Second-tier systems replace legacy controls

Windows Woes Cured at Canner
The legacy control system at McCall Farms in Effingham, S.C., was based on “whitebox” PCs and an older Microsoft Windows 95 operating system that was no longer supported by Microsoft or the control system vendor. It tended to “crash” when operators entered production line changes, causing process data to be lost and production to stop. Legacy systems that crash usually get replaced.

McCall Farms produces a variety of canned fruits and vegetables under the Margaret Holmes brand, and distributes them throughout the southern U.S. in major national retail stores and major supermarket chains. Its canning process is continually moving with minimal allowances for equipment downtime and wasted product. Plus, according to a McCall Farms mandate, all products must be canned within ten hours of harvest.

The canning process starts with fresh fruits and vegetables bought from farmers across the southeastern U.S. Each type of produce has its own unloading area where it goes through a series of wash tanks for thorough cleaning before being sent to a blancher.

FIGURE 2: CANNED TOUCHSCREEN
Touchscreens allow operators all over the plant to request data, see trends, and change parameters.

Non-edible material and bad product are removed in the inspection area. The good product is sent to a filler, which puts the correct amount of food in each can. The cans are sealed and sent to a continuous feed rotary cooker where they undergo cooking and cooling according to FDA standards. In the final stage, cans are brought to the label line where they’re labeled, wrapped, palletized and readied for direct shipment to purchasers. This entire process is automated. Without automation, everything would have to be manually set with rheostats and toggle switches.

The assorted can and pouch types McCall Farms uses require up to four line changes a day. “An on-line change would sometimes cause computers to lock up, stopping the process and generally causing us to lose any food in production at that time,” Jason Durant, McCall Farms’ electrical engineer said. “Obviously, this was not a good situation and made us reluctant to make changes unless absolutely necessary.”

Fortunately, part of the solution was already on hand. Several years ago, as part of a project to fully automate its 500,000 square foot plant, McCall Farms installed 4,000 feet of Profibus I/O wiring, numerous touchscreen HMIs, and racks with 70 nodes of Beckhoff IP 20-rated I/O.

Unfortunately, the canning process was still being run by the clunky legacy system, and software had become a source of nagging problems. "We were programming with our previous software, but it was continuing to cause process errors that we could no longer tolerate," he said. "Complicating things even more, the program had been discontinued four years earlier and would only run on an outdated Windows operating system."

The company bouight Beckhoff TwinCAT control software and Beckhoff Industrial PCs to replace the legacy system. “The ‘buy’ part of the equation wasn’t very difficult,” says Durant. “TwinCAT licensing was about 25 percent less expensive than other offerings we looked at and, even better, there aren’t yearly upgrade fees.”

Durant was able to expand to as many Profibus drops as required in an area and could easily add or retrofit drops as needed. There are currently three Profibus trunks –each using a separate Beckhoff C3640 PC running TwinCAT for plant-wide process control. At present, the Profibus trunks have a combined total of just over 100 drops.

The software changeover was not difficult. "Our process has programming with a lot of sub-routines. Beckhoff worked with me for a couple months to figure out the best route for making a painless conversion to TwinCAT."

The User Defined Function Block (UDFB) capability of TwinCAT turned out to be a major timesaver. "There was a lot of repetitiveness in our programming and the rungs were exactly the same with just different bits on them," he explained. "I made a UDFB, copied the block and changed the bit rather than having to create entire rungs over and over again. There were several operations that previously took three rungs to get one function done. Instead of using three rungs, I used the UDFB. It cut hundreds of rungs worth of logic down to 70 rungs, and amounted to about a 30 to 40 percent savings in programming time.”

TwinCAT adheres to the IEC-1131 environment. “We used three of the programming languages and tailored them to the area of the facility we were controlling. These are Sequential Function Chart, Structured Text and Ladder Logic programming. As a person with no ‘formal’ programming experience, I found TwinCAT to be very easy to use.”

The change-over has been so seamless that McCall Farms machine operators don’t know that they’ve switched to TwinCAT. “It has been ‘business as usual’ for all involved without any major problems,” Durant said. “We upgraded our existing HMI program, which directly links into TwinCAT via an OPC Server connection, but the operators didn’t notice any difference with the possible exception of some improved graphics.”

Recently, McCall Farms avoided considerable expense from potentially wasted product because TwinCAT runs in the Microsoft Windows kernel, and is independent of other operating system processes. A whitebox computer in their cook room locked up and Durant was unable to make an immediate corrective action. But, because TwinCAT runs in the kernel mode, the process was able to complete without problem, resulting in zero loss of product.