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Transducer Properties in Magnetostrictive Delay Lines
Gunnar Svala, Telefonaktiebolaget L M Ericsson, Stockholm
l)en mekaniska omvandlaren vid en
maynetostrik-tiv fördröjningsledning har underkastats en
elementär analys, som resulterat i ett ekvivalent schema och
motsvarande "fyrpolekvationer". Det visar sig att
effektiviteten hos omvandlaren kan återföras till den
elektromekaniska kopplingskoefficienten k, som är
en materialkonstant och en faktor kd, som änger den
magnetiska kretsens effektivitet. En enkel metod för
uppmätning av produkten kd k2 anvisas. Vidare har
inducerade spänningen i mottagars polen beräknats
för rektangulär strömpuls under vissa förenklade
antaganden. Beräkningsmetoden har visat god
överensstämmelse med uppmätta värden.
Magnetostrictive delay lines have long been used
as memories in digital computer technique and are
well known from numerous publications. The
present paper will be concerned solely with certain
considerations which have proved of value of
assessing the properties of transmission media and
transducers in magnetostrictive delay lines.
An extremely schematic magnetostrictive delay
line is shown in fig. 1. The actual transmission
medium consists of a wire or tape, the material being
chosen especially for its magnetostrictive properties
and for its ability to propagate mechanical
oscillations with low attenuation. The existing
magnetostrictive materials are usually well adapted in the
latter respect, whereas in many applications their
delay/temperature coefficient cannot be disregarded.
The two terminations of the transmission medium
should be suitably damped in order to avoid
reflected waves which might set up disturbing signals.
This is essential since the transmitter transducer,
when electrically excited, inevitably transmits
mechanical waves in both directions and the receiver
transducer is not in itself capable of fully extracting
an incident wave even under ideal conditions. The
terminations can be arranged in practice by
surrounding" the wire or tape with a mechanical
damping" material such as foamed plastic or rubber with
continuously increased pressure along the length of
the tape. Transmitting and receiving transducers
consist of short, preferably identical, coils which
should fit tightly round the mechanical line. Finally,
the receiver transducer at all events must be given a
polarizing magnetic field in order that a voltage
may be induced into its winding by the mechanical
wave travelling down the line. The magnetic field
can be arranged most simply by placing a small
permanent magnet at such a distance from the receiver
coil as to produce the maximum voltage output,
corresponding" to the optimum biasing field. Separate
biasing of the transmitter coil may be dispensed
with, if very strong current pulses are applied.
538.(552 : 681.112
Elementary theory of pulse transmission along
a magnetostrictive delay line
The propagation of waves in the mechanical line
may be described by an electrical analogy. The
mechanical force in a cross-section of the line is
comparable to an electric voltage, and the particle
velocity in the line to an electric current. The
specific characteristic impedance of the line is then
found to be
zk = VqË (1)
where E is Young’s modulus and o the density, using
the MKSA system. The absolute impedance of a line
of cross-sectional area a will be
Z, = a
(2)
The propagation coefficient y will be described by
the equation
and the phase velocity will obviously be
(3)
(4)
For the sake of simplicity the propagation
coefficient has been assumed to be purely imaginary. In
actual fact damping will, of course, occur on
account of deformation work in the material, which
may be reflected in a complex value of E.
A wave advancing through the line will be described
by the equation
P = Zk V (5)
where P is the force in the cross-section (the
voltage) and V is the particle velocity (the current).
With small signals the magnetostrictive properties
of a material may be expressed by the coefficient of
magnetostriction * = i«c« the relative change
in length in relation to the change in induction at
constant mechanical pressure. The converse effect
(the Villari effect) may be characterized by the
Fig. 1. Schematical drawing of a magnetostrictive delay
line.
.92 ELTEKNIK 1959
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