Abstract:
A position detecting device in which a count value is incremented or decremented on the basis of a pulse output from an encoder every predetermined shift amount of a control target to thereby detect a current value representing a current position of the control target as the count value, is characterized by comprising: initialization signal output means for outputting an initialization signal representing an initialization position at plural positions of a movable range of the control target when the movable range of the control target is restricted, or at plural positions of an operating range of integer times of a period when the movable range of the control target is not restricted and the same state is repeated at the period concerned; current value storing means for storing, in association with identification information for identifying each initialization signal, the current value of the control target at each position where the initialization signal is output by each initialization signal output means; and count value setting means for moving the control target to a position where some initialization signal is output from the initialization signal output means at the initialization time of the count value, and achieving the current value corresponding to identification information for identifying the initialization signal from the current value storage means, and setting the current value thus achieved as the count value.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Technical Field of the Invention  
         [0002]     The present invention relates to a position detecting device, and particularly to a position detecting device used to detect the position of a control target in a servo device.  
         [0003]     2. Description of the Related Art  
         [0004]     A servo device (servo circuit) for servo-driving a motor in accordance with an instruction signal from a controller has been hitherto used in a lens driving operation of a television lens, a pan/tilt driving operation of a camera platform for supporting the television camera or the like. The servo circuit uses a position detecting sensor for detecting the position of a control target to be driven by the motor, and a potentiometer or encoder has been hitherto used as such a position detecting sensor. JP-A-7-230031 describes a position detecting device using a potentiometer.  
         [0005]     However, in connection with a case where a potentiometer is used as a position detecting sensor and a case where an encoder is used as a position detecting sensor, the servo circuit required to have high-precision control has the following drawbacks, respectively. That is, when the potentiometer is used as the position detecting sensor, a high-precision potentiometer is needed, which increases the cost. Furthermore, even the high-precision potentiometer has a limitation in the linearity of an output voltage to a detected position (detected angle), and thus the precision of the position detection is restricted. Furthermore, when digital processing is needed, it is required to convert the output voltage of the potentiometer to a digital signal by an A/D converter, and thus the A/D converter is also required to have high precision, resulting in increase in cost.  
         [0006]     On the other hand, for example when an incremental type encoder (hereinafter referred to merely as encoder) is used as the encoder, it can carry out high-precision position detection cheaply without any drawback imposed on the potentiometer. However, in order to make the count value of the number of pulses output from the encoder the absolute value (the value indicating the absolute value) corresponding to the position, it is required to carry out an original-point returning operation in which a control target is temporarily moved to the end of a movable range when power is turned on and the count value is initialized to zero, or when the encoder is an encoder with a zero position signal (Z-phase), the control target is moved until the zero position signal thereof is detected and the count value is initialized to zero. Therefore, there is a drawback that a time needed for the original-point returning operation is needed when power is turned on.  
         [0007]     An absolute type encoder for outputting the absolute value corresponding to the position as in the case of the potentiometer is known as needing no original-point returning operation. When the absolute type encoder is used, it is required to have high resolution, and thus it has a drawback that the cost is increased. Furthermore, there is also a limit in the resolution of the absolute type encoder, and also it needs parallel outputs. Therefore, it has a drawback that there are required cables and input terminals whose numbers correspond to the number of bits. The present invention has been implemented in view of the foregoing situation.  
       SUMMARY OF THE INVENTION  
       [0008]     An object of the invention is to provide a position detecting device which can carry out high-precision and high-resolution position detection by using an incremental type encoder and also carry out initialization processing in a short time when power is turned on, so that position detection can be effectively performed just after the power is turned on.  
         [0009]     The object can be attained by adoption of the following constitution, thereby achieving the invention.  
         [0010]     (1) A position detecting device in which a count value is incremented or decremented on the basis of a pulse output from an encoder every predetermined shift amount of a control target to thereby detect a current value representing a current position of the control target as the count value, is characterized by comprising: initialization signal output means for outputting an initialization signal representing an initialization position at plural positions of a movable range of the control target when the movable range of the control target is restricted, or at plural positions of an operating range of integer times of a period when the movable range of the control target is not restricted and the same state is repeated at the period concerned; current value storing means for storing, in association with identification information for identifying each initialization signal, the current value of the control target at each position where the initialization signal is output by each initialization signal output means; and count value setting means for moving the control target to a position where some initialization signal is output from the initialization signal output means at the initialization time of the count value, and achieving the current value corresponding to identification information for identifying the initialization signal from the current value storage means, and setting the current value thus achieved as the count value.  
         [0011]     According to the present invention, the incremental type encoder is used, and thus the position detection can be carried out inexpensively with high precision and high resolution. Furthermore, in the initialization of the count value of the encoder when power is turned on, the control target is moved to a position at which a desired initialization signal is output without moving the control target greatly, whereby the count value can be properly set to the predetermined value corresponding to the position of the control target. Therefore, the initialization of the count value can be performed in a short time, and the position detection can be effectively carried out just after power is turned on.  
         [0012]     (2) The position detecting device is preferably characterized in that in the invention of (1), each initialization signal output from the initialization signal output means is a zero position signal output from the encoder. There is an encoder outputting a Z-phase pulse representing a zero position signal, and the zero position signal is used as the initialization signal.  
         [0013]     (3) The position detecting device is preferably characterized in the invention of (1) or ( 2 ), the identification information for identifying each initialization signal output from the initialization signal output means is an output value of a position detecting sensor for outputting a signal whose value corresponds to the position of the control target, and also an output value when each initialization signal is output. The output value of the position detecting sensor for outputting the absolute value corresponding to the position is used as the identification information for identifying the initialization signal. The position detecting sensor may be designed to have such a precision level that it can identify the initialization signal, and thus it is unnecessary to use a high-precision and high-resolution. Accordingly, there is little disadvantage that the cost is increased.  
         [0014]     (4) The position detecting device is preferably characterized in the invention of (3), a potentiometer or an absolute type encoder may be used as the position detecting sensor.  
         [0015]     (5) The position detecting device is preferably characterized in that in the invention of (3), the count value setting means detects the output value of the position detecting sensor when initialization of the count value is started, and moves the control target to a position where an initialization signal identified by the nearest value out of the values of the identification information stored in the current value storage means is output. When the position detecting sensor is used as in the case of (3), the control target can be moved to the nearest position at which the initialization signal is output to initialize the count value by detecting the current value of the control target (which is not necessarily coincident with the original current value) with the position detecting sensor when the initialization of the count value is started. Accordingly, the initialization of the count value can be carried out in a shorter time.  
         [0016]     (6) The position detecting device is preferably characterized in that in any one of (1) to (5), it further comprises initial setting means for storing a count value as the current value of the control target into the current storage means along with identification information, the count value concerned corresponding to a count value achieved when an initialization signal is output by the initialization signal output means while moving the control target after the control target is moved to a predetermined position and the count value is set to zero when data to be stored in the current value storage means are created. The present invention is equipped with the means for storing in advance the current value when the initialization signal is output.  
         [0017]     (7) The position detecting device is preferably characterized in that the invention according to any one of (1) to (6) is applied to position detection of a control target in a servo device.  
         [0018]     (8) The position detecting device is preferably characterized in that the invention according to any one of (1) to (6) is applied to position detection for a pan operation or a tilt operation in a camera platform for supporting a television camera and carrying out the pan operation or the tilt operation on the television camera.  
         [0019]     According to the present invention, the position detection is carried out by using the incremental type encoder, and thus the position detection can be performed inexpensively with high precision and high resolution. Furthermore, the initialization of the count value of the encoder can be carried out in a short time. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is a diagram showing one embodiment of the construction of a servo circuit to which the present invention is applied.  
         [0021]      FIGS. 2A  to C are diagrams showing output signals of a rotary encoder, and  FIG. 2D  is a diagram showing an output signal of a potentiometer.  
         [0022]      FIG. 3  is a flowchart showing the procedure of initialization.  
         [0023]      FIG. 4  is a diagram showing an example of an output voltage of the potentiometer and the output timing of a Z-phase pulse with respect to a movable range of a control target.  
         [0024]      FIG. 5  is a diagram showing corresponding data generated in the initialization.  
         [0025]      FIG. 6  is a flowchart showing the processing procedure of initialization of an A/B-phase count value. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]     Preferred embodiments of a position detecting device according to the present invention will be described hereunder with reference to the accompanying drawings.  
         [0027]      FIG. 1  is a diagram showing the construction of a servo circuit to which the present invention is applied. The servo circuit shown in  FIG. 1  controls a desired control target  18  by a motor  16 , and it sets the position of the control target  18  as a control amount and controls the control target  18  so that the control amount is equal to a target value supplied by an instruction signal. An application range of this servo circuit is not limited to a specific field. Particularly when it is applied to a camera platform system for supporting a television system or television camera, a movable lens group of the television lens, a pan mechanism of the camera platform system, a tilt mechanism, etc. are control targets, and an instruction signal is supplied from a predetermined controller.  
         [0028]     In  FIG. 1 , CPU  10  is supplied with the instruction signal, and a target value indicating a position to which the control target  18  should be moved is given by the instruction signal. A control signal for driving the motor  16  is output from CPU  10  to a D/A converter  12 , and the control signal is converted from a digital value to an analog value (voltage value) by the D/A converter  12  and then output to a driving circuit  14 . The driving circuit  14  applies the driving voltage corresponding to the voltage of the control signal to the motor  16 , whereby the motor  16  is driven in a rotational direction and at a speed, the rotational direction and the speed corresponding to the value of the control signal output from CPU  10 . The control target  18  is connected to the output shaft of the motor  16  through a gear train or the like, and operates interlockingly with the motor  16 .  
         [0029]     A rotary encoder  20  is an incremental type rotary encoder, and it is provided as a position detecting sensor for detecting the current position of the control target  18  with high precision and high resolution. The rotational shaft of the rotary encoder  20  is connected to rotational shaft of the motor  16  or the control target  18  directly or through a gear train or the like. The output signal of the rotary encoder  20  comprises A-phase, B-phase and Z-phase as shown in  FIGS. 2A, 2B ,  2 C (the abscissa axis represents a rotational angle). For example, as shown in  FIGS. 2A and 2B , a predetermined number of pulses per revolution are output from the A-phase and the B-phase every predetermined rotational amount so that the pulses of the A-phase and the B-phase have a phase difference of 90 degrees. Furthermore, as shown in  FIG. 2C , one pulse per revolution is output as a zero position signal from the Z-phase. The output signals of these phases which are output from the rotary encoder  20  are supplied to CPU  10 .  
         [0030]     A potentiometer  22  is a position detecting sensor for detecting a rough position of the control target  18 . As described in detail later, it specifies the position of the control target  18  at which the pulse of the zero position signal is output from the Z-phase of the rotary encoder  20 , and it is also used to identify the zero position signal output at plural positions (or the positions). The rotational shaft of the potentiometer  22  is connected to the rotational shaft of the motor  16  or the control target  18  directly or through a gear or the like. For example, a voltage from 0V to 5V is substantially linearly output from the potentiometer  22  in accordance with the rotational angle of the rotational shaft as shown in  FIG. 2D , and the voltage signal is converted to a digital value by an A/D converter  24  and then supplied to CPU  10 .  
         [0031]     CPU  10  has the function of a counter, and counts a count value of the counter on the basis of the pulses of the A-phase and the B-phase input to CPU  10 . The count value counted by the counter on the basis of the pulses of the A-phase, the B-phase is called as an A/B-phase count value. Here, the phase relationship between the pulses of the A-phase and B-phase is reverse between a case where the rotational shaft of the rotary encoder  20  is forwardly rotated and a case where it is reversely rotated. The counter increments or decrements the A/B-phase count value one by one every time the pulse of A-phase, B-phase is supplied. When the pulse of the A-phase is advanced with respect to the pulse of the B-phase by 90 degrees (in the case of the forward rotation), the A/B-phase count value is incremented. Conversely, when the pulse of the B-phase is advanced with respect to the A-phase by 90 degrees (in the case of the reverse rotation), the A/B-phase count value is decremented.  
         [0032]     As described in detail later, when the control target  18  is set at a predetermined position (end), the A/B-phase count value is initialized to zero. Accordingly, the A/B-phase count value at each time point at which the count is carried out by the counter as described above indicates the absolute value corresponding to the position of the control target  18 . The A/B-phase count value when the initialization is carried out as described above will be hereinafter referred to as the current value of the control target  18 . The value which is achieved by converting the voltage signal output from the potentiometer  22  to the digital signal by the A/D converter  24  and then input to CPU  10  indicates the absolute value corresponding to the current position of the control target  18 . However, this value is not required to be made coincident with the A/B-phase count value, and this value will be hereinafter referred to as an A/D conversion value of the potentiometer  22 .  
         [0033]     CPU  10  sets as the current value of the control target  18  the A/B-phase count value achieved by the counter as described above, successively achieves the control signal of the value corresponding to (for example, proportional to) the difference between the target value given on the basis of the instruction signal and the current value, and outputs the control signal to the D/A converter  12  as described above. Accordingly, the motor  16  is rotated so that the difference between the target value and the current value is reduced, and also the rotational speed of the motor  16  decreases as the difference between the target value and the current value is reduced. The control signal which varies with the variation of the current value is successively supplied from CPU  10  to the driving circuit  14 , whereby the control target  18  is moved to the position indicated by the target value and stopped there. Furthermore, when the target value of the instruction signal is varied, the position of the control target  18  is changed in connection with the variation of the target value.  
         [0034]     Next, the initializing procedure of the servo circuit thus constructed will be described with reference to the flowchart of  FIG. 3 . The servo circuit is initialized as desired, for example when products are shipped, the position precision of the control target is lowered or the like. When initialization is executed, for example, a predetermined is pushed to instruct CPU  10  to execute initialization. When the instruction of executing the initialization is supplied to CPU  10 , CPU  10  outputs a control signal for rotating the motor  16  in a predetermined direction through the D/A converter  12  to the driving circuit  14 , and drives the motor  16  to one end of a rotatable range. That is, CPU  10  moves the control target  18  to one end of the movable range (this end will be hereinafter referred to as a start end) (step S 10 ). Then, it is judged whether the start end is detected or not (step S 12 ). Whether the start end is detected or not may be judged by providing a signal from a sensor (micro-switch, a photointerruptor or the like) for detecting arrival of the control target  18  at the start end) and using the signal. Furthermore, it may be judged by means for detecting that the rotation of the motor  16  is forcedly stopped or the like (the same is applied to detection of a terminal end described below). During the period when NO is judged in this judgment processing, the processing of step S 10  and the processing of step S 12  are repeated. If YES is judged, the processing is shifted to the processing of the next step S 14 .  
         [0035]     When the control target  18  is designed to be rotatable like a pan mechanism of a camera platform and operable endlessly with no start end and no terminal, a range in which the control target  18  makes one revolution (the range from 0 degree to 360 degrees) is regarded as a movable range of the control target  18 , and when the control target is located at predetermined positions, these positions are regarded as a start end and a terminal. In this case, a sensor for detecting the positions of the start end and the terminal is provided, and CPU  10  detects it on the basis of the sensor that the control target  18  moves to the start end and the terminal. However, desired positions may be set to the start end and the terminal without using the sensor as described above. In this case, an operator operates the controller to control the control target  18  so that the control target  18  is set to a desired position serving as an end.  
         [0036]     When the control target  18  arrives at the start end and YES is judged in step S 12 , CPU  10  clears the A/B-phase count value to zero (step S 14 ). Then, CPU  10  alters the value of the control signal output to the driving circuit  14 , reverses the rotation direction of the motor  16  to drive the control target  18  to the other end (hereinafter referred to as the terminal), and also starts the count of the A/B-phase count value (step S 16 ).  
         [0037]     Subsequently, CPU  10  judges whether a pulse representing the zero position signal is detected in the Z-phase output signal supplied from the rotary encoder  20  (step S 18 ). Here, if NO is judged, it is judged whether the terminal is detected (step S 22 ). If NO is judged, the above processing is repeated from the step S 18 . That is, the judgment of the step S 18  is repeated until the control target  18  arrives at the terminal.  
         [0038]     When YES is judged in step S 18 , CPU  10  associates the A/B-phase count value (D value) at that time with the A/D conversion value (A value) achieved from the potentiometer  22  through the A/D converter  24 , and stores them into a built-in memory (step S 20 ). The memory is a non-volatile memory such as EEPROM or the like, and data stored therein are held even when the memory is powered off.  
         [0039]     The A/B-phase count value (D value) and the A/D conversion value (A value) of the potentiometer  22  when the pulse of the Z-phase of the rotary encoder  20  is detected are stored in the memory as described above, the processing is shifted to the judgment processing of step S 22 .  
         [0040]     Here, the reduction gear ratio between the rotary encoder  20  and the motor  16  is determined so that the rotary encoder  20  is rotated by at least plural times during the period when the control target  18  moves from the start end to the terminal, and the rotary encoder  20  is connected to the motor  16 , etc. so that the above reduction gear ratio is achieved. Accordingly, the pulse of the Z-phase is detected at least twice or more until YES is judged in step S 22 . Furthermore, the potentiometer  22  is connected to the motor  16 , etc. so that it makes substantially one rotation until the control target  18  moves from the start end to the terminal. The frequency of the rotation of the potentiometer  22  during the movement of the control target  18  from the start end to the terminal is not limited to a specific value. However, the A/D conversion value of the potentiometer  22  when the Z-phase pulse is output from the rotary encoder  20  is used as identification information for discriminating the pulse concerned from other pulses. Therefore, if the A/D conversion value of the potentiometer  22  when the Z-phase pulse is output is overlapped with the A/D conversion value when another Z-phase pulse is output in the case where the potentiometer  22  is rotated twice or more during the movement of the control target  18  from the start end to the terminal, these data are invalid as data to be stored in the memory. Accordingly, when there is no reason why the frequency of the rotation of the potentiometer  22  is increased, it is reasonable to set the rotation frequency to once as in the case of this embodiment.  
         [0041]      FIG. 4  is a diagram showing an example of the timing of the Z-phase pulse output from the rotary encoder  20  and the output voltage of the potentiometer  22  during the movement of the control target  18  from the start end to the terminal when the initialization processing is carried out. In this example, the potentiometer  22  is rotated once and the output voltage varies from the minimum value (0V) to the maximum value (5V) during the movement of the control target  18  from the start end to the terminal. However, the position detectable range of the potentiometer  22  is limited to a range of about 340 degrees out of the range of 360 degrees, and thus a portion where the output voltage is unvaried exists in the neighborhood of the start end and the terminal of the control target  18 . Furthermore, it is assumed in the example of  FIG. 4  that the control target  18  is rotated in the one-rotation range, and the start end is set to 0, and the terminal is set to 360 degrees.  
         [0042]     On the other hand, the Z-phase pulse of the rotary encoder  20  is output at each position of the control target  18  which is represented by Z 1  to Z 8  located at an equal interval. Actually, the Z-phase pulse maybe output in the neighborhood of the start end and terminal of the control target  18  in the range where the output of the potentiometer  22  is unvaried, however, in this case the Z-phase pulse is invalidated. If the rotational amount of the potentiometer  22  is reduced with respect to the movable range of the control target  18  by one rotation or more so that the output voltage of the potentiometer  22  varies over the movable range from the start end to the terminal of the control target  18 , the range where the output voltage of the potentiometer  22  is unvaried can be eliminated. In this case, all the Z-phase pulses output in the movable range of the control target  18  can be validated.  
         [0043]     CPU  10  reads out the A/D conversion value (A value) of the potentiometer  22  and the A/B-phase count value (D value) and stores them into the memory at each timing of Z 1  to Z 8  at which the z-phase pulse (valid pulse) is detected. Accordingly, corresponding data as shown in Fig. are generated. The A/B-phase count value (D value) at the position of each of Z 1  to Z 8  on the assumption that the A/B-phase count value when the control target  18  is located at the start end is set to 0, that is, the current value of the control target  18  stores the A/D conversion vale (A value) of the potentiometer  22  as identification information, for example, like a case where when the A/D conversion value (A value) of the potentiometer  22  is equal to 1000H (H means hexadecimal numeral), the A/B-phase count value (D value) of the rotary encoder  20  is equal to 1500H on the basis of the data achieved when the control target  18  is at the position Z 1 .  
         [0044]     As described above, the corresponding data are successively generated and stored in the memory while the control target  18  is moved to the terminal, and then when the control target arrives at the terminal and YES is judged in the step S 22  of  FIG. 3 , the initialization is finished.  
         [0045]     Next, the procedure of the initialization of the A/B-phase count value at the power-on time when the servo circuit is started to be used will be described with reference to the flowchart of  FIG. 6 . When power is turned on, CPU  10  reads out the A/D conversion value of the potentiometer  22  (step S 30 ). The position of the control target  18  when power is turned on is set to the position of the control target  18  when the previous use thereof was finished, and thus it is uncertain. Subsequently, CPU  10  compares the current A/D conversion value of the potentiometer  22  detected in step S 30  with the A/D conversion value (A value) of the potentiometer  22  stored as the corresponding data in the memory, and it finds out the A value nearest to the current A/D conversion value, specifies a point-blank position at which the Z-phase pulse is output, and also rotates the motor  16  in that direction (step S 32 ). CPU  10  judges whether the pulse representing the zero position signal is detected in the Z-phase output signal supplied from the rotary encoder  20  (step S 34 ). If NO is judged, this judgment is repeated. On the other hand, if YES is judged, the rotation of the motor  16  is stopped at that position, and the D value corresponding to the A value which is set as the nearest value in step S 32  is read out from the corresponding data of the memory and set as the A/B-phase count value (step S 36 ). Then, the initialization processing is finished, and subsequently the processing is shifted to the normal processing of driving the motor  16  according to the instruction signal and controlling the position of the control target  18 .  
         [0046]     As described above, according to the initialization processing, when the A/B-phase count value when the control target is set to the start end is set to zero like the case where the initial setting is carried out, the A/B-phase count value at the position at which the Z-phase pulse is detected in step S 34  is reproduced. That is, when the A/B-phase count value when the control target  18  is set to the start end is set to 0 and the A/B-phase count value counted by the pulses of the A-phase and B-phase is represented as the current position indicating the position of the control target  18 , the current value at the position at which the Z-phase pulse is detected in step S 34  is initialized as the A/B-phase count value. Accordingly, after the initialization, the A/B-phase count value is incremented/decremented from the initialized value on the basis of the pulses of the A-phase and B-phase, whereby the detection of the current value of the control target can be started, and the servo circuit can be effectively used.  
         [0047]     In the initialization procedure of  FIG. 6 , after power is turned on (after the initialization processing is started), the A/D conversion value of the potentiometer  22  is first read out, and the A/D conversion value thus read out is compared with the A/D conversion value (A value) of the potentiometer  22  which is stored as the corresponding data in the memory to find out the A value nearest to the current A/D conversion value and move the control target  18  toward the position concerned. However, the present invention is not limited to this embodiment. For example, after power is turned on, the control target  18  may be moved in a desired direction (for example, in a predetermined direction). In this case, after the control target  18  is moved, the A/D conversion value of the potentiometer  232  at the position where the Z-phase pulse is detected is read out, and the A-value of the corresponding data which is coincident with the A/D conversion value concerned is found out. Then, the D value corresponding to the A value may be read out from the corresponding data, and set as the A/B-phase count value.  
         [0048]     As described above, in the above embodiment, the movable range of the control target  18  is limited, and it has the start end and the terminal. When the movable range of the control target  18  is not limited, for example when the same state is periodically repeated, an operation range which is integer times as large as the above period is regarded as a movable range, and the initial setting and the initialization processing of the A/B-phase count value can be carried out.  
         [0049]     In the above embodiment, the pulse representing the zero position signal of the Z-phase of the rotary encoder  20  is used as the initialization signal representing the initialization position of the A/B-phase counter value. However, the present invention is not limited to this embodiment, and the initialization signal may be output (generated) by another means. For example, a switch, a photointerruptor or other signal generating means which outputs an ON-signal or the like when the control target  18  is moved to a predetermined position corresponding to an initialization position may be disposed at each of plural positions in the movable range of the control target  18 , whereby the initialization signal can be output as in the case of the Z-phase pulse.  
         [0050]     Still furthermore, in the above embodiment, the Z-phase pulse of the rotary encoder  20  which is output as the initialization signal at plural times in the movable range of the control target  18  is identified, and also the A/D conversion value of the potentiometer  22  is used to specify the Z-phase-pulse outputting nearest position to the position of the control target  18  when power is turned on. However, there may be used a position detecting sensor for outputting the absolute value corresponding to the position of the control target  18  as in the case of an absolute type encoder in place of the potentiometer  22 .  
         [0051]     Still furthermore, in the above embodiment, the Z-phase-pulse outputting nearest position to the position of the control target  18  when power is turned on is set as the initialization position of the A/B-phase count value, and the control target  18  is moved to the position concerned. However, after power is turned on, the control target  18  may be moved in any direction and the first Z-phase-pulse detecting position maybe set as the initialization position of the A/B-phase count value. In this case, it is unnecessary to specify the Z-phase pulse outputting nearest position to the position of the control target  18  when power is turned on, and plural initialization signals output in the movable range of the control target  18  may be identified. That is, in the above embodiment, the A/D conversion value of the potentiometer  22  is used as the identification information for identifying the Z-phase pulse output at the plural positions of the movable range of the control target  18 . At the initialization time, the current value of the control target  18  when each Z-phase pulse (initialization signal) is output in association with the A/D conversion value is stored as the corresponding data in the memory. At the initialization time of the A/B-phase, the current position of the control target which corresponds to the A/D conversion of the potentiometer  22  when the Z-phase pulse is output is read out from the corresponding data of the memory and set as the A/B-phase count value. However, such identification information may be a value other than the A/D conversion value of the potentiometer  22 . For example, when signal generating means is provided as means for outputting an initialization signal at plural initialization positions of the movable range of the control target, the initialization signal containing identification information such as the number of the initialization signal or the like maybe generated by the signal generating means.  
         [0052]     Still furthermore, as described in the above embodiment, the position detecting device for detecting the position of the control target  18  in the servo circuit may be applicable to circuits other than the servo circuit.  
         [0053]     This application is based on Japanese Patent application JP 2004-096450, filed Mar. 29, 2004, the entire content of which is hereby incorporated by reference. This claim for priority benefit is being filed concurrently with the filing of this application.