Measuring and testing – Speed – velocity – or acceleration – Acceleration determination utilizing inertial element
Reexamination Certificate
1999-10-01
2001-05-15
Moller, Richard A. (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Acceleration determination utilizing inertial element
C073S514380
Reexamination Certificate
active
06230566
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to sensors, particularly low &mgr;G accelerometer sensors, and more particularly to micromachined rocking accelerometers with capacitive pickoff having low frequency (<500 Hz), high sensitivity (&mgr;G), and low power consumption and cost.
Sensors of various types are being utilized for security and environmental monitoring needs. Network of sensors are being utilized for improved range, sensitivity, directionality, and data readability. It is desirable to use sensor networks composed of individual sensors and associated electronics which are small in size and low in power consumption so as to be powered by a battery that is both small and with a long life. Smaller, low power sensors allow the use of more sensors per network, thus increasing the sensitivity of the network.
As pointed out above, a variety of sensors have been developed for various applications. However, because of low power and small size requirements or certain applications, sensors such as fiber optic devices which require laser power or sensor technologies that require heaters cannot be utilized. Recently micromachined accelerometer type sensors have been developed, but these sensors typically have mG sensitivities, and due to the stiffness of support structures, are designed to operate at high frequency (>5 kHz), and thus are not applicable for applications requiring both high sensitivity and low frequency operation, such as for monitoring large scale structures, to detect ground motion, or passing vehicles. The prior micromachined accelerometers are exemplified by U.S. Pat. No. 4,928,203; No. 5,337,606; No. 5,456,111; and No. 5,659,195, and rely on relatively small mass surface techniques, or wafer bonding to produce a larger proof mass; and use an electrode moved in a semi-linear fashion with respect to another electrode.
The present invention satisfies the above-mentioned requirements for small, low power, low cost sensors, but which operates at low frequency (<500 Hz) and has high sensitivity (&mgr;G). The present invention involves a micromachined low frequency rocking accelerometer with capacitive pickoff fabricated by deep reactive ion etching, and involves a central silicon proof mass suspended by a thin (2 &mgr;m), surface micromachined polysilicon tether. During movement of the sensor, the tethered mass moves relative to the surrounding packaging, and this deflection is measured capacitively by means of plate capacitors or interdigitated finger capacitors. Also, the accelerometer may include a feedback control which enables a greater dynamic range.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a sensor having low frequency and high sensitivity.
A further object of the invention is to provide a micromachined accelerometer type sensor fabricated by deep reactive ion etching and surface micromachining.
A further object of the invention is to provide an accelerometer constructed by a combination of surface and bulk micromachining/etching techniques which has high sensitivity at low frequencies.
Another object of the invention is to provide a rocking accelerometer which includes a proof mass suspended by a tether, such that upon acceleration the tethered mass moves and this deflection is measured by plate or interdigitated finger capacitors.
Another object of the invention is to provide an accelerometer which consists of a central silicon proof mass suspended by a thin polysilicon tether and having a pair of capacitor elements located on opposite ends of the proof mass and adapted to cooperate with corresponding capacitor elements, such that a rocking motion of the proof mass causes a capacitance change in the oppositely located capacitors.
Another object of the invention is to provide an accelerometer which uses a tethered proof mass which may be asymmetric or symmetric.
Another object of the invention is to provide feedback control for an accelerometer by the use of additional actuating capacitive plates, which gives a greater dynamic range of the accelerometer.
Another object of the invention is to provide an array of accelerometer sensors to provide for simultaneous measurement in orthogonal axes.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings. Basically, the invention involves a sensor which includes an accelerometer fabricated by a combination of surface and bulk micromachining and deep reactive etching. The accelerometer includes a low frequency proof mass with capacitive pickoff. More specifically the accelerometer comprises a pair of spaced capacitors with one electrode of each capacitor mounted on a central silicon proof mass suspended by a thin polysilicon tether, whereby deflection or rocking motion of the tethered proof mass causes a change in the capacitance of the spaced capacitors, which is transmitted by interface electronics to a point of use. The capacitors may be of a plate type or of an interdigitated finger type, with one plate or set fingers mounted to the tethered proof mass and the other plate or set of cooperating fingers being mounted to surrounding packaging. The central silicon proof mass is fabricated by deep reactive ion etching and the tether is fabricated using surface micromachining techniques. The accelerometer is designed for low frequency (<500 Hz) and high sensitivity (&mgr;G), and the proof mass may be asymmetric or symmetric. In the rocking configuration, the device acts as a set of differential capacitors, but may be used in a linear configuration wherein the two moving capacitors are compared to a reference capacitor on a substrate or packaging structure. Arrays of these accelerometer sensors may be used to provide for simultaneous measurement in orthogonal axes. Also, an additional capacitive plate may be used for feedback control of the sensor, which gives the sensor a greater dynamic range and is not limited by the maximum deflection of the structure.
REFERENCES:
patent: 4928203 (1990-05-01), Swindal et al.
patent: 5337606 (1994-08-01), Bennett et al.
patent: 5456111 (1995-10-01), Hulsing
patent: 5524489 (1996-06-01), Twigg
patent: 5659195 (1997-08-01), Kaiser et al.
patent: 5905203 (1999-05-01), Flach et al.
patent: 6082197 (2000-07-01), Mizuno et al.
Lee Abraham P.
McConaghy Charles F.
Simon Jonathon N.
Carnahan L. E.
Moller Richard A.
The Regents of the University of California
Thompson Alan H.
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