Abstract:
A light emitter is operable to emit light. A scale comprises a transparent main body and a plurality of marks. The transparent main body has a first face and the second face which is opposite to the first face. The plurality of marks is provided on at least one of the first face and the second face and formed at a predetermined interval, and adapted to reflect or intercept the light emitted from the light emitter. A light detector is operable to detect light reflected by the marks or light passing through a plurality of regions each of which is defined between adjacent ones of the marks. The main body of the scale is formed with a plurality of through holes each of which connects the first face and the second face at one of the regions.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to an encoder and a printer using the same. 
   Printers have various motors such as a paper feed motor for driving a feed roller that conveys print paper or a print object and a carriage motor for driving a carriage having a print head. DC motors are widely used as such motors to reduce noise. Printers having DC motors are equipped with an encoder composed of a scale having marks or slits disposed at specified intervals and a sensor that senses the marks or slits of the scale to output given signals to control the positions and speeds of the DC motors. 
   For example, to control a paper feed motor, printers have a disc-shaped scale having multiple slits arranged at specified intervals and a sensor constructed to sandwich each slit between a light-emitting device and a light-receiving device. This type of scale is constructed to rotate with a feed roller. This type of sensor generally outputs two signals with a phase difference of 90° (for example, refer to Japanese Patent Publication No. 2001-232882). The motor is controlled by sensing changing points of the levels of the two signals output from the sensor. 
   Among the optical encoders, an optical encoder that has graduations attached to a transparent glass substrate, and allows light reflected by the graduations to pass through a space between the graduations is known (see Japanese Patent Publication No. 2001-232882). 
   In order to improve print quality, more accurate control is required for motors mounted to printers. For more accurate control, encoders have to output signals with higher resolution. There may be two methods for outputting higher-resolution signals from encoders: a method of increasing the diameter of the disc-shaped scale while maintaining the intervals of the slits and a method of decreasing the interval of the slits while maintaining the diameter of the scale. 
   However, printers that need to be compact cannot have a large-diameter scale. To provide the space for the scale, the mechanical structure of the printers becomes complicated. In contrast, narrowing the interval between slits makes it difficult to manufacture the scale itself. 
   Since an ink mist occurs in an apparatus using ink, such as a printer, if the interval between the graduations is narrow, a portion, through which light passes, may significantly change due to the ink mist, and thus control may be made unstable. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the invention to provide an encoder that, even though wastes, such as an ink mist and so on, are attached to a scale, can prevent output signals from being made unstable. It is another object of the invention to provide a printer that can perform stable control with high accuracy. 
   In order to achieve the above objects, according to the embodiment of the invention, there is provided an encoder comprising: 
   a light emitter, operable to emit light; 
   a scale comprising:
         a transparent main body which has a first face and the second face which is opposite to the first face; and   a plurality of marks provided on at least one of the first face and the second face and formed at a predetermined interval, and adapted to reflect or intercept the light emitted from the light emitter; and       

   a light detector operable to detect light reflected by the marks or light passing through a plurality of regions each of which is defined between adjacent ones of the marks, 
   wherein the main body of the scale is formed with a plurality of through holes each of which connects the first face and the second face at one of the regions. 
   The number of the through hole may be no more than one third of a total number of the regions. 
   The number of the through hole may be no less than one tenth of a total number of the regions. 
   The marks may be arranged on the first face in a first direction; and 
   a width in the first direction of the through hole may be wider than the interval between the marks. 
   The marks may be arranged on the first face in a first direction; and 
   a width in the first direction of the through hole may be narrower than the interval between the marks. 
   According to the invention, there is also provided a printer operable to print information on a printing medium comprising: 
   a motor having a rotatable shaft; 
   the encoder described above, wherein the scale is rotated in conjunction with the rotation of the shaft, and the light detector is operable to output a signal in accordance with the rotation of the scale; 
   a controller, which controls 
   the rotation of the shaft based on the signal output from the detector. 
   The motor may be operable to rotate a roller adapted to feed the printing medium. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and advantages of the present invention will become more apparent by describing in detail preferred exemplary embodiments thereof with reference to the accompanying drawings, wherein: 
       FIG. 1  is a schematic perspective view of a printer according to a first embodiment of the invention; 
       FIG. 2  is a schematic side view of a part for paper feeding of the printer of  FIG. 1 ; 
       FIG. 3  is a schematic diagram of a carriage of  FIG. 1  and a sensor mechanism of a PF drive roller of  FIG. 2 ; 
       FIG. 4  is a block diagram showing the schematic structure of a controller of the printer and its peripherals; 
       FIG. 5  is a block diagram showing the structure of a speed control unit for a PF motor in a DC unit of  FIG. 4 ; 
       FIG. 6  is a graph of an example of target speed curves drawn from a target speed table; 
       FIG. 7  is an enlarged view of part Z in  FIG. 6 ; 
       FIG. 8  is a schematic diagram of a part related to the rotary encoder in  FIG. 3 ; 
       FIG. 9  is a front view of the rotary scale in  FIG. 3 ; 
       FIG. 10  is a side view of the rotary encoder in  FIG. 3 ; 
       FIGS. 11A to 11C  are partial cross-sectional views showing a structure of a rotary scale of  FIG. 3 . 
       FIG. 12  is a schematic diagram showing the relationship between the board in  FIG. 10  and its peripherals; 
       FIG. 13  is an electric circuit diagram of the rotary encoder of  FIG. 3 ; 
       FIG. 14  shows signal waveforms generated by the rotary encoder; 
       FIG. 15  shows signal waveforms generated by the rotary encoder when the rotating direction is changed; 
       FIG. 16  is an electric circuit diagram of a rotary encoder according to a second embodiment of the invention; 
       FIG. 17  shows signal waveforms generated by the rotary encoder according to the second embodiment. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   Hereinafter, an encoder and a printer using the same according to an embodiment of the invention will be described in detail with reference to the accompanying drawings. Moreover, the configuration of the printer will first be described, and the configuration of the encoder will be described, together with the description of the printer. In addition, as regards the description of the printer, a control method of a printer will also be described. 
   First Embodiment 
   (Schematic Structure of Printer) 
     FIG. 1  is a schematic perspective view of a printer  1  according to a first embodiment of the invention;  FIG. 2  is a schematic side view of a part for paper feeding of the printer  1  of  FIG. 1 ;  FIG. 3  is a schematic diagram of a carriage  3  of  FIG. 1  and a sensor mechanism of a PF drive roller  6  of  FIG. 2 . 
   The printer  1  of the first embodiment is an inkjet printer that ejects ink to print paper P or a print object to thereby execute printing. Referring to  FIGS. 1 to 3 , the printer  1  includes a carriage  3  having a print head  2  that ejects ink droplets; a carriage motor (CR motor)  4  that drives the carriage  3  in a main scanning direction MS; a paper feed motor (PF motor)  5  that feeds the print paper P in a subscanning direction SS; a PF drive roller  6  connected to the PF motor  5 ; a platen  7  opposed to the nozzle surface (the lower surface in  FIG. 2 ) of the print head  2 ; and a chassis  8  on which these components are mounted. In this embodiment, the CR motor  4  and the PF motor  5  are both a direct-current (DC) motor. 
   As shown in  FIG. 2 , the printer  1  further includes a hopper  11  on which the print paper P before printing is placed; a paper feed roller  12  and a separation pad  13  for taking the print paper P placed on the hopper  11  into the printer  1 ; a paper sensor  14  that senses the passage of the print paper P taken into the printer  1  from the hopper  11 ; and a delivery drive roller  15  that ejects the print paper P from the printer  1 . 
   The carriage  3  can be moved in the main scanning direction MS by a guide shaft  17  supported by a support frame  16  fixed to the chassis  8  and a timing belt  18 . Specifically, the timing belt  18  runs between a pulley  19  and a pulley  20  under a specified tension, the pulley  19  being partly secured to the carriage  3  and being fixed to the output shaft of the CR motor  4 , and the pulley  20  being rotatably fixed to the support frame  16 . The guide shaft  17  sidably holds the carriage  3  so as to guide the carriage  3  in the main scanning direction MS. The carriage  3  further has an ink cartridge  21  in addition to the print head  2 , in which various inks to be supplied to the print head  2  are housed. 
   The paper feed roller  12  connects to the PF motor  5  with a gear (not shown), and is driven by the PF motor  5 . As shown in  FIG. 2 , the hopper  11  is a plate-like member on which the print paper P can be placed, which can be oscillated about a rotation shaft  22  at the top by a cam mechanism (not shown). The oscillation by the cam mechanism springily brings the lower end of the hopper  11  into and out of pressure contact with the paper feed roller  12 . The separation pad  13  is made of a high-friction member and is opposed to the paper feed roller  12 . As the paper feed roller  12  rotates, the surface of the paper feed roller  12  and the separation pad  13  come into pressure contact with each other. Accordingly, when the paper feed roller  12  rotates, the uppermost of the print paper P placed on the hopper  11  passes through the contact between the surface of the paper feed roller  12  and the separation pad  13  toward the delivery side; the second and later upper print paper P are stopped by the separation pad  13 . 
   The PF drive roller  6  connects to the PF motor  5  directly or with a gear (not shown). As shown in  FIG. 2 , the printer  1  further has a PF driven roller  23  that feeds the print paper P with the PF drive roller  6 . The PF driven roller  23  is rotatably held at the delivery side of a driven-roller holder  24  that is rotatable about a rotation shaft  25 . The driven-roller holder  24  is urged counterclockwise (in the drawing) by a spring (not shown) so that the PF driven roller  23  is constantly urged to the PF drive roller  6 . When the PF drive roller  6  is driven, the PF driven roller  23  also rotates with the PF drive roller  6 . 
   As shown in  FIG. 2 , the paper sensor  14  is composed of a sensing lever  26  and a sensor  27 , and is disposed in the vicinity of the driven-roller holder  24 . The sensing lever  26  is rotatable about a rotation shaft  28 . When the print paper P completes passing below the sensing lever  26  from the passing state shown in  FIG. 2 , the sensing lever  26  turns counterclockwise. When the sensing lever  26  turns, the light from a light-emitting portion of the sensor  27  toward a light-receiving portion is interrupted to thereby sense the passage of the print paper P. 
   The delivery drive roller  15  is disposed on the delivery side of the printer  1 , and connects to the PF motor  5  with a gear (not shown). As shown in  FIG. 2 , the printer  1  further includes a delivery driven roller  29  for delivering the print paper P together with the delivery drive roller  15 . Like the PF driven roller  23 , the delivery driven roller  29  is also constantly urged toward the delivery drive roller  15  by a spring (not shown). When the delivery drive roller  15  is driven, the delivery driven roller  29  also rotates with the delivery drive roller  15 . 
   Referring to  FIG. 3 , the printer  1  further includes a linear encoder  33  having a linear scale  31  and a sensor  32  for determining the rotational position of the CR motor  4  (the position of the carriage  3  in the main scanning direction MS) and the rotational speed of the CR motor  4  (the speed of the carriage  3 ); and a rotary encoder  36  having a rotary scale  34  and a sensor  35  for determining the rotational position of the PF motor  5  in the subscanning direction SS (the position of the print paper P in the subscanning direction SS) and the rotational speed of the PF motor  5  (the feeding speed of the print paper P). 
   The linear scale  31  is shaped in a long straight line, and is mounted to the support frame  16  in parallel with the main scanning direction MS. The linear scale  31  has marks  31   a  at specified intervals. The sensor  32  has a light-emitting device and a light-receiving device (not shown), and is mounted to the carriage  3 . The linear encoder  33  outputs a specified output signal in such a manner that the light emitted from the light-emitting device toward the linear scale  31  is reflected by the marks  31   a , and the light-receiving device receives the reflected light. Unlike a rotary scale  34  to be described below, the linear scale  31  does not have a main body portion formed of a transparent member. However, the linear scale  31  may have a main body portion formed of a transparent member. 
   The rotary scale  34  is shaped like a disc, and is mounted to the PF drive roller  6  so as to rotate therewith. Specifically, when the PF drive roller  6  makes a turn, the rotary scale  34  also makes a turn. The sensor  35  is fixed to the chassis  8  with a bracket (not shown). Alternatively, the rotary scale  34  may be connected to the PF drive roller  6  with a gear or the like. However, mounting the rotary scale  34  directly to the PF drive roller  6  so as to rotate therewith allows one-to-one correspondence of the rotation amount of the rotary scale  34  and that of the PF drive roller  6  without errors such as play at the engaging portion of a gear. The details of the structure of the rotary encoder  36  will be described later. 
   (Schematic Structure of Controller of Printer) 
     FIG. 4  is a block diagram showing the schematic structure of a controller  37  of the printer  1  and its peripherals. 
   As shown in  FIG. 4 , the controller  37  includes a bus  38 , a CPU  39 , a ROM  40 , a RAM  41 , a character generator (CG)  42 , a nonvolatile memory  43 , an interface (I/F) dedicated circuit  44 , a DC unit  45 , a PF-motor drive circuit  46 , a CR-motor drive circuit  47 , a head drive circuit  48 , and an application-specific integrated circuit (ASIC)  51 . The controller  37  is configured such that the CPU  39  and the ASIC  51  receive output signals from the linear encoder  33  and the rotary encoder  36 . 
   The CPU  39  performs operations for executing the control programs of the printer  1  stored in the ROM  40  and the nonvolatile memory  43  and other necessary operations. The ROM  40  stores control programs for controlling the printer  1  and data necessary for processing. For example, the ROM  40  stores a target speed table that contains target rotational speeds for the rotational positions of the CR motor  4  and the PF motor  5 . 
   The RAM  41  temporarily stores programs that the CPU  39  is executing and data during operation. The CG  42  stores dot patterns expanded corresponding to print signals input to the I/F dedicated circuit  44 . The nonvolatile memory  43  stores various data that needs to be stored after the printer  1  is turned off. The I/F dedicated circuit  44  has a parallel interface circuit, which can receive print signals sent from a computer  50  via a connector  49 . The ASIC  51  controls the CR motor  4  and the PF motor  5  via the DC unit  45 , and controls the print head  2  via the head drive circuit  48 . 
   The DC unit  45  is a control circuit for controlling the speed of the DC motor. The DC unit  45  performs various operations for controlling the speed of the CR motor  4  and the PF motor  5  according to the control instruction sent from the CPU  39  and signals output from the ASIC  51  via the I/F dedicated circuit  44 , and outputs motor control signals to the PF-motor drive circuit  46  and the CR-motor drive circuit  47  on the basis of the calculations. 
   The PF-motor drive circuit  46  controls the driving of the PF motor  5  according to the motor control signal from the DC unit  45 . This embodiment adopts a pulse width modulation (PWM) control to control the PF motor  5 . Thus the PF-motor drive circuit  46  outputs a PWM driving signal. Similarly, the CR-motor drive circuit  47  controls the CR motor  4  in response to the motor control signal from the DC unit  45 . 
   The head drive circuit  48  drives the nozzles of the print head  2  under the control instruction sent from the CPU  39  or the ASIC  51  via the I/F dedicated circuit  44 . 
   The bus  38  is a signal line that connects the foregoing components of the controller  37 . The bus  38  interconnects the CPU  39 , the ROM  40 , the RAM  41 , the CG  42 , the nonvolatile memory  43 , and the I/F dedicated circuit  44  to enable exchange of data. 
   (Structure of PF-Motor Speed Control Unit) 
     FIG. 5  is a block diagram showing the structure of a speed control unit  53  for the PF motor  5  in the DC unit  45 ;  FIG. 6  is a graph of examples of a target speed curve drawn from the target speed table stored in the ROM  40  of  FIG. 4 ; and  FIG. 7  is an enlarged view of part Z in  FIG. 6 . 
   As has been described, the DC unit  45  serves as a control circuit for controlling the speed of the CR motor  4  and the PF motor  5 . The structure of the speed control unit  53  for the PF motor  5  in the DC unit  45  will be described hereinbelow. A speed control unit for the CR motor  4  in the DC unit  45  has the same structure as the speed control unit  53 . 
   As shown in  FIG. 5 , the speed control unit  53  includes a location-deviation operating section  56 , a target-speed operating section  57 , a speed-deviation operating section  58 , a comparing element  59 , an integrator element  60 , a differentiating element  61 , an adding section  62 , and a D/A converter  63 . In other words, this embodiment employs a proportional, integral, and derivative (PID) control to control the PF motor  5 , in which the present rotational speed of the PF motor  5  is converged to a target rotational speed by a combination of comparing control, integral control, and derivative control. The location-deviation operating section  56  and the speed-deviation operating section  58  receive specified signals from the ASIC  51 . 
   As has been described, the ASIC  51  receives a signal output from the rotary encoder  36 . The ASIC  51  outputs a present-rotational-position signal (a print-paper-P present-position signal) Pc corresponding to the present rotational position of the PF motor  5  responding to an output signal from the rotary encoder  36 , and a present-rotational-speed signal (a print-paper-P present-feed-speed signal) Vc corresponding to the present rotational speed of the PF motor  5  responding to an output signal from the rotary encoder  36 . 
   The location-deviation operating section  56  receives the present-rotational-position signal Pc and a target-stop-position signal Pt corresponding to the next stop position of the print paper P in the subscanning direction SS. The location-deviation operating section  56  calculates and outputs a location-deviation signal dP corresponding to location deviation that is the difference between the input present-position signal Pc and the target-stop-position signal Pt. The target-stop-position signal Pt is input from the CPU  39 . 
   The target-speed operating section  57  receives the location-deviation signal dP. The target-speed operating section  57  calculates and outputs a target-rotational-speed signal (a print-paper-P target-feed-speed signal) Vt corresponding to the target rotational speed of the PF motor  5  on the basis of the input location-deviation signal dP. More specifically, the target-speed operating section  57  reads a target-rotational-speed signal Vt corresponding to the location-deviation signal dP from the target speed table stored in the ROM  40  and outputs it. 
   The solid line of  FIG. 6  shows an example of a target speed curve created from the target speed table store in the ROM  40 . The target speed curve created from the target speed table has an accelerating region, a constant-speed region, and a decelerating region toward a target stop position X. The target speed table provides the target-rotational-speed signal Vt so as to correspond to the location-deviation signal dP in a specified range of values. Accordingly, the target speed curve is actually in the form of steps, as shown in  FIG. 7 , so that the target rotational speed is held constant even if the location-deviation signal dP varies slightly. Rotational speed in the constant-speed region depends on print mode. For example, the ROM  40  also stores target-speed tables corresponding to the dotted line and the two-dot chain line in  FIG. 6 . The ROM  40  also stores a target-speed table corresponding to various target stop positions. 
   The speed-deviation operating section  58  receives the target-rotational-speed signal Vt and the present-rotational-speed signal Vc. The speed-deviation operating section  58  outputs a speed deviation signal dV that is the difference between the input target-rotational-speed signal Vt and the present-rotational-speed signal Vc. The speed deviation signal dV output from the speed-deviation operating section  58  is input to the comparing element  59 , the integrator element  60 , and the differentiating element  61 . The comparing element  59 , the integrator element  60 , and the differentiating element  61  respectively output a comparing-control-value signal QP, an integral-control-value signal QI, and a derivative-control-value signal QD calculated from the input speed deviation signal dV by a specified calculating expression. 
   The adding section  62  receives the comparing-control-value signal QP output from the comparing element  59 , the integral-control-value signal QI output from the integrator element  60 , and the derivative-control-value signal QD output from the differentiating element  61 . The adding section  62  adds the control value signals QP, QI, and QD to output a PID-control-value signal □Q that is digital data, to the D/A converter  63 . The D/A converter  63  converts the digital PID-control-value signal □Q to analog data, and outputs it. The analog data output from the D/A converter  63  is input to the PF-motor drive circuit  46  as a motor control signal. 
   (Structure of Rotary Encoder) 
     FIG. 8  is a schematic diagram of a part related to the rotary encoder  36  of  FIG. 3 ;  FIG. 9  is a front view of the rotary scale  34  in  FIG. 3 ;  FIG. 10  is a side view of the sensor  35  in  FIG. 3 ;  FIGS. 11A  to C are partial cross-sectional views showing a structure of the rotary scale of  FIG. 3 ;  FIG. 12  is a schematic diagram showing the relationship between a board  68  disposed to the sensor  35  shown in  FIG. 10  and its peripherals. 
     FIG. 13  is an electric circuit diagram of the rotary encoder  36  of  FIG. 3 ; and  FIG. 14  shows signal waveforms generated by the rotary encoder  36  by the normal rotation of the rotary scale  34 , wherein (A) shows level signal waveforms amplified by a first amplifier  74  and a third amplifier  76  shown in  FIG. 13 ; (B) shows a signal waveform output from a first-differential-signal generating circuit  78  shown in  FIG. 13 ; (C) shows level signal waveforms amplified by a second amplifier  75  and a fourth amplifier  77  shown in  FIG. 13 ; (D) shows a signal waveform output from a second-differential-signal generating circuit  79  shown in  FIG. 13 ; (E) shows a signal waveform output from an exclusive OR circuit  80  shown in  FIG. 13 ; (F) shows a signal waveform output from a row-B-signal generating circuit  71  shown in  FIG. 13 ; (G) is a signal waveform output from a row-C-signal generating circuit  72  shown in  FIG. 13 ; and (H) is a signal waveform output from a row-D-signal generating circuit  73  shown in  FIG. 13 .  FIG. 15  shows signal waveforms generated by the rotary encoder  36  when the rotating direction of the rotary scale  34  is changed, wherein (A) shows a signal waveform output from the exclusive OR circuit  80  shown in  FIG. 13 ; (B) shows a signal waveform output from the row-B-signal generating circuit  71  shown in  FIG. 13 ; (C) shows a signal waveform output from the row-C-signal generating circuit  72  shown in  FIG. 13 ; and (D) shows a signal waveform output from the row-D-signal generating circuit  73  shown in  FIG. 13 . 
   The rotary scale  34  is, for example, a plastic thin plate and is formed in a disc shape shown in  FIG. 9 . As shown in  FIG. 11A , the rotary scale  34  has a main body portion  34   a  formed of polyethylene terephthalate (PET), and marks  34   b  serving as graduations. The main body portion  34   a  is transparent so as to allow light to pass therethrough. In this embodiment, the thickness of the main body portion  34   a  is significantly thin, for example, 180 μm. Moreover, in  FIGS. 11A to 11C , the marks  34   b  are shown thick, but are actually set in a range of several μm to 20 μm. The marks  34   b  are formed by attaching a non-transmissive material to a surface of the main body portion  34   a  using printing or deposition. For this reason, light does not pass through the marks  34   b.    
   In the rotary scale  34 ,  180  slits  65 , each forming the space between the marks  34   b , are formed in a direction perpendicular to the paper of  FIG. 9 . The 180 slits  65  are arranged at the same positions of the rotary scale  34  in a radial direction at regular angular intervals. That is, the 180 slits  65  are arranged at the regular angular intervals along an outer circumference of the rotary scale  34 . An interval between adjacent slits  65  and the width of each of the slits  65  in an arrangement direction of the slits  65  (a circumferential direction of the rotary scale  34 ) are substantially equal to each other. In  FIG. 9 , for convenience, the slits  65  are displayed in the circumferential direction on a magnified scale, but the 180 slits  65  are actually formed in one round, and thus the width of each of the slits  65  in the circumferential direction is made significantly small. A through hole  34   c  that has a width W 2  equal to the width W 1  of the slit  65  is formed to correspond to the slit  65  for every three slits  65  among the slits  65 . The through hole  34   c  prevents the occurrence of diffused reflection or refraction due to a decrease in the amount of light passing through the slit  65  caused by the ink mist attached to the slit  65 . 
   As shown in  FIG. 11B , the rotary scale  34  may have the through hole  34   c  that has a width W 3  larger than the width W 1  of the slit  65 . Further, as shown in  FIG. 11B , the number of through holes  34   c  to be provided may be a fourth of all the slits  65 , not a third of all the slits  65  (see  FIG. 11A ). If the width W 3  of the through hole  34   c  becomes larger than the width W 1  of the slit  65 , light  34   c  passing through the periphery of the mark  34   b  rarely enter the main body portion  34   a . If light  34   d  enters the main body portion  34   a , light  34   d  enters a deep part of the main body portion  34   a  due to a refractive index when incident. Then, a light-receiving range of a light-receiving element  69 , which is described below, changes by the position of the light-receiving element  69 , and thus the output signals are rarely stabilized. The structure shown in  FIG. 11B  does not have such problems. 
   The rotary scale  34  may have a structure shown in  FIG. 11C . That is, the through hole  34   c  may have a width W 4  smaller than the width W 1  of the slit  65 . With this configuration, the strength of the main body portion  34   a  can be kept. Light passing through the periphery of a boundary portion  34   e  between the mark  34   b  and the slit  65  is incident on the main body portion  34   a  from the top surface. Therefore, light that is received by the light-receiving element  69  can be stabilized, and a light-receivable region can be prevented from being expanded. 
   Preferably, the through holes  34   c  are respectively provided to correspond to slits  65  of a third to a tenth of all the slits  65 . If the through holes are respectively provided between marks of a tenth or more of all the marks, more wastes pass through the scale, and thus the wastes are rarely attached to the rotary scale  34 . Meanwhile, if the through holes  34   c  are respectively provided between marks of a third or less of all the marks, the strength of the rotary scale  34  can be kept. Moreover, in view of strength balance, the through holes  34   c  are preferably provided at predetermined regular intervals. 
   The rotary scale  34  rotates with the PF drive roller  6 , as described above. That is, when the PF drive roller  6  makes a turn, the rotary scale  34  also makes a turn. When the peripheral length of the PF drive roller  6  is one inch, the resolution of the single rotary scale  34  is 180 (=1 in./180) dpi. The rotary scale  34  may be connected to the PF drive roller  6  with a gear or the like, as described above, so that, e.g., the rotary scale  34  makes two turns when the PF drive roller  6  makes a turn. 
   Referring to  FIG. 10 , the sensor  35  has a substantially rectangular parallelepiped housing. The sensor  35  has a recess  66  from one side (the left side in  FIG. 10 ) toward the center of the housing. A light-emitting element  67  or a light emitter is disposed on one of two opposing surfaces (two vertically opposing surfaces in  FIG. 10 ) of the recess  66 , while a board  68  is disposed on the other surface. The board  68  has a plurality of light-receiving elements  69  or sensing elements (see  FIG. 12 ), so that the portion of the board  68  serves as the photoreceiver (sensing portion) of the sensor  35 . The sensor  35  holds part of the outer periphery of the rotary scale  34  in the recess. Thus the outer periphery of the rotary scale  34 , that is, the portion of the rotary scale  34  where the slits  65  are formed is located between the light-emitting element  67  and the light-receiving elements  69 . 
   The light-emitting element  67  is, for example, a light-emitting diode, which emits light having a good straight-forwarding performance. 
   Referring to  FIG. 12 , the board  68  has the light-receiving elements  69  arranged in four rows along the rotating direction of the rotary scale  34 . Hereinafter, the four rows of the light-receiving elements  69  are referred to as rows A, B, C, and D from the top of  FIG. 12 . The light-receiving elements  69  are, for example, a photodiode, which output signals of a level according to the amount of received light. Moreover, in  FIG. 12 , the main body portion  34   a  formed of the transparent member is not shown. 
   Assuming that the light-emitting element  67  emits parallel rays onto the board  68 , as shown in  FIG. 12 , light and dark portions (light and shade) are formed on the surface of the board  68  at the same intervals as that of the slits  65  along the outer periphery of the rotary scale  34 . Specifically, the portions of the board  68  corresponding to the slits  65  are irradiated with the light from the light-emitting element  67 . The portions of the board  68  corresponding to the interval between the slits  65  of the rotary scale  34  are shielded from the light of the light-emitting element  67 . Thus, one cycle of the light and dark portions formed on the surface of the board  68  (hereinafter, referred to as a light and shade cycle T) corresponds to the arrangement pitch of the slits  65  of the rotary scale  34 . In other words, when the light-emitting element  67  irradiates the board  68  with parallel rays, the light and shade cycle T formed on the surface of the board  68  is the same as the pitch of the slits  65 . Accordingly, when the rotary scale  34  rotates at equal speed, the light and shade cycle T formed on the surface of the board  68  becomes substantially constant. 
   When the light emitted from the light-emitting element  67  is not parallel rays, or is diffused light, the light and shade cycle T formed on the board  68  is narrow at the portion of the board  68  closest to the light-emitting element  67 , and is wider with an increasing distance from the light-emitting element  67 . Thus, in that case, even when the rotary scale  34  rotates at equal speed, the light and shade cycle T does not become constant. 
   The light-receiving elements  69  in rows A to D are each disposed over a plurality of light and shade cycles T (three cycles in  FIG. 12 ) of the board  68 .  FIG. 12  shows the arrangement relationship among the light-receiving elements  69  in the case where the light from the light-emitting element  67  is parallel light. Each of the light-receiving elements  69  has a light-receiving surface of a size approximately one quarter of the light and shade cycle T formed on the board  68 . In other words, each of the light-receiving elements  69  in each row has a size equal to one quarter of the light and shade cycle T. As shown in  FIG. 11 , a plurality of sets of four light-receiving elements  69  of a first light-receiving element A 1  ( 69 ) (B 1  ( 69 ), C 1  ( 69 ), or D 1  ( 69 )); a second light-receiving element A 2  ( 69 ) (B 2  ( 69 ), C 2  ( 69 ), or D 2  ( 69 )); a third light-receiving element A 3  ( 69 ) (B 3  ( 69 ), C 3  ( 69 ), or D 3  ( 69 )); a fourth light-receiving element A 4  ( 69 ) (B 4  ( 69 ), C 4  ( 69 ), or D 4  ( 69 )) corresponding to the light and shade cycle T is disposed in each of rows A to D from the left in the drawing. 
   The light-receiving elements  69  in four rows are disposed with a slight displacement with each other in the rotating direction of the rotary scale  34 . More specifically, the four rows of light-receiving elements  69  are displaced one sixteenth of the light and shade cycle T with each other in the rotating direction of the rotary scale  34 . Referring to  FIG. 12 , when the PF motor  5  rotates in the normal direction (in the direction in which the print paper P is fed to the delivery side) (when the rotary scale  34  rotates in the normal direction), the rotary scale  34  rotates from the left to the right of the drawing. In this case, row B is formed in a position shifted to the right of the light-receiving elements  69  in row A by one sixteenth of the light and shade cycle T. Row C is formed in a position shifted to the right of the light-receiving elements  69  in row A by two sixteenths of the light and shade cycle T. Row D is formed in a position shifted to the right of the light-receiving elements  69  in row A by three sixteenths of the light and shade cycle T. 
   In other words, referring to  FIG. 12 , for example, the light-receiving element A 1  ( 69 ) at the left end of row A, the light-receiving element B 1  ( 69 ) at the left end of row B, the light-receiving element C 1  ( 69 ) at the left end of row C, and the light-receiving element D 1  ( 69 ) at the left end of row D are displaced with each other in that order by one sixteenth of the light and shade cycle T (one cycle of light and shade) along the moving direction of the light and shade formed by the slits  65 . 
   When the rotary scale  34  rotates with the PF drive roller  6 , the slits  65  move between the light-emitting element  67  and the light-receiving elements  69  of the sensor  35 . As the slits  65  moves, the light-receiving elements  69  output signals at a level depending on the amount of received light. More specifically, the light-receiving elements  69  corresponding to the slits  65  output high-level signals, while the light-receiving elements  69  corresponding to the interval between the slits  65  output low-level signals. Thus the light-receiving elements  69  output signal at a level varied in a cycle depending on the moving speed of the slits  65 . 
   Referring to  FIG. 13 , the sensor  35  that configures the rotary encoder  36  includes a row-A-signal generating circuit  70  or first signal generating means having a plurality of row-A light-receiving elements  69 , a row-B-signal generating circuit  71  or second signal generating means having a plurality of row-B light-receiving elements  69 , a row-C-signal generating circuit  72  or third signal generating means having a plurality of row-C light-receiving elements  69 , and a row-D-signal generating circuit  73  or fourth signal generating means having a plurality of row-D light-receiving elements  69 . 
   The row-A-signal generating circuit  70  includes the row-A light-receiving elements  69 , the first to fourth amplifiers  74 ,  75 ,  76 , and  77 , the first differential-signal generating circuit  78 , the second differential-signal generating circuit  79 , and an exclusive OR circuit  89 . 
   As shown in  FIG. 12 , a plurality of sets of four light-receiving elements  69 , the first light-receiving element A 1  ( 69 ), the second light-receiving element A 2  ( 69 ), the third light-receiving element A 3  ( 69 ), and the fourth light-receiving element A 4  ( 69 ) corresponding to the light and shade cycle T is arranged in row A. The first amplifier  74  connects to the row-A first light-receiving elements A 1  ( 69 ) in parallel. The first light-receiving elements A 1  ( 69 ) each output a signal at a level responsive to their respective received light amount. The first amplifier  74  amplifies the level signals output from the first light-receiving elements A 1  ( 69 ). 
   Similarly, the second amplifier  75  connects to the A-row second light-receiving elements A 2  ( 69 ) in parallel. The second amplifier  75  amplifies the level signals output from the second light-receiving elements A 2  ( 69 ), and outputs them. The third amplifier  76  connects to the row-A third light-receiving elements A 3  ( 69 ) in parallel. The third amplifier  76  amplifies the level signals output from the third light-receiving elements A 3  ( 69 ), and outputs them. The fourth amplifier  77  connects to the row-A fourth light-receiving elements A 4  ( 69 ) in parallel. The fourth amplifier  77  amplifies the level signals output from the fourth light-receiving elements A 4  ( 69 ), and outputs them. 
   As shown in  FIG. 12 , the first light-receiving elements A 1  ( 69 ) and the third light-receiving elements A 3  ( 69 ) are each formed on the board  68  in such a manner as to be displaced a half of the light and shade cycle T with respect to each other. Accordingly, as shown in  FIG. 14(A) , the signal waveform amplified by the first amplifier  74  and the signal waveform amplified by the third amplifier  76  are displaced a half of the light and shade cycle T with respect to each other. Similarly, the second light-receiving elements A 2  ( 69 ) and the fourth light-receiving elements A 4  ( 69 ) are each formed on the board  68  in such a manner as to be displaced a half of the light and shade cycle T with respect to each other. Accordingly, as shown in  FIG. 14(C) , the signal waveform amplified by the second amplifier  75  and the signal waveform amplified by the fourth amplifier  77  are displaced a half of the light and shade cycle T with respect to each other. The time of the cycle TL of the signal waveforms output from the amplifiers  74 ,  75 ,  76 , and  77  is the same as that of the light and shade cycle T. 
   The first amplifier  74  and the third amplifier  76  output amplified level signals to the first-differential-signal generating circuit  78 . The level signal amplified by the first amplifier  74  is input to a noninverting input terminal of the first-differential-signal generating circuit  78 , while the level signal amplified by the first-differential-signal generating circuit  78  is input to an inverting input terminal of the first-differential-signal generating circuit  78 . 
   When the level of the signal input to the noninverting input terminal (the signal output from the first amplifier  74 ) is higher than that of the signal input to the inverting input terminal (the signal output from the third amplifier  76 ), the first-differential-signal generating circuit  78  outputs a high-level signal; when the level of the signal input to the noninverting input terminal is lower than that of the signal input to the inverting input terminal, the first-differential-signal generating circuit  78  outputs a low-level signal. Thus the first-differential-signal generating circuit  78  outputs a digital-waveform signal. In other words, as shown in  FIG. 14(B) , the first-differential-signal generating circuit  78  outputs a digital-waveform signal with a duty of approximately 50% substantially in the same cycle as that output from the third light-receiving element A 3  ( 69 ). 
   The second amplifier  75  and the fourth amplifier  77  output amplified level signals to the second-differential-signal generating circuit  79 . The level signal amplified by the second amplifier  75  is input to a noninverting input terminal of the second-differential-signal generating circuit  79 , while the level signal amplified by the fourth amplifier  77  is input to an inverting input terminal of the second-differential-signal generating circuit  79 . 
   When the level of the signal input to the noninverting input terminal (the signal output from the second amplifier  75 ) is higher than that of the signal input to the inverting input terminal (the signal output from the fourth amplifier  77 ), the second-differential-signal generating circuit  79  outputs a high-level signal; when the level of the signal input to the noninverting input terminal is lower than that input to the inverting input terminal, the second-differential-signal generating circuit  79  outputs a low-level signal. Thus the second-differential-signal generating circuit  79  outputs a digital-waveform signal. In other words, as shown in  FIG. 14(D) , the second-differential-signal generating circuit  79  outputs a digital-waveform signal with a duty of approximately 50% substantially in the same cycle as that of the level signal output from the fourth light-receiving element A 4  ( 69 ). 
   As shown in  FIG. 12 , the first light-receiving elements A 1  ( 69 ) and the second light-receiving elements A 2  ( 69 ) are each formed on the board  68  in such a manner as to be displaced a quarter of the light and shade cycle T with respect to each other. Accordingly, the output signal of the first-differential-signal generating circuit  78  shown in  FIG. 14(B)  and the output signal of the second-differential-signal generating circuit  79  shown in FIG.  14 (D) are displaced a quarter of the light and shade cycle T with respect to each other. 
   The output signal of the first-differential-signal generating circuit  78  and the output signal of the second-differential-signal generating circuit  79  are input to the exclusive OR circuit  80 . When both of the two inputs are on a high level or a low level, the exclusive OR circuit  80  outputs a low-level signal; when only one of the two inputs is on a high level, it outputs a high-level signal. Specifically, as shown in  FIG. 14(E) , the exclusive OR circuit  80  outputs a signal S 1  with a cycle about a half of that of the level signal of the light-receiving elements  69 . When the rotating direction of the rotary scale  34  is changed at time t 0 , the exclusive OR circuit  80  outputs the signal S 1  shown in  FIG. 15(A) . 
   The output signal of the exclusive OR circuit  80  is output from an output terminal  81  of the rotary encoder  36 . The output signal of the exclusive OR circuit  80  (the output signal of the row-A-signal generating circuit  70 ) S 1  corresponds to a first output signal. 
   Since the internal structures of the row-B-signal generating circuit  71 , the row-C-signal generating circuit  72 , and the row-D-signal generating circuit  73  are the same as that of the row-A-signal generating circuit  70 , drawings thereof and descriptions will be omitted. The row-B signal generating circuit  71 , the row-C-signal generating circuit  72 , and the row-D-signal generating circuit  73  respectively output signals S 2 , S 3 , and S 4  with a cycle approximately a half of the level signal of the light-receiving elements  69  shown in  FIGS. 14(F) ,  14 (G), and  14 (H). When the rotating direction of the rotary scale  34  is changed at time t 0 , the row-B-signal generating circuit  71 , the row-C-signal generating circuit  72 , and the row-D-signal generating circuit  73  respectively output signals S 2 , S 3 , and S 4  shown in  FIGS. 15(B) ,  15 (C), and  15 (D). 
   As has been described, the light-receiving elements  69  in row B are displaced to the right of the light-receiving elements  69  in row A by a sixteenth of the light and shade cycle T. The light-receiving elements  69  in row C are displaced to the right of the light-receiving elements  69  in row A by two sixteenths of the light and shade cycle T. The light-receiving elements  69  in row D are displaced to the right of the light-receiving elements  69  in row A by three sixteenths of the light and shade cycle T. Therefore, as shown in  FIGS. 14(E) to 14(H) , when the rotary scale  34  rotates in the normal direction, the phase of the output signal S 2  of the row-B-signal generating circuit  71  is basically delayed a sixteenth of the light and shade cycle T behind the phase of the output signal S 1  of the row-A-signal generating circuit  70 . The phase of the output signal S 3  of the row-C-signal generating circuit  72  is basically delayed two sixteenths of the light and shade cycle T behind the phase of the output signal S 1  of the row-A-signal generating circuit  70 . The phase of the output signal S 4  of the row-D-signal generating circuit  73  is basically delayed three sixteenths of the light and shade cycle T behind the phase of the output signal S 1  of the row-A-signal generating circuit  70 . 
   As shown in  FIG. 13 , the output signal S 2  of the row-B-signal generating circuit  71  is output from an output terminal  82  of the rotary encoder  36 ; the output signal S 3  of the row-C-signal generating circuit  72  is output from an output terminal  83  of the rotary encoder  36 ; and the output terminal S 4  of the row-D-signal generating circuit  73  is output from an output terminal  84  of the rotary encoder  36 . In other words, the rotary encoder  36  has four output terminals  81 ,  82 ,  83 , and  84 . The output signal S 2  of the row-B-signal generating circuit  71  corresponds to a second output signal; the output signal S 3  of the row-C-signal generating circuit  72  corresponds to a third output signal; and the output signal S 4  of the row-D-signal generating circuit  73  corresponds to a fourth output signal. 
   Referring back to  FIG. 8 , the four output terminals  81 ,  82 ,  83 , and  84  connect to the controller  37  with four signal lines  86 ,  87 ,  88 , and  89 , respectively. 
   (Method for Controlling Printer) 
   The printer  1  with this arrangement reciprocates the carriage  3  driven by the CR motor  4  in the main scanning direction MS while feeding the print paper P taken from the hopper  11  into the printer  1  with the paper feed roller  12  and the separation pad  13  in the subscanning direction SS with the PF drive roller  6  driven by the PF motor  5 . While the carriage  3  is reciprocating, the print head  2  jets out ink drops to print on the print paper P. Upon completion of printing to the print paper P, the print paper P is delivered to the outside of the printer  1  with the delivery drive roller  15  and so on. 
   When the print paper P is fed in the subscanning direction SS, the PF motor  5  rotates the PF drive roller  6 . On rotation of the PF drive roller  6 , the rotary scale  34  rotates with the PF drive roller  6 . On rotation of the rotary scale  34 , the rotary encoder  36  outputs the four signals S 1 , S 2 , S 3 , and S 4 . The output signals S 1 , S 2 , S 3 , and S 4  are input to a predetermined processing circuit (e.g., the ASIC  51 ) of the controller  37 . To control the PF motor  5  and so on, the rotational position and speed of the PF motor  5  are determined from the output signals S 1 , S 2 , S 3 , and S 4  of the rotary encoder  36 . 
   A method for determining the rotational position and speed and rotating direction of the PF motor  5  will be described in sequence. 
   A method for determining the rotational position of the PF motor  5  will first be described. The rotational position of the PF motor  5  is determined using edges E 1 , E 2 , E 3 , and E 4  at which the levels of the output signals S 1 , S 2 , S 3 , and S 4 , shown in  FIGS. 14(E) to 14(H) , change (rise and fall). In other words, the rotational position of the PF motor  5  is determined by counting the number of the edges E 1 , E 2 , E 3 , and E 4  output from the rotary encoder  36 . The four output signals S 1 , S 2 , S 3 , and S 4  are expressed as output signals S hereinbelow, if collectively expressed. The four edges E 1 , E 2 , E 3 , and E 4  are expressed as edges E, if collectively expressed. 
   When the PF motor  5  rotates in both of the normal and reverse directions, the rotational position of the PF motor  5  is determined from the determination on the rotating direction, to be described later, and the number of the edges E. Here a case where the PF motor  5  rotates only in one direction will be described. 
   For example, where the PF motor  5  rotates in the normal direction, the edges E are input when the edges E 1 , E 2 , E 3 , and E 4  are output from the rotary encoder  36  in that order, as shown in  FIGS. 14(E) to 14(H) , so that the rotational position of the PF motor  5  can be determined appropriately by a predetermined processing circuit (e.g., the ASIC  51 ) of the controller  37 . 
   The cycle of the output signals S is approximately a half of that of the level signal of the light-receiving elements  69 . The signals S 1 , S 2 , S 3 , and S 4  are basically sequentially output with a phase difference of one sixteenth of the light and shade cycle T. Accordingly, when the rotational speed of the PF motor  5  increases to output high-frequency signals S from the rotary encoder  36 , a phenomenon in which the edges E 1 , E 2 , E 3 , and E 4  are not output in that order, e.g., two edges E overlapped or the order of the output edges E are reversed, because of the characteristic of the electrical circuit of the rotary encoder  36 . To determine the rotational position of the PF motor  5  using the four output signals S under such a phenomenon due to the high-frequency signals, the structure of a processing circuit for determining the rotational position is complicated or the processing load on the processing circuit is increased. 
   Accordingly, in this embodiment, when the PF motor  5  rotates at or below a specified rotational speed at which the foregoing problems due to high-frequency signals do not occur, a predetermined processing circuit determines the rotational position of the PF motor  5  using all the four output signals S. That is, the processing circuit determines the rotational position of the PF motor  5  by counting the number of the edges E of each of the four output signals S. On the other hand, when the PF motor  5  rotates at or over a specified rotational speed at which the foregoing problems due to high-frequency signals can occur, a predetermined processing circuit determines the rotational position of the PF motor  5  using the two output signals S 1  and D 3  or the two output signals S 2  and S 4 . That is, the processing circuit determines the rotational position of the PF motor  5  by counting the number of the respective edges E 1  and E 3  of the output signals S 1  and S 3 , or by counting the number of the respective edges E 2  and E 4  of the output signals S 2  and S 4 . 
   Thus, in this embodiment, the predetermined processing circuit for determining the rotational position switches (selects) between determining the rotation position using the four output signals S and determining it using two output signals S according to the rotational speed of the PF motor  5 . The switching (selection) by the processing circuit is made according to the information on the rotational speed of the PF motor  5  determined from the output signals S of the rotary encoder  36  or the instruction from the CPU  39  based on the print mode information sent from the computer  50  or the like. 
   The PF motor  5  is controlled on the basis of the information on the rotational position of the PF motor  5  determined from the four or two output signals S. For example, the PF motor  5  is PID-controlled on the basis of the rotational position of the PF motor  5  determined by the ASIC  51 . 
   The rotating direction of the PF motor  5  is determined as follows: the rotating direction of the PF motor  5  is determined from the edges E of one output signal S and the output level of the other output signals S at that time. For example, as shown in  FIG. 15 , if the output signals S 2 , S 3 , and S 4  are at low levels when the edge E 1  at the rising of the output signal S 1  is detected, it is determined that the PF motor  5  rotates in the normal direction. If the output signals S 2 , S 3 , and S 4  are at high levels when the edge E 1  at the rising of the output signal S 1  is detected, it is determined that the PF motor  5  rotates in the reverse direction. If the output signal S 1  is at a high level and the output signals S 3  and S 4  are at low levels when the edge E 2  at the rising of the output signal S 2  is detected, it is determined that the PF motor  5  rotates in the normal direction. On the other hand, if the output signal S 1  is at a low level and the output signals S 3  and S 4  are at high levels when the edge E 2  at the rising of the output signal S 2  is detected, it is determined that the PF motor  5  rotates in the reverse direction. Similarly, the rotating direction of the PF motor  5  is determined using the edges E 3  and E 4  of the output signals S 3  and S 4  and the output level of the other output signals S. 
   Accordingly, if the above-described problems due to high-frequency signals such that the signals are output with two edges E overlapped with each other or the order of the edges E is reversed occur, a processing circuit of the controller  37  (for example, ASIC  51 ) cannot appropriately determine the rotating direction of the PF motor  5 . 
   Accordingly, in this embodiment, like the detection of the rotational position, when the PF motor  5  rotates at a speed less than the predetermined rotation speed, or equal to or less than the predetermined rotational speed, and the problems due to the high-frequency signals do not occur, the processing circuit that detects the rotating direction detects the rotating direction using all the four output signals S and the four edges E. That is, the rotating direction of the PF motor  5  is detected by the output level of another output signal S when any one edge E among the edges E is detected. Further, when the PF motor  5  rotates at a speed that exceeds the predetermined rotational speed or is equal to or more than the predetermined rotational speed, and the problems due to the high-frequency signals occur, the predetermined processing of detecting the rotating direction detects the rotating direction of the PF motor  5  using two signals of the output signals S 1  and S 3  or two signals of the output signals S 2  and S 4 . That is, the rotating direction of the PF motor  5  is detected by the edges E 1  and E 3  of the output signals S 1  and S 3 , and the output level of another output signal S when one edge E is detected, or by the edges E 2  and E 4  of the output signals S 2  and S 4 , and the output level of another output signal S when one edge E is detected. 
   Thus, in this embodiment, the processing circuit for determining the rotating direction switches (selects) between determining the rotating direction using four output signals S and determining the rotating direction using two output signals S, depending on the rotational speed of the PF motor  5 . The switching (selection) by the processing circuit is made according to the instruction from the CPU  39  based on the information on rotational speed of the PF motor  5 , as described above. 
   Printer  1  is controlled on the basis of the information on the rotating direction of the PF motor determined using four or two output signals S. 
   For example, the rotational position of the PF motor  5  is determined from the information on the rotating direction, and the PF motor  5  is PID-controlled on the basis of the determination. 
   Next, the detection method of the rotation speed of the PF motor  5  will be described. The rotation speed of the PF motor  5  is detected using a time (cycle) from a rising edge (or falling edge) E of each output signal S to a next rising edge (or falling edge) E. For example, the rotation speed of the PF motor  5  is detected using the cycles T 1 , T 2 , T 3 , and T 4  shown in (E) to (H) of  FIG. 14 . 
   For this reason, even if two edges E are output to overlap each other or a sequence of the output edges E is reversed, a predetermined processing circuit (for example, the ASIC  51 ) of the control circuit  37  that detects the rotation speed can appropriately detect the rotation speed of the PF motor  5 . 
   In this embodiment, the rotation speed of the PF motor  5  is detected using all the four output signals S, regardless of the rotation speed of the PF motor  5 . Further, a predetermined control of the printer  1  is performed on the basis of information about the rotation speed of the PF motor  5  detected using the four output signals S. For example, the PID control of the PF motor  5  is performed on the basis of information about the rotation speed of the PF motor  5  detected by the ASIC  51 . 
   As described above, when the PF motor  5  rotates at the speed less than the predetermined rotation speed or equal to or less than the predetermined rotation speed, the ASIC  51  detects the rotation position of the PF motor  5  using the four output signals S. Meanwhile, when the PF motor  5  rotates that is equal to or more than the predetermined rotation speed or exceeds the predetermined rotation speed, the ASIC  51  detects the rotation speed of the PF motor  5  using the two output signals S. For this reason, as shown in  FIG. 7 , when the rotation speed is equal to or more than the predetermined rotation speed V 1 , for example, only the target rotation speeds corresponding to the rotation positions detected from the output signals S 1  and S 3  are set in the target speed table. Further, if the rotation speed is less than the predetermined rotation speed V 1 , the target rotation speeds corresponding to the rotation positions detected from the output signals S 1 , S 2 , S 3 , and S 4  is set in the target speed table. With this configuration, the amount of data of the target speed table can be reduced. 
   (Main Effects of First Embodiment) 
   As described above, in the first embodiment, the rotary encoder  36  has the through holes  34   c  that are provided to correspond to some of all the slits  65  for each predetermined interval. Therefore, a plurality of slits  65  can be formed, without worrying the wastes, such as the ink mist and so on, or the strength. 
   In addition, the rotary encoder  36  outputs four output signals S from the level signals output from the light-receiving elements  69  arranged in four rows on one board  68 . The signals S are generated from the level signal waveforms of the four light-receiving elements A 1  ( 69 ) to A 4  ( 69 ), B 1  ( 69 ) to B 4  ( 69 ), C 1  ( 69 ) to C 4  ( 69 ), and D 1  ( 69 ) to D 4  ( 69 ) arranged at intervals corresponding to one quarter of the light and shade cycle T on the board  68 . Therefore, the output signals S have double the frequency of the level signals and the turning points of all the signals correspond to the turning points of the level signals of the light-receiving elements  69 . In other words, the cycles T 1  to T 4  of the signals S are a half of the cycle TL of the level signal waveform, and the edges E are generated in one-to-one correspondence with the light-receiving elements  69 . The rotary encoder  36  can therefore obtain such a resolution that slits are provided at intervals of one eighth of the interval of the slits  65  on the rotary scale  34 . In other words, the rotary encoder  36  can obtain a resolution of the position and speed eight times higher than that with the slits  65 . 
   As a result, a rotary scale  34  of the same size and accuracy as conventional ones can provide a resolution of the position and speed eight times as high as the conventional ones. In other words, the rotary encoder  36  can output high-resolution output signals S. Also a rotary scale  34  smaller than conventional ones can provide a resolution of the position and speed equal to the conventional ones. 
   In this embodiment, according to the rotation speed of the PF motor  5 , the control of the printer  1  on the basis of the two output signals of the output signal S 1  and the output signal S 3  or the two output signals of the output signal S 2  and the output signal S 4 , or the control of the printer  1  on the basis of the four output signals of the output signals S 1 , S 2 , S 3 , and S 4  is switchably (selectably) performed. For this reason, when the problems due to the high-frequency signals do not occur even through the control is performed using the four output signals S, the control of the printer  1  can be performed with higher resolution on the basis of the four output signals S. Further, in a case where the problems due to the high-frequency signals occur when the control is performed using the four output signals S, the control of the printer  1  can be performed using the two output signal S 1  and the output signal S 3  or the two output signals of the output signal S 2  and the output signal S 4 , whose phases are sifted from each other by an eighth of a brightness cycle T. For this reason, the problems due to the high-frequency signals can be suppressed, and the configuration of a circuit that processes the output signals from the rotary encoder  36  can be simplified. 
   In this embodiment, when the rotation speed of the PF motor  5  is equal to or more than the predetermined speed, or exceeds the predetermined speed, the rotation position and the rotation direction of the PF motor  5  are detected from the two output signals of the output signal S 1  and the output signal S 3  or the two output signals of the output signal S 2  and the output signal output from the rotary encoder  36 , and the control is performed on the basis of the detection result. Further, when the rotation speed of the PF motor  5  is less than the predetermined speed, or is equal to or less than the predetermined speed, the rotation position and the rotation direction of the PF motor  5  are detected from the four output signals S output from the rotary encoder  36 . 
   In case of the PF motor  5 , the positional accuracy of the PF motor  5  is demanded at the time of the stop, not at the time of the rotation. In this embodiment, before the PF motor  5  that rotates the rotation speed less than the predetermined speed or equal to or less than the predetermined speed stops, the rotation position or the rotation direction of the PF motor  5  can be detected from the four output signals S, and the control of the PF motor  5  can be performed on the basis of the detection result. Further, when the PF motor  5  rotates at a speed that is equal to or more than the predetermined speed or exceeds the predetermined speed, the rotation position or the rotation direction of the PF motor  5  is detected from the two output signals, and the control of the PF motor  5  is performed on the basis of the detection result. Even in this case, there is no problem in view of the positional accuracy. 
   In this embodiment, the rotation speed of the PF motor  5  is detected from the four output signals S output from the rotary encoder  36 , regardless of the rotation speed of the PF motor  5 , and the control is performed on the basis of the detection result. For this reason, the accurate control of the PF motor  5  based on the more rotation speed information can be performed. 
   Second Embodiment 
     FIG. 16  is an electric circuit diagram of a rotary encoder  36  according to a second embodiment of the invention; and  FIG. 17  shows signal waveforms generated by the rotary encoder  36  by the normal rotation of a rotary scale  34  according to the second embodiment, wherein (A) shows level signal waveforms amplified by a first amplifier  74  and a third amplifier  76  shown in  FIG. 16 ; (B) shows a signal waveform output from a first-differential-signal generating circuit  78  shown in  FIG. 16 ; (C) shows level signal waveforms amplified by a second amplifier  75  and a fourth amplifier  77  of  FIG. 16 ; (D) shows a signal waveform output from a second-differential-signal generating circuit  79  of  FIG. 16 ; (E) shows a signal waveform output from an exclusive OR circuit  80  shown in  FIG. 16 ; (F) shows a signal waveform output from a row-B-signal generating circuit  71  shown in  FIG. 16 ; (G) shows a signal waveform output from a row-C-signal generating circuit  72  shown in  FIG. 16 ; (I) shows a signal waveform output from a row-D-signal generating circuit  73  shown in  FIG. 16 ; (I) shows a signal waveform output from a first exclusive OR circuit  91  of  FIG. 16 ; and (J) shows a signal waveform output from a second exclusive OR circuit  92  of  FIG. 16 . 
   Although the configurations of the rotary scale  34  of the rotary encoder  36  are identical, the first embodiment and the second embodiment are different in the structure of the electric circuit of the rotary encoder  36 . Because of the difference in the structure of the electric circuit, signals output from the rotary encoder  36  are also different. Since the other structures of the second embodiment are identical to those of the first embodiment, the difference will be principally described. In the second embodiment, components identical to those of the first embodiment are given the same reference numerals and descriptions thereof will be simplified or omitted. Illustrations and descriptions on components identical to those of the first embodiment will be omitted. 
   Referring to  FIG. 16 , the rotary encoder  36  of this embodiment includes the row-A-signal generating circuit  70 , the row-B-signal generating circuit  71 , the row-C-signal generating circuit  72 , and the row-D-signal generating circuit  73  which are described in the first embodiment. The row-A-signal generating circuit  70 , the row-B-signal generating circuit  71 , the row-C-signal generating circuit  72 , and the row-D-signal generating circuit  73  output the output signal S 1 , S 2 , S 3 , and S 4  shown in  FIGS. 17(E) to 17(H) , respectively. In addition, the rotary encoder  36  of this embodiment includes a first output exclusive OR circuit  91  and a second output exclusive OR circuit  92 . 
   The first output exclusive OR circuit  91  receives the signal S 1  output from the row-A-signal generating circuit  70  and the signal S 3  output from the row-C-signal generating circuit  72 . The first output exclusive OR circuit  91  generates a first output exclusive OR signal S 11  that is the exclusive OR of the output signal S 1  and the output signal S 3 , and outputs it. In other words, the first output exclusive OR circuit  91  generates and outputs the first output exclusive OR signal S 11  with a cycle approximately a half of the cycle of the output signals S 1  and S 3 , as shown in  FIG. 17(I) . 
   The second output exclusive OR circuit  92  receives the signal S 2  output from the row-B-signal generating circuit  71  and the signal S 4  output from the row-D-signal generating circuit  73 . The second output exclusive OR circuit  92  generates a second output exclusive OR signal S 12  that is the exclusive OR of the output signal S 2  and the output signal S 4 , and outputs it. In other words, the second output exclusive OR circuit  92  generates and outputs the second output exclusive OR signal S 12  with a cycle approximately a half of the cycle of the output signals S 2  and S 4 , as shown in  FIG. 17(J) . 
   The output signals S 1  and S 2  are out of phase with each other by one sixteenth of the light and shade cycle T. Accordingly, the first output exclusive OR signal S 11  and the second output exclusive OR signal S 12  are also out of phase with each other by one sixteenth of the light and shade cycle T, as shown in  FIGS. 17(I) and 17(J) . 
   The rotary encoder  36  of this embodiment also has four output terminals  81 ,  82 ,  83 , and  84  as in the first embodiment. Referring to  FIG. 15 , the signal S 1  of the row-A-signal generating circuit  70  (the exclusive OR circuit  80 ) is output from the output terminal  81 , while the signal S 3  of the row-C-signal generating circuit  72  is output from the output terminal  82 . The first output exclusive OR signal S 11  output from the first output exclusive OR circuit  91  is output from the output terminal  83 , while the second output exclusive OR signal S 12  output from the second output exclusive OR circuit  92  is output from the output terminal  84 . In place of the output signal S 1  of the row-A-signal generating circuit  70  and the output signal S 3  of the row-C-signal generating circuit  72 , the signal S 2  of the row-B-signal generating circuit  71  and the signal S 4  of the row-D-signal generating circuit  73  may be output from the rotary encoder  36 . 
   As in the first embodiment, the four output terminals  81 ,  82 ,  83 , and  84  connect to the controller  37  via the four signal lines  86 ,  87 ,  88 , and  89 , respectively (refer to  FIG. 8 ). 
   In this embodiment, the signals output from the rotary encoder  36  are different from those from the rotary encoder  36  of the first embodiment. Thus, a method for determining the rotational position and speed and the rotating direction of the PF motor  5  is different from that of the first embodiment. The method for determining the rotational position and speed and rotating direction of the PF motor  5  will be described in sequence. 
   The method for determining the rotational position of the PF motor  5  will first be described. The rotational position of the PF motor  5  is determined by counting the number of the edges E 1  and E 3  of the output signals S 1  and S 3  shown in  FIGS. 17(E) and 17(G) , respectively, or the edges E 11  and E 12  of the first output exclusive OR signal S 1  and the second output exclusive OR signal S 12  shown in  FIGS. 17(I) and 17(J) , respectively. 
   More specifically, in this embodiment, when the PF motor  5  rotates at the rotational speed less than the predetermined rotational speed or equal to or less than the predetermined rotational speed, and the problems due to the high-frequency signals do not occur, a predetermined processing circuit (for example, the ASIC  51 ) that detects the rotational position detects the rotational position of the PF motor  5  by counting the number of the edges E 11  and E 12  of the high-frequency first and second exclusive OR signals S 11  and S 12 . Further, when the PF motor  5  rotates at the rotational speed that is equal to or more than the predetermined rotational speed or exceeds the predetermined rotational speed, and the problems due to the high-frequency signals occur, the predetermined processing circuit that detects the rotational position detects the rotational position of the PF motor  5  by counting the number of the edges E 1  and E 3  of the low-frequency output signals S 1  and S 3 . 
   Thus, in this embodiment, a predetermined processing circuit for determining the rotational position switches (selects) between determining the rotational position using the first output exclusive OR signal S 11  and the second output exclusive OR signal S 12  of high frequency and determining the rotational position using the output signals S 1  and S 3  of low frequency. The switching (selection) of the processing circuit is made according to instruction from the CPU  39  based on the information on the rotational speed of the PF motor  5  and so on, as in the first embodiment. 
   The printer  1  is controlled on the basis of the information on the rotational position of the PF motor  5  determined from the first output exclusive OR signal S 11  and the second output exclusive OR signal S 12  or two output signals S 1  and S 3 . The PID control of the PF motor  5  is made on the basis of the information such as the rotational position of the PF motor  5  determined by the ASIC  51 . 
   Next, the detection method of the rotation direction of the PF motor  5  will be described. The rotation direction of the PF motor  5  is detected from the edge E 1  of the output signal S 1  and/or the edge E 3  of the output signal S 3 , and the output level of the output signal S 3  and/or the output signal S 1  when the edge E 1  and/or the edge E 3  is detected. Alternatively, the rotation direction of the PF motor  5  is detected from the edge E 11  of the first exclusive OR signal S 11  and/or the edge E 12  of the second exclusive OR signal S 12 , and the output level of the second exclusive OR signal S 12  and/or the first exclusive OR signal S 1  when the edge E 11  and/or the edge E 12  is detected. The view for the detection of the rotation direction of the PF motor  5  is the same as the first embodiment, and the specified description thereof will be omitted. 
   In this embodiment, like the detection of the rotation speed, when the PF motor  5  rotates at the rotation speed less than the predetermined rotation speed or equal to or less than the predetermined rotation speed, and the problems due to the high-frequency signals do not occur, a predetermined processing circuit (for example, the ASIC  51 ) that detects the rotation direction detects the rotation direction of the PF motor  5  using the high-frequency first and second exclusive OR signals S 11  and S 12 . Further, when the PF motor  5  rotates at the rotation speed that is equal to or more than the predetermined rotation speed or exceeds the predetermined rotation speed, and the problems due to the high-frequency problems occur, the predetermined processing circuit that detects the rotation direction detects the rotation direction of the PF motor  5  using the low-frequency output signals S 1  and S 3 . 
   In such a manner, in this embodiment, according to the rotation speed of the PF motor  5 , the predetermined processing circuit that detects the rotation direction switches (selects) whether to detect the rotation position using the high-frequency first and second exclusive OR signals S 11  and S 12  or to detect the rotation position using the low-frequency output signals S 1  and S 3 . Switching (selection) at the predetermined processing circuit is performed, for example, by an instruction from the CPU  39  on the basis of the information about the rotation speed of the PF motor  5 . 
   Further, a predetermined control of the printer  1  is performed on the basis of the information about the rotation position of the PF motor  5  detected using the first and second exclusive OR signals S 11  and S 12  or the two output signals S 1  and S 3 . For example, the rotation position of the PF motor  5  is detected on the basis of the information about the rotation direction, and the PID control of the printer  1  is performed on the basis of the detection result. 
   A method for determining the rotational speed of the PF motor  5  will next be described. The rotational speed of the PF motor  5  can be determined using the time (period) from the edge E at which the output signals S 1  and S 3  (or the first output exclusive OR signal S 11  and the second output exclusive OR signal S 12 ) rise (or fall) to the edge E at the next rising (or falling). For example, the rotational speed of the PF motor  5  can be determined using times T 1 , T 3 , T 11 , and T 12  shown in  FIGS. 17(E) ,  17 (G),  17 (I), and  17 (J), respectively. Accordingly, the problems due to high-frequency signals, as described in the first embodiment, do not occur in determining the rotational speed. 
   Thus, in this embodiment, the rotational speed of the PF motor  5  is determined using the first output exclusive OR signal S 11  and the second output exclusive OR signal S 12  of high frequency irrespective of the rotational speed of the PF motor  5 . Thus more rotational-speed information can be obtained from the first output exclusive OR signal S 11  and the second output exclusive OR signal S 12 . 
   The printer  1  is controlled on the basis of the information on the rotational speed of the PF motor  5  determined using the first output exclusive OR signal S 11  and the second output exclusive OR signal S 12 . The PID control of the PF motor  5  made on the basis of the information such as the rotational speed of the PF motor  5  determined by the ASIC  51 . 
   As described above, in the second embodiment, since the structure of the rotary scale  34  is the same as the first embodiment, a plurality of slits  65  can be formed, without worrying the wastes or the strength. In addition, the rotary encoder  36  generates four output signals S 1 , S 2 , S 3 , and S 4  from the level signals output from the light-receiving elements  69  arranged in four rows on one board  68 , of which it outputs two output signal S 1  and S 2 . In this embodiment, the rotary encoder  36  generates the first output exclusive OR signal S 11  having double the frequency of the output signals S 1  and S 3  from the output signals S 1  and S 3  and outputs it, and generates the second output exclusive OR signal S 12  having double the frequency of the output signals S 2  and S 4  from the output signals S 2  and S 4  and outputs it. The rotary encoder  36  can therefore obtain a resolution of position and speed eight times as high as with the slits  65  on the rotary scale  34  using the first output exclusive OR signal S 11  and the second output exclusive OR signal S 12 . 
   As a result, the rotary scale  34  of the same size and accuracy as conventional ones can obtain a resolution of the position and speed eight times as high as the conventional ones. In other words, the rotary encoder  36  can output high-resolution output signals. Also a rotary scale  34  smaller than conventional ones can obtain a resolution of the position and speed equal to the conventional ones. 
   In the second embodiment, according to the rotation speed of the PF motor  5 , the control of the printer  1  on the basis of the high-frequency first and second exclusive OR signals S 11  and S 12  or the control of the printer  1  on the basis of the low-frequency output signals S 1  and S 3  is switchably (selectably) performed. For this reason, when the problems due to the high-frequency signals do not occur even though the control is performed on the basis of the high-frequency first and second exclusive OR signals S 11  and S 12 , a predetermined control of the printer  1  can be performed with higher resolution on the basis of the first exclusive OR signal S 11  and the second exclusive OR signal S 12 . In addition, when the problems due to the high-frequency signals occur, the control of the printer  1  can be performed on the basis of the output signal S 1  and the output signal S 3 , whose phases are sifted from each other by an eighth of the brightness cycle T. For this reason, the problems due to the high-frequency signals can be suppressed, and the configuration of a circuit that processes the output signals from the rotary encoder  36  can be simplified. 
   In the second embodiment, when the rotation speed of the PF motor  5  is equal to or more than the predetermined speed or exceeds the predetermined speed, the rotation position and the rotation direction of the PF motor  5  are detected from the high-frequency first and second exclusive OR signals S 11  and S 12 , and the control is performed on the basis of the detection result. Further, when the rotation speed of the PF motor  5  is less than the predetermined speed or is equal to or less then the predetermined speed, the rotation position and the rotation direction of the PF motor  5  are detected from the low-frequency output signals S 1  and S 3 , and the control is performed on the basis of the detection result. 
   In case of the PF motor  5 , the positional accuracy of the PF motor  5  is demanded at the time of the stop, not at the time of the rotation. In this embodiment, before the PF motor  5  that rotates at the rotation speed less than the predetermined speed or equal to or less than the predetermined speed stops, the rotation position or the rotation direction of the PF motor  5  is detected from the high-frequency first and second exclusive OR signals S 11  and S 12 , and the control of the PF motor  5  can be performed on the basis of the detection result. Therefore, the positional accuracy of the PF motor  5  at the time of the stop can be increased. Further, when the PF motor  5  rotates at the rotation speed that is equal to or more than the predetermined speed or exceeds the predetermined speed, the rotation position or the rotation direction of the PF motor  5  is detected from the low-frequency output signals S 1  and S 3 , and the control of the PF motor  5  is performed on the basis of the detection result. Even in this case, there is no problem in view of the positional accuracy. 
   Other Embodiments 
   While preferred embodiments of the invention have been described, it is to be understood that the invention is not limited to those but various modifications and changes may be made without departing from the spirit and scope of the invention. 
   In the above-described embodiments, the rotary encoder  36  includes the rotary scale  34  having the transparent main body portion  34   a  formed of PET, the marks  34   b  attached to one surface of the main body portion  34   a , and the through holes  34   c  formed in some of the slits  65 . However, the main body portion  34   a  may be formed of transparent resin or a glass substrate, in addition to PET. Further, the marks  34   b  may be formed on both surfaces of the main body portion  34   a , not one surface thereof. Further, the marks  34   b  are attached by deposition, such as sputtering or the like, or printing, but the marks  34   b  may be provided by plating or exposure using a resist. In addition, in case of using a method of printing the marks  34   b , in addition to printing by an ink jet printer, other general printing methods can be used. Alternatively, the marks  34   b  may be buried in the main body portion  34   a.    
   The through holes  34   c  may be provided at irregular intervals, not at regular intervals. For example, two through holes  34   c  may be successively provided, and then another two through holes  34   c  may be successively provided at an interval from the two through holes  34   c . Further, the through holes  34   c  may be provided only in a predetermined angular range of the rotary scale  34 , not in other angular ranges. In addition, in the above-described embodiments, each of the through holes  34   c  is a straight hole having the same width from the top to the bottom. However, each of the through holes  34   c  may be formed such that a side close to the mark  34   b  is wider and an opposing side is narrower or vice versa. 
   In addition, the rotary encoder  36  includes the disc-shaped rotary scale  34  and the sensor  35  that senses the light passing through the slits  65  formed along the outer periphery thereof. Alternatively, the rotary encoder  36  may be of a reflection type that detects light reflected by a plurality of marks formed along the outer periphery of the rotary scale  34 . 
   The structure of the invention may be applied to the linear encoder  33  that determines the rotational speed and position of the CR motor  4 . Specifically, the linear encoder  33  may be constructed such that a plurality of light-receiving elements is arranged on a board to which the light from light-emitting elements is reflected by the marks  31   a , as in  FIG. 12 , and the level signals of the light-receiving elements are integrated together through the circuit shown in  FIG. 13  or  16 . This arrangement enables the linear encoder  33  to output a plurality of signals with a resolution higher than that of the marks  31   a . The encoder may not necessarily be of an optical type but may be of magnetic or another type. 
   In the foregoing embodiments, the rotary encoder  36  outputs one output signal from the level signals of, e.g., the four (=2 2 ) light-receiving elements A 1  ( 69 ) to A 4  ( 69 ). Alternatively, the rotary encoder  36  may generate one output signal from the level signals of 2n+1 (n is an integer of 1 or above) sets of light-receiving elements  69 , in which case the frequency of the output signal is 2n times that of the level signals of the light-receiving elements  69 . In this case, for example, the light-receiving elements  69  in row A and the light-receiving elements  69  in row C may be disposed on the board  68  with a displacement of one 2n+2th of the light and shade cycle T, and the light-receiving elements  69  in row B and the light-receiving elements  69  in row D may be disposed on the board  68  with a displacement of one 2n+2th of the light and shade cycle T. 
   In the foregoing embodiments, the four light-receiving elements A 1  ( 69 ) to A 4  ( 69 ), B 1  ( 69 ) to B 4  ( 69 ), C 1  ( 69 ) to C 4  ( 69 ), and D 1  ( 69 ) to D 4  ( 69 ) are disposed next to each other in the range corresponding to the light and shade cycle T. However, they may not necessarily be disposed next to each other. For example, the first second light-receiving element A 2  ( 69 ), the third light-receiving element A 3  ( 69 ), and the fourth light-receiving element A 4  ( 69 ) in row A may be disposed in a position in which a distance integer times of the light and shade cycle T is added to the first position shown in  FIG. 11 . The same arrangement is possible for rows B, C, and D. Furthermore, while rows A, B, C, and D are arranged with a displacement of one sixteenth of the light and shade cycle T with each other, they may be displaced at a pitch in which a distance integer times of the light and shade cycle T is added to one sixteenth of the light and shade cycle T. 
   While the foregoing embodiments use the four light-receiving elements A 1  ( 69 ) to A 4  ( 69 ), B 1  ( 69 ) to B 4  ( 69 ), C 1  ( 69 ) to C 4  ( 69 ), and D 1  ( 69 ) to D 4  ( 69 ) to generate the signals S, for example, the output signal S 1  may be generated only with the first light-receiving element A 1  ( 69 ). Specifically, the output signal S 1  can be generated by generating a signal displaced from the signal detected by the first light-receiving element A 1  ( 69 ) by one half, one quarter, and three quarters, and inputting them to the amplifiers  74 ,  75 ,  76 , and  77 . The signals S 2 , S 3 , and S 4  can be generated similarly. 
   In the foregoing embodiments, the output-signal generating circuits  70 ,  71 ,  72 , and  73  of four rows output signals that change at a duty of approximately 50%. Alternatively, the output-signal generating circuits  70 ,  71 ,  72 , and  73  may output at a duty other than 50%, in which case the four light-receiving elements A 1  ( 69 ) to A 4  ( 69 ) may be disposed at intervals with a displacement other than one quarter of the light and shade cycle T, or at intervals in which a displacement integer times of the light and shade cycle T is added to the displacement. 
   In the first embodiment described above, according to the rotation speed of the PF motor  5 , the control of the printer  1  on the basis of the two output signals or the control of the printer  1  on the basis of the four output signals is switchably performed. Further, in the second embodiment, according to the rotation speed of the PF motor  5 , the control of the printer  1  on the basis of the high-frequency first exclusive OR circuit S 11  and so on or the control of the printer  1  on the basis of the low-frequency output signal S 1  and so on is switchably performed. Besides, according to the rotation position of the PF motor  5 , it may be configured on the basis of which signals to switchably perform the control of the printer  1 . 
   For example, as shown in  FIG. 6 , when the rotation position of the PF motor  5  is in a range of the target stop position X from a predetermined rotation position X 1  before the PF motor  5  stops (that is, in a range of a predetermined range from the target stop position X) or when the rotation position of the PF motor  5  is out of the range, it may be configured on the basis of which signals to switchably perform the control of the printer  1 . 
   More specifically, when the rotation position of the PF motor  5  is in the predetermined range from the target stop position X of the PF motor  5 , the rotation position or the rotation direction of the PF motor  5  is detected from the four output signals S or from the high-frequency first and second exclusive OR signals S 11  and S 12 , and the control of the printer  1  is performed on the basis of the detection result. Further, when the rotation position of the PF motor  5  is out of the predetermined range from the target stop position X of the PF motor  5 , the rotation position or the rotation direction of the PF motor  5  is detected from the two output signals S, and the control of the printer  1  is performed on the basis of the detection result. With this configuration, the positional accuracy of the PF motor  5  at the time of the stop can be increased. Further, when the rotation position of the PF motor  5  is out of the predetermined range from the target stop position X of the PF motor  5 , a processing at the control unit  37  is simplified. 
   In each of the embodiments described above, as for the detection of the rotation speed of the PF motor  5 , all the four output signals S or the high-frequency first and second exclusive OR signals S 11  and S 12  are used, regardless of the rotation speed of the PF motor  5 . Besides, according to the rotation speed of the PF motor  5 , the signals to be used for the detection of the rotation speed of the PF motor  5  may be switched. For example, when the PF motor  5  rotates at a speed less than a predetermined rotation speed or equal to or less than the predetermined rotation speed, the rotation speed of the PF motor  5  is detected using the four output signals S. Meanwhile, when the PF motor  5  rotates at a speed that is equal to or more than the predetermined rotation speed or exceeds the predetermined rotation speed, the rotation speed of the PF motor  5  may be detected using the two signals of the output signals S 1  and S 3  or the two signals of the output signals S 2  and S 4 . Further, when the PF motor  5  rotates at a speed less than the predetermined rotation speed or equal to or less than the predetermined rotation speed, the rotation speed of the PF motor  5  is detected using the high-frequency first and second exclusive OR signals S 11  and S 12 . Meanwhile, when the PF motor  5  rotates at a speed that is equal to or more than the predetermined rotation speed or exceeds the predetermined rotation speed, the rotation speed of the PF motor  5  may be detected using the low-frequency output signals S 1  and S 3 . 
   In the above-described embodiments, the configuration of the invention has been described by way of the printer  1 . However, the encoder of the invention can be applied various fields, such as robots, machine tools, measurement, medical instruments, OA instruments, and so on. In addition, the arrangement of the invention can also be applied to multifunction printers, scanners, automatic document feeders (ADFs), copiers, facsimiles and so on.