1 Development of Programmable Logic Controllers (PLC)
In 1968, General Motors (GM) in the United States proposed the need to replace relay control devices. In the manufacturing industry, characterized by changes in geometric shapes and mechanical properties, and in the process industry, where raw materials are transformed into products through physical and chemical changes, there was a significant demand for feedback control primarily based on discrete signals, especially in manufacturing. This included a large amount of sequential control based on logical conditions, which performed sequential actions according to timing: in addition, there were control actions for chain protection based on logical relationships that were unrelated to sequence or timing; and a large number of state variables primarily based on discrete data collection and monitoring, such as limit alarms for various signals, pulses, timers, counters, and analog signals. Due to these control and monitoring requirements, PLCs evolved to replace relays and primarily perform sequential control. The following year, Digital Equipment Corporation developed the first generation of programmable logic controllers, which met the requirements of GM’s assembly line. With the development of integrated circuit technology and computer technology, the fifth generation of PLC products is now available.
A Programmable Logic Controller (PLC) is a new generation of industrial control devices, a product of the combination of automatic control, computer, and communication technology, specifically designed for controlling industrial production processes. Due to the complexity of the control objects, the special nature of the operating environment, and the long-term continuity of operation, PLCs have distinct design characteristics: high reliability, wide adaptability, communication capabilities, easy programming, and modular structure. Given the importance of coal transportation systems in power plants, PLC control is adopted to develop towards unmanned monitoring and management, aiming to reduce personnel and increase efficiency while improving management levels.
2 Characteristics and Components of PLCs
A Programmable Logic Controller (PLC) is a digital operation electronic system designed for application in industrial environments. It uses programmable memory to store instructions for executing 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. It is an industrial control computer that changes control functions through programming, with application fields including digital logic control, motion control, closed-loop process control, and data processing. Its characteristics include:
(1) High reliability and strong anti-interference capability;
(2) Variable control programs and simple programming;
(3) Comprehensive functions, including mathematical operations, storage, and digital and analog input/output;
(4) Compact size, all standard modules, and arbitrary expansion combinations.
The main components of a Programmable Logic Controller include four parts:
(1) Central Processing Unit (CPU). The CPU consists of an arithmetic unit, a controller, and registers, connected to memory and I/O interface circuits via address, data, and control buses.
(2) Memory. The memory is a semiconductor circuit with memory functions, used to store system programs and user programs. It is mainly divided into Read-Only Memory (ROM) and Random Access Memory (RAM).
(3) Input/Output Interface. The I/O interface is the connection circuit between the PLC host and external devices. To improve anti-interference capability, the input and output interfaces generally have optical isolation circuits, which consist of light-emitting diodes and phototransistors. Input signals from the field, such as push buttons, limit switches, and sensor outputs (switching signals and analog signals like pressure, flow, temperature, voltage, and current), must be sent to the PLC through input modules, converting them into digital signals that the CPU can accept and process. The output module receives the digital signals processed by the CPU and converts them into control signals that can be accepted by field actuators, such as solenoids, indicator lights, and motors.
(4) Power Supply. The PLC’s power supply includes the system power and backup. PLCs generally use a 220V AC power supply, and the power supply module converts the external AC input into the DC power required by the PLC’s CPU, memory, and I/O interfaces through rectification, filtering, and voltage regulation circuits.
3 Functions and Working Principles of PLCs
The main functions of a Programmable Logic Controller include:
(1) Logic control function. The PLC uses AND, OR, and NOT instructions to replace relay contacts in series, parallel, and other logical connections for switch control.
(2) Timing/Counting control function. The PLC uses timers and counters to control operations that can only proceed to the next step after a certain operation is completed on time.
(3) Data processing function. The PLC can perform data transmission, comparison, shifting, number system conversion, arithmetic and logical operations, as well as encoding and decoding.
(4) Sequential control function. The PLC uses step instructions to control processes with multiple sequential operations, allowing the next operation only after the previous one is completed.
(5) A/D and D/A conversion functions, completing the conversion between analog and digital signals through A/D and D/A modules.
(6) Motion control function. The PLC can perform single-axis or multi-axis control through high-speed counting modules and position control modules.
(7) Expansion functions. Additional intelligent units can be connected to increase the number of input/output points and enhance the PLC’s control capabilities.
(8) Process control function. The PLC’s PID control module achieves closed-loop control of physical quantities such as temperature, pressure, speed, and flow.
(9) Remote control function. Remote I/O units connect various input/output devices located at a distance to the PLC host for remote control.
(10) Communication networking function. The PLC can connect with other PLCs or a master computer to achieve large-scale system control.
(11) Monitoring function. The PLC can monitor the operational status of the system, issue alarms for abnormal conditions, display information, diagnose faults, and automatically terminate operations.
The working principle of a PLC:
The CPU of the PLC continuously executes user programs to complete control functions in a scanning manner. The so-called scanning working mode means that the CPU reads and executes the program sequentially from the first instruction to the last. During the scanning cycle, the CPU must complete the following tasks:
(1) Input processing phase
At the beginning of each scanning cycle, the current values of the digital input points are read and written into the input image register.
(2) Program execution
During the program execution phase of the scanning cycle, the CPU executes the program starting from the first instruction until the last instruction is completed. Whether in the main program or during the execution of interrupt programs, direct I/O instructions allow direct access to input and output points. If interrupts are used in the program, the interrupt program is stored as part of the program and is executed only when an interrupt occurs.
(3) Processing communication requests
During the information processing phase of the scanning cycle, the CPU processes any information received from the communication port.
(4) Executing self-diagnostic tests of the CPU
During the scanning cycle, the CPU checks its hardware and user memory, as well as the status of all I/O modules.
(5) Output processing phase
At the end of each scanning cycle, the CPU sends the data in the output image register to the digital output points.
(6) Input and output image registers
4 Classification of PLCs
PLC devices come in various forms and functionalities.
(1) By capacity, they can be divided into three types: “small”, “medium”, and “large”.
Small PLCs. These PLCs are smaller in scale, with I/O points generally ranging from 20 to 128. Their main functions include logical operations, counting, shifting, etc., and they use dedicated programmers.
Medium PLCs. Their I/O points typically range from 129 to 512, with memory below 8K, suitable for switch logic control, process parameter detection and adjustment, data processing, A/D and D/A conversion, networking communication, and remote I/O functions.
Large PLCs. Their I/O points exceed 513, with 513 to 896 points classified as large machines and over 896 points as super-large machines. They have advanced functions, in addition to the capabilities of small and medium PLCs, including PID calculations and high-speed counting. They are equipped with CRT displays and standard computer keyboards, similar to industrial control computers, with computing, control, and adjustment functions.
(2) From a structural perspective, PLCs can be divided into three categories based on hardware structure.
Integrated structure: All components of the PLC are housed in a single enclosure, with input/output terminal connections and power input on the upper and lower sides of the chassis, respectively, and corresponding LEDs indicating input/output status.
Modular PLC: Larger medium and some small PLCs use a modular structure with a higher number of input/output points.
Stackable PLC.