PLC (Programmable Logic Controller)
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Programmable logic controllers (PLCs) are used in every aspect of industry to expand and enhance production. Where older automated systems would use hundreds or thousands of relays, a single PLC can be programmed as a replacement. The functionality of the PLC has evolved over the years to include capabilities beyond typical relay control. This book teaches and demonstrates the basics of the Allen-Bradley MicroLogix 1000 programmable logic controller. Information is provided to help the reader get and operate an inexpensive MicroLogix 1000 and associated hardware and software. Examples with ladder diagrams and circuit diagrams.
The material is quite suitable for anyone wishing to study this interesting subject and does not require a lot of mathematical knowledge. Obviously, access to suitable computer software such as Pneusim Pro™ or Bytronics™ simulation software will be a great help. You do need to have a reasonable knowledge of computer technology and a good background understanding of industrial processes. SYLLABUS Design characteristics: unitary, modular, rack-mounted Input and output devices: mechanical switches, non-mechanical digital sources, transducers, relay. Communication links: twisted pair, coaxial, fibre optic, networks. Internal architecture: CPU, ALU, storage devices, memory, opto-isolators, input and output units, flags, shift, registers Operational characteristics: scanning, performing logic operations, continuous updating, mass 1/O copying Outcome Assessment Criteria Describe the design characteristics of typical programmable logic devices. Describe different types of input and output device. Describe the types of communication link used in programmable logic control systems. Describe the internal architecture of a typical programmable logic device. 1. Investigate the design and operational characteristics of programmable logic control systems Describe the operational characteristics of the CPU. The PLC has its origins in the motor manufacturing industries. Manufacturing processes were partially automated by the use of rigid control circuits, electrical, hydraulic and pneumatic. It was found that when ever a change had to be made, the system had to be rewired or reconfigured. The use of wiring boards on which connections could be changed by unplugging them and changing them around followed. With the development of micro-computers it was realised that if the computer could switch things on or off and respond to a pattern of inputs, then the changes could be made by simply reprogramming the computer and so the PLC was born. There are still many applications of automated systems with permanent connections to perform a single control action. Often the system uses logic components to produce the correct action (electronic and pneumatic). The PLC mimics this process by performing the logical operations with the programme rather than with real components. In this way cost savings are produced as fewer components are needed and more flexibility is introduced as programmes can be changed more easily than reconfiguring a hard ware system. Programming is covered in Outcomes 2 and 3. A Programmable Logic Controller is a mini computer specifically designed for industrial and other applications. Examples are: • Pneumatic machines. • Hydraulic machines. • Robots. • Production processes. • Packaging Lines. • Traffic Lights and signalling systems. • Refining processes. 2. ARCHITECTURE AND TERMINOLOGY The PLC activates its output terminals in order to switch things on or off. The decision to activate an output is based on the status of the system’s feed-back sensors and these are connected to the input terminals of the PLC. The decisions are based on logic programmes stored in the RAM and/or ROM memory. They have a central processing unit (CPU) , data bus and address bus. A typical unitary PLC is shown below. THE CPU The next diagram shows the internal structure of the Central Processing Unit in its simplest form. It usually contains (but sometimes it is external and separate) an Arithmetic Logic Unit. This is the part that performs operations such as adding, subtracting, multiplying, dividing and comparing. The Buffers act as switches that isolate the lines on either side if required. A, B and C are latches that passes the data from one side to the other when told to do so. Digital data is passed around through busses. The busses were originally 4 parallel lines but as technology progressed this become 8, then 16 and now 32. Digital numbers and how they are put onto busses is explained in outcome 2. The busses are connected to memory chips. In a memory chip, digital numbers are stored in locations. The number is the data and the location is the address. Data can be sent to or brought from memory locations by either writing it or reading. The lines labelled R and W are signal lines that makes the CPU read or write. A REGISTER is a temporary memory location where data is put to be manipulated and then taken away. The CLOCK line is pulsed at a regular rate to synchronise the operations. Currently this has reached a rate measure in Giga Hertz (1000 million times a second) . The Reset line when activated resets the programme Counter to Zero. The operations are carried out to a set of instruction (the programme) and these are decoded in the ID (Instruction Decoder) MEMORY The PLC has RAM (Random Access Memory) and ROM (Read Only Memory). The programme, when written and entered, is stored in the RAM. The ROM contains permanent programmes such as that required to monitor the status of the inputs and outputs and to run diagnostic tests. TESTING The PLC has certain diagnostic, monitoring and testing facilities within the software. Light Emitting Diodes (LED) shows the status of the inputs and outputs. It is also possible to fix a bank of switches to the input side and test a programme by setting the switches to a certain state and seeing if the appropriate output action is taken. The most advanced method connects the PLC to a computer with appropriate software and runs a complete simulation of the system being controlled showing the status of everything. PROGRAMMING METHODS The P.L.C. is programmed with logical commands. This may be done through a programming panel or by connection to a computer. There are several types of programming panels varying in complexity from a simple key pad to a full blown hand held computer with graphics screen. Computers are able to run programming software with graphics, simulators, diagnostics and monitoring. This could be a laptop carried to the site or a main computer some distance away. Often the programme is developed and tested on the computer and the programme is transferred to the PLC. This could be by a communication link, by a magnetic tape, compact dusc or more likely with an EEPROM. The EEPROM is a memory chip to which the programme is written. The chip is then taken to the PLC and simply plugged in. The memory cannot be overwritten but it can be erased by exposure to UV light and reused.
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