Programming Logic Control

Programming Logic Control


               ·      What is a PLC?

A PLC (Programmable Logic Controller) is an assembly of solid state digital logic elements designed to make logical decisions as per the program written on it, which would provide desired outputs.
The PLC is designed to perform logic functions previously accomplished by electromechanical relays, Timers and Counters. Also, PLC packs millions of such logic functions on one chip, which is the advantage it has over its predecessors.

               ·        Role of PLC

The PLC is the part of the Control System that directly controls the application process.
According to the program stored in its memory, the PLC accepts data from the input devices connected to it, and uses this data to monitor the controlled system.
When the program calls for some action to take place, the PLC sends data to the output devices connected to it, to cause that action to take place.
 The PLC may be used to control a simple, repetitive task, or it may be connected to other PLCs, or to a host computer in order to integrate the control of a complex process.

               ·        Operation of PLC

When a process is controlled by a PLC it uses Inputs (sensors, switches etc.,) to make decisions & update outputs to drive valves, actuators etc. The control loop is a continuous cycle of the PLC reading inputs, solving the ladder logic, & then changing the outputs. Like any computer, this doesn’t happen instantly.
When the power is turned ON, initially the PLC does a quick sanity check to ensure that the hardware is working properly. If there is a problem, the PLC will halt & indicate that there is an error. For example, if the PLC backup battery is low & power was lost, the memory will be corrupt & this will result in a fault.
If the PLC passes the sanity check, it will then scan (read) all the inputs. After the input values are stored in memory, ladder logic will be scanned (solved) using the stored values-not the current values. This is done to prevent logic problems when input change during ladder logic scan. When the ladder logic scan is complete, the outputs will be scanned (the output values will be changed).

               ·        IO Modules

Input and output (I/O) modules connect the PLC to sensors and actuators. These provide isolation for the low-voltage, low-current signals that the PLC uses internally from the higher-power electrical circuits required by most sensors and actuators. Wide range of I/O modules are available including: digital (logical or binary) I/O modules and analogue (continuous) I/O modules.

               ·        Digital Inputs

Inputs come from sensors that translate physical or chemical phenomena into electrical signals. The simplest form of inputs are digital/discrete in AC/DC.
In smaller PLCs the inputs are normally built in and are specified when purchasing the PLC. For larger PLCs the inputs are purchased as modules, or cards, with 8, 16, 32, 64, 96 inputs of the same type on each card.

               ·        Analogue Inputs

Analogue input cards convert continuous signals via an A/D converter into discrete values for the PLC. Resolution or accuracy can be important in choosing an applicable card

               ·        Digital Outputs

Output modules rarely supply any power, but instead act as switches. External power supplies are connected to the output card and the card will switch the power on or off for each output. A common choice when purchasing output cards is relays, transistors or Triacs.
Relay are the most flexible output devices. They are capable of switching both AC and DC outputs. But, they are slower, cost more, and they will wear out after millions of cycles.  

               ·        Analogue Outputs

Analogue output cards convert digital values from the PLC to continuous signals via a D/A converter. Resolution can be important in choosing an applicable card.


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