Abstract:
A scale has a first incremental track with first incremental patterns including first light and dark patterns formed at equal intervals in first periods, an absolute track with absolute patterns representing absolute positions, and a second incremental track with second incremental 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 
       [0001]    This application is based on and claims the benefit of priority from prior Japanese Patent Application No. 2007-161776, filed on Jun. 19, 2007, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an absolute position encoder. 
         [0004]    2. Description of the Related Art 
         [0005]    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. 
         [0006]    In the case of photoelectric encoders, the incremental encoders have incremental tracks with incremental patterns including equally spaced light and dark regions. 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 incremental patterns with equally spaced light and dark regions, and then detecting relative travel distances from the origin. However, prior to the measurement, a scale must be moved to right and left directions in order to read origin detection patterns. 
         [0007]    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 at a current position when powered on. However, the absolute encoders have a lower detection accuracy than the incremental encoders. 
         [0008]    As such, an absolute position 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., Japanese Patent Laid-Open No. (HEI) 7-286861. 
         [0009]    In an encoder of this type, if the pitch of incremental patterns is 40 μm, then the absolute patterns must have a position accuracy of not more than ±20 μm. However, in encoders so configured, it is difficult to form highly accurate patterns since incremental patterns and absolute patterns have a different line width and different density of patterns, respectively. 
       SUMMARY OF THE INVENTION 
       [0010]    An absolute position encoder according to the present invention comprises: a scale having formed therein a first incremental track with first incremental patterns including first light and dark patterns formed at equal intervals in first periods, an absolute track with absolute patterns representing absolute positions, and a second incremental track with second incremental patterns including second light and dark patterns formed at equal intervals in second periods longer than the first periods; a light source emitting a measurement light to the scale; a photodetector receiving the measurement light reflected at or transmitted through the scale; and a signal processing circuit processing the received light signal of the photodetector to detect an absolute position of the scale. 
         [0011]    In one aspect, the signal processing circuit may be configured to generate a reference position signal, the reference position signal having periods of the least common multiple between a pitch of the first incremental patterns and a pitch of the second incremental patterns, determine which one of the periods of the reference position signal the scale is located in based on a signal obtained from the absolute patterns, and detect an absolute position of the scale based on the determination result, a signal obtained from the second incremental patterns, and a signal obtained from the first incremental patterns. 
         [0012]    In addition, in another aspect, an absolute position encoder according to the present invention comprises: a scale having formed therein a first incremental track with first incremental patterns including first light and dark patterns formed at equal intervals in first periods, an absolute track with absolute patterns representing absolute positions, and second incremental tracks, each with second incremental patterns including second light and dark patterns formed at equal intervals in second periods longer than the first periods, and each being arranged on the upper and lower sides of the first incremental track, respectively; a light source emitting a measurement light to the scale; a photodetector receiving the measurement light reflected at or transmitted through the scale; and a signal processing circuit processing the received light signal of the photodetector to detect an absolute position of the scale. 
         [0013]    In one aspect, the signal processing circuit may generate a reference position signal, the reference position signal having periods of the least common multiple between a pitch of the first incremental patterns and a pitch of the second incremental patterns, determine which one of the periods of the reference position signal the scale is located in based on a signal obtained from the absolute patterns, and detect an absolute position of the scale based on the determination result, an average of signals obtained from the second incremental patterns, and a signal obtained from the first incremental patterns. 
         [0014]    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 signals that change at 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 in encoders. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      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; 
           [0016]      FIGS. 2A and 2B  are a plan view illustrating a configuration of the scale  12  in  FIG. 1 ; 
           [0017]      FIG. 3  is a plan view illustrating a configuration of the photodiode array  14  in  FIG. 1 ; 
           [0018]      FIGS. 4A and 4B  are a conceptual diagram illustrating operations of the absolute position length-measurement type photoelectric encoder according to the first embodiment; 
           [0019]      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; 
           [0020]      FIGS. 6A and 6B  are a plan view illustrating a configuration of the scale  12  in  FIG. 5 ; 
           [0021]      FIG. 7  is a plan view illustrating a configuration of the photodiode array  14  in  FIG. 5 ; 
           [0022]      FIGS. 8A through 8D  illustrate operations of the signal separation circuit in  FIG. 5 ; 
           [0023]      FIG. 9  is a schematic diagram illustrating an entire configuration of an absolute position length-measurement type photoelectric encoder according to a third embodiment of the present invention; 
           [0024]      FIG. 10  is a plan view illustrating a configuration of the scale  12  in  FIG. 9 ; 
           [0025]      FIG. 11  is a plan view illustrating a configuration of the photodiode array  14  in  FIG. 9 ; 
           [0026]      FIG. 12  is a conceptual diagram illustrating operations of the absolute position length-measurement type photoelectric encoder according to the third embodiment of the present invention; 
           [0027]      FIG. 13  is a schematic diagram illustrating an entire configuration of an absolute position length-measurement type photoelectric encoder according to a fourth embodiment of the present invention; 
           [0028]      FIG. 14  is a plan view illustrating a configuration of the scale  12  in  FIG. 13 ; 
           [0029]      FIG. 15  is a plan view illustrating a configuration of the photodiode array  14  in  FIG. 13 ; and 
           [0030]      FIG. 16  illustrates a variation of the embodiments. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0031]    Embodiments of the present invention will now be described in detail below with reference to the accompanying drawings. 
       First Embodiment 
       [0032]      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 . 
         [0033]    The light-emitting element  11  is a light source, such as a laser diode, that emits a coherent light. As illustrated in  FIG. 2A , the scale  12  is configured to form the following tracks on a transparent glass substrate: a first incremental track  301  with first incremental patterns  31  formed at an arrangement pitch P 1  (e.g., 40 μm) that include equally spaced light and dark regions; an absolute track  302  with general absolute patterns  32  that represent absolute positions in pseudo-random patterns (in this case, M-sequence codes); and a second incremental track  303  with second incremental patterns  33  formed at an arrangement pitch P 2  (e.g., 50 μm) slightly larger than that of the first incremental patterns. 
         [0034]    A relationship between the first incremental patterns  31  and the second incremental patterns  33  will be described in detail below.  FIG. 2B  illustrates an enlarged region (region A) of the scale  12 . The pitch P 2  of the second incremental patterns  33  is slightly larger than the pitch P 1  of the first incremental patterns  31 . A reference position signal is configured by a phase difference between the first incremental patterns  31  and the second incremental patterns  33 . A reference position signal has such a phase that changes at predetermined periods along the sweeping direction. In this case, each of the periods of the reference position signal is configured to be the least common multiple between the pitch P 1  of the first incremental patterns  31  and the pitch P 2  of the second incremental patterns. For example, provided that the pitch P 1  of the first incremental patterns  31  is 40 μm and the pitch P 2  of the second incremental patterns is 50 μm, each of the periods of the corresponding reference position signal is 200 μm. 
         [0035]    With this configuration, substantially the same line width and density may be provided for the first incremental patterns  31  and the second incremental patterns  33 . In addition, since both the first incremental patterns  31  and the second incremental patterns  33  can be formed with a transcription method using a step-and-repeat scheme in lithography technology, highly accurate scale patterns may be formed in a more simple fashion. In contrast, since none of the regions in the absolute patterns  32  is the same throughout the length, the absolute patterns  32  are difficult to form in an accurate fashion throughout the length of the encoder. 
         [0036]    In this case, it is assumed here that, as in the conventional art, an encoder having only first incremental patterns  31  and absolute patterns  32 , without any reference position signal. According to this encoder, for example, provided that the arrangement pitch of the first 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 . 
         [0037]    As in this embodiment, if such a reference position signal is generated that changes at periods larger than the arrangement pitch P 1  of the first incremental patterns  31 , such a position accuracy is sufficient for the absolute patterns  32  that is set to the same level as the periods of the reference position signal. In this way, the absolute patterns  32  may accept larger position errors. For example, if each of the periods of the reference position signal is five times larger than P 1 , i.e., 200 μm, then those position errors up to ±100 μm may be accepted in the absolute patterns  32  throughout the length of the scale  12 . This means that the arrangement pitch P 1  of the first incremental patterns  31  can be determined regardless of the accuracy of the absolute patterns  32 . Therefore, this embodiment may provide a more minute pitch of the first incremental patterns  31 , which would provide improved accuracy in the encoder. 
         [0038]    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 . 
         [0039]    As illustrated in  FIG. 3 , the photodiode array  14  comprises a first INC photodiode array  41 , an ABS photodiode array  42 , and a second INC photodiode array  43 , corresponding to the first incremental track  301 , the absolute track  302 , and the second incremental 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 . 
         [0040]    The first 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 first incremental patterns  31  to output a quadrature sine wave signal  410  with a phase difference of 90°. The second INC photodiode array  43  detects light and dark signals based on the second incremental patterns  33  to output a quadrature sine wave signal  430  with a phase difference of 90°. The ABS photodiode array  42  outputs a signal  420  resulting from sweeping light and dark signals based on the absolute patterns  32  in a direction of length-measurement. 
         [0041]    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 , relative position detection circuits  23  and  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 absolute position composition circuit  29 , and a reference position generation circuit  45 . 
         [0042]    The noise filter/amplifier circuit  21  removes any noise in an analog output signal  410  (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  410 . The A/D converter  22  converts the analog output signal output from the noise filter/amplifier circuit  21  to a digital signal. Through an arctan operation on the amplitude of the resulting digital signal (digitized two-phase 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 . 
         [0043]    The noise filter/amplifier circuit  27  removes any noise in an analog output signal  430  provided by the second INC photodiode array  43 . Then, the noise filter/amplifier circuit  27  amplifies and outputs the analog output signal  430 . The A/D converter  28  converts the analog output signal output from the noise filter/amplifier circuit  27  to a digital signal. Through an arctan operation on the amplitude of the resulting digital signal (digitized two-phase 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 above-mentioned reference position signal D 3  may be obtained through a phase comparison between the resulting relative position signals D 2  and D 2 ′ at the reference position generation circuit  45 . 
         [0044]    The noise filter/amplifier circuit  24  removes any noise in an analog output signal (absolute position signal)  420  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 . 
         [0045]    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 . Alternatively, the absolute position detection circuit  26  outputs such an absolute position signal D 1  through a correlation operation between a designed value of the absolute patterns and the detected signal. 
         [0046]    Based on the absolute position signal D 1 , relative position signal D 2 , and reference position signal D 3 , the absolute position composition circuit  29  calculates minute absolute positions of the scale  12 . 
         [0047]    Referring now to  FIG. 4A  and  FIG. 4B , operations of the absolute position composition circuit  29  will be described below. The absolute position signal D 1  has information for absolute positions of the scale  12 . The absolute patterns  32  are formed with a predetermined accuracy with respect to the reference position signal D 3 . Thus, it is possible to determine which one of periods of the reference position signal D 3  the scale  12  is located in, by obtaining absolute positions from the absolute position signal D 1  (( 1 ) of  FIG. 4A ). 
         [0048]    After the one of the periods is determined for the reference position signal D 3 , 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 first incremental patterns  31  (( 2 ) of  FIG. 4B ). 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 first incremental patterns  31 . 
         [0049]    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 signal D 3 , based on the absolute position signals D 1  obtained from the absolute patterns  32 . Then, elaborate absolute position information of the scale  12  may be obtained, according to the reference position signal D 3  and the relative position signal D 2  based on the first incremental patterns  31 . Therefore, this embodiment may provide a more minute pitch of the incremental patterns, which would provide improved accuracy in the absolute length-measurement encoder. 
       Second Embodiment 
       [0050]    Referring now to  FIGS. 5 through 8 , 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 8 , the same reference numerals represent the same components as the first embodiment and detail description thereof will be omitted herein. 
         [0051]      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. 6B , it comprises, instead of the absolute track  302  and the second incremental track  303  of the first embodiment, an ABS/incremental integrated track  304  with ABS/incremental integrated patterns  34 , wherein those two types of patterns, i.e., the absolute patterns  32  and the incremental patterns  33  are integrated into one track. As illustrated in  FIG. 6A , the ABS/incremental integrated patterns  34  are formed with the following two types of patterns integrated and arranged in one track: absolute patterns  32  representing pseudo-random patterns and second incremental patterns  33  arranged at an arrangement pitch P 2  larger than the arrangement pitch P 1  of the first incremental patterns  31 . Specifically, the ABS/incremental integrated patterns  34  are configured to form patterns only in those regions where both the absolute patterns  32  and the second incremental patterns  33  exist. In addition, the ABS/incremental integrated patterns  34  include absolute pattern areas  32 ′ corresponding to the absolute patterns  32  and second incremental pattern areas  33 ′ corresponding to the second incremental patterns  33 . 
         [0052]    In this embodiment, as described above, since the scale  12  involves only two tracks, the scale  12  may be easily made smaller in comparison to the first embodiment where three tracks are involved. 
         [0053]    As illustrated in  FIG. 7 , corresponding to the scale  12  configured as above, the photodiode array  14  includes an INC photodiode array  41  and an ABS/incremental photodiode array  44  corresponding to each of the first incremental patterns  31  and the ABS/incremental integrated patterns  34 . 
         [0054]    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  440  based on the ABS/incremental 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 of separating a signal provided from the second incremental pattern areas  33 ′ from another provided from the absolute pattern areas  32 ′ in the ABS/incremental integrated patterns  34 . Specifically, the separation circuit  201  has a low-pass filter, a subtraction circuit, etc., not illustrated. For example, a signal from the second incremental pattern areas  33 ′ is first removed with the low-pass filter from a light and dark signal of the ABS/incremental integrated patterns  34 , thereby obtaining an absolute light and dark signal. Then, the obtained absolute light and dark signal is subtracted from the light and dark signal of the ABS/incremental integrated patterns  34 , thereby obtaining a second incremental light and dark signal. 
         [0055]    Referring now to  FIG. 8 , a method for processing signals of the ABS/incremental integrated patterns  34  will be described in detail below.  FIG. 8A  illustrates a relationship between the ABS/incremental photodiode array  44  and the ABS/incremental integrated patterns  34 .  FIG. 8B  illustrates a light and dark signal  440  of the ABS/incremental integrated patterns  34  obtained at the ABS/incremental photodiode array  44  through a sweeping operation in a measurement-axis direction. The vertical axis represents signal strength and the horizontal axis represents sweeping direction. It can be seen that a signal A contains both an absolute light and dark signal and an incremental light and dark signal.  FIG. 8C  illustrates a light and dark signal after a low-pass filter process. Through the low-pass filter process, the incremental light and dark signal is removed and only an absolute light and dark signal B is left. As such, the absolute light and dark signal B may be separated and obtained. The absolute light and dark signal B is sent to the absolute position detection circuit  26 .  FIG. 8D  illustrates a signal C resulting from subtraction of the absolute light and dark signal B in  FIG. 8C  from the light and dark signal A of the ABS/incremental integrated patterns  34  in  FIG. 8B  through a subtraction process. In the signal C, signal strength becomes zero in those regions where no absolute pattern area  32 ′ exists, which makes no contribution to the incremental position detection. Thus, only the incremental light and dark signal C may be separated. This incremental light and dark signal C obtained from the separation is sent to a relative position detection circuit  202 . 
         [0056]    The separated signal C from a second incremental pattern area  33 ′ is input to the relative position detection circuit  202 , which in turn outputs a relative position signal D 2 ′. A reference position signal D 3  may be obtained through a phase comparison between the obtained relative position signals D 2  and D 2 ′ at a reference position signal generation circuit  203 . The operations of the absolute position composition circuit  29  are the same as the first embodiment. 
         [0057]    According to this configuration, not only the reduction in track width, but also the absolute position detection may be achieved using a smaller number of types of photodiode arrays than the number of types of patterns. As a result, improved accuracy in measurement as well as reduction in size of devices and costs may be achieved. 
       Third Embodiment 
       [0058]    Referring now to  FIGS. 9 through 12 , an absolute position length-measurement type photoelectric encoder according to a third embodiment of the present invention will be described below. In  FIGS. 9 through 12 , the same reference numerals represent the same components as the first embodiment and detail description thereof will be omitted herein. 
         [0059]      FIG. 9  is a schematic diagram illustrating an entire configuration of the third embodiment, and  FIG. 10  illustrates a plan configuration of the scale  12 . As illustrated in  FIG. 10 , the scale  12  is configured to form the following tracks on a transparent glass substrate: a first incremental track  301  with first incremental patterns  31  formed at an arrangement pitch P 1  (e.g., 40 μm) that include equally spaced light and dark regions; an absolute track  302  with general absolute patterns  32  that represent absolute positions in pseudo-random patterns (in this case, M-sequence codes); and a second incremental track  303 A with second incremental patterns  33 A as well as a second incremental track  303 B with second incremental patterns  33 B, each of the second incremental tracks being formed at an arrangement pitch P 2  (e.g., 50 μm) slightly larger than the arrangement pitch P 1  and arranged on the upper and lower sides of the first incremental patterns  31 , respectively. 
         [0060]    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 . 
         [0061]    As illustrated in  FIG. 11 , the photodiode array  14  includes a first INC photodiode array  41 , an ABS photodiode array  42 , and second INC photodiode arrays  43 A and  43 B, corresponding to the first incremental patterns  31 , the absolute patterns  32 , and the second incremental patterns  33 A and  33 B, 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 . 
         [0062]    The first 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  410  with a phase difference of 90°. Each of the second INC photodiode arrays  43 A and  43 B has four sets of photodiode arrays, each with a phase difference of 90°, respectively, and detects light and dark signals based on each of the incremental patterns  33 A and  33 B to output quadrature sine wave signals  430 A and  430 B with a phase difference of 90°. The ABS photodiode array  42  outputs a signal  420  resulting from sweeping a light and dark signal based on the absolute patterns in a direction of length-measurement. 
         [0063]    As illustrated in  FIG. 9 , the signal processing circuit  20  of this embodiment has a configuration similar to the first embodiment for signal processing based on the first incremental patterns  31  and the absolute patterns  32 . This embodiment is different from the first embodiment in that two types of configurations are provided for signal processing based on each of the second incremental patterns  33 A and  33 B. 
         [0064]    Specifically, the noise filter/amplifier circuit  27 ′ removes any noise in an analog output signal  430 B provided by the second INC photodiode array  43 B. Then, the noise filter/amplifier circuit  27 ′ amplifies and outputs the analog output signal  430 B. The A/D converter  28 ′ converts the analog signal output from the noise filter/amplifier circuit  27 ′ to a digital signal. Through an arctan operation on the amplitude of the resulting digital signal (signal with a phase difference of 90°), a 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 . A average relative position signal D 2 ave may be obtained through averaging of the resulting relative position signals D 2  and D 2 ″ at an averaging circuit  30 . A reference position signal D 3  may be obtained through a phase comparison between the resulting average relative position signals D 2 ave and D 2  at a reference position generation circuit  45 , as is similar to the former embodiment. 
         [0065]    Based on the absolute position signal D 1 , the relative position signal D 2 , and the reference position signal D 3 , an absolute position composition circuit  29  calculates minute absolute positions of the scale  12 . 
         [0066]    Referring to  FIG. 12 , operations of the absolute position composition circuit  29  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 signal D 3 , it is possible to determine which one of periods Pr the scale  12  is located in for the reference position signal D 3  by obtaining absolute positions from the absolute position signal D 1  (( 1 ) of  FIG. 12 ). 
         [0067]    After the one of the periods Pr is determined for the reference position signal D 3 , a value of the reference position signal D 3  is detected. Then, it is possible to determine which period the scale  12  is located in for the first incremental patterns  31  (( 2 ) of  FIG. 12 ). 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 first incremental patterns  31 . 
         [0068]    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 signal D 3 ave based on the absolute position signals D 1  obtained from the absolute patterns  32 . Then, the relative position signal D 2 ′ based on the second incremental patterns  33 A and the relative position signal D 2 ″ based on the second incremental patterns  33 B are averaged to compensate errors in position reading. In this way, the absolute position information of the scale  12  may be obtained from the relative position signal D 2  based on the first incremental patterns  31 . Therefore, this embodiment may provide a more minute pitch of the incremental patterns  31  and achieve error compensation by averaging the reference position signals, which would provide improved accuracy in the absolute length-measurement type encoder. 
       Fourth Embodiment 
       [0069]    Referring now to  FIGS. 13 through 15 , an absolute position length-measurement type photoelectric encoder according to a fourth embodiment of the present invention will be described below. In  FIGS. 13 through 15 , the same reference numerals represent the same components as the above-mentioned embodiments and detail description thereof will be omitted herein. 
         [0070]      FIG. 13  a schematic diagram illustrating an entire configuration of the fourth embodiment, and  FIG. 14  illustrates a plan configuration of the scale  12 . This embodiment differs from the third embodiment in that, as illustrated in  FIG. 14 , it comprises, instead of the absolute track  302  and the second incremental tracks  303 A and  303 B, an ABS/incremental integrated track  304 A with ABS/incremental integrated patterns  34 A and an ABS/incremental integrated track  304 B with ABS/incremental integrated patterns  34 B, wherein those two types of patterns, i.e., the absolute patterns  32  and the second incremental patterns  33  are integrated into one track, respectively. As illustrated in  FIG. 14 , the ABS/incremental integrated patterns  34 A and  34 B are each formed with the following patterns arranged in one track: absolute patterns  32  representing pseudo-random patterns, and the second incremental patterns  33 A and  33 B, each arranged at an arrangement pitch P 2  larger than the arrangement pitch P 1  of the first incremental patterns  31 . Specifically, the ABS/incremental integrated patterns  34 A and  34 B are each configured to form patterns only in those regions where both the absolute patterns  32  and the second incremental patterns  33  exist. In addition, the ABS/incremental integrated patterns  34 A and  34 B each include absolute pattern areas  32 ′ corresponding to the absolute patterns  32  and second incremental pattern areas  33 ′ corresponding to the second incremental patterns  33 . 
         [0071]    The scale  12  is, as a whole, configured to form three tracks in such a way that the ABS/incremental integrated tracks  304 A and  304 B are formed on the upper and lower sides of the first incremental track  301 . In this embodiment, as described above, since the scale  12  involves only three tracks, the scale  12  may be easily made smaller in comparison to the third embodiment where four tracks are involved. 
         [0072]    In addition, as illustrated in  FIG. 15 , corresponding to the scale  12  configured as above, the photodiode array  14  includes an INC photodiode array  41  and ABS/incremental photodiode arrays  44 A and  44 B corresponding to each of the first incremental patterns  31  and the ABS/incremental integrated patterns  34 A and  34 B. 
         [0073]    As illustrated in  FIG. 13 , the signal processing circuit  20  of this embodiment has a configuration similar to the first embodiment for signal processing based on the first incremental patterns  31  and for separation of an incremental light and dark signal and an absolute light and dark signal from the ABS/incremental integrated patterns  34 A and  34 B. This embodiment is different from the second embodiment in that two types of separation systems are provided for separating an incremental light and dark signal and an absolute light and dark signal from the ABS/incremental integrated patterns  34 A and  34 B. 
         [0074]    The signal from the second incremental patterns that is separated from the ABS/incremental integrated patterns  34 A at a separation circuit  201  is input to a relative position detection circuit  202 , which in turn outputs a relative position signal D 2 ′. Similarly, the signal from the second incremental patterns that is separated from the ABS/incremental integrated patterns  34 B at a separation circuit  201 ′ is input to a relative position detection circuit  202 ′, which in turn outputs a relative position signal D 2 ″. 
         [0075]    An average relative position signal D 2 ave may be obtained through averaging of the resulting relative position signals D 2  and D 2 ″ at an averaging circuit  204 A. The average relative position signal D 2 ave is output to the reference position composition circuit  45 . 
         [0076]    On the other hand, the signal B from the absolute patterns that is separated from the ABS/incremental integrated patterns  34 A at a separation circuit  201  is input to an absolute position detection circuit  26 , which in turn outputs an absolute position signal D 1 . 
         [0077]    Similarly, the signal from the absolute patterns that is separated from the ABS/incremental integrated patterns  34 B at a separation circuit  201 ′ is input to an absolute position detection circuit  26 ′, which in turn outputs an absolute position signal D 1 ′. An average absolute position signal D 1 ave may be obtained through averaging of the resulting absolute position signals D 1  and D 1 ′ at an averaging circuit  204 B. The average absolute position signal D 1 ave is output to an absolute position composition circuit  29  together with the output signal D 2  from the relative position detection circuit  23 . 
         [0078]    As can be seen from the above, according to this embodiment, an absolute position of the scale  12  is detected in relation to the ABS/incremental integrated patterns  34 A and  34 B, based on the absolute position signal D 1 ave obtained from the absolute patterns  32 . Then, the reference position signal D 2 ′ based on the ABS/incremental integrated patterns  34 A and the reference position signal D 2 ″ based on the ABS/incremental integrated patterns  34 B are averaged to compensate errors in position reading. In this way, the absolute position information of the scale  12  may be obtained from the relative position signal D 2  based on the first incremental patterns  31 . Therefore, this embodiment may provide a more minute pitch of the incremental patterns  31  and achieve error compensation by averaging the reference position signals, which would provide improved accuracy in the absolute length-measurement type encoder. In addition, this embodiment may achieve the reduction in number of types of photodiode arrays. 
       Other Embodiments 
       [0079]    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. 
         [0000]    For example, although the above-mentioned embodiments have been described in the context of a transmissive type photoelectric encoder, as illustrated in  FIG. 16 , 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 scale  12 .