Piezomembrane constrained in the center and its employment to provide an acoustic howler

Abstract

 A piezoelectrictric transducer device with a piezomembrane coupled, by means of a central pin, to a loading plate, which in turn is constrained on its external circumferance, is employed to provide an acoustic howler in which the loading plate is integrant part of the howler casing.

Description

[0001] This invention relates to a piezoelectric transducer and in particular to the use one may do of it, to provide an acoustic howler.
At the present time, in the telephonic ambit, piezoelectric transducers find a large employment as telephone transmitters and as acoustic howlers, well-known as "buzzers".
Piezoelectric transducers offer several advantages over the traditional ones based on magnetodynamic and electrodynamic effect.
In fact they are basically voltage drived and offer a higher electric impedance, furthermore the absence of magnets and copper-wire coils, allows a much lower weight and a lower cost.
At the present time the most common embodyment of piezoelectric transducer comprises the employment of a piezomembrane formed by a thin metal disk constrained along its external circumference on which a disk of piezoelectric ceramic is overlapped and closely connected; this disk is also quite thin.
Piezoelectric ceramic disk tends, by means of two electrodes connected to its main surface, to oscillate in its radial mode contracting and expanding its diameter when an alternating voltage is applied to it. Ceramic disk radial expansion is opposed by the metal disk. Internal stresses owed to ceramic are not symmetric as regards the ceramic and metal interface plane and therefore the membrane is subjected to flexural stresses such as to what happens in the case of a bimetal subjected to temperature changes. The piezomembrane oscillates basically in a flexional mode presenting resonances frequencies which vary according to the elasticity modules of the metal and of the piezoceramic, the geometric sizes and the type of constraint.
In general the transducer is designed so that it has the first, and in some cases, also the second resonance frequency in the bands of interest (100-5000 Hz).
Flat enough band are obtained by using one or more resonating cavities coupled with the membrane.
As we have already seen, the type of constraint affects the resonance frequency of the piezoceramic.
A yielding constraint allows to lower advantageously the frequency, radius being equal, but since in practice this constraint is obtained by coupling cords of elastomers which unavoidably dissipate elastic energy, the type of constraint determines after all not only the resonance frequency but also the acoustic efficiency of the piezomembrane.

[0002] It is an object of the present invention to provide a new way of constraining the piezoceramic which offers the advantage to be highly efficient and to be cheap to construct.

[0003] It is a further object of the present invention to provide a piezoceramic device, the harmonic spectrum of which allows the largest design freedom concerning the electromechanical dimensioning of the piezomembrane.
It is still further object of the present invention to provide an example of employment of the aforesaid device to embody an acoustic howler which shows the substantial possibilities to improve the spectrum according to the particular structure.
The aforesaid objects are obtained by means of a pin passing through the center of a plate of plastic material (loading plate), which in turn, is constrained along the external circumference. In this device the piezomembrane behaves like a generator of vibromotive force while the plate, which oscillates due to the piezoelectric membrane, is the means by wich acoustic energy is irradiated by air coupling.

[0004] In order to get a better description, the invention is now explained by referring to the accompaning drawings, in which:

Fig. 1 is a section view of the piezoelectric membrane-loading plate system constrained on the pin;

Figs. 2-3 are graphical representations of the flexional modes of the piezomembrane constrained in the centre;

Fig. 4 is a graphical representation of deformed sectors in the considered system;

Fig. 5 is a top section view and a longitudinal section view of the acoustic howler;

Fig. 6 is a section view of the acoustic howler which shows electric connections of the piezoceramic.

[0005] In order to better explain the invention and therefore without restricting its generality, the referred piezoceramic is considered to be formed of a nickel-alloy 0,1 mm thick (t) and a radius of a = 15 mm disk to which sticks a piezoelectric ceramic, 0,1 mm (d) thick and a radius of b = 12 mm disk.
When, as in traditional case, the piezomembrane is constrained along the rim by means of an elastic joint, the first symmetric resonance mode of said piezomembrane corresponds to a frequency of about 1300 Hz, quite in the middle of the telephonic band.
On the contrary the second flexional mode corresponds to a frequency of about 5080 Hz and therefore out of the efficient band. When this piezomembrane is stiffly constrained in the center (see Fig. 2), the first symmetric mode considerably lowers to almost 700 Hz, while the second symmetric mode (see Fig. 3) lowers to about 3160 Hz, as it results from the diagram of electric impedance measured by means of the Hewlett-Packard impedance bridge HP 4194A (table 2-A).
In this diagram the second resonance results to be about 2650 Hz and corresponds to an antisymmetric mode which we do not consider here.
Fig. B in table 2 shows the electric impedance of the piezoceramic coupled to a 1 mm thick and 20 mm radius plate of plastic material (Makrolan).
As we can notice the piezomembrane coupled to the plastic plate, by means of the central pin, constitutes an oscillating system which shows, in the band of interest besides its resonance frequencies, also the one of the plate (about 1600 Hz). The frequencies change a little due to modes coupling. The related system has been observed by means of node analysis with ANSYS calculus code, which uses finite-element method. Fig. 4 shows deformed lines of a sector of the related system, 10° large with reference to the first three resonance modes previously pointed out by means of the impedance diagram.
The figure refers to the case where the piezoceramic is external to the constraint structure, contrarily the case of Fig. 1; however this difference does not change the results, as it is easy to realise.
By considering the three profiles of the system deformation, it's possible to notice that at the first resonance frequency of 700 Hz the piezoceramic oscillates basically as it were rigidly constrained in the center in its first symmetric mode; anyway it is also able to let the plastic loading plate oscillate, which vibrates without presenting nodal circles and reverse phase areas.
At the second resonance frequency the piezoceramic oscillates with a nodal circle such as it were completely free to oscillate while the loading plate has no noxious reverse phase areas, which, as it is known, decrease substantially the irradiation efficiency.
Finally at the third frequency the piezomembrane has two nodal circles likewise what, would happen in the second symmetric mode if it was rigidly constrained in the center; also in this case the loading plate has no noxious nodal circles.
It's possible to predict the resonance frequencies of the system either by means of the aforesaid simulation method or by suitable equivalent circuits.
The advantages of such a system are various, compared with what concerns the system of the membrane constrained on the rim. First of all, from the point of view of the design the system has three resonance modes in the efficiency band, with deformed lines of the loading plate without nodal circles and therefore potentially with high efficiency.
Thiknesses of metal and of ceramic being equal, the piezomembrane constrained on the rim has its second symmetric mode also in the band; however without a nodal circle, only if its diameter is > 40mm. By the point of view of the structure, the constraint in the center dissipates much less elastic energy and it's simplyer to construct; it does not need the employment of elastomers, glues or elastic seals, and it's substantially cheaper and makes it possible to carry out at the same time electric connections, as it will be seen in the next paragraph. According to the aforesaid principles an acoustic howler has been built, the technical drawing of which is represented in Fig. 5.
Again, only for illustrative clearness and without restricting the generality and the range of possible applications, the loading plate is considered to be 0.47 mm thick and to have a diameter of 43 mm (Fig. 5). It is integrant part of the howler casing and the employed piezomembrane is the one previously described. The loading plate has a pin joint therewith, inside and at the center of the casing, which makes the piezoceramic to be fixed. In fact said piezoceramic after having been perforated is fixed on the pin by means of cold rivetting or by means of ultrasonic welding. The casing and therefore the loading plate are mold in ABS by means of thermoplastic technique. Even the casing cover is mold in ABS; it's formed by a disk which hasaa hole in the center. The cover is fixed at the bottom of the casing by means of ultrasonic welding.
The cover hole, together with the air volume contained in the casing, provides a resonant cavity which produces a further resonance with respect to the aforesaid resonances of the piezomembrane - loading plate system. As it's possible to notice in Fig. 5 the section of said loading plate is slightly conical, this is in order to increase its crosswise rigidity and to avoid noxious, not symmetric deformations of the plate.

[0006] The electric contacts of the piezoceramic are made by means of nickel silver springs, which have a buttonhole fitting in the loading plate pin (Fig. 6). The piezomembrane, which has a hole in the center, as it has been said, is inserted on the pin between the two springs; after the rivetting, said springs assure a good electric contact with the metal plane. Finally, a good electric insulation between the two electrodes of the piezoceramics is assured, being the pin of ABS.
Springs are constrained to the casing by means of suitable placings, and they lean out of said casing in order to provide the electric terminal of the transducer.
Electroacoustic characteristics of the howler are showed in Tab. I, while in Tab. 3-A, is showed its transfer characteristic, finally octave thirds spectrum is showed in Tab. 3-B tested at a distance of 30 cm, with the microphone placed on the axis of the device on the side of the hole of the resonant chamber when said device is drived with a square wave two tone (650-950Hz) signal with a peak to peak voltage of 26 V; this is a typical signal for telephone ringers.
The embodied acoustic howler has competing characteristics with the ones actually on the market, but compared with these it offers besides the aforesaid advantages, also the one to be easily producible by means of automatic lines without using adhesives.
Furthermore as the spectrum depends essentially on the loading plate characteristics, there is the largest design choice about electromechanic characteristics of the piezomembrane.
The acoustic system loading plate-piezomembrane fits very well for improving the spectrum, as regards the particular appliance either as acoustic howler or as telephone transmitter or as other acoustic transducer such as microphone and loudspeaker.

TAB.1

TELEPHONIC ACOUSTIC HOWLER CCP-10
30cm distant acoustic pressure typical value with two tones excitation (680-950Hz 26 Vpp) * telephonic standards.	85 dBA
120 Hz static capacitance	65 nF ± 25%
Band minimum impedance 100-5000Hz	590 Ω ± 10%
1 Khz impedance	>2.6 KΩ
Maximum working voltage	30 Vpp
Operating temperature	-20 a +70 C°

* The measure is executed fixing the buzzer in the center of a 10x10cm, 1,6 mm thick phenolic material tablet with ⌀ = 12mm central hole (the arrangement is hanged in the air).

Claims

1. Piezoelectric transducer device for electroacoustic employment characterized by the fact that the piezomembrane is provided coupled with a plate of plastic material (loading plate) by means of a pin passing through the center, said loading plate being constrained along the external circumferance, said piezomembrane working as the vibromotive force generator and said plate, which oscillates dub to the piezoelectric membrane, providing the means which irradiates acoustic energy by air coupling.

2. Piezoelectric transducer device for electroacoustic employment according to claim 1 characterized by the fact that the piezomembrane - loading plate system provides one oscillating system which produces resonance modes in the phonic band with deformed lines in the loading plate, without nodal circles.

3. Method of embodying acoustic howler using a piezoelectric transducer according to any of the previous claims characterized by the fact that the loading plate is an integrant part of the howler casing and it is provided of a central pin to fix the piezoceramic, said casing and the loading plate being mold by means of thermoplastic technique.

4. Method of embodying an acoustic howler using a piezoelectric transducer according to any of the previous claims characterized by the fact that the cover hole together with the air volume contained in the casing provide a resonant cavity which produces a further resonance with respect to the typical resonances of the piezomembrane-loading plate system.

5. Method of embodying an acoustic howler using a piezoelectric transducer according to claims 3 and 4, characterized by the fact that the section of the loading plate is slightly conical.

6. Method of embodying an acoustic howler using a piezoelectric transducer according to claims 3 and 4, characterized by the fact that the electric contacts of the piezoceramic are made by means of nickel silver springs, which have a buttonhole fitting in the pin of the loading plate, the piezomembrane holed in the center beeing inserted on the pin between the two springs.

Drawing