Definition of Programmable Logic Controller?
PLC (Programmable Logic Controller) is a digitally powered electronic device. It records the instructions given in its programmable memory and controls various machines or processes through digital or analog I / O modules, executing logic, sequencing and actions accordingly
PLC (Programmable Logic Controller)
PLC is used to troubleshoot all of the above faults that occur in a hard wire control circuit.
A PLC performs relay switching operations such as counting, comparing and calculations.
It also processes digital and analog signals. This PLC control system is very flexible.
That is, you can easily modify the control logic in less time by changing the program of the PLC.
With this, the fault in the circuit can be detected very easily in a short time. Thus PLC can be used for many types of process control.
PLC is very useful for controlling the repetitive processes that take place in the industry, in sequence.
Currently all types of manufacturing plants and all the divisions there use PLC technology. Computers act as the heart of a PLC.
But not all computers can function as PLC. This is because PLC is a special computer designed to perform special functions.
PLCs are programmed with relay schematic diagrams. Computer languages like Basic and Fortran are not used in these
Block Diagram of Programmable Logic Controller
The input module converts high power signals (220 V AC or DC) received from input devices such as push buttons, limit switches, level sensors and proximity sensors into low power signals that are compatible with digital circuits in the CPU.
Such an input module converts all input signals into binary format with address. It then records all the input data in the input image table in RAM memory.
The CPU is considered to be the brain of the PLC. It executes and controls all the operations of the PLC in accordance with the given program instructions. To execute a program instruction, the processor first obtains input data from the input image table, solves its logic instructionally, and records the solution in the output image table. This process is called scan.
The Ouput Module acts as a signal amplifier. It converts the low power signals available from the output image table into high power signals suitable for operating output devices such as contactor, solenoids, relay and indicator lamps.
system Memory (ROM) permanently stores information necessary for the operating system. Application memory consists of two parts (1) User Program Memory (2) variable Data Memory. User program memory is used to record the user’s program instructions.
Data Memory is used to record the information file of the timer and counter, the status file of the input and output, and the values of the physical variables used in the process being controlled.
Programming device is used to record the user program in the PLC’s memory. There are two types of programming devices. One of them is a handheld device with an LCD display.
These types of devices display only one line of the program at a time. On the other hand, a keyboard 6, Cathode Ray Tube (CRT) 20 6001 SUULL system is used for programming.
This will display 4 or 6 lines of the program. Sometimes a personal computer (PC) is used to program in a PLC
WORKING PRINCIPLE OF PLC (Programmable Logic Controller )
When the power supply is supplied to the PLC and it is converted to RUN mode, the processor starts scanning the control program in the program memory.
The controller first obtains the status of all the inputs from the input image table and then executes the logic function according to the control programs.
It then registers the solution in binary format in the output image table. This output information is then sent to the output modules. Output modules convert binary format output data to fit output devices and run them
A PLC program is an organization that contains one or more instructions for performing a specific task. These programs are stored in the PLC’s memory with the help of a programming device
There are three types of programming methods for programming a PLC
- Ladder logic diagram method
- Statement listing method
- Function block diagram method
1. Ladder Logic Diagram Method
It is the most widely used programming language in PLCs. Its structure is similar to that of the line diagram used in hard-wired control circuits
The vertical line to the left of the ladder diagram indicates power or energized conductors. As such, the vertical line to the right indicates the neutral or return path
The ladder diagram is first read from left to right, from top to bottom. The horizontal lines in the ladder diagram are called rung or network. In a network (Rung), many types of control elements can be used. But only one output element should be used
Each rung must have an input element, followed by an input or control element, followed by an output element. A ladder diagram can have multiple rungs. Each of its input and output elements is given a separate number and alphabetical address
Usually, the address of the input element is preceded by the letter 1 followed by the numbers. Similarly, in the address of the output element, the letter 0 is given first followed by the numbers
I: 0/0, I: 0⁄1, I: 0⁄2 and I: 0⁄3 are the inputs in the above model. These can be the switch or push button or the contact of the contractor. O: 0/0 and O: 0⁄1 are the outputs. These can be coils of relay or contractors The first rung contains instructions for addresses such as I: 0/0, I: 0⁄1 and O: 0/0. It is connected in AND logic mode. That is, both I: 0/0 and I: 0⁄1 are close but only the output of O: 0/0 is energized
The second rung is I: 0⁄2, I: 0⁄3 and O: 0⁄1. It is connected in OR logic mode. So either output I: 0⁄2 or I: 0⁄3 is close but even output relay O: 0⁄1 is energized
2. Statement Listing Method
All actions that need to be performed in this manner are, in turn, referred to as statements. The actions to be taken are indicated on the left side of the statement. These are called OP code. All the information required for the action is given on the right hand side of the statement. These are called operand
Network 1 Operation Performed
LD I:0/0 AND
O I:0/3 OR
3. Function Block Diagram Method
In this mode, each rung is represented by a box. Each action is named after its function. The processes are also referred to as rectangular shapes. Inputs are indicated on the left side of the rectangle and outputs on the right side. The function block diagram shown in the figure above represents the ladder diagram and the same functions mentioned in the statement listing
1.The size is small.
3.Easy to install.
4.With modern technology