Method and apparatus for detecting a reference position of a rotating scale

Abstract: In the reference-position detection method and ap­ paratus for detecting a reference position of a scale, first and second patterns are disposed on the scale so that respective detection timings shift inversely to each other in response to a deviation in attitude of the scale. The first and second patterns are detected with timings dif­ ferent from each other by corresponding first and second sensors, and the reference position of the scale is deter­ mined according to outputs of respective sensors. Hence, even if the attitude of the scale changes from a predeter­ mined attitude due to an inclination of the scale or a deviation in position of the scale, it is possible to ex­ actly detect the reference position of the scale. Accord­ ingly, it becomes also possible to increase the performance of a displacement measuring apparatus.

Description: This invention relates to a reference-position detection method according to the preamble of claim 1 and a reference-position detection apparatus according to the preamble of claim 2. Heretofore, as a measuring instrument which can measure the displacement of an object to be measured in the unit of submicrometers, there has been known an optical encoder in which a laser beam is irradiated on a diffraction grating (scale) linked to an object to be measured, an interference light is formed by superimposing a pair of diffracted lights produced by the diffraction grating with each other, and the displacement of the diffraction grating, that is, the displacement of the object to be measured is measured according to a signal obtained by photoelectrically converting the interference light. A generic reference-position detection method and a generic reference-position detection apparatus is respectively shown in the US-A-4 064 435. This reference shows an optical encoder apparatus, in which the scale is arranged on a side parallel to the rotating axis. Two optical encoder means arranged diametrically opposite each other detect a reference position in connection with polygonal mirror facets. The second encoder is only used for coding of key points so as to allow all polygonal mirror facets to be untilted. As a result the auxiliary encoder is only a flag to detect in which direction of the shaft. Furthermore, a reference-position detection apparatus is known from the DE-B-2 454 915. This known reference-position detection apparatus includes a circular scale rotatable relative to two objectives of an optical system, the objectives being arranged so as to be able to scan respective portions of the circular scale. The optical system displays images of its scanned portions on a common display. The detection of the reference-position is effected by inverse shifting of the images of the reading positions of the circular scale by moving the objectives of the optical system until the graduation lines of the numbered circular scale coincide at a separating edge. In combination with the movement of the two objectives, a scale is also pivoted to supply interpolation values for the reading obtained from the circular scale. The apparatus is only intended for reading off numeral values, and thus not suited for quickly rotating components. Besides, in this method, the scale is evaluated at two exactly opposite positions. This can only eliminate eccentricity errors of the scale as against the rotation axis. If the rotation axis is displaced or tilted together with the scale as against the detecting means (objectives), the objectives have to be manually readjusted. According to the Navy Technical Disclosure Bulletin, Vol. 3, No. 7, July 1978, pp. 51-54 three light beams are used to provide an averaging effect which shall cancel errors due to mechanical tolerances, bearing play and the like. The functioning of these shaft angle encoders is not to be compared with the problems according to the invention, as, compared with the subject matter of the application here additionally a rotating component is provided. The photosensitive cells are further activated at the same timing. Besides, the shaft angle encoder has to be arranged on the end of the shaft due to the light-emitting diode being mounted centrally on the face. In the method disclosed by JP-A-62-200223 (1987), a mark for detecting a reference position formed on a scale is optically detected, and the reference position of the scale is detected with a very high resolution. Although this method can be applied to a linear scale and a rotary scale, problems sometimes arise, especially when this method is applied to a rotary scale mounted to a small rotating mechanism. A mark, or marks, consisting of a rectangular reflective film is, for example, provided on a rotating scale, and a rectangular or an elliptic beam spot extended in the direction of the radius of the scale is formed on the scale by irradiating a laser beam on the scale. A reflected light beam produced when the mark crosses the beam spot is then detected by two photosensors disposed in different positions so that detection timings for the reflected light beam are different to each other, levels of output signals from the two photosensors are compared with each other, and the reference position of the scale is detected with excellent accuracy based on the levels of the output signals. However, if a deviation of the scale or an inclination due to a slanting of the rotation shaft occurs in the rotating scale during its rotation, the detection timings for the reflected light by the two photodetectors shift in the same manner, as shown in FIG. 1, FIG. 2 and FIG. 3, and the reference position is misdetected. In FIGS. 1 and 2, there is shown a laser diode 1, a collimating lens 2, a half-mirror 3, a cylindrical lens 4, a rotating scale 5, a mark 6 for detecting a reference position, a rotation shaft 7 of the rotating scale 5, and photosensors 8 and 9. Light from the laser diode 1 is made a parallel light by the collimating lens 2, and the parallel light is directed to the half-mirror 3. The parallel light reflected by the half-mirror 3 is converted into a linear light beam by the cylindrical lens 4, and a linear beam spot extended in the direction of the radius of the scale is formed on the scale 5. When the mark 6 passes through the linear spot, the light beam reflected by the mark 6 is directed to the photosensors 8 and 9 via the cylindrical lens 4 and the half-mirror 3. The mark 6, the linear beam spot and the photosensors 8 and 9 are set so that levels of output signals A and B from the photosensors 8 and 9 coincide with each other only when the center line of the mark 6 and the center line of the linear beam spot coincide with each other, and detection of the reference position of the rotating scale 5 is performed according to a coincidence of output signals A and B from the photosensors 8 and 9. Now, if the rotating scale 5 shifts 0.5 µm in the direction of the X-X′ axis which orthogonally crosses the rotation shaft of the scale and is displaced from the position indicated by the full line to the position indicated by the dotted line, as shown in FIG. 1, detection timings of the reflected light beam from the rectangular mark 6 on the rotating scale 5 by the photosensors 8 and 9 shift as shown in FIG. 3. That is, the timing t₁ at which levels of the output signals A and B of the photosensors 8 and 9 coincide with each other shifts to t₁'. Pulses depicted at an upper portion of FIG. 3 are reference (origin) signals corresponding to the reference position of the scale 5, produced according to a coincidence of levels of the output signals A and B. The reference signal is misdetected by the deviation of the scale 5. The shift from the timing t₁ to the timing t₁′ corresponds to the shift of the reference position of the scale 5 by 0.5 µm along the circumferential direction of the scale 5. Hence, when the diameter of the scale 5 is 20 mm, an error of {tan⁻¹(0.5/10000)x3600} ≅ about 10 angular seconds is produced. On the other hand, if the rotation shaft 7 of the scale 5 is slanted to tilt the scale 5 as shown in FIG. 2, the incident positions of the light beam reflected by the mark 6 on the photosensors 8 and 9 are shifted, and output timings of the output signals A and B from the photosensors 8 and 9 are shifted. For example, when a lens having a focal length f = 5 mm is used as the cylindrical lens 4, the error in detecting the reference position becomes tan⁻¹{5 x tan(1/60 x 2)/10} x 60 = 20 angular seconds , for the tilt of the rotation shaft 7 of 20 angular seconds. In general, when a rotating scale is mounted to a small rotating mechanism using a bearing and the like, it is very difficult to prevent deviation of the shaft of 0.5 um or less, or prevent slanting of the shaft of 10 angular seconds or less. Accordingly, in conventional methods, it is difficult to exactly detect a reference position of a rotating scale. It is the object underlying the subject matter of the application to provide a reference-position detection method and a reference-position detection apparatus which are capable of exactly detecting a reference position of a scale also when the scale has a deviation in attitude. According to the application this object is achieved for the method by the features of the claim 1 and for the apparatus by the features of the claim 2. Advantageous further developments are set out in the dependent claims. The features, concrete methods and configurations of the present invention will become more apparent from the embodiment to be described later. FIGS. 1 and 2 are explanatory diagrams showing a conventional method for detecting a reference position; FIG. 3 is a graphic diagram for showing a problem in the conventional method for detecting a reference position; FIGS. 4A - 4D are schematic diagrams showing an embodiment of the present invention; and FIGS. 5A - 5C are graphic diagrams for showing the effect according to the method of the present invention. FIGS. 4A and 4B are schematic diagrams showing an embodiment of the present invention. FIG. 4A shows the configuration of an optical system, and FIG. 4B shows the arrangement of a mark pattern for detecting a reference position, on a rotating scale. FIG. 4C is a partial schematic diagram which sees the optical system shown in FIG. 4A from the direction Y' - Y in FIG. 4A. FIG. 4D is an explanatory diagram showing the detection principle of the mark pattern by the optical system shown in FIG. 4A. A laser diode is used for a light source 1 in this case. A collimating lens 2 makes the laser light beam from the light source 1 a parallel light beam. A beam splitter 3a divides the light beam from the collimating lens 2 into two light beams. There is also shown a reflecting mirror 10 and beam splitters 3b and 3c. A rotating scale 5, as an object to be detected, along the circumferential direction of which a diffraction grating 100 is formed, is rotated making the center 0 of a rotating shaft 7 the center of rotation. Cylindrical lenses 4a and 4b, generatrices of which nearly coincide with the direction of the radius of the rotating scale 5, focus light beams from the beam splitters 3b and 3c in the circumferential direction (tangential direction) of the rotating scale 5, shape the beams into an elliptical shape extended in the direction of the radius of the rotating scale 5, and irradiate the beams perpendicularly on portions separated from each other on the rotating scale 5. Patterns 6a and 6b for detecting the reference (origin) position consist of rectangular reflecting films made of Al, Au or the like, and are disposed facing each other relative to the center O of rotation on a straight line passing through the center O of rotation of the rotating scale 5, as shown in FIG. 4B. The patterns 6a and 6b are provided so as to be located on radii (distances from the center O) r a and r b , respectively. Photosensors 8 and 9 have photosensing members having an identical size, and receive the light beams reflected by the rectangular patterns 6a and 6b via the cylindrical lenses 4a and 4b and the beam splitters 3b and 3c, respectively. In the present embodiment, as shown in FIG. 4C, the photosensors 8 and 9 are disposed with being shifted to the same side relative to the optical axis 11 of the cylindrical lenses 4a and 4b. The distances from the optical axis 11 to the centers of photosensors 8 and 9 are equal to each other. The cylindrical lenses 4a and 4b have the same focal length, and are diposed so that they form beam spots which have the same dimensions and shape on the rotating scale 5. The mark patterns 6a and 6b also have the same dimensions. The widths of the mark patterns 6a and 6b in the direction of their movements are set to about 1/2 of the widths of the beam spots. Accordingly, as shown in FIG. 4D, if the mark pattern 6b passes through, for example, the corresponding beam spot while moving from the right to the left in FIG. 4D, the amount of light incident upon the photosensor 9 gradually increases from the moment when the left-side edge of the mark pattern 6b passes through the optical axis 11, reaches the maximum value when the mark pattern 6b passes through a predetermined position, and subsequently decreases as the mark pattern 6b leaves the predetermined position. In FIG. 4C, if the rotating scale 5 is rotating so that the mark pattern 6b moves from the right to the left in FIG. 4C at near the optical axis 11, the mark pattern 6a moves from the left to the right in FIG. 4C at near the optical axis 11. As described above, the light beam reflected from the mark pattern 6b starts being incident upon the photosensor 9 from the position where the mark pattern 6b verges on the optical axis 11. On the other hand, since the mark pattern 6a is situated at a side opposite to the mark pattern 6b relative to the center O of rotation of the rotating scale 5, the light beam reflected from the mark pattern 6a starts being incident upon the photosensor 8 from the moment when the mark pattern 6a is situated at a position separated from the optical axis 11. Accordingly, the detection timings of the mark patterns 6a and 6b by the photosensors 8 and 9 are shifted to each other, and the photosensor 8 starts generating a photoelectric conversion signal earlier than the photosensor 9 does. Now, in FIG. 4D, if the rotating scale 5 is shifted, for example, to the left, the mark pattern 6b approaches the optical axis 11 although the rotating scale 5 have not rotated a predetermined amount. Hence, the timing for the light beam reflected from the mark pattern 6b to be incident upon the photosensor 9 is advanced. At this time, since the mark pattern 6a is shifted to a direction being separated from the optical axis 11, the timing for the light beam reflected from the mark pattern 6a to be incident upon the photosensor 8 is delayed. Accordingly, as will be described later, the detection timings of the mark patterns 6a and 6b by the photosensors 8 and 9 are shifted in reverse directions to each other. The situation is identical when the rotating scale 5 is tilted due to slanting of the rotation shaft 7. As described above, in the present embodiment, laser light beam from the light source 1 is made a nearly parallel light beam by the collimating lens 2, the light beam is directed to the cylindrical lenses 4a and 4b disposed at sides opposite to each other relative to the rotation shaft 7 via the beam splitters 3a, 3b and 3c and the mirror 10, and two elliptic beam spots are formed on portions separated from each other on the rotating scale 5 by the cylindrical lenses 4a and 4b. Photosensing timings (pattern detection timings) of respective reflected beams produced when the rectangular patterns 6a and 6b provided on the rotating scale 5 pass through respective beam spots by the photosensors 8 and 9 are shifted to each other. FIG. 5A is an explanatory diagram showing output signals A and B obtained from the two photosensors 8 and 9 at this time. The photosensor 8 starts detecting the pattern 6a from the time t A , and the photosensor 9 starts detecting the pattern 6b at the time t B . At this time, the time t₁ corresponding to the cross point (a point where levels of the two output signals coincide with each other) of the illustrated curves of the output signals A and B corresponds to the reference position (origin point) of the rotating scale 5. The levels of output signals from the photosensors 8 and 9 change in accordance with the positional relationship between the beam spot on the scale 5 corresponding to the one photosensor 8 and the pattern 6a, and the positional relationship between the beam spot on the scale 5 corresponding to the other photosensor 9 and the pattern 6b. Now, if the center O of the rotating scale 5 is deviated in the direction of the X-X′ axis as shown in FIG. 1, the photosensing timing of the photosensor 8 delays to produce the signal A₂ as shown in FIG. 5B. On the contrary, the photosensing timing of the photosensor 9 advances to produce the signal B₂. As a result, respective signal outputs as shown in FIG. 5B can be obtained. At this time, the time t₂ corresponding to the cross point of the output signals A₂ and B₂ becomes identical to the time t₁ in FIG. 5A. Similarly, when slanting of the rotation shaft 7 such that the rotating scale 5 rotates around the Y-Y′ axis occurs, the photosensing timing of the photosensor 8 delays to become the time t A3 , but the photosensing timing of the photosensor 9 advances to become the time t B3 , as shown in FIG. 5C. As a result, the time t₃ corresponding to the cross point of the signal outputs A₃ and B₃ becomes identical to the time t₁ in FIG. 5A. That is, in any case, the time corresponding to the cross point of the signals A and B from the photosensors 8 and 9 does not change. Particularly, in the present embodiment, a light beam from the single light source 1 is divided into two light beams, which are directed to respective detection systems (3c, 4b, 6b and 9: 3b, 4a, 6a and 8). Hence, the present embodiment has the feature that if the light emission intensity of the light source 1 changes, the detection timing never shifts as shown in FIG. 5C. As described above, in the present embodiment, by separately providing the rectangular patterns 6a and 6b at predetermined positions on the line passing through the center O of rotation on the rotating scale 5 and disposing the two photosensors 8 and 9 with being shifted to each other at the same side from the optical axis 11 of the cylindrical lenses 4a and 4b, an error produced when detecting the reference position (origin position) is removed using output signals obtained from the two photosensors 8 and 9 even if the rotating scale is displaced from a normal state (position), and hence highly accurate detection becomes possible. Although, in the present embodiment, the positions of the photosensors 8 and 9 are shifted from the optical axis 11, the photosensors 8 and 9 may be disposed so that their centers coincide with the optical axis 11, and the patterns 6a and 6b may be formed at positions shifted by a predetermined amount to each other at the same side relative to a line passing through the center O of rotation. In the present embodiment, the distances from the center O of rotation of the two rectangular patterns 6a and 6b are slightly changed from each other for the purpose of obtaining one reference-position signal for every rotation of the rotating scale 5. The patterns 6a and 6b are disposed so that the reflected light beams from the patterns 6a and 6b are received only by separate photosensors 8 and 9, respectively. The diffraction grating 100 formed on the rotating scale 5 is a gradation scale for detecting the amount of rotation of the rotating scale 5, and is read by a scale-reading apparatus (not illustrated). Signals from the photosensors 8 and 9 are also input to a processing circuit within this apparatus. In this circuit, a reference signal corresponding to the reference position of the rotating scale 5 is generated according to the coincidence of levels of the respective signals. According to the reference signal, an absolute signal indicating the absolute position of the rotating scale can be formed from an incremental signal obtained by reading the diffraction grating 100. Reading of the diffraction grating 100 is performed by irradiating a laser beam on the diffraction grating 100, having + 1st-order diffraction light beams generated by the diffraction grating 100 interfere with each other, and performing photoelectric conversion of an interference light formed by the interference. By the photoelectric conversion, pulses are sequentially generated in accordance with the displacement (rotation) of the diffraction grating 100, and incremental signals are provided. Since the configuration of a scale-reading apparatus which performs such a reading is shown, for example, in the above-described Japanese Patent Public Disclosure (Kokai) No. 62-200223 (1987), a detailed explanation thereof will be omitted in the present application. An incremental encoder provided with a scale and a reading apparatus of this kind performs displacement measurement with an extremely high resolution. The reference-position detection method and apparatus according to the present invention can sufficiently exhibit their performances even when used in such an encoder. Although, in the embodiment shown in FIGS. 4A and 4B, the patterns for detecting the reference position are reflective patterns formed by reflective films made of Al, Au or the like, transparent patterns the circumferences of which are surrounded by reflective films or light-absorbing films may be provided. Furthermore, the shape of the patterns is not limited to a rectangular shape, but the patterns may have various shapes, such as a circular shape and the like. Three-dimensional marks may also be provided. Moreover, magnetized patterns which are magnetized in a predetermined direction may also be used with other optical patterns of this kind. Detection of the magnetized patterns may be performed by a magnetic sensor. In addition, the scale as an object is not limited to an optical scale as shown in FIGS. 4A and 4B. Accordingly, for the gradation scale, a magnetic scale as well as an optical scale having many aligned slits may be used, as well as not using a diffraction grating at all. In the reference-position detection method and apparatus for detecting a reference position of a scale, first and second patterns are disposed on the scale so that respective detection timings shift inversely to each other in response to a deviation in attitude of the scale. The first and second patterns are detected with timings different from each other by corresponding first and second sensor, and the reference position of the scale is determined according to outputs of respective sensors. Hence, even if the attitude of the scale changes from a predetermined attitude due to an inclination of the scale or a deviation in position of the scale, it is possible to exactly detect the reference position of the scale. Accordingly, it becomes also possible to increase the performance of a displacement measuring apparatus.

Claims: A reference-position detection method for detecting a reference position of a scale (5) moving in a predetermined attitude comprising the steps of supplying a first pattern (6a) and a second pattern (6b) to said scale (5), detecting said first pattern (6a) with a first sensor (8) and detecting said second pattern (6b) with a second sensor (9) and determining the reference position of said scale (5) according to the detection made in the detection step, characterized in that the detecting of said first pattern (6a) is performed at a first timing, the detecting of said second pattern (6b) is performed at a second timing different from said first timing, wherein said first timing and said second timing shift inversely to each other in response to a deviation in attitude of said scale (5) from its predetermined attitude. A reference-position detection apparatus for detecting a reference position of a scale (5) moving in a predetermined attitude having a first pattern (6a) and a second pattern (6b) provided on said scale (5) to be detected for determining the reference position of said scale (5), and having a first sensor (8) for detecting said first pattern (6a) and a second sensor (9) for detecting said second pattern (6b) characterized in that said first pattern (6a) is detected by said first sensor (8) at a first timing, said second pattern (6b) is detected by said second sensor (9) at a second timing different from said first timing, wherein one of said first pattern (6a) or said first sensor (8) is so arranged with respect to said second pattern (6b) or said second sensor (9), respectively, that said first timing and said second timing shift inversely to each other in response to a deviation in attitude of said scale (5) from its predetermined attitude. A reference-position detection method according to claim 1, characterized in that said first pattern (6a) and said second pattern (6b) are so supplied to said scale (5), which is rotating, that the line connecting said first pattern (6a) and said second pattern (6b) passes through the center of said rotating scale (5), and said first pattern (6a) is situated at a side opposite to said second pattern (6b) relative to the center of rotation. A reference-position detection method according to one of claims 1 or 3, characterized in that said detecting step comprises the steps of directing a radiant beam to said rotating scale (5), receiving a first reflected beam from said first pattern (6a) by said first sensor (8) and receiving a second reflected beam from said second pattern (6b) by said second sensor (9). A reference-position detection method according to claim 4, characterized in that said directing step comprises the steps of dividing a radiant beam from a light source (1) into a first radiant beam and a second radiant beam, irridating said first radiant beam and said radiant second beam on portions of said rotating scale (5) which are different from each other, and in that said first radiant beam and said second radiant beam are used for detecting said first pattern (6a) and said second pattern (6b). A reference-position detection method according to claim 5, characterized in that said detecting step comprises the step of irradiating said first radiant beam and said second radiant beam on said rotating scale (5) so that the line connecting respective incident positions of the radiant beams substantially passes through the center of rotation of said rotating scale (5), said first radiant beam is incident on a side opposite to said second radiant beam relative to the center of rotation. A reference-position detection method according to one of claims 5 or 6, characterized in that said first radiant beam and said second radiant beam are irradiated in the form of linear beam spots extending in the radial direction of said rotating scale (5). A reference-position detection method according to one of claims 1 or 3 to 7, characterized in that said detecting step comprises the steps of generating a first signal with a level dependant on the relative position of said first pattern (6a) and said first sensor (8), and generating a second signal with a level dependant on the relative position of said second pattern (6b) and said second sensor (9). A reference-position detection method according to claim 8, characterized in that said determining step comprises the steps of detecting when the level of said first signal and the level of the second signal coincide and generating a signal indicating the reference position according to the coincidence of the levels of said first signal and said second signal. A reference-position detection method according to claim 9, characterized in that said signal indicating the reference signal and an incremental signal generated by reading a gradation (100) on said rotating scale (5) are used to measure said deviation of attitude of said rotating scale (5). A reference-position detection apparatus according to claim 2, characterized in that said first pattern (6a) and said second pattern (6b) are so supplied to said scale (5), which is rotating, that the line connecting said first pattern (6a) and said second pattern (6b) passes through the center of said rotating scale (5), and said first pattern (6a) is situated at a side opposite to said second pattern (6b) relative to the center of rotation. A reference-position detection apparatus according to claim 11, characterized in that said first pattern (6a) and said second pattern (6b) are supplied to said rotating scale (5) with different distances to the center of rotation. A reference-position detection apparatus according to one of claims 11 or 12, characterized in that said first pattern (6a) and said second pattern (6b) are marks each extending in the radial direction of said rotating scale (5). A reference-position detection apparatus according to claim 13, characterized in that said marks are reflective films. A reference-position detection apparatus according to one of claims 2 or 11 to 14, characterized by a light source (1), the radiant beam of which is directed to said rotating scale (5), wherein the first sensor (8) and the second sensor (9) receive a first reflected beam and a second reflected beam, respectively. A reference-position detection apparatus according to claim 15, characterized in that said light source (1) is a laser. A reference-position detection apparatus according to one of claims 15 or 16, characterized by a beam splitter (3a) dividing said radiant beam of said light source (1) in a first radiant beam and a second radiant beam, a first projecting system for directing said first radiant beam to said rotating scale (5) and a second projecting system for directing said second radiant beam to said rotating scale (5). A reference-position detection apparatus according to claim 17, characterized in that said first projecting system and said second projecting system include respective cylindrical lenses (4a, 4b) and beam splitters (3b, 3c) and in that said first radiant beam and said second radiant beam are projected through said cylindrial lenses (4a, 4b) to form linear beam spots extending in the radial direction of said rotating scale (5). A reference-position detection apparatus according to claim 17 or 18, characterized in that said first sensor (8) receives a reflected beam from said first pattern (6a) and said second sensor (9) receives a reflected beam from said second pattern (6b), wherein said first sensor (8) generates a first signal with a level dependant on the relative position of said first pattern (6a) and said first sensor (8), and said second sensor (9) generates a second signal with a level dependant on the relative position of said second pattern (6b) and said second sensor (9). A reference-position detection apparatus according to claim 19, characterized in that a reference position signal is generated, when the levels of said first signal and said second signal coincide. A reference-position detection apparatus according to one of claims 11 to 20, characterized in that a gradation (100) is further provided on said rotating scale (5) which is read to generate an incremental signal. A reference-position detection apparatus according to one of claims 11 to 21, characterized in that said first sensor (8) and said second sensor (9) photoelectrically detects said first pattern (6a) an said second pattern (6b).

Inventors: ISHII SATOSHI, ISHIZUKA KOH, NISHIMURA TETSUHARU, TSUKIJI MASAAKI, ISHII, SATOSHI, ISHIZUKA, KOH, NISHIMURA, TETSUHARU, TSUKIJI, MASAAKI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1989
Main classification: G01D5
IPC codes: G01D5