Up-sampled filtering for servo demodulation

An apparatus, method, and system for providing a fine adjustment for transducing head positioning in a hard disk drive (HDD). The apparatus, method, and system include reading a positioning error field wherein the resulting signal is a substantially sinusoidal position error signal (PES), filtering the PES to remove low frequencies and attenuate high frequencies, sample the filtered PES at a multiple of the channel frequency, filter the higher frequency harmonics, down sample the PES, and provide a signal proportional to the amplitude of the down sampled PES. This signal is the reference signal to the head positioning servo.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates in general to the field of information storage, and more particularly to disk drive servo control systems for controlling the disk arm assembly movement across the surface of a rotating disk.

2. Description of the Related Art

Hard disk drives (HDD) typically comprise at least one disk having a magnetic medium for storing information, a spindle, a controller for controlling disk rotational speed, a transducing head (for reading and writing data), a servo actuator assembly including a positioning controller for positioning the head over the appropriate disk track, and data channels for transmitting data to and from the disk. The transducing head reads data from and writes data to the disk in data blocks having either fixed or variable length. A data block comprises a preamble (for acquiring timing signals), timing bits, a position error field, address bits, data bits, and error correction bits. Data blocks are recorded in sectors in concentric tracks. A track may comprise several sectors. The number of sectors may depend on the radial location of the track on the disk.FIG. 1shows a typical HDD as described above.FIG. 2shows the transducing head positioned over a data track on the disk.

Conventionally, the transducing head is mounted on an arm and the arm move across the surface of the rotating disks to access the proper track. The positioning controller directs the transducing head to the correct data track and maintains the head position to align with the centerline of the data track. The position error field consists of a prerecorded series of magnetic bit flux reversals, and when read by the transducing head provides a substantially sinusoidal position error signal (PES) with a frequency of one quarter of the baud rate. (Baud rate refers to the number of distinct events per second in a modulated signal, and in the case of a read channel, refers to the channel frequency.) The PES includes higher order harmonic frequencies. The bit pattern is typically a repetition of “11001100” known as the 2T pattern (some in the field refer to the pattern as the 4T pattern). The “T” in the 2T pattern represents the channel bit period. The recorded position error field flux transitions are spaced generally to one side or the other of the centerline of the tracks.

The servo controller demodulates the PES and uses the amplitude of the PES to position the head over the centerline of the data track. If the amplitude is above a certain level, the controller positions the head to one side of the track centerline, and if the amplitude is below a certain level, the controller positions the head to the other side.

The PES amplitude is used as an error correction signal to the servo controller and varies with the distance of the head from the centerline and with the sampling phase offset. If the position error fields are written asynchronously with the timing recovery field, there will be a sampling phase offset when the PES is demodulated. This sampling phase offset is random, and provides an additional source of PES amplitude variation.

Referring now toFIG. 3, the head reads the position error field in words of 4 bits (the 2T pattern) as the PES301and provides the PES along with any associated noise to an analog continuous time filter (CTF)302that is set in the differentiation mode. Electronic and magnetic sources contribute to the noise. The CTF, when set in the differentiation mode, has the response curve shown inFIG. 4, wherein the CTF filters any low frequencies, and attenuates frequencies higher than 0.25 of the baud rate. The ideal filter would block all energy over 0.5, but a filter having such a sharp cut-off is difficult to construct in view of the cost, size, and energy constraints in HDD read channels.

FIG. 5shows the frequency response spectrum for the sine wave produced by the 2T pattern before filtering. This spectrum has a fundamental frequency at 0.25 baud and a third harmonic at 0.75 baud. Harmonic frequencies cause interference with the fundamental frequency when the analog signal is sampled at intervals of T because any energy outside the Nyquist band of 0 to 0.5 baud will fold back into the Nyquist band. Here the third harmonic is at 0.75 baud, and is outside the Nyquist band. Depending on the sampling phase, this out-of-band energy will either constructively or destructively interfere with the fundamental frequency. The analog-to-digital converter (ADC)303,FIG. 3, samples the PES and the energy contained in the third harmonic reflects to the fundamental frequency, either adding to or subtracting from the energy of the fundamental frequency depending on the sampling phase offset.

A second harmonic may be present if the transducing head has a non-linear transfer function. For example, a magneto-resistive transducing head is non-linear because a positive pulse produces a different amplitude than a negative pulse. This non-linearity also affects PES amplitude variation.FIG. 6shows the frequency response spectrum of the PES when a transducing head having 25% asymmetry is used. In this example, the second harmonic caused by the transducing head asymmetry is stronger than the third harmonic.

Referring again toFIG. 3, the CTF302passes the filtered PES to the ADC303, which samples the filtered signal at the channel frequency. The ADC then passes the sampled signal to a 4-point discrete Fourier transform (DFT)304for producing the PES amplitude signal305. This amplitude signal provides a position error signal to the actuator position controller. Any variation in the amplitude signal not representing an offset from the track centerline will produce a positioning error. The variation due to the sampling phase offset is such a variation. As shown in the example inFIG. 7, the output of the DFT varies by +4.5% depending upon the sampling phase offset.

Therefore, a need exists for a method and apparatus that reduce the effects of sampling phase offset on positioning a transducing head over the centerline of a data track in an HDD.

SUMMARY OF THE INVENTION

To address the stated need and fulfill other desired objectives, in accordance with one embodiment of the invention, a position servo controller provides an ADC that samples a PES at twice the baud rate so the samples are spaced at T/2 intervals, a bandpass finite-impulse-response (FIR) filter, and a down converter to restore the signal to the original frequency. Sampling the PES at twice the baud rate has the effect of spreading the frequency lobes in the frequency domain. The FIR filter has notches at the exact frequencies of the harmonics, thereby eliminating the higher harmonics while passing the fundamental frequency.

DETAILED DESCRIPTION OF EMBODIMENTS

The inventive apparatus and method provide a PES amplitude signal for transducing head positioning, wherein the PES amplitude signal is not sensitive to the sampling phase offset. If the position error fields are recorded to a disk asynchronously with the timing recovery field, there will be a sampling phase offset when the PES is demodulated, and this offset is random.

As shown inFIG. 1, a hard disk drive system100comprises a disk having a magnetic disk106. The motor107spins the disk at a constant speed and under control of the motor controller108. Data110passes through a read/write channel103to a transducing head105. The servo actuator104positions the transducing head105over the proper data track, and data is transmitted to or from the read/write channel103. A reference frequency109provides timing signals to the read/write channel103. Skilled practitioners in the art will recognize that other configurations are possible.

FIG. 2shows a planar view of the disk surface, specifically, the relationship between the transducing head205, the data tracks202, the positioning arm206, and the servo actuator207. The transducing head205records data in concentric tracks202on a disk201. A data track202has a number of sectors, each sector having a preamble204and data203. The preamble204contains the position error field as well as other timing acquisition signals. The transducing head205provides the recorded data203and the preamble204to a read channel for further processing over data channel208.

The servo actuator207positions the positioning arm206and the transducing head205over a data track202in response to a servo actuator signal209. The servo actuator signal is an analog signal whose amplitude determines the direction and amount of the servo movement. Therefore, any noise in the servo actuator signal results in a dithering movement of the transducing head205.

Referring now toFIG. 8, the transducing head reads the position error field as a position error signal801. The PES contains a noise component originating as media and electronic noise, and is a natural part of the PES. The PES and associated noise passes to a CTF802that is set in the differentiation mode. The CTF has the frequency response curve shown inFIG. 4and filters low frequencies, and attenuates frequencies higher than 0.25 baud.

An analog-to-digital (ADC) converter803samples the filtered PES at 2× the baud rate. This oversampling spreads the frequency response to expose the higher frequency harmonics. A bandpass filter then filters these higher frequency harmonics. The bandpass filter may be either analog or digital. One example of a digital bandpass filter is a Finite Impulse Response (FIR) filter. Referring again toFIG. 8, the bandpass FIR804filters the sampled signal.

In order to attenuate harmonics of the sampled signal, the bandpass FIR804comprises two cascaded FIRs having coefficients substantially equal to

[12,1,12,0,-12,-1,-12]
and [1, 1, 1] resulting in combined impulse response substantially equal to

[12,1+12,1+22,1+12,0,⁢-⁢(12),-(1+12),-(1+22),-(1+12)].
FIG. 9shows the response curve of the cascaded bandpass FIR filter805having notches at the harmonic frequencies of the 2× sampled signal.FIG. 10shows the results of the 2× sampled signal after passing though the bandpass FIR804. The bandpass FIR804has removed the out-of-band energy that was contained in the harmonic frequencies.

The bandpass filter FIR804may be the equivalent of cascading a sinusoidal filter with a simple filter for additional filtering. The simple filter may be a comb filter or a cascade of more than one comb filter.

Referring again toFIG. 8, the converter805down samples the signal and passes the down sampled signal to the discrete Fourier transform (DFT)806for generating the analog amplitude signal807. This amplitude signal807controls the servo actuator to position the transducing head over a data track centerline.

FIG. 11compares the PES amplitude signal of the inventive apparatus with the prior art as a function of sampling phase, showing the inventive system is relatively insensitive to the sampling phase.

The above description describes individual circuitry to implement an apparatus to provide a signal for controlling the position of a transducing head across a rotating disk, the apparatus may be implemented for execution on a computational processor using the method described inFIG. 12. The apparatus reads the position error field previously recorded on a HDD1201and provides the PES to the processor. The processor filters the PES to remove low frequencies and to attenuate the high frequencies1202. The processor then samples the PES at twice the baud rate1203and removes any higher order harmonic frequencies1204. The sampled PES is then down converted to the baud rate. A discrete Fourier transform then provides a PES amplitude signal1206for controlling a position servo. The PES amplitude signal determines the direction and magnitude of any correctional movements needed to position the transducing head over the centerline of a data track.

Therefore, the foregoing is illustrative only of the principles of the invention. Further, those skilled in the art will recognize that numerous modifications and changes are possible. The disclosure of the foregoing embodiments does not limit the invention to the exact construction and operation shown. Accordingly, all suitable modifications and equivalents fall within the scope of the invention.