Abstract:
The present invention recognizes that the limiting structures to size reduction for an optical encoder were the laser diode/photo diode package and the photodetector with amplifier package(s). The invention includes removing the discrete components from the laser diode cover package and mounting the much smaller discrete components on a base framework. The photodetector with amplifier package of the invention have an altered photodetector/amplifier geometry including shortening the leads between them. This resulted in significant size reduction of the encoder, a reduction in required gain and resulting increase in bandwidth. The entire base framework is temperature cooled by disposing two thermo-electric coolers between the base framework and an adjacent heat sink. Such positioning of the two coolers between the base framework and it&#39;s adjacent heat sink stabilizes both the laser diode&#39;s and the photo detector&#39;s outputs. In addition this novel design stabilizes the entire footprint/base of the encoder from thermal effects, whatever their origin.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates in general to an optical rotary encoder and more specifically to a miniaturized optical encoder having an in situ laser beam generator, a novel heat dissipation system and a wave plate phase delay means. 
     2. Description of the Related Art 
     For some time now, optic encoder means have been used with great success for precise positioning applications. During this time it has been widely recognized by those skilled in the art that smaller is better. A smaller position tracking mechanism has obvious packaging and operational advantages such as for instance a smaller platform, increased beam intensity for diverging beam applications such as in the present invention, more responsive positioning system due to momentum reduction, reduced motor size requirements, etc. Limiting factors for farther size reduction are the physical size of the laser diode and photodetector packages, heat dissipation and optical component orientation and adjustment requirements. 
     Temperature parameter compensation is addressed in: U.S. Pat. No. 5,652,426 which is directed to an optical encoder that obtains displacement information by means of a twice-diffracted beam and a twice-transmitted beam to thereby compensate for changes of wavelength of a light beam due to temperature change. 
     Wave plate compensation is addressed in: U.S. Pat. No. 5,596,403 which is directed to an angular position measuring system having a source assembly including a laser driver, a conventional linear polarizer and a rotating half-wave plate. U.S. Pat. No. 5,677,768 discloses an encoder having a stationary and a rotating quarter-wave plate. A signal processor (for removing an unwanted component due to the rotating wave plate introducing a frequency shift proportional to the angular rotation) is mentioned but not described. 
     None of the above references addresses the temperature dissipation or wave plate orientation problems encountered when size reduction of optical encoders of the related art is attempted. Nor do the references address the problem that the physical size limitations of the laser diode and photo detector packages hampers further size reduction of optical encoder apparatus. 
     Accordingly it would be desirable to have a miniature optical encoder apparatus that is not limited by the size of the laser diode and photodetector packages. It would further be desirable if the miniature optical encoder apparatus included heat dissipation means for removing and dissipating heat from the entire apparatus. It would be further desirable if the miniature optical encoder apparatus included a combination of a wave plate phase delay means and signal processing means to provide for phase delay compensation/adjustment. 
     As has been mentioned earlier, the focus of this invention is to optimize and miniaturize the size of optical encoder apparatus. One of the necessary parameters that has to be addressed to miniaturize an optical encoder is of course heat control including most importantly heat dissipation. The invention recognizes that heat control plays an equally important role in optical encoders than as just a parameter to be controlled or optimized for miniaturization. For instance, uneven heating (including that due to uneven heat dissipation) anywhere in an optical encoder and especially in the base of the optical encoder can lead to degeneration of the optics due to uneven expansion or contraction of one or more elements and especially the base upon which the elements are mounted in the optical encoder apparatus, and degeneration of the epoxy attachment means for all of the encoder elements. In encoders of the Relevant Art, the heat generated by the laser diode is addressed because of course excess heat at the laser diode can interfere with its performance. For this reason, heat control mitigating elements are employed in typical laser diode packages. However, the present invention recognizes that heat comes from other sources besides the laser diode. Another heat source that has not been addressed by the Relevant Art is the heat that stems from the electric motor and drive shaft for movement of the rotary grating or other tracking means the optical encoder employs. Another source of heat is the ambient temperature of the environment in which the encoder is employed. Another source of heat are the electronic circuits that are both integral with and mounted on the optical encoder. 
     Accordingly, it would be desirable to have heat control apparatus that controls the heat profiles of the entire optical encoder architecture so that the laser diode performance is not affected, and that misalignment of the optics of the encoder will not result from heat induced material distortion. It would further be particularly desirable if the base mounting plate of the optical encoder were to be heat controlled so as to be operated in a heat range wherein laser diode output noise sources will be minimized. 
     SUMMARY OF THE INVENTION 
     Briefly the present invention recognizes that the limiting structures to size reduction for an optical encoder are the laser diode/photo diode package and the photodetector with amplifier packages. The invention includes removing the discrete components from the laser diode cover package and mounting the much smaller discrete components on a base framework. The photo detector with amplifier packages of the invention have an altered photodetector/amplifier geometry including reducing the length of the connection wire. This resulted in significant size reduction of the encoder, a reduction in required gain and resulting increase in bandwidth. The entire base framework is temperature cooled by disposing two thermoelectric coolers between the base framework and an adjacent heat sink. Such positioning of the two coolers between the base framework and it&#39;s adjacent heat sink maintains the base framework at a controlled temperature regimen so as to stabilize all elements(both optical and electronic) mounted on the base framework, most importantly stabilizing both the laser diode&#39;s and the photodetecto&#39;s outputs. In addition this novel design stabilizes the entire footprint/base of the encoder from thermal effects, whatever their origin. A preferred embodiment of the invention includes a combination of a wave plate and electronic tuning to adjust phase delay. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be understood and further advantages and uses thereof more readily apparent, when considered in view of the following detailed description of the exemplary embodiments, taken with the accompanying drawings in which: 
     FIG. 1 is an exploded isometric view of the miniature interferometric optical encoder further illustrating the location and orientation of the discrete components; 
     FIG. 2 is a plan view of the optics block of the miniature interferometric optical encoder illustrating the location and orientation of the discrete components and the light paths within the encoder in operation; 
     FIG. 3 is a schematic view of the grating and the way the grating diffracts the incident laser beam into multiple beams of differing order according to the teachings of the invention; 
     FIG. 4 is a block diagram of the encoder electronics and the interconnections with the output of the encoder unit operating components. 
     FIG. 5 is a schematic diagram of the A+B; A−B phase tuning circuit; 
     FIG. 6 is another embodiment of the laser diode mounting block; 
     FIG. 6A is another embodiment of the laser diode mounting block; 
     FIG. 6B is an elevational view of a vertical cavity surface emitting laser (vcsel) and a prism illustrating how polarization can be adjusted by rotating the vcsel relative to the prism; 
     FIG. 7 is a schematic view showing how the prism  64  with polarizing coating  65  can be eliminated by coating a polarizing film directly on photo detector  66  of FIG. 2, and 
     FIG. 8 is a schematic view showing how a small detector eliminates the pin hole  42  of FIG.  2 . 
     FIG. 9 is a schematic diagram illustrating how one photodetector with a two-element array and a rhombic prism combination can replace two photodetectors and prism combination all according to the teachings of the invention; 
     FIG. 9A is a three-dimensional view of the rhombic prism of FIG.  9 ; 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings and to FIGS. 1, and  2  in particular, there are shown an isometric view and a plan view of the miniature interferometric optical encoder  10 , constructed according to the teachings of the invention. Optical encoder unit  10  includes encoder optics block  12 , rotary grating  14 , first and second thermo-electric (TE) coolers  16  and  18 , heat sink  22 , end cover  24 , and cap  26 . Encoder optics block  12  includes and has disposed thereon laser diode module  32  which includes laser diode chip source  34  and photo diode  36 . Encoder optics block  12  further includes and has disposed thereon first and second pinholes  38  and  42 , first and second mirrors  44  and  46 , first and second wave plates  48  and  52 , first light shield  54  (FIG. 1) and second and third lightshields  56 ,  58  (FIG.  2 ), beam splitter plate  62 , polarizing coated prism  64  and first and second photodetectors with amplifiers  66  and  68 . Encoder optics block  12  further includes and has disposed thereon temperature sensor  72  for regulating thermo-electric coolers  16  and  18 , a printed circuit board (PCB)  74  dedicated to the photodetector amplifiers  66  and  68  respectively and a miscellaneous component wiring printed circuit board (PCB)  76 . Thermal electric coolers  16 ,  18  respectively, operate to transfer heat in either direction between optics block  12  and heat sink  22 , to provide for regulating and stabilizing the temperature of optics block  12 . 
     Referring now to FIGS. 2 and 3, there are illustrated the operating principles of the miniature grating interferometric rotary encoder  10 , constructed according to the teachings of the invention. Referring now in particular to FIG. 2, in operation, the horizontally polarized light output  102  from laser diode chip source  34  passes through pinhole  38 , light shield  58  and quarter wave plate  48 , by which time the horizontally polarized light output  102  then becomes near circularly polarized. At this stage, the laser light polarization cannot be all along or perpendicular to the grating groove directions but rather must be partially along or perpendicular to the grating groove directions. This is essential in order to get two phase delayed optical outputs. Theoretically any other polarization direction will work, but in order to generate two equal amplitude outputs, it is preferred to use either a circularly polarized or linearly polarized direction with 45° orientation. Referring now in particular to FIG. 3, the surface normal reference line is shown at N, the spatially and temporally coherent circularly polarized light  103  illuminates reflective radial grating  14 . The grating  14  diffracts the incident laser beam into one pair of beams of first order  106  and  108 , and possibly other beams of higher order depending on wavelength and grating constant. The zeroth order beam  104  is reflected back and mostly blocked by the pinhole  38  due to the divergence of the beam. 
     The diffraction angle θ d  relative to the surface normal reference line N is given by 
     
       
         sinθ d =sinθ in +mλ/d 
       
     
     where, m is the diffraction order number, θ in  is the angle of the incident laser beam with respect to the surface normal reference line N of the grating surface, λ is the laser wavelength, and d is the grating constant. If d is small enough (d&lt;2λ), only orders with m=−1 and m=+1 are diffracted, producing two diffracted beams  106  and  108 , at angles θ=θ −1  and θ=θ +1 , respectively. In our case, θ in =0°, d =0.95 μm, and θ d =+45°. All diffracted beams are mutually coherent both spatially and temporally and have different phase changes upon diffraction. They are able to interfere with one another once they are overlapped along the same direction. When the grating (14) moves along the direction of the arrow for a distance d within certain time interval t, the phase difference φbetween beams  106  and  108  is modulated by the grating motion, 
     
       
         φ(t)=ωt=2πd/λ(sinθ +1 −sin −1 θ −1 ). 
       
     
     Therefore the grating movement modulates the phase variation of the two beams and thereby produces moving interference patterns. In order to produce interference effects, the two diffracted beams,  106  and  108 , must be recombined. This is accomplished by the two mirrors  44 ,  46  (FIG. 2) respectively, and by beam splitter plate  62  that partially transmits and partially reflects incident light beams  106  and  108  respectively. The interference occurs when the two beams  106  and  108  are recombined at beam splitter plate  62  whereby the two beams  106  and  108  are made to be as collinear as possible. The interference fringe separation L depends on the angle δ between the two overlapped beams which is given by 
     
       
         L=λ/2sin(δ/2). 
       
     
     A small area of the interference fringe on one side of the beam splitter plate transmits through pinhole  42 . A polarizing prism  64  having a polarizing coating on the diagonal side  65  transmits light polarized in a first predetermined direction and reflects light polarized in a second predetermined direction and thereby splits the recombined beam into first and second orthogonal polarized beams. First and second photodetectors  66  and  68 , each having their own built in amplifiers intercept the first and second orthogonal polarized beams respectfully and convert the optical signals into voltage signals suitable for counting and other electronic processing. 
     The electronic processing of the invention is set forth with reference to FIGS. 4 and 5 wherein there are illustrated a block diagram of the encoder electronics and the interconnections with the output of the encoder unit operating components and a schematic diagram of the A+B; A−B phase tuning circuit. Referring to FIG. 4, the phase A and phase B blocks are the raw signals from detectors  66  and  68  of encoder  10 . These encoder raw signals are then amplified through two amplifier blocks  202 ,  204 , with output A and B. Outputs A and B are then fed into A+B, A−B circuit blocks  206 ,  208  which generate output A′ and B′ with exact 90° degree phase difference. The normalization circuit blocks  212 ,  214  generate stable A″ and B″ signals which are independent of the input signal variations. Signals A″ and B″ are then fed into interpolation stage  216 ,  218  which generate interpolated high resolution signals A′″ and B′″. Signals A′″ and B′″ are then fed into digitizer blocks  222 ,  224  respectively, and are then fed into the display counter block  226 . 
     The purpose and function of the second waveplate  52  is to bring the phase delay between signals A and B as close to 90 as feasible with a wave plate. This phase delay adjustment by the second waveplate  52  is important because the A+B, A−B circuits  206 ,  208  respectfully, (which generate output A′ and B′ with exact 90° degree phase difference) will not work properly, that is they will not be able to compensate when the initial phase difference between signals A and B is small (such as when the phase difference approximates 0° or 180°). The reason for this difficulty in compensation by the A+B, A−B circuits  206 ,  208  respectfully, is that the DC offsets of A+B will be much larger (or much smaller depending on the phase difference) than that of A−B or A−k*B, thus one channel might need tremendous gain to bring the two signals A′ and B′ equal. But in all practicality, an amplification circuit can only provide gain to a certain reasonable extent. In addition to the high gain problem there is a noise problem associated with high gain; one channel (that requiring high gain) will be much noisier than the other channel. Therefore the circuit will not work properly when the input signals&#39; phase difference is so small such that the amplification circuit cannot provide enough gain. The invention therefore adds waveplate  52  in one beam path to change the phase difference between the signals A and B closer to 90°. There is no stringent requirement on the wave plate orientation as long as the phase difference is small such that the circuit can provide enough gain to approximate 90° phase difference with an acceptable signal/noise ratio. By combining the waveplate  52  and the phase tuning circuits  206 ,  208  respectively, the invention teaches a simple method and the required apparatus to get 90° phase delayed signals. 
     Referring now to FIG. 5, there is shown a schematic diagram of phase tuning circuit  230 , including three operational amplifiers (e.g. OP 27 ) shown at adder blocks  232 ,  234  respectively, and inverter with gain block  236  which are interconnected as shown to provide A+B and A−k*B functions. 
     When the two channels A and B have the same amplitudes A 0  but non−90° phase difference, a method of combining A and B to generate 
     A′=A+B and B′=A−B can be used. This can be expressed as: 
     
       
         A′=A 0  sin(θ+δ)+A 0  sin θ 
       
     
     
       
         =A 0  sin( 74 +δ/ 2)cos(δ/2)αsin(θ+δ/2) 
       
     
     
       
         B′=A 0  sin(θ+δ)−A 0  sinθ 
       
     
     
       
         =A 0  cos(θ+δ/2)sin(δ/2)αcos(θ+δ/2) 
       
     
     where θ is the overall phase angle of one channel, and δ is the phase angle difference between the two channels. The new outputs A′ and B′ have exactly 90° phase difference. But this method does not apply to cases where the amplitudes of A and B are not equal, A 0 ≠B 0 , which is common in real applications. We add a gain factor k in one of the channels, e.g. B′=A−k*B instead of A−B, to expand the phase tuning range of this circuit. 
     A′=A 0  sin(θ+δ)+B 0  sin θ 
     B′=A 0  sin(θ+δ)−k*B 0  sin θ 
     In this way the circuit can handle with non-90° phase difference and unequal amplitudes. By adjusting the gain factor k, the phase difference between A′ and B′ can be easily tuned to 90°. Referring now to FIG. 5, the gain factor k is adjusted by varying the POT R 8  shown at  242 , which may for example be a 20K variable resister. POT R 7  shown at  244 , which may for example be a 50K variable resister, is adjusted to match the channel B′ output level with that of channel A′. 
     Assembly: The assembly procedure is mount or assemble the discreet encoder components on encoder cold block  12  in the following order: First the laser diode module  32  and mirrors  44 , 46  are mounted on encoder optics/cold block  12 . Second, mount beam splitter plate  62  without fixing its position. Third, assemble first and second thermoelectric coolers  16 ,  18  respectively and radial grating  14  on he at sink  22 , and attach the heat sink  22  to encoder optics/cold block  12  with the thermoelectric coolers  16 ,  18  disposed there between. Forth, adjust encoder optics/cold block  12 &#39;s position relative to heat sink  22  and attached radial grating  14  while monitoring the interference pattern of recombined beam  70  until a good pattern is observed. The fifth and final assembly step is to fine tune the beam splitter plate  62 &#39;s angular position to get closer to the center of the interference pattern i.e. to provide the largest interference pattern. 
     The machined encoder optics/cold block  12  and heat sink  22  have high enough accuracy to provide an accurate alignment structure for the encoder constructed according to the teachings of the invention. The accuracy of the typical CNC machine is about a few micrometers and angular accuracy is about 1 milli-radians for current encoder size. The alignment tolerance required for a grating interferometer using a collimated laser beam can be estimated to be 
     
       
         δ&lt;λ/D 
       
     
     where δ is the angle that produces the interference pattern spacing D. This equation was developed in patent application Ser. No. 08/728,625, entitled “MINIATURE INTERFEROMETRIC ROTARY ENCODER” assigned to the same assignee as the present invention, which is hereby referred to and incorporated herein. This equation gives an estimate on the angular alignment error tolerance. For the laser wavelength of μ=0.67 μm and D=1 mm, the interfering beams must be co-linear to a precision &lt;0.67 mill-radians. Based on the CNC machined structure and fine tuning of the beam splitter plate  62 , plus the beam divergence, all the errors generated when the laser diode module  32  and mirrors  44 ,  46  are mounted on encoder optics/cold block  12  can be compensated for by fine tuning the beam splitter plate  62 . This is an advantage over the related art in that this mounting procedure dispenses with the necessity for accurate mounting tolerances during the assembly process when the laser diode module  32  and mirrors  44 ,  46  are mounted on encoder optics/cold block  12 . 
     What has been disclosed and described then is the ability to put the laser diode chip source  34  for generating a beam of light directly on the optical encoder means rather than bringing the light source in by means of a fiber optic cable. This was accomplished by using a chip for the laser diode module  32  directly, including both the laser diode chip source  34  and the photo diode  36 , without a conventional surrounding enclosure. The overall size and in particular the footprint of optics/cold block  12  was reduced significantly. Furthermore, disposing photo diode  36  at 110° to the laser diode chip source  34 &#39;s output axis reduces reflection light to the laser diode chip  36 . 
     The size of the laser diode module  32  may be further reduced by integrating the laser diode  34  and the photo diode  36  on one chip module  30  as shown in FIG. 6 with approximate sizes of ½ millimeter wide by less than one millimeter long. This can reduce the encoder&#39;s length, width and height because the laser diode  34  and the photo diode  36  are the limiting factors for the size of the encoder when they are installed insitu according to the teachings of the invention. The electrical leads for laser module chip  30  may be disposed through chip  30  by standard chip fabrication. Prism  37  is used to couple lazer diode  34 &#39;s output to photo detector  36 . 
     Additional embodiments of the invention include the elimination of quarter wave plate  48 . The purpose of quarter wave plate  48  is to generate a circular beam of polarized light before it reaches and strikes grating  14 . This can also be accomplished by using a linearly polarized light with polarization oriented along and at 45° relative to the groove direction of grating  14 . This can be accomplished by a 45° rotation of laser chip  34  along laser output axis  76  as shown in FIGS. 6 and 6A. 
     Another embodiment to replace laser diode  34  and photodiode  36  of FIG. 2 is shown in FIG.  6 B. Referring now to FIG. 6B, laser module  250  includes photodiode  252 , laser (VCSEL) diode  254 , and prism  256 . The laser polarization state can be adjusted by rotating the VCSEL laser diode  254  relative to prism  256 , thereby providing a readily adjusted polarization state in a compact laser diode module  250 . 
     Referring now to FIG. 7 there is shown how the prism  64  with polarizing coating  65  can be eliminated by coating a polarizing film directly on photo detector  66  (FIG. 3) with a suitable polarizing coating film. Incident light beam  70  has two phase delayed optical components of light, P &amp; S. Photo detector  114  has a polarizing coating  110  which transmits the P component of light into photo detector  114  and reflects the S component of light  118  over to photo detector  122 . Photo detector  122  then has anti-reflection coating  124  transmitting S component of light into photo detector  122 . In this manner, photo detector  114  detects and generates a voltage output for P component of incident beam  70  and photo detector  122  gives a voltage output relative to S component of incident light beam  70 . This will eliminate prism  64  while still detecting both phase delayed optical components of incident light beam  70 . This will reduce the number of optical components inside and mounted on optic block  12 . 
     Referring now to FIG. 8, there is shown another embodiment of photo detector  114  of FIG. 7 wherein now polarized coating  110  of FIG. 7 is now replaced by small area detector  132  disposed on a support. This small area detector  132  functions as a pinhole and detector to detect the P polarization component and reflects the S component of incident beam  70  over to photo detector  122 . Detector  132  then combines three functions (pinhole, polarization detector, polarization reflector) of pinhole  42 , prism  64  and photodetector  66  (see FIG.  2 ). In this maimer, pinhole  42  and prism  64  (FIG. 2) can be eliminated and again optics/cold block  12  will have fewer components to be mounted. 
     Referring now to FIGS. 9 and 9A, there is shown photo detector  145  having a two element array  145   a  and  145   b  respectively, with built-in amplifiers. Due to the limitations of the wire bonding configurations, it is preferable to bond the photodetector chips in one plane. That is why both elements  145   a  and  145   b  are shown disposed in the same plane  146 . There is also shown on top of plane  146  disposed a right angle prism  142  having black painted coating  142 A defining opening  142  or an additional pinhole (not shown) and rhombic prisms  144  having a polarizing coating  152  on one face  144 A. Polarized laser beam  147  will be reflected by the polarizing coating  152  to one photo detector  145 A and the other polarization beam  148  will transmit through this small coated area  152  of face  144 A and be totally internally reflected at face  144 B of the prism to the second photo detector  145 B. This combination will eliminate one photo detector, one prism, and one pinhole from having to be mounted on encoder optics/cold block  12 . Right angle prism  142  and rhombic prism  144  must be made of the same material to prevent diffraction of the light polarized beam  147 .