GB897740A - A method and device for determining the dimensions of an object - Google Patents

Abstract

897,740. Measuring thickness photo-electrically. ZEISS-STIFTUNG, CARL, [trading as ZEISS, CARL [firm of]]. Oct. 29, 1958 [Oct. 31, 1957; Sept. 10, 1958], No. 34676/58. Class 40(3). The thickness of an object is determined by photo-electric detection of the movement of the image of a mark projected first on to the surface of a gauge of known thickness and then on to the object. In the arrangement of Fig. 1 the mark is a slit in a diaphragm 3 projected on to the gauge or object and reflected through lenses 7, 8 to the junction of photo-cells 9, 10 the output of which feeds a differential amplifier 14. Initially the gauge is put in place. If the mark image does not coincide with the gap between the cells the cell assembly is displaced vertically until it does when the image of a scale 18 is projected on to a glass plate 21. The workpiece now replaces the gauge. If the workpiece and gauge have not the same thickness the amplifier output drives motor 15 which moves the cell assembly until the mark image falls on the cell junction when the image of scale 18 gives the object thickness directly. The cell assembly may be mechanically coupled to switch arrangements to control machine tools or to classify workpieces. In the arrangement shown in Fig. 2 motor 31, controlled by the photo-cell output, adjusts light deflecting plates 32, 35 simultaneously to equalise the light falling on the two cells and at the same time adjusts stylus 37 to mark a rotating drum. The adjustment of these plates may be coupled to adjust an electric bridge circuit the unbalance voltage of which will indicate thickness. In the arrangement of Fig. 3 (not shown) light reflected from the object passes by two paths chopped alternately to a single cell which gives an A.C. ouput unless the thickness is correct. In a still further arrangement a double image eyepiece in which the images coincide when the gauge is in position but are brought back to coincidence automatically when a workpiece of differing thickness is inserted is used. Fig. 4 shows the device for measuring the thickness of transparent layer 60. The image from the layer-support junction is adjusted by rotation of plate 76 by motor 77 fed from the output of differential photo-cells 70. At the same time variation of the upper surfaces are taken into account by a similar arrangement responding to the image produced at the top surface thus the relative rotation of plate 76, 79 will give an indication of layer thickness.