Programmable Logic Controller (PLC), a digital operation controller with a microprocessor for automatic control, can load control instructions into memory at any time for storage and execution. A programmable controller is composed of functional units such as a CPU, instruction and data memory, input/output interfaces, power supply, and digital-to-analog conversion. Early programmable logic controllers only had the function of logic control, so they were named programmable logic controllers. Later, with continuous development, these initially simple computer modules now have various functions including logic control, timing control, analog control, and multi-machine communication, and their names have been changed to programmable controllers

Programmable logic controllers used in industry are already equivalent to or close to the mainframe of a compact computer. Their advantages in scalability and reliability have made them widely applied in various industrial control fields. Whether in computer direct control systems, centralized and distributed control systems (DCS), or fieldbus control systems (FCS), various types of PLC controllers are always widely used



The programmable logic controller is essentially a computer specifically used for industrial control. Its hardware structure is basically the same as that of a microcomputer. The basic composition is shown in Figure 1, and the detailed description of the basic composition is as follows:



Power supply

The power supply is used to convert alternating current into the direct current required inside the PLC. Most PLCS are powered by switch-type regulated power supplies.

Central processing unit

The central processing unit (CPU) is the control center of a PLC and also its core component. Its performance determines the performance of the PLC.

The central processing unit is composed of a controller, an arithmetic unit and registers. All these circuits are concentrated on a single chip and are connected to the input/output interface circuits of the memory through the address bus and control bus. The function of the central processing unit is to handle and run user programs, perform logical and mathematical operations, and control the entire system to make it coordinated.

Memory

Memory is a semiconductor circuit with memory function. Its role is to store system programs, user programs, logical variables and some other information. Among them, the system program is the program that controls the PLC to achieve various functions, which is written by the PLC manufacturer and solidified in the read-only memory (ROM), and cannot be accessed by users.

Input unit

The input unit is the input interface that connects the PLC to the controlled device, serving as a bridge for signals to enter the PLC. Its function is to receive signals sent by the master command components and detection components. The input types include DC input, AC input, and AC-DC input.

Output unit

The output unit is also a connecting component between the PLC and the controlled device. Its function is to transmit the output signals of the PLC to the controlled device, that is, to convert the weak current signals sent by the central processing unit into level signals to drive the actuating elements of the controlled device. The types of output include relay output, transistor output and gate output.

In addition to the above-mentioned components, PLC also has various external devices depending on the model. Their functions are to assist in programming, achieve monitoring and network communication. Common external devices include programmers, printers, cassette tape recorders, computers, etc



PLCS can be classified into three categories:

1. Integrated PLC

Integrated PLC integrates power supply, CPU, input/output interfaces and other components in a single chassis, featuring a compact structure, small size and low cost.

2. Modular PLC

Modular PLC divides each component of the PLC into several separate modules, such as the CPU module, input/output module, power supply module (some are contained in the CPU module), and various functional modules.

3. Stackable PLC

Combining the features of the integral PLC and the modular PLC constitutes what is called a stacked PLC. The CPU, power supply, input/output interfaces, etc. of the stackable PLC are also independent modules, but they are connected by cables, and each module can be stacked layer by layer. In this way, the system can not only be flexibly configured but also be made compact in size




Functional characteristics


1. High reliability. Since most PLCS adopt single-chip microcomputers, they have a high degree of integration. Coupled with the corresponding protection circuits and self-diagnostic functions, the reliability of the system is improved.

2. Programming is easy. The programming of PLCS mostly adopts relay control ladder diagrams and command statements, which are much fewer than the instructions of microcomputers. Except for medium and high-end PLCS, the general small PLCS only have about 16. Because the ladder diagram is vivid and simple, it is easy to master and convenient to use. Even without professional computer knowledge, programming can be carried out.

3. Flexible configuration. Because PLC adopts a modular structure, users only need to simply combine it to flexibly change the functions and scale of the control system. Therefore, it can be applied to any control system.

4. The input/output function modules are complete. One of the greatest advantages of PLC is that it has corresponding templates for different field signals (such as DC or AC, switch quantity, digital or analog quantity, voltage or current, etc.), which can be directly connected to industrial field devices (such as buttons, switches, sensor current transmitters, motor starters or control valves, etc.), and connected to the CPU motherboard through a bus.

5. Easy to install. Compared with computer systems, the installation of PLCS neither requires a dedicated computer room nor strict shielding measures. When in use, simply connect the detection device correctly to the actuator and the I/O interface terminals of the PLC, and it can work normally.

6. Fast running speed. Since the control of PLC is carried out by program control, both its reliability and operating speed are incomparable to those of relay logic control.







There are a wide variety of PLC products. The models of PLCS vary, corresponding to differences in their structural forms, performance, capacity, instruction systems, programming methods, prices, etc., and the applicable occasions also have different focuses. Therefore, the rational selection of PLC is of great significance for improving the technical and economic indicators of the PLC control system.

1.PLC model

The selection of a PLC should mainly be comprehensively considered from aspects such as its model, capacity, I/O module, power supply module, special function module, and communication networking capability. The basic principle for choosing a PLC model is to strive for the best performance-to-price ratio under the premise of meeting functional requirements and ensuring reliability and convenient maintenance. When making a selection, the main considerations should be the reasonable structural type, the choice of installation method, the corresponding functional requirements, the response speed requirements, the system reliability requirements, and the possibility of uniformity of the model as much as possible.

2. Structural type

PLC mainly has two structural types: integral type and modular type.

The average price of each I/O point of the integral PLC is lower than that of the modular type, and its volume is relatively small. It is generally used in small control systems where the system process is relatively fixed. The functional expansion of modular PLCS is flexible and convenient. They offer a wide range of choices in terms of the number of I/O points, the ratio of input points to output points, and the types of I/O modules. Moreover, they are easy to maintain and are generally used in more complex control systems.

3. Installation method

The installation methods of PLC systems are divided into centralized type, remote I/O type and distributed type with multiple PLCS networked.

Centralized systems do not require the setting up of remote I/O hardware for drivers, and the system has a fast response and low cost. Remote I/O type is suitable for large-scale systems. The distribution range of the system's devices is very wide. Remote I/O can be installed separately near the on-site devices. The connection lines are short, but additional drivers and remote I/O power supplies need to be added. The distributed system where multiple PLCS are networked is suitable for situations where each device is independently controlled but also needs to be interconnected. Small PLCS can be selected, but an additional communication module must be added.

4. Functional Requirements

Generally, small (low-end) PLCS have functions such as logical operations, timing, and counting, which can meet the requirements of devices that only require switch control.

For systems that mainly rely on switch control with A small amount of analog control, enhanced low-end PLCS that can be equipped with A/D and D/A conversion units and have the functions of addition and subtraction arithmetic operations and data transmission can be selected. For control that is more complex and requires functions such as PID operation, closed-loop control, and communication networking, medium or high-end PLCS can be selected based on the scale and complexity of the control. However, medium and high-end PLCS are relatively expensive and are generally used in large-scale process control and distributed control systems, etc.

5. Response speed

PLC is a general-purpose controller designed for industrial automation. The response speed of PLCS of different grades generally meets the needs within their application scope. If a PLC is to be used across different scopes, or if certain functions or signals have special speed requirements, the response speed of the PLC should be carefully considered. A PLC with high-speed I/O processing capabilities can be selected, or a PLC with fast response modules and interrupt input modules can be chosen, etc.

6. Reliability

The reliability of the PLC in general systems can all be met. For systems with high reliability requirements, it should be considered whether to adopt redundant systems or hot standby systems.

7. Try to keep the models as uniform as possible

An enterprise should strive to achieve uniformity in the models of PLCS. The following three aspects are mainly considered:

1) The models are uniform, and their modules can be used as backups for each other, facilitating the procurement and management of spare parts.

2) The models are uniform, and their functions and usage methods are similar, which is conducive to the training of technical forces and the improvement of technical levels.

3) The models are uniform, their external devices are universal, resources can be shared, and it is easy to communicate through networking. After being equipped with an upper computer, it is easy to form a multi-level distributed control system

If you need information about our products, please contact us at info@arcsecobot.com