GB1592416A

GB1592416A - Piezoelectric device for converting mechanical energy to electrical energy

Info

Publication number: GB1592416A
Application number: GB52860/77A
Authority: GB
Original assignee: Kureha Corp
Current assignee: Kureha Corp
Priority date: 1976-12-28
Filing date: 1977-12-20
Publication date: 1981-07-08
Also published as: JPS5394083U; DE2757099B2; JPS5831427Y2; FR2376523A1; FR2376523B1; DE2757099A1; DE2757099C3

Description

(54) PIEZOELECTRIC DEVICE FOR CONVERTING MECHANICAL ENERGY TO ELECTRICAL ENERGY (71) We, KUREHA KAGAKU KOGYO KABUSHIKI KAISHA, a Japanese Company, of No. 8, Horidome-cho, l-chome, Nihonbashi, Chuou-ku, Tokyo, Japan, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a piezoelectric device for converting mechanical energy to electrical energy.

It is well known in the art that polarized polyvinylidene fluoride or polyvinyl fluoride have piezoelectric characteristics. A variety of electro-acoustic transducers or keyboards employing these piezoelectric polymer films have been proposed. Most of the electroacoustic transducers and keyboards employing piezoelectric polymer films utilize the property that the films are readily deformed or vibrated by application of very small stess because the films are thin and flexible. However, this very property prevents the thin polymer film from being free-standing. In addition, the vibration of the film caused by application of stress may cause noise in the resulting electrical signal.

Accordingly, the free vibration characteristic of the thin polymer film may be disadvantageous depending on the use of the film.

If the thickness of the film is increased, then its free-standing characteristics can be improved. However, the piezoelectric polymer material is, in general, high in crystallization, and has a relatively high Young's modulus of elasticity. Accordingly, if the thickness of the piezoelectric polymer material is increased, its flexibility as a piezoelectric element is decreased, and in addition the vibration by application of stress is also increased.

An object of the present invention is therefore to eliminate or minimise the aforementioned drawbacks of the conventional piezoelectric devices employing piezoelectric polymer films.

Accordingly, the invention resides in a piezoelectric device for converting mechanical energy to electrical energy comprising a piezoelectric polymer film having two surfaces, each surface being provided with an electrode, connecting wires extending from said electrodes, and two elastic sheets arranged so that said piezoelectric polymer film is sandwiched between said elastic sheets with said elastic sheets and film forming a unitary structure, the Young's modulus of elasticity of said elastic sheets being lower than that of the said piezoelectric polymer film and the thickness of said elastic sheets being greater than that of said piezoelectric polymer film.

In the accompanying drawings, Figure 1 is a cross-sectional view illustrating a piezoelectric device according to one example of the present invention, Figure 2 is a cross-sectional view illustrating the piezoelectric device shown in Figure 1 when employed as a key in an electronic musical instrument, Figure 3a is a schematic illustration showing the piezoelectric device of said one example when employed as a counter or a selecting meter, and Figure 3b is a schematic illustration showing the piezoelectric device of said one example when employed as a material metering assembly.

Referring to the drawings, in Figure 1 the reference numeral 1 designates a polyvinylidene fluoride film which has a piezoelectric characteristic as a result of polarization. Provided on opposite sides of the film 1 are electrodes 2 and 2' respectively which are formed of, for example, aluminium, gold, silver, nickel or chromium, by means of vacuum evaporation, plating or coating. Relatively thick elastic sheets 3 and 3' having a Young's modulus of elasticity lower than that of the piezoelectric film are provided on the electrodes 2 and 2' respectively to form a unitary structure. Connecting wires 4 and 4' extend from the electrodes 2 and 2' respectively.

The elastic sheets are normally secured to the piezoelectric film with an adhesive.

However, in the case where the electrodes are formed on the piezoelectric polymer film with an electrically conductive paint or adhesive, the elastic sheets may be coupled to the piezoelectric polymer film by the adhesive forces of these electrodes.

It is preferable to produce the elastic sheets 3,3' from rubber such as natural rubber, synthetic rubber, silicon rubber, fluororubber, ethylene-propylene rubber.

foamed polyurethane, or foamed rubber, or from foamed plastic. In addition, it is necessary that the thickness of the elastic sheet be greater than that of the piezoelectric polymer film. However, it is more preferable to determine the thickness of each elastic sheet so that its elasticity calculated as a product of the Young's modulus of elasticity and its thickness is greater than the elasticity of the piezoelectric polymer film. It is possible to arrange that the two elastic sheets are formed of different materials and of different thicknesses.

Referring to Figure 2, the device described above is indicated at A and is placed on a flat base board B so that, when a piano-touch stress P is applied thereto, the piezoelectric polymer film 1 is slightly deformed. Piezoelectricity is thereby generated across the electrodes 2 and 2' so that, if the wires 4 and 4' are connected to the electrodes, an electrical signal can be obtained in response to the magnitude of the stress applied to the film. Therefore, the device can be employed as a key in an electronic musical instrument, according to the principle described above. In this case, even if a strong impact is applied to the device A, or the key, the impact sound is absorbed by the sheets 3 and 3'. Accordingly, noises due to such impact are minimal. In addition, since the piezoelectric polymer film is reinforced by the elastic sheets, the device will not be damaged even if strong impacts are repeatedly applied thereto.

Furthermore, the free-standing characteristics and elasticity restoring force of the device shown in Figure 1 are increased by the use of the elastic sheets. This permits the use of the device, for instance, in the case where an upper end of th piezoelectric device A is fixedly secured to a post C so that a solid body E conveyed by a conveyor D is brought to be in contact with a lower end of the device A as shown in Fig. 3a. The number of times the piezoelectric device is deflected or the amount of displacement of the device A can be detected. Accordingly, the device shown in Figure 1 can be utilized as a counter for counting the number of articles or as a selecting means for selecting articles according to their dimensions.

Another application of the device according to said one example of the invention is shown in Fig. 3b where powder F having a predetermined thickness is conveyed by a conveyor. Two piezoelectric devices A and A2 are provided at different distances above the conveyor in such a manner that only one of the piezoelectric elements A and A2 is in contact with the powder F.

The assembly of the piezoelectric devices can then be employed as a material metering assembly for controlling the thickness of powder.

In the piezoelectric device according to the present invention, the cushion effect of the elastic sheets provided on both surfaces thereof and properties such as improved free-standing characteristics and elasticity restoring forces permit greater application of piezoelectric polymer film elements.

WHAT WE CLAIM IS: 1. A piezoelectric device for converting mechanical energy to electrical energy, comprising a piezoelectric polymer film having two surfaces, each surface being provided with an electrode, connecting wires extending from said electrodes, and two elastic sheets arranged so that said piezoelectric polymer film is sandwiched between said elastic sheets with said elastic sheets and film forming a unitary structure, the Young's modulus of elasticity of said elastic sheets being lower than that of said piezoelectric polymer film and the thickness of said elastic sheets being greater than that of said piezoelectric polymer film.

2. A device as claimed in Claim 1, wherein the material of said elastic sheets is natural rubber, synthetic rubber, silicon rubber, fluororubber, ethylene-propylene rubber, foamed polyurethane, foamed rubber or a foamed plastic.

3. A device as claimed in Claim 1 or Claim 2, wherein the elasticity of each of said elastic sheets, represented by the product of the Young's modulus of elasticity and thickness thereof, is higher than that of said piezoelectric polymer film.

4. A piezoelectric device comprising the combination and arrangement of parts substantially as hereinbefore described with reference to and as shown in Figure 1 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. or adhesive, the elastic sheets may be coupled to the piezoelectric polymer film by the adhesive forces of these electrodes. It is preferable to produce the elastic sheets 3,3' from rubber such as natural rubber, synthetic rubber, silicon rubber, fluororubber, ethylene-propylene rubber. foamed polyurethane, or foamed rubber, or from foamed plastic. In addition, it is necessary that the thickness of the elastic sheet be greater than that of the piezoelectric polymer film. However, it is more preferable to determine the thickness of each elastic sheet so that its elasticity calculated as a product of the Young's modulus of elasticity and its thickness is greater than the elasticity of the piezoelectric polymer film. It is possible to arrange that the two elastic sheets are formed of different materials and of different thicknesses. Referring to Figure 2, the device described above is indicated at A and is placed on a flat base board B so that, when a piano-touch stress P is applied thereto, the piezoelectric polymer film 1 is slightly deformed. Piezoelectricity is thereby generated across the electrodes 2 and 2' so that, if the wires 4 and 4' are connected to the electrodes, an electrical signal can be obtained in response to the magnitude of the stress applied to the film. Therefore, the device can be employed as a key in an electronic musical instrument, according to the principle described above. In this case, even if a strong impact is applied to the device A, or the key, the impact sound is absorbed by the sheets 3 and 3'. Accordingly, noises due to such impact are minimal. In addition, since the piezoelectric polymer film is reinforced by the elastic sheets, the device will not be damaged even if strong impacts are repeatedly applied thereto. Furthermore, the free-standing characteristics and elasticity restoring force of the device shown in Figure 1 are increased by the use of the elastic sheets. This permits the use of the device, for instance, in the case where an upper end of th piezoelectric device A is fixedly secured to a post C so that a solid body E conveyed by a conveyor D is brought to be in contact with a lower end of the device A as shown in Fig. 3a. The number of times the piezoelectric device is deflected or the amount of displacement of the device A can be detected. Accordingly, the device shown in Figure 1 can be utilized as a counter for counting the number of articles or as a selecting means for selecting articles according to their dimensions. Another application of the device according to said one example of the invention is shown in Fig. 3b where powder F having a predetermined thickness is conveyed by a conveyor. Two piezoelectric devices A and A2 are provided at different distances above the conveyor in such a manner that only one of the piezoelectric elements A and A2 is in contact with the powder F. The assembly of the piezoelectric devices can then be employed as a material metering assembly for controlling the thickness of powder. In the piezoelectric device according to the present invention, the cushion effect of the elastic sheets provided on both surfaces thereof and properties such as improved free-standing characteristics and elasticity restoring forces permit greater application of piezoelectric polymer film elements. WHAT WE CLAIM IS:

1. A piezoelectric device for converting mechanical energy to electrical energy, comprising a piezoelectric polymer film having two surfaces, each surface being provided with an electrode, connecting wires extending from said electrodes, and two elastic sheets arranged so that said piezoelectric polymer film is sandwiched between said elastic sheets with said elastic sheets and film forming a unitary structure, the Young's modulus of elasticity of said elastic sheets being lower than that of said piezoelectric polymer film and the thickness of said elastic sheets being greater than that of said piezoelectric polymer film.

5. A key for an electronic musical

instrument including a device as claimed in any one of the preceding Claims.

6. A counter or selection mechanism including a device as claimed in any one of Claims 1 to 4.

7. A material metering assembly including a device as claimed in any one of Claims 1 to 4.