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An active structure consists of a structure provided with a set of actuators
and sensors coupled by a controller; if the bandwidth of the controller includes
some vibration modes of the structure, its dynamic response must be considered.
If the set of actuators and sensors are located at discrete points of the
structure, they can be treated separately. The distinctive feature of smart
structures is that the actuators and sensors are often distributed and have a
high degree of integration inside the structure, which makes a separate
modelling impossible.
Moreover, in some applications like vibroacoustics, the behavior of the
structure itself is highly coupled with the surrounding medium; this also
requires a coupled modelling.
From a mechanical point of view, classical structural materials are entirely
described by their elastic constants relating stress and strain, and their
thermal expansion coefficient relating the strain to the temperature. Smart
materials are materials where strain can also be generated by different
mechanisms involving temperature, electric field or magnetic field, etc... as a
result of some coupling in their constitutive equations. The most celebrated
smart materials are briefly described below:
SMAs allow one to recover up to 5% strain from the phase change induced by
temperature. Although two-way applications are possible after education, SMAs
are best suited for one-way tasks such as deployment. In any case, they can be
used only at low frequency and for low precision applications, mainly because
of the difficulty of cooling. Fatigue under thermal cycling is also a problem.
SMAs are little used in vibration control.
They have a recoverable strain of 0.1% under electric field; they can be
used as actuators as well as sensors. They are two broad classes of
piezoelectric materials used in vibration control: ceramics and polymers. The
piezoplymers are used mostly as sensors, because they require high voltages
and they have a limited control authority; the best known is the
polyvinylidene fluoride (PVF2). Piezoceramics are used extensively
as actuators and sensors, for a wide range of frequency including ultrasonic
applications; they are well suited for high precision i the nanometer range (1
nm = 10-9 m). The best known piezoceramic is the Lead Zirconate
Titanate (PZT).
Magnetostrictive materials have a recoverable strain of 0.15% under
magnetic field; the maximum response is obtained when the material is
subjected to compressive loads. Magnetostricitive actuators can be used as
load carrying elements (in compression alone) and they have a long life time.
They can also be used in high precision applications.
The range of available devices to measure position, velocity, acceleration
and strain is extremely wide, and there are more to come, particulary in
optomechanics. Displacements can be measured with inductive, capacitive and
optical means (laser interferometer); the latter two have a resolution in the
nanometer range. Piezoelectric accelerometers are very popular but they cannot
measure a d.c. component. Strain can be measured with strain gages,
piezoceramics, piezopolymers and fiber optics. The latter can be embedded in a
structure and give a global average measure of the deformation.
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