Low-cost accelerometer

Details Low-cost accelerometer Low-cost accelerometer

1999-08-24

2002-01-08

Moller, Richard A. (Department: 2856)

Measuring and testing

Speed, velocity, or acceleration

Acceleration determination utilizing inertial element

Reexamination Certificate

active

06336365

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to methods and devices for monitoring the acceleration of objects.
2. Discussion of Related Art
Piezoelectric materials are commonly used in transducer and actuator applications. A piezoelectric material generates an electric field in response to applied mechanical force, and generates mechanical force in response to an applied electric field. Transducer applications take advantage of the former of these properties, and actuator applications take advantage of the latter. Examples of piezoelectric materials include quartz crystal (which is a naturally occurring crystal commonly used in oscillators), and certain polycrystaline ceramics, e.g., barium titanate, lead metaniobate, lead [Pb] zirconate titanate (PZT), and the like. These types of ceramics are commonly referred to as piezoceramics.
Piezoceramic elements for use as actuators or transducers may be fabricated by precasting and firing a quantity of piezoceramic material into a desired shape, e.g., a rectangle or circle. After being formed, each element is typically subjected to a treatment called prepolarization during which the dipoles of the element are aligned in a chosen direction. This polarization of the element's dipoles causes the element to exhibit its piezoelectric properties. One way to prepolarize a piezoceramic element is to attach a pair of electrodes to opposing surfaces of the element, and to apply a strong electric field across the electrodes while keeping the element at a temperature just below its Curie point. When a piezoceramic element is prepolarized in this manner, the element experiences a permanent increase in dimension in the direction of the applied electric field, i.e., between the electrodes, and experiences a permanent decrease in dimension perpendicular to the direction of the applied electric field, i.e., parallel to the surfaces on which the electrodes are disposed.
After a piezoceramic element has been prepolarized, when a dc voltage of the same polarity as the prepolarizing voltage (but of a lesser magnitude) is applied between the element's electrodes, the element experiences further expansion in the direction of the applied voltage and further contraction perpendicular to the direction of the applied voltage. Conversely, when a dc voltage of the opposite polarity (but of a lesser magnitude) as the prepolarizing voltage is applied between the element's electrodes, the element experiences contraction in the direction of the applied voltage and expansion perpendicular to the direction of the applied voltage. In either case, the piezoceramic element returns to its original shape after the dc voltage is removed from the electrodes. Therefore, such a piezoceramic element can be used as an actuator insofar as the voltage applied across the element's plates cause the element's physical shape to undergo corresponding changes.
This phenomenon also works in reverse. That is, after a piezoceramic element has been prepolarized, when a tension force is applied to the element in a direction parallel to the prepolarization field and/or a compression force is applied to the element perpendicular to the direction of the prepolarization field, the element is caused to expand in the perpendicular direction and contract in the parallel direction. This expansion and contraction, in turn, causes a voltage of the same polarity as the prepolarizing voltage (but of a lesser magnitude) to appear between the electrodes. Conversely, when a compression force is applied to the element in a direction parallel to the prepolarization field and/or a tension force is applied to the element perpendicular to the direction of the prepolarization field, the element is caused to contract in the parallel direction and expand in the perpendicular direction. This contraction and expansion, in turn, causes a voltage of the opposite polarity (but of a lesser magnitude) as the prepolarizing voltage to appear between the electrodes. Therefore, such a piezoceramic element can be used as a transducer insofar as the physical forces applied to the piezoceramic element cause corresponding voltages to be generated between the electrodes.
An example of a prior art acceleration-sensing device
100
which employs a pair of piezoceramic elements as a transducer is shown in FIG.
1
. Such a device is disclosed in U.S. Pat. No. 5,631,421, which is hereby incorporated herein by reference. As shown in
FIG. 1
, the acceleration-sensing device
100
includes a pair of support members
102
a
and
102
b
, a piezoceramic beam
104
, and a pair of electrodes
106
a
and
106
b
. The piezoceramic beam
104
includes two distinct piezoceramic portions
104
a
and
104
b
, with a bottom surface
114
of the portion
104
a
being mated with a top surface
116
of the portion
104
b
. The beam
104
is sandwiched between the pair of support members
102
a
and
102
b
, and the electrodes
106
a
and
106
b
are attached, respectively, to a top surface
112
of the portion
104
a
and a bottom surface
118
of the portion
104
b
. Each of the portions
104
a
and
104
b
is polarized vertically in a direction perpendicular to the top and bottom surfaces of the portions
104
a
and
104
b
, but the two portions
104
a
and
104
b
are polarized in opposite directions.
In the device
100
, a center portion
108
of the beam
104
is held stationary by the support members
102
a
and
102
b
, and ends
110
a
and
110
b
of the beam
104
are permitted to move freely in response to acceleration of the support members
102
a
and
102
b
. The beam
104
is therefore caused to flex when an object (not shown) to which the support members
102
a
and
102
b
are attached is subjected to acceleration. When the ends
110
a
and
110
b
of the beam
104
flex upward in such a situation, the portion
104
a
of the beam
104
is subjected to compression forces and is caused to contract (i.e., shorten), and the portion
104
b
is subjected to tension forces and is caused to expand (i.e., lengthen). Because the portions
104
a
and
104
b
are polarized in opposite directions, however, the voltage generated (in response to these compression and tension forces) between the top and bottom surfaces of the respective portions is of the same polarity. Therefore, the voltage produced between the electrodes
106
a
and
106
b
when the ends
110
a
and
110
b
of the beam
104
flexes upward is equal to a sum of the voltages generated between the top and bottom surfaces of the respective portions
104
a
and
104
b.
Conversely, when the ends
110
a
and
110
b
of the beam
104
flex downward, the top portion
104
a
of the beam
104
is subjected to tension forces and is caused to expand, and the bottom portion
104
b
is subjected to compression forces and is caused to contract. Therefore, because the portions
104
a
and
104
b
are polarized in opposite directions, the voltage produced between the electrodes
106
a
and
106
b when the ends
110
a
and
110
b
of the beam
104
flex downward is also equal to a sum of the voltages generated between the top and bottom surfaces of the respective portions
104
a
and
104
b
, but is of an opposite polarity as the voltage produced when the ends
110
a
and
110
b
flex upward.
Thus, because the beam
104
flexes in proportion to the acceleration of the object (not shown) to which the support members
102
a
and
102
b
are attached, the signal generated between the electrodes
106
a
and
106
b
(as a result of the portions
104
a
and
104
b
of the piezoceramic beam
104
expanding and contracting when the beam
104
flexes) is indicative of the acceleration of the object.
FIG. 2
is a diagram showing another example of a prior art acceleration-sensing device
200
which employs a pair of piezoceramic elements as a transducer. The device of
FIG. 2
is disclosed in U.S. Pat. No. 5,063,782, which is hereby incorporated herein by reference. As shown in
FIG. 2
, the acceleration-sensing device
200
includ

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