Multi-track servo recording system

Abstract: This invention relates to a magnetic recording head assembly 10 and method enabling an end user to record a unique servo control track on a magnetic recording tape. The invented head assembly includes, in addition to a plurality of sets of read/write transducers, a servo write head having an extended transverse width for uniformly recording a servo signal across the width of a servo track section, and a servo erase head having a plurality of alternating erasing and non-erasing sections adapted to controllably erase portions of the previously recorded servo signal, resulting in unique servo control tracks which are subsequently read in a servo system to control the transverse position of read/write head assembly.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to servo systems for controlling the transverse position of a plurality of read/write transducers for multi-channel recordinq and playback of data and to heads for recording the servo tracks on media. 2. Description of the Prior Art It is very common to provide magnetic recording head assemblies having a plurality of recording/playback transducers positioned transverse to the intended path of a magnetic recording medium for writing data on and reading data from parallel tracks on the medium. It is also well known to include servo information on at least some of the tracks to enable control of the lateral position of the head assembly, thereby dynamically maintaining the respective transducers of the assembly centered on a respective track. The track widths can then be made significantly narrower, and the areal density of recording similarly increased. A number of techniques for providing the servo information-containing tracks have been previously employed, each technique having certain attributes and certain deficiencies. Thus, for example, it is known to provide dedicated servo tracks on the medium at the time of manufacture. Such a technique, of course, prevents an end user from writing the servo information on an unformatted medium, and may result in unacceptable inaccuracies depending upon the variability of transducer spacings in various drives. Accordingly, it is desirable to enable an end user to write the servo pattern on the medium while it is in the recorder/drive. An end user may then preformat, i.e., add the servo information to either a blank medium or utilize a medium which was either intentionally or unintentionally erased. Previously-known methods available to the user have included the use of a dedicated drive for servo recording only. Such a method is generally unacceptable due to the cost and inconvenience, particularly to users of small systems where only one drive can be afforded. Other methods require the drive to include a separate auxiliary head, which adds complexity to the drive, is a significant additional cost, and cannot probably be done within typical space envelopes of most drives. One technique for providing the servo tracks by way of a separate, auxiliary head is disclosed in U.S. Patent No. 4,979,051 (Eggebeen) of which WO-A-8909466 is a family member. The head assembly there depicted is primarily intended for use with multi-layer magnetic recording tape in which the servo information is buried, i.e., recorded on an underlying, high-coercive force layer, while actual user data is recorded directly thereover in an upper layer having a lower coercive force. The head assembly is shown to include two components: a first servo write transducer having a write face which extends the full width of the magnetic recording tape with which it is to be used, and a second servo write transducer which is spaced from the first and has a write face of the same overall width, but which contains a plurality of alternating contact and non- contact portions. A second servo signal is to be applied to the contact portions so as to continuously write that signal over the first servo signal. FR-A-2,428,888 teaches using a single head both for writing servo track information, and for erasing data. Both the servo writing and erasing functions are performed through the use of continuously applied frequencies, although the frequencies may change from track to track to distinguish between tracks. Use of discontinuous servo blocks is taught in The Complete Handbook of Magnetic Recording, Finn Jorgenson, 3rd Edition, 1988, Tabs Books Inc., at pages 388-392. In that system, servo blocks are written at periodic intervals part way across data tracks around a diskette. As the read head moves along a data track, it intercepts the servo blocks, allowing the positioning mechanism to recalibrate. However, this has the disadvantage that it interrupts the read/write process and only periodically recalibrates. With ever increasing track densities, discontinuous servoing of this type is increasingly inadequate. SUMMARY OF THE INVENTION According to this invention there is provided a system for writing a plurality of tracks of dedicated servo information as claimed in claim 1 herein. In contrast to any of the techniques previously employed, and, in particular, to avoid the need for a buried servo approach as envisioned in the '051 patent acknowledged above, the present invention is directed to a system for writing at least one track of dedicated servo information within a servo section extending along a magnetic recording medium, thereby enabling immediate writing and reading of data on at least one data track in a plurality of adjacent data sections under control of servo information read back from the servo track. This system comprises a head assembly including a servo write transducer and a servo erase transducer. The servo write transducer has a write face with a first predetermined width (W1), while the servo erase transducer is longitudinally displaced from the servo write transducer, and has an erase face which includes a plurality of alternating erasing and non-erasing portions, the total width of said erase face substantially corresponding to the first predetermined width (W1), and in which each of the erasing portions has a second predetermined width (W2) separated from an adjacent erasing portion a distance equal to said second predetermined width (W2). The system further comprises means for applying a servo signal to the servo write transducer, so as to write the servo signal on the medium, extending substantially continuously over the designated servo section. The width of the servo section thereby substantially corresponds in width to the first predetermined width. Means are also included for applying periodic bursts of an erase signal to the alternating erasing portions of the servo erase transducer to periodically erase a portion of the servo signal along at least one zone extending along said servo section. This produces distinguishable transitions between the periodically erased zones and adjacent continuously written zones, which transitions define at least one servo track extending along said servo section. In a preferred embodiment of this system, the outer edges of the two outermost erasing portions are inwardly offset from the edges of the servo section one-half of the second predetermined width, such that the resultant distinguishable transitions defining the center lines of each said servo track are integral multiples of the first predetermined width from a corresponding data track within each of the data sections. BRIEF DESCRIPTION OF THE DRAWING Figure 1 is an isometric view of a magnetic head assembly of the system of the present invention; Figure 2 is an isometric view of the head assembly of Figure 1, showing the placement of only those transducers used to record the servo tracks within a designated servo section; Figure 3 is an isometric view of the head assembly of Figure 1, showing the placement of only those transducers used for reading and writing data on data tracks within designated data sections; Figure 4 is a conceptual view of the servo write and erase transducers of the head assembly of Figure 1 and the resultant recorded servo tracks on a magnetic recording medium; Figures 5A, 5B, and 5C are successive views showing the same servo section and six data sections, but in which the head assembly is differently positioned in each view such that one set of data transducers is always in position to read one track of the servo data while the other two sets are positioned to each read or write tracks in the data sections; Figure 6 is a schematic diagram of the servo write subsystem, including the servo write and erase transducers of the head assembly of Figure 1; Figure 7 is a schematic diagram of the servo control system of the present invention showing the closed loop and head assembly positioning assembly used to locate and follow previously written servo tracks; and Figure 8 is a view of an alternative system configuration as it would be recorded on media. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT As noted above, Figure 1 is a partially cut-away isometric view of a head assembly 10 of the system of the present invention. As there is seen, the assembly 10 includes a support base 12 within which are mounted a plurality of transducers. Of particular interest to the present invention is a servo write transducer 14 and a servo erase transducer 16. The servo write transducer 14 has an extended transverse gap width (W1) equal to the width of a desired servo section. Such a section, as is described in more detail hereinafter, may typically contain a plurality of servo tracks, each having a width (W2). In contrast, the servo erase transducer 16, while having a total width equal to that of first transducer 14 (i.e., W1), is divided into sections 18, with each section 18 being equal to the width of one of the desired servo tracks (W2), each of the sections 18 being separated from the adjacent section by a similar width (W2). The head assembly further includes a plurality of read and/or write transducers. These transducers are connected to the support base 12, and, in the preferred embodiment shown in Figure 1, include three sets 20, 28, and 36 of read-write-read transducers, each set being spaced apart from an adjacent set by a distance equal to an integral multiple of the transverse width (W1) of the aforementioned servo section established by the width of the first servo write transducer 14. Thus, the first set 20 includes a centermost located write transducer 22 and, located on either side thereof, read transducers 24 and 26. The first set of heads 20 is positioned in line with the lower edge of the first servo write transducer 14, such that the read transducers 24 and 26 will automatically be positioned to read the servo track at the lower edge of the servo section. Similarly, the second set 28 of read/write transducers 30, 32, and 34, respectively, is positioned in transverse alignment with the first set, but spaced away therefrom a distance equal to the width of the servo section. Accordingly, those transducers will be positioned to read corresponding data tracks within an adjacent section of data tracks. Finally, the third set 36 of read/write transducers 38, 40, and 42, respectively, is located in transverse alignment with the respective transducers of the first and second sets, but is spaced transversely from the second set, a distance equal to two times the width of the servo section. The manner in which such variously-spaced transducers are utilized to read data tracks within respective sections is disclosed in U.S. Pat. No. 5,008,765, and will be discussed in more detail hereinafter. In any event, a magnetic tape 44, passing along a transport path adjacent the head assembly may, in the first instance, pass in the direction of the arrow from right to left, such that a servo track extending the width of the entire servo section is first recorded by the servo write transducer 14 and, is subsequent partially erased by the servo erase transducer 16. The resultant servo tracks may thereafter be played back by the respective read transducers in one of the sets of transducers, while data is being recorded and/or played back by the read/write transducers of the other sets of transducers. The use of two read transducers in each set enables data retrieval in either direction of tape travel. The magnetic gaps of all of the respective transducers, whether they be provided for recording, playing back, or erasing, are in each case orthogonal to the direction-of the travel of the media and in substantially the same plane as the media. As is shown in greater clarity in Figure 2, the servo write transducer 14 can write a pattern having a first predetermined width (W1), which is the width of the respective servo section. Specifically, the servo write transducer 14 is preferably energized with a high-frequency drive signal so as to uniformly record only that signal pattern throughout the width of the servo section. Subsequently, as the medium 44 passes the servo erase transducer 16, the erasing portions 18 erase respective stripes from the previously continuously-recorded signal pattern. These erased stripes correspond to the width (W2) of each of the erasing portions 18. Preferably, the erasure is provided by applying a high frequency signal to the servo erase transducer 16. The frequency of that pattern is much higher than can be recorded and played back and thereby erases the written pattern. Also, the servo write erase transducer 16 is preferably switched on and off at a rate which is a sub-multiple of the servo write frequency, such that effective erasure only occurs while the servo erase transducer 16 is presented with a gated-on signal. The total result of the servo pattern produced by the combined actions of the servo transducers 14 and 16 is a pattern having a substantial checkerboard-like configuration as shown in more detail in Figure 4. It may further be noted in Figure 2 that the alternating erasing and non-erasing portions of the servo erase transducer 16 are transversely situated so as to leave a continuous one-half track width (W2/2) portion of the recorded servo signal along both edges of the servo section. The respective sets of read and write transducers 20, 28, and 36 are shown in more detail in Figure 3. As may there be noted, the respective read and write transducers of each respective set are positioned along a common center line so as to record and read data along a track centered about that center line. The respective write transducers 22, 32, and 40 have a width (W2) slightly wider than that of the respective read transducers 24, 26, 30, 34, 38, and 42. This variation in width is typically done to accommodate transverse shifting of the recording medium without loss in the signal output produced by the read transducers. As discussed in more detail hereinafter with respect to Figure 7, an advantage of the head assembly 10 and the servo mechanism utilized with it is that the width of the read transducers may now be considerably wider than previously, and, for a given minimally-acceptable width, now allow a significantly greater transverse track density, i.e., significantly narrower tracks without loss in the playback signal amplitude. The manner in which the servo tracks are recorded within a servo section 46 of the magnetic recording tape 44 is set forth in more detail in the conceptual view of Figure 4, where the resultant recorded patterns are shown adjacent the associated write and erase transducers 14 and 16, respectively. The action of the servo write transducer 14 produces a continuously-recorded pattern 48 which extends uniformly from one edge to the other edge of the servo section 46. Next, the servo erase transducer 16, being made up of the four subsections 18, periodically erases portions 50 of the previously continuously-recorded signals. As was previously noted, each of the sections 18 are one track wide and are offset from the desired tracks by one-half track width. Thus, within track 1, for example, there is established a center line along which a transition between the continuously recorded patterns and the erased portions exists. A similar transition along a center line exists for track 2, albeit of opposite polarity. The same alternating polarity situation exists for tracks 3 through 8. Referring now to Figures 5A, 5B, and 5C, it may be seen that the magnetic recording tape 44 is preferably configured with seven sections within each of which tracks of servo and/or user-related data may be recorded and played back. Thus, the centermost section 46 may be seen to have recorded thereacross eight tracks of servo data, while on either side there are three sections 52, 54, 56, 58, 60, and 62 within each of which may be recorded and played back eight tracks of user-related data. In Figure 5A, it may be seen that the data transducer set 36 is positioned along the upper edge of the servo section so as to play back the uppermost servo track within that section. Accordingly, the read transducer 38 of that set is used in the forward direction of tape movement to playback the servo track while the opposite read transducer 42 is used to play back the servo data while the tape is moving in the opposite direction. Additionally, during write operations, the output of both read transducers 38 and 42 are averaged to provide a servo signal corrected for azimuthal errors between the tape and head assembly. The set 36 is laterally positioned so as to successively read the full width servo recording followed by the periodic bursts of half-signal intensity due to the erased portions 50, so long as the head assembly is positioned with the appropriate read transducer centered along the center line of a given servo track. The resultant output signal of the read transducers is a burst of signal of full amplitude while over the recorded servo information 48, followed by a burst of half signal amplitude, while over the portion 50 in which half of the signal has been erased. so long as the respective center lines of the read transducers are exactly over a respective edge of the erased portions, one-half of the gap of the playback transducers will see a recorded signal while the lower half will not respond to the erased portions. The amplitude of signal bursts out of the transducers will, thus, be exactly two to one when the transducer is correctly positioned at the center line of each of the respective servo tracks. However, as the medium 44 moves transversely due to undesirable effects of edge slitting tolerances, imperfect positioning of the head assembly, etc., the ratio of amplitude bursts will change, this in turn providing an error signal which may be used to reposition the magnetic head assembly 10 by means of the aforementioned servo mechanism as discussed in more detail hereinafter. As shown in the subsequent Figures 5B and 5C, the head assembly 10 of the system of the present invention may be variously positioned so that either the set of transducers 28 are positioned within the servo section as in Figure 5B, or so that the third set 20 of transducers is positioned within the servo section as shown in Figure 5C. It will be readily apparent that as each of those respective sets of transducers are so positioned, that the remaining sets are appropriately positioned to record and playback data from the data-containing track sections 52 through 62. It may there also be appreciated that if the entire head assembly 10 is stepped down one increment of track width, the data read transducer will read the lower half of an erased portion and the full width recorded signal so as to maintain the head assembly to be centered on track 2 of all of the respective sections. A similar situation as noted will apply to tracks 3 through 8. Figure 6 illustrates schematically the circuit for writing the continuous servo pattern 48 and for erasing the subsequent periodic bursts 50 as shown in Figure 4. As shown in Figure 6, this circuitry 53 includes an oscillator 55 which generates a basic clock frequency of 6 MHz. In response to a write-enable command at terminal 57, a signal is provided through the divider 59 to provide a signal of 400 kHz, which drives a servo write amplifier 61. The output of the amplifier 61 is coupled to the servo write transducer 14 of the magnetic head assembly 10. The output from the oscillator 55 is similarly coupled directly to an AND gate 63, together with a further divided output at 10 kHz from a second divider 64. The resultant output from the AND gate 63 is a gated burst of high-frequency signals which is applied to an erase amplifier 68. The output thereof is applied to the servo erase transducer 16. There are situations, particularly when recording long wavelength information, when the magnetic field recorded in the media extends beyond the width of the write transducer. This phenomena is commonly referred to as fringing and is usually undesirable. Depending on how severe the problem, this fringing effect may interfere with recording on adjacent tracks. The head assembly 10 accounts for such an eventuality. If the variables of wavelength, speed and magnetic field strength cause a fringing problem, the magnetic head assembly 10 can be positioned, for instance as in Figure 5B. In this position, the output of the data read transducers 30 and 34 can be averaged to provide the lateral head assembly positioning servo information while the data write transducer 32 is positioned along the lower edge of the servo section 46 to erase any fringing magnetic flux recorded as a result of an earlier servo writing pass. If the magnetic head assembly 10 is stepped up a distance (W2), it can be seen how using the outputs of the data read transducers 24 and 26 can be averaged to provide servo information to position the data write transducer 32 such that it can effectively erase the fringe fields along the upper edge of the servo section. Figure 7 illustrates schematically the entire closed loop lateral head assembly positioning servo system in the playback mode. With the magnetic head assembly 10 positioned as shown in Figure 5C, the output of the data read transducer 24 (assuming forward motion of the media) is presented to the multiplexer 72 via the preamp 70. It is this signal of alternating bursts of full amplitude and, nominally, one-half amplitude that are selected for servo control. The servo information is then processed through a filter and demodulator 78, analog to digital converter 80, and digital signal processor 82 in order to compute the resultant position error signal. This position error signal is then sent through the driver module 84 to power amplifier 86, causing the lateral head assembly positioning actuator 88 to move the head assembly support structure 90, thereby repositioning the magnetic head assembly 10 such that the appropriate read transducer is centered over the center line of a respective servo track. It should be evident from Figure 7 that the components of the servo mechanism contained in the block 92 are exactly the same as the elements above them and are used when the media is moving in the reverse direction. The details of the servo mechanism themselves are well known to those skilled in the art and are not part of the invention claimed herein. When originally positioning the head assembly transversely for purposes of recording the servo track within the servo band on previously unrecorded media, traditional edge seeking methods, such as those disclosed in U.S. Pat. No. 4,422,111 (Moeller and Wolff) and transverse positioning methods, such as disclosed in U.S. Pat. No. 4,313,143 (Zarr), may be utilized. An alternate configuration for placing the servo information according to the present invention is set forth in Figure 8. As may there be seen, the previously-described servo section 46, within which are positioned eight servo tracks, may be split into two components such that there are two servo subsections 94 and 96, each section containing four servo tracks. Associated with the two servo sections 94 and 96 will then be 12 sections, each containing four data tracks. Other combinations with more or fewer servo tracks, or more than two servo sections within the respective data sections and corresponding to more or fewer data tracks, can similarly be envisioned and are within the scope of the present invention.

Claims: A system for writing a plurality of tracks of dedicated servo information within a servo region on a magnetic recording medium and for writing data on and reading data from a plurality of data tracks in each of a plurality of adjacent data regions under control of the servo information read back from the servo tracks, the system comprising: a multi-channel head assembly (10) having: a support base (12); a servo write transducer (14) connected to the support base (12) and having a write face with a first predetermined width (W1) for writing a servo signal extending substantially uniformly over a designated servo region on the medium, the width of the servo region thereby substantially corresponding in width to said first predetermined width, a servo erase transducer (16) connected to the support base (12) and displaced from the servo write transducer (14) in a direction substantially orthogonal to the width of the servo write transducer (14), the servo erase transducer (16) having an erase face including a plurality of alternating erasing (18) and non-erasing portions, the total width of the erase face substantially corresponding to the first predetermined width (W1), and each erasing portion of the servo erase transducer (16) having a second predetermined width (W2) for enabling at least one of the erasing portions to erase a portion of the written servo signal, thus providing distinguishable transitions between the erased and remaining portions of the servo signal defining at least one servo track extending along the servo region, and a plurality of sets (20, 28, 36) of read/write transducers (22, 24, 26; 30, 32, 34; 38, 40, 42) connected to the support base (12), each set (20, 28, 36) of read/write transducers (22, 24, 26; 30, 32, 34; 38, 40, 42) being separated from an adjacent set (20, 28, 36) by a distance equal to an integral multiple of the first predetermined width (W1) in a direction substantially parallel to the width of the servo write transducer (14), each transducer within each set having a read/write face with a transverse gap width corresponding to the second predetermined width (W2) for writing on and reading from the medium a plurality of data tracks, each of which data tracks substantially corresponding in width to the second predetermined width (W2) and extending over a plurality of data regions having a width equal to the first predetermined width (W1) and abutting each other and the servo region; and movement means for moving the head assembly (10) after the servo signal is written and partially erased to provide the distinguishable transitions, the movement means moving the head assembly (10) relative to the medium to enable one of the read/write transducers (22, 24, 26, 30, 32, 34, 38, 40, 42) to access a given servo track in the servo region while the other read/write transducers (22, 24, 26, 30, 32, 34, 38, 40, 42) access a given data track in the data regions, and subsequently dynamically repositioning the head assembly (10) in response to the distinguishable transitions played back from the given servo track to maintain a predetermined response to the transitions, thereby enabling each of the other read/write transducers (22, 24, 26, 30, 32, 34, 38, 40, 42) to be maintained centered along a respective data track in a corresponding data region. A system according to claim 1, wherein each erasing portion is separated from an adjacent erasing portion a distance equal to the second predetermined width (W2). A system according to claims 1 or 2, wherein circuit means (53) is provided for applying periodic bursts of an erase signal to the alternating erasing portions of the head assembly (10) to periodically erase a portion of the servo signal along at least one zone in the servo region, resulting in distinguishable transitions between the periodically erased zones and adjacent continuously written zones, which transitions define at least one servo track in the servo region. A system according to claim 3, wherein the circuit means comprises: a) a servo signal controller (53) for producing a servo control write signal and a servo control erase signal; b) a servo write amplifier (61) responsive to the servo control write signal for providing a servo transducer write signal to the servo write transducer 14; and c) an erase amplifier (68) responsive to the servo control erase signal for providing a servo transducer erase signal to the servo erase transducer (16). A system according to claim 3, wherein the servo signal controller (53) includes means (55, 59) for applying a signal having a first frequency to the servo write amplifier (61) to cause writing of the first frequency as the servo signal and means (57, 59, 63, 64, 68) for applying a signal having a series of bursts at a second frequency to the servo erase amplifier (68) to cause the periodic erasure of the servo signal, so that discontinuities between the partially erased zones and continuously written zones define the center of each of a plurality of servo control tracks extending across said servo region. A system according to any of claims 1-5, wherein the outermost erasing portions are inset from the respective outer edges of the erase width (W1) by one-half of the second predetermined width (W2), so that the resultant distinguishable transitions written thereby will define the center of each servo track. A system according to any of claims 1-6, wherein the servo write transducer (14) and the servo erase transducer (16) are each divided into at least two subregions, each subregion having a write or erase face, respectively, which is a predetermined width such that the combined widths of all the subregions of each servo transducer is equal to the first predetermined width (W1), thereby providing some of the plurality of servo tracks in each of the subregions. A system according to any of claims 1-7, wherein the movement means further comprises means for deriving an error signal from the ratio of an output signal obtained from a given read transducer (22, 24, 26, 30, 32, 34, 38, 40, 42) of one of the sets (20, 28, 36) of transducers while that transducer is over a continuously written portion of the servo region to an output signal obtained from the given read transducer while that transducer is over a periodically erased portion of the servo region. A system according to any of claims 1-8, wherein the movement means moves the head in steps of a distance equal to an integral multiple of the first predetermined width (W1).

Inventors: DY BENNETH G, MOLSTAD RICHARD W, SCHWARZ THEODORE, YOUNGQUIST ROBERT J, DY, BENNETH G., MOLSTAD, RICHARD W., SCHWARZ, THEODORE, YOUNGQUIST, ROBERT J., Dy, Benneth G., c/o Minnesota Mining and, MOLSTAD, RICHARD W., C/O MINNESOTA MINING AND, Schwarz, Theodore, c/o Minnesota Mining and, YOUNGQUIST, ROBERT J., C/O MINNESOTA MINING AND
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1992
Main classification: G11B5
IPC codes: G11B21, G11B5