Electronic Design

Micro-Structured Transducer Accurately Measures Pressure

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.

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