Environmental Engineering Reference
In-Depth Information
Figure 20: Set-up of the control system.
loading. However, this can also be tuned by changing the frequency spectrum of
the disturbances which is controlled by the pitch system.
The second type of dynamic effects to take into account, is the unsteadiness
2.
of the aerodynamics. This is expressed by the parameter
k
, called the reduced
frequency:
w
b
( 46)
k
=
V
in which
is the frequency of the disturbances,
V
the undisturbed airspeed and
b
the half cord of the aerofoil. With it, frequencies of disturbances can be scaled to
the dimensions of the blade and the wind speed. The aerodynamic delay, the phase
between a sine on the fl ap and the resulting lift forces, is dependent on this reduced
frequency [ 112 ].
The blade is designed to match the frequencies that were derived from these
considerations. The target fi rst fl apping frequency was determined to be 19.2 Hz
and in the actual blade the eigenfrequency was 12.5 Hz. This was easily compen-
sated for by changing the airspeed and the frequencies of the disturbances to which
the blade is subjected. The blade was tuned by changing the internal structure, viz. the
number of glass-epoxy plies and the presence of a spar. A spar was added in the tip
because here the actuator slots were cut out. The spar adds additional stiffness and
strength and can be used as mounting point for the actuators.
The blade was produced using vacuum infusion in a closed mould and after
assembly of the sensors and actuators, it was mounted on the pitch system and
connected to the control hardware. Several tests were conducted:
Ω
•
Feedforward on disturbances with a sinusoid signal.
•
Feedback control on a sinusoid signal with a strain sensor at the root.
•
Feedback control on a spectrum angle of attack disturbances that resembles the
turbulence that an actual blade experiences.
Feedback control on a step on the angle of attack (simulating gusts or tower
•
shadow) .
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