Indexed optical encoder, method for indexing an optical encoder, and method for dynamically adjusting gain and offset in an optical encoder

Abstract

An optical encoder may include an encoder disk, an illumination system, and a detector to detect light diffracted from the encoder disk. The encoder disk may include a signal track comprising a diffraction grating, and an index track comprising a reflective index mark, wherein a width of the index mark is larger than a pitch of the diffraction grating. An indexing method may include providing an encoder disk, providing an illumination system to direct light to the encoder disk, providing a detector structured to detect light diffracted from the encoder disk, calculating an estimated count of quadrature states from a rising edge of an index pulse to a middle of the index interval, and calculating the quadrature state at an approximate center of the index pulse. A dynamic parameter correction method may include calculating a target gain and offset and correcting values based on the target gain and offset.

1 . An optical encoder comprising:
 an encoder disk comprising:   a signal track comprising a diffraction grating formed as a ring on the encoder disk; and   an index track comprising a reflective index mark, wherein a width of the index mark is larger than a pitch of the diffraction grating;   an illumination system structured to direct light to the encoder disk; and   a detector structured to detect light diffracted from the encoder disk.   
     
     
         2 . The optical encoder of  claim 2 , wherein the index track comprises:
 a first index track formed as a ring on the encoder disk; and   a second index track formed as a ring on the encoder disk.   
     
     
         3 . The optical encoder of  claim 2 , wherein the first index track comprises a first index mark provided at an index angular coordinate; and
 the second index track comprises a second index mark provided at the index angular coordinate;   wherein either:   the first index track is non-reflective except for the first index mark, which is reflective, and the second index track is reflective except for the second index mark, which is non-reflective; or the second index track is non-reflective except for the second index mark, which is reflective, and the first index track is reflective except for the first index mark, which is non-reflective.   
     
     
         4 . The optical encoder of  claim 1 , wherein the encoder disk further comprises:
 a glass substrate;   a low reflective layer provided on the substrate; and   a high reflective layer provided on the low reflective chrome layer;   wherein the high reflective chrome is formed in a pattern such that a portion of the low reflective layer is visible.   
     
     
         5 . The optical encoder of  claim 4 , wherein the low reflective layer comprises a low reflective chrome layer. 
     
     
         6 . The optical encoder of  claim 4 , wherein the high reflective layer comprises a high reflective chrome layer. 
     
     
         7 . The optical encoder of  claim 4 , wherein the low reflective layer has a reflectivity of approximately 5% on a side opposite the glass substrate. 
     
     
         8 . The optical encoder of  claim 4 , wherein the high reflective layer has a reflectivity of approximately 59% on a side facing the glass substrate and a reflectivity of approximately 65% on a side opposite the glass substrate. 
     
     
         9 . The optical encoder of  claim 4 , wherein the glass substrate comprises an optical absorption material applied to a face of the glass substrate. 
     
     
         10 . The optical encoder of  claim 1 , wherein the illumination system comprises a light-emitting diode. 
     
     
         11 . The optical encoder of  claim 1 , wherein the illumination system comprises:
 a multimode VCSEL laser structured to emit laser light.   
     
     
         12 . The optical encoder of  claim 1 , wherein the detector further comprises two offset detectors structured to detect light diffracted from the signal track and output a quadrature signal. 
     
     
         13 . The optical encoder of  claim 2 , wherein the detector further comprises:
 a first index detector structured to detect light reflected from the outer index track and output an outer index signal;   a second index detector structured to detect light reflected from the inner index track and output an inner index signal; and   a comparator structured to output an index pulse based on the outer index signal and the inner index signal.   
     
     
         14 . An encoder disk for use in an optical encoder, the encoder disk comprising:
 a signal track comprising a diffraction grating formed as a ring on the encoder disk; and   an index track comprising a reflective index mark, wherein a width of the index mark is larger than a pitch of the diffraction grating.   
     
     
         15 . The encoder disk of  claim 14 , wherein the index area comprises:
 a first index track formed as a ring on the encoder disk; and   an second index track formed as a ring on the encoder disk.   
     
     
         16 . The encoder disk of  claim 15 , wherein
 the first index track comprises a first index mark provided at an index angular coordinate; and   the second index track comprises a second index mark provided at the index angular coordinate;   wherein either:   the first index track is non-reflective except for the first index mark, which is reflective, and the second index track is reflective except for the second index mark, which is non-reflective; or   the second index track is non-reflective except for the second index mark, which is reflective, and the first index track is reflective except for the first index mark, which is non-reflective.   
     
     
         17 . The encoder disk of  claim 14 , further comprising:
 a glass substrate;   a low reflective layer provided on the substrate; and   a high reflective layer provided on the low reflective layer;   wherein the high reflective layer is formed in a pattern such that a portion of the low reflective layer is visible.   
     
     
         18 . The encoder disk of  claim 17 , wherein the low reflective layer comprises a low reflective chrome layer. 
     
     
         19 . The encoder disk of  claim 17 , wherein the high reflective layer comprises a high reflective chrome layer. 
     
     
         20 . The encoder disk of  claim 17 , wherein the low reflective layer has a reflectivity of approximately 5% on a side opposite the glass substrate. 
     
     
         21 . The encoder disk of  claim 17 , wherein the high reflective layer has a reflectivity of approximately 59% on a side face the glass substrate and a reflectivity of approximately 65% on a side opposite the glass substrate. 
     
     
         22 . The encoder disk of  claim 17 , wherein the glass substrate comprises an optical absorption material applied to a face of the glass substrate. 
     
     
         23 . An indexing method for use with an optical encoder, the indexing method comprising:
 providing an encoder disk, comprising:   a signal track comprising a diffraction grating formed as a ring on the encoder disk; and   an index track formed as a ring on the encoder disk, the index track comprising an index mark provided at an index angular coordinate;   providing an illumination system structured to direct light to the encoder disk;   providing a detector structured to detect light diffracted from the encoder disk, the detector comprising:   two offset detectors structured to detect light diffracted from the signal track and output a quadrature signal; and   an index detector structured to detect light reflected from the index track and output an index pulse;   calculating an estimated state count k est  of quadrature states from a rising edge of the index pulse to a middle of the index interval;   calculating Q kest , wherein Q kest  is the quadrature state at k est  and corresponds to the quadrature state at an approximate center of the index pulse; and   determining an offset correction.   
     
     
         24 . The indexing method of  claim 23 , wherein the calculating an estimated state count k est  comprises:
 calculating k est  as K/2 in a case that K is even; or   calculating k est  as ((K+1)/2) in a case that K is odd;   wherein K is the quadrature count from the beginning to the end of the index pulse.   
     
     
         25 . The indexing method of  claim 24 , wherein Q kest  is equal to Q (kest mod 4) , wherein Q kest− is the quadrature state at kest and Q (kest mod 4)  is the quadrature state at (k est  mod 4). 
     
     
         26 . A method of dynamically adjusting gain and offset in an optical encoder, the method comprising:
 providing an encoder disk comprising a diffraction grating;   illuminating the encoder disk with light;   providing a detector structured to detect light diffracted from the diffraction grating and output a first fine count channel;   calculating a first target gain and first target offset for the first fine count channel; and   applying a correction to data sampled from the first fine count channel based on the first target gain and first target offset.   
     
     
         27 . The method of  claim 26 , wherein calculating a target gain and offset comprises:
 determining a minimum value and maximum value in a set of data from the first fine count channel;   calculating a moving average minimum based on the minimum value and minimum values from a plurality of prior sets of data from the first fine count channel;   calculating a moving average maximum based on the maximum value and maximum values from the plurality of prior sets of data from the first fine count channel;   calculating the first target gain according to the equation G cal =(ave_max A −ave_min A )/2, where G cal  is the first target gain, ave_max A  is the moving average maximum, and ave_min A  is the moving average minimum; and   calculating the first target offset according to the equation OFFSET cal =(ave_max A +ave_min A )/2, where OFFSET cal  is the first target offset.   
     
     
         28 . The method of  claim 26 , wherein applying a correction to data sampled from the first fine count channel comprises:
 calculating a gain G A  of a data sample according to the equation G A =(max A −min A )/2, wherein max A  is a maximum of the data sample and min A  is the minimum of the data sample;   calculating an offset OFFSET A  of the data sample according to the equation OFFSET A =(max A +min A )/2;   calculating a gain correction factor GF A  according to the equation GF A =G cal /G A , where G cal  is the first target gain;   calculating an offset correction factor ΔOFF A  according to the equation ΔOFF A =OFFSET cal −OFFSET A , where OFFSET cal  is the first target offset; and   correcting fine counts according to the equation CFC A =(GF A *FC A )+ΔOFF A , where FC A  is raw fine count data from the first fine count channel and CFC A  is the corrected fine count data.   
     
     
         29 . The method of  claim 26 , wherein the detector is structured to output a second fine count channel, and the method further comprises:
 calculating a second target gain and second target offset for the second fine count channel; and   applying a correction to data sampled from the second fine count channel based on the second target gain and second target offset.   
     
     
         30 . The method of  claim 29 , wherein the detector is structured to output a coarse count, and the method further comprises applying a correction to the course count based on the correction to data sampled from the first fine count channel and data sampled from the second fine count channel. 
     
     
         31 . The method of  claim 30 , wherein applying a correction to the coarse count comprises:
 defining a quantization operator Q(x) as:
 Q(x)=0 for x<0, and 
 Q(x)=1 for x≧0; 
   calculating a compensated state pair {A′, B′} according to the equation:
   { A′, B′}={Q (CFC A −ΔOFF A ),  Q (CFC B , −ΔOFF B )}; 
   where CFC A  is corrected data sampled from the first fine count channel, CFC B  is corrected data sampled from the second fine count channel, ΔOFF A  is an offset correction value for the first fine count channel, and ΔOFF B  is an offset correction value for the second fine count channel;   comparing the coarse count to the compensated state pair;   adjusting the coarse count to match the compensated state pair.   
     
     
         32 . The method of  claim 26 , wherein a plurality of detectors are provided, each of the plurality of detectors being structured to detect light diffracted from the diffraction grating and output a first fine count channel. 
     
     
         33 . The method of  claim 27 , wherein the determining a minimum and maximum value in a set of data from the first fine count channel comprises:
 determining the three highest values and the three lowest values in the set of data;   rejecting the minimum and maximum value if any of the three highest values differ from a median of the three highest values by greater than a predetermined amount or if any of the three lowest values differ from a median of the three lowest values by greater than a predetermined amount.   
     
     
         34 . The method of  claim 27 , wherein the determining a minimum and maximum value in a set of data from the first fine count channel comprises:
 rejecting the minimum and the maximum if the minimum differs from the moving average minimum by more than a predetermined amount or if the maximum differs from the moving average maximum by more than a predetermined amount.   
     
     
         35 . The optical encoder of  claim 2 , wherein the first index track is provided outside the signal track in a radial direction, and the second index track is provided inside the signal track in a radial direction. 
     
     
         36 . The encoder disk of  claim 15 , wherein the first index track is provided outside the signal track in a radial direction, and the second index track is provided inside the signal track in a radial direction.