Massachusetts Institute of Technology

Joseph H Haritonidis

Stephen D Senturia

Mehran Mehregany

David J Warkentin

Roger T Howe

Martin Arnold Schmidt

A micromachined diaphragm is positioned across a gap from an end of an optic fiber. The optic fiber and the diaphragm are integrally mounted. The end of the optic fiber provides a local reference plane which splits light carried through the fiber toward the diaphragm. The light is split into a transmitted part which is subsequently reflected from the diaphragm, and a locally reflected part which interferes with the subsequently diaphragm reflected part. The interference of the two reflective parts forms an interference light pattern carried back through the fiber to a light detector. The interference pattern provides an indication of diaphragm deflection as a function of applied pressure across the exposed side of the diaphragm. A detection of magnitude and direction of diaphragm deflection is provided by use of a second fiber positioned across the gap from the diaphragm. The second fiber provides an interference pattern in the same manner as the first fiber but with a phase shift. An opening allowing communication between ambient and the gap enables use of the interferometer sensor as a shear stress measuring device.

Shear stress measuring device

Pressure transducer apparatus

A thin diaphragm receives pressure across one side and faces a beam splitter on the other side. The beam splitter is integrally attached to the diaphragm and serves as a local optical reference plane for the entire assembly. Coherent light from a light source is partially reflected at the beam splitter. The remainder of the light is reflected from the diaphragm. The reflected beams recombine at a detection point and have a phase difference which is a function of the amount of deflection of the diaphragm. The detected recombined beams are indicative of the deflection of the diaphragm. Optical calibration of the aseembly is a function of the distance between the diaphragm and beam splitter which remains as predefined because the beam splitter is integral with the diaphragm. A vent in the small cavity formed between the diaphragm and beam splitter enables the diaphragm to sense small pressures with increased sensitivity. Through micro-fabrication techniques, the diaphragm is made sensitive to dynamic and static pressure. Detection of the thermal expansion of the assembly as well as deflection of the diaphragm enables a sensed pressure measurement as a differential between the detected thermal expansion and deflection.

Optical micropressure transducer

A microbridge is used for the accurate measuring of time varying shear forces in the presence of fluctuating pressure. A microdimensioned plate is suspended by arms to form a microbridge. The microdimensions enable the smallest turbulence scales of interest to be sensed uniformally throughout the entire surface of the plate. The cavity beneath the microbridge is so small that a viscous drag is created in the air within the cavity and dampens normal movement of the plate. The microdimensions in conjunction with the damping effect of the cavity enable the sensor to be substantially insensitive to pressure and thus sense lateral forces independent of normal forces. The microbridge sensor is fabricated by surface micromachining. A sacrificial layer is deposited over a substrate. A structural layer is deposited and patterned to form the plate and support arms over the sacrificial layer. The cavity is formed by a selective etchant removing the sacrificial layer and leaving the rest of the microbridge structure suspended above the substrate. In a differential capacitance readout scheme, a conducting layer in the plate of the microbridge is capacitively coupled with conductors in the substrate. A sensed change in capacitive coupling generates an indication of plate deflection and thereby shear stress independent of vertical movement. Optic readout schemes may also be employed and are readily incorporated in the fabrication process. A mounting member presses the microbridge sensor into a holding plate which fits in a matching slot flush with the target wall.

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Brookline, MA, United States of America

Joseph H Haritonidis