Patent GB-1017458-A (GB)
Title: Improvements in or relating to tape driving systems
IPC: H02P7/288
Filed: 1964-02-20
Abstract: 1,017,458. Driving magnetic tape. AMPEX CORPORATION. Feb. 20, 1964 [March 22, 1963], No. 7215/64. Heading G5R. [Also in Divisions G3 and H2] A feeding arrangement for a magnetic tape comprises supply and take-up reels with a tape extending between them, a drive capstan in constant non-sliding engagement with the tape, a bi-directional drive motor directly coupled to the capstan, and circuit means for selectively energizing the drive motor, the circuit means including means for providing selected energizing pulses to the motor during acceleration and deceleration. Referring to Fig. 1, the tape passes between reels 12 and 13 and through vacuum chambers 21 and 22 which provide buffer loops and control the drives of the reels. The tensions provided by the vacuum chambers are nearly equal so that a low tension difference exists across the head 17 and the tape guides 37-40 have low friction. The capstan 24 has a high friction surface, e.g. of rubber and is directly coupled to a low armature inertia motor 44 such as a D.C. motor having a planar rotor with printed circuit conductors thereon. Control of motor drive.-The motor 44 is accelerated and decelerated by pulses from a pulse generator 46 controlled by a control circuit 47. After acceleration the motor may run at a constant speed under the control of a servo amplifier 52 provided with an input from a tachometer 49 and "with a reference signal provided by unit 56 under the control of control circuit 47. The pulse generator may comprise a capacitor 61, Fig. 2, coupled to a charging circuit 62 and adapted to be discharged through the motor, shown as an inductance 66 and resistance 69, via a silicon-controlled rectifier 65 or 79. The rectifier keeps the circuit open for half a cycle, so that at the end of a pulse the capacitor 61 is partly charged in the reverse direction in preparation for delivering a pulse of opposite polarity. Since the pulses for starting and stopping are identical the system provides predictable start and stop times and distances. The control circuit 47 may include interlock means for preventing a second command affecting the system whilst a first command is being executed; means for operating a brake at the end of a decelerating pulse; and means for disabling the servo system whilst pulses are applied to the motor, Figs. 4 and 5 (not shown). When operating under servo control, part of the operating current for the motor may be supplied from a constant current source. The pulses may be applied to the motor through the servo system, Fig. 6 (not shown), or the output of the servo amplifier may be used, when saturated by a pulse input, to trigger a pulse generator supplying the motor, Fig. 7 (not shown). Incremental operation.-During recording a start pulse from generator 160, Fig. 8, accelerates the motor which continues under the control of servo 161 until stopped by a stop pulse from generator 162 after a delay determined by a one-shot multivibrator 163, the servo 161 being enabled and disabled by the start and stop pulses. Data may be recorded with the tape stationary or when the tape is at speed and the incremental distance may be controlled by adjusting the delay provided by unit 163. During playback the motor is started as before until data pulses are read in parallel by circuits 165 to provide a single strobe pulse which passes through switch 168 to the stop pulse generator 162. An alternative form of incremental drive for recording uses the circuit of Fig. 2 with both rectifiers 65 and 79 fired so that a full sine wave cycle of current is supplied to the motor. The first half-wave accelerates the motor and the second half-wave decelerates the motor, and the electrical losses can be balanced against frictional losses to ensure that the capstan comes to rest.