handbook of modern sensors

352 10 Pressure Sensors
(A)
(B)
Fig. 10.12. Construction of a VRP sensor for low-pressure measurements: (A) assembly of the
sensor; (B) double E-core at both sides of the cavity.
The VRP sensor’s output is proportional to the reluctance in each arm of the
inductive Wheatstone bridge that uses the equivalent inductive reactances x 1,2 as the
active elements. The inductance of a coil is determined by the number of turns and
the geometry of the coil. When a magnetically permeable material is introduced into
the field flux, it forms a low-resistance path-attracting magnetic field. This alters the
coil’s self-inductance. The inductance of the circuit, and subsequently its reactance,
is inversely proportional to the magnetic reluctance; that is, x 1,2 = k/d where k is a
constant and d is the gap size. When the bridge is excited by a carrier, the output
signal across the bridge becomes amplitude modulated by the applied pressure. The
amplitude is proportional to the bridge imbalance, and the phase of the output signal
changes with the direction of the imbalance. The ac signal can be demodulated to
produce a dc response.
10.8 Optoelectronic Sensors
When measuring low-level pressures or, to the contrary, when thick membranes are
required to enable a broad dynamic range, a diaphragm displacement may be too
small to assure sufficient resolution and accuracy. In addition, most of piezoresistive
sensors, and some capacitive, are quite temperature sensitive, which requires
an additional thermal compensation. An optical readout has several advantages over
other technologies, namely a simple encapsulation, small temperature effects, high