Relay control panel (left) requires lots of wiring to handle a few On/Off type switching functions. PLC control panel (right) contains small Square D PLCs controlling mail tray sleeving, weighing, label application, verification, and routing for a mail tray handling system.

Programmable logic controllers (PLCs), originally developed to replace relays in machine control, have steadily grown into more sophisticated devices with many new capabilities.

Today’s low-end controllers, called micro PLCs, still perform relay-replacement functions. But they also offer many functions that were once available only in large PLCs, such as math calculations, jumps, subroutines, and analog control. Other additions include increased memory, alarms, and diagnostics, plus highspeed counters, input latches, and emergency interrupts. Further, these micro PLCs provide the added functions in a smaller package, making them desirable for machine control applications where space is limited.

The PLCs of a decade ago came in only a few sizes, which often forced a user to buy a larger unit than necessary. A user needing only a few more input/output (I/O) points than a typical 60 I/O unit had to buy another block of at least 20 I/O.

Today, more sizes are available, making it easy to match the number of I/O required by an application. Stand-alone units, called block or fixed I/O PLCs, generally handle 16 to 80 I/O and can be expanded to 256 I/O. Micro modular PLCs have separate CPUs, power supplies, and I/O modules. These modular PLCs typically can be expanded to as many as 512 I/O or more.

Micro PLCs of 5 years ago had scan times of over 10msec/K of logic (program steps) compared to today’s range of 0.5 to 5 msec/K. Operating speeds of high-speed counters, formerly 1 to 2 kHz in blocktype PLCs, now reach 10 kHz.

Meanwhile, advances in microelectronics and surface mount technologies have dropped the cost and size of a micro PLC by 30 to 50%. As a result of cramming more functions in less space, micro PLCs are now used in applications that were previously performed either by relay controls or by larger, more expensive PLCs. Such applications include compressors and pumps, packaging and labeling equipment, curing ovens, and machinery used in textile and food industries.

Relays or small PLCs?

Many simple control applications use either hard-wired relay panels or micro PLCs. Relay controls are generally used for simple sequencing and switching a small number of devices On or Off. Though relay control panels can be wired by technicians with little training, large systems become very complex. Generally, relay control is economical where the number of relays is small. The advent of micro PLCs has driven the cost breakeven point down to 5 to 10 relays.

Micro PLC from Mitsubishi provides any combination of 16 inputs and outputs — dc, ac, triac, transistor, or relay. It offers many functions including high-speed counting, timing, floating point math, and diagnostics. Ribbon connector links PLC to remote terminal strip (bottom). RAM capacity ranges up to 8,000 steps.

By comparison, micro PLCs are better suited to handling complex sequencing functions and switching small control devices. Cost savings go up rapidly as the number of devices or functions increases. For example, a small PLC costing a few hundred dollars can perform the same relay functions (and more) than several control relays costing $25 to $100 each.

Compared to relay controls, micro PLCs:
• Are easier and less expensive to install (wiring a PLC typically costs ¼ to ½ as much as a comparable relay panel).
• Allow control system modifications through programming — no wiring changes are required.
• Require less maintenance because there are no moving contact points to deteriorate.
• Handle larger tasks in a smaller envelope.
• Simplify troubleshooting. A technician can determine the point of failure by checking the control system status on a programming terminal.

At the other end of the control spectrum, a micro PLC can often do the same job performed by a larger PLC, but at less cost and in a smaller size. This is especially true where the number of I/O points is less than 256.

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