Patent GB-1059170-A (GB)
Title: Magnetic flux responsive sensing device
IPC: B61H13/04
Filed: 1964-05-22
Abstract: 1,059,170. Magnetic playback heads; selective signalling. UNIMATION Inc. May 22, 1964 [July 22, 1963], No. 21238/64. Headings G4A, G4H and G5R. A magnetic playback head comprises a core 10, Fig. 1, provided with windings 13-16 arranged in an oscillator circuit so that the presence of a magnet 28 of one polarity in the air gap 18 causes the transistor 34 to oscillate whereas the presence of a magnet 29 of opposite polarity does not allow the circuit to oscillate. When the transistor 34 oscillates, its output is rectified and used to cut off transistor 60 and thereby provide a negative output at terminal 64. The magnets 28 may be positioned in grooves 26 in a rotatable non- magnetic drum 24 to provide command signals for a computer and a core 10 may be provided for each row of magnets so that all the bits in a selected groove 26 may be read simultaneously. The core 10 comprises equal limbs 11 and 12 with windings 13-16. The collector of transistor 34 is connected to a negative supply through windings 13 and 15, and the currents thereby induced in feedback windings 14 and 16 are equal and opposite so that the bias on the base of the transistor is insufficient to allow oscillation. With a magnetic field of correct polarity in the gap 18 the limb 12 saturates and winding 14 produces a larger bias voltage than winding 16 and thereby allows the transistor 34 to oscillate. A butt joint 70 in the core 10 allows some latitude in balancing the two halves. In a modification, Fig. 5 (not shown), the core 10 is provided with a bias winding fed with an adjustable current so that the device will respond to a field of the correct polarity exceeding a predetermined strength. In a further arrangement, Fig. 6, the bias winding is connected between the collectors of two transistors 132 and 134 connected in a flipflop circuit. A positive gate pulse may be applied to transistors 132 and 134 to cut both of them off, and whilst they are cut off the transistor 34 will oscillate or not according to the polarity of the field in the air gap. If transistor 34 oscillates, then a positive bias is applied to transistor 132 so that at the end of the gate pulse transistor 132 is cut off and transistor 134 conducts. Current then flows through the bias winding to maintain oscillations irrespective of the field in the air gap. Similarly, if transistor 34 fails to oscillate during the gate pulse, at the end of this pulse transistor 132 conducts and transistor 134 is cut off, and a current flows through the bias winding to prevent oscillation of transistor 34. Thus the head is only sensitive during gate pulses and the output at terminal 162 remains constant between gate pulses. In a modification, Fig. 8 (not shown), the drum is provided with magnetic teeth which pass between the polepieces of the head, and in another modification, Fig. 9 (not shown), instructions are recorded as transverse magnetizations of a magnetic tape which passes between the polepieces. Selective signalling.-An input shaft 180, Fig. 7, carries annular permanent magnets 188 and 190 ; a disc 182, and discs 184 and 186 which are notched to provide staggered teeth 184a and 186a. With a tooth 184a adjacent the core 170, as shown, the magnetic field in the core is such that transistor 34 oscillates and thereby produces an output at terminal 64. With a tooth 186a adjacent the core 170 the magnetic field through the core positively prevents oscillation of transistor 34. As the shaft 180 rotates, pulses are produced for each increment of rotation. A plurality of units may be arranged on the shaft 180 to give a plural digit output.