Detection of mechanical defects in a disc drive using injected test signals

• US 5,654,841 A
• Filed: 07/07/1995
• Issued: 08/05/1997
• Est. Priority Date: 07/07/1995
• Status: Expired due to Fees

• Current Assignee: Seagate Technology LLC (Seagate Technology Holdings Plc)
• Original Assignee: Seagate Technology plc (Seagate Technology Holdings Plc)
• Inventors: Hobson, Daniel E., Hampshire, Randall D., Felts, Wayne L.
• Primary Examiner(s): Psitos, Aristotelis
• Assistant Examiner(s): WONG, KIN C
• Application Number:
  US08/498,621
• Time in Patent Office: 760 Days
• Field of Search: 360/75, 360/77.05, 360/78.14, 360/130.34, 360/135, 360/48, 369/4, 369/32, 369/38, 369/43, 369/44.25, 369/44.26, 369/44.28, 370/100.1, 371/20.4, 371/5.4, 371/5.5, 395/275, 395/425, 395/500, 324/210, 324/212
• US Class Current: 360/75
• CPC Class Codes: G11B 5/59627 Aligning for runout, eccentricity or offset compensation G11B5/5534, G11B5/59677, G11B5/59688 take precedence

Abstract

A method and apparatus for testing for mechanical defects in a disc drive during manufacturing test is disclosed. Resonances in the disc drive are excited by injecting a sinusoidal test signal of known magnitude and frequency into an incoming servo position error (SPE) signal in the servo loop to provide a modified SPE. While the test signal is injected, the modified SPE is sampled and filtered to characterize the test frequency in the modified SPE which is compared to a resultant frequency characterized in the incoming SPE. The presence of resonances in the disc drive structure will result in an increase in the gain of the loop as indicated by the characterized resonant frequencies from the incoming and modified SPE. When the gain exceeds predetermined criteria, an error flag is set indicative of a failure condition for the disc drive, enabling the disc drive to be removed from test for subsequent remedial actions.

Claims

1. In a disc drive having a disc and an actuator adjacent the disc, the actuator having a transducer positionably adjacent any selected one of a plurality of tracks on the disc surface, the disc drive having a servo control circuit operable as a servo loop for generating a correction signal in response to an incoming servo position error signal received from the transducer, the servo position error signal indicative of the relative position of the transducer to the selected track, the correction signal controlling the output of current from an actuator driver to an actuator coil to position the transducer with respect to the selected track in response to the servo position error signal, wherein the servo position error signal is obtained from the transducer in each of a succession of time intervals, a method for detecting mechanical defects in the disc drive comprising the steps of:

injecting a test signal having a selected frequency and magnitude into the incoming servo position error signal to generate a modified servo position error signal;

using the modified servo position error signal to generate a correction signal to control the current supplied to the actuator coil by the actuator driver;

sampling and accumulating data from the incoming servo position error signal to characterize the magnitude of a resonant signal in the incoming servo position error signal, the resonant signal having a frequency substantially equal to the test signal frequency;

determining a ratio of the magnitude of the resonant signal in the incoming servo position error signal to the magnitude of the test signal in the modified servo position error signal; and

providing an indication when the ratio exceeds a preselected limit.

2. The method of claim 1, wherein the steps of sampling and accumulating data comprises the step of performing a discrete fourier transform over a plurality of time intervals.

3. In a disc drive of the type having a disc and an actuator including a coil and a transducer, the disc drive having a servo control circuit operable as a servo loop for generating a correction signal in response to servo position error information received from the transducer, the servo position error information indicative of the relative position of the transducer to a selected track on the disc, the correction signal controlling the output of current from an actuator driver to the actuator coil in order to position the transducer with respect to the selected track in response to the servo position error information, wherein the servo position error information is obtained from the transducer in each of a succession of time intervals, a method for detecting mechanical defects in the disc drive comprising the steps of:

in each of a predetermined number of said time intervals,

receiving servo position error information from the transducer;

adding selected components of a digital representation of an injection test signal to the servo position error information to generate modified servo position error information, the injection test signal comprising a sinusoidal signal having a preselected frequency and magnitude;

using the modified servo position error information to generate a modified correction signal;

using the modified correction signal to control the output of current by the actuator driver to the actuator coil;

sampling and accumulating the servo position error information to characterize a first signal in the servo position error information, the first signal comprising a sinusoidal signal having a frequency and magnitude, the frequency of the first signal having a frequency substantially that of the injection test signal; and

sampling and accumulating the modified servo position error information to characterize a second signal in the modified servo position error information, the second signal comprising a sinusoidal signal having a frequency and magnitude substantially that of the injection test signal;

thereafter, comparing the magnitude of the second signal to the magnitude of the first signal to determine the gain of the servo loop;

comparing the gain of the servo loop to a predetermined maximum limit; and

setting an error flag at such time that the gain of the servo loop exceeds the predetermined maximum limit.

4. The method of claim 3, wherein the step of sampling and accumulating the servo position error information comprises the step of performing a discrete fourier transform to characterize the first signal in the servo position error information.

5. The method of claim 3, wherein the step of sampling and accumulating the modified servo position error information comprises the step of performing a discrete fourier transform to characterize the second signal in the modified servo position error information.

6. In a disc drive of the type having a disc and an actuator adjacent the disc, the actuator having a coil and a transducer, the disc drive further having a servo control circuit operable as a servo loop for generating a correction signal to position the transducer with respect to the disc in response to an incoming servo position error signal received from the transducer, the servo position error signal indicative of the relative position of the transducer to a selected track on the disc, the correction signal controlling the output of current from an actuator driver to an actuator coil, wherein the servo position error signal is obtained from the transducer in each of a succession of time intervals, the improvement comprising:

injection means for injecting a test signal of known frequency and magnitude into the incoming servo position error signal to generate a modified servo position error signal;

characterization means, responsive to the incoming servo position error signal, for characterizing the magnitude of a resonant signal resident in the incoming servo position error signal, the resonant signal having a frequency substantially that of the test signal; and

determining means, responsive to the characterization means, for determining the gain of the servo loop using the magnitude of the resonant signal and the magnitude of the test signal in the modified servo position error signal and providing a signal indicative of the gain exceeding a preselected limit;

wherein the modified servo position error signal is used to generate a correction signal for controlling the current supplied by the actuator driver to the actuator coil.

7. The improved disc drive of claim 6, wherein the gain of the servo loop is determined by dividing the magnitude of the resonant signal by the magnitude of the test signal.

8. The improved disc drive of claim 6, wherein the characterization means uses a discrete fourier transform to characterize the magnitude of the resonant signal by sampling and accumulating data from the resonant signal over preselected number of said time intervals.