Abstract:
An electrostatic encoder comprising receiving coupling electrodes formed on a scale, and extending in a predetermined direction, transmitting coupling electrodes provided on a sensor head, and located to face the receiving coupling electrodes, digital electrodes provided on the scale to extend from the receiving coupling electrodes in a direction perpendicular to the predetermined direction, and arranged at a predetermined pitch, two pairs of interdigital electrodes provided on the sensor head, and located to face the digital electrodes, a voltage applying portion configured to apply an alternating voltage to the transmitting coupling electrodes, and a potential difference detecting portion configured to detect a potential difference between the interdigital electrodes of each of the two pairs of interdigital electrodes. The two pairs of interdigital electrodes are arranged at the same pitch, and the each pair of interdigital electrodes are spaced apart from each other by a predetermined distance in the predetermined direction.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2004-229471, filed Aug. 5, 2004, the entire contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an electrostatic encoder. 
   2. Description of the Related Art 
   Various kinds of encoders for use in controlling an actuator have been proposed. Of those encoders, an electrostatic encoder is superior in that: it can be made thinner, and the sensor itself is relatively cheap. As a method of manufacturing such an electro-static encoder, for example, the method disclosed in U.S. Pat. No. 3,961,318 is known. According to this method, a sensor head and a scale can be each formed by using a simple wiring board, as a result of which a very thin encoder can be manufactured at a relatively low cost. 
   BRIEF SUMMARY OF THE INVENTION 
   According to an aspect of the present invention, there is provided an electrostatic encoder comprising: 
   a scale; 
   a sensor head provided to face the scale, and movable relative to the scale in a predetermined direction; 
   a pair of receiving coupling electrodes formed on the scale, and extending in the predetermined direction; 
   transmitting coupling electrodes provided on the sensor head, and located to face the pair of receiving coupling electrodes; 
   digital electrodes provided on the scale to extend from the receiving coupling electrodes in a direction perpendicular to the predetermined direction, and arranged at a predetermined pitch; 
   two pairs of interdigital electrodes provided on the sensor head, and located to face the digital electrodes of the scale; 
   a voltage applying portion configured to apply an alternating voltage to the transmitting coupling electrodes; and 
   a potential difference detecting portion configured to detect a potential difference between the interdigital electrodes of each of the two pairs of interdigital electrodes of the sensor head, 
   wherein the two pairs of interdigital electrodes of the sensor head are arranged at the same pitch, and the each pair of interdigital electrodes are spaced apart from each other by a predetermined distance in the predetermined direction. 
   According to another aspect of the present invention, there is provided an electrostatic encoder comprising: 
   a scale; 
   a sensor head provided to face the scale, and movable relative to the scale in a predetermined direction; 
   a pair of receiving coupling electrodes formed on the scale, and extending in the predetermined direction; 
   a pair of transmitting coupling electrodes provided on the sensor head, and located to face the pair of receiving coupling electrodes; 
   digital electrodes provided on the scale to extend from the receiving coupling electrodes in a direction perpendicular to the predetermined direction, and arranged at a predetermined pitch; 
   two pairs of interdigital electrodes provided on the sensor head, and located to face the digital electrodes of the scale; 
   a voltage applying portion configured to apply an alternating voltage to the transmitting coupling electrodes; and 
   a potential difference detecting portion configured to detect a potential difference between the interdigital electrodes of each of the two pairs of interdigital electrodes of the sensor head, 
   wherein the two pairs of interdigital electrodes of the sensor head are arranged at the same pitch, and the each pair of interdigital electrodes are spaced apart from each other by a predetermined distance in the predetermined direction. 
   Advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
       FIG. 1  is a view showing the entire structure of an electrostatic encoder according to a first embodiment of the present invention; 
       FIG. 2  is a view showing the structure of a scale in the electrostatic encoder according to the first embodiment of the present invention; 
       FIG. 3  is a view showing the structure of a sensor head in the electrostatic encoder according to the first embodiment of the present invention; 
       FIG. 4  is a schematic view which schematically shows outputs of an A-phase voltmeter and a B-phase voltmeter, which are obtained after phase detection; 
       FIG. 5A  is a view showing the structure of a sensor head in an electrostatic encoder according to a second embodiment of the present invention; and 
       FIG. 5B  is a vertical sectional view taken along line A-A′ in  FIG. 5A . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The best mode for carrying out the present invention will be explained with reference to the accompanying drawings. 
   First, an electrostatic encoder according to a first embodiment of the present invention will be explained. As shown in  FIG. 1 , the electrostatic encoder according to the first embodiment comprises a scale  10  and a sensor head  20 . The sensor head  20  is located to face the scale  10 . The sensor head  20  can be displaced relative to the scale  10  in either of the directions indicated by the double-headed arrow in  FIG. 1 . 
   In the scale  10 , as shown in  FIG. 2 , an interdigital electrode  11  is formed on a glass board  12 . The interdigital electrode  11  comprises a first electrode  13  and a second electrode  14 . The first electrode  13  comprises a basal portion  13 A and digital portions  13 B, and the second electrode  14  comprises a basal portion  14 A and digital portions  14 B. The digital portions  13 B are arranged at a constant pitch λ, and the digital portions  14 B are also arranged at the constant pitch λ. It should be noted that in the entire region of the scale  10 , each of the basal portions  13 A and  14 B of the interdigital electrode  11  is divided into a plurality of regions by slits  15  located at appropriate regular intervals in the direction of relative displacement of the sensor head  20  against the scale  10 , that is, the interdigital electrode  11  is divided into a plurality of regions by the slits  15 . Also, a thin insulating layer not shown is formed on the surface of the interdigital electrode  11 . 
   In the sensor head  20 , as shown in  FIG. 3 , a number of electrodes are formed on a resin board  21 . As those electrodes, a pair of feed electrodes  22  and  23 , A-phase interdigital electrodes  24  and  25 , B-phase interdigital electrodes  26  and  27  and sub-electrodes  28  and  29 . 
   To be more specific, the A-phase digital electrodes  24  and  25  are located in parallel with the B-phase digital electrodes  26  and  27 , and digital portions of the A-phase digital electrodes  24  and  25  and digital portions of the B-phase digital electrodes  26  and  27  are arranged in the same pitch. In this case, the digital electrodes  24  and  25  the digital electrodes  26  and  27  are located with a phase difference of fourth the pitch λ in the direction of the relative displacement of the sensor head  20  against the scale  10 , the direction indicated by an arrow in  FIG. 3 . The pitch of the digital portions of the digital electrodes  24 ,  25 ,  26  and  27  is half the pitch λ of the digital portions  13 B and  14 B of the interdigital electrode  11  of the scale  10 . 
   It should be noted that when the electrostatic encoder is actually used, the above electrodes on the sensor head  20  face the interdigital electrode  11  of the scale  10 , and the feed electrodes  22  and  23  of the sensor head  20  faces the basal portions  13 A and  14 B of the interdigital electrode  11 , respectively. 
   Furthermore, as shown in  FIG. 3 , which contains a connection diagram, the feed electrodes  22  and  23  are connected to an alternating voltage source  31 ; the A-phase interdigital electrodes  24  and  25  are connected to an A-phase voltmeter  32 ; the B-phase interdigital electrodes  26  and  27  are connected to a B-phase voltmeter  33 ; and sub-electrodes  28  and  29  are connected to a compensation signal source  34 . 
   Then, the operation of the encoder according to the first embodiment will be explained. 
   From the alternating voltage source  31 , an alternating voltage is applied to the feed electrodes  22  and  23  of the sensor head  20 . As a result, due to electrostatic induction of the basal portions  13 A and  14 A of the interdigital electrode  11  of the scale  10 , which face the feed electrodes  22  and  23 , an alternating electric field generates at the digital portions  13 B and  14 B of the interdigital electrode  11 , which are electrically connected to the basal portions  13 A and  14 A of the interdigital electrode  11 . In regions which the digital portions  13 A and  14 B of the interdigital electrode  11  face, the A-phase interdigital electrodes  24  and  25  and the B-phase interdigital electrodes  26  and  27  are present. Therefore, the voltmeters  32  and  33  connected to the interdigital electrodes  24 ,  25 ,  26  and  27  detect a periodic potential change which depends on a relative displacement amount of the sensor head  20  against the scale  10 . Needless to say, the potential change includes the change of a potential component which varies in accordance with the frequency of the alternating voltage source  31 . However, the potential change caused only by the relative displacement of the sensor head  20  against the scale  10  can be measured by performing detection, and eliminating the above component. 
     FIG. 4  schematically shows the outputs of the A-phase voltmeter  32  and B-phase voltmeter  33 , which are obtained after the phase detection, in the case where the sensor head  20  is displaced relative to the scale  10  in the direction indicated by the arrow in  FIG. 3 . 
   The A-phase interdigital electrodes  24  and  25 , and the B-phase interdigital electrodes  26  and  27  are located at a phase difference of fourth the pitch λ, corresponding to 90° in phase. Therefore, the output phase of the B-phase voltmeter  33  lags that of the A-phase voltmeter  32  by 90°. On the other hand, in the case where the sensor head  20  is displaced in the direction opposite to that indicated by the arrow in  FIG. 3 , the output phase of the B-phase voltmeter  33  leads that of the A-phase voltmeter  32  by 90°. In such a manner, when the outputs of the A-phase voltmeter  32  and B-phase voltmeter  33  are both measured, the direction of the above displacement can be detected. Also, when a phase interpolation circuit is provided to obtain the phase angle of a Lissajous waveform of the outputs of the A-phase voltmeter  32  and B-phase voltmeter  33 , the displacement amount of the sensor head  20  against the scale  10  can be determined with a higher resolution compared with the pitch of the interdigital electrode  11  of the scale  10 . 
   In such a manner, the electrostatic encoder according to the first embodiment can detect the above displacement direction, and has a high resolution. 
   In the first embodiment, an alternating voltage is applied such that the phase of the alternating voltage at the feed electrode  22  is opposite to that at the feed electrode  23 . However, even if one of the feed electrodes  22  and  23  is grounded, the function of the electrostatic encoder is still ensured. 
   However, in the electrostatic encoder having the above structure, the alternating voltage applied to the feed electrodes  22  and  23  may induce an alternating voltage to the interdigital electrode  11  of the scale  10  which may generate a noise, and has an adverse effect on electronic equipment provided in the vicinity of the electrostatic encoder. The greater the total area of the interdigital electrode  11 , where the alternating voltage is induced, the more clearly the above phenomenon occurs. Furthermore, when the area of the interdigital electrode  11  of the scale  10  is great, the electrostatic encoder is easily influenced by noise generated by the electronic equipment in the vicinity of the electrostatic encoder. This is a problem, especially in the case where the scale  10  is greatly long with respect to the sensor head  20  in the displacement direction. 
   In the scale  10  in the first embodiment, as described above, each of the basal portions  13 A and  14 A of the interdigital electrode  11  is divided into a plurality of regions by the slits  15  at the appropriate regular intervals. Thus, an alternating electric field generates only at those parts of the digital portions  13 B and  14 B of the interdigital electrode  11  of the scale  10 , which face the feed electrodes  22  and  23  of the sensor head  20 . That is, application of a voltage through the feed electrodes  22  and  23  does not occur at part of the scale  10  which does not overlap the sensor head  20 . This structural feature can therefore reduce the adverse effect of the electrostatic encoder on the electronic equipment provided in the vicinity of the electrostatic encoder. Furthermore, since the part of the interdigital electrode  11  which does not overlap the sensor head  20  is electrically disconnected by the slits  15 , the influence of the noise generated from the electronic equipment provided in the vicinity of the electrostatic encoder can be reduced. Accordingly, the above displacement can be measured with a higher accuracy. 
   Moreover, in the case where the feed electrodes  22  and  23  of the sensor head  20  are located close to the A-phase interdigital electrodes  24  and  25  and the B-phase interdigital electrodes  26  and  27 , the alternating voltage applied to the feed electrodes  22  and  23  directly influences the A-phase interdigital electrodes  24  and  25  and the B-phase interdigital electrodes  26  and  27 . That is, the alternating voltage gives offsets to the outputs of the A-phase voltmeter  32  and B-phase voltmeter  33 , which are obtained as schematically shown in  FIG. 4  after the phase detection, thus reducing the accuracy of measuring the displacement. To restrict this reduction of the measurement accuracy, in the sensor head  20 , the sub-electrodes  28  and  29  are respectively provided between the feed electrode  22  and the A-phase interdigital electrodes  24  and  25 , and between the feed electrode  23  and the B-phase interdigital electrodes  26  and  27 . For example, when the sub-electrodes  28  and  29  are grounded, the influence of the voltages of the -feed electrodes  22  and  23  upon the A-phase interdigital electrodes  24  and  25  and the B-phase interdigital electrodes  26  and  27  can be restricted. Furthermore, a certain voltage opposite in phase to that of the feed electrode  22  is applied to the sub-electrode  28 , and a certain voltage opposite in phase to that of the feed electrode  23  can be applied to the sub-electrode  29  so as to cancel the influence of the voltages of the feed electrodes  22  and  23  upon the A-phase interdigital electrodes  24  and  25  and B-phase interdigital electrodes  26  and  27 . In this case, preferably, the voltages to be applied should be adjusted such that after the phase detection, the outputs of the A-phase voltmeter  32  and B-phase voltmeter  33  are set at 0, with the scale  10  removed from the sensor head  20 . In such a manner, when the sub-electrodes  28  and  29  are provided, and a potential control is properly carried out, offsets of the outputs of the A-phase voltmeter  32  and B-phase voltmeter  33 , which are obtained as schematically shown in  FIG. 4  after the phase detection, are restricted, and lowering of the measurement accuracy is also restricted. 
   Next, an electrostatic encoder according to a second embodiment of the present invention will be explained. In the electrostatic encoder according to the second embodiment, the scale  10  is identical to that shown in  FIG. 2 , but the sensor head  20  is different from that in  FIG. 3 . 
   In the sensor head  20  used in the second embodiment, as shown in  FIGS. 5A and 5B , the structures of the feed electrodes  22  and  23  and the sub-electrodes  28  and  29  are the same as those in  FIG. 3 , but the structures of the A-phase interdigital electrodes and B-phase interdigital electrodes are different from those in  FIG. 3 . To be more specific, the A-phase interdigital electrodes are separately arranged from the B-phase interdigital electrodes in  FIG. 3 , whereas in the second embodiment, A-phase interdigital electrodes and B-phase interdigital electrodes consist of four phase strip-shaped electrodes  41 ,  42 ,  43  and  44 , interdigitatingly arranged at a pitch of λ/4 as shown in  FIG. 5A . The output of each of the strip-shaped electrodes  41 ,  42 ,  43  and  44  of each group has one of four phases, respectively. Furthermore, wiring electrodes  45 ,  46 ,  47  and  48  having different phases in output are formed on a reverse surface of the board  21 . The strip-shaped electrodes  41 ,  42 ,  43  and  44  are connected to the wiring electrodes  45 ,  46 ,  47  and  48  at their appropriate portions through contact holes  49 . To be more specific, the strip-shaped electrodes  41 ,  42 ,  43  and  44  arranged at a constant pitch are successively connected to the wiring electrodes  45 ,  46 ,  47  and  48  through the contact holes  49 , in units of one group, i.e., four strip-shaped electrodes, from the first four of the strip-shaped electrodes  41 ,  42 ,  43  and  44  from an upper side in  FIG. 5A . However, to keep the figure simple, only the first four of all the strip-shaped electrodes are denoted by reference numerals  41 ,  42 ,  43  and  44 . To be more specific, the wiring electrodes  45 ,  47 ,  46  and  48  correspond to phases of 0°, 90°, 180° and 270°, respectively. The wiring electrodes  45  and  46 , corresponding to the phases of 0° and 180°, are connected to the A-phase voltmeter  32 , and the wiring electrodes  47  and  48 , corresponding to the phases of 90° and 270°, are connected to the B-phase voltmeter  33 . 
   Due to the above connection, the A-phase voltmeter  32  and the B-phase voltmeter  33  output two signals whose phase difference is 90°. By virtue of this structural feature, the displacement of the sensor head  20  against the scale  10  can be measured as in the first embodiment. In the second embodiment, it is indispensable that wiring of the sensor head  20  is provided to have a two-layer structure, unlike the first embodiment. In this regard, the first embodiment is slightly more advantageous than the second embodiment in terms of manufacturing cost. However, in the second embodiment, since the A-phase and B-phase interdigital electrodes are arranged alternately, even if the sensor head  20  or the scale  10  is slightly inclined, thereby losing parallelism, the displacement of the sensor head  20  against the scale  10  can be stably measured. 
   The present invention will be explained by referring to the above embodiments; however, it is not limited to the embodiments. Needless to say, various modifications and applications can be made without departing from the subject matter of the present invention. For example, it should be noted that in the above embodiments, by virtue of the interdigital electrodes of the scale  10 , the output signal is large, and the S/N ratio of the signal is improved. However, even if one of the set of the basal portions  13 A and the digital portions  13 B and the set of the basal portions  14 A and the digital portions  14 B, which are formed as shown in  FIG. 2 , e.g., the set of the basal portions  14 A and the digital portions  14 B, is omitted, the resulting structure can still function as an electrostatic encoder. In this case, since the feed electrode  23  of the sensor head  20  can also be omitted, the width of the sensor  20  can be shortened. Therefore, although this electrostatic encoder is slightly inferior in function to those according to the above embodiments, it is advantageous where a smaller electrostatic encoder is required. 
   Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, and representative devices shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.