Abstract: In the finishing stage of textiles, humidity directly affects the quality of production and the benefits of enterprises. Microwave is a relatively advanced online detection method currently under research. This paper discusses the mechanism of water on microwave and the principle of microwave moisture measurement, proposes a solution to overcome the shortcomings of traditional drying methods that cannot achieve online measurement, introduces the hardware and software design of the microwave moisture detection system, and focuses on the hardware implementation of data processing and transmission based on ARM7 microprocessor. Key words: Microwave, Moisture Detection, On-line, Textile Introduction With the development of science and technology, people have put forward new and higher requirements for the processing of textile materials. The textile printing and dyeing industry accounts for a significant proportion of my country's national economic income and is one of the country's major industries. However, my country still imports billions of US dollars worth of high-end clothing fabrics annually for processing into export garments. This is mainly due to the backwardness of my country's printing and dyeing technology and equipment, especially the low level of automation and the inability of online detection of process parameters in dyeing and finishing equipment, resulting in low overall product quality, grade, and added value. The grade of textiles largely depends on the level of post-dyeing and finishing treatment. As a key process parameter in dyeing and finishing, online humidity detection has received much attention, as it directly affects the reliability and rationality of the process. Therefore, the state has clearly stated that it is imperative to upgrade and transform traditional industries such as textiles with advanced technology and improve printing and dyeing finishing technology. While my country has a certain foundation in humidity measurement technology research, its application in actual production still lags behind that of advanced countries worldwide. Although mature products are already available abroad, these detection systems and sensors face problems such as high price, poor compatibility, and operational difficulties when applied to production lines in domestic enterprises, hindering their widespread promotion and application in China. Therefore, it is imperative to research and develop online humidity detection systems for fabrics in China. Microwave humidity measurement is currently the most promising method, superior to infrared and radiation methods. The latter can only measure the sample surface or pose radiation hazards, while microwave methods can measure the humidity of the entire sample or perform localized detection, requiring very little microwave radiation that is harmless to humans. Applying microwave detection to online humidity detection systems for fabrics and integrating it into actual production to control equipment has practical significance for ensuring product quality, saving energy, and improving enterprise efficiency. 1. Mechanism of Water's Effect on Microwaves and Frequency Selection Water is a dielectric material. Water molecules have a nonlinear structure, resulting in a constant dipole moment. Therefore, water is a polar substance with strong polarization characteristics. In a microwave electric field, water molecules undergo orientation polarization: on the one hand, they continuously gain energy from the electric field, converting it into potential energy and storing it (measured by metric); on the other hand, due to the inertia of water molecules, the orientation polarization motion lags behind changes in the external electric field, a phenomenon known as relaxation. The change in electric field also causes water molecules to proceed in the opposite direction, that is, water molecules continuously release energy, and the energy release is measured. The former is manifested as a phase shift of the microwave signal, and the latter is manifested as an attenuation of the microwave signal. This characteristic is characterized by the dielectric constant (permeability) of the water molecule, that is: (1) The and in the formula are respectively (2) (3) In the above formula, is the relaxation time. is the dielectric constant of infinite frequency (optical frequency). is the static dielectric constant, corresponding to extremely low frequency [1]. At low or high frequencies, is a constant, and its asymptotic values ​​are and respectively. It is very slow to change near the relaxation frequency. Its imaginary part is very small at low or high frequencies, but the change region becomes larger. Because in the low frequency region, polarization can be fully carried out, and the loss per cycle is proportional to the frequency. In the high frequency region, since polarization cannot be carried out, the loss also disappears with the increase of frequency. In fact, only the polar loss disappears, while in the millimeter wave band, due to the polarization of electrons and atoms, the total loss remains at a very small constant value. At the relaxation frequency, it reaches a maximum value (4). The above formula contains three constants, and. It is produced by electronic polarization and atomic polarization, and is therefore independent of temperature. As the temperature increases, the disorder increases, and it decreases with the temperature. It is inversely proportional to temperature. The relaxation process is related to temperature, physical, chemical and biological properties of the matrix. The relaxation frequency of water is in the microwave band, so water absorbs microwaves thousands of times more than dry solids. This significant difference indicates that the amount of water in the material is the basic factor that determines the dielectric constant of wet materials. This is the physical basis for microwave humidity measurement based on the dielectric constant of the material. In the low frequency band, wet materials have increased attenuation due to ionic conductivity. It has a good correlation with humidity, but a poor correlation with humidity. However, when the frequency reaches above 10 GHz, it is only related to humidity and not to the material [1]. Therefore, this frequency band is selected for microwave humidity measurement. 2 Microwave Moisture Measurement Principle and Steps Microwave is a band of electromagnetic waves between ordinary radio waves (long wave, medium wave, short wave, ultra-short wave) and infrared rays. It is the band with the shortest wavelength and the highest frequency in radio waves. Its frequency range is from 300MHz (wavelength 1m) to 300GHZ (wavelength 0.1mm). According to the different wavelengths, microwave bands can usually be divided into four bands: decimeter wave, centimeter wave, millimeter wave and submillimeter wave. Since the absorption of microwaves by water and dry solids is very different, and microwaves have strong penetrability to objects. The macroscopic effect of water molecules on microwaves is to cause microwave energy to decay. The change in energy obtained by the microwave receiver reflects the amount of water content in the matrix. The principle of microwave moisture measurement is: to use the characteristics of microwaves to detect the humidity of fabrics. Microwaves are emitted by the transmitting antenna. When this wave encounters the fabric, it will be absorbed, causing the microwave power to change. The receiving antenna receives the microwaves passing through the fabric and converts them into electrical signals. After passing through the signal conditioning circuit, the power loss of the microwave is measured. The humidity can be obtained from the relationship curve between humidity and microwave attenuation, thus realizing microwave detection [2]. The microwave transmitting/receiving device is shown in Figure 1. The measurement steps of the microwave transmitting/receiving device are as follows: First, under certain conditions, the relationship curve between the moisture content of the standard sample and the microwave attenuation is measured—the calibration curve. Then, under the same conditions, the microwave attenuation of the sample to be tested is measured, and the corresponding humidity can be found from the calibration curve. The standard sample used to create the calibration curve is called the calibration sample, and its moisture content is given by the classic method for measuring moisture content, the "drying method." Therefore, this humidity measurement method belongs to relative measurement. 3 System Overall Design and Implementation 3.1 System Circuit Design The online humidity detection device adopts a dual-channel detection method, mainly composed of five parts: signal source, measurement section, signal acquisition and processing, digital display, and host computer interface. Its basic block diagram is shown in Figure 2. Figure 2 Humidity Measurement System Structure Block Diagram The signal source uses a point-frequency microwave source with a built-in isolator to prevent microwave reverse transmission. In the data acquisition circuit, the receiving/transmitting antenna uses a low-sidelobe lens antenna with a waveguide coaxial converter, and the signal amplification circuit uses a DC amplifier. The microprocessor uses the S3C44B0X based on the ARM7TDMI core. It includes an 8-channel multi-ADC, 8 address spaces, a 4-way set-associative 8KB cache, and unused cache memory can be used as on-chip SRAM. It supports 5-bit, 6-bit, 7-bit, and 8-bit serial data transmission/reception, a 16-bit watchdog timer, and a multi-master I2C bus based on interrupt operations, fully meeting the system requirements. The ARM7TDMI exchanges data with the host industrial computer via the USBN9603. This design uses a non-multiplexed 8-bit parallel bus mode. Because DMA is not used in this mode, DACK is connected high. The CPU selects the USB controller via CS1 generated by the decoder, and the USBN9603 sends an interrupt request to the CPU via EXINT0. The connection between the ARM7TDMI and the USBN9603 is shown in Figure 3. Figure 3 shows the connection diagram between ARM7TDMI and USBN9603. The measurement process is as follows: The microwave emitted by the signal source passes through a variable attenuator and a coaxial transmission line, and is then split into two paths by the T-type connector: one path passes through isolator 2 and a signal amplification circuit, directly inputting to the microprocessor; the other path is transmitted to the transmitting antenna through a waveguide converter, then through the fabric, and received by the receiving antenna. The received signal is then input to the microprocessor through isolator 3 and a signal amplification circuit. Finally, after processing, the two signals are used to calculate the humidity using a characteristic curve, which is then sent to the display circuit for display, completing the data exchange with the host industrial control computer and achieving the purpose of humidity measurement. 3.2 Software Design In this system, the software adopts a modular structure design and is written in C language. The entire program mainly includes subroutine modules such as the main program, A/D conversion, data acquisition and processing, table lookup, display, and USB interface control. The overall design scheme of the system program is: using the S3C44B0X initialization subroutine as the main program, calling other subroutines to process the detected data, and storing and displaying the results. The flowchart of the system main program is shown in Figure 4. Figure 4 shows the main program flowchart of the system. In this system, the host computer mainly completes real-time communication and database access functions with the microprocessor of the humidity measurement system through the USB interface. After the system starts, the operator inputs the set humidity range into the system through the human-machine interface of the host industrial control computer. The system compares the humidity measured by the humidity measurement system with the set value. If it exceeds the set range, the system outputs a control command, connects the corresponding device, and issues an alarm. The USB interface control program includes several parts: USB read/write, interrupt initialization, USB initialization, and interrupt service routine. Read/write operations on the USB controller mainly involve setting the internal address register to specify the starting address of the data to be read/written; interrupt initialization mainly involves initializing the interrupt controller and interrupt mode, and specifying the interrupt input pin; USB initialization requires using the USB read/write function to set the control register inside the USB controller; the interrupt service routine handles the interrupts generated by the USB controller, reads the data from the USB internal FIFO, and establishes the correct event flag to notify the main loop program to process it. The flowchart is shown in Figure 5. Figure 5. Flowchart of USB Interface Control Program 4. Conclusion This paper mainly introduces an online fabric humidity detection system, suitable for dyeing and finishing equipment such as dyeing machines, pad dyeing machines, and drying rooms. The system employs an algorithm that performs three measurements on a calibration point and then takes the arithmetic mean of the three measurements to eliminate mismatch errors. Furthermore, a special measurement method and dual-channel hardware circuit design enable the system to achieve high measurement accuracy and reliability. The analysis and research of the microwave measurement principle and the design of the entire measurement system have been completed, and the measurement method has been experimentally verified using existing laboratory resources. The research results show that the principle of the system is feasible and reasonable, and it is of great significance for improving the online detection technology level of process parameters in dyeing and finishing equipment. References [1] Li Ying. Microwave and millimeter wave sensors and non-electrical quantity detection. Electronic Industry Press, 1991 [2] Chen Liqiu. Automation of dyeing and finishing industry. China Textile Press, 2005 [3] Sun Shiping, Mao Zongyuan. Research on microwave test system for sand and gravel moisture content based on lock-in amplification. China Instrument and Meter, 2006.9 [4] Wang Shuping et al. Design and research of a microwave humidity measurement experiment. Physical Experiment, 1997.2 [5] Zuo Chunying, Ding Yanmei. Microwave humidity measurement principle and its application. Journal of Microwave, 2005.4