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2.3 Electrodes
As mentioned above, the electrodes should be as no-
invasive and no-annoying as possible for the driver.
Therefore, using standard metal electrodes seems to
be not the better option. In addition, as cited in
(Wheelwright, 1962),during long-term monitoring,
the hidrogel used with this kind of electrodes can
cause irritation and allergy problems.
So, in this system instead of using standard metal
electrodes, electrodes made of textiles, also called
textrodes, are chosen. In this way, not only irrita-
tion and allergy problems are solved but also a higher
comfort for the driver is achieved. However, the
main drawback of the textrodes is that the electrode
impedance, Z
ep
, shows a strong capacitive behavior,
(Beckmann et al., 2010). In addition, as the textrode is
not directly in contact the skin, this capacitive behav-
ior depends on the exerted pressure and factors related
to the clothing of the driver like material, thickness or
number of layers.
2.4 Demodulator & Acquisition
Although there are several demodulation techniques,
a switching demodulator is used. Switching demod-
ulators are based on a switch controlled by a square
signal. The frequency of this square signal is the same
that the signal generated by the microcontroller in the
signal generator block. After the switching demodu-
lator, the signal is driven to a third-order Sallen-Key
low-pass filter (LPF). Then using this output signal
from the LPF, the measured voltage is acquired. In
addition, by a high-pass filter (HPF) and a basic cir-
cuitry, the relative variations of the measured voltage
are also amplified and acquired. These voltage vari-
ations should be amplified before recording because
of their low amplitude and also the poor accuracy
that the 10-bit Analog-to-Digital Converter (DAC) of
the microcontroller can provide. Finally, the acquired
data are sent from the microcontroller to a computer
by a mini USB-Serial UART development module.
Thus, any software such as Matlab or LabVIEW can
be used later to process and to estimate the impedance
value.
3 MEASUREMENTS
To check that the biodevice works properly, several
measurements are carried out. These measurements
can be classified into three groups according to:
• Comparison to a reference signal.
• Configuration of electrodes, i.e, the placement of
the driving and sensing electrodes in the car seat
and steering wheel.
• Influence of the thickness of clothing.
Note the measurements were done in a simulation
environment where there are no interferences caused
by the state of the road or the car engine vibrations.
3.1 Comparison to a Reference Signal
In this group, several subjects are monitored by the
designed device and also by a commercial one made
by BIOPAC Systems. The commercial device ac-
quires at a sampling frequency of 1 kHz the ventila-
tion signal using a piezoresistive thoracic band. Then,
this signal is used as reference signal to verify the cor-
rect operation of the designed device.
It is worth mentioning that in this case, only the
proper behavior of the AFE is checked in fact. In-
stead of using the signal generator block and the de-
modulator block mentioned above, a National Instru-
ments Data Acquisition (DAC) module is utilized. By
this module, a single frequency sine wave of 300 kHz
is generated and In-phase Quadrature (IQ) demodu-
lation is done to obtain the real and imaginary part
of the signal at a sampling frequency of 25 Hz. Fi-
nally, using a Labview application, the magnitude and
the phase of the estimated impedance are saved. Note
that instead of generating a signal of 62.5 kHz, a sig-
nal of 300 kHz is applied. There are two reasons to
apply a higher frequency. First, using this DAC mod-
ule, the hardware limitation is less strong as the one
imposed by the generator and the demodulator blocks
described above. Second, the higher the frequency a
better response of the system is achieved because of
the capacitive behavior of the textile electrodes.
3.2 Configuration of Electrodes
In the second group of measurements, the biodevice is
checked according the placement of electrodes. Thus,
whereas the frequency of the injection signal is fixed
to 62.5 kHz, the place of driving and sensing elec-
trodes is changed, giving three configurations:
• Steering Wheel-steering Wheel Configuration.
• Steering Wheel-back Seat Configuration.
• Back Seat-back Seat Configuration.
In the steering wheel-steering wheel configura-
tion, a driving electrode and a sensing electrode are in
contact with the right hand of the driver. In the same
way, the other pair of driving-sensing electrodes and
the left hand are also in contact.
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