Digital Photoelectric Length Measure

Abstract

Universal devices in several modifications under the general name of optical length measures, which include the vertical ocular devices IZV-1, IZV-2, and IZV-21, screen-type device IZV-3, automated device IZV-5 with digital readings, horizontal screen-type device IKU-2, and horizontal automated device IZG-5 with digital reading, are now found in general use for the purpose of measurements of linear dimensions [1, 2]. The operation of ocular length measures is based on the following types of interpolation instruments: spiral eyepiece microscope, optical micrometer with plane-parallel plate, as well as an optical micrometer with wedge compensator for display of fractions of a period of a measure. The use of such interpolators together with screen-type reading devices makes it possible to reduce operator fatigue and increase measurement precision. However, the presence in the interpolators of powerful illuminators with short service period levels these advantages and screen-type length measures have not found wide use. Moreover, a multistage system of reading of the numerical value of the measured quantity is used in all of these length measures. The full periods of a multidigit working measure, and then tenths, hundredths, and thousandths of a period are first read, and, finally, tenthousandths of a millimeter are read “approximately.” Under favorable conditions, an experienced observer may read with sufficient confidence down to a twentieth of an interval of a scale of the most precise digit. An estimate of tenths of an interval as an “approximation” is achieved quite easily [2]. The measurement systems of the latest automated length measures with digital reading are constructed with the use of measures in the form of periodic structures, such as diffraction gratings, raster scales or interference fields, the light flux from which is detected by means of photoelectric detectors and then analyzed by means of electronic systems. Amplitude, phase, or extremal transducers for readings fractions of a period of the measure serve as the interpolators in these devices. A decrease in the discreteness of the reading is achieved by decreasing the period of the original measure or by increasing the interpolation index. Diffraction gratings are now manufactured with minimal period of 1 μm. Amplitude interpolators have not found wide use due to a low interpolation index. Phase interpolators are considered quite promising though are structurally complex, which also makes it impossible to obtain interpolation indices greater than 200. Moreover, such interpolators require highly skilled operators. The use of diffraction gratings is limited by their rather high cost, low range of measurement down to 300 mm, difficulty of adjustment and maintenance of the primary transducer, low degree of reliability, and low maintainability. They are, therefore, manufactured in small lots, basically for metrological institutes and laboratories [3]. Raster extremal interpolators seem more promising. An extremal computational interpolation measurement system has been developed at the Bauman Moscow State Technical University. With the use of the system, several models of digital