Introduction to PLC:

Programmable logic control or PLC is the most commonly used industrial automation technique in the world. It is universally applied for factory automation, process control and manufacturing systems. Programmable logic control originated from the creation of computerized versions of relay control systems used to control manufacturing and chemical process systems. The programming is done using a special technique called ladder logic, which allows sequences of logical actions to be set up, inter-linked and timed. A standard task in logic control is batch control and sequencing in a process system. PLC allows as controlling an automated process. Automation

1. Gain complete control of manufacturing process.

2. Achieve helps a manufacturing facility to- consistency in manufacturing

3. Improve quality & accuracy

4. Work in difficult or hazardous environments

5. Increase productivity

6. Shorten the time to market7. Lower the cost of quality, scrap & rework8. Offer greater product variety9. Quality charge over from one product to another10. Control inventory

A PLC is a user friendly, microprocessor specialized computer that carries out control functions of many types and levels of complexity. Its purpose is to monitor crucial process parameters and adjust process operations accordingly. It can be programmed, controlled and operated by a person unskilled in operating computers. Essentially, a PLC's operator draws the lines and devices of ladder diagrams with a keyboard onto a display screen. The resulting drawing is converted into computer machine language and run as a user program.

Architecture of PLC:

Basic Structure of PLC

PLC scan cycle :A PLC works by continually scanning a program. We can think of this scan cycle as consisting of 3 important steps. There are typically more than 3 but we can focus on the important parts and not worry about the others. Typically the others are checking the system and updating the current internal counter and timer values.

PLC operation

Advantage of PLC:

Programmable Logic Controllers have been gaining popularity on the factory floor and will probably remain predominant for some time to come. Most of this is because of the advantages they offer. With the growth in technology PLCs are now capable of advanced data manipulations, communications and process controls. Benefits of using PLC in industries are1. The PLC is a hardened industrial computer designed to withstand the harshfactory environment.2. PLSs is reusable they contain a changeable program that eliminators extensiveand component changes.3. PLCs offer easy troubleshooting4. PLCs feature easy installation and small size.

Some of the used Instructions:

Examine if Closed (XIC)

Use the XIC instruction in your ladder to determine if a bit is on. When the instruction is executed, if the bit addressed is on (1), then the instruction is evaluated as true. When the instruction is executed, if the bit addressed off (0),then the instruction is evaluated as false.

Examine if Open (XIO)

Use an XIO instruction in your ladder program to determine if a bit is off. When the instruction is executed, if the bit addressed is off (0), then the instruction is evaluated as true. When the instruction is evaluated as false.

Timer On-Delay (TON)

Use the TON instruction to turn an output on or off after the timer has been on for a preset time interval. The TON instruction begins to count time base intervals when rung conditions become true. As long as rung conditions remain true, the timer adjusts its accumulated value (ACC) each evaluation until it reaches the preset value (PRE). The accumulated value is reset when rung conditions go false, regardless of whether the timer has timed out.