Due to the continuity of production, the color coated plate production unit needs to timely reflect the changes of process parameters in the production process and monitor the coating thickness of the color coated plate. Once any deviation is found, it shall be adjusted immediately to ensure that the production process is stable and controlled, so as to reduce the manufacturing cost, reduce the probability of defective or waste products and comprehensively improve the product quality.In theory, the following three methods can be used for on-line detection of coating film thickness of color coated plate: first, the infrared spectrum absorption method is used to detect the coating film thickness. Its detection principle is to quantitatively detect the infrared spectrum according to the characteristic element functional groups of the organic coating, but the infrared characteristic functional groups of the organic coatings provided by different color and different coating suppliers are different, If you want to accurately detect the coating thickness, each paint supplier needs to establish a detection model for each color of paint; For users, this means that at least 500 infrared detection working models need to be established, and the working models need to be inspected and maintained regularly. Therefore, the on-line detection of coating film thickness by infrared spectral absorption method has no practical application value. Second, the laser thickness measurement on-line detection system is adopted. The principle of laser thickness measurement is to drill a small hole with a specific shape with a diameter of about 0.5mm on the color coated plate by controlling the laser light source, then measure the length of the small hole slope with a microscope, and then calculate the coating thickness through a mathematical model. This method is a destructive test, which destroys the integrity of the coating, The corrosion resistance of the color coated plate decreases during use. Third, the eddy current on-line detection system is adopted. This method does not need to make hundreds of detection models or punch holes. It is a relatively effective on-line detection system for coating thickness of color coated plate; However, the detection system is contact detection, and the film thickness detection can be implemented only when the unit is shut down. Therefore, the application of eddy current on-line detection system is also limited and can not implement on-line real-time detection.