Disc drive with compensation for non-repeatable runout

An NRRO compensation circuit controls a head position in a disc drive. The circuit has a first frequency circuit providing a first output corresponding to NRRO and a first characteristic sensing circuit that senses the first output and that generates a first control output that adjusts a first NRRO compensation gain. A first control circuit included in the NRRO compensation circuit receives the first control output and the first NRRO compensation gain and provides a first NRRO compensator output.

FIELD OF THE INVENTION

The present invention relates generally to disc drives, and more particularly but not by limitation to reducing non-repeatable runout error.

BACKGROUND OF THE INVENTION

At very high tracks per inch (TPI) in disc drives, non-repeatable runout (NRRO) becomes a major issue to achieve high performance tracking. NRRO comes from many sources such as electronic noises, disc flutter, spindle motor, arm/suspension modes, media defects, external vibrations, etc. Some of the sources have relatively high energy at certain narrow frequency bands (e.g., disc flutter and actuator resonance modes). There can be components of NRRO in several frequency ranges. At some outside diameter (OD) tracks, the sum of the NRRO energy at these multiple frequency ranges can comprise a significant portion of total NRRO energy. This may result in a long settling time at the end of seeking, increased write unsafe number, and large track following PES.

There is a desire to reduce disc drive NRRO in one or more narrow frequency bands without incurring the cost of increasing runout at other frequencies.

SUMMARY OF THE INVENTION

Disclosed is an NRRO compensation circuit that controls a head position in a disc drive.

A runout input that includes a characteristic of non-repeatable runout (NRRO) is coupled to a control circuit. The control circuit receives the runout input and provides a first NRRO compensator output responsive to the runout input. The control circuit has a first NRRO compensation gain adjusted as a function of the characteristic.

These and other features and benefits that characterize embodiments of the present invention will be apparent upon reading the following detailed description and review of the associated drawings.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

A self-tuning compensation circuit addresses the non-repeatable runout (NRRO) in disc drives. A self-tuning algorithm has the capability of detecting the amplitude of NRRO, and adjusting the attenuation or gain of the compensator in real-time based on the NRRO distribution. In one preferred arrangement, NRRO is detected in the position error signal (PES) using a filter F, and then an automatic tuning algorithm adjusts the NRRO compensation gain such that the NRRO is attenuated in a desired frequency range. When there is little NRRO energy located at this frequency, the self-tuning function will gradually remove the NRRO compensator to keep the servo loop as usual, avoiding the water-bed effect. Embodiments of such circuits are described below in connection with examples shown inFIGS. 1–10.

FIG. 1is an oblique view of a disc drive100in which embodiments of the present invention are useful. Disc drive100includes a housing with a base102and a top cover (not shown). Disc drive100further includes a disc pack106, which is mounted on a spindle motor (not shown) by a disc clamp108. Disc pack106includes a plurality of individual discs, which are mounted for co-rotation in a direction indicated by arrow107about a central axis109. Each disc surface has an associated disc head slider110which is mounted to disc drive100for communication with the disc surface. In the example shown inFIG. 1, sliders110are supported by suspensions112which are in turn attached to track accessing arms114of an actuator116. The actuator shown inFIG. 1is of the type called a rotary moving coil actuator and includes a voice coil motor (VCM), shown generally at118. Voice coil motor118rotates actuator116with its attached heads110about a pivot shaft120to position heads110over a desired data track along an arcuate path122between a disc inner diameter124and a disc outer diameter126. Voice coil motor118is driven by servo electronics130based on signals generated by heads110and a host computer (not shown). As explained in more detail below in connection with examples illustrated inFIGS. 2–10, the servo electronics130includes one or more self-tuning NRRO compensation circuits.

As the number of tracks per inch (TPI) increase, non-repeatable runout (NRRO) becomes a major issue to achieve a high performance tracking. NRRO comes from many sources such as electronic noises, disc flutter, spindle motor, arm/suspension modes, media defects, external vibrations, etc. Some of them have relatively high energy at certain narrow frequency bands (e.g., disc flutter and actuator resonance modes). For example, in one disc drives with four discs, a large amount NRRO is observed at 970 Hz, 1280 Hz, 1785 Hz and 2000 Hz ranges. At some outside diameter (OD) tracks, the sum of the NRRO energy at these four frequency ranges can reach 20–30% of total NRRO energy. This can result in a long settling time at the end of seeking, increased write unsafe number, and large track following PES.

FIG. 2Aschematically illustrates a first embodiment of a disc drive circuit132that includes an NRRO compensation circuit134. The disc drive circuit132controls a position of a head185in the disc drive.

The disc drive circuit132includes a summing junction184that provides a position error signal (PES)186. A head interface circuit190provides a feedback output188that provides an indication of head position to the summing junction184. The head interface circuit190also is connected to a data bus191for communicating data between the head185and a host system (not illustrated). The host system, such as a computer (not illustrated) provides a desired reference position181for the head185. A controller194receives the position error signal186and provides a head position control output196. The head position control output196is preferably a current driving a voice coil motor198to position the head185. The controller194preferably controls the head position control output196as a function of a sum of the position error signal186and a first NRRO compensator output210.

A runout input131includes a characteristic of non-repeatable runout (NRRO). The runout input131can be received from a device such as an accelerometer mounted on the disc drive or from the position error signal186.

The NRRO compensation circuit134receives the runout input131and provides the first NRRO compensator output210responsive to the runout input131. The control circuit134has a first NRRO compensation gain adjusted as a function of the characteristic in the runout input131. The NRRO compensator output210couples to the controller194. The circuit134ofFIG. 2Ais explained in more detail below in connection with an example illustrated inFIG. 2B.

FIG. 2Bschematically illustrates a second embodiment of a disc drive circuit133that includes an NRRO compensation circuit134. InFIG. 2B, reference numbers that are the same as reference numbers used inFIG. 2Aidentify the same or similar features.

InFIG. 2B, the NRRO compensation circuit134includes a first frequency circuit136that provides a first output138corresponding to NRRO. The first output138provides an indication of a frequency at which NRRO noise is currently occurring. The first frequency circuit136can obtain NRRO noise data by a connection to the position error signal186, by connection to an accelerometer sensing environmental vibration or from another source of real time NRRO noise data. The first frequency circuit136can include a filter, a peak detector or another means of separating NRRO noise from other signal components in real time.

The NRRO compensation circuit134inFIG. 2Balso includes a first control circuit144that receives the first control output138and that provides the first NRRO compensator output210. The first control circuit144can be an amplifier with adjustable gain, an attenuator with adjustable attenuation or another circuit that can provide adjustment when there is NRRO present in a selected frequency range. The first frequency circuit136selectively provides the NRRO noise at the first output138. When the noise is above the threshold level, the gain of the first control circuit144is adjusted accordingly. The circuit134ofFIG. 2Bis explained in more detail below in connection with an example illustrated inFIG. 2C.

FIG. 2Cschematically illustrates a third embodiment of a disc drive circuit135that includes an NRRO compensation circuit134. InFIG. 2C, reference numbers that are the same as reference numbers used inFIGS. 2A,2B identify the same or similar features.

The NRRO compensation circuit134inFIG. 2Cincludes a first frequency circuit136that provides a first output138corresponding to NRRO. The first output138provides an indication of a frequency at which NRRO noise is currently occurring. The first frequency circuit136can obtain NRRO noise data by a connection to the position error signal186, by connection to an accelerometer sensing environmental vibration or from another source of real time NRRO noise data. The first frequency circuit136can include a filter, a peak detector or another means of separating NRRO noise from other signal components in real time.

The NRRO compensation circuit134includes a first characteristic sensing circuit140that senses the first output138and that generates a first control output142that adjusts a first NRRO compensation gain. The first characteristic sensing circuit140can sense amplitude, power, duration or other characteristics to ascertain whether NRRO noise is above a selected threshold level where NRRO compensation is desired.

The NRRO compensation circuit134also includes a first control circuit144that receives the first control output138and the first output and that provides the first NRRO compensator output210. The first control circuit144can be an amplifier with adjustable gain, an attenuator with adjustable attenuation or another circuit that can provide adjustment when there is NRRO present in a selected frequency range. The first frequency circuit136selectively provides the NRRO noise at the first output138. The first characteristic sensing circuit140adjusts the first control output142based on whether the noise is above a selected threshold level. When the noise is above the threshold level, the gain of the first control circuit144is adjusted by the first control output142.

FIG. 3schematically illustrates a fourth embodiment of a disc drive circuit180that includes a self-tuning NRRO compensation circuit182. The disc drive circuit180comprises a servo circuit that controls a position of a read/write head185.

The disc drive circuit180inFIG. 3is similar to the disc drive circuit132inFIG. 2Cand reference numbers used inFIG. 3that are the same as reference numbers used inFIG. 2Cidentify the same or similar features. InFIG. 3, the position error signal186and the first NRRO compensator output210are summed at a summing junction211, also called a summing node211, in the controller194. The controller194controls based on the output213of the summing node211rather than on the position error signal186alone.

A first bandpass filter200, also called a first NRRO detector200, (comparable to the first frequency circuit136inFIG. 2C) receives the position error signal186and providing a first filter output E at202that selectively reproduces a first portion of the position error signal186in a first preselected frequency band that corresponds with a first NRRO frequency peak. The first bandpass filter200detects the energy of the noise due to NRRO within a certain frequency range defined by the pass band of the first bandpass filter200. The first bandpass filter200preferably comprises a second order filter, and is discussed in more detail below in connection with an example illustratedFIG. 5.

A first amplitude sensing circuit204(comparable to the first characteristic sensing circuit140inFIG. 2C) receives the first filter output202and provides a first gain control output206. In one preferred embodiment, the amplitude sensing circuit204includes an envelope detector that senses an amplitude of an envelope of the first filter output202. A first gain control circuit208(comparable to the first control circuit144inFIG. 2C) receives the first filter output202and also receives the first gain control output206. The first gain control circuit208provides the first NRRO compensator output210to the controller194. The first NRRO compensator output210reproduces the first filter output202multiplied by a gain G that is adjusted or set by the first gain control output206. Gain G is a time-varying gain being adjusted by first amplitude sensing circuit204. In one preferred arrangement, the first gain control circuit208comprises a multiplier.

In the arrangement shown inFIG. 3the voice coil motor198is considered a plant P to be servo controlled by the controller C at194. The position error signal (PES)186is the error between the sensed head position188and the reference signal181. The first amplitude sensing circuit204automatically tunes the first gain control output206to increase the gain G when the first filter output202increases, and to decrease the gain G when the first filter output202decreases.

In a preferred arrangement, the first amplitude sensing circuit204includes a recursive algorithm that tunes the first gain control output206in real time during normal disc drive operation. The recursive algorithm, for example, can be a function of

g⁡(t)={α*⁢g⁡(t-1)+β*⁡(e-e0),when⁢⁢e>e0α*⁢g⁡(t-1),⁢when⁢⁢e≤e0⁢Equation  1
where t is a number of a time increment, g is the gain, α (alpha) has a value between 0 and 1, β (beta) is a tuning rate greater than 0, e is the first filter output and e0is a selected level of e for turning the automatic tuning on and off.

The recursive algorithm in Equation 1 has an initial condition value g(0) at the start of a seek operation, and the initial condition value g(0) is preferably adjusted as a function of track number. The initial value g(0) can be set based on the available information of the NRRO location and amplitude. For example, if the NRRO at a OD track has a very large disc flutter mode, g(0)=1 can be chosen to reduce the transient of the NRRO cancellation. If there is no such kind of information, it can be simply set to zero.

The self-tuning algorithm (Equation 1) has the following properties:1. For g(0)=0 (i.e., no NRRO control action is used), when a large NRRO signal |e|>e0is detected, the gain g(t) increases. The NRRO compensator will be gradually added in the servo loop.2. As time goes on, when g(t) reaches a value that equals to

β⁡(e-e0)1-αEquation  2
it will stop increasing. This implies that g(t) is bounded for α<1, and the upper bound can be adjusted by the design parameters α and β.3. When the NRRO is small (|e|≦e0), the self-tuning law becomes g(t)=α* g(t−1). Since α<1, the gain g(t) will gradually decrease to zero, this NRRO compensator can be removed from the servo loop automatically.

As an example, a disc drive is found to have a mechanical resonance, also called a frequency peak, near 1280 Hz that can be non-repeatably excited by some actuations of the voice coil motor198or by some mechanical vibrations coupled in from the disc drive mounting environment. The mechanical resonance causes non-repeatable runout error (NRRO). In this example, the filter200has a pass band with a fixed center frequency at 1280 Hz to correspond with the observed frequency peak of the non-repeatable runout error. When there is a low amplitude E at the output of the filter200, then the amplitude sensing circuit204senses this low amplitude and provides a lower level of the gain G, effectively shutting off the NRRO compensation at 1280 HZ when there are low levels of NRRO at 1280 Hz. When there is a high amplitude E at the output of filter200, however, the amplitude sensing circuit204senses this high amplitude and provides a higher level of the gain G, effectively turning on the NRRO compensation at 1280 Hz when there are high levels of NRRO at 1280 HZ. The “self-tuning” characteristic of the self-tuning NRRO compensation circuit182is not a frequency-tuning characteristic, but is a gain-tuning characteristic. The gain-tuning characteristic adjusts the amount of compensation for NRRO at a particular frequency based on real time measurement by the first amplitude sensing circuit204of the amount of NRRO present in the position error signal186. The positioning of the head185is optimized for conditions ranging from no NRRO noise at 1280 Hz to high levels of noise at 1280 HZ. This technique can be applied to a frequency other than 1280 and can also be applied to multiple frequencies as explained in more detail below in connection withFIG. 4.

The circuit182can be realized using hardware, firmware, software, analog or digital circuits, standard or custom integrated circuits or combinations thereof.

FIG. 4schematically illustrates a fifth embodiment of a disc drive circuit230that includes multiple self-tuning NRRO compensation circuits182,183,187. InFIG. 4, reference numbers that are the same as reference numbers used inFIG. 3identify the same or similar features. InFIG. 4, a second self-tuning NRRO compensation circuit183is provided. The second self-tuning NRRO compensation circuit183has a pass band that is different than the pass band of the first self-tuning NRRO compensation circuit182, but is otherwise similar in construction to the first NRRO compensation circuit182.

The second NRRO compensation circuit183includes a second bandpass filter232that receives the position error signal186and that provides a second filter output E2at234that selectively reproduces a second portion of the position error signal186in a second preselected frequency band that corresponds with a second NRRO frequency peak that is different than the first NRRO frequency peak. A second amplitude sensing circuit236receives the second filter output234and provides a second gain control output238. A second gain control circuit240receives the second filter output234and receives the second gain control output238. The second gain control circuit240provides a second NRRO compensator output242to the controller194. The second NRRO compensator output242reproduces the second filter output234multiplied by the second gain control output238.

InFIG. 4, the controller194controls the head position control output196as a function of a sum of the position error signal186and the first NRRO compensator output210and the second NRRO compensator output242. A summing node244in the controller194sums, or adds, the inputs from the position error signal186, the first and second NRRO compensator outputs210,242and also additional NRRO compensator outputs246from additional NRRO compensator circuits187that differ from first and second NRRO compensator circuits182,183by the frequency of the pass bands.

FIG. 5illustrates a response shape of a bandpass filter that can be used as one of the bandpass filters182,183or187discussed above in connection withFIGS. 3–4. InFIG. 5, a vertical axis260represents the gain of the bandpass filter in decibels and a horizontal axis262represents frequency in Hertz on a logarithmic scale. For implementation simplicity, a second-order filter with a frequency response shape shown at264inFIG. 5is sufficient for the NRRO detection. Depending on the nature of the NRRO distribution that requires compensation, the 3 dB pass band266of the bandpass filter F can be chosen narrower or wider. If the energy of NRRO is located within very narrow frequency range, e.g. the NRRO caused by suspension resonance mode or disc flutters, a bandpass filter (NRRO detector) may be chosen with a 50–100 Hz width 3 dB pass band.

In general, the sensitivity function of a closed-loop servo system gives a measure of how much attenuation the servo system provides at certain frequency to reject the external disturbances. The NRRO compensator design re-shapes the servo loop such that more attenuation is placed at the frequency where more NRRO disturbances are located, when NRRO disturbances are actually present. The NRRO compensation is dynamic and applied at times when it is actually useful, and in proportion to the amount of NRRO actually detected.

In order to generate a notch-like shape in the sensitivity function at a low frequency (below gain cross frequency), a high loop gain is used in the servo open loop. For example, in a sample drive, the gain cross frequency is about 1900 Hz. The following filter F(z) can be used to design a NRRO compensator at 1280 Hz,

To generate a notch-like shape in the sensitivity function at a high frequency (between gain-cross frequency and phase-cross frequency), a high phase margin is desirable at the compensation frequency of the servo open loop. In another sample drive, the phase cross frequency is about 4000 Hz, to design a NRRO compensator at 2000 Hz, the following filter can be used

FIG. 6schematically illustrates a sensitivity function of a disc drive circuit with and without multiple bandpass filters. InFIG. 6, a vertical axis270represents the sensitivity function in decibels, and a horizontal axis272represents the frequency in Hertz on a logarithmic scale. A sensitivity function is a ratio of the incremental change in system response of the servo system to an incremental change in system noise and disturbances over a range of operating frequencies. A solid line276represents a sensitivity function of a servo system when two NRRO compensation circuits at 1280 Hz and 2000 HZ are both active due to NRRO noise. A dashed line278represents the sensitivity function of the same servo system when the NRRO circuits are inactive because there is no NRRO noise.

It can be seen fromFIG. 6that two NRRO compensators enhance the attenuation capability of servo system at frequency 1280 Hz and 2000 Hz. As the compensator gain g(t) is adjusted through a self-tuning algorithm (such as Eq. 1), the depth of the notch-like shape inFIG. 6is changed in real-time to reduce the amplification of the PES in other frequencies when there is little NRRO located at 1280 Hz and 2000 Hz.

FIG. 7schematically illustrates a timing diagram showing the gain G(t) at286and the filter output E(t) at288as functions of time t at284. E(t) is expressed in equivalent microinches (μin) of displacement of the read/write head on a vertical axis282. G(t) is expressed as a dimensionless ratio on a vertical axis280. The filter output288has an amplitude envelope290(dashed lines) with an envelope time constant, and the first gain control output (206inFIGS. 3–4) has a tuning time constant that is faster than the envelope time constant. It can be seen inFIG. 7that variations in the envelope290are slow relative to variations in the gain286. This different in time constants allows the control of the gain G(t) to keep up with the bandpass filter in real time.

In the example illustrated inFIG. 7, four compensators have been used to cancel the NRRO at 970 Hz, 1280 Hz, 1785 Hz and 2000 Hz.FIG. 7shows the filtered signal E(t)=PES*F(z) of the 970 Hz NRRO compensator. It can be seen that the NRRO amplitude detected by filter F(z) at 970 Hz frequency is time varying. An interesting characteristic of this NRRO signal is that the changing rate of the NRRO amplitude envelope290is slow. This property enables the self-tuning algorithm to sense and adjust the compensation in real time. In this example, the initial gain g(0) is chosen as zero. As the signal e increases, the gain g(t) increases quickly to add in the NRRO control action. From the time 15 ms the 970 Hz NRRO becomes small, and the control gain g(t) reduces rapidly. From time 25 ms–50 ms, the gain goes up again to attenuate the NRRO at this frequency.

FIG. 8schematically illustrates a sensitivity function of a disc drive circuit with four bandpass filters.FIG. 8plots a sensitivity function measured in a sample disc drive for the cases of no NRRO compensation (dashed line300) and with the self-tuning NRRO compensation (solid line302). The four notch-like shapes304,306,308,310can be seen on the closed-loop response at the desired frequency ranges, which provides 6–10 dB attenuation. The amplitude of sensitivity function302increases 1–2 dB at other frequencies312from 1.5 Khz to 4 Khz due to the water-bed phenomenon. The “water-bed” phenomena occurs when a control system that has already been optimized over a wide frequency range is modified to perform better in a narrow frequency range. The modification has the effect of improving performance in the narrow frequency range, but in so doing, deteriorates performance over a wider frequency range. The present arrangement evades the water-bed phenomena because the performance improvement in the narrow frequency range is not active except at times it is actually useful due to NRRO.

FIG. 9schematically illustrates a position error signal as a function of frequency, with (solid line324) and without (dashed line326) multiple bandpass filters. InFIG. 9, a vertical axis320represents the position error signal (expressed in equivalent microinches of read/write head displacement), and a horizontal axis322represents frequency in Hertz.

Since the NRRO at the selected frequencies are much larger than other frequency, attenuating the NRRO at these frequencies results in an overall tracking performance improvement. The key to the success of this self-tuning technique is to place the compensator at the frequency where large NRRO may happen.

FIG. 10schematically illustrates variation of runout, from inside to outside diameter tracks, using circuits with and without multiple bandpass filters. InFIG. 10a vertical axis330represents runout in microinches, and a horizontal axis332represents track number ranging from a track number50,000at an insider diameter (ID) to a track number0at an outside diameter (OD) of a disc. Runout readings marked with small circles334are runout readings without use of a NRRO compensation circuit. Runout readings marked with small asterisks336are runout readings using a NRRO compensation circuit such as the one illustrated inFIG. 4with four bandpass filters.

At OD tracks, about 0.3 μinch improvement is achieved. At ID tracks, 0.1 μinch improvement is observed. This difference is due to the fact that the drive has much more NRRO located at outer diameter cylinders than inner diameter cylinders. The average NRRO reduction on this head is about 12.4%.

In summary, an embodiment of a disc drive circuit (such as180) has a controller (such as194) that receives a position error signal (such as186) and that provides a head position control output (such as196). A bandpass filter (such as200) also receives the position error signal and provides a filter output (such as202) that selectively reproduces a portion of the position error signal in a preselected frequency band that corresponds with a NRRO frequency peak. An amplitude sensing circuit (such as204) receives the filter output and provides a gain control output (such as206). A gain control circuit (such as208) receives the filter output and the gain control output. The gain control circuit provides a NRRO compensator output (such as210) to the controller. The NRRO compensator output reproduces the filter output multiplied by a gain G set by the gain control output.

It is to be understood that even though numerous characteristics and advantages of various embodiments of the invention have been set forth in the foregoing description, together with details of the structure and function of various embodiments of the invention, this disclosure is illustrative only, and changes may be made in detail, especially in matters of structure and arrangement of parts within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. For example, the particular elements may vary depending on the particular application for the non-repeatable run out compensation while maintaining substantially the same functionality without departing from the scope and spirit of the present invention. In addition, although preferred embodiments described herein are directed to a disc drive with a thin film read/write head, it will be appreciated by those skilled in the art that the teachings of the present invention can be applied to other types of storage devices with non-repeatable runout, without departing from the scope of the present invention.