Detection and measurement of head disc interference using read back signal (without acoustic emission sensor or laser doppler vibrometer)

• US 6,008,640 A
• Filed: 09/30/1997
• Issued: 12/28/1999
• Est. Priority Date: 04/28/1997
• Status: Expired due to Fees

• Current Assignee: Seagate Technology LLC (Seagate Technology Holdings Plc)
• Original Assignee: Seagate Technology plc (Seagate Technology Holdings Plc)
• Inventors: Tan, Alan C., Ngwe, Myint, Liu, Joseph C.
• Primary Examiner(s): STRECKER, GERARD R
• Application Number:
  US08/940,929
• Time in Patent Office: 819 Days
• Field of Search: 324/210, 324/212, 360/25, 360/31, 360/53, 360/75, 369/53, 369/55, 73/570
• US Class Current: 324/212
• CPC Class Codes: G11B 2005/001 Controlling recording characteristics of record carriers or transducing characteristics of transducers by means not being part of their structure
• CPC Class Codes: G11B 33/10 Indicating arrangements; Warning arrangements G11B15/04, G11B19/04, G11B27/34, G11B27/36 take precedence
• CPC Class Codes: G11B 5/012 Recording on, or reproducing or erasing from, magnetic disks G11B17/00, G11B19/00 take precedence
• CPC Class Codes: G11B 5/455 Arrangements for functional testing of heads; Measuring arrangements for heads

Abstract

A method and apparatus for detecting and measuring head-disc interference, in a disc drive, for the purpose of taking preventative and corrective measures, to prevent disastrous head crashes and permanent data loss, is disclosed. The method and apparatus detect and measure the frequency modulation (FM) of a read back signal generated by longitudinal head oscillation, as a result of head-disc interference. The longitudinal head oscillation can be ascribed to the non-linear response of the air bearing supporting the head on the disk surface. The response exhibits a vibration frequency in the head in the 100 to 250 kHz range. The more severe the interference contact is, the higher the vibration amplitude which then leads to read signal frequency modulation (FM). A method and apparatus is described to detect the FM modulation and thereby to detect the severity of the head disk interference for proximity recording. A pre-written pattern on the disk can be read back to monitor for FM, and a modulation index, β, is generated by observing the resulting signal. By demodulating the modulated read back signal, the severity of the interference can be determined. This method can be applied as a diagnostic tool at a manufacturing test station, in a laboratory test setup, or in a disk drive using the disk drive'"'"'s own read/write channel electronics, without the need for an external sensor.

Claims

1. A method of detecting longitudinal head oscillations caused by head-disc interference in a disk drive, said method comprising the steps of:

(a) writing a constant frequency test pattern on a selected track of a magnetic disc, said test pattern being initially void of frequency modulation;

(b) reading said test pattern and generating a read back signal; and

(c) detecting spurious frequency modulation, in the range of about 120 kHz to 250 kHz, in said read back signal as an indication of a presence of longitudinal head oscillations caused by head disc-interference.

5. A method of detecting longitudinal head oscillations as described in claim 1, wherein:

said step (b) comprises reading said test pattern by using a differential probe; and

said step (c) comprises detecting said frequency modulation using a spectrum analyzer.

6. A method of detecting longitudinal head oscillations as described in claim 1, wherein:

said steps (b) and (c) comprises reading and detecting said test pattern by using a modulation analyzer capable of detecting a frequency modulation, a modulation index, a peak frequency deviation and modulation frequency, and said method further comprises;

detecting said frequency modulation, said modulation index, said peak frequency deviation and said modulation frequency on said modulation analyzer, said modulation index being a first indication of the degree of severity of said head-disc interference; and

determining a second indication of said degree of severity of said head-disc interference by determining a longitudinal head vibration velocity and peak longitudinal head oscillation displacement.

7. A method of detecting longitudinal head oscillations as described in claim 1, wherein:

said steps (b) and (c) comprises reading said test pattern and detecting frequency modulation by utilizing a disk drive'"'"'s read/write channel electronics, comprising a phase lock loop circuit used as an FM demodulator.

8. A method of detecting longitudinal head oscillations as described in claim 7, wherein said method further comprises the step of:

employing a threshold comparing setup for taking operative corrective action in using said disc drive, said setup comprising a plurality of threshold signal magnitude values for being compared against a signal magnitude associated with said detected frequency modulation, said plurality of threshold signal magnitude values being related to degrees of head-disc interference severity.

2. A method of detecting longitudinal head oscillations caused by head-disc interference in a disk drive, said method comprising the steps of:

(a) writing a constant frequency test pattern on a selected track of a magnetic disc, said test pattern being initially void of frequency modulation;

(b) reading said test pattern and generating a read back signal; and

(c) detecting spurious frequency modulation in said read back signal as an indication of a presence of longitudinal head oscillations caused by head disc-interference;

(d) analyzing said detected frequency modulation for purposes of determining a degree of severity of said head-disc interference by performing tasks of measuring and recording a voltage magnitude of sidebands, a main carrier frequency and a modulating frequency (f_m) associated with said detected frequency modulation;

(e) determining a peak frequency deviation (Δ

f) of said frequency modulation; and

(f) determining a modulation index (β

) as a first indication of said degree of severity of said head-disc interference.

3. A method of detecting longitudinal head oscillations as described in claim 2, wherein:

said step (e) of determining a peak frequency deviation comprises determining coefficients of a Bessel function; and

said step (f) of determining said modulation index comprises computing a ratio of said determined peak frequency deviation and said measured modulation frequency, namely computing β

=Δ

f/f_m.

4. A method of detecting longitudinal head oscillations as described in claim 2, said method further comprising the steps of:

(g) determining a peak period deviation, a longitudinal head vibration velocity and a displacement amplitude of said longitudinal head oscillation, said displacement amplitude being directly proportional to said modulation index β

determined in said step (f) and being a second indication of said degree of severity of said head-disc interference.

9. A method of detecting and measuring severity of longitudinal head oscillation caused by head-disc interference in a disc drive, said method comprising the steps of:

(a) writing a constant frequency test pattern on a selected track of a magnetic disc;

(b) reading said test pattern and generating a read back signal;

(c) detecting frequency modulation in said read back signal;

(d) analyzing said detected frequency modulation for purposes of determining the degree of severity of said head-disc interference by performing tasks of measuring and recording a voltage magnitude of sidebands, a main carrier frequency and a modulating frequency (f_m) associated with said detected frequency modulation;

(e) determining a peak frequency deviation (Δ

f) of said frequency modulation; and

(f) determining a modulation index (β

) as a first indication of said degree of severity of said head-disc interference; and

(g) determining a peak period deviation, a longitudinal head vibration velocity and a displacement amplitude of said longitudinal head oscillation, said displacement amplitude being directly proportional to said modulation index β

determined in said step (f) and being a second indication of said degree of severity of said head-disc interference.

10. A method of detecting and measuring severity of longitudinal head oscillations as described in claim 9, wherein:

said step (e) of determining a peak frequency deviation comprises determining coefficients of a Bessel function; and

said step (f) of determining said modulation index comprises computing a ratio of said determined peak frequency deviation and said measured modulation frequency, namely computing β

=Δ

f/f_m.

11. A method of detecting and measuring severity of longitudinal head oscillations as described in claim 9, wherein:

said step (b) comprises reading said test pattern by using a differential probe; and

said step (c) comprises detecting said frequency modulation using a spectrum analyzer.

12. A method of detecting and measuring severity of longitudinal head oscillations causing head-disc interference in a disc drive, said method comprising the steps of:

(a) writing a constant frequency test pattern on a selected track of a magnetic disc;

(b) reading said test pattern and generating a read back signal;

(c) detecting frequency modulation in said read back signal by using a modulation analyzer and generating a modulation index, a peak frequency deviation and modulation frequency, said modulation index being a first indication of the degree of severity of said head-disc interference; and

(d) determining a second indication of said degree of severity of said head-disc interference by determining a longitudinal head vibration velocity and peak longitudinal head oscillation displacement of said longitudinal head oscillation.

13. A method of detecting longitudinal head oscillations causing head-disc interference in a disc drive, said method comprising the steps of:

(a) writing a constant frequency test pattern on a selected track of a magnetic disc;

(b) reading said test pattern and generating a read back signal;

(c) detecting frequency modulation, in the range of about 120 kHz to 250 kHz, in said read back signal by utilizing a disk drive'"'"'s read/write channel electronics, comprising a phase lock loop circuit used as an FM demodulator.

14. A method of detecting longitudinal head oscillations as described in claim 13, wherein said method further comprises the step of:

employing a threshold comparing setup for taking operative corrective action in using said disc drive, said setup comprising a plurality of threshold signal magnitude values for being compared against a signal magnitude associated with said detected frequency modulation, said plurality of threshold signal magnitude values being related to degrees of head-disc interference severity.

15. A method of preventing head crashes in a disc drive due to longitudinal head oscillations causing head-disc interference, said method comprising the steps of:

(a) writing a constant frequency test pattern on a selected track of a magnetic disc;

(b) reading said test pattern and generating a read back signal;

(c) detecting frequency modulation, in the range of about 120 kHz to 250 kHz, in said read back signal by utilizing a disk drive'"'"'s read/write channel electronics, comprising a phase lock loop circuit used as an FM demodulator; and

(d) employing a threshold comparing setup for taking operative corrective action in using said disc drive, said setup comprising a plurality of threshold signal magnitude values for being compared against a signal magnitude associated with said detected frequency modulation, said plurality of threshold signal magnitude values being related to degrees of head-disc interference severity.

16. A method for evaluating impact of a magnetic media disc'"'"'s surface texture and operational head fly height in a disc drive on head-disc interference, said method comprising the steps of:

(a) providing a magnetic media disc having a known surface texture roughness;

(b providing a known thickness of said magnetic media disc;

(c) providing a known fly height between said magnetic media disc and a head element;

(d) writing a constant frequency test pattern on a selected track of said magnetic media disc;

(e) reading said test pattern and generating a read back signal;

(f) detecting frequency modulation in said read back signal;

(g) analyzing said detected frequency modulation for purposes of determining the degree of severity of said head-disc interference by performing tasks of measuring and recording a voltage magnitude of sidebands, a main carrier frequency and a modulating frequency (f_m) associated with said detected frequency modulation;

(h) determining a peak frequency deviation (Δ

f) of said frequency modulation; and

(i) determining a modulation index (β

) as a first indication of said degree of severity of said head-disc interference;

(j) determining a peak period deviation, a longitudinal head vibration velocity and a displacement amplitude of said longitudinal head oscillation, said displacement amplitude being directly proportional to said modulation index β

determined in said step (i) and being a second indication of said degree of severity of said head-disc interference; and

(k) making a record containing said peak period deviation and said displacement amplitude in said step (j), said known surface texture roughness, said known thickness of said magnetic media disc, and said known fly height, as provided in said steps (a), (b), and (c), as information relating to said head-disc interference by said provided magnetic media disc operating at said fly height.

17. A disc drive apparatus, said apparatus comprising:

a head disc assembly, said disc assembly having a constant frequency test pattern written on a selected track of a magnetic disc;

a read/write channel electronic circuit comprising a phase lock loop circuit used as an FM demodulator, said phase lock loop circuit being electronically coupled to said head disc assembly for reading said test pattern, said phase lock loop circuit detecting frequency modulation, in the range of about 120 kHz to 250 kHz, during reading of said test pattern; and

said phase lock loop circuit having output means for use in displaying said detected frequency modulations.

18. A disc drive apparatus, as described in claim 17, said apparatus further comprising:

a threshold comparing circuit setup for producing operative corrective action in using said disc drive, said setup comprising electronic means for storing a plurality of threshold signal magnitude values for being compared against a signal magnitude associated with said detected frequency modulation, said plurality of threshold signal magnitude values being related to degrees of head-disc interference severity, said electronic means comprising logic decision generating circuitry having a memory for storing at least one threshold amplitude value of a base frequency modulation, an early warning circuit portion for alerting a user of a severity of head-disc interference and further having a resume operation circuit portion for resuming normal operations of said disc drive if a an amplitude of a detected frequency modulation is less than a stored threshold amplitude value.