Sensors and Actuators - Fachbereich Physik der Universität ...

sensitivity factor and the gauge factor are identical. Quelle: http://www.astromed.com/knowledge/dcbridgestrain.html
The sensing element of the polymer thick-film pressure sensor consists of a circular edge-clamped epoxy-glass
(or flexible polymer) diaphragm an which four PTF resistors connected into a Wheatstone-bridge configuration
are screened and cured. In order to maximize the loss of the bridge balance and the voltage output, two
resistors must be positioned near the centre of the diaphragm and the other two resistors near the edges. The
pressure to be measured induces a strain in the diaphragm and the resistors change their values, two
resistances increase and two decrease, similarly to the silicon-based version. A very simple membrane structure
can be used for relative pressure measurements (pressure differences related to the ambient pressure).
Absolute pressure sensor needs a closed cavity with a constant pressure or vacuum in it. Conventional
CERMET thick-film versions can also be fabricated using ceramic or enamelled steel substrates and resistor
pastes.
Piezo Resistive A piezo resistive sensor is a resistor circuit constructed on a thin silicon
wafer. Physically flexing or distorting the wafer a small amount changes its resistance. This
type of sensor is usually used as a pressure sensing device such as a manifold pressure
sensor, although it may also be used to measure force or flex in an object such as the
deceleration sensor located in the SRS air bag center sensor. One of the most important
piezo resistive sensors is the manifold pressure sensor which monitors the air intake volume
for Electronic Fuel Injection (EFI). The signal it sends to the ECU determines the basic fuel
injection duration and ignition advance angle. Within the sensor is a silicon chip combined
with a vacuum chamber. One side of the chip is exposed to the intake manifold pressure
and the other side to the internal perfect vacuum in the chamber. A change in the intake
manifold pressure causes the shape of the silicon chip to change, with the resistance value
of the chip fluctuating in relation to the degree of deformation. An integrated circuit converts
the fluctuation to a voltage signal that is sent to the ECU, where the air-fuel ratio is
regulated. The sensor has three external terminals: one for power, one for ground and one
to provide the voltage signal to the computer. The voltage signal varies with the pressure in
the intake manifold. Another use for this same type of sensor is to sense turbocharger
boost. On turbocharged engines, the sensor is used to measure pressures that are higher
than atmospheric pressure and to supply corresponding voltage signals to the ECU. To
prevent engine damage, the ECU can cut off the fuel being injected if the manifold pressure
becomes too high.
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