Bimorph：A Piezoelectric Actuator with Smart Design and Control
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.
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.
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.
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.
According to operation of the bimorph, the bending displacement depends on the following parameters：
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.
The geometric size of the piezoelectric ceramics. Thinner and longer ceramics results in larger relative displacement, whereas thicker and shorter ceramics causes smaller displacement.
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.