Abstract:
A digital encoder control method of a digital control apparatus has a driving unit, a frequency signal generating unit for generating a pulse signal of a frequency according to a driving velocity of a driven member which is driven by the driving unit, an edge detecting unit for detecting a rising-up edge and a falling-down edge of the pulse signal, and a period data detecting unit for counting period data between edges detected by the edge detecting unit. Each time the edges are detected by the edge detecting unit, the period data between the same edges as the detected edges is outputted as control velocity data of the driving unit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method of updating velocity data in case of performing a servo control on the basis of a digital encoder signal. More particularly, the invention relates to a digital encoder control method suitable for applying to a positioning servo for controlling a feed position by a paper feed control of a recording apparatus. 
     2. Related Background Art 
     An ink jet recording apparatus is widely used as a means which is installed into a printer, a facsimile, or a copying apparatus and records an image (including characters and symbols) onto a recording medium such as paper, plastic thin sheet (OHP or the like), or the like on the basis of image data. 
     The ink jet recording apparatus performs the recording by emitting an ink droplet from a recording head onto the recording medium and has features such that a compact size of recording means can be easily realized, an image of a high precision can be recorded at a high speed, running costs are low, and noises are small since it is a non-impact type. The ink jet recording apparatus also has an advantage such that a color image can be easily recorded by using inks of multicolors. 
     As driving sources of the ink jet recording apparatus, there are a carriage motor for reciprocatively driving a carriage on which the recording head is mounted, an ASF motor for feeding the recording medium, a recovery system motor for performing a head cleaning or the like, a paper feed motor for feeding the recording medium every recording scan, and the like. Hitherto, a stepping motor has often been used as such a driving source because low costs can be easily realized, a control is simple, and the like. 
     The noise level of the ink jet recording apparatus is low because it is a non-impact type. However, the usage of a DC motor as such a driving source is increasing for the purpose of realizing a further silent operation or the like. In this case, an encoder is generally used for obtaining control data of the DC motor. 
     FIG. 6 shows a model diagram of the encoder. According to the encoder, a detector  703  detects light emitted from an LED  701  through a code wheel  702  and generates a signal. Light transmitting portions  704  and light non-transmitting portions  705  are arranged on the code wheel  702  at predetermined intervals. Photodiodes  706 ,  707 ,  708 , and  709  are arranged in the detector  703  at predetermined intervals. The light detected by the photodiodes  706 ,  707 ,  708 , and  709  is converted into an electric signal A  710 , an electric signal *A  711 , an electric signal B  712 , and an electric signal *B  713 , which are output. The outputted electric signals  710 ,  711 ,  712 , and  713  are outputted as differential outputs, Channel A  716  and Channel B  717 , by comparators  714  and  715 , respectively. 
     FIG. 7 shows waveforms of the differential output signals. A signal which is inverted at a cross point of an electric signal A  801  and an electric signal *A  802  becomes a Channel A  803 . When a velocity is constant, a duty ratio of the Channel A  803  is ideally equal to 50%. However, the duty ratio changes due to various factors. An important one of the factors is a sensitivity difference of the photodiodes. 
     FIG. 8 shows waveforms of the differential output signals in the case where there is a difference between the sensitivities of the photodiodes. The sensitivity of the photodiode appears as an amplitude difference of the electric signals. FIG. 8 shows a Channel A  903  in the case where an amplitude of an electric signal A  901  is smaller than that of an electric signal *A  902 . It will be understood from FIG. 8 that the sensitivity difference of the photodiodes changes a duty ratio of an output signal. However, it does not exert an influence on a period of the Channel A. Because of the reasons as mentioned above, generally, the period of the output signal of the encoder has the highest precision. 
     Position data and velocity data are obtained as control data of the DC motor from the encoder signal. To obtain more accurate data as velocity data, generally, a one-edge sampling system for counting and employing, for example, a period in a range from a rising-up edge to another rising-up edge of an output signal is used. 
     However, when the velocity data is obtained by the one-edge sampling system, the velocity data is not updated unless the output signal of the encoder elapses one period. That is, the number of times of update of the velocity data is only ½ as compared to that of a both-edge sampling system and is only ¼ as compared to that in case of sampling both edges of two phases, i.e. the Channel A and the Channel B. 
     For example, when considering a paper feed control of the ink jet recording apparatus, the paper is fed at a high speed at the beginning and a servo control is performed at a low speed from a position that is slightly before a stop position. After that, the control mode is shifted to a stop mode at a position just before a target stop position, thereby stopping the paper at the target position. In this case, the operation for stabilizing the low-speed servo control at the position which is slightly before the stop position, largely influences on a stop precision of the paper. 
     As mentioned above, when the paper is fed at a low speed, a change in encoder signal naturally becomes slow and an updating interval of the velocity data in the one-edge sampling system also becomes long. Therefore, a case where the velocity data obtained at a servo period is not updated from previously obtained velocity data occurs, and thus a problem such that the servo operation is not stable occurs. 
     If the both-edge sampling system or the like is used in order to solve the above problem, although the updating interval of the velocity data becomes short, the precision of the velocity data deteriorates due to the above-mentioned reasons, and thus a problem such that the servo operation is not stable similarly occurs. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a digital encoder control method which can realize the stable servo operation of a short velocity data updating interval at a precision that is equal to that of a conventional one-edge sampling system. 
     Another object of the invention to provide a digital encoder control method comprising the steps of: arranging driving means; arranging frequency signal generating means for generating a pulse signal of a frequency according to a driving velocity of a driven member which is driven by the driving means; arranging edge detecting means for detecting a rising-up edge and a falling-down edge of the pulse signal; and arranging period data detecting means for counting period data indicative of a period between the edges detected by the edge detecting means, wherein each time the edges are detected by the edge detecting means, the period data of the period between the same edges as the detected edges is outputted as control velocity data of the driving means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a whole perspective view showing a schematic structure of a serial type ink jet printer as an embodiment of the invention; 
     FIG. 2 is a block diagram for explaining the first embodiment of the invention; 
     FIG. 3 is a timing chart for explaining the first embodiment of the invention; 
     FIG. 4 is a block diagram for explaining the second embodiment of the invention; 
     FIG. 5 is a timing chart for explaining the second embodiment of the invention; 
     FIG. 6 is a model diagram of a digital encoder; 
     FIG. 7 is a diagram of waveforms of encoder differential outputs; and 
     FIG. 8 is a diagram of waveforms of encoder differential outputs in the case where there is a sensitivity difference between photodiodes. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     A preferred embodiment of the invention will now be described in detail hereinbelow with reference to the drawings. An example of a serial type ink jet printer in which a recording head having a detachable ink tank has been mounted will be described here. 
     FIG. 1 is a whole perspective view showing a schematic structure of the serial type ink jet printer in the first embodiment of the invention. In the diagram, reference numeral  1  denotes a recording head having an ink tank and  2  indicates a carriage on which the recording head  1  is mounted. 
     A guide shaft  3  is inserted into a bearing portion of the carriage  2  in a state where it is slidable in the main scanning direction. Both edges of the shaft  3  are fixed to a chassis  14 . A driving force of a driving motor  5  is transferred to the carriage  2  through a belt  4  as carriage drive transfer means which is in engagement with the carriage  2 , so that the carriage  2  moves in the main scanning direction. 
     In a recording standby mode, recording papers  15  are stacked onto a paper feed base  6 . At the start of the recording, the recording papers are fed one by one by a paper feed roller (not shown). To convey the fed recording paper, a conveying roller  10  is rotated by a driving force of a conveying motor  7  as a DC motor through a gear train (motor gear  8 , conveying roller gear  9 ) as transfer means. The recording paper  15  is conveyed only by a proper feed amount by the conveying roller  10  and pinch rollers  11  which are rotated with being pressed onto the conveying roller  10  by a pinch roller spring (not shown). 
     Slits of a code wheel (rotary encoder film  16 ) inserted into the conveying roller gear  9  with a pressure are detected and counted by an encoder sensor  17 , whereby a conveying amount is managed, and thus a feed amount may be controlled at a high precision. 
     FIG. 2 shows a block diagram for explaining the first embodiment of the invention. In FIG. 2, by the driving of a motor  115 , an encoder  101  outputs two signals of phase A and phase B to an encoder signal control unit  102 . An edge detecting unit  103  for detecting edges of the encoder signals is provided in the encoder signal control unit  102 . In the edge detecting unit  103 , each edge of each phase is detected, that is, a rising-up edge detection  104  of phase A, a falling-down edge detection  105  of phase A, a rising-up edge detection  106  of phase B, and a falling-down edge detection  107  of phase B are independently performed, so that a signal synchronized with each edge is generated. 
     The signals synchronized with the respective edges are sent to edge interval counters  108 ,  109 ,  110 , and  111 , respectively. Each edge interval is independently counted. The edge detection signals are sent from the edge detecting unit  103  to the edge interval counters  108 ,  109 ,  110 , and  111  and each time the edge interval is determined, the velocity data in a velocity data storing unit  112  is overwritten. When a servo period of a predetermined interval comes, a servo controller  113  reads (Read) the velocity data storing unit  112  in order to obtain the velocity data necessary for the servo control. On the basis of the obtained velocity data, position data, and the like, the servo controller  113  executes an arithmetic operation and outputs optimum motor control data to a motor driver  114 . The motor driver  114  outputs a driving signal to the motor  115  in accordance with the inputted control data, thereby driving the motor  115 . 
     FIG. 3 shows a timing chart for explaining the first embodiment of the invention. With respect to phase A  301  and phase B  302  of the encoder signal, the following edge detection signals are generated: that is, a phase A rising-up edge detection signal  303  synchronized with a rising-up edge of phase A  301 ; a phase B rising-up edge detection signal  304  synchronized with a rising-up edge of phase B  302 ; a phase A falling-down edge detection signal  305  synchronized with a falling-down edge of phase A  301 ; and a phase B falling-down edge detection signal  306  synchronized with a falling-down edge of phase B  302 , respectively. An interval of each edge signal is independently counted by each counter (not shown). 
     If a rising-up edge interval (a) of phase A is determined due to the generation of the phase A rising-up edge detection signal  303 , the counter (not shown) which counts the phase A rising-up edge interval is reset and the determined phase A rising-up edge interval (a) is latched into a velocity data latch  307 . Subsequently, if a rising-up edge interval (b) of phase B is determined due to the generation of the phase B rising-up edge detection signal  304 , the counter (not shown) which counts the phase B rising-up edge interval is reset and the determined phase B rising-up edge interval (b) is overwritten into the velocity data latch  307 . After that, if the phase A falling-down edge detection signal  305  is generated, the counter (not shown) which counts a phase A falling-down edge interval is reset, a falling-down edge interval (c) of phase A is overwritten into the velocity data latch  307 . If the phase B falling-down edge detection signal  306  is generated, the counter (not shown) which counts a phase B falling-down edge interval is reset, and a falling-down edge interval (d) of phase B is overwritten into the velocity data latch  307 . If the phase A rising-up edge detection signal  303  is generated again, the counter (not shown) which counts the phase A rising-up edge interval is reset, and a determined rising-up edge interval (e) of phase A is latched into the velocity data latch  307 . The operation similar to that mentioned above is repeated after that. 
     When the servo controller requests the reading of the velocity data at a timing of a servo period  308 , velocity data (d) in the velocity data latch  307  is latched into a Read Latch  309 . The servo controller reads the velocity data in the Read Latch  309 . 
     As will be obviously understood from FIG. 3, according to the invention, the velocity data is updated four times for one period of time of the encoder and the velocity data certainly becomes the count data between the same edges. Thus, even if the servo control is performed at a low speed, the stable servo control can be performed. 
     Second Embodiment 
     FIG. 4 shows a block diagram for explaining the second embodiment of the invention. By the driving of a motor  211 , an encoder  201  outputs two signals of phase A and phase B to an encoder signal control unit  202 . An edge detecting unit  203  for detecting edges of the encoder signal is provided in the encoder signal control unit  202 . The edge detecting unit  203  generates a signal synchronized with each edge each time each edge of each phase is detected, that is, each time a rising-up edge and a falling-down edge of phase A and a rising-up edge and a falling-down edge of phase B are detected. The signals synchronized with the edges are sent to an edge interval detecting unit  204  and edge intervals are counted. 
     When the edge detection signals are outputted from the edge detecting unit  203  and the edge interval is determined, the velocity data latched so far in the edge interval detecting unit  204  is latched into a first edge interval history unit  205 . Similarly, the velocity data latched in the first edge interval history unit  205  is latched into a second edge interval history unit  206 . The velocity data latched in the second edge interval history unit  206  is latched into a third edge interval history unit  207 . 
     Each velocity data latched in the edge interval detecting unit  204 , first edge interval history unit  205 , second edge interval history unit  206 , and third edge interval history unit  207  is sent to an edge interval adder unit  208 . A result of addition of four velocity data is stored as velocity data at that time. That is, in the edge interval adder unit  208 , the velocity data is updated each time the edge detection signals are outputted from the edge detecting unit  203 . 
     A servo controller  209  reads the velocity data in the edge interval adder unit  208  when the servo period of a predetermined interval comes. On the basis of the obtained velocity data, position data, and the like, the servo controller  209  executes an arithmetic operation and outputs the optimum motor control data to a motor driver  210 . The motor driver  210  outputs a driving signal to the motor  211  in accordance with the inputted control data, thereby driving the motor  211 . 
     Although the addition of the velocity data is executed in the encoder signal control unit  202 , the servo controller  209  can also read the velocity data latched in each of the edge interval detecting unit  204 , first edge interval history unit  205 , second edge interval history unit  206 , and third edge interval history unit  207  and add them in a software manner. 
     FIG. 5 shows a timing chart for explaining the second embodiment of the invention. With respect to phase A  401  and phase B  402  of the encoder signal, each time each edge of each phase is detected, an edge detection signal  403  synchronized with each edge is generated. Intervals between the edge signals are counted by a counter (not shown). When the edge interval (a) is determined due to the generation of the edge detection signal  403 , the counter (not shown) which counts the edge intervals is reset and the determined edge interval (a) is latched into a velocity data latch  404 . When the edge detection signal  403  is generated again and the edge interval (b) is determined, the counter which counts the edge intervals is reset and the determined edge interval (b) is overwritten into the velocity data latch  404 . The velocity data (a) is shifted to a first edge interval history  405 . 
     After that, when the edge detection signal  403  is generated again, the counter is reset. The edge interval (c) is latched into the velocity data latch  404 , the velocity data (b) is latched into the first edge interval history  405 , and the velocity data (a) is latched into a second edge interval history  406 , respectively. When the edge detection signal  403  is further generated, the counter is reset. The edge interval (d) is latched into the velocity data latch  404 , the velocity data (c) is latched into the first edge interval history  405 , the velocity data (b) is latched into the second edge interval history  406 , and the velocity data (a) is latched into a third edge interval history  407 , respectively. 
     In a manner similar to that mentioned above, each time the edge detection signal  403  is generated, the latching operation is repeated. Each time the edge detection signal  403  is generated, a result of an addition of values in the velocity data latch  404 , first edge interval history  405 , second edge interval history  406 , and third edge interval history  407  is stored into a velocity addition latch  408 . 
     When the servo controller requests the reading of the velocity data at the timing of a servo period  409 , the data in the velocity addition latch  408  is latched into a Read Latch  410 . The servo controller reads the velocity data in the Read Latch  410 . 
     As will be obviously understood from FIG. 5, according to the invention, the velocity data is updated four times for one period of time of the encoder. The velocity data certainly becomes count data between the same edges. Thus, even if the servo control is executed at a low speed, the stable servo control can be performed. 
     As described above, according to the embodiment, when the velocity data is obtained from the output signal from the digital encoder, the velocity data updating interval is set to a short interval which is ½ or ¼ as large as that of the one-edge sampling system at a precision that is equivalent to that obtained in case of the one-edge sampling system. Particularly, the servo operation in case of performing the servo control at a low speed can be stabilized.