How Do I Choose the Best Programmable Logic Controller?

Programmable logic controller is a kind of digital operation electronic system specially designed for application in industrial environment. It uses a programmable memory that stores instructions for performing logical operations, sequence control, timing, counting, and arithmetic operations within it. It controls various types of mechanical equipment or production through digital or analog input and output. process. ^[1]

Chinese name: PLC
Foreign name: Programmable Logic Controller
Short name: PLC

Appear time: 1969
Features: Reliable and programmable
Subject: computer

Introduction to Programmable Logic Controllers

Programmable logic controller (PLC), a digital operation controller with a microprocessor for automatic control, can load control instructions into the memory at any time for storage and execution. Programmable controller is composed of CPU, instruction and data memory, input / output interface, power supply, digital analog conversion and other functional units. Early programmable logic controllers only had logic control functions, so they were named programmable logic controllers. Later, with continuous development, these computer modules with simple functions had already included logic control, timing control, analog control, Various functions such as multi-machine communication have been renamed Programmable Controller. However, due to the conflict between the abbreviated PC and the Personal Computer, and people's habits, people often use The term "programming logic controller" still uses the PLC abbreviation. ^[2]

Nowadays, the programmable logic controller used in the industry has been equivalent to or close to the host of a compact computer. Its advantages in scalability and reliability make it widely used in various industrial control fields. No matter in the computer direct control system or the centralized distributed control system DCS or the field bus control system FCS, there are always a large number of PLC controllers. There are many PLC manufacturers, such as Siemens, Schneider, Mitsubishi, and Delta. Almost all manufacturers involved in the field of industrial automation will have their PLC products available. ^[2]

Development history of programmable logic controllers

Programmable Logic Controller Origin

The development of production technology requirements in the American automobile industry has promoted the generation of PLC. In the 1960s, when General Motors of the United States adjusted the factory production line, it found that relays and contactor control systems were difficult to modify, bulky, noisy, inconvenient to maintain, Poor reliability, so the well-known "General Ten" bidding index was proposed. ^[3]

In 1969, the United States Digital Equipment Corporation developed the first programmable controller (PDP-14), and after trial on the production line of General Motors Corporation, the effect was remarkable; in 1971, Japan developed the first programmable controller (DCS -8); In 1973, Germany developed the first programmable controller; in 1974, China began to develop programmable controllers: In 1977, China promoted PLC in the field of industrial applications. ^[3]

The original purpose was to replace mechanical switching devices (relay modules). However, since 1968, the functions of the PLC have gradually replaced the relay control board, and modern PLCs have more functions. Its use extends from single process control to control and monitoring of the entire manufacturing system. ^[4]

Programmable logic controller development

Microprocessors appeared in the early 1970s. People soon introduced it to programmable logic controllers, which enabled programmable logic controllers to add computing, data transfer, and processing functions, and completed industrial control devices with computer characteristics. The programmable logic controller at this time is the product of the combination of microcomputer technology and the conventional control concept of the relay. After the development of personal computers, in order to facilitate and reflect the functional characteristics of programmable controllers, programmable logic controllers were named Programmable Logic Controller (PLC). ^[5]

In the mid and late 1970s, programmable logic controllers entered a stage of practical development, and computer technology has been fully introduced into programmable controllers, making their functions leap. Higher operation speed, ultra-small size, more reliable industrial anti-interference design, analog operation, PID function and high cost performance have established its position in modern industry. ^[5]

In the early 1980s, programmable logic controllers have been widely used in advanced industrial countries. The number of countries producing programmable controllers in the world is increasing, and the output is increasing. This indicates that the programmable controller has entered a mature stage. ^[5]

The 1980s to the mid-1990s were the fastest developing period for programmable logic controllers, with an annual growth rate of 30 to 40%. During this period, PLCs have greatly improved their analog processing capabilities, digital computing capabilities, human-machine interface capabilities, and network capabilities. Programmable logic controllers have gradually entered the field of process control, replacing some applications in the process control field. Dominant DCS system. ^[5]

At the end of the 20th century, the development characteristics of programmable logic controllers were more adapted to the needs of modern industry. During this period, mainframes and ultra-small computers were developed, various special function units were born, and various human-machine interface units and communication units were produced, making it easier to match industrial control equipment using programmable logic controllers. ^[5]

Basic structure of programmable logic controller

A programmable logic controller is essentially a computer dedicated to industrial control. Its hardware structure is basically the same as that of a microcomputer. Its basic composition is shown in the figure below. The basic structure is described in detail below: ^[6]

The basic components of a PLC

Programmable logic controller power

The power supply is used to convert the AC power into the DC power required inside the PLC. Currently, most PLCs use a switching regulator power supply. ^[6]

Programmable logic controller central processing unit

The central processing unit (CPU) is the control center of the PLC and the core component of the PLC. Its performance determines the performance of the PLC. ^[6]

The central processing unit is composed of a controller, an arithmetic unit and a register. These circuits are concentrated on one chip, and are connected to the input / output interface circuit of the memory through an address bus and a control bus. The role of the central processing unit is to process and run user programs, perform logical and mathematical operations, and control the entire system to coordinate it. ^[6]

Programmable logic controller memory

Memory is a semiconductor circuit with a memory function, and its role is to store system programs, user programs, logical variables, and other information. The system program is a program that controls the PLC to implement various functions. It is written by the PLC manufacturer and is solidified into a read-only memory (ROM), which cannot be accessed by users. ^[6]

Programmable logic controller input unit

The input unit is the input interface between the PLC and the controlled equipment, and it is the bridge for the signal to enter the PLC. Its function is to receive the signals from the master command element and the detection element. Input types are DC input, AC input, and AC / DC input. ^[6]

Programmable logic controller output unit

The output unit is also the connection part between the PLC and the controlled device. Its role is to transmit the PLC output signal to the controlled device, that is, convert the weak current signal sent by the central processing unit into a level signal, and drive the execution element of the controlled device . The output types are relay output, transistor output, and thyristor output. ^[6]

In addition to the above parts of PLC, there are various external devices according to the different models. Its role is to help programming, monitoring and network communication. Commonly used external devices are programmers, printers, cassette tape recorders, computers, etc. ^[6]

Working principle of programmable logic controller

When a programmable logic controller is put into operation, its working process is generally divided into three phases, namely, input sampling, user program execution, and output refresh. The completion of the above three phases is called a scan cycle. During the entire operation period, the CPU of the programmable logic controller repeatedly executes the above three stages at a certain scanning speed. ^[5]

Programmable logic controller input sampling

In the input sampling phase, the programmable logic controller sequentially reads all input states and data in a scanning manner and stores them in the corresponding cells in the I / O image area. After the input sampling is completed, it enters the user program execution and output refresh phases. In these two stages, even if the input status and data change, the status and data of the corresponding cell in the I / O image area will not change. Therefore, if the input is a pulse signal, the width of the pulse signal must be greater than one scan period to ensure that the input can be read in under any circumstances. ^[5]

Programmable logic controller (2 photos)

Programmable logic controller user program execution

During the execution of the user program, the programmable logic controller always scans the user program (ladder) in order from top to bottom. When scanning each ladder diagram, always scan the control circuit composed of contacts on the left side of the ladder diagram, and perform logic operations on the control circuit composed of contacts in the order of left, right, top, and bottom. , And then according to the result of the logical operation, refresh the state of the corresponding bit of the logic coil in the system RAM storage area; or refresh the state of the corresponding bit of the output coil in the I / O image area; or determine whether to execute the ladder diagram. Prescribed special function instructions. ^[5]

That is, during the execution of the user program, only the state and data of the input points in the I / O image area will not change, while other output points and soft devices are in the I / O image area or the system RAM storage area. Both the status and the data may change, and the ladder diagrams above the program execution result will work on the ladder diagrams that use these coils or data; on the contrary, the ladder diagrams below The status or data of the refreshed logic coil can only affect the programs arranged above it in the next scan cycle. ^[5]

If the immediate I / O instruction is used during program execution, I / O points can be accessed directly. Even if I / O instructions are used, the value of the input process image register will not be updated, the program will take the value directly from the I / O module, and the output process image register will be updated immediately, which is somewhat different from the immediate input. ^[5]

Programmable logic controller output refresh

When the scan user program ends, the programmable logic controller enters the output refresh phase. During this period, the CPU refreshes all output latch circuits according to the corresponding state and data in the I / O image area, and then drives the corresponding peripherals through the output circuits. At this time, it is the real output of the programmable logic controller. ^[5]

Summary of Programmable Logic Controllers

According to the description of the above process, the characteristics of the PLC working process can be summarized as follows: ^[7]

PLC adopts the working method of centralized sampling and centralized output, which reduces the influence of external interference. ^[5]

The working process of PLC is a process of cyclic scanning. The length of the cyclic scanning time depends on factors such as the instruction execution speed and the length of the user program. ^[5]

The effect of output on input is lagging. PLC adopts the working method of centralized sampling and centralized output. After the sampling phase is over, the change of input state will not be received until the next sampling period. Therefore, the length of this lag time depends on the length of the cycle period. In addition, the factors that affect the lag time include the input filter time and the lag time of the output circuit. ^[5]

The content of the output image register depends on the result of the scan execution of the user program. ^[5]

The content of the output latch is determined by the data in the output image register during the last output refresh. ^[5]

The actual output state of the PLC is determined by the contents of the output latch. ^[5]

Programmable logic controller features

(1) High reliability. Because most PLCs use a single-chip microcomputer, they have a high degree of integration, coupled with corresponding protection circuits and self-diagnostic functions, which improve the reliability of the system. ^[8]

(2) Easy programming. Most of the PLC programming uses relays to control ladder diagrams and command statements. The number is much smaller than that of microcomputer instructions. Except for medium and high-end PLCs, there are only about 16 small PLCs in general. Because the ladder diagram is simple and simple, it is easy to grasp and easy to use, and it can be programmed without even requiring computer expertise. ^[8]

(3) Flexible configuration. Because PLC adopts the building block structure, users only need to simply combine to flexibly change the function and scale of the control system, so it can be applied to any control system. ^[8]

(4) The input / output function modules are complete. One of the biggest advantages of PLC is that for different field signals (such as DC or AC, switching, digital or analog, voltage or current, etc.), there are corresponding templates to connect with industrial field devices (such as buttons, switches , Sensor current transmitter, motor starter or control valve, etc.) directly connected, and connected to the CPU motherboard through the bus. ^[8]

(5) Easy installation. Compared with computer systems, the installation of a PLC requires neither a dedicated machine room nor strict shielding measures. When using, you only need to properly connect the detection device with the I / O interface terminals of the actuator and the PLC, and then it can work normally. ^[8]

(6) Fast running speed. Because PLC control is performed by program control, relay logic control is incomparable regardless of its reliability or operating speed. ^[8]

In recent years, the use of microprocessors, especially with the large-scale use of single-chip microcomputers, has greatly enhanced the capabilities of PLCs, and made the difference between PLCs and microcomputer control systems smaller and smaller, especially for high-end PLCs. ^[8]

Programmable logic controller system integration

In the manufacturing industry, there are a large number of open-loop sequential control based on switching values. It performs sequential action numbers and sequential actions according to logical conditions; in addition, there is control of chain protection actions according to logical relationships that are independent of sequence and timing; and A large number of switching quantities, pulse quantities, timings, counters, analog limit alarms and other state quantities are mainly-discrete data collection monitoring. Because of these control and monitoring requirements, PLC has developed into a product that replaces relay circuits and performs sequence control. PLC manufacturers have gradually added various communication interfaces to the original CPU template, and the field bus technology and Ethernet technology have also developed synchronously, making the application scope of PLC more and more extensive. PLC has the advantages of stability and reliability, cheap price, complete functions, flexible and convenient application, and convenient operation and maintenance. This is the fundamental reason for it to occupy the market for a long time. ^[5]

The hardware of the PLC controller itself adopts a building block structure. It has a motherboard, digital I / O templates, analog I / O templates, and special positioning templates, barcode identification templates and other modules. Users can use the expansion on the motherboard as needed. Or use bus technology to equip remote I / O slaves to get the desired number of I / Os. ^[5]

PLC realizes various I / O control, and also has the ability to output analog voltage and digital pulse, which makes it can control various servo motors, stepper motors, variable frequency motors, etc. that can receive these signals, plus touch screen HMI support, Schneider PLC can meet your needs at any level in process control. ^[5]

Programmable logic controller selection rules

In the design of a programmable logic controller system, the control scheme should be determined first, and the next step is the engineering design selection of the programmable logic controller. The characteristics of process flow and application requirements are the main basis for design selection. The programmable logic controller and related equipment should be integrated and standard. According to the principle that it is easy to form a whole with the industrial control system and easy to expand its functions, the selected programmable logic controller should be put into operation in the relevant industrial field. Performance, mature and reliable system, system hardware, software configuration and function of the programmable logic controller should be adapted to the device scale and control requirements. Familiar with the programmable controller, function chart and related programming language is helpful to shorten the programming time. Therefore, when selecting and estimating the engineering design, the process characteristics and control requirements should be analyzed in detail, and the control tasks and scope determination needs should be specified. Operation and action, then according to the control requirements, estimate the number of input and output points, the required memory capacity, determine the function of the programmable logic controller, the characteristics of external equipment, etc., and finally choose a programmable logic controller and design with a higher cost performance Corresponding control system. ^[5]

Programmable logic controller points estimation

Appropriate margin should be considered in the estimation of I / O points, usually based on the statistical input and output points, and then 10% to 20% of the expandable margin is added as the input and output point estimation data. When ordering, the input and output points must be rounded according to the product characteristics of the manufacturer's programmable logic controller. ^[5]

Programmable logic controller memory capacity

The memory capacity is the size of the hardware storage unit that the programmable controller itself can provide, and the program capacity is the size of the storage unit used by the user application item in the memory, so the program capacity is less than the memory capacity. At the design stage, since the user application has not yet been compiled, the program capacity is unknown at the design stage and needs to be known after the program is debugged. In order to have a certain estimation of the program capacity when designing and selecting, the estimation of memory capacity is usually used instead. ^[5]

There is no fixed formula for estimating the memory capacity. Many formulas have given different formulas in the literature. Generally, they are based on 10 to 15 times the number of digital I / O points and 100 times the number of analog I / O points. The number is the total number of words in the memory (16 bits is a word), and the remaining amount is considered according to 25% of this number. ^[5]

Programmable logic controller control function selection

The selection includes selection of characteristics such as arithmetic function, control function, communication function, programming function, diagnostic function, and processing speed. ^[5]

1.Computational functions

The operation functions of simple programmable logic controllers include logic operations, timing, and counting functions; the operation functions of ordinary programmable logic controllers also include operation functions such as data shifting and comparison; more complex operation functions include algebraic operations and data transfer; There are also analog PID calculations and other advanced calculation functions in large programmable logic controllers. With the emergence of open systems, all programmable logic controllers have communication functions. Some products have communication with lower-level computers, some products have communication with co-located or upper-level computers, and some products also have communication with factory or enterprise networks. Function for data communication. The design and selection should be based on the requirements of practical applications, and reasonable selection of required computing functions. Most applications require only logic operations and timing counting functions. Some applications require data transfer and comparison. When used for analog detection and control, only algebraic operations, numerical conversions, and PID operations are used. To display data, operations such as decoding and encoding are required. ^[5]

Control function

Control functions include PID control operations, feedforward compensation control operations, ratio control operations, etc., which should be determined according to control requirements. Programmable logic controllers are mainly used for sequential logic control. Therefore, in most cases, single-loop or multi-loop controllers are often used to solve the analog control. Sometimes dedicated intelligent input and output units are also used to complete the required control functions. Programming logic controller processing speed and saving memory capacity. For example, PID control unit, high-speed counter, analog unit with speed compensation, ASC code conversion unit, etc. are used. ^[5]

Communication function

Large and medium-sized programmable logic controller systems should support multiple fieldbuses and standard communication protocols (such as TCP / IP), and should be able to connect with the plant management network (TCP / IP) when needed. The communication protocol should conform to the ISO / IEEE communication standard and should be an open communication network. ^[5]

The communication interface of the programmable logic controller system should include serial and parallel communication interfaces, RIO communication ports, commonly used DCS interfaces, etc .; large and medium-sized programmable logic controller communication buses (including interface equipment and cables) should be 1: 1 redundant configuration The communication bus should comply with international standards, and the communication distance should meet the actual requirements of the device. ^[5]

In the communication network of the programmable logic controller system, the higher-level network communication rate should be greater than 1Mbps, and the communication load should not be greater than 60%. The main forms of communication network of programmable logic controller system are as follows: ^[5]

1) PC is the master station, and multiple programmable logic controllers of the same model are slave stations, forming a simple programmable logic controller network; ^[5]

2) One programmable logic controller is the master station, and other programmable logic controllers of the same model are slave stations, forming a master-slave programmable logic controller network; ^[5]

3) The programmable logic controller network is connected to a large DCS as a DCS subnet through a specific network interface; ^[5]

4) Dedicated programmable logic controller network (dedicated programmable logic controller communication network of each manufacturer). ^[5]

In order to reduce the CPU communication task, according to the actual needs of the network composition, communication processors with different communication functions (such as point-to-point, fieldbus, etc.) should be selected. ^[5]

4, programming function

Offline programming mode: The programmable logic controller and the programmer share one CPU. When the programmer is in programming mode, the CPU only provides services to the programmer and does not control field devices. After the programming is completed, the programmer switches to the running mode, and the CPU controls the field devices and cannot be programmed. Offline programming can reduce system cost, but it is not convenient to use and debug. On-line programming mode: The CPU and programmer have their own CPUs. The host CPU is responsible for on-site control and exchanges data with the programmer within a scanning cycle. The programmer sends the online program or data to the host, and the next scanning cycle The host runs according to the newly received program. This method has a higher cost, but the system is easy to debug and operate. It is often used in large and medium-sized programmable logic controllers. ^[5]

Five standardized programming languages: Sequential Function Diagram (SFC), Ladder Diagram (LD), Function Block Diagram (FBD), three graphical languages, statement list (IL), and structured text (ST). The selected programming language should comply with its standard (IEC6113123). At the same time, it should also support multiple language programming forms, such as C, Basic, etc. to meet the control requirements of special control occasions. ^[5]

5.Diagnostic function

The diagnostic functions of the programmable logic controller include hardware and software diagnostics. The hardware diagnosis determines the hardware fault location through the logic judgment of the hardware. The software diagnosis is divided into internal diagnosis and external diagnosis. The diagnosis of the internal performance and functions of the PLC by software is internal diagnosis, and the diagnosis of the information exchange functions of the programmable logic controller CPU and external input and output is external diagnosis. ^[5]

The strength of the diagnostic function of the programmable logic controller directly affects the requirements for the technical ability of the operation and maintenance personnel, and affects the average maintenance time. ^[5]

6, processing speed

The programmable logic controller works in a scanning manner. In terms of real-time requirements, the processing speed should be as fast as possible. If the signal duration is less than the scan time, the programmable logic controller will not be able to scan the signal, resulting in the loss of signal data. ^[5]

The processing speed is related to the length of the user program, the processing speed of the CPU, and the quality of the software. Programmable logic controller contacts have fast response and high speed. The execution time of each binary instruction is about 0.2 ~ 0.4Ls, so it can meet the application requirements of high control requirements and corresponding fast requirements. The scan cycle (processor scan cycle) should meet: the scan time of the small programmable logic controller is not greater than 0.5ms / K; the scan time of the large and medium-sized programmable logic controller is not greater than 0.2ms / K. ^[5]

Programmable logic controller controller type

Programmable logic controllers are divided into two types according to the structure: integral type and modular type. According to the application environment, they are divided into two types: field installation and control room installation. 64-bit, etc. From the application point of view, it can usually be selected according to the control function or the number of input and output points. ^[5]

The overall programmable logic controller has a fixed number of I / O points, so the user has less room for selection and is used for small control systems; the modular programmable logic controller provides a variety of I / O cards or plug-in cards, so users can The I / O points of the control system are reasonably selected and configured, and the function expansion is convenient and flexible. It is generally used for large and medium-sized control systems. ^[5]

Programmable logic controller input and output type

The switching value mainly refers to the input and output, and refers to the auxiliary point of a device, such as the auxiliary point of the relay of the transformer's temperature controller (the transformer is displaced after overtemperature), and the valve cam switch Auxiliary points (displacement after valve switching), auxiliary points (displacement after contactor action), thermal relay (displacement after thermal relay action), these points are generally transmitted to the PLC or comprehensive protection device, power supply Generally it is provided by PLC or comprehensive protection device. It does not have power itself, so it is called passive contact, also called PLC or comprehensive protection device input. ^[5]

1.Digital quantity

Physical quantities that are discrete in time and quantity are called digital quantities. A signal representing a digital quantity is called a digital signal. Electronic circuits that work under digital signals are called digital circuits. ^[5]

For example, when the number of parts output from an automatic production line is recorded with an electronic circuit, a signal is given to the electronic circuit for each part sent, so that it is counted as 1, and the signal added to the electronic circuit is usually 0 when no part is sent out. number. It can be seen that the signal of the number of parts is discontinuous in time and quantity, so it is a digital signal. The smallest quantity unit is one. ^[5]

2.Analog

Physical quantities that are continuous in time or value are called analog quantities. The signal representing the analog quantity is called an analog signal. An electronic circuit working under an analog signal is called an analog circuit. ^[5]

For example, the voltage signal output by a thermocouple during operation is an analog signal. Because the measured temperature cannot jump suddenly under any circumstances, the measured voltage signal is continuous in time and quantity. Moreover, any value of this voltage signal during the continuous change process has a specific physical meaning, that is, a corresponding temperature. ^[5]

Programmable logic controller conversion principle

1. A digital-to-analog converter is a system that converts digital signals into analog signals. Generally, it can be achieved by low-pass filtering. The digital signal is decoded first, that is, the digital code is converted to a corresponding level to form a stepped signal, and then low-pass filtered. ^[5]

According to the theory of signals and systems, a digital stepped signal can be considered as the convolution of an ideal impulse sampling signal and a rectangular pulse signal. According to the convolution theorem, the spectrum of a digital signal is the frequency spectrum of the impulse sampling signal and the rectangular pulse spectrum Sa function). In this way, using the inverse of the Sa function as the spectral characteristic compensation, the digital signal can be restored to the sampled signal. According to the sampling theorem, the spectrum of the sampled signal is ideally low-pass filtered to obtain the spectrum of the original analog signal. ^[5]

In general implementation, these principles are not directly based on, because sharp sampling signals are difficult to obtain, so the two filtering (Sa function and ideal low-pass) can be combined (cascaded), and because the filtering characteristics of these systems are physical It is not achievable, so it can only be done approximately in a real system. ^[5]

2. Analog-to-digital converter is a system that converts analog signals into digital signals. It is a process of filtering, sample-holding, and encoding. ^[5]

The analog signal undergoes band-limit filtering, a sample-and-hold circuit, and becomes a ladder-shaped signal, and then passes through an encoder to make each level in the ladder-shaped signal into a binary code. ^[5]

Programmable logic controller selection model

There are many types of PLC products. The different types of PLCs correspond to their structural forms, performance, capacity, instruction systems, programming methods, prices, etc., and the applicable occasions also have different focuses. Therefore, the reasonable selection of PLC is of great significance for improving the technical and economic indicators of the PLC control system. ^[5]

PLC Programmable logic controller PLC model

The choice of PLC should mainly be considered from the aspects of PLC model, capacity, I / O module, power module, special function module, communication and networking capabilities. The basic principle of PLC model selection is to strive for the best performance-price ratio under the premise of meeting the functional requirements and ensuring reliability and convenient maintenance. The selection should mainly take into account reasonable structural types, choice of installation methods, corresponding functional requirements, response speed requirements, system reliability requirements, and the model as uniform as possible. ^[5]

Programmable logic controller structure

PLC mainly has two types of structure: monolithic and modular. ^[5]

The average price of each I / O point of the integral PLC is cheaper than the modular type, and the volume is relatively small. It is generally used in small control systems with relatively fixed system processes. The modular PLC's function expansion is flexible and convenient. I / O points, the ratio of input points to output points, the type of I / O modules, etc., have a large choice, and easy maintenance, generally in more complex control systems. ^[5]

Programmable logic controller installation

The installation methods of the PLC system are divided into centralized, remote I / O, and distributed distribution of multiple PLCs. ^[5]

The centralized type does not need to be set to drive remote I / O hardware, and the system has a fast response and low cost. The remote I / O type is suitable for large systems. The system has a wide range of device distribution. The remote I / O can be distributed and installed near field devices. The line is short, but additional drivers and remote I / O power are needed; the distributed distribution of multiple PLCs is suitable for occasions where multiple devices are independently controlled and connected to each other. A small PLC can be selected, but a communication module must be added. ^[5]

Programmable logic controller functional requirements

Generally small (low-end) PLCs have functions such as logic operations, timing, and counting, which can be satisfied for devices that only require switching control. ^[5]

For the system with switch control and a small amount of analog control, an optional low-end PLC with A / D and D / A conversion units, addition and subtraction arithmetic operations, and data transfer functions can be selected. For more complicated control, it is required to realize PID calculation, closed-loop control, communication networking and other functions. Depending on the size and complexity of the control, a mid-range or high-end PLC is selected. However, medium and high-end PLCs are more expensive and are generally used in large-scale process control and distributed control systems. ^[5]

Programmable logic controller response speed

PLC is a general-purpose controller designed for industrial automation. The response speed of different grades of PLC can generally meet the needs in its application range. If you want to use the PLC across ranges, or some functions or signals have special speed requirements, you should carefully consider the response speed of the PLC. You can choose a PLC with high-speed I / O processing functions, or use a fast response module and interrupt input. PLC of the module. ^[5]

Programmable logic controller reliability

For general system PLC reliability can be satisfied. For systems with high reliability requirements, consider whether to use redundant systems or hot standby systems. ^[5]

Programmable logic controller models as uniform as possible

An enterprise should try to unify the PLC models. The following three aspects are considered: ^[5]

1) The model is unified, and its modules can be used as backups for each other, which facilitates the procurement and management of spare parts. ^[5]

2) The model is unified, and its functions and use methods are similar, which is conducive to the training of technical forces and the improvement of technical level. ^[5]

3) The model is unified, the external equipment is universal, the resources can be shared, and it is easy to communicate with the network. It is easy to form a multi-level distributed control system when equipped with a high-level computer. ^[5]

Programmable Logic Controller Application Overview

Programmable logic controller open loop control

Open-loop control of switching value is the most basic control function of PLC. The PLC's instruction system has powerful logic operation capabilities, and it is easy to implement various logic control methods such as timing, counting, sequence (stepping). Most PLCs are used to replace traditional relay contactor control systems. ^[9]

Programmable logic controller analog closed loop

For the analog closed-loop control system, in addition to the digital input and output, there must be analog input and output points in order to sample the input and adjust the output to achieve continuous adjustment of temperature, flow, pressure, displacement, speed and other parameters And control. Current PLCs not only have this function for large and medium-sized machines, but also some small machines. ^[9]

Digital control of programmable logic controller

When the control system has a rotary encoder and a pulse servo device (such as a stepping motor), it can use PLC to realize the function of receiving and outputting high-speed pulses to achieve digital control. The more advanced PLC has also developed a digital control module to achieve the curve The interpolation function has recently introduced a new type of motion unit module, which can also provide a programming language for digital control technology, making PLC's digital control easier. ^[9]

Programmable logic controller data acquisition monitoring

Since PLC is mainly used for field control, collecting field data is a very necessary function. On this basis, connect the PLC to a host computer or touch screen. You can observe the current values of these data and perform statistical analysis in time. Some The PLC has a data recording unit, which can be inserted into the unit with a memory card of a general personal computer to save the collected data. Another feature of PLC is that there are many self-test signals. With this feature, the PLC control system can implement white diagnostic monitoring, reduce system failures, and improve system reliability. ^[9]