Men at Work

Programmable Logical Controllers (PLC’s) and what they can do for you!

Robocop "Come quietly or there will be trouble!" 1 Input, AC/DC 5V ~ 15 V, Opto-Isolated 1 Output, relay, 230V, 5A Input/Output isolation: 5000V Power supply: 10 ~ 15VDC Power consumption: (typical) ± 125mA Terminator "Hasta la vista, baby. I’ll be back!" 8 Inputs, AC/DC 5V ~ 15 V, Opto-Isolated 8 Outputs, relay, 230V, 5A Input/Output isolation: 5000V Power supply: 10 ~ 15VDC Power consumption: (all outputs active) ± 480mA Stormtrooper "On your feet, soldier!" 6 Inputs, 1.5VDC (low) / 4.8VDC (high) / 0 – 30VDC (Analogue) 4 Outputs, relay, 230V, 5A Input/Output isolation: ??? Power supply: 10 ~ 20VDC Power consumption: (typical) ± 320mA Commando "Lock ‘n load troops!" 7 Inputs, AC/DC 5V ~ 25 V, Opto-Isolated / 0 – 10VDC (Analogue) 7 Outputs, relay, 230V, 5A Input/Output isolation: ??? Power supply: 10 ~ 20VDC Power consumption: (typical) ± 2.5W

So, just what is a PLC? Student: (puts up his hand and waves frantically) "Mr. D’Alamo, Mr. D’Alamo!" Teacher: "Yes James, what is it?" Student: "Mr. D’Alamo sir, do you know what a Pee-eL-Cee is?" Teacher: "Certainly, my young man. Now please be quiet so I can explain to you. Listen carefully, class…" A programmable logic controller (PLC) is a microcomputer, typically used in automation processes, the control of machinery and the securing of property, etc. PLC’s are used in many different industries and machines, such as packaging and automation lines (i.e. automotive industry), electrical and water control (i.e. municipal water-treatment plants and electricity supply substations) and various security- and safety applications. Unlike other computers, a PLC is designed with multiple input- and output arrangements (typically called "I/O ports; zones or -channels") in order to do a specific work. Its’ fast-scanning of these ports one after another, many times a second, let’s it measure and make decisions in split seconds, (milliseconds). Being able to "run" through its program at many cycles per second (frequency, measured in Hertz), it can do actions so fast, that to our humans it actually appears to be doing many tasks, all at once (multi-tasking). In other words, the PLC can look at, control and automate, for example, a nursery. It will constantly measure the humidity, ground-moisture and temperature levels and open various valves to irrigate areas that need watering. It will simultaneously switch on many different fans or heaters to keep temperatures within a certain range and will even sound alarms if any serious problems arise, or a human needs to be contacted. Inputs are used to "read" (or measure certain values) from, while outputs are used to "write" (or operate a certain action or actions) to. This is done after the PLC has compared the status of the input, to the program in its memory, which tells it what to do and also, when to do it. The "logic" in Logical Controller, therefore derives from the logical steps of (1) read, (2) compare and then (3) write, if necessary. A simple logical "if" and "then" sequence is followed, i.e. "if" this switch is pressed, "then" activate that electrical fan, for example. More intricate operations are also possible where two (or more) actions might be needed to cause a reaction from the controller, e.g. "if" the water level is low, "and" it is day time, "then" switch on that pump, "else" ignore! Picture a PLC as an Octopus with it’s many arms reaching out over a certain distance. Half the arms can be used to "feel" around under rocks and in crevasses, etc. The other half of its arms can be used to "grab" prey in order for the octopus to eat. The "feeling" part of the octopus’s arms, can be constituted the ‘inputs’ of the PLC and the "grabbing" part of it’s arms, can be compared to the ‘outputs’ of the PLC. The head of the octopus, can be likened to the internal microprocessor of the PLC, loaded with a program (software) in order for it to take certain actions, as and when is required. The PLC can sense a multitude of things through various sensors and wiring connected to its inputs, and take action and operate different equipment, connected to its outputs, based on the program in its memory banks. Just like the octopus doesn’t even need to "see" underneath rocks in order to catch a meal, likewise a PLC does not need to be close to its in- or outputs. It can be, and usually is, several meters, even tens of meters away from its sensors, while still operating effectively b.m.o. hard-wired strands that connect it to various equipment and sensors. PLC’s typically read limit switches, analog-process variables (such as temperature and pressure variations), and the status of complex positioning systems. On the output side, PLCs operate electric motors, pneumatic- or hydraulic cylinders, magnetic relays, solenoids, valves, etc. In fact, the first PLC was built in the 1960’s, after the General Motors Company requested an electronic solution to a hard-wired, relay system they had. This replaced some bulky and outdated operation equipment of theirs, with a much more compact and intelligent unit that could be programmed many times over (as their needs changed) and did not need rewiring every time this happened. So was born a re-programmable, computerised controller system that was cost-effective, hardworking and reliable and the foundation thus laid for today’s modern PLC’s. Typically, a single PLC can be programmed to replace hundreds of relays and/or other, labour-intensive, expensive or otherwise redundant equipment that were previously used to do a certain task. The latest PLC’s can read both Analogue- (4~20mA) and Digital (0; +5V) signals, otherwise the analogue signal is converted by analogue to digital (A-D) equipment inserted in the sensing side of the controller to enable it to "understand" the value/s. These analogue and digital signals are the values that sensors typically produce to "tell" the PLC what is going on at the input side of it. Outputs are usually a bank of relays that can be controlled individually from one another, although triac- and transistor-switches can also be found on the output side of certain PLC’s in order to drive electrical equipment. Inside the PLC, programs are typically written in a special language, from either a special programmable keyboard, custom-made to "speak" to the PLC, or otherwise on a personal computer (PC), then uploaded by cable-connection. These languages take on various forms, like "Ladder diagrams"; "Function block diagrams"; "Structured text"; "Instruction lists" and "Sequential function charts". New languages that are faster, more effective and (more) easily understood by us humans, are constantly being developed and tried with newer PLC’s. Unfortunately, PLC’s do not understand these languages directly and have to convert them into a "Machine Code" of one form or another in order to operate on their instructions. These programs (the PLC’s specific set of operating instructions) are stored inside the PLC, either in a battery backed-up RAM or some other non-volatile, flash memory. This has become necessary because of the need for the PLC to retain its program, as well as its last status during power losses, very important in most industries and controller applications! Where PLC’s need to interact with people for the purpose of programming and configuration or just everyday control, a certain kind of Human-Machine Interface (HMI), has to be employed. These can take on various forms, but are usually a specifically designed keyboard or otherwise a colourful window with graphically displayed functions, in a dedicated PLC-program on a PC. From here, the PLC is programmed, re-programmed and its internal program scrutinized (debugged) for any unwanted, erroneous or ill-effective commands that can then be replaced by better orders. Working in diverse conditions and tasked with various important jobs, PLC’s have to operate in unforgiving environments over extended temperature ranges, be immune to electrical noise and radio interference, etc. and be resistant to most vibrations and unexpected impacts of some sort. Just think of controllers dedicated to monitor and react to critically ill patients in hospital, PLC’s used to measure and control the life support of astronauts in space and microcomputers used for their reliability in the dangerous world of mining, to name a few examples. Where smaller PLC’s (8; 12 and 20 I/O) are usually found in minor applications, external modules can be added-on, to produce a "larger" (40 - 100 I/O) unit, if needed. Otherwise bigger controllers with several hundreds of I/O are used in large factories and may even be connected to other PLC’s over Fibre-optic and Coaxial, or Ethernet networks with speeds up to 100 Mbit/s. One of the latest addition to controllers, is their own, built-in GSM Cellular phone so that users can communicate with, and program it by SMS text. People can also be notified to their personal cellphones, of the various actions of the PLC, or the status of a certain in- or output, a great addition to microcomputers and definitive bonus for ordinary people who might use a PLC in and around the home to complement their domestic security. Having a "butler" that never sleeps, doesn’t drink or steal, automatically switches on the lights, remind you to feed the pets and are ready at a moment’s notice to guard your property and summon help in an emergency, is an invaluable extra for citizens in this crime-ridden country of ours.