Figure 88. Piezoelectric amplifier.
speed moving film camera.
More desirable is a
actuated devices may have a
multichannel oscillograph since many simultaneous
duration of about 1 second;
curves can be obtained. The recorder must have a
therefore, the time constant of the
suitable frequency response. If filters are included to
gage circuit must be of about 100
eliminate noise from the record, they must be so chosen
seconds for adequate accuracy.
that they will not significantly distort the data.
Since suitable capacitors are not
available in sizes larger than about
c. Prepare Gages.
1/10 microfarad, this means that the
(1) Piezoelectric gage.
input resistance must be of the
(a) The piezoelectric gage is a device
order of 1,000 megohms.
The
which contains discs of quartz
piezoelectric gage is capable of
(crystal) and collecting electrodes
very high frequency response and is
(steel) so disposed that mechanical
very rugged.
force applied to the gage results in
However, they are difficult to use in
the generating of an electric charge
the field because of their extremely
proportional to the force.
The
high-Ii input impedance and the
chamber gas pressure acts upon a
maintenance they require. Barium
piston which compresses the
titanate gages are not satisfactory
crystal. A typical gage used in
for these applications.
propellant
actuated
device
(b) The gage must be designed so that
evaluation generates a charge of
it has a frequency response high
about 0.01 microcoulomb per 500
enough that the pressure-time curve
pounds of force. A capacitor is
is not distorted.
connected across the terminals of
the gage, and the voltage across
(c) Piezoelectric gages have been used
the capacitor is measured to
more
extensively
in
testing
determine the force applied to the
propellant actuated devices than
gage. This capacitor must be of
any other transducer since they are
laboratory precision grade.
A
the only type that will consistently
pressure
pulse
in
propellant
stand up under high rates of
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