Basic knowledge of programmable controller_Programmable logic controller principle and application

The development of PLC is also closely related to the development of high technology such as computer technology, control technology, digital technology, and communication network technology. It is the development of these high and new technologies that has promoted the development of programmable controllers.

This article mainly introduces the basic knowledge of the programmable controller. First of all, it introduces the development and characteristics of the programmable controller. Secondly, it introduces the principle of the programmable controller and finally explains the application of the programmable controller.

Programmable Controller (PC) or PLC is an electronic system for digital operation. It is designed specifically for use in industrial environments. It uses a memory that can be programmed to perform instructions that store logical operations and sequence control, timing, counting, and arithmetic operations, and controls various types of inputs through digital or analog inputs (I) and outputs (O). Type of mechanical equipment or production process. The programmable controller is developed on the basis of electrical control technology and computer technology, and gradually developed into a new type of industrial control device that integrates automation technology, computer technology, and communication technology with the microprocessor as the core.

The programmable controller is mainly composed of a central processing unit, an input interface, an output interface, and a communication interface. The CPU is the core of the programmable controller. The I/O component is an interface circuit between the field device and the CPU, and the communication interface. Used to connect with the programmer and host computer. For integral programmable controllers, all components are housed in the same enclosure; for modular programmable controllers, the individual functional components are individually packaged, called modules or templates, and each module is connected via a bus to the rack or rail. on. The number of modules that can be placed on each rack varies with the different series of products produced by different manufacturers, which is generally 3-10 pieces. The number of racks that can be expanded is also different, typically 2-8 racks. The distance between the basic rack and the extension rack should not be too long, generally not exceeding 10M.

Programmable controller components

1, the central processing unit cpu

The CPU reads the state of the peripheral device through the input device, and is processed by the user program, and controls the peripheral device through the output device according to the processing result. The general medium-sized programmable controller is mostly a dual-microprocessor system. One is a word processor, which is a main processor. It processes byte operation instructions, controls the system bus, internal counters, internal timers, and monitors the scan time. Unified management of the programming interface while coordinating the bit processor and input and output. Another bit processor, also called a Boolean processor, is a slave processor. Its main role is to process bit manipulation instructions and implement programmable controller programming language to machine language conversion under the management of a machine operating system.

The CPU processing speed refers to the time it takes for the programmable controller to execute 1000 basic instructions.

2, memory

Memory stores system programs, user programs, and work data.

The memory used by the programmable controller consists essentially of PROM, EPROM, EEPROM and RAM.

3, input / output components

Input/output components are also called I/O modules. The programmable controller can detect various parameters of the controlled object or the controlled production process through the I/O interface, and uses these field data as the information basis for the programmable controller to control the controlled object. At the same time, the programmable controller sends the processing result to the controlled device or industrial production process through the I/O interface to achieve control.

4, programming device and programming software

The programmable controller performs its control function by sequentially executing the programs in the memory.

5, power components

1, the initial stage: from the first PLC to the mid-1970s

Because the first-generation PLC is to replace the relay, the main function is the logic operation and timing and counting functions. The CPU consists of small and medium-sized digital integrated circuits. The main products are: 084 from MODICON, PDQ-IL from AB, PDP-14 from DEC, and SCY-022 from Hitachi. In the first stage, Ladder Language was used as a programming method. Although it was boring, it formed the programming standard of the factory.

2. Expansion phase: from the mid 1970s to the end of the 1970s

At this stage, the control functions of PLC products have been greatly expanded. The extended functions include data transmission, data comparison and calculation, and analog calculations. The products at this stage include MODICON's 184,284,384, Siemens' SIMATIC S3 series, and Fuji Electric's SC series.

3, communication stage: the late 1970s to the mid 80s

This stage is related to the development of computer communications, forming a distributed communications network. However, because each manufacturer is in its own right, the communication system has its own specifications. Because in a very short period of time, PLC has rapidly expanded from the automotive industry to other industries, as a substitute for relays into the food, beverage, metal processing, manufacturing and papermaking industries. Second, product features have also been greatly developed. At the same time, the reliability is further improved. The products at this stage include Siemens' SIMATIC S6 series, GOULD's M84, 884, Fuji Electric's MICRO, and TI's TI 530.

4. Opening phase: starting from the mid-1980s

As the International Organization for Standardization has proposed a reference model OSI for open system interconnection, PLC has made great progress in its open function. Mainly manifested as the opening of the communication system, so that the products of each manufacturer can communicate, and the communication protocol begins to be standardized, benefiting users. In addition, PLC began to adopt standardized software systems, increased high-level language programming, and completed the standardization of programming languages. The products at this stage include Siemens' S7 series, AB's PLC-5, SLC500, DeVerson's V80 and PPC11, Canada's ONLINECONTROL company and CCDC's OPENPLC.

1, high reliability, strong anti-interference ability

High reliability and strong anti-interference ability are one of the most important features of PLC.

2, simple programming, easy to use

At present, the programming language used by most PLCs is the ladder diagram language, which is a production-oriented, user-oriented programming language. Ladder diagram and electrical control circuit diagram similar, image, intuitive, do not need to master computer knowledge, it is easy for the majority of engineering and technical personnel to master. When the production process needs to be changed, the program can be changed on the spot, which is convenient and flexible. At the same time, the operation and use of the PLC programmer is also simple.

3, perfect function, strong universality

Modern PLC not only has logic operation, timing, counting, sequence control and other functions, but also has many functions such as A/D and D/A conversion, numerical calculation, data processing, PID control, communication networking and so on. At the same time, due to serialization and modularization of PLC products, there are a variety of hardware devices available for users to choose from, which can form a control system that satisfies various requirements.

4, simple design and installation, easy maintenance

Because PLC replaced the hardware of the traditional electrical control system with software, the control cabinet's design, installation and wiring workload has been greatly reduced. Most of the PLC's user programs can be simulated in the lab, shortening the application design and debugging cycle. In terms of maintenance, due to the extremely low failure rate of the PLC, the maintenance workload is very small; and the PLC has a strong self-diagnosis function. If a fault occurs, the cause can be quickly identified according to the instructions on the PLC or the fault information provided on the programmer. , maintenance is extremely convenient.

5, small size, light weight, low energy consumption

Because PLC adopts the integrated circuit, its structure is compact, volume is small, the energy consumption is low, so it is the ideal control device that realizes the electromechanical integration.

1, scanning works

When the PLC is running, the control tasks are performed by executing the user program that reflects the control requirements. Many operations need to be performed, but the CPU cannot perform multiple operations at the same time. It can only operate in a time-sharing manner (serial operation). Perform one operation at a time, one by one in sequence. Since the processing speed of the CPU is very fast, it appears from the macro point of view that the results appearing outside the PLC seem to be completed simultaneously (in parallel). This serial work process is called the PLC's scan operation.

When the user program is executed in the scan mode, the scan starts from the first program. In the case of no interruption or jump control, the user program is executed one by one according to the sequence in which the program is stored until the end of the program. Then start the scan from the beginning and repeat it again and again.

The scanning operation mode of PLC is obviously different from the working principle of electrical control. The electrical control device adopts a hard logic parallel working mode. If a coil of a relay is energized or de-energized, all the normally open and normally closed contacts of the relay will immediately act simultaneously regardless of the position of the control line; The PLC adopts the scanning working mode (serial working mode). If a soft relay coil is turned on or off, all of its contacts will not act immediately and must wait until it is scanned. However, due to the fast scanning speed of the PLC, there is usually no difference between the PLC and the electrical control device in the I/O processing results.

2, PLC scanning process

In addition to executing the user program, the PLC scanning process requires internal processing and communication services to be completed during each scan. As shown in Figure 2-11, the entire scanning process includes internal processing, communication services, input sampling, program execution, and output refresh. The time required for the entire process to scan and execute is called the scan period. The scan cycle is related to the CPU operating speed, PLC hardware configuration and user program length. The typical value is 1~100ms.

In the internal processing phase, a PLC self-test is performed to check if the internal hardware is normal, reset the watchdog timer (WDT), and perform some other internal processing.

In the communication service phase, the PLC communicates with other smart devices and responds to commands entered by the programmer to update the display contents of the programmer.

When the PLC is in the STOP state, only internal processing and communication service work is completed. When the PLC is in the RUN state, in addition to the completion of internal processing and communication service work, input sampling, program execution, and output refresh operations are also completed.

The scanning operation mode of PLC is simple and intuitive, which facilitates the design of the program and provides guarantee for reliable operation. When the instruction scanned by the PLC is executed, the result is immediately used by the instruction to be scanned later, and it is also possible to monitor whether or not each scan exceeds the specified time by using a watchdog timer set internally in the CPU to avoid internal failure due to the CPU. Make program execution into an infinite loop.

At present, PLCs at home and abroad have been widely used in metallurgy, petroleum, chemicals, building materials, machinery manufacturing, electric power, automobiles, light industry, environmental protection, culture and entertainment, and other industries. With the continuous improvement of the PLC's performance-to-price ratio, its application fields have continued to increase. expand. From the application type, the application of PLC can be roughly classified into the following aspects:

1, switch logic control

Utilize PLC's basic logic operation, timing, counting and other functions to realize logic control, which can replace traditional relay control, used for stand-alone control, multi-group control, automatic production line control, etc. For example: Machine tools, injection molding machines, printing machinery, assembly lines , plating line and elevator control. This is the most basic application of PLC, and it is also the most extensive application field of PLC.

2, motion control

Most PLCs have single-axis or multi-axis position control modules that pull stepper motors or servo motors. This function is widely used in various mechanical devices such as motion control for various machine tools, assembly machines, and robots.

3, process control

Large and medium-sized PLCs have multiple analog IO modules and PID control functions, and some small PLCs also have analog input and output. So PLC can realize analog quantity control, and PLC with PID control function can form closed loop control, use in process control. This feature has been widely used in boilers, reactors, water treatment, brewing and closed-loop position control and speed control.

4, data processing

Modern PLCs have functions such as mathematical operations, data transfer, conversion, sorting, and look-up tables, which can collect, analyze, and process data. At the same time, these data can be transmitted to other smart devices through a communication interface, such as computer numerical control (CNC). ) Equipment for processing.

5, communication networking

PLC communication includes communication between PLC and PLC, PLC and host computer, PLC and other intelligent devices. PLC system and general-purpose computer can be directly or through communication processing unit and communication conversion unit to form a network to realize information exchange. It can constitute a "centralized management, decentralized control" multi-level distributed control system to meet the needs of the development of factory automation (FA) systems.

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