USRE22258E - High frequency wattmeter - Google Patents

Description

Jan. 26'; 1943. E. MITTELMANN HIGH FREQUENCY WATTMETER Original Filed April 22, 1938 lllllllllllllllllllll mo 1s .10 as Reiaued Jan. 26, 1943 UNITED STATES A PATENT OFFICE I mun rumor- 311:: wamm'rm Eugen Mittelmann, Chime, Ill. Original No. 2,240,955, dated May 6, 1941, Serial .No.203,661,.April 22, 1938.

issue Mayli, 1942, Serial No. 441,921;

' tria April 26, 1937 Application for re- In Aus- 15 Claims. (Cl. I'll-95) This invention relates to a method of and an apparatus for measuring the power which is absorbed by. the load only in the output circuit of a high frequency generator.

While ,my invention is applicable to the measurement or determination of the power absorbed by any load in the output circuit of a high frequency generator, the invention has special applicatlon to the determination of power absorbed by a patient from the output circuit of a short wave or high frequency therapy machine.

v An object of my invention is to provide a method of and apparatus for the measurement or determination of the load component oi the total power output of an oscillatory circuit, which component may be referred to as that portion of the total power output which is actualiy used to energize the load, as distinguished from those portions of the total power output which constitute power losses in, or radiation losses zrom, the circuit or apparatus. In the appiicatiou of short wave therapy, only a fraction oi? the totai high frequency power output of the apparatus is absorbed by the patient undergoing treatment,

andthe remainder is lost through radiation,

leakage, etc. Known measuring devices with which I am familiar indicate merely the total power output oi the output circuit, which is included the heating device or electrodes.

An object of my invention is, accordingly, to measure the power absorbed by the patient independently of the total power output of the circuit.

Other objects and advantages oi my invention will be readily understood by reierenceto the accompanying drawing, wherein is illustrated diagrammatically the output circuit of a short wave or high frequency generator embodying my invention.

Referring to the accompanying drawing- Figure l is a diagrammatic view of the secondary or outputv circuit of a high frequency zenerator, showing the instrumentalities which -I utilize in carrying out my invention; and

Figure 2 is a similar view to Figure 1 in which a modified form of instrumentality is utilized.

In both of the figures of the drawing, A represents a source of high frequency oscillations coupled to a secondary or output circuit B by means of a transformer C, all in a manner well known in the art. The apparatus forming the The resonance output circuit B is shown as including conductors l and 2 connecting the secondary of the transformer C withsuitable electrodes 5 which are spaced apart, and between which the object 6 is positioned. The object 5 represents a load, absorbing'power from the output circuit B. It is the power absorption of this load or object which is to be measured.

Ifthe high frequency is to he applied as treatment to a patient, then it is to be understood that the object G-is intended to represent the patient's body or some portion thereof positioned between the electrodes, as is well known and generally understood in this art. The output circuit in apparatus of this character generally has a variable condenser 4 bridged across the circuit for the purpose of tuning the circuit to resonance.

The efiect of placing a patients bodlfior some portion thereof, between the electrodes in course, that other power absorbing elements already exist in the circuit and are inherent thereto as characteristics thereof.

In accordance with my invention, I utilize a measuring instrument which operates on the principle of the square law, that is, in which the deflection oi the pointer is proportional to the square oi the'voltage or current. any standard or well mown device of this type may m utilized. That which is shown in Figure l is of the hot wire thermocouple and galvanometer type, while that shown in Figure 2 is of the dynamometer type having a moving coil and an exciting coil and utilized in connection with an electronic tube operating in the square portion of its characteristic.

In Figure l the thermocouple 9 is, bridged across the output circuit conductors I and 2 by means of conductors 'l-I, which include the condensers H.

The thermocouple 8 is connected to the terminals of the galvanometer 12 by means of conductors lt-Jiin the usual manner in instruments of this character. The galvanometer 12 has a pointer II which moves over a calibrated dial lie, the deflection of the pointer being proportional to the square of the voltage across the condenser l, or the load in parallel therewith. This dial is calibrated in equal divisions, and, in the present instance, I have shown the dial as calibrated in divisions from 0 to 100.

In accordance with my invention, I provide a shunt circuit around the galvancmeter, which shunt circuit includes a variable resistance ll for varying the sensitivity'of the galvanometer. This shunt circuit includes the conductors I! which are, connected to the terminals of the galvanometer. One of these conductors is connected to a terminal of the resistance I4 and the other is connected to a contact member or wiper l8 movable over the resistance ll. Alongside the path of the wiper I8 is a scale or dial II which is preferably calibrated in divisions from to 100, corresponding to the dial of the galvanometer. The wiper carriesa pointer l! which cooperates with the dial II to indicate the resistance values in. terms of the galvanometer readings.

Since it is desired that the shunt resistance ll vary the sensitivity of the galvanometer in certain mathematical ratios, as will hereinafter be xplained, it is necessary to first calibrate the dial ll of the resistance in accordance with those ratios rather than in terms of the true values of the resistance l4.

It indicates the same value as'that indicated by the galvanometer II, the sensitivity of the galvanometer will be of the value required to cause the galvanometer to indicate the true value of the power absorbed by the body or load I. It will be seen from latter discussions that the resistance is conveniently constructed in such amanner that .the ohmage gradient thereof will permit use of a dial l'l having linear divisions.

The principle upon which the measurement of the power absorbed by the object is accomplished by the above describedapparatus may be explained as follows:

The secondary resonance circuit B (of Figure 1) under no load and load condltions'may be denoted by two equivalent circuits. respectively,

as follows: (i) a source of high frequency voltage in series with a simple coil and a variable condenser and having a single resistance m in parallel with-the condenser; and (2) the same circuit with a second resistance, R1 in parallel with'the resistance Rm, The Be may be defined as that resistance in parallel with the condenser which causes a power loss equal to the no load losses of the circuit, and the resistance R1 may bedeflned as that resistance inparaliel with the condenser which absorbs an amount of some the secondary-circuit, i. e., across the variable condenser, under no load, is es, and the relation between on and R0 is:

ec=k.Ro (1) where k is a factor of proportionality dependent upon the resonance frequency, the capacitance or inductance, and the value of the high frequency source of voltage, and represents the current through the equivalent resistance Ru. From this equation it will be evident that the resonance voltage of the circuit at no load is proportional to the equivalent parallel circuit-loss resistance R0 of the circuit.

When a load orobject is introduced into the circuit between the electrodes 5-4, additional reactive and resistive elements are coupled into the circuit. The circuit is once more tuned to changed from so toanew value e1.

, ance introduced into the circuit by the addition This resistance is so 0811- of the load or the introduction of the body 6 may,

, brated, asfwili hereinafter appear, that when the wiper It is moved to a point such that the pointer resonance to compensate for the change in reactance of the circuit effected by the addition of the load, or the insertion of the object. The

resonance voltage of the circuit is thereby The resistas previously noted, be represented by the equivalent parallel load resistance R1; The voltage and frequency of the source being constant, and the capacitance of the circuit having been kept constant by the adjustment of the condenser to tune I the circuit to the same resonance frequency, 1. e.,

' the frequency of the source or generator, the resoequation: 7

R+Ri From theabove two-equations, it will be seen that i RFRTH V power equal to the power absorbed by'the load or object. The variable condenser of this equivalent circuit is, ofcourse, equivalent in all re-- spects to the capacity represented by the electrodes 5-5 (in Figure 1) shunted by the variable condenser], and other contributory capacitive factors inherent in the circuit. The inductance represents all inductive reactances present as.

part of the characteristics thereof.

The first equivalent circuit represents the I secondary circuit B with no load. or the object 6 not in position between the electrodes 'l-l.

Under these no load conditions thereare certain losses in and from the circuit B, i. e.. leakage losses, radiation losses, 'etc., all of which absorb power from the" circuit.

power absorbed at no load is a function of the equivalent parallel resistance Bo. Rs is. of course, a constant for any given arrangement and spacins of the electrodes. The voltage in Since the circuit is at resonance and all reactive factors or reactancesare balanced, the total nance voltage c1 of the circuitis proportional to the combined resistance of the equivalent parallel resistance R0 and R1, according to the following certain limits, which are rarely exceeded in practice, will not, at the high frequency used, alter or have any effect upon the equivalent no load resistance Rn.

By accurate control of the voltage ofthe source. of high frequency, either manually, or by voltage stabilizer, or by any other method, the no load resonance voltage may be maintained constant for any given no load circuit conditions. Thus, with en constant and R0 constant, Equation 3 becomes: I

l ti flei) or R1 is purely a function of or (4)- Keeping this inmind', and recalling that the power absorbed by R1 is thatwhich is to be determined, it can be seen that if we can-measure the resonance voltage of the circuit under load;

and if we know the equivalent parallel resistance added totl'ie circuit by the introduction Of the load. we can determine the power absorbed by the load by the well known power equation:

1 P v whereE isin volts, Risinohms. and rm in watts.

Since a: is a direct function of R1 when theno load resonance voltage and no load circuit conditions are maintained constant, then under 1. those conditions the equivalent parallel resistances of the different loads will be-determined by the values of 81. V

For given electrodes, the spacing within directly or in absolute voltage values.

conditions and the ratio is a constant for any given load, the equivalent parallel load resistance determine the value of the equivalent parallel load resistance R1 for any given load introduced into the circuit.

Every change in the amount of power supplied to the secondary circuit B so changes both the no load and the load voltages, that the conditions of Equation 3 are always met. Hence, if we adlust resistance 14 to a value which will render the sensitivity inversely proportional to R1 determined by Equation 3, the galvanometer will indicate the power absorbed by the load in accordance with Equation 5. The resistance ll having been, adjusted to this corresponding value, the galvanometer, it calibrated in power units, will indicate the power absorbed by the same load for any value of power supplied by the high frequency source-this measurement oi the power'absorption being, of course, limited only by the range of equivalent parallel load resistance'Ri for eachload need not be actually measured or known, and that the no load resonance voltage co and the resonance load voltage e1 need not be measured 7 All that is necessary is to determine by'measurement, in a manner to be described hereinafter, the ratios or quantities'aflecting the setting of the resistance H for any given load. The import of this is that the measurement of the power absorbed by the load or body 6 becomes a simple expedient. By.

merely determining,'trom the change in the resonance voltage caused by the addition 01 a given load to the circuit, the ratio corresponding to R1, we can, it we know any resonance voltage E applied to the given load under'the same electrode or circuit conditions, determine the power absorbed by that given load for any amount power input. This can be seen from a simple use of the power Equation 5 which becomes P. E R1 (6) where P1 is the power absorbed by the load or body 6 at any resonance load voltage E and R1 is the equivalent parallel resistance of the given load and is constant for that given load.

, All measurements contemplated by my invention are made in practice by means oi a hot wire law measuring device.

In the galvanometer II, the deflection 6 repvoltage caused by the insertion of a load to the raonanco load voltage. I! we adjust the sensitivity v of the galvanometer after introduction of the load so that the sensitivity is proportional to the reciprocal of the equivalent parallel resistance oi the load,-as indicated by the followingequation:

l I a K I (8) where K is a factor of proportionality, then by substituting for a in Equation 7, the value of :7 determined by Equation 8, we obtain:

6 K'- KI'F: (9)

The voltage E applied to the galvanometer is proportional to the square of the resonance voltage E of .the circuit, or

I hieg 10 where K4 is a factor of proportionality including K, K and a factor E; and E.

It can be seen by comparingEquation 10 with Equation! that the deflection of the galvanometer is directly proportional to P1, the power absorbed by the load. All that remains, therefore, is to calibrate the galvanometer in terms oi power units, i. e., watts, and to calibrate the scale ll of the resistance It in such manner that upon adjustment it will cause the sensitivity values of the resistance ll, forany given setting is no part of this invention, but it sufiices to state that the equation for shunting galvanometers to alter the sensitivity thereof maybe used for calculation; that is, for any desired value of R, J the resistance of the shunt could be determined from the following equation:

resented by the pointer 13 thereoils proportional to the voltage measured and the sensitivity of the galvanometer or Y where R4 is the resistance 01 the galvanometer, R. is the resistance of the shunt and a is the sensitivity factor oi the galvanometer.

Since R1 for different loads is a direct function 0! e1. (assuming constant no load resonance voltage and no load circuit conditions) the proper sensitivity control setting of the resistance II for a particular load can be determined by adjustment of the voltage en to the same value under any no load circuit condition and then noting the reading corresponding to the resonance load voltage oi as indicated by the galvanometer after the load has been introduced and the secondary output circuit B retuned.

According to Equation 3, R1 is always proportional to the ratio of proportionality between and, since is a constant. the relation of the settings of the resistance R- to the resonance casts v tion of the load represented by the equivalent load voltages may be properly determined from v a measurement of the variation in resonance voltage from any no load value en to any load value a; for a known load. Advantage is taken R1 The procedure in calibrating the resistance It is as follows:

The equivalent parallel resistance R0, repre senting the circuit resistance without a load or object between the electrodes, is 'flrstdetermined in terms of the resonance voltage 60. This is done by tuning the secondary output circuit B to resonance by means of variable condenser l. The galvanometer will indicate a maximum voltage, which by suitable input control means, for instance, means varying the anode voltage of the oscillating tubes of the generator, is adjusted'so that the pointer I3 is deflected to the full scale reading which, for illustration, we will assume to be 100. 'This reading of the meter corresponds to, or is indicative of, en. The resistance ll being maximum, the scale I1 is marked zero A calibrating load is then placed between the electrodes and the circuit is again tuned to resonance by the condenser l. The actual power absorbed by the calibrating loadmay be determined according to any known method, as, for example, by a calorimeter. For purposes of illustration it may be assumed that the calibrating load absorbs 60 watts ofpower, and that the galvanometer pointer indicates the value of 25' for the resonance load voltage e1. The change in the reading of the galvanometer from the value of 100 to the value of 25 signifies the value of eo-ei (which represents the sensitivity of the gaivanometer) tobeproportional to r in being the equivalent we resistance of the load.

The scale I1. is now marked opposite the po-' sition .of the pointer II to indicate the proper parallel resistance R1. The galvanometer l2-will register the correct value of the power absorbed 1 by the load for any value of high frequency voltage applied to the-circuit B by adjustment of the high frequency source, for instance, by adjustment oi the anode voltage of the oscillating tubes of the generator A. name any amount of power can be supplied to theload by such adjustment of the high frequency generator A, Once the proper setting of the resistance ll has been determined from the change in the resonance voltage effected by the introduction of the load, the galvanometer I: will correctly indicate the power absorption regardless or the value of the power input. Having determined onevalue of the resistance It which converts the galvanometer from a voltage indicating instrument into a correct reading watt meter and marked the location of the wiper on the dial I I, the dial can now be calibrated into proper divisions throughout its range by computation and without the necessity of utilizing other known loads. It is to be understood that the calibrating procedure lust describedis that which .is used in the factory to calibrate the apparatus.

The instrument having been calibrated, the procedure for the measurement of the power absorbed by an unknown load or object placed in the circuit is very simple. The circuit B is tuned to resonance by 'cqndenser 4 before the object or loadis placed in the circuit, and the output control of generator A so adjusted that the Pointer ll of the galvanometer will indicate a certain reading, for instance, a. full scale reading. The voltage applied by the circuit is changed by this adiustment while the frequency is maintained constant. The object is placed between theelectrode's, and the circuit B again tuned to'resonance. 1 The value. registered by the galvanometer is noted, and the wiper I! of the resistance is then adjusted until the pointer It arrives at a mark on the dial I'I corresponding to the value indicated by the galvanometer. adjustment of the resistance ll changes the sensitivity of the galvanometer proportionally to the equivalent parallel loss conductance register the true value of the power absorbed adjustment has been made such as to cause the ratio position of the wiper II. The scale II for ready voltage of the circuit 8 caused by he introduc- 7 by the object. Any desired value of power may be administered, or supplied, to the object by so varying the output control generator as to vary the value of the high frequency voltage applied to the circuit 13. The correct values of 1 power by the object will be indicated, no matter what amount of power is supplied to the output circuit B since, the circuit remains resonant fortthe entire run. The resistance ll isset once for any run; that is, for any particular loador object, and is not thereafter altered duringthat run,'even though the power input, and consequently the power absorption, is varied during the rim. Thus any desired amount of power may be administered, or supplied. to the load or object and the value of the power absorbed by the load or object will be indicated in brated'meter II.

In Figure 2 of the drawing there is shown another form of instmmentality. An electronic I tube E is bridged across the conductors i and 2 of the output circuit and is convicted with Thepower units by the call- I once.

themovingcoilllofaeterl 'oithe dynamometer type. The excitation coil 2| of the galvanometer is in the circuit 12 which includes' the variable resistance 23 so that the sensitivity of said galvanometer can be varied by adjusting the re istance and thereby the exciting current.

circuit, the combination of a square law measurlng device connected across the oscillatory circuit, said device including a meter, the indicating element of which is adapted tobe deflected proportionally to the square of the voltage oi the oscillatory. circuit, and a variable resistance in parallel with the meter of said device {or varying the sensitivity of the meter, said resistance being calibrated for adjustment in accordance with the change in the voltage of the oscillatory circuit as indicated by the meter upon the introduction of the load and when so adjusted to proportion the sensitivity of the meter to the equivalent load resistance which is added to the circuit by the load. said meter having a dial calibrated to indicate, upon said adjustment of the resistance, the true value of the power absorbed by the load. 7

2. An apparatus for measuring the power absorbed by a load from an oscillatory circuit, the combination of a hot wire thermocouple and a aalvanometer for measuring the voltage across the circuit, said galvanometer indicating the voltage component of the power absorbed by the load, and a calibrated resistance in parallel with said galvanometer for changing the sensitivity thereof in definite relation to said voltage component oi the power absorbed so that the corresponding value of the power absorbed by the load portional to the equivalent conductance added to the circuit by the load.

4. The method of measuring the power absorbed by a load from a high frequency output circuit energizing the load, including the steps of impressing upon an electrical measuring instrument a voltage proportional to the square of the no load resonance voltage of said circuit, inserting the load into the circuit and thereby adding an equivalent conductance into the circuit which affects a change in the voltage impressed upon the instnnnent, and then adjusting the sensitivity of said instrument to a value proportional to said equivalent conductance inserted into the circuit by the load whereby said instrument then the power abindicates a value proportional to 5. In a device for measuring the power absorbed by a load frolna high frequency supply circuit, said device comprising a meter connected to said circuit and calibrated to indicate the change in the voltage of the circuit caused by the addition of the load to the circuit, and adjustm'ent means for determining the sensitivity or said meter, said adjustment means being cali- 'brated to adjust the sensitivity of the meter in accordance with the indicated change in voltage to cause the meter to indicate the power absorbed by the load.

6. In a device for measuring the power absorbed by a load from the output circuit of a high frequency variable power source, said device comprising means for deriving from said circuit is indicated by the galvanometer, said relation being defined by the formula 1 I P1=E E corresponds to the sensitivity of the galvanometer following the adjustment of the calibrated resistance, R1 corresponds to the equivalent parallel resistance inserted into said circuit by said load, and P1 represents the power absorbed by the load, as indicated by the galvanometer following the adjustment of the calibrated resist- 3. In an apparatus for determining the power absorbed by a load from an oscillatory circuit. the combination of an electrical device having a square law characteristic, a variable resistance in parallel with said device, said resistance detel-mining the sensitivity of said electrical device, and resistance adjustment means including a scale calibrated to adjust theresistance to a value such that the sensitivity of the device is proan electrical quantity proportional to the voltage across the circuit, clectro-responsive measuring means for indicating the value of S8.ld quantity, and means for thereafter ad ustmg the sensitivity of said measuring means according to a previously calibrated factor of proportionality between the indicated value of the quantity and the known power absorption of a calibrating load causing said quantity to be of equal value, whereby said measuring means then indicates the value of" the power absorbed by the load for any value of power supplied by said source.

'1. In a device for measuring the fractional amount of the total output of power from a high frequency variable power source that is absorbed by the load in the output circuit of said source, said device comprising a variable impedance for tuning the circuit to resonance under no load and load conditions, nlcans foroeriving from said circuit unidirectional electrical enel-glzat ons proportional to the resonance voltages of the circuit under no load and load conditions, electroresponsive measuring means responsive to said electrical energizations and indicating the values of said energizations under no load and load conditlons, and means for varying the sensitivity of said measuring means, following the introduction of the load and the retuning of the circuit, in inverse proportion to an equivalent circuit rmistance causing equal change in said energize.- tions whereby said 'electro-responsive measuring means thereafter indicates the value of the power absorbed by the load, and variations thereof as the power supplied by the source is Varied.

8. In a device for measuring the power absorbed by a load in the tunable output circuit of a high frequency variable power source, said device comprising means for deriving from said circuit an electrical quantity proportional to the square of the resonance voltage of the circuit under load, electro-responsive-measuring means for indicating the value 01' said'quantity. and

means for adjusting the sensitivity of the measuring means and calibrated to proportion the sensitivity of said measuring means to an equivalent circuit resistance causing said quantity to be of thesame valueas that indicated by the measuring means for the particular load, whereby said measuring means indicates the value of the power absorbed by the load for any value of powersup plied by said source. 1

9. In a device for measuring the power absorbed by a load in the tunable output circuit of a high frequency variable power source, said device comprising means for deriving from said I circuit no load and load voltages proportional, re-

spectively, to the resonance voltage of the circuit under no load and loadconditions, electro-responing said variation in the magnitude of the derived load voltage, indicates the value of the power abeases across the circuit, electroeresponsive measuring means for indicating a value having a mathematical relation to said quantity, and means for thereafter adjusting the sensitivity of said measuring means according to a previously calibrated factor of proportionality between the indicated value-and the known power absorption of-a calibration load causing an identical indication,

whereby said measuring means then indicates the a value of the power absorbed by the load for any value of power supplied by said source.

13. In,a device for measuringthe power absorbed by a load in the tunable output circuit of a high frequency variable power source, said device comprising means for deriving from said circuit an electrical quantity proportional to the square of the resonance voltage of the circuit sorbed by the load for any value of power supplied by said source.

10. In a device for measuring the power absorbed by a load in the output circuit of a high frequency variable power source, electro-responsive measuring means including a meter for measuring the change in voltage across the output circuit produced by the introduction of the load to provide a calibration number indicative of the equivalent conductance of the load. and means for then presetting the sensitivity of the measuring means according to saidcalibration number,

, said means being calibrated to indicate, when its sensitivity is sopreset, the value of the power absorbed by the load for any amount of power input.

11. In a device for measuring the power absorbed by a load from a high frequency supply circuit, said device comprising a meter connected to said circuit and calibrated to indicate a value corresponding to the change in the voltage of the circuit caused by the addition or the load to the circuit, and adjustment means for altering the sensitivity of said meter, said adjustment means being calibrated to adjust the sensitivity of the meter in accordance with the indicated value to cause the meter to indicate the power absorbed by the load.

12. In a device for measuring the power ab,-

, sorbed by a load from the output circuit of a high frequency variable power source, said device comprising means for deriving from said circuit an electricai quantity proportional to the voltage under load, electro-responsive measuring means for indicating a value proportional to said quantity, and means for adjusting the sensitivity of the measuring means and calibrated to proportion the sensitivity 0! said measuring means to the value of an equivalent circuit resistance causing said quantity to be of the same value as that indicated by the measuring means for the particular load, whereby said measuring means indicates the value of the power absorbed by the .load for any value of power supplied by said source.

14. In a device for measuring the power absorbed by a load in the output circuit of a high frequency variable power source, electro-responsive means including a meter for indicatinga value corresponding to the change in voltage' across the output circuit produced by the introduction of the load to provide a calibration number indicative of the equivalent conductance of to indicate, when its sensitivity is so adjusted, the value of power absorbed by the load for any amount of power input.

15. The method of measuring the power absorbed by a load in a high frequency output circuit energizing the load, including the steps of taking readings on an electrical instrument indicative of the resonant voltages of the circuit before and after inserting the load, adjusting the sensitivity of an electrical instrument indicating a value proportional to the square of the voltage across the circuit in accordance with an index determined from said readings to render the sensitivity of said instrument proportional to the equivalent loss conductance inserted into the high frequency output circuit by said load, said instrument thereby indicating a quantity proportional to the power absorbed by said load.

EUGEN LUI'I'ELMIANN.

Images