This article was printed in CONTROL's May 2009 edition.
By Dan Hebert, Senior Technical Editor
In many hybrid industries such as food processing, PLCs have ruled the roost for years. However, there are some hybrid applications where PLCs don’t provide enough processing power, data handling and communications. In those instances, PC-based control can be the best alternative.
For example, McCain Foods in New Brunswick, Canada, is the world’s largest producer of french fries and other oven-ready frozen food products. Its 20,000 employees and 55 production facilities in 12 countries span six continents. McCain processes 1 million pounds of potatoes each hour and sells one-third of the world’s frozen french fry products.
McCain prides itself on being ahead of the curve in product creation and delivery. It was the first company to create a french fry that would stay as fresh as when it was made—even after being frozen and shipped worldwide.
This forward-looking philosophy has not only helped McCain create frozen foods, but also has enabled the company to embrace technological innovations in its manufacturing facilities. One such innovation was first implemented at its Easton, Maine, plant and is now serving as the model for all the company’s frozen food processing plants.
Reducing Refrigeration Costs
Precise temperature control is critical in food processing. In fact, refrigeration costs in a typical food processing plant can be up to 70% of total energy consumption. McCain had designed and implemented an automated control system in its Easton plant,but the company was looking for even better ways to manage energy distribution, cut usage and reduce greenhouse gas emissions.
So, McCain turned to systems integrator TechCold International (TCI), also based in New Brunswick, to assist in updating its refrigeration control systems. As early as 2002, TCI and McCain agreed the Easton plant would be a good facility at which to develop and customize a new kind of controller. This special controller would connect to the existing infrastructure and target specific areas for energy management. The new controller also would work alongside McCain’s existing PLCs, but provide the algorithms to make the cooling process more efficient.
This is one of the many industrial refrigeration compressors used in McCain Food's processing plant. TCI’s PC-based controls help optimize operation and reduce energy consumption.
Using a second controller along with a PLC would also provide a level of redundancy. If the controller was not accessible for any reason, the control system could simply revert to PLC-based control with no disruption to plant operations.
The result of those discussions was a two-year partnership that provided the impetus for the design and development of a prototype solution that became TCI’s enhanced refrigeration control system. McCain has since standardized many of its plants on the TCI prototype, which was greatly modified in the ensuing years. The TCI system is now deployed in 20 McCain facilities in Canada, the United States, England and some European Union countries. The company is also looking to expand the TCI system into its plants in South Africa, Australia and New Zealand.
PC-Based Control is the Answer
The partnership began with a kick-off meeting between TCI and personnel from the Easton plant. “We were met with some skepticism,” says Ernie Adsett, TCI’s president. “McCain already had a sophisticated control system based on an Allen-Bradley ControlLogix PLC, which they considered to be top of the food chain. They weren’t sure we could significantly cut their energy consumption and deliver an acceptable return on investment by adding another layer of control.”
TCI’s idea was to take the McCain facility from simply using a PLC-based system to one with a more intelligent, finer level of PC-based control. McCain was interested in using the new control system not only to reduce energy use, but also to operate its equipment more efficiently.
TCI had two goals for the PC-based enhanced refrigeration control system. The first was to reduce energy consumption and demand by usingadvanced control algorithms. The second was to provide a standardized system common to all McCain plants.
To accomplish the first goal, TCI found that PC-based control was the best option. TCI’s advanced refrigeration control scheme is written in a language that’s easily executed by a PC. Executing the advanced control algorithm in a PLC may have been possible, but it would have been unwieldy and impractical.
“We’re using a high-speed PC-based industrial processor based on a Microsoft Windows environment. The PC allows us to create a sophisticated model of the complete refrigeration system in real time to determine capacity and efficiencies,” notes Adsett.
“We can optimize five to 10 compressors working on a certain line, run all different scenarios and come up with a set of compressors that is the most efficient. We can then switch to these compressors by using all the equations to generate efficiencies, something a PLC can’t easily do,” adds Adsett.
The second goal, standardization across many different types of food processing facilities, also required a PC. Standardization reduces servicing and implementation costs, and it also provides easier ongoing remote monitoring and upgrades.
If the new refrigeration control scheme were implemented in a PLC, then standardization across different plants would be nearly impossible. That’s because different plants standardized on various PLCs, and each plant would want the refrigeration control scheme to reside in its existing PLC.
If the scheme were executed in one PLC, then that PLC would have to communicate with each plant’s PLC infrastructure—a nearly impossible task given that most PLCs don’t support multiple communication options.
On the other hand, a PC is ideally suited to communicating with virtually any PLC and with just about any type of controller. In this case, those communications are primarily OPC-based. The PC-based controller also facilitates remote monitoring and upgrades as communication via the Internet is standard for a PC, but often difficult with a PLC.
Communications Are Key
TCI found that most process plants have already standardized on a particular brand of centralized controllers, so its refrigeration control system had to interface seamlessly with almost any controls manufacturer’s products.
“Device communications in industrial plants can be daunting because there are so many device manufacturers and communication protocols. Though we can develop communication drivers, we knew this would be a distraction from our core business of developing control algorithms for reducing energy use in refrigeration systems,” explains Adsett.
TCI realized early on that the best way to deal with the communication problem was to implement OPC client capabilities in its PC-based controller. The PC could then connect to third-party OPC servers for interface with customer control systems.
“We chose to standardize on Kepware's OPC server. The KEPServerEX application is a single OPC server with driver plug-ins for hundreds of PLCs and devices, allowing the TCI system to connect to the widest variety of control systems,” notes Adsett.
Figure 2 illustrates how a typical system allows parallel operations flow between an existing control system, typically a PLC, and TCI’s PC-based refrigeration control system. The existing PLC-based controller performs most real-time control activities, while the TCI system executes higher-end modulations and activations of the refrigeration equipment.
The TCI Refrigeration Processor is a industrial PC-based controller capable of executing a complex refrigeration control scheme. Via OPC, the controller can communicate with virtually any brand of PLC.
Communications with existing PLCs are via OPC over a variety of protocols. To communicate with Rockwell Automation’s Allen-Bradley ControlLogix PLCs, the protocol is EtherNet/IP. AutomationDirect’s DirectLogic PLCs communicate with the PC via their own proprietary ECOM protocol. TCI has also used Modbus TCP and Modbus RTU to communicate from the PC to other control components.
In addition to communications with PLCs and other control components, an Ethernet port allows access to remote clients within or outside the plant. Via this port, the PC permits system operations to be viewed by an Internet browser. Again, this level of communication capability is easily implemented with a PC, but virtually impossible with a PLC.
ROI is the Measure of Success
The initial annual cost savings for McCain with the new system was 22% at one cold storage plant. The breakdown for energy use and savings with the initial prototype are seen in Figure 3. Different savings are realized in different plants, and the system depicted is for a cold storage plant.
Dramatic savings in energy use result from the advanced refrigeration control scheme executed in the PC. Running the scheme in a PLC would not be practical.
“As part of the return on investment (ROI) calculations, each plant is analyzed to determine potential savings based on present operation, geographical location and other factors. Typically, the TCI system can realize a payback period of one to two years,” explains Adsett. “The prototype was our basic strategy, and we’ve improved with software upgrades. The solution grows as our clients’ needs change.”
TCI manages upgrades with the PC via the Internet. With remote access, its employees can view the same controls that clients have at their sites and manage accordingly.
“We strongly believe in being a good partner, so we don’t install systems and walk away,” says Adsett. “We do spot checks on all of our clients’ plants and develop logs of how they’re doing. Subsequent and ongoing fine-tuning allows maximum system efficiency, and remote access allows us to deliver software upgrades in a cost-effective manner.”