History, Basic Structure & Classification of Programmable Logic Controller (PLC)

Introduction

Programmable Logic Controller (PLC) is a digital computing controller with a microprocessor for automatic control. It can load control instructions into memory at any time for storage and execution. The programmable controller consists of CPU, instruction and data memory, input/output interface, power supply, digital analog conversion and other functional units. The early programmable logic controller only had the function of logic control, so it was named programmable logic controller. Later, with the continuous development, these computer modules with simple functions at the beginning have various functions including logic control, timing control, analog control, multi-machine communication, etc., and the name was changed to programmable controller (PLC). However, because its abbreviation PC conflicts with the abbreviation of personal computer (Personal Computer), and because of habit, people still often use the name of programmable logic controller and still use the abbreviation PLC.

The programmable logic controller used in industry is 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. Whether in a computer direct control system, a centralized distributed control system DCS, or a field bus control system FCS, there are always a large number of PLC controllers. There are many PLC manufacturers, such as Siemens, Schneider, Mitsubishi, Delta, etc. Almost all manufacturers involved in the field of industrial automation will have their PLC products.

Development History

Origin

The development of production technology requirements of the American automobile industry has promoted the emergence of PLC. In the 1960s, when General Motors adjusted the factory production line, it was found that the relay and contactor control systems were difficult to modify, large in size, noisy, inconvenient to maintain, and poor in reliability, so it proposed the famous “General Ten” bidding indicators. 3
In 1969, the American Digital Equipment Company developed the first programmable controller (PDP-14), which was tested on the production line of General Motors and achieved remarkable results; 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 industrial applications.
The original purpose was to replace mechanical switching devices (relay modules). However, since 1968, the functions of PLCs have gradually replaced relay control boards, and modern PLCs have more functions. Its use extends from single process control to the control and monitoring of the entire manufacturing system.

Development

The microprocessor appeared in the early 1970s. People quickly introduced it into programmable logic controllers, which added functions such as calculation, data transmission and processing to the programmable logic controllers, completing industrial control devices with real computer characteristics. At this time, the programmable logic controller is a product of the combination of microcomputer technology and conventional control concepts of relays. After the development of personal computers, in order to facilitate and reflect the functional characteristics of programmable logic controllers, programmable logic controllers were named Programmable Logic Controller (PLC).
In the mid-to-late 1970s, programmable logic controllers entered the practical development stage, and computer technology was fully introduced into programmable controllers, making their functions leap forward. Higher computing speed, ultra-small size, more reliable industrial anti-interference design, analog quantity calculation, PID function and extremely high cost performance have established its position in modern industry.
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 programmable controllers have entered a mature stage.
From the 1980s to the mid-1990s, programmable logic controllers developed fastest, with an annual growth rate of 30-40%. During this period, the PLC’s ability to process analog quantities, digital computing capabilities, human-machine interface capabilities, and network capabilities were greatly improved. Programmable logic controllers gradually entered the field of process control and replaced the DCS system that dominated the field of process control in some applications.
In the late 20th century, the development of programmable logic controllers was characterized by being more adapted to the needs of modern industry. During this period, large 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.

Basic structure

The programmable logic controller is essentially a computer dedicated to industrial control. Its hardware structure is basically the same as that of a microcomputer. The basic composition is described in detail as follows:

Power supply

The power supply is used to convert AC power into DC power required by the PLC. Most PLCs are powered by switching regulated power supplies.

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.
The central processing unit consists of a controller, an arithmetic unit, and registers. These circuits are concentrated on a chip and connected to the input/output interface circuit of the memory through the address bus and the control bus. The function 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

Memory

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

Input unit

The input unit is the input interface between the PLC and the controlled device. It is the bridge for signals to enter the PLC. Its function is to receive signals from the main command element and the detection element. The input types include DC input, AC input, and AC/DC input. 6

Output unit

The output unit is also the connecting component between the PLC and the controlled device. Its function is to transmit the output signal of the PLC to the controlled device, that is, to convert the weak current signal sent by the central processor into a level signal to drive the actuator of the controlled device. The output types include relay output, transistor output, and thyristor output. 6
In addition to the above parts, PLC also has a variety of external devices depending on the model. Its function is to help programming, realize monitoring and network communication. Commonly used external devices include programmers, printers, cassette recorders, computers, etc. 6

Classification of mechanical architectures

PLC (Programmable Logic Controller) mechanical architectures can generally be classified into four main types based on their physical structure and design. These are:

1. Compact (Fixed) Architecture

• Description: All components of the PLC (CPU, power supply, and I/O) are housed in a single, fixed enclosure.
• Key Features:
  • Limited expandability due to the fixed number of I/O points.
  • All-in-one design makes it compact and portable.
• Applications: Ideal for small-scale or standalone applications, such as packaging machines, conveyor systems, or HVAC systems.
• Advantages:
  • Cost-effective for smaller systems.
  • Simple to install and configure.
• Disadvantages:
  • Limited scalability and flexibility.
  • Replacement of individual components may require replacing the entire unit.

2. Modular Architecture

• Description: The PLC consists of separate modules (CPU, I/O, power supply, communication) that are connected via a shared backplane or individual wiring.
• Key Features:
  • Allows users to customize the system by selecting only the required modules.
  • Can be rack-mounted or designed as a standalone system with interconnected modules.
• Applications: Medium to large-scale automation projects where customization and flexibility are important.
• Advantages:
  • Flexible and customizable design.
  • Easier to upgrade by replacing individual modules.
• Disadvantages:
  • Requires careful planning for compatibility and integration of modules.

3. Stackable or Rack-Mounted Architecture

• Description: Stackable PLCs combine the features of both integral and modular PLCs. The CPU, power supply, I/O interfaces, and other components are separate modules that connect via cables and can be stacked on top of each other. This structure allows for flexible configuration and can be made compact in size.
• Key Features:
  • Modules are plugged into a shared rack or chassis with a common bus system.
  • The rack provides both power and communication pathways between modules.
  • Highly scalable, as additional modules can be added to the rack for expanded functionality.
• Applications: Suitable for large-scale systems with complex I/O requirements, such as manufacturing plants or process industries.
• Advantages:
  • Easy to expand and maintain.
  • Good for systems requiring a large number of I/O points.
• Disadvantages:
  • Larger physical footprint.
  • Higher cost compared to other architectures.

4. Distributed Architecture (Decentralized PLCs)

• Description: The system uses multiple PLCs or I/O modules distributed across a network, communicating with each other and with a central controller.
• Key Features:
  • Components are located close to the field devices to reduce wiring complexity.
  • Communication is typically achieved through fieldbus protocols like Ethernet/IP, Profibus, or Modbus.
• Applications: Large-scale systems with geographically dispersed components, such as oil refineries, water treatment plants, and large production lines.
• Advantages:
  • Reduces wiring complexity and costs.
  • Allows better control over geographically spread systems.
• Disadvantages:
  • Requires robust communication infrastructure.
  • Higher complexity in system design and troubleshooting.

Each type of PLC mechanical architecture serves specific needs based on the scale, complexity, and flexibility required for the application. While Rack-Mounted and Modular architectures are favored for scalability and adaptability, Compact systems are ideal for small-scale projects. Distributed Architecture excels in managing complex systems with dispersed locations.

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