Electricity: measuring and testing – Magnetic – Displacement
Reexamination Certificate
1998-08-07
2001-02-27
Oda, Christine K. (Department: 2862)
Electricity: measuring and testing
Magnetic
Displacement
C360S266100, C360S266400
Reexamination Certificate
active
06194892
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to micro-actuators such as those considered for computer system hard disk drives. More particularly, it relates to a position sensor for a micro-actuator to achieve higher bandwidth control.
BACKGROUND OF THE INVENTION
The present invention enables more precise control of a micro-actuator than in previous micro-actuator systems. For example, the present invention may be advantageously employed to position the read/write heads of a hard disk drive (HDD).
An inductive position sensor for an electromagnetic linear actuator was described in a paper by H. Guckel, T. Earles, J. Klein, D. Zook, and T. Ohnstein entitled “Electromagnetic Linear Actuators with Inductive Position Sensing” appearing in
Sensors and Actuators A
53 (1996) at pages 386-391, incorporated herein by reference. The sensor used the self inductance of the coils for position sensing of the plunger. This system employs wire wound coils rather than an integrated coil. Also, the actuator was a solenoid plunger rather than a silicon micromachined device.
Information storage in HDD systems is arranged in concentric “tracks” upon the disks; information density increases when the concentric tracks can be placed closer together. The conventional parameter used to describe this characteristic is “tracks-per-inch” (TPI) which refers to the number of tracks measured along the radius of the disk.
There are several limitations in conventional HDD systems which prevent TPI from increasing much beyond today's state-of-the-art. The actuator arm (
FIG. 1
) which rotates about its pivot point determines the position of the read/write head. As TPI increases, the size of the rotation angle corresponding to a single track obviously decreases. Friction, ball slippage, and other nonlinear phenomena within the pivot and its bearings, and within flexible electrical wiring or spring mechanisms, makes control to smaller rotation angles very difficult, if not impossible. Furthermore, flexure or mechanical resonances within the actuator arm itself introduce small movements in the position of the read/write head which are essentially independent of the actuator arm's rotational movement within its bearings. Although these small movements are not serious for a low capacity HDD, they are serious limitations if one wishes to increase TPI.
In addition to limitations within the actuator arm, support bearings, and other associated structures, other disturbances also occur due to interactions between the actuator arm, read/write head, and the rotating disk. Windage effects, spindle motor eccentricities, and spindle motor cogging are examples of mechanical “run out” errors which limit practical TPI levels for today's HDD systems.
Another limitation is errors which occur when the “master” tracks are defined for the HDD. Servo write “errors” are imperfections in the concentric track locations and therefore become another limitation which the servo system must accommodate.
If a HDD is fabricated with higher bandwidth servo control, many of the limitations from friction and ball bearings can be overcome. However mechanical resonances of the arm itself not only represent small movements, but they also prevent fabrication of higher bandwidth servo control which would help overcome these other limitations.
If one attempts to increase the bandwidth beyond the limitations imposed by the mechanical resonances, the control system becomes unstable and operation is impossible. Conventional methods for reducing mechanical resonance limitations are to increase the thickness, strength, weight, and manufacturing precision of the actuator arm. Obviously this is counter to the design goals for low power, low cost HDD systems.
SUMMARY OF THE INVENTION
The present invention describes an improved micro-actuator position sensor which provides an accurate position signal for higher bandwidth control of a micro-actuator. The position sensor allows closed loop control of the micro-actuator position. Preferred embodiment micro-actuator sensors include a piezo-resistive stress sensor integrated within the springs, a capacitance sensor and magnetic reluctance sensor.
Embodiments of the present invention are described in detail as integrated sensors for a micro-actuator used in hard disk drives. The sensor is integrated with a micro-actuator at the tip of the conventional actuator arm to improve the precision seeking capability of the actuator arm. By using a combination of voice coil actuator and micro-actuator, the tracking density can be increased. The dual actuator system requires a dual feedback loop to accomplish the tracking. The in-situ position sensor according to the present invention can be used to provide the displacement information to be included in the feed back loop to the actuator control electronics. Using one or more of the above methods of sensing, the micro-actuator can be precisely driven in position so that the read/write head can be adjusted, over a small range, from track to track.
One prior art approach, described by Denny K. Miu et al in IEEE Transactions on Industrial Electronics, Vol. 42, No. 3., June 1995, incorporated herein by reference, includes a silicon micromachined rotary actuator at the end of a rotary actuator. However, this does not describe micro-actuator position sensors or the advantages that may be gained by the use of a sensor.
An advantage of the present invention is higher Bandwidth over prior art designs through closed loop control using a position sensor.
An additional advantage of the present invention is an in-situ position sensor. The position sensor can be integrally formed with the micro-actuator using compatible IC batch processes.
An additional advantage of the present invention is the position sensor in a preferred embodiment provides self aligned measurements.
REFERENCES:
patent: 5499161 (1996-03-01), Hosseinzadeh et al.
patent: 5940250 (1999-08-01), McNeil et al.
Miu et al. Jun. 1995 IEEE Transactions on Industrial Elecctronic vol.42 No. 3 Silicon Micromachined.
Imamura et al. Sep. 1998 IEEE ASME vol. 3 No.3 MEMS-Based Head/Actuator/Slider.
Hirano et al. Sep. 1998 IEEE/ASME vol. 3 No. 3 High Bandwidth for Microactuators for HDD.
Aggarwal et al. Jun. 1997 Microactuators for HDD p. # 3979-3984 American control Conference .
H. Guckel, et al., “Electromagnetic Linear Actuators with Inductive Position Sensing for Micro Relay, Micro Valve and Precision Positioning Applications”, The 8thInternational Conference on Solid State Sensors and Actuators and Eurosensors IX, Stockholm Sweden, Jun. 25-29, 1995, pp. 324-327.
H. Guckel, et al., “electromagnetic linear actuators with inductive position sensing”, Sensors and Actuators A53 (1996), pp. 386-391.
Denny K. Miu, “Silicon Microstructures and Microactuators for Compact Computer Disk Drives”, Silicon Microstructures and Microactuators for Compact Computer disk Drives, 1995. pp. 1-12.
Charles S. Smith, “Piezoresistance Effect in Germanium and Silicon”, Physical Review, vol. 94, No. 1, Apr. 1, 1954.
J. W. Gardner, “Microsensors: principles and applications”, Library of Congress Cataloging-in-Publications Data, 1994. pp. 178-183 and p. 196.
Congdon Philip A.
Heaton Mark W.
Lin Tsen-Hwang
Masten Michael K.
Oda Christine K.
Petersen Bret J.
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
Zaveri Subhash
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