Disk drive with phase-quadrature servo pattern and demodulated position error signal insensitive to timing and phase-misalignment errors

[0001] 1. Field of the Invention

[0002] This invention relates generally to magnetic recording hard disk drives, and more particularly to disk drives that use phase-quadrature servo patterns as part of the head-positioning servo control system.

[0003] 2. Description of the Related Art

[0004] In a magnetic recording hard disk drive, the read/write head is moved across the data tracks of the rotating disk and maintained on the data tracks by an actuator, typically a rotary voice-coil-motor (VCM) actuator. The head detects a position-error-signal (PES) pattern from angularly-spaced servo sectors on the disk and generates a servo signal that is demodulated to provide the PES. The disk drive servo control system receives the PES and generates a control signal to the actuator to maintain the head on track and move it to the desired track for reading and writing of data. Each read/write head is attached to the end of a head carrier or air-bearing slider that rides on a cushion or bearing of air above the rotating disk. The slider is attached to a relatively flexible suspension that permits the slider to “pitch” and “roll” on the air bearing, with the suspension being attached to the end of the VCM actuator arm.

[0005] One type of PES pattern is a phase-quadrature pattern that has two generally like patterns radially-spaced in the servo track, but with one pattern phase shifted along-the-track from the other pattern. The pattern is called a quadrature pattern because it includes four bands spaced along-the-track, with the third and fourth bands radially spaced by one-half track from the first and second bands. The conventional servo-signal demodulator for a phase-quadrature pattern provides a PES that is a function of two amplitudes, with the two amplitudes directly corresponding to the servo signals from the two phase-shifted patterns. However, the conventional demodulator does not account for clock errors in reading the servo pattern and for phase-misalignment errors between the two phase-shifted patterns.

[0006] What is needed is a disk drive with a phase-quadrature PES pattern and a servo signal demodulator that is insensitive to clock errors and phase misalignment errors.

[0007] The invention is a disk drive with a phase-quadrature PES pattern and a servo signal demodulator that is insensitive to clock errors and phase-misalignment errors. Each servo sector has radially-spaced servo tracks that contain a phase-quadrature pattern of servo blocks. A first band in each servo sector contains two generally like patterns radially-spaced in the servo track with the second pattern phase-shifted 90 degrees along-the-track from the first pattern. A second band in each servo sector is spaced along-the-track from the first band and also has two patterns like those in the first band, but with the phase shift of the second pattern being in the opposite direction to the phase shift of the second pattern in the first band.

[0008] The servo signal from each band is the sum of a sine function and a cosine function. The demodulator calculates the sine and cosine amplitudes of the servo signals from each band and uses these four values to calculate the true amplitudes of the servo signal. The true amplitudes of the servo signal are the amplitudes after removal of clock errors and phase-misalignment errors. The PES is then calculated from the true amplitudes and used by the control system to generate the control signal to the actuator.

[0009] For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures.

[0020] FIG. 1 is a block diagram of a disk drive according to the present invention. The disk drive uses servo positioning information located in angularly-spaced servo sectors for positioning the read/write heads. The disk drive, generally designated as 102, includes data recording disk 104, a voice coil motor (VCM) 110 as the actuator, an actuator arm 106, a suspension 107, a head carrier or air-bearing slider 108, a data recording transducer 109 (also called a head, recording head or read/write head), read/write electronics 113, servo electronics 112, and servo control processor 115.

[0021] The recording head 109 may be an inductive read/write head or a combination of an inductive write head with a magnetoresistive read head and is located on the trailing end of slider 108. Slider 108 is supported on the actuator arm 106 by a suspension 107 that enables the slider to “pitch” and “roll” on an air-bearing generated by the rotating disk 104. Typically, there are multiple disks stacked on a hub that is rotated by a disk motor, with a separate slider and recording head associated with each surface of each disk.

[0022] Data recording disk 104 has a center of rotation 111 and is rotated in direction 130. The disk 104 has a magnetic recording layer with radially-spaced concentric data tracks, one of which is shown as track 118. The disk drive in FIG. 1 is illustrated as a zone-bit-recording (ZBR) disk drive because the data tracks are grouped radially into a number of annular data zones or bands, three of which are shown as bands 151, 152 and 153, but the invention is fully applicable to a disk drive that does not use ZBR, in which case the disk drive would have only a single data band. Each data track has a reference index 121 indicating the start of track. Within each band, the tracks are also circumferentially divided into a number of data sectors 154 where user data is stored. If the disk drive has multiple heads, then the set of tracks which are at the same radius on all disk recording layers is referred to as a “cylinder”.

[0023] Each data track also includes a plurality of circumferentially or angularly-spaced servo tracks. The servo tracks are also aligned radially so that they extend across the data tracks in a generally radial direction, as represented by radially-directed servo sectors 120. The servo positioning information in each servo sector typically includes a servo timing mark (STM), a track identification (TID) code, and a pattern of servo blocks that are decoded to provide a head position-error-signal (PES). The pattern of servo blocks, also called the PES pattern, may be high-frequency bursts of magnetic transitions as in conventional disk drives, or discrete magnetized islands or blocks separated by nonmagnetic regions as proposed for future disk drives.

[0024] The servo positioning information in the servo sectors is detected by the read head, sent to the read/write electronics 113, and input to the servo electronics 112 as a clocked data stream. The read head is controlled by a read clock. The servo electronics 112 provides digital signals to servo control processor 115. The servo control processor 115 provides a control signal 196 to VCM driver 198 that controls current to the VCM 110 to position the head 109.

[0025] Within the servo electronics 112, the STM decoder 160 detects the STM from the clocked data stream. Once an STM has been detected, an STM found signal is generated. The STM found signal is used to adjust timing circuit 170, which controls the operating sequence for the remainder of the servo sector. After detection of an STM, the track identification (TID) decoder 180 receives timing information from timing circuit 170, reads the TID, which is typically Gray-code encoded, and then passes the decoded TID information to servo control processor 115. Subsequently, the analog servo signal 183 from the clocked data stream is converted to a digital servo signal by digitizer 185 in servo electronics 112 and passed to the servo control processor 115 as a digital servo signal 187. The servo signal from the clocked data stream from R/W electronics 113 is the analog signal from the read head as the read head detects the PES pattern of servo blocks.

[0026] The servo control processor 115 is typically a programmable controller or microprocessor with associated memory 190. The processor 115 includes a stored demodulator program 192 that calculates a PES from the digitized servo signal, and a stored control program 194 that uses the PES to calculate the actuator control signal 196 sent to VCM driver 198. The control program 194 recalls from memory a set of parameters based on the static and dynamic characteristics of the “plant” being controlled, i.e., the VCM 110. The control program 194 is well-known and is essentially a matrix multiplication algorithm, and the controller parameters are coefficients used in the multiplication and stored in the memory 190 of processor 115.

[0027] FIG. 2 shows the PES pattern of servo blocks in a portion of a typical servo sector according to the present invention. The servo blocks in the sector are arranged in four circumferentially-spaced bands B1, B2, B3 and B4, with three typical servo tracks ST1, ST2 and ST3 being depicted. While only three servo tracks are depicted, the servo sector with bands B1–B4 extends radially across all the data tracks. For conventional magnetic recording disks, the shaded blocks and white blocks FIG. 2 represent high-frequency bursts of magnetic transitions magnetized in opposite directions, respectively. For example, for horizontal recording the shaded blocks represent magnetization in one direction along-the-track in the plane of the disk, and the white blocks represent magnetization in the opposite direction. For perpendicular recording the shaded blocks represent magnetization in one direction perpendicular to the plane of the disk, and the white blocks represent magnetization in the opposite direction. For disks with discrete magnetized islands or blocks separated by nonmagnetic regions, as proposed for future disk drives, the shaded blocks can represent magnetization in one direction, either in the plane or perpendicular to the plane of the disk, and the white blocks can represent the nonmagnetic regions, or vice-versa.

[0028] Servo track ST1 in band B1 includes a first pattern P1 of blocks along-the-track, and a second pattern P2 of blocks along-the-track but shifted along-the-track a distance of approximately one-half the circumferential width of a servo block. This is shown in B1 as a −90 degree phase shift. Servo track ST1 in band B2 includes a first pattern P1 of blocks along-the-track, and a second pattern P2 of blocks along-the-track. P1 and P2 in bands B1 and B2 are identical, but P2 in B2 is shifted from P1 in the direction opposite to the shift of P2 in B1. This is shown in B2 as a +90 degree phase shift. The PES pattern of servo blocks in bands B3 and B4 is identical to the previously-described pattern in B1 and B2, except that the pattern in B3 and B4 is shifted radially by one-half the radial width of a servo track. The PES pattern of servo blocks shown and described with respect to FIG. 2 is called a phase-quadrature PES pattern.

[0029] FIG. 3A depicts the phase quadrature of patterns P1, P2 from band B2 and the associated analog voltage signal V(t) as the patterns pass read head 200. The two signals from the two patterns P1, P2s are said to be in “phase quadrature” if the timing difference between them is equal to one quarter of the period (or if the phase difference is 90 degrees). The signal from the read head is the linear superposition of the contribution from the two patterns, and can be written as:
V(t)=C2 sin(ωt)+S2 cos(ωt)  Eq. (1)
where C2 is the amplitude of the sine function from pattern P1 and S2 is the amplitude of the cosine function from pattern P2. FIG. 3B is the associated phase diagram showing vector V and its C2 and S2 components.

[0030] The C2 and S2 amplitudes are the measured amplitudes from the measured or detected signal V(t) and are used to calculate the PES, with the term (C2-S2) representing radial position of the head 100 in the servo track. The C2 and S2 amplitudes can be determined by multiplying both sides of Eq. (1) by sin(ωt) or cos(ωt), respectively, and integrating over the period from 0 to T, resulting in the following equations:
C2=(2/T) INTEGRAL [V(t) sin(ωt)dt]  Eq. (2A)
S2=(2/T) INTEGRAL [V(t) cos(ωt)dt]  Eq. (2B)

[0031] The prior art servo signal demodulator assumes that the C2 and S2 measured amplitudes are the true amplitudes A and B, respectively, from the sine and cosine signals generated by the two patterns P1, P2, respectively, and thus generates or calculates A and B directly from equations 2A–2B. The calculated A and B terms are then used to calculate the PES, typically as A−B or atan (A/B).

[0032] However, C2 and S2 would be the true amplitudes A and B, respectively, only if there were no clock errors when reading P1 and P2 and no timing errors when P1 and P2 were servowritten. However, if there is a clock error θ when the head reads the patterns, as shown in FIG. 4A, then the true amplitudes A and B may be calculated by rotating the C and S coordinate system by the angle θ in the phase diagram shown in FIG. 4B. This results in the following equations for C2 and S2:
C2=A cos θ−B sin θ  Eq. (3A)
S2=B cos θ+A sin θ  Eq. (3B)

[0033] In addition to a clock error, there may be a misalignment of the two patterns P1, P2 along-the-track so that the phase shift between the two patterns is not precisely 90 degrees (i.e., not precisely one-half the circumferential width of a block). This may be the result of an error when the servo tracks are written. The result would be a phase misalignment φ, as shown in FIG. 5A. In the presence of the phase misalignment φ then the true amplitudes A and B are related to C2 and S2 as shown in the phase diagram of FIG. 5B. This results in the following equations for C2 and S2:
C2=A cos φ  Eq. (4A)
S2=B+A sin φ  Eq. (4B)

[0034] When both a clock error θ and a phase misalignment φ are present, as depicted in FIG. 6A, then the true amplitudes A and B are related to C2 and S2 as shown in the phase diagram of FIG. 6B. This results in the following equations for C2 and S2:
C2=A cos(θ+φ)−B sin θ  Eq. (5A)
S2=A sin(θ+φ)+B cos θ  Eq. (5B)

[0035] The above description of the effect of clock and phase misalignment errors on the PES has been explained with respect to band B2 (FIG. 2). The same effect will occur in band B1, resulting in the following equations correlating the C and S amplitudes with the true amplitudes A and B in band B1:
C1=A cos(θ+φ)+B sin θ  Eq. (6A)
S1=A sin(θ+φ)−B cos θ  Eq. (6B)

[0036] From equations 5A–5B and 6A–6B, the true amplitudes A and B can be derived from the measured C and S amplitudes when both bands B1 and B2 pass the read head. The following equations result:
A=SQRT{0.25[(S1+S2)²+(C1+C2)²]}  Eq. (7A)
B=SQRT{0.25[(S1−S2)²+(C1−C2)²]}  Eq. (7B)

[0037] Equations 7A–7B enable the PES to be calculated from the true amplitudes A and B, with clock errors and phase misalignment errors removed. The calculated PES represents the radial position of the read head in a servo track, and is typically calculated from A and B by one of the following equations:
PES=A−B  Eq. (8A)
PES=atan(A/B)  Eq. (8B)
The servo control processor (FIG. 1) uses the PES as the input to the control program to generate the actuator control signal 196 to reposition the read head in the servo track.

[0038] For ease in explanation, the present invention has been explained using supposedly distinct “sine” and “cosine” functions. However, as is well known, a cosine function and a sine function are identical, except for a 90 degree shift in the origin. Thus the invention could have been explained using two sine functions with a 90 degree shift between them, or two cosine functions with a 90 degree shift between them, but such an explanation would have been unnecessarily complicated. Thus reference to these terms in the claims is not to be considered limiting.

[0039] While the present invention has been particularly shown and described with reference to the preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope of the invention. Accordingly, the disclosed invention is to be considered merely as illustrative and limited in scope only as specified in the appended claims.