A SERVO CONTROL SYSTEM FOR CENTRING A TRANSDUCER HEAD ON A DATA TRACK ON A MAGNETIC RECORD

Abstract: A servo control system (Figure 3) for data track following of the read/write head in a magnetic recording system uses a pattern of seg­ ments (51,52,53, 61,62,63, Figure 1) of servo information slanted across the data tracks. The time of flight of the servo segment from occur­ rence of a reference pulse (40) until the peak servo signal is read by the head is an indication of the position of the head in a direction perpendicular to the desired track. This time is compared with the known time of flight if the head were precisely over the centreline of the desired track and the time difference is used to generate a control signal to position the head over the track centreline. In an embodiment where each segment is slanted across only one track, the reference pulse is the beginning of the servo signal and the time measurement is the number of fixed-frequency magnetic transitions read by the head in the servo segment. When the system is used in flexible disk drives, the servo segments are written in one of the data sectors on the flexible disk by the user's disk drives.

Description: This invention relates to a servo control system for centring a transducer head on a data track on a magnetic record. In order to substantially increase the track density in magnetic record disk files (or 'drives'), it is necessary to incorporate a servo control system to centre and maintain the transducer head over the tracks during read or write operations. The movement of the head to a desired track is referred to as track accessing or 'seeking', while the maintaining of the head over the centre line of the desired track during read or write operations is referred to as track 'following'. While the need for a servo control system for track following exists in any disk file with a relatively high track density, it is especially critical in the case of flexible disks because such disks are subject to non-uniform distortion due to temperature and humidity changes, spindle motor runout and other effects. There are several known servo control techniques which utilise the 'time of flight' of various servo patterns past the read/write head to indicate the position of the head relative to the centreline of the desired track. In one such technique, as disclosed in US-A-3 812 533, the servo signals for a group of data tracks are angularly staggered so that the time between a reference pulse and a servo signal in the desired track identifies that track within the group of tracks. This technique is used on disk files using a rigid disk which incorporates prerecorded servo signals on equally angularly spaced sectors which extend out radially from the disk centre. As the disk rotates, the head receives sampled track position signals as the servo sectors pass beneath the head. While the use of pre-recorded servo sector signals and the technique described in US-A-3 812 533 to utilise those servo sector signals are applicable to rigid disk files, they are not generally suitable for use with flexible disk drives because flexible disks are generally available only in unrecorded form. Any recording of servo information in angularly spaced sectors would significantly increase the cost of the flexible disks. Furthermore, the time required to record servo information in multiple sectors on each data track would be prohibitive if recorded by the user's disk drive. US-A-4 149 198 discloses a disk file head positioning system which utilises two dedicated servo tracks and associated dedicated positioning servo heads radially spaced on opposite sides of the read/write head. The servo heads and the information recorded in the servo tracks are both slanted relative to the tracks. The time of arrival of pulse trains caused by the two servo heads reading servo information on their respective servo tracks are combined to generate a signal indicative of the position of the read/write head relative to the data tracks. An article entitled 'Servo System for Magnetic Recording Based on Time Comparison' by E G Gruss et al, IBM Technical Disclosure Bulletin, volume 23, Number 2 (July 1980), pages 787 to 789, discloses the use of intersecting servo markings slanted relative to the data tracks. A comparison of the times of intersection of the head with the servo markings indicates the position of the head relative to the track centrelines. US-A-4 346 413, US-A-4 454 549 and US-A-4 488 187 all disclose servo patterns slanted relative to the tracks and various means, other than time of flight measurement, to determine the position of the head relative to the track centrelines. EP-A-97768 discloses the use of a servo pattern comprising a group of servo segments slanted relative to the data tracks in which each segment consists of magnetic transitions which increase in frequency in the radial direction. The position of the head relative to the centreline of the desired data track is determined not by the time of flight of the servo segment, but by the number of magnetic transitions actually read by the head. The present invention provides a servo control system for centring a transducer head on a data track on a magnetic record in which each segment of a group of recorded segments of servo information extends in a slanting manner across at least one of a plurality of data tracks, each segment comprising a plurality of magnetic transitions progressively staggered in a radial direction with respect to the data tracks, the segments within a group being spaced from each other in a direction substantially perpendicular to the data tracks, the system comprising detection means for detecting a peak amplitude of the envelope of the signal produced by the head as it passes over such a servo information segment, time measuring means to measure the duration of the time from a reference time to the detection of said peak amplitude and means to position the head in accordance with the duration of the measured time. While each servo segment may be slanted across more than one data track, in an embodiment in which each servo segment spans only one data track the reference pulse is the servo signal above a predetermined threshold. In this embodiment the measured time is essentially the time from the beginning of tile servo signal to the peak amplitude of the servo signal. In an embodiment especially applicable to flexible disk drives, the slanted servo segments each comprise magnetic transitions of a single frequency and are written on the flexible disk by the user's disk drive prior to the recording of data. How the invention can be carried out will now be described by way of example, with reference to the accompanying drawings in which: Figure 1 is an illustration of the pattern of slanted servo segments on a portion of a magnetic recording disk; Figure 2 is an illustration of one slanted servo segment showing the head at various radial positions within the segment and a time scale showing the reference pulse and the times of peak servo signal for the head at these radial positions; Figure 3 is a block diagram of a servo control system embodying the invention, showing the means for measuring the time from a reference pulse to the peak amplitude of the signal from a slanted servo segment; Figure 4A shows a slanted servo segment covering only one data track and five positions of the head within the data track; and Figure 4B is an illustration of the servo signals read by the head at the five radial positions shown in Figure 4A. Referring first to Figure 1, a pattern of servo information used with one embodiment of the present invention, is illustrated on a portion of a data disk in which, for convenience in explanation, portions of the concentric circular data tracks are represented as straight lines. The portion of the disk shown in figure 1 has an outside perimeter 20, and a plurality of data tracks, represented by data track centrelines 1 through 12. The last data track (not shown) is located close to the inside perimeter of the disk. Also shown in Figure 1 is a representation of a read/write head 30 shown centred on track 6. The servo information recorded on the disk in Figure 1 is identified as representative segments 51, 52 and 53 which are located within band 50, and representative segments 61, 62 and 63 which are located within an adjacent angularly spaced band 60. Bands 50 and 60 occur within a specific time gate 66 during rotation of the disk. In the case of a flexible disk, there are typically 8 or 9 data sectors angularly spaced around the disk and formatted on the blank disk by the disk drive before the disk is used. With flexible disk drives, the servo segments 51 etc (and time gate 66) preferably occur in only one of the data sectors. Each of the segments of servo information is slanted relative to the tracks and, in this example, spans the centrelines of two adjacent tracks. For example, segment 51 in band 50 provides servo information for positioning the head at the centreline of either track 2 or 3, and segment 62 in adjacent band 60 provides information for positioning the head at the centreline of either track 4 or 5. In the example of Figure 1, the open-loop tolerance of the head positioning means, which may be a finely calibrated stepper motor, is at least plus or minus one-half track. Thus, if the head 30 were commanded to move to track 2 the stepper motor has sufficient accuracy to position the head within one-half track of the track 2 centreline. The information contained in each of the servo segments consists of magnetic transitions of a single frequency, which is the same for each track. The method of writing a typical servo segment, such as segment 52, will now be explained. When the disk is rotating at its operating speed, current pulses of a constant frequency are applied to head 30. Simultaneously, the disk file stepper motor (not shown) moves the head 30 in a radially inwardly direction with a constant velocity when the sector of the disk which is to contain the segments of servo information is beneath the head, i.e at the beginning of time gate 66. The head 30 thus writes the servo segment 52 on the disk. With each occurrence of time gate 66, i.e with each rotation of the disk, a new servo segment in band 50 is written in this manner. When all of the servo segments in band 50 are written on the disk, the servo segments in band 60 are written in a similar manner, the difference being that the writing of the segments in band 60 occurs after a short time delay within time gate 66 to permit band 60 to be angularly spaced from band 50. The frequency of the signal can be selected to be between the 1F and 2F operating frequencies of the disk file, which in the case of one type of conventional flexible disk drive corresponds to a 1F frequency for 'all zeros' of 125 Khz and a 2F frequency for 'all ones' of 250 Khz. While in this method of writing the servo segments only one servo segment is written per revolution of the disk, if the bands 50 and 60 are separated by a short time delay then two servo segments, for example segments 51 and 62, can be written in one pass of the servo sector past head 30. In addition, the servo segments could be divided into more than two bands within the servo sector. Also depicted in Figure 1 is a reference pulse 40 on the same time scale as time gate 66. The reference pulse 40 is used to start a counter which measures the time from occurrence of the reference pulse to the peak amplitude of the servo signal read by head 30. In the case of a flexible disk drive, the reference pulse may be the index pulse generated by rotation of the spindle motor. Alternatively, the beginning of the data tracks on the disk may contain a signal which is read by the head to generate the reference pulse. The function of reference pulse 40 may be better understood by reference to Figure 2. As shown in Figure 2, a typical servo segment 52 is shown slanted across the centrelines of tracks 5, 6, 7 and 8. The servo segment 52 is used only when centring the head over either track 6 or track 7, since it is assumed for purposes of this explanation that the open-loop tolerance of the head positioning means, e.g a stepper motor, is one-half track. Thus the maximum excursion of head 30 radially within segment 52 would be midway between tracks 5 and 6 and midway between tracks 7 and 8, as shown by positions 1 min and 1 max , respectively. The time scale of Figure 2 shows occurrence of the reference pulse at t ref and occurrence of the peak amplitude of the servo signal for the position of head 30 at the minimum and maximum radial locations within segment 52 as t min and t max , respectively. The time occurrences t min and t max are shown at the peak of signal envelopes 55 and 57 which are representative of the output of the read/write head amplifier in the manner explained below. Referring now to figure 3, a block diagram depicts the circuitry required to correctly position the head 30 to the centreline of the desired track in response to the measured time of the peak servo signal from reference pulse 40. A disk file incorporating a servo control system embodying this invention includes a microprocessor 70, a suitable memory device such as an electrically erasable programmable read only memory (EEPROM) 72, a timer or counter 74, stepper motor 76, read/write head 30, readback signal amplifier 78, rectifier 80, filter 82, and differentiator 84. Also depicted in Figure 3 above amplifier 78, rectifier 80, filter 82 and differentiator 84 are signal waveforms representative of the outputs of each of those respective circuit elements. The microprocessor 70 is connected by suitable address and bus lines to EEPROM 72 and receives an input from counter 74. Other inputs to microprocessor 70 are the reference pulse 40 (which in the example of a flexible disk drive is the index pulse from the spindle motor) and a track command signal from the data controller (not shown) which is an indication of the desired data track. The operation of the servo control system can be understood by considering the function of the above-described components during operation with reference to Figures 2 and 3. For purposes of this explanation, it will be assumed that it is desired to maintain head 30 over the centreline of track 6. The data controller first signals microprocessor 70 with the identification of the desired track, namely track 6. The microprocessor 70, based upon the known present track location of head 30, then commands the stepper motor 76 to step a predetermined number of microsteps, based upon the calibration of stepper motor 76, in order to reach track 6 centreline. Typically this track seeking is performed open-loop, i.e without any servo feedback. After the stepping has been completed and head 30 is at track 6 within the half-track tolerance the index pulse is received by microprocessor 70 as the disk rotates. Upon receipt of the index pulse, microprocessor 70 then starts counter 74 to begin the measurement of elapsed time. When servo segment 52 passes head 30, the signal read by head 30 is amplified by amplifier 78 which produces an output waveform similar to that shown in Figure 3 above amplifier 78. The amplified signal is then rectified by rectifier 80 and filtered by filter 82. The signal is next differentiated by differentiator 84 which produces the digital pulse shape shown in Figure 3 above differentiator 84. The step change of the differentiated pulse from positive to negative occurs at the peak signal amplitude read by head 30 from the passing servo segment 52 and is output to counter 74 to stop the measurement of elapsed time. For purposes of this example, let it be assumed that the head 30 has been positioned by stepper motor 76 to the actual radial position 1 a , as shown in Figure 2. In that case the peak readback signal will occur at time t a , as shown in Figure 2. The desired time, t d , corresponds to the time of occurrence of the peak signal if head 30 were precisely over the centreline of track 6. The microprocessor 70 then reads the value stored in counter 74, which corresponds to the time t a , and computes the difference t a -t d , which corresponds to a value of head position error. Microprocessor 70 then addresses a lookup table in EEPROM 72 to determine the number of microsteps needed to generate a control signal to cause stepper motor 76 to centre the head 30 over the centreline of track 6. The lookup table stored in EEPROM 72 is merely a compilation of microstep values, each microstep value corresponding to a particular value of time difference, t a -t d . In the preferred embodiment there is only one sector of servo information on the disk so that the above-described time of flight track centring procedure occurs once per revolution of the disk. The procedure is repeated until the head is centred over track 6, which typically occurs within one disk revolution. An example is illustrated in Figures 4A and 4B in which each slanted servo segment spans only one track. Such a servo segment is shown with five different radial positions of head 30 designated 'A' through 'E'. The difference between the single track servo segment of Figure 4A and that of a servo segment which spans more than one track, such as the segment 52 in Figure 2, is that with the single track segment there is only one possible radial position for head 30 where a completely symmetrical readback signal from amplifier 78 can occur. Each of the five different radial positions, A through E, will result in the corresponding signal out of filter 82 (Figure 3) as shown in Figure 4B. The single track servo pattern is usable only with disk files which have head actuator open-loop tolerances of less than one-half track. Thus, if head 30 is commanded to the centreline of the track it will be actually positioned somewhere between radial positions A and E. The desired time occurrence of the peak signal amplitude is t c , as shown in Figure 4B, which corresponds to the track centreline. One of the advantages of the single track slanted servo segment is that an external reference pulse is not required. This is because the leading edge of the pulse from differentiator 84 can be used to trigger the start of counter 74. The crossing of the differentiator 84 pulse from positive to negative stops counter 74. The leading edge of the differentiator 84 pulse, i.e. the first signal from the servo segment read by head 30 which is above a predetermined threshold, indicates the time when the servo segment is essentially first aligned with head 30. The use of this time as the reference pulse is not possible with a servo segment which spans more than one track since the signal profile shown at Position C of Figure 4B could occur at more than one radial position in such a segment. With the use of single track servo segments the magnetic transitions recorded within each servo segment can function as the counter, thus eliminating the need for an external timer, such as counter 74. For example, if the head were located at radial position C in Figure 4A the head 30 would read all the transitions in the servo segment above a predetermined amplitude threshold. The amplifier 78 would then provide a separate digital output to a register (not shown) which would continue until the differentiator 84 terminated the input to the register when the peak signal was read. The value in the register would be accessed by microprocessor 70 and used to determine the control signal to the head actuator in the manner previously described. While the invention has been described with specific application to magnetic recording disk files and in particular to flexible disk drives, the basic concept of the invention is applicable to other types of recording systems, such as magnetic tape drives.

Claims: A servo control system for centring a transducer head on a data track on a magnetic record in which each segment of a group of recorded segments (51,52,53, 61,62,63) of servo information extends in a slanting manner across at least one of a plurality of data tracks, each segment comprising a plurality of magnetic transitions progressively staggered in a radial direction with respect to the data tracks, the segments within a group being spaced from each other in a direction substantially perpendicular to the data tracks, the system comprising detection means (84) for detecting a peak amplitude of the envelope of the signal produced by the head as it passes over such a servo information segment, time measuring means (70,74) to measure the duration of the time from a reference time (40) to the detection of said peak amplitude and means (70,76) to position the head in accordance with the duration of the measured time. A system as claimed in claim 1, for use with a magnetic record in which alternate ones of the servo information segments in the direction substantially perpendicular to the data tracks on the record are disposed in first and second bands, the second band being spaced along the data tracks from the first band, the system including means for determining with which of the bands of servo information segments the head is aligned. A system as claimed in claim 1 or claim 2 for use with a magnetic record in which each servo information segment extends over a single individual data track and comprises magnetic transitions of a single frequency, in which the time measuring means comprises means for counting from the reference time the number of magnetic transitions read by the head as it passes over such a servo information segment. A system as claimed in claim 3, including means to provide the reference time by determining when the head produces a signal above a predetermined threshold level as it passes over such a servo information segment. A magnetic record disk drive including a servo control system as claimed in any preceding claim, and a magnetic record disk having a plurality of concentric data tracks and a group of radially spaced recorded servo information segments, each segment extending in a slanting manner across at least one of the data tracks. A disk drive as claimed in claim 5, in which the record disk is flexible and the data tracks are divided into a plurality of data sectors and the servo information segments are recorded in one of the data sectors, the disk drive including a servo control system as claimed in any of claims 1 to 3 and means for generating an index pulse indicative of the beginning of the data tracks on the record disk, the reference time being the time at which such an index pulse is generated. A disk drive as claimed in claim 5 or claim 6, including means for recording the servo information segments on the magnetic record disk by moving the head radially and supplying it with a write signal of a substantially constant frequency.

Inventors: JEN, SHEN NMN, KABELAC, WILLIAM JOHN
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1986
Main classification: G11B5
IPC codes: G11B21, G11B5