Ⅰ Definition 

The programmable logic controller is referred to as PLC. With the development of science and technology, a new type of industrial control device developed to meet the needs of multiple varieties and small batch production.

programmable logic controller

(1) The field input interface circuit is integrated by the optical coupling circuit and the input interface circuit of the microcomputer. It is the input channel of the interface interface between the PLC and the field control.

(2) The field output interface circuit is integrated by the output data register, the strobe circuit and the interrupt request circuit, and it functions as the PLC to output corresponding control signals to the execution components of the field through the field output interface circuit.

The commonly used I/O are as follows:

Switching quantity: according to the voltage level, there are 220VAC, 110VAC, 24VDC; according to the isolation method, there are relay isolation and transistor isolation.

Analog quantity: According to the signal type, there are current type (4-20mA, 0-20mA), voltage type (0-10V, 0-5V, -10-10V), etc. According to the accuracy, there are 12bit, 14bit, 16bit, etc.. In addition to the above general IO, there are special IO modules, such as thermal resistance, thermocouple, pulse and other modules.

The number of I/O points determines the module specifications and quantity. I/O modules can be more or less, but the maximum number is limited by the basic configuration capabilities that the CPU can manage, that is, by the maximum number of base plates or rack slots.

In 1987, the International Electrotechnical Commission (IEC) promulgated the PLC standard draft as follows:

"Programmable Controller (PLC) is an electronic system of digital operation, designed for industrial applications. It uses programmable memory to store instructions for performing logical operations, sequential control, timing, counting and arithmetic operations, and controls various types of machinery or production processes through digital and analog inputs and outputs. Programmable controllers and their peripherals should be designed in accordance with the principle of easy integration with industrial systems and easy expansion of their functions."

Ⅱ Structure and product

From the PLC hardware structure form, PLC can be divided into 5 basic structure forms: the overall fixed I/O type, the basic unit plus the expansion type, the modular type, the integrated type and the distributed type. 

The composition of PLC:

basic composition of PLC

(1) Central Processing Unit (CPU)

The central processing unit (CPU) is the control center of the PLC. It is the core of the PLC and functions as the nerve center. Each PLC has at least one CPU. It receives and stores the user program and data entered from the programmer according to the functions given by the PLC system program; checks the status of the power supply, memory, I/O, and watchdog timer, and can diagnose syntax errors in the user program. When the PLC is put into operation, it first receives the status and data of each input device on the spot by scanning, and stores them in the I/O image area respectively, and then reads the user program one by one from the user program memory. After the command is explained, the result of the execution of logic or arithmetic operation specified by the instruction is sent to the I/O image area or data register. After all the user programs are executed, finally the output status of the I/O image area or the data in the output register is transferred to the corresponding output device, and the cycle runs until it stops.

In order to further improve the reliability of PLC, double CPUs are used to form a redundant system for large PLCs, or a voting system with three CPUs is used. In this way, even if a certain CPU fails, the entire system can still operate normally.

CPU speed and memory capacity are important parameters of the PLC. They determine the working speed of the PLC, the number of I/Os, and the software capacity, etc., thus limiting the control scale.

(2) Memory

The system program memory is the memory for storing system software. The user program memory is for storing PLC user program applications. The data memory is used to store the intermediate state and information when the PLC program is executed, and it is equivalent to the

PC memory.

(3) Input and output interface (I/O module)

The interface between the PLC and the electrical circuit is completed through the input and output part (I/O). The I/O module integrates the I/O circuit of the PLC. Its input register reflects the state of the input signal and the output point reflects the state of the output latch. The input module transforms the electrical signal into a digital signal and enters the PLC system, while the output module is the opposite. I/O is divided into digital input (DI), digital output (DO), analog input (AI), analog output (AO) and other modules.

(4) Communication Interface

The main function of the communication interface is to realize data exchange (communication) between PLC and external equipment. The communication interface has various forms, the most basic are standard serial interfaces such as UBS, RS-232, RS-422/RS-485. It can be connected by multi-core cable, twisted pair, coaxial cable, optical cable, etc.

(5) Power supply

The PLC power supply provides working power for the PLC circuit, which plays a very important role in the entire system. A good and reliable power supply system is the most basic guarantee for PLC. Generally, the AC voltage fluctuates within +10% (+15%). You can connect the PLC directly to the AC grid without taking other measures. The power input types are: AC power (220VAC or 110VAC), DC power (24VDC commonly used).

Ⅲ Features

1. High reliability and strong anti-interference ability

PLC replaces a large number of intermediate relays and time relays with software, and only a small amount of hardware related to input and output is left. The wiring can be reduced to 1/10~1/100 of the relay control system. The faults caused by poor contact contact are also greatly reduced.

High reliability is the key performance of electrical control equipment. Because PLC adopts modern large-scale integrated circuit technology and strict production technology to manufacture. The internal circuit adopts advanced anti-interference technology, and has high reliability. For example, the F series PLC produced by Mitsubishi Corporation has a mean time between failures of up to 300,000 hours. Some PLCs that use redundant CPUs have longer MTBF. For the PLC's external circuit, the use of PLC to form the control system, compared with the relay contactor system of the same scale, the electrical wiring and switch contacts have been reduced to hundreds or even one-thousandths, and the failure is greatly reduced. In addition, the PLC has a hardware failure self-detection function, which can send out alarm information in time when a failure occurs. In the application software, the user can also program the fault self-diagnosis program of the peripheral device, so that the circuits and equipment other than the PLC in the system also obtain fault self-diagnosis protection. In this way, it is not surprising that the entire system has extremely high reliability.

2.Complete hardware, complete functions and strong applicability

The development of PLC to today has formed a series of large, medium and small scale products, and has been standardized, serialized, modularized. It has equipped with a complete variety of hardware devices for users to choose. Users can flexibly and conveniently choose system configuration to form systems with different functions and different scales. The installation and wiring of PLC is also very convenient. Generally, wiring terminals are used to connect external wiring. PLC has strong load capacity, can directly drive general solenoid valves and AC contactors, and can be used in industrial control occasions of various scales. In addition to logic processing functions, most modern PLCs have perfect data computing capabilities and can be used in various digital control fields. The emergence of PLC functional units in large numbers makes PLC penetrate into various industrial controls such as position control, temperature control, and CNC. Coupled with the enhancement of PLC communication capabilities and the development of man-machine interface technology, it has become very easy to use PLC to form various control systems.

3.Easy to learn and use

As a general industrial control computer, PLC is an industrial control equipment for industrial and mining enterprises. Its interface is easy, and the programming language is easy to be accepted by engineering and technical personnel. The graphic symbols and expressions of the ladder language are quite close to the relay circuit diagram, and the functions of the relay circuit can be easily realized with only a small amount of PLC logic control instructions. It opens the door for people who are not familiar with electronic circuits, computer principles and assembly language to use computers for industrial control.

4.Easy to transform

The design, installation, and commissioning of the system are small, easy to maintain and easy to modify. PLC ladder diagram program generally adopts sequential control design method. This programming method is very regular and easy to master. For complex control systems, the design time of the ladder diagram is much less than the design time of the relay system circuit diagram.

PLC replaces wiring logic with storage logic, greatly reducing the external wiring of the control equipment and greatly shortening the design and construction cycle of the control system. At the same time, it is easier to maintain. More importantly, it is possible to change the production process of the same equipment through changing procedures. This is very suitable for multi-variety, small-batch production occasions.

5.Small size, light weight, low energy consumption

Taking the ultra-small PLC as an example, the bottom size of the newly produced varieties is less than 100mm, which is only the size of a few relays, so the volume of the switch cabinet can be reduced to 1/2~1/10 of the original. It weighs less than 150g and consumes only a few watts. Because of its small size, it can be easily installed inside the machine, making it an ideal controller for mechatronics.

Ⅳ Main functions

PLC functions

PLC uses microelectronics technology to complete various control functions. Under the action of field input signals, according to pre-input procedures, it controls field actuators and performs actions according to certain rules. Its main functions are as follows.

1.Sequence logic control

This is the most basic and most extensive application field of PLC, which is used to replace the relay control system to realize logic control and sequence control. It can be used for single-machine control or multi-machine control and also for the control of automated production lines. The PLC controls the mechanical moving parts to perform corresponding operations according to the operation buttons, limit switches and other command signals given on the spot .

2.Sport control

In the machining industry, programmable logic controllers are integrated with computer numerical control (CNC) to complete the motion control of machine tools. Many PLC manufacturers have provided single-axis or multi-axis position control templates for dragging stepper motors or servo motors. In most cases, the PLC sends the data describing the target position to the template, and the template moves one axis or several axes to the target position. When each axis moves, the position control template maintains appropriate speed and acceleration to ensure smooth movement. It has been used to control centerless grinding, stamping, segmented blanking of complex parts, hobbing, and abrasion applications.

3.Timing control

PLC provides users with a certain number of timers, set timer instructions. Each timer CPM1A of OMRON company can realize the timing control of 0·1~999·9s or 01~99·9s. SIEMENS's S7-200 series can provide timers with time base units of 0·1s/0`01s and 0·00s, realizing the timing system from 0·001s to 3276·7s. The timing time can be extended in a certain way. The timing accuracy is high, and the timing setting is convenient and flexible. PLC also provides high-precision clock pulses for accurate real-time control.

4.Counting control

The counters provided by PLC for users are divided into ordinary counters, reversible counters (increasing and decreasing counters), high-speed counters, etc., which are used to complete counting control commands for different purposes. When the counter value is equal to the counter setting value, or in a certain value range, a control command is issued. The count value of the counter can be read during operation and can also be modified during operation.

5.Step control

PLC provides users with a certain number of shift registers, which can be used to easily complete the step control function. After one process is completed, the next process is automatically carried out. After one work cycle ends, it automatically enters the next work cycle. Some PLCs are also specially equipped with step control instructions to make step control more convenient.

6.Data processing

Most PLCs have different levels of data processing functions, such as F2 series, C series, S7 series PLCs, etc., which can complete data operations such as addition, subtraction, multiplication, division, power, extraction, etc., logical operations such as word AND, word OR, word exclusive OR, negation, shift, data comparison and conversion of transferred values.

7.Analog, digital and digital, analog conversion

In process control or closed-loop control systems, there are continuous changes in physical quantities such as temperature, pressure, flow, velocity, displacement, current, and voltage. In the past, PLC was used for logic control, and the control of these physical quantities mainly relied on instrument control or distributed system DCS. Both large and medium-sized PLCs have physical processing functions, and many small PLCs also have physical quantity processing functions, which are very convenient for programming and use.

8.Communication and networking

Most PLCs have communication capabilities and can communicate between PLC and computer, and between PLC and PLC. Through these communication technologies, it is easier for PLC to form a factory automation (FA) system. It can also be connected to external devices such as printers and monitors to record and monitor relevant data.

Ⅴ Frequently asked questions

1. What does a programmable logic controller do?

A PROGRAMMABLE LOGIC CONTROLLER (PLC) is an industrial computer control system that continuously monitors the state of input devices and makes decisions based upon a custom program to control the state of output devices.

2. What are the 4 main components of a PLC?

The basic components include a power supply, central processing unit (CPU or processor), co-processor modules, input and output modules (I/O), and a peripheral device.

3. Which type of controller is used in PLC?

A PLC is s general purpose controller. Often these are used in mechanized automation, but it very broad in application. PID usually refers to a form of closed-loop control; named for the terms Proportional, Integral and Derivative. PID controllers are often used in temperature control.

4. What are the three types of PLC?

PLC are divided into three types based on output namely Relay output, Transistor output, and Triac Output PLC. The relay output type is best suited for both AC and DC output devices. Transistor output type PLC uses switching operations and used inside microprocessors.

5. Is PLC hard to learn?

Someone with a good bit of programming experience will probably find the process easier than someone with little to no experience. On average, the expectation is that usually within a week or so one should be able to grasp the basics of PLC programming no matter what language is used.

6. What are the 5 PLC programming languages?

The 5 most popular types of PLC Programming Languages are:

• Ladder Diagram (LD)

• Sequential Function Charts (SFC)

• Function Block Diagram (FBD)

• Structured Text (ST)

• Instruction List (IL)

7. What is PLC block diagram?

Block diagram of PLC- Programmable logic controller. PLC contains mainly three unit CPU, INPUT and OUTPUT. CPU:-CPU contains a processor. CPU read and executes programming instruction which is programmed by programmer. CPU controls all activity by receiving input, and as per program control all output.

8. Can I learn Plc on my own?

There are many companies that provide you with video based training for specific PLCs as well as general courses for programming. Most of these give you access to a tutor and you can learn very much at your own rate. Use online training videos through sites like Udemy and even YouTube.

9. Which PLC is mostly used in industry?

Most common used PLC in process industries are Siemens. Most of American process industries used Allen Bradley controllers. Its same in American manufacturing industries. In asian countries, Mitsubishi, Toshiba, Fuji and Omron are used in manufacturing industries.

10. What are the major areas of application of PLC?

PLCs are often referred to as industrial PCs. The PLC is also commonly used in civil applications such as in washing machines and for controlling traffic signals and elevators. They are used in many industries to monitor and control production processes and building systems.