Piezoelectric transducer and flat panel speaker with improved frequency response and method of manufacture

Abstract

A transducer comprises two piezoelectric layers attached to a substrate, two protective layers, two masses, and two stiffeners arranged at the ends of the bimorph. The transducer is made using a composite bagging process and autoclave curing. The transducer is attached to a flat panel using a parallelogram-shaped double-sided tape to form a loudspeaker or a display panel with haptic feedback. The improved transducer enables a full-range flat panel speaker with uniform frequency response over its operating frequency.

Claims

1. A transducer, comprising: a substrate; two piezoelectric layers having a similar size to the substrate, the two piezoelectric layers attached to the two opposite surfaces of the substrate; four electrodes electrically connecting the two piezoelectric layers, the electrodes extending beyond the substrate; and two protective layers fully covering the piezoelectric layers; whereby when a voltage source is applied to the electrodes, two opposite electric fields are generated across the two piezoelectric layers, causing expansion or contraction of the piezoelectric layers, resulting in a bending motion of the transducer.

2. The transducer of claim 1, wherein the transducer is manufactured using a composite bagging process and autoclave curing.

3. The transducer of claim 1, wherein the two piezoelectric layers have the same polarization direction.

4. The transducer of claim 1, wherein the substrate and protective layers are glass fiber composite.

5. The transducer of claim 1, wherein the electrodes are copper foils.

6. A transducer, comprising: a substrate; two piezoelectric layers having a similar size to the substrate, each layer attached to an opposite surface of the substrate; at least two electrodes electrically connecting the two piezoelectric layers; two protective layers completely covering the piezoelectric layers; and two stiffeners arranged at the two ends of the piezoelectric layers, the stiffeners overlapping the piezoelectric layers; whereby when a voltage source is applied to the electrodes, two opposite electric fields are generated across the two piezoelectric layers causing expansion or contraction of the piezoelectric layers, resulting in bending motion of the transducer.

7. The transducer of claim 6, wherein the transducer is manufactured using a composite bagging process and autoclave curing.

8. The transducer of claim 6, wherein the piezoelectric layers have the same polarization direction.

9. The transducer of claim 6, wherein the stiffeners are a unidirectional carbon fiber composite.

10. The transducer of claim 9, further including two masses positioned at each end of the transducer.

11. The transducer of claim 9, wherein thickness and overlap between the stiffeners and the piezoelectric layers are optimized using DOE to minimize resonance.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A complete understanding of the present technology disclosed may be obtained by reference to the accompanying drawings, when considered in conjunction with the subsequent detailed description, in which:

(2) FIG. 1A is a cross sectional view of a stacked structure of a piezoelectric transducer according to this technology disclosed;

(3) FIG. 1B is a cross sectional view of the transducer of FIG. 1A after the curing process;

(4) FIG. 2A is a perspective view of a flat panel speaker using the transducer of FIG. 1A;

(5) FIG. 2B is a bottom view of FIG. 2A;

(6) FIG. 3 is a graph showing comparison of frequency response of the transducer of FIG. 1A with and without the stiffeners;

(7) FIG. 4 is a bottom view of FIG. 2B without the panel;

(8) FIGS. 5A-5J illustrate examples of adhesive shapes for the transducer of FIG. 4;

(9) FIG. 6 shows comparison of frequency response between a rectangle adhesive of FIG. 4 and a proposed adhesive shape of FIG. 5C; and

(10) FIG. 7 is a perspective view of a flat panel speaker using the transducers of FIG. 1A.

DETAILED DESCRIPTION

(11) The structure of a piezoelectric transducer according to the disclosed technology is illustrated in FIG. 1A and FIG. 1B, showing the unique stacked structure. The transducer 100 includes two active piezoelectric layers 102, 104 attached to the two opposite surfaces of a thin substrate 106 having approximately the same size. The piezoelectric layers 102, 104 have the same polarization direction. They can be made of PZT ceramic (lead zirconate titanate), although other substrate materials are contemplated.

(12) Two protective layers 108, 110 are arranged one at the top and one at the bottom of the piezoelectric layers 102, 104, respectively. The protective layers 102, 110 extend beyond the length of the piezoelectric layers 102, 104. Between the piezoelectric layers 102, 104 and the substrate 106 are two electrodes 112, 114. Between the piezoelectric layers 102, 104 and the protective layers 108, 110 are another pair of electrodes 116, 118. The electrodes 112, 114, and 116, 118 may be fabricated from thin strips or foils of copper, aluminum, or carbon graphite. The electrodes 112, 114, and 116, 118 may be arranged on the same end or on different ends of the piezoelectric layers 102, 104. Alternatively, electrodes 112, 114 and 116, 118 may be formed as a single continuous strip.

(13) The substrate and protective layers 106, 108, 110 can be made of an epoxy, polyimide, or composite fabric (e.g., carbon, Kevlar or glass fiber mixed with uncured epoxy). Two masses 120, 122 are positioned at two ends of the transducer 100.

(14) Stiffeners have the purpose of providing rigidity to the structure as well as shaping the dynamic response of the transducer. Two stiffeners 124, 126 are arranged on the top of the masses 120, 122. Alternatively, the stiffeners can be positioned between the masses 120, 122 and the top protective layer 108.

(15) The stiffeners 124, 126 have a small overlap with the piezoelectric layers 102, 104. This small overlap ensures that the transducer has bending stiffness with a smooth transition between the piezoelectric layers 102, 104 and the masses 120, 122. The masses 120, 122 may be fabricated from a heavy material such as steel, copper, or ceramic. The stiffeners 124, 126 may be fabricated from a fiber fabric, such as unidirectional carbon, Kevlar, or glass fiber, mixed with uncured epoxy.

(16) Using an industrial autoclave system, the entire stacked structure is then cured at high temperature (180-200 degrees C.) under vacuum condition for four to six hours using a composite bagging process. The curing temperature of the stacked structure is then slowly ramped down to ambient temperature, whereby the epoxy resets and integrally bonds the stacked layers together as shown in FIG. 1B.

(17) Autoclave curing of composites applies a combination of vacuum and external pressure. The vacuum removes air as well as volatiles trapped within a laminate, and the external pressure suppresses any remaining vapors into the resin matrix to prevent void formation. One of the several benefits of using an autoclave curing process is that it can produce large volumes, making it ideal for large-scale manufacturing runs. It also offers a great deal of precision, making it ideal for custom parts production.

(18) Autoclave curing makes it possible to produce very strong, uniform components, especially relative to their weight. Autoclave-cured parts also are more resistant to chemical or heat damage. This makes them ideal for marine, aerospace, or industrial applications.

(19) The electrodes 112, 114 are pressed against each other creating electrical connection between them. Electrode 116 is electrically connected to electrode 118 in the same manner. When a voltage signal is applied to the electrodes 112, 114, and 116, 118, opposite electric fields are applied across the piezoelectric layers 102, 104, causing expansion or contraction of the layers, depending on the polarity of the electric fields. When layer 102 contracts and layer 104 expands, and vice versa, this induces bending motion and vibration in the transducer 100.

(20) In one implementation, the transducer 100 is a bimorph actuator which includes two piezoelectric layers. Alternatively, it can be a multi-morph actuator, which includes more than two piezoelectric layers, e.g., four, six layers, constructed using the same principle.

(21) Turning now to FIG. 2A, a flat panel speaker is illustrated that uses the transducer illustrated in FIG. 1B. The flat panel speaker is in the form of a lightweight thin panel 204, providing a mounting surface for attaching piezoelectric transducer or multiple transducers. The panel is preferably a rigid, solid flat surface. The panel has a thin form factor and is preferably lightweight to allow mounting in almost any desired location. The panel is also low cost.

(22) In some applications, the transducer may be applied to the back of a micro-LED or OLED panel or display to produce stereo sound to accompany video programming. Because the disclosed technology is lightweight, the substrate does not require any cumbersome supporting structure, as is needed with electrodynamic speakers such as those including magnets and drivers. The present technology can be applied directly to a supporting substrate with adhesive.

(23) In another implementation, any lightweight, rigid, panel of a desired size may be used to provide optimal performance. The present technology is designed to work effectively with panels fabricated from various materials, including ABS, acrylic, aluminum, steel, composite materials, plywood, foamboard, cardboard, and paper. All these panel materials may provide satisfactory results.

(24) The flat panel speaker 200 includes a transducer 202 attached to a thin panel 204 using adhesive 206 near center of the transducer 200. Adhesive 206 may be in the form of can be double-sided tape. In one implementation, the adhesive 206 is attached off-center to the panel. When connected to a voltage source, the transducer 202 bends and transfers its inertial force to the panel 204, inducing vibration of the panel 204. In a loudspeaker application, the voltage source is an audio-modulated electrical signal, while in a haptic feedback application, the voltage source can be an AC sine signal.

(25) The stiffeners 124, 126, shown in FIGS shown in FIGS. 1A and 1B, are among elements in the transducer 100 that have strong influence on dynamic behavior of the transducer 100. Material, thickness, and the amount of overlap between the stiffeners 124, 126 and the piezoelectric layers 102, 104 can be optimized using FEM (finite element method) simulation and DOE (design of experiments).

(26) FIG. 3 shows experimental result of frequency response curves of the panel speaker 200. As illustrated in FIG. 3, the curve 302 represents a panel excited by a transducer shown in FIG. 1A (with stiffeners 124, 126), while the curve 304 represents a panel excited by a transducer shown in FIG. 1A with the stiffeners 124, 126 removed (without the stiffeners). It can be observed that curve 304 has a dip 306 at approximately 190 Hz, while curve 302 is more uniform in the range of 100-250 Hz. This result demonstrates that adding stiffeners 124, 126 can completely remove the resonance associated with the transition area between the masses 120, 122 and the piezoelectric layers 102, 104, thus improving the sound quality in low-frequency range.

(27) Similarly, the masses 120, 122 may be optimized to obtain the desired frequency response. Generally, heavier mass can help move the frequency response curve to the left, extending frequency response further into low frequency range.

(28) FIG. 4 illustrates the bottom view of FIG. 2B without the panel 204. Typically, the adhesive 204 has a rectangle shape and covers the whole width (in vertical direction) of the transducer 202. With this configuration, frequency response of the panel 204 usually has large peaks and dips, due to resonance of the transducer 202.

(29) In one implementation, to solve this problem, the present technology uses different slanted shapes for the adhesive 206 as shown in FIGS. 5A to 5I. All these shapes are arranged at an angle different from 90 degrees to the longer edge of the transducer 202. This allows them to span longer along the longer edge of the transducer 202 (in horizontal direction) compared to a rectangular one, as shown in FIG. 4, of the same area. This arrangement was found to help attenuate resonance, thus minimizing peaks and dips in frequency response. The angle and shape of the adhesive 206 can be optimized using DOE.

(30) The optimal angle for the adhesive 206 preferably falls between 45 and 60 degrees. Some of the disclosed adhesive shapes do not cover the whole width of the transducer 202, for example, the ones illustrated in FIGS. 5B, 5D, and 5E. These implementations may have non-uniform cross-sections or irregular shapes as shown in FIGS. 5C, 5E, and 5G. They may comprise several discrete elements as shown in FIGS. 5D, 5H, 5J. These characteristics help improve the frequency response further.

(31) FIG. 6 shows an experimental result comparison of frequency response between typical adhesive shape of FIG. 4 and a proposed adhesive shape of FIG. 5B. The frequency response 602 of the adhesive in FIG. 4 has a peak 604 at approximately 1 kHz and a dip 606 at approximately 2 kHz, while the frequency response 608 of the adhesive in FIG. 5B is significantly more uniform in the frequency range between 700 Hz and 5 kHz. The adhesive shapes disclosed in the present technology improve the sound quality in the mid-frequency range.

(32) FIG. 7 illustrates a flat panel speaker which uses the transducers illustrated in FIG. 1B.

(33) The flat panel speaker 700 includes two transducers 702, 704 attached to a thin panel 706 using double-sided tape as illustrated in FIG. 5B. The transducer is preferably arranged horizontally, while the panel 700 is arranged vertically in its final use configuration, in an OLED display application, for example. In this orientation, when the double-sided tape is used as the mounting adhesive is installed off-center, there will be a permanent rotational moment applied to the double-sided tape proportional to the masses of the transducers 702, 704.

(34) Under higher temperature conditions, the moment can induce rotation of the transducers 702, 704, resulting in creep deformation and failure of the double-sided tape. Creep deformation or tape creep may be defined as unwanted movement of the transducer along the panel 700 due to gravity effects and rotational momentum. To avoid this possible failure, the transducers 702, 704 are preferably installed vertically on the panel, as shown in FIG. 7. This vertical orientation helps to minimize the rotational moment applied to the double-sided tape and eliminates the possibility of creep deformation and failure of the double-sided tape. In higher temperature applications where creep deformation may still be a problem, alternative attachment methods may be used.

(35) Since other modifications and changes in the material, shape, size, number of the parts, and arrangement of the parts will be apparent to those skilled in the art, it has to be understood that the technology disclosed is not considered limited to the above-described implementations of this technology disclosed, and covers all changes and modifications which do not constitute departures from the true spirit and scope of this technology disclosed.