Programmable Logic Control(PLC)

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Programmable logic controllers are the most widely used electronic devices in the control of production and assembly process in most automated factories due to its simplicity and versatility.
A programmable logic controller (PLC) is a user-friendly, microprocessor-based, specialized computer carrying out control functions of many types and levels of complexity in industrial applications.
In the coming sections the introduction to company and company profile can be overviewed. Functions of different departments in the company are also mentioned. After that a brief discussion of PLC, its advantages and disadvantages are given.

Control engineering has evolved over time. In the past humans were the main method for con-
trolling a system. More recently electricity has been used for control and early electrical control was
based on relays. These relays allow power to be switched on and off without a mechanical switch. It is common to use relays to make simple logical control decisions. The development of low cost computer has brought the most recent revolution, the Programmable Logic Controller (PLC). The advent of the PLC began in the 1970s, and has become the most common choice for manufacturing controls.PLCs 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.

? Cost effective for controlling complex systems.
? Flexible and can be reapplied to control other systems quickly and easily.
? Computational abilities allow more sophisticated control.
? Trouble shooting aids make programming easier and reduce downtime.
? Reliable components make these likely to operate for years before failure.

??????????????????????? PLC is a digitally operating electronic apparatus which uses a programmable (through dedicated language such as Relay Ladder Programming) memory for the internal storage of instructions for implementing? specific functions? such? as logic, sequencing, timing, counting and arithmetic to control through digital or analog input output modules, various types of machines and processes.
The programmable controllers (PC) have developed as the heart of the automation of the industries. The PCs offer a number of advantages over the conventional relay control logic in many aspects like reliability, repeatability, programmability, compact design etc. Small controllers play an important role in the automation where such controllers are implemented in huge and complicated processes and plants. In such situations the communication among the PCs is of vital importance along with other aspects.

1. Main module based around 8031 microcontroller.
2. 16 digital Inputs and digital outputs with proper isolation and signal conditioning.
3. Twenty-five hex keyboard and 7-segment display interface.
4. Closed loop PI controller for DC motor of 20 HP.

The general block diagram of the system is shown in fig1. The system is divided in following different cards: 1. 8031 CPU card 2. Display and Keyboard card 3.Digital input card 4.Digital output Card 5.Closed Loop PI Controller which includes ADC and DAC. These cards are fitted on the back plane through 96-pin EURO connector. Thus it is completely modular and open ender for future expansion.
1. 8031 CPU CARD
Its general block diagram is shown in fig.
The EPROM requirement for monitor program is less than 4K bytes but additional 4K bytes of ROM memory space are given for future expansion of the system. No. of digital inputs and outputs is 16. This has caused to select Intel 8031 microcontroller as the CPU.
The Intel 8031 microcontroller is interfaced to two EPROMs (total 8K bytes), a SRAM 6116 (2K bytes), keyboard and display controller 8279 two programmable peripheral interface chips 8255 alongwith necessary Reset, Clock, and Chip Selection logic circuit.
Its block diagram is shown in fig.2.2 . Five keys hex keyboard a common cathode 7-segment displays are interfaced to corresponding control signals of 8279 from EURO connector.
Its block diagram is shown in fig.2. Digital inputs are sensed through limit switches and carried on the card through PUT 2.5 to opto isolators MCT2E. Its output is connected to port pins of 8255 via 96-pin EURO connector. Total no. of digital inputs are 16.
Its block diagram is shown in fig.2.The digital outputs are taken from 8255 ports. These are used to make on and off the relevant 10 VA solenoid valve through proper signal conditioning circuit. These outputs are also used to change the armature voltage of 24 VDC motor (Lens rotation motor) in the step of 24 V and 15 V through proper signal conditioning. Total no. of digital outputs are 16.


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