US 3689162 A
A displacement sensing device using superimposed dissimilar circular moire patterns. Groups of circles in one of the patterns alternately have a greater and a smaller pitch than those of the other pattern.
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Description (OCR text may contain errors)
United States Patent Ferguson Sept.5,l972
 APPARATUS FOR THE RELATIVE  References Cited POSITIONING OF TWO OBJECTS BY OTHER PUBLICATIONS MEANS OF A BEAM OF RADIATION Inventor: Eric Tapley Ferguson, Emmssingel,
Netherlands Field of Search ..250/237 G; 356/169, 172', 350/162 M. Stecher, The Moire Phenomenon, Amer. J. of Physics, Vol. 32, No. 4, April 1964, pp. 247- 57.
G. Oster, Moire Optics: A Bibliography, J.O.S.A. Vol. 55, 10/65, pp. 1,329- 30. T'heocaris & Kuo, Interference Patterns of Zonal Gratingsz... Z. Ang. Math. Phys 17, 90- 107 1966 Primary Examiner-Ronald L. Wibert Assistant ExaminerJeff Rothenberg Att0rneyFrank R. Trifari ABSTRACT A displacement sensing device using superimposed dissimilar circular moire patterns. Groups of circles in one of the patterns alternately have a greater and a smaller pitch than those of the other pattern.
4 Claims, 7 Drawing Figures PAIENTEDSEP 5 I 12 3.689.162
l fig.3 fig.6
fig.7 V Z, Q Kong AGEN T APPARATUS FOR THE RELATIVE POSITIONING OF TWO OBJECTS BY MEANS OF A BEAM OF RADIATION This is a continuation of application Ser. No. 811,177, filed Mar. 27, 1969, now abandoned.
The invention relates to an apparatus for the relative positioning of two objects by means of a beam of radiation which interacts with part of each of the objects.
Relative positioning of two objects is used, for example, in the manufacture of color television display tubes. Color television display tubes are known which have three electron guns and a shadow mask disposed between the display screen and the guns. The screen is covered with a plurality of phosphor dots arranged in clusters of three. When electrons impinge on the dots, the three dots of each cluster fluoresce in different colors, usually red, green and blue. The shadow mask, which is made of a plate of material impermeable to electrons, is perforated in a manner such that each hole corresponds to a dot cluster. Electrons from a given gun must impinge on one dot only of each cluster. Therefore not only are the dots on the screen to be uniformly distributed but also the holes in the shadow mask sheet must be evenly distributed over the sheet to ensure the desired marking action.
The holes in the mask sheet, which preferably is made of metal, are usually made by means of an etching process. For this purpose the sheet is coated with a so-called photo-lacquer and subsequently photographic masks containing a suitable dot pattern are clamped against both faces of the sheet. The mask sheet is exposed through the photographic mask. The lacquer coating may, for example, have the property of becoming insoluble in a given solvent after being exposed to short-wave light. The unexposed part of the lacquer is removed by means of this solvent. The resulting uncovered part of the mask sheet is removed, for example, by etching in a special etching solution.
Obviously, corresponding images in the two photographic masks must be relatively centered with a high degree of accuracy, for example within nm, for the holes made in the sheet by etching to have the desired shapes and positions. The dots or, as the case may be, special alignment marks on the photographic masks may be observed with a microscope. However, it is difficult to bring the microscope into close proximity to the masks.
It is an object of the invention to obviate this disadvantage. For this purpose, an apparatus of the aforementioned kind is characterized in that on each of the objects is provided a pattern of concentric equidistant circles which intercept the beam of radiation, the pitch of the pattern on one object being different from that on the other object and the beam of radiation interacting with the patterns which nearly engage one another. Preferably, the spacing between adjacent circles in each pattern is one half of the pitch of the respective pattern.
When the two patterns are concentrially superimposed, a moire pattern of concentric circles is produced. On one object a pattern of, say, several hundreds of concentric circles which intercept the beam of radiation are provided so that the spacing between each two adjacent circles is equal to the width of each circle. The other object is also provided with a pattern of concentric circles which are also spaced by distances equal to their width. The pitch S of the circle on one object, however, differs from that of the circles on the other object by, for example, a few say a percent. Hence, the spacing between adjacent moire circles is (S/a).
It should be noted that moire patterns are known. However, the known moire patterns have disadvantages. Moire patterns of straight lines produced from two patterns of straight parallel lines of slightly different pitches or slightly different directions have the disadvantage that displacement of one of the patterns may readily give rise to an error of an entire period, for in the case of displacement each pattern is identical to itself.
Two equal patterns of possible equidistant, concentric circles have the disadvantage that in the case of correct adjustment no moire pattern is visible. The moire pattern becomes visible only on relative displacement of the two patterns by at least one half of a pitch, and this means an adjustment of the centering mark. In addition, from the moire pattern the sign of the displacement in a given direction cannot be deduced because the observer cannot see which of the two patterns is nearer to the source of radiation.
In the apparatus in accordance with the invention even with exact centering at least one moire ring is visible which changes its position and shape on decentering. When the relative displacement of the patterns on the objects is A, where A is assumed to be smaller than S, the shape of the moire pattern remains substantially unchanged, but the moire rings are displaced through a distance of 100 (Ala) in the direction of displacement of the pattern having the smaller pitch.
One or both circle patterns may be marked, for example, by omitting one of the circles which intercept the radiation or by filling up a space. An extremely sensitive adjustment mark is obtained by causing one of the circles of the moire pattern to coincide with the omitted reference circle. A deviation of 1/40 part of the pitch of the moire pattern is visible with the naked eye at the location of the reference circle.
In order that the invention may be readily carried into effect, an embodiment thereof will now be described, by way of example, with reference to the accompanying drawing, in which:
FIG. 1 is a sectional view of two patterns of circles in accordance with the invention,
FIG. 2 shows a moire pattern produced on irradiation of two mutually centered patterns of the kind shown in FIG. 1, and
FIGS. 3 and 4 show part of the moire pattern produced on decentering of the circle patterns of FIG. 1
FIGS. 5, 6 and 7 show circled patterns where one or more of the circles have been removed to provide an indexing device.
FIG. 1 is a cross-sectional view of two circle patterns 1 and 2. The patterns are nearly in engagement with one another. The spacing is small with respect to the lack of definition due to diffraction and with respect to the source of radiation which irradiates the patterns. The pitch of the pattern 1 is slightly different from that of the pattern 2. On irradiation of the mutually centered patterns alternately light and dark moire rings are visible FIG. 2 When the pattern of smaller pitch is displaced in the direction shown in FIG. 3, the moire rings are displaced. At a comparatively large displacement a moire pattern similar to that of FIG. 4 is found from which the direction of displacement is immediately recognized.
In a practical embodiment both patterns comprised 200 circles. The width of each circle was 40 pm in one pattern and 39.6 pm in the other. The spacing between adjacent circles was also 40 and 39.6 pm respectively. In one of the patterns as shown in FIG. 5 and FIG. 7 one circle had been omitted/The two patterns could be relatively centered to within 2 pm with the unaided eye.
A high sensitivity is also obtainable if one of the patterns contains groups of circles alternately having a positioning of two objects using circular moire patterns comprising a pattern of concentric circles on each of the objects, the concentric circles on one of the objects being equidistantly spaced and the concentric circles on the other object forming at least one group of equidistantly spaced circles, the pitch of the pattern on one object being different from the pitch of the pattern on the other object, the pattern on one object nearly engaging the pattern on the other object, and indexing means comprising a predetermined optical nonuniformity in at least one of the patterns, whereby the relative positions of the two objects can be determined when said patterns intercept a beam of radiation.
2. An apparatus as claimed in claim 1, wherein the spacing between the adjacent circles in each pattern is one-half of the pitch of the respective pattern.
3. An apparatus as claimed in claim 1, wherein the optical non-uniformity comprises in one of the patterns a light transmitting in place of a light absorbing circle.
4. An apparatus as claimed in claim 1 wherein the optical non-uniformity comprises groups of alternate circles in one pattern having a greater and smaller pitch respectively, then the pitch of the other pattern.