Tape position-dependent, standstill tape tension control system

A magnetic tape transport having a pair of reel motors for bidirectionally driving a tape between a pair of reels within a cassette housing, and a pair of tension control transistors for controllably varying supply voltages across the respective reel motors. For holding the tape under tension when it is at rest, against the possibility of tape displacement regardless of varying tape roll diameters on both reels, the output pulses of a tape speed sensor are directed through a bidirectional counter into a digital to analog converter, so that the latter provides a voltage output that varies linearly in magnitude as the tape travels from one extremity toward the other. This variable voltage output from the converter is applied through a tension control circuit to the pair of tension control transistors whenever the tape is stopped. Standstill tape tension is thus controlled by varying the voltages impressed to both reel motors according to the current ratio of tape roll diameters on both reels.

BACKGROUND OF THE INVENTION 
This invention relates to magnetic tape transports, sometimes referred to 
as tape units, tape drives, tape decks, etc., for use with a replaceable 
tape assembly such as that known as a tape cassette or cartridge, and more 
particularly to digital magnetic tape transports used as a subsystem to 
enable a host system to obtain access to data on the magnetic tape. Still 
more particularly, the invention pertains, in such tape transports, to a 
system for controlling the tension of the tape, especially when it is at a 
standstill. 
Cassette tape transports, particularly to those operating in streaming 
mode, as contrasted with start/stop mode, have come to find extensive use 
as peripherals of computer systems. U.S. Pat. No. 4,163,532, filed by 
Sakai and assigned to the assignee of the instant application, discloses 
one such streaming cassette tape transport, or streamer. This prior art 
streamer comprises a pair of reel motors, to be drivingly coupled one to 
each reel of a tape cassette, for bidirectionally transporting the tape 
between the two reels under the direction of a motor control circuit 
forming a part of a tape speed control servo. Also included in the tape 
speed control servo is a tape speed sensor comprising a roll for 
frictional engagement with the tape, and an encoder for generating a 
series of pulses representative of the actual tape speed. The motor 
control circuit causes one of the reel motors to be driven in response to 
the tape speed sensor output pulses for constant speed tape transportation 
in each direction. 
The tape must of course travel under proper, constant tension between the 
reels in order to enable the transducer to correctly write or read data on 
the tape. To this end the motor control circuit additionally comprises 
tension control means which afford constant tape tension in the face of 
varying diameters of tape rolls on both reels. The tape must be held under 
tension when it is not only traveling but also at rest, in order to 
prevent the tape from slackening due to external forces. 
Conventionally, the pair of reel motors were both energized in opposite 
directions for holding the tape under tension when it is at rest. The sum 
of the magnitudes of currents flowing through both reel motors during such 
times was kept the same regardless of variable tape diameters on both 
reels, so that a current of greater magnitude flowed through whichever of 
the reel motors associated with the reel carrying a greater amount of 
tape. This conventional scheme was effective in the sense that tape 
tension could be controlled according to tape roll diameters on both 
reels, though to a limited degree. 
When too much difference existed between the tape roll diameters, as in 
tape positions near the beginning and end of the tape, the prior art 
system was not necessarily capable of causing the reel motors to be 
energized accordingly. The tape tended to travel slowly in the worst case 
because of imbalance in the magnitudes of currents flowing through both 
reel motors, especially when the tape was exceptionally light weight. 
SUMMARY OF THE INVENTION 
The present invention aims, in tape transports of the kind defined, at 
holding the tape under proper tension when it is at a standstill, against 
the risk of tape displacement no matter what the ratio of tape roll 
diameters on both reels may be. 
Briefly, the invention may be summarized as a tape tension control system 
for a tape transport for use with a tape assembly, comprising a pair of 
reel motors, a pair of tension control elements connected one between each 
reel motor and power supply means, each tension control element being 
capable of controllably varying a voltage applied from the power supply 
means to one of the reel motors in response to a variable voltage signal, 
and tape position sensor means for ascertaining a current position of the 
tape with respect to a transducer. Also included are tension control means 
connected between the tape position sensor means and the pair of tension 
control elements for applying to the tension control elements the variable 
voltage signals having magnitudes determined by the tape position when the 
tape is stopped, the variable voltage signals being such that the voltage 
applied to one of the reel motors decreases linearly, and the voltage 
applied to the other of the reel motors increases linearly, as the tape 
travels from a first toward a second extremity thereof, and vice versa. 
Typically, the tape position sensor means comprise a familiar tape speed 
sensor which provides a tape speed pulses indicative of the traveling 
speed of the tape, and a counter which bidirectionally counts the tape 
speed pulses from either extremity of the tape toward the other, so that 
wherever the tape is stopped, the count of the bidirectional counter 
represents the current position of the tape. The counter count is 
subsequently translated into an equivalent voltage by a digital to analog 
converter, so that the voltage output of the converter linearly varies 
with a predetermined gradient from one extremity of the tape toward the 
other. 
Connected next to the digital to analog converter is a tension control 
circuit comprising first circuit means for applying, when the tape is 
stopped, to one of the first tension control elements a variable voltage 
signal having a gradient equal in direction to the gradient of the voltage 
output of the converter, and second circuit means for applying, also when 
the tape is stopped, to the other tension control element a variable 
voltage signals having a gradient opposite in direction to the gradient of 
the voltage output of the converter. 
Thus, whenever the tape is stopped, standstill tape tension is controlled 
by varying the voltages impressed to both reel motors according to the 
current position of the tape and hence to the current ratio of tape roll 
diameters on both reels. There is therefore no danger of the tape 
accidentally loosening or traveling when it should be at a standstill. 
The above and other features and advantages of this invention and the 
manner of realizing them will become more apparent, and the invention 
itself will best be understood, from a study of the following description 
and appended claims, with reference had to the attached drawings showing a 
preferable embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention may be best embodied in the streaming tape transport 
of FIG. 1 which presupposes the use of the so-called digital cassette 
based on the standard audio cassette developed by Philips. Generally 
designated 1, the tape cassette has a housing 2 of relatively flat boxlike 
shape within which there are mounted a file reel 3 and a takeup reel 4 for 
rotation about spaced parallel axes. A length of magnetic tape 5, shown 
wound on the file reel 3, extends between the two reels along a predefined 
guide path. The cassette housing 2 has three windows 8, 9 and 10 formed in 
one edgewall thereof to expose parts of the tape 5 along the guide path. 
The tape transport has a magnetic transducer or read/write head 11 which 
partly intrudes into the cassette housing 2 through the window 8 for data 
transfer with the tape 5. A read/write circuit 12 of any known or suitable 
design is coupled to the head 11 for reading and writing data on the tape. 
For bidirectionally running the tape 5 between the reels 3 and 4, there are 
provided a pair of reel motors 13 and 14 which preferably are controllable 
speed, direct current motors. The reel motors 13 and 14 have drive 
spindles 3a and 4a which make driving engagement with the hubs of the 
reels 3 and 4, respectively, when the tape cassette 1 is loaded in 
position within the tape transport. The polarities of the reel motors 13 
and 14 are so determined as to rotate in a direction for winding up the 
tape 5; that is, the file reel motor 13 rotates in a clockwise direction, 
as viewed in FIG. 1, and the takeup reel motor 14 in a counterclockwise 
direction. 
The reel motors 13 and 14 are controllably driven by a motor control 
circuit 15 through a closed loop servomechanism. The tape speed a control 
servo includes a tape speed sensor 16. As disclosed in Sakai U.S. Pat. No. 
4,163,532, supra, the tape speed sensor 16 comprises a roll 17 and a pulse 
generator or encoder 18. The speed sensor roll 17 makes frictional contact 
with the tape 5 through the window 10 in the cassette housing 2. As this 
roll rotates with the travel of the tape 5, the encoder 18 
photoelectrically generates a series of discrete tape speed pulses at a 
recurrence rate proportional with the tape speed. The tape speed pulses 
are sent over a line 19 to the motor control circuit 15. 
Additionally, a beginning-of-tape (BOT) and end-of-tape (EOT) sensor 22 is 
connected to the motor control circuit 15. The BOT/EOT sensor 22 detects 
the standard BOT and EOT markers, not shown, of the tape 5. Typically, 
such markers are transparent end zones of the otherwise opaque tape 5. For 
sensing such transparent end zones, the sensor 22 is shown to comprise a 
light source 23 and a photodetector 24 disposed opposite each other across 
the tape 5. The noted window 9 in the cassette housing 2 is utilized 
toward this end. The BOT/EOT sensor 22 sends its output to the motor 
control circuit 15 over a line 25. 
The motor control circuit 15 is itself under the control of a host system, 
not shown, to which the illustrated tape transport is to be connected as a 
data storage subsystem. The host system supplies various tape 
transportation commands to the motor control circuit 15 over a bus 26. 
Normally, such commands include a tape run command, forward run command, 
reverse run command, fast run command, and stop command. 
FIG. 2 is a detailed illustration of the motor control circuit 15 shown 
together with the reel motors 13 and 14. These reel motors are both 
connected to a common 12-volt supply terminal 28 on one hand and, on the 
other hand, to a common grounding terminal 29 via a feedback resistor 
R.sub.3. A speed control transistor 27 is shown connected between the reel 
motors 13 and 14 and the supply terminal 28 for controlling the magnitude 
of the supply voltage across both reel motors and hence the traveling 
speed of the tape 5, although two such transistors could be connected in 
series with the respective motors for the same purpose. 
Two switching transistors Q.sub.1 and Q.sub.2, hereinafter referred to as 
the direction control transistors, are connected between the respective 
reel motors 13 and 14 and the grounding terminal 29. Also, the serial 
circuits of two tape tension control transistors Q.sub.3 and Q.sub.4 and 
two resistors R.sub.1 and R.sub.2 are connected between the respective 
reel motors 13 and 14 and the grounding terminal 29. Alternatively, 
however, the two tension control transistors Q.sub.3 and Q.sub.4 could be 
replaced by a single transistors connected to both reel motors via diode 
switches, as in the aforementioned Sakai patent. 
The motor control circuit 15 includes a controller 30, which may take the 
form of a programmable microprocessor in practice, for controlling the 
speed control transistor 27, direction control transistors Q.sub.1 and 
Q.sub.2, and tension control transistors Q.sub.3 and Q.sub.4, either 
directly or indirectly. The controller 30 directly controls the direction 
control transistors Q.sub.1 and Q.sub.2, turning the file reel motor 
switching transistor Q.sub.1 off, and the takeup reel motor switching 
transistor Q.sub.2 on, for forward tape travel from file reel 3 to takeup 
reel 4, and the file reel motor switching transistor Q.sub.1 on, and the 
takeup reel motor switching transistor Q.sub.2 off, for reverse tape 
travel. Both transistors Q.sub.1 and Q.sub.2 are turned off for stopping 
the tape. 
The controller 30 also takes part in the speed control and tension control 
of the tape 5. First, for tape speed control, the controller 30 is 
connected to the base of the speed control transistor 27 via a serial 
circuit of a duration modulated pulse generator 31, a voltage converter 32 
and a transistor driver circuit 33. For tape tension control, on the other 
hand, the controller 30 is connected to the bases of both tension control 
transistors Q.sub.3 and Q.sub.4 via a serial circuit of a digital to 
analog converter (DAC) 34 and a tension control circuit 35. 
Typically, the controller 30 takes the form of a microprocessor comprising 
a central processor unit and both random-access- and read-only-memories. 
Functionally speaking, however, the controller 30 may be thought of as 
being constructed as shown in FIG. 3. The controller 30 is herein shown to 
include a speed control data creation circuit 51 connected between the 
tape speed sensor 16, FIG. 1, and the duration modulated pulse generator 
31, FIG. 2. 
During writing or reading of data on the tape 5 by the head 11, the speed 
control data creation circuit 51 constantly inputs the tape speed pulses 
from the tape speed sensor 16 over the line 19, determines the actual 
traveling speed of the tape from the recurrence rate of the incoming 
pulses, and creates speed control data necessary for feedback control of 
tape transportation at a desired constant speed. The speed control data is 
supplied over a line 36 to the duration modulated pulse generator 31, 
which then responds by generating a series of speed control pulses having 
their durations modulated accordingly. These pulses have a fixed cycle of, 
typically, sixty-four microseconds but are subject to change in duty 
ratio. The voltage converter 32, FIG. 2, translates the incoming duration 
modulated speed control pulses into a unidirectional voltage with a 
magnitude determined by the pulse durations. Inputting this voltage, the 
driver circuit 33 causes conduction through the speed control transistor 
27 to a corresponding degree, with the consequent application of the 
supply voltage of controlled magnitude to either of the reel motors 13 and 
14. 
With reference back to FIG. 3 the controller 30 is also shown to include a 
direction control circuit 52 for directional control of tape 
transportation. Inputting the various tape run commands from the unshown 
host system over the bus 26, the direction control circuit 52 puts out the 
signals for direct on/off control of the direction control transistors 
Q.sub.1 and Q.sub.2, as has been set forth previously. 
For tape tension control the controller 30 is shown to comprise a 
bidirectional counter 53, two registers 54 and 55, two selector switches 
56 and 57, and a switch control circuit 58. The controller 30 determines 
tape tension control data when the tape is at rest, on the bases of the 
diameters of tape rolls on the reels 3 and 4, FIG. 1, of the tape cassette 
1 according to this invention. Toward this end the bidirectional counter 
53 is shown to have inputs connected to the tape speed sensor 16 by way of 
the line 19, and to the BOT/EOT sensor 22 by way of the line 25. Reset 
each time the sensor 22 senses the BOT and EOT markers, the counter 53 
counts the tape speed pulses in an increasing direction during forward 
tape travel and in a decreasing direction during reverse tape travel. The 
tension control data is sent to the DAC 34, FIG. 2, via the switches 56 
and 57 and the line 37. 
The first selector switch 56 chooses between the tension control data from 
the counter 53 and the output from the first register 54 under the control 
of the switch control circuit 58, for delivery to the DAC 34. The first 
register 54 has stored therein digital data representative of a relatively 
high voltage of, say, 2.8 volts. Relying on the outputs from the BOT/EOT 
sensor 22, the tension control data from the counter 54, and the tape run 
commands over the bus 26, the switch control circuit 58 ascertains whether 
the current tape position is known or not and causes the first selector 
switch 56 to choose the 2.8 volt register 54 when the current tape 
position is unknown. The DAC 34 on inputting the digital 2.8 volts data 
will put out a voltage of the same magnitude. 
The second register 55, on the other hand, stores digital data 
representative of a lower voltage of, say, 2.5 volts. The second selector 
switch 57 will cause this 2.5 volt data to be delivered to the DAC 34 over 
the line 37 during tape travel, either forward or reverse, instead of the 
output from the first selector switch 56. The DAC 34 on inputting the 2.5 
volts data will put out a voltage of that magnitude. 
As will be understood by referring to FIG. 2 again, the DAC 34 delivers an 
analog equivalents of the various digital tension control data from the 
controller 37 to the tension control circuit 35 over a line 38. The 
tension control circuit 35 generates tension control signals in the form 
of variable voltage signals from the tension control data as well as from 
a motor current signal supplied thereto over a line 39 connected to one 
extremity of the feedback resistor R.sub.3. The tension control signals 
are sent over lines 40 and 41 to the bases of the tension control 
transistors Q.sub.3 and Q.sub.4. The emitters of these tension control 
transistors are coupled to the tension control circuit 35 by way of 
respective lines 46 and 47 for feedback purposes. 
Seen at 43 in FIG. 2 is a motor braking circuit. It comprises a resistor 96 
connected in parallel with the reel motors 13 and 14 via respective diodes 
44 and 45. 
As illustrated in detail in FIG. 4, the tension control circuit 35 
comprises a first 71 and a second 72 operational amplifier. Both 
operational amplifiers have their noninverting inputs connected to a 
reference voltage source 73, and their outputs to the bases of the tension 
control transistors Q.sub.3 and Q.sub.4 by way of the lines 40 and 41, 
respectively. The inverting input of the first operational amplifier 71 is 
connected to the DAC output line 38 via a resistor 74, and the inverting 
input of the second operational amplifier 72 to the DAC output line 38 via 
another resistor 75 and an inverting amplifier 76. This inverting 
amplifier 76 comprises an operational amplifier 78 and two resistors 79 
and 80. The resistor 79 is connected between the DAC output line 38 and 
the inverting input of the operational amplifier 78 whereas the other 
resistor 80 is connected between the inverting input and output of the 
operational amplifier 78. The noninverting input of the operational 
amplifier 78 is connected to the reference voltage source 73. 
Also included in the tension control circuit 35 is a feedback circuit 81 
having its input connected to the feedback line 39, FIG. 2, and its output 
to the fixed contact a of a selector switch 82. Comprising an amplifier 
and resistors, the feedback circuit 81 puts out a signal indicative of the 
variable magnitude of the current flowing through either of the reel 
motors 13 and 14, the output signal being utilized for feedback control of 
tape tension during tape transportation. 
The selector switch 82 has another fixed contact b to which is connected a 
bias source 83. This bias source puts out a bias voltage obtained by 
dividing the reference voltage from the source 73 by two resistors 84 and 
85, such a bias voltage being utilized according to the invention for tape 
tension control when the tape is at rest. 
The selector switch 82 selects either of its contacts a and b in response 
to a signal indicative of whether the tape is traveling or at a 
standstill, such a signal being supplied over a line 42, FIG. 3, branching 
off from the tape run commands bus 26. The contact a is chosen when the 
tape is running, and the contact b when it is at rest. It will be noted 
from FIG. 2 that the signal line 42 is also connected to the transistor 
driver circuit 33 for purposes yet to be described. 
The output of the selector switch 82 is connected to a buffer amplifier 86 
and thence to the inverting input of the first operational amplifier 71 
via a resistor 87 and to the inverting input of the second operational 
amplifier 72 via a resistor 88. 
The emitter of the first tension control transistor Q.sub.3 is connected to 
the inverting input of the first operational amplifier 71 via a parallel 
connection of resistor 89 and capacitor 90 to form a feedback circuit. The 
emitter of the second tension control transistor Q.sub.4 is likewise 
connected to the inverting input of the second operational amplifier 72 
via a parallel connection of resistor 91 and capacitor 92 to form a 
feedback circuit. 
The typical constants of the various pertinent parts of the FIG. 4 
circuitry are as follows: 
______________________________________ 
Reference voltage source 73 
2.5 volts 
Resistors 74 and 75 30 kilohms 
Resistors 79, 80 and 84 
12 kilohms 
Resistor 85 43 kilohms 
Resistor 87 10 kilohms 
Resistor 88 6.2 kilohms 
Resistors 89 and 91 12 kilohms 
Capacitors 90 and 92 0.01 microfarads 
Resistor R.sub.1 12 ohms 
Resistor R.sub.2 15 ohms 
Supply voltage of 12 volts. 
amplifiers 71, 72, 78 and 86 
______________________________________ 
Operation 
Assume that the controller 30, FIGS. 2 and 3, has now received from the 
host a forward tape run command dictating the forwarding of the tape 5 for 
writing or reading. Then the direction control circuit 52 of the 
controller 30 will respond by turning the first direction control 
transistor Q.sub.1 off, and the second direction control transistor 
Q.sub.2 on. Then the takeup reel motor 14 will start rotation, being 
energized through the closed supply circuit comprising the supply terminal 
28, speed control transistor 27, motor 14, second direction control 
transistor Q.sub.2, feedback resistor R.sub.3 and grounding terminal 29. 
As the tape 5 thus starts traveling forwardly, the speed sensor 16, FIG. 1, 
will begin delivering tape speed pulses to the controller 30 over the line 
19. The speed control data creation circuit 51 of the controller 30 will 
then respond by creating speed control data accordingly, for delivery to 
the duration modulated pulse generator 31, FIG. 2. The duration modulated 
speed control pulses from the generator 31 will be translated by the 
voltage converter 32 into an equivalent unidirectional voltage for 
controlling the collector-emitter resistance of the speed control 
transistor 27. There will thus be completed a tape speed control servo 
loop whereby the supply voltage across the takeup reel motor 14 will be 
controlled so as to hold constant the traveling speed of the tape. 
Tape tension during such forward tape travel is controlled by energizing 
the file reel motor 13 in a direction opposite to the rotational direction 
of the takeup reel motor 14. The supply voltage across the file reel motor 
13 must be varied according to the varying diameters of the tape rolls on 
both reels. The tension control circuit 35 makes such control of the 
supply voltage in cooperation with the controller 30 in the following 
manner: 
As has been set forth with reference to FIG. 3, the functional diagram of 
the controller 30, the selector switch 57 connects the 2.5 volts register 
55 to the output line 37 during forward, as well as reverse, tape travel. 
Thus the DAC 34, FIG. 2, supplies a voltage of 2.5 volts to the tension 
control circuit 35 over the line 38. This output voltage of the DAC 34 
remains unchanged throughout tape travel from BOT to EOT, and vice versa, 
as indicated by the dashed line designated A in FIG. 5. 
In the tension control circuit 35, FIG. 4, the switch 82 connects the 
feedback circuit 81 to the inverting inputs of both operational amplifiers 
71 and 72. Since the voltage on the DAC output line 38 is constant at 2.5 
volts as aforesaid, the output from the first operational amplifier 71 
changes with the voltage from the feedback circuit 81 during forward tape 
travel. The voltage from the feedback circuit 81 gradually increases with 
the progress of forward tape travel, so that the output from the first 
operational amplifier 71 gradually decreases, resulting in turn in a 
gradual increase in the collector-to-emitter voltage of the first tension 
control transistor Q.sub.3 and, therefore, in a gradual decrease in the 
voltage across the file reel motor 13. This gradual decrease in the 
voltage across the file reel motor is essential for constant tape tension 
during forward tape travel, because then the tape roll on the file reel 3 
of the tape cassette 1, FIG. 1, decreases in diameter and hence in weight, 
imposing a progressively less load on the file reel motor 13. 
The second tension control transistor Q.sub.4 as well as the resistor 
Q.sub.2 is short circuited during forward tape travel by the second 
direction control transistor Q.sub.2, which is then conductive as 
aforesaid. The second tension control transistor is therefore independent 
of the output from the second operational amplifier 72 of the tension 
control circuit 35. 
During reverse tape travel, on the other hand, the file reel motor 
switching transistor Q.sub.1 is on, and the takeup reel motor switching 
transistor Q.sub.2 off. As during forward tape travel the selector switch 
57, FIG. 3, of the controller 30 connects the 2.5 volts register 55 to the 
output line 37, so that the DAC 34, FIG. 2, supplies a constant voltage of 
2.5 volts to the tension control circuit 35. 
In the tension control circuit 35, FIG. 4, the 2.5 volts output from the 
DAC 34 is impressed to the inverting input of the inverting amplifier 76, 
to the noninverting input of which is applied the 2.5 volts output is from 
the reference voltage source 73. The resulting 2.5 volts output from the 
inverting amplifier 76 is impressed to the inverting input of the second 
operational amplifier 72. Thus, as during forward tape travel, the output 
from the second operational amplifier 72 changes with the voltage from the 
feedback circuit 81, which is held connected to the inverting inputs of 
both operational amplifiers 71 and 72 by the switch 82. It is therefore 
apparent that the second operational amplifier 72 functions to cause a 
gradual increase in the collector-to-emitter voltage of the second tension 
control transistor Q.sub.4 and hence a gradual decrease in the voltage 
across the takeup reel motor 14, thereby holding the tape under constant 
tension during reverse tape travel. 
For holding the tape under tension when it is at rest according to the 
invention, the driver circuit 33, FIG. 2, applies a constant voltage to 
the speed control transistor 27 in response to the signal on the line 42 
indicative of whether the tape is traveling or not. Consequently, both 
reel motors 13 and 14 can be energized from the supply terminal 28 even 
when the tape is at rest. 
In the controller 30, FIG. 3, the second selector switch 57 also responds 
to the signal on the line 42 by connecting the first selector switch 56 to 
the output line 37. It is understood that the current tape position is now 
known, so that it is the bidirectional counter 53, rather than the 2.8 
volts register 54, that is thus connected to the controller output line 
37. As is conventional in the art, the bidirectional counter 53 is reset 
when the BOT/EOT sensor 22, FIG. 1, senses the BOT or EOT marker during 
the initialization process following the loading of the tape cassette 1 in 
the tape transport, and thereafter continuously puts out a count 
corresponding to the current tape position. When the tape is stopped in 
any arbitrary position, therefore, the output from the counter 53 
indicates that tape position. 
Inputting this output from the counter 53 over the line 37, the DAC 34, 
FIG. 2, produces a voltage signal having a magnitude corresponding to the 
count of the counter and hence to the current tape position. Thus, as 
graphically indicated at B in FIG. 5, the output voltage of the DAC 34 
increases linearly from, say, 2.4 volts at BOT to, say, 3.2 volts at EOT. 
This variable voltage signal is applied to the tension control circuit 35 
over the line 38 for controlling tape tension when the tape is at rest. 
The selector switch 82, FIG. 4, of the tension control circuit 35 also 
responds to the signal on the line 42 by connecting the bias voltage 
source 83 to the buffer amplifier 86 when the tape is stopped. The tension 
control feedback loop is now opened; instead, a constant bias voltage is 
applied to the inverting inputs of both operational amplifiers 71 and 72. 
FIG. 6 graphically represents the voltages produced by the two operational 
amplifiers 71 and 72 in response to the variable voltage signal from the 
DAC 34 and the constant bias voltage from the source 83, against tape 
position from BOT to EOT. It will be noted that the output voltage of the 
first operational amplifier 71 decreases linearly from BOT to EOT because 
of the prepositioned inverting amplifier 76, whereas that of the second 
operational amplifier 72 increases linearly from BOT to EOT. 
Let it be assumed that the tape has been stopped at BOT. The output voltage 
of the DAC 34 is the lowest when the tape is in this tape position, 
according to the voltage gradient B in FIG. 5. The output voltage of the 
first operational amplifier 71 is therefore the highest now, as will be 
seen from the graph of FIG. 6, and gradually decreases toward EOT. Thus, 
as the output voltage of the first operational amplifier gradually 
decreases from BOT to EOT, so does the voltage with which the file reel 
motor 13 is energized. The file reel motor is thus energized with a 
decreasing voltage from BOT to EOT because the diameter, and hence the 
weight, of the tape roll on the file reel 3 decreases in that direction. 
The output voltage of the DAC 34 when the tape is at rest is directed 
through the inverting amplifier 76 to the second operational amplifier 72, 
so that the output voltage of the second operational amplifier is the 
lowest when the tape is stopped at BOT, and linearly increases toward EOT. 
Thus, as indicated by the dashed line in FIG. 6, the output voltage of the 
second operational amplifier 72 has a gradient opposite in direction to 
that of the output voltage of the first operational amplifier 71. The 
takeup reel motor 14 is therefore energized with an increasing voltage 
from BOT to EOT because the tape roll diameter on the takeup reel 4 
increases in that direction. 
It will thus be appreciated that, whatever position the tape may be stopped 
in, it can be held under proper tension as both reel motors 13 and 14 are 
energized with voltages determined by the variable voltage output from the 
DAC 34 and hence by that tape position. The tape position is accurately 
ascertained by bidirectionally counting the output pulses of the tape 
speed sensor 16 from either extremity of the tape, which sensor is a 
standard component of tape transports of the kind under consideration. 
Further, the variable voltage corresponding to the variable tape position 
is obtained merely by directing the count of the bidirectional counter 53 
into the DAC 34. Thus the tape transports of standard design require only 
minor alterations in electronics for incorporating the teachings of this 
invention. 
The tape position is unknown from the loading of the tape cassette 1 to the 
detection of the BOT or EOT marker by the sensor 22. During such times, as 
well as when the tape position subsequently becomes unknown for some 
reason or other, the switch control circuit 58 of the FIG. 3 controller 30 
will cause the selector switch 56 to choose the 2.8 volts register 54. 
Then the DAC 34, FIG. 2, will put out a constant voltage of 2.8 volts, as 
indicated C in FIG. 5, until the current tape position becomes known. 
Despite the foregoing detailed disclosure, it is not desired that the 
invention be limited by the exact showing of the drawings of the 
description thereof. The following, then, is a brief list of possible 
modifications, alterations and adaptations of this invention which are all 
believed to fall within the scope of the invention: 
1. The output voltage from the DAC 34 could be increased from BOT to EOT 
during tape travel, too, instead of being held constant as in FIG. 5, for 
tape tension control when the tape was traveling. In this case, however, 
the increase in DAC output voltage might be made a little steeper than 
that when the tape was at rest. 
2. The feedback resistor R.sub.3, FIG. 2, could be omitted if the motor 
current signal, needed for feedback control of tape tension during tape 
travel, was obtained from the supply side of the reel motors 13 and 14. 
3. The tape speed sensor 16, FIG. 1, might be replaced by a revolution 
sensor or sensors of either or both of tape cassette reel hubs, although 
in this case the resulting reel revolution data would have to be amended 
according to the amount of travel of the tape for providing the desired 
tape speed data.