Monday 18 May 2015

Electrical Automation: PLC Scada Training Institute For Btech/Diploma Students

Programmable Logic Controller (PLC)

A programmable logic controller (PLC) or programmable controller is a digital computer used for automation of industrial processes, such as control of machinery on factory assembly lines. Unlike general-purpose computers, the PLC is designed for multiple inputs and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed or non-volatile memory. A PLC is an example of a real time system since output results must be produced in response to input conditions within a bounded time, otherwise unintended operation will result.
Hence, a programmable logic controller is a specialized computer used to control machines and processes.  It therefore shares common terms with typical PCs like central processing unit, memory, software and communications.  Unlike a personal computer though the PLCis designed to survive in a rugged industrial atmosphere and to be very flexible in how it interfaces with inputs and outputs to the real world.
The components that make a PLC work can be divided into three core areas.
  • The power supply and rack
  • The central processing unit (CPU)
  • The input/output (I/O) section
PLCs come in many shapes and sizes.  They can be so small as to fit in your shirt pocket while more involved controls systems require large PLC racks.  Smaller PLCs (a.k.a. “bricks”) are typically designed with fixed I/O points.  For our consideration, we’ll look at the more modular rack based systems.  It’s called “modular” because the rack can accept many different types of I/O modules that simply slide into the rack and plug in.
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Figure 1 Power supply and Rack
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Figure 2 Backplane
Rack
The rack is the component that holds everything together.  Depending on the needs of the control system it can be ordered in different sizes to hold more modules.  Like a human spine the rack has a backplane at the rear which allows the cards to communicate with the CPU.  The power supply plugs into the rack as well and supplies a regulated DC power to other modules that plug into the rack.  The most popular power supplies work with 120 VAC or 24 VDC sources.
The CPU
The brain of the whole PLC is the CPU module.  This module typically lives in the slot beside the power supply.  Manufacturers offer different types of CPUs based on the complexity needed for the system.
The CPU consists of a microprocessor, memory chip and other integrated circuits to control logic, monitoring and communications.  The CPU has different operating modes.  Inprogramming mode it accepts the downloaded logic from a PC.  The CPU is then placed in run modeso that it can execute the program and operate the process.
Since a PLC is a dedicated controller it will only process this one program over and over again.  One cycle through the program is called a scan time and involves reading the inputs from the other modules, executing the logic based on these inputs and then updated the outputs accordingly.  The scan time happens very quickly (in the range of 1/1000th of a second).  The memory in the CPU stores the program while also holding the status of the I/O and providing a means to store values.
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Figure 3 Components of a PLC
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