Microfabricated, silicon-based capacitive actuator/sensor devices have been developed as prototypes of compact, low-power transducers that would be used to detect the presence (and perhaps eventually measure the thickness) of ice on aircraft lift and control surfaces. These transducers would be mounted flush with surfaces, so that they would not perturb airflows. Transducers of this type could also be used in such diverse applications as detecting ice in refrigerators for triggering defrosting cycles and detecting ice on roadways to trigger warning signals for drivers.

Figure 1. This Microfabricated Actuator/Sensor Device is used to detect ice on the upper (outer) diaphragm surface. The diaphragm is deflected electrostatically by application of voltage to the driving electrode. The amount of deflection (which decreases in the presence of ice) is measured capacitively via the sensing electrode.

Figure 1 presents a simplified cross-sectional view to illustrate the basic device configuration and ice-detection principle. The main supporting structure is a low-thermal-expansion glass wafer containing a rectangular hole 1 to 3 mm on a side and about 2 to 3 µm deep on its upper surface. The glass wafer is capped by a 7-µm-thick silicon wafer that has been heavily doped with boron to make it highly electrically conductive. The part of the wafer over the hole constitutes both a diaphragm and one electrode of a three-electrode parallel-plate capacitor. The other two capacitor electrodes are concentric rectangular patches of aluminum, about 0.3 µm-thick, on the bottom of the hole; the inner patch serves as the driving electrode, while the outer one serves as the sensing electrode, as explained below. Vent holes (not shown in the figure) between the hole and the outside prevent spurious deflection of the diaphragm by changes in ambient air pressure.

In use, the upper (outer) surface of the diaphragm and the rest of the doped silicon wafer is mounted flush with an aircraft or other surface of interest. A dc potential is applied between the driving electrode and the diaphragm to deflect the diaphragm slightly into the hole by electrostatic attraction. The amount of deflection is inferred from the change in capacitance between the diaphragm and the sensing unit; for this purpose, the diaphragm and sensing electrode are connected as the terminals of an input capacitor in a frequency-modulation circuit with a nominal frequency of 48 MHz, and the change in capacitance is determined from the change in frequency.

Figure 2. The Change in Capacitance as a function of driving voltage was measured in an experiment on a prototype device, with and without ice on the outer surface. In this device, the diaphragm was square, 1 mm on a side.

A deposit of ice on the outer diaphragm surface stiffens the diaphragm, reducing the deflection and thus the change in capacitance for a given driving voltage (see Figure 2). The presence of ice can thus be inferred from the reduction in the change in capacitance for a given applied potential.

Future versions of these devices may afford the capability to determine the thickness of ice according to the following principle: The diaphragm would be designed to vibrate at a suitable resonance frequency. In operation, the resonance would be excited and its frequency measured, and the thickness of ice would be determined from any deviation from a nominal (no-ice) resonance frequency.

This work was done by Russell G. DeAnna of the U. S. Army Research Laboratory and Mehran Mehregany and Shuvo Roy of Case Western Reserve University for Lewis Research Center. Inquiries concerning rights for the commercial use of this invention should be addressed to

NASA Lewis Research Center, Commercial Technology Office, Attn: Tech Brief Patent Status, Mail Stop 7 ~3, 21000 Brookpark Road, Cleveland, Ohio 44135

Refer to LEW-16633.

NASA Tech Briefs Magazine

This article first appeared in the October, 1998 issue of NASA Tech Briefs Magazine.

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