Biomedical Engineering Reference
In-Depth Information
Fig. 9.3
BLA of the (
left
) commercial device and (
right
) prototype device.
Bold black line
:BLA;
blue dashed line
: total variance (noise
stochastic non-linear distortion);
red dotted line
: noise
variance;
green dash-dot line
: variance of the stochastic non-linear distortion with respect to one
multisine realization. The values of the noise and variance in (
left
)are
+
+
20 dB enhanced for clarity
of observation (same Y-scale)
impedance (i.e. a calibration tube). The measurement of air pressure
P
and air flow
Q
dV/dt
(with
V
as the air volume) during the FOT lung function test is done at
the mouth of the patient. The FOT lung function tests were performed according to
the recommendations described in [
116
]. The FOT excitation signal was kept within
a range of a peak-to-peak size of 0
.
1-0
.
3 kPa, in order to ensure patient comfort and
safety.
The BLA of both the commercial and the prototype devices are given in Fig.
9.3
.
Notice the noise lines in Fig.
9.3
-(left) are increased with 20 dB for illustrative pur-
poses only. In Fig.
9.3
-(right), a significant improvement is done with respect to the
power of the low-frequency signal generator, namely an increase in amplitude with
10 dB and a flat amplitude curve at low frequencies in the BLA. The drawback is the
relatively high noise amplitudes. From this it follows that the noise in the prototype
is about 20 dB higher, partly due to the less amount of tubing between the actuator
and the mouthpiece and mechanical friction. The improved prototype device can
send reliably a multisine excitation as low as 0.625 Hz, as shown by Fig.
9.4
.The
prototype has been further analyzed both experimentally and theoretically.
In order to validate the correctness of the measurement, a known impedance
is required. A calibration tube with the characteristics shown in Fig.
9.5
has been
measured by means of the prototype device, and the impedance results are given in
Fig.
9.6
. The tube has been simply added to the device at the same position where the
mouthpiece is placed. The theoretical impedance has been also calculated, in order
to validate the measured data. The systematic difference in amplitude between the
=
