Apparatus for maintaining a predetermined quantity of an elongated medium at a predetermined zone

934,173. Automatic control systems. ELECTRIC & MUSICAL INDUSTRIES Ltd. Nov. 16, 1959 [Nov. 26, 1958], No. 38032/58. Class 38 (4). A predetermined length of magnetic tape is maintained in each of the bins 5, 6, by means of spools 1, 2, driven by reversible electric motors 3 and 4, a pair of vacuum capstan rollers 9, 10, being continuously rotated in opposite directions by electric motors 11, 12. Suction is selectively applied to the rollers from a vacuumsource 13. The quantities of tape in the bins is determined by electrodes 14, 14a, and capacitymeasuring devices 15, 16. Command signals from a source 17 control the application of suction to one or other of the rollers 9 or 10, and condition the motors 3 and 4 to drive the spools in a direction corresponding to the direction of feed of the tape. When the apparatus is required to drive the tape in a left-hand direction by means of roller 9 into bin 5, a positive signal is applied from source 17 to OR-gate 18 which opens the gate to feed a positive signal to AND-gate 19. The gate 19 also receives a positive signal from the device 15 if the bin 5 is full, the gate then opening to energize the motor 3 in a direction to empty the bin, an amplifier 20 feeding back a positive signal to the gate 18. The gate 18 remains open, therefore, even if the command signal is reversed, until the bin is empty and the signal from the device 15 changes its sign. The tape is driven in a right-hand direction into bin 6 by means of roller 10 when source 17 supplies a negative signal to OR-gate 21 which passes a negative signal to AND-gate 22. If the bin 5 is empty, the device 15 also passes a negative signal to the gate 22, whereupon the gate opens and causes the motor 3 to feed tape into the bin 5. The amplifier 23 feeds back a negative signal to the gate 21 which remains open even if the signal from the source 17 is reversed. When the bin 5 is full, the gate 22 closes to de-energize the motor 3. An arrangement similar to that shown for use with spool 1 and bin 5 is provided for use with spool 2 and bin 6 Specification 930, 664 is referred to.