Patent Publication Number: US-2007120048-A1

Title: Reflective encoder module

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. A linear encoder measures the distance between the present position of a moveable carriage and a reference position that is fixed with respect to the moveable carriage as the moveable carriage moves along a predetermined path.  
      Optical encoders utilize a light source and a photodetector to measure changes in the relative position of the carrier that includes an encoding pattern. In a transmissive encoder, the carrier includes a pattern consisting of a series of alternating opaque and transparent bands. The light source is located on one side of the carrier on which this pattern is located, and the photodetector is located on the other side of the carrier. The light source and photodetector are fixed relative to one another, and the carrier moves between the light source and the photodetector such that the light reaching the photodetector is interrupted by the opaque regions of the pattern. The position of the carrier is determined by measuring the transitions between the light and dark regions observed by the photodetector.  
      In a reflective encoder, the light source and photodetector are located on the same side of the carrier, and the encoding pattern consists of alternating reflective and absorbing bands. The light source is positioned such that light from the light source is reflected onto the photodetector when the light is reflected from the reflective bands.  
      Transmissive encoders have a number of advantages over reflective encoders in terms of tolerance, cost of code strips, and contrast ratios. In a transmissive encoder, the light from the light source is collimated before it reaches the carrier, and hence, the light leaving the carrier is also collimated. The detection assembly needs only to image this collimated light onto the detector surface.  
      In a reflective encoder, the distance between the carrier and the detector is critical as either the pattern itself or the light source as seen in the reflected light from the reflective bands is imaged into the detector. Hence, if there is an error in the carrier to detector module distance, the image will be out of focus and errors will result. In addition, the bands for reflective encoders have a contrast ratio determined by the ratio of the reflectance of the reflective and absorptive regions. This ratio tends to be less than the ratio of the absorbance of the clear and opaque regions of a transmissive code strip.  
      Unfortunately, transmissive encoders require that the two separate components, the light source and photodetector, be mounted and aligned with one another at the time of assembly of the encoder. This increases the burden on the manufacturer of the final product that incorporates the encoder. Reflective encoders, in contrast, are constructed from a single emitter-receiver element that is packaged together with the various optical components for imaging the light source onto the photodetector. Hence, the manufacturer only has to mount and align one component. Ideally, the manufacturer would like to have a reflective encoder that has the relaxed tolerances associated with a transmissive encoder.  
     SUMMARY OF THE INVENTION  
      The present invention includes an encoder having a carrier that passes through an opening in a mounting body. The carrier includes an encoding region having a plurality of clear and opaque regions, the carrier having first and second surfaces. The clear and opaque regions of the carrier pass through the opening in the mounting body when the carrier moves relative to the mounting body. A light emitter generates a light signal that passes through the carrier, the light emitter being located adjacent to the first side of the carrier and separated therefrom. A light reflector is attached to the mounting body at a position such that the light reflector directs the light signal through the second surface of the carrier. A photodetector measures light leaving the first surface of the carrier when one of the clear regions passes through the light signal as the carrier moves relative to the mounting body. The photodetector is located on the same side of the carrier as the light emitter. The light emitter and photodetector can be attached to the mounting body. The light reflector can include one or more mirrors positioned such that collimated light generated by the light emitter passes through the carrier at right angles.  
    
    
     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 reflective encoder.  
       FIG. 4  is a top view of an encoder showing the carrier and the underlying emitter-detector module.  
       FIG. 5  is a cross-sectional view through line  5 - 5  shown in  FIG. 4 .  
       FIG. 6  illustrates an embodiment of an encoder according to the present invention in which the light emitter is placed as close to the photodetector as possible.  
       FIG. 7  is a top view of a carrier with the light emitter and photodetector positioned in a radial manner under the encoding bands.  
       FIGS. 8 and 9  are cross-sectional views of additional embodiments of an encoder according to the present invention.  
       FIG. 10  is a top view of a linear encoder according to one embodiment of the present invention.  
       FIG. 11  is a cross-sectional view through line  11 A- 11 A shown in  FIG. 10 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION  
      The manner in which the present invention provides its advantages can be more easily understood with reference to  FIGS. 1-3 , which illustrate some typical encoder designs. The encoder can be divided into an emitter/detector module  15  and a carrier that includes the encoding pattern. Module  15  includes an emitter  11  that illuminates a portion of the carrier  12 . The illuminated pattern on the carrier is viewed by a detector  13 . 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 . The carrier  12  includes opaque bands  16  and transparent bands  17 . When carrier  12  moves between emitter  11  and detector  13 , the light beam is interrupted by the opaque bands on the carrier. 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 bands 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 carrier and the focal point of lens  25 . The distance between the photodetector and the lens determines the size of the code pattern image on the photodetector.  
      In general, the collimator is constructed from two separate sub-modules that are provided to the manufacturer of the completed 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 . These distances can be controlled to a high level of precision by the sub-module manufacturer. 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 carrier includes reflective bands  18  and absorptive bands  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 band on the carrier. The light from the emitter is reflected or absorbed by the bands on the carrier. 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 bands to the photodiodes, a second lens  27  can be included to adjust the size of the pattern image to the size of the photodetectors in a manner analogous to that described above.  
       FIG. 3  illustrates another form of a reflective encoder that will be referred to as an imaging encoder in the following discussion. 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 pattern on the carrier onto the detector  14 . In addition, the light source is processed by lens  22  such that the pattern is uniformly illuminated in the region imaged onto the detector.  
      The present invention combines the benefits of the single module feature of a reflective encoder with the advantages of a transmissive encoder. The present invention can be used to construct both rotational encoders in which the carrier is a code disk and linear encoders in which the carrier is a code strip. To simplify the following discussion, the present invention will first be explained in terms of rotational encoders. The manner in which the present invention is used to construct a linear encoder will then be discussed in more detail.  
      Refer now to  FIGS. 4 and 5 , which illustrate a rotational encoder  40  according to one embodiment of the present invention.  FIG. 4  is a top view showing the code disk  41  and the underlying emitter-detector module  55 .  FIG. 5  is a cross-sectional view through  20  line  5 - 5  shown in  FIG. 4 .  
      Code disk  41  has an area at the middle and a series of clear and opaque bands in a region  44  along its outer edge. An exemplary transparent band is shown at  42 , and an exemplary opaque band is shown at  43 . Code disk  41  rotates about shaft  46 . The present invention includes an encoder having a carrier that passes through an opening in a mounting body. The carrier includes an encoding region having a plurality of clear and opaque regions, the carrier having first and second surfaces. The clear and opaque regions of the carrier pass through the opening in the mounting body when the carrier moves relative to the mounting body. A light emitter generates a light signal that passes through the carrier, the light emitter being located adjacent to the first side of the carrier and separated therefrom. A light reflector is attached to the mounting body at a position such that the light reflector directs the light signal through the second surface of the carrier. A photodetector measures light leaving the first surface of the carrier when one of the clear regions passes through the light signal as the carrier moves relative to the mounting body. The photodetector is located on the same side of the carrier as the light emitter. The light emitter and photodetector can be attached to the mounting body. The light reflector can include one or more mirrors positioned such that collimated light generated by the light emitter passes through the code strip at right angles. The central region of code strip  41  between shaft  46  and the plurality of opaque bands is also transparent in this embodiment. In the view shown in  FIG. 4 , the code disk is positioned such that one of the transparent regions overlies the emitter-detector module  55 .  
      Refer now to  FIG. 5 . Code disk  41  is suspended such that the bands pass through a gap  48  in a mounting body  56 . Emitter-detector module  55  is located below code disk  41  within this gap. Emitter-detector module  55  includes a light emitter  51  and a photodetector  52 . Light emitter  51  preferably includes a light source such as an LED and a collimating lens. Photodetector  52  can be constructed from one or more photodiodes and, optionally, a lens to adjust the magnification of the code disk image on the photodiodes. To simplify the drawing, the various lenses have been omitted in light emitter  51  and photodetector  52 .  
      The upper surface of gap  48  includes a mirror  57 . Mirror  57  reflects the collimated light  58  generated by light emitter  51  back down through the band region of code disk  41 . Mirror  57 , in effect, creates a virtual image of a collimated light source above code disk  41 . Hence, encoder  40  behaves substantially the same as a conventional transmissive encoder while allowing the light emitter and photodetector to be packaged in a single emitter-detector module  55  that is mounted on mounting body  56 . It should be noted that the distance between code disk  41  and emitter-detector module  55  is not critical, since the various lens that have critical spacings with respect to the light emitter and photodetector are packaged in emitter-detector module  55 .  
      The angle of incidence of the collimated light on code disk  41  is preferably 90 degrees to better simulate a collimated source directly above photodetector  52 . The arrangement shown in  FIG. 5  can only approximate such an arrangement. However, by moving the emitter  51  closer to photodetector  52  as shown in  FIG. 6 , this condition can be substantially satisfied.  FIG. 7  illustrates an embodiment of an encoder according to the present invention in which light emitter  51  is placed as close to photodetector  52  as possible. It should be noted that light emitter  51  can be placed below the region  44  of code disk  41  that includes the opaque and clear bands if light emitter  51  is aligned with photodetector  52  such that each clear band allows the light from light emitter  51  to reach mirror  57  when photodetector  52  is also under the clear band. Such an arrangement is shown in  FIG. 7 , which is a top view of code disk  41  with the light emitter and photodetector positioned in a radial manner under the code disk.  
      Refer now to  FIG. 8 , which is a cross-sectional view of another embodiment of an encoder according to the present invention. Encoder  70  provides the desired normal incidence of the collimated light on code disk  41  by replacing mirror  57  with a mirror  59  that is angled with respect to the plane of code disk  41  such that the light leaving mirror  59  strikes code disk  41  at  90  degrees. In this embodiment, the mounting body  86  is similar to mounting body  56  discussed above except for the portion used to mount mirror  59 .  
      While the arrangement shown in  FIG. 8  solves the problem of providing normal incidence for the collimated light, the arrangement complicates the optics in light emitter  81 . To provide a collimated beam that leaves the light emitter at the correct angle, either the plane lens in light emitter  81  must be tipped or the collimating lens must be off center. Such an arrangement places constraints on the fabrication of emitter-detector module  55 .  
      Refer now to  FIG. 9 , which is a cross-sectional view of another embodiment of an encoder according to the present invention. Encoder  80  provides the desired normal incidence of the collimated light on the code strip while utilizing a more conventional optical arrangement in which the collimated light leaves light emitter  91  at an angle that is normal to the surface of the LED and code disk  41 . Encoder  80  utilizes two mirrors shown at  61  and  62  in a mounting body  96  to redirect the collimated light from light emitter  91  in emitter-detector module  55 A back through code disk  41  at right angles to the plane of code disk  41  and photodetector  52 . Since mirrors  61  and  62  reflect the collimated light, the distance between the mirrors is not critical, and hence, light emitter  91  can be placed under the clear interior region of code disk  41 .  
      The above-described embodiments of the present invention have been directed to shaft encoders in which the angle of rotation of a shaft such as shaft  46  shown in  FIG. 4  is measured. However, embodiments of the present invention directed to providing a measurement of a linear displacement can also be constructed using an analogous mounting body and an emitter-detector module. Refer now to  FIGS. 10 and 11 , which illustrate one embodiment of a linear encoder according to the present invention.  FIG. 10  is a top view of a linear encoder  90  that measures the displacement of a linear code strip  95  with respect to a mounting body  96  having a light emitter  91  and light detector  52  arranged in an emitter-detector module similar to that discussed above with reference to  FIG. 9 .  FIG. 11  is a cross-sectional view through line  11 A- 11 A shown in  FIG. 10 . The code strip includes a series of clear and opaque bands in a region  94  along one edge of the code strip. An exemplary opaque band is shown at  93 . It should be noted that the same emitter-detector module and mounting body used in the embodiments shown in  FIGS. 6 and 8  could also be utilized in encoder  90 .  
      In the above-described embodiments of the present invention, the encoding pattern carrier includes a top and bottom surface and the emitter-detector module is placed under the code strip while the reflector is positioned above the top surface. However, these designations are arbitrary. Embodiments of the present invention in which the emitter-detector module is placed above the carrier and the reflector below the code strip can also be constructed in a manner analogous to that described above.  
      The light emitters used in the above embodiments of the present invention are typically an LED with a collimating lens. However, it will be appreciated that other forms of light emitter can be utilized. For example, semiconductor lasers provide collimated light signals without the need for a collimating lens.  
      Similarly, the photodetectors discussed in the above embodiments are typically constructed from photodiodes. However, it will be appreciated that other forms of photodetector can be utilized provided the photodetector provides an electrical signal that measures the amount of light received by the photodetector. For example, semiconductor-based photodetectors based on phototransistors can be utilized.  
      The above-described embodiments of the present invention utilize a single photodetector for measuring the light passing through the encoding pattern carrier. However, embodiments of the present invention that utilize multiple photodetectors positioned such that the resulting signals provide a measure of the direction of travel of the carrier and/or interpolate the distance traveled to an accuracy greater than that of a single band on the carrier can also be utilized in place of the single photodetector described above. Such detector arrangements are known to the art, and hence, will not be described in detail here. For the purposes of the present discussion, it is sufficient to note that such detectors operate by forming an image of the code pattern on the surface of a detector array having a plurality of adjacent photodetectors whose areas correspond to the bands in the encoding pattern image that is projected onto the individual photodetectors.  
      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.