Apparatus and method to calibrate one or more transducers in a noisy environment

A method to calibrate a transducer, whereby the transducer provides a first signal, and whereby a reference provides a reference signal. The first signal is sampled at a sampling rate comprising a reference frequency, and a digital measured first signal waveform is formed. The reference signal is sampled at the sampling rate, and a digital measured reference signal waveform is formed. The real and imaginary components of the measured first signal waveform are determined at (P) harmonics of the reference frequency. A filtered first signal waveform is formed using those real and imaginary components. The real and imaginary components of the measured reference signal waveform are determined at (P) harmonics of the reference frequency. A filtered reference signal waveform is formed using those real and imaginary components. A transfer function is formed using the filtered first signal waveform and the filtered reference signal waveform. A calibration curve is formed using that transfer function.

FIELD OF THE INVENTION

This invention relates to an apparatus and method to calibrate one or more transducers in a “noisy” environment. In certain embodiments, this invention relates to servo track following a moving magnetic tape having one or more servo edges of dissimilar recorded servo signals, and, more particularly, to calibrating one or more servo sensors.

BACKGROUND OF THE INVENTION

Automated media storage libraries are known for providing cost effective access to large quantities of stored media. Tape cartridges containing a moveable magnetic tape are often used in automated data storage libraries. Tape media, such a magnetic tape, is a common medium for the storage of data to be utilized by a computer. Magnetic tape has found widespread use as a data storage medium because it provides a relatively inexpensive solution for storing large amounts of data.

Magnetic tape data storage typically provides one or more prerecorded servo tracks to allow precise positioning of a tape head with respect to those prerecorded servo tracks. Servo sensors disposed on the tape head are used to track the recorded servo tracks. The tape head comprises one or more read/write elements precisely positioned with respect to those servo sensors. One example of a magnetic tape system is the IBM 3590, which employs magnetic tape having prerecorded servo patterns that include three parallel sets of servo edges, each servo edge being an interface between two dissimilar recorded servo signals, each set of servo edges comprising one servo edge on each of opposite lateral sides of a middle recorded servo signal.

In certain embodiments, the tape head includes a plurality of servo sensors for each servo edge, with the result that the tape head may be stepped between those servo sensors, each positioning the read/write elements at different interleaved groups of data tracks. Typically, for a given servo pattern of a set of two servo edges, the outer servo signals are recorded first, and the center servo signal is recorded last, to provide the servo edges. The nominal separation distance between the servo edges of each set of servo edges is a certain distance, but there is variation in the magnetic separation between the servo edges, for example, due to the variation of the width of the physical write element which prerecords the servo pattern, due to variation in the magnetic characteristics of the physical write element, etc. The variation may occur between servo tracks in a single magnetic tape, and may occur between prerecording devices and therefore between magnetic tapes.

To reduce the apparent difference of the edge separation distance of the prerecorded servo tracks from nominal, the prerecording of the servo tracks is conducted at different amplitudes so as to attempt to compensate for the physical difference and provide a magnetic pattern that is closer to nominal. Thus, the difference in physical distance and the amplitude compensation may tend to offset each other with respect to the apparent distance between the servo tracks. These actions may provide an adequate signal for track following at the servo edges.

However, to increase track density, a servo sensor may be indexed to positions laterally offset from the linear servo edges to provide further interleaved groups of data tracks. The indexed positions are determined by measuring the ratio between the amplitudes of the two dissimilar recorded servo signals. Thus, when the amplitudes of the recorded servo signals are varied to compensate for physical distance variations, track following the prerecorded servo edges at the offset indexed positions becomes less precise. As the result, the data tracks may vary from the desired positions, i.e. be “squeezed” together, such that writing on one track with a write element that is subject to track misregistration (TMR) may cause a data error on the immediately adjacent data track.

The tape path of the above IBM 3590 is a guided tape path. In such a guided tape path embodiment, the magnetic tape can be moved in a first direction and an opposing second direction along a first axis, i.e. along the longitudinal axis of the tape. Movement of that tape along a second axis orthogonal to the first axis, i.e. the lateral axis of the tape, is minimized. Limiting the lateral movement of the magnetic tape results in generating minimal guiding noise, and therefore, the step from a first ratio of servo signals to a second ratio is readily discernible.

Another approach, however, is required for open channel guiding in which the magnetic tape can move laterally a distance which is substantially greater than the separation between index positions, thereby introducing substantial noise into the calibration process. The guiding signal to noise ratio thus becomes negative, with the guiding noise being far larger than the step from one ratio to another, making it difficult to gather data points with a monotonically increasing or decreasing slope to conduct a calibration of the servo ratios.

SUMMARY OF THE INVENTION

Applicants' invention includes an apparatus and method to calibrate one or more servo sensors with respect to one or more index positions laterally offset from one or more servo edges recorded on a magnetic tape in an environment where that magnetic tape is subject to movement along two different axes. Applicants' method utilizes a magnetic tape having at least one set of parallel linear servo edges. In certain embodiments, each servo edge comprises an interface between two dissimilar recorded servo signals, and each set of servo edges comprises one of the servo edges on each of opposite lateral sides of a middle recorded servo signal. Applicants' method calibrates one or more servo sensors with respect to one or more servo index positions laterally offset from the one or more linear servo edges, where those one or more index positions are determined by the ratios of the detected dissimilar recorded servo signals.

The servo system comprises at least one servo sensor disposed on a tape head. That tape head can be moved in a first direction and an opposing second direction along a first axis. The magnetic tape is caused to move along a tape path primarily in a third direction and an opposing fourth direction along a second axis, where the first axis and the second axis are substantially orthogonal. By substantially orthogonal, Applicants mean the angle formed between the first axis and the second axis is about 90 degrees, plus or minus about 10 degrees. As noted above, in non-guided embodiments the tape is also subject to movement along the first axis as it is caused to move unidirectionally, or optionally bidirectionally, along the second axis.

As the tape moves along both the first and second axes, the tape head is movable along the first axis such that the one or more servo sensors detect the recorded servo signals. The servo system also comprises a servo detector in communication with each of the one or more servo sensors. Each of those servo sensors provides an analog signal to the servo detector which provides an analog servo signal comprising a ratio of the detected recorded frequencies. Applicants' apparatus further includes an independent position sensor to measure the position of the tape head with respect to the tape path. That independent position sensor provides an analog IPS signal comprising that measurement.

Applicants' apparatus further includes a servo loop for positioning the tape head laterally with respect to the magnetic tape, and servo logic in communication with the servo detector, the independent position sensor, and the servo loop. That servo logic track follows the sensed servo signals at specific servo signal ratios, sometimes called position error signals, corresponding to specific lateral displacements from the linear servo edges, i.e. specific index positions.

Applicants' method operates the servo loop to modulate the lateral position of the tape head and, thereby, the servo sensor, using a sinusoidal positioning signal. In certain embodiments, Applicants' method operates the servo loop to laterally position the servo sensor to measure the servo signals at continually altered digital set points of the ratios of the sensed servo signals. These set points are altered at the sample rate of the servo loop, and are altered to inject the sinusoidal positioning signal, whereby the servo loop track follows the linear servo edges with the servo loop at the continually altered digital set points.

The sinusoidal positioning signal comprises a reference frequency that is precisely known. In certain embodiments, that reference frequency is selected such that the reference frequency, and major harmonics thereof, each differs from intrinsic operational frequencies of the track following servo system and/or tape system.

Applicants' method digitally forms a sampled IPS signal waveform by sampling the provided IPS signal at the predetermined sampling rate. Applicants' method then determines the real and imaginary components of that measured IPS signal waveform at the reference frequency and, optionally, at (P) harmonics of that reference frequency. Applicants' method then forms a filtered IPS signal waveform using those real and imaginary components.

Similarly, Applicants' method digitally forms a sampled servo signal waveform by sampling the provided servo signal at the predetermined sampling rate. Applicants' method then determines the real and imaginary components of that sampled servo signal waveform at the reference frequency and, optionally, at (P) harmonics of that reference frequency. Applicants' method then forms a filtered servo sensor ratio waveform using those real and imaginary components.

Applicants' method then correlates the filtered IPS signal waveform with the filtered servo sensor ratio waveform and determines therefrom a plurality of measured datapoints comprising independent position sensor lateral positions corresponding to detected ratios of the recorded servo frequencies. Applicants' method then calculates a transfer function using these measured datapoints and an (n)th order curve fitting algorithm. That transfer function is thereafter used to calculate expected position error signals for the servo loop at one or more laterally offset servo index positions with respect to the sensed servo edge(s).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention is described in preferred embodiments in the following description with reference to the Figures, in which like numbers represent the same or similar elements. The invention will be described as embodied in an apparatus and method to calibrate servo sensors tracking servo signals recorded on a magnetic tape. The following description of Applicant's apparatus and method is not meant, however, to limit Applicant's invention to magnetic tapes or to data processing applications, as the invention herein can be applied generally to calibrating transducers in an electrically noisy environment.

FIG. 1shows magnetic tape data storage system100. Control unit110receives and transmits data and control signals to and from a host device102via an interface105. The control unit110is coupled to a memory device107, such as a random access memory for storing information and computer programs. An example of a host device102comprises an IBM RS/6000 processor.

A multi-element tape head190includes a plurality of read/write elements to record and read information onto and from a magnetic tape197, and servo sensors to detect servo signals comprising prerecorded linear servo edges on the magnetic tape197. In certain embodiments, magnetic tape head190comprises a thin-film magneto-resistive transducer. In an illustrative embodiment, tape head190may be constructed as shown in FIG.2B. The length of the tape head190substantially corresponds to the width of the tape197. In certain embodiments tape head190includes thirty-two read/write element pairs (labeled “RD” and “WR”) and three sets of servo read elements (e.g. LS1 272, RS6 298) corresponding to the three servo areas AB (FIG.2A), BC (FIG.2A), and CD (FIG.2A). In the illustrated embodiment, the thirty-two read/write element pairs are divided into groups of eight, adjacent groups being separated by two tracks occupied by a group of four servo sensors. Each group of four servo sensors may be referred to as a “servo group”, e.g. servo group255, servo group265, and servo group275.

In the illustrated embodiments, tape head190includes left and right modules separately fabricated, then bonded together. Write and read elements alternate transversely down the length of each module (i.e., across the width of the tape), beginning with a write element in position on the left module and a read element in the corresponding position on the right module. Thus, each write element in the left module is paired with a read element in the corresponding position on the right module and each read element in the left module is paired with a write element in the corresponding position on the right module such that write/read element pairs alternate transversely with read/write element pairs.

A tape reel motor system (not shown inFIG. 1) moves the tape197along a tape path195in a first direction, and optionally in an opposing second direction, along a first axis, i.e. the longitudinal axis of the tape, while it is supported by a tape deck for reading and writing. The tape deck does not precisely hold the tape in position laterally. Rather, open channel guiding may be employed in which the magnetic tape can move laterally a distance which is substantially greater than that between index positions, thereby introducing substantial guiding noise into the calibration process. The guiding signal to noise ratio thus becomes negative, with the guiding noise being far larger than the step from one ratio to another, making it difficult to gather data points with a monotonically increasing or decreasing slope to conduct a calibration of the detected servo signal ratios.

A servo track follower150directs the motion of the magnetic tape head190in a lateral or transverse direction relative to the longitudinal direction of tape motion. The control unit110is coupled to one or more tape reel motors and controls the direction, velocity and acceleration of the tape197in the longitudinal direction.

The data tracks on the tape197are arranged in parallel and are parallel to the linear servo edges. Thus, as the servo track follower150causes the servo sensors of the magnetic tape head to track follow a linear servo edge or a servo index position laterally offset from a servo edge, the read/write elements track a parallel group of the data tracks. If it is desired to track another parallel group of data tracks, the magnetic tape head190is indexed laterally to another servo edge or to another servo index position, or a different servo sensor is aligned with the same or a different servo edge or servo index position.

When the magnetic tape head190is to be moved to a selected index position, an index controller170is enabled by the control unit110, receiving a lateral position signal from an independent position sensor180and transmits an appropriate signal to servo logic160to select the appropriate servo track, while the control unit110transmits an appropriate signal to a servo gap selector130to select the appropriate servo sensor. The independent position sensor180is discussed in the incorporated U.S. Pat. No. 5,946,159, where it is called a non-servo position sensor, and indicates the lateral mechanical position of the tape head190with respect to the tape path195.

Over the course of longer distances of longitudinal tape movement, the open channel guiding system will allow the tape to move laterally with respect to the tape deck. In accordance with the present invention, the independent position sensor180, in limited distances of tape movement, accurately tracks the lateral mechanical position of the tape head190, and therefore of the servo sensor(s), with respect to the magnetic tape197and of the servo edges. The logic160operates the servo track follower150in accordance with the present invention to calibrate the servo index positions as sensed by the servo sensor with respect to the parallel sets of linear servo edges, as will be explained. The logic160may comprise a programmed PROM, ASIC or microprocessor.

The tape system100may be bidirectional, in which ones of the read/write elements are selected for one direction of longitudinal tape movement, and others of the read/write elements are selected for the opposite direction of movement. The control unit110additionally selects the appropriate ones of the read/write elements by transmitting a signal to a read/write gap select unit120.

Once a servo edge or edges are selected, the servo gap selector130provides the servo signals to a servo detector140, which information is employed by servo logic160to position the tape head190to track follow the detected edges. In accordance with the present invention, servo logic160employs the servo information sensed by the servo detector140and the mechanical positioning information from the independent position sensor180to calibrate the track following servo. The track following servo logic is also implemented in the servo logic160employing the sensed servo signals to determine the ratios of the sensed servo signals, which are employed in accordance with the present invention to calibrate the servo index positions of the track following servo150.

Referring toFIG. 2A, a plurality, for example, three, parallel sets of linear servo edges250,260and270are illustrated, each servo edge comprising an interface between two dissimilar recorded servo signals, each set of servo edges comprising one of the servo edges on each of opposite lateral sides of a middle recorded servo signal. As an example, a corresponding plurality of laterally offset servo sensors, i.e. servo sensor groups255(FIG.2B),265(FIG.2B),275(FIG.2B), are disposed on tape head190to sense the servo signals at each corresponding edge. Additional pluralities of servo sensors, i.e. sensors272,274,276,278, may be provided to allow positioning of the tape head at additional data tracks.

Referring toFIG. 3A, the typical magnetic tape format of servo signals to form linear servo edges312and314comprising an interface between two dissimilar recorded servo signals is illustrated. One set of servo edges comprises outer bands320and322, having a recorded pattern of a constant amplitude signal of a single first frequency, on either side of an inner band310of the other servo signal, having a recorded pattern alternating between a constant amplitude burst signal318of a single second frequency and a zero amplitude null signal316. Typically, the servo signals320,310and322are provided with servo guard bands324and326to protect the outer bands320and322from noise resulting from the data track areas302and304.

It is desirable that the servo edges are separated by a predetermined nominal distance350employed for prerecording the servo signals. Typically, the outer servo signals320,322are recorded first, and the center servo signal310is recorded last, to provide the servo edges312,314. There is, typically, variation in the magnetic separation350between the servo edges, for example, due to the variation of the width of the physical write element which prerecords the servo pattern, due to variation in the magnetic characteristics of the physical write element, etc. The variation may occur between servo tracks in a single magnetic tape, and may occur between prerecording devices and therefore between magnetic tapes.

To reduce the apparent difference of the edge separation350distance of the prerecorded servo tracks from nominal, the prerecording of the servo signals is conducted at different amplitudes so as to attempt to compensate for the physical difference and provide a magnetic pattern that is closer to nominal. Additionally, three servo sensors are employed to simultaneously sense the three servo tracks. Thus, the difference in physical distance and the amplitude compensation may tend to offset each other with respect to the resultant apparent distance between the servo tracks. These actions may provide an adequate signal for track following at the servo edges.

However, to increase data track density, in the embodiment ofFIG. 3Afour servo index positions, i.e. index positions 0, 1, 2, and 3, are calibrated. These index positions are laterally offset with respect to the sensed servo edges of the set of linear servo edges. Index position 0 corresponds to sensor placement330over tape track position340. Similarly, index positions 1, 2, and 3, respectively, correspond to sensor placements332,334, and336, respectively, over tape track positions342,344, and346, respectively. The relative positions of these four index positions are: 0, 2, 1, 3.

As an example, the servo index positions may be offset laterally about one quarter the width of the inner band310away from the servo edge in either direction, providing four index positions. The indexed positions are determined by measuring the ratios between the amplitudes of the two dissimilar recorded servo signals, e.g., as measured by the servo detector140ofFIG. 1, and mapping those ratios into physical distances in microns. The servo logic160operates the servo track follower150to track follow at the desired measured ratio. For example, the measured ratio will be the ratio between the sum of the sensed outer band signal320plus the inner band signal318, and the sensed outer band signal320, giving effect to the null316. The illustrations and descriptions herein employ this ratio.

Alternatively, the measured ratio may be the ratio between the outer band signal320at frequency F1and the inner band signal318at frequency F2. In order to center the data read/write elements at each of the servo index positions, the ratios must be measured precisely. Thus, when the amplitudes of the recorded servo signals are varied to compensate for physical distance variations, the measured ratios are distorted and track following the prerecorded servo edges at the offset indexed positions becomes less precise. As the result, the data tracks may vary from the desired positions, for example, squeezed together, such that writing on one track with a write element that is subject to track misregistration (TMR) may cause a data error on the immediately adjacent data track.

FIG. 3Billustrates another embodiment of displaced index positions that may be employed with the present invention. This embodiment includes six index positions, i.e. index positions 0, 1, 2, 3, 4, and 5. At the “0” or “1” index positions, the servo element is located at position360centered on servo edge312or at position361centered on servo edge314. Additional index positions are provided which are aligned such that a servo element is displaced from an edge312or314in either direction. As the result, the number of index positions becomes six. The relative positions of these six index positions are: 2, 0, 4, 3, 1, 5.

In order to center the data read/write elements in the “2” and “5” index positions, the servo read element must be located at position362or at position365, and will read a minimum signal that has an amplitude ratio of about ⅚ of the maximum signal, and to center the data read/write elements in the “3” and “4” index positions, the servo read element must be located at position363or at position364, and will read a minimum signal that has an amplitude ratio of about ⅙ of the maximum signal.

To track follow an edge or edges, once a servo edge or edges are selected, the servo gap selector130ofFIG. 1provides the servo signals to a servo detector140, which digitally detects the servo signals at a predetermined sample rate, and provides servo signal ratios of each of the selected servo sensors. The servo logic160employs the servo signal ratios to determine the displacement from the edges and operates the servo loop servo track follower150to position the tape head190to track follow at the desired displacement from the edges.

FIG. 4Aillustrates examples of distortion of the measured ratios between the sensed servo signals of one linear servo edge, at various lateral positions of the servo sensors, in a guided tape system. Referring additionally toFIGS. 3A and 3Bregarding servo sensor placement, in an ideal relationship, the ratio of signals varies linearly from a value of “1” when the servo sensor is at position P(A), which is centered on and senses only the outer band320or outer band322, to a value of “0” when the servo sensor is at position P(B), which is centered on and senses only the inner band310. Straight line410graphically illustrates such an ideal relationship.

Curve420comprises a graphical representation of more typical ratio of servo signals, measured in a guided system, where the center recorded servo signal310generating the edges is of a relatively weak amplitude.FIGS. 4B and 4Cillustrate wave forms of the analog signal from the servo transducer at, respectively, positions332and336of FIG.3A. Thus, inFIG. 4B, the bursts440and460formed while the servo transducer is at position332ofFIG. 3Afrom the combination of the first frequency and the second frequency burst is at a high amplitude, but the burst450formed from the combination of the first frequency and the null signal is at a very low amplitude because only a small portion of the servo transducer is positioned over the first frequency. InFIG. 4C, the bursts470and490formed while the servo transducer is at position336ofFIG. 3Afrom the combination of the first frequency and the second frequency burst is at a high amplitude, as is the burst480formed from the combination of the first frequency and the null signal, because the servo transducer is positioned primarily over the first frequency.

Referring again toFIG. 4A, curve430comprises a graphical representation of more typical ratio of servo signals as a function of servo sensor location, measured in a guided system, where the center recorded servo signal310generating the edges is of a relatively strong amplitude. As those skilled in the art will appreciate, curves420and430do not define linear relationships between the ratios of measured servo signals and servo sensor placement. In light of the differing, and complex, relationship between the ratio of measured servo signals as a function of servo sensor location, employing the same ratio setting to position the tape head at various servo index positions for each of the linear edges may result in track misregistration.

As discussed above, the tape deck does not precisely hold the tape in position laterally. Rather, open channel guiding may be employed whereunder the magnetic tape can move laterally a distance which is substantially greater than that between index positions, e.g., index positions340-346of FIG.3A and index positions370,312,372,374,314, and376ofFIG. 3B, thereby introducing substantial noise into the calibration process. The guiding signal to noise ratio thus becomes negative, with the guiding noise being far larger than the step from one ratio to another, making it difficult to gather data points with a monotonically increasing or decreasing slope to conduct a calibration of the detected servo signal ratios.

In one embodiment of the present invention, servo logic160is provided with digital signal processor165, and is coupled to the servo detector140, the servo track follower150, and the independent position sensor180, all of FIG.1. The logic160operates the servo loop, comprising servo gap selector130, servo detector140, and servo track follower150, to inject a defined signal to modulate the lateral position of the head and, thereby, a plurality of servo sensors.

Referring now toFIG. 5A, Applicants' method to calibrate one or more servo sensors begins at step505. In step510, Applicants' method operates the servo loop to move off-track to begin calibration of the servo sensor(s). In certain embodiments, step510is performed by logic160. In certain embodiments, Applicants' method calibrates one or more servo sensors across a range which includes four servo index positions, i.e., positions340-346of FIG.3A. In certain embodiments, Applicants' method calibrates one or more servo sensors across a range which includes six index positions, i.e. positions370,312,372,374,314, and376of FIG.3B. In certain embodiments, Applicants' method calibrates one or more servo sensors across a range which includes more than six servo index positions.

In step512, Applicants' method operates the servo loop to move in the direction of the servo pattern, such as pattern250(FIG.2A), or260(FIG.2A), or270(FIG.2A), in order to find and lock to the pattern. In certain embodiments, step512is performed by logic160. Applicants' method transitions from step512to step514wherein Applicants' method determines whether the servo pattern has been found. In certain embodiments, step514is performed by logic160. If Applicants' method determines in step514that the servo pattern has not been found, then Applicants' method transitions from step514to step510to repeat the movement off-track.

Alternatively, if Applicants' method determines in step514that the servo pattern has been located, Applicants' method transitions from step514to step516wherein Applicants' method follows the servo pattern on the moving tape by causing the servo loop to laterally position the servo sensor to detect the servo signals at continually altered digital set points of the ratios of the sensed servo signals. In certain embodiments, step516is performed by logic160. The set points are altered at a predetermined sampling rate, and are altered to inject a predetermined positioning signal, whereby the servo loop track follows the linear servo edges, e.g., edges312or314ofFIGS. 3A and 3Bat each of the parallel sets of linear servo edges250(FIG.2A),260(FIG.2A),270(FIG.2A), with the corresponding plurality of laterally offset servo sensors, i.e. sensor groups255(FIG.2B),265(FIG.2B),275(FIG.2B), at the continually altered digital set points.

In certain embodiments, this predetermined positioning signal comprises a sinusoidal pattern having a known reference frequency. In certain embodiments, the positioning signal is recorded in firmware disposed in logic160. In certain embodiments, the positioning signal comprises a sinusoidal pattern having a varying frequency. In certain embodiments, the positioning signal is varied according to an algorithm disposed in logic160.

In certain embodiments, the reference frequency is set in “firmware” disposed in DSP165. In certain embodiments, the reference frequency is set by the user during system initialization. In certain embodiments, the reference frequency is set by field service personnel during system initialization. In certain embodiments, the reference frequency is modified by Applicants' method in order to further refine the calibration of the servo sensors disposed in the system.

In certain embodiments, the positioning signal comprises a reference frequency selected such that the reference frequency, and major harmonics thereof, each differs from intrinsic operational frequencies of the track following servo system and/or of the tape drive. As examples, the positioning signal is selected so that the reference frequency and major harmonics thereof differ from the frequencies of the tape reels197ofFIG. 1, the tape motor(s), and the cooling, fans, and any resonant frequencies of the servo system.

In certain embodiments, the positioning signal is modulated such that the ratio of servo signals detected by the one or more servo sensors being calibrated, i.e. the ratio of the detected amplitudes of F1and F2, varies from about 0.1 to a ratio of about 0.9. Those detected servo signals are dominated by the sinusoidal pattern and not by the tape movement. The frequency of the positioning signal, i.e. the reference frequency, is known precisely, and signals not having that reference frequency, or harmonics of that reference frequency, comprise noise in the measurement.

Independent position sensor (“IPS”)180measures the position of tape head190with respect to the tape path195. In step520, Applicants' method samples the analog IPS signal at the selected sample rate. In certain embodiments, step525is performed by servo logic, such as logic160. Applicants' method transitions from step520to step525wherein Applicants' method digitally forms a measured IPS waveform using the data of step520. In certain embodiments, step525is performed by servo logic, such as logic160.

Referring now toFIG. 6, waveform640comprises the measured IPS signal waveform formed in step525. The Y axis corresponding to curve640is found at the right side of graph600. The digitized measured IPS signal waveform of step525is dominated by the sinusoidal pattern having the reference frequency, and not by the tape movement. The frequency of the reference frequency is known precisely, and anything that is not at that reference frequency, or its harmonics, comprises noise in the measurement.

Applicants' method transitions from step525to step530wherein Applicants' method determines the real and imaginary components, at selected frequencies, of the measured IPS waveform of step525. In certain embodiments of Applicants' invention, those real and imaginary components are formed using a Goertzel algorithm. In certain embodiments, Applicants' method uses a Goertzel algorithm defined by equation (1)H⁢⁢fi⁡(z)=[1-ⅇ(2⁢⁢π⁢⁢fi/fs)⁢z-1]/1-2⁢⁢cos[2⁢⁢π⁢⁢fi/fs]⁢z-1+z-2(1)
where fiis the frequency of interest, and fsis the sampling frequency. In certain embodiments, Applicants' method includes second order recursive Goertzel filter I.
In certain embodiments, DSP component165includes a Goertzel filter.

In certain embodiments, the measured IPS waveform of step525comprises the input to a Goertzel filter, and the real and imaginary components of step530comprise the output of that Goertzel filter. In certain embodiments, servo logic160(FIG. 1) provides the waveform of step525to a Goertzel filter in DSP component165(FIG.1).

In certain embodiments, the selected frequencies comprise (P) harmonics of the reference frequency. As (P) increases, Applicants' method can identify more complex distortions of the detected servo signal ratios as a function of IPS location. In certain embodiments, Applicants' method utilizes as many of 6 harmonics of the reference frequency, i.e. (P) equals 6. On the other hand, in certain embodiments Applicants' have found an insubstantial increase in useful information beyond the fourth harmonic.

In step535, Applicants' method uses the real and imaginary components of step530to form a filtered IPS signal waveform. The real components of the measured IPS waveform determined in step530are used in step535to determine the magnitudes M(i) for that IPS waveform at the selected frequencies, where (i) is an integer between 1 and (P). The imaginary components of the measured IPS waveform are used in step535to determine the phase φ(i) of that IPS waveform at the selected frequencies, wherein (i) is as described above.

For example, in embodiments wherein Applicants' method uses the first four harmonics of the reference frequency, (i) is an integer greater than or equal to 1 and less than or equal to 4. M(1)comprises the magnitude of the measured IPS waveform at the reference frequency. As those skilled in the art will appreciate, the first harmonic is identical to the fundamental frequency, i.e. the reference frequency. M(2)comprises the magnitude of the measured IPS waveform at the second harmonic of the reference frequency, M(3)comprises the magnitude of the measured IPS waveform at the third harmonic of the reference frequency, and M(4)comprises the magnitude of the measured IPS waveform at the fourth harmonic of the reference frequency.

Similarly, φ(1)comprises the phase of the measured IPS waveform at the reference frequency, φ(2)comprises the phase of the measured IPS waveform at the second harmonic of the reference frequency, φ(3)comprises the phase of the measured IPS waveform at the third harmonic of the reference frequency, and φ(4)comprises the phase of the measured IPS waveform at the fourth harmonic of the reference frequency.

In step535, Applicants' method forms a filtered IPS signal waveform using the magnitudes M(i), phases φ(i), and the equation:Filtered⁢⁢IPS⁢⁢Signal⁢⁢Waveform=⁢M(1)*cos⁡[ϕ(1)+(2)⁢(π)⁢(t)/x]+⁢M(2)*cos⁡[ϕ(2)+(4)⁢(π)⁢(t)/x]+⁢M(3)*cos⁡[ϕ(3)+(6)⁢(π)⁢(t)/x]+⁢M(4)*cos⁡[ϕ(4)+(8)⁢(π)⁢(t)/x]
where (t)/x comprises a fraction having a value greater than or equal to 0 and less than or equal to 1, where multiplying that fraction by (n)π gives a point within the (n)π period.

Thus, Applicants' method in steps520,525,530, and535, processes the measured IPS signal wherein the tape guiding noise is dominant, to form a filtered IPS signal waveform wherein the signal of interest comprise a sinusoidal pattern at a known reference frequency. In this filtered waveform, the noise, not only the tape guiding noise, but also noise resulting from tape defects, has been effectively removed.

In step540, Applicants' method samples, at the selected sampling rate, one or more servo signals. The sampling of step540is performed concurrently with the sampling of step520. In certain embodiments, the one or more servo sensors each provide an analog signal to a servo detector, such as servo detector140(FIG.1), and the servo detector forms one or more servo signals comprising ratios of the detected first frequency and detected second frequency. The servo detector then provides those one or more servo signals to the servo logic, such as logic160(FIG.1). In certain embodiments, the servo sensors provide the servo signals comprising the ratios of detected signals.

Applicants' method transitions from step540to step545wherein Applicants' method digitally forms one or more measured servo signal waveforms using the data obtained in step540. Referring again toFIG. 6, waveforms610,620, and630comprise the measured servo signal waveforms formed in step545. The Y axis corresponding to curves610,620, and630, is found at the left side of graph600. As those skilled in the art will appreciate, in the embodiment ofFIG. 6Applicants' method includes using three servo sensors and one IPS sensor.

Applicants' method transitions from step545to step550wherein Applicants' method determines the real and imaginary components of each of the measured servo signal waveforms of step545. In certain embodiments, step550includes using a Goertzel algorithm as described above. In certain embodiments, step550includes using a Goertzel filter as described above.

Applicants' method transitions from step550to step555wherein Applicants' method forms one or more filtered servo signal waveforms using the real and imaginary components determined in step550. When forming, for example, the (j)th filtered servo signal waveform using the real and imaginary components of the (j)th measured servo signal waveform at the (P) harmonics of the reference frequency, Applicants' method in step555uses the magnitude M(i)(j)determined from real components of the (j)th measured servo signal waveform at the (i)th harmonic of the reference frequency, and the phase φ(i)(j)determined from the imaginary components of that (j)th measured waveform at the (i)th harmonic of the reference frequency, wherein (i) is an integer greater than or equal to 1 and less than or equal to (P).

The DC component of the measured servo signal waveforms is also kept so as to provide an offset between the plurality of servo sensor channels. M(0)(j)comprises that DC component for the (j)th measured servo signal waveform. Further in step555, Applicants' forms the (j)th filtered servo signal waveform using the equation:(j)⁢th⁢⁢filtered⁢⁢servo⁢⁢signal⁢⁢waveform=⁢M(0)⁢(j)+⁢M(1)⁢(j)*cos⁡[ϕ(1)⁢(j)+(2)⁢(π)⁢(t)/x]+⁢M(2)⁢(j)*cos⁡[ϕ(2)⁢(j)+(4)⁢(π)⁢(t)/x]+⁢M(3)⁢(j)*cos⁡[ϕ(3)⁢(j)+(6)⁢(π)⁢(t)/x]+⁢M(4)⁢(j)*cos⁡[ϕ(4)⁢(j)+(8)⁢(π)⁢(t)/x]
where (t)/x comprises a fraction having a value greater than or equal to 0 and less than or equal to 1, where multiplying that fraction by (n)π gives a point within the (n)π period.

Thus, Applicants' method in steps540,545,550, and555, processes the detected servo signals wherein the tape guiding noise is dominant, to form one or more filtered servo signal waveforms wherein the signal(s) of interest comprise a sinusoidal pattern at a known reference frequency. In these filtered waveforms, the noise, not only the tape guiding noise, but also noise resulting from tape defects, has been effectively removed.

Applicants' method transitions from step555to step560. Referring now toFIG. 5B, in step560, Applicants' method correlates each of the one or more filtered servo signal waveforms of step555with the filtered IPS signal waveform of step535. As those skilled in the art will appreciate, one such correlation comprises graphing the one or more filtered servo signal waveforms against the filtered IPS signal waveform. Such a correlation comprises graphing a first sinusoidal function against a second sinusoidal function.

As those skilled in the art will appreciate, a Lissajous pattern comprises a graph of a first sinusoidal function having a first frequency plotted on the y axis combined with a second sinusoidal function having a second frequency plotted on the x axis, i.e. Y and X are both periodic functions of time t given by equations such as X=sin(w*n*t+c) and Y=sin w*t. Different patterns may be generated for different values of n (period shift) and c (phase shift).

If a linear relationship exists between sampled servo signals and measured IPS positions, then graphing the filtered IPS waveform formed in step535against the one of the filtered servo signal ratio waveforms of step555would give a straight line Lissajous pattern. For example referring toFIG. 9, if curve910comprises the filtered IPS waveform, and curve920comprises one of the filtered servo signal ratio waveforms, and graphing curve910against curve920gives straight line curve930, then a linear relationship exists between the detected servo signal ratios and the measured IPS location. As those skilled in the art will appreciate, curve910and920have the same frequency, but are phase shifted by 180 degrees. Graphing sinusoidal functions having identical frequencies and phase shifted by 0 degrees similarly gives a straight line curve.

Graphing differing periodic functions against one another gives more complex curves. Referring toFIG. 10, waveform1010differs from waveform1020. Graphing waveform1010and against waveform1020gives curve1030. As those skilled in the art will appreciate, the mathematical relationship between curve1010and curve1020cannot be accurately expressed as a linear function. On the other hand, a more complex function such as a quadratic function might adequately define the relationship between curves1010and1020.

FIG. 7comprises graph700wherein the filtered servo signal waveforms of step550, wherein (P) equals 3, are plotted against the filtered IPS waveform. Curve710comprises the filtered servo signal waveform derived from measured servo signal waveform610(FIG. 6) plotted against the filtered IPS waveform derived from measured IPS signal waveform640(FIG.6). Similarly, curves720and730, respectively, comprise the filtered servo signal waveforms derived from measured servo signal waveforms620(FIG. 6) and630(FIG.6), respectively, plotted against the filtered IPS waveform.

As those skilled in the art will appreciate, curves710,720, and730, are more complex than curve930or curve1030. In certain embodiments, Applicants' method utilizes a second order curve fitting algorithm to model curves710,720, and730. In other embodiments, Applicants' method utilizes a third order curve fitting algorithm to model curves710,720, and730. In yet other embodiments, Applicants' method uses an (n)th order data analysis algorithm to model curves710,720, and730, wherein (n) is 4, 5, or 6.

In step562, Applicants' method forms (N) X/Y datapoint arrays using the correlation of step560. Each X/Y array comprises a plurality of measured datapoints DPACT(i). Each such datapoint DPACT(i)comprises the IPS signal actually measured for a given measured servo signal. In certain embodiments, step562is performed by servo logic160(FIG.1).

In step566, Applicants' method forms (N) transfer functions using, where each of those (N) transfer functions is formed using one of X/Y data arrays of step562and an (n)th order regression algorithm. In step564, that (n)th order is selected. In certain embodiments, step564further includes establishing a highest order curve fitting algorithm, i.e. (n)MAX. In certain embodiments, a second order curve fitting algorithm is used. In certain embodiments, a third order curve fitting algorithm is used. In certain embodiments, one or more higher order curve fitting algorithms are used wherein (n) is 4, 5, or 6.

Applicants' method transitions from step566to step568wherein Applicants' method forms (N) calibration curves using the (N) transfer functions of step566. As those skilled in the art will appreciate, a calibration curve is formed for each servo sensor being calibrated. Each of those (N) calibration curves includes a plurality of calculated datapoints DPCAL(i). Referring now toFIG. 8, curves815,825, and835, are derived using the transfer functions of step566.FIG. 8further recites three sets of measured datapoints810,820, and830.

Applicants' method transitions from step568to step570wherein Applicants' method compares the measured datapoints DPACT(i)for a given servo sensor with the calculated datapoints DPCAL(i)for that servo sensor. In certain embodiments, Applicants' method determines the residual error RE(i)for each actual datapoint by computing the difference between DPACT(i)and DPCAL(i). In certain embodiments, Applicants' method establishes a maximum residual error REMAX, and compares each RE(i)to that REMAX. In certain embodiments, Applicants' method determines an average residual error REAVGusing the values for RE(i), and compares REAVGto REMAX.

Applicants' method transitions from step570to step572wherein Applicants' method, based upon the comparison of step570, determines if the transfer function of step566models the actual data measured, i.e. determines if the calibration curves formed using that transfer function adequately match the actual datapoints DPACT(i). If Applicants' method determines in step572that the transfer function does not model the measured datapoints, then Applicants' method transitions from step572to step574. For example, in certain embodiments Applicants' method determines that the transfer function of step566models the measured datapoints if the REAVGis less than or equal to REMAX.

In step574Applicants' method determines if (n) equals (n)MAX. If Applicants' method determines in step574that (n)MAXhas not been reached, then Applicants' method transitions to step576wherein (n) is incremented. Thereafter, Applicants' method transitions from step576to step566wherein a new transfer function is determined using a higher order data regression analysis. In the event no value for (n)MAXis established in step564, then Applicants' method returns a finding of “YES” in step574and transitions to step578.

In certain embodiments, Applicants' method includes establishing a maximum number (M) of calibration attempts. If Applicants' method determines in step574that (n)MAXhas been reached or has not been established, then Applicants' method transitions from step574to step578wherein Applicants' method determines if (M) calibration attempts have been made. If Applicants' method determines in step578that the maximum number of calibration attempts have not been made, then Applicants' method transitions from step578to step516(FIG.5A).

If Applicants' method determines in step578that (M) calibration attempts have already been made, then Applicants' method transitions from step578to step580wherein an error message is provided. In the event no value for (M) has been established, then Applicants' method returns a finding of “YES” for step578.

If Applicants' method determines in step572that the transfer function of step566adequately models the measured datapoints, then Applicants' method transitions to step582wherein that transfer function is saved for subsequent use. In certain embodiments, the transfer function is saved in a memory device, such as memory107(FIG.1).

Applicants' method transitions from step582to step584wherein Applicants' method determines if the one or more sensors require calibration using a different servo pattern. If Applicants' method determines in step584that the one or more sensors requires calibration using a different servo pattern, then Applicants' method transitions from step584to step510. Alternatively, if no further calibration is required, then Applicants' method transitions from step584to step590and ends.

As those skilled in the art will appreciate, the transfer function saved in step582is subsequently used to calculate one or more expected position error signals (“PES”) for the servo loop at the laterally offset servo index positions with respect to the sensed first set of linear servo edges. These calculated PES signals are used to track follow during, for example, a read operation, a write operation, and erase operation, and the like.

In certain embodiments, one or more individual steps of Applicants' method summarized inFIGS. 5A and 5Bmay be combined, eliminated, or reordered.

Applicants' method has been described above in embodiments wherein one or more servo sensors are calibrated. Applicants' method, however, is not limited to calibrating servo sensors. Rather, Applicants' method can be used to calibrate transducers in an electrically noisy environment.

Referring toFIG. 11, in step1105Applicants' method calibrates a transducer with respect to a reference signal, where that transducer produces a first signal. In step1110, the transducer provides that first signal. In step1115, the reference provides the reference signal. In step1120, Applicants' method establishes a sampling rate at a reference frequency.

In step1125, Applicants' method samples the first signal at the sampling rate. In step1130, Applicants' method samples the reference signal at the reference frequency. In step1135, Applicants' method digitally forms a measured first signal waveform using the data of step1125. In step1140, Applicants' method digitally forms a measured reference signal waveform using the data of step1130.

In step1145, Applicants' method determines at (P) harmonics of the reference frequency the real and imaginary components of the measured first signal waveform using the embodiments described above with respect to steps530(FIG. 5A) and550(FIG.5A). In step1150, Applicants' method forms a filtered first signal waveform using the embodiments described above with respect to steps535(FIG. 5A) and555(FIG.5A).

In step1155, Applicants' method determines at (P) harmonics of the reference frequency the real and imaginary components of the measured reference signal waveform using the embodiments described above with respect to steps530(FIG. 5A) and550(FIG.5A). In step1160, Applicants' method forms a filtered reference signal waveform using the embodiments described above with respect to steps535(FIG. 5A) and555(FIG.5A).

In step1165, Applicants' method correlates the filtered reference signal waveform of step1160with the filtered first signal waveform of step1150to form a transfer function using the embodiments described above with respect to steps560(FIG.5B),562(FIG.5B),564(FIG.5B), and566(FIG.5B). In step1170, Applicants' method forms a calibration curve for the transducer using the transfer function of step1165. In certain embodiments, one or more individual steps of Applicants' method summarized inFIG. 11may be combined, eliminated, or reordered.

Applicants' invention further includes an article of manufacture comprising a computer useable medium having computer readable program code disposed therein for calibrating a transducer in an electrically noisy environment. Applicants' invention further includes a computer program product usable with a programmable computer processor having computer readable program code embodied therein for calibrating a transducer in an electrically noisy environment.