Page 393 - Mechatronics with Experiments

October 9, 2014 8:1
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JWST499-c06
JWST499-Cetinkunt
SENSORS 379 254mm×178mm
connected to an LVDT or a linear potentiometer to get a voltage signal proportional to the
pressure.
Δx = k ⋅ ΔP (6.156)
p
V = k ⋅ Δx (6.157)
out vx
= (k ⋅ k ) ⋅ ΔP (6.158)
p vx
= k ⋅ ΔP (6.159)
pv
The pressure sensors shown in Figure 6.48 measure the relative pressure between p and
1
p , that is the pressure difference between them, Δp = p − p . If either p or p is the
2 1 2 1 2
vacuum pressure (absolute zero pressure), then the sensor measures the absolute pressure.
6.8.2 Strain-Gauge Based Pressure Sensor
The pressure induced deformation of the diaphragm is measured by strain gauges on it.
The strain on the diaphragm is proportional to the pressure. The resistance of the strain
gauge changes in proportion to its strain. Using a Wheatstone bridge circuit, a proportional
output voltage is obtained from the strain gauge. The relationship between pressure and
strain gauge voltage output is (Figure 6.49),
Δx = k ⋅ ΔP (6.160)
p
= k ⋅ Δx (6.161)
1
V out = k ⋅ (6.162)
2
= k ⋅ ΔP (6.163)
An example of a strain-gauge based differential pressure sensor is the Model P2100
series by Schaevitz Inc. The sensor has two pressure input ports and can measure the
pressure differential between the two ports up to 5000 psi. The natural frequency of the
sensor is 4 kHz for pressures up to 75 psi and 15 kHz for pressures upto 1000 psi. For
instance, such a sensor is appropriate to measure the pressure differential between two
sides of a hydraulic cylinder.

p

Dead volume, V

Strain Diaphragm
gauges
Signal conditioning
electronics
Lead
wires Excitation voltage in
Electrical signal out FIGURE 6.49: Pressure sensor
using a diaphragm and strain
gauge. Pressure is proportional
to the strain induced on the
diaphragm. The strain is
measured as a proportional
p voltage output by a Wheatstone
reference bridge circuit.