difference between plc and relay pdf

Difference Between Plc And Relay Pdf

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A programmable logic controller PLC or programmable controller is an industrial digital computer that has been ruggedized and adapted for the control of manufacturing processes, such as assembly lines , robotic devices, or any activity that requires high reliability, ease of programming, and process fault diagnosis. Programs to control machine operation are typically stored in battery-backed-up or non-volatile memory.

PLCs have a lot more functions that just use like relay contacts, though they were first made to replace relays. PLCs provide a grater flexibility than the relays and are easy to upgrade. Despite these PLC add a lot more like HMI integration, Soft signal transfer, better diagnostics, Historian, trends etc and definitely better in ease of use and maintenance.

Difference between Relays and PLCs

In this tutorial, I am explaining each and every point considering the difference between them, the advantages of PLC over relay, and vice-versa. A relay is an electro-mechanical operator switch. So, it provides a software-based solution. For the controlling electro-mechanical system, the basic functions of both Relays and PLCs are the same in the industries. PLC requires less time, less wiring. PLC is easy to maintain.

Thus a PLC can perform many functions with the same hard wiring. A simple architecture of PLC is shown in figure below. But for performing many functions using Relays we need to have so many hard wiring. Basic functions of both Relay and PLC is same that is controlling electro-mechanical systems in the industries. NO keeps the circuit open and NC keeps the circuit closed. If they are ON they do opposite job.

As such, a large number of the inputs and outputs are analog like a mA signal or V signal. In Literary meaning, a Distributed Control System DCS refers to a control system usually of a process or manufacturing system, in which the controller elements are not central in location like the brain but are distributed throughout the system with each component sub-system controlled by one or more controllers. Historically a PLC was in discrete control of manufacturing processes. Whole discrete logic used to be implemented with relay circuitry. Most of the inputs and outputs for discrete control are binary, meaning they have only two states: On and Off.

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These transistors have current amplification capability and are used as switches in a PLC. More and more devices today are of the NPN category as they are easier to manufacture and perform better under most conditions. A transistor output is a solid-state device that handles small to medium amount of current. A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. This means that relays allow one circuit to switch a second circuit which can be completely separate from the first.

Post a Comment. Friday, October 18, 2. Posted by Blogger Name. Listed below the comparison of PLC vs Relay logic circuits. Relays usually used for protection applications Example: over-current , under-voltage ,Etc.

There are many advantages that a PLC system holds over a relay system. Today, we will dive into some of the advantages of PLCs. One of the first and simplest advantages is reliability. The internal relay systems of a PLC are solid state. This means that the relay function is not mechanical like conventional relay systems and components. Traditional mechanical relays wear out much faster than the electronics in a solid state relay. Every time a mechanical relay opens and closes, the contacts will arc slightly.

PLC is operated on the digital system. Relay is operated on the analog system. PLC consists of more programming functions like timer, counter, memory, etc. Relay gives only one fault detection function.

Programmable logic controller

Before the advent of solid-state logic circuits, logical control systems were designed and built exclusively around electromechanical relays. Instead, digital computers fill the need, which may be programmed to do a variety of logical functions. As an acronym, it meant Mod ular Di gital Con troller, and later became the name of a company division devoted to the design, manufacture, and sale of these special-purpose control computers. Other engineering firms developed their own versions of this device, and it eventually came to be known in non-proprietary terms as a PLC , or P rogrammable L ogic C ontroller. The purpose of a PLC was to directly replace electromechanical relays as logic elements, substituting instead a solid-state digital computer with a stored program, able to emulate the interconnection of many relays to perform certain logical tasks.

Relay logic is a hard wired control system using instrumentation, switches, timers, relays, contactors, motors and actuators. Traditional machine and process automation was accomplished using relay logic. Automating a machine using relay logic requires a mass of wiring and magnitude of devices to perform even the simplest of tasks. The advent of the microprocessor meant that relay logic control functionality could be programmed and stored in a computer. Relay logic in a PLC is the method of formulating logical expressions in order to automate machinery and processes in industrial applications.

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Programmable Logic Controllers (PLC)


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