M A G N E T O M E T E R W I T H A P E R M A L L O Y T R A N S D U C E R

Yu. N. D e n i s o v , A. G, K o m i s s a r o v , a n d P. T. S h i s h l y a n n i k o v

UDC 691.317.444

Magnetometers with permal loy transducers are widely used for measuring magnet ic field strengths of the order

of units or tens of oersteds [1-4]. A compensat ion measuring method is normal ly used in such instruments, The mea- sured magnet ic field H x is balanced by field H b of a compensat ion coi l with known parameters. The value of this f ield is determined from the current which must be transmitted through the compensation coi l in order to obtain a

comple t e balance. The permal loy transducer serves only as a null detector for determining the instant of comple te

compensation.

In measuring stationary magnet ic fields, the external f ie ld within the transducer is modulated in addit ion with a low-ampl i tude al ternating magnet ic field. In this case the instant of comple te compensat ion can be determined

by the symmetry of the emf pulses which are induced in the signal winding during the transducer's al ternating mag- net izat ion, and by the disappearance of odd harmonics or a maximum number of even harmonics in these pulse trains.

In the instrument described above the measured magnet ic field is modulated by a re la t ive ly high-frequency sinusoidal f ield H03, which we shall henceforth ca l l a switching f ield with ampl i tude How >> Hc, where H e is the co-

ercive force of the permal loy core. When the total field Hs = H x + H b + H03 passes through values :~ H e during peri- od Too = 2~r/03, the transducer's magnet ic polari ty is reversed twice and emf pulses E(t) of different polari t ies are in-

duced in the signal winding. These pulses are separated by T03/2 for H' 2 = H x + H b = 0, or else they are displaced addi t ional ly by ~ At, depending on the value and sign of the uncompensated f ield strength H'r. , For H003 >> Hc, the E(t) pulses are bel l -shaped, and their ampl i tude Vp and duration r at level Vp /2 can be ca l cu la t ed f rom the formulas

Vp ~ o~Sl~maxH0~, (1)

Bs (2) .~ (0.5 1) t,maxmH00~ ,

where co is the frequency of the switching field, w is the number of turns in the signal winding, S is the transducer's

cross section, #max is the maximum permeabi l i ty determined by the mater ia l and shape of the core, B s is the satura- tion induction of the core.

The relationship of d isplacement At to the uncompensated field strength H ' s for H ' s << H0w can be determined from the relationship

2H~ At - - - (3)

coHom

Thus, the train of emf pulses V(t) induced in the signal winding and, therefore, the ampl i tude of harmonics in this train are a function of H'I;.

Let us determine the first harmonic ampl i tude of function V(t)

where a 1 and b 1 are Fourier coefficients represented by the formulas

T

2 al = ~ j E(t)cos cot dr,

0

(4)

(5)

Translated from Izmer i t e l ' naya Tekhnika, No. 7, pp. 25-28, July, 1968. Original ar t ic le submitted August l0 , 1966.

885

Fig. 1

T (0

2 bl == -T~ j E(t) sin cot dr.

o

(6)

In order to simplify calculations we shall consider the E(t) pulses to be rectangular with amplitude Vp and dura- tion r . Such an approximation is fully permissible, since the nature of the relationship of amplitude Vlp to H' Z is not affected. The first harmonic amplitude of function V(t) in this case is equal to

4 Vp sin co~ H'~ V,p= ~ ~ .cos .Z/o~ ~ . (7)

It will be seen from (7) that VIp has a maximum for H' Z = 0. However, it is difficult to establish the instant of complete balance between the measured magnetic field and that of the solenoid by the maximum of the first har- monic of the induced signal train. In fact, although the amplitude of signal E(t) increases with a rising field switch- ing frequency w, any attempt to increase the transducer's sensitivity only by raising this frequency will not lead to the desired result, since the emf induced in the signal winding by the modulating field will increase simultaneously with E(t). Moreover, the frequency and phase of this stray pickup coincide with those of the useful signal's first har- monic, so that the signal to noise ratio may even drop at a sufficiently high switching field frequency.

In order to separate the useful signal from the pickup the instrument's magnetic field HZ is modulated in addi- tion at a low frequency with field Hf2 = H0flsin fit, with H0f<< H0w. In this case (7) changes to

4 v cow + sin at (8) Vlp n p s i n - ~ - c o s H0 ~ + Vi,

where V i is the induced amplitude. For small values of H' Z this formula can be converted to

4 cow [ H'xH0~ sinQt \] V,p= vp sin - - ; (9) + V i H02o~

It will be seen from (9) that, by using the first harmonic envelope of the pulse train V(t) as the useful signal, it becomes possible to eliminate the effect of induction even for a high switching field frequency. The amplitude detector output signal is equal to

Ud(O = 4 k Vp t/~

where k is the total transmission factor from the signal winding to the output of the amplitude detector and rp = 0for H'Z > 0 or Ir for H '2 < 0. Thus, by adopting a second modulation it becomes possible not only to eliminate the ef- fect of induction, but also to make the signal suitable for phase discrimination.

The block schematic of a magnetometer is shown in Fig. 1. A magnetic field with frequency w = 20 kHz and amplitude How = 27.85 A / m is used for alternating magnetization of transducer 1. The frequency and amplitude of the low-frequency modulation amounts to 400 Hz and 1.6 A / m respectively.

The high-frequency tuned amplifier 3 picks out the first harmonic of Vp(t). The amplitude of pulses E(t) is comparable to that of pickup V i and amounts to about 20 inV. This amplifier's output signal amplitude is modulated according to (8). Amplitude detector 4 separates envelope Vd(t), and the narrow-band low-frequency amplifier picks out the first harmonic of that envelope and transmits it to phase detector 5. The dc voltage is fed from the detector's output to the pointer null detector 7 for indicating the magnitude and sign of H'Z.

In order to facilitate the setting of the required current I c, which flows through the compensating winding, the instrument is provided with a negative feedback circuit. The phase detector output signal is fed to the control input of direct current I c adjustable source 2. This current is controlled by the voltage drop V c = IcR r across the stable reference resistor R r, which is connected in series with the compensation winding. The value of R r is selected in such a manner that the value of V c in volts is recorded on digital voltmeter 9 and corresponds to the tested magnetic field strength in oersteds.

886

The generators of the al ternating magnet iza t ion and modulation currents 8 aild 9 (reference frequency) have

the same circuits and differ only by the values of their reactances. The required values of the switching and modu- lating fields H0a~ and H0f~ are set with rheostats R2 and Ra, respectively, Rheostat R 1 serves to set the required phase

relationship between the signal and the reference voltage, which are fed to the phase detector,

The input high-frequency tuned ampl i f ier consists of two emi t te r followers which are coupled through a series resonant c i rcui t tuned to frequency ~o. The vol tage gain of the.ze two stages amounts to 12. The narrow-band low-

frequency ampl i f ier comprises a buffer emi t te r follower and a tuned ampl i f ier with an LC resonator in its col lec tor circuit.

The adjustable supply source of the compensation solenoid consists of a pa ra l l e l -ba lanced dc ampl i f ier whose output vol tage can be set both manual ly and au tomat ica l ly (within certain limits) by the phase detector 's output sig- nal. The maximum compensating current I c amounts to 80 mA, thus providing variations of the f ield in the range

of :k8188.1 A / m for the existing parameters of the compensation winding. The reference resistance value is Rr = 50 ft.

The core consists of a 6-10 mm long 0.08 mm wire which is made of permal loy 80 NKhS, enc losed in a quartz cap i l l a ry tube, and annealed in a hydrogen atmosphere according to a specified technology, The signal co i lhas 40 turns of 0.15 P~L (tinned enamel) wire wound direct ly on a 8 mm long quartz cap i l l a ry tube. The remaining coils are wound on a common Texto l i te former 28 mm long and 6 mm in diameter . The alternating magnet iza t ion and modulat ion coils each consists of a s ing le - layer 60 turn winding of 0.2 P~L wire. The mul t i layer compensation coi l is wound with 800 turns of 0.4"/ m m P~V-2 wire.

The instrument is fully transistorized and has the following characterist ics: a) the instrument's sensitivity is 0.16 A / m for a null detector 's deflect ion over 0,1 of the scate and a nonuniformity of the tested magnetic field along the core not exceeding 400 (A /m) / cm;b ) its measurement precision amounts to 0.08%~0A6 A / m with its re-

cording instrument consisting of digi ta l vol tmeter VIE2116 (or any other class 0.01% dc voltmeter); c) its magnet ic f ield strength range is +8188.1 A / m . This range can be extended, if necessary, by changing the parameters of the

compensat ion winding; d) for measuring f ield strength with a precision of several percent , the instrument has an in- corporated pointer vo l tmeter with ranges up to 79.6, 198.9, 795.8, 1989,4, and 7957.5 A / m .

The magnetometer described above is a convenient and sufficiently universal instrument suitable for measuring

weak magnet ic fields in various installations or leakage fields produced by electromagnets . Owing to the smal l size of its permat loy transducer the instrument can be used for measuring not only uniform, but also re la t ive ly irregular magnet ic fields,

L I T E R A T U R E C I T E D

1. I . M . Kelly, Rev, Sci. Instr., No. 22 (1951).

2. O .D . Adams, R. W. Dressel, and F. E. Towsley, Rev. Sci. Instr., No. 21 (1950). 3. B.P. Peregud, Pribory i Tekh. ~ksperim., No. 3 (1957).

4. K . N . Shorin, Yu. N. Metal 'n ikov, G. M. Bozin, and L. V. Eremin, Pribory i Tekh. ~ks~erim., No. 4 (1958)o

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