WO1998052195A1

WO1998052195A1 - Method for use in a disk drive to calibrate demodulator gain without a/d conversion of cpes signal

Info

Publication number: WO1998052195A1
Application number: PCT/US1998/008335
Authority: WO
Inventors: Kelly Douglas Wright
Original assignee: Iomega Corporation
Priority date: 1997-05-13
Filing date: 1998-04-24
Publication date: 1998-11-19
Also published as: TW454179B

Abstract

A servo system for use in a disk drive in controlling the position of a read/write (R/W) head with respect to tracks of a disk voids inadvertent over-writing of data on a neighboring track. The disk drive generates a position error signal (PES) indicative of the error, if any, in the position of the head. The servo system includes a controller (10) and a sign comparator (42). The controller is programmed to perform a calibration sequence to determine a value of PESOFFSET corresponding to a 1/4-track head position error in PES, and in subsequent track following operations to use this known relationship between PES and PESOFFSET to accurately control and monitor the position of the R/W head with respect to a given data track. PES and PESOFFSET are added to form a composite position error signal (CPES). The sign comparator is coupled to the controller and provides a signal to the controller indicative of the sign of CPES.

Description

METHOD FOR USE IN A DISK DRIVE TO CALIBRATE DEMODULATOR GAIN WITHOUT A/D CONVERSION OF CPES SIGNAL

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. Patent Application Serial No. 08/855,150, filed May 13, 1997.

FIELD OF THE INVENTION

The present invention relates generally to servo loop calibration techniques, and more particularly to a servo loop calibration technique that saves cost by eliminating the need for an A/D conversion of the composite position error signal (CPES) in a disk drive in order to calibrate the gain of the head position transducer, commonly known as the demodulator.

BACKGROUND OF THE INVENTION The presently preferred embodiments of the invention relate to a servo loop calibration method and circuit useful in accurately controlling the track following in a disk drive. Therefore, although the invention may be used in other applications, the background of the invention will be discussed with reference to a disk drive.

A closed loop control system may be configured as shown in Figure 1. In this example, a summing device 14 and controller circuit 10 are utilized to control a physical plant or process, which in this case is a magnetic recording system 12. In the system depicted in Figure 1, the controlled variable Y(t) is compared to a reference input X(t) . The reference input X(t) may be the position of a particular track on a disk, and the controlled variable may be the position of the read/write head of the magnetic recording system. The comparison between the reference input and the controlled variable at the summing device 14, which could be a differential amplifier, multiplier, or some other electrical or mechanical signal processing device, produces an error signal E(t) that corresponds to the difference between the input X(t) and the output Y(t) . This error signal is provided as an input to the controller 10. Controller 10 is a dynamic system added to the loop to stabilize and enhance the closed-loop system characteristics. The output of controller 10 is an actuating signal A(t), which changes as a function of the error signal E(t) . The actuating signal A(t) is developed to correct the recording system 12, e.g., to control the position of a read/write (R/W) head over a desired track. The controller circuit 10 contains the control strategy for the entire servo control system. Within the control circuit 10 is a compensation component (s) . Typical goals of a controller are (1) to position a read/write head over a given disk track as quickly as possible and (2) to hold it over the track as accurately as possible. The position of the read/write transducer is controlled by the servomechanism in accordance with signals from the read/write electronics of the system.

Many presently commercially available hard disk drives position the read/write (R/W) head at a 1/4 track position in order to calibrate the demodulator gain, but both their method of getting the head to a 1/4 track position and the manner in which the resulting information is processed by the drive is different from the method and system disclosed herein. Specifically:

1. To position the head at a 1/4 track position, the drive's servo off of just the (A-B) signal. The present inventor is not aware of any hard disk drives that use alternating samples from (A-B) and (C-D) as in the present invention, nor has the present inventor ever seen the alternating sample method of the present invention described anywhere before.

2. To determine that the head is positioned at the 1/4 track position, hard disk drives examine the magnitude of A/D samples from both (A-B) and (C-D) and look for the condition | (A-B) j = j (C-D) j . But again this is different from using a sign comparator instead of an A/D, as in the present invention.

As discussed in greater detail below, one known servo system employs an A/D convertor to feed digitized samples of a composite position error signal (CPES) to a control processor. A primary goal of the present invention is to save cost by replacing the A/D convertor with a less expensive sign comparator. In the highly competitive computer storage industry, the use of an A/D converter to provide feedback to the control processor adds to the production cost of disk drive units. While A/D devices are relatively inexpensive on a per-unit basis, the requirement of such a device for each disk drive can add hundreds of thousands or even millions of dollars to the manufacturer's total production costs, depending on the number of units produced. Thus, the need for this A/D convertor is a disadvantage to manufacturers of disk drives and other devices requiring motor control.

SUMMARY OF THE INVENTION

The present invention provides a servo system for use in a disk drive in controlling the position of a read/write (R/W) head with respect to tracks of a disk so as to avoid inadvertent over-writing of data of a neighboring track. The disk drive includes means (14A, 32) for generating a position error signal (PES) indicative of the error, if any, in the position of the head (see Fig. 3) . The inventive servo system comprises a controller (10) and a sign comparator (42) , and the controller is programmed to perform a calibration sequence to determine a value of the offset signal (PESOFFSET) corresponding to PES when the read/write head is at the l/4-track head position, and in subsequent track following operations to use this known relationship to accurately control and monitor the position of the read/write head with respect to a given data track. The sign comparator is coupled to the controller and provides signals to the controller indicative of the sign of the CPES. In the presently preferred embodiment of the invention, the controller is programmed to alternately obtain samples of (A-B) signals (50) and (C-D) signals (52) , and to determine a first sum of outputs of the sign comparator corresponding to the (A-B) samples and a second sum of outputs of the sign comparator corresponding to the (C-D) samples. The controller is further programmed to determine a value of the offset signal (PESOFFSET) that will cause the first and second sums to be equal.

The present invention also provides a servo method for use in a disk drive. The method includes steps for performing an initial demodulator calibration procedure whereby a demodulator gain or slope (K_DEM0D) is determined. The width of the read/write head is an integral term of the demodulator gain. The calibration procedure comprises (1) alternately obtaining, via the R/W head, position samples from the (A-B) and (C-D) signals; (2) generating a position error signal (PES) from said samples, wherein PES includes a square wave component; (3) activating the track following control system with said PES that includes a square wave component which will cause the average track following head position to converge to a 1/4 track position and will result in the square wave component of PES having an amplitude that alternates between a positive value and a negative value that corresponds to a l/4-track head position error; (4) forming a composite position error signal (CPES) by summing the PES with an offset signal (PESOFFSET) , wherein PESOFFSET has a square wave component whose amplitude is adjustable and whose phase is opposing the phase of the square wave component of PES, (5) monitoring the sign of CPES and determining a first sum of outputs of the sign comparator corresponding to the (A-B) samples and a second sum of outputs of the sign comparator corresponding to the (C-D) samples; and (6) determining the amplitude of the square wave component of PESOFFSET that will cause the first and second sums to be substantially equal which establishes a known relationship between the 1/4 track PES and PESOFFSET magnitudes; and subsequently, during normal track following operations, using this known relationship to accurately control and monitor the position of the R/W head with respect to a given data track. In this manner, the position of the R/W head with respect to the data tracks is controlled so as to avoid inadvertent over-writing of the data on a neighboring track.

Other features of the invention are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 schematically depicts a servo control system.

Figure 2 schematically depicts a known format of a servo burst pattern of a disk.

Figure 3 schematically depicts how a disk drive servo system may be modified in accordance with the present invention to replace an A/D converter with a simple sign comparator for monitoring the sign of the composite position error signal.

Figures 4A-4C depict exemplary waveforms useful in explaining the operation of the present invention. In particular, Figure 4A depicts the idealized demodulator characteristic of the preferred embodiment; Figure 4B illustrates the slope of the exemplary idealized demodulator characteristic; and Figure 4C depicts the square wave component of the position error signal.

Figures 5A-5C depict additional waveforms useful in explaining the operation of the present invention. Figure 5A is an exemplary plot of frequency of CPES values corresponding to (A-B) and (C-D) samples; Figure 5B illustrates how the frequency plots of Figure 5A are changed via the square wave component of the PESOFFSET signal; and Figure 5C illustrates how the integral or sum of the sign comparator outputs intersect at the point at which the magnitude of the PESOFFSET signal equals the magnitude of the PES signal corresponding to a one-quarter track error.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention is especially suited for use in conjunction with a disk having a servo pattern as (partially) depicted in Figure 2. As shown, a read/write

(R/W) head or transducer 16 is utilized to read servo burst signals ("A", "B" , "C" , "D" ) that are recorded in servo sectors of the disk. The servo burst signals are arranged as shown, in a burst pattern generally denoted by reference numeral 20, such that the signals ("A") of the first set, denoted 22A, are disposed on one side of each track and the signals ("B") of a second set, denoted 22B, are situated on a side of the track opposite to "A" . The "A" and "B" burst signals do not overlap their respective tracks. In contrast, a third set 24A of "C" bursts and a fourth set 24B of "D" signals overlap the tracks such that, for example, the "C" signals overlap every other track (such as tracks 26A and 26C) whereas the "D" signals overlap the tracks not covered by the "C" signals, as shown. Thus, in the example of Figure 2, a first track 26B is marked by adjacent burst signals "A" and "B" and further by overlapping burst signals "D" . Similarly, a second track 26C is marked by adjacent bursts "A" and "B" and overlapping "C" bursts . A third track 26D is like the first track 26B insofar as the layout of the "A", "B" , "C" and "D" burst signals are concerned.

In an ideal case where the R/W head itself is centered over the center line of track 26C only the upper half of the R/W head would pass over an "A" burst, only the lower half of the R/W head would pass over a "B" burst, the whole width of the head would pass over a "C" burst, and the "D" burst would be missed entirely. The amplitude of the signal detected by the demodulator from a given burst is proportional to how much of the R/W head's width passes over that burst. Thus, the PES for a sample obtained from the "A" and "B" bursts is formed by taking the average amplitude detected from the "A" burst and subtracting the average amplitude detected from the "B" burst, represented by (A-B) . (C-D) is similarly a difference of average amplitudes detected from the "C" and "D" bursts. Thus, in the ideal case, where the R/W head 16 reads a difference signal (A-B) that is equal to zero, it is inferred that the head 16 is tracking directly over the second track 26C. (In addition, a grey scale track number may be embedded in servo data (not depicted herein) to unambiguously identify the track number. This can help the controller to be able to distinguish tracks, such as track 26A and track 26C, that are surrounded by an identical layout of servo burst signals. See the repeating nature of the idealized demodulator characteristic shown in Fig. 4A. )

Figure 3 schematically depicts the general architecture of the servo system utilized in Iomega

Corporation's (the assignee of the present invention) Zip™ Disk Drive. In addition, Figure 3 illustrates how this arrangement may be modified in accordance with the present invention to save cost. In particular, as illustrated by the dash- line block 42, the prior servo system may be modified by replacing the analog-to-digital (A/D) convertor 40 with a simple sign comparator 42. Cost is saved because the sign comparator 42 is a significantly less expensive component than the A/D convertor 40. Thus, a servo control system in accordance with the present invention comprises the following components: a controller 10, which may be implemented with a microprocessor, digital signal processor (DSP) , or the like; a pair a digital-to-analog (D/A) convertors 30A, 30B; a position error signal (PES) demodulator 32; a zero order hold circuit 34; an analog compensator circuit 36; an actuator 38; the simple sign comparator 42; amplifiers 44A, 44B and 44C; and summing devices 14A, 14B and 14C. The simple sign comparator 42, e.g., may output a single digital bit of information which is a "1" value when its input is zero or positive and a "0" value when its input is negative. In this respect, it is significantly less complicated, and expensive, than the A/D convertor 40 since the latter outputs a multiple digital bit representation of the actual value (magnitude) of its input.

The servo control system of Figure 3 may be implemented as an electromechanical circuit. The actual track position ("Ptrk") is input to summing device 14A along with the head position ("Phd") . The position error ("Perr") is equal to Ptrk - Phd. The position error, Perr, is a mechanical signal input to the demodulator 32. The demodulator 32 outputs an electrical signal, i.e., the PES signal, which is proportional to the head position error. As discussed in greater detail below, the demodulator 32 has a gain, K_DEM0D. The PES signal output by the demodulator 32 is input to the summing device 14B and summed with the PESOFFSET signal generated by the controller 10. Samples of the PES signal are stored temporarily in the zero order hold (ZOH) device 34. The output of the ZOH is referred to as the composi te position error signal, or CPES. According to the present invention, the sign (+ or -) of each sample of the CPES signal is monitored by the sign comparator 42. The CPES signal is input to the analog compensator 36. The output of the analog compensator circuit, V_TF, is summed with a seek signal, V_SEEK, and fed via amplifier 44C to actuator 38. The general mode of operation of a servo control system of the kind depicted in Figure 3 will be apparent to one of ordinary skill in designing disk drive servo systems, and so the non-essential details of the system are not described herein. The focus of the following discussion of the operation of the preferred embodiment is on the use of the sign comparator 42, and the output thereof to the controller 10, to enable the controller to generate the correct PESOFFSET signal so as to cancel the square wave component present in PES during a calibration sequence, while track following at a 1/4 track position. The demodulator 32 is defined by a "characteristic" function that determines the output PES value for a given input Perr value. The particular characteristic that is applicable at a given time depends upon whether the Perr value is based on an (A-B) sample or a (C-D) sample. Given the demodulator characteristic depicted in Figure 4A, where the (A-B) and (C-D) characteristics are denoted 50 and 52, respectively, and the -(C-D) characteristic may be obtained by inverting the (C-D) characteristic, then it may be assumed:

1. Taking alternating samples from (A-B), then from (C-D) will cause track following to occur at the -1/4 track position because that is the only position in the demodulator characteristic where alternating samples generate the correct polarity for stable negative servo feedback and also have an average PES value of zero. With PESOFFSET held at zero, CPES will have a square wave component of the type depicted in Figure 4C. Alternatively, taking alternative samples from (A-B) then from - (C-D) will cause track following to occur at the +1/4 -track position. In this case, CPES will have a square wave component like that shown in Figure 4C but the positive pulses will correspond to the (A-B) samples and the negative pulses will correspond to the (C-D) samples.

2. The square wave component of CPES described above can be eliminated by outputting a corresponding inverted phase square wave on PESOFFSET. When the amplitude of the PESOFFSET square wave corresponds to the 1/4 -track position, it will equal the amplitude of the PES square wave signal and will cancel the square wave component of PES.

3. The PESOFFSET amplitude corresponding to the l/4-track position is detected, in accordance with the present invention, without the use of an A/D convertor. Instead, a simple sign comparator that provides a signal indicative of the sign (positive or negative) of CPES is used in conjunction with the random noise that exists on the PES signal. When the square wave amplitude of the PESOFFSET signal is too small, and when track following on (A-B) and (C-D) , the distribution of CPES samples will be as shown in Figure 5A, which illustrates an exemplary (A-B) distribution 60 and (C-D) distribution 62. In this figure, the width, W, of the distribution is due to random noise in the position error signal. As the amplitude of the PESOFFSET signal is increased, the distribution 60 of (A-B) sample values will move in a positive direction, and the distribution 62 of (C- D) sample values will move a negative direction. When the PESOFFSET amplitude is at the value corresponding to a 1/4- track head position, both the (A-B) distribution 60 and the (C-D) distribution 62 should be coincident with each other each having a mean value of zero. If the PESOFFSET square wave amplitude is increased beyond the value corresponding to l/4-track head position, then the two distributions will switch places as depicted in Figure 5B. If the sign of CPES on the (A-B) samples is summed and the sign of CPES on the (C-D) samples is summed, the characteristic depicted in Figure 5C will be obtained. The value of PESOFFSET magnitude that corresponds to 1/4 track head position is where the two curves cross. Any offsets in the reference value of the sign comparator or in an integrator that may be present in the analog compensator will cause the two curves to cross above or below the horizontal ( j PESOFFSET j ) axis, but the crossing still defines the 1/4 track PESOFFSET value. Once the value of PESOFFSET magnitude that corresponds to the 1/4 track position is known for a given head 16 and demodulator 32 (Figs. 2 and 3) , it can be used to calibrate subsequent offtrack operations for that head. If the demodulator gain is calibrated using the technique described above and if any offsets in the sign comparator are negligible then the "zero value" of PESOFFSET that will cancel the analog integrator's input offset voltage, and thus center the read/write head 16 on the data track is :

PESOFFSET "ZERO" VALUE = 1/2 {( | PESOFFSET j where ∑SGN(C-D)=0)

- ( j PESOFFSET j where ∑SGN (A-B) =0) } . In addition, it should be noted that the magnitude of the square wave on PESOFFSET that corresponds to the 1/4 track position can be obtained either from the point at which the two curves of Figure 5C cross each other or, assuming symmetrical curves and non zero offsets in the sign comparator, or analog compensator's, integrator, from the average of the two PESOFFSET magnitude values described above, i.e., from the midpoint between where the two curves cross the x axis ( j PESOFFSET | axis).

Finally, it should be noted that while a servo circuit and calibration method for controlling the position of a R/W head with respect to a magnetic recording disk have been described, there are other embodiments of the invention and the above disclosure should not be read as a limitation on the scope of the invention, but rather as illustrative thereof. For instance, the above method for demodulator gain calibration can be implemented without obtaining the cost advantage of the sign comparator by using the A/D and controller to simulate the simpler sign comparator. Additionally, this method may also be used to calibrate the demodulator gain of any tape drive that uses servo bursts similar to those depicted in Figure 2.

Claims

I c laim :

1. A servo system for use in a disk drive in controlling the position of a read/write (R/W) head with respect to tracks of a disk so as to avoid inadvertent writing of data to an incorrect track, said disk bearing a plurality of first, second and third information tracks and servo burst signals, wherein servo signals ("A") of a first set (22A) of signals are respectively situated on a first side of said second track (26C) , servo signals ("B") of a second set (22B) of said servo signals are respectively situated on a second side of said second track, servo signals ("C") of a third set (24A) are respectively disposed to overlap said second track, and servo signals ( "D" ) of a fourth set (24B) are respectively disposed to overlap said first and third tracks (26B, 26D) , whereby, when an (A-B) servo signal read via said head is equal to zero, said head is assumed to be approximately centered over said second track and, when a (C-D) servo signal read via said head is equal to zero, said head is assumed to be approximately halfway between said first and second or second and third tracks, and the disk drive comprising means (14A, 32) for generating a position error signal (PES) indicative of the error, if any, in the position of the head, the claimed servo system comprising: a controller (10) ; and a sign comparator (42) ; wherein said controller is programmed to perform a calibration sequence to determine a value of an offset signal (PESOFFSET) corresponding to a 1/4 track head position error evident in said PES, the sum of PES and

PESOFFSET comprising a composite position error signal (CPES) , and in subsequent track following operations to use this known relationship between PES and PESOFFSET to accurately control and monitor the position of the R/W head with respect to a given data track, and wherein said sign comparator is coupled to said controller and provides a signal to said controller indicative of the sign of said CPES.

2. A disk drive as recited in claim 1, wherein said CPES is formed as a sum of PESOFFSET and PES, and has an amplitude related to the error in the position of said R/W head.

3. A disk drive as recited in claim 2, wherein said controller is programmed to generate said PES signal by alternately obtaining samples of said (A-B) signals and said (C-D) signals and then to cause track following to occur at the 1/4 track position during a calibration procedure by use of said alternating samples.

4. A disk drive as recited in claim 3, wherein said CPES comprises a square wave component having an amplitude that alternates between a positive value and a negative value during a calibration procedure.

5. A disk drive as recited in claim 4, wherein said controller is programmed to alternately obtain samples of said (A-B) signals and said (C-D) signals, and to determine a first sum of outputs of said sign comparator corresponding to said (A-B) samples and a second sum of outputs of said sign comparator corresponding to said (C-D) samples .

6. A disk drive as recited in claim 5, wherein said controller is further programmed to determine a value of said offset signal (PESOFFSET) that will cause said first and second sums to be equal .

7. A disk drive as recited in claim 5, wherein said controller is further programmed to determine a value of said offset signal (PESOFFSET) that will cause said first and second sums to be zero.

8. A disk drive as recited in claim 6 or claim 7, wherein said means for generating PES comprises a demodulator (32) that outputs PES as a function of a read/write head position error (Perr) , and wherein said controller is further programmed to determine a slope (K_DEM0D) of said PES.

9. A servo method for use in a disk drive in controlling the position of a read/write (R/W) head with respect to tracks of a disk, said disk bearing first, second and third information tracks and servo burst signals, wherein servo signals of a first set (A) of signals are respectively situated on a first side of said second track (26C) , servo signals of a second set (B) of said servo signals are respectively situated on a second side of said second track, servo signals of a third set (C) are respectively disposed to overlap said second track, and servo signals of a fourth set (D) are respectively disposed to overlap said first and third tracks (26B, 26D) , whereby, when an (A-B) servo signal read via said head is equal to zero, said head is assumed to be approximately centered over said second track and, when a (C-D) servo signal read via said head is equal to zero, said head is assumed to be approximately halfway between said first and second or second and third tracks, and the disk drive including a demodulator to generate a position error signal (PES) indicative of the error, if any, in the position of the head, the claimed method comprising: performing an initial demodulator calibration procedure whereby a demodulator gain or slope (K_DEM0D) is determined, said calibration procedure comprising: (1) alternately obtaining, via said R/W head, samples of said (A-B) and (C-D) signals;

(2) generating a position error signal (PES), wherein PES is a function of a R/W head position error (Perr) and includes a square wave component having an amplitude that alternates between a positive value and a negative value that corresponds to a 1/4 track head position error; (3) forming a composite position error signal

(CPES) by summing said PES with an offset signal (PESOFFSET) , wherein PESOFFSET has a square wave component with an amplitude that is adjustable, and a phase that is in opposition to the square wave component in

PES;

(4) causing track following to occur at either the -1/4 track position or the +1/4 track position by use of alternating (A-B) and (C- D) samples;

(5) monitoring the sign of CPES and determining a first sum of outputs of said sign comparator corresponding to said (A-B) samples and a second sum of outputs of said sign comparator corresponding to said (C-D) samples; and

(6) determining the amplitude of the square wave component of PESOFFSET that will cause said first and second sums to be substantially equal ; and subsequently, during normal track following operations, using this known relationship between PES and PESOFFSET to accurately control and monitor the position of the R/W head with respect to a given data track; whereby the position of said read/write head with respect to said tracks is controlled so as to avoid inadvertent over-writing of data on a neighboring track.

10. A method as recited in claim 9, wherein PES has an amplitude related to the error in the position of said R/W head.

11. A method as recited in claim 9, further comprising determining a value of PESOFFSET that will cause said first and second sums to be zero.

12. A method as recited in claim 9, further comprising determining a slope (K_DEM0D) of said PES.