DOI 10.1007/s11018-016-1069-z Measurement Techniques, Vol. 59, No. 9, December, 2016

LINEAR AND ANGULAR MEASUREMENTS DIGITAL PHOTOELECTRIC RASTER COMPARATOR

V. N. Yanushkin, Yu. B. Kolyada, and N. T. Krushnyak

UDC 681.2.531.71

The possibility of creating comparators for verification of raster grids is considered. A high-precision extremal computing-interpolation photoelectric digital raster system for measurement of linear displacements with high precision, reliability and performance indicators is used as the calibration instrument. Keywords: comparator, verification, raster system, precision, reliability, performance.

Modern requirements imposed on the precision of measurements of linear and angular quantities are satisfied basically by means of photoelectric methods and instruments. The use of rasters in measurement science has made it possible to solve in a novel way many problems involving high-precision measurement of linear and angular displacements in machine construction and automated processes, and increase precision, performance, and reliability in this field. In the overwhelming majority of control and measurement instruments (up to 90%) that function as components of coordinated motion measurement systems, photoelectric raster transducers are used as devices that satisfy all the precision and performance requirements the most exhaustively [1]. The design of such transducers consists of parallel-linear plane raster grids with strokes and pupils of identical width. In the junction, a periodic information scale is formed in a space of fine periodic rasters with error in the steps 1/n1/2 times that of the rasters [2]. The values of the shifts in relative movement of the grids in the direction perpendicular to the strokes are counted off along this scale. Depending on the junction angle of the strokes of the conjoined rasters, we consider moire or obstructive junctions of raster grids, an approach that is widely used in measurements. Junctions of rasters produce the effect of optical amplification (magnification) described in 1874 by Rayleigh: G = g/[2sin(ϕ/2)], where G and g are the steps of the compound bands of the moire pattern and the raster grids, respectively, and ϕ is the junction angle of the strokes of the raster grids. Still another important property of a raster junction, the fact that a “combination of two rasters may prove to be a sensitive indicator of weak signals,” is noted in [3]. In addition to raster phase transducers, a new type of instrument, or extremal raster transducer, the elements of which are incorporated into phase instruments to increase the precision, have now become quite competitive. The basic advantage of phase transducers is that they make it possible to perform measurements in a dynamic mode, while the advantage of extremal transducers lies in their maximal precision. Comparators for verification of raster measures must, therefore, be designed on the basis of the types of extremal photoelectric computing and interpolating digital measurement systems that are used in transducers for linear and angular measurements [4]. The precision of the system is determined by two elements of the return circuit, the digit-to-analog transducer and the raster compensation guidance interpolator. These elements form a

Bauman Moscow State Technical University, Moscow, Russia; e-mail: [email protected]. Translated from Izmeritel’naya Tekhnika, No. 9, pp. 19–21, September, 2016. Original article submitted April 14, 2016. 0543-1972/16/5909-0929 ©2016 Springer Science+Business Media New York

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Fig. 1. Functional diagram of digital photoelectric raster comparator: MH (V) and MH (C), verifying and calibrating measuring heads, respectively.

multi-mechanism that combines the most important functions needed for measurement of linear and angular displacements: raster, compensation, interpolation, and guidance. In addition, these elements greatly increase the precision and reliability of the measurement system [5, 6]. Different types of comparators, such as stroke and end, longitudinal and transverse, vertical and horizontal, and parallel and serial comparators as well as comparators with perflectometers, have now been developed and are in widespread use. Structural variants of such devices and their technical and metrological characteristics must be taken into account when designing raster comparators and different versions of such comparators. There is no clear-cut definition of a comparator-type device (a “comparator” is a device that performs comparisons), hence single-coordinate measuring instruments constructed on the basis of the following principles belong to the class of such comparator-type devices: The device consists of two parts, one of which is linked to the measurement object and comprises a sighting device while the other contains a calibrating measuring device. The first part of the comparator is used to determine the boundaries of a measured segment of the object, while the second part is used to compare this value with that obtained in the calibrating device. The measurement process consists in a simultaneous relative displacement of the verified object and a calibrating device rigidly linked to it (or a sighting and a measuring device) by one and the same magnitude [7]. The operating principle of comparators consists in measurements of particular intervals between specified points of compared measures (or scales), a verifying scale and a calibrating scale. In designing a photoelectric raster comparator, it is necessary to take into account a number of circumstances in order to decrease the structural-type errors that arise in the course of measurements [7]. Such a comparator must be constructed according to the design of a horizontal longitudinal comparator with fixed verifying and calibrating measuring heads (verifying and calibrating measuring instruments) along with sequentially arranged moveable raster measures rigidly bound together. The IZA-2 device, which is commonly used to compare precision scales and grids and the distances between lines in laboratories at institutes and industrial enterprises, may be considered a prototype. The sequential arrangement of the raster measures along a common line makes it possible to reduce the comparator error caused by a violation of the Abbé principle. To reduce the parallax error, which in some textbooks is referred to as the error due to violation of the second Abbé principle, the verifying and calibrating measuring instruments must be placed at minimal distances from the verifying measures, which is always realized in photoelectric raster measuring instruments, since the gap between the measurement and indicator rasters of the transducers is roughly equal to the raster step, i.e., 20–100 μm [8]. In raster junctions, the boundaries between the compared segments are determined at extremal points of illuminance of the spatial scale, in other words at the maximum or minimum of the signals adopted as the start or end of the step (period) of the raster. The error in establishing such points may be calculated only experimentally. A study using a novel and specially developed two-channel raster interference head was carried out for this purpose. The experiments were performed on a test bench with local temperature stabilization. The limiting extremum guidance error of the illuminance of the junction of the rasters amounted to 0.004 μm. The technique and results of the studies are presented in [9]. 930

In photoelectric raster systems, the basic requirements imposed on the design of the comparator are quite simple to realize in practical applications. The measurement and indicator grids of the verifying and calibrating measuring instruments are situated next to each other separated by a gap equal in magnitude to a step of the measure. Such an arrangement virtually excludes any parallax error. The strokes of the conjoined rasters are parallel, which can be easily controlled by a guidance microscope, while the obstructive junction is only weakly sensitive to rotations of the measures. The construction of a digital photoelectric raster comparator consists of several blocks as represented in Fig. 1. The construction comprises a mount with moveable platform resting on aerodynamic guides borrowed from the BV 6065 device, which is intended for measurements of deviations from rectilinearity. A 150 mm displacement created at a speed of 20 or 80 mm/sec is performed by an electric drive. In the vertical plane, the deviation from rectilinearity is not more than 0.1 μm, and in the horizontal plane, not more than 0.2 mm. Raster measurement measures of verifying and calibrating measuring instruments are mounted sequentially and collinearily on a fixed platform. The measuring heads of these measuring instruments, that is, MH(V) and MH(C), are clamped down on the mount. The letters A, B, C, and Aʹ, Bʹ, Cʹ denote the circuits linking MH(V) and MH(C) to the electronic signal processing blocks, which are produced in a single circuit. The structural implementation of the measuring heads as an electronic block does not greatly vary as a function of the type of extremal computing-interpolation photoelectric digital raster measurement system, which is constructed according to the same operating principle [4]. Consequently, the elements of this system are novel, optimal in terms of composition, and universal in terms of application [10]. In designing the comparator, special attention must be devoted to the calibrating measuring instruments in order to ensure verification precision. All the accumulated experience in the development of digital photoelectric raster transducers must be employed. Because of structural features, the illuminators incorporated into extremal raster transducers do not exert a substantial effect on the precision, since they are easily controlled through the introduction of local feedback (for example, in the amplifier) or by means of a stable supply voltage. The astatic compensation servo system implemented in the interpolation part of the circuit is not very sensitive to the form and amplitude of the signals and always functions only on the linear segment of the characteristics, constantly maintaining maximal sensitivity to any variations in the measured parameter [11]. The precision of a calibrating measuring instrument is determined to a significant extent by the raster measure, which in the operation of a measuring instrument is realized by means of the spatial scale formed by the raster junctions. In turn, the precision of this scale depends on the precision with which the rasters are manufactured and the certification precision. The error of a raster junction (spatial scale) is composed of the sum of the average local errors [2]. In raster measuring instruments, the error may be reduced by decreasing the step of the measure and by increasing the interpolation index. An example illustrating the use of raster measures with 20 μm step and interpolation index 1000 points in each period is presented in [12]. We have developed additional methods of correction of the errors of transducers to achieve increased interpolation indices [13]. The use of obstructive junctions of rasters and the absence of an analyzing diaphragm together produces an increase in the contrast of the information field, makes it possible to obtain sufficient energy for the signal, and enables the use of weak-current light sources, which, in turn, reduces the temperature error. The totality of these strategies enables us to construct a calibrating measuring instrument that satisfies the requirements of the problem. REFERENCES 1. 2. 3. 4. 5.

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