METHOD OF, AND APPARATUS FOR, AVOIDING A MAGNETIC TRANSDUCER CRASH

Abstract: The likelihood of an impending crash of a magnetic transducer or head with a rotating magnetic medium is determined by dynamically and periodically calculating overwrite efficiency and a control operation is initiated when an impending crash is determined to be likely.

Description: This invention relates to a method of, and apparatus for, avoiding an impending crash of a magnetic transducer against a rotating magnetic record medium. Magnetic record disks have been used for years for high capacity random access storage. One or more magnetic transducers, called magnetic heads, are disposed adjacent a magnetic recording surface on the disk to write data on and read data from the recording surface. A slider supports one or more magnetic heads. The slider is lightly biased to cause the heads to move toward the recording surface when the disk is stationary; but as the disk is brought up to operating speed, an air bearing is generated which moves each slider and hence the head(s) away from the recording surface toward a preselected flying height. As disk storage capacity is increased by decreasing track width and transducing gaps, the head(s) must fly closer and closer to the recording surface in order to insure adequate signal strength. This increases the hazard of so-called "head crashes" which wipe out previously recorded data and also may damage the recording surface. These crashes occur when the head and slider scrape the recording surface of the moving disk. The effect of head/media spacing on the amplitude of magnetic readback signals is described by R. L. Wallace, Jr. in "The Reproduction of Magnetically Recorded Signals", The Bell System Technical Journal, Vol. 30, Oct. 1951, pp. 1145-1173. This publication includes the Wallace equation which expresses the dependence of the readback voltage on various recording parameters including the head/disk spacing. Modulation of the read signal envelope to measure variations in head/disk spacing is described in Shi et al, "Use of Readback Signal Modulation to Measure Head/Disk Spacing variations in Magnetic Disk Files", Technical Report No. 11, The Center for Magnetic Recording Research, University of California, San Diego, December 1985. The readback signal modulation technique is used in conjunction with a lasser-doppler vibrometer to measure simultaneously the spacing variation and disk vibrations that result from various laboratory induced impacts directly on the disk of an operating disk file. While this apparatus is useful as a laboratory tool in the research and development of magnetic disk files, the disk file structure must be altered to accommodate the lasser-doppler vibrometer and the equipment required is very costly. "Effect of Flying Height Variation on Offtrack Data Handling" by Morris et al, IEEE Trans. on Magnetics MAG-17 No. 4, July 1981, pp. 1372-1375 infers flying height variations by modulation of the magnetic head read signal and correlates the flying height variation to offtrack data handling capability. "Head Flight Height Monitoring" by Baudet et al, IBM Technical Disclosure Bulletin, Vol. 11, No. 12, May 1969, p. 1650, describes an arrangement using the modulation of the read signal envelope to measure variations in head/disk spacing. The control factor described there is based on a comparison of the amplitude of an instantaneously detected read signal level with a continuously modified or running average amplitude of previously detected levels. Hence, this may not signal an impending head crash if, as we have found, the deterioration in flying height preceding a head crash is usually gradual. Also of interest is EP-A-0256356 (an intermediate document falling within the scope of Article 54(3)) which discloses a method for measuring the head/recording surface clearance n situ. In a first embodiment a readback signal is sensed periodically, first at normal operating speed at a first flying height, and thereafter at substantially zero flying height following reduction in disk speed. These signals are then used in an algorithm to calculate whether the first flying height has deteriorated to a degree indicative of an impending head crash. While this arrangement operates satisfactorily, writing and reading of data is undesirably interrupted during this periodic disk speed reduction. In an alternative embodiment, a signal is obtained which has non-zero amplitude for at least two different frequencies. This can be achieved either with a single write signal having multiple harmonics, or by writing several passes over the same track, each time writing a different wavelength. Instantaneous measurement of the ratio of the two corresponding lines in the spectrum of the readback signal is used to indicate the flying height of the slider. The invention seeks to provide a self-diagnostic method and apparatus that (a) reliably, dynamically and periodically indicate the likelihood of an impending head crash without requiring the speed of the disk to be reduced, (b) is based upon contemporaneous readback signal inputs so as to eliminate any errors due to differences in ambient conditions that may otherwise exist if the respective signals are sensed at widely spaced time intervals, and (c) is based upon signal overwrite efficiency, rather than upon the readback signal amplitude at the particular instant the amplitude is being measured. According to one aspect, the invention provides a method of avoiding an impending crash of a magnetic transducer against a magnetic recording medium rotating at a substantially constant velocity due to deterioration in flying height of the transducer below a predetermined acceptable value, the method comprising the steps of: writing over a predetermined area of said recording medium, one signal at a predetermined amplitude and at a frequency of f₁; reading back said one signal at frequency f₁ and storing its spectral amplitude; overwriting said predetermined area with another signal at said predetermined amplitude but at a higher frequency f h ; then reading back the component of said one signal remaining after said overwriting and storing its spectral amplitude; calculating the ratio of the spectral amplitudes of said one signal as read before said overwriting and said remaining component of said one signal as read after said overwriting; and initiating a corrective control operation when said ratio is indicative of the flying height being below said acceptable value. According to another aspect, the invention provides a method of avoiding an impending crash of a magnetic transducer against a rotating magnetic recording medium due to deterioration in flying height of the transducer below a predetermined acceptable value while the medium is being rotated at substantially constant velocity, the method comprising the steps of: over a predetermined area of said recording medium, writing one signal at a predetermined amplitude and at a frequency f₁ and then overwriting with another signal at said predetermined amplitude but at a higher frequency f h ; following the overwriting at frequency f h , sensing said one and other signals as read back from said predetermined area to produce first and second readback signals, respectively; calculating the ratio of the amplitudes of said second signal to said first signal; and using said ratio to initiate a corrective control operation when said ratio denotes that the flying height is below said acceptable value. Other aspects of the invention are defined in the claims appended hereto. The term "flying height" is used herein to define the height of the transducer above the exposed surface of the recording medium. When the term frequency f₁ or f h is hereinafter used, it is to be understood that it respectively connotes 2f₁ or 2f h magnetic field reversals per second on the recording medium. A preferred embodiment comprises writing one signal at a predetermined amplitude and at a frequency f₁ on a predetermined dedicated track on the recording medium, then reading back said one signal at that same frequency f₁ and storing its spectral amplitude. The dedicated track is then overwritten with another signal at said predetermined amplitude but at a higher frequency f h , then the component of said one signal remaining after said overwriting is read back and its spectral amplitude stored. The ratio of the spectral amplitudes of said one signal as read before overwriting and said remaining component as read after overwriting are calculated and used to initiate a desired control operation when the ratio denotes that flying height is below the acceptable value. According embodiment comprises writing one signal at a frequency f₁ on a dedicated track of the recording medium and then overwriting with another signal at a higher frequency f h . Readback signals from this dedicated track are sensed following the writing at f₁ and following the overwriting at f h . The ratio of the amplitudes of the readback signal following overwriting at f h to that following writing at f₁ is calculated and used to initiate a desired control operation when the ratio denotes that the flying height is below the acceptable value. Thus, both embodiments are based upon contemporaneous readback measurements indicative of signal overwrite efficiency. These methods may be employed with any rotating recording medium comprising either epoxy based or thin film disks. How the invention can be carried out will now be described by way of examples with reference to the accompanying drawings, in which:- Fig. 1 is a schematic view of an apparatus embodying the invention; Fig. 2 is a plot of overwrite modulation ratio versus transducer flying height; Fig. 3 is a schematic view of another apparatus embodying the invention; and Fig. 4 is a plot of overwrite amplitude ratio versus transducer flying height. As schematically illustrated in Fig. 1, one apparatus embodying the invention comprises a plurality of magnetic transducers or heads 1a-d connected via respective preamplifiers 2a-d and lines 3a-d to a matrix card 4. Card 4 contains means, including an automatic gain control amplifier and detection electronics, for writing data on and reading data from a conventional rotating recording medium (not shown). The apparatus as thus far described is conventional and commonly used in multiple-head disk files, such as the IBM 3380 direct access storage device. The invention is embodied by connecting the lines 3a-d via a multiplexer 5, a bandpass filter/detector 6, and an analog-to-digital converter (ADC) 7 to a digital control unit 8. Digital control unit 8 comprises a microprocessor 9 and a read only memory (ROM) 10. Microprocessor 9 may be a dedicated microprocessor, such as the well-known Model 80286 made by Intel Corporation. This microprocessor contains a plurality of registers, including R1 and R2. ROM 10 may comprise two portions, 10a,10b. Portion 10a contains the program instructions concerning the digital values of the spectral amplitudes of the signals at frequencies f₁ and f h and instructions as to what to do when the predesignated test track of the recording medium is accessed. Portion 10b constitutes a lookup table that contains a series of values corresponding to overwrite modulation ratios as measured when the transducer is flown at various known flying heights while the recording medium is rotated at a constant normal operational velocity. In operation, assume that the magnetic recording medium is rotating at constant velocity and that the flying height condition for a particular transducer, say 1b, is to be measured. For sake of illustration, a control unit 11, such as the IBM 3880, will be operative, according to a preselected instruction, to periodically interrupt its normal operation and will signal a control director 12 to position that transducer 1b at the predesignated dedicated test track on the medium. Control director 12 will then via a line 13 instruct the circuitry in the matrix card 4 to write the following sequence of signals on the test track and then read them back: One signal at a predetermined amplitude will be written at a frequency f₁ via matrix card 4, then read back. Circuitry on matrix card 4 will condition multiplexer 5 to cause this readback signal as tapped off via the branch of line 3b to be transmitted via multiplexer 5 and filter/detector 6 and converted by ADC 7 and its spectral amplitude stored in register R1 of microprocessor 9. Then the test track will be immediately overwritten with another signal at a higher frequency f h ; whereupon the component of said one signal written at frequency f₁ remaining after such overwriting will be read back and, via components 3b,5,6,7, its spectral amplitude will be stored in register R2. Microprocessor 9 will then calculate the ratio of the spectral amplitudes of said one signal before and after the overwriting as stored in R1,R2 and compare that ratio against the ratios in lookup table 10b. The microprocessor 9 will then signal control director 12 via OK/ OK line 14 whether the ratio is above or below a preselected value (see x on Fig. 2). If the ratio is above the value x, a crash is considered to be imminent. This will bring up a OK signal in line 14, in response to which the control director 12 will signal the control unit 11 to perform a desired control operation; e.g., initiating transfer of data from the medium accessed by said selected transducer 1b or shutting down operation of all recording media accessed by the transducer array. The embodiment of Figs. 3 and 4, like that in Figs. 1 and 2, employs measurement of overwrite efficiency to indicate likelihood of a transducer crash. It differs structurally in slight respects from that of Figs. 1 and 2 in that it comprises bandpass filters/detectors 6a, 6b and ADCs 7a,7b in lieu of the single filter/detector 6 and ADC 7 of Figs. 1 and 2. Register R1, as heretofore, stores the spectral amplitude of the frequency f₁ signal as read back after overwriting at frequency f h ; but register R2 stores the spectral amplitude of the signal overwritten at frequency f h . In operation, one signal at frequency f₁ is written on the predesignated dedicated test track by the transducer, e.g., 1b, selected for flying height change test. Then said one signal is overwritten with another signal at a higher frequency f h . After this overwriting, the residual signal at frequency f₁ is then read back via multiplexer 5, filter/detector 6a and ADC 7a, and its amplitude stored in register R1. The signal at frequency f h is then read back via components 5,6b,7b and its amplitude is stored in register R2. Microprocessor 9 now calculates the ratio of the amplitude as stored in R2 to that as stored in R1, then compares the ratio against overwrite amplitude values in the lookup table 10b′. As before, if the ratio exceeds a certain value x′ (see Fig. 4), indicating flying height is below an acceptable value,then a OK signal in line 14 will signal the control director 12 to initiate the desired control operation. It should be noted that the test operation according to either of the above-described embodiments can be accomplished in not more than four revolutions of the recording medium. Although the disclosed method requires dedication of one track per recording surface as a test track, the effect on throughput capability is minimal (probably less than one-tenth of one percent). This throughput reduction is deemed insignificant and acceptable in view of the increased likelihood that an impending transducer crash can be avoided by the self-diagnostic techniques herein disclosed. It should further be noted that, as disclosed, there is (as there must be) access to the readback signals from the preamplifiers 2a-d before they have undergone any gain compression or automatic gain control. The required signals are already available on the matrix card 4 as used in the IBM 3380 disk file. If desired, the electronics (depicted as components 5,6,7,8 herein) needed to implement either embodiment may be incorporated on the matrix card 4. The control director 12 is preferably that presently used in the IBM 3380 but modified so as to allow the diagnostic tests to be performed periodically. The ratio of f h to f₁ preferably closely matches the ratio of frequencies used in the particular storage device, and would thus be substantially 2.666 for the IBM 3380 disk file. This ratio, in any event should be a nonintegral value to avoid harmonic effects. Moreover, as will be apparent, the test operations may be performed at any time; e.g., before release of the product from manufacturing, to detect and replace any slider which is flying its associated transducer below a predetermined nominal acceptable flying height, as well as periodically during customer use.

Claims: A method of avoiding an impending crash of a magnetic transducer against a magnetic recording medium rotating at a substantially constant velocity due to deterioration in flying height of the transducer below a predetermined acceptable value, the method comprising the steps of: writing over a predetermined area of said recording medium, one signal at a predetermined amplitude and at a frequency f₁; reading back said one signal at frequency f₁ and storing its spectral amplitude; overwriting said predetermined area with another signal at said predetermined amplitude but at a higher frequency f h ; then reading back the component of said one signal remaining after said overwriting and storing its spectral amplitude; calculating the ratio of the spectral amplitudes of said one signal as read before said overwriting and said remaining component of said one signal as read after said overwriting; and initiating a corrective control operation when said ratio is indicative of the flying height being below said acceptable value. A method of avoiding an impending crash of a magnetic transducer against a rotating magnetic recording medium due to deterioration in flying height of the transducer below a predetermined acceptable value while the medium is being rotated at substantially constant velocity, the method comprising the steps of: over a predetermined area of said recording medium, writing one signal at a predetermined amplitude and at a frequency f₁ and then overwriting with another signal at said predetermined amplitude but at a higher frequency f h ; following the overwriting at frequency f h , sensing said one and other signals as read back from said predetermined area to produce first and second readback signals, respectively; calculating the ratio of the amplitudes of said second signal to said first signal; and using said ratio to initiate a corrective control operation when said ratio denotes that the flying height is below said acceptable value. A method as claimed in claim 1 or claim 2, wherein said predetermined area constitutes at least a portion of at least one allocated track on the magnetic medium. A method as claimed in any preceding claim, wherein said writing, sensing, calculating and initiating steps are performed periodically according to a selectable timing sequence. A method as claimed in any preceding claim 1, including the further steps of providing a plurality of magnetic transducers for reading indicia from a single recording medium, and multiplexing said writing, sensing, calculating and initiating steps periodically among different ones of said transducers for initiating said control operation when the flying height of any one of said transducers is below said acceptable value. A method as claimed in any preceding claim, wherein the ratio of f h to f₁ closely matches the ratio of frequencies used in said magnetic recording medium. A method as claimed in any preceding claim, wherein the ratio of f h to f₁ is a nonintegral value. A method as claimed in any preceding claim, wherein the ratio of the calculated spectral amplitudes is compared with that in a lookup table to determine flying height acceptability. Apparatus for performing a control operation in response to an indication of an impending crash of a magnetic transducer (1a-d) against a rotating magnetic recording medium due to deterioration in flying height of the transducer below a predetermined acceptable value, said apparatus comprising: means (4) for writing, over a predetermined area of said recording medium, one signal at a predetermined amplitude and at a frequency f₁; means (3a-d,4,5,6,7,8,9) for reading back said one signal at a frequency f₁ and storing its spectral amplitude; means (4) for overwriting said predetermined area with another signal at said predetermined amplitude but at a higher frequency f h ; means (3a-d,4,5,6,7,8,9) for reading back the component of said one signal remaining after said overwriting and storing its spectral amplitude; means (8,9) for calculating the ratio of said spectral amplitudes as read back before and after said overwriting; and means (8,9,10) for initiating the control operation when said ratio denotes that the flying height is below said acceptable value. Apparatus for performing a control operation responsively to an indication of an impending crash of a magnetic transducer (1a-d) against a rotating magnetic recording medium due to deterioration in flying height of the transducer below a predetermined acceptable value, said apparatus comprising: means (4) for writing, over a predetermined area of said recording medium, one signal at a predetermined amplitude and a frequency f₁ and then overwriting that signal with another signal at said amplitude but at a higher frequency f h ; means (3a-d,4,5,6a,6b,7a,7b,8,9) for reading back said one and other signals from said predetermined area after overwriting at frequency f h to produce first and second readback signals, respectively; means (8,9) for calculating the ratio of the amplitudes of said second signal to said first signal; and means (8,9,10) for initiating the control operation when said ratio denotes that the flying height is below said acceptable value. Apparatus as claimed in 9 or claim 10, having a plurality of magnetic transducers (1a-d) for reading indicia from a single recording medium, and multiplexing means (5) for causing said means (3a-d,4,5,6,7,8,9) for writing, sensing, calculating and initiating to be operated periodically among different ones of said transducers for initiating said control operation when the flying height of any one of said transducers is below said acceptable value. Apparatus as claimed in any of claims 9 to 11, wherein said initiating means includes a lookup table (10b;10b') and means (10a;10a') responsive to a comparison of flying height versus said ratio in said table to denote flying height acceptability.

Inventors: HOYT, ROGER FRANKLIN, MILLMAN, STEVEN EDWARD
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1988
Main classification: G11B5
IPC codes: G01B7, G11B19, G11B5, G11B21