Magnetic disk device and flying height detection method of magnetic head

Abstract

To provide a magnetic disk device and a method for detecting a flying height of a magnetic head capable of accurately detecting the flying height of the magnetic head without being affected by "thermal fluctuation" and temperature characteristics.SOLUTION: A magnetic disk device detects the fluctuation of measured values of the flying height measuring part based on the change of "the correspondence relation between a change amount of the heater power and the measured value of the flying height measuring part."SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The embodiments relate to a magnetic disk device and a method for detecting the flying height of a magnetic head in the magnetic disk device. [Background technology]

[0002] In a magnetic disk device equipped with a magnetic disk and a magnetic head for writing / reading data to / from the magnetic disk, an electric heater is provided on the magnetic head, and the heat generated by the electric heater causes the magnetic head to thermally expand toward the magnetic disk, thereby controlling the flying height of the magnetic head relative to the magnetic disk. The flying height of the magnetic head is also called spacing.

[0003] In the manufacturing process of this magnetic disk drive, a signal pattern of a specific frequency is written onto the magnetic disk as data for measuring the flying height of the magnetic head, and a flying height measuring unit that measures the flying height of the magnetic head based on a read signal of the magnetic head for the flying height measuring data is mounted on the signal processing circuit of the magnetic disk drive. A harmonic sensor circuit that uses the triple harmonic method (THM) is known as a flying height measuring unit.

[0004] Then, when the magnetic disk drive is delivered to the user, the user or an engineer reads the flying height measurement data written on the magnetic disk with the magnetic head and checks the measurement value of the flying height measurement unit based on the read signal to determine whether the flying height of the magnetic head has deviated from the optimal state at the time of manufacture. If the measurement value deviates to some extent from the optimal state, the user or an engineer will take action to adjust the flying height of the magnetic head to the optimal state or to notify the user of the abnormality. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2004-303318 A [Patent Document 2] JP 2008-217841 A [Patent Document 3] JP 2012-190519 A Summary of the Invention [Problem to be solved by the invention]

[0006] The flying height measurement data magnetically written to the magnetic disk is subject to a phenomenon known as "thermal fluctuation," in which the magnetism deteriorates over time due to the influence of surrounding heat.

[0007] For this reason, even if the flying height of the magnetic head is measured by a flying height measuring unit when the magnetic disk drive is delivered to the user, the measured value will include deviations (fluctuations) caused by "thermal fluctuations." Not only deviations caused by "thermal fluctuations," but also deviations caused by the temperature characteristics of the signal processing circuit are included in the measured value.

[0008] If the measurement values ​​include deviations caused by "thermal fluctuations" or temperature characteristics, the flying height of the magnetic head cannot be adjusted to an optimal state.

[0009] An object of the embodiments is to provide a magnetic disk device and a method for detecting the flying height of a magnetic head, which can accurately detect the flying height of a magnetic head without being affected by "thermal fluctuation" or temperature characteristics. [Means for solving the problem]

[0010] The magnetic disk device of the embodiment includes a magnetic head that writes and reads data to a magnetic disk and includes a heater that generates heat when power is supplied thereto and expands toward the magnetic disk when the heater generates heat; a flying height measurement unit that measures the flying height of the magnetic head based on a read signal of the magnetic head for flying height measurement data written to the disk; and a controller that controls the power supplied to the heater. The controller detects fluctuations in the measurement value of the flying height measurement unit based on a change in the "correspondence between the amount of change in the supplied power and the measurement value of the flying height measurement unit." [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram illustrating a configuration of an embodiment. [Diagram 2] FIG. 2 is a diagram showing a configuration of a main part of a magnetic head according to an embodiment. [Diagram 3] FIG. 3 is a diagram showing how the thermal expansion and flying height H of the magnetic head change depending on the value of the heater power P in one embodiment. [Figure 4] 4 is a block diagram showing a configuration of a signal processing circuit appearing in FIG. 1. [Diagram 5] FIG. 5 is a diagram showing flying height measurement data Dw and its read signal Dr in one embodiment. [Figure 6] FIG. 6 is a diagram showing another example of the flying height measurement data Dw and its read signal Dr in one embodiment. [Figure 7] FIG. 7 is a diagram showing a state in which a deviation occurs in a measurement value H of a harmonic sensor circuit in one embodiment due to "thermal fluctuation." [Figure 8] FIG. 8 is a diagram showing, for reference, how the measurement value H of the harmonic sensor circuit changes depending on the environmental temperature Tx in one embodiment. [Figure 9] FIG. 9 is a diagram showing a first correspondence relationship between a measurement value H of the harmonic sensor circuit at a first time point and an amount of change Px of the heater power P from a reference value Po in one embodiment. [Figure 10]FIG. 10 is a diagram showing a second correspondence relationship between the slope So at a first time point and the amount of change Px of the heater power P from the reference value Po in one embodiment. [Figure 11] 11 is a diagram showing a third correspondence relationship between the slope So in FIG. 10 and the measurement value H in FIG. 9 in one embodiment. [Figure 12] FIG. 12 is a diagram showing the correspondence relationship between the slope S1 at a first time point and the amount of change Px of the heater power P in one embodiment. [Figure 13] FIG. 13 is a diagram showing a correspondence relationship between a measurement value H and a slope S1 at a first time point in one embodiment. [Figure 14] FIG. 14 relates to one embodiment and is a diagram showing how the average value H of each measurement value of the flying height measurement unit based on the read signals Dr of a plurality of magnetic heads changes with the environmental temperature Tx as a parameter. [Figure 15] FIG. 15 is a graph showing the rate of change At of the coefficient Ao in one embodiment. [Figure 16] FIG. 16 is a graph showing a rate of change Bt of the coefficient B 0 in an embodiment. [Figure 17] FIG. 17 is a flowchart showing processing of a controller in one embodiment. [Figure 18] FIG. 18 is a diagram showing, in one embodiment, a measurement value Hoa monitored at a first point in time while changing the heater power P in a decreasing direction from a reference value Pо, and a measurement value Hob monitored at a second point in time while changing the heater power P in a decreasing direction from the same reference value Pо. [Figure 19] FIG. 19 is a diagram showing, in one embodiment, a measurement value Hoa monitored at a first time point while changing the heater power P in a decreasing direction from a reference value Pо, and a measurement value Hob monitored at a second time point while changing the heater power P in a decreasing direction from a reference value Pо' at the second time point. [Figure 20] FIG. 20 is a diagram showing how the gradients S11 obtained at a first point in time are distributed among the magnetic heads, and how the gradients S12 obtained at a second point in time are distributed among the magnetic heads, according to one embodiment. [Figure 21] FIG. 21 is a diagram showing how the amount of variation ΔH detected by the controller 20 is distributed according to the individual characteristics of the magnetic head 10. In FIG. [Figure 22] FIG. 22 is a diagram showing how the amount of variation ΔH detected by the conventional method varies greatly depending on the individual characteristics of the magnetic head 10. In FIG. [Figure 23] FIG. 23 is a diagram showing how the detected fluctuation amount ΔH is distributed according to the temperature characteristics of the magnetic head and the environmental temperature Tx in one embodiment. [Figure 24] FIG. 24 is a diagram showing how the amount of variation ΔH detected by the conventional method is distributed according to the temperature characteristics of the magnetic head and the environmental temperature Tx. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, an embodiment will be described with reference to the drawings. As shown in Fig. 1, the magnetic disk device 1 includes a disk-shaped magnetic disk 2 as a recording medium. The center of the magnetic disk 2 is attached to the rotating shaft of a spindle motor (SPM) 3, and rotates in a predetermined direction by receiving power from the spindle motor 3. An arm-shaped actuator 4 is disposed near the magnetic disk 2. Note that the spindle motor 3 may be configured to have multiple magnetic disks 2 attached to it, not just one. In this configuration, magnetic heads 10 are provided in a number corresponding to the number of magnetic disks 2. The spindle motor 3 is driven by power supplied from an SVC 6.

[0013] The actuator 4 is pivotally supported at a position where its base end is removed from the magnetic disk 2, and its tip end extends to the vicinity of the center of the magnetic disk 2. A voice coil motor (VCM) 5 is disposed at a position corresponding to the base end of the actuator 2, and the power of the voice coil motor 5 rotates the tip end of the actuator 4 in the radial direction between the inner and outer peripheries of the magnetic disk 2.

[0014] A magnetic head 10 is disposed at the tip of the actuator 4. As shown in Figures 2 and 3, the magnetic head 10 has a slider 11 at its tip and a head element 12 on the underside of the slider 11 (the side facing the magnetic disk 2), and moves (seeks) in the radial direction of the magnetic disk 2 as the actuator 4 rotates. A plurality of servos on which track information and positioning information are written are disposed at equal intervals around the circumference of the magnetic disk 2, and the spaces between these servos are used as data areas. When the magnetic disk 2 rotates, the slider 11 is lifted by the wind pressure caused by the rotation, and causes the magnetic head 10 to float in a direction away from the magnetic disk 2.

[0015] Head element 12 includes a write element 13 that writes magnetic data to magnetic disk 2, a read element 14 that reads magnetic data from magnetic disk 2, and an electric heater (also simply called a heater) 15 for generating heat. Write element 13 and read element 14 are exposed on the lower surface of head element 12 and face surface 2a of magnetic disk 2. Electric heater 15 is disposed inside head element 12.

[0016] A controller (CPU) 20, which is the center of control of the magnetic disk device 1, is connected to an SVC (servo combo) 6 that drives the spindle motor 3 and voice coil motor 5, a signal processing circuit 21, a flash ROM 23, a DRAM 24, and a hard disk controller (HDC) 25. A head amplifier 22 is connected to the signal processing circuit 21, and a host computer 100 is connected to the hard disk controller (HDC) 25. The head amplifier 22 includes a preamplifier 22a that supplies power P to the electric heater 15 in response to an instruction from the controller 20, and a preamplifier 22b that amplifies a read signal output from the read element 14. The head amplifier 22 supplies a write signal (write current) in response to write data supplied from the signal processing circuit 21 to the write element 13, and amplifies the read signal output from the read element 14 by the preamplifier 22a and supplies it to the signal processing circuit 21.

[0017] The signal processing circuit 21 includes a write channel (WDC) 21a that performs analog conversion processing on write data supplied from the controller 20 and supplies the analog data to the head amplifier 22, a read channel (RDC) 21b that performs digital conversion processing on read signals supplied from the head amplifier 22 and supplies the digital data to the controller 20, and a flying height measuring unit, such as a harmonic sensor circuit 21c, that measures the flying height H of the magnetic head 10 based on the output (read signal Dr described below) of the read channel 21b. The flying height H of the magnetic head 10 is specifically the distance between the bottom surface of the head element 12 of the magnetic head 10 and the surface 2a of the magnetic disk 2. The flying height H is also called spacing H.

[0018] 4, the head amplifier 22 includes a preamplifier 22b consisting of an amplifier 31 and a boost band pass filter (BST BPF) 32. The read channel 21b of the signal processing circuit 21 includes, from the input side to the output side, a high pass filter (HPF) 41, a variable gain amplifier (VGA) 42, a cut off filter (CTF) 43, an analog to digital converter (ADC) 44, and a finite impulse response (FIR) 45.

[0019] The harmonic sensor circuit 21c extracts the first harmonic component (fundamental wave component) and the third harmonic component from the read signal Dr corresponding to the flying height measurement data Dw described below, out of the output signal of the read channel 21b, and measures the flying height H of the magnetic head 10 by the well-known THM (Triple Harmonic Method) processing based on the ratio of the amplitude value of the first harmonic component to the amplitude value of the third harmonic component.

[0020] The hard disk controller 25 controls the writing / reading of data together with the controller 20, and also functions as an interface that controls the transfer of data between the controller 20 and the host computer 100, including error correction processing.

[0021] The controller 20 executes control of writing / reading data, servo control required for positioning the magnetic head 10 with respect to a target track on the magnetic disk 2, control of the power P supplied to the electric heater 15, etc. The controller 20 also executes a process of detecting a variation from the initial state of the measurement value H of the harmonic sensor circuit 21c based on a change in the "correspondence between the amount of change in the power P supplied and the measurement value (measured flying height) H of the harmonic sensor circuit 21c." The initial state of the measurement value H refers to the measurement value H immediately after the flying height measurement data Dw is written to the magnetic disk 2.

[0022] The flash ROM 23 is a non-volatile memory and stores various data and the control program executed by the controller 20. The flash ROM (storage means) 23 stores a characteristic table 23a including various characteristic data used to detect the fluctuation amount ΔH described later. The DRAM 24 is a volatile memory and functions as a work area when the controller 20 executes the control program in the flash ROM 23.

[0023] [Thermal expansion of head element 12] When the magnetic disk 2 rotates, the slider 11 acts to create a spacing between the head element 12 and the surface 2a of the magnetic disk 2. In order to achieve high density recording on the magnetic disk 2, it is necessary to reduce this spacing and bring the distance between the head element 12, the surface 2a of the magnetic disk 2, and the magnetic disk 2 as close as possible. When power is supplied to the electric heater 15 of the head element 12, the electric heater 15 generates heat, and the heat causes the head element 12 to thermally expand toward the magnetic disk 2. This thermal expansion causes the write element 13 and the read element 14 to protrude toward the magnetic disk 2 and approach the surface 2a of the magnetic disk 2. By controlling the power P supplied to the electric heater 15, the spacing between the head element 12 and the surface 2a of the magnetic disk 2, that is, the flying height H of the magnetic head 10, can be adjusted to an appropriate state. Hereinafter, the power P supplied to the electric heater 15 is referred to as heater power P.

[0024] 3 shows how the thermal expansion of the head element 12 and the flying height H change according to the value of the heater power P. When the heater power P is 0, the flying height H is H3. When the heater power P is gradually increased from 0, the head element 12 thermally expands toward the magnetic disk 2, and the flying height H decreases to H2. When the heater power P is further increased, the head element 12 thermally expands further, the flying height H becomes 0, and the head element 12 enters a touchdown state in which it is in contact with the surface 2a of the magnetic disk 2. When the heater power P is reduced from this touchdown state to P2, the thermal expansion of the head element 12 decreases and it moves away from the surface 2a of the magnetic disk 2, and the flying height H becomes H1 (= target value Hs, for example) which is greater than 0.

[0025] The flying height H of the magnetic head 10 by the slider 11 is not constant due to the influences of individual differences in the magnetic head 10, the seek position of the magnetic head 10 in the radial direction of the magnetic disk 2, changes in the environmental temperature Tx, etc. For this reason, the general method is to adjust the flying height H to a target value Hs based on the heater power P when the magnetic head 10 is in a touchdown state.

[0026] [Flying height measurement data Dw and read signal Dr] In order to enable the flying height H of the magnetic head 10 to be measured and adjusted, for example, at the user's site, a rectangular signal pattern repeating "0" level and "1" level at a specific frequency as shown in Fig. 5 is written to the magnetic disk 2 as flying height measurement data Dw during the manufacturing process of the magnetic disk device 1. When the magnetic head 10 reads this flying height measurement data Dw, a read signal Dr whose level changes like a sine wave as shown in Fig. 5 is obtained.

[0027] The flying height measurement data Dw is not limited to that shown in Fig. 5, and a rectangular signal pattern irregularly repeating "0" and "1" levels at a specific frequency as shown in Fig. 6 may be written to the magnetic disk 2 as the flying height measurement data Dw. When the magnetic head 10 reads this flying height measurement data Dw, a read signal Dr having a waveform including distortion at the peak level while changing in a sinusoidal manner as shown in Fig. 6 is obtained. The flying height measurement data Dw shown in Fig. 6 is disclosed in, for example, US9502063B1 as bit pattern data that enhances the spectrum of the third harmonic component.

[0028] [Amount of change in measurement value H from the initial state ΔH] The flying height measurement data Dw written to the magnetic disk 2 is subject to a phenomenon called "thermal fluctuation," in which the magnetism of the data deteriorates over time due to the influence of the surrounding heat. Even if the harmonic sensor circuit 21c measures the flying height H of the magnetic head 10 in a state where this "thermal fluctuation" occurs, the measured value H will contain a deviation caused by the "thermal fluctuation."

[0029] 7 shows how a fluctuation ΔH (nm) caused by "thermal fluctuation" occurs in the flying height H measured by the harmonic sensor circuit 21c based on the read signal Dr of the magnetic head 10 for the flying height measurement data Dw at an environmental temperature Tx = 70° C. In other words, although the actual flying height H of the magnetic head 10 is constant, the measured value H of the harmonic sensor circuit 21c decreases over time.

[0030] Moreover, the signal path (the preamplifier 22a of the head amplifier 22 and the read channel 21b of the signal processing circuit 21) from the magnetic head 10 until the read signal Dr reaches the harmonic sensor circuit 21c has temperature characteristics. This temperature characteristic also affects the measurement value H of the harmonic sensor circuit 21c. Figure 8 shows how the measurement value H of the harmonic sensor circuit 21c changes depending on the environmental temperature Tx (degC).

[0031] [Formula for calculating the amount of variation ΔH from the initial state of the measured value H] The amount of change ΔH from the initial state of the measurement value H of the harmonic sensor circuit 21c can be obtained by calculating the following equation (1) based on the harmonic components of the read signal Dr of the magnetic head 10 for the flying height measurement data Dw immediately (first point in time) after the flying height measurement data Dw is written to the magnetic disk 2, and the harmonic components of the read signal Dr of the magnetic head 10 for the flying height measurement data Dw a predetermined time after the flying height measurement data Dw is written to the magnetic disk 2 (second point in time). ΔH=K(ln(V11 / V13)- ln(V01 / V03))+Cr+Ct …(1) K is a characteristic coefficient (λ / 4π) based on the wavelength λ of the signal of the flying height measurement data Dw. ln means natural logarithm. V11 is the amplitude value of the first harmonic component of the read signal Dr after the predetermined time. V13 is the amplitude value of the third harmonic component of the read signal Dr after the predetermined time. V01 is the amplitude value of the first harmonic component of the read signal Dr immediately after the writing. V03 is the amplitude value of the third harmonic component of the read signal Dr immediately after the writing. Cr is a correction value related to "thermal fluctuation". Ct is a correction value related to the temperature characteristics of the preamplifier 22a, the read channel 21b, etc.

[0032] However, since the effect of "thermal fluctuation" changes with the passage of time and the environmental temperature Tx, it is difficult to appropriately determine the correction value Cr for "thermal fluctuation". Since the temperature characteristics of the preamplifier 22a and the read channel 21b also vary, it is also difficult to appropriately determine the correction value Ct for the temperature characteristics.

[0033] [Slope of the measured value H at the first time point (first slope) Sо] 9 shows a first correspondence relationship between the monitored measurement value H and the amount of change Px of the heater power P from the reference value Po of the heater power P, which is obtained by reading the measurement value H of the harmonic sensor circuit 21c at a reference value Po when the measurement value H of the harmonic sensor circuit 21c is "0" at a first time point (a situation not yet affected by "thermal fluctuation") immediately after the flying height measurement data Dw is written to the magnetic disk 2 during the manufacturing process of the magnetic disk device 1, and the measurement value H of the harmonic sensor circuit 21c based on the read signal Dr is monitored. In the example of FIG. 9, when the amount of change Px=-60[DAC], the measurement value H=1[nm].

[0034] The measurement value H of the harmonic sensor circuit 21c changes quadratically with the change amount Px of the heater power P. The reason for the quadratic change is that as the head element 12 of the magnetic head 10 approaches the surface 2a of the magnetic disk 2, the head element 12 becomes more susceptible to the cooling effect from the magnetic disk 2, and the amount of thermal expansion of the head element 12 decreases.

[0035] 9, the measurement value H of the harmonic sensor circuit 21c for the amount of change Px of the heater power P from the reference value Po can be obtained as a quadratic approximation value by the following formula (2). Ao and Bo are coefficients that change according to the environmental temperature Tx. That is, by substituting the value of the amount of change Px into formula (2), which is a quadratic approximation formula, the measurement value (quadratic approximation value) H of the harmonic sensor circuit 21c when the heater power P is a predetermined value (reference value Po+amount of change Px) can be obtained. H=Ao×Px ∧2 +Bo×Px…(2)

[0036] The measurement value (second-order approximation value) H obtained by formula (2) is differentiated once to obtain the gradient (first gradient; also called the slope value) So of the change in the measurement value (second-order approximation value) H. A second correspondence relationship between the obtained gradient So and the amount of change Px of the heater power P from the reference value Po is shown in Fig. 10. The gradient So of the measurement value (second-order approximation value) H of the harmonic sensor circuit 21c when the heater power P is a predetermined value (reference value Po + amount of change Px) can be obtained by the following formula (3). Sо = 2 × Ao × Px + Bo…(3)

[0037] Based on the first correspondence relationship [formula (2)] between the measurement value (second-order approximation value) H and the amount of change Px of the heater power P from the reference value P0, and the second correspondence relationship [formula (3)] between the slope So and the amount of change Px of the heater power P from the reference value P0, it is possible to derive the "third correspondence relationship between the slope So and the measurement value (second-order approximation value) H" shown in Fig. 11. Data of this third correspondence relationship is stored in the flash ROM 23 together with data of formula (2) representing the first correspondence relationship and data of formula (3) representing the second correspondence relationship.

[0038] [Slope of measurement value H between two points (second slope) S1] The measurement value (second-order approximation value) H of the harmonic sensor circuit 21c when the heater power P is changed between a first predetermined value (a value at which the change Px from the reference value Po is, for example, "-20 [DAC]") and a second predetermined value (a value at which the change Px from the reference value Po is, for example, "-100 [DAC]") is obtained from the above formula (2).

[0039] For example, by substituting the above-mentioned "-20[DAC]" into the formula (2) as the value of the amount of change Px, the measurement value (second-order approximation value) H of the harmonic sensor circuit 21c when the heater power P is at the first predetermined value can be calculated as 0.306[nm]. Alternatively, by substituting the above-mentioned "-100[DAC]" into the formula (2) as the value of the amount of change Px, the measurement value (second-order approximation value) H of the harmonic sensor circuit 21c when the heater power P is at the second predetermined value can be calculated as 1.855[nm]. From -80[DAC], which is the difference between the heater power P between the first and second predetermined values, and 1.549[nm], which is the difference between H, the gradient S1 between the two measurements can be calculated as -0.0194. S1 is treated as the gradient of the heater power P with respect to the average value -60[DAC] of the first and second predetermined values. The operation of this example is repeated with the first predetermined value being between 0 and -70 [DAC] and the second predetermined value being between -80 and -150 [DAC] without changing the difference between the first and second predetermined values, and the relationship between the average value of the first and second predetermined values, which is -40 to -110 [DAC], and the slope S1 is shown in Figure 12. In Figure 12, if the average value of the first and second predetermined values ​​is treated as Px, the correspondence relationship between this slope S1 and the amount of change Px is approximately the same as the second correspondence relationship between the slope So at the first time point and the amount of change Px (Figure 10).

[0040] Fig. 13 shows the correspondence relationship between the slope S1 and the measurement value (second-order approximation value) H. This correspondence relationship between the slope S1 and the measurement value (second-order approximation value) H (Fig. 13) is approximately the same as the third correspondence relationship between the slope So at the first time point and the measurement value (second-order approximation value) H (Fig. 11).

[0041] Therefore, by calculating the slope S1, it is possible to obtain the measurement value (second-order approximation value) H of the harmonic sensor circuit 21c when the heater power P is, for example, the average value of the first predetermined value and the second predetermined value [= (first predetermined value + second predetermined value) / 2].

[0042] [Measurement of the change ΔH from the initial state of the measured value H] At a second point in time (when the magnetic disk 2 has already been affected by "thermal fluctuation") a predetermined time after the flying height measurement data Dw was written to the magnetic disk 2, the heater power P is set to a first predetermined value (reference value Po+change amount Pa) and the flying height measurement data Dw is read by the magnetic head 10, and the measurement value H of the harmonic sensor circuit 21c based on the read signal Dr is captured as a first captured value Ha, and the heater power P is set to a second predetermined value (reference value Po+change amount Pb) and the flying height measurement data Dw is read by the magnetic head 10, and the measurement value H of the harmonic sensor circuit 21c based on the read signal Dr is captured as a second captured value Hb. The second predetermined value (reference value Po+change amount Pb) is lower than the first predetermined value (reference value Po+change amount Pa).

[0043] The gradient (second gradient) S1 of the change in the measurement value H between the first captured value Ha and the second captured value Hb can be calculated by the following formula (4). The data of formula (4) is stored in the flash ROM 23. S1={[K(ln(V11a / V13a)- ln(V01 / V03))+Cr+Ct] -[K(ln(V11b / V13b)- ln(V01 / V03))+Cr+Ct]} / (Pa-Pb) =K[ln((V11a×V13b) / (V13a×V11b)))] / (Pa-Pb)…(4)

[0044] V11a is the amplitude value of the first harmonic component of the read signal Dr of the magnetic head 10 when the heater power P is a first predetermined value (reference value Po + change amount Pa). V13a is the amplitude value of the third harmonic component of the read signal Dr of the magnetic head 10 when the heater power P is a first predetermined value (reference value Po + change amount Pa). V11b is the amplitude value of the first harmonic component of the read signal Dr of the magnetic head 10 when the heater power P is a second predetermined value (reference value Po + change amount Pb). V13b is the amplitude value of the third harmonic component of the read signal Dr of the magnetic head 10 when the heater power P is a second predetermined value (reference value Po + change amount Pb).

[0045] From equation (4), it can be seen that in the process of calculating the slope S1 at the second point in time, it is not necessary to take into account the correction value Cr related to “thermal fluctuation”, and it is also not necessary to take into account the correction value Ct related to the temperature characteristics of the preamplifier 22a, the read channel 21b, etc. When the amount of change ΔH of the measurement value H of the harmonic sensor circuit 21c from the initial state is zero, the slope S1 at the second point in time is the same as the slope So at the first point in time, regardless of the presence or absence of "thermal fluctuation".

[0046] Therefore, by comparing the slope So at the first point in time with the slope S1 calculated from equation (4) at the second point in time, the amount of change ΔH from the initial state of the measurement value H of the harmonic sensor circuit 21c can be accurately detected without being affected by “thermal fluctuation” or temperature characteristics. The amount of change ΔH from the initial state of the measurement value H of the harmonic sensor circuit 21c can be expressed as the difference between the value of the heater power P when calculating the slope Sо and the value of the heater power P when calculating the slope S1, based on the relationship between equation (3) representing the slope Sо at the first point in time and equation (4) representing the slope S1 at the second point in time.

[0047] If the value of the heater power P when determining the slope S1 is P1, this value P1 can be calculated by the following formula (5). P1={K[ln((V11a×V13b) / (V13a×V11b)))] / (Pa-Pb)-Bo} / (2×Ao)…(5)

[0048] As described above, V11a is the amplitude value of the first harmonic component of the read signal Dr of the magnetic head 10 when the heater power P is the first predetermined value (reference value Po+change amount Pa). V13a is the amplitude value of the third harmonic component of the read signal Dr of the magnetic head 10 when the heater power P is the first predetermined value (reference value Po+change amount Pa). V11b is the amplitude value of the first harmonic component of the read signal Dr of the magnetic head 10 when the heater power P is the second predetermined value (reference value Po+change amount Pb). V13b is the amplitude value of the third harmonic component of the read signal Dr of the magnetic head 10 when the heater power P is the second predetermined value (reference value Po+change amount Pb).

[0049] In the formula (2) expressing the first correspondence relationship between the measurement value H and the amount of change Px, the difference between the measurement value H calculated by substituting the value P1 for the amount of change Px and the measurement value H calculated by substituting the average value of the amounts of change Pa and Pb [=(amount of change Pa+amount of change Pb) / 2] for the amount of change Px is the amount of change ΔH from the initial state of the measurement value H. This amount of change ΔH can be calculated by the following formula (6). ΔH=Ao×{P1 ∧2 -[(Pa+Pb) / 2] ∧2}+Bo×[P1-(Pa+Pb) / 2]…(6)

[0050] When the fluctuation amount ΔH is obtained by calculating the formulas (5) and (6), the heater power P is set to a first predetermined value (reference value Po+change amount Pa) and the reading operation of the magnetic head 10 is performed, and the heater power P is set to a second predetermined value (reference value Po+change amount Pb) and the reading operation of the magnetic head 10 is performed, while the amplitude value V11a of the first harmonic component and the amplitude value V13a of the third harmonic component of the read signal Dr when the heater power P is the first predetermined value (reference value Po+change amount Pa) are extracted from the harmonic sensor circuit 21c, and the amplitude value V11b of the first harmonic component and the amplitude value V13b of the third harmonic component of the read signal Dr when the heater power P is the second predetermined value (reference value Po+change amount Pb) are extracted from the harmonic sensor circuit 21c. In the process of finding the amount of variation ΔH using formula (6), there is no need to change the heater power P until the magnetic head 10 reaches a touchdown state, and the heater power P only needs to be changed at two points, the first and second predetermined values, thereby shortening the time required to detect the amount of variation ΔH. Since there is no need to repeat the reading operation of the magnetic head 10 while changing the heater power P many times, the amount of variation ΔH can be detected without imposing a burden on the performance of the magnetic disk device 1.

[0051] If the detected amount of variation ΔH exceeds an allowable threshold, a user or an engineer can take steps to adjust the flying height H of the magnetic head 10 or to report an abnormality. However, the above formulas (5) and (6) do not include any correction for the temperature characteristics of thermal expansion of the magnetic head 10.

[0052] Figure 14 shows how the average value of each monitored measurement value H of the harmonic sensor circuit 21c based on the read signals Dr of multiple magnetic heads 10 prepared as samples changes with the environmental temperature Tx (=-5, 5, 30, 60, 75 [degC]) as a parameter (temperature characteristic of the average thermal expansion of each magnetic head 10), as monitored in the same manner as the process described in Figure 9.

[0053] Specifically, the heater power P when the measurement value H of the harmonic sensor circuit 21c based on the read signals Dr of the multiple magnetic heads 10 becomes "0" is set as a reference value Po, and the heater power P is significantly changed in a decreasing direction from each reference value Po while the flying height measurement data Dw is read by each magnetic head 10. Each measurement value H of the harmonic sensor circuit 21c based on the read signals Dr of each magnetic head 10 is monitored, and the relationship between the average value of each monitored measurement value H and the change amount Px of the heater power P from the reference value Po is shown for each environmental temperature Tx.

[0054] In this example, the reference value Po of the heater power P is calculated for each environmental temperature Tx, so that the “deviation” that occurs in the average value of each measured value H can be regarded as a “deviation” caused by the temperature characteristics Ct of the preamplifier 22a, the read channel 21b, etc. in formula (1) but a “deviation” caused by the temperature characteristics of the average thermal expansion of each magnetic head 10.

[0055] 15 shows that the coefficient Ao of formula (2) expressing the first correspondence relationship between the measurement value (second-order approximation value) H and the amount of change Px changes at a rate At determined according to the environmental temperature Tx. This rate of change At increases as the environmental temperature Tx increases in the range from a negative value to approximately 40 [degC], and decreases as the environmental temperature Tx increases in the range from approximately 40 [degC] to approximately 75 [degC]. When the environmental temperature Tx=30 [degC], the rate of change At=“1”.

[0056] 16 shows that the coefficient Bo in formula (2) changes at a rate Bt determined according to the environmental temperature Tx. This rate of change Bt decreases as the environmental temperature Tx increases in the range from a negative value to approximately 10 [degC], and increases as the environmental temperature Tx increases in the range from approximately 10 [degC] to approximately 75 [degC]. When the environmental temperature Tx=30 [degC], the rate of change Bt=“1”.

[0057] In the manufacturing process of the magnetic disk device 1, the environmental temperature Tx=30 [degC] at which the rates of change At and Bt are both "1" is defined as the reference environmental temperature for determining the formula (2).

[0058] The rates of change At and Bt can be approximated to the second order based on the environmental temperature Tx when determining the amount of change ΔH of the measurement value H from the initial state, and the temperature characteristics Aa, Ab, Ac, Ba, Bb, and Bc of the magnetic head 10, as shown in the following mathematical expressions (7) and (8). At = Aa × Tx ∧2 + Ab × Tx + Ac…(7) Bt=Ba×Tx ∧2 + Bb×Tx+Bc…(8) Representative values ​​of the temperature characteristics Aa, Ab, Ac, Ba, Bb, and Bc are measured before the manufacturing process of the magnetic head device 1, and data of the measured representative values ​​is stored in the characteristic table 23a in the flash ROM 23.

[0059] [Controller 20 processing] The process executed by the controller 20 is shown in the flowchart of FIG. First, in the manufacturing process of the magnetic disk device 1, when an engineer sets the test mode (YES at M1) immediately after the flying height measurement data Dw is written to the magnetic disk 2 (first point in time), the controller 20, similar to the process shown in FIG. 9, reads the flying height measurement data Dw with the magnetic head 10 while greatly changing the heater power P in the decreasing direction from the reference value Po (M2), monitors the first measurement value H of the harmonic sensor circuit 21c based on the read signal Dr, obtains a second-order approximation of the monitored first measurement value H, and obtains a slope (first slope) So of the change in the first measurement value (second-order approximation) H relative to the change amount Px of the heater power P from the reference value Po by performing a single differentiation operation on the second-order approximation value (M3).

[0060] In conjunction with this process, the controller 20 stores in the flash ROM 23 data of formula (2) expressing a first correspondence relationship between the first measurement value (second-order approximation value) H and the amount of change Px of the heater power P from the reference value Pо, data of formula (3) expressing a second correspondence relationship between the slope So and the amount of change Px of the heater power P from the reference value Pо, and data of a third correspondence relationship (FIG. 11) between the slope So and the measurement value (second-order approximation value) H (M4). Next, when the manufactured magnetic disk device 1 is delivered to the user, if a detection mode is set by an engineer or user a predetermined time (second point in time) after the writing of the flying height measurement data Dw to the magnetic disk 2 (NO in M1, YES in M5), the controller 20 sets the heater power P to a first predetermined value (reference value Pо+change amount Pa) and reads the flying height measurement data Dw with the magnetic head 10, captures the measurement value H of the harmonic sensor circuit 21c based on the read signal Dr as the first captured value Ha, and sets the heater power P to a second predetermined value (reference value Pо+change amount Pb) and reads the flying height measurement data Dw with the magnetic head 10, and captures the measurement value H of the harmonic sensor circuit 21c based on the read signal Dr as the second captured value Hb (M6).

[0061] Next, the controller 20 obtains a slope (second slope) S1 of the change in the measurement value H between the first captured value Ha and the second captured value Hb by using the formula (4) in the flash ROM 23 (M7). Then, the controller 20 detects a variation ΔH from the initial state of the measurement value H of the harmonic sensor circuit 21c by comparing the slope So in the third correspondence relationship (FIG. 11) in the flash ROM 23 with the obtained slope S1 (M8).

[0062] Specifically, the controller 20 fits the slope S1 to the slope So of the third correspondence relationship (FIG. 11) and refers to the third correspondence relationship, thereby reading out the measurement value H corresponding to the slope S1 from the data of the third correspondence relationship. Furthermore, the controller 20 fits the slope S1 to the slope So of the formula (3) in the flash ROM 23 and executes the calculation of the formula (3), thereby obtaining the amount of change Px corresponding to the slope S1. Next, the controller 20 substitutes the obtained amount of change Px into the formula (2) in the flash ROM 23, thereby calculating the measurement value (second-order approximation value) H corresponding to the slope S1. Then, the controller 20 detects the difference between the calculated measurement value (second-order approximation value) H and the measurement value H read out from the data of the third correspondence relationship as the amount of change ΔH from the initial state of the measurement value H of the harmonic sensor circuit 21c.

[0063] The detected variation ΔH does not include deviations due to "thermal fluctuation" or temperature characteristics of the preamplifier 22a, the read channel 21b, etc., and deviations due to the temperature characteristics of the magnetic head 10 are also removed by correction based on the correction data. Therefore, the variation ΔH from the initial state of the measurement value H of the harmonic sensor circuit 21c can be accurately detected without being affected by "thermal fluctuation" or temperature characteristics.

[0064] If the detected amount of variation ΔH exceeds an allowable threshold, the controller 20 notifies a user or an engineer of this fact via the host computer 100. The user or engineer who receives this notification then performs the task of adjusting the flying height H of the magnetic head 10.

[0065] [Verification of effectiveness] 18 shows the measurement value Hoa of the flying height measurement unit monitored while the heater power P is changed in a decreasing direction from the reference value Pо at a first point in time when the heater power P is not yet affected by "thermal fluctuation", in correspondence with the amount of change Px in the heater power P, and also shows the measurement value Hob of the flying height measurement unit monitored while the heater power P is changed in a decreasing direction from the same reference value Pо at a second point in time (after the first point in time) when the heater power P is already affected by "thermal fluctuation", in correspondence with the amount of change Px in the heater power P. In this example, the measurement value Hoa does not have any deviation due to "thermal fluctuation", while the measurement value Hob includes a deviation due to "thermal fluctuation".

[0066] 19 shows the measurement value Hoa of the flying height measurement unit monitored while the heater power P is changed in a decreasing direction from a reference value Pо at a first point in time when the heater power P is not yet affected by "thermal fluctuation", in correspondence with the amount of change Px in the heater power P, and also shows the measurement value Hob of the flying height measurement unit monitored while the heater power P is changed in a decreasing direction from the reference value Pо' at the second point in time when the heater power P is already affected by "thermal fluctuation", in correspondence with the amount of change Px in the heater power P. In this example, there is no difference between the slope So at the first point in time and the slope S1 of the change at the second point in time.

[0067] 20 shows how the slopes S11 obtained for the multiple magnetic heads 10 at a first point in time before the magnetic heads 10 are affected by "thermal fluctuation" are distributed among the magnetic heads 10, and how the slopes S12 obtained for the multiple magnetic heads at a second point in time after the magnetic heads have already been affected by "thermal fluctuation" are distributed among the magnetic heads. In this example, regardless of the time that has passed since the flying height measurement data Dw was written, the slopes S11 and S12 converge to approximately the same value.

[0068] Fig. 21 shows how the variation ΔH detected by the controller 20 is distributed according to the individual characteristics of the magnetic head 10. Fig. 22 shows how the variation ΔH detected by the conventional method is largely distributed according to the individual characteristics of the magnetic head 10.

[0069] Fig. 23 shows how the variation ΔH detected in one embodiment is distributed according to the temperature characteristics of the magnetic head and the environmental temperature Tx (=-5, 5, 30, 60, 75 [degC]). Fig. 24 shows how the variation ΔH detected by the conventional method is distributed according to the temperature characteristics of the magnetic head and the environmental temperature Tx (=-5, 5, 30, 60, 75 [degC]).

[0070] [Variations] In the above embodiment, the temperature characteristic of the magnetic head 10 is stored in the characteristic table 23a, but in addition to the temperature characteristic, the characteristic table 23a may also store the seek position characteristic of the magnetic head 10 in the radial direction of the magnetic disk 2. In this case, the controller 20 uses the temperature characteristic and the seek position characteristic as correction data when detecting the slope S1 and the fluctuation amount ΔH. In addition, the above embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and spirit of the invention, and are included in the scope of the invention and its equivalents described in the claims. [Explanation of symbols]

[0071] 1...magnetic disk device, 2...magnetic disk, 4...actuator, 10...magnetic head, 11...slider, 12...head element, 13...write element, 14...read element, 15...electric heater, 20...controller, H...floating height, ΔH...variation in measured value, P...heater power.

Claims

1. a magnetic head which writes and reads data to and from a magnetic disk, includes a heater which generates heat when power is supplied thereto, and expands toward the magnetic disk when the heater generates heat; a flying height measuring unit that measures the flying height of the magnetic head based on a read signal of the magnetic head corresponding to flying height measurement data written on the magnetic disk; A controller for controlling power supplied to the heater; Equipped with The controller: detecting a fluctuation in the measurement value of the flying height measurement unit based on a change in the "correspondence between the amount of change in the power supply and the measurement value of the flying height measurement unit"; Magnetic disk device.

2. The controller: At a first time point, the magnetic head reads the flying height measurement data while decreasing the supplied power from a reference value, monitors a first measurement value of the flying height measurement unit based on the read signal, and obtains a first slope of a change in the first measurement value relative to an amount of change in the supplied power by performing a first differentiation operation on a second-order approximation of the first measurement value, and obtains a "third correspondence relationship between the first slope and the first measurement value" based on "a first correspondence relationship between the amount of change in the supplied power and the first measurement value" and "a second correspondence relationship between the amount of change in the supplied power and the first slope", at a second time point, the supply power is set to a first predetermined value, the flying height measurement data is read by the magnetic head, and a measurement value of the flying height measurement unit based on the read signal is captured as a first captured value, the supply power is set to a second predetermined value lower than the first predetermined value, the flying height measurement data is read by the magnetic head, and the measurement value of the flying height measurement unit based on the read signal is captured as a second captured value, and a second slope of change between the first and second captured values ​​of the measurement value of the flying height measurement unit with respect to the amount of change of the supply power between the first and second predetermined values ​​is obtained; A deviation between the first inclination and the second inclination is detected as a fluctuation in a measurement value of the flying height measuring unit.

2. The magnetic disk drive according to claim 1.

3. the first time point is a time point when a test mode is set immediately after the flying height measurement data is written to the magnetic disk, the second time point is a time point when a detection mode is set after a predetermined time has elapsed since the flying height measurement data was written onto the magnetic disk; 3. The magnetic disk drive according to claim 2.

4. the flying height measuring unit extracts a first harmonic component and a third harmonic component from a read signal of the magnetic head for the flying height measurement data, and measures the flying height of the magnetic head by a Triple Harmonic Method (THM) process based on a ratio between an amplitude value of the first harmonic component and an amplitude value of the third harmonic component.

3. The magnetic disk drive according to claim 1.

5. The controller applies a correction based on a temperature characteristic of the magnetic head when detecting the amount of fluctuation at the second point in time.

3. The magnetic disk drive according to claim 2.

6. a magnetic head which writes and reads data to and from a magnetic disk, includes a heater which generates heat when power is supplied thereto, and expands toward the magnetic disk when the heater generates heat; a flying height measuring unit that measures the flying height of the magnetic head based on a read signal of the magnetic head corresponding to flying height measurement data written on the magnetic disk; A controller for controlling power supplied to the heater; In a magnetic disk device comprising: detecting a fluctuation in the measurement value of the flying height measurement unit based on a change in the "correspondence between the amount of change in the power supply and the measurement value of the flying height measurement unit"; A method for detecting the flying height of a magnetic head.

7. At a first time point, the magnetic head reads the flying height measurement data while decreasing the supplied power from a reference value, monitors a first measurement value of the flying height measurement unit based on the read signal, and obtains a first slope of a change in the first measurement value relative to an amount of change in the supplied power by performing a first differentiation operation on a second-order approximation of the first measurement value, and obtains a "third correspondence relationship between the first slope and the first measurement value" based on "a first correspondence relationship between the amount of change in the supplied power and the first measurement value" and "a second correspondence relationship between the amount of change in the supplied power and the first slope", at a second time point, the supply power is set to a first predetermined value, the flying height measurement data is read by the magnetic head, and a measurement value of the flying height measurement unit based on the read signal is captured as a first captured value, the supply power is set to a second predetermined value lower than the first predetermined value, the flying height measurement data is read by the magnetic head, and the measurement value of the flying height measurement unit based on the read signal is captured as a second captured value, and a second slope of change between the first and second captured values ​​of the measurement value of the flying height measurement unit with respect to the amount of change of the supply power between the first and second predetermined values ​​is obtained; A deviation between the first inclination and the second inclination is detected as a fluctuation in a measurement value of the flying height measuring unit.

7. The method for detecting the flying height of a magnetic head according to claim 6.

8. At the second time point, by fitting the second gradient to the first gradient of the second correspondence relationship and referring to the second correspondence relationship, reading out a measurement value corresponding to the second gradient from the second correspondence relationship; determining an amount of change in the supplied power corresponding to the second gradient by applying the second gradient to the first gradient of a formula expressing the second correspondence relationship and executing an operation of the formula; calculating a measurement value corresponding to the second gradient by substituting the amount of change thus obtained into a mathematical expression that represents the first correspondence relationship; a difference between the calculated measurement value and a measurement value read from the third correspondence data is detected as a variation of the measurement value of the flying height measuring unit from an initial state; 8. The method for detecting the flying height of a magnetic head according to claim 7.

9. the first time point is a time point when a test mode is set immediately after the flying height measurement data is written to the magnetic disk, the second time point is a time point when a detection mode is set after a predetermined time has elapsed since the flying height measurement data was written onto the magnetic disk; 8. The method for detecting the flying height of a magnetic head according to claim 7.

10. the flying height measuring unit extracts a first harmonic component and a third harmonic component from a read signal of the magnetic head for the flying height measurement data, and measures the flying height of the magnetic head by a Triple Harmonic Method (THM) process based on a ratio between an amplitude value of the first harmonic component and an amplitude value of the third harmonic component.

8. The method for detecting the flying height of a magnetic head according to claim 6.

11. When detecting the amount of fluctuation at the second point in time, a correction based on the temperature characteristics of the magnetic head is applied.

8. The method for detecting the flying height of a magnetic head according to claim 7.

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