Patent GB-574941-A (GB)
Title: Electrical indicating system
IPC: G01V3/10
Filed: 1943-01-06
Abstract: 574,941. Inductance measurements; locating conductors ; magnetic tests. HAZELTINE CORPORATION. Jan. 6, 1943, No. 277. Convention date, Dec. 31, 1941. [Class 37] A system for indicating variations of a coupling effect between input and output terminals of an impedance network, particularly for locating conducting or magnetic metal objects buried a short distance underground, includes variable means adapted to balance out a component of coupling between the input and output terminals. As shown in Fig. 1 an impedance network has input terminals M, N and output terminals O, P, and includes a variable impedence adapted to balance out a component of coupling between these input and output terminals. This variable impedance comprises exploring inductors 10, 11 with their axes normal to each other so that the coupling between them is normally inappreciable but is increased in the presence of a mass to be detected. Oscillations are applied to the terminals M, N and so to the inductor 10 by an oscillator comprising the triode section of a triode-pentode 12 and a tuned circuit comprising a condenser 13 and inductor 14, to the mid-tap of which is supplied the anode voltage for the valve 12 through a resistor 15. An inductor 18 is coupled to the inductor 14 and connected to the input terminals M, N and through leads 19 to the exploring inductor 10. Oscillations induced in the inductor 11 are applied through a transformer 25, 26 to the output terminals O, P and through amplifiers 23, 23a to the input of the pentode section of the valve 12 and thereby vary the effective reactance in the tuned circuit 13, 14 and alter the frequency of the oscillator. A phase shift of an odd multiple of 90 degrees is provided in the feed-back circuit of the oscillator, which includes the inductors 10,11 by tuning the secondary winding 26 by means of a condenser 33 to the nominal frequency of the oscillations generated by the triode section of the valve 12. Variations of frequency of these oscillations are determined by heterodyning with oscillations produced by a second oscillation-generator comprising the triode section of a diode-triode-pentode 37 and a circuit 35, 36 tuned to the seventh harmonic of the nominal frequency of the first oscillator. Unidirectional operating potential for the triode section of the valve 37 is supplied through a resistor 39 connected to the mid-tap of the inductor 35. The oscillations produced by this second oscillator and the seventh harmonic of those produced in the first oscillator are applied to an amplifier which includes the pentode section of the valve 37. This is effected through a doubletuned transformer 41 and through a condenser 43 which may be comprised by the inter-electrode capacitance of the valve 37. The output from the pentode section of the valve 37 is applied through a tuned transformer 45 to the diode rectifier including the anode 46 within the envelope of the valve 37 wherein the oscillations derived from the first and second oscillators are heterodyned. The load circuit of the rectifier includes resistors 47, 48 by-passed by condensers 49, 50. The rectified beat note of the heterodyned oscillations, which is developed across the resistor 48, is coupled by a condenser 51 to the input circuit of the triode section of a diode-triode-pentode valve 52 for amplification. The output of the triode section of the valve 52 is coupled to the pentode section, through a resistor 53 and condenser 54, for further amplification. The anode of the pentode section is connected to the primary winding 55 of an audio-frequency transformer, to the secondary winding 56 of which headphones 57 are adapted to be connected by a plug and jack. For providing a visual indication a meter 60 is connected through a valve 68 and a frequency-responsive network comprising resistors 61, 62, 63 connected in series with a condenser 64 between the anode and cathode of the pentode section of the valve 52. The voltage developed across the resistor 61 is rectified in the diode rectifier including the anode 67 and the rectified voltage is applied to the input circuit of the duplex-triode amplifier 68, the triode sections of which are in parallel. The filament circuits of the valves are controlled by a three-position switch 72. The filament of the valve 23 is connected directly to a contact c of the switch 72 and the remaining filaments are connected to a contact b, the filaments of the valves 12, 37 being connected through a choke 73. In the position shown all the filaments are connected, while in a second position the contact c is disconnected from the supply 71 and the valve 23 is de-energized. In order to reduce the normal coupling between the inductors 10,11 to an imappreciable value, an inductor 21 is connected in series with the inductor 10 and an inductor 28 is connected in series with the inductor 11. The inductors 21, 28 are positioned so as to have normally substantially no coupling. Between these inductors is a metal disc 75 rotatable about its centre 76 by means of an insulating arm 77 and a screw 78. A variable coupling is thus provided between the inductors 21, 28 and enables any residual coupling between the inductors 10, 11 to be balanced out. The resistors 62, 63 are shunted by condensers 65, 66 of such values that the resistor 62 is by-passed at an intermediate audio frequency and the resistor 63 is by-passed at high audio frequencies. This provides a frequencyresponsive network such that a signal having an amplitude varying in accordance with frequency is supplied to the diode section of the valve 52. This is amplified in valves 52, 68 to give a visual frequency indication on the meter 60. For preliminary adjustment the contact c of the switch 72 is opened, thus opening the filament circuit of the valve 23 and rendering the feedback circuit inoperative. The frequency of the second oscillator is adjusted by a condenser 81 so that it is exactly equal to the seventh harmonic frequency of the first oscillator. The contact c is then closed and the adjusting screw 78 is adjusted until no beat note is heard in the headphones. In a modification, Fig. 4, the variable impedance means comprises a single exploring inductor 10<1> connected in one arm of a reactance bridge, another arm of which comprises a fixed inductor 85. Oscillations are supplied to the inductor 10<1> from the first oscillator through the terminals M, N. A valve 86 has its input circuit connected across one diagonal of the bridge and its output circuit coupled through a transformer 87 to the terminals O, P. An auxiliary adjusting means including inductors 21<1>, 28<1>, and 751 is connected in series with the inductor 10<1> in order to vary the inductance of this arm of the bridge through a small range for initial adjustment. U.S.A. Specification 2,312,687 is referred to.