PLC is an abbreviation for "Programmable Logic Controller". A PLC is a computer that is specifically designed to operate reliably in harsh industrial environments such as high temperatures, wet, dry, and/or dusty conditions.
PLCs are used to automate industrial processes such as the assembly line of a manufacturing plant, an ore processing plant, or a waste water treatment plant.
Advantages of PLC
PLCs have many of the same features as your home computer. They both have a power supply, a CPU (Central Processing Unit), inputs and outputs (I/O), memory, and operating software (though the operating software is different).
Dick Morley invented PLCs in 1964. PLC has since transformed the industrial and manufacturing sectors. PLC functions include timing, counting, calculating, comparing, and processing various analog signals.
The main advantage of a PLC over a "hard-wired" control system is that you can change a PLC after you've programmed it for little cost (just the programmer's time). In a hard-wired control system, you must essentially rip out wires and start over (which is more expensive and takes longer).
Let's take a look at an example to better understand this benefit. Assume you have a light that is linked to a switch. In general, the light has two modes of operation: ON and OFF. You are now tasked with ensuring that when you turn on the switch, the light glows only after 30 seconds.
We're stuck with this hard-wired setup. The only way to accomplish this is to completely rewire our circuit in order to include a timing relay. That's a lot of trouble for such a minor change.
This is where a programmable logic controller comes in, which doesn't require any extra wiring or hardware to ensure a change. Rather, a simple change in code is required to program the PLC to only turn on the light 30 seconds after the switch is turned ON.
As a result, incorporating multiple inputs and outputs is simple when using a PLC.
Features of PLC
PLCs have many of the same features as your home computer. They both have a power supply, a CPU (Central Processing Unit), inputs and outputs (I/O), memory, and operating software (though the operating software is different).A PLC's operation is extremely simple.
The processor makes decisions based on a ladder logic program that the user has written. The PLC must communicate with the various field devices it is tasked with monitoring and controlling in order to use the program properly.
It then compares the field devices' actual conditions to what the program instructs them to do, and updates the output devices accordingly.
The Order of Operations
The following is the operational sequence:
- The input switch has been depressed.
- The input module inserts a "1" into the input data table. The ladder logic program notices the "1" and inserts a "1" into the output data table.
- The output data table activates the associated point in the output module.
- The output device is turned on.
The Scan Cycle
PLCs operate by scanning programs continuously and repeating this process many times per second. When a PLC boots up, it checks the hardware and software for errors, also known as a self-test. If there are no issues, the PLC will begin the scan cycle. The scan cycle consists of three steps: input scan, program execution, and output scan.
Input Scan: To put it simply, the PLC takes a snapshot of the inputs and solves the logic. The PLC examines each input card to determine whether it is ON or OFF and stores this information in a data table for later use. This speeds up the process and eliminates cases where an input changes from the start to the end.
Program Execution (or Logic Execution): The PLC executes a program one instruction at a time, using only a memory copy of the ladder logic program's inputs. For example, the program's first input is set to ON. Because the PLC knows which inputs are ON/OFF from the previous step, it can decide whether to turn on the first output.
When the ladder scan is finished, the outputs are updated using the temporary values in memory. The PLC updates the output status based on which inputs were ON during the first step and the results of running a program during the second step. The process is now restarted by the PLC initiating a self-check for faults.
Logic Scan
Relay logic is used to model ladder logic programs. Each element in the ladder will switch as quickly as possible in relay logic. Program elements can only be examined one at a time, in a predetermined order. The ladder logic scan starts at the top of the ladder.
It interprets the top output first, followed by the output branched below it, at the end of the rung. It solves branches on the second rung before proceeding along the ladder logic rung.
Module of Power Supply
This module is responsible for supplying power to the entire PLC system. It converts the available alternating current (AC) power to the direct current (DC) power required by the CPU and I/O module. PLCs are typically powered by a 24V DC supply. Only a few PLCs make use of an isolated power supply.
Memory and CPU Module
A CPU module contains a central processor, as well as ROM and RAM memory. An operating system, drivers, and application programs are all stored in ROM memory. RAM memory is where programs and data are stored.
The CPU is the PLC's brain, and it is equipped with an octal or hexagonal microprocessor. . It replaces timers, relays, and counters because it is based on a microprocessor. A PLC can incorporate two types of processors: single bit and word processors.
To perform logic functions, a single bit processor is used. Word processors, on the other hand, are used to process text, numerical data, control, and record data. The CPU reads sensor input data, processes it, and then sends the command to controlling devices.
As previously discussed, a DC power source is required, as are voltage signals. Other electrical parts in the CPU are used to connect cables used by other units.
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PLC Varieties
PLCs are classified into two types: fixed / compact PLCs and modular PLCs.
Miniature PLC
There would be numerous modules within a single case. It has a fixed number of internal and external I/O modules and cards. As a result, it lacks the ability to expand the modules. The manufacturer would decide on every input and output.
PLC that is modular
This type of PLC allows for multiple expansions via "modules," hence the name Modular PLC. The number of I/O components can be increased. It is simpler to use because each component is self-contained.
PLCs are classified into three types based on their output: relay output, transistor output, and triac output. The relay output type is best suited for devices with both AC and DC outputs. PLCs with transistor outputs use switching operations and are found inside microprocessors.
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The Evolution of PLCs
Because many early PLCs were incapable of displaying logic graphically, it was instead represented as a series of logical expressions in a Boolean format (akin to Boolean algebra).
Ladder logic became more popular as programming terminals evolved because it was a familiar format used for electro-mechanical control panels. Although more modern formats such as state logic and Function Block diagrams exist, they are not as widely used as ladder logic.
One possible explanation is that programmers prefer ladder logic to structured text programming because it is more visually appealing. PLCs were programmed using proprietary programming panels or special-purpose programming terminals until the mid-1990s, when dedicated function keys representing the various logical elements were introduced.