Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
11~45243
1 Background and Summar~of the Invention
This invention relates to the field of magnetic telegraphophones,
and more specifically to magnetic tape units employing one or more
rotating heads which record and/or reproduce machine-convertible in-
formation while moving in transducing relationship with a stationary
magnetic web or tape, this information being oriented as magnetic
domains to form information tracks which extend generally transverse
to the longitudinal tape length.
Rotating head magnetic tape units are widely known. In one form
a generally cylindrical mandrel or drum includes a rotating headwheel
which carries one or more read/write heads. The magnetic tape engages
the mandrel at one point, makes a helical wrap about at least a portion
of the mandrel, and exits the mandrel at a point which is both axially
and circumferentially spaced from the entrance point. The angle of
helical tape wrap can vary in accordance with design choice, but is
usually between 180 and 360. The headwheel rotates so as to sweep
its magnetic heads transversely across the tape. The angle at which
the head enters and exits the tape may vary, in accordance with design
choice, from slightly less than 90 to a small angle, such as 15.
Another form of device is one wherein the headwheel is associated
with a tape guiding structure which bends the tape transversely into
an arcuate shape that conforms to the circumferential shape of the
headwheel. In this device the tape travels in a generally straight
line past the headwheel, and is transversely bent by the associated
guides as it enters the headwheel area.
The present invention finds utility with either aforementioned
type of device, and has been found particularly useful with the helical
wrap device.
A major problem encountered in the aforementioned devices is that
of establish-ing and maintaining accurate positional alignment between
the path of the headwheel and the tape's data track. This is parti-
cularly true when a data track is written on one tape transport or
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1 unit and later read by another tape unit.
To facilitate servo control of the tape position, so as to main-
tain this head/track alignment, the tape is provided with one or more
longitudinal servo tracks. These tracks function to identify the
position at which the rotating head should enter and/or exit the tape
in order for the head to trace the proper transverse path across the
tape. The prior art teaches means whereby the rotating head itself,
or alternatively a stationary head or heads, reads the servo track
or tracks and provides a head/track alignment error signal which is
then used to fine-position the tape to correct this positional error.
The stepping of the tape from one data track to an adjacent
track requires that the take-up spool be rotated an incremental amount.
The step-distance by which the tape is moved is a function of
both the angular distance through which the take-up spool rotates,
and the tape radius thereon. The tape distance between adjacent
data tracks is uniform throughout the entire tape length. However,
if equal angular steps are made by the take-up spool, to access an
adjacent track, positional error will occur as the tape radius on
the take-up spool varies.
Once the tape has been stepped it must be held taut, as by
maintaining the tape under tension. This is accomplished by the
force exerted due to motor stop-lock energization and the force ex-
erted by associated tape buffers. Unbalance in these forces causes
the tape to creep away from the stop-lock position, and thereby
produce stop-lock position error.
The present invention executes a requested tape-step and pro-
vides unique structural means, responsive to the particular take-up
spool tape radius associated with that step, to adjust the take-up
motor's stop-lock torque in order to prevent the above-mentioned
creeping of the tape away from its stop-lock position, as well as to
adjust the step distance which is executed between adjacent data tracks,
such that the execution of a step, at any position along the length
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1 Qf tape, brings the rotating head into accurate alignment with the
underlying tape's data track so that only minimum fine positioning
is thereafter required.
More specifically, the present invention first executes the re-
quested step. Once the step has been executed, the moving of the tape
away from its stop-lock position is sensed and the take-up spool
motor's stop-lock torque is adjusted to the proper magnitude. This
torque is applied to the tape at the particular take-up spool radius
associated with that step, and produces a force equal and opposite to
the tape force derived from the vacuum column buffer. In addition,
the execution of a step is effective to enable a network which is
responsive to head/track alignment error. This network modifies the
step request to an adjacent data track in accordance with the take-up
spool tape radîus information. This modification adaptively controls
the length of the take-up spool's incremental stepping as the tape
radius thereon varies.
The foregoing and other features and advantages of the invention -
will be apparent from the following more particular description of ~ -
preferred embodiments of the invention, as illustrated in the accom-
panying drawing.
Brief Description of the Drawing
FIGURE 1 is a view showing a rotating head magnetic tape unit
whose take-up spool DC motor is controlled in accordance with the
present invention,
FIGURE 2 is a view of the tape-wrap mandrel of FIGURE 1, showing
a helical wrap of tape thereabout, and showing the centrally located
headwheel which carries a magnetic head or transducer,
FIGURE 3 is a view showing a flat section of tape, and diagram-
matically showing two of the many transverse data tracks and the two
individual servo track indicia which identify the physical location
of these two data tracks,
FIGURE 4 is a view of the take-up spool of FIGURE 1 and is use-
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1 ful in explaining the manner in which an unbalance in the take-up
motor's stop-lock torque and the vacuum column force tends to cause
the tape to creep off the desired stop-lock or track position,
FIGURE 5 is a view of the take-up spool of FIGURE 1 and is useful
in explaining the manner in which a unit rota~ion of the take-up
spool produces a tape step distance at the rotating head which is
dependent upon the take-up spool's tape radius,
FIGURE 6 is a schematic showing of a further embodiment of the
present invention,
FIGURE 7 shows a digital integration network which functions as
the integrator of FIGURE 6 to adjust the stop-lock torque in accordance
with the spool's tape radius,
FIGURE 8 is a state diagram useful in explaining the operation
of the network of FI6URE 7,
FIGURE 9 shows a digital network which functions as the portion
of FIGURE 6 which functions to adjust the take-up spool's rotational
step distance in accordance with the spool's tape radius,
FIGURE 10 is a state diagram useful in explaining the operation
of the network of FIGURE 9, and
FIGURE 11 is a movement diagram showing an example of a step which
was too long, requiring a minus delta tape movement step, and resulting
in a new step calculation for the next step.
Description of the Preferred Embodiments
The present invention will be described in the environment of a
rotating head magnetic tape unit. This general type of magnetic tape
unit transduces data from a length of magnetic tape while the tape is
stationary. Specifically, magnetic tape 10 (FIGURE 3) includes a
plurality of inclined data tracks 11, 12 which are swept by the ro-
tating head while the tape is stationary. Once a given data track is
transduced, that is, either written or read by the rotating head, the
tape is incremented or stepped to an adjacent data track.
While the present invention is described in the environment of a
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1 rotatjng head magnetic tape unit, and particularly the type wherein
the tape forms a helical wrap about a mandrel having a centrally lo-
cated headwheel, as shown in FIGURE 2, the present invention is not
to be restricted thereto. As is well known to those of skill in the art,
a stationary length of magnetic tape may be transduced by a moving
head, such as a rotating head or, alternatively, by a head which does
not require relative movement during transducing, such as a magneto-
restrictive head. -
With reference to FIGURE 1, a length of magnetic tape 10 extends
between supply spool 13 and take-up spool 14. The tape path extending
between these two spools includes transducing station 15 whereat a
helical wrap of tape is formed about a stationary mandrel having a
centrally located headwheel 16 carrying head 17, as more specifically
shown in FIGURE 2. This tape path includes a single tape buffer in
the form of vacuum column 18. A loop of tape 19 is maintained in the
vacuum column and its position is sensed by a loop position sensor,
not shown. This loop position sensor, which may be of the type des-
cribed in United States Patent 3,122,332 to F.G. Hughes, Jr., provides
an input signal to loop position servo 20 to control supply spool DC
motor 21, thereby maintaining loop 19 at an optimum position as the ~ -
tape moves in incremental step-by-step fashion from supply spool 13 to
take-up spool 14.
A preferred magnetic transducer or head configuration to be used
as head 17 of FIGURE ? useful in establishing a stable hydrodynamic
air film at the head/tape interface, is described in the copending
Canadian application of F.R. Freeman, W.R. Golz and W.K. Taylor,
Serial Number 188,739, filed December 21, 1973 and commonly assigned.
As the tape passes through the tape path of FIGURE 1, it is air-
bearing supported at 22, 23 and 24. The side edges of the tape are
preferably compliantly guided, particularly at air bearings 23 and 24.
By way of example, this compliant guiding may be the continuous com-
pliant guide described in the co-pending U.S. application of M.L. Nettles,
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1 Serial Number 335,609, filed February 26, 1973 and commonly assigned
now U.S. Patent 3,850,358 issued November 26, 1974.
FIGURE 3 is a view showing a flat section of tape, such as the
tape section shown in FIGURE 2, wherein two of the many transverse
data tracks and two specific servo track indicia 25 and 26 are shown.
Indicia 25 and 26 reside in servo track 27 and serve to identify the
centerline of the two data tracks 11 and 12, respectively. When the
section of tape is properly positioned relative to the mandrel, head-
wheel 16 of FIGURE 2 traverses a data track in exact alignment and
coincidence therewith. Movement of head 17 is at a constant speed,
as controlled by constant speed headwheel motor 28.
The exemplary showing of FIGURE 3 is intended to be quite general
since the particular data field formats used in data tracks 11 and 12
and in servo track 27 are not important to the present invention.
Suffice it to say that head/track alignment error detection network 29
of FIGURE 1 responds to the position of indicia 25 and 26 in servo
track 27 to originate a head/track alignment error or conductor 30.
The details of network 29 are not disclosed since this network may
take many forms, in accordance, for example, with the format of servo
track 27. Furthermore, such a network may receive its input infor-
mation either from a stationary head which reads the tape's servo
track 27, or alternatively, this servo track may be read by the rota-
ting head itself as it enters and/or leaves the tape. Examples of
acceptable networks of this type can be found in the co-pending
Canadian application of G.A. Hart et al, Serial Number 186,740, filed
November 26, 1973 and commonly assigned; U.S. Patent 3,666,897, issued
on May 30, 1972, to J.D. Harr; and the co-pending Canadian application
of W.S. Buslik et al, Serial Number 169,268, filed April 13, 1973
and commonly assigned.
Turning for the moment to FIGURE 4, this figure is useful in
explaining the manner in which the tape adjacent transducing station
15 is maintained in a stable stop-lock position when the stop-lock -
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1 torque of motor 31 is adiusted as a function of the variable tape
radius on spool 14. As will be appreciated by those of skill in the
art, vacuum column 18 maintains a substantially constant force on the
left-hand end of the tape section passing through processing station
15. This force is in a direct;on to pull the tape to the left, re-
moving it from the transducing station. This force is transmitted to
spool 14 and tends to rotate this spool in a counterclockwise direction.
The stop-lock torque of motor 31 is in a direction tending to produce
clockwise rotation of spool 14. This torque is applied at a variable
radius R, depending upon the quantity of tape wrapped about spool 14.
Thus, in order for the length of tape traveling through processing
station 15 to remain at a stable stop-lock position, the motor torque,
applied to the tape at a radius R, must be equal and opposite to the
vacuum column force. ~;
FIGURE 5 is useful in explaining a second variable phenomenon
associated with variable tape radius on take-up spool 14. Rotational
units of movement of motor 31 and spool 14 are sensed by movement
feedback transducer 32, FIGURE 1. In the preferred embodiment of the
present invention, this transducer is a digital transducer which pro-
vides an output pulse for each unit of rotation of motor 31 and spool
14. In FIGURE 5, this unit of rotation is identified by reference
numeral 33. As can be seen from this figure, the tape moves a smaller
step distance when the tape radius is at 34 than it will when a greater
wrap of tape exists on spool 14, such as indicated at 35. The pro-
portions in FIGURE 5 have been exaggerated for clarity. Normally,
distance 33 is much smaller than that indicated and, in fact, trans-
ducer 32 in the digital form may provide as many as 500 to 1,000 incre- -
ment pulses for one 360 rotation of spool 14.
Turning again to FIGURE 1, the function of the servo apparatus
associated with take-up spool 14 is to step the tape incrementally,
such that one data track is replaced by an adjacent data track, in
alignment with rotating headwheel 16 of FIGURE 2. Furthermore, the
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1 alignment between the tape data track and the head is seryo controlled
and stabilized.
More specifically, the head/track alignment is maintained by aservomechanism which energizes DC motor 31 and operates to provide fine
control of the tape's position. Such a network is not disclosed in
FIGURE 1 and may, for example, take the form disclosed in the two above-
mentioned co-pending Canadian applications or U.S. Patent 3,666,897.
Also, the embodiment of the present invention disclosed in FIGURE 6
provides such a means to fine position the tape as a result of head/
track alignment error detecting Network 29.
As has been mentioned, servo control of motor 31 functions to incre-
ment or step the tape one data track at a time. Gross tape movement is
controlled by position servo 36 which receives position feedback informa-
tion derived from transducer 32. After the initial gross step is executed,
fine tape positioning is achieved by using the output of network 29. The
input signal to this network is derived from the tape's servo track 27, as
this servo track is read by rotating head 17.
When a command is received to move to the next data track, input con-
conductor 37 is enabled. As a result, network 38 is effective to place a
command step distance by means of conductor 39 to summing terminals 40 and
41. The magnitude of the step command is designated S~ ~S. This magnitude
is determined in a unique manner by operation of the present invention, as
will be apparent. A second input to junctions 40 and 41 appears on con-
ductor 42 and is provided by the output of transducer 32. Junction 40 is
effective to compare the actual motor position, on conductor 42, to the
command position on conductor 39. As a result of this comparison, a posi-
tion error signal appears on conductor 43. This position error signal
is applied as an input to position servo 36, thereby energizing DC motor
31 in a manner to reduce thi,s error to zero.
Summing junction 41 also responds to the discrepancy between the
motor's actual position and the command position. This junction is
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l effective to enable stop-lock source 45 and memory network 38 only
when the position error has been redueed substantially to zero, this
state indicating that position servo 36 has completed the execution of
a requested tape step. Once stop-lock source 45 is enabled, it is there- -
after effective to maintain motor 31 at a stable stopped position. The
torque of this motor now resists the force of vacuum column 18 tending
to pull tape through transducing station 15 and off spool 14. In
addition, network 38 is enabled to begin the calculation of a new
S + QS to be used in a subsequent step.
Considering first the operation of stop-lock source 45, once this
source is enabled, it receives control input information by way of
conductor 44. Should there be an inequality between the force applied
to the tape from vacuum column 18 and the force applied by the stop-
lock torque of motor 31, the tape will tend to creep away from the
desired stop-lock position. This movement results in a position error
signal appearing on conductor 43. This position error signal controls
position servo 36 to return the tape to its desired stop position.
This signal also controls the magnitude of stop-lock source 45 such
that the stop-lock torque of motor 31 is increased or decreased, as
necessary. For example, should there be a tendency for vacuum column
18 to cause take-up spool 14 to rotate in a counterclockwise direction,
the sense of the position error signal will be such as to increase the
magnitude of stop-lock source 45, thereby increasing the stop-lock
torque of motor 31 and resulting in a balance between this tape force
and the vacuum column's tape force.
Considering now the operation of head/track alignment error
detection network 29, the completion of the requested tape step enables
memory network 38. Thereafter, this network responds to any misalign-
ment which may exist between the track of head 17 and the new data track.
If the executed step S + ~S was of the proper magnitude, exact coinci-
dence will exist between the head track and the data track, and no
head/track alignment error signal will exist on conductor 30. However,
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jf slight misalignment occurs, this misalignment is detected by~S
calculation network 46. This network now computes a new as WhiCh is
added to or subtracted from the prior S+l~S executed step. Note that
S+~S is defined in terms as units of motor rotation, rather than tape
length. Thus, as is readily apparent from the above description of
FIGURE 5, the actual step movement of the tape for a given step move-
ment of rotation of motor 31 is directly related to the tape radius
wrapped about the take-up spool. That is, the greater the radius the
greater the length of tape step. Network 46 facilitates the adaptive -~ -
definition of a motor step length such that substantially exact co-
incidence is effected between the head track and the data track at
the end of an executed step. This results from the fact that memory
38 at all times contains an updated definition of the motor's step
distance, a new step being defined at the end of every step execution.
FIGURE 6 is a schematic showing of a further embodiment of the
present invention wherein motor 31 is servo controlled to maintain
spool 14 at a stable stop-lock position and to execute an S+~S step
which results in substantially coincidence between transducing station
15 and the desired tape data track or field.
In this embodiment, movement of motor 31 and spool 14 is detec-
ted or transduced by two-phase digital tachometer 50 whose output signal
appears on conductor 51 as a pulse for every unit of rotational movement,
this output bearing both distance and direction information.
When a command is received to move the tape to the next adjacent
data track, conductor 59 becomes active. Conductor 59 enables reference
pulse generator 52. The output of pulse generator 52 is exemplified
by curve 53. This signal is applied to summing junction 54 where it is
summed with the output of digital-to-analog converter (DAC) 55 appearing
on conductor 56. DAC 55 responds to position feedback information
30 present on conductor 51 and provides an output waveform generally
s~milar to that rePresented by curve 57.
The request to execute a step also enables gate 58, by way of
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1 conductor 59. When gate 58 is enabled, the S+ as content of register60 is transferred to counter 61, causing this counter to increment or
count up. The magnitude contained in register 60 is the quantity S+ as
discussed in relation to memory 38 of FIGURE 1.
The magnitude of this count establishes the instantaneous magnitude
of the DAC output 56 at the instant that an execute step command is
received. As a result of closed-loop servo operation, the output of
DAC 55 substantially follows the step profile defined by curve 53. As
motor 31 moves, the feedback information present on conductor 51 causes
counter 61 to count down. At the end of the requested step execution,
the output of counter 61 is substantially zero.
Conductor 59 enables timer 62 when a step is requested. At the
end of the time interval, normally slightly less than a full revolution
of headwheel 16, conductor 63 enables gates 64 and 65. With gate 64
enabled, the output of DAC 55 is connected to position error integrator
66. With gate 65 enabled, any head/track alignment error which exists
at the end of the executed step is applied as a as quantity to ~S add/
subtract network 68, by way of conductor 69.
In addition, fine positioning of tape 10 is achieved by way of
conductor 70. This conductor applies any head/track alignment error
on conductor 30 to the input of DAC 55, thereby causing fine positioning
of spool 14 in order to produce the required exact coincidence between
the path of headwheel 16 and the new data track.
During the time of this fine posit1oning, network 68 is computing
a as quantity which is either added to or subtracted from the previous
S+ ~S executed step contained in register 60.
With reference to FIGURE 11, assume that the tape is initially at
rest at the position identified as Tl and that the executed step
S~ ~ S is equal to 76 pulses from tachometer 50. In the example shown,
the position of the adjacent track T2 has been overshot, and a ~S move-
ment of -3 tachometer pulses is nècessary to fine position the tape to
bring track T2 into exact coincidence with headwheel 16. At this time
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1 reg~ster 60 contains the count of 76. The aB quantity on conductor 69
is 3. Network 68 is now effective to subtract the quantity 3 from the
s+as quantity in register 60. As a result, the content of register 60
changes to the quantity 73. This quantity 73 designates the length
of the step to be executed to the next adjacent tape track, the next
time an execute step command is received.
Now that tape 10 has been properly positioned, if vacuum column
force-71 is not exactly balanced by the stop-lock torque of motor 31,
tape 10 will tend to creep away from the desired stop-lock position.
In the structure of FIGURE 6 a nominal stop-lock source 72 of constant
magnitude is applied to motor 31. Integrator 66 is adaptively effec-
tive to increase or decrease the stop-lock torque of motor 31 to produce
the required balance in forces on the length of tape passing through
transducing station 15. More specifically, a tendency of the tape to
creep away from the desired stop-lock position results in rotation of
tachometer 50. As a result of this rotation, a count is entered into
counter 61 and an output appears from DAC 55. This output is inte-
grated by integrator 66 and an output appears on conductor 73. This
output is summed with the output of source 72 to control the stop-
lock energization of motor 31. For example, should the stop-lock ener-
gization of motor 31 be too large at the end of an executed step,
reel 14 will rotate counterclockwise. As a result, the output of DAC
55 is integrated by integrator 66 to produce a voltage on conductor
73 which is opposite in polarity to that of source 72. The stop-
lock energization of motor 31 is thereby reduced. In addition, the
output of DAC 55 is effective to reposition motor 31 to the desired
rest position, reducing the count in counter 61 to zero. The tape
is now repositioned at the desired position with lower stop-lock ener-
gization of motor 31, and the tape is stabilized in this condition.
FIGURE 7 shows a digital integration network which functions much
as integrator 66 of FIGURE 6. In this figure, DAC 55 has been desig-
nated by the legend step execution DAC. This DAC receives binary
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1 position error input 62, as from counter 61 of FIGURE 6. The output
of DAC 55 energizes motor 31, usually by way of a power amplifier, not
shown. Nominal stop-lock source 72 provides a steady-state fixed
polarity energization of motor 31. In the structure of FIGURE 7,
combinational logic network 80 is sensitive to the operational status
of the magnetic tape unit and to the status of bistable latches 81,
82 and 83. A binary number resides in counter 84 which is effectively
an integration of the tendency of the tape to move away from the de-
sired stop-lock position, this tendency resulting from an inequality
between the motor's stop-lock torque and the force of the vacuum column.
The content of counter 84 controls stop-lock DAC 85 to thereby energize
motor 31 in a manner to readjust its stop-lock torque to the required
value. :
FIGURE 8 is a state diagram useful in explaning the operation
of the network of FIGURE 7. State 86 defines the binary 000 state
of latches 81, 82 and 83. The network of FIGURE 7 is initialized to
this state at the beginning of a period of operation.
When condition 87 occurs, the three latches move to state 88,
namely binary 001. The transition from state 86 to state 88 occurs
when combinational logic network 80 receives synchronizing information
from headwheel 16 indicating that the headwheel is in the tape's data
track area. Subsequently, the three latches may move to state 89 or
90, depending upon the conditions defined at 91 and 92. Both of these
conditions require that the synchronizing information received by logic
network 80 indicate that the head is in the servo track area. If the
tape has moved away from the desired stop-lock position, a position
error exists and the latches are moved to state 90, namely binary 011.
However, if no position error exists, namely, the execution of the
step has produced a stable stop-lock state, the three latches pass
through states 89 and 93 to state 86.
Assuming that a position error does in fact exist, then one of
the states 94 or 95 is entered, dependent upon the direction of this
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1 pqsition error. If the error is in a clockwise direction, counter
84 is incremented by conductor 96, whereas if the direction of error
is counterclockwise, the counter is decremented by conductor 97.
The binary state of latches 81, 82 and 83 is decoded by state
decode network 98 and is applied as an input to network 80 by way of
conductors 99. As with the previous embodiments of the present invention,
network 80 is enabled by conductor 100 only after a step has been ex-
ecuted. In addition, the apparatus of FIGURE 7 is clocked to step
through the state diagram of FIGURE 8 by means of clock 101.
FIGURE 9 shows a digital network which functions similar to regis-
ter 60 and network 68 of FIGURE 6 as a step modifying network to adjust
the take-up spool's rotational step distance in accordance with the
spool's tape radius. FIGURE 10 is a state diagram useful in explaining
operation of FIGURE 9.
A portion of the structure of FIGURE 9 is generally similar to the
structure of FIGURE 7 in that combinational logic network 102 functions
with three bistable latches 103, 104 and 105 and state decode network
106. With reference to FIGURE 10, the three latches are initialized
to the binary 000 state 107 at the beginning of machine operation.
Thereafter, the latches move to state 108 upon receiving an execute
step command. Transition to state 109 requires that head synchronizing
data from headwheel 16 indicate that head 17 is in the tape's data
track area. Thereafter, the latches make a transition to state 110
when synchronizing information indicates that the head is in the servo
track area. If a position error exists, indicating that head/track
alignment error should be ignored since the tape has not been posi-
tioned properly, the three latches pass from state 110 to state 107.
However, if the tape is properly positioned, the latches pass to
binary 110, identified as state 111. When the latches are in this
state, conductor 11? is effective to control register 113 in a manner
to cause this register to store the S+ ~S contents of network 114.
The content of network 114 is equal to the magnitude of the JUSt ex-
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1 ecuted S+ ~ S step, on conductor 115, and the ~S error quantity on
conductor 116. This ~S quantity is a function of the magnitude of the
head/track alignment error sensed by head 17 reading the tape's serYo
track. The direction of this error is compared to the direction of the
just executed step to determine if the ~ quantity should be added to
or subtracted from the just executed step magnitude S+~S on conductor
115. This new step command distance is stored in register 113 for use
when the next execute step command is received. The new quantity S~ S
contained in register 113 is calculated to produce a step providing
substantially exact coincidence between the path of headwheel 17 and the
next adjacent tape data track.
Should a new command to execute a step exist before the network
of FIGURE 9 has had time to compute a new s+as quantity, the latches
of FIGURE 9 enter state 11~ and a delay is provided by way of states
119 and 120. This delay is associated with rotation of headwheel 17
and the headwheel synchronizing signals required by conditions 121
and 122 to insure that this new step is executed before the three latches
enter the 107 state. As a result of this delayed operation, no new
s+as calculation is made and the old S+ ~S content of register 13 is
used for two successive steps.
While the invention has been particularly shown and described with
reference to preferred embodiments thereof, it will be understood by
those skilled in the art that various changes in form and details may be
made therein without departing from the spirit and scope of the invention.
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