Plate Flatness Meter Based on Moire Fringes

A plate flatness meter based on the laser moire method was developed. A large grating is installed above a plate transfer line, a laser beam is directed through a concave lens onto the plate on the transfer line. Moire fringes that conform to the surface profile of the plate are formed on the plate. The flatness of the plate as i t is transferred can be measured to a high accuracy of +l. ODD( 2 0 1. 2. B A C K G R O U N D In recent years, increasingly severe flatness requirements have been demanded of plates in view of elimination of leveling work for cost reduction and of introduction of automatic welding machines at the end users. For example, some bridge ggirder plates must meet wave height requirements of 2 mm or less over the entire length of 1500 mm. No flatness meters are commercially available that can measure the flatness of plates to such a high degree of accuracy. To determine their flatness to such close tolerances, plates must be examined by inspectors with straightedges. Under this method. the inspector stops the plate and measures the entire surface of the plate with straightedge, expending much time and labor. In view of the available production capacity, there is a limit to the quality assurance of plates by this inspection method. Given the urgent need for high-accuracy in-line flatness meter. We tackled the development of a plate flatness meter based on the laser moire method . 3. P R I N C I P L E 3. 1 Principle of moire topography The principle of moire topography is illustrated in Fig.1. When the object to be measured is viewed from the observation point E,the surface of the object appears bright where radial rays in a group with S as the center (transmitted light) intersect with radial rays in another group with E as the center(ref1ected light). As evident from Fig. 1 , these intersections are formed as equal distances from the grating surface. Since the grating pitch P is small in reality,the intersections appear continuous laterally and look bright in the sections Z = ZI, 22. ... in the object and look dark in the intermediat portions. Namely, contourlines are obtained. N in Fig. 1 is referred to as the fringe order of the sequence of the light section in question from the grating surface. The depth Zn for the fringe order N = n and the contour line resolution A Z n are given by F i g . I . P r i n c i p l e o f m o i r e t o p o g r a p h y . In moire topography, contour lines are revealed on the surface of the object to be measured, so that the relative elevation of the contour lines. that is, the surface irregularities of the object, are not known. Various methods are thus devised for knowing the surface irregularities of the object. The plate flatness meter has the method of uniquely assigning the fringe orders ( relative frings orders, to be more exact),regardless of the surface irregularity judgment involved, by setting the relative angle between the transfer roll plane and the light sections larger than the maximum angle of inclination of plate shape. This method is simply called the incline moire method hereinafter. 3.2 Configuration of plate flatness meted system The configuration of the plate flatness meter system in shown in Fig. 2. To provide distance( lift-off) of at least 500 mm between the grating surface and transfer roll surface for collision as the source of light. The laser beam is directed onto the large grating through a beam expander and concave lens. The main specifications of the optical system are given in Table 1. An original image of moire fringes formed on a 2,400 mm wide plate with surface waves of approximately 2 mm height is shown in Fig. 3. The image is composed of 512 x 512 x 8 bits. avoidance, a highly directional laser was employed I I K . i. O r ~ # ~ n , i I ~ m ~ i x ~ ~ ( 1 1 m 1 1 1 r 1 t r ~ n ~ ~ s