Adaptive Servo Schemes using Micromachined Accelerometers for Increased Disturbance Rejection for Magnetic Hard Disk Drives


Hard disk drive track widths of 1 um are needed to achieve 10 GBit/in^2. Since it is desired to keep the tracking error below an eigth of the track width, the magnetic hard disk drive servo must maintain the tracking error below 0.12um at the above track width.  In a low disturbance environment, some hard disk drive manufactures believe that they can achieve this tracking error accuracy without the addition of new sensors and/or actuators.  However, conventional hard disk drives are used in a wide variety of environemtns which subject them to a significant amount of shock and vibrations.  External disturbances excite lightly damped internal resonances in the hard disk drive which can cause the tracking error to easily exceed acceptable levels.  Hence, a new direct adaptive servo scheme is presented which ultilzes a micromachined accelerometer to improve the external disturbance rejection capability of the track following servo in hard disk drives.  The objective of the control scheme is to minimize the variance of the position error signal in the presence of external disturbances.  The adaptive controller acts as an add-on compensator to an existing fixed compensator.  The micromachined accerlerometer is mounted on the base casting of the hard disk drive in order to obtain a measure of the external disturbance.

A magnetic hard disk drive was subjected to a number of different external vibration tests: rotational about the center of the actuator pivot and translational in the x, y, and z directions.  For a 10 tap adaptive FIR controller, experimental  results display 15 dB attenuation and 89% reduction in the variance of the position error signal for the random external rotational disturbance.  Similar performances were obtained in the x and y directions; while in the z direction, 15 dB attenuation and 53% reduction in the variance of the position error signal were experimentally verify for a random external disturbance.The rates of convergence of the FIR filter parameters averaged approximately 50msec.

The hard disk drive was also subjected to external shocks about the center of rotation of the actuator bearing. Half sine waves of 3 rad/sec^2 in magnitude and 2.7 msec in duration were sued as the external shoscks.  The maximum magnitude of the postion error signal as a result of the shock was reduced by 68% using the adaptive accelerometer servo.  The duration of the effect of the shock was also decreased by 10 msec.

Overall, the adaptive accelerometer control scheme successfully reduced the effects of external vibrations and shocks on the hard disk drive.  Under these external disturbances, the hard disk drive was not able to read nor write withou the utilization of th emicromachined accerlerometer based adaptive control scheme.

Satinderpall Pannu
Roberto Horowitz
Publication date: 
May 31, 1998
Publication type: 
Ph.D. Dissertation
Pannu, S. S. (1998). Adaptive Servo Schemes Using Micromachined Accelerometers for Increased Disturbance Rejection for Magnetic Hard Disk Drives. United States: University of California, Berkeley.

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