Micro PLCs

So do you need a micro PLC or a PAC (programmable automation controller)? What's the difference, and, if you do want a programmable logic controller, what designates a "micro PLC," and do traditional monikers still apply?

It may be that the functionality a controller brings from automation design through implementation, use, repair, and upgrades is more important than the type of controller. As for size, all bets are off on traditional measures such as input/output (I/O) count; just get the functions needed for the application, perhaps with a bit more for any possible expansion.

In general PACs are PLC-like in form factor and function with industrially hardened PC-based logic and capabilities, more open, with easier, built-in communications, and often with more programming flexibility than PLCs.

Even so, PLCs have continued to advance in ease of use, communications, and other functionality. Mid-year 2004, ARC Advisory Group noted, "PLCs have now become so much of a commodity that suppliers of PLCs often do not know the end use of the nano- and micro-PLCs that they sell through distributors." Control Engineering's November 2004 PLC research (to be updated in the December issue) noted, "Micro and medium are the most widely installed types of PLCs."

Small-sized PLCs pack in features unheard of a few years ago, suggests Sydney Brooks, senior technical support specialist, Panasonic Electric Works Corp. of America (formerly Aromat, NAiS). The new compact FP-X brick-style PLC, for instance, has I/O cassettes that stack atop the PLC; it has eight built in high-speed counters, for encoders, proximity or photoelectric sensors, compared to competitors that have two, on average, Brooks said.

As with other PLCs, "Most micro PLCs have adopted the IEC 61131-3 programming standard, which defines a standard instruction set, datatypes, and programming environment," explains Mark DeCramer, Wago product manager, advanced electronics. Often-covered advantages include reusable code, processor-independent programming, code portability across PLC brands, and, DeCramer suggests, a faster learning curve. "If you're not using a micro PLC supported by IEC 61131-3 programming, the money you're saving on your micro PLC hardware is being spent elsewhere on excessive engineering and training costs."

PAC, PLC, or Both?

The application should help determine if a PLC or PAC (or both) should be used, suggests Gricha Raether, data acquisition product manager, National Instruments.

"Today's control processes rely on myriad signals and data, ranging from analog and digital I/O devices to high-resolution, high-speed cameras, and multi-axis motion controllers," Raether says. "Applications like high-speed production, real-time machine condition monitoring, high-precision control, and complex process control require high-speed data acquisition, advanced analysis, and processing algorithms to be executed deterministically."

High-end PLCs, he admits, can satisfy some of these requirements, but "engineers need computational resources, such as floating-point processors and substantial memory, to handle these signals efficiently. PACs integrate this off-the-shelf hardware with a real-time operating system to provide a cost-efficient platform for control engineers." NI products include PACs and logic boards, not PLCs.

When selecting a smaller PLC versus a larger PAC, says Tim Roberts, staff product specialist and team leader, low end control products, Schneider Electric, "consider the size and breadth of the application, and determine if the smaller PLC like a 'nano PLC' is suitable. A nano PLC can be an option for complex machine applications while a PAC is usually the preferable option for complex processes." Roberts says, "If a smaller PLC is suitable for the application, then determine if the PLC has adequate I/O count (discrete and analog), PID loops, adequate memory, communications capabilities, and processing speed that is fast enough. Physical dimensions should be a consideration as well if space is an issue." Applications show advantages and adaptability of today's small controllers.

Automated Painting

North Eastern Ohio Co., an OEM of automated painting machines, required near real-time coordination of multi axis motion with digital and analog outputs. The prior setup included a motion card and I/O card(s) installed in a Real Time Unix-based industrial PC. Code resided in the PC and on the motion card.

The upgrade needed to maintain the existing motion platform because of an installed base, and it added I/O-based logic that could independently operate non-robotic functions, like paint filling. The new application combined an Ethernet-based stand-alone motion controller, Microsoft Windows PC, and Wago Ethernet Programmable Fieldbus Coupler. Artomation by Digital Coating Devices Inc. supplied and integrated the controller/HMI. Chuck Greene, vice president, product development, outlined benefits in this application:

• Motion controller was same manufacturer so motion software didn't change.

• Microsoft Windows PC cut the cost of PC by more than 50 percent.

• Wago I/O connections provided direct communication to motion using Modbus protocol. Cost of I/O modules fell 10-20 percent for standard equipment and as much as 40 percent on machines that require expanded I/O connections, which can be added at any voltage in the field, using a power feed module. Communications to multiple field devices included RS-485.

"We were able to write a configuration utility that allows us to add I/O points without re-writing software. Simply configure and go," Greene notes. Wago 842 PFC (programmable field controller) uses IEC 61131-3 standard programming and functions on its own. "Motion communicates via Modbus to notify I/O devices which process to run. I/O runs process independently of motion and PC, allowing the operation to load paint, test spray gun setup, etc." For applications that don't require motion, the Wago controller functions on its own; using a specialty module it reads an encoder on the conveyor and triggers the correct guns on or off as parts move by.

Wastewater Express

Standing water is not something that you want to see or smell while waiting for your train (or working in an industrial environment). Better to ensure any runoff "takes the express" to an appropriate location. Toronto Transportation Co. (TTC) has installed ITT Flygt's Logimac PLC-based pumping systems at all of its stations, after seeking to standardize controllers. ITT Flygt Canada sought reliable, cost-effective powerful control, with flexible communications, and strong service and support.

Designed for use with duplex, triplex, and quadruplex pumping stations, a Logimac system includes a PLC running a standard industry-specific program that ITT Flygt Canada developed for identification of faulty equipment and safe and reliable operation of the pumping station, and an operator interface. The system monitors and controls overheating of the pump motor stator and ball bearings, liquid infiltration inside the pump motor stator and pump junction box, and detection of faulty level regulators. It also monitors inputs, including switches or pressure sensors that measure incoming flow volume. If wastewater surpasses a pre-determined level, the PLC sends a command to one or more pumps to begin pumping to a treatment plant.

"The Logimac system is powerful enough to calculate pumped volume based on incoming flow," says Luc-Réjean Lepine, product manager, ITT Flygt Canada. "By automatically bringing more pumps into operation when needed, the system prevents overflow of untreated wastewater to the rivers, enabling better wastewater treatment." Lepine says GE Fanuc's micro PLCs benefit ITT Flygt Canada and its customers. Currently used VersaMax Micro PLC is expandable up to 84 I/O points and used with variable-frequency drives and soft starters. "These controllers are powerful, reliable, and cost-effective," Lepine says. "They are also easily customizable, which is important to us and our customers." Industry-standard, easy-to-use programming also enables ITT Flygt Canada to commission pumping stations quickly.