Magneto-resistive removable hard disk cartridge and disk drive and quadrature embedded servo bursts

Abstract: A disk drive comprises a magneto-resistive read head (20) and an inductive write head (22) for use with removable hard disk cartridges. A four-bust embedded servo is written and read to guide the servo-positioning of the magneto-resistive read head over data tracks (24) recorded on the removable hard disk (12).

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates generally to computer memory and more specifically to removable hard disk cartridges and disk drives. 2. Description of the Prior Art The recording and reading of rotating magnetic hard disks has conventionally included the use of inductive read/write heads. However, as track densities have increased, conflicts have arisen between optimum read head design and optimum write head design. Higher recording frequencies require less inductance and therefore fewer coil turns in the head. The narrower recorded data tracks produce fainter pickup signals, so more sensitivity is required and this means more coil turns are needed in the head. Magneto-resistive materials have recently been employed in magnetic disk recording read heads. Such materials experience a change in their resistance in the presence of magnetic flux. A change of two percent in the resistance is typical. However, the change in resistance is not linear and a magnetic bias is required to move the center point of operation into a small area that is linear. Such bias is typically provided by a small permanent magnet positioned nearby. Magneto-resistive (MR) heads are easily saturated by ordinary disk recording levels and for this reason, hard disks recorded by ordinary inductive read/write (R/W) heads are not always compatible with MR head disk drives. In fixed disk drives this is not a problem because the read and write heads and hard disk are part of a permanent set. In removable hard disk drives this can be a significant problem area. MR heads typically have narrow pickup widths. Within a comparatively widely-written data track, the narrow read width is not a problem. In fact, some extra degree of disk run out and track misregistration can be tolerated as the MR head can be allowed to wander within a data track radially without producing a concomitant read-amplitude variation. Prior art embedded servos are a problem with MR heads because of the narrow read width. A typical prior art embedded servo consists of two bursts, an "A" burst and a B" burst, that head a block on a data track. The "A" burst is offset radially one-half of a track width, and the "B" burst is offset radially in the opposite direction one-half of a track width and longitudinally behind the "A" burst. An inductive R/W head passing the "A" and "B" bursts while at center track will glance both bursts equally and will produce output pulses for each of equal amplitude. If the R/W head is off center, the pulses will not be equal in amplitude, and which one has the greater amplitude will indicate which radial direction the R/W head is from dead center. A servo system is conventionally employed to keep the R/W head at track center in real time, even if the recorded track is not perfectly circular or perfectly concentric with the disk hub. A typical MR head has a read width so narrow that the "A" or "B" servo bursts may not be seen at all as the head moves from track center by more than 25% of a track width. Further complicating the use of MR heads with embedded servo features is the fact that MR heads often do not present symmetrical responses, thus comparative "A" and "B" burst amplitudes are not necessarily representative of the MR head position between the bursts. In fact, a properly positioned MR head can fail to recognize its good position by responding differently to bursts that are on opposite sides and/or that have opposite polarities. The asymmetry of MR element off-track performance can be adequately modeled as the area of the stripe of MR material subjected to a magnetic field as the element passes over an isolated track written with an inductive head. The magnetic field propagates through the MR stripe normal to the angle of MR bias. This angle of incidence along with a nearby permalloy shield casts a magnetic shadow over the element. Such a shadow causes the asymmetry in sideways reading that an MR element shows when it moves off track in opposite directions. Thus the degree of asymmetry is dependent on the MR stripe height and bias angle. Such subjects are discussed by A. Wallash, et al., in "Dependence of magneto-resistive head read back characteristics on sensor height", published in the Journal of Applied Physics , 69(8), 15 April 1991. Simply moving the "A" and "B" bursts such that they are offset radially from track center so that they can be detected by the MR head at a certain track offset is not completely satisfactory because the detection of other track offsets by the MR head will be compromised. The MR head can get lost in the blank area that would be created between adjacent tracks. EP-A-0 524 839 discloses a magnetooptical drive. A floating slider supports an inductive transducer head, and on the top of this head, a Hall effect sensor for sensing the direction of the flux of a rotatable permanent magnet. WO-A-93 18507 discloses a removable cartridge disk drive, using an inductive head for reading and writing to and from a recording media rotating inside of the cartridge. US-A-5 151 294discloses a method for forming a protective film on a slider air bearing surface in a magnetic recording disk file. This document mentions that recent advances in read/write head technology have resulted in possible use of magnetoresistive heads in disk files. There is no suggestion as to the use of such MR heads in removable cartridges. US-A-4 729 048 discloses a combination magnetic transducer apparatus, for hard disk drives. This apparatus comprises a MR head, and two inductive heads. The MR head is used for tracking purposes, while the inductive heads are used for writing and reading to and from the disk. The prior art describes magneto-resistive read heads only in fixed hard disk drives. The problems that are encountered in removable hard disk applications have so far been left unaddressed, even by those advanced in the development of the art, e.g., IBM. SUMMARY OF THE PRESENT INVENTION It is therefore an object of the present invention to provide a magneto-resistive removable hard disk cartridge system. It is another object of the present invention to provide hard disk cartridge system that provides for the interchange-ability of hard disk cartridges between disk drives. It is an object of the present invention to provide hard disk cartridge system that provides consistent recording levels on the hard disk cartridges. It is a further object of the present invention to provide an embedded servo method for magneto-resistive hard disk recording. The invention is defined in the appended claims. Briefly, an embodiment of the present invention comprises a disk drive with a magneto-resistive read head and an inductive write head for use with removable hard disk cartridges. A four-burst embedded servo is written and read to guide the servo-positioning of the magneto-resistive read head over data tracks recorded on the removable hard disk. An advantage of the present invention is that a magneto-resistive hard disk drive is provided that accepts removable cartridges. Another advantage of the present invention is that an embedded servo method is provided for magneto-resistive hard disk recording. These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment which is illustrated in the drawing figures. IN THE DRAWINGS Fig. 1 is a perspective view of a removable disk cartridge and disk drive embodiment of the present invention; Fig. 2 is a cutaway view of the disk cartridge in place in the disk drive of Fig. 1; Fig. 3 is a diagram of an embedded servo burst and data block for a typical data track shown in the hard disk of Fig. 2; Fig. 4 is a graph of the typical position error signal (PES) read versus off-track position of a MR head tested, with a width of one-half track, at a density of 4,200 tracks per inch; Fig. 5 is a graph of the normalized burst amplitudes of the disk drive of Fig. 1 and geometry of Fig. 4 plotted against off-track position of the MR head; and Fig. 6 is a block diagram of the electronics used to support the recording and reading operations of the heads included in the disk drive of Fig. 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 1 illustrates a removable cartridge disk drive embodiment of the present invention, referred to herein by the general reference numeral 10. Disk drive 10 accepts a removable cartridge 12 that includes a magnetic hard disk 14. Fig. 2 shows disk drive 10 in cutaway with the removable cartridge 12 in position for disk storage read/write operation. A hub 16 included in disk drive 10 engages the hard disk 14 and rotates it at a controlled speed on a hub 16. An actuator arm 18 carries a magneto-resistive (MR) read head 20 for reading and an inductive head 22 for writing a plurality of data tracks 24 with embedded servo features on the surface of hard disk 14. Since MR read head 20 is subject to damage by electrostatic discharge (ESD), it is preferable to ground hard disk 14 through hub 16 to disk drive 10 and to ground MR read head 20 also to disk drive 10. Overall, the voltage difference between the MR read head 20 and hard disk 14 is preferably controlled to not exceed two or three volts. A coating of carbon-like material, such as diamond, or other such hard and wear resistant material is preferably placed over MR read head 20 to protect it from its regular contact with hard disk 14. Such coatings for wear protection are conventional and therefore an illustration of this simple measure is unnecessary here. Fig. 3 illustrates a typical data track 24 which comprises a servo frame 26 followed by a data block 28 which is 80% of the nominal track width. The ideal center of all the data tracks 24 are circles concentric with one another and hub 16. However, in practice, the data tracks 24 will have some amount of wobble, or run out, and it is the job of a servo system to not only position the heads 20 and 22 over an appropriate data track 24, but to cause the actuator 18 to be moved such that the heads 20 and 22 will follow the eccentricities. In general, it may be necessary to position the heads at an offset to the center of a track to optimize the writing of data on the track. Each servo frame 26 begins with an "A" burst 30, which will produce a di-bit pulse series 31 in the MR head 20 at a time t0. This is followed by a "B" burst 32, which will produce a di-bit pulse series 33 at the center time t1. At a time t2, a "C" burst 34 will produce a di-bit pulse series 37. The fourth burst time period, a time t3, has a "D" burst 36, which will produce a di-bit pulse series 37. Given that the MR head 20 has a response that results in a read width so narrow that only radially adjacent bursts can be simultaneously detected, the relative radial position of the MR head 20 anywhere over data track 24 can be ascertained. For di-bit pulse series 31, 33, 35 and 37, a servo burst frequency of five megahertz and a length of 4.6 microseconds has produced good results. If the MR head 20 is off track by +50% of a full track width, "A" burst 30 and adjacent track "B" burst 32 will produce equal amplitude t0 and t1 di-bit pulse series 31 and 33. No t2 di-bit pulse series 35 will be present and t3 di-bit pulse series 37 will be stronger than normal. Other track offsets can similarly be determined and an appropriate servo signal can be formulated using conventional techniques to move actuator 18 radially inward or outward. Fig. 4 graphs a position error signal (PES) expected for various off-track positions of the MR head 20. Fig. 5 is a graph of the expected and normalized burst amplitudes of di-bit pulse series 31, 33, 35 and 37, plotted against various off-track positions of the MR head 20. A linear portion of at least of each "A" through "D" burst is available for servo-positioning over the entire width of track 24. Each inductive write head 22 and MR read head 20 will typically have a response or signal sensitivity that varies from one production unit to the next. Therefore, it is preferable to equip disk drive 10 with some form of non-volatile memory that can be used to store a digital value representing a write current amplitude for driving inductive write head 22. By controlling the write current, the read back signal quality becomes more consistent and the population of MR read heads that can be used in production is increased. Therefore, as illustrated in Fig. 6, disk drive 10 preferably has connected to heads 20 and 22 a quadrature servo burst detector 60 for interpreting di-bit pulse series 31, 33, 35 and 37, a read back amplifier 62 that provides a recorded data output, a write circuit 64 that provides an input for data to be recorded, a memory 66 with which to store a representation of a write current amplitude, and a variable gain amplifier 68 that varies the output current applied to write head 22 in accordance to the value stored in the memory 66.

Claims: A rotating disk drive for the magnetic storage of computer data on a rotating magnetic hard disk (14) comprising a plurality of concentric data tracks (24) and a servo track pattern with positioning bursts, within a removable cartridge (12), comprising: an actuator arm (18) carrying an inductive head (22) for writing on said rotating magnetic disk; cartridge receiving means proximate to the head for accepting and ejecting said removable cartridge, characterized in that it further comprises a magneto-resistive read head (20) for reading information from said rotating magnetic disk; in that the reading width associated with the magneto-resistive read head is less than the recording width of the inductive head on the magnetic disk, and in that over the entire width of a track, a linear portion of the burst amplitude of at least one burst is detected by said read head for servo-positioning said read head. The drive of claim 1, wherein said magneto-resistive read head (20) is arranged for reading information from a same track of said rotating magnetic disk where said inductive head (22) writes. The drive of claim 1 or 2, further comprising a protective layer of hard material placed over the magneto-resistive read head for protecting it from contact with said rotating magnetic disk (14). The drive of claim 1, 2 or 3, further comprising read back means (62) connected to the magneto-resistive read head for reading a track of data previously recorded by an inductive head with said removable cartridge hard disk. The drive of one of claims 1 to 4, further comprising spindle rotation means connected to said cartridge receiving means for rotating said removable cartridge hard disk at a first speed while reading a track of data previously recorded by an inductive head at a second speed with said removable cartridge hard disk, wherein said first and second speeds are independent. The drive of one of claims 1 to 5, wherein the magnetic disk comprises on its surface a plurality of concentric data tracks (24) with embedded servo-features, said embedded servo-features comprising four servo bursts (30, 32, 34, 36) for each data block (28). The drive of claim 6, wherein said servo bursts have half of the width of said data track. The drive of claim 6, wherein the width of said data block is 80% of the nominal width track. The drive of claim 6, wherein the width of said burst is 0.625 of the width of said data block. The drive of one of claims 1 to 9, wherein said servo bursts are written a four times the track radial density. The drive of one of claims 1 to 10, wherein the magneto-resistive read head is grounded to the disk drive. The drive of one of claims 1 to 11, wherein the hard disk is grounded to the disk drive. The drive claim 11, wherein the hard disk has a hub (16) and is grounded through the hub. The drive of claims 11 and 12, wherein the drive comprising means for maintaining under two or three volts the voltage difference between the magneto-resistive disk and the hard disk. A removable cartridge (12) containing a rotatable magnetic hard disk (14) having on its surface a plurality of concentric data tracks (24) and a servo track pattern with positioning bursts, characterised in that over the entire width of a track, a linear portion of the burst read amplitude of at least one burst is available for servo-positioning the magneto-resistive read head of the diskdrive according to claim 1. The cartridge of claim 15, wherein four servo bursts (30, 32, 34, 36) are provided for each data block (28) on a track. The cartridge of claim 15 or 16, wherein said servo bursts have half of the width of said data track. The cartridge of claim 17, wherein the width of said data block is 80% of the nominal width track. The cartridge of claim 17, wherein the width of said burst is 0.625 of the width of said data block. The cartridge of one of claims 15 to 19, wherein said servo bursts are written a four times the track radial density. A process for positioning a magneto-resistive read head (20) in a disk drive receiving a magnetic hard disk (14) within a removable cartridge (12), the disk having a plurality of concentric data tracks (24) and a servo track pattern with positioning bursts, a linear portion of the burst amplitude of at least one burst being available over the entire width of a track for servo-positioning said read head the process comprising: rotating the magnetic hard disk (14) within the removable cartridge (12); analyzing a read-back signal obtained by the magneto-resistive read head passing over the servo track pattern, and positioning the magneto-resistive read head (20) according to the analyzed signal.

Inventors: COMBE ANTHONY DAVID, COMBE, ANTHONY DAVID, Combe, Anthony David, Myrica Ltd.
Applicants: IOMEGA CORP, IOMEGA CORPORATION
Application year: 1995
Main classification: G11B5
IPC codes: G11B5, G11B23, G11B25