Combined PC/PLC systems: the best of both worlds

Dairy Foods, June, 1998 by Jack Mans

PLCs have been used for task-specific plant control functions that could operate independently from other plant activities, for example, machine control. PCs have been used in office and scientific applications.

The historical drawbacks of the PLC have been its limited process flexibility and non-standard communication protocols. PC shortcomings included high cost and lack of software.

Today, however, the two factions are merging. The PLC is being viewed as the "distributed" component of a distributed control system while the PC platform manages communications, processes data and maintains the process/plant database. Over the past several years, process control software for PCs has reached maturity and has tended to standardize exclusively on X86 and Pentium processors.

The growth of this hybrid control philosophy of PCs linked to PLCs will continue due to the freedom to share software applications between platforms. This is similar to the expansion of the PC industry when microprocessor architectures were standardized.

Benefits to the dairy processor from these hybrid systems include:

* Better control of the manufacturing process because all aspects of the process are subject to real-time computer control.

* Improved decision-making capability because operators are better able to see plantwide implications of process changes.

* Lower cost because plant managers have access to standardized, compatible hardware/software available from a variety of vendors instead of being limited to vendor-specific control systems.

* Easier maintenance because control system modules will simply be swapped out and serviced by the vendor or third-party maintenance companies.

* Straightforward upgrading because improvements in PLC/PC systems and software can be rapidly deployed in the plant control system.

SOURCE: R. J. PHILIPS & ASSOC., GREAT FALLS, VA. PHONE: 703-406-0072

RELATED ARTICLE: Hybrid System at Centennial Farms

A combined PLC/PC system installed last year at Kroger's Centennial Farms plant, Atlanta, illustrates the technology.

The old computer-based control system was equipped with stand-alone, microprocessor-based controllers on each of the four CIP units and on the standardizing system, all interfaced to the operator via hardwired switches. Kroger replaced the entire process and CIP control system with a new PLC/PC-based system. PCs or human-machine interfaces (HMIs) replaced all hardwired switches.

A major benefit is the system's easily expandable open architecture. The plant is planning to add more extensive supervisory control and data acquisition (SCADA) capabilities and a future project will add a local area network (LAN) and software to provide this information directly to the front office on a real-time basis for developing more comprehensive management reports, including a plant utilization report and identification of product losses by area.

"The hybrid PLC/PC system is working well for us at Centennial, and we plan to install similar systems at other plants," says Bob Kosman, Kroger vice president, dairy group manufacturing div.