Disc drive apparatus with tracking servomechanism

Abstract: A disc drive apparatus with a tracking servomechanism, comprising at least one disc medium having data surfaces where servo signal and data signal are recorded; transducer means having a first head for reproducing the data signal from the data tracks in a playback mode, and a second head for recording the data signal on the data tracks in a recording mode; a rotary arm for supporting the transducer means at one end thereof; actuator motor means for driving the rotary arm in a manner to move the transducer means over the data tracks; servo signal processing means for receiving and processing the servo signal reproduced by the transducer means and outputting first and second servo control signals; and positioning control means for controlling the actuator motor means in the playback and recording modes respectively in accordance with the first and second servo control signals so that the first head of the transudcer means is positioned precisely on any selected one of the data tracks in the playback mode and also that the second head is positioned precisely on any selected one of the data tracks in the recording mode.

Description: The present invention relates to a tracking servomechanism for a disc drive apparatus having a playback head and a recording head individually attached to a rotary support arm. The conventional head displacement means known heretofore for use in a disc drive apparatus are generally classified into a linear type which moves a support arm linearly to displace a head in the radial direction of a disc, and a rotary type which rotates a support arm to displace a head in the radial direction of a disc. As a means of realising a compact and lightweight structure of the drive apparatus, the rotary type is considered to be preferable. Meanwhile, a magnetoresistive type head capable of exhibiting excellent playback sensitivity characteristics even in a narrow track width has been developed to meet the requirement of achieving a higher track density. However, the magnetoresistive type head cannot function as a recording head and, therefore, when such magnetoresistive type head is employed as a read head, it is necessary to provide a recording head in addition thereto. Despite the need for two heads, it is still possible to produce a high density recording/reproducing disc drive apparatus of a compact and lightweight structure by using a rotary support arm and attaching a playback head and a recording head individually to such support arm. Figure 1 schematically illustrates the structure of a conventional disc drive apparatus equipped with a rotary actuator, in which a base end of a support arm 1 is held rotatably by a rotary shaft 2. This shaft 2 is rotated by a driving force of a motor (not shown) to control the position of the support arm 1. A slider 3 is attached to a distal end of the support arm 1, and a magnetoresistive type playback head Hp and, a coil type recording head Hr are secured to the slider 3 as illustrated in figure 2. The playback head Hp and the recording head Hr are disposed on a centre line C1 of the support arm 1 while being spaced apart by a predetermined distance from each other, in such a manner that the recording head Hr is set in a front position while the playback head Hp is set in a rear position. The playback head Hp and the recording head Hr are so arranged as to be both in a just-tracking state at an innermost servo signal track Ti on a disc 4. Therefore at any other track, the recording head Hr is placed in an off-tracking state when the playback head Hp is at a just-tracking position. In such known disc drive apparatus equipped with a rotary actuator as shown in figure 1, the angle formed by the slider 3 and the tangential direction C to each track is changed in accordance with the track position. Figure 1 represents an exemplary case where a directional coincidence is attained therebetween at the innermost track position Ti. The above angle gradually increases in accordance with the displacement of the slider 3 toward the periphery of the disc and finally reaches a maximum value  at the outermost track position T o . In an exemplary case of figure 2 where the recording head Hr and the playback head Hp are so disposed on the slider 3 that the respective centres thereof are positioned exactly at the centre line C2 of the innermost track Ti, when the rotary arm is controlled for aligning the centre of the playback head Hp with the centre line of any other track, the centre of the recording head Hr comes to be positioned at a point deviated from the centre line C2 of the track. When a tracking servo control is executed while a tracking servo signal prerecorded on the disc is read out there-from by the playback head Hp, the centre of the playback head Hp can be exactly positioned with respect to the track centre, but there exists a positional deviation between the centre of the recording head Hr and any other track centre except the innermost track. Consequently, if a recording mode is executed in a state where merely the playback head Hp alone is aligned with the track centre line as shown in figure 3, a newly recorded data track is formed with a positional deviation from the preceding data track. When the signal recorded in this manner is reproduced, there arises a problem that some noise or read error is induced since the playback head Hp is so controlled that the centre is continuously positioned on the track centre line C2 while the recording head Hr scans the off-track position of the data signal at any track other than the innermost track Ti. There is known a discrete track disc where servo tracks and data tracks are formed alternately in the circumferential direction on a magnetic disc, and both tracks are composed of a magnetic layer while any other region is composed of a non-magnetic layer. In such discrete track disc, the width of the data track is smaller than the width of the recording head as shown in figure 4, so that a signal recording operation can be performed without causing any exudation or incomplete erasure in a recording mode. However, even by the use of such a discrete track disc, when the off-track deviation becomes so great that the end of the recording head Hr scans the data track, some exudation or incomplete erasure is caused in the recording mode which detracts from the merit of using a discrete track disc. It is an object of the present invention to provide a disc drive apparatus with a tracking servo-mechanism which enables a recording head to scan each data track in a just-tracking state and consequently to record data signal properly even by the use of a discrete track disc as a recording medium. According to the present invention, there is provided a disc drive apparatus with a tracking servo-mechanism comprising: at least one disc medium having data surfaces on which a plurality of concentric tracks are formed, each of said tracks being divided into plural data tracks and plural servo tracks, wherein data signals are recorded and reproduced by transducer means on and from said data tracks, and servo signals are prerecorded and reproduced by said transducer means on and from said servo tracks; said transducer means having a first head for reproducing said data signals from said data tracks in a playback mode, and a second head for recording said data signals on said data tracks in a recording mode, the width of said first head being narrower than that of said second head; a rotary arm for supporting said transducer means at one end thereof, said rotary arm being rotatably held around an axis; actuator motor means for driving said rotary arm in a manner to move said transducer means over said data tracks; servo signal processing means for receiving and processing said servo signals reproduced from said servo tracks and outputting a servo control signal; and positioning control means for receiving said servo control signal and controlling said actuator motor means in the playback and recording modes in accordance with said servo control signal; characterised in that the centres of said servo tracks deviate from the centres of said data tracks by an amount depending on the track address of the corresponding concentric tracks, and corresponding to the amount of the deviation in the disc radial direction between said first and second heads of the transducer means, such that the second head of said transducer means is positioned precisely on any selected one of said data tracks while the first head of said transducer means is positioned precisely on the corresponding one of said servo tracks. The invention will be further described by way of non-limitative example with reference to the accompanying drawings, in which:- Fig. 1 schematically illustrates the structure of a known disc drive apparatus; Fig. 2 illustrates a positional state of heads disposed with respect to track position in a conventional example; Fig. 3(a) illustrates a scanning state of heads in a recording mode in the conventional example; Fig. 3(b) illustrates a recording state in the conventional example; Fig. 4 illustrates a positional state of heads disposed with regard to the rotational positions of a support arm in the conventional example; Fig. 5 illustrates a format on a disc employed in the present invention; Fig. 6 illustrates a servo pattern of a servo area in the disc format; Fig. 7 graphically shows the off-tracking characteristic of a fine address servo; Fig. 8(a) illustrates a pattern of an innermost track; Fig. 8(b) illustrates a pattern of a track other than the innermost track; and Fig. 9 illustrates how a shift length is calculated in an embodiment of the present invention. Hereinafter a preferred embodiment of the present invention will be described with reference to the accompanying drawings. Fig. 5 illustrates a format on a disc employed in a disc drive apparatus to which the present invention is applied. This format is based on a sector servo system where a multiplicity of tracks T of mutually different radii are formed around the rotation centre of the disc 4, and servo areas 5 are formed at equal intervals on each track T of the same radius. In the disc 4 based on such sector servo system, one sector is constituted by the servo area 5 and a data area 6 posterior thereto, and several tens of sectors are existent in such a track. Fig. 6 shows one type of possible servo pattern of the servo areas 5, and Fig. 7 graphically shows the off-tracking characteristics of a fine address servo. In Fig. 6, a sector head ST with a recorded sector start signal is formed in a head portion of the servo area 5, and address portions AD1 and AD2 with recorded track addresses are formed posterior to the sector head ST. Immediately anterior to the address portion AD1, there is formed a portion X where a burst signal representing an odd track is recorded; and immediately posterior to the address portion AD2, there is formed a portion Y where a burst signal representing an even track is recorded. The track address is detected by comparing the levels of X and Y with each other and adopting the address of the portion AD1 when X > Y or the address of the portion AD2 when X < Y. Meanwhile, when X = Y, it is recognized that the playback head Hp is positioned between the tracks. Posterior to the address portions A, B, C where signals A, B, C are recorded respectively. The portions A and B are formed at different positions in the longitudinal direction of the track while being spaced apart slightly from the track centre line C2 and are provided integrally with the portions A and B of the adjacent track. Meanwhile the portion C is formed continuously with the corresponding portion of the adjacent track so as to be picked up without fail by the playback head Hp regardless of the scanning course thereof. A calculation of (A - B)/C is executed and then a value FA proportional to the off-track deviation from the track center line Claim 2 as shown in Fig. 7 is computed to detect a fine address FA. In Fig. 7, TP stands for a track pitch, and TW for a track width. The present address of the playback head Hp can be obtained by combining the aforementioned track address with the fine address thus detected. Fig. 8 illustrates the detail of a preferred disc format pattern according to the present invention, wherein each servo area 5 includes a servo track T S , and each data area 6 includes a data track T D . The servo track T S and the data tracks T D denoted with oblique lines are composed of a magnetic layer, while any other region is composed of a nonmagnetic layer. The servo track T S comprises a track address portion T A and three fine address portions A, B, C. A track address signal is recorded in the track address portion T S , while burst signals of one frequency are recorded in the fine address portions A, B, C. The off-track deviation from the centre line C3 of the servo track T S is obtained by calculating an expression (A - B)/C in accordance with the burst signal outputs picked up, and when the width d3 of the fine address portions A and B is rendered equal to the width d2 of the playback head Hp, the servo characteristics can be enhanced to bring about a satisfactory result. The data track T D is a region for recording a data signal, and its width is determined to be smaller than the width d1 of the recording head Hr but greater than the width d2 of the playback head Hp. At the innermost track Ti, as shown in Fig. 8(a), the center line C3 of the servo track T S becomes coincident with the center line C2 of the data track T D . However, at any track other than the innermost track Ti, as shown in Fig. 8(b), the center line C3 of the servo track T S is shifted from the center line C2 of the data track T D . Such shifted position is denoted by S in Fig. 9 and is set by calculating a shift length X of the recording head Hr in the radial direction of the disc with respect to the playback head Hp at each rotational position of the support arm 1 and then shifting the center line C3 by such calculated length X in the reverse direction. The shift length X can be obtained by calculating an equation X = ℓ sin , in which ℓ denotes the distance between the playback head Hp and the recording head Hr, and  denotes the skew angle formed by the track center line C2 and the center line C1 of the support arm 1. The shift length X becomes greater toward the outermost track. Now the function of the above constitution will be described below. When the playback head Hp scans the servo track T S in a playback mode, the playback head Hp picks up a track address signal and a burst signal. The present track address being scanned is identified from the track address signal thus picked up, while the off-track deviation is detected from the burst signal, whereby the playback head Hp is kept scanning the servo track T S in a just-tracking state. At the innermost track Ti, as shown in Fig. 8 (a), the playback head Hp and the recording head Hr are not shifted in the radial direction of the disc, so that the center line C3 of the servo track T S and the center line C2 of the data track T D are positionally coincident with each other. Therefore, both the playback head Hp and the recording head Hr scan the data track Tp in a just-tracking state. Meanwhile at any track other than the innermost track Ti, the playback head Hp and the recording head Hr are shifted in the radial direction of the disc as shown in Fig. 8(b), but since the center line C3 of the servo track T S is shifted from the center line C2 of the data track T D correspondingly to the above shift length, the playback head Hp scans the off-track position while the recording head Hr scans the just-track position. Consequently, the recording head Hr keeps scanning the just-track position on the data track T D and therefore records data signal properly on the data track T D without any exudation or incomplete erasure in the recording mode. Meanwhile the playback head Hp scans the off-track position on the data track T D . However, since the playback head Hp is formed to be narrower than the data track width, the end of the playback head Hp never protrudes outside the data track T D to eventually induce none of impediments to the signal reproduction. As described hereinabove, in a magnetic disc apparatus where a playback head and a recording head are attached individually to a rotary support arm and a discrete track disc is employed as a recording medium, the head positioning system of the present invention calculates a shift length of the recording head in the radial direction of the disc with respect to the playback head at each rotational position of the support arm, and then shifts the center of the servo track in the reverse direction by the calculated shift length from the center of the data track. Therefore the recording head scans the data track in a just-tracking state to eventually ensure a proper data signal recording operation without causing any exudation or incomplete erasure in the recording mode. Meanwhile the playback head scans the data track with an off-track deviation. However, since the playback head is generally formed to be narrower than the data track, it is possible for the playback head to scan the data track without protrusion therefrom, hence attaining an advantageous effect that eliminates any impediment to the signal reproduction.

Claims: A disc drive apparatus with a tracking servo-mechanism comprising: at least one disc medium (4) having data surfaces on which a plurality of concentric tracks (T) are formed, each of said tracks (T) being divided into plural data tracks (T D ) and plural servo tracks (T S ), wherein data signals are recorded and reproduced by transducer means (Hr, Hp) on and from said data tracks (T D ), and servo signals are prerecorded and reproduced by said transducer means (Hr, Hp) on and from said servo tracks (T S ); said transducer means having a first head (Hp) for reproducing said data signals from said data tracks (T D ) in a playback mode, and a second head (Hr) for recording said data signals on said data tracks (T D ) in a recording mode, the width of said first head (Hp) being narrower than that of said second head (Hr); a rotary arm (1) for supporting said transducer means (Hp,Hr) at one end thereof, said rotary arm (1) being rotatably held around an axis (2); actuator motor means for driving said rotary arm (1) in a manner to move said transducer means (Hp,Hr) over said data tracks (T D ) ; servo signal processing means for receiving and processing said servo signals reproduced from said servo tracks (T D ) and outputting a servo control signal; and positioning control means for receiving said servo control signal and controlling said actuator motor means in the playback and recording modes in accordance with said servo control signal; characterised in that the centres of said servo tracks (T S ) deviate from the centres of said data tracks (T D ) by an amount depending on the track address of the corresponding concentric tracks (T), and corresponding to the amount of the deviation in the disc radial direction between said first and second heads of the transducer means (Hp, Hr), such that the second head (Hr) of said transducer means (Hp, Hr) is positioned precisely on any selected one of said data tracks (T D ) while the first head (Hp) of said transducer means (Hp, Hr) is positioned precisely on the corresponding one of said servo tracks (T S ). A disc drive apparatus according to claim 1, wherein said disc medium (4) is a magnetic disc, and magnetic material is provided only on said data tracks (T D ) and servo tracks (T S ). A disc drive apparatus according to claim 1 or 2, wherein said first head (Hp) is of a magentoresistive type.

Inventors: HOSHIMI SUSUMU, SUZUKAWA HIROYUKI, HOSHIMI, SUSUMU, SUZUKAWA, HIROYUKI, HOSHIMI, SUSUMU, C/O SONY MAGNETIC PRODUCTS INC., SUZUKAWA, HIROYUKI, C/O SONY CORPORATION
Applicants: SONY CORP, SONY CORPORATION
Application year: 1990
Main classification: G11B5
IPC codes: G11B21, G11B5