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Dedicated I/O delivers faster

Everyone is aware of how important it is to choose the right combination of hardware and software to deliver optimal results in automation applications. Putting this piece of well-worn wisdom to the test, Metro Weighing and Automation (www.metrowa.com), based in Taylor, MI, discovered that a new I/O-based system with logic and communications performed better than its existing PLCs for weighing and packaging systems.

Though MWA produces five distinct packaging systems, each is a custom unit configured to meet a customer's unique needs, says Philip Harrison, an MWA executive in charge of the control hardware and software in those systems. Essentially, a packaging system consists of hoppers, feeders, weigh scales and conveyors that must be carefully synchronized to ensure that packages aren’t overfilled or underfilled and that they are moved into place under the hoppers quickly.

"Typically, you have ten or twelve controls that govern the equipment that feeds product into the scale hopper," says Harrison. "You have to be able to control how fast the feeders run, and quickly adjust feed rates and cutoffs. Ninety percent of the control functions are performed through a touchscreen PC running IntelliPack software, which we developed. The remaining control—primarily the case movement—was handled through the PLC."

MWA’s systems performed well, but Harrison was frustrated by communications and processing bottlenecks in his systems’ PLCs that forced him to develop inelegant software workarounds. The problem, he says, is that PLCs simply didn’t support the necessary data structures or the deterministic processing that the packaging applications demand. In addition, the proprietary PLC networking protocols used were too slow, aggravating the situation.

"The PLCs we were using had turnaround times in the thirty- to fifty-millisecond range. The PLC vendor’s paradigm is to make control on the inside of the device quick, but if you have an outside process that has to get to those data, that device is a last-class citizen," says Harrison. Following standards to get the data would have been more effective, he suggests.

Determinism sought

"Our software was both polling inputs in the background and setting outputs as it made decisions," says Harrison. "But even with optimized communication, the communication lines were clogged, and the software was unable to adjust feeder speeds deterministically. We had to design our software to actually pause for up to 300 milliseconds in cases where the I/O queue got too large. During that time, the software would be blind to incoming scale data until it could make another I/O call."

The problem prompted Harrison to seek other means of enabling control. Opto 22’s (www.Opto22.com) Snap Ultimate I/O has standard 10-Mbps and 100-Mbps Ethernet communications, which allow data exchange with enterprise databases without middleware.

"A PLC is a single-thread device," says Anthony J. Dern, an Opto 22 engineer. "But the control side of our devices involves multitasking. So, we can do the things that need to be done very quickly and deterministically."

Instead of using PLCs, MWA now mounts the Ultimate I/O bank and the Ultimate Brain controller on its packaging machines. An Ethernet crossover communicates between PCs running MWA’s Intellipack software and the I/O devices. MWA also uses Opto 22’s Terminal-G70, an Ethernet-enabled, graphics-based operator interface with Opto 22 Snap Ethernet I/O and SNAP Ultimate I/O systems.

The difference in performance between these I/O systems and the previous PLCs is huge, says Harrison. MWA had been using two PLC models that took 20 to 50 ms for an I/O call. Opto 22’s Ultimate Brain takes 0.5 to 1.5 ms to perform the same function. Additionally, standards, sample source and available OPC drivers eliminated the need for proprietary protocols.

The I/O system facilitates physical maintenance of the packaging systems and simplifies the control panel. If a scale indicator fails, for example, technicians previously would have had to pry a circuit board off the unit, replace it and close the case. Now, it’s a matter of popping a self-contained module in and out, says Harrison.
MWA also simplified its system programming through use of Opto 22’s ioControl, a graphical, flowchart-based programming tool for industrial automation, remote monitoring and data acquisition applications. Dern says, "Within the graphic programming package, you define each task by building a flowchart of what needs to be accomplished. The instructions themselves are located inside each block of the flowchart."

Harrison says the faster I/O system enabled MWA to make the new version of Intellipack much more adaptable to dynamic conditions. As a result, the systems are easier to preconfigure and more flexible for the users. Intellipack Version 1, designed for PLCs, could not monitor conditions fast enough to make intelligent decisions, he says.

"This forced us to combine decision-making with preset values that we had to set up on a utilities screen at the factory after running a mix of the customers parts," says Harrison. "As a result, if the customer introduced a new part that wasn’t similar to something we tested, we would have to go out into the field and make adjustments accordingly."

The speed of communications via Opto 22 Ultimate I/O, however, allows the new version of Intellipack software to quickly "learn" a new part and adapt to it automatically.

"All we have to enter on the utilities screen is information about the feeders being used. Intellipack Version 2 analyzes the parts and makes precise decisions to obtain optimal speed and accuracy," Harrison says. "One customer bought three packaging lines from us, essentially replacing their entire existing packaging operations."

While the customer’s old system should have been packing 15 cartons/min, complex setup procedures and other problems resulted in producing less than half that number.

"But with our system, all MWA has to do is touch a part number and start the system, and they get fifteen boxes per minute all day long," Harrison says.

More information is available:

Packaging equipment:Metro Weighing and Automation, 313/299-9600. www.metrowa.com.

I/O systems, Opto22, 800/321-6786. www.opto22.com. Circle No. 250.

Reprinted from Control Engineering

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