Patent GB-979471-A (GB)
Title: Vehicle sensing systems
IPC: G08G1/01
Filed: 1962-07-26
Abstract: 979,471. Locating magnetic bodies; measuring short time intervals. D. A. PEARS. (Thompson Ramo Wooldridge Inc.). July 26, 1962, No. 28828/62. Headings G1N and G1U. [Also in Divisions G4 and and H1] A vehicle sensing system comprises at least two magnetic field sensors 10, 11, Fig. 1 spaced apart a predetermined distance in the direction of vehicular traffic along a vehicle path each of the sensors being responsive to variations in the earths magnetic field produced by the presence of the vehicle, and logic and computer circuits 15 for producing a composite signal from the output signals of the two sensors. The system may be used to detect vehicle presence in a parking stall, or along a road, vehicle presence signals being supplied to a counter 16; or, for the purpose of measuring vehicle speed and traffic density, these being indicated in meters 17, 18. Flux gate magnetometers, Fig. 4 which constitute the sensors are energized by the transitor oscillator portion 23 of oscillator detector 14, via screened cables 47, 48, and the signals arising from the presence of a vehicle returned over screened cables 49, 50 are detected by circuits tuned by capacitors 53, 54 to tone the oscillator frequency and including diode rectifiers 55, 56. The magnetometers are brought by bias to a working point corresponding to the dashed "BIAS LEVEL" line of Fig. 5 so that there is substantially no change in output signal to a field change in one direction but a substantial response to a field change in the opposite direction. In the logic and computer circuit 15, Fig. 6, a gating diode 64 is rendered conductive by a signal on terminal A arising from the first traffic sensor 10 and actuates counter 16 through its actuating coil 65. The subsequent signal from sensor 11 over line 13 opens contacts 76 of relay 73 to release the counter, further actuation being prevented until the termination of the signal on B. Integrating circuits 81, 82 and 77, 78 provide a measure of vehicle speed and traffic density; the former being based on the time period during which diode 64 is "ON", for a single vehicle, i.e. the period between the initial pulses from A and B and the latter on the time period on an integrated basis for which diode 64 is "ON". The optimum spacing of the sensors is discussed and examples are given. A mathematic analysis is included and component values for the integrating circuits are exemplified. An arrangement using two flux gate magnetometers for detecting the presence of a vehicle in which the outputs are applied to the counter through an OR gate is described with reference to Fig. 8 (not shown), the composite signal produced by the "OR" gate being amplified and clipped. A suitable flux gate magnetometer comprises a strip 90 Fig. 11 of molybdenum permalloy of cross-section 0.001 inch by 0.062 inch spirally wrapped inside a length of ceramic tubing, the ceramic being capable of withstanding temperatures of 2500‹F. so that the assembly may be hydrogen annealed. Output windings 94, 95, are wound on top of energizing windings 92, 93. The selection of an optimum frequency is described with reference to Fig. 13 (not shown) and an arrangement in which a magnetometer is encapsulated in a plastic block is described with reference to Fig. 14 (not shown). The capsule is inserted in a small hole in the vehicle path with the axis of the saturable element perpendicular to the Earth's surface. Component values are given. The system may effect traffic control.