Abstract:
An absolute position length-measurement type encoder includes a scale having an incremental track, an absolute track, and a reference position track. The incremental track has incremental patterns including first light and dark patterns formed at equal intervals in first periods. The absolute track has absolute patterns representing an absolute position. The reference position track has reference position patterns including second light and dark patterns formed at equal intervals in second periods longer than the first periods. A light source emits a measurement light to the scale. A photodetector receives the measurement light reflected at or transmitted through the scale. A signal processing circuit processes the received light signal of the photodetector to detect an absolute position of the scale.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims the benefit of priority from prior Japanese Patent Application No. 2007-104122, filed on Apr. 11, 2007, 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 absolute position length-measurement type encoder. 
     2. Description of the Related Art 
     Incremental encoders and absolute encoders are known as devices for measuring travel distances of objects. The incremental encoders measure relative travel distances and absolute encoders allow for absolute position length-measurement. 
     In the case of photoelectric encoders, the incremental encoders have incremental tracks with incremental patterns including equally-spaced light and dark patterns. Based on these patterns, the incremental encoders count light and dark signals to detect relative travel distances. In addition, the incremental encoders may detect absolute travel distances by detecting origin detection patterns provided separately from the above-mentioned pattern with equally-spaced light and dark patterns, and then detecting relative travel distances from the origin. However, prior to the measurement, a scale must be moved to right and/or left directions in order to read origin detection patterns. 
     On the other hand, the absolute encoders have absolute tracks with absolute patterns representing pseudo-random codes such as M-sequence codes and detect absolute positions resulting from reading the absolute patterns for a corresponding object. Unlike the incremental encoders, the absolute encoders does not require any origin detection based on origin detection patterns and may start measurement from the very position when powered on, without moving the scale. However, the absolute encoders have a lower detection accuracy than the incremental encoders. 
     As such, an absolute position length-measurement type encoder is known where an incremental track with equally-spaced incremental patterns and an absolute track with absolute patterns representing pseudo-random codes are positioned in parallel on one scale, as disclosed in, e.g., JP H7-286861A. This encoder first detects the absolute position after powered on by reading absolute patterns on the absolute track. Then, the encoder detects a relative travel distance from that position by reading the incremental patterns on the incremental track. In this way, an absolute position length-measurement type encoder may be obtained that covers the shortcomings of each of the incremental and absolute encoders, while enjoying advantages of both encoders. 
     However, in encoders so configured, it is more difficult to form minute absolute patterns with respect to incremental patterns without positional errors, as incremental patterns have more minute light and dark pitches. In addition, as the entire length of a scale becomes longer, it is more difficult to maintain a relative phase relation between absolute patterns and incremental patterns throughout the scale. 
     Therefore, it is difficult to provide smaller absolute position length-measurement type encoders in which both absolute and incremental patterns are used. 
     SUMMARY OF THE INVENTION 
     An absolute position length-measurement type encoder according to the present invention comprises: a scale having an incremental track formed therein with incremental patterns including first light and dark patterns formed at equal intervals in first periods, an absolute track formed therein with absolute patterns representing absolute positions, and a reference position track formed therein with reference position patterns including second light and dark patterns formed at equal intervals in second periods longer than the first periods; a light source for emitting a measurement light to the scale; a photodetector for receiving the measurement light reflected at or transmitted through the scale; and a signal processing circuit for processing a received-light signal of the photodetector to detect an absolute position of the scale. 
     According to this encoder, the absolute patterns does not need to be formed precisely in relation to the incremental patterns with light and dark patterns formed therein at first periods, but rather it is sufficient to form the absolute patterns with a predetermined accuracy with respect to the reference position patterns that are formed at second periods larger than the first periods. Accordingly, the absolute patterns may accept larger position errors with respect to the incremental patterns, which may lead to more minute incremental patterns as well as improved accuracy encoders. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating an entire configuration of an absolute position length-measurement type photoelectric encoder according to a first embodiment of the present invention: 
         FIG. 2  is a plan view illustrating a configuration of the scale  12  in  FIG. 1 ; 
         FIG. 3  is a plan view illustrating a configuration of the photodiode array  14  in  FIG. 1 ; 
         FIG. 4  illustrates operations of the absolute position length-measurement type photoelectric encoder according to the first embodiment; 
         FIG. 5  is a schematic diagram illustrating an entire configuration of an absolute position length-measurement type photoelectric encoder according to a second embodiment of the present invention; 
         FIG. 6  is a plan view illustrating a configuration of the scale  12  in  FIG. 5 ; 
         FIG. 7  is a plan view illustrating a configuration of the scale  12  in  FIG. 6  without hatching; 
         FIG. 8  is a plan view illustrating a configuration of the photodiode array  14  in  FIG. 5 ; 
         FIG. 9  is a schematic diagram illustrating details of the configuration of the ABS/reference position integrated scale  34  in the scale  12  illustrated in  FIG. 5 ; 
         FIG. 10  is a schematic diagram illustrating details of the configuration of the ABS/reference position integrated scale  34  in the scale  12  in  FIG. 6 ; 
         FIG. 11  illustrates a configuration of the scale  12  of an absolute position length-measurement type photoelectric encoder according to a third embodiment of the present invention; 
         FIG. 12  illustrates a configuration of the scale  12  of an absolute position length-measurement type photoelectric encoder according to a fourth embodiment of the present invention; 
         FIG. 13  illustrates a configuration of the scale  12  of an absolute position length-measurement type photoelectric encoder according to a fifth embodiment of the present invention; 
         FIG. 14  illustrates a configuration of the scale  12  of an absolute position length-measurement type photoelectric encoder according to a sixth embodiment of the present invention; 
         FIG. 15  illustrates a configuration of the scale  12  of an absolute position length-measurement type photoelectric encoder according to a seventh embodiment of the present invention; 
         FIG. 16  illustrates a configuration of the scale  12  of an absolute position length-measurement type photoelectric encoder according to a eighth embodiment of the present invention; and 
         FIG. 17  illustrates a variation of the embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the present invention will now be described in detail below with reference to the accompanying drawings. 
     First Embodiment 
       FIG. 1  is a schematic diagram illustrating an entire configuration of an absolute position length-measurement type photoelectric encoder according to a first embodiment of the present invention. The absolute position length-measurement type photoelectric encoder according to this embodiment comprises a light-emitting element  11 , a scale  12 , a lens  13 , a photodiode array  14 , and a signal processing circuit  20 . 
     The light-emitting element  11  is a light source, such as a laser diode, that emits a coherent light. As illustrated in  FIG. 2 , the scale  12  is configured to form the following tracks on a transparent glass substrate: an incremental track  301  with incremental patterns  31  formed at an arrangement pitch Pi (e.g., 40 μm) that include equally-spaced light and dark regions, and an absolute track  302  with general absolute patterns  32  that represent absolute positions in pseudo-random patterns (in this case, M-sequence codes). 
     In addition to this, the scale  12  further comprises a reference position track  303  with reference position patterns  33 , each of which has a width of Wr in a direction of length-measurement. The reference position patterns  33  have a predetermined phase relation to the absolute patterns  32  and are formed at an arrangement pitch Pr (&gt;Pi) with equally-spaced light and dark regions. That is, the absolute patterns  32  represent absolute positions of the equally-spaced patterns in the reference position patterns  33 . The arrangement pitch Pi of the incremental patterns  31  is set to be smaller than the arrangement pitch Pr of the reference position patterns  33 , e.g., by a factor of an integer. In this embodiment, for illustration, it is assumed that Pi=4Pr. For example, if Pi=40 μm, then Pr is set to 160 μm. 
     The incremental patterns  31  and the reference position patterns  33  may easily be formed in an accurate fashion throughout the whole length of the encoder, since both are formed at equally-spaced arrangement pitches (Pi, Pr) throughout the length, respectively. In contrast, the absolute patterns  32  are difficult to be formed in an accurate fashion throughout the length of the encoder, since none of the regions in the absolute patterns  32  is the same throughout the length. 
     In this case, It is assumed here that, as in the prior art, an encoder has only incremental patterns  31  and absolute patterns  32 , without any reference position pattern  33 . According to such the encoder, for example, provided that the arrangement pitch of the incremental patterns  31  is 40 μm, then the absolute patterns  32  must have an accuracy less than one-half of the arrangement pitch, i.e., less than ±20 μm, throughout the length of the scale  12 . 
     As in this embodiment, if the reference position patterns  33  are formed at an arrangement pitch Pr larger than the arrangement pitch Pi of the incremental patterns  31 , such a position accuracy is sufficient for the absolute patterns  32  that is set to the same level as the arrangement pitch Pr of the reference position patterns  33 . In this way, the absolute patterns  32  may accept larger position errors. For example, if the arrangement pitch Pr of the reference position patterns  33  is four times larger than Pi, i.e., 160 μm, then the absolute patterns  32  may accept position errors up to ±80 μm throughout the length of the scale  12 . This means that the arrangement pitch Pi of the incremental patterns  31  can be determined regardless of the accuracy of the absolute patterns  32 . For example, the arrangement pitch Pi may be smaller than the positional accuracy of the absolute pattern  32 . Therefore, this embodiment may provide a more minute pitch of the incremental patterns  31 , which would provide improved accuracy in the encoder. 
     The light-emitting element  11  emits the scale  12 . Then, the irradiated light transmitted through the scale  12  is projected through the lens  13  onto the photodiode array  14 . 
     As illustrated in  FIG. 3 , the photodiode array  14  comprises an INC photodiode array  41 , an ABS photodiode array  42 , and a reference position photodiode array  43 , corresponding to the incremental track  301 , the absolute track  302 , and the reference position track  303 , respectively. Each of the photodiode arrays  41  to  43  is configured to arrange photodiodes therein at a respective arrangement pitch corresponding to each of the corresponding patterns  31  to  33 . 
     The INC photodiode array  41  has four sets of photodiode arrays, each with a phase difference of 90°, respectively, and detects light and dark signals based on the incremental patterns  31  to output a quadrature sine wave signal with a phase difference of 90°. The ABS photodiode array  42  outputs signals resulting from sweeping light and dark signals based on the absolute patterns in a direction of length-measurement. In addition, a dimension WPDR (WPDR&gt;Pr+Wr) in a direction of length-measurement is set for the reference position photodiode array  43  such that at least one or more reference position patterns  33  can be detected. The reference position photodiode array  43  outputs signals resulting from sweeping light and dark signals based on the reference position patterns  33  in the direction of length-measurement. 
     Now returning to  FIG. 1 , further description will be given below. By way of an example, a signal processing device  20  comprises a noise filter/amplifier circuit  21 , an A/D converter  22 , a relative position detection circuit  23 , a noise filter/amplifier circuit  24 , an A/D converter  25 , an absolute position detection circuit  26 , a noise filter/amplifier circuit  27 , an A/D converter  28 , a reference position detection circuit  29 , and an absolute position composition circuit  30 . 
     The noise filter/amplifier circuit  21  removes noises in an analog output signal (a quadrature signal with a phase difference of 90°) provided by the INC photodiode array  41 . Then, the noise filter/amplifier circuit  21  amplifies and outputs the analog output signal. The A/D converter  22  converts the analog output signal output from the noise filter/amplifier circuit  21  to a digital signal. Through an arc-tangent calculation on the amplitude of the resulting digital signal (with a phase difference of 90°), the relative position detection circuit  23  outputs a relative position signal D 2  that indicates a relative travel distance and a travel direction of the scale  12 . 
     The noise filter/amplifier circuit  24  removes noises in an analog output signal (absolute position signal) provided by the ABS photodiode array  42 . Then, the noise filter/amplifier circuit  24  amplifies and outputs the analog output signal. The A/D converter  25  converts the analog output signal output from the noise filter/amplifier circuit  24  to a digital signal. In this case, the converted digital signal includes data of M-sequence codes represented by the absolute patterns  32 . 
     The absolute position detection circuit  26  has a table (not illustrated) that indicates a relationship between the M-sequence codes and absolute positions represented by the M-sequence. The absolute position detection circuit  26  refers to the table to output an absolute position signal D 1  that indicates an absolute position of the scale  12 . 
     The noise filter/amplifier circuit  27  removes noises in an analog output signal provided by the reference position photodiode array  43 . Then, the noise filter/amplifier circuit  27  amplifies and outputs the analog output signal. The A/D converter  28  converts the analog signal output from the noise filter/amplifier circuit  27  to a digital signal. Then, the reference position detection circuit  29  outputs a reference position signal D 3  that indicates reference positions of the reference position patterns included in the digital signal. 
     Based on the absolute position signal D 1 , relative position signal D 2 , and reference position signal D 3 , the absolute position composition circuit  30  calculates minute absolute positions of the scale  12 . Referring to  FIG. 4 , operations of the absolute position composition circuit  30  will be described below. The absolute position signal D 1  has information for absolute positions of the scale  12 . Since the absolute patterns  32  are formed with a predetermined accuracy with respect to the reference position patterns  33 , it is possible to determine which one of periods Pr the scale  12  is located in for the reference position patterns  33  by obtaining absolute positions from the absolute position signal D 1  (( 1 ) of  FIG. 4 ). 
     After the one of the periods Pr is determined for the reference position patterns  33 , the amount of signal for the reference position signal D 3  is detected. Then, it is possible to determine which period the scale  12  is located in for the incremental patterns  31  (( 2 ) of  FIG. 4 ). Since the incremental patterns  31  and the reference position patterns  33  are formed with equally-spaced light and dark patterns, respectively, it is easy to maintain a position accuracy between these patterns at a high level even if the ratio of the arrangement pitches Pr and Pi is high. Therefore, by determining the period in which the scale  12  is located for the reference position patterns  33  and detecting the amount of signal for the reference position signal D 3 , it is possible to determine which period the scale  12  is located in for the incremental patterns  31 . Thereafter, absolute positions of the scale  12  may be calculated and output by counting light and dark regions of the relative position signal D 2  obtained from the incremental patterns  31 . 
     As can be seen from the above, according to this embodiment, an absolute position of the scale  12  is detected in relation to the reference position patterns  33 , based on the absolute position signals D 1  obtained from the absolute patterns  32 . Then, precision absolute position information of the scale  12  may be obtained, according to the reference position signal D 3  based on the reference position patterns  33  and the relative position signal D 2  based on the incremental patterns  31 . The absolute patterns  32  are not required to have a position accuracy comparative to the incremental patterns  31  formed in a minute manner, but rather it is sufficient to form the absolute patterns  32  with a predetermined position accuracy with respect to the reference position patterns  33  with a larger arrangement pitch. Therefore, this embodiment may provide a more minute pitch of the incremental patterns  31 , which would provide improved accuracy in the encoder. 
     Second Embodiment 
     Referring now to  FIGS. 5 through 10 , an absolute position length-measurement type photoelectric encoder according to a second embodiment of the present invention will be described below. In  FIGS. 5 through 10 , the same reference numerals represent the same components as the first embodiment and detail description thereof will be omitted herein. 
       FIG. 5  is a schematic diagram illustrating an entire configuration of the second embodiment, and  FIG. 6  illustrates a plan configuration of the scale  12 . This embodiment differs from the first embodiment in that, as illustrated in  FIG. 6 , it comprises, instead of the absolute patterns  32  and the reference position patterns  33 , an ABS/reference position integrated track  304  with ABS/reference position integrated patterns  34 , wherein these two types of patterns are integrated into one track. As illustrated in  FIG. 6 , the ABS/reference position integrated track  304  is formed with the following two types of patterns arranged in one track: absolute patterns  32 ′ representing pseudo-random patterns and reference position patterns  33 ′ arranged in gaps between the absolute patterns  32 ′, at an arrangement pitch Pr larger than the arrangement pitch Pi of the incremental patterns  31 . Besides, the reference position patterns  33 ′ have hatching in  FIG. 6 , which is for clarity of illustration as the absolute patterns  32 ′ and the reference position patterns  33 ′ can be easily distinguished from each other. In an actual scale, as illustrated in  FIG. 7 , the absolute patterns  32 ′ and the reference position patterns  33 ′ are formed with the same material on the scale  12 , different only in their shapes. In this embodiment, as described above, since the scale  12  involves only two tracks therein, the scale  12  may be easily made smaller in comparison to the first embodiment where three tracks are involved therein. 
     In addition, corresponding to the scale  12  configured as above, the photodiode array  14  includes an INC photodiode array  41  and an ABS/reference position photodiode array  44  corresponding to each of the incremental track  301  and the ABS/reference position integrated pattern track  304 , as illustrated in  FIG. 8 . 
     Further, as illustrated in  FIG. 5 , the signal processing circuit  20  of this embodiment has a configuration similar to the first embodiment for signal processing ( 21  to  23 ) based on the incremental patterns  31 . On the other hand, the signals based on the ABS/reference position integrated patterns  34  as mentioned above are different from the first embodiment in that they are input to a separation circuit  201  via the noise filter/amplifier circuit  24  and the A/D converter  25 . The separation circuit  201  has a function for separating a signal provided from the reference position patterns  33 ′ from another provided from the absolute patterns  32 ′ in the ABS/reference position integrated patterns  34 . Such separation between these signals may be achieved through a correlation calculation between a signal based on the patterns  34  and a designed value of the reference position patterns  33 ′. That is, as a result of the correlation calculation, those signals may be obtained that are based on the reference position patterns  33 ′. As a correlation calculation, both multiplication type and subtraction type may be employed. The separated signal provided from the reference position patterns  33 ′ is input to the reference position detection circuit  29 , which in turn outputs a reference position signal D 3 . 
     Alternatively, those signals may be obtained that are based on the absolute patterns  32 ′ as a result of calculation of a correlation between a signal based on the patterns  34  and a designed value of the absolute patterns  32 ′. 
     Referring now to  FIGS. 9 and 10 , an exemplary configuration of the ABS/reference position integrated patterns  34  according to this embodiment will be described below.  FIG. 9  illustrates a first configuration. In the first configuration, the absolute patterns  32 ′ and the reference position patterns  33 ′ illustrated in the upper part of  FIG. 9  are integrated in one track, which results in a configuration as illustrated in the lower part of  FIG. 9 . When these patterns are integrated, some of the absolute patterns  32 ′ and the reference position patterns  33 ′ overlap each other (in the regions indicated by arrow A). In the first configuration, the absolute patterns  32 ′ are omitted in the overlapping regions and each of the reference position patterns  33 ′ is formed at each of the positions (indicated by arrow A) instead. In this way, when the absolute patterns  32 ′ are omitted (erased) in the regions indicated by arrow A, absolute positions may be detected as if the absolute patterns  32 ′ were not omitted, as long as the designed values of the reference position patterns  33 ′ (including information for the position of each arrow A) are known to the absolute position detection circuit  26 . 
       FIG. 10  illustrates a second configuration. In this case, as illustrated in  FIG. 10(   a ), if there is a region where some of the absolute patterns  32 ′ and the reference position patterns  33 ′ overlap each other, as illustrated in  FIG. 10(   b ), the absolute patterns  32 ′ are reduced in size and formed to eliminate any overlapping regions, instead of omitting such overlapping regions. 
     Third Embodiment 
       FIG. 11  illustrates a configuration of an absolute position detection type encoder according to a third embodiment of the present invention. The entire configuration is substantially the same as the first embodiment ( FIG. 1 ) and illustration thereof is omitted here. 
     This embodiment is different from the first embodiment in that the two types of tracks, reference position tracks  303 A and  303 B, are provided on opposite sides of the incremental track  301  (correspondingly, two reference position photodiode arrays  43  are also provided in the photodiode array  14 , while not illustrated). According to this configuration, if the scale  12  is tilted (yawing), those errors due to the yawing may be compensated by averaging signals based on each of the two types of patterns, reference position patterns  33 A and  33 B. 
     Fourth Embodiment 
       FIG. 12  illustrates a configuration of an absolute position detection type encoder according to a fourth embodiment of the present invention. The entire configuration is substantially the same as the first embodiment ( FIG. 1 ) and illustration thereof is omitted hire. 
     This embodiment is different from the above-mentioned embodiments in that ABS/reference position integrated tracks  304 A and  304 B similar to the second embodiment are formed on opposite sides of the incremental track  301 . According to this configuration, if the scale  12  is tilted (yawing), those errors due to the yawing may be compensated by averaging signals based on each of the two types of patterns, ABS/reference position patterns  34 A and  34 B. 
     Fifth Embodiment 
       FIG. 13  illustrates a configuration of an absolute position detection type encoder according to a fifth embodiment of the present invention. The entire configuration is substantially the same as the first embodiment ( FIG. 1 ) and illustration thereof is omitted here. 
     The fifth embodiment relates to a modification of the first embodiment. Specifically, A piece of the reference position pattern  33  is not formed of a single pattern as shown in  FIG. 1  but is formed of plural patterns S 1 -S 4  that are arranged in an unequal interval. 
     The plural reference position patterns  33 , each of which is formed of such the unequally-arranged patterns S 1 -S 4 , are arranged with a width Wr, respectively, and with a pitch Pr. Such the patterns S 1 -S 4  are disclosed in JP H07-318371A, for example, and used as a origin detection pattern therein. Also in this embodiment, the reference position pattern  33  with such the patterns S 1 -S 4  serves to allow larger position errors of the absolute pattern  32  as described above, and may also be used as a origin detection pattern. 
     Sixth Embodiment 
       FIG. 14  illustrates a configuration of an absolute position detection type encoder according to a sixth embodiment of the present invention. The entire configuration is substantially the same as the first embodiment ( FIG. 1 ) and illustration thereof is omitted here. The sixth embodiment is a modification of the second embodiment. Specifically, a piece of the reference position  33 ′ in the second embodiment is not formed of a single pattern as shown in  FIG. 6  and  FIG. 7 , but is formed of plural patterns S 1 -S 4  that are arranged in an unequal interval. The other configurations are the same as the second embodiment. 
     Seventh Embodiment 
       FIG. 15  illustrates a configuration of an absolute position detection type encoder according to a seventh embodiment of the present invention. The entire configuration is substantially the same as the first embodiment ( FIG. 1 ) and illustration thereof is omitted here. 
     The seventh embodiment is a modification of the third embodiment. Specifically, a piece of the reference position  33 A and  33 B in the third embodiment is not formed of a single pattern as shown in  FIG. 11 , but is formed of plural patterns S 1 -S 4  that are arranged in an unequal interval. The other configurations are the same as the third embodiment. 
     Eighth Embodiment 
       FIG. 16  illustrates a configuration of an absolute position detection type encoder according to a eighth embodiment of the present invention. The entire configuration is substantially the same as the first embodiment ( FIG. 1 ) and illustration thereof is omitted here. The seventh embodiment is a modification of the fourth embodiment. Specifically, a piece of the reference position  33 ′ in the fourth embodiment is not formed of a single pattern as shown in  FIG. 12 , but is formed of plural patterns S 1 -S 4  that are arranged in an unequal interval. The other configurations are the same as the fourth embodiment. 
     Other Embodiments 
     Although the embodiments of the present invention have been described as above, the present invention is not intended to be limited to the disclosed embodiments and various other changes and additions may be made thereto without departing from the scope of the invention. For example, although the above-mentioned embodiments have been described in the context of a transmissive type photoelectric encoder, as illustrated in  FIG. 17 , the light-emitting element  11  may be positioned at the same end as the lens  13  and the photodiode array  14  as a reflective type optical system from the light-emitting element  11 .