PLC - Programmable Logic Controllers

What Is A Programming Logic Controller?
Programming Logic Controller, PLC, is basically a digital computer which controls machinery and automates industrial processes. In general, PLC’s take a multiple number of inputs and outputs and are resistant to high temperature, vibration, noise and physical impact. These characteristics differentiate the PLC from a normal general-purpose computer. Another advantage of PLC is that the executable programs to control the operation of machines are saved in batteries or non-volatile memories.

Programming Logic Computers can be considered to be a very good example of a real time system since the output of the system controlled by PLC depends on the input conditions and also, the intended output is expected to be obtained within the specified time, the failure of which produces unexpected and harmful results.

Specialties of PLC
The most special characteristic of the Programmable Logic Controllers is its resistance to extreme conditions of dust, moisture, heat, electrical noise etc. Unlike general-purpose computers, the programming logic computers can be used to connect with actuators and sensors which can monitor and operate certain physical machines like electric motors, hydraulic cylinders etc.

Prior to the invention of PLC’s, thousands of relays and timers were used in automotive manufacturing industries. But the efficient and cost-beneficiary PLC’s have been used to replace such thousands of relays.

In recent years, Programming logic computers have grown to accommodate sequential relay control, motion control, distributed control systems, process control and networking. Modern PLC’s have developed in such a way that its storage facilities and data handling techniques along with their processing speed and power are being compared with common desktop computers.

PLC Vs Other Real Time Systems
PLCs suit well to a wide range of automation tasks. Considering processes where the cost of automation of the product can be lesser compared to the development and maintenance of the product. Also, the changes made to the product in the near future would greatly affect the performance and operation of the products.

PLCs contain extensible input and output devices and functionalities which can be useful in assisting and controlling the embedded machinery. Most of the representations of the Programming Logic Controllers are preferably expressed in the Ladder logic or the Function chart notations.

Compared to various real time systems, like a custom-built controller design, which perform the same operations like a PLC, the cost is indefinitely low. Another important issue in the case of mass-produced goods, customized control systems is comparatively more economic because of the low cost of the application components, which can rather be chosen instead of a "generic" solution.

User Interface
Programming Logic Controllers, in general, is intended to interact and communicate with the people for the purpose of configuration or simple examples like alarm reporting. A Human-Machine Interface or more simply, Graphical User Interface has been in the use, for this purpose. A simple Graphic User Interface which is more user-friendly simply uses buttons and lights.

Evolution Of PLC Programming
Initially, the PLC programs were learnt by electricians who learnt them while they were on the job. Such programs were written in the ladder logic. These programs diagrammatically represent the functionality of relay logic. On the passage of time, the PLC programs slowly got introduced to certain programming languages. Some of the modern PLC’s use BASIC and C.

More recently, different PLC manufacturers rely on certain programming methods on their own and this has some side effects too. Though the principal concepts are the same in all methods of programming, there might be slight differences in addressing, memory allocation and retrieval, data handling etc. This leads to the fact that the PLC programs cannot be interchanged between different manufacturers.

Thus, the evolution of PLC’s in the place of large clusters of relays in the field of automotive machinery has led to a large amount of cost benefits.