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APPLICATIONS INFORMATION
0
200
400
600
800
1000
1200
1400
1600
40
35
30
Taxiway crack
25
20
15
Aliasing
10
Engine spin-up
(fundamental + resonant
5
modes)
Aux. Power Unit (920 Hz)
0
Frequency (Hz)
Figure 4: 2-D FFT of the time domain data shown in Figure 2. Salient features have been labeled.
peak amplitudes, but appear negligible in the overall 1-
D result since this energy is spread out over a wide
frequency range. A closer look at the 150-375Hz
signals, along with the similarly-shaped features at
higher frequencies, shows that they all start off at
multiples of approximately 64Hz and 156Hz. That is, 64,
128, 156, 312, … . Note that not all of these overtones
are visible at t=0; the higher modes are visible mainly
as they cross fixed resonant modes in the aircraft
structure itself. Some examples of these mode
crossings are near 750Hz, 900Hz, 1050Hz and 1200-
1300Hz. The 150-375Hz signals are most likely the
n/rev contributions of one engine being spun up,
meaning they are engine-speed dependent and may not
be reliable for e.g. energy harvesting, but may be useful
920Hz all the way up to 1600Hz. This is the impulse
response as the craft crosses a large crack or other
discontinuity in the taxiway, creating a mild mechanical
impact. Other sudden mechanical shock or impacts will
produce a similar response. Finally, note the signal
between 1200-1450Hz at approximately 13-15
seconds. As the engine speed continues to increase,
frequency features in the inverse (decreasing) direction
are evident. This is a good example of an aliasing
artifact caused by a very strong signal beyond the
maximum measurable frequency of the device. As
mentioned in the Aliasing appnote, any signal energy
remaining beyond the Nyquist (folding) frequency will
be mirrored over this frequency and appear as a
spurious signal below the Nyquist frequency.
to estimate engine RPM remotely.
The next series of plots shows data taken from the
A couple other features are of note in this plot: At
approximately 28 seconds, a brief horizontal
‘broadband’ feature is evident, most prominently from
same location during flight at cruising altitude. The APU
contribution is still visible in the Z axis, but the response
is now dominated by “wind noise” at 400-600Hz (Y
REVISION N0. 001
REVISION DATE: 08-24-2010
www.mide.com
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