Today’s micro- and nano-structured sensors boast excellent
linearity, resolution, and wide measurement ranges.
As a result, microelectromechanical systems (MEMS) are
playing an increasing role in many more applications. This
design describes a novel use of such a sensor—a non-contact
differential variable-reluctance transducer (NCDVRT)—in the
pressure port of a system.
The application involves the selection of a stainless-steel
membrane and provides a simple pressure sensor that can work
in a rugged environment while providing measurement repeatability
and excellent resolution. The NCDVRT used, which is
from Micro-Epsilon, offers a wide dynamic range from millibars
to several kilobars in industrial and R&D environments.
An NCDVRT works on the principle of inductance ratio. The
device incorporates two coils: a sensing element and a compensation
element (Fig. 1). Placing a ferrous or highly conductive
material near the transducer’s face changes the sense coil’s
reluctance, while the compensation coil acts as a reference.
Ferrous targets change the sense coils’ reluctance by altering
the magnetic circuit’s permeability. Conductive targets (such
as stainless steel) operate by the interaction of eddy currents
induced in the target’s skin with the field around the sense coil.1
A high-frequency sine-wave excitation voltage drives the
coils, and a sensitive demodulator measures their differential
reluctance. Differencing the two coils’ outputs provides a sensitive
measure of the position signal, while canceling out variations
caused by temperature.
In the fabricated pressure sensor, the NCDVRT was rigidly
fixed 0.14 mm from one side of a 1-mm thick stainless-steel
membrane (Fig. 2). This distance was selected in order to
operate the transducer in its linear region.1 Teflon material was
employed to fix the NCDVRT to avoid any possible spurious
signal pickup. The other side of the diaphragm was used as the
pressure port.
This pressure sensor can measure the positional movement
of the diaphragm with an accuracy of ±2 µm, corresponding to
a pressure of 0.1 bar. The transducer’s output was connected to
a demodulator that provides an ac excitation to the NCDVRT’s
primary coil and measures the modulated synchronous rectified
secondary coil output voltage to determine its moving core
position, which is proportional to the applied pressure.
Figure 3 shows the linearity of the measurements made by
the NCDVRT sensor.
Reference:
1. “Microstain NC-DVRT,” http://www.microstrain.com/ncdvrt.aspx.
