Patent GB-857232-A (GB)
Title: Electronic circuit providing an indication of the occurrence of predetermined values and changes in values of input signals
Inventor: BOWER JOHN L
IPC: H03K5/26
Filed: 1958-06-30
Abstract: 857,232. Electric direction detection. NORTH AMERICAN AVIATION Inc. June 30, 1958, No. 20846/58. Class 40 (1). A circuit adapted to receive two or more binary signals has means for detecting changes in a signal from either value to the other, and further means producing an output on one terminal when certain changes in one signal coincide with certain values of the other or vice versa, and on another terminal when other signal changes coincide with other signal values. The invention is described as applied to apparatus for detecting the direction of movement of a magnetic tape 1, Fig. 1, a track 2 of which co-operates with reading heads 3, 4 spaced apart by an odd number of quarter wavelengths. The outputs from the heads, Fig. 2, are applied to flip-flops 5, 6 and converted thereby into square waves, Fig. 3. The circuit 7 determines the direction of motion of the heads relative to the track 2, motion to the right producing changes of signal such as occur when moving to the right in Fig. 3 and motion to the left producing changes encountered by moving to the left in Fig. 3. The signal values are termed α and α<SP>1</SP> for flip-flop 5 and # and (31 for flip-flop 6, α and # being zero and α<SP>1</SP> and #<SP>1</SP> being -10 volts. The changes of signal value are likewise termed a and b for a downward change and a<SP>1</SP> and b<SP>1</SP> for an upward change. Movement to the right produces an upward change b<SP>1</SP> when the other signal is at α, a downward change a when the other signal is at #, a downward change b when the other signal is at α<SP>1</SP> and an upward change a<SP>1</SP> when the other signal is at #<SP>1</SP>. Movement to the left produces a succession of different combinations and the two successions are detected by the circuit of Fig. 4. Transistor 11 inverts the output from flip-flop 5 and transistor 15 inverts that from flip-flop 6. Line 12 is high when α exists, line 13 when α<SP>1</SP> exists. Line 16 is high when # exists and line 17 when #<SP>1</SP> exists. A change a is detected by condenser 18 and resistor 19 and a corresponding signal a is produced. Similar differentiating circuits 20, 21; 22, 23 and 24, 25 produce signals b, a<SP>1</SP> and b<SP>1</SP>. The signal #<SP>1</SP> when present prevents a negative a signal from passing through diode 27. When # is present the a signal passes to provide a a# signal signifying that the flip-flop 5 has changed downwardly while flip-flop 6 is at the upper value. By similar gating arrangements signals α<SP>1</SP>b, a<SP>1</SP>#<SP>1</SP> and αb<SP>1</SP> are also extracted when present and applied through a transistor 28 to a terminal 29 and thence to the " add " input of counter 8. These combinations occur only during movement to the right and the pulses appearing at terminal 29 are passed to counter 8, Fig. 1, to count positively (say). Other combinations a#<SP>1</SP>, αb, a<SP>1</SP># and α<SP>1</SP>b<SP>1</SP> are produced by similar diode gating circuits and pass corresponding signals to terminal 38. Since these are the combinations appearing during leftward movement, the pulses are passed to the " subtract " input of the counter 8. In a modification, the inverters 11 and 15 are omitted, the flip-flops being arranged to have two complementary outputs representing respectively α and α<SP>1</SP>, # and #<SP>1</SP>. These outputs are differentiated and diode-gated as before. In another modification transistorized flip-flops 5 and 6 are described, each having complementary output leads, the signals on which are differentiated and gated with the values on other leads to produce outputs to " right " and " left " terminals as before. These outputs are combined and amplified in four transistor stages to produce pulses on a single terminal for application to the counter. The signals on the left and right terminals are used to set or re-set a transistor flip-flop, the outputs from which indicate to the counter whether the amplified pulses are to be added or subtracted. In an alternative arrangement three reading heads are used, producing signals phase-displaced by 60 degrees so that instead of four combinations appearing for right- or left-hand movement, there are six such combinations. These are detected as described above, a greater resolution being obtained by using three heads than by two. Any other number of heads may be used.