Patent GB-989665-A (GB)
Title: Improvements in or relating to tape handling apparatus
Inventor: SHARPE TERENCE CLIFFORD
IPC: G11B15/56
Filed: 1963-02-27
Abstract: 989,665. Photo-electric control systems. DECCA Ltd. June 20, 1963 [Feb. 27, 1963], No. 7926/63. Heading G1A. [Also in Divisions G5 and H3] Handling apparatus for magnetic tape comprises a bin formed of two parallel sheets of transparent material between which the tape is scrambled; a light source and a diffusing screen on one side of the bin and a sensing photo-cell system on the other side; a monitor photo-cell system adjacent the light source; circuit means for adjusting the outputs of the photo-cell systems to equality when the bin contains a predetermined quantity of tape; and a pair of trigger circuits adapted to respond respectively to predetermined positive and negative differences between the outputs of the two photo-cell systems. As shown in Fig. 3 the bin may comprise two parallel sheets 30 and 31 of rough cast glass spaced apart by a distance slightly greater than the width of the tape and joined together by a U shaped strip 32 of rigid material. A diffusing screen 33 is arranged on one side of the bin and illuminated by a fluorescent lamp 34 backed by a mirror 36. The light output of the lamp is detected by photo-cells 35 and the light passing through the bin by photo-cells 37, the cells preferably being of the selenium barrier type. As shown in Fig. 4 the photo-cells in each set are connected in series and a variable resistance 39 or 41 is connected across each output so that the outputs can be adjusted to be equal with a suitable amount of tape in the bin. The outputs of the cells are fed to a differential chopper amplifier comprising transistors 45 and 46 the bases of which are fed anti-phase outputs from a multivibrator 47. The output is taken from an adjustable potentiometer 48; which provides an adjustment for setting the amount of tape to give zero output, and fed to an amplifier formed by transistors 50-53 and provided with an adjustable feed-back resistor 54 which varies the gain. The amplifier output passes to a phase sensitive detector including transistor 56 which is driven from the multivibrator 47, and the output of the detector is a series of positive or negative pulses according to whether there is too little or too much tape in the bin. These pulses are integrated by resistances and capacitors 58-61 and applied to a differential amplifier formed by transistors 62 and 63, the base of transistor 63 being maintained at a constant voltage by a Zener diode 64. The amplifier provides an output from the collector of transistor 62 or 63 according to the polarity of the input, and these are fed separately to two Schmitt trigger circuits 65 and 66. The trigger circuits are identical except that they have different sensitivities, as less tape has to be removed from the bin than need be added to it to give the same change in light transmitted. The output of the triggers on leads 80 and 81 are used to operate servo systems controlling the feeding of tape into and the drawing of tape out of the bin. The triggers have backlash, and the backlash must be less than the dead zone between the levels of tape demanding feed into and out of the bin; the width of this dead zone czn be controlled by resistance 54.