Implementation of the ‘smart’ rotor concept - Wind Power Generation and Wind Turbine Design - page 477

for a fi n [61] and later to a small helicopter rotor [59]. But according to Boller [50]
the solution is only applicable to small rotors.
As with concepts for smart wings, some general considerations can be deducted
from this. The fi rst one is that generally the goal of adding smart features to heli-
copter blades is vibration control and sometimes quasi-static blade tracking. But
not replacing the current fl ight control system. For vibration control most research
is aimed at active twist or trailing edge fl aps, actuated by piezoelectric driven
mechanisms. With piezoelectrics high actuation frequencies are attainable.

3 Adaptive materials

In the previous section on aerodynamic features, two types of adaptive materials
are repetitively implemented: piezoelectrics and SMA material. Others can also
be discarded for adaptive features on wind turbine blades because of the need for
heavy coils (magnetostrictives, magneto-rheological fl uids), too low bandwidth
(electroactive and SMPs).

3.1 Piezoelectrics

Piezoelectric materials exhibit a coupling between mechanical deformations and
dielectric effects. Actually, the term piezo is derived from the Greek word for
squeeze or press. Moreover, the piezoelectric effect was fi rstly discovered in crys-
tals which gave of a surface charge when strained. This is known as the direct
effect. But the effect also works in the inverse manner: the material will strain
under the application of an electric fi eld. This is known as the converse effect.
These effects, as well as the governing equations have been documented by
many [62-67]. They can be written in many forms [62, 66], depending on the
electric and mechanical boundary conditions, but here they have written in two
forms which are relevant to the use of actuator and strain sensor and the appropriate
constants that describe the material will be discussed:

E

e = S s + d E

( 1 )

i ij j i m

s

= + ( 2 )
with: i,j =1,2…,6 and m,k = 1,2,3 The subscripts are related to the material's coor-
dinate system and the superscripts denote under which constant boundary condition
the parameter is assessed - s for constant stress and E for constant electric fi eld. In
eqs (1) and (2) the following variables defi ne the electric and mechanical state of
the piezoelectric materials.
e i : Strain component.
s i : Stress component.
D i : Electric displacement component. Electric displacement is related to the amount
of charge q on the electrodes: q = Ú Ú DdA
E i : Applied electric fi eld component.

Dd s eE

m i i ik k