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By: onki (95 month(s) ago)


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Topics to be Covered:

Topics to be Covered History of PLC Need for PLC Definition of PLC Components of PLC Advantages of PLC Dis advantages of PLC Applications of PLC Additional capabilities of a PLC PLC Vs Computer

History of PLC :

History of PLC Introduced in the late 1960’s. Developed to offer the same functionality as the existing relay logic systems. Programmable , reusable and reliable 1.Could withstand a harsh industrial environment. 2.They had no hard drive, they had battery backup. 3.Could Start in Seconds. 4.Used ladder logic for programming.

Need for PLC:

Need for PLC Hardwired panels were very time consuming to time, debug and change. The following requirements for computer controllers to replace hardwired panels 1.Solid-state not mechanical. 2.Easy to modify input and output devices. 3.Easily programmed and maintained by plant electricians. 4.Be able to function in an industrial environment.

Definition of a PLC:

Definition of a PLC A Programmable Logic Controller (PLC) is a specialized computing system used for control of industrial machines and processes. A PLC is a computer designed to work in an industrial environment . It uses a programmable memory to store the instructions and specific functions that include On/Off control, timing counting, sequencing, arithmetic and data handling. PLCs are equipped with special input/output interfaces .

PLC Basic Architecture:

PLC Basic Architecture An open architecture design allows the system to be connected easily to devices and programs made by other manufactures. A closed architecture or proprietary system, is one whose design makes it more difficult to connect devices and programs made by other manufactures. Note : When working with PLC systems that proprietary in nature you must be sure that any generic hardware or software you use compatible with your particular PLC

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The structure of PLC is based on the same principle as those employed in computer architecture

PLC System:

PLC System

Components of PLC :

Components of PLC 1.Processor. 2.Memory unit . 3.Power supply. 4.I/O Modules. 5.Programming device.


Processor: Processor is the central processing unit (CPU) of the programmable controller, it executes the various logic and sequencing functions by operating on the PLC inputs to determine the appropriate output signals. The CPU consists of one or more microprocessors similar to that of PCs and other data processing equipment but are designed to facilitate I/O transactions with different ranges with corresponding clock speeds

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Is the “brain “ of PLC. Consists of a microprocessor for implementing the logic and controlling the communications among the modulus. Designed, so the desired circuit can be entered in relay ladder logic form. The processor accepts the input data from various sensing devices, executes the stored user program and sends appropriate output commands to control devices.

2. Memory::

2. Memory: Memory unit is connected to the CPU, which contains the programs of logic, sequencing and I/O operations . It also holds data files associated with these programs, including I/O stations bits, counter and timer constants, and other variable and parameter values. This memory unit is referred to as the user or application memory because its contents are entered by the user and also directs the execution of control program and coordinates I/O operations with the aid of system memory .

3. Power Supply::

3. Power Supply: A power supply of 120v alternating current(ac) is typically used to drive the PLC (some units operate on 240 v ac). The power supply converts the 120v ac into direct current (dc) voltages of ±5V. These low voltages are used to operate equipment that many have much higher voltage and power ratings than the PLC itself. The power supply often includes a battery backup that switches in automatically in the event of an external power source failure.

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Supplies DC power to other modules that plug into the rack. In large PLC systems, this power supply doe not normally supply power to the field devices. In small and micro PLC systems , the power supply is also used to power field devices.`

4. I/O Modulus::

4. I/O Modulus: The input/output module provides the connections to the industrial equipment or process that is not to be controlled. Inputs to the controller are signals from limit-switches , push-buttons, sensors , and other on/off devices. Outputs from the controller are on/off signals to operate motors, valves, and other devices required to actuate the process. Many PLCs are capable of accepting continuous signals from analog sensors ad generating signals suitable for analog actuators.

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Forms the interface by which input field devices are connected to the controller. The terms “field” and “real world” are used to distinguish actual external devices that exist and must be physically wired into the system.

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Forms the interface by which output field devices are connected to the controller. PLCs employ an optical isolator which uses light to electrically isolate the internal components from the input and output terminals.

5. Programming Device::

5. Programming Device: The PLC is programmed by means of a programming device . The programming device is usually detachable from the PLC cabinet so that it can be shared among different controllers . Different PLC manufacturers provide different devices , ranging from simple teach pendant type devices , similar to those used in robotics, to special PLC programming keyboards and CRT displays. PCs and laptops now widely used for programming a much faster and a PC / laptop are also smaller now.

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The software allows users to create , edit .document, store and trouble shoot programs . The personal computer communicates with the plc processor via serial or parallel data communications link

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Hand – held programming devices are sometimes used to program small PLCs. They are compact, inexpensive and easily to use, but are not able to display as much logic on screen as a computer monitor. Hand-held units are often used on the factory floor for trouble shooting, modifying programs and transferring programs to multiple machines .

PLC Advantages :

PLC Advantages Less and simple wiring. Increased Reliability. More Flexibility. Lower Cost . Faster Response. Easier to troubleshoot. Remote control capability. Communication Capability. Handles much more complicated systems.

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PLC Disadvantages In contrast to microcontroller systems that have what is called an open architecture, most PLCs manufacturers offer only closed architectures for their products . PLC devices are proprietary, which means that parts and software from one manufacturer can t easily be used in combination with parts of another manufacturer, which limits the design and cost options.

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PLC Applications Originally hardwired arrays of relays were used to control the operation of heavy machines that contain motors and other high power devices. PLCs were originally used to substitute the switching relay networks used in industrial applications, but now they can also be used implement other tasks such as timing, delaying counting, calculating, comparing and processing of analog signals.

Additional capabilities of a PLC Analog control Arithmetic functions Matrix functions Data processing and reporting:

Additional capabilities of a PLC Analog control Arithmetic functions Matrix functions Data processing and reporting

PLC Vs Computer:

PLC Vs Computer Plc Designed for extreme industrial environments Can operation in high temperature and humidity High immunity to noise Integrated command interpreter (proprietary) No secondary memory available (in the PLC) Optimized for Single task Computer Designed mainly for data processing and calculation Optimized for speed Can’t operate in extreme environments Can be programmed in different languages Lost of secondary memory available Multitasking capability

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