Coordinates transformation method for pointer gauge reading by machine vision

Abstract

Abstract In this report we present a machine vision algorithm for pointer gauge reading based on coordinates transformation. Since a pointer gauge is a polar representation of a linear scale, it is possible to transform its polar reading to the original linear counterpart. Once the picture of a pointer gauge is captured digitally, we can assign the polar positions to each pixel of the image. Those polar coordinates can be plotted on a rectangular frame to form a transformed image. The scale lines and the pointer are transformed to point vertically (or horizontally). Locations of the scale lines and pointer can be obtained from their pixel histograms maxima. The main advantage of our algorithm is its simplicity. We can read the pointer orientation and scale lines position of a pointer gauge directly from its geometrical structure. The complication of feature extraction process from the conventional algorithm, e.g., Hough transformation, can be avoided. We have tested the algorithm with the ideal and real image of pointer gauges. In the latter case, even though there are some noises on the transformed image, the locations of pointer and scale lines can still be easily found. Due to its simplicity, our algorithm is fast. We expect that our method can be useful in practice as an alternative machine vision algorithm when the real time pointer gauge reading system, e.g., the automatic dial gauge tester, is needed.