Programmable Logic Controllers (PLC) – Essay Example
Topic: Architecture and operational characteristics of CPU Introduction Programmable Logic Controller (PLC) is a computer used in industries to supervise input variables and give corresponding outputs based upon the computational logic stored in it. PLC supports standard I/O connections and is compatible with standard signal levels. It is considered as a central part of any process control system.
Keywords: Input sensing devices, communication module, CPU, memory, output load devices, power supply.
fig.: Internal structure of a typical PLC
1) Input/output module: Input module consists of sensing devices such as switches and various types of sensors as per requirement. Output generally consists of load devices such as motors, valves, pumps, etc.
2) Input/output communication module: The communication module is typically termed as bus. It consists of data bus, address bus, control bus. It is used to carry signals between CPU-input module and CPU-output module.
3) CPU: CPU is the central processing unit. It gathers signals, does arithmetic computations and works the stored logic upon signals to give output. It is also used to store data and retrieve whenever needed. CPU is selected based upon 16/32/64 bit system requirement. The logic or program in the form of ladder diagram or likewise is stored in CPU memory. CPU memory consists of ROM (for storing fixed data), and RAM (which stores temporary variables). RAM is also known as operating memory. ROM can be erasable and/or programmable. PLC can have memory sizes ranging from 1kb to 20kb.
4) Power supply: Most PLC controllers work either at 24 VDC or 220 VAC. ( PLC manual Web site, copyright 2008, October 2, 2008, )
Processes in a PLC:
a) Read the inputs: Inputs are continuously monitored and any changes whatsoever are sensed by the input sensing devices and then recorded in CPU memory.
b) Execute the program: RAM memory inside CPU is divided into input memory and output memory. The stored variables are fetched as and when required to run the logic stored in ladder diagram. The output generated is stored in output memory from where it is transported to the output module through communication devices.
c) Update the outputs: The state of output depends on the output variables generated as a result of program execution. Continuous monitoring of inputs facilitates updating outputs after sensing any change in input parameter.
Automation process in industries becomes easy with the use of PLC because PLCs offer advantages of being rugged, and immune to vibration and heavy noise in industrial vicinity. Few disadvantages it has are; initial installment takes considerable time and it takes long to eliminate errors if occurred in the system.
1) PLC manual Web site, copyright 2008, October 2,2008,
2) kernow.curtin.edu.au Web site, September 5,2008, Curtin University of Technology, Perth, October 2,2008,
3) Electronicspal Web site, mikroElektronika, October2,2008,
4) Plcs.net Web site, October 2,2008,