Patent GB-699853-A (GB)
Title: Improvements in or relating to systems for detecting movements of objects by their effect on the electrostatic capacity of a conductor
IPC: G08B13/26
Filed: 1949-06-14
Abstract: 699,853. Detecting movements of objects; frequency measurements. McSORLEY D.J., ROBINS, J. T., BEHAR, D., and TARBAT, VISCOUNT. Sept. 14, 1950 [June 14, 1949; Aug.14, 1950], Nos. 15797/49 and 20128/50. Class 37. Apparatus for detecting movements of objects by their effect on the capacitance of a conductor comprises a system designed to respond to relatively high rates of change of capacitance caused by such movements and having means for preventing it from responding to relatively low rates of change of capacitance such as might otherwise cause spurious operation of the system. In the apparatus shown in Fig. 1, two valve oscillators 1, 2 are tuned to slightly different frequencies so as to produce a beat frequency of, say, 1000 cycles per second. Changes in capacitance of the conductor 4 due to movements of objects in its vicinity cause changes in the frequency of the oscillations generated by the oscillator 1. The outputs of the oscillators 1, 2 are combined in a frequency-changer 5 comprising a double triode in which the triode 6 operates as a detector and the triode 7 as an amplifier. The output of the frequency-changer 5 is applied to a frequency discriminator consisting of two tuned transformers T1, T2, the secondary windings of which are connected through the rectifiers 12, 13 of a double diode 14 to two discriminating relays 15, 16. A signal relay 19 is responsive to the secondary current of either of the transformers T1, T2, but requires a greater current to operate it than is required to operate the relays 15, 16. The transformer T1 is tuned to a frequency slightly above the nominal beat frequency whilst the transformer T2 is tuned slightly below the nominal beat frequency giving a double peak response characteristic. The relays 15, 16 have contacts 20, 21 controlling a reversible motor 22 which operates a variable condenser C3 for adjusting the frequency of the oscillator 1. When the oscillators 1, 2 are adjusted to give the nominal beat frequency the response of the transformers T1, T2 is greatly attenuated on account of the double-peak characteristic. A small change in capacitance of the wire 4 causes the frequency of the oscillator 1 to vary and the response of one of the transformers T1, T2 increases, operating the corresponding relay 15, 16 and causing the motor 22 to adjust the tuning condenser C3 to correct the deviation of the beat frequency from the nominal value. If the change of capacitance is slow, such as is caused by change of atmospheric temperature or humidity, the tuning motor 22 compensates for the change and thus prevents the response of the discriminator from rising sufficiently to operate the signal relay 19. A rapid change of capacitance caused by movement of a person, &c., in the vicinity of the conductor 4 causes operation of the signal relay 19 before the motor 22 can adjust the condenser C3 sufficiently to compensate for the disturbance. In a modification, Figs. 3, 4 and 5 (not shown), the automatic tuning device is a reactance valve controlled by a variable D.C. voltage supplied to it under the control of the discriminator, delay means being interposed between the discriminator and the reactance valve.