1. Introduction Yarn evenness is one of the important indicators for evaluating yarn quality. It not only plays a decisive role in the appearance and intrinsic quality of textiles, but also directly affects the stability of the textile production process. Therefore, the analysis and measurement of yarn evenness is of great significance for the control and improvement of textile quality. The analysis of yarn evenness has been a hot topic in the textile industry since the 1930s. Various indicators describing yarn unevenness have been proposed, including the unevenness coefficient, coefficient of variation, unevenness index, variation length curve, wavelength spectrum, and autocorrelation function. Corresponding testing methods and equipment have also emerged. Applying these methods and equipment to improve yarn quality is a major aspect of yarn evenness practice. The application of stochastic process theory and the establishment of a mathematical model for yarn evenness have important guiding significance for the theory and practice of yarn evenness and the development of testing instruments. A representative work in this area is Rao's paper published in JTI in 1961. Based on Martindale's work, this paper gave a rigorous mathematical definition and corresponding mathematical model of an ideal yarn, and derived the spectral density formula for an ideal yarn. The main methods for testing yarn evenness include length measurement and weighing, visual inspection, and yarn evenness testing. Yarn evenness testing instruments are further divided into two types: capacitive and photoelectric. The capacitive method uses an air capacitor as a detector. Utilizing the fact that the dielectric constant of fiber is greater than that of air, the yarn passes through an electric field between metal plates, causing a change in capacitance. This change is essentially proportional to the amount of fiber in the yarn cross-section. This method is unaffected by the shape of the yarn cross-section; however, uneven moisture content in the yarn itself and, for blended yarns, uneven blending ratios will affect the accuracy of the results. The photoelectric method uses a beam of light to project the yarn onto a photodetector, thereby outputting an electrical signal representing the yarn's thickness. This method is simple and easy to implement and is unaffected by the yarn's own parameters. Its disadvantage is that it can only inspect fine yarns throughout the entire textile process, is greatly affected by external factors, and, especially since the yarn's cross-section is not circular, its accuracy is relatively low. 2. Capacitive Evenness Tester: The capacitive evenness tester (or simply evenness tester) is a standard instrument for evaluating yarn quality. It is used to measure the coefficient of variation and the number of defects in the yarn, serving as a basis for evaluating yarn quality. The evenness tester is also an important means of quality control. Analyzing the spectrograms generated by the evenness tester can help determine the causes of unevenness, providing reliable information for improving yarn quality and reducing production costs. Existing evenness testers mainly include the Swiss Zeilwig-Ust AG models UTⅠ-B, UTⅡ-B, and UTⅢ-B for short fibers, and UTⅠ-C, UTⅡ-C, and UTⅢ-C for filaments. Japanese metrology companies also offer models such as KET80B and KET80C. Domestically produced capacitive evenness testers include models YG130C, YG131, YG133, YG133A, YG131C, YG135M, and YG139. These instruments utilize the principle that as yarn passes through an air capacitor composed of parallel metal plates, the capacitance of the capacitor changes accordingly with the change in the mass of a section of yarn between the plates. The unevenness of yarn fineness is obtained from the change in capacitance. Capacitive yarn evenness testers are widely used in textile enterprises, mainly because they offer significant advantages in process adjustment and product quality control. Using the curves and spectrograms of their software system, yarn quality problems caused by poor equipment condition or process issues can be easily identified and adjusted promptly. However, feedback from users indicates that capacitive yarn evenness testers lack sufficient calculation accuracy and cannot accurately pinpoint the problem, often only identifying a specific piece of equipment rather than a specific component; they are also ineffective in addressing yarn structural unevenness. Furthermore, the correlation between the indicators measured by the capacitive yarn evenness tester and the fabric appearance quality is poor: often the yarn CV value is good while the fabric quality is problematic, or vice versa. Therefore, as a modern measuring instrument, the capacitive yarn evenness tester needs improvement. 3. Photoelectric Yarn Evenness Tester: For textile companies, the traditional method involves winding yarn around a blackboard and visually judging the yarn grade—i.e., the evenness of the yarn on the blackboard. However, the Electronic Inspection Board (EIB) developed by Lawson-Hemphill in the United States uses a CCD camera to continuously scan the yarn diameter, inputting the information into a computer. After processing, it obtains an image of the yarn's appearance, the scanned diameter, and the diameter variation curve. Based on pre-set yarn morphology values ​​or defect sizes, it determines whether the tested yarn contains defects. A defect refers to a specific yarn segment whose diameter is higher or lower than a specified level value. Before testing, parameters such as the level value, segment length, and the number of times the diameter exceeds or falls below the level value must be determined. When the "higher than" option is selected, it means that only yarn diameters scanned by the CCD that exceed the level value are entered into the computer and converted into data for software analysis. The same applies when the "lower than" option is selected. The set micro-segment length value is a binary value, meaning that any value greater than or equal to the set value and less than the next higher length value is counted under this binary value. For example, if 1mm and 5mm are two adjacent length values, and a certain level is chosen, setting the binary value of the yarn micro-segment length to 1, then when the yarn diameter is greater than or equal to that level, the number of times the yarn micro-segment is greater than or equal to 1mm and less than 5mm is counted in this binary value. Traditional manual blackboard yarn winding differs from the EIB yarn winding principle, although the underlying principle is the same. When manually winding yarn on a blackboard, it's often necessary to consider whether to use the front or back side as a sample when judging yarn grade. In EIB testing, both the front and back sides can be inspected simultaneously on the board. EIB also overcomes the drawback of insufficient correlation between capacitive yarn evenness testers and actual fabric quality. However, because it cannot be well applied to process adjustment, EIB's guiding significance for enterprise quality control in actual production is not significant to date. 4. A new type of capacitive and CCD combined evenness tester. The national standard GB/T398-93 for cotton natural yarn stipulates that the evenness of cotton natural yarn can be tested using either blackboard evenness or the coefficient of variation of evenness. Since blackboard evenness is easily affected by human factors, and the capacitive evenness tester can obtain objective test data, it is more concrete and convincing. Therefore, the national standard stipulates that when the two methods of evaluating evenness are inconsistent, the coefficient of variation of evenness shall prevail. However, in production practice, yarn that meets the quality requirements for evenness as measured by the capacitive evenness tester may not necessarily meet the quality requirements for the fabric surface. Production practice shows that the periodic unevenness of the yarn, whether on the blackboard evenness or the fabric surface, has a much greater impact than the increase in the unevenness rate indicates. Practice has proven that even small differences in periodic unevenness can often lead to a downgrade in blackboard evenness and a downgrade in fabric surface quality; therefore, relying solely on the coefficient of variation of capacitive evenness is insufficient. The EIB photoelectric yarn appearance inspection and analysis instrument from Lawson Corporation of the United States overcomes the shortcomings of traditional blackboard yarn evenness testing. Its cotton yarn standards have been approved by the U.S. Department of Agriculture, making it a replacement for traditional blackboard yarn evenness testing. For this reason, we conceived of a capacitive and CCD combined yarn evenness testing instrument. Its structure is shown in Figure 1. The yarn under test is tensioned by two constant devices with speeds V1 and V2, where V2 > V1. The speed signals from both devices are transmitted to a speed-difference tension constant controller. The difference between the calculated tension and the predetermined tension value is fed back to the two constant speed devices via the speed-difference tension constant controller. This ensures that V1 maintains the required yarn transmission speed, while adjusting V2 to maintain the predetermined tension value. The yarn speed and tension data are transmitted to a computer, which also instructs the speed-difference tension constant controller when needed. Two detection devices, a CCD charge-coupled device and a capacitive sensor, are respectively set within the detection area. The two detection devices are isolated from each other to block interference from light and electrical signals. The computer acquires image information from a CCD charge-coupled device via an image acquisition card, presenting it as a two-dimensional digital image. Image recognition and data processing are then performed to obtain data such as yarn diameter, defects, neps, unevenness, and fuzz. Simultaneously, the computer processes information from a capacitance sensor regarding yarn diameter and proportions. Since the air capacitance value (excluding yarn) is known, the yarn composition ratio can be deduced from the measured capacitance data and the yarn diameter measured by the CCD sensor. Furthermore, the impurity content can be inferred from the presence of substances with unexpected dielectric constants. The computer can also simulate the effect of fabric woven from the tested yarn according to a specific pattern. This is essentially a computer-generated display of the weaving effect at a specific warp and weft point, based on a weaving pattern. The combined capacitance and CCD evenness tester overcomes the shortcomings of capacitive evenness testers (poor correlation with actual fabric quality) and photoelectric evenness testers (limited guidance for enterprise quality control and process adjustment), making it more suitable for textile enterprise management and quality control. 5. Conclusion As a novel type of stem uniformity meter, the capacitive-CCD combined detection stem uniformity meter integrates the advantages of both capacitive and photoelectric stem uniformity meters. However, due to space limitations, this paper does not analyze the experimental data obtained using the capacitive-CCD combined detection stem uniformity meter; this will be discussed in future articles.