Patent Application
20110317302
This application is based upon and claims the benefit of priority from Japanese Patent Application No, 2010-144146, filed Jun. 24, 2010; the entire contents of which are incorporated herein by reference.
Embodiments described herein relate generally to a technique of measuring the flying height of a head incorporated in a disk storage apparatus.
In most disk storage apparatuses (hereinafter referred to as “disk drives” in some cases), a representative example of which is the hard disk drive, a head reads or writes data from or to a disk used as a recording medium, while it is flying above the disk. The flying height of the head influences the data recording and reproducing characteristic of the disk drive, and should therefore be set to an optimal value.
A general architecture that implements the various features of the embodiments will now be described with reference to the drawings. The drawings and the associated descriptions are provided to illustrate the embodiments and not to limit the scope of the invention.
Various embodiments will be described hereinafter with reference to the accompanying drawings.
To accomplish achieving a high recording density, a technique of reducing the head flying height with respect to the disk has been developed in recent years. The dynamic flying height (DFH) control is broadly practiced as a technique of controlling the head flying height. The DFH control is a heat control to maintain the head flying height at a constant value. More specifically, a current is made to flow in the heater coil mounted on the head slider, thereby heating the distal end of the slider, on which the read/write element of the head is mounted. So heated, the distal end of the slider undergoes thermal expansion.
The head flying height is known to change as the ambient temperature changes and as the head moves in the radial direction of the disk. The higher the ambient temperature, the smaller the head flying height will be. Conversely, the lower the ambient temperature, the larger the head flying height will be. In most cases, if the head flying height is large, the error rate during the reproduction of data from the disk will be high.
In view of this, a technique has been recently proposed, in which the head flying height is measured at high precision all the time the disk drive is operating, and the DFH control maintains the head flaring height at a constant small value.
An effective method of measuring the head flying height at high precision is to record measurement signals on the disk and to calculate the head flying height from the measurement signals read by the head from the disk. During the magnetic recording, a phenomenon called either the superparamagnetic effect or thermal decay may occur. This phenomenon causes the magnetization of the recording layer of the disk to deteriorate with time, ultimately degrading the signals recorded in the recording layer. As a result, the measurement signals are also degraded with time, possibly making measuring errors in the method of measuring the head flying height.
In general, according to one embodiment, a disk storage apparatus includes a read module, a calculation module, and a compensating module. The read module is configured to read first and second signals recorded on a disk, from which to measure the flying height of a head. The calculation module is configured to calculate the flaring height of the head from the first and second measurement signals read by the read module. The compensating module is configured to compensate for the flying height calculated by the calculation module, which includes error resulting from the superparamagnetic effect.
The embodiment will be described with reference to the accompanying drawings.
[Configuration of the Disk Drive]
As seen from
The HDA has a disk 1, a spindle motor (SPM) 2, an arm 3, and a voice coil motor (VCM) 4. The disk 1 is a recording medium for achieving perpendicular magnetic recording. The arm 3 holds a head 10. The spindle motor 2 rotates the disk 1. The arm 3 and the VCM 4 constitute an actuator. The VCM 4 drives the arm 3, whereby the head 10 is moved to a designated position above the disk 1.
The head 10 has a slider, a read head element and a write head element. The slider is the main unit of the head 10, and holds both the read head element and the write head element. The read head element reads data 100 from the disk 1. (The data 100 contains measurement signals, which will be described later.) The write head element writes data 110 to the disk 1. (The data 110 contains measurement signals, which will be described later.)
A heater coil (not shown), which is indispensable to DFH control, is mounted on the slider. In the DFH control, a current is made to flow in the heater coil, thereby heating the distal end of the slider, on which the write head element is mounted. So heated, the distal end of the slider expands, reducing the flying height. Conversely, the amount of heat applied to the distal end of the slider may be decreased, contracting the distal end of the slider. In this case, the flying height is increased. The head-disk assembly (HDA) includes a temperature sensor that starts detecting the temperature in the disk drive when the power switch of the disk drive is turned on.
The head amplifier IC 11 has a read amplifier and a write driver. The read amplifier amplifies any read signal the read head element has output, and then supplies the read signal to the read/write (R/W) channel 12 provided in the HDC 15. The write driver receives any write signal current 110 output from the R/W channel 12 and corresponding to write data, and supplies the write signal current 110 to the write head element. The head amplifier IC 11 includes a heating driver configured to make a current flow in the heater coil during the DFH control.
The HDC 15 includes a disk controller 13 and a microprocessor (CPU) 14, in addition to the R/W channel 12. The R/W channel 12 includes a read channel and a write channel. The read channel processes any signal representing read data. The write channel processes any signal representing write data. The R/W channel 12 further includes a harmonic sensing control (HSC) module 16. The HSC module 16 calculates a flying height of the head 3 from the flying height measurement signals (also called “measurement signals” in some cases) recorded on the disk, as will be explained later.
The disk controller 13 performs an interface control, controlling the data transfer between a host system (not shown) and the R/W channel 12. The disk controller 13 includes a DFH control module, which performs DFH control, controlling the current supplied to the heater coil through the head amplifier IC 11. The CPU 14 is the main controller of the disk drive, and performs the servo control to position the head 4 at a desired position over the disk 1 and the data read/write control. The CPU 14 also performs a process of correcting the flying height measured by the HSC module 16, and cooperates with the DFH control module to control the flying height of the head 10.
[Method of Measuring the Flying Height]
The principle of measuring the flying height of the head 10 will be explained first.
The relationship that flying height d has with the amplitude A of a signal the head 10 reproduces from a signal recorded at wavelength λ is given by the following equation (1), which is called Wallance's equation:
d=(−λ/2π)·log A+C (1)
where A=C exp(−2πd/λ), C is an indefinite constant that does not depend on the flying height d, and log is the natural logarithm.
Since the equation has constant C, flying height d cannot be directly obtained. Therefore, amplitude A0 is measured while the head 10 remains contacting the disk 1. The relationship that relative flying height d0 has with the amplitude A0 is determined as: d0=(−λ/2π) log A0+C.
The absolute flying height d the head 10 has with respect to the disk 1 is obtained by the following equation (2), from the relationship between d and amplitude A defined in the equation (1) and the relationship between d0 and amplitude A described above:
d−d
0=(−λ/2π)·(log A−log A0) (2)
As seen from the equation (2), the indefinite constant C is cancelled. As a result, if d0=0, the absolute flying height d can be obtained by the following equation (3):
d=(−λ/2π)·(log A−log A0) (3)
In the disk drive, however, the gain of the read amplifier incorporated in the head amplifier IC 11 changes because of, for example, changes in temperature. That is the signal amplitude A changes due to a factor other than the flying height. To eliminate such a change in the signal amplitude A, thereby to make the disk drive more practical, the flying height d can be calculated by the following equation (4) having amplitude ratio Afa/Afb between two frequency components Afa and Afb of the same reproduced signal, obtained at two frequencies fa and fb, respectively:
d=K·log(Afa/Afb)+C (4)
where Afa/Afb=[Ca exp(−2πd/λa)]/[Cb exp(−2πd/λb)], λa=v/fa and λb=v/fb at the circumferential speed v of the disk 1, K=1/[2π(1/λb−1/λa)]=v/[2π(fb−fa)], and Ca and Cb are indefinite constants that do not depend on the flying height d.
Amplitude components A0fa and A0fa are measured while the head 10 remains contacting the disk 1. The relationship that relative flying height d0 has with the amplitude components AOfa and A0fa is determined as: d0=K log(A0fa/A0fa)+C. On the other hand, the absolute flying height the disk 10 has with respect to the disk 1 is obtained by the following equation (5), from the relationship of the frequency components Afa and Afb and the relative flying height d0:
d−d
0
=d=K·[log(Afa/Afb)−log(A0fa/A0fb)] (5)
Here, the indefinite constant C is cancelled. Thus, C=0 in most cases. Hereinafter, C will be regarded as zero (C=0). The frequencies fa and fb used to measure the amplitude of the signal can be set to any values. However, a signal is usually used repeatedly, which has a specific frequency. The amplitude component Af1 of the fundamental wave f1 of the signal and the amplitude component of the third harmonic f3 of the signal are utilized. In order to extract these amplitude components at the frequencies f1 and f2, respectively, the signal is subjected to discrete Fourier transformation (DFT).
The principle of measuring the flying height of the head 10 has been explained. If the flying height is measured the disk drive in accordance with the principle, however, the following events will take place.
To cancel the indefinite constant C, thereby to find the absolute flying height d, it is necessary to obtain not only the amplitude components Afa and Afb at the flying height, but also the amplitude components A0fa and A0fb at time the head 10 remains contacting the disk 1. Any measurement reproduced from the disk 1 inevitably changes in amplitude. Therefore, the amplitude components A0fa and A0fb at time the head 10 must be measured again, while keeping the head 10 in contact with the disk 1.
If the head 10 frequently contacts the disk 1, either the head 10 or the disk 1 will be worn, possibly reducing the performance of the disk drive. This is why each measurement signal is recorded only once, not recorded again, in most cases. Nonetheless, the disk 1 is gradually demagnetized as time passes, because of the phenomenon known as the superparamagnetic effect or thermal decay, as pointed out above. After a long lapse of time from the recording of the flying height measurement signal, the signal changes due to the superparamagnetic effect, making a flying height measuring error.
To be more specific, the amplitude components A0fa and A0fb measured while the head 10 is contacting the disk 1 increases α times and β times, changing to αAfa and βAfb, respectively. The relative flying height dr measured in this condition is: dr=K log(αAfa/βAfb)=K log(Afa/Afb)+K log(α/β). That is, a flying height measuring error, K log(α/β), is made. The error K log(α/β) is a parameter that does not depend on the flying height measured. The error changes, depending on only the time passed after the recording, no matter how the flying height of the head 10 changes.
A method of measuring the flying height in this embodiment will be explained with reference to
In the disk drive, the CPU 14 measures the flying height of the head 10 at regular time intervals. As shown in
More precisely, as shown in
After the disk drive has been shipped from the manufacturer, the CPU 10 causes the head 10 to read the measurement signals S1 and S2 from the disk 1. The measurement signals S1 and S2 thus read are supplied to the R/W channel 12. The HSC module 16 of the R/W channel 12 calculates the mean value of the flying heights of the head 10 from the measurement signals S1 and 32, in accordance with the measuring principle described above (Blocks 21 and 22). Assume that correct flying height FH(t) is measured at time t. Further assume that flying height FH1(t) and flying height FH2(t) are obtained from a measurement signal S1 and a measurement signal 32, respectively.
Then, the CPU 14 acquires measuring error data from a flash memory (Block 23). The measuring error data represents the flying height measuring errors E1 and E2 and the ratio G of error E2 to error E1 (G=E2/E1). Using the measuring error ratio G, the CPU 14 corrects the flying height calculated by the HSC module 16, calculating correct flying height FH(t) (Block 24).
This process of measuring the flying height will be described in detail, with reference to
Assume that during the test of the disk drive manufactured, the disk drive is so operated that the head 10 has a flying height FH having value 200, after measurement signals S1 and 22 have been recorded on the disk. As described above, the flying height FH having value 200 gradually decreases with time (log time) because of the superparamagnetic effect.
The flying height 201 calculated from measurement signal S1 is therefore detected as a measuring error E1 at time (t) with respect to the value 200. Similarly, the flying height 202 calculated from measurement signal S2 is detected as a measuring error E2 at time (t) with respect to the value 200. The ratio G of error E2 to error E1 (G=E2/E1) is calculated and saved in the flash memory as measuring error data.
In this embodiment, the CPU 14 cancels the flying height measuring error from the flying height calculated by the HSC module 16, thereby calculating correct flying height FH(t). Thus, the CPU 14 can obtain correct flying height FH(t) by the following equation (6):
FH(t)=(G×FH1(t)−FH2(t))/(G−1) (6)
where G=E2/E1.
Correct flying height FH(t) corresponds to the flying height the head 10 has at time t after the ratio G has been obtained, as can be seen from
The flaring height measuring errors E1 and E2 can be handled as independent events that do not depend on the flying height of the head 10 FH1(t) is the flying height obtained from measurement signal S1 at time t in a given flying state of the head 10. Hence, FH1(t) is the sum of the correct flying height FH(t) and the measuring error E1(t) resulting from the superparamagnetic effect, i.e., FH1(t)=FH(t)+E1(t). Similarly, FH2(t) is the flying height obtained from measurement signal S2 at time t in a given flying state of the head 10, and is the sum of the correct flying height FH(t) and the measuring error E2(t) resulting from the superparamagnetic effect, i.e., FH2(t)=FH(t)+E2(t). Since E2(t)/E1(t)=G at time t in a given flying state of the head 10, E1(t) and E2(t) cancel each other, the correct flying height FH(t) can be calculated, as seen from the equation (6).
In this embodiment, it is desired that the difference between the measuring errors E1 and E2 be as large as possible, if G is greater than 1 in the case where the measurement signals S1 and S2 recorded on the disk 1 are used to measure the flying height. As stated above, the measuring error E1 results from the superparamagnetic effect in the process of calculating the flying height from measurement signal S1. The measuring error E2 results from the superparamagnetic effect in the process of calculating the flying height from measurement signal S2. If G is less than 1, too, it is desired that the difference between the measuring errors E1 and 12 be as large as possible.
In
As described above, the amplitude components Af1 of the fundamental wave of the single-frequency signal f1 and the amplitude components Af3 of the third harmonic wave f3 thereof are measured, thereby finding the relative flying height d, i.e., d=K log(Af1/Af3), where K=v/[2(f3−f1)]>0. The superparamagnetic effect is more prominent at the ±DC magnetization part than at the magnetization transition part. In any signal reproduced by the head 10, the superparamagnetic effect is observed as a concave at the ±DC magnetization part. This means that the amplitude component Af1 of the fundamental wave f1 decreases, while the amplitude component Af3 of the third harmonic wave f3 increases. Thus, the measured flying height gradually decreases.
In view of the mechanism of making flying height measuring errors because of the superparamagnetic effect, the shorter the ±DC magnetization part, or the higher the frequency of the signal, the smaller will be the flying height measuring error resulting from the superparamagnetic effect. As evident from
This is why a signal of 7T-cycle and a signal of 32T-cycle, for example, are used as measurement signals S1 and S2, respectively, in the present embodiment. As has been stated the ratio G (G=E2/E1) of the measuring error E2 to the measuring error E1, both resulting from the superparamagnetic effect is obtained while the flying height of the head 10 remains constant, after the measurement signals S1 and S2 have been recorded during the manufacture of the disk drive. In this case, G=E2/F1>1. However, G may be greater than 1 (G>1), as will be described below.
A process of correcting the flying height will be explained, which is performed if the measured flying heights FH1(t) and FH2(t) obtained from the measurement signals S1 and S2, respectively, have errors resulting from any other factors than the superparamagnetic effect.
Assume that the flying heights FH1(t) and FH2(t) have errors α(t) and β(t) resulting from any other factors than the superparamagnetic effect. Then, the measured flying height FH1(t) is FH(t)+E1(t)+α(t), and the measured flying height FH2(t) is FH(t)+E2(t)+β(t). The corrected flying height HFa(t) can be expressed by the following equation (7):
FHa(t)=FH(t)+(G+α(t)−β(t))/(G−1) (7)
where FH(t) is the measured flying height not corrected yet, and G is the above-mentioned ratio E2/E1.
As can be understood from the equation (7), in a limited condition, FHa(t) infinitely increases (∞) as the ratio G approaches 1, and becomes α(t) as the ratio G increases infinitely. The larger the ratio G, the smaller the corrected errors will be. Hence, it is desired that the measurement signals S1 and S2 should be used in combination so that the difference between the measuring errors E1 and E2 resulting from the superparamagnetic effect may be as large as possible. Of the 7T-cycle signal 400, 24T-cycle signal 401 and 32T-cycle signal 402 shown in
The relative flying height FH1 and FH2 measured from the measurement signals S1 and S2 may not change in accordance with the changes of the actual flaring height of the head 10. In this case, the measured relative flying height FH1 is K1 log(Af11/Af31), where K1=v/[2π(f31−f11)], and the measured relative flying height FH2 is K2 log(Af12/Af32), where K2=v/[2π(f32−f12)]. Here, K1 and K2 are coefficients, f11 is frequency 1/T, f31 is frequency 3/7T, f12 is frequency 1/24T, and f32 is frequency 3/24T. Further, T is a cycle, and Af is amplitude component of frequency f.
In this instance, the measured flying height differs from the absolute flying height measured while the head 10 remains contacting the disk 1.
Then, the coefficient K1 or the coefficient K2, or both are corrected, equalizing the changed FH1c of the measured value 600 obtained from measurement signal S1 to the changed FH2c of the measured value 601 obtained from measurement signal S2.
Here, the change FH1c of the measured flying height value FH1 is K1 [log(Af11c/Af31c)−log(Af11d/Af21d)]. The change FH2c of the measured value FH2 is K2 [log(Af12c/Af32c)−log(Af12d/Af32d)].
In order to correct the coefficients K1 and K2, the absolute flying height may be measured while the head 10 is flying. Then the coefficients K1 and K2 may be corrected to accord with the absolute flying height thus measured.
The changes in the flying heights measured in given different flying states of the head 10 are used, obtaining a correction coefficient Ka for the 24T-cycle signal, and a correction coefficient Kb for the 32T-cycle signal.
The flying height value FH1 measured from the 7T-cycle signal used as measurement signal is K1 log(Af11/Af31), where K1=v/[2π(f31−f11)]. The flying height value FH2 measured from the 24T-cycle signal used as measurement signal is Ka K2 log(Af12/Af32), where K2=v/[2π(f32−f12)]. Further, the flying height FH3 measured from the 32T-cycle signal used as measurement signal is Kb K3 log(Af13/Af33), where K3=v/[2π(f33−f13)]. Here, f11=frequency 1/7T, f31=frequency 3/7T, f12=frequency 1/24T, f32=frequency 3/24T, f13=frequency 1/32T, and f33=frequency 3/32T. T is cycle, and Af is an amplitude component of frequency f.
As has been described, the flying height the head 10 can be measured at high precision at all times in this embodiment, without frequently bringing the head 10 into contact with the disk 1. Therefore, neither the head 10 nor the disk 1 will be worn, preventing a decrease in the performance of the disk drive. Moreover, any flying height measured is corrected, achieving nigh-precision measuring of the head flying height at all times even if the flying height measured has an error resulting from the degradation the measurement signals recorded on the disk undergo with time because of the superparamagnetic effect, or thermal decay. The embodiment can thus prevent the measuring errors from increasing with time, and can measure the flying height of the head at high precision at all times.
Hence, in the disk drive according to this embodiment, the DFH control module incorporated in the disk controller 13 can reliably control the flying height of the head 10 to a small value, in accordance with the flying height measured at high precision.
The various modules of the systems described herein can be implemented as software applications, hardware and/or software modules, or components on one or more computers, such as servers. While the various modules are illustrated separately, they may share some or all of the same underlying logic or code. While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-144146 | Jun 2010 | JP | national |
