Abstract:
Alignment of an optical encoder with respect to a codewheel. An alignment jig is used to align the encoder with respect to the codewheel shaft by mating with the codewheel shaft and with reference features on the encoder, which may comprise pins or receiving means for pins. One or more of the pins may be tapered to speed assembly.

Description:
TECHNICAL FIELD  
       [0001]     Embodiments in accordance with the invention relate generally to optical encoders and more particularly to rotary optical encoders Still more particularly, embodiments in accordance with the invention relate to alignment of optical encoders with respect to code wheels.  
       BACKGROUND  
       [0002]     An encoder is a motion detector which provides feedback to a motor control system. A typical encoder design consists of an emitter/detector module operating in a transmissive, reflective, or imaging configuration. When operated in conjunction with a codewheel, the encoder translates rotary motion into a digital output.  
         [0003]     As the resolution of the detector increases, so do the requirements for proper alignment of the encoder with respect to the codewheel.  
       SUMMARY  
       [0004]     In accordance with the invention, an optical encoder adapted for use with an alignment jig is provided. The alignment jig mates with the shaft holding the codewheel. The alignment jig has reference features which mate with matching features in the optical encoder, providing proper radial and tangential alignment of the encoder with respect to the codewheel shaft.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]     The invention will best be understood by reference to the following detailed description of embodiments in accordance with the invention when read in conjunction with the accompanying drawings, wherein:  
         [0006]      FIG. 1  shows a transmissive optical encoder known to the art,  
         [0007]      FIG. 2  shows a reflective optical encoder known to the art,  
         [0008]      FIG. 3  shows an imaging optical encoder known to the art,  
         [0009]      FIG. 4  is a side view of an encoder and codewheel,  
         [0010]      FIG. 5  is a diagram of mounting errors, and  
         [0011]      FIG. 6  shows an encoder, codewheel, and alignment jig. 
     
    
     DETAILED DESCRIPTION  
       [0012]     The invention relates to optical encoders, and the alignment of rotary optical encoders with respect to codewheels. The following description is presented to enable one skilled in the art to make and use the invention, and is provided in the context of a patent application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the generic principles herein may be applied to other embodiments. Thus, the invention is not intended to be limited to the embodiments show but is to be accorded the widest scope consistent with the appended claims and with the principles and features described herein.  
         [0013]     With reference now to the figures and in particular with reference to  FIGS. 1 through 3 , representative optical encoders for use with the invention are shown.  
         [0014]     In the transmissive type of encoder shown in  FIG. 1 , encoder  100  consists of light source  110  and detector  120 . Codewheel  130  containing a pattern of transparent and opaque regions interrupts the light passing from source  110  to detector  120 . Various technologies may be used in this type of encoder. Source  110  may be any light emitter, such as an incandescent bulb, although a light emitting diode is preferred. A laser diode may also be used. Detector  120  is preferably a photodiode, although other technologies such as photocells or photoresistive elements may also be used. Codewheel  130  may be a transparent material such as plastic or glass with opaque regions placed on it, or may be an opaque material such as a metal disc with holes placed in it to allow the passage of light.  
         [0015]      FIG. 2  shows a reflective encoder  200 , with source  210  and detector  220 . Codewheel  230  reflects light from source  210  to detector  220 . Technology tradeoffs for source  210  and detector  220  are similar to those in transmissive type encoders. Codewheel  230  contains regions which reflect light from source  210  to detector  220 , and regions which do not reflect light from source  210  to detector  220 . This reflection need not be in the nature of a focused or sharp reflection; it is usually in the nature of a specular reflection. Codewheel  230  may be made of a nonreflective material with reflective elements, or it may be made of a reflective material with masked nonreflective areas.  
         [0016]      FIG. 3  shows an imaging encoder. In the imaging encoder, emitter  310  and codewheel  330  are similar to those used in reflective encoders such as shown in  FIG. 2 . In the imaging encoder, an imaging array  320  is used rather than a simple photodetector such as used in simpler transmissive and reflective encoder designs.  
         [0017]      FIG. 4  shows a side view of an encoder and codewheel. Shaft  410  supports codewheel  420 . Shaft  410  is rotatably mounted to base  430 , usually through use of a bearing, not shown. Encoder  440  is also mounted to base  430 . The alignment of encoder  440  to codewheel  420  is of vital importance to the proper operation of encoder  440 . As the resolution of codewheel  420  and encoder  440  increase, this alignment becomes more important.  
         [0018]      FIG. 5  is a diagram of two important mounting errors. Shaft  510  supports codewheel  520 . Ideally, an encoder will be positioned at a particular radial distance  530  from the center of the shaft. The encoder should also be tangential  540  to this radial line  540 . A grossly exaggerated tangential error is shown as line  550 . The two errors important to the instant invention are radial error and tangential error.  
         [0019]     According to the present invention, and shown in  FIG. 6 , alignment jig  600  includes central post  610  with recess  620  for receiving shaft  410 . When alignment jig  600  is lowered, and shaft  410  engages with recess  620  in post  610 , alignment post  630  on alignment jig  600  engages alignment recess  640  in encoder  440 . In the preferred embodiment, alignment post  630  is matched by an additional post, not shown, at the same radial distance as post  630 . While more than two matching alignment posts and recesses may be used, two post-recess pairs have been found suitable to provide the needed alignment.  
         [0020]     With the alignment posts on alignment jig  600  engaged in the corresponding alignment recesses in encoder  440 , encoder  440  is held in precise radial and tangential alignment to shaft  410 . While this alignment is provided by the use of alignment jig  600 , encoder  440  may be fixed in position to base  430 . This fixing may be made through the use of fasteners such as screws, through adhesives including but not limited to cyanoacrylates or epoxies, or combinations. As an example, screws could be used to hold encoder  440  in place on base  430  while an adhesive sets.  
         [0021]     While in the preferred embodiment posts are present on alignment jig  600  which mate with recesses in the encoder, posts may be provided on the encoder, and the recesses on the alignment jig. One post may be provided on the alignment jig, and one may be provided on the encoder.  
         [0022]     To assist in positioning, a portion of the alignment pins may be tapered. Additionally, recess  620  on alignment shaft  610  may be flared to ease its engagement with shaft  410 . Flaring may also be used on the alignment recesses in encoder  440 . The use of tapering on pins and flaring on recesses may aid in the ease of engaging alignment jig  600  with shaft  410  and encoder  440 .  
         [0023]     The foregoing detailed description of the present invention is provided for the purpose of illustration and is not intended to be exhaustive or to limit the invention to the precise embodiments disclosed. Accordingly the scope of the present invention is defined by the appended claims.