Abstract : Based on the requirements of the automatic chemical dosing system in the chemical workshop of a thermal power plant, this paper introduces a practical monitoring system composed of PLC, frequency converter, and upper-level monitoring configuration software, employing a variable-structure humanoid intelligent control algorithm. This system boasts advantages such as high reliability, strong adaptability, and powerful functionality. Keywords : PLC; frequency converter; monitoring system; humanoid intelligent control 1 Introduction The chemical dosing system in the chemical workshop of a thermal power plant consists of two main parts: feedwater ammonia addition and hydrazine addition, and boiler water phosphate coordinating treatment. The purpose of feedwater hydrazine addition is to remove residual oxygen. Adding an appropriate amount of ammonia to the feedwater is to control the pH value within a certain alkaline range (generally 8.8–9.2) to prevent corrosion of copper and iron pipes. Boiler water phosphate coordinating treatment mainly ensures that the pH value, conductivity, and phosphate content of the boiler water are within acceptable ranges, preventing boiler tube wall corrosion and scaling, improving heat transfer efficiency, and preventing boiler tube rupture due to uneven heat transfer. With the increasing demands for production automation and efficiency, the existing manual dosing systems in the chemical workshops of thermal power plants urgently need upgrading due to their long processing time, heavy workload, reliance on human error, and difficulty in achieving continuous control. Automatic dosing monitoring systems based on computer and automation technologies can solve these problems. 2. Composition of the Automatic Dosing Monitoring System As shown in Figure 1, the automatic dosing monitoring system consists of a host computer, measuring instruments, a PLC (Programmable Logic Controller), a frequency converter, and a manual/automatic switching switch. Each local unit receives analog and switching signals from the field instruments, samples them through the input module, and converts them into digital signals for the Mitsubishi FX2N series PLC. The PLC receives digital signals and executes corresponding control programs based on the status of control switches. Simultaneously, it communicates directly with the Mitsubishi FR-A500 series frequency converter via RS-485 bus, outputting data to the actuators for automatic dosing and dispensing. Furthermore, the PLC communicates with a host computer. The host computer uses KingSCADA, a general-purpose professional configuration software, to write system software, enabling real-time monitoring of water and steam quality. It manages the monitoring results and controls the frequency converter via the PLC, thereby controlling the dosing pump to add chemicals according to actual operating conditions, effectively regulating water quality and maintaining the boiler under normal operating conditions. The local PLC can also be connected to an embedded touchscreen display terminal. By installing the system software written in the configuration software into this terminal, the system can be conveniently controlled locally. The host computer can also be connected to the power plant's management information system, allowing users to browse relevant information about the dosing system via the network. [b][align=center]For more details, please click: Development and Application of Automatic Dosing Monitoring System in Thermal Power Plants[/align][/b]