THE VARIABLE RELUCTANCE SENSOR
as a pressure sensor and accelerometerThe variable reluctance sensor is strain-based, wherein a
magnetic circuit is formed, and the parameter input causes
mechanical deflection of the spring member as a function
of pressure, force, or acceleration.To provide a static output capability, variable reluctance
sensors require an oscillator and demodulator system
internally limiting operational temperatures from -40 C
to +120 C.The spring member is comprised of magnetic, high-permeability
material and is centrally located between two coils as shown.The Variable Reluctance Differential Pressure Sensor
The coils are sealed from the measurand by nonmagnetic
welded stainless steel barriers. In the case of the differential
pressure transducer, the difference in pressure between
the two sides of the spring member will cause distortion
of the spring member towards the magnetic pole piece
on the low-pressure side of the spring member resulting
in modulation of the inductance (L) of the two coils.
The electrical configuration of the variable reluctance sensor
is that of an inductive half-bridge driven by an alternating
voltage source in the range of 1 KHz to 10 KHz. The centrally
disposed spring member results in an inductive push-pull
arrangement where deflection of the spring member reduces
the inductance of one coil and increases the inductance
of the other creating a difference in coil impedance.The variation in the magnetic reluctance produces the
effective inductance modulation as a function of the
parameter input.
As shown below, the variable reluctance principle may
also be used to fabricate accelerometer structures.The Variable Reluctance Accelerometer
This sensing method is very-well suited to the high
accuracy measurement of static and slowly-varying
phenomena a stable null-bias of low magnitude as well
as repeatable thermal errors.
The thermal-sensitivity shift of the variable reluctance
sensor varies with temperature in much the same way
as the LVDT inductance varies with temperature, in that,
the TCR of the coils increases the coil resistance with
increasing temperature, thereby decreasing the current
and the magnetic flux generated.The use of a constant-current drive can be used to fix
the current at a constant value irrespective of temperature.
A series resistor, as used in sensitivity compensation of
LVDT sensors, can also be used in series with a voltage
source to simulate a constant-current drive in variable
reluctance applications.The series resistor is low thermal coefficient of resistance
(TCR) and forms a voltage divider in series with the coils
where the voltage applied to the coils increases with
increasing temperature, thereby resulting in a constant
current and constant-flux condition.Other active means may also be employed to maintain a
constant coil current using linearized thermistors and the
like.
The mass of the sensor is relatively high and the sensitivity
to thermal transients is correspondingly low. The large
diameter spring element used in low pressure range designs
results in a low spring member resonance and high-acceleration
sensitivity in comparison to the strain-gaged sensor designs.The pressure cavities required in differential pressure designs
form Helmholtz cavities, limiting the usefulness of the variable
reluctance design for higher-frequency measurement.High accuracy with exceptional stability and 100,000 hours
MTBF are key attributes of this sensor technology.The diagram shows the signal-processing block circuit for
this sensor type.The Variable-Reluctance Circuit Geometry
The variable-reluctance pressure sensor is available for
use at pressures as low as 0.1” of water to in excess
of 10,000 psi.
Some reluctance pressure units are available with
replaceable or interchangeable diaphragms to facilitate
the replacement of an over-ranged diaphragm or to
simply install a more flexible diaphragm for lower-range
use. The replaceable diaphragms are clamped between
the two pole pieces. This geometry results in high
stresses concentrated in the clamping zones and lower
long-term stability than in the integrated and welded
noninterchangeable diaphragm variable-reluctance
capsules that are also commercially available.This extract is taken from 'The Art of Practical & Precise
Strain Based Measurement' by Jim Pierson.
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