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1 Introduction
With the steady improvement and enhancement of properties
of the piezoelectric ceramics, its application range is gradually enlarged.
New piezoceramic elements come out in succession, and occupy the market
that conventional electronic components and devices used to share in many
areas, due to its original superiority. It has been widely used in electronics,
optics, heat, acoustics, and etc., and has become an important functional
material in defense industry, civil industry and people’s daily life.
As the basis of modern information
technology, the acquisition of information and action are two major aspects.
Based on the research of high performance electronic ceramics, a series
of actuators has been developed in the world, using the piezoelectric—converse-piezoelectric
property of the piezoceramics, including unimorph, bimorph, stack and
moonie. Among them, the piezoelectric bimorph has become popular because
of its perfect design, easy-to-control process, high performance and simple
application design.
Based on the operational
theory of the bimorph, a series of piezoelectric bimorph actuators has
been developed using different materials and combined technologies, to
satisfy various demands in defense and civil industries.
2
Theory of Operation
Piezoelectric bimorph is a bending element that generates
horizontal displacement at the drive of electric field using the converse
piezoelectric effect. Dual stacked-element structure is used for the device.
The two layers of piezoceramic plates are arranged in the direction of
polarization and are closely bonded to the middle electrode, for which
special material is used. It generates corresponding vibration amplitude
when driven by an alternate power supply, and it can be used for the application
of micro-displacement actuator.
The core technologies for
this series of products are the design of bimorph, the selection of material,
fabrication process and system matching. Breakthrough has been made in
these respects by combining the technology for special high performance
electronic ceramics with process equipments for electronic ceramics preparation.
The product has been accepted on the international market. And its applications
are increasingly extended. It has become the core and basis of many high-tech
products. Many products derived from this technology have found applications
in the advanced loom, micro-infusion pump, piezoelectric motor, and optical
switches, optical circuit converter for optical communications, which
covers the renovation of technologies from traditional industries to information
industry, and involves areas from civil industry to defense industry.
The
performance of bimorph, in terms of the piezoceramic material, is dependent
upon d33, dielectric constant, density and strength of the
piezoelectric ceramics on the whole, and in terms of processing of the
element, it is determined by the thickness and strength of the bond of
piezoceramic plate to middle electrode, choice of material for middle
electrode, the state of cutting shape and the matched test apparatus.
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3
Modes of Applications
Usually, a 3-terminal
input is used for driving power supply (shown in Fig. A)：the
positive piezoelectrics is connected to the positive terminal of
the piezoceramics, the negative terminal is connected to zero, and
the middle electrode layer is connected to alternating “positive
voltage or zero”, while the conversion frequency is set based on
different requirements. Generally, a 1Hz conversion frequency is
used. When the middle electrode layer is at zero voltage, the middle-layer
and negative terminal are at isopotential and the piezoceramics
at the negative side does not operate. The piezoceramics at the
positive side is at the positive electric field. By the action of
converse piezoelectric effect, the ceramics is lengthened along
the polarization orientation (thickness or 33 orientation), which
causes shortening at 31 orientation (length orientation). Since
the negative terminal side tightly bonded to the ceramics restricts
its shortening, the positive terminal can only bend to its own side.
Similarly, when the middle electrode is at the positive voltage,
the negative terminal operates, which results in bending to the
negative terminal side. The operational mode is very much like that
for heated bimorph. This power-connected driving mode is called
serial connection, in which piezoceramics at one side operates.
Since the piezoceramics is a capacitive element, the electric energy
absorbed in the piezoelectric ceramics will be slowly released in
a short period after power off. Two resistors are connected in parallel
between two electrodes to consume the absorbed energy, so that the
bimorph can rapidly return to its original position after power
off.
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The other way of connection
is to incorporate two sides of piezoceramics into one terminal.
Take the middle as another terminal, and input frequency- and voltage-adjustable
alternating current. When the piezoceramic electrode is at the positive
piezoelectrics in half cycle of the power, piezoceramics at both
positive and negative side bend to the positive orientation; whereas,
when the piezoceramic electrode is at the negative piezoelectrics
in half cycle of the power, the piezoelectric ceramic plates at
both sides bend to the negative orientation. This mode of connection
is called parallel connection, in which piezoceramics at both sides
can operate simultaneously.
By comparison, larger displacement and
thrusting force, approximately 1 time higher than that for serial
connection, can be gained for parallel connection, as simultaneous
operation at both sides is performed in this mode of connection.
In practice, the driving method is chosen according to the actual
use and working conditions.
When
in operation, the bimorph is mechanically clamped at its end, while
the tip part swings (vibrates) to-and-fro as a cantilever.
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4、Conclusion
According to operation of the bimorph, the bending
displacement depends on the following parameters：
a)
d31 and dielectric constant of piezoelectric ceramics.
The larger the d31 of the piezoceramics is, the greater the
displacement is. Usually, the d31 is above 300×10-12C/N,
and the dielectric constant is 4500.
b)
The geometric size of the piezoelectric ceramics. Thinner and longer
ceramics results in larger relative displacement, whereas thicker and
shorter ceramics causes smaller displacement.
c)
The material of the middle electrode layer. The thicker and harder
the middle electrode is, the smaller the displacement is.
Driving
voltage. In a certain range, the displacement is directly proportional
to the piezoelectrics.
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