Abstract:
A position encoder and a method for estimating absolute position using two or more diffractive grating tracks of differing periods to generate interference fringe patterns on a multi-track sensor. Detectors, corresponding to the diffractive grating tracks, detect the interference fringes. A first processing circuitry coupled to the detectors extract phase signals from the signals from the detectors. A second processing circuitry then estimates the cycle counts of the track signals based on the phase signals from the first processing circuitry. The absolute position is estimated by combining the cycle count, the fractional fringe value, and the grating period.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Provisional Application No. 60/066,514, which was filed Nov. 25, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to position encoders, and more particularly, to a multi-track encoder and method for measuring absolute position for position sensing applications. 
     2. Description of Related Art 
     Position sensors and precise measurement devices generally require a detector that converts relative motion or position of two elements into an electrical signal. The electrical signal is then processed to determine the position and/or displacement. 
     Existing diffractive optical encoders for position sensing use the interference pattern from a diffractive grating to produce sinusoidal signals on a detector inserted in the fringe pattern. The sinusoidal signals are then electronically interpolated by an interpolator to detect the position and/or displacement. 
     SUMMARY OF THE INVENTION 
     The present invention is a multi-track absolute encoder, hereinafter referred to as a “Multi-Dimensional Encoder” or MDE, and a method for measuring absolute position, using two or more periodic tracks (e.g., diffractive gratings) of slightly differing periods to generate fringe patterns on an appropriately designed multi-track sensor. 
     The present invention is a position sensing device having a plurality of signal generators that provide signals having different periods in response to movement. A first processing circuitry extracts a position phase from each periodic signal and a second processing circuitry combines the position phases to estimate absolute position. 
     According to one embodiment of the present invention, the diffractive grating tracks are positioned to be illuminated by a light source. Two or more detectors are positioned to detect the interference fringes from the corresponding diffractive grating tracks. A first processing circuitry, such as an interpolator, is coupled to the detectors for obtaining phase signals from the detectors. The interpolator is coupled to a second processing circuitry, such as a track combiner processor, which is responsive to a combination of the phase signals and identifies the fringe count of one or more of the grating tracks from which the combination of phase signals is estimated to have originated. 
     The number of tracks and their periods are selected so that the two or more measured phases form a unique vector (in the mathematical sense) for each position in the measurement range. That is, the periods of the grating tracks are selected to be different such that the combination of phase signals from the interference fringes as the grating tracks are moved relative to the detectors defines unique combinations of phases over a range of movement. By mapping this unique vector back into fringe count, the grating displacement can be estimated to within one fringe. Since the output of any one of the tracks is suitable for fine position sensing within each fringe, the combination of the fringe count and the fine phase gives an absolute position reading without any memory of prior position. 
     The absolute encoder of the present invention is similar to current encoders manufactured by MicroE of Natick, Massachusetts. However, the present invention incorporates additional diffractive grating tracks, a multi-channel detector, which is a detector with parallel linear arrays which act like MicroE&#39;s standard “phased array” detector, and a special processing algorithm to convert the multiple measured phases into a single absolute position estimate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood by reference to the attached figures in which: 
     FIG. 1A is a simplified diagram of the side view of the multi-dimensional absolute encoder of the present invention; 
     FIG. 1B is a simplified functional diagram of a collimated beam, wavefront compensator, multi-track diffractive grating, and a detector, in accordance with the present invention; 
     FIG. 2A is a schematic diagram of the detector, interpolator, combiner processor, and output buffer, in accordance with the present invention; 
     FIG. 2B is a schematic diagram of the detector, interpolator, fringe counter, and output buffer in an incremental encoder; 
     FIG. 3A is an example of a signal output from the detector; 
     FIG. 3B is an example of the fractional fringe value signal output from the interpolator; 
     FIG. 3C is an example of the cycle count estimated by the combiner processor; 
     FIG. 3D is an example of the estimated absolute position; and 
     FIG. 4 illustrates phase signals that might be provided by the interpolator to the combiner, in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is the first purely diffractive absolute encoder. It is unique in the way it uses its multiple tracks. Unlike other absolute encoders, it neither uses its multiple tracks as binary increments to the absolute range (i.e., like the classic code disks of geometric encoders), nor does it simply use an “N, N−1” cycle approach to create a simple beat frequency to cover the desired measurement range, nor does it use a non-diffractive pseudo-random track. The present invention will be described in more detail below, with reference to FIGS. 1A-4. 
     From a mechanical point of view, the present invention comprises two or more “standard” MicroE encoders packaged into a single encoder head, along with an appropriately designed multi-track grating, as shown in FIGS. 1A and 1B. For example, a G1100 series encoder, manufactured by MicroE of Natick, Massachusetts, is such a “standard” MicroB encoder. Details of the operation of these encoders can be found in U.S. Pat. Nos. 5,486,923 and 5,559,600, assigned to MicroE, the assignee of the subject application, and which are hereby incorporated by reference. 
     Referring to FIG. 1A, a preferred embodiment of the present invention includes a light source  100 , a wavefront compensator  300 , a mirror  900 , a multi-track diffractive grating  400 , and a detector  500 . As shown in FIG. 1A, a light source  100 , such as a laser diode, is used to provide a quasi-monochromatic, spatially coherent light. The diverging laser beam is collimated by a collimating lens  200  to provide a collimated beam  2 . The collimated beam  2  then passes through a wavefront compensator  300 , such as the triple wavefront compensator shown in FIGS. 1A and 1B. It is to be noted that although FIG. 1B shows a three-track system, more than three tracks may be employed within the spirit of the present multi-dimensional encoder invention. 
     A mirror  900  then deflects the light beams  311 ,  312 ,  321 ,  322 ,  331 ,  332  such that they are directed to pass through a multi-track diffractive grating  400 , such as the three-track diffractive grating  400  with triple linear diffractive gratings  410 ,  420 ,  430  shown in FIG.  1 B. The multi-track diffractive grating  400  is positioned such that there is a one-to-one matching of the light beams  311 ,  312 ,  321 ,  322 ,  331 ,  332  with the grating tracks  410 ,  420 ,  430 , as shown in FIG.  1 B. The multi-track diffractive grating  400  can be either radial or linear. A linear grating  400  is shown for simplicity. The grating tracks  410 ,  420 ,  430  can be either combined into one device, such as the multi-track diffractive grating  400  shown in FIG. 1B, or the tracks  410 ,  420 ,  430  can be individual devices. 
     The multi-track diffractive grating  400  diffracts the light beams  311 ,  312 ,  321 ,  322 ,  331 ,  332  into discrete orders. The grating tracks  410 ,  420 ,  430  diffract and redirect the light beams  311 ,  312 ,  321 ,  322 ,  331 ,  332 . After passing through the grating tracks  410 ,  420 ,  430 , the light beams  313 ,  314 ,  323 ,  324 ,  333 ,  334  propagate to the detector  500 . 
     The diffracted orders of the light beams  313 ,  314 ,  323 ,  324 ,  333 ,  334  interfere to form linear sinusoidal fringes. The period of these fringes is determined by the grating track period and the wavefront compensator  300 . The periods of the grating tracks  410 ,  420 ,  430  are different from each other and are selected to provide the greatest measurement range given the available measurement accuracy. 
     The detector  500  then receives the interference fringes of the light beams  313 ,  314 ,  323 ,  324 ,  333 ,  334  created by the diffractive grating tracks  410 ,  420 ,  430 . The detector  500  shown in FIG. 1B is a detector having three parallel linear detector arrays  510 ,  520 ,  530 , where one of the detector arrays  510 ,  520 ,  530  corresponds to each grating track  410 ,  420 ,  430 . 
     As shown in FIG. 2A, the output signals  511 / 512 ,  521 / 523 ,  531 / 532  from each detector array  510 ,  520 ,  530  are processed by processing circuitry, such as an interpolator  600 , using standard interpolation techniques, creating two or more periodic phase signals, each of which represents the fractional fringe position for its respective grating track  410 ,  420 ,  430 . The interpolator  600  uses multi-phased signals to interpolate between periods of periodic signals. The output signals  511 / 512 ,  521 / 522 ,  531 / 532  are analog sine/cosine signals. The interpolator may be a digital processor. 
     The interpolator  600  may also include front end processing, including compensation for gain differences, amplitude offsets, and phase offsets, to ensure correct phase signals before interpolation, such as the error compensation techniques found in the MicroE MC2000 motion control board used with the MicroE G1100 series encoders. 
     In an incremental encoder, the fringes produced by a single track grating increase in phase linearly with grating displacement. As phase is measurable modulo 2π only, a fringe counter is normally included in a processor of an incremental encoder to keep track of the number of cycles that have gone by, as shown in FIG.  2 B. 
     In order to calculate the non-modulo position of the grating, the fringe count (integer cycles)  655  and the fractional cycle estimate from an interpolator are combined. By multiplying this mixed number estimate of fringes by the a priori knowledge of the fringe period, the fringe count can be converted to physical displacement, relative to the fringe counter initialization point. 
     No cycle counter is required in this processor as would be in an incremental encoder. Each phase signal is fed to a “track combiner” processor  700 , as shown in FIG.  2 A. The track combiner processor  700  may be a digital processor and this processor  700  estimates the cycle count(s) of one or more of the track signals, based only on the immediate phase values available. The cycle count  710  and the fractional fringe value  630  (digital phase values), combined with the a priori knowledge of the grating period, permit an estimate of the absolute position  810 . As an example, the cycle count  710  would provide the upper bits, while the fractional fringe value  630  would provide the lower bits, for example the lowest 12 bits of the estimated phase position. See FIG.  3 D. 
     Output signals from the system of FIG. 2A are illustrated in FIGS. 3A-3D. FIG. 3A provides an example of the signal  511 / 512  output from the detector  500 . FIG. 3B is an example of the fractional fringe value signal  630  output from the interpolator  600 . FIG. 3C provides an example of the cycle count  710  estimated by the “track combiner” processor  700 . FIG. 3D is an example of the estimated absolute position. 
     An MDE of the present invention can be implemented in either linear or rotary encoder applications and can be used with any position sensing technology based on periodic signals. For example, the MDE approach could be applied to the multi-track periodic capacitive sensors that have been used in incremental encoders, making this technology suitable for a true absolute encoder. When used in a 360 degree rotary application with the diffractive technology described in the above-referenced U.S. Pat. Nos. 5,486,923 and 5,559,600, which always produces an even number of fringes around a disk, the MDE of the present invention will require an extra, binary indicator track to indicate in which 180 degree sector the sensor is. 
     From the above description of the present invention, it should be clear that the key principle of operation for the absolute MDE is the understanding of how to create and process the two or more sets of diffractive fringes. 
     In order to create an absolute encoder of the present invention, the fringe counter (position memory) used in incremental encoders, such as the one shown in FIG. 2B, is eliminated, but there is still a need to estimate the integer number of fringes. This estimate is obtained by the fact that each track  410 ,  420 ,  430  on the multi-track diffractive grating  400  has a slightly different period, so the fringes produced by the tracks  410 ,  420 ,  430  change phase with grating position at a slightly different rate from each other. 
     In this regard, the present invention is similar to any beat frequency phenomenon. However, the present invention differs from a simple beat frequency approach in the number of fringes used, the way frequencies are selected, and the processing algorithm applied to the resulting fringe phases. 
     In the present invention, it is desired to uniformly spread the measured information in an N-dimensional space, where N is the number of grating tracks. This uniform spread is achieved by the following steps: 
     1) Select a convenient base period, P, for which the encoders work well and which will provide the required resolution, r. For example, MicroE G1100 series encoders typically have: 
     5 microns&lt;2P&lt;60 microns, and, if 12 bit interpolation is used, 
     P≦4096×r. 
     2) N is selected based on the accuracy, A, of to the measurement of each individual track&#39;s phase and the desired range, R, using the following formula: 
     
       
         N=Ceiling [log(R/P)/Alog(2)+1], 
       
     
     where A is the number of bits of accuracy and the Ceiling function returns the next highest integer. 
     3) Finally, the remaining N−1 periods are selected by choosing physically convenient periods close to the base period, applying the criterion that the number of fringes that each track  410 ,  420 ,  430  produces in the range is both integer and has at least one factor that is not common to all other tracks  410 ,  420 ,  430 . That is, while a track  410 ,  420 ,  430  may share factors on a pairwise basis with the other tracks  410 ,  420 ,  430 , it must have at least one factor that is not shared with all the other tracks  410 ,  420 ,  430 . In order to make all N periods convenient for fabrication, the numerical value of the range can be adjusted slightly. 
     As an example of the present invention, consider an absolute encoder required to measure a 32 mm range with a 12 nanometer resolution. Also assume that phase can be measured with 8-bit accuracy. For this system, the base period can be chosen to be 12.4 microns, which is both between 5 and 60 microns and less than 4096 times the 12 nanometers resolution. 
     Since the measurement range, R, is 32 mm, the formula provides the number of tracks, N, as 3. Finally, we can select the other two fringe periods to be 12.3 and 12.6 microns. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Example parameters 
               
             
          
           
               
                   
                 Track Period 
                 Number of Fringes 
                   
               
               
                   
                 (microns) 
                 in 32.0292 mm 
                 Factors 
               
               
                   
                   
               
               
                   
                 12.3 
                 2604 
                 2, 3, 7, 31 
               
               
                   
                 12.4 
                 2583 
                 3, 7, 41 
               
               
                   
                 12.6 
                 2542 
                 2, 31, 41 
               
               
                   
                   
               
             
          
         
       
     
     As seen in the attached table, there is no common factor between the number of fringes for all three tracks, and the numeric value of the range has been adjusted to be 32.0292 mm. 
     FIG. 4 illustrates phase signals φ 1 , φ 2 , and φ 3  such as might be provided by interpolator  600  to the “track combiner” processor  700  in FIG.  2 A. It should be noted that the period of each phase signal differs from the others, and that at any point, M, along the range of movement, the instantaneous phases, φ 1M , φ 2M , and φ 3M , differ from one another uniquely. The phase periods shown in FIG. 4 cover only a portion of the range. It is to be noted that FIG. 4 is for illustration only and not meant to precisely depict the relative phase relationships. 
     There are several ways to process the phases produced by an MDE encoder of the present invention. The conceptually easiest approach is to use a large look-up table. That is, since the set of N phases (“N-vector”) produced by the N gratings is unique within the range, these N-vectors can be precalculated for each possible position and the answer may simply be looked up on the table. This approach can be preformed very quickly, but requires a large read-only storage memory. 
     At the other extreme, one may use an algorithm for converting the measured N-vectors into absolute position with no memory requirements. Although the algorithm is computationally efficient, by definition it has a higher processor requirement than the simple look-up table approach. Intermediate processing stratagems, in which the algorithm solution is used to reduce the size of the look-up table are possible (and possibly preferred, depending on the available resources) where the algorithm can be determined and performed computationally efficiently. 
     In other words, for example, the first part of the processing may be performed using the algorithm or analytically, and the remaining part of the processing can then be done using a look-up table. For example, the algorithm may be used to project the point inside the cube defined by the tip of the N-vector. The look-up table may then be used to find the intercepts. 
     The algorithm described above presumes an understanding of the concept of N-dimensional space, where N can be greater than 3. For the sake of clarity, the algorithm will be described in further detail below by referring to the 3-dimensional example discussed above, but those skilled in the art will understand its extension to higher dimensional spaces. 
     The 3-vector formed from the three measured phases traces a series of parallel line segments confined in a cube. The sides of the cube are each one cycle long. As long as the grating substrate moves within the predefined range, these line segments are non-repeating. Each line segment can be mapped to a particular fringe in each of the tracks  410 ,  420 ,  430 . The goal of this algorithm is to determine the fringe number (count) for a pre-selected “base” track from the 3-vector. 
     The first step in the algorithm is to project the point inside the cube defined by the tip of the 3-vector down its line segment to find its intercept with the plane associated with one face of the cube, specifically the face for which the base track phase equals 0. The following formulae are used: 
     
       
         I x =Round[mod(p 1 −p 3 *T 3 /T 1 )*Lx)]/Lx, 
       
     
     and 
     
       
         I y =Round[mod(p 2 −p 3 *T 3 /T 2 )*Ly)]/Ly, 
       
     
     where the third track has been designated the base track, I x  and I y  are the coordinates of the intercept in the P 3  plane, pn is the phase from the nth track, Tn is the period of the nth fringe, and the L&#39;s are greatest common factors between the tracks  410 ,  420 ,  430  when taken pairwise. Note that the mod function (here used with a base of unity) restricts the intercepts to be within the face of the unit cube and the Round function eliminates the ambiguities from measurement errors. It can be shown that these intercept points lie along identifiable parallel line segments in the unit square that forms one side of the cube. 
     The second step in the algorithm is to project these calculated intercepts back along their line segments to find where they in turn intercept one axis: 
     
       
         I=mod(Ix−Iy*Ly/Lx) 
       
     
     The third step in the algorithm is to calculate what has been defined as the “overlap”. The overlap is the number of beat cycles through which the combined periodic signals have passed. The total number of overlaps OV is the ratio of the range to the beat period. It can be shown that for the MDE: 
     
       
         Beat=Lx*Ly/|Lx−Ly|, 
       
     
     and 
     
       
         OV=|Lx−Ly|, 
       
     
     and the overlap value for a particular data point is 
     
       
         Overlap=mod ov (I*Lx), 
       
     
     where the base for the mod function is OV. The function ‘Overlap’ should be tested to see that it is a monotonic function of I. If it is not, the selection of various grating periods (and thus the common factors, L) must be adjusted on an ad hoc basis until this condition is met. The look up table approaches for estimating the absolute position are not subject to this condition. 
     Finally, this information can be combined to estimate the position: 
     
       
         P=[(Overlap+1)*Beat+Iy*Ly+p 3 ]*T 3 , 
       
     
     where each term in the above estimate is calculated from the instantaneous measurement of the 3 fringe phases. 
     Although the present invention has been described by way of particular examples, it is to be understood that invention is not limited to the particular examples described. For example, although the present invention has been described generally in terms of diffractive optical encoders, the present invention may also apply to other types of encoders, including geometric optical encoders, capacitative displacement encoders, and magnetic displacement encoders. Moreover, while the functional components of the present invention are described and illustrated as distinct components, it is to be understood that they may be combined into a single component or assembly, or distributed among several components or assemblies, within the spirit of the present invention. 
     The terms and expressions which have been employed herein are used as terms of description and not of limitation. There is no intention in the use of such terms and expressions of excluding equivalents of the features shown and described, or portions thereof, it being recognized that various modifications are possible within the scope of the invention claimed.