WO1997017621A1

WO1997017621A1 - Method and apparatus for measuring the smoothness of a magnetic disk

Info

Publication number: WO1997017621A1
Application number: PCT/US1996/018093
Authority: WO
Inventors: Michael Inbar
Original assignee: Phase Metrics
Priority date: 1995-11-08
Filing date: 1996-11-08
Publication date: 1997-05-15

Abstract

A tester for testing the smoothness of a magnetic disk comprising a magnetic head (14) which can write and read data onto a disk. The magnetic head (14) generates a read signal that corresponds to the magnetic field of the disk. The read signal is a signal which varies at a frequency. When the head (14) strikes an asperity, the contact will produce a mechanical resonance in the head (14) and modulate the frequency of the read signal. The frequency modulated read signal is detected by a phase lock loop circuit (20) which generates an error signal. The error signal is provided to a digital signal processor (36) to measure the magnitude and number of asperities in the disk.

Description

METHOD AND APPARATUS FOR MEASURING THE

SMOOTHNESS OF MAGNETIC DISK RECORDING SURFACES

USING CONVENTIONAL MAGNETIC READ HEADS AND

UTILIZING FREQUENCY DOMAIN MEASUREMENT TECHNIQUES

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

The present invention relates to a test device that measures the smoothness of a magnetic disk typically used in a hard disk drive.

2. DESCRIPTION OF RELATED ART

Hard disk drives contain magnetic heads that magnetize and sense the magnetic field of a rotating disk. The heads are integrated into a slider that is mounted to a suspension arm. The suspension arm is typically cantilevered from an actuator arm assembly. The sliders have aerodynamic characteristics that create an air bearing between the heads and the rotating disk. The air bearings prevent the heads from sliding on the disk surface and creating mechanical wear on the head components. The stability of the air bearing is a function of the smoothness and flatness of the disk. Any asperities on the disk surface may induce undesirable contact between the head and the disk. For this reason, disk and disk drive manufacturers measure the surface finish of the disks before installation into a disk drive unit.

Disk certification testers typically contain a piezoelectric transducer that is "flown" over the disk. Any disk asperities will strike the piezoelectric transducer and induce a resonance in the transducer.. The mechanical energy is converted to an electrical signal by the transducer. The size and number of asperities can be measured by analyzing the electrical signal produced by the piezoelectric transducer.

After certification for smoothness, the disk is then placed in a second tester to measure the magnetic characteristics of the disk. This is performed by writing and subsequently reading data onto the disk with a magnetic recording head. The read signal generated by the head is analyzed to locate any disk defects. Having two separate test stations requires valuable production floor space. Additionally, transferring the magnetic disks between test stations may damage or introduce contaminants to the disks during the transfer process. It would be desirable to provide a disk certification tester which can measure the magnetic characteristics and smoothness of a disk in a single station.

SUMMARY OF THE INVENTION

The present invention is a disk certification tester for testing the magnetic characteristics and the smoothness of a magnetic disk in a single station. The station includes a magnetic head which can write and subsequently read data onto a disk. The head is typically integrated into a head gimbal assembly that is cantilevered above or below the surface of the disk. The magnetic head generates a read signal that corresponds to the magnetic field of the disk. The read signal is typically an alternating current signal which varies at a frequency. When the head strikes an asperity in the disk, the contact will produce a mechanical resonance in the head gimbal assembly. The movement of the head will modulate the frequency of the read signal. The frequency modulated read signal is detected by a phase lock loop circuit which generates an error signal. The error signal is a function of the frequency and amplitude modulation. The error signal is provided to a digital signal processor which analyzes the signal to measure the magnitude and number of asperities in the disk. The magnetic head can also be used to measure the magnetic characteristics of the disk by detecting and analyzing the magnitude of the read signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages of the present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, wherein:

Figure 1 is a top view of head gimbal assembly being resonated by an asperity in a rotating disk;

Figure 2 is a schematic of a smoothness detection circuit of the present invention;

Figure 3 is a graph showing a read signal and a corresponding error signal generated by the smoothness detection circuit.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings more particularly by reference numbers, Figure 1 shows a head gimbal assembly ("HGA") 10 "flying" above a rotating disk 12. The HGA 10 includes a magnetic head 14 integrated into a slider 16. The slider 16 is mounted to a suspension beam 18 that is cantilevered above the disk 12. The rotating disk generates a flow of air that passes under the slider 16. The slider 16 has aerodynamic characteristics that induce the formation of an air bearing between the head 14 and the disk 12. The air bearing may have a height on the order of microns.

The disk surface may have bumps, contaminants and other disparities which cause contact between the head 14 and the disk 12. Any contact between the disk 12 and the head 14 will induce a corresponding resonant translational movement of the HGA 10 as indicated by the arrows. When installed into a hard disk drive unit, the continuous contact created by the disk asperities may result in an undesirable wear of the head or a head crash. Additionally, the asperities may create unwanted noise in the read signal of the drive. For this reason, it is desirable to measure the magnitude and number of asperities on the disk surface. The present invention senses the contact between the head 14 and the disk 12 by detecting and measuring the mechanical resonance of the HGA 10 with the read signal of the magnetic head 14.

Figure 2 shows a schematic of a phase lock loop ("PLL") circuit 20 that detects the mechanical resonance of the HGA 10. The magnetic head 14 generates a read signal that corresponds to the magnetic field of the disk 12. The read signal is typically an alternating current with a frequency wl. The mechanical resonances of the magnetic head 14 modulates the frequency of the read signal to produce a frequency modulated signal.

The frequency modulated read signal is provided to the input of a phase detector circuit 22 on line 24. The phase detector 22 has another input connected to the output of a voltage controlled oscillator ("VCO") 26 on line 28. The voltage controlled oscillator 26 generates an ac signal which has a frequency w2.

The phase detector circuit 22 generates an error signal that has an amplitude which corresponds to the difference between the frequency wl of the read signal and the frequency w2 of the oscillator signal. The difference in frequency corresponds to the phase shift created by the frequency modulation of the read signal . The amplitude of the error signal will typically increase with an increase in the phase shift of the read signal.

The error signal is also provided to a low pass filter 30 on line 32. The filter 30 removes undesirable noise in the error signal and the higher harmonics of the signal at point 32. The filtered error signal is provided to the input of the VCO 26 on line 34. The frequency of the oscillator signal provided to the phase detector 22 on line 28 is a function of the amplitude of the error signal.

The error signal is provided to a digital signal processor ("DSP") 36 on line 38. The digital signal processor 36 analyzes the error signal to determine whether the phase shift in the read signal is created by an asperity in the disk. The frequencies at which the head gimbal assembly resonates is known before the disk is certified. The DSP 36 can compare the error signal from the phase lock loop circuit 20 with a known pattern, value, etc. to determine whether the frequency modulation of the read signal is the result of an asperity, or the result of another source of modulation that is not related to the smoothness of the disk.

Figure 3 shows a read signal that is frequency modulated by an HGA 10, that is resonated by an asperity on the disk 12. The phase lock loop circuit 20 produces an error signal that corresponds to the phase shift of the read signal. As shown in Fig. 3, the HGA 10 will dampen after the initial resonant movement of the head 14. The dampening movement of the HGA will cause a decaying frequency modulation of the read signal. The decaying modulation will produce a resulting gradual decrease in the amplitude of the error signal generated by the phase lock loop circuit 20. In the preferred embodiment a disk is certified, if the disk has an average number of asperities that are below an average threshold value, and contains no asperities that exceed a single asperity threshold value. The DSP 36 can analyze the error signal to determine whether the disk meets the average and single asperity criteria required to certify the disk. The DSP 36 can integrate the error signal to determine the average energy generated by the disk asperities. If the integrated error signal is less than an average threshold value the disk passes the first step of the certification process. The DSP 36 also compares the peak magnitude generated by each asperity with another threshold value to determine whether any single asperity exceeds a manufacturing tolerance. The disk may be certified if the phase lock loop circuit 20 provides error signals below both threshold values.

In operation, the magnetic head 14 writes data onto the tracks of the disk 12. The head 14 then reads the data from the disk 12. The DSP 36 and other circuitry (not shown) may measure various magnetic characteristics of the disk with the read signal generated by the head. By way of example, the voltage amplitude of each peak may be compared to a threshold value. If the peak value is below the threshold value, the tester may provide an error indication. This test measures magnetic characteristics of the disk 12. Additionally, the phase lock loop circuit 20 and DSP 36 utilize the same read signal to measure the asperities of the disk. The present invention thus provides a system which can test the magnetic characteristics and smoothness of the disk with the same test station.

While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention not be limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those ordinarily skilled in the art.

Claims

What is claimed is:

1. An apparatus for measuring a smoothness of a disk that rotates relative to a head gimbal assembly which has a magnetic head that generates a read signal at a frequency, wherein the frequency of the read signal is modulated when the magnetic head makes contact with the disk, comprising: a phase lock loop circuit that detects the modulation in the frequency of the read signal and generates an error signal that corresponds to the frequency modulation, and, a digital signal processor that analyzes said error signal of said phase lock loop circuit.

2. The apparatus as recited in claim 1, wherein said digital signal processor integrates said error signal and compares said integrated error signal with an average threshold value.

3. The apparatus as recited in claim 1, wherein said digital signal processor compares a magnitude of said error signal with a single asperity threshold value.

4. The apparatus as recited in claim 1, wherein said phase lock loop circuit includes a phase detector coupled to the magnetic head, a filter coupled to said phase detector, and a voltage controlled oscillator coupled to said filter and said phase detector, said digital signal processor being coupled to an output of said filter.

5. An apparatus for measuring a smoothness of a disk that rotates relative to a head gimbal assembly which has a magnetic head that generates a read signal at a frequency, wherein the frequency of the read signal is modulated when the magnetic head makes contact with the disk, comprising: phase lock loop circuit means for detecting the modulation in the frequency of the read signal and generating an error signal that corresponds to the frequency modulation; and, digital signal processor means for analyzing said error signal of said phase lock loop means.

6. The apparatus as recited in claim 5, wherein said digital signal processor means integrates said error signal and compares said integrated error signal with an average threshold value.

7. The apparatus as recited in claim 5, wherein said digital signal processor means compares features of said error signal with a single asperity threshold value.

8. The apparatus as recited in claim 5, wherein said phase lock loop means includes a phase detector coupled to the magnetic head, a filter coupled to said phase detector, and a voltage controlled oscillator coupled to said filter and said phase detector, said digital signal processor being coupled to an output of said filter.

9. An apparatus for measuring a smoothness of a disk that rotates relative to a head gimbal assembly which has a magnetic head that generates a read signal at a frequency, wherein the frequency of the read signal is modulated when the magnetic head makes contact with the disk, comprising: a voltage controlled oscillator that has an input and that generates an oscillator signal at an oscillator frequency on an output; a phase detector that has a first input coupled to the magnetic head and a second input connected to said output of said voltage controlled oscillator, said phase detector generates an error signal on an output, wherein said error signal corresponds to a difference between the read frequency and said oscillator frequency; a low pass filter that has an input connected to said output of said phase detector and an output connected to said input of said voltage controlled oscillator; and, a digital signal processor that is connected to said output of said filter and analyzes said error signal.

10. The apparatus as recited in claim 9, wherein said digital signal processor integrates said error signal and compares said integrated error signal with an average threshold value.

11. The apparatus as recited in claim 10, wherein said digital signal processor compares a magnitude of said error signal with a single asperity threshold value.

12. A method for measuring an asperity of a disk that rotates relative to head gimbal assembly which has a magnetic head that generates a read signal at a read frequency, comprising the steps of: a) placing the head gimbal assembly adjacent to the disk so that the asperity in the disk produces contact between the disk and the head gimbal assembly and induces a mechanical oscillation of the head gimbal assembly, wherein the mechanical oscillation causes a frequency modulation of the read signal; b) detecting said frequency modulation of the read signal; and, c) analyzing said frequency modulation of the read signal to measure the asperity.

13. The method as recited in claim 12, wherein said frequency modulation of the read signal is detected by generating an error signal that is a function of the read frequency of the read signal and an oscillator frequency of an oscillator signal that is generated by a voltage controlled oscillator which receives said error signal.

14. The method as recited in claim 13, wherein said frequency modulation of the read signal is analyzed by integrating said error signal and comparing said integrated error signal with an error threshold value.

15. The method as recited in claim 14, wherein said frequency modulation of the read signal is analyzed by comparing a magnitude of said error signal with a single asperity threshold value.