PROGRAMMABLE LOGIC CONTROLLER

Posted by : Michael Panjaitan            ID  : 112110109

PROGRAMMABLE LOGIC CONTROLLER

A programmable controller is a computer designed for use in machines. A Programmable Logic Controller or as usually known as PLC or Programmable Controller is a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures. 

It doesn’t like a 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-up or non-volatile memory.

Most likely you encounter PLC based controls every day without even realizing it. In simple applications such as;  car wash, elevator, even amusement parks .

For  more complex application is water and waste water treatment plants, manufacturing assembly lines, machinery, and bottling lines, just to name a few. 

PLC also has the advantage of taking up less panel space and consuming less energy.

Based on that name, the concept of PLC is :

1.        Programmable

It shows that the ability in the memory for stored the program that has made with easily to change the functions and the useful.

2.        Logic

It shows that the ability in processing the input aritmatically and logically (ALU), which are done the operation of comparing, summing, multiplying, dividing, subtracting, negations AND, OR, and etc.

3.        Controller

It show that the ability in controlling and managing the process so that’s why resulting that the output that they want.

 

PLC was planned for changing the sequential relay circuit in a control system. Beside that it can programmable, this devices also can be controllable, and can be operated by the people who doesn’t have a knowledge to operate computer specifically.

This PLC has a programmable communication which easy to understand and can be operated if the data has been inputted. This devices work based on the inputs and depends of the condition of the certain time and then it will be  -ON or -OFF the outputs.

And  “1" will be state that the condition that expected had fulfilled and “0” states that the condition that expected still not fulfilled. PLC also can be implemented for controlling the system which have a lot of output.

How PLC examining the interactions of four internal areas :

a. CPU

  CPU or central processing unit this is the brain or in other worlds where the decision making takes places, controls others areas.

b. Memory

  User's control program stored in nonvolatile section of memory. Also I/O status and data. The program is stored in non volatile memory so it is not lost in the event of power failure.

c. Communications ports

Load user's program from PC, also exchange data with external devices, including other PLCs.

d. Input /Output

Handles interface of signals to rear world devices.

PLC I/O

The input and output connections interface to the real world devices as seen in the schematic example. Conditions and the status of the inputs are monitored, decision made by the user’s control program executing via the CPU, and outputs activated based on the outcome.

PLC Ladder Logic

A PLC is programmed using applications software running on a PC. The most widely use programming method, carried over from relay based control system, is Ladder Logic. Made up contacts, coils, and the other functions that are arranged as rungs on a ladder.

Program Scan :

It is helpful when programming a PLC to understand how the Ladder Logic programs is ‘scanned’. Once the PLC is in RUN mode, the CPU executes in the order shown in the flow diagram.

a.        Status of the inputs devices are read and stored in the data registers.

b.        Housekeeping of any peripheral devices.

c.         ‘Scan’ the user’s ladder logic left to right, sequencing through the ‘rungs’

d.        Compute the result and write updates to the outputs.

e.        Do diagnostics and if all is well, repeat the scan.

PLC is an example of a hard real time system since output results must be produced in response to input conditions within a limited time, otherwise unintended operation will result.

Video :

What is a PLC? Learn the basics. Featuring

Direct LOGIC Programmable Logic Controller

https://www.youtube.com/watch?v=VQf_smqzAEA&feature=player_detailpage