Patent Publication Number: US-2007120047-A1

Title: Ring-configured photodiode array and optical encoders using the same

Description:
BACKGROUND OF THE INVENTION  
      Encoders provide a measurement of the position of a component in a system relative to some predetermined reference point. Encoders are typically used to provide a closed-loop feedback system to a motor or other actuator. For example, a shaft encoder outputs a digital signal that indicates the position of the rotating shaft relative to some known reference position that is not moving. Optical encoders utilize a light source and a photo detector to measure changes in the position of an encoding pattern that is carried on a code disk or strip.  
      In a transmissive encoder, the encoding disk/strip includes a series of alternating opaque and transparent bands. The light source is located on one side of the encoding pattern, and the photodetector is located on the other side of the encoding pattern. The light source and photodetector are fixed relative to one another, and the encoding pattern moves between the photodetector and the detector such that the light reaching the photodetector is interrupted by the opaque regions of the encoding pattern. The position of the encoding pattern is determined by measuring the transitions between the light and dark regions observed by the photodiode.  
      In a reflective encoder, the light source and photodetector are located on the same side of the encoding pattern, and the encoding pattern consists of alternating reflective and absorbing stripes. The light source is positioned such that light from the light source illuminates the encoding pattern and the light leaving the encoding pattern is then used to generate an image that is superimposed on one or more photodiodes that are fixed relative to the motion of the encoding pattern.  
      In a shaft encoder, the encoding pattern is typically part of a disk that is connected to the shaft and rotates past the photodetectors as the shaft moves. The encoding pattern consists of an annulus having a center that is coincident with that of the shaft. The photodetectors are mounted on a surface that is fixed such that the encoding pattern moves past the photodetectors. The photodetectors must be aligned with respect to the encoding pattern during the assembly of the part that utilizes the encoder.  
      There is a trend toward smaller parts having encoders with increased resolution. This trend requires increased accuracy in the alignment of the encoding pattern with the photodetector array. Providing the needed alignment substantially increases the cost of assembly of small high-resolution encoders.  
     SUMMARY OF THE INVENTION  
      The present invention includes a photodiode array and an encoder utilizing the same. The encoder includes the photodiode array, a code wheel, a light source, and an optical system. The photodiode array includes a plurality of photodiodes arranged in a ring, each photodiode includes an annular sector of the ring, each photodiode generates a signal determined by the optical radiation incident on the photodiode, the photodiode array is characterized by an array center. The code wheel includes an annular array of alternating bands disposed about a code wheel axis, the code wheel moving relative to the photodiode array about the code wheel axis. The light source illuminates the code wheel and the optical system forms an image of the code wheel having alternating light and dark bands on the photodiode array. In one aspect of the invention, the photodiodes are arranged in a plurality of groups around the ring. Each group includes a plurality of photodiodes in which each photodiode is assigned to a class. For each photodiode of a particular class in one of the groups, there is a corresponding photodiode of that class at a position diametrically opposed to that photodiode in the ring. In another aspect of the invention, the encoder includes a plurality of sum circuits, each sum circuit generating a signal related to a sum of the signals from all of the photodiodes in one of the classes. The encoder can be constructed in either a transmissive or reflective mode. In addition, encoders that utilize a plurality of channels can be constructed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  illustrates a transmissive encoder.  
       FIG. 2  illustrates one type of reflective encoder.  
       FIG. 3  illustrates another form of imaging encoder.  
       FIG. 4  is a top view of the encoding disk.  
       FIG. 5  is a cross-sectional view through line  5 - 5  shown in  FIG. 4 .  
       FIG. 6  illustrates a prior art two-channel encoder  200 .  
       FIG. 7  is a graph of the amplitude of the output of each photodetector as a function of position of the code disk/wheel image.  
       FIG. 8  illustrates two logic channel signals that are 90 degrees out of phase.  
       FIGS. 9 and 10  illustrate a photodiode array according to one embodiment of the present invention and the corresponding code wheel, respectively.  
       FIG. 11  is a cross-sectional view of a transmissive encoder for measuring the position of a shaft.  
       FIG. 12  illustrates a photodiode array in which only a portion of the ring is actually populated with photodiodes.  
       FIG. 13  is a cross-sectional view of a reflective imaging encoder according to one embodiment of the present invention.  
       FIGS. 14 and 15  illustrate another embodiment of a reflective encoder according to the present invention.  
       FIGS. 16 and 17  illustrate a photodiode array  180  and a corresponding code wheel  182  for a single channel encoder.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION  
      Refer now to  FIGS. 1-3 , which illustrate some typical encoder designs. The encoder can be divided into an emitter/detector module  15  and a code wheel or code strip. Module  15  includes an emitter  11  that illuminates a portion of the code strip  12 . A detector  13  views the illuminated code strip. The emitter typically utilizes an LED as the light source. The detector is typically based on one or more photodiodes.  FIG. 1  illustrates a transmissive encoder. In transmissive encoders, the light from the emitter is collimated into a parallel beam by a collimating optic such as lens  24 . Code strip  12  includes opaque stripes  16  and transparent stripes  17 . When code strip  12  moves between emitter  11  and detector  13 , the light beam is interrupted by the opaque stripes on the code strip. The photodiodes in the detector receive flashes of light. The resultant signal is then used to generate a logic signal that transitions between logical one and logical zero.  
      The detector can include an imaging lens  25  that images the collimated light onto the photodiode. Lens  25  can be used to adjust the size of the light stripes to match the size of the photodiode or photodiodes in the detector. When used in this manner, the photodetector is placed at a point between the code strip and the focal point of lens  25 . The distance between the photodetector and the lens determines the size of the code strip image on the photodetector.  
      In general, the collimator is constructed from two separate sub-modules that are provided to the manufacturer of the encoder. The first sub-module includes the light source consisting of emitter  11  and lens  24 . The second sub-module consists of photodetector  13  and lens  25 . Since the light is collimated, the only critical distances are those between emitter  11  and lens  24  and between lens  25  and photodetector  13 . The sub-module manufacturer can control these distances to a high level of precision. Hence, the tolerances that need to be maintained by the encoder manufacturer are substantially reduced in transmissive designs.  
       FIG. 2  illustrates one type of reflective encoder. In reflective encoders, the code strip includes reflective stripes  18  and absorptive stripes  19 . The emitter includes an optical system such as a lens  21  that images the emitter light source into the detector when the light strikes a reflective stripe on the code strip. The light from the emitter is reflected or absorbed by the stripes on the code strip. The output from the photodetector is again converted to a logic signal. In embodiments in which the photodetector includes a plurality of photodiodes that provide a signal that depends on matching an image of the strips to the photodiodes, a second lens  27  can be included to adjust the size of the code strip image to the size of the photodetectors in a manner analogous to that described above.  
       FIG. 3  illustrates another form of imaging encoder. An imaging encoder operates essentially the same as the reflective encoder described above, except that module  15  includes imaging optic  23  that forms an image of the illuminated code strip on the detector  14 . In addition, the light source is processed by lens  22  such that the code strip is uniformly illuminated in the region imaged onto the detector.  
      To simplify the following discussion, a multi-channel transmissive encoder will be used. Refer now to  FIGS. 4 and 5 , which illustrate a transmissive encoder.  FIG. 4  is a top view of the encoding disk and  FIG. 5  is a cross-sectional view through line  5 - 5  shown in  FIG. 4 . Code disk  41  includes an encoding pattern having a series of pie-shaped bands in an annular region around the outside edge of the code disk. The code disk  41  rotates about shaft  45  such that the encoding pattern remains fixed radially with respect to shaft  45 . The encoding pattern consists of alternating opaque and transmissive bands. Exemplary opaque bands are shown at  43 , and exemplary transparent bands are shown at  42 .  
      Referring to  FIG. 5 , the encoder includes a light source  51  and a light receiver  55  that are mounted on a body  56  that is fixed relative to the moving code disk. Light source  51  typically includes an LED  57  and a collimating lens  58  positioned such that the light leaving light source  51  is collimated. The light illuminates a number of adjacent bands.  
      If the light is perfectly collimated, the light pattern under code strip  44  is a series of light and dark bands having the same size as the bands on the code disk. This pattern is imaged onto a detector array  52  in receiver  55  by a lens  59  such that the size of the band pattern seen by the detector array is matched to the size of the individual detectors in the array.  
      In multichannel encoders, the properly magnified band pattern is imaged on a photodetector array. An image of one portion of the band pattern is generated on the photosensitive area of a photodiode in an array of photodiodes. To simplify the following discussion, drawings depicting the image of the encoding pattern and the surface area of the photodetectors on which the image is formed will be utilized. In each drawing, the image of the encoding pattern will be shown next to the photodiode array to simplify the drawing. However, it is to be understood that, in practice, the image of the encoding pattern would be projected onto the surface of the photodiode array. In addition, to further simplify the drawings, the light source and any collimating or imaging optics are omitted from the drawings.  
      Refer now to  FIG. 6 , which illustrates a prior art two-channel encoder  200 . Encoder  200  includes an encoding pattern that is imaged to form an image  221  that is viewed by a detector array  222 . The image  221  of the encoding pattern consists of alternating “white” and “black” bands. For the purposes of this example, it will be assumed that when a “white” band is imaged on the detector, the detector outputs its maximum signal value, and when a “black” band is imaged on the detector, the detector outputs its minimum value. It will also be assumed that the detector outputs an intermediate value when only a portion of a “white” band is imaged onto the detector.  
      Detector array  222  is constructed from 4 photodetectors labeled A, A′, B, and B′. Each photodetector views a portion of image  221  that has an area that is one half the area of one band in the image. The A′ and B′ detectors are positioned such that the A′ and B′ detectors generate the complement of the signal generated by the A and B detectors, respectively. The outputs of the A, A′, and B photodetectors are shown in  FIG. 7 , which is a graph of the amplitude of the output of each photodetector as a function of position of the encoding pattern image. To simplify  FIG. 7 , the output of the B′ photodetector has been omitted.  
      The signals generated by these detectors are combined by detector circuits  231  and  232  to generate two logic channel signals that are 90 degrees out of phase as shown in  FIG. 8 .  FIG. 8  illustrates the channel A and channel B signals when the encoding pattern is moving in one direction. If the encoding pattern were to move in the opposite direction, the channel B signal would lead the Channel A signal; however, the two signals would still be 90 degrees out of phase.  
      Circuits for converting the photodiode output signals to the channel signals shown in  FIG. 6  are known in the art, and hence, will not be discussed in detail here. For the purposes of this discussion, it is sufficient to note that the channel signal corresponding to a pair of photodiode output signals such as A and A′ switches between logical one and logical zero at the points at which the output of detector A is equal to the output of detector A′.  
      The two channel signals provide a measurement of the direction of motion of the image of the code strip relative to the detector array. In addition, the two channel signals define  4  states that divide the distance measured by one black and one white band into quarters. The 4 states correspond to a two-bit binary number in which the first bit is determined by the value of the channel A signal and the second bit is determined by the value of the channel B signal.  
      It should be noted that the detector array can be viewed as an arc of a circle having the same radius and center as the circle containing the code disk band image. If the centers of these two circles are displaced from one another, or if the planes of the two circles are not aligned, the operation of the encoder is degraded significantly. The present invention provides an encoder array that is more tolerant of such alignment errors.  
      Refer now to  FIGS. 9 and 10 , which illustrate a photodiode array according to one embodiment of the present invention and the corresponding code wheel, respectively. Photodiode array  100  is constructed from an annular ring of photodiodes  101 . Each photodiode has a truncated pie shape consisting of an area bounded by two circles having the same center and different radii and two radii of the larger circle.  
      Photodiode array  100  is configured to operate in the two-channel mode discussed above. The image of the entire encoding pattern is projected onto the photodiode array  100  such that each band on the encoding pattern covers two of the photodiodes on photodiode array  100 . The photodiodes are divided into 4 classes in a manner analogous to that described above with reference to  FIGS. 6-8 . All of the photodiodes within a class are connected together. Hence, all of the A photodiodes are summed to form a sum A signal and all of the A′ photodiodes are summed to form a sum A′ signal. The sum A and sum A′ signals are then input to an A detector circuit that provides a channel A signal analogous to that described above. Similarly, the B and B′ signals are summed and input to a B detector circuit to generate a channel B signal. The channel A and channel B signals are then processed as described above. The corresponding code wheel is shown in  FIG. 10 . Code wheel  110  has alternating opaque and clear regions. Exemplary opaque and clear regions are shown at  111  and  112 , respectively.  
      The combination of the code wheel and photodiode array can be used to construct a transmissive encoder. Refer now to  FIG. 11 , which is a cross-sectional view of a transmissive encoder  120  for measuring the position of a shaft  121 . Code wheel  100  is attached to the shaft and moves therewith. Code wheel  110  is illuminated with collimated light from a light source  124 . Light source  124  can be constructed from a number of LEDs and collimating lenses. It should be noted that light source  124  need only illuminate the encoding pattern on the code wheel, and hence, can include an opening for shaft  121 . A lens  122  can be used to adjust the image of the code wheel such that the image matches the physical size of the photodiode array chip.  
      The outputs of the various photodiodes in photodiode array  101  are input to a controller  123  that sums the outputs of each class of photodiodes and combines the summed signals to provide the channel A and channel B signals discussed above. While controller  123  is shown as being separate from photodiode array  100 , it is to be understood that embodiments in which photodiode array  100  and controller  123  are part of the same silicon integrated circuit substrate can also be constructed. Such single chip solutions are particularly attractive in designs that utilize CMOS photodiodes, since the control circuitry can be fabricated in the same CMOS process as the photodiodes.  
      Referring again to  FIG. 9 , the photodiodes are distributed around the ring. Ideally, the rotation axis of the image of the code wheel intersects the photodiode array at the center of the photodiode array. However, the present invention compensates for misalignment between the centers of the code disk image and photodiode array. Hence, so long as the axis intersects the photodiode array at a point proximate to the center of photodiode array, the encoder will function. It can be shown from computer simulations that this embodiment is more resistant to misalignment of the centers of the code wheel image relative to the center of the photodiode array than encoders that utilize a single sector of photodiodes. In addition, this arrangement compensates for errors due to any eccentricity of the code wheel pattern relative to the photodiode pattern.  
      In the above-described embodiments of the present invention, the photodiode array consists of a fully populated ring of photodiodes. However, embodiments in which the ring is only partially populated can be constructed. Refer now to  FIG. 12 , which illustrates a photodiode array in which only a portion of the ring is actually populated with photodiodes. Photodiode array  150  includes 4 groups of photodiodes arranged such that each group is diagonally across from a corresponding group. This arrangement provides significantly better performance than the prior art single group arrangements discussed above while requiring fewer photodiodes to be operational in the chip on which the photodiode array is constructed. It should be noted that the precise positions on the ring structure at which photodiodes are to be placed can be decided after a fully populated ring structure is constructed and tested.  
      In addition to providing tolerance for alignment errors, the present invention also improves the signal-to-noise ratio of the signals used to provide the channel signals. By summing the outputs of a large number of photodiodes, the effective area of the photodiodes contributing to each signal is increased. Hence, an improved signal-to-noise ratio is obtained. There are many applications in which very small encoders are desired. As the size is reduced, the area of silicon available for each photodiode also decreases, and hence, the signal-to-noise ratio increases in prior art devices. The ring photodiode array of the present invention provides a mechanism for improving the signal-to-noise ratio and thereby allowing smaller encoders to be constructed.  
      The above-described embodiments of the present invention have been directed toward a transmissive encoder. However, reflective encoders can also be constructed utilizing a photodiode array according to the present invention. Refer now to  FIG. 13 , which is a cross-sectional view of a reflective encoder according to one embodiment of the present invention. Encoder  170  measures the movement of shaft  121 . A reflective code wheel  171  is attached to shaft  121  and moves with the shaft. The bottom surface of code wheel  171  includes a pattern of alternating reflective and absorptive bands. A light source  174  illuminates the bottom surface. The band pattern is imaged onto a ring shaped photodiode array  175  by lens  172 . Since the lens provides the imaging of the stripe pattern onto the photodiode array, light source  174  can be a diffuse source that illuminates the striped regions of the code wheel. A controller  176  performs the summing operations on the signals from the various classes of photodiode.  
      Refer now to  FIGS. 14 and 15 , which illustrate another embodiment of a reflective encoder according to the present invention.  FIG. 14  is a cross-sectional view of encoder  200 , and  FIG. 15  is top view of emitter-detector module  203  shown in  FIG. 14 . Encoder  200  includes a code wheel  201  having the encoding pattern thereon. Code wheel  201  is attached to shaft  202  and rotates therewith. An emitter-detector module  203  illuminates code wheel  201  and detects the light reflected therefrom. Emitter-detector module  203  includes a chip  210  that contains a ring-configured detector  204 . A light source  205  is mounted on this chip in a region not utilized for the detector. These components are mounted on a substrate  207  and are encapsulated in a layer  206  of clear medium such as an epoxy resin. The top surface  208  of the encapsulating medium is molded to form a lens that directs the light from light source  205  at the encoding pattern and images the returning light onto detector  204 .  
      The above-described embodiments of the present invention have been directed to two channel encoders. However, the present invention can be used to implement other types of encoders. For example, in a one channel encoder, the photodiode array consists of alternating photodiodes that provide the signals A and A′. Refer now to  FIGS. 16 and 17 , which illustrate a photodiode array  180  and a corresponding code wheel  182  for a single channel encoder. In this case, the photodiode array areas are twice as large as those discussed above for the same number of code wheel bands, and the image of the code wheel on the photodiode array is such that the light from one white band covers the A detector when the shadow of the adjacent black bands covers the A′ detector.  
      Various modifications to the present invention will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Accordingly, the present invention is to be limited solely by the scope of the following claims.