Biomedical Engineering Reference
In-Depth Information
layer and aluminum foil were used as electrodes, while the top PET layer and the
bottom plastic film protected the active segment.
The sensors were characterized by the self-made set-up as shown in Figure 8.23.
This consists of a controller, load cell, amplifier, and oscilloscope. The controller
manages the movement of the load cell with specific frequency and force. The force
on the sensor surface is detected by the load cell when the load cell hits the sensor,
and the electrical signals from the pressure sensor are enhanced by an amplifier
and then collected and recorded by an oscilloscope.
Figure 8.24 shows the response of the sensor to the external force. The peak in
Figure 8.24a from load cell is in proportion to the force when the load cell contacts
the surface of the sensor. Corresponding to the external force, two opposite peaks
in Figure 8.24b come from the sensor, which correspond to the exerting force and
releasing force, respectively.
The sensitivity of the fabricated sensors was determined in a series of measure-
ments by the set-up, and all the sensors were hit at a frequency of 1Hz. The
measured peak-to-peak voltages are plotted against the applied force for all the sen-
sors in Figure 8.25. The voltages from the sensors varied linearly with the applied
forces, confirming linear piezoelectricity. The highest sensitivity of 60.5mV/N is
found with the sensor based on P(VDF-TrFE) (77/23) web, as shown in Table 8.4.
This result is in a good agreement with having more
-phase in P(VDF-TrFE)
(77/23), as seen from characterizations of XRD and FTIR spectra.
To determine the reliability of the sensor under various conditions, the responses
of the sensor to external forces with different frequencies were characterized.
Figure 8.26 shows the sensor's reliable response to a given frequency and force. Due
to the dynamic response of the measurement circuit, the output voltage becomes
a little higher as the frequency of the force increases. However, the signals remain
in a one-to-one relationship with external force up to 20Hz. This result suggests
that the sensors are reliable for the measurement of dynamic pressure force, and
have promising applications in various fields, especially for health monitoring.
β
8.5.4
Nanogenerator Based on Electrospun PVDF Nanofiber Web [4]
Kap Jin Kim
Since, Sections 8.4.2 and 8.5.1 it was found that increasing the number of the
stacked PVDF layers can increase the amplitude of the piezoelectric output signal,
the electrospun PVDF nanofiber web can also be utilized as a generator to charge
the capacitor or the rechargeable battery. Figure 8.27a shows the schematic circuit
to charge the capacitor by rectifying the alternating current generated from the
electrospun PVDF nanofiber web by imparting a sinusoidal force to it. Figure 8.27b
shows the voltage build-up curves of capacitors with different capacitance val-
ues. From the charge curve, it is shown that this nanoweb generator can be
applied practically to low-power-consuming devices in spite of very low generating
power.
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