Inductive sensor for slow varying magnetic fields

A ELSEVIER

Sensors and Actuators A 59 0997! 219 "21

Inductive sensor for slow varying magnetic fields P,D. Popa *, N. Rezlescu, R. Anghelache T¢clmi¢¢l Imtitute of Ph)~ics, 47 Mangcron Blvd., 6600 Imi, gwmmia

Abstract The usual inductive sensorscan not be used in slowlyvat)'ing(variation rate almost zero) magneticfieldsdue m their lack of sensitivity. We descfitx an inductive sensorcapable of detecting slow variationsof the magneticfiled. The device is based on the magnetic domains reordering due to external variations of the applied magnetic field. The sensor contains a coil with ferromagneticcore, A continuousvariation of the applied field determines the sequential reorderingof the magneticdomainsin the core and, con~quently, the variation in steps of the magnetic induction, therefore, the coil will generate a pulse train. The number and amplitude of pulsesare not influencedby the speed of the variation. To obtain largeramplitude pulsesthe magnetic core of the sensor is ferromagnetic,relatively thin layer, {10-50 ttmL with high permcabilityand moderate coercitive field. Such layers may he amorphousribbons or laminated ribbons thermally and mechanicallytreated. Along with an electronic amplifier and analyser of the generated pulses,the described sensor may b¢ used in a vainly of applicationsfeatured by a slow variation of a magneticfield. ~ [997 ElsevierScience S.A. Keywords: Magnetic lipid sensor: Inductive; Low frequencT

I. Introduction

The inductive sensors have a very simple functional principle based on the law of electromagnetic induction. Because the induced electromotive voltage is proportional to the flux variation rate, slow variations (i.e. variation rate almost zero) in the applied field do not generate a signal strong enough to be detected with the classical sensors [I,2]. To obtain a useful signal at the output of an inductive sensor in no matter how slow a varying magnetic field, we describe here a way of constructing such a sensor and a way of analysing its output.

2. Principle of operation It is widely known that the ferromagnetic materials have a magnetic domain structure given by the spontaneous magnetisation, A variation in the applied field modifies the domains configuration by rearrangement of the domain frontiers and by the reorientation of the magnetisation. A contir.uous variation in the applied field modifies discontinuously the domains configura*Correspondingauthor. 0924-4247/97/$17.00~ 1997ElsevierScieae¢S.A. All fightsreserved, Pll S0924-4247{97101446-5

tion and the magnetisation varies in steps. In a coil with ferromagnetic core the continuous variation of the applied field results in voltage pulses [3,4]. In a classical inductive sensor, with a soft ferromagnetic core and a relatively large section, the electrical pulses given by the above described phenomena are extremely weak. This is due to the mobility of the frontiers and to the magnetic screening in the core's hulk. The transfer function of the sensor is practically continuous and is given by /,:~ C .dH ' dt

{1)

where U is the output voltage of the sensor, H is the intensity of the applied magnetic field and C~ is a coefficient dependent on the structure and core. The above o:lation shows that, in principle, a classical in. ductive sensor is not useful in no matter how slow a varying applied field. The inductive sensor for slowly varying field (ISSVF) may be constructed such as the high amplitude output voltages to be favoured. The ISSVF response to a slow variation DH of the applied field will be a voltage pulse train. The sign of the pulses depends on the variation sign of the applied field. The number of the pulses delxnds on the absolute value of the field variation.

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P.D. Popa er al. &,tl~or~am/Actuators

,.! 59 (1~)97) 219 221

The amplitude of the pulses depends on the construct-

ing way of the sensor. The sensor's transfer function may be written

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where N is the number of pulses obtained from a variation ~H of the applied field. U* is the mean amplitude of the pulses, C, and C~ are coefficients depending on structure and field. Because time does nnt enter in Eqs. (2) and (3), the sensor is useful no enattcr how slow the field variation is.

3. Design considerations For the realisation of an ISSVF it is needed to optimise the coil core's construction, Experimental results suggest thai the optimal dimensions of the core are given by a thickness of d= 10-50 lain and width / given by l / d = 50-200. i.e, ribbon shape. The magnetic properties of the core's material strongly influence the sensor performance. A soft magnetic material with large permeability and small coercitive field, typical for a chssical sensor, is not appropriate for an ISSVF, becau~ C., is too large and C~ is too small. To obtain pulses with a large amplitude it is necessary a material featured by moderate coercitive field, average permeability and with longitudinal orientation of the magnetic domains. We have sampled a large batch of magnetic materials. Good results were obtained with amorphous and cold laminated permalloy. Fe-Si and Fe with reduced carbon content ribbons. The laminated ribbons need annealing and, eventually, a mechanical treatment. The optimising treatments are material and application specific. If the sensor has a more complex magnetic circuit, the external elements may be constructed from other magnetic materials. The sensor's coil has the role of converting the rapid and weak variations of the core's magnetic flux in electrical pulses of a,iequately high amplitude (above 10 mV) at the output. To obtain these results it is needed for a short coil to be made of many turns and its inductance and capacity need to be small. The resonance frequency of the coil with core must be as large as possible (above 20 kHz). Under these conditions the output signal will contain short, high amplitude pulses without supplemental oscillations.

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Fig. 1, Transfi2rftmctionI'or~mISSVF. the domain of values for which the core's permeability is high, If the core exhibits a relatively large hysterezis (due to a larger coercitive magnetic fieldJ tile output sigmd depends also Oll the previous magnetis:Ltion. The ISSVF AC (frequency around I Hz)response, when the core has a larger hysterezis, is plotted in Fig. 2. One can notice that, when the sign of the field variation changes. tile sensors exhibits an insensitivity domain which increases when the amplitude of the AC field decreases. In a weak AC field such a sensor gives zero output, At higher amplitudes of the AC field the insensitivity zones appear also due to tile core saturntion, even for the materials ~vith no hystcrezis. In a rapidly changing magnetic lield, the ISSVF behaves like a dnssical sensor, In a low frequency field the output signal contains a si~usoidal component along the pulses. The two components have the same polarity and they sum up (Fig. 3) (freqt,ency around 31) Hz). At higher frequencies the sinusoidal component enhances and the pulses undergo attenuation. Although the ISSVF construction does not differ essentially from that of a eh|ssical inductive sensor, the output signal is very different and requires a more special analysis. First, a signal containing pulse trains has a large frequency spectra and requires u I.'trge bandwidth amplifier. Second, the signal is charucterised by the number ::rid the sign of some pulses and not by their amplitude. These facts require that the device used

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4. Experimental results

In Fig. 1 we give the ISSVF output pulse amplitudes versus the applied field. One can notice that the amplitude is small in weak and strong fields and greater in

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Fi~. 2. ISSVF response wi0x hystcrczisillAC field

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Fig. 3. ISSVF respon'~cin lo~; fi'cqucnc}licld. to translate the output should notice the moment of emergence for the first impulse, its sign, the number of the pulses of the same sign, the moment of the sign changing etc.. thus, it is needed a digital device v, ith logical circuits. A simple example of using the above described sensors is the construction of a body rotations counter. On the respective body it should be attached a permanent magnet. The rotation will give a variation of the magnetic field in the neighbourhood. A ISSVF placed in this field will generate a signal similar with the one given in Fig. 4. After a large band~idth amplification. the resulting signal is applied in a two-state logical circuit such as a trigger Schntitt. Each chang: in the pulses sign will trigger the circuit in the alternati~,'e state. Thus, the output of the circuit will be a rectangular signal which will be applied to a counter. The device will work no matter how slow is the body rotation.

5, Conclusions The application domain of ISSVF is larger than that of the classical inductive sensors. However. they cannot

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Fig. 4. Sigmd diagr.tm of a rotation counter width bc used as analogue transducers but they can be successfully used by extending the domain of applications to the slowest processes: in such cases they can replace other :?nso~.

References []i H,N. Norton. Handh,,.k .t Tmn~dun'r~ h,r Elet tronfi .th,asttrm~ .".~wu,m.~.Premi~:eflail. EnglewoodCliIT,~,NJ. 1%9. p. 184. [2) I.YLI.Wortg ~utd W.E. Ott, Ft+nl'tl~m ('ircuit.~. Dt'~li~tlam/..|pplioath,n. MeGra,.~-HilI.N¢~+York, 1976, 0.71. t3] E.P. Wohlfimh. Hamlhm~k t!l MJ*gm'tic MarerhaL~. Vol.3. El.;¢t.icr. Amsterdam, [987. p.52. [4] P Ripka. Noi~ and stability of magnetic sensors. J. Magn. .llagl~..thiner.. 1571199¢,)424 429,