Web transport system
CROSS REFERENCE TO RELATED APPLICATIONS 
     This patent application is related to the following commonly assigned, copendinng patent applications: 
     1. Web Transport Capstan Control System, Ser. No. 682,849, filed May 3, 1976, by Robert Perrine Harshberger, Jr. 
     2. Switching Motor Control System, Ser. No. 682,372, filed May 3, 1976, by Robert Perrine Harshberger, Jr. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to web transport systems having a capstan controlling web motion along a web path and more particularly to a professional audio magnetic tape transport system. 
     2. Description of the Prior Art 
     Web transport systems are required for a large variety of applications which range from heavy industrial applications to motion picture projectors and magnetic tape transports. Such systems are typically utilized where comparatively rapid, precise control over web motion is required. These systems typically employ one or more capstans engaging the web material to control its motion along a web path and a web storage system disposed to supply and take up web material moving along the web path. Vacuum chambers or tension arms may be employed to provide buffer loops adjacent the web storage system to permit web acceleration along the web path to exceed the capabilities of the web storage system for a short time. 
     The exact requirements of a web transport system depend on particular applications and even requirements of magnetic tape transports vary widely according to their intended usage. For example, digital tape transports must provide closely controlled acceleration intervals with moderate speed control during steady state operation. For audio and instrumentation tape transports acceleration is less important but a very precise constant speed must be maintained. 
     In particular, audio tape transports have typically provided small diameter capstans of 1/2 inch diameter or less with a large inertia rotating motor assembly and a pinch roller to maintain the tape in frictional, nonslip engagement with the capstan. The small capstan must rotate rapidly to drive tape at a nominal speed and the high rotational energy of the capstan rotor assembly resulting from the high speed and high inertia helps to maintain rotating speed more constant. These audio transports thus accelerate tape very slowly and frequently provide two loop tension buffers adjacent the reels. One typical audio system has no buffering between the capstan and a take-up reel and only a tension sensor positioned between the head assembly and a supply reel to permit the supply reel to be energized to maintain constant tape tension across the head assembly during steady speed operation. 
     Although audio tape transports have heretofore provided high energy, slow acceleration capstan control systems, these systems have had severe disadvantages. For example, several seconds and several inches of tape may be required to accelerate to a nominal play speed. An operator wishing to begin playing a given recording location at a given instant in time must preposition the given recording location on the tape several inches upstream from the read head and then turn the transport on several seconds before the desired play start time. A misjudgment of either tape distance or acceleration time would result in a failure to begin playing at the desired location at the desired time. 
     SUMMARY OF THE INVENTION 
     A high performance, but compact and economical web transport system in accordance with the invention includes a web storage system disposed to supply and take up web material moving bidirectionally along a web path, a single drive capstan positioned in a nonslip engagement with web material to bidirectionally control web motion along the web path, a capstan drive system coupled to energize the capstan in accordance with transport operating conditions, a pair of web loop tension arms positioned along the web path on opposite sides of the capstan to maintain buffer loops of web material as well as web tension in accordance with torque applied to the tension arms, a pair of torque transducers coupled to apply torque to the tension arms in response to tension command signals, and a torque control system coupled to generate tension command signals which vary in accordance with web transport system operating conditions. 
     More particularly, in a professional audio transport system the web is a magnetic tape and the storage system includes a pair of tape reels having tape wound thereon in a helical coil and a reel energization system controlling reel motion to tend to maintain the tension arms at nominal positions between long and short loop limit positions. The nominal positions are selected to orient each tension arm perpendicular to the line of tape when an intermediate diameter tape pack is stored by an adjacent reel. Under this arrangement the torque moment arm of the segment of tape going to the reel is a maximum and decreases with deviations of tape pack from the intermediate diameter. However, if the distance between the points of tangency of the tape with the tape pack and tension arm is greater than the variations in tape pack radius, the tension variations tend to be quite small and the requirement for constant tension guides between the tension arms and reels to assure a constant wrap angle of tape about the tension arms can be avoided. Furthermore, variations in tape tension at the two tension arms tend to be equal to help maintain equal tension across the capstan. The orientation of the tension arms relative to symmetrical idler and capstan rollers in such that torque applied to the tension arm by a tape segment between the arm and roller increases as a tension arm rotates inwardly to increase buffer loop length. The torque transducers are constructed to apply increased torque as loop length increases. These torque changes tend to cancel each other to maintain constant tension in the tape. 
     Additional advantages result from controlling tension arm torque in accordance with tape direction to compensate for head friction and further maintain balanced tension across the capstan. With tension thus balanced a large, hard surface capstan of ceramic material such as aluminum oxide may be employed without need for a pinch roller or large tape wrap angle. The problems associated with maintenance and susceptibility to damage of a rubberized capstan are thus avoided. The large capstan diameter reduces the required acceleration energy to permit acceleration to nominal speed in 0.5 second or less as well as capstan control over high shuttle speeds. In one mode of operation the tension arm adjacent the capstan may be given a high torque energization to cause the arm to engage the capstan as a pinch roller. Capstan control of tape motion may thus be maintained even in a play edit operating mode wherein the take-up reel is deenergized and tape accumulates between the capstan and the take-up reel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the invention may be had from a consideration of the following detailed description taken in conjuction with the accompanying drawings in which: 
     FIG. 1 is a combination plan view and block diagram representation of a professional audio magnetic tape transport in accordance with the invention; 
     FIG. 2 is a plan view, partly broken away, of a portion of the transport shown in FIG. 1 having a tension arm and torque transducer; 
     FIG. 3 is a schematic diagram of an energization control circuit for controlling the energization of the torque transducer shown in FIG. 2; and 
     FIG. 4 is a diagram illustrating the relationship between tape tension and tension arm torque for one of the tape loop tension arms. 
    
    
     DETAILED DESCRIPTION 
     As shown in FIG. 1, a professional audio magnetic tape transport 10 in accordance with the invention includes a supply reel 12 storing a first helical coil 14 of magnetic tape 16 and a take-up reel 18 sotring a second helical coil 20 of magnetic tape 16. The reels 12, 18 are mounted in side-by-side relationship for rotation about reel axes 22, 24 respectively which lie along a horizontally extending reel ine 26. A supply reel energization system 28 is coupled to rotationally energize the supply reel 12 in response to a tension arm position signal 30 received from a position sensor 32. Position sensor 32 is coupled to sense the rotational position of a supply side tape loop tension arm 34 and generate the tension arm position signal 30 as an indication thereof. The reel energization system 28 responds to the position signal 30 to bidirectionally energize the reel 12 and tape coil 14 to maintain tension arm 34 at a nominal central position intermediate long and short loop limit positions. Similarly, a reel energization system 40 receives a tension arm position signal 42 from a position sensor 44 which indicates the rotational position of a take-up side tape loop tension arm 46 and bidirectionally energizes the take-up reel 18 and tape coil 20 to tend to maintain the tension arm 46 at a central nominal position during steady speed motion of tape 16 along a web path extending between the supply tension arm 34 and take-up tension arm 46. 
     The tension arms 34, 46 are mounted for rotation about tension arm axes 48, 50 respectively which are positioned equidistant from the reel axes 22, 24 respectively along a tension arm line 52 which extends parallel to the reel line 26. This spacing between the axes 48 and 50 is less than the spacing between the axes 22 and 24. 
     An idler roller 54 and a capstan roller 56 are positioned for rotation about roller axes 58 and 60 respectively. The roller axes 58 and 60 are positioned equidistant from the reel axes 22 and 24 respectively along a roller line 62 that extends parallel to the reel line 26 and arm line 52 with the arm line 52 positioned between the roller line 62 and reel line 26. The distance between the roller axes 58 and 60 is greater than the distance between the arm axes 48 and 50 but less than the distance between the reel axes 22 and 24. The rollers 54 and 56 have cylindrical tape engaging surfaces with substantially equal diameters to provide symmetry for the tape path defined by the tape 16. The rollers 54 and 56 have relatively large diameters in excess of 1.5 inches (3.31 cm) and preferably of approximately 2.375 inches (6.03 cm). This relatively large diameter permits the capstan 56 to control motion of the tape 16 along the tape path in all modes of operation including high speed forward and rewind shuttle modes as well as normal record/reproduce speeds. Capstan motion is controlled by a capstan energization system 64 in response to operator control signals communicated through a control panel 66. The capstan energization system 64 may include a direct coupled D.C. capstan motor and tachometer providing a capstan rotor assembly having moderate inertia for good control over steady state speed operation. However, the large diameter of capstan 56 permits the capstan motor of capstan energization system 64 to provide nominal tape speeds at moderate motor speeds so that excessive energy is not required for acceleration of capstan 56. A combination of precise steady state speed control and rapid acceleration times less than 0.5 seconds may thus be attained with the present arrangement. A transducer head assembly 68 including an erase head 70, a write head 72, and a read head 74 is positioned along the path of tape 16 between the rollers 54 and 56 for frictional engagement with the same side of the tape 16 as the rollers 54 and 56. Recording circuits 76 control the recording of information on or reproducing of information from the magnetic tape 16 as it moves past the head assembly 68. Edge guide surfaces 78, 80 and 82 positioned along the path of tape 16 between the rollers 54 and 56 guide and position the tape 16 as it passes the head assembly 68. A pair of solenoid actuated lifter arms 84, 86 are positioned to selectively lift the tape 16 from the head assembly 68 during high speed shuttle operations to reduce head wear on the head assembly 68. 
     A rotary switch 90 permits operator selection of nominal play/record tape speeds while a toggle switch 92 turns the transport 10 on and off. Capstan 56 has a cylindrical capstan handle 94 which extends out of the plane of the tape path for easy manipulation by an operator. The handle 94 thus makes it possible for the capstan roller 56 to control motion of tape 16 along the tape path even in response to direct operator control. An operator may thus preposition the tape 16 by hand by merely rotating the capstan 56 and without touching the reels 12, 18 which are automatically energized to supply and take up tape 16 as necessary. 
     Guide rollers 102, 104 are mounted on the tape loop tension arms 34 and 46 for rotation about guide axes 106, 108 which are spaced apart from the arm axes 48 and 50 respectively. The tape 16 passes directly from the associated respective tape coils 14 and 20 along straight line tape paths 110, 112 to the guide rollers 102 and 104 respectively without intermediate guiding. From guide roller 102 the tape 16 passes directly to idler roller 54 where it has an approximately 143° wrap around angle and passes the head assembly 68 to the capstan roller 56. The tape 16 has an approximately 134° wrap around angle about capstan 56 to complete the symmetry of the tape path before passing directly to gude roller 104 and then directly along straight line tape path 112 to tangentially engage the outer circumference of helical tape coil 20. 
     The torque exerted on a tape loop tension arm 34 or 46 by the tape 16 is equal to the tape tension times the distance between a first straight line tape segment of tape on one side and the axis of rotation plus the tape tension times the distance between a second straight line segment of tape on the other side and the axis of rotation. For example, a first portion of the torque exerted on tension arm 34 about axis 48 is equal to the tape tension times the distance between the straight line tape path 116 and axis 48. It may be observed that this distance is fairly small for the nominal position of arm 34 wherein the straight line 116 passes reasonably close to axis 48. However, as arm 48 moves inwardly toward a longer loop position, this moment arm length, which is measured perpendicular to the line 116, becomes longer as indicated by alternate tape line 118 and alternate moment arm 120. 
     As long as the tension arm 34 remains in its nominal position in fixed relationship to roller 54, the torque exerted on arm 34 about axis 48 by the tape segment passing along tape path 116 remains constant and does not induce changes in tape tension. 
     On the other hand, the angle of tape path segment 110 changes with tape pack diameter and therefore does induce changes in tape tension. In prior art systems these tension changes can be as much as three to one and thus a constant tension tape guide is placed along the tape path segment 110 to prevent changes in tape tension with a diameter of coil 14. These large changes in tape tension could easily cause slippage of tape 16 on capstan 56. However, as explained more fully below, so long as the distance between the tangent points of tape path segment 110 with guide 102 and coil 14 is greater than the changes in coil radius and the distance between line segment 110 and axis 48 is at a maximum when the tension arm 34 is in its nominal position and the coil 14 has an intermediate diameter half way between its typical largest and smallest diameters, the variations in tape tension are small and the extra guide can be eliminated to simplify the tape path and reduce cost. 
     In addition to the tape tension uniformity provided by proper positioning of tension arms 34 and 46, tape tension adjacent capstan 56 on opposite sides thereof may be further balanced by selectively controlling the torque applied to tension arms 34 and 46 by a tension arm energization system 124. The tension arm energization system 124 includes electromagnetic torque transducers applying torque to the tension arms 34 and 46 about axes 48 and 50 respectively in response to tension control signals. The tension may thus be controlled in response to tape direction or other tape transport operating conditions to neutralize variations in tape tension at the capstan 56 which result from frictional forces of head assembly 68 and depend on tape direction. Thus, by providing constant tape tension regardless of tape coil diameter and tape direction, across capstan 56, there with be no tape slippage relative to capstan 56 even though the tape wrap angle is relatively small and the capstan 56 does not have a high friction resilient rubber coating thereon. For example, the capstan 56 may have a hard nonresilient tape engaging surface of a ceramic material such as aluminum oxide. The problems of damage and special cleaning requirements for rubberized capstan surfaces are thus avoided and a convenient tape path configuration that does not require a large wrap around angle of tape 16 about capstan 56 may be employed. 
     Referring now to FIG. 2, an electromagnetic torque transducer 126 in the form of a variable reluctance rotary solenoid is illustrated behind a partly broken away cover plate 127 as including a magnetic armature 128 and rotor 130 coupled by rivets or screws 131 to apply torque to the supply side tape loop tension arm 34 about tension arm axis 48. Armature 128 provides a continuous laminated magnetic path to opposite sides of rotor 130 and is magnetically energized by coils 132, 134 which are in turn energized by a tension control signal 136 in the form of a controlled current. Alternate laminations terminate along straight lines 137 and 138 which are tangential to the curvature of armature 128 adjacent the rotor 130. Thus, as arm 34 and rotor 130 rotate clockwise toward a shorter loop position the effective air gap between armature 128 and rotor 130 increases and the torque applied to arm 34 by transducer 126 decreases for a given applied tension control current signal. Accordingly, a counterclockwise rotation tends to produce an increased applied torque which compensates the increase of effective moment arm of straight tape line segment 116 about axis 48 for counterclockwise rotation of arm 34. Constant tape tension at arm 34 is thus maintained for different rotational positions of arm 34 to further insure balanced tape tension across opposite sides of capstan roller 56 to guard against tape slippage. The torque transducer 126 thus replaces the spring mechanism that is typically associated with a tape tension arm to provide a torque that may be conveniently controlled and varied in accordance with tape operating conditions of the tape transport 10. Electromagnetic relationships of torque transducer 126 are described generally in Rotors, Herbert C., Electromagnetic Devices, First Ed., pp. 204-205, John Wiley &amp; Sons, Inc. (New York, 1941). 
     Referring now to FIG. 3, there is shown the energization control circuit 140 for controlling the energization of the supply torque transducer solenoid 126 and the take-up torque transducer solenoid 142. Torque transducer solenoid 126 has one terminal coupled to +20 volts and a second terminal at which the tension control signal 136 is provided coupled to a drive amplifier 144 which energizes torque transducer solenoid 126 in response to a command voltage provided through a 16K coupling resistor 146. A diode 148 is coupled in parallel with torque transiducer solenoid 126. 
     similarly, torque transducer solenoid 142 has one terminal coupled to +20 volts and a second terminal coupled to a tension control signal 150 provided in the form of a current signal by a drive amplifier 152 in response to a voltage provided through a 16K coupling resistor 154. A diode 156 is connected in parallel across torque transducer solenoid 142. Because the drive amplifiers 144 and 152 are identical, only the drive amplifier 152 is shown in detail. It includes an operational amplifier 158 with a high open loop voltage gain coupled in a noninverting configuration with the noninverting input coupled through a 100K resistor 160 and through an adjustable terminal of a 10K potentiometer 162 to coupling resistor 154. The potentiometer 162 permits fine adjustment of the torque produced by torque transducer solenoid 142. For a preferred method of adjustment, the tape transport is operated in a forward mode with intermediate tape pack diameters on both the supply and take-up reels. The potentiometer within drive amplifier 144 (not shown) which is equivalent to potentiometer 162 is adjusted to provide a desired tape tension across the head assembly 68 as indicated by a tape tension sensor, which is an item of test equipment and not a part of the tape transport 10. The potentiometer 162 is then adjusted to provide substantially no tape tension differential adjacent opposite sides of the capstan 56. 
     The inverting input of amplifier 158 is coupled through a 100K resistor 164 to a 1 ohm current sensing resistor 166. Amplifier 158 thus provides a positive output signal until the current through resistor 166 provides a voltage potential at the inverting input equal to the voltage potential at the non-inverting input. Amplifier 152 thus operates as a voltage to current converter to produce a current through solenoid 142 in the form of tension control signal 150 which is linearly proportional to an applied voltage signal. The output of amplifier 152 is coupled to the base of an npn transistor 168 whose emitter is in turn coupled to the base of an npn transistor 170. The collector current of transistor 170 provides the tension control signal 115 for torque transducer solenoid 142. Voltage divider networds 174 and 176 provide the controlled voltages for driver amplifier 144 and driver amplifier 152 respectively. The voltage divider network 174 includes a 1K resistor 178 coupled between +5 volts and a summing junction 180 providing the control voltage for the tension control signal 136. Junction 180 is directly coupled to a logic inverter gate 182, coupled through a 510 ohm resistor 184 to a logic inverter gate 186, coupled through a 240 ohm resistor 188 to a logic inverter gate 190, coupled through a 360 ohm resistor 192 to a logic inverter gate 194, coupled through a 330 ohm resistor 196 to a logic inverter gate 198 and coupled through a 220 ohm resistor 200 to a logic inverter gate 202. Similarly, the voltage divider 176 includes a 1K resistor 210 coupled between +5 volts and a summing junction 212. The summing junction 212 is directly coupled to a logic inverter 214, coupled through a 430 ohm resistor 216 to an inverter 218, coupled through a 270 ohm resistor 220 to an inverter 222, coupled through a 430 ohm resistor 224 to an inverter 226, coupled through a 270 ohm resistor 228 to an inverter 230 and coupled through a 220 ohm resistor 232 to an inverter 234. 
     The coupling resistors and logic inverter gates operate as a special digital to analog converter to provide voltages at the summing junctions 180 and 212 in response to applied digital signals. 
     An AND gate 240 has its output connected to inverter 186 and inverter 218, and AND gate 241 has its output connected to inverter 190 and inverter 222, and AND gate 242 has its output connected to an inverter 194 and an inverter 226 and an AND gate 243 has its output connected to the inputs of an inverter 198 and an inverter 230. 
     The complement of a servo on signal, SVO, is coupled to the input of inverter gates 182 and 214. 
     In a standby mode when the servos are to be inoperative, the signal SVO goes low or false, making its complement go high and the output of inverter gates 182 and 214 to go low and pull the summing junctions 180 and 212 down to ground potential. This commands zero current flow through the torque transducer solenoids 126 and 142 to prevent the generation of any torque in the tension arms. Under normal operating conditions the servo on signal, SVO, is high making its complement low to disable inverter gates 182 and 214 to enable the other inverter gates to control the current command signals, in accordance with the operating status of tape transport 10. 
     The complement of a stop command signal, STC is communicated through an inverter gate 246 to one input of a NAND gate 248 while the complement of a motion sense signal, MTS, is applied to the other input of NAND gate 248. The output, STOPPED, of NAND gate 248 thus goes low to indicate a stopped condition when a tape stop condition has been commanded and tape motion has actually stopped. A NAND gate 250 receives the STOPPED signal as well as a complement play edit command signal, PEC, to generate an output signal, PEC + STOPPED. This signal is inverted by an inverter gate 252 to disable the AND gates 240-243 in response to a play edit command or a commanded stop condition and to enable the AND gates 240-243 during a normal tape motion condition. 
     In the event of a stopped condition, the output of NAND gate 250 goes high and is connected to the input of inverter gate 200 and through an AND gate 254 to the input of an inverter 234. The inverter gates 202 and 234 are coupled through relatively small 220 ohm resistors to the respective summing junctions 180 and 212 to provide equal, but relatively small command voltages to the drive amplifiers 144 and 152 to command relatively small torques for the tape loop tension arms during a standby stopped condition. In the event of a play edit command, the same small torque is applied to the supply reel tension arm 34 but AND gate 254 blocks the signal to inverter gate 234 so that summing junction 212 receives a maximum 5 volt command signal and a maximum torque is applied to tape loop tension arm 46 causing it to be driven against capstan roller 54 to act as a pinch roller with the tape 16 pinched between the guide roller 104 and the capstan roller 156. 
     In the event of a normal tape motion condition, the output of NAND gate 250 goes low to enable the AND gates 240-243. One of these AND gates is fully activated in response to a tape direction signal, TDR, and a tape width command signal, TWC. If 1/4 inch tape is being used signal TWC is high to enable AND gates 211 and 243 while if 1/2 inch tape is being used signal TWC is low to enable AND gates 240 and 242. Signal TDR enables gates 241 and 243 for a forward motion direction while AND gates 240 and 242 are enabled for a reverse motion direction. 
     If it is assumed for purposes of illustration that half inch tape is being utilized and tape motion is in a forward direction, then AND gate 242 will be fully enabled to provide a high output signal. AND gate 240 would be enabled for a reverse direction. It will be noted that for the take-up tension arm the summing junction 212 is coupled to ground through a 430 ohm resistor 216 or 224 for either direction of motion and applied torque, and hence taped tension, remains the same for both directions of motion of tape 16. The tension is maintained the same because the tape path friction does not modify the tape tension on the take-up side of the capstan in accordance with tape direction. However, the head assembly 68 does modify tape tension on the supply side of the capstan in accordance with tape motion direction. Thus, for forward motion inverter gate 226 activates a 430 ohm resistor 224 to command a medium torque for the take-up reel while an inverter gate 194 activates a 360 ohm resistor 192 to command a slightly smaller torque for the supply reel. On the supply side of the capstan 56 the tension adjacent the capstan 56 equals the supply tension arm 34 tension plus the head friction tension to balance the somewhat higher tension arm tension on the opposite take up side. On the other hand, for reverse direction motion of tape 16, AND gate 240 is activated to enable 430 ohm resistor 216 and maintain the same tension at the take-up tension arm while inverter gate 186 enables a larger resistor 184 to increase the tension at the supply reel tension arm. However, this higher tension at the supply tension arm 34 is decreased by the friction across head assembly 68 and the tension adjacent the capstan 56 on the supply side remains equal to the tension on the take-up side. Equal tape tensions are thus maintained adjacent capstan 56 on opposite sides thereof for both directions of motion notwithstanding the different friction effects of the head assembly 68 for different directions of motion of tape 16 along the tape path between the tension arms 34, 46. In the event that 1/4 inch tape is used, the same principles apply except that all of the tape tensions are reduced somewhat. 
     Referring now to FIG. 4, the idler roller 54 has a radius R t  = 1.193 inches, the guide roller 102 has a radius R g  = 0.406 inch, and the distance between the axes 58 and 106 of roller 54 and guide roller 102 respectively is 1.6758 inches. The distance between guide roller axis 106 and supply reel axis 22 is 8.255 inches. Tension arm 34 has a nominal position of approximately 30° with respect to the vertical. For this orientation, the angle θ between the center line between axes 58 and 106 and the straight line 116 of tape between guide 102 and roller 54 equals 72.58°, the angle ψ between the center line between axes 58 and 106 and the vertical is 6.17°, the angle σ between the straight line 116 of tape and horizontal is 11.25° and the angle φ between the center line between axes 58 and 106 and a perpendicular line from straight tape path line 116 to axis 58 is 17.42°. The moment arm R1 which extends perpendicular to the straight tape path line 116 between guide roller 102 and idler roller 54 can thus be readily calculated to have a length of 1.338 inch and the torque exerted on tension arm 34 about axis 48 by the tape segment along line 116 is equal to 1.338 times the tape tension force at guide roller 102. 
     The torque exerted about tension arm axis 48 by the segment of tape along line 110 can be determined in accordance with the reel radius r. Helical coil 14 is shown at its intermediate radius in FIG. 4 wherein the straight line tape segment along line 110 which is tangential to both tape coil 14 and guide roller 102 is perpendicular to a radial line Ra extending between tension arm axis 48 and guide roller axis 106 and intersects line Ra at the point of tangency with guide roller 102. For this intermediate reel pack diameter, the moment arm R2 is equal to the tension arm radius, Ra = 1.24 inch, less the guide roller radius Rg = 0.406 inch or 0.834 inch. The total torque exerted about axis 48 is thus 1.338 t + 0.834 t where t is the tension in tape 16. 
     However, as indicated by alternate tape path 260 and alternate moment arm 262 for larger tape pack diameters and alternate tape path 264 and alternate moment arm 266 for smaller tape pack diameters, the moment arm length decreases for tape pack diameters that are either greater than or less than the intermediate tape pack diameter that produces the perpendicular tape path 110-tension arm center line Ra relationship. However, as long as the tape path length 110 between tangential points on guide roller 102 and tape coil 14 remains greater than the total change in the tape pack radius Rr , the changes in moment arm length and hence tape tension required for a torque balance will remain acceptable. It has been found that tension per unit torque varies less than ± 7% for tape pack radii varying from 2.25 to 7 inches with the intermediate radius at which the perpendicular tension arm 34 tape path segment 110 relationship exists selected to be about 4.5 inches. Furthermore, since both a large reel pack and a small reel pack will tend to produce a larger tape tension, the tension remains balanced on both sides of the capstan adjacent thereto for all reel pack diameter conditions when a 7 inch coil of tape is used. 
     The proper positioning and selection of tension arms 34, 46 and their torque transducer solenoids 126, 142 thus cooperate in multiple ways to control and balance tape tension across the drive capstan roller 56. Positioning the arms perpendicular to the intermediate pack tape line greatly reduces changes in tape tension with reel tape pack radius and also provides automatic compensation for changes that do occur because a large tape pack radius produces a change similar in magnitude and polarity as the change caused by a corresponding small tape pack radius in the other reel. Proper shaping of the torque transducer solenoid air gaps permits torque balancing to maintain constant tape tension at the tension arms notwithstanding changes in torque applied by the tape to the tension arms for changes in arm position. Furthermore, the actively powered and controlled torque transducers permit compensation for tension changes at the capstan dependent on operating conditions such as tape motion direction and this compensation can be applied immediately when desired without waiting for changes in tension arm position to produce changes in tape tension. 
     While there has been shown and described a particular arrangement of a web transport system in accordance with the invention for the purpose of enabling a person of ordinary skill in the art to make and use the invention, it will be appreciated that the invention is not limited thereto. Accordingly, any modifications, variations or equivalent arrangements within the scope of the appended claims should be considered to be within the scope of the invention.