Estimation of deformation for a glass sheet on a noncontact air conveyor with film pressure feedback

Abstract

Recently, air film conveyors equipped with porous pads have been developed to transport large, thin glass substrates in a contactless state. The deformation of the glass substrates should be monitored for the inspection process on the production line. Although laser sensors can be applied to detect the deformation along a given path, there are some problems such as poor efficiency and low real-time performance. For the purpose of estimating the deformation of the floating glass sheet indirectly, in this paper a film pressure feedback method is proposed and studied via theoretical analysis and experimental verifications. First, a theoretical model including the characteristics of the porous media and gap flow is established, and the model is solved by the finite volume method to obtain the film pressure distribution. Then, an experimental apparatus is built to measure the film pressure with different degrees of deformation of the glass sheet. Comparisons of the calculated film pressure with the measured data indicate that deviations gradually appear for the outer units as the deformation degree enlarges. Thus, we use four inner pressure points as the feedback and apply a proportional–integral algorithm to adjust the deformation shape in terms of a quadratic curve. Furthermore, we verify the method via a deformation measurement experiment. We find that the estimation method can afford an accuracy of 3% in cases where the glass sheet is symmetrically distributed, or asymmetrically distributed but with an asymmetry coefficient of less than 0.2. Finally, we find that the realistic boundary conditions, i.e. the film pressure in the groove and the edge deflection, greatly affect the estimation results, and an allowable measurement error to ensure adequate accuracy of the estimation is provided.