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.”