handbook of modern sensors

10.7 VRP Sensors 351
(A)
(B)
Fig. 10.11. Variable reluctance pressure sensor: (A) basic principle of operation; (B) an equivalent
circuit.
reluctance pressure (VRP) sensor uses a magnetically conductive diaphragm to modulate
the magnetic resistance of a differential transformer. The operation of the sensor
is very close to that of the magnetic proximity detectors described in Chapter 5. Figure
10.11A illustrates a basic idea behind the magnetic flux modulation. The assembly of
an E-shaped core and a coil produces a magnetic flux whose field lines travel through
the core, the air gap and the diaphragm. The permeability of the E-core magnetic
material is at least 1000 times higher than that of the air gap [10], and, subsequently,
its magnetic resistance is lower than the resistance of air. Since the magnetic resistance
of the air gap is much higher than the resistance of the core, it is the gap which
determines the inductance of the core-coil assembly. When the diaphragm deflects,
the air gap increases or decreases depending on the direction of a deflection, thus
causing the modulation of the inductance.
To fabricate a pressure sensor, a magnetically permeable diaphragm is sandwiched
between two halves of the shell (Fig. 10.12). Each half incorporates an E-core/coil
assembly. The coils are encapsulated in a haÈd compound to maintain maximum
stability under even very high pressure. Thin pressure cavities are formed at both
sides of the diaphragm. The thickness of the diaphragm defines a full-scale operating
range; however, under most circumstances, total deflection does not exceed
25–30 µm, which makes this device very sensitive to low pressures. Further, due to
thin pressure cavities, the membrane is physically prevented from excessive deflection
under the overpressure conditions. This makes VRP sensors inherently safe devices.
When excited by an ac current, a magnetic flux is produced in each core and the air
gaps by the diaphragm. Thus, the sensors contain two inductances and can, therefore,
be thought of as half of a variable reluctance bridge where each inductance forms
one arm of the bridge (Fig. 10.11B). As a differential pressure across the diaphragm
is applied, the diaphragm deflects, one side decreasing and the other increasing, and
the air-gap reluctances in the electromagnetic circuit change proportionally to the
differential pressure applied. A full-scale pressure on the diaphragm, although very
small, will produce a large output signal that is easily differentiated from noise.