Patent Publication Number: US-4549808-A

Title: Movable aperture photoelectric measuring instrument

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a photoelectric length measuring instrument for measuring changes in position of an object, of the type wherein an optical system is provided for projecting the image of an aperture onto the object, and for projecting a reflected image of this aperture back from the object to the instrument. 
     German Pat. No. 20 23 265 discloses a prior art photoelectric measuring system for the determination of changes in position of an object, in which a periodically oscillating illuminated aperture is provided for scanning the object to be measured. An optical system is provided for imaging the oscillating image of the aperture projected onto the object back on to the oscillating aperture. This oscillating aperture simultaneously functions as a source of illumination light and as a scanning element. The oscillating aperture periodically scans the position of the aperture image in its oscillation plane. The position of this image represents a measure of the change of the position of the object to be measured. In order to allow relatively large position deviations of the object to be measured, the oscillating system is arranged slidably in the field of view of the reproducing optical system. For the determination of the displacement path, this oscillating system is mechanically coupled with a length measuring system. 
     In this prior art measuring system, the oscillating aperture is fastened to a vibrating or oscillating string. This oscillating system requires for its excitation an electro-mechanical oscillator. The amplitude of oscillation is regulated by means of a velocity sensor which supplies an input signal to a servo system which symmetrically adjusts the oscillating aperture with respect to the perpendicular which extends from a mirror connected to the object to be measured and passes through the center of the reproduction objective lens. Furthermore, a separate length measuring system is required for the determination of the displacement of the oscillating aperture. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an improved photoelectric measuring system of the general type described above which provides substantially simplified construction without a reduction in measuring accuracy. 
     According to this invention, a photoelectric length measuring instrument of the general type described above is provided with means for defining at least one measuring graduation mounted to the at least one aperture to move with the aperture. A scanning unit is mounted to scan the at least one measuring graduation to measure the position of the graduation and thereby of the aperture. In one preferred embodiment of this invention, the image of the light emitting aperture is reflected off of the object being measured and is projected onto a second, receiving aperture rather than onto the first aperture. The second receiving aperture is aligned with the first aperture such that the position of the second aperture at which the reflected image of the first aperture coincides with the second aperture is a measure of the position of the object being measured. In this embodiment, a measuring graduation is mounted to the first and second apertures to move with these apertures, and a scanning unit is mounted to scan the measuring graduation to measure the position of the graduation and thereby of the first and second apertures. This invention provides the important advantage that it makes possible an improved measuring instrument having a large measuring range which operates at high accuracy and can be fabricated economically. Further advantageous features of this invention are set forth in the dependent claims defined below. 
     The invention itself, together with further objects and attendant advantages, will best be understood by reference to the following detailed description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of a measuring instrument which incorporates a first preferred embodiment of this invention. 
     FIG. 2 is a waveform diagram of three signals generated by the embodiment of FIG. 1. 
     FIG. 3a is a plan view of a first aperture plate suitable for use in the embodiment of FIG. 1. 
     FIG. 3b is a sectional view taken along lines 3b--3b of FIG. 3a. 
     FIG. 4 is a plan view of a second aperture plate suitable for use in the embodiment of FIG. 1. 
     FIG. 5 is a plan view of a third aperture plate suitable for use in the embodiment of FIG. 1. 
     FIG. 6 is a plan view of a fourth embodiment of an aperture plate suitable for use with this invention. 
     FIG. 7a is a sectional view of a fifth preferred embodiment of this invention taken along lines 7a--7a of FIG. 7b. 
     FIG. 7b is a sectional view taken along line 7b--7b of FIG. 7a. 
     FIG. 7c is side elevational view taken along line 7c--7c of FIG. 7a. 
    
    
     DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
     Turning now to the drawings, FIG. 1 is a schematic representation of a measuring instrument which incorporates a first preferred embodiment of this invention. This measuring instrument includes a transparent plate 1 which acts as a carrier for an opaque, photoimpermeable diaphragm 3. This diaphragm 3 refines an aperture 2 and a measuring graduation 4. The aperture 2 is illuminated by means of a lamp 5. Light from the lamp 5 passes through lenses 6a, 6b and a beam splitter 7 before impinging on the aperture 2. The light rays emerging from the aperture 2 pass through a reproduction objective lens 8 and are projected onto a plane mirror 9, which is fastened to an object (not shown) to be measured. The image of the aperture 2 which is reflected by the plane mirror 9 passes through the reproduction objective lens 8 and is re-imaged in the focal plane of the reproduction objective lens 8 in the plane of the plate 1. The aperture 2 serves as a scanning element for this reflected image of the aperture 2. Light rays which pass from the mirror 9 through the aperture 2 are projected by the objective 6b, the beam spliter 7, and the lens 10 onto a photosensor 11. This photosensor serves to evaluate the amplitude of the light passing from the mirror 9 through the aperture 2 during scanning. 
     The graduation 4 is scanned photoelectrically with the aid of a scanning plate 12 that defines at least one division field which corresponds to the graduation 4. A corresponding number of photoelements 13 measure the amplitude of light which passes from the lamp 5 by means of a mirror 14 through the graduation 4 and the scanning plate 12. 
     As shown in FIG. 1, the normal axis to the plane mirror 9 encloses an angle φ with the optical axis 15 of the autocollimator. In order to allow the determination of the magnitude of this angle φ, the transparent plate 1, with the aperture 2 and the graduation 4 thereon, is slidably arranged obliquely to the longitudinal extent of the aperture 2. As the plate 1 moves in the X direction, there is a certain position X 1  at which the aperture 2 is positioned such that the reflection of the aperture 2 generated at the focal plane of the lens 8 coincides with the position of the aperture 2, and thereby allows the reflected light rays to pass via the beam splitter 7 onto the photosensor 11. During the scanning operation, displacement of the plate 2 brings about a modulation of the light beam falling on the photosensor 13. The photosensor 13 generates a periodic output signal S 2  which is a measure of the position of the plate 2 and can be evaluated in a known manner and applied to a counter which acts to count cycles of the signal S 2  . As shown in FIG. 2, The output signal S 1  of the photosensor 11 indicates when the reflected image of the aperture 2 coincides with the position of the aperture 2. The output signal S 2  of the photosensor 13 is a periodic signal as described above indicative of the movement of the plate 2. The position X 1  of the plate 2 at which the aperture image coincides with the aperture can be determined from the relation X 1  =Z 1  ·C, in which Z 1  signifies the number of cycles (light-dark periods) of the periodic signal S 2  generated by the photosensor 13 and C signifies the grid constant of the division 4. The angle φ  can then be determined according to the following relationship: 
     
         φ=arctan X.sub.1 /f, 
    
     wherein f equals the focal length of the reproduction objective lens 8. For small angles, the following approximation obtains: 
     
         φ=X.sub.1 /f=Z.sub.1 ·C/f. 
    
     The focal length f and the grid constant C can be expediently chosen in such a way that one counting step of the counter (one cycle of the signal S 2 ) corresponds to the desired measuring step of the angle Δφ as follows: 
     
         Δφ=C/f. 
    
     If need be, the period of the graduation 4 can be subdivided accordingly to known methods by the factor N. For example, several scanning fields can be arranged phase shifted with respect to one another on the scanning plate 12 and associated photosensors can be used to measure smaller measuring steps Δφ=C/(N·f). The zero position X 0  of the measuring magnitude X can be obtained or reproduced by scanning reference marks 16 associated with the graduation 4 on the plate 1 by means of a photosensor (not shown). In FIG. 2, the signal S 3  indicates the manner in which the output of this photosensor peaks at the selected zero position X 0 . 
     As shown in FIGS. 3a and 3b, the transparent plate 1 which defines the aperture 2, the graduation 4, and the reference mark 16 is in this embodiment fastened to a carrier 17 which is linearly guided in a frame 18 by means of spheres 19. The movement of the plate 1 in the X direction (perpendicular to the longitudinal extent of the aperture 2) is an oscillatory movement which is driven by a motor 20. This motor 20 rotates an eccentric 21 that serves to shift the carrier 17 periodically in the X direction. 
     FIG. 4 shows an alternative embodiment of an aperture plate suitable use in the instrument of FIG. 1. As shown in FIG. 4, a plate 1 1  is provided which defines an aperture 2 1 , a graduation 4 1 , and a reference mark 16 1 . This plate 1 1  is fastened to a carrier 17 1 . A motor 20 1  is provided which rotates a crank 22 1 . This crank 22 1  is coupled to the carrier 17 1 , which is supported by two revolving rocker arms 23 1 . With this linkage, the motor 22 1  drives the carrier 17 1  in a two dimensional, periodic, translatory movement in the X and Y directions of a predetermined coordinate system. In this embodiment, the rocker arms 23 1  are the same length as the crank 22 1  and are arranged parallel to the crank 22 1 . The rocker arms 23 1  and the cranks 22 1  are pivotally mounted to articulate on the carrier 17 1 . 
     FIG. 5 shows a schematic representation of a third aperture plate suitable for use in this invention. As shown in FIG. 5, a plate 1 2  is provided which defines two elongated apertures 2 2  &#39;, 2 2  &#34; which are oriented perpendicularly to one another. The plate 1 2  also defines a graduation 4 2  and a reference mark 16 2 . The plate 1 2  is mounted on a carrier 17 2  which is linearly guided in a frame 18 2  by means of spheres 19 2 . The plate 1 2  is guided along the direction oriented at an angle π/4 with respect to a predetermined X and Y coordinate system. The plate 1 2  is oscillated along this direction by means of a motor 20 2  which rotates a crank 22 2 . This crank 22 2  is connected to the carrier 17 2  by a connecting rod 24 2  which is articulated on the carrier 17 2 . The aperture 2 2  &#39; is oriented in the X direction and the aperture 2 2  &#34; is oriented in the Y direction of the coordinate system, so that the apertures 2 2  &#39;, 2 2  &#34; move successively obliquely through their longitudinal extents through the optical axis of the measuring instrument as the plate 1 2  is displaced periodically by the motor 20 2 . By means of the graduation 4 2  and the reference mark 16 2 , the position of the two apertures 2 2  &#39;, 2 2  &#34; is measured, so that the angular position of the plane mirror connected with the object to be measured can be determined in both coordinate directions. 
     FIG. 6 shows a schematic representation of a fourth embodiment of an aperture plate suitable for use with the invention. As shown in FIG. 6, a plate 1 3  is provided which defines two apertures 2 3  &#39;, 2 3  &#34; oriented perpendicularly to one another. This plate 1 3  is mounted on a carrier 17 3 , which is moved in a periodic pattern by means of a motor 20 3 . This motor 20 3  rotates a crank 22 3  which is coupled in an articulated manner to the carrier 17 3 . The carrier 17 3  is further supported by two revolving rocker arms 23 3 , such that the carrier 17 3  undergoes a two dimensional, periodic, translatory movement in the X and Y directions of a given coordinate system as the crank 22 3  is rotated by the motor 20 3 . The rocker arms 23 3  are the same length as the crank 22 3  and are arranged parallel to the crank 22 3 . The rocker arms 23 3  and the crank 22 3  are articulated on the carrier 17 3 . A graduation 4 3  &#39; and a reference mark 16 3  &#39; are mounted on the plate 1 3  and are associated with the aperture 2 3  &#39;. An additional graduation 4 3  &#34; and an associated reference mark 16 3  &#34; are also mounted on the plate 1 3  and associated with the aperture 2 3  &#34;. The longitudinal extent of the aperture 2 3  &#39; is oriented in the X direction and the longitudinal extent of the aperture 2 3  &#34; is oriented in the Y direction of the coordinate system, so that the apertures 2 3  &#39;, 2 3  &#34;, during the translatory displacement of the plate 1 3  in the X and Y directions move successively transversely to their longitudinal extents through the optical axis of the measuring system. By means of the graduations 4 3  &#39;, 4 3  &#34; and the associated reference marks 6 3  &#39;, 6 3  &#34; the movements of the two apertures 2 3  &#39;, 2 3  &#34; are measured. In this way, it is possible to determine the angular position of the plane mirror connected with the object to be measured in both coordinate directions. 
     FIGS. 7a, 7b, and 7c show a measuring instrument which comprises a casing 25. This casing 25 serves to mount a collimator tube 26 which serves as a light emitter and two collimator telescopes 27 and 28 which serve as light receivers (FIG. 7c). The casing 25 serves to mount a carrier 17 4  on which is secured a transparent plate 1 4  oriented perpendicularly to the collimator tube 26 and the two collimator telescopes 27, 28. A motor 20 4  is mounted within the casing 26 and serves to rotate a crank 22 4 . This crank 22 4  is pivotally coupled to the carrier 17 4 , and two revolving rocker arms 23 4  are also interconnected between the casing 25 and the carrier 17 4 . When the crank 22 4  is rotated by the motor 20 4 , the carrier 17 4  undergoes a two dimensional, periodic, translatory movement in the X and Y directions of a given coordinate system. As before, the rocker arms 23 4  have the same length as the crank 22 4  and are arranged parallel to the crank 22 4 . Similarly, the rocker arms 23 4  and the crank 22 4  are articulated on the carrier 17 4 . As shown in FIG. 7b, the plate 1 4  defines a first pair of a light emitting aperture 29&#39; and a light receiving aperture 29&#34; as well as a second pair of a light emitting aperture 30&#39; and a light receiving aperture 30&#34;. The apertures 29&#39;, 29&#34; are aligned such that both are oriented with their longitudinal extent oriented in the X direction. Similarly, the apertures 30&#39;, 30&#34; are aligned with respect to one another such that their longitudinal axis extends in the Y direction of the given coordinate system. A graduation 4 4  &#39; and an associated reference mark 16 4  &#39; are associated on the plate 1 4  with the apertures 29&#39;, and 29&#34;, and a graduation 4 4  &#34; and an associated reference mark 16 4  &#34; are defined by the plate 1 4  in association with apertures 30&#39;, 30&#34; . 
     As shown in FIG. 7a, the light emitting apertures 29&#39;, 30&#39; are illuminated in the collimator tube 26 by a lamp 5 4  which directs light through a lens 31. The light rays which emerge from the emitting apertures 29&#39;, 30&#39; pass through an objective lens 32 onto a plane mirror (not shown) which is fastened to an object to be measured. The image of the emitting aperture 29&#39; reflected by the plane mirror is reproduced or projected by means of a reproduction objective 33 in the collimator telescope 27 onto the receiving aperture 29&#34;, and the reflected image of the light emitting aperture 30&#39; reflected by the plane mirror is reproduced by means of a reproduction objective lens 34 and the collimator telescope 28 onto the light receiving aperture 30&#34;. The light rays which pass through the receiving apertures 29&#34; are collected (in a manner not shown) by a lens onto a photosensor; similarly, the light rays which pass through the receiving aperture 30&#34; are collected by a lens 35 onto a photosensor 36. 
     The graduation 4 4  &#39; is scanned by means of a scanning plate 12 4  and a photosensor 13 4 . The graduation 4 4  &#39; and the scanning plate 12 4  are illuminated by a light-emitting diode 37. The graduation 4 4  &#34; is scanned by an analogous scanning unit (not shown). By means of the graduations 4 4  &#39;, 4 4  &#34; and the associated reference marks 16 4  &#39;, 16 4  &#34;, the movements of the aperture pairs 29&#39;, 29&#34;; 30&#39;, 30&#34; are measured. In this way, the angular position of the plane mirror about axes parallel to the aperture pairs 29&#39;, 29&#34;; 30&#39;, 30&#34; in directions parallel to the coordinate directions are determined. 
     The aperture pair 29&#39;, 29&#34; and the aperture pair 30&#39;, 30&#34; can in an alternate embodiment in each case consist of a single aperture. 
     In each of the preferred embodiments described above, the apertures 2, 29 and 30 as well as the associated graduations 4 and reference marks 16 on the transparent plate 1 are generated by vaporizing a photoimpermeable layer onto the plate, and by then forming patterns in this layer by photolithographic techniques. 
     The measuring instrument shown in FIGS. 7a, 7b, and 7c is particularly advantageous when large measuring ranges are required. This is because the direct current constituent of the signal S 1  which arises through the reflection on the back of the plate 1 and the aperture forming layer is largely excluded. That is, the minimum value of the signal S 1  is more nearly equal to zero. 
     Of course, it shoud be understood that a wide range of changes and modifications to the preferred embodiments described above will be apparent to those skilled in the art. It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, which are intended to define the scope of this invention.