Abstract:
An apparatus for optically detecting rotational position information of a rotating object, includes a light source, a detector which is located at a position to receive scattered light from the rotating object when the rotating object is irradiated with a light beam from the light source, and outputs a frequency signal based on the scattered light, a signal processing system for detecting rotational position information by performing signal processing for the frequency signal from the detector, and a rotation control system for controlling rotation of the rotating object. The rotation control system preliminarily rotates the rotating object in detecting the rotational position information.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an optical rotational position information detecting apparatus mainly used for an apparatus for outputting rotational position signals required as clock signals for a hard disk drive, a photosensitive drum rotational position signal output apparatus for a copying machine, or the like.  
         [0003]     2. Related Background Art  
         [0004]      FIG. 1A  is a plan view of a conventional data writing/reading apparatus used for a data processing apparatus such as a computer. A magnetic disk  2  formed by, for example, coating a disk with a magnetic material is placed on an HDD (Hard Disk Drive)  1  serving as a data writing/reading apparatus. The HDD  1  has a magnetic head arm  4  having a magnetic head slider  3  mounted on its distal end. The magnetic head slider  3  writes an information signal on the magnetic disk  2 . A voice coil motor  5  is mounted on the rear end portion of the magnetic head arm  4 .  
         [0005]      FIG. 1B  is a plan view of the magnetic disk  2 .  FIG. 1C  is a view for explaining a servo pattern. A plurality of sectors, each consisting of a servo pattern region and data region, are arranged outside a disk hub  2   a  of the magnetic disk  2 .  
         [0006]     In this arrangement, the magnetic disk  2  is set on the HDD  1 , and the rotational position of the magnetic head arm  4  is forcibly set to a position corresponding to a desired track by a rotary positioner (not shown). The magnetic head arm  4  is then driven in the track direction to write a servo pattern of an information signal with a resolution 1/2 a data track using the magnetic head slider  3  on the distal end of the arm.  
         [0007]     When a data signal is to be written on the circular magnetic disk  2  by using the magnetic head slider  3 , a servo track signal as information for positioning the magnetic head slider  3  must be accurately written in advance. For this purpose, a magnetic signal must be written at a desired position on the magnetic disk  2  by accurately detecting position information of the rotational direction of the magnetic disk  2  as well as position information of the magnetic head slider  3  in the track direction which is the radial direction of the magnetic disk  2 .  
         [0008]      FIG. 1D  is a perspective view of the HDD  1  having a magnetic clock head  7  that is used independently of a magnetic head  6  for writing information to accurately detect the rotational direction of the magnetic disk  2 . According to this scheme, the magnetic clock head  7  enters the HDD  1  through an opening portion  8  and writes a clock signal of a rotational direction on an outermost peripheral portion of the magnetic disk  2 . The rotational position of the magnetic disk  2  is then detected while the clock signal is read by the magnetic clock head  7 , and a servo track signal is written on each track using the magnetic head  6  for writing information.  
         [0009]     The service life of this magnetic clock head  7  is short because it is used to write clock signals on several ten thousand HDDs  1  in a short period of time. That is, the magnetic clock head  7  itself is a consumable item, and hence maintenance such as replacement is required, resulting in an increase in cost.  
         [0010]     In addition, since the gap between the magnetic disk  2  and the magnetic clock head  7  must be kept very small, these members may contact each other for some cause. This structure is therefore structurally undesirable in efficiently and economically mass-producing HDDs  1 .  
         [0011]     As a means for solving this problem, a laser Doppler scheme of detecting the rotational position information of a rotating object by irradiating it with a laser beam is disclosed in Japanese Patent Application Laid-Open No. 7-29229. According to this laser Doppler scheme, since it is only required to irradiate the disk hub  2   a  of the magnetic disk  2  with a laser beam, no special part such as a scale needs to be bonded to the magnetic disk  2 . In addition, owing to noncontact detection, the detecting unit does not wear.  
         [0012]      FIG. 1E  is a view showing the arrangement of a laser Doppler velocimeter. This device measures the moving velocity of a moving object by using the Doppler effect that when the moving object is irradiated with a laser beam, the frequency of light scattered by the moving object shifts in proportion to the moving speed. In this device, a laser source  11 , collimator lens  12 , beam splitter  13 , and mirrors  14   a  and  14   b  are arranged. An object K to be measured, which moves in the direction indicated by the arrow at a velocity V, is placed in the reflection direction of the two mirrors  14   a  and  14   b , and a condenser lens  15  and photodetector  16  are arranged on the optical path of light reflected by the object K.  
         [0013]     In this arrangement, a laser beam emitted from the laser source  11  is collimated into a parallel light beam L 1  by the collimator lens  12  and strikes the beam splitter  13  to be split into two light beams L 2  and L 3 . These light beams are reflected by the mirrors  14   a  and  14   b  and strike the object K, which is moving at the velocity V, at an incident angle e. Scattered light from the object K is detected by the photodetector  16  via the condenser lens  15 .  
         [0014]     The frequency of the scattered light beams originating from the two light beams respectively undergo Doppler shifts +Δf and −Δf. Letting λ be the wavelength of a laser beam, Δf is given by
 
 Δf =( V  sin θ)/λ  (1)
 
         [0015]     The scattered light beams having undergone the Doppler shifts +Δf and −Δf interfere with each other to cause brightness changes on the light-receiving surface of the photodetector  16 . A frequency F at this time is given by
 
 F =2 Δf =(2 V  sin θ)/λ  (2)
 
         [0016]     If the Doppler frequency F of the photodetector  16  is measured according to equation (2), the velocity V of the object K can be obtained.  
         [0017]     When the object K is a rotating object, the velocity V of the object K is given by
 
 V =2 πrW /60  (3)
 
 where r is the irradiation radius and W (rpm) is the rotational velocity. 
 
         [0019]     Equation (2) is finally rewritten into
 
 F =( πrW  sin θ)/(15λ)  (4)
 
         [0020]     If equation (4) is converted into a pulse count N for one revolution, equation (4) is rewritten into
 
 N =(4 πr  sin θ)/λ  (5)
 
         [0021]     By detecting this pulse signal, rotational position information can be detected.  
         [0022]     (1) It is, however, known that the above conventional optical rotational position information detecting means for detecting rotational position information by using a Doppler signal causes dropouts that are portions in which signal components are statistically omitted. It is therefore difficult to accurately identify a rotational position.  
         [0023]     ( 2 ) In the above prior art, an NRRO (Non-Repeatable Run Out) corresponding to about 0.1 μm occurs when the magnetic disk  2  of the HDD  1  rotates. To write a stable servo signal, it is very important to form a clock signal while minimizing the influence of this NRRO.  
       SUMMARY OF THE INVENTION  
       [0024]     It is an object of the present invention to solve the above problem (1) and provide an optical rotational position information detecting apparatus which accurately detects rotational position information in a circumferential direction on a rotating object that continuously rotates.  
         [0025]     It is another object of the present invention to solve the above problem (2) and provide an optical rotational position information detecting apparatus which can perform accurate position detection by minimizing the influence of the run out of a rotating object. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]      FIG. 1A  is a plan view of a conventional HDD;  
         [0027]      FIG. 1B  is a plan view of a magnetic disk;  
         [0028]      FIG. 1C  is a view for explaining a servo pattern on the magnetic disk;  
         [0029]      FIG. 1D  is a perspective view of the HDD using a magnetic clock head;  
         [0030]      FIG. 1E  is a view showing the arrangement of a laser Doppler velocimeter;  
         [0031]      FIG. 2A  is a perspective view of a hard disk drive according to the first embodiment;  
         [0032]      FIG. 2B  is a graph showing a Doppler signal corresponding to a rotational position;  
         [0033]      FIGS. 3A and 3B  are graphs showing processed signals in the first embodiment;  
         [0034]      FIG. 4  is a view showing the flow of storage of dropouts;  
         [0035]      FIG. 5  is a flow chart showing signal processing;  
         [0036]      FIGS. 6A and 6B  are graphs showing processed signals in the second embodiment;  
         [0037]      FIG. 7  is a flow chart showing signal processing;  
         [0038]      FIG. 8  is a flow chart showing signal processing according to the third embodiment;  
         [0039]      FIG. 9  is a view showing the arrangement of the main part of the fourth embodiment;  
         [0040]      FIG. 10  is a view for explaining the positional relationship associated with the detection direction of an LDV optical head;  
         [0041]      FIG. 11  is a view for explaining a non-repeatable run out;  
         [0042]      FIG. 12  is a graph showing write angle deviations at LDV angles and magnetic head positions;  
         [0043]      FIG. 13  is a graph showing write angle deviations at LDV angles; and  
         [0044]      FIG. 14  is a graph showing magnetic head positions and write angle deviations. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0045]     The present invention will be described in detail with reference to the embodiments shown in  FIGS. 2A  to  14 .  
         [0046]      FIG. 2A  is a perspective view showing an HDD applied to a servo track writer according to the first embodiment. A magnetic disk  22  is placed on an HDD  21 . The HDD  21  has a magnetic head  23   a  which is mounted on the distal end of a magnetic head arm  23  and has a slider serving as a magnetic recording head for writing an information signal on the magnetic disk  22 . An LDV (Laser Doppler Velocimeter) optical head  24  for detecting the rotational velocity of the magnetic disk  22  by irradiating a disk hub  22   a  of the magnetic disk  22  with a laser beam is placed above the HDD  21 .  
         [0047]     The output of the LDV optical head  24  is sequentially connected to a signal processing logic circuit  25 , rotational position detecting circuit  26 , sector servo pattern writing circuit  27 , and magnetic head  23   a . The output of a track direction position control circuit  28  is connected to the sector servo pattern writing circuit  27  and a rotary positioner (not shown).  
         [0048]     With this arrangement, the track direction of the magnetic head  23   a  is sequentially controlled by the rotary positioner (not shown) in accordance with each track to write a servo track signal for each track on the magnetic disk  22  on the basis of the rotational position information of the magnetic disk  22  which is obtained from the signal processing logic circuit  25  for processing a Doppler signal from the LDV optical head  24 .  
         [0049]      FIG. 2B (a) shows the amplitude of a Doppler signal from the LDV optical head  24  which corresponds to the rotational position. This signal amplitude exhibits repeatability with respect to the rotational position as long as the laser beam irradiation region of the disk hub  22   a  remains unchanged.  FIG. 2B (b) shows a Doppler signal A, a waveform-shaped signal B obtained by converting the Doppler signal A, and a rotational position origin signal (rotational position index) C. The frequency of the Doppler signal A is proportional to the rotational velocity as long as the radius of the disk hub  22   a  at the laser beam irradiation position always remains the same.  
         [0050]      FIGS. 3A and 3B  are graphs showing a signal processing method for a dropout portion of a Doppler signal in the first embodiment. Referring to  FIG. 3A , a signal ( 1 ) indicates the envelope (repeatability) of a Doppler signal, and a signal ( 2 ) is a comparate signal. This comparate signal ( 2 ) is obtained by removing noise components from the Doppler signal ( 1 ) upon providing a threshold with hysteresis. A dropout can be detected by using the phenomenon that the comparate signal ( 2 ) cannot follow the Doppler signal ( 1 ) at a dropout portion. However, the signal amplitude is small at the position of a dropout, and the phase and amplitude of the Doppler signal ( 1 ) change even with slight vibrations. For this reason, even if this signal is waveform-shaped, it repeatability cannot be ensured.  
         [0051]      FIG. 3B  is a graph showing signal processing for identifying a dropout portion. First of all, a 1/4 frequency signal ( 5 ) of a PLL oscillation signal ( 7 ) is synchronized (phase-locked) to a Doppler comparate signal in the interval between a rotational position origin signal which is not a dropout portion and the first dropout portion. Subsequently, a dropout identification signal ( 3 ) including a dropout unstable portion like the one shown in  FIG. 3A  is stored by counting the pulses of the PLL oscillation signal ( 7 ) from the rotational position origin signal (rotational position index).  
         [0052]     At a dropout portion, the voltage value of a VCO (not shown) used for the PLL oscillation signal ( 7 ) is fixed and oscillated at an oscillation frequency immediately preceding the dropout. Since the dropout portion corresponds to a distance of several 10 μm, the rotational velocities before and after the dropout can be regarded as almost equal. By counting the pulses of the PLL oscillation signal ( 7 ) during the dropout, therefore, the dropout identification signal ( 3 ) exhibiting good repeatability of the dropout start and avoidance timings can be generated.  
         [0053]     As is obvious from a signal ( 4 ) and the signal ( 5 ) in  FIG. 3B , the phase of a Doppler signal before a dropout often differs from that after the dropout. To prevent such a phase shift in signal processing, the PLL oscillation signal ( 7 ) obtained by multiplying the frequency of a Doppler comparate signal by two or more (four) is used as a signal for detecting dropout avoidance.  
         [0054]     Assume that the PLL reference signal obtained by this method is the signal ( 4 ) obtained by ORing the signals ( 2 ) and ( 3 ). A phase comparison signal ( 6 ) is almost synchronized to the rising point of the signal ( 4 ) after the dropout avoidance, and the number of pulses of the PLL oscillation signal ( 7 ) switched from the dropout identification signal ( 3 ) is stored. At a portion other than a dropout, the phase comparison signal ( 6 ) synchronizes (phase-locks) the 1/4 frequency signal ( 5 ) of the PLL oscillation signal ( 7 ) to the Doppler comparate signal ( 4 ).  
         [0055]     There are several 10 dropout portions during one revolution. For this reason, the numbers of PLL oscillation signal pulses in regions which are not dropouts and dropout regions are sequentially stored, with reference to the rotational position origin, so as to cover no dropout uncertain regions, and dropout portions corresponding to one revolution are identified, thereby determining the final dropout identification signal ( 3 ).  
         [0056]      FIG. 4  shows the flow of operation of sequentially storing dropouts. First of all, a count value from the index of the PLL oscillation signal ( 7 ) to a first dropout D.O. 1  is stored at the first revolution. With this operation, a PLL oscillation signal free from the instability of D.O. 1  can be obtained. A count value from the index of the PLL oscillation signal ( 7 ) corresponding to a next dropout D.O. 2  is stored after the above processing is performed at the second revolution by using the dropout identification signal ( 2 ), and the PLL oscillation signal in the interval between the index and dropout D.O. 2  is stabilized. This makes it possible to obtain the PLL oscillation signal ( 7 ) free from the instability of the dropouts D.O. 1  and D.O. 2 .  
         [0057]     Likewise, n D.O. portions that exist during one revolution are sequentially stored at D.O. 3 , D.O. 4 , . . . , D.O.n, i.e., the third revolution, fourth revolution, . . . , nth revolution, thereby determining a last dropout identification signal corresponding to one revolution.  
         [0058]     As described above, if a dropout portion can be fixed, instability, i.e., random signal phase changes before and after a dropout, can be eliminated. If a dropout identification signal ( 9 ) from the rotational position origin can be determined, the PLL reference signal ( 4 ) that is stable throughout one revolution can be generated by ORing the Doppler comparate signals ( 2 ) and ( 3 ) detected in real time. In addition, the PLL phase comparison signal ( 6 ) that is stable throughout one revolution is generated from the PLL oscillation signal ( 7 ) and signal ( 3 ).  
         [0059]     By performing PLL oscillation using the PLL reference signal ( 4 ) and phase comparison signal ( 6 ), therefore, the PLL oscillation signal ( 7 ) that is stable throughout one revolution can be generated, thus obtaining a stable signal equivalent to an encoder signal.  
         [0060]     PLL oscillation pulses for one revolution can be determined by counting them in response to several rotational position origins as triggers. Subsequently, a rotational position origin can be detected/generated by counting the number of PLL oscillation pulses. This will obviate the necessity for an external origin signal. In addition, in this case, the same signal processing is always performed at the same rotational position, closed processing (closed loop) is performed for one revolution, thus making signal processing for PLL oscillation more stable and realizing good signal repeatability.  
         [0061]      FIG. 5  is a flow chart showing a signal processing logic R 3 , in which an origin signal is written as an external original signal on the magnetic disk  22  by the magnetic head  23   a  before a servo track signal is written, and the origin signal is read by the magnetic head  23   a . In this case, after a dropout identification signal for one revolution is preliminarily determined by rotating the magnetic disk  22 , an origin signal can be generated by the signal processing logic R 3  of the LDV optical head  24 . Therefore, no external origin signal is required to write a servo track signal. In setting an external original signal, stable switching to the origin signal can be performed by the signal processing logic R 3  by selecting a portion which is not a dropout region. According to this method, since no external origin sensor is required, a servo track write with a simple arrangement can be implemented, thus providing a practical system.  
         [0062]      FIGS. 6A and 6B  are graphs showing a signal processing method according to the second embodiment. This signal processing method is used to accurately detect a rotational position even if a Doppler signal undergoes a dropout state.  
         [0063]     Referring to  FIG. 6A , a signal ( 1 ) is a Doppler signal, and a signal ( 2 ) is a comparate signal. This comparate signal ( 2 ) is set at HI at a zero level, i.e., a falling point, of the signal ( 1 ) and set at LO at a threshold with hysteresis, i.e., a rising point, of the signal ( 1 ), thus removing noise components from the Doppler signal ( 1 ). A signal ( 3 ) is obtained by advancing the Doppler signal ( 1 ) by 3/16 phase and setting a threshold to be smaller than that of the comparate signal ( 2 ).  
         [0064]      FIG. 6B  is a graph showing signal processing performed upon detection of a dropout portion. In detecting a dropout, when the signal ( 2 ) is kept at H level, a dropout start is detected, and when the signal ( 2 ) goes to L level, a dropout end is detected. In addition, when the signal ( 2 ) is set at H level at a rising point of the signal ( 3 ), a dropout start is detected. When the signal ( 2 ) is set at L level, a dropout end is detected. This makes it possible to always detect a dropout at the timing preceding a rising point of the signal ( 2 ) by 3/16 phase. Note that a signal ( 4 ) is a dropout detection signal.  
         [0065]     First of all, a 1/8 frequency signal ( 5 ) of a PLL oscillation signal ( 7 ) is synchronized (phase-locked) to a Doppler comparate signal in the interval between a rotational position original signal corresponding to a non-dropout portion and the first dropout portion. At a dropout portion, i.e., when the dropout detection signal ( 4 ) is at H level, the voltage value of a VCO (Voltage-Controlled Oscillator) (not shown) used for the PLL oscillation signal ( 7 ) is fixed and oscillated at an oscillation frequency immediately preceding the dropout. More specifically, a signal ( 6 ) generated by resetting the 1/8 frequency signal of the PLL oscillation signal ( 7 ) as a PLL phase comparison signal using the dropout detection signal ( 4 ) is used to prevent the signal ( 2 ) serving as a PLL reference signal and the phase comparison signal ( 6 ) from rising at the time of the dropout. This makes the VCO keep oscillating without frequency adjustment of the PLL oscillation signal ( 7 ).  
         [0066]     Since the dropout portion corresponds to a distance of several 10 μm, the rotational velocities before and after the dropout can be regarded as almost equal. In this case, the comparison between the signals ( 2 ) and ( 5 ) reveals that the phase of the Doppler signal ( 1 ) before the dropout differs from that after the dropout. In this embodiment, to preliminarily detect this phase shift by signal processing, detection is performed after dropout avoidance at the timing preceding a rising point of the comparate signal ( 2 ) by 3/16 phase. The signal ( 6 ) to be synchronized to the comparate signal ( 2 ) is generated at the third pulse of the 8-times frequency-multiplied PLL oscillation signal ( 7 ) after a falling point of the detection signal ( 4 ). The PLL oscillation signal ( 7 ) is then frequency-divided by  8  until the next dropout, thereby achieving phase locking.  
         [0067]     The PLL phase comparison signal ( 6 ) obtained by this method may shift from the PLL reference signal ( 2 ) by a quantization error (±1/16 phase) at a rising point after dropout avoidance. However, as the PLL multiplication number is increased, the error can be neglected.  
         [0068]     When the PLL phase comparison signal ( 6 ) is generated without preliminary detection, the quantization error becomes a positive 1/8 phase, and the quantization error accumulation always increases. In contrast to this, if dropouts are preliminarily detected as in this embodiment, quantization errors can be assigned to positive and negative values. This makes it possible to bring the average of quantization error accumulations to zero without increasing them in one direction.  
         [0069]     In general, the number of dropouts for one revolution is 100 or less. If, therefore, the PLL multiplication number is  128 , a variation in the number of PLL oscillation pulses for one revolution (rotation detection error) is not ±1/2 pulse or more at maximum in terms of the number of pulses of the Doppler signal ( 1 ). This makes it possible to set a gate at the position of a rising point of the Doppler signal ( 1 ) at the same rotational position with the number of PLL oscillation pulses and generate a rotational position original signal on the basis of a rising point of the Doppler signal ( 1 ) having passed through the gate. If the number of pulses of the PLL oscillation signal ( 7 ) is reset to coincide with this rotational position origin signal, a rotational position can be detected by reading the number of PLL oscillation pulses, thus implementing a noncontact type rotational position detecting system requiring no scale.  
         [0070]      FIG. 7  is a flow chart showing a signal processing logic R 1 , in which an origin signal is written as an external original signal on the magnetic disk  22  by the magnetic head  23   a  before a servo track signal is written, and the original signal is read by the magnetic head  23   a . In this case, the magnetic disk  22  is preliminarily rotated to check the number of PLL oscillation signal pulses for one revolution, and a gate is set at the position of a rising point of the Doppler signal ( 1 ) at the same rotational position. With this operation, since an origin signal can be generated by the signal processing logic R 1  using the LDV optical head  24 , no external origin signal is required to write a servo track signal. By reading the number of PLL oscillation pulses in this manner, a rotational position is detected.  
         [0071]      FIG. 8  is a flow chart showing a signal processing logic R 2  according to the third embodiment, in which an origin signal is written as an external original signal on a magnetic disk  22  by a magnetic head  23   a  before a servo track signal is written, and the original signal is read by the magnetic head  23   a . In this case, the magnetic disk  22  is preliminarily rotated to check the number of PLL oscillation pulses for one revolution, and at the same time, a dropout portion corresponding to a rotational position is checked and roughly stored with the number of PLL oscillation pulses.  
         [0072]     Subsequently, an index signal for each of sectors (about 50 in general) is set at a rising point of a Doppler signal ( 1 ) independently of an external origin signal. If the index position coincides with the stored dropout portion, an index signal is set at a portion which is not the most recent prior dropout. When the number of PLL oscillation pulses is reset in accordance with this index signal, a reset PLL oscillation signal ( 7 ) does not include any instability of signal processing due to a dropout but includes only a quantization error (±1/16 phase) of a PLL phase comparison signal ( 6 ). By reading both the count number of pulses of this index rotational signal and the count number of PLL oscillation pulses, a rotational position can be accurately detected.  
         [0073]     In the second embodiment, a quantization error accumulation for one revolution can become a rotational position detection error. In contrast to this, this embodiment is configured to disperse a quantization error accumulation to a fraction of the number of sectors (about 50), and hence is effective especially when a strict rotational position detection accuracy is required. By using this method, even in the Doppler signal ( 1 ) including dropouts, rotational position detection with very high repeatability can be realized without excessively increasing the frequency of the PLL oscillation signal ( 7 ). In this embodiment, when a sector index signal is identified, no external origin signal is required.  
         [0074]      FIG. 9  is a perspective view showing the main part of the fourth embodiment, in which the present invention is applied to a system for detecting the outer surface moving distance of a photosensitive drum  31 . The output of an LDV optical head is connected to the photosensitive drum  31  via a signal processing logic circuit  33 , outer surface rotational position detecting circuit  34 , and rotational driving control circuit  35 .  
         [0075]     Conventionally, the outer surface moving amount of the photosensitive drum  31  is detected by using a rotary encoder. In this method, however, if the outer surface of the photosensitive drum  31  is eccentric with respect to the rotational axis, the detected moving amount from the rotary encoder deviates from the actual outer surface moving amount of the photosensitive drum  31  in proportion to a radial error. Therefore, a mechanical arrangement with no eccentricity is required. For this reason, in this embodiment, a signal from the LDV optical head is detected by the outer surface rotational position detecting circuit  34  via the signal processing logic circuit  33  to make the rotational driving control circuit  35  control the rotation of the photosensitive drum  31 . As described above, since an outer surface moving amount is detected by using a light beam from the LDV optical head, rotational position information from which eccentricity is removed can be quickly detected.  
         [0076]     This embodiment presents a method of controlling the rotational driving of the photosensitive drum  31 . For example, the accuracy of final transfer characteristics can be improved by performing feedback to driving control on another transfer system or feedback to control on an exposure process. This processing can be performed in the same manner as a signal processing logic R 3  in  FIG. 12 . In addition, a signal from a low-resolution rotary encoder (not shown) is used as an external signal to accurately detect an outer surface rotational moving amount.  
         [0077]     The fifth embodiment will be described next. An apparatus of this embodiment has the same arrangement as that of the first embodiment in  FIG. 2A , and hence an illustration of this arrangement will be omitted. An RRO (Repeatable Run Out) and NRRO (Non-Repeatable Run Out) are present on a magnetic disk  22 , and the RRO occurs in the same manner at the same rotational position. Even if, therefore, this RRO is present as a detection error, since identical detection errors are superimposed on the respective tracks, no write error occurs when a servo track signal is written. On the other hand, detection errors due to the NRRO are randomly superimposed on the respective tracks. To accurately write a servo track signal, therefore, it is important to minimize the detection errors due to the NRRO.  
         [0078]      FIG. 10  is a plan view showing the positional relationship between the magnetic disk  22 , a magnetic head  23   a , and the detection direction of an LDV optical head  24 . Assume that the direction of the magnetic head  23   a  with respect to the center of the magnetic disk  22  is the x-axis. In this case, letting Ex be a vibration component in the x-axis direction with respect to a vibration width E of the NRRO, Ey be a vibration component in the y-axis direction, α be the angle of a perpendicular line in an LDV detection direction F with respect to the x-axis, and r be the radius of the magnetic disk  22  at an LDV detection position D, a detection error component S of the NRRO which exerts an influence in the LDV detection direction F is given by
   S=−Ex ·sin α+ Ey ·cos α  (6) 
         [0079]     A radial error component V of a run out NR which exerts an influence in the perpendicular direction of LDV detection is given by
 
 V=Ex ·cos α+ Ey ·sin α  (7)
 
         [0080]     Therefore, letting R be the distance between the rotational center of the magnetic disk  22  and the magnetic head  23   a , an LDV detection error E due to the NRRO on the magnetic head  23   a  is given by
 
 E =( R/r )· S ·( r+V )/ r   (8)
 
         [0081]     In this case, since V≦E&lt;&lt;r, equation (8) can be approximated as follows:
 
 E ∝( R/r )· S =( R/r )·(− Ex ·sin α+ Ey ·cos α)  (9)
 
         [0082]     If this write position deviation is converted into a write angle deviation ω, then
 
ω= W/R =(1 /r )·(− Ex ·sin α+ Ey ·cos α)− Ey/R   (10)
 
         [0083]     In general, the NRRO has a direction-independent vibration width E, and its angle β takes a random value satisfying 0≦β&lt;2π. This indicates that the center of the magnetic disk  22  randomly deviates within the range indicated by the hatched portion in  FIG. 11 .  
         [0084]     In this case, the vibration component Ex of the run out NR in the x-axis direction and the vibration component Ey in the y-axis direction are expressed with E and β as follows:
 
 Ex=E ·cos β,  Ey=E ·sin β  (11)
 
         [0085]     A substitution of equation (11) into equation (10) yields
 
ω=( E/r )·{−cos β·sin α+sin β·(cos α− r/R )}  (12)
 
         [0086]     In this case, since the angle β can take a random value within the range of 0≦β&lt;2π, the write angle deviation |ω| due to the NRRO at the mounting angle a of the LDV optical head  24  can take a value that is equal to the maximum value at an angle β in equation (13):
 
|ω|=|( E/r )·{−cos β·sin α+sin β·(cos α− r/R )}|  (13)
 
         [0087]     A condition required for the angle β in equation (13) to take a maximum value is dA/dβ=0 according to equation (14) below:
 
 A =−cos β·sin α+sin β·(cos α− r/R )  (14)
 
         [0088]     Therefore, dA/dβ can be written into
 
 dA/d β=sin β·sin α+cos β·(cos α− r/R )=cos(β−α)−( r/R )·cos β=sin(β+π/2−α)−( r/R )·cos β
 
         [0089]     According to equations (15), equation (16) is established:
 
 c =sin(π/2−α)− r/R =cos α− r/R 
 
 d =cos(π/2−α)=sin α
 
sin Φ= c /( c   2   +d   2 ) 1/2 
 
cos Φ= d /( c   2   +d   2 ) 1/2   (15)
 
 dA/d β=( c   2   +d   2 ) 1/2 ·sin(β+Φ)  (16)
 
         [0090]     If dA/dβ=0, then β=−Φ according to equation (16).  
         [0091]     According to equations (15), a condition required for the angle β in equation (13) to take a maximum value is expressed by
 
sin β=− c /( c   2   +d   2 ) 1/2 
 
cos β= d /( c   2   +d   2 ) 1/2   (17)
 
         [0092]     A substitution of equation (17) into equation (14) yields
 
 A={−d ·sin α− c ·(cos α− r/R )}/( c   2   +d   2 ) 1/2 =−{sin  2  α+(cos α− r/R ) 2 }/{sin  2  α+(cos α− r/R ) 2  } 1/2 ={sin  2 α+(cos α− r/R ) 2 } 1/2   (18)
 
         [0093]     With operation, a maximum value ωmax of a write angle deviation |ω| by NRRO is given by
 
ωmax=|−( E/r )·{sin  2  α+(cos α− r/R ) 2 } 1/2 |=( E/r )·{1+( r/R ) 2 −2·( r/R )·cos α} 1/2   (19)
 
         [0094]     As is obvious from equation (19), if R is a constant, ωmax is minimum when α=0 rad.  
         [0095]     Consider specific numerical values for the 6.3-cm (2.5-inch) HDD  21 . Assuming that r=10 mm, E=0.1 μm, and 14 mm≦R ≦30 mm (if a=R/r, then 1.4≦a≦3), the write angle deviation ωmax is obtained as shown in FIGS.  12  to  14 . The range of R indicates the movement of the magnetic head  23   a  in the disk track radial direction.  
         [0096]      FIG. 12  shows three-dimensionally the write angle deviation ωmax at a position a of the magnetic head  23   a .  FIG. 13  shows the relationship between the LDV angle α and the write angle deviation ωmax at a=1.4, 2.2, and  3 .  FIG. 14  shows the relationship between the position a of the magnetic head  23   a  and the write angle deviation ωmax with α=0, π/6, π/3, π/2, 2π/3, 5π/6, and π rad.  
         [0097]     According to these results, even when the position a of the magnetic head  23   a  moves within the range of 1.4 to 3, the write angle deviation ωmax becomes minimum when α=0 rad, and becomes maximum when α=π rad. The value of the write angle deviation ωmax does not depend on the sign of α as long as the absolute value of the LDV angle α remains the same.  
         [0098]     As is obvious from the above description, when the detection direction of the LDV optical head  24  is near a direction perpendicular to a straight line connecting the center of the rotating object and the magnetic head  23   a , and the detection position is on the magnetic head  23   a  side with respect to the center of the rotating object, a clock signal can be formed by the LDV optical head  24  which is least affected by a detection error due to the run out of the rotating object.  
         [0099]     As described above, by rotating the rotating object before the rotational position information detecting means identifies rotational position information, information for identifying rotational position information can be detected. This makes it possible to accurately detect a stable rotational position even with a frequency signal that causes dropouts.  
         [0100]     In addition, by setting the detection direction of the rotation detection position information detecting means to be near a direction perpendicular to a straight line connecting the center of the rotating object and the information recording head, and also setting the detection position on the information recording head side with respect to the center of the rotating object, accurate position detection can be performed with minimum influence of the run out of the rotating object, and a stable servo track signal can be written in the laser Doppler scheme.