US2919413A

US2919413A - Means for examining a substance

Info

Publication number: US2919413A
Application number: US609671A
Authority: US
Inventors: Thomas G Charles
Original assignee: ASEA AB
Current assignee: ABB Norden Holding AB
Priority date: 1954-07-22
Filing date: 1956-09-13
Publication date: 1959-12-29
Anticipated expiration: 1976-12-29

Description

Dec. 29, 1959 T. G. CHARLES MEANS FOR EXAMINING A SUBSTANCE Filed Sept. 13, 1956 I I l I 20 J T IN VEN TOR.

T/zomas G Charz'es.

United States Patent 2,919,413 MEANS FO A SUBSTANCE Thomas G. Charles, Vasteras, Sweden, assignor to Allmiinna Svenska Elektriska Aktiebolaget, Vasteras, Sweden, 2 Swedish corporation Application September 13, 195.6,, Serial No, 609,671 Claims priority, application Sweden July 22, 1954 10, Claims. (Cl. 331-65),

This invention is a. continuation-in-part (as to all common subject matter) (If the QQPending application Serial No. 521,504, filed July 12, 1955, now abandoned, by Mr. Sverre Walter and myself for improvements in metal. etector or magnetic bore-hole logger, which plication will now be permitted to lapse without prejudic, in view of its being superseded by this present application It is know that the change in the, magnification factor of. a resonant circuit au e by a b dy moved i o a magneti or elect ic field fr m the. circui is ependent on the ubs ance in the dy, nd t has been Proposed o tili this fle t in he construc ion of metal dete or r r -hole g r and quiva nt as we as means for ng f c l r or T be u ef l in pract ce. a device of this sort must have a very high sensitivity to ang in the magnifi ation f c r- In a k w device ith high ns ivity, th esonan circuit is a parallel-resonant circuit used as grid circuit n scank n sci l The ankl n oscillator consists of two cascaded resistance-coupled vacuum ub Wi h a f db k ne t n ite he anode f th second tube to the grid oi the first tube. The feedback circuit contains a large series resistance which together with the resonance impedance of the parallel-resonant circuit between the grid and the cathode of the first tube constitutes atvoltage divider determining the feedback ratio of the oscillator. As the resonance impedance of the circuit is proportional to the magnification factor of the circuit, the feedback ratio is dependent on the magnification factor, and if the oscillator works on a linear part of the tube characteristics and near the critical feedback ratio, the amplitude of oscillation is very sensitive to changes in the magnification factor. Through a measurement of the changes in the amplitude of oscillation it is in this way possible to detect a piece of metal fed into the coil of the resonant circuit, even if the diameter of the piece of metal is less than one thousandth the diameter of the coil.

The known device is, however, very sensitive to changes in the tube characteristics, and the sensitivity mentioned above is attainable only if the oscillator is provided with means for automatic amplitude stabilization, which, however, should be provided with a certain time-delay so that transient changes in the amplitude can develop. The amplitude stabilization has to be very powerful, so that one or more amplifying stages are needed in this circuit in addition to the two tubes in the oscillator.

It is the object of the present invention to provide a means of the kind described which is simpler in design and has higher s nsi iv ty ha p e ously kno n i ce It is another object of the invention to provide a means of the kind described which has the stability necessary for'its use as bore-hole logger without the need for any stabilizing .or compensating arrangements.

A further object is to provide an oscillator design for a means of the kind described-having excellent stability and sensitivity and still utilizing one single vacuum tube.

A still further object of the invention is to provide an sc l ates desi n fornreens 'e the kind d scribed. in which the s ngl os illa o tube may he re ied on even for p ificati n of he osc llat ol age wi nt ny p itm nt the sens i y at he. sc llato car caused y lea s connec ed o said amp ifi d v ta e- The i v h c s cha acte zed in th t e sonan circ o the oscillator is a series-resonant circuit and that the iced: back ratio of the oscillator is, determined by the ratio betwe he ol ag acr s one r acti e eleme in he. resonant circuit and the input voltage to said circuit.

i ce he ra i be en he vol g ac o he o l o the capacitor in a series resonant circuit and the input voltage to the circuit at resonance equals the magnif caticn a tor o he circ it, he feedb k at o of he sci lator always is proportional to the magnif cation factor in such an ar a g me and the f e bac ra o an he ade larg r th ne The a plifi r cons i u ng p t the os lla or herefore ne ne ha any h gh vo tage g and may w rk as a p e p wer p ie t p ob l m of nen-lin ar ies and depen en y n nhe chara t istics t us eing mal t is e pecia ly adv a eou to utilize a cathode connected vacuum tube as oscillator t r w h t e reso nt c uit The invention will best be understood from they descrip= ticn he e with refe enc to th c mpan i d w ng, n hi h igs- 1 and 2 sh w Pref rred cnns o the nv tion- Th u damen a d sign t o to is the same in the two ennb tt nent s, b the ube n i 2 is i zed'bc h as oscillato t be and. a amp fi r tithe or the output v lta e. he arran e n n Fig- 2 is further provided with a circuit for amplitude stabilization,

In ig. l the nume al 1n desig a es a tube ha in a cathod 1 2 d 12 and Plate 13- The Pla e s an ct t to th p n termina of a a ery 1 and the ca h de i n series with wo. at ode res stors 11 and 8 co nected to, the inu term nal of he battery 16 an to g nd te minal 5- A co 20 ndtwg series c nne capacitors Aland 2.2 a e co nec d be ween th g id .2 ct het be and ro d 5, the comrncn ernina o the capac ors 2i nd .22 be n can tes ed to the junction. po nt of he cathod r s ors" 1.8- n pnt term na 25 is ia a enab i g capacitor 26 canneeted to he cathode 11 not he tube. an is sn'nn s d to be onnecte to. a voltme e e or e sensi ver measurem n o i d a o oi he a nl e f oscillatio which is a m as re on the subst nce influenc n the esnant c r u h capac tor 22 shu t n th ca h d resister 1 is ar e cn npered to e can or 21 and its ac nce i lo mpared to h esi tance in t esi er 17 a the wo kin fre uency of th vi e tube 19 ogethe with the. bat e y in and t e athode p ance co s ting f esi crlfiin s ie w h the p ra el nnection o resis or 1 an capac or 22 he earde a a athode-connec ed amplifier lea ed wi h a eries-r sonan ircui can 'st g Qt 991 2!) and the ap citor 1. Because 9. t e teedbac m th essr e'nt'circ i tn he rid 2 of he u e the shn n r a e: ent orks e a c l n s i ct t e ta t at the ta e g n em he'gri 12 t0 the c es e capa itor 22 is less than one. This is so because the necessary voltage gain is provided by the series-resonant circuit. 7

It is a Well-known fact that the ratio between the voltage across the coil in a series-resonant circuit and the input voltage to the circuit equals the magnification factor of the circuit at resonance. A series resonant circuit thus can provide a voltage gain larger than one, in certain cases several hundred times. The voltage of the coil is however in quadrature to the input voltage of feedback voltage. It has been found, however, that the device obtains its highest sensitivity when the phase displacement in the capacitor 22 is somewhat incomplete, so that the oscillator works at a frequency deviating somewhat from the resonance frequency of the series resonant circuit. The purpose of the resistor 18 is to add to the internal resistance of the tube to provide a series resistance suflicient for the phase displacement of the output voltage. The resistor 18 will even attenuate the output voltage of the amplifier so that the total gain in the oscillator equals one Without any saturation in the amp plifier. The sole purpose of resistor 17 is to pass direct current through the tube so that the correct working point on the tube characteristic is obtained and it may be substituted for instance by a suitable inductance.

Tests have shown that the arrangement according to Fig. 1 in spite of its simplicity has an excellent sensitivity to changes in the magnification factor of the resonant circuit arising when a substance to be tested is moved into the magnetic field of the coil 20 or the electric field of the capacitor 21. The reason is that the feedback ratio of the oscillator is directly proportional to the magnification factor of the circuit, at the same time as the excellent stability in the cathode-connected amplifier makes it possible to let the oscillator work very near to its critical feedback ratio. Even if the arrangement according to Fig. 1 has excellent stability, it is often advantageous to stabilize the amplitude of oscillation so that slow changes in the characteristics of the oscillator does not disturb its function. In this way it is among other things possible to avoid adjustments of the device when the measuring member, ie the coil 20 or the capacitor 21, changes its position in relation to the surroundings.

An arrangement satisfying this requirement is shown in Fig. 2, in which the numerals from Fig. 1 are used on equivalent elements. The pentode tube together with the cathode resistor 17 and the capacitor 22 Work as a cathode-follower and feeds the series-resonant circuit comprising the coil and the capacitor 21. The control grid 12 of the tube is connected to the coil 20 via a coupling capacitor 23, which has for its purpose to separate the direct potential of the grid from ground. The suppressor grid 41 is connected to the cathode 11, and the screen grid 42 is connected through a resistor 43 to the positive terminal of the direct current source 16 and through a capacitor 44 to the ground terminal 15. The cathode resistor 18 in Fig. 1 is in Fig. 2 substituted by a plate resistor 19, from which an alternating voltage is taken out via a coupling capacitor 27 to a rectifier of voltagedoubler type, comprising

rectifier elements

31 and 32 together with a capacitor 33 being loaded by a resistor 34. The voltage across the capacitor 33, which is negative, is added to a positive voltage taken out from a potentiometer 36, which in series with a resistor 37 is connected to the battery 16. The voltage difierence is fed to the grid 12 of the vacuum tube via a time delaying network comprising a series resistor 38 and a shunt capacitor 39. A resistor 40 is connected between the grid 12 and the capacitor 39 in order that the capacitor shall not short-circuit the resonant circuit. The direct voltage fed to the grid is negative, and its magnitude is dependent on the amplitude of oscillation. An increase in the amplitude of oscillation causes an increased negative grid voltage, which in turn causes a decrease in the voltage gain in the tube. An increase or decrease in the amplitude of oscillation is thus counteracted, and through a suitable adjustment of the bias voltage from the potentiometer 36, the oscillator may be made to work very near to its sensitivity maximum at all times. The

timedelay network

38, 39 makes, however, the stabilizing circuit inefilective when transient changes in the amplitude occurs. When a material to be tested passes through the electric or magnetic fields from the resonant circuit, a voltage surge is thus fed to the output terminal via the capacitor 26 each time an inhomogeneity having loss properties differing from those of the base material passes the measuring device. This voltage surge may be measured or indicated by an instrument 45 or any other suitable device, so that a measurement or indication of the inhomogeneity is obtained.

As the tube is working as a cathode-follower the plate voltage of the tube has very small influence on the amplitude of oscillation in the oscillator, and this influence is reduced in the same degree as the amplification factor ,0. of the tube is increased. Across the plate resistor 19 it is therefore possible to obtain an amplified signal, and if the tube is a pentodc, this amplification may be made very large. The necessary degree of amplitude stabilization thus can be obtained without the introduction of any separate amplifying stages and the output signal on the terminal 25 will also be so large that it directly can be fed to a measuring or indicating means, for instance a sensitive sudden change relay.

The embodiments of the invention shown are at present regarded as the most suitable, but the design of the oscillator may of course be varied within the range of the invention. For instance it is possible to utilize a transistor instead of the vacuum-tube shown.

I claim as my invention:

1. Means for examining a substance, comprising an oscillator having a series resonant tank circuit arranged to produce an electromagnetic field in the substance under test, said oscillator comprising a vacuum tube having a cathode, a control grid and a plate, a resistance means connected in series with said tube in such a way that it has a linearizing effect upon the plate-current-gridvoltage characteristic of said tube, an impedance means forming no part of said series resonant circuit and being connected in the cathode circuit of said tube, said series resonant tank circuit consisting of a coil and a capacitor being series connected across said impedance means. the junction point between said coil and said capacitor being connected to said control grid.

2. Means according to claim 1, in which said vacuum tube is a pentode.

3. Means according to claim 1, in which said impedance means is a capacitor shunted by a resistor.

4. Means according to claim 1, comprising means for measuring or indicating the amplitude of oscillation in said oscillator, connected across at least a part of said resistance means.

5. Means according to claim 1, in which said resistance means is connected in the cathode circuit of said tube.

6. Means according to claim 1, in which said resistance means is connected in the plate circuit of said tube.

7. Means according to claim 1, comprising a time-delayed amplitude-stabilizing feedback in said oscillator.

8. Means for examining a substance, comprising a vacuum tube having a cathode, a control grid and a plate, a source of direct current having a positive terminal and a negative terminal, a resistance means having a tap, connected between said negative terminal and said cathode, a coil and a first capacitor being series connected between said negative terrninal and said tap on said resistance means, said control grid being connected to the junction point between said coil and said first capacitor, a second capacitor connected between said negative terminal and said tap, an output terminal connected to said cathode.

9. Means for examining a substance, comprising a vacuum tube having a cathode, a control grid and a plate, a source of direct current, a resistance means, an impedance means consisting of a resistor shunted by a capacitor, said tube and said direct current source being connected in series through said resistance means and said impedance means, said resistance means being connected to said plate, said impedance means being connected to said cathode, a series resonant circuit comprising a coil and a capacitor shunting said impedance means, means includ- I References Cited in the file of this patent ing a coupling capacitor connecting said control grid to an intermediate point in said series resonant circuit, a recti- UNITED STATES PATENTS fying means fed by an alternating voltage existing across 2,018,080 Martiensen Oct. 22, 1935 said resistance means and having an output, a time delay- 5 2,376,610 Millington May 22, 1945 ing means including a series resistor and a shunt capacitor 2,408,029 Bazzoni et a1. Sept. 24, 1946 connecting said output to said control grid. 2,501,834 Wheeler Mar. 28, 1950 10. Means according to claim 9, in which a source of 2,550,607 Shoemaker Apr. 24, 1951 bias voltage for said control grid is series connected with 2,679,007 Sands May 18, 1954 7 said output from said rectifying means. 10 2,772,393 Davis Nov. 27, 1956