This disclosure relates to disk storage systems. Disk storage systems typically contain a rotating disk on which information (usually in the form of digital data) can be recorded (written) and/or from which recorded information can be retrieved (read).
In systems of the type mentioned above, the information is generally recorded in a plurality of concentric circular paths or tracks on the disk. The head that writes data to and/or reads data from the disk must follow particular ones of these tracks in order to write data to or read data from the disk. (To simplify the further discussion herein, it will generally be assumed that the disk is already recorded with information and that the above-mentioned head is a read head via which information is read from the disk. It will be understood, however, that this disclosure is also applicable to writing information to a disk, and that the head can include write as well as read capabilities. Another assumption that will generally be made to simplify the following discussion is that the disk is a magnetic disk that records information based on how various regions (areas) of the disk are magnetically polarized. Again, however, it will be understood that the disk can alternatively record information in other ways such as by optically detectable means.)
In order to read desired information from the disk, the read head must be properly located over the track containing that desired information. To facilitate such read head positioning, the disk is also recorded with several radially extending and angularly spaced wedges of information that contain track-identifying information, and also information that can be used to control the read-head-positioning mechanism to optimally center the read head over the desired track, especially in the direction that is radial of the disk.
To save space on the disk for user data, only some of the above-mentioned wedges may be so-called full servo wedges. A full servo wedge includes a full complement of information such as track and sector identifying information for each track, as well as so-called servo information for helping the head-control circuitry to radially position the head over the desired track. As a space-saving measure, such full servo wedges may be angularly interspersed (or inter-digitated) with so-called short servo wedges. A short servo wedge may contain only servo information for helping to ensure that the read head remains radially centered over the track it is already currently reading data from.
This disclosure relates to improving the performance of disk storage systems that employ short servo wedges.
FIG. 1 shows a representative portion of a known arrangement 100 of information recorded on a memory disk 20 in a disk storage system. Only a representative portion of disk 20 is shown in FIG. 1. Also, the actual, somewhat arcuate shape of recorded information arrangement 100 has been made straight (i.e., from left to right across FIG. 1) to simplify the depiction and the following discussion. Directions that are radial of disk 20 are vertical as viewed in FIG. 1. Planar rotation of disk 20 causes information 100 to move from right to left (which is the circumferential or angular direction of disk 20). Read head 10 (which is part of the disk storage system containing disk 20) is able to move in a direction that is radial of disk 20 (i.e., up or down in FIG. 1), but can be assumed for present purposes not to move in directions that are circumferential or angular of disk 20. A servo mechanism (not shown in FIG. 1) controls the radial position of read head 10, based at least in part on information read from disk 20. The recorded information arrangement 100 shown in FIG. 1 has the so-called null servo position error signal format. (Position error signal is frequently abbreviated PES.) Other formats for information on disks in disk storage systems are also known, but the present disclosure is especially useful in connection with the null servo PES format.
FIG. 1 shows portions of three, representative, radially adjacent, recorded information tracks on disk 20. These tracks are arbitrarily referenced data tracks R, S, and T, respectively. Note again that the representative portions of tracks R, S, and T shown in FIG. 1 and other FIGS. throughout this disclosure would in reality actually be somewhat arcuate (e.g., concave up or concave down as viewed in FIG. 1) because the track portions shown are radially adjacent segments of longer, concentric circular tracks on the disk. FIG. 1 shows read head 10 positioned over track S and therefore reading information along the axis indicated by arrow 12 as that information moves from right to left under read head 10. It will be appreciated that arrow 12 in FIG. 1 does not indicate motion of read head 10, but rather the direction of information reading as rotating disk 20 travels under read head 10. The dimension of the effective portion of read head 10 that is radial of disk 20 is preferably no greater than the radial dimension (width) of a track, so that when read head 10 is properly centered over a track, read head 10 is only influenced by the information recorded in that track and not by information recorded in the radially adjacent tracks.
FIG. 1 also shows the information in a representative portion of one full null servo PES wedge 110 and a representative portion of one short null servo PES wedge 300. Data tracks R, S, and T are in a data wedge 200 between full null servo PES wedge 110 and short null servo PES wedge 300. This pattern may continue in a closed loop series all the way around disk 20. For example, this closed loop series may comprise a first full null servo PES wedge, a first data wedge, a first short null servo PES wedge, a second data wedge, a second full null servo PES wedge, a third data wedge, a second short null servo PES wedge, a fourth data wedge, and then back to the first full null servo PES wedge. Alternatively, a pattern like the one just described may be longer before it gets back to the first full null servo PES wedge. As still another alternative, a pattern like the one described may have more than one short null servo PES wedge, each of which is followed by a data wedge, before the next full null servo PES wedge is encountered.
FIG. 1 shows that each full null servo PES wedge 110 may include (in the order read by read head 10) a preamble area 120, a servo sync mark (“SSM”) area 130, a track/sector identification (“ID”) area 140, a PES area 150, and a repeatable run-out (“RRO”) area 160. Preamble area 120 and PES area 150 are of special importance for present purposes and will be described in more detail below. FIG. 1 further shows that each short null servo PES wedge 300 includes a PES area 150′ similar to PES area 150.
Preamble area 120 may include bands of disk 20 material having alternating magnetic polarity that extend radially of the disk, continuously across all of the tracks on the disk. FIG. 2 includes a depiction of a representative portion of such a preamble area 120. Thus, as shown in FIG. 2, each band 122+ extends radially across all of tracks R, S, T, etc., and has “positive” magnetic polarity. Interspersed or inter-digitated between bands 122+ are bands 122−, which are similar to bands 122+ but which have magnetic polarity (e.g., “negative” magnetic polarity) that is opposite to the magnetic polarity of bands 122+. As read head 10 passes over the successive bands 122 of preamble area 120, the output signal of read head 10 oscillates in response to the different polarities of the successive bands 122. The frequency and phase of a sampling clock signal that is used for subsequently sampling the read head 10 output signal (as will later be described in this specification) are locked to the frequency and phase of the read head 10 output signal as read head 10 passes over preamble area 120. Note that because bands 122 are radially continuous across all of tracks R, S, T, etc., this frequency and phase locking is equally effective regardless of how well read head 10 is currently aligned in the radial direction with any particular track R, S, T, etc.
Servo sync mark (“SSM”) area 130 contains a special pattern of recorded information that the circuitry receiving the output signal of read head 10 can recognize; and that when recognized, tells the circuitry when to expect read head 10 to be subsequently reading track/sector ID information 140, PES information 150, RRO information 160, data 200, etc.
Once the frequency and phase of the above-mentioned sampling clock signal have been locked to the preamble area 120 information, the PES area 150 that is part of the same full null servo PES wedge 110 can be used to provide information for ensuring that read head 10 is centered (in the radial direction) over a particular track like any of tracks R, S, T, etc. FIG. 1 shows that in PES area 150 a PES A+ subarea is aligned with track R, a PES A− subarea is aligned with track S, and a PES A+ subarea is aligned with track T. This pattern of alternating PES A+ and PES A− subareas continues for tracks above and below representative tracks R, S, and T.
Downstream from the PES A area, PES area 150 includes a PES B+ subarea that straddles (i.e., is equally distributed across) radially adjacent, radial halves of each of tracks R and S. A PES B− subarea similarly straddles tracks S and T. Another PES B+ subarea similarly straddles track T and the next track down. This pattern of alternating PES B+ and PES B− subareas continues above and below what is representatively depicted in FIG. 1.
FIG. 2 also shows the pattern of information that may be recorded on the disk in PES area 150. For example, a PES A+ subarea may begin (on the left) with a band 152+ having positive magnetic polarity, followed by a band 152− of negative magnetic polarity, which is followed in turn by another band 152+ having positive magnetic polarity, and so on. On the other hand, a PES A− subarea may start (on the left) with a band 152 of negative magnetic polarity, followed by positive polarity band 152+, then a negative polarity band 152−, and so on. A PES B+ subarea may be similar to a PES A+ subarea, except, of course, that it exactly straddles two radially adjacent tracks. A PES B− subarea may be similar to a PES A− subarea, except that it exactly straddles two radially adjacent tracks.
If read head 10 is exactly centered (in the radial direction) over a track, the signal from read head 10 as it passes over the PES A region of full null servo PES wedge 110 will be an oscillating signal of maximum strength (amplitude) corresponding to either the PES A+ or the PES A− pattern, depending on whether the track being read is aligned with a PES A+ or a PES A− subarea. After passing over the PES A area, a read head 10 that is exactly centered over a track as mentioned in the preceding sentence will produce an output signal having essentially zero amplitude as it passes over the PES B area. This is so because in the PES B area, the read head 10 in this situation is always exactly straddling 152+ and 152− subareas. The effects of such exactly straddled subareas 152+ and 152− on read head 10 substantially cancel one another.
If instead of being radially centered over a track as read head 10 passes over the PES A and PES B areas, read head 10 is somewhat radially offset from the center of a track, the amplitude of the read head 10 output signal will be correspondingly diminished as it passes over the PES A area. This read head 10 output signal amplitude reduction will be due to some contribution from each of two radially adjacent PES A areas, one of which will be a PES A+ subarea and the other of which will be a PES A− subarea. The amount of this read head 10 output signal amplitude reduction can be used as a measure of how far from the center of a track read head 10 is.
Such a non-centered read head 10 will also have an output signal of non-zero amplitude as it continues over the PES B area of full null servo PES wedge 110. This non-zero amplitude will be due to the fact that read head 10 no longer exactly straddles radially adjacent PES B+ and PES B− subareas, but rather is being more strongly influenced by one or the other of such subareas. The phase of this non-zero amplitude read head 10 output signal (relative to the phase of the immediately preceding preamble area 120) can be used to indicate whether read head 10 needs to be moved radially in or radially out in order to become radially centered over the track it is trying to read. For example, the phase of the above-mentioned non-zero amplitude signal (relative to the preamble area 120 phase) may be 0° if a PES B+ subarea is the predominant influence on read head 10. Alternatively, the phase of the above-mentioned non-zero amplitude signal (relative to the preamble area 120 phase) may be 180° if a PES B− subarea is the predominant influence on read head 10. Of course, these are only examples, and the phase relationship between the preamble and PES B areas depends on such factors as the angular spacing between these areas, the patterns in which information is recorded in these areas, etc.
Thus, to briefly summarize the foregoing discussion, the information gathered from a read head 10 passing over a full null servo PES wedge 110 can be used to determine whether read head 10 is properly centered over the track it is trying to read (PES A area read head 10 output signal at full amplitude and PES B area read head 10 output signal at or near zero amplitude), or is in need of some radial adjustment in order to become radially centered over the track. In the latter case (i.e., radial adjustment of read head 10 needed), the amount of that radial adjustment can be derived from the amount by which the PES A read head 10 output signal amplitude is reduced from full amplitude; and the direction of the needed radial adjustment can be derived from the phase of the PES B output signal (relative to the phase of the immediately preceding preamble area 120).
The foregoing operations are highly effective within a full null servo PES wedge like 110. As mentioned earlier in this disclosure, however, to conserve space on a disk, short null servo PES wedges like 300 may be interspersed with full null servo PES wedges like 110. The recorded information in each known short null servo PES wedge 300 may be another instance 150′ of what is shown at 150 in FIGS. 1 and 2. In particular, each known short null servo PES wedge 300 typically includes only PES A and PES B areas, and does not include other information such as is shown in areas 120, 130, 140, and 160 in FIG. 1 or in area 120 in FIG. 2. Because a short null servo PES wedge 300 is angularly relatively far from the preceding full null servo PES wedge 110, the phase information for the PES B area of a short null servo PES wedge 300 relative to the phase of the preamble area 120 of the preceding full null servo PES wedge 110 can be less reliable than the relative phase information for the preamble and PES B areas within a full null servo PES wedge like 110. Also, because of the relatively great angular distance between a full null servo PES wedge 110 and the next short null servo PES wedge 300, when read head 10 reaches the wedge 300, it can be radially off the center of the track it is trying to read by more than half the radial width of a track. Considerations such as these can make use of known recorded information arrangements such as are illustrated by FIG. 1 (i.e., recorded information arrangements 100 in which short null servo PES wedges like 300 are interspersed with full null servo PES wedges like 110) less reliable than would be desirable in dealing with information read from a short null servo PES wedge 300. The present disclosure substantially eliminates this problem.
The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the inventor hereof, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
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