programmable logic control system

PROGRAMMABLE LOGIC CONTROL SYSTEM

PLC APPLICATIONS IN FLUID POWER CONTROL

1. Introduction

Programmable logic controller (PLC) is one of the important micro-processor based controller, As we aware, microprocessor has a tremendous impact on industrial control and instrumentation due to its high reliability and flexibility at the design and implementation ges. The decreasing cost of microprocessors with increasing facilities in them are acting as catalyst in their widening scope of applications. In recent years, PLCs are being used in place of electro-mechanical relays or cam-operated logic controllers to control fluid power systems. 

2. What is a Programmable Logic Controller (PLC) ?

• Definition: A programmable logic controller (PLC) can be defined as a digital electronic device that uses a programmable memory to store instructions and to implement functions such as logic, sequencing, timing, counting, and arithmetic in order to control machines and processes.

• In simple terms, a PLC is a user-friendly electronic computer designed to perform logic functions such as AND, OR, and NOT for controlling the operation of industrial equipment and processes.

• Thus a PLC consists of solid-state digital logic elements for making logic decisions and providing corresponding outputs.

• Basically, PLCs are designed as a replacement for hard-wired electro-mechanical relays to control fluid power systems.

3. PLCs Vs Computers

PLCs are similar to general-purpose computers. But PLCs have certain features which are specific to their use as controllers. Some of the important features of PLCs are:

1. PLCs are rugged and designed to withstand vibrations, temperature, humidity, and noise.

2. The interfacing for inputs and outputs is inside the controller.

3. They are easily programmed and have an easily understood programming language. Programming is primarily concerned with logic and switching operations.

4. Advantages of PLCs over Electromechanical Relays

The PLCs replace electromechanical relays due to their following advantages :

1. PLCs are more reliable and faster in operation.

2. They are smaller in size and can be more readily expanded,

3. They require less electrical power.

4. They are less expensive when compared to electromechanical relays for the same number of control functions.

5. Hard-wired elèctromechanical relays lack flexibility. For instance, when system operation requirements change, then the relays have to be rewired.

6. PLCs have very few hardware failure when compared to electro-mechanical relays.

7. Special functions such as time-delay actions and counters, can be easily performed using PLCs.