Recent Development for Surface Distortion Measurement

Small distortions on the surface of sheet metal stamping parts may have significant impact on the optical appearance of the final product after finish treatment. Recently, there are considerable research activities in the development of optical methods for surface distortion measurement because of the fullfield and nondestructive nature of these methods. Optical methods such as Lights and Mirrors, Defracto, 3D-Digitazition for contour measurement, 3D-Digitazition for surface distortion measurement etc. are emerging as strong candidates for industrial inspections of surface distortion. Because each technique has always its unique advantages and disadvantage for different applications, the selection of one technique depends on the particular objective as well as limitations of each technique. This paper will give a through review of the above optical methods for surface distortion measurement. In particular, the recent development “3D-Digitazition for surface distortion measurement” will be discussed in details. The fundamental and methodology are demonstrated by example of surface distortion measurement on door panel of a car body. Introduction: The demands for greater quality on the surface sheet metal stamping parts has created a need for better techniques of nondestructive and not-contacting surface inspection. Nowadays, different technologies have been used for body surface inspection. They can be summarized as follows: Lights and mirrors, “D Sight” from Defracto and 3D digitization for measuring surface shape. This method of Lights and Mirror has been widely used to inspect surface distortion by observing (through human eyes) the reflected light from mirror-like surface. If the surface to be inspected is not smooth enough, a thin oil film should be spread on the surface so that the applied light can be reflected from the surface. The measuring sensitive of this technique greatly depends on the observation direction, the bigger the angle is, the higher the sensitivity. The advantage of this method is its simplification. It is able to find a small surface distortion, such as a dent or a surface low. However, it is a qualitative method and impossible to indicate the depth of the lows, a success entirely depends on observing and illuminating direction and the experiences of the operator. Moreover, the method is suited only for a mirror-like surface, for the surfaces like aluminum and sheet-metal material, a surface pre-processing (e.g. applying a oil on the surface) are required. The technique of Defracto technique has also been used for detecting a surface distortion by automotive industries. Although the measuring sensitivity of this technique is very high (it has a capability to find a surface distortion less than 25 microns in depth), the results obtained depend greatly on the experiences of the operator. Similar to Light and mirror method, the measuring sensitive greatly depends on the observing and illuminating direction, and it is a qualitative method and suited only for a mirror-like surface Nowadays, the demands for greater quality on the surface sheet metal stamping parts has created a need for a technique of quantitative measurement of surface distortion. The mission of the measurement is not only to find the surface distortion on sheet metal stamping parts, but also to improve the manufacturing quality, e.g. to improve the quality of the die-sets. 3D digitization for measuring surface shape is a relatively new technique to deliver surface geometry quantitatively and has been applied for contour and 3D-shape measurement. In this paper, we have developed this technique for surface distortions measurement so that the surface low, highs and dents can be evaluated quantitatively. Fundamental of 3D Digitization: 3D digitization is a technique for 3D shape (contour) measurement based on the optical triangulation. Fig. 1 shows the fundamental of spot of light triangulation. A relationship between a displacement ∆x (usually called distortion) and the surface depth change ∆z can be derived as following: ∆x = M • ∆a (1) Optical light