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MAGNETOFLUIDIC MATERIAL AS AN ACTIVE MEDIUM FOR ACOUSTICAL SENSORS
M. Sarbu, M. Piso
To cite this version:
M. Sarbu, M. Piso. MAGNETOFLUIDIC MATERIAL AS AN ACTIVE MEDIUM FOR ACOUSTICAL SENSORS. Journal de Physique Colloques, 1990, 51 (C2), pp.C2-903-C2-906.
�10.1051/jphyscol:19902210�. �jpa-00230534�
ler Congres F r a n ~ a i s d'Acoustique 1990
MAGNETOFLUIDIC MATERIAL AS AN ACTIVE MEDIUM FOR ACOUSTICAL SENSORS
M.A. SARBU and
M . I .PIS0
Researches Institute for Electrical Engineering (ICPE),
Bd. T.Vladimirescu 45,
79623Bucharest, Romania
The first part of the paper presents an example o+
a composite magnetofluidic material. Mathematical modelling of the behaviour in periodical external fields is given in the second part. The third part presents an application of this material t o an electrical1 y tunable acoustical filter.
Magnetic fluids are often employed as passive components of some classes of sensors; as active media, the domain restricts to some particular applications. The purpose of the paper is to introduce a composite magnetofluidic system, endowed with special features that provide the capability to construct various classes of
sensors; moreover, this material could be priorlly used as active medium for acoustical and vibrations sensors and
transducers.
One of the relevant properties of the magnetic fluids is the s o called magnetofluidic levitation, /I/ of a nonmagnetic material in a magnetic fluid. If a nonmagnetic mass is embedded in a magnetic fluid under the action of a magnetic field, a volume force appears:
f = (M V) H
where M is the inner fluid magneti sation and V H is the gradient of the external magnetic field. This particular feature provides the capability to achieve the composite magnetof luidical material to be described below.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19902210
COLLOQUE DE PHYSIQUE
I. MAGNETOFLUIDICAL MATERIAL WITH NONMAGNETIC INCLUSIONS
The present material i s made of a mixture of magnetic fluid and nonmagnetic particles o n e t o t w o orders of magnitude larger in t h e characteristic dimensions of t h e magnetic +luid particles (O,1..1 p m vs. 40..90 A), made of heavy conducting metals. This medium i s included in a box (fig. 1). By fixing t h e appropriate values of exterior magnetic field and gradient, a stabile volume configuration of nonmagnetic particles in the magnetic fluid i s t o be obtained.
Fig. 1
The inner parameters of such kind of a system (dielectric constant, magnetic permeability, conductance) a r e perturbed by external fields / 2 / , as t h e acoustical
one. In t h e same way, t h e inner properties of t h e system could be smooth modified by performing usual electromagnetic methods
11. MATHEMATIC&L MODEL
In t h e case t h e upper described box i s acted by an inertial (or acoustical) field, t h e spatial distribution of nonmagnetic particles i s to be modified. In order t o
describe this, t h e nonmagnetic particles system i s considered as a set of 2-dimensional harmonic oscillators identical square lattices, with a given distribution of rigidities C k 3 ; t h e damping coefficients t C 3 are t h e s a m e in t h e first order,and t h e masses too:
m.. = m
-
kii
-
kin = ki C.. = CL l LJ
The rigidities distribution over i i s exponential, in t h e adiabatic approximation of t h e magnetic field spatial distribution.
The case of interest i s for time dependent external +ield. Here, each particle performs a forced oscillation on t h e external field frequency. Due t o t h e particular rigidities distribution, t h e system posesses a wide eigenf requencies spectrum. In t h e first order, t h e
ei genval ues are t h e f 01 lowing:
For a large number of particles, t h e frequency spectrum could be considered a s continuum; practically, for a finite frequency domain of external excitations, t h e system i s linear in a good approximation.
T h e upper and t h e lower thresholds of the linear time-response domain depends directly on t h e product of t h e levitation magnetic field and gradient. Also, t h e internal losses and the damping coef+icients Could be varied by means of dilution of t h e base magnetic fluid. These properties enables one t o set up appropriate inner parameters in order
t o get wide time-response properties.
111. APPLICATION
Among various applications of this active material in sensors and transducers, an electrical tunable acoustical filter h a s been achieved (fig. 2).
--
Fig. 2COLLOQUE DE PHYSIQUE
The composite magnetof l u i d i c m a t e r i a l i s included i n a c y l i n d r i c a l box made o f a r a d i a l magnetised permanent magnet. The s p a t i a l d i s t r i b u t i o n and the peak magnitude of t h e microscopic r i g i d i t i e s a r e c o n t r o l l e d by t h e mean o f a c y l i n d r i c a l c o i l i n DC. The frequency response o f t h e system could be smooth c o n t r o l l e d i n t h i s way.
The f u t u r e a p p l i c a t i o n s of t h i s a c t i v e m a t e r i a l include microminiature a c o u s t i c a l and v i b r a t i o n s e l e c t i v e sensors.
The authors a r e indebted t o I. Anton, L. Vekas and F. T. Tanasescu f o r t h e p r e p a r a t i o n of t h e magnetic f l u i d s employed and f r u i t f u l discussions. This work i s supported on
a National Comitee f o r Science and Technology grant.
1. R.L. Bailey, J. Magn.Magn.Mater. 39, 178 (1983)
2. H. Piso, A. Aciu, i n "Proceedings of t h e SENSORS'88 I n t e r n a t i o n a l Congress", Wrnberg ( 1988)
