A method for measuring the oil film thickness on metal sheet before stamping based on infrared absorption and correlation filtering

Stamping and forming is a manufacturing process widely used in the production of automobile body, a thin oil film needs to be coated on metal sheet before stamping to prevent surface from damage and cracking. The thickness of the oil film is a key factor affecting product quality, so high-precision measurement of oil film thickness on the metal sheet is a technical challenge that needs to be solved urgently. To address this issue, a method based on infrared absorption and correlation filtering was proposed to measure the thickness of the oil film on metal sheet before stamping. And a reflection–absorption measurement system using dual-source and common optical path was designed. The correlation filtering algorithm is constructed by using the ratio of the signal which the infrared light is normal absorbed and complete absorbed, which fundamentally eliminates errors caused by the system and measurement environment and avoids the introduction of additional errors from separately measuring reference signals. The system calibration is completed by nonlinear least squares fitting, the coefficients of determination (R²) of the fitted model is 0.9908. Static measurement experiments were conducted to analyze the measurement accuracy and repeatability, the RMSE of the measurement results is 0.0776 g/m². The results demonstrated that the proposed measurement method can provide an effective means for high-precision measurement of oil film thickness on metal sheet. And the designed measurement system with good repeatability and has the condition for application in automotive body stamping production sites.