Apparatus for measuring dimension of article

Abstract: A scale has marks of a point-symmetrical shape which are arranged in matrix. The scale and an article to be measured in dimension are positioned without any relative movement. An image sensor unit detects a predetermined portion of the article and the marks of the scale corresponding to the predetermined portion of the article selectively and successively, and the image sensor unit generates output signals in accordance with the detected results of the article and the scale. The output signals are processed to calculate the dimension of the article. An apparatus for measuring the dimension of an article is provided which can eliminate the occurrence of parallax, resulting in satisfactorily improved precision of the measurement. In the apparatus, an article and a marker are disposed so that the optical distance between the article and a reading unit is equal to the optical distance between the marker and the reading unit.

Description: FIELD OF THE INVENTION The invention relates to an apparatus for measuring dimension, angle, distance, size etc. (defined "dimension" simply hereinafter) of an article, and more particularly to, an apparatus for improving precision in measuring dimension of an article regardless of a portion of the article to be measured. BACKGROUND OF THE INVENTION A conventional apparatus for measuring dimension of an article is described in the JP-B 5-85004. The conventional apparatus for measuring dimension of an article comprises X- and Y- transparent scale plates having marks for scales, a mount plate for placing a rectangular sheet article to be measured thereon, a light equipment for radiating light to the article placed on the transparent scale plates, and X-, Y- and origin image sensors for receiving light transmitted through the X- and Y- transparent scale plates. In operation, the rectangular sheet article is placed on the mount plate, such that first and second orthogonal sides of the rectangular sheet article are positioned on inner portions of the X- and Y- transparent scale plates, and a vertex defined by the first and second orthogonal sides is positioned on an inner portion of a crossing area of the X- and Y- transparent scale plates. Then, light is radiated from the light equipment to the X- and Y-transparent scale plates, so that light transmitted through outer portions of the X- and Y- transparent scale plates and an outer portion of the crossing area is received by X-, Y-and origin image sensors. In accordance with output signals of the X-, Y- and origin image sensors, a position of the vertex defined by the first and second orthogonal sides of the rectangular sheet article, and positions of two other vertexes defined by the second side and a third side of the rectangular sheet article, and by the first side and a fourth side of the rectangular sheet article are determined relative to the scales of the X- and Y- transparent scale plates. Consequently, lengths of the first to fourth sides of the rectangular sheet article, lengths of diagonal lines of the rectangular sheet article and vertex angles of the rectangular sheet article are obtained in accordance with the calculation of using the positions of the three vertexes of the rectangular sheet article. In the conventional apparatus for measuring dimension of an article, however, there are disadvantages in that, for instance, a distance between two arbitrary points on the rectangular sheet article is impossible to be measured, because the positions of the vertexes are detected by using the X- and Y- transparent scale plates, and any pattern defined on the rectangular sheet article can not be measured, even if the pattern is positioned in the vicinity of the first and second sides, because each of the X- and Y- image sensors can not discriminate the pattern from the marks for the scales due to the structure in which the rectangular sheet article is positioned directly on the X- and Y- scale plates. The document DE 33 23 836 A1 discloses a method for determining a coordinate on a surface of an article by providing a two-dimensional reference surface in a predetermined distance to the surface of the article. A portion on said reference surface opposite the coordinate to be determined is imaged by means of opposite objectives having the same optical axis. A mirror system is used to guide the beams to a sensor unit by interconnecting a half mirror. The document EP 0 685 709 A2 which is effective under Articles 54 (3) and 54 (4) EPC for the contracting states Germany, Switzerland and Liechtenstein, discloses an apparatus for measuring dimension of an article with X- and Y-tables moving in X- and Y-directions, said article being placed on one of said X- and Y-tables. A scale having marks arranged in matrix is arranged at an angle of 90° relative to the X- and Y-tables, and a half mirror is arranged at an angle of 45° relative to said scale and said article. An image sensor unit selectively detects a portion of said article and said marks of said scale corresponding to a portion of said article, and according to output signals of said image sensor unit, the dimensions of said article are calculated. A relative movement between the table and the image sensor unit is possible. SUMMARY OF THE INVENTION Accordingly, it is an object of the invention to provide an apparatus for measuring dimension of an article in which a distance between two arbitrary points on an article can be measured, even if the two points are far from, or off sides or edges of the article. It is a further object of the invention to provide an apparatus for measuring dimension of an article in which marks for a scale and a pattern defined on the article are precisely discriminated from the others to increase precision in measuring the dimension of the article. This object is solved by an apparatus as defined in claim 1. According to the invention, an apparatus for measuring dimension of an article, comprises: a table having a first flat surface on which the article to be measured is placed; a marker plate disposed on a second flat surface parallel to the first flat surface, the marker plate having a marker representing a unit of measurement including a coordinate, a dimension or an angle; half mirror means disposed on a third flat surface located at a position where the optical distance between the article and the half mirror means is equal to the optical distance between the marker and the half mirror means; and reading means for reading a value corresponding to an arbitrary point on the article and a value, of the marker on the marker plate, corresponding to the arbitrary point by taking advantage of light transmitted through and light reflected from the half mirror means. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be explained in more detail in conjunction with appended drawings, wherein; Figs. 1A to 1H are explanatory diagrams showing an apparatus for measuring dimension of an article as disclosed in EP 0 685 709 A, in which Fig. 1A shows the apparatus, Fig. 1B shows a scale used in the apparatus, Fig. 1C shows marks for the scale, Fig. 1D shows an image sensor unit used in the apparatus, Fig. 1E shows an article to be measured in the apparatus, and Figs. 1F to 1H show the relation of CCD sensors of the image sensor unit to a mark of the scale in the apparatus, Figs. 2 to 4 are explanatory diagrams showing apparatuses for measuring dimension of an article in embodiments according to EP 0 685 709 A2, Fig. 5 is an explanatory diagram showing an apparatus for measuring the dimension of an article according to the invention, and Figs. 6 and 7 are explanatory diagrams showing conventional apparatuses for measuring dimension of an article. DESCRIPTION OF THE PREFERRED EMBODIMENTS An apparatus for measuring dimension of an article according to EP 0 685 709 A2 will be explained in Figs. 1A to 1H for illustration purposes. In Fig. 1A, the apparatus comprises an X-table 1a moving in the X-direction indicated by an arrow, a Y-table 1b moving in the Y-direction orthogonal to the X-direction, on which an article 2 is placed to be measured in dimension, a transparent scale plate 4 provided on the Y-table 1b to have a scale on one surface thereof, an image sensor unit 3 having a focusing system (not shown) to be focused on the article 2 and the scale of the scale plate 4 selectively, a calculation unit 5 for calculating dimension of the article 2 by receiving output signals of the image sensor unit 3, and light equipments 7a, 7b and 7c for radiating light through the scale plate to the article 2. In the apparatus, the Y-table 1b and the scale plate 4 are designed , such that the article 2 is placed on the Y-table 1b to have a distance d greater than a focus depth of the focusing system of the image sensor unit 3 from the scale plate 4. Fig. 1B shows the transparent scale plate 4 having marks 41 for the scale arranged in matrix. In Fig. 1C, the marks 41 which are enlarged are of a width W and a height W, and arrayed by a pitch P1. Fig. 1D shows a view area V x v on the marks 41 of the scale plate 4 or the article 2 covered by CCD sensors 31 of the image sensor unit 3, wherein the CCD sensors 31 are arranged in matrix by a pitch P 2 . Fig. 1E shows the article 2 of, for instance, a metal plate having apertures 21 and 22 formed by a distance D between the central points O 1 and O 2 thereof. In Figs. 1 to 1H, the CCD sensors 31 of the image sensor unit 3 detect the mark 41 of the scale plate 4, so that a detected signal S is supplied to the calculation unit 5, wherein a waveform of the output signal S depends on the relation between the width W of the mark 41 and the pitch P 2 of the CCD sensors 31 of the image sensor unit 3, and the relative position of the CCD sensors 31 to the mark 41. In order to locate a specific one of the marks 41 having the same size and configuration, the conditions must be met as set out below. M < P 1 /2 where M is an approximate position precision of the X-and-Y tables 1a and 1b, even if a scale other than one used in the invention is used. 2P 1 + W < V This means that more than two marks 41 must be covered in the X-and Y- directions by the image sensor unit 3. W < P 1 /2 This means that a ratio of the pitch P 1 to the width W must be less than two, so that the influence of the article 2 to an image of the marks 41, that is the deterioration of resolution is suppressed to be low. 2P 2 ≤ W This means that the width W must be larger relative to the pitch P 2 of the CCD sensors 31 by a predetermined value, that is, two times. Consequently, the central point of each mark 41 is precisely obtained in accordance with the processing of the output signals S. The relation as shown in Fig. 1G does not meet the above condition to generate the output signal S which is of a pulse-like waveform, while the relations as shown in Figs. 1F and 1H meet the above condition to provide the output signals S which are of a step-shaped waveform. The step-shaped waveform is advantageous in processing the output signals S, and discriminating dust on the scale plate 4 from the marks 41. In measuring dimension of the article 2, the calculation unit 5 uses the central points of the marks 41. Therefore, the width W of the marks 41 is permissible to have deviation, because the central points thereof are not deviated due to the structure in which the marks 41 are square to be symmetrical in regard to the central points. In this sense, the marks 41 are not limited to be square, but point-symmetrical shapes such as circle, etc. As discussed above, one specific optimum example is as follows, W = 80µm, P 1 =400µm, V = 1000µm, M =100µm, and P 2 = 2µm. As described in Fig. 1A, the article 2 is separated on optical axis from the marks 41 of the scale plate 4 by the predetermined distance preferably largely greater than the focus depth. In one specific example, it is preferable that the inequality d > W / 2N.A. is satisfied, when the number of aperture is N.A., and the width of the marks 41 is W. In operation, the X- and Y- tables 1a and 1b are controlled successively to move in the X- and Y- directions, respectively, so that the aperture 21 of the article 2 is positioned below the image sensor unit 3. At this state, the aperture 21 and the marks 41 of the scale plate 4 covering the aperture 21 are focused separately by the focusing system of the image sensor unit 3. Thus, the aperture 21 and the marks 41 are detected separately by the CCD sensors 31 of the image sensor unit 3, so that output signals S are supplied therefrom to the calculation unit 5, in which the central point O 1 of the aperture 21 is calculated in accordance with addresses of the marks 41. Then, the X-and Y- tables 1a and 1b are moved in the X- and Y-directions, respectively, so that the aperture 22 of the article 2 is positioned below the image sensor unit 3. In the same manner as in the aperture 21, the central point O 2 of the aperture 22 is calculated in accordance with addresses of the marks 41 in the calculation unit 5. Then, the distance D between the central points O 1 and O 2 of the apertures 21 and 22 is calculated in accordance with the calculated positions thereof in the calculation unit 5. An image sensor unit 3 may be moved in the X- and Y-directions, while an article 2 and a scale may be stationary, and a magnetic detecting system may be adopted in place of the optical detecting system as adopted above. Fig. 2 shows an apparatus for measuring dimension of an article according to EP 0 685 709 A2, wherein like parts are indicated by like reference numerals as used Fig.1, provided that the scale plate 4 having the marks for the scale is provided on the opposite side to the article 2. In the apparatus, the X- and Y- tables are simply shown by a table 1, and the image sensor unit 3 as shown in Fig. 1A is replaced by image sensor units 3A and 3B which are provided at the same positions on the opposite sides relative to the table 1. In operation, the article 2 and the scale plate 4 are moved to take the same positions relative to the image sensor units 3A and 3B in accordance with the X- and Y- movements of the table 1. Thus, output signals of the image sensor units 3A and 3B are supplied to the calculation unit 5, in which the output signals are processed to calculate dimension of the article 2. Fig. 3 shows a further apparatus for measuring dimension of an article, wherein like parts are indicated by like reference numerals as used in Figs. 1 and 2. In the apparatus, the scale plate 4 has the scale 4A on the surface facing the article 2 to increase precision in measuring dimension of the article 2 for the reason why both the article 2 and the scale 4A are detected through the scale plate 4 by the image sensor unit 3, so that the influence of a refractive index of the scale plate 4 is commonly applied to the detecting lights of the article 2 and the scale 4A. The optical conditions for the image sensor unit 3 of the scale 4A and the article 2 are different to each other. Accordingly, an image of the article 2 is diffracted at the scale plate 4, so that the optical error is occurred by this diffraction. Fig. 4 shows still a further apparatus for measuring dimension of an article wherein like parts are indicated by like reference numerals as used in Figs. 1 to 3. In the apparatus, a half mirror 6 is provided at an angle of 45° relative to the X- and Y- table 1 and the scale table 4 having the scale 4A, and shutters 8A and 8B are provided to shut one of light paths for the scale 4A and the article 2. The shutters 8A and 8B may be omitted when other means such as using different wavelengths of lights for the scale 4A and the article 2 is adopted. In operation, a predetermined portion of the article 2 and the relevant marks of the scale 4A are selectively detected in accordance with the switch-over of the shutters 8A and 8B by the image sensor unit 3. In Figs. 2 and 4, optical distances can be equal to decrease error. Fig. 5 shows an apparatus for measuring the dimension of an article according to the present invention. This apparatus comprises: a table 1 having a horizontal surface on which an article to be measured is placed; a marker plate unit 7, disposed parallel to the table 1, comprising a marker plate 4 provided on the side of a reading unit 3 described below and a half mirror 6 provided on the side of the article 2; a reading unit 3, such as a CCD camera, for reading a value corresponding to an arbitrary point on the article 2 and a value, of a marker 4A on the marker plate 4, corresponding to said arbitrary point by taking advantage of light transmitted through and light reflected from the half mirror 6; and a calculation unit 5 for calculating the marker values read by the reading unit 3. The marker plate unit 7 is disposed so that the optical distance d 1 , between the article 2 and the half mirror 6 is equal to the optical distance, d 2 , between the marker 4A and the half mirror 6. The marker plate 4 has, on its surface, a coordinate marker 4A as shown in Fig. 3 and is formed of a material through which visible light can be passed, for example, glass. The marker . 4A may also be one representing a dimension, an angle or the like. The half mirror 6 comprises a dichroic mirror of which the light reflectance and transmittance vary depending upon the wavelength. The reading unit 3 is constructed so as to be movable in X-and Y- directions. A value corresponding to an arbitrary point on the article 2 and a value, of a marker 4A on the marker plate 4, corresponding to said arbitrary point are read by taking advantage of light transmitted through and light reflected form the half mirror 6. In the apparatus having the above construction, when the reading unit 3 is moved in X- and Y- directions, it reads a arbitrary point of the article 2 on the table 1 by taking advantage of light transmitted through the half mirror 6 and, at the same time, a value, of the marker 4A on the marker plate 4, corresponding to the arbitrary point is read by taking advantage of light reflected from the half mirror 6. The reading unit 3 outputs the read marker 4A values into the calculation unit 5 which calculates the input marker 4A values to determine the dimension of the article 2. In this case, the article 2 and the marker 4A are disposed so that the optical distance between the article 2 and the reading unit 3 is equal to the optical distance between the marker 4A and the reading unit 3. Therefore, even though the reading unit 3 undergoes some change resulting in inclined optical path, the relationship between the article 2 and the reading point of the marker 4A through the half mirror 6 is not broken, enabling the precision of the measurement to be improved. Although the apparatus of the present invention has been described with reference to an embodiment where the reading unit 3 is moved, the apparatus may be constructed so that the table 1 and the marker plate unit 7 are moved instead of the reading unit 3. Further, according to the above embodiment, the marker plate 4 and the half mirror unit 6 are integral with each. other to constitute the marker plate unit 7. They, however, may be independent of each other. As described above, in the apparatus for measuring the dimension of an article according to the present invention, an article to be measured and a marker are disposed so that the. optical distance between the article and a reading unit is equal to the optical distance between the marker and the reading unit. This can eliminate the occurrence of parallax, resulting in satisfactory improved precision of the measurement. Although the invention has been described with respect to specific embodiment for complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modification and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth. The features disclosed in the foregoing description, in the claims and/or in the accompanying drawings may, both separately and in any combination thereof, be material for realising the invention in diverse forms thereof.

Claims: An apparatus for measuring the dimension of an article, comprising: a table (1) having a first flat surface on which said article (2) to be measured in placed; a marker plate (4) disposed on a second flat surface parallel to the first flat surface, said marker plate (4) having a marker (4A) representing a unit of measurement including a coordinate, a dimension or an angle; characterized in that half mirror means (6) are disposed on a third flat surface located at a position where the optical distance between the article (2) and the half mirror means (6) is equal to the optical distance between the marker and the half mirror means; and reading means (3) are provided for reading a value corresponding to an arbitrary point on the article (2) and a value, of the marker (4A) on the marker plate (4), corresponding to said arbitrary point by taking advantage of light transmitted through and light reflected from the half mirror means (6). The apparatus as defined in claim 1, wherein, in the measurement, the table (1), the marker plate (4) and the half mirror means (6) are stationary with the reading means (3) being moved in a one- or two-dimensional direction. The apparatus as defined in claim 1, wherein in the measurement, the reading means (3) is stationary with the table (1), the marker plate (4) and the half mirror means (6) being moved in a one- or two-dimensional direction.

Inventors: HOSHIYAMA HIROKI, HOSHIYAMA, HIROKI, HOSHIYAMA, HIROKI, C/O JAPAN EM CO., LTD.
Applicants: JAPAN EM KK, JAPAN EM CO., LTD.
Application year: 1995
Main classification: G01B11
IPC codes: G01B11