High precision linear displacement measurements can be made with Moire Fringes, as shown in figure below. Both of the strips are transparent (or reflective), with black lines at measured intervals. The spacing of lines determines the accuracy of the position of measurements. The stationary strip is offset at an angle so that the strips interfere to give irregular patterns. As the moving strip travels by stationary strip then the pattern will move up, or down, depending upon speed and direction of motion.
A device to measure the motion of Moire Fringes is shown in next figure. A light source is collimated by passing it through a narrow slit to make it one slit width. This is then passed through the fringes to be detected by light sensors. At least two sensors are needed to detect the bright and dark locations. Two sensors, close enough, can act as a quadrate pair, and same method used for quadrature encoder which can be used to determine direction and distance of motion.
These are used in high precision applications over long distances, often meters. It can be purchased from many suppliers but cost will be high.Typical applications include Co-ordinate measuring machine (CMMs).
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