CAPSTAN DRIVE AND SERVO SYSTEM

All tape motion in the Mod 10 is initiated by the capstan, which is driven by a DC motor. Wnen the motor is running, a tachometer

generates a DC voltage that is used to control the tape velocity through the capstan servo system (see Figure 14.)

The strobe disc will be on the front of the capstan and will have two patterns, one for 50"Hz and one for 60 Hz. The inner pattern will be for 60 Hz. It is to be viewed with the corresponding AC light (such as fluorescent) the pattern on this disc appears to stand still \·vhen

the capstan motor of the tape transport is operating at the correct speed.

The strobe pattern will be present only on machines operating at 12.5, 25 , or 37 • 5 inche s per second. Thu s ^I proper operation of this

component of the tape transport can be under continuous visual inspec-tion by the operator. Any departure from a stainspec-tionary pattern (i. e. ^I precession of the strobe lines in either a clockwise or counterclock-wise direction) indicates a speed variance of the capstan drive. (See Section VI for calculating the percentage of such variance.)

Two ramp generators are used in the capstan servo. One controls the forward and reverse speeds at nominal velocity ^I and the other controls the revv"ind speed. The forward/reverse ramp generator uses a Zener diode as a precise voltage reference. The rewind ramp generator uses the regulated +5 volt level as a voltage reference. Resistors RI and R2 in Figure 14 in combination with R3 and R4 , function as a

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INPUTS

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RI

REV

R2 RAMP

REWIND -..., GEN t-o----""

SERVO AMPLIFIER

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R4

R3

Figure 14. Capstan Drive and Servo System

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CAPSTAN

MOTOR

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summing network to control the capstan speed. Current through R3 is generated in the tachometer ^I and R4 provides feedback from the capstan motor ^I proportional to the motor current. The current feedback is

generated by sensing the voltage across a O. 1 ohm resistor (R5) in series with the motor. When the motor is running, the sum of currents in R3 and R4 is equal to the sum of the currents in Rl and R2.

Rl establishes tape motion ^I in the appropriate direction. The dis-tances traveled during acceleration or deceleration are such that an IBM -compatible interrecord gap is generated. Forward and reverse commands generate currents through Rl having opposite polarities.

Symmetry of the start and stop times and distances is readily achieved through the potentiometers in the forward and reverse inputs to the ramp generator. A potentiometer is also used to adjust the capstan drive servo amplifier offset so that no tape motion occurs unless the tape transport has received a motion command.

Figure 15 shows the relative timing of commands to the capstan servo ^I the ramp function generated, and the resulting tachometer output seen by the servo amplifier.

In the rewind ramp generator ^I the rise time has nominally a 1 second time constant. This provides a time interval that permits the tape to accelerate to 150 ips without exceeding the storage capacity of the servo arms. Fall ^t~me is nominally aD. 5 second ramp and it assures that the storage ann capacity is not exceeded as the tape slows and halts.

When the system is in the ready state, the tape is held motionless by the balanced tension (eight ounces) in the storage arms and the friction in the capstan drive motor. Although the wrap on the capstan varies slightly with the arm position for the takeup reel, it is nominally 180 degrees. The area of tape in contact with the capstan and the tension on the tape prevent any relative motion between capstan and tape.

REEL SERVO SYSTEM

Two identical servo systems control the supply and takeup reels in the Mod 10. Storage of appropriate lengths of tape to perm!t accel-eration and decelaccel-eration is provided by the buffer arms, which permit the capstan to start and stop the tape without having to start and stop the reels in the same short time. Storage of tape by the arms is suf-ficient to permit the system to operate at the nominal tape speed without program restrictions.

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Figure 15. Timing Diagram of Commands to Capstan SelVo

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Operation of the reel servo system is diagrammed in Figure 16.. A light-sensing circuit provides ann -position infonnation to the servo amplifier ^I which drives the reel motor. As tape is delivered to the ann or taken from it ^I the arm· moves up or down and the position of the mask betv.reen the light source and the light-sensing element changes. When the mask is at the appropriate position, the output of the light-sensing circuitry no longer provides an error signal to the amplifier. Enclosure of the light source prevents ambient light from affecting system performance.

Reel motors are driven by linear amplifiers ^I stabliized for all operating situations and sequences. During the Rewind mode the amplifier gain is increased and the output stage operating voltage is raised to offset the increase in back emf generated by the reel motors at higher rpm.

An offset signal is fed to each servo amplifier during the unload cycle to bias each servo swing arm close to its respective stop.. This assures gentle handling of tape as it unloads from the fixed reel. It also prevents violent impact of the ·anns against their stops.

Spring tension on the servo arms is balanced at all times by torque in the reel motors. Should power fail or servo operation be interrupted ^I the springs pull the arms out and into contact with limit switches that turn off all reel servo and capstan functions. If power fails during high-speed rewind ^I the reel motors are shorted by contacts on the ready relay. The resulting strong dynamic braking effect stops the reels vvithout damage or spilling of the tape.

Potentiometer adjustments are provided on the transport board to permit proper sett.ing of each s'wing arm position. ..l1 .. potentiometer adjustment is also provided to set the gain of each reel servo amplifier ^I compensating for the normal manufacturing tolerances in components.

CONTROL ELECTRONICS

The control portion of the transport electronics printed circuit board (see Figure 17) ree eives its primary inputs from the operator^l s control panel or the remote controller. In addition, it responds to control signals from the photosense assembly and from the servo amplifiers

(during the rewind sequence).

The internal control circuitry outputs signals to the remote tape con-troller and to the operator's control panel (in the form of indicator lights). Within the transport, it provides signals to the servo ampli-fiers and to the data electronics printed circuit board.

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