**Abstract**: The principle of online detection of paperboard thickness using the displacement differential method based on laser displacement sensors and eddy current displacement sensors, as well as the software and hardware design of the measurement system, are introduced. This measurement principle completely eliminates the influence of radial runout of the papermaking machine's reel drum on the measurement results. By calibrating each sensor and using the least squares method for linear fitting of calibration data, the measurement accuracy of the system is further improved.
**Keywords**: Paperboard thickness; Laser displacement sensor; Eddy current displacement sensor
**0 Introduction**
The measurement of paper thickness falls under the category of thickness measurement for thin non-metallic materials. In the thickness measurement of paper in high-humidity environments, contact methods are prone to scratching the surface being measured and result in larger measurement errors. Among non-contact methods, such as capacitance, infrared, and radiation techniques, instability in the detection values occurs due to the influence of factors like paper moisture content and density, which fails to meet the precision requirements for production quality control. Since both laser displacement sensors and eddy current displacement sensors offer non-contact measurement with high precision and stability, and are not affected by factors such as the humidity and density of the material being measured, the displacement differential principle based on these two types of sensors has been adopted to achieve high-precision online measurement of paper thickness in high-humidity conditions. A secondary instrument centered on the MCS-51 microcontroller completes functions such as data acquisition, scale transformation, thickness measurement calculations, display and storage of measurement results, communication, and printing, fully satisfying the requirements for quality control and management in actual production.