The present invention relates to positioning actuators of electromechanical systems, and more particular to highly fast and precise servo positioning systems such as those employed in disc drives.
Disc drives of the type known as xe2x80x9cWinchesterxe2x80x9d disc drives, or hard disc drives, are well known in the industry. Such disc drives magnetically record digital data on a plurality of circular, concentric data tracks on the surfaces of one or more rigid discs. The discs are typically mounted for rotation on the hub of a brushless DC spindle motor. In disc drives of the current generation, the spindle motor rotates the discs at speeds of up to 15,000 RPM.
Data are recorded to and retrieved from the discs by an array of vertically aligned read/write head assemblies, or heads, which are controllably moved from track to track by an actuator assembly. The read/write head assemblies typically consist of an electromagnetic transducer carried on an air bearing slider. This slider acts in a cooperative pneumatic relationship with a thin layer of air dragged along by the spinning discs to fly the head assembly in a closely spaced relationship to the disc surface. In order to maintain the proper flying relationship between the head assemblies and the discs, the head assemblies are attached to and supported by head suspension tabs.
The actuator assembly used to move the heads from track to track has assumed many forms historically, with most disc drives of the current generation incorporating an actuator of the type referred to as a rotary voice coil actuator. A typical rotary voice coil actuator consists of a pivot shaft fixedly attached to the disc drive housing base member closely adjacent the outer diameter of the discs. The pivot shaft is mounted such that its central axis is normal to the plane of rotation of the discs. An actuator bearing housing is mounted to the pivot shaft by an arrangement of precision ball bearing assemblies, and supports a flat coil which is suspended in the magnetic field of an array of permanent magnets, which are fixedly mounted to the disc drive housing base member. On the side of the actuator bearing housing opposite to the coil, the actuator bearing housing also typically includes a plurality of vertically aligned, radially extending actuator head mounting arms, to which the head suspensions mentioned above are mounted. When controlled DC current is applied to the coil, a magnetic field is formed surrounding the coil which interacts with the magnetic field of the permanent magnets to rotate the actuator bearing housing, with the attached head suspensions and head assemblies, in accordance with the well-known Lorentz relationship. As the actuator bearing housing rotates, the heads are moved radially across the data tracks along an arcuate path.
The movement of the heads across the disc surfaces in disc drives utilizing voice coil actuator systems is typically under the control of closed loop servo systems. In a closed loop servo system, specific data patterns used to define the location of the heads relative to the disc surface arc prerecorded on the discs during the disc drive manufacturing process. The servo system reads the previously recorded servo information from the servo portion of the discs, compares the actual position of the actuator over the disc surface to a desired position and generates a position error signal (PES) reflective of the difference between the actual and desired positions. The servo system then generates a position correction signal which is used to select the polarity and amplitude of current applied to the coil of the voice coil actuator to bring the actuator to the desired position. When the actuator is at the desired position, no PES is generated, and no current is applied to the coil. Any subsequent tendency of the actuator to move from the desired position is countered by the detection of a position error, and the generation of the appropriate position correction signal to the coil.
One problem with servo-controlled actuator systems is that errors can result from vibrations produced at resonance frequencies of the actuator. As the operating frequency of the actuator system is varied, resonance can result at natural vibrational modes of the actuator and intervening mechanical components. This resonance causes excitation of the actuator, which can result in excessive settling time and reduced tracking ability of the supported heads.
It has been known to address this problem by providing gain stabilizing filters such as electronic notch filters within the servo control loop. These notch filters are placed in the downstream portion of the control loop to filter out the signal information within the band reject frequency range of the notch and thus help minimize excitation of the actuator. This allows the servo control system to effectively ignore lightly damped structural actuator resonances. At the resonances very little control is applied by the servo controller.
While notch filters are an effective solution to the problem of resonance, vibrational modes of actuator systems can be difficult to predict, especially as these systems become more mechanically complex. As a result, designers often must measure the resonance of a drive in an attempt to predict the resonance properties of a production lot of similar drives. Measurement has been complicated by confusion caused by resonance at levels above what is known as the Nyquist frequency, which is equal to half the frequency of the sampling control system. When a mechanical resonant frequency lies above the Nyquist frequency, it will appear as an alias in the control spectrum below the Nyquist limit. It is therefore often difficult to distinguish true resonance frequencies which are below the Nyquist from mere aliases. If notch filters are applied at aliases, the control system can become overly desensitized.
Some designers distinguish true resonant frequencies from aliases by substituting a frequency generator for the drive crystal in a test drive, thereby changing the servo system sample frequency. When the sample frequency is varied, change in alias frequencies can be observed while true resonance frequencies remain constant, and notch filters can be placed at true resonance frequencies only. This method ordinarily requires permanent and/or time-consuming customization of the drive.
There is a continuing need in the industry to identify and characterize resonances of a drive without such modifications, or even of a significant sample of a production lot of disc drives.
The present invention identifies a resonance by changing a spindle speed, which has the effect of changing a corresponding sampling rate and Nyquist frequency. The invention discriminates (1) between plausible frequencies of true resonance and/or (2) aliased resonance from non-aliased resonances. This is accomplished by monitoring whether and/or how a resonance shifts during this speed change. Methods of the present invention monitor resonances by deriving a resonance indicator such as an apparent frequency (below the Nyquist frequency), a gain or error magnitude (compared to a derived threshold at a calculated frequency of interest), or similar criteria tested by values conventionally illustrated on a Bode plot.
The present invention further comprises steps or structural features for configuring a controller to attenuate or similarly limit an unwanted frequency component of an actuator control signal. In this way, resonances in the actuator can be reduced, and servo speed and accuracy thereby enhanced. Further features and benefits of the present invention will be apparent upon a review of the following figures and their accompanying explanation.