Abstract: Methods and apparatus for providing improved gain and bandwidth and robust damping of mechanical resonances for a servo control system by providing a collocated or nearly collocated feedback loop that operates in parallel with the ordinary feedback loop used for position or velocity feedback.

Abstract: The present invention senses angular acceleration of a Head Disk Assembly ("HDA") in a disk drive servo system and provides an acceleration feedback signal. A filter network filters the acceleration feedback signal and supplies a filtered signal for nullifying tracking error caused by shock, vibration and windup disturbances to the system.

Abstract: A disk drive system wherein the radial position of a sensing head is servoed using signals derived from the sensing of servo signals recorded in circular tracks on a disk surface and disposed in circumferentially spaced sets thereof, each set comprising radially aligned signals in a succession of tracks, where the derived signals vary cyclically as a function of the radio position of the head. The DC offsets of the derived signals relative to a reference level are removed by causing a constant velocity radial displacement of the head and, for each of the derived signals, measuring the durations of the positive and negative half-cycles of the derived signal relative to the reference level and adding to the derived signal an offset voltage sufficient to equalize the durations.

Abstract: An automatic gain control circuit in which the gain of the gain controlled amplifier is controlled in an inverse exponential relationship with the amplitude of the output of said amplifier to provide a loop bandwidth that is essentially constant over variations in the strength of the input signals. The circuit is used in a magnetic disk drive to control amplification of detected servo signals to provide position and velocity error signals.

Abstract: Methods for writing and detecting servo positioning dibit transitions on a magnetic data surface having circular tracks with two magnetic polarizations, the transitions each having a radial extent greater than the spacing between adjacent centerlines. The transitions are written by first prepolarizing the magnetic surface to a first polarization and then writing a second polarization followed by a first polarization and not writing when the write head passes a previously written transition in an adjacent track. The transitions are detected by qualifying the detection means when the sensed servo signal exceeds a threshold amplitude corresponding to a first polarization and detecting the crossing by the sensed servo signal of a second threshold corresponding to the transition to a second polarization.

Abstract: An automatic gain control circuit for controlling amplification of two independent input signals in which first and second attenuators are connected to receive the input signals and each attenuator is selectively switchable to vary its output between maximum and minimum attenuations such that the sum of the outputs of the attenuators are held at a substantially constant reference level. The circuit is used in a disk drive with a data surface comprising circular data tracks divided into sectors each of which includes a data field and a servo field containing embedded servo signals, an embedded signal sensor for sensing embedded servo signals on a data surface and a head positioning system for positioning a read/write head over a selected data track. The servo fields in the data tracks contain first and second embedded servo signals of substantially equal amplitude offset relative to each other and disposed on alternate sides of the track centerlines.

Abstract: Methods and apparatus for automatically identifying and adapting to changes in the overall gain of the mechanical plant in a digital servo control system to maintain a constant bandwidth.

Abstract: A magnetic disk drive wherein the radial position of a sensing head being positioned by a servo system is ascertained by the sensing of servo signals arranged in successive groups of tracks on a disk surface, each of the tracks in a group being of a different track type and said servo signals being in relative circumferential positions according to the track types of the tracks containing those signals. The sensed servo signals are linearly related to the radial position of the head relative to the track containing the corresponding servo signal, the linear relationship extending over a radial distance of a plurality of tracks. The position is ascertained alternately from differences in the amplitudes of pairs of sensed servo signals associated with the destination track and differences in the amplitudes of pairs of sensed servo signals associated with the track traversed immediately prior to the destination track.

Abstract: A magnetic disk drive having a dedicated servo surface containing circular tracks of dedicated servo signals and data surfaces containing circular data tracks divided into sectors each of which includes a data field and a servo field containing embedded servo signals, a dedicated servo signal sensor for sensing servo signals on said dedicated surface, an embedded signal sensor for sensing embedded servo signals on a data surface and a head positioning system for positioning a read/write head over a selected data track. The dedicated servo tracks comprise successive groups of four tracks, each of a different track type, and the dedicated servo signals are recorded in circumferential positions according to the track types of the tracks. The dedicated signal sensor spans two dedicated servo tracks and its output is linearly related to the radial position of said servo signal sensor relative to the track containing the corresponding recorded servo signal.

Abstract: A disk drive wherein the radial position and velocity of a sensing head being positioned by a servo system is estimated from the sensing of servo signals arranged in successive groups of tracks on a disk surface, each of the tracks in a group being of a different track type and said servo signals being in relative circumferential positions acording to the track types of the tracks containing those signals. The sensed servo signals are linearly related to the radial position of the head relative to the track containing the corresponding servo signal, the linear relationship extending over a radial distance of a plurality of tracks.

Abstract: A magnetic disk drive having a disk surface containing circular tracks containing servo transitions, a head positioning system for positioning a servo signal sensor over selected servo tracks, each servo track containing a series of frame radially aligned with the frames in other tracks, each frames containing a synchronizing field. The synchronizing fields contain first and second dibit transitions, the second dibit transitions being contained in every synchronizing field, said second dibit transitions being contained in every synchronizing field in a given track, and the first dibit transitions being contained in fewer than every synchronizing field in the rack, but in at least one of any two successive synchronizing fields. The time period between the sensing of the first and second dibit transitions in a frame is shorter than the time period between the sensing of any other pair of dibit transitions.

Abstract: A disk drive comprising circular data tracks divided into sectors each of which includes a data field and a servo field containing embedded servo signals, an embedded signal sensor for sensing embedded servo signals on a data surface and a head positioning system for positioning a read/write head over a selected data track. The servo fields in the data tracks contain first and second embedded servo signals of substantially constant amplitude high-frequency bursts offset relative to each other and disposed on alternate sides of the track centerlines. The sensed signals corresponding to the first and second embedded servo signals are sampled and compared to provide a track error signal for positioning the read/write head over a selected data track.

Abstract: A magnetic disk drive wherein the radial position of a sensing head being positioned by a servo system is ascertained from the sensing of servo signals arranged in successive groups of tracks on a disk surface, each of the tracks in a group being of a different track type and said servo signals being in relative circumferential positions according to the track types of the tracks containing those signals. The sensed servo signals are linearly related to the radial position of the head relative to the track containing the corresponding servo signal, the linear relationship extending over a radial distance of a plurality of tracks, are amplified by a gain controlled amplifier, and are gated to respective detectors corresponding to different track types. The amplifier is controlled by a gain error signal derived from comparison of the sum of the outputs of the detectors to a reference level.

Abstract: For a disk drive having a servo positioning system for moving and positioning a read/write head in track seeking and following operations, a velocity estimator connected to derive velocity signals selectively from a plurality of position signals, the phasing of said position signals being such that at least one of the signals at any given radial position of ther head varies linearly with radial head displacement, that is, with a constant change in signal level per unit of radial displacement. The phase of the position signals are such that as one position signal is about to leave its linear operating region, another is in its linear operating region. The velocity estimator comprises a plurality of sections each selectively switched to receive and differentiate a position signal and connected so that there is not current surge resulting form such switching.

Abstract: A dynamic filter circuit used to process analog position error information in a disk drive head-positioning servo system is disclosed. The filter circuit monitors the velocity of the disk drive carriage and dynamically increases the gain and bandwidth of the filter as the carriage velocity increases. Specifically, an operational amplifier in feedback configuration is used in which the feedback networks are switched in response to the velocity signal. At low carriage velocity the filter has low gain and reduced bandwidth and therefore helps to decrease sensitivity of the system to noise. At high carriage velocities the gain and bandwidth of the filter are increased to minimize the effects of missing servo data.

Abstract: A two-terminal integral-cycling solid state relay comprises a first semiconductive switching device having a control electrode for controlling the conductivity between two main electrodes, the latter being attachable to a cyclic power source in series with a load. A novel control circuit is utilized for providing a firing signal to the control electrode in response to an energization signal, only when there is approximately zero instantaneous voltage between the main electrodes of the first switching device at the beginning of a power signal cycle. The control circuit includes a memory so that when the relay is open, the first semiconductive switching device will stop conducting between its main electrodes only at the end of the power signal cycle when the instantaneous current through the main electrodes is substantially zero.