example is a PID loop in a PLC. With classic OPC DA, the only way to read that data is to make one tag for each element of the structure.”

Weber says that works when you only need a few variables from the structure. If you need the entire structure, and your system has many such structures, this is not the most efficient way for an OPC client and server to pass data. “The OPC Complex Data initiative is all about providing a more efficient means for passing large numbers of Complex Data structures between an OPC client and server,” he says. 

TIGHTLY INTEGRATED CONTROL LOOPS

ADVERTISEMENT Across sophisticated processes, the ability to seamlessly commujnicate among devices and integrate disparate business and control systems is a growing priority. With its ability to address any object-oriented device, OPC Comples Data will help companies make it happen.

Steve Redcay, product specialist at Phoenix Contact's Automation Systems Group. Middletown, Pa., provides an example, using Complex Data with variable-frequency drives (VFDs). “A structure is a collection of different-type variables under a single name,” he explains. “Some simple parameters of a VFD could be 'RampUpTime,’ 'RampDownTime,’ 'MaxSpeed’ and 'MinSpeed.’” As John Weber said, current OPC data would require an independent tag for each parameter. This means there would be four tags for each VFD in the system. As the number of VFDs increases, the program becomes less efficient and there is more likelihood of programming mistakes."

“One structure can contain all the parameters for the VFD, so to 'parameterize' the VFD we need only call one variable. Also each parameter can be addressed separately using an identifier, such as 'VFD_1.RampUpTime.’ This is a more efficient way of passing large or complex groups of data.”

Phoenix Contact is already using the Complex Data in its products. “We recently added Complex Data to our Ethernet infrastructure,” says David Skelton, Director of Automation at Phoenix Contact. “For example, all of the conditions and set up of a managed Ethernet switch are shared and can be maintained through traditional control and HMI platforms. This alternative allows the maintenance department to continue to provide first response while the IT department provides second-level support. The support of Complex Data just makes it easier to implement this type of system with fewer errors.”

Not Just For the New and Smart
Once again, as in the case of OPC XML it’s not just new, smart devices that will benefit from this new spec. Scott Saunders, director of strategic marketing at Moore Industries, says Complex Data can be used on all OPC-based equipment. “This will allow applications to not only read process variable data from sensors, but also complex binary arrays and patterns that can be used for preventive maintenance,” he predicts. “Since most of the 'smarts’ reside at the sensor level, the data can be analyzed locally and sent up the IT food chain where it can be viewed in a useful format, instead of waiting for the data mining programs to interpret the confounding patterns and anomalies.”

Complex Data can be used for many predictive maintenance and CMMS applications, such as oil analysis, vibration, power spectrums, or thermal images generated by infrared thermography. Today, OPC DA clients might be able to read vibration waveform data from an OPC server, but probably have difficulty interpreting and using the data. Complex Data will allow OPC client applications to read and decode any type of data being generated by OPC-based measurement and control systems on the plant floor.

Randy Kondor, OPC Product Manager at Matrikon, Edmonton, Alberta, Canada, says that Complex Data will make it much easier to communicate with DCSs. “While OPC DA is a good match for the simple data structure of most PLCs, Complex Data is an excellent match for the 'Object’ or 'Tag’ concept of a DCS,” he explains. “Control loops can be treated as a single entity, and equipment will follow suit. This will enable much tighter integration between the DCS and data-consuming applications such as HMIs, Process Historians and advanced process control applications.”