Abstract:
In an optical encoder, a plurality of light sources is controlled on and off so as to use light rays to irradiate an optical grating of a scale from a plurality of different directions. The light rays are received by a plurality of photoreceptor elements. Operations are performed using signals output from the photoreceptor elements. Accordingly, the relative position between the photoreceptor elements and the scale is detected precisely.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to optical encoders. Particularly, the present invention relates to optical encoders with enhanced resolution for measuring angles and displacements. 
   2. Description of the Related Art 
   An optical encoder includes a main scale having a first optical grating, and an index scale having a second optical grating. Disposed opposite the main scale is a light source for irradiating the main scale with light, and a photoreceptor element that receives light via the optical grating of the main scale and the optical grating of the index scale. A photoreceptor element array that also functions as an index scale has been used in this type of optical encoder. 
     FIG. 9  is a schematic diagram showing the construction of a conventional photoelectric encoder. As shown in  FIG. 9 , a detecting-side grating substrate  232  includes photoreceptor elements  258  disposed at a regular pitch. As shown in  FIG. 10 , each of the photoreceptor elements  258  includes a first signal lead-out layer  252  composed of a light-blocking, conductive material such as a metal film, a PN semiconductor layer  254  that converts light into an electric signal, and a second signal lead-out layer  256  composed of a light-transmitting, conductive material such as In 2 O 3 , SnO 2 , Si, or a mixture thereof, laminated in that order on a light-transmitting base  250  composed of, for example, glass. The photoreceptor elements  258  are disposed opposite a main scale  224 , and the photoreceptor elements  258  form slits. 
   Light reaches the PN semiconductor layer  254  via the second signal lead-out layers  256  of the photoreceptor elements  258 , and is photoelectrically converted at the boundary between an N-type amorphous silicon film  260  and a P-type amorphous silicon film  262 . The resulting signals are extracted to the outside from output terminals  264  and  266 . 
   In this type of optical encoder, a light-emitting-side grating substrate  230  is formed integrally with light-emitting elements  212 , and the detecting-side grating substrate  232  is formed integrally with the photoreceptor elements  258 , serving to reduce the number of parts and to reduce size and weight. 
     FIG. 11  shows the relationship between an example of a pattern of photoreceptor photodiodes in the encoder and a pattern of detected bright and dark light. Photodiodes S 1  to S 4  are repeatedly subject to signal phase shifts of 0°, 90°, 180°, and 270° with respect to a bright and dark pattern of light represented by a sine wave. Signals generated by the photodiodes S 1  to S 4  are input to a current-voltage converter circuit (not shown). Then the signals from the current-voltage conversion are shifted by 90° with respect to each other. By differential amplification, analog sine-wave voltage signals of two phases are obtained. For example, phase A (S 1 –S 3 ) and phase (S 2 –S 4 ) having phases of 0° and 90°, are obtained. 
   Actually in the encoder, the analog sine-wave voltage signals are input into a comparator, and the resulting digital signals are fed into a counter circuit or the like. 
   In a conventional encoder, in order to further enhance resolution, the pitch of the scale and the bright and dark regions of the photoreceptor elements must be further reduced. 
   However, when the scale pitch is reduced, a considerable decrease in precision occurs. This is because the amplitudes of signals obtained by the photoreceptor elements become smaller, causing noise or affecting the hysteresis of the comparator used for digitization, resulting in a considerable decrease in precision. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to overcome one or more of the problems described above, and to provide an optical encoder in which the resolution is enhanced without reducing the scale pitch or the photoreceptor element pitch. 
   Accordingly, in one aspect, the present invention provides an optical encoder including a scale having an optical grating; a plurality of photoreceptor elements that are movable with respect to the scale and that are disposed in relation to a pitch of the optical grating; light source means having at least two light sources for irradiating the photoreceptor elements through the scale with light rays from at least two different directions; and control means for switching light-emitting status of the at least two light sources; wherein the control means obtains relative-position information of the scale and the photoreceptor elements by processing information obtained from the light-emitting status of the light sources when the light-emitting status of the light sources is switched. 
   According to another aspect of the present invention, an optical encoder with enhanced resolution is provided. The optical encoder includes a scale having an optical grating and a plurality of movable photoreceptor elements. Note that each movable photoreceptor element is positioned based on a pitch of the optical grating. The optical encoder further includes first and second light sources for providing light to the photoreceptor elements such that light is provided in a first direction by the first light source and in a second direction by the second light source. 
   Also included in the optical encoder is a switch for controlling the light-emitting status of the first light source and the second light source. This switch is capable of using the light emitting status of the first and the second light sources to acquire relative-position information of the scale and the photoreceptor elements. 
   Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments (with reference to the attached drawings). 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram showing the construction of an optical encoder according to a first embodiment of the present invention. 
       FIG. 2  is a perspective view of light-emitting elements. 
       FIG. 3  is a diagram showing the relationships between scale positions and signals in cases where the respective light-emitting elements are turned on. 
       FIG. 4  is a diagram showing the construction of an optical encoder according to a second embodiment of the present invention. 
       FIG. 5  is a perspective view of light-emitting elements. 
       FIG. 6  is a diagram showing the relationships between scale positions and signals in cases where the respective light-emitting elements are turned on. 
       FIG. 7  is a diagram showing the construction of an optical encoder according to a third embodiment of the present invention. 
       FIG. 8  is a diagram showing a case where the balance of powers of lights emitted by signal light sources is changed. 
       FIG. 9  is a diagram showing the construction of a conventional optical encoder. 
       FIG. 10  is a sectional view of a detecting-side grating substrate. 
       FIG. 11  is a diagram showing the relationship between an example of a pattern of a photodiode array and a bright and dark pattern of detected light. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Now, preferred embodiments of the present invention will be described in detail with reference to  FIGS. 1 to 8 . 
     FIG. 1  is a diagram showing the construction of an optical encoder according to a first embodiment. In  FIG. 1 , the optical encoder includes two light-emitting elements  11  and  12  disposed in parallel to each other. The optical encoder also includes both an encoder scale having an optical grating and having a movable member at a middle part, and a photoreceptor  14  having photodiodes S 1  to S 4  on a surface thereof, disposed opposing the light-emitting elements  11  and  12  across the encoder scale  13 . 
   As shown in  FIG. 2 , the light-emitting elements  11  and  12  have light-emitting windows  11   a  and  12   a , receive voltages through wires  11   b  and  12   b , respectively, and also receives a common voltage through a common electrode  15 . One advantage of the present invention is that at least two light-emitting windows  11   a  and  12   a  are provided so that light-emitting states are controlled independently of each other. In contrast, in the related art, windows are provided for light-emitting elements and lights are emitted simultaneously at multiple points. 
   The light-emitting elements  11  and  12  are positioned such that lights received on the photoreceptor  14  are mutually shifted in position by 45°. Thus, the intensity of light received on the photoreceptor  14  when the light-emitting element  11  is turned on is as indicated by  11 ′ in  FIG. 1 , and the intensity of light received on the photoreceptor  14  when the light-emitting element  12  is turned on is as indicated by  12 ′. 
     FIG. 3  is a diagram showing the relationships between positions of the encoder scale  13  and signal outputs in cases where the light emitting elements  11  and  12  are turned on, respectively. In  FIG. 3 , part (a) also shows the relationship between an analog waveform and digitally counted values obtained by multiplying one cycle of the analog waveform by four. 
   When the encoder scale  13  attached to the movable member is moved, a pattern of bright and dark regions moves over the photoreceptor  14 . On the photoreceptor  14 , a set of photodiodes S 1  to S 4  is arranged so as to divide each cycle of the bright and dark pattern by four, and by processing the divided parts of the bright and dark pattern, two-phase signals including phase-A signals (S 1 –S 3 ) and phase-B signals (S 1 –S 3 ) are output. 
   For the light distribution of the state  11 ′ with the light-emitting element  11  turned on, signal values shown in part (a) of  FIG. 3  are output from processing circuits for phase A and phase B. On the other hand, for the light distribution of the state  12 ′ with the light-emitting element  12  turned on, signal values shown in part (b) of  FIG. 3  are output from the processing circuits for phase A and phase B. 
   When the bright and dark pattern moves over the photoreceptor  14 , the light-emitting element  11 , which is temporally shifted by 90° in phase, is turned on, and signals of phase A and phase B by the encoder scale  13  are obtained. Thus, the amount of movement can be detected by counting the number of wave cycles of phase A and phase B. When the encoder scale  13  is at a halt at a certain point P 1 , signal levels take two points a in part (a) of  FIG. 3 . 
   When the light-emitting element  11  is turned off and the light-emitting element  12  is turned on, the positional relationship between the light-emitting elements  11  and  12  and the encoder scale  13  changes as shown in part (b) of  FIG. 3 . Thus, the relationship between positions and signals also changes; more specifically, the signals at points a in part (a) of  FIG. 3  change to points b in part (b) of  FIG. 3 . This is equivalent to moving the encoder scale  13  by 45° in the arrow direction. 
   With regard to signals output from the signal processing circuits, when the light-emitting element  11  is on, points a are High for both phase A and phase B. On the other hand, when the light-emitting element  12  is on, points b are Low for phase A and High for phase B. The switching for phase B indicates that when the encoder  13  actually stops moving after further moving by 45°, the signal for phase A switches. That is, phase A and phase B reside in a 45° to 90° region within the 90° region at High level, so that the resolution becomes twice as high. 
   If the signal for phase A remains high, phase A and phase B exist within a 0° to 45° range in the above 90° region at high level. 
   Table 1 below shows the relationship between counter values, and digital signal level changes after switching of light-emitting elements, and position. 
   
     
       
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
               Counter value 
               Change in digital signal 
               Position in one cycle 
             
             
                 
             
           
           
             
               0 
               No change 
                0°–45° 
             
             
               0 
               Change 
               45°–90° 
             
             
               1 
               No change 
                90°–135° 
             
             
               1 
               Change 
               135°–180° 
             
             
               2 
               No change 
               180°–225° 
             
             
               2 
               Change 
               225°–270° 
             
             
               3 
               No change 
               270°–315° 
             
             
               3 
               Change 
               315°–360° 
             
             
                 
             
           
        
       
     
   
   By switching between the light-emitting elements  11  and  12  as described above, the present invention can double the resolution of conventional art systems by reflecting a result obtained to another bit of counter value. 
     FIG. 4  is a diagram showing the construction of an optical encoder according to a second embodiment. In the first embodiment, the two light-emitting elements  11  and  12  are provided and switched to achieve a resolution that is twice as high compared with the related art. In the second embodiment, light-emitting elements  21  and  22  are further arranged on both sides of the light-emitting elements  11  and  12  to achieve a resolution that is four times as high compared with the related art. 
   In  FIG. 4 , lines  11 ′,  12 ′,  21 ′, and  22 ′ represent the intensities of lights received on the photoreceptor  14  when the light-emitting elements  11 ,  12 ,  21 , and  22  are turned on, respectively. The light-emitting elements  11 ,  21 ,  12 , and  22  are positioned such that lights received thereby on the photoreceptor  14  are shifted in position by 22.5°. 
     FIG. 5  is a perspective view of the light-emitting elements in the second embodiment. The light-emitting elements  11 ,  12 ,  21 , and  22  have light-emitting windows  11   a ,  12   a ,  21   a , and  22   a , and are connected to wires  11   b ,  12   b ,  21   b , and  22   b  for supplying voltages, respectively. 
     FIG. 6  shows the relationship between the positions of the encoder scale  13  and phase-A signals in cases where the light-emitting elements  11 ,  12 ,  21 , and  22  are turned on. When the light-emitting element  11  is on, and when the encoder scale  13  stops at a certain point P 2 , signal a is obtained as a phase-A voltage. At this time, the voltage is at High level. Then, the light-emitting element  21  is turned on, whereby a signal c is obtained. Furthermore, as the light-emitting elements are switched to turn on the light-emitting elements  12  and  22  sequentially, the voltage changes to Low level when the light-emitting element  22  is switched on. The state where the light-emitting element  22  is on corresponds to the state where the encoder scale  13  is moved by 67.5°. That is, a point at which signal level changes correspond to a movement of the encoder scale by 67.5°. Thus, it is understood that P 2  is in a range of 22.5° to 45° of the region of the counter value 1. 
   Table 2 summarizes similar relationships. 
   
     
       
             
             
             
           
             
             
             
             
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
                 
               Change in digital signal after switching 
                 
             
             
               Counter 
               of light-emitting elements 
               Position in one 
             
           
        
         
             
               value 
               11→21 
               21→12 
               12→22 
               22→11 
               cycle 
             
             
                 
             
             
               0 
               No change 
               No change 
               No change 
               No change 
                  0°–22.5° 
             
             
               0 
               No change 
               No change 
               Change 
               No change 
                22.5°–45° 
             
             
               0 
               No change 
               Change 
               No change 
               No change 
                 45°–67.5° 
             
             
               0 
               Change 
               No change 
               No change 
               No change 
                67.5°–90° 
             
             
               1 
               No change 
               No change 
               No change 
               No change 
                 90°–112.5° 
             
             
               1 
               No change 
               No change 
               Change 
               No change 
               112.5°–135° 
             
             
               1 
               No change 
               Change 
               No change 
               No change 
                 135°–157.5° 
             
             
               1 
               Change 
               No change 
               No change 
               No change 
               157.5°–180° 
             
             
               2 
               No change 
               No change 
               No change 
               No change 
                 180°–202.5° 
             
             
               2 
               No change 
               No change 
               Change 
               No change 
               202.5°–225° 
             
             
               2 
               No change 
               Change 
               No change 
               No change 
                 225°–247.5° 
             
             
               2 
               Change 
               No change 
               No change 
               No change 
               247.5°–270° 
             
             
               3 
               No change 
               No change 
               No change 
               No change 
                 270°–292.5° 
             
             
               3 
               No change 
               No change 
               Change 
               No change 
               292.5°–315° 
             
             
               3 
               No change 
               Change 
               No change 
               No change 
                 315°–337.5° 
             
             
               3 
               Change 
               No change 
               No change 
               No change 
                37.5°–360° 
             
             
                 
             
           
        
       
     
   
   Thus, according to this embodiment, one cycle of waveform can be divided by 16. 
     FIGS. 7 and 8  show the construction of an optical encoder according to a third embodiment. In the first embodiment, the two light-emitting elements  11  and  12  are provided and switched to achieve a resolution that is twice as high compared with the related art. In the third embodiment, light-emitting powers of the two light-emitting elements  11  and  12  are changed and the lights are combined to produce a signal. 
   Referring to  FIG. 7 , when the light-emitting element  11  and the light-emitting element  12  are turned on individually, patterns of bright and dark occur at positions shifted by 90° corresponding to one wave cycle on the photoreceptor  14 . The bright and dark pattern indicated by  11 ′ is achieved on the photoreceptor  14  when only the light-emitting element  11  is turned on while the light-emitting elements  11  and  12  and the encoder scale  13  are in a certain positional relationship. When the light-emitting element  12  is then switched on, the bright and dark pattern is shifted by 90° on the photoreceptor  14 , as indicated by  12 ′. 
   When the light-emitting elements  11  and  12  are simultaneously caused to emit light at a power of 1/√2 compared with the related art, signals output from the processing circuits are combined as indicated by  13 ′. This is equivalent to the signal in a case where the photoreceptor  14  is shifted by 45° with respect to the light-emitting element  11 . 
     FIG. 8  shows a case where the balance of light-emitting powers of the light-emitting elements  11  and  12  is changed. As shown in  FIG. 8 , a signal in a case where the photoreceptor  14  is shifted by 30° with respect to the light-emitting element  11  can be obtained by setting a ratio such that the light-emitting power of the light-emitting element  11  is cos(30°)=√ 3/2 and the light-emitting power of the light-emitting element  12  is sin(30°)=½. As for other points, similarly, signals corresponding to shifts in light-emitting position can be obtained by changing the light-emitting powers of the light-emitting elements  11  and  12 . 
   Thus, after the encoder scale  13  is stopped, by changing the balance between the light-emitting elements  11  and  12  as if the device is moving, and finding a point where the digital signal level changes, the stop position can be detected at a desired resolution. 
   Optical power can be changed by stabilizing optical power while detecting it. Or, the optical power can be controlled based on current values assuming a substantially linear relationship between optical power and current. Although not discussed, optical power may also be changed by using other methods. 
   Although this embodiment relates to a transmissive optical encoder, the same advantages can be achieved by a reflective optical encoder, with the light-emitting elements and the photoreceptor element disposed on the same side. 
   According to one aspect of the present invention, by switching between or changing the power of light sources, a resolution much higher than that of a conventional optical encoder can be achieved. 
   While the present invention has been described with reference to what are presently considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.