1.  Field of the Invention
The present invention relates to a position detecting apparatus and a positioning apparatus for an object such as a magnetic head of a hard disk drive, and an information recording apparatus using the same.  The present invention is particularly preferably applicable to an apparatus for manufacturing a hard disk drive (to be referred to as an HDD hereinafter) for use in a computer, and especially an apparatus for accurately writing a servo track signal in an internal hard disk of an HDD. 
2.  Related Background Art
FIG.  1 is a view for explaining a conventional apparatus for writing a servo track signal in an internal hard disk of an HDD. 
Referring to FIG.  1, a hard disk drive HDD comprises a hard disk HD, a slider SLID, a magnetic head arm ARM, a voice coil VOIC, a spindle OHD of the hard disk HD, and a rotating shaft OARM of the magnetic head arm ARM. 
A magnetic recording medium is vapor-deposited on the surface of the hard disk HD.  The hard disk HD is constantly rotated at a high speed about the spindle OHD.  A magnetic head is arranged close to the surface of the hard disk HD.  The magnetic head is incorporated into a substantially rectangular parallelpiped member called the slider SLID attached to the end portion of the arm ARM having the rotational center OARM outside the hard disk HD.  The magnetic head can be relatively moved in a substantially radial direction on the hard disk HD by rotating the arm ARM by the voice coil VOIC. 
Accordingly, magnetic information can be written in or read out from an arbitrary position (track) on the surface of the hard disk by the rotating hard disk HD and the pivoting magnetic head. 
A method of performing magnetic recording on the surface of the hard disk HD is as follows.  First, the hard disk is divided into a plurality of concentric annular tracks having different radii with respect to the hard disk rotational center OHD.  Each annular track is also divided into a plurality of circular arcs.  Finally, information is recorded on and reproduced from a plurality of arcuated regions along the circumferential direction in a time-series manner. 
Recently, it is being demanded to increase the recording capacity of a hard disk and therefore increase the density of information to be recorded on a hard disk.  As a means for increasing the density of information to be recorded on a hard disk, it is effective to narrow the width of concentrically divided tracks and thereby increase the recording density in the radial direction. 
The recording density in the radial direction is expressed by a track density TPI (track/inch) per inch and currently is about 8,000 TPI.  This means that the track pitch is approximately 3.125 .mu.m. 
To detect this fine track pitch, it is necessary to position the magnetic head with a resolution of about 1/50 (0.06 .mu.m) of the track width in the radial direction of the hard disk HD and to write servo track signals beforehand.  An important technique in this case is to sequentially write servo track signals while positioning the magnetic head with a high resolution within a short time period. 
FIG.  2 is a schematic view showing the structure of a conventional positioning apparatus for writing servo track signals.  Referring to FIG.  2, this positioning apparatus comprises a pushrod PROD, an arm ARM2' for the pushrod PROD, an arm ARM1 a positioning motor MO, a rotary encoder RE for detecting the rotational amount of the rotating shaft of the motor MO, a signal processor SP for analyzing the detection output from the rotary encoder RE and generating a positioning command signal for a servo track signal write position of a magnetic head, and a motor driver MD for driving the motor MO in accordance with the command signal from the signal processor SP. 
Conventionally, as shown in FIG.  2, the magnetic head arm ARM1 is pressed laterally against the cylindrical surface of the pushrod PROD.  While feedback control is performed by a system including the rotary encoder RE, the signal processor SP, and the motor driver MD, the motor MO rotates the arm ARM2' to position the pushrod PROD by sequentially finely feeding it, thereby sequentially writing servo track signals. 
Recently, a method assuming more accurate positioning was also invented.  This method accurately measures the movement of a magnetic head arm by an optical means, instead of mechanically pressing the magnetic head arm.  FIG.  3 shows an example of an apparatus of this sort. 
Referring to FIG.  3, this apparatus comprises a laser light source LA, a mirror M, a beam splitter BS, a retroreflector CC such as a corner cube provided on the magnetic head arm ARM1, and a photodetector PD. 
In this apparatus, the laser light source LA, the mirror M, the beam splitter BS, and the retroreflector CC constitute a Michelson interferometer.  The photodetector PD detects the interference light of light beams L1 and L2 propagating via the retroreflector CC and the mirror M, respectively, and obtains position information of the magnetic head arm ARM1.  On the basis of the obtained detection signal, the signal processor SP generates a command to control a current flowing from a voice coil motor driver VCMD to a voice coil VOIC, thereby directly moving and appropriately controlling the magnetic head arm.  Consequently, even if the press force of the pushrod PROD varies due to, e.g., vibrations while the hard disk is being rotated and the position of the slider SLID moves from an accurate position, this movement is corrected to maintain accurate head positioning. 
In such an apparatus, however, the retroreflector CC such as a corner cube must be mounted on the magnetic head arm.  This readily poses a problem of securing the space or a problem of a change in the gap between the slider and the hard disk caused by an increase in weight.