Bistable mechanical sensors capable of threshold detection...

Details Bistable mechanical sensors capable of threshold detection... Bistable mechanical sensors capable of threshold detection...

1999-11-30

2001-12-11

Kwok, Helen (Department: 2856)

Measuring and testing

Speed, velocity, or acceleration

Acceleration determination utilizing inertial element

C073S651000, C200S181000

Reexamination Certificate

active

06327909

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to the field of acceleration and shock sensors, and more particularly to bistable threshold sensors.
BACKGROUND OF THE INVENTION
Micro-Electro-Mechanical Systems (MEMS) integrate mechanical elements, such as microsensors and microactuators, and electronics on a common substrate through the utilization of microfabrication technology. MEMS are typically micromachined using integrated circuit (IC) compatible batch-processing techniques that selectively etch away parts of a silicon wafer or add new structural layers. MEMS range in size from several micrometers to many millimeters. These systems sense, control, and actuate on a micro scale and function individually or in arrays to generate effects on a macro scale.
Microsensors, such as acceleration and shock sensors, are known in the prior art. While shock sensors come in many shapes and sizes, they typically involve the use of a suspended structure to detect vibration with peak excursions of that structure closing an electrical contact to indicate that a shock has occurred or a threshold has been exceeded. Acceleration sensors typically use a resonant structure to detect motion. One type of detector includes a silicon mass suspended by silicon beams with ion implanted piezoresistors on the beams to sense motion. Another type of detector uses capacitance changes to detect movement of the beam. Another type employs a shift in a physical load to produce a shift in the structure's resonant frequency.
A conventional shock sensor is shown in FIG.
1
. Shock sensor
10
includes substrate
11
, insulating layer
12
, conductive cantilever
13
having a free end and fixed end, and contact conductor
14
. Voltage is applied to the conductive cantilever
13
that serves as the top electrode. Contact conductor
14
serves as the bottom electrode. A shock with sufficient magnitude causes the free end of conductive cantilever
13
to touch contact conductor
14
completing the circuit. Current is detected by an ammeter (
17
). Once the circuit is completed, however, it immediately opens again. Thus, the detector must be continuously monitored to detect a shock. This can be a problem for one time use applications that require a sensor to disable itself when an extreme vibration is detected. Another problem is that the amplitude of a shock that causes the cantilever to close the electrical contact is fixed by the material properties and geometry of the sensor. Thus, variation of the detection threshold can only be made by physical modification of the distance between the electrodes.
An example of a conventional acceleration sensor is provided by U.S. Pat. No. 4,855,544. It discloses acceleration sensor
20
including a cantilevered beam
23
having an integral end mass at the free end of the beam as shown in FIG.
2
. Prior art acceleration sensor
20
further includes substrate
21
, insulating layer
22
, and bottom conductor
24
. Acceleration causes deflection of the free end of beam
23
from a relaxed condition. As acceleration increases, beam
23
will move to an increasingly strained condition with the free end moving towards bottom conductor
24
. Movement of beam
23
is typically detected by a capacitance measurement. One problem with this prior art accelerometer is that there is no simple mechanism for threshold detection. Thus, it cannot automatically disable itself to eliminate large amplitude noise at a specific frequency.
In light of the foregoing, there is a need for bistable threshold sensors that can detect extremes in acceleration and that allow the detection threshold to be electrically modified.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to bistable micromechanical sensors that can detect extremes in acceleration and that allow the detection threshold to be electrically modified that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
In accordance with the purposes of the present invention, as embodied and broadly described, the invention provides a bistable threshold sensor including a substrate, a resonant structure over the substrate with a fixed portion and a free portion as a first electrode, a ground conductor layer on the substrate as a second electrode, and an insulating layer over the ground conductor. The sensor further includes a contact conductor on a portion of the insulating as a third electrode, wherein the free portion of the resonant structure contacts the contact conductor when the resonant structure is in a deflected position, and a voltage source for providing a bias voltage between the first electrode and the second electrode.
In another embodiment, the invention provides a method for threshold detection including providing a bistable threshold sensor having a detection threshold. The sensor includes a resonant structure as a first electrode, wherein the resonant structure has an elastic restoring force, a ground conductor as a second electrode, a contact conductor as a third electrode, wherein a gap exists between the first and second electrode, a bias voltage between the first and a second electrode creating a nonlinear electrostatic force, and wherein the threshold is determined by the elastic restoring force, the gap, and the electrostatic force. The method further includes the step of electrostatically locking the resonant structure in a deflected position once the detection threshold is reached.
The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description serve to explain the principles of the invention.


REFERENCES:
patent: 4638669 (1987-01-01), Chou
patent: 5001933 (1991-03-01), Brand
patent: 5431051 (1995-07-01), Biebl et al.
patent: 5638946 (1997-06-01), Zavracky
patent: 6046659 (2000-04-01), Loo et al.
patent: 6057520 (2000-05-01), Goodwin-Johansson
patent: 6127744 (2000-10-01), Streeter et al.

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