Reluctance is the opposition to current flow that is caused by magnetic circuits. Inductance is the property of a coil of wire to build magnetic flux lines. The amount of inductance in a coil depends on the number of turns of wire in the coil, the amount of current flowing in the wire, and the type of core that is placed inside the coil of wire or the position of a core when it is inside a coil. An easy way to change the amount of inductance in the coil is to change the position of a movable core inside the coil.

The movable core can be attached to the diaphragm in a manner similar to the pressure sensors previously discussed. When the pressure moves the diaphragm, the core will move and change the amount of inductance in the coil of wire. When the amount of inductance changes, the amount of inductive reactance also changes. If the coil is placed in an ac circuit, the change of inductance can be used to change the voltage drop across the inductor or it can be used in an oscillator circuit to change the frequency of the circuit. These types of sensors are also called variable reluctance, since the opposition to inductance is called reluctance.

The Variable Reluctance Sensor - as a pressure sensor and accelerometer

The 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 DegC to +120 DegC.

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 measure and 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 no interchangeable diaphragm variable-reluctance capsules that are also commercially available.

Summary - Two coils are wired in opposition to form two legs of an AC circuit, A diaphragm made of a magnetic material is placed between the two coils, Pressure form the measured process is applied to one side of diaphragm while the other side is exposed to a reference pressure ie. atmospheric pressure. Changes in process pressure will cause the diaphragm to flex and move towards one of the coils and away from the other. As the diaphragm moves, the relative inductance of the coils changes which change the circuit output, thus can be measured as pressure

Advantages - Rugged and stable in performance, High output and dependable overload protection
Disadvantages - Frequency response is limited, No linear output at times