PLC

PLC

What is PLC?

PLC also known as programmable logic controllers are basically digital computers designed especially for machine control, factory assembly lines and automation. The industrial automation techniques make use of PLC’s the most. The control of these devices vary from the general purpose computers, however, they were designed for output arrangements and multiple inputs.

Programming Languages used to program PLC

The programming of these devices is done using specialized computer languages that vary from IEC 61131-3 standard, ladder logic, SFC also known as sequential function charts, FBD also known as function block diagram, Structured Text or ST to basic B and C languages, that can be saved later in non-volatile memory or batteries.

History Of PLC

Before the introduction of PLC, the controlling, sequencing, and safety interlocking logic of the automobiles manufactured mainly comprised of cam trimmers, close loop controllers, relays and drum sequencers. Due to the reason that these could number up to hundreds or thousands at times, the upgradation process of these facilities yearly was very expensive and time consuming, because it required the electricians to rewire the relays individually in order to change their characteristics. As digital computers were general purpose programmable devices, they were soon used for industrial purposes. In the earlier days, it was important to have programmers, power quality, cleanliness and environmental control.

Block Diagram

The PLC is a specialized computer and it incorporates all the basic parts that a general computer would own such as the CPU (central processing unit), input and output interfacing and memory. 

1. CPU (Central Processing Unit)- It is the main part of the PLC also known as the control potion of PLC.  It understands the commands retrieved from the memory and works accordingly. Currently, the CPU is a microprocessor based system. It is addressed in the processor module of the modularized systems.
2. Memory- The memory of PLC’s is two types i.e. RAM and ROM. The ROM comprises of the program commands that need to be interpreted by the CPU and act on the Ladder Logic Program stored in RAM. On the other hand, RAM is kept alive by making use of an on board battery so as to secure the ladder logic program even when the system is removed. The battery can either be Nickel-cadmium or standard dry cell. The present day PLC consists of EERPOM (Electrically Erasable Programmable Read Only Memory) that does not need any battery.  Just like the CPU, the memory is addressed in the processor module of the systems.
3. Input Units- These input units can be of different types. They can discrete, accept or analog signals of different voltages and current levels. The current controllers provide discrete signals of both DC and AC voltages from TTL to 250 VDC and 5 to 250 VAC.  Input levels like ±10 VDC ±5 VDC and 4-20 ma are accepted by analog  input units, while discrete units provide the CPU as 1 or 0. The number of bits that represent the input current and voltage depends on the unit resolutions. A defined number of magnitude and sign bits are contained in this number.
4. Output Units- The functioning of the output units is pretty much the same as the input units, with the concession that the units are either sourcing or sinking discrete voltages, analog or current. These signals are presented to the CPU directly and the output is usually transistors for TTL or triacs or DC voltages for AC voltage outputs.

An Extension

The input and output lines of every PLC is limited. However, the number can be increased if needed with the help of some additional modules by system extensions through extension lines. Every module may or may not contain extensions for both input and output lines. In addition, the extension modules can have outputs and inputs of a different nature from that of the PLC controller.

Applications of PLC

In order to meet the harsh environmental conditions in the industries, PLC’s were designed to be extremely sturdy and durable so as to withstand humidity, extreme temperatures, electrical noises and vibrations. The logic controllers are the most common equipment used to monitor and control numerous actuators and sensors and are hence different from the other computers in the I/O arrangements. The PLC cycle comprises of

• Reading and then storing the present value of every input.
• Changing the physical outputs so as to match the output table values saved in the memory.
• Executing the instructions in the program sequentially, while storing and updating outputs or variables to the memory.

However, the order of the performance of tasks varies from one software to the other, nevertheless, it is performed in an order. This implies that the outputs, inputs and programs are isolated from one another. In case any physical input is changed during the scan, it does not change the value of the input until the next scan takes place. Substantially, the output values will not be changed until the next cycle. The scans are done at a faster pace, i.e. microseconds. The PLC’s are commonly used for industrial purposes, however, their unique strengths make them liable of being used for other applications as well. A BAS also known as Builiding automation system may use the PLC for a boost to the system response and processing power. It may comprise of a variety of sensors used as inputs that range from simple limit switch to a mesh netowrk. Different types of input/output ports allow the building operations and maintenance workforce interface with PLC to adjust and monitor the behavior from a central point, that is known as SCADA system.

Manufacturers of PLC

The Power Line communication systems are manufactured by companies that include- Addon technology, , Alstom, Corinex, Advantech Corporations, Ariane Controls, AutomationDirect Controls, Beckhoff, DS2, Devolo, Echelon, FF-Automation Oy, Keyence, Hitachi, Gigle Networks, Ilevo, Mitsubishi, Maxim integrated products, Motorola, Omron, Panasonic, Rockwell Automation, SiConnect, Siemens, Schneider Electric, SIPRO, SpiDCom, Telkonet, Thomson SA, WAGO, ON Semiconductor, Phoenix contact and Yamar Electronics Ltd.