Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2001-05-18
2003-02-11
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S323010, C310S323020, C310S323040
Reexamination Certificate
active
06518689
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to piezoelectric motors. More particularly, the present invention relates to a unique configuration for mounting piezoelectric elements in a piezoelectric wave motor.
2. Description of Related Art
Piezoelectric motors are utilized in a wide variety of applications in modern society, such as autofocusing camera lenses and automatic control units for window blinds. Piezoelectric motors are particularly well-suited for any application which requires a motor having a compact size (e.g. as small as the size of a fingertip), quiet operation, high torques at low speeds, quick response, and which is not affected by magnetic fields.
A typical design of an existing piezoelectric motor is shown in
FIG. 1
designated by reference numeral
10
. Motor
10
is a rotary motor that includes a disk-shaped stator
12
having a comb-tooth top surface
14
and flat bottom surface
16
. Motor
10
also includes a thin piezoelectric ring
18
, which is bonded to bottom surface
16
of stator
12
with an adhesive material, such as an epoxy resin. Dispersed around ring
18
are individual segments of piezoelectric ceramic which have been electrically poled in alternating opposite directions (indicated by “+” and “−”) along an axis of poling which is perpendicular to the plane of stator
12
.
Piezoelectric motor
10
further includes a disk-shaped rotor
20
, here shown as a geared rotor, which, together with stator
12
and piezoelectric ring
18
bonded to the stator, are mounted onto a shaft
22
which extends upwardly from the center of a rigid base
24
. A spring washer
26
, bearing
28
, and e-clip
30
function to hold rotor
20
in pressure contact with top surface
14
of stator
12
. A thin friction liner
32
is placed between stator
12
and rotor
20
to reduce sliding energy losses during the operation of motor
10
. (It is known in the art to attach friction liners to rotors and further references to rotors herein will be understood to include a possible friction liner.)
A high frequency a.c. voltage drive source
34
is also provided to drive motor
10
. A first electrical lead
36
supplies a first a.c. voltage signal (typically, V
o
sin &ohgr;t) to a first set of poled segments on ring
18
, and a second electrical lead
38
supplies a second a.c. voltage signal (V
o
cos &ohgr;t) to a second set of poled segments on ring
18
, which are displaced along the stator from the first set as is know in the art. A third electrical lead
40
is connected to ground.
In operation, the a.c. voltage signals from drive source
34
cause the poled segments of piezoelectric material in ring
18
to expand and contract in such a manner that a traveling wave is generated in stator
12
. The comb tooth top surface
14
of stator
12
amplifies this traveling wave, and the crests of the amplified traveling wave move in an elliptical motion such that a tangential force is created at the wave crests. As the wave crests contact rotor
20
, this tangential force causes movement of rotor
20
to thereby drive motor
10
.
Motor
10
has all of the desirable features which are generally associated with piezoelectric motors (e.g. compact size, quiet operation, high torques at low speeds, quick response, and not affected by magnetic fields); however, there are still several shortcomings associated with the design and operation of motor
10
.
First, the expansions and contractions of the individual segments (i.e. individual piezoelectric ceramics—typically referred to as elements) of ring
18
create alternating tensile and compressive stresses in the elements. Because piezoelectric elements are ceramics, and are typically weak in tension these alternating tension stresses promote the growth of cracks within the elements. Over time, these cracks will decrease motor reliability and may eventually lead to the failure of motor
10
.
Second, motor
10
is driven by the expansions and contractions of the poled segments of piezoelectric element ring
18
, which expansions and contractions are transverse to the element's axis of poling. (Expansions and contractions transverse to the piezoelectric element's axis of poling are commonly referred to as being in the “d
31
direction”). It is well known in the art that expansions and contractions parallel to the element's axis of poling (commonly referred to as the “d
33
direction”) are approximately twice the magnitude of those in the d
31
direction for a given electrical field. Thus, motor
10
does not fully utilize the piezoelectric properties of the elements.
Third, in order to transmit forces to the stator
12
, piezoelectric element ring
18
is directly bonded to stator
12
such that shear stress is placed on the bond as the segments of ring
18
expand and contract in such a manner that a traveling wave is generated in stator
12
. Over time, this shear stress on the bond between ring
18
and stator
12
may lead to the failure of motor
10
.
SUMMARY OF THE INVENTION
Accordingly, it is an object of this invention to provide a piezoelectric motor in which the mounting of the piezoelectric elements is configured to reduce operating stresses in the piezoelectric elements by reduction of tensile stresses in the elements, shear stresses in the element bond, or both. It is a further object of this invention to provide a piezoelectric motor in which deformation of piezoelectric elements in the d
33
direction may be utilized to produce motion.
In order to address these objects, the present invention utilizes a series of slots formed in the stator transverse to the desired wave motion. Piezoelectric elements are contained within these slots. Upon imposing an appropriate electric field, the elements deform, exerting forces perpendicular to the sides of the slots (i.e., in the direction of the desired wave motion). Because the elements so mounted impart forces resulting from their deformation in a direction perpendicular to the surface of the slots, shear forces imposed upon the elements are minimized or eliminated.
In a preferred aspect of the invention, piezoelectric elements contained in slots may be compressively fitted into the slots in the stator. Because the elements have an initial resting compression, the magnitude of tension experienced by these elements in operation is minimized or eliminated. Further, the elements may be compressed to such an amount that no tensile forces exist in the elements during operation.
In another preferred aspect, the piezoelectric elements contained in slots may be operated in the d
33
mode. Operation in this mode maximizes the deformation achieved from an element for a given electric field, thus maximizing relative motion of the stator.
In a further preferred aspect, the individual piezoelectric elements contained in slots are stacked together to form what will be referred to as piezoelectric stacks. As the term is used herein, a piezoelectric stack consists of a plurality of piezoelectric elements mechanically connected side-to-side, with the elements being poled and electrically connected so that when an electric field is imposed, the deformation of the stacked elements is additive. Stacking may readily be accomplished where the elements are operated in the d
33
mode so that contiguous portions of adjacent elements may be electrically connected (operated at the same potential).
In one aspect, the present invention may be a rotary piezoelectric motor having a design similar to the motor illustrated in
FIG. 1
, with the exception that the stator and piezoelectric element ring are replaced with a new stator and utilizing piezoelectric stack assemblies. The replacement stator and piezoelectric stack assemblies of the present invention generally comprise a stator, typically disk-shaped with a generally circular surface, the stator having a plurality of levers dispersed along and extending outwardly from the periphery of the stator surface. (As used herein, lever are projections either affixed or bonded to the stator or c
Dougherty Thomas M.
Honeywell Federal Manufacturing & Technologies, LLC
Hovey & Williams, LLP
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