One of South Africa’s highest capacity maize and flour mills, requiring the constant monitoring and control of 10 000 tag points, is being run on just five PCs. Three for scada and two to replace the complex logic of 10 PLCs.
Perhaps there is a lesson to be learnt here, where focusing on common sense and logic will usually win the day against technology hype in more ways than one.
Pietermaritzburg-based Tiger Milling is one of South Africa's largest producers of maize and wheat flour. But since 1992, the plant had been struggling with a highly complex and costly system consisting of 10 PLCs monitoring some 10 000 tags. A rigid system that could not cost-effectively cope with the company's changing needs.
Changes to the PLC software were costly and had to be done by experts. The whole system worked in isolation of the rest of the company's information processes and, last but not least, the mill experienced 50-60 h of downtime each month. This was not because of mill failure but rather due to the inability of the control equipment to cope with unforeseen circumstances. This was the result of not being able to simulate a realistic-enough environment to check on how changes to the PLC's control software would cope with the reality of the mill. Simulation proved to be as difficult and costly an exercise as the PLC software changes.
"Faced with these mission-critical challenges, we decided to look for solutions that fitted more the practical environment of our needs on the shop floor than technology hype," says Neil Ledingham, Engineer, "and we have succeeded. Our staff complement during the week is now as low as two or three while the mill has the potential to be controlled at weekends from home and with no one in attendance. Our monthly downtime has also been completely eliminated. For a mill this size and of this complexity, I would say that we have found the scada equivalent of the golden goose.
"We looked for solutions that would put us in control of our own business cost-effectively and that would also cope with our changing needs. This meant systems that were open and easy to change and reconfigure." To convert their plans into reality, Tiger Milling sought the help of system integrator Buhler which structured a cost-effective alternative to the previous, complex approach. "We bypassed the L in PLC by using InControl and standard PCs," says Ralph Hauselmann, Manager of the Automation and Electronic (AE) department at Buhler. We then used transceivers that would connect the existing PLCs into an Ethernet. A few RJ45 plugs, a standard office type hub and some lengths of UTP cable later and we had the start of an exciting new solution." Initial reaction to the proposed approach prompted comments regarding the hardware and software fragility of PCs as well as their questionable versatility and speed in the realtime world of thousands of tags.
"While there was an initial speed problem, this has long since been overcome," says Hauselmann. "And as to the inadequacy of PCs to do the job, that has of course, been disproved with Tiger Milling's uninterrupted production for over a year now. The other major benefits are system flexibility, the extension of Tiger Milling's realtime environment into their IT facility and versatile simulation." Simulation of Tiger Milling's plant got to be so real that trainees couldn't tell whether they were interacting with the real thing or not. "With the soft PLC approach, simulation has become a precise science," says Hauselmann. "We simply use one PC to simulate the desired conditions and another to play its conventional role. Using InControl, we can then simulate a virtually infinite number of scenarios on demand and at very little cost.
The result is better operator understanding and the proven reduction in downtime. While the plant's centralised control and data repository made system maintenance easier, it also provided an unexpected and highly significant benefit that was to pay for the installation in a matter of months.
"With centralised control of this nature," says Hauselmann, "we were able to power up and shut down sections of the plant in an orderly manner. This resulted in important savings in power consumption as well as reduced plant maintenance." Today, Tiger Milling's pragmatic approach has given them a flexible and cost-effective system that is in their control and that does not work in isolation of the rest of the company's IT facilities. "InTouch and especially InControl, have allowed our realtime environment to become an integrated and networked part of our IT facility," says Ledingham. "This, in turn, allows for a more seamless integration of our whole supply chain."
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