Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
1999-10-12
2002-05-21
Ramirez, Nestor (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
Reexamination Certificate
active
06392329
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to vibrating motors using piezoelectric actuators. More particularly the present invention is directed to vibrating motors and methods of mechanically mounting the piezoelectric actuators in motors to transfer vibrational energy through an attached plate and flexible membrane to a work surface.
2. Description of the Prior Art
Piezoelectric and electrostrictive materials develop a polarized electric field when placed under stress or strain. Conversely, they undergo dimensional changes in an applied electric field. The dimensional change (i.e., expansion or contraction) of a piezoelectric or electrostrictive material is a function of the applied electric field.
A typical prior ceramic device such as a direct mode actuator makes direct use of a change in the dimensions of the material, when activated, without amplification of the actual displacement. Direct mode actuators typically include a piezoelectric or electrostrictive ceramic plate sandwiched between a pair of electrodes formed on its major surfaces. The device is generally formed of a sufficiently large piezoelectric and/or electrostrictive coefficient to produce the desired strain in the ceramic plate. However, direct mode actuators suffer from the disadvantage of only being able to achieve a very small displacement (strain), which is, at best, only a few tenths of a percent.
Indirect mode actuators are known in the prior art to provide greater displacement than is achievable with direct mode actuators. Indirect mode actuators achieve strain amplification via external structures. An example of an indirect mode actuator is a flextensional transducer. Prior flextensional transducers are composite structures composed of a piezoelectric ceramic element and a metallic shell, stressed plastic, fiberglass, or similar structures. The actuator movement of conventional flextensional devices commonly occurs as a result of expansion in the piezoelectric material which mechanically couples to an amplified contraction of the device in the transverse direction. In operation, they can exhibit several orders of magnitude greater displacement than can be produced by direct mode actuators.
The magnitude of the strain of indirect mode actuators can be increased by constructing them either as “unimorph” or “bimorph” flextensional actuators. A typical unimorph is a concave structure composed of a single piezoelectric element externally bonded to a flexible metal foil, and which results in axial buckling or deflection when electrically energized. Common unimorphs can exhibit a strain of as high as 10% but can only sustain loads that are less than one pound. A conventional bimorph device includes an intermediate flexible metal foil sandwiched between two piezoelectric elements. Electrodes are bonded to each of the major surfaces of the ceramic elements and the metal foil is bonded to the inner two electrodes. Bimorphs exhibit more displacement than comparable unimorphs because under the applied voltage, one ceramic element will contract while the other expands. Bimorphs can exhibit strains up to 20% (i.e. about twice that of unimorphs), but, like unimorphs, typically can only sustain loads which are less than one pound.
A unimorph actuator called “THUNDER”, which has improved displacement and load capabilities, has recently been developed and is disclosed in U.S. Pat. No. 5,632,841. THUNDER (which is an acronym for THin layer composite UNimorph ferroelectric Driver and sEnsoR), is a unimorph actuator in which one or more pre-stress layers are bonded to a thin piezoelectric ceramic wafer at high temperature. Cooling the composite structure asymmetrically stress biases the ceramic wafer due to the difference in thermal contraction rates of the ceramic layer and the pre-stress layers (or substrate). In other words, as the substrate(s) and adhesive cool they contract more than the ceramic to which they are bonded. This places the ceramic layer under compression and the substrate in tension. Because the ceramic layer in compression is bonded to the substrate(s) in tension, the assembled actuator assumes its normal arcuate shape. This prestress condition which compresses the ceramic layer enables the ceramic to be less susceptible to cracking as well as increasing the amount of deformation and resultant strain that the actuator may experience.
In operation a THUNDER actuator may be energized by an electric power supply via a pair of electrical wires which are typically soldered to the metal prestress layers (substrates) or to the electroplated faces of the ceramic layer. When a voltage of a first polarity is applied across the ceramic layer, the ceramic contracts (in the direction of the tension in the substrate), which causes the actuator to relax and flatten (position
99
in FIG.
4
). When a voltage of an opposite polarity is applied across the ceramic layer, the ceramic layer expands (increasing the tension in the substrate), which causes the actuator to become more concave (position
101
in FIG.
4
). By applying an alternating voltage, the ceramic layer in the actuator can cyclically contract and expand, which causes the actuator to alternately become more and less concave (as illustrated by positions
99
and
101
in FIG.
4
).
In practice, these actuators
100
have been used to directly drive a pressure plate
8
or other mechanism in a prior art cyclic motor as shown in FIG.
1
. Typically, the convex face
100
a
of the actuator
100
would directly push (in the direction of arrow
7
) against a plate
8
at the lowest point of its curvature, and the plate
8
would maintain contact with the actuator
100
, returning to its rest position through the use of a spring mechanism
6
.
FIG. 2
illustrates another device using multiple stacked actuators
100
. Each actuator
100
has its edges
11
mounted in slots
67
in the sidewalls
70
of a housing
72
. The actuators
100
and their electrical connections are electrically isolated from each other using spacers
33
, typically TEFLON™ insulators, that are mounted to a spring
6
biased drive shaft
32
. The drive shaft
32
may be further mounted to a pressure plate
8
or other motion translating means (not shown).
A problem with the above described mounting methods for a direct drive actuator is that the force against the actuator
100
was concentrated on one point, or at least in a very small area of the actuator
100
. This would cause the ceramic
10
in the actuator
100
or the whole actuator
100
to break due to point load concentration. The actuator
100
would then lose most of its effectiveness because it could not generate as much force or displacement with a cracked ceramic
10
.
Another problem with prior art actuator mounting methods is that a single actuator typically could not generate sufficient force for higher output applications. This is especially true of applications where the pressure plate against which the actuator acted was spring mounted. The actuator dissipated a large amount of its useful force in trying to overcome the spring mechanism. The force generated for some applications would also fracture the ceramic layer of the actuator.
Another problem with prior art actuator mounting methods is that even a stack of actuators acting against a pressure plate typically could not generate sufficient force for higher output applications.
Another problem in applications using multiple, stacked actuators was that the spacers add weight to the motor as well as opposing the motion of the actuators, dissipating the useful force and displacement in the stacked actuators. The actuators also dissipated a large amount of their useful force in trying to overcome the spring mechanism. The force generated for some applications would also fracture the ceramic layer of the actuator.
Another problem with prior mounting methods for single or multiple actuators was that where a lightweight motor was desired, the use of a housing in which to mount actuator edges would add extra weight to the motor.
SUM
Bryant Timothy D.
Face, Jr. Samuel A.
Rogers, Jr. Glenn F.
Bolduc David J.
Clark Stephen E.
Face International Corp.
Medley Peter
Ramirez Nestor
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