Requirements
A higher yield of Grade A cheddar and Monterey Jack has become the rule at Golden Cheese, the world's largest integrated cheese maker, thanks to the new process control system for the dairy and whey plant.
Q: What could possibly improve the largest, most sophisticated fully integrated cheese and whey processing facility in the world?
A: A new process control system to boost Grade A production to 97 percent or higher, minimize spills, and increase throughput by 10 million pounds per year. In effect, over 2,500 machines, which were operating independently, are now fully integrated into a single continuous system.
According to George Bennett, Production Manager of Golden Cheese, "We saw an opportunity to automate the art of cheese making with SIMATIC PCS_ (process control system) and APT (advanced process technology software) to a level of precision unheard of in this industry. Several years of planning, and a relatively modest investment of $1.5 million is allowing us to ramp up to a production increase of 'best-of-fair cheese' every day." In addition to the obvious benefits of increased production of Grade A cheese, the flexibility to continuously fine tune the process appealed to Bennett. Secondary advantages include reduced maintenance and repair, longer life of equipment, and less waste treatment through recovery of rinses.
Solution
Automating the Art
As a mechanical engineer, Bennett first encountered the "art of cheese making" only a few years ago. In his words, "Everybody said cheese making is an art. So, I asked, why not automate that art?" He stresses that the key factors of yield, quality and flexibility only result from the tighter control and trending tools made possible by the new integrated system.
Bennett suggests that variation is a culprit in cheese making, and it's hard to pinpoint. "If quality is an issue, I can change that. If the process is going up and down, forget it."
The distributed process control in place prior to the new PCS system was powerful in itself, but not up to the tasks envisioned by Golden's management.
The thousand inputs and outputs hooked up throughout the cheese plant were frozen in place, based upon the programming required to make changes and existing hard wiring. "Our biggest obstacle was being locked into a set way of operating. Flexibility just wasn't there."
Now, thanks to APT software, even the operator can make adjustments on the fly. A wealth of easy-to-read graphical trend and information is available via an Ethernet and PROFIBUS communications system.
Where It All Begins
The cheese making process begins by adding color, or a "rennet" starter bacteria (a dry powdered solution mixed in-house) into the vats. Bacteria cultures are produced to develop specific strains of starter-the secret recipe.
After transfer of milk from the outside storage silos into the plant, 11 vats are filled at 15-minute intervals, followed by the addition of salt. (Golden successfully operates the vats today with an average of only two people.)
At all critical stages, including vats, curd handling, whey handling and starter handling, the PLC driver is back-up. In each of these areas the PLC is considered a true confidence builder, continuously checking key measures of the countless events which occur. Valves and other mechanical devices are looked to for feedback. Confirmations from each device are essential.
"If a particular valve doesn't open, we know we have a fault, because the PCS displays it," says Bennett. By design, the PLC always has a path to follow. If it doesn't know where to go, it's because we didn't build the logic in. With this system, we'll keep adding dimensions as our learning curve goes up."
With ladder logic, the level of precision on place could not have been achieved. "Without the APT software, troubleshooting would have been a nightmare. Only the mastermind who wrote it could troubleshoot the system since few people understand hexadecimal programming," says manager Dermot O'Brien.
"The system is so fast that we had to build in 10 seconds here and there to monitor and confirm feedbacks. The redundancy in the plant was poor-now we have it with the hardware," adds O'Brien.
Connectivity
As part of the overall PCS concept, PLCs are connected through a 10 Mbaud standard Ethernet bus conforming to IEEE 802.3, which forms the lifeline throughout the plant. The bus ensures manufacturers' neutrality, when it comes to interfacing other devices, providing true open architecture. A standardized PROFIBUS field bus is used to connect peripheral devices to the PLCs.
The process is broken down into a series of islands or cells assigned to PLCs, which handle both fast binary processing (interlocks, sequential controls) and loop controls, such as level, temperature and more, with one common configuring technique.
Serial buses transfer data to and from the actual process or field. Intelligent, remote I/O are distributed throughout the process, providing significant savings for cabling, also allowing fast start-up and troubleshooting. The individual PLCs, or process cells, are then networked through the Ethernet bus so that all plant areas can be accessed using a single window approach.