PLCs have grown in power and functional capabilities. But, small, simple PLCs are still best for replacing relays and performing other machine control functions in a minimal space.
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.
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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Counting and timing
In addition to simple relay-type (On/Off) operations, micro PLCs perform other functions such as timing and counting, analog signal processing, and motor positioning control. To do this, the PLCs contain counters, timers, registers, and sequencers. A highspeed counter is essential, for example, for position control. This device counts pulses from a rotary encoder (pulses/rev), or a linear encoder (pulses/in.). By comparing the pulse count with a set value, the PLC monitors the machine position so it can initiate action at a specific point.
Most micro PLC counters handle up to 9,999 counts, though some are limited to 999. Newer versions have increased counting capabilities to 32,000 or more.
A timer typically operates after a preset delay, (usually a few seconds or less). This function is useful in applications where, for example, a fast-moving part comes to a sudden stop, but requires a short delay to become completely stationary before an operation is performed. Micro PLC timers typically handle intervals of 100 msec, but others accommodate as little as 1 msec.
Some micro PLCs have a special timer called a real-time clock that indicates the time of day, week, or month. Applications include date stamping on food labels and measuring production rates. A battery backup lets the clock function even when power is interrupted.
A shift register keeps track of parts as they move through a manufacturing process. Each part is represented by a bit (binary number) that is in an On or Off state. As the part moves from one operation to the next, the bit shifts position. This feature is used, for example, in a multi-stage assembly operation to track a defective part (with a missing component) to a point where the part is rejected.
Block and modular configurations
Micro PLCs come in three basic forms: dedicated block, expandable modular, and a combination of the two. Block configurations contain the CPU, power supply, and a fixed number of I/O. They use little space and are cost-effective where most or all of the available I/O are used, but they are limited in I/O flexibility.
Modular PLCs divide the CPU, power supply, and I/O into separate modules. These are more flexible, but tend to be more expensive and use more panel space.
A block-type PLC typically costs about half as much as a comparable modular PLC. Because of their lower cost and smaller size, block-type PLCs are commonly used for simple relay replacement and in embedded machine control applications on small machines.
Modular micro PLCs offer more I/O flexibility, plus more program capacity and special functions. Thus, they are used in more complex applications requiring a mix of I/O types and special functions such as analog processing and high-speed positioning.
However, the trend for many applications is toward systems that combine the cost savings of block-type units with the flexibility of modular extensions. This type of modular PLC contains the CPU, power supply, and I/O in a base unit, plus plug-in expansion modules containing as little as 4 or 8 I/O. In another approach, newer block designs let one unit act as the CPU and others act as I/O extensions.
Choosing a PLC
When selecting any PLC (micro or other), first consider the number of I/O required, then the input signal voltage, output type, and type of memory.
Number of I/O includes those required to perform both machine control and operator control panel functions. Input signals typically come from proximity sensors and limit switches on the controlled machinery, plus control panel pushbuttons, selector switches, and keypads. Output modules perform On/Off switching to control devices such as contactors, relays, and solenoid valves, plus control panel displays.
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Some applications require analog I/O. Analog inputs typically come from temperature and pressure transducers. Analog outputs control adjustable-speed drives, valves, and heating elements.
Input voltage and output type. Though 110-Vac input is usually available, the trend is to use 24 Vdc for control voltage and sensor inputs because it is safer and less expensive.
Output circuits typically use relays, which can handle both ac and dc loads. But long-life, non-contact components, such as triacs (ac) and transistors (dc), are also available.
Memory type. To perform relay-replacement functions, micro PLCs require only modest amounts of program memory, generally from 800 to 2,000 programming steps (words). More complex applications require up to 8,000 steps or more.
Control programs are stored in random access memory (RAM), with battery backup, which saves the programs if power is switched off.
Some PLCs offer read-only-memory (ROM), which requires no battery backup, either on a separate cartridge or in the form of newer built-in Flash- PROM.
The most common programming languages for micro PLCs are ladder logic and Boolean (List), which let a user display and debug ladder diagrams for relaytype control systems. Ladder programs are familiar to machine operators and are widely used in the U.S., whereas Boolean is familiar to control engineers and is commonly used in other countries. Other languages include C, which lets a user create customized function blocks, State, a sequence-based language that handles multistep operations such as engine transfer lines, and Grafcet or Sequential Function Chart, both of which have a flowchart format that simplifies programming for multistep operations.
Micro PLC applications range from simple standalone control to those that require several micro PLCs connected to a higher level supervisory controller such as a PC or larger PLC. In the latter case, the units are typically connected by RS-232 or RS-485 serial communication links. RS-232, the most common, is satisfactory for distances up to 50 ft. RS-485 is used for distances to 3,000 ft. n