Note: Descriptions are shown in the official language in which they were submitted.
~1~96`~13~7
1 BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of magnetic
recordings, and more specifically, to magnetic tape units
employing one or more rotating heads which record and/or
reproduce machine-convertible information while moving in
transducing relationship with a magnetic web media or tape,
this information being orientated as magnetic domains to
form information tracks which extend generally traverse to
the longitudinal tape length.
2. Description of the Prior Art
Rotating head magnetic tape units are widely
known. In one form a generally cylindrical guide, sometimes
referred to as a mandrel or drum, includes a rotating head
wheel which carries one or more magnetic read/write heads.
The magnetic tape engages the guide means, at one point,
makes a helical wrap about at least a portion of said guide
means, and exits the guide means at a point which is both
axially and circumf~rencially spaced from the entrance
point. The angle of helical wrap can vary in accordance
with design choice, but is usually between 180 degrees and
360 degrees. The head wheel rotates so as to sweep its
magnetic head or
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1 heads traversely across the tape. The angle at which the
2 head enters and exits the tape may vary, in accordance with
3 de_ign choice, from slightly less than 90 degrees to a
4 small angle such as 15 degrees.
Another type o device is one wherein the head
6 wheel is associated with a tape guiding structure which
7 bends the tape traversely into an arcuate shape that conforms
8 to the circumferencial shape of the head wheel. In this
9 device the tape travels in a generally straight line past
the head wheel, and is traversely bent by the associated
11 guides as it enters the head wheel area.
12 The present invention finds utility with either
13 aforementioned type of device, and has been found particularly
14 useful with the helical wrap device.
A major problem encountered in the aforementioned
16 device is that of maintaining accurate positional alignment
17 (that is registration) between the path of the head wheel
18 carrying the transducing head or heads and skewed obli~ue
19 data tracks on the media. The skewed condition is particularly
true when a data track is written in one tape unit and later
21 read by another tape unit. The skew or misalignment
22 phenomenom between tapes written and read on different tape
23 transport units stems from the fact that the angle at which
24 the oblique data track is recorded on the writing unit
differs from the angle at which the reading unit accesses
26 ~that is traverses) the recorded track.
27 Another source for the skew problem is media
28 deormity. Generally, the media which is used ~or ~ata
. P
O
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1 recording is flexible and is somewhat sensitive to changes
2 due to temperature, humidity, pressure, time, etc. Although
3 the recording angle of a data track is within a prescribed
4 range at the time of recordation, any changes in the afore-
mentioned parameters (that is temperature, time, pressure,
6 humidity, etc.) will tend to deform the media (that is the
7 media will expand or contract); which results in changing
8 the angle at which the data track was originally recorded.
9 With a change or deviation in the recordation angle, when an
attempt is made to recover the prerecorded data the recording
11 head or heads cannot faithfully trace the deformed data tracks
12 and, as such, the information is lost.
13 As a stop gap measure to the misalignment problem,
14 it is often required that the recording media be stored
under stringent conditions. For example, it is often required
16 that the media be stored in a storage area having prescribed
17 temperature and humidity control. Also, in some situations,
18 the recording media is assigned a useful lifespan at the end
19 of which the data, recorded on the media, has to be trans-
ferred to other recording media or re-recorded on the same
21 media. These requirements impose relatively high maintenance
22 costs on the customer while in some cases do not guarantee
23 faithful and/or accurate retrieval of prerecorded data.
24 Still another means which the prior art adapts to
solve the aforementioned skew or misalignment problem is a
26 static means as opposed to a dynamic means. Generally, in
27 the prior art rotating head device the magnetic media is
~8 guided onto the rotating head via an entry guide and is
- ^ .
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1 guided away from the rotatiny head via an exit guide. In
2 the prior art, skew is corrected by manually adjusting the
3 rotating head and/or manually adjusting entry and/or exit
4 guides. This adjustment effectively changes the angle which
the rotating head accesses (i.e. enters) a recorded track on
6 the recording media and effectuates limited skew adjustment.
7 The problem with this type of skew correcting scheme is that
8 it is either done once in the factory prior to shipment or
g requires the services of a skilled technician to conduct the
1~ adjustment. A more detailed description of a device which
11 uses manual means for correcting skew is disclosed in U.S.
12 Patent 3,697,676, issued to Orville I. Protas.
13 In another form o~ prior art static adjustment,
14 the means ~or shifting the head wheel and/or the tape guides
are automatic. With this type of device, the adjustment is
16 done without external or manual intervention. Although this
17 approach is a significant improvement over the a~orementioned
18 manual prior art skew adjustment, its defect is that there
19 are situations wherein it does not afford the recovery of
skewed data, for example, in situations wherein a data
21 track is so skewed that a bowed trajectory is formed within
22 the intermediate portion of the oblique data track. In
23 this situation the s~ewed data cannot be satis~actoril~r
24 retrieved using the prior art static scheme of adjustment;
since the ad~ustment is made at a time prior to the beginning
26 of the head trace across the selected data track.
27 Probably, one o the main defects with the prior
28 art static skew correction scheme, be it manual or automatic,
e
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1 is that movable guides are more difficult to control and,
2 as a result, add complexity and cost to the recording device.
3 Moreover, the response time (that is the time required for
4 repeated skew adjustment) is significantly longer. This is
S due to the fact that more time is required for shifting the
6 mass of the guides and/or head wheel.
7 SUMMARY
':
8 In accordance with the present teachings, skew
9 error between data tracks recorded on magnetic media and a
magnetic transducer is corrected dynamically by a close
11 loop controlled servo system. More specifically, an offset
12 number is placed into a stop/iock counting means of the
13 close loop control system when the magnetic transducer is
14 not in transducing relationship with the magnetic media
~that is the magnetic transducer is not under the tape).
16 The stop/lock counting means outputs a series of pulses as
17 the offset number is decremented to zero. The pulses are
18 converted to analog voltages which, in turn, energize a motor
19 which rotates a take-up spool (capstan) on which the media
is wound so as to pass said magnetic transducer. With
21 zero count in the stop/lock counter, the magnetic transducer
22 is positioned in registry with the selected da~a tracX on
` 23 the magnetic media due to lateral translation of said media.
24 In another feature of the present teaching, when
the magnetic transducer is in transducing relationship with
-26 the media (i.e. under the media) a relatively low value
27 digital number, ~or example, "a digital one" is loaded into u
28 the stop/lock ~ounter. In like fashion, the "digit~l one"
:
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1 is decxemented to zero. This process (i.e. the loading/
2 decrementing) is continuous for a selected number of times
3 or until the magnetic transducer exits the magnetic media.
4 As the stop/lock counter is loaded and decremented, a
pulse is outputted which controls the take-up spool motor to
6 continually step the media a finite distance which, in effect,
7 continually alters the effective angle at which data is
8 transduced from the magnetic media. This continual motion
9 generates a new head/tape trajectory which brings the
magnetic transducer into registry with the skew stripe on
11 said media.
12 The foregoing and other features and advantages
13 of the present teachings will become apparent from the following
14 more particular description of the preferred embodiment as
shown in the accompanying drawings.
16 BRIEF DESCRIPTION OF_T~IE DRAWING9
17 FIGURE 1 shows a rotating head ma~netic tape
1~ transport whose take-up spool DC motor is controlled in
19 accordance with the present teaching.
FIGURE 2 shows a simple form of the tape's
21 longitudinal-servo tracks and traverse data track format.
22 The data tracks in this figure are unskewed.
23 FIGURE 3 depicts a length of magnetic media with
24 skewed and unskewed data tracks thereon. Data recorded in
the skewed aata tracks are retrievable with the device of
26 the present teachings.
27 FIGURE 4 is a conceptual representation showing
2~ the magnetic media unwrapped from its mandrel support with
.
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1 normal and deskewed head paths and circuit means according
2 to the present teaching for controlling the media so that
3 the head will recover skewed data.
4 FIG~RE 5 depicts à series of curves which show
- 5 the events which occur as the head makes one revolution to
6 sweep out a traverse track across the media. The curve
7 also shows the point in real time when events occur in
8 accordance with the present teachings.
9 DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIGURE 1 discloses a helically wxapped rotating
11 head magnetic device incorporating the dynamic deskewing
12 of the present teachings. This rotating head magnetic tape
13 unit includes a tape processing station 10 in the f~rm of
14 a 2-section mandrel 11, having an intermediate rotating head
wheel 12, which carries magnetic tr~nsducer or head 13.
16 Although a single transducer or head is shown in the figure,
17 it is contemplated within the scope of this invention that
1~ a plurality of heads may be used. A length of flexible
1~ media 14 is helically wrapped about the center of mandrel
11 and head 13 traces traverse paths, hereinafter called
21 tracks, across this length of media. A media supply is
22 contained on supply spool 15. The spool is controlled by
23 direct current motor 16. As media leaves spool 15, a length
24 of the media is maintained in vacuum column 17. This
vacuum column serves to maintain one end of the processing
2~ station media under constan~ tension. Media loop 18~
27 contained with the vacuum column, is positioned~monitored
~ u
28 ~y loop position servo 19. This servo in turn controIs the '!_
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1 tape energization of motor 16 to maintain an optimum loop
2 length within the column. This loop position sensor,
- 3 which may be of the type described in U.S. Patent 3,122,332
4 to F. G. Hughes, Jr., provides bi-directional and variable
magnitude energization of mo~or 16, thereby maintaining
6 loop 18 at an optimum position as the tape moves in either
7 direction relative to supply spool lS. The other end of
8 the tape length 14, which extends through the tape processing
9 station 10, is maintained under tension by take-up spool 20
and direct current spool motor 21. To facilitate guidance
11 of media 14, entry guide 30 and exit guide 32 are positioned
12 at the entry point and exit point respectively, of processing
; 13 ætation 10. Both the entry and exit guide function to
1~ isolate tape perturbations from the processing station.
Although the present invent:ion will be described
16 in a rotating head environment, this should not be regarded
17 as a limitation on the scope of the invention since the
18 invention may be practiced in any technology where a trans-
19 ducing means has to be aligned with a track on a substan-
tiall~ flexible media.
21 Referring again to FIGURE 1, head wheel 12 is
22 driven by drivin~ means 27. In the preferred embodiment
23 of this invention, driving means 27 is a DC motor. As
24 transducer or head 13 is rotated by head wheel 12, the
trajectory which the head traces is divided into two
26 sections, namely, arc A (shown in broken lines) and arc B
27 (shown in solid lines). When transducer 13 is traversing
28 arc A, the transducer is in transducing relationship with
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1 media 14. Stated another way, when transducer 13 is traversing
; 2 arc A, the transducer is under the media. Transducing
3 relationship, as used herein, means that the transducer may
4 write data onto the media or read data from the media.
As the transducer traverses arc B, the transducer is not
6 in transducing relationship with the media.
7 ~s will be explained subsequently, the period of
8 time which elapses when the head is orbiting arc A and ~ is
9 used for setting up the conditions which adjust the media to
enable reproduction of data recorded in skewed data tracks
11 on the media.
12 In order to determine the speed (velocity) and
13 direction in which head wheel 12 is rotating, timing means
14 28, hereinafter referred to as tachometer 28, is attached
to the shaft of driving means 27. Tachometer 28 outputs
16 positional pulses on conductor 34. The pulses, in turn,
17 are transmitted to head track alignment positionin~ means 36.
18 Head track alignment positioning means 36, in turn, outputs
19 control pulses on conductor 23. The pulses on conductor
23 energize DC motor 21 to rotate clockwise or counter-
21 clockwise. The rotational motion of the motor is transferred
22 to take-up spool 20 which transfers a stepping motion to
23 media 14. The stepping motion of the media results in
24 the generation of a new head track trajectory which enables
the reco~ery of skewed data. Any motion of DC motor 21 is
26 monitored by tachometer 25 which outputs a feedback pulse
27 on terminal 26. The feedback pulse is fed into head/track
28 a~ignment positioning means 36. As will be explained sub-
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1 sequently, the close loop servo which controls DC motor 21
2 together with the control pulses on conductor 34 defines
3 the skew correction scheme of the present teaching.
4 In order to initialize the skew correction
circuitry, a control signal is issued on terminal 38 prior
6 to the activation of the skew correcting circuitry.
7 Referring now to FIGURE 2, a diagrammatic view
8 of the tapes oblique data tracks and the servo format is
9 shown. In this representation, data tracks 36 and 40 are
normal (~hat is free from skew). In this arrangement, head
11 13 is shown moving in the direction of arrow 34 along the
12 ideal head path identified by broken line 35. This head
13 path is called ideal in that it coincides with the center
14 of traverse data track 36. Thus, as head 13 follows path
35, the data contained within track 36 will be accurately
16 transduced. For head 13 to select the ideal head path,
17 prerecorded information is obtained from servo track 42
18 and 44 respectively. This servo information is utilized
19 by the head control means (not shown) for correct head
track positioning. A more detailed description of the
21 positional scheme may be obtained from U.S. Patent 3,845,500
22 issued to Gary Hart and commonly assi~ned.
23 Referrin~ now to FIGURE 3, in which previously
24 described elements will be identified by common numerals,
the relationship between unskewed data track 36 and skewed
26 data track 36' is shown. In the normal situation, (i.e.
27 the unskewed situation) head 13 traverses ideal head path
28 35 and faithfully reproduces information recorded in unskewed
P
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1 data track 36. However, absent the present invention
2 whenever unskewed data track 36 is skewed and adapts the
3 form as shown in skewed data track 36', any head which
traverses ideal head path 35 cannot recover data recorded
in skewed data track 36'. However, by implementing the
6 mechanism of the present invention, head 13 will shift an
7 offset distance 116 from the ideal head path 35 and traverse
8 a new trajectory depicted as head path 35'. ~s head means
9 13 traverses trajectory 35', data which is recorded in
skewed data track 36' will be recovered. Although the skewed
11 data track 36' is shown shifted to the left of unskewed
12 data track 36, the skewed track can be shifted to the right
13 of unskewed data track 36 and the present invention would
14 also ~e applicable and effective in recovering the skewed
data.
16 FIGURE 4 shows the working area of the tape
17 straddled by entry and exit guides 3~) and 32 respectively.
18 In this figure the tape is shown unwound from the mandrel.
19 Tension, as depicted by arrow 50, is supplied via vacuum
column 17. Additionally, ideal head path 35r together with
21 the skewed head paths 35', are shown. As was previously
22 mentioned, the skewed head path can be to the right or left
23 of the ideal head path. In accordance with the present
24 invention and in relationship to FIGURE 1, the detail of
one embodiment of FIGURE l's head/track alignment positioning
26 means 36 which positioned transducing means 13 to traverse
27 the skewed head path 35 ~o as to recover skewed recorded
28 data is shown. Essentially, DC motor 21 (FIGURE 1 and
t
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1 FIGURE 4 ) which drives take-up spool 20 is controlled by a
2 close loop servo system, whereby controlled information to
3 the close loop servo system is derived from DC motor 27
4 which positions magnetic means 13 relative to media 14 and
the DE-skew command signal appearing on conductor 52. As
6 will be explained subsequently, activation of the DE-skew
7 command signal will activate the deskewing apparatus or
8 circuitry of the present invention. More specifically, and
9 with reference to FIGURE 4, FIGURE 1, and FIGURE 3, the tape
transport unit utilizes the servo information recorded in
11 servo tracks 42 and 44`to position head 13 so as to traverse
12 ideal head path 35. After several attempts, if head 13
13 is unable to recover data from a selected traverse data
14 track due to skewing, a command signal is outputted on
lS conductor 52. This signal enables tne head/track alignment
16 positioning means 36 (FIGURh 1) details of which are shown
17 in FIG~R~ ~ to perEorm the skew correction routine. In
18 addition to the deskew command si~nal, the position of head ;
19 13, relative to the media 14, is ascertained. This is done
via tachometer 28.
21 Tachometer 28 (FIGU~E 1) is a conventional see-
22 through optical tacllometer, which includes an optical disc
23 with one or more optical tracks thereon. Associated with
24 said disc is illuminating means and light receiving means
and circuitry which outputs signals indicative of direction
26 and ~elocity of head wheel 12. More particularly, the
27 optical disc of tachometer 28 has two tracks thereon. ,~
28 One of said trac~s generates a signal when head 13 is in
:
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1 arc B of the head trajectory shown in FIGURE 1. This signal
2 is known as the ~-angle signal which appears on conductor
3 54 of FIGURE 4 and is shown in Curve A of FI~URE 5. The
4 other track generates a plurality of pulses known as digital
gain control pulses hereinafter called DGC pulses (see
6 Curve B of FIGURE 5) which appears on conductor 56 of FIGURE
7 4. Incidentally, the DGC pulses are generated when the head
8 is in transducing relationship with media 14 (i.e. when the
9 head is traversing arc A, FIGURE 1). The signals which are
outputted on terminal 54 and 56, and shown as Curve A and
11 B (FIGURE 5), together with the DE-skew command signal which
12 appears on terminal 52 are the control signals which drive
13 the close loop servo which controls DC motor 21.
14 With the ~-angle signal on t:erminal 54 and the
DE-skew command signal on terminal 52 first "AND" circuit
16 means 58 is activated. When this occurs, a load offset
17 slgnal appears on conductor 60. With the load offset signal
18 on terminal 60 a digital number is loaded into Up/Down
19 counter 62 (UP/DN CTR 62). The counter is a conventional
Up/Down counter which is under the co~trol of counter control
21 means 64. The combinàtion of Up/Down counter 62 and counter
22 control means 64 is called Stop/Lock counter means 66. Stop/
23 Loc~ counter means 66 generates control pulses on terminals
24 68 which are fed into digital-analog-converter 70 hereinafter
called DAC 70. DAC 70 converts the digital pulses into
26 analog pulses or analog voltages and supply said analog
27 voltages to power amplifier means 72 via conductor 74. The
28 analog voltages are multiplied bv the gain of power amplifier
s
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1 72 and is then applied to DC motor 21 via conductor 76.
i 2 The signs of these analog voltages are either positive or
3 negative. The analog voltages, in turn, energize DC motor
4 21 forcing it to rotate in either a clockwise or a counter-
clockwise direction depending on whether the sign which
6 is supplied to DAC 70 from counter control means 64 via
7 conductor 78 is either positive or negative. Of course,
8 the sign which is generated by counter control means 64
9 depends on whether DC motor 21 is rotating clockwise or
counterclockwise.
11 Still referring to FIGURE 4, Stop/Lock counter
12 means 66 controls DC motor 21 via DAC 70 and power amplifier
13 72 so that the motor is at rest (i.e., remains stationary)
14 when the count in Up~Down counter 62 is "0". Any rotational
movement ~clockwise or counterclockwise) of DC motor 21
16 Eorces Stop/Lock counter means 66 to control the motor so
17 that the count in Up/Down coun~er 62 stands at substantially
18 "0". Also, c~unter control means 6~ controls Up/Down counter
19 62 in such a way that it never counts down past "0".
Counter control means 64 generates the Stop/Lock sign bit
21 which appears on conductor 78 and also the count-up and
22 count-down pulses which appear on terminal 80 and 82,
23 respectively. The count-up and count-down pulses signify
;~ 24 when Up/Down counter means 62 should count-up or count-down.
Also, when the contents of Up/Down counter 62 is approximatel~
26 "0", a signal is outputted on conductor 84. The signal on
27 conductor 84 informs counter control 64 that the counter
28 is now sitting in an e~uilibrium state.
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1 As was mentioned previously, whenever the load
2 offset signal appears on conductor 60, a load offset number
3 is loaded into Up/Down counter 62. In one embodiment of
4 the present teaching, digital numeral "9" is loaded into
the counter. However, this number is only representàtive
6 rather than a limitation on the scope of the present teaching.
7 In essence, a range of digital numbers can be loaded into
8 the counter. The limiting parameter is that the number
9 which is loaded into the counter has to be of a magnitude
so that it is decremented substantially to "0" at a point
11 in time when head 13 is traversing arc B of FIGURE 1 (i.e.,
12 when the head substantially enters into transducing re-
1~ lationship with media 14). Still stated another way, the
14 number must be such that the total time in which said
number is decremented to "0" is less than or e~ual to the
16 time required for head 13 to traverse the ~-angle. Of
17 couxse, this will depend on the rotational speed of the head
18 wheel. It is also within the skill of the art to generate
19 an automatic scheme and circuitry which will determine
automatically the magnitude of the digital number which
~1 is loaded into the Up/Down counter. Of course, in such
22 an automatic scheme, the magnitude of the digital number
23 is related to the degree of media skew.
24 In one embodiment of the present teaching, the
head wheel is rotating at a peripheral speed of approximately
26 1,000 inches per second. The media is being operated in
27 a stepping mode, (i.e.j the media is substantially stationary
28 while the head is rotating in its orbit). At this speed,
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1 the head re~uires approximately 11 milliseconds to complete
- 2 its orbit. Of the 11 milliseconds, 8 milliseconds is spent
3 for the head to traverse the data track on the media while
4 3 milliseconds is spent for the head to traverse the ~-angle
~i.e., arc B, FIGURE 1). In this environment, the number
6 which is loaded into the Up/Down counter 62 must be one
7 which can be decremented to "0" within the time interval of
8 substantially 3 milliseconds.
9 Simultaneously, with loading the selected digital
number into Up/Down counter 62, a signal is outputted on
11 conductor 86. This signal from conductor 86 is used to
12 deactivate third "AND" circuit means 88 via inverter circuit
13 means 90. Third "AND" circuit means 88 is inhibited during
14 the period or instant of time when a digital number is placed
into Up/Down counter 62 so that any ~lignal in the feedback
16 loop of DC motor 21 will not upset or alter the counter
17 settin~.
18 As was melltioned earlier, the system is in equi-
19 librium when the count in the ~p/Down counter 62 is "0"
and DC motor 21 is at rest. I~ the count in Up/Down counter
21 62 is greater than "0" then an analog voltage is applied
22 to DC motor 21 via conductor 76. This voltage, depending
~` 23 on its sign, tends to rotate the motor in a clockwise or
.
24 counterclockwise direction. As the motor begins to move,
; 25 conventional tachometer 25 which is of the type previously
26 disclosed, outputs a two-phase signal identified as ~ 1 and
27 ~ 2 on terminals 92 and 94, respectively. Direction detection
28 means 96 receives the ~ 1 and ~ 2 pulses and ~enera~es
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1 forward and backward pulses which appear on conductors 100
2 and 98, respectively. These pulses, together with an
3 additional pulse outputted from "OR" circuit means 102
4 on terminal 104, control counter control means 64.
Still referring to FI&URE 4, the direction
6 detection means converts the tach signals into pulses that
7 represent units of displacem`ent in backward or forward
8 direction. The direction detection means is of the type
9 disclosed in an article entitled "Logical Motion and
Direction Detection", by H. C. Jackson, which is disclosed
11 in IBM TECHNICAL BULLETIN Volume 14, No. 12, May, 1972,
12 page 3672. The forward and backward pulses on terminals
13 98 and 100 are used to activate second "O~" circuit means
14 106 which, in turn, output a feedback signal on terminal
108 as DC motor 21 rotates.
16 Referring now to FIGURE 5, various curves showing
17 the point in time when the control signals are applied to
18 the circuitry shown in FIGURE 4 are depicted. Also, the
19 analog voltages which are applied to DC motor 21 together
with the head trajectory are shown. The head trajectory
21 is generated as a result of translational motion impart
22 to the media by the DE-skew circuitry of the present
23 disclosure via the take-up spool and DC motor 21. As was
24 stated previously, the dynamic skew correcting scheme of
the present invention has to be effective to correct skew
26 within a time interval equivalent to at least one head
27 revolution. Of course, the use of the dynamic skew correcting
28 scheme, of the present invention, may be less fre~uent than
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1 one head's revolution. Curve A in FIGURE 5 depicts the
2 period o~ time which it takes the head to make one revolution.
3 Down portion 110 of curve A identifies the period of time
4 when head 13 is traversing arc A (i.e., in transducing
relationship with the media, see FIGURE 1). During this
6 period of time, conductor 54 (FIGURE 4~ is in a down state,
7 (i.e., inactive). Up portion 112 of curve A depicts the
8 period of time when head 13 is traversing arc B, (i.e.,
9 in the ~-angle) and the head is not in transducing relation-
ship with the tape. During this period of time, conductor
11 54 is in an active state and a number can be loaded into
12 Up/Down counter 62.
13 Curve C depicts the analog voltage which is
1~ applied to DC motor 21 via concluctor 76. Section 114 of
Curve C depicts the analog voltages which are applied when
16 the head is in the ~-angle. This voltage reaches a maximum
17 peak and is then reduced back to essentially 0. As will be
18 explained subsequently, the number which is loaded into
19 Up/Down counter 62 at the instance when the ~-angle signal
is active was decremented to "0" as the motor begins to
21 move in a clockwise or counterclockwise direction and tach
22 25 begins to feed back decrementing pulses via the feedback
23 loop previously described and shown in FIGURE 4. As the
24 head exits the ~-angle and enters into transducing relation-
ship with the tape, the DGC pulses (see curve B of FIGURE 5
2G begins to appear on conductor 56 tFIGURE 4). For each DGC
27 pulse appearing on conductor 56, a corresponding low digital
28 nume~al, e.g., "1" is loaded into Up/Down counter 62. For
BO976007 - 19 -
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1 eac~ "l" loaded into Up/Down counter 62 a corresponding
2 analog voltage is applied to DC motor 21 via conductor 76.
3 These analog voltages are shown as portion 115 of Curve C
- 4 in FIGURE 5. When these voltages are applied to motor 21,
the motor attempts to move. A signal is fed back by tach
6 25 via the feedback loop and decrements the counter to
7 l~0~. This process (i.e., load a small digital number and
8 decrement to "0") is continued repeatedly until the head
9 exits the tape. Stated another way, for every DGC pulse
which appears on conductor 56 (see curve B) a corresponding
11 analog voltage is applied to DC motor 21. (See portion 115
12 of Curve C). The voltage then decays to zero due to
13 decrementation of the number which is placed into the
14 Up/Down counter. The load decrement process continues
for a number of times until the head exits the tape. OE
16 course, the process may be performed only once in the time
17 period when the head is txaversln~ arc A (FIGURE 1) without
18 departing from the scope of the invention.
-19 For each DGC pulse which appears on conductor
56, DC motor 21 is subsequently rotated a finite distance
21 (clockwise or counterclockwise) which results in stepping
22 media 14 an incremental distance.
23 In relating curves A, B, and C (FIGURE 5) with
24 the head trajectory as depicted in FIGURE 3 and FIGURE 4,
offset distance 116 is generated as section 114 of curve C
26 is applied to DC motor 21 and the tape is stepped. Head
27 trajectory 35' is generated as the DGC pulses are applied to
28 conductor 56 ~FIGURE 4) which, in turn, generates the
t
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1 plurality of analog voltages which are applied to DC motor
2 21. Head trajectory 35', as is depicted in FIGURE 3, is
3 an electrical model of the head tra~ectory. However, due
4 to mechanical lag of the mechanical components, e.g.,
motor, take-up spool guides in FIGURE 4 the head trajectory
6 35' is rounded as is shown in Curve D of FIGURE 5O In
7 other words, the head trajectory tends to be rounded by
8 joining the peaks of the electrical responses as shown in
g Curve D of FIGURE 5. In other words, the head trajectory
tends to be rounded by joining the peaks of the electrical
11 responses as shown in Curve D of FIGURE 5. This ends the
12 detailed description of the present invention.
13 OPERATION
14 In operation prior to activatin~ the automatic
skew control mechanism of the present invention, a command
16 signal is issued on terminal 38. This signal will reset
17 Up/Down collnter 62 to a "0". In other words, this signal
18 will initialize the DE-skew correction circuitry.
1~ Suppose the tape transport unit is in the read
mode; it will read prerecorded data from prerecorded tracks
21 on media 14. If a selected track is skewed, as that shown
22 in FIGURE 3, then the tape transport unit will make several
23 attempts to read the prerecorded informaticn. If after a
24 predetermined number of attempts the tape transport unit
~5 is unable to read the prerecorded data, the device Inot
26 shown) which controls the tape drive will issue a deskew
27 signal on conductor 52. With the issuance of this si~nal,
28 as the rotatin~ head enters the ~-angle, a si~nal i~ outputted
P
O
~ BO976007 ~1 - o
1 on conductor 54. With both signals concurrently on
2 terminals 54 and 52, "AND" circuit means 58 becomes active
3 and the offset number is loaded into Up/Down counter 62.
4 Simultaneously, with loading the digital offset number
into Up/Down counter 62, an inhibiting signal which de-
- 6 activates the feedback loop of the dynamic deskewing circuitry
7 is issued on conductor 86. Stated another way, whenever
8 Up/Down counter 62 is being loaded, the feedback loop is
9 deactivated so that the number which is loaded into the
10 counter is not changed prematurely.
11 With the number loaded in Up/Down counter 62,
12 digital pulses are outputted on conductors 68. The pulses
13 are converted :into analog voltages by DAC 70 and fed into
14 power amplifier 72. The voltages are multiplied by the
15 gain of power amplifier 72 and, in turn, are applied to DC
16 motor 21 via terminal 76. This voltage will rotate DC
17 motor 21 in a clockwise or counterclock~lise direction.
18 As DC motor 21 begins to move, pulses are fed back by
19 tachometer 25 over conductors 92 and 94. These pulses
20 are fed into direction detection means 96 which outputs
21 pulses on terminals 98 and 100 indicative of forward
22 rotation or backward rotation. The forward and/or backward
23 pulses are fed into counter controller 64. Counter controller
24 64 then outputs pulses which decrement the count in Up/Down
25 counter 62. Counter control 64 also outputs a sign bit to
26 the DAC. As the motor continues to move, the pulses on
27 terminal 98 or lnO continue to decrement the number previousl~
28 placed in Up/Down counter 62 until the counter contains a
count of "Q". P
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,
6~37
1 At this time, the motor should be standing still,
2 i.e., not in motion. During this time interval, the media
3 is displaced a distance substantially equivalent to offset
4 distance 116 (FIGURE 3). As a result of the media dis-
placement, head 13 is now positioned in line with deskew
6 head path 35'. At this point in time, head 13 should be
7 at the beginning of new head trajectory 35'. Also, the
head should be at a point where it is substantially ready
g to transduce data from media 14.
At this point in time, the DGC pulses begin to
11 appear on conductor 56, with the first pulse on conductor
12 56 and the deskew command signal active, second "AND"
13 circuit means 118 is activated and a si~nal is outputted
14 on conductor 120 which loads a substantially low value
digital number into Up/Down countcr 62. As with the load
16 decrement scheme previously described, the low value di~ital
17 number Up/Down counter 62 will gen~rate a voltage which
18 tends to rotate DC motor 21. Feedback signal will be
19 generated which decrements the counter back to "zero".
This will step the tape an incremental distance. The ~;
21 load decrement operation will continue for a predetermined
22 number of times at the end of which the new head trajectory,
23 as depicted by 35' in FIGURE 3 or Curve D in FIGURE 5
.
24 will be ~enerated. The new head trajectory 35' will enable
the tape transport unit to read data which is skewed.
26 While the inven~ion has been particularly shown
27 and described with reference to a preferred embodiment
. 2~ thereof, it will be understood by those sk;lled in the
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!3~
1 art that various changes in form and details may be made
2 therein without departing from the spirit and scope of
3 the invention.
4 What is claimed is:
11
12
13
14
17
~8
19
21
22
23
24
26
27
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