Thin Film, Thermoplastic Piezoelectric Switches

The invention relates to piezoelectric switches.

It is well known in the art that an oriented polyr-methyl glutamate film or a polar thermoplastic film such as a polyvinylidene fluoride or polyvinyl fluoride film which is polarized under a high d.c. voltage electric field, has a high piezoelectric property. Piezoelectric switches, which comprise a piezoelectric thermoplastic film element with electrodes attached to both surfaces thereof, are subjected to a stress to produce a piezoelectric field for a switching operation. Such switches are disclosed in Japanese Opened Patent Application No. 47-37244.

In such conventional piezoelectric switches, the electric field is temporarily obtained only at the moment when the stress is applied to or removed from the piezoelectric film element, and is not held permanently. Further, the resulting electric field is indicated as a differential value according to changes in stress. Accordingly, the conventional piezoelectric switches cannot be used as switches for remove sensors which require an absolute value.

U.S. Pat. No. 3,935,485 to Yoshida et al describes a piezoelectric switch utilizing a thermoplastic film element. The switch shown is an example of the conventional piezoelectric switches mentioned above. Only a momentary output signal can be obtained by a single push operation since the film element is defined by a push button.

It is also known in the prior art to provide a piezoelectric switching system wherein a mechanical oscillator is brought into contact with an inorganic piezoelectric element for providing a continuous electrical output. U.S. Pat. No. 3,749,947 to Kawata et al describes an example of such a piezoelectric switching system. In this case, when a push button is actuated to bring the mechanical oscillating element into contact with the inorganic piezoelectric element, a continuous electric signal is provided between the electrodes. In FIGS. 2 and 4 of the Kawata et al patent there is shown a piezoelectric switching system wherein a driving element 13 which is a mechanical piezoelectric transducer is attached to a resonating member 14. The intermediate points of the resonating member are brought into contact with the inorganic piezoelectric members 4A through 4N by means of a push button to individually actuate the latter piezoelectric transducers.

In accordance with this invention, there is provided an analog operation switch comprising a piezoelectric thermoplastic film element with electrodes attached to both surfaces, an oscillator provided in the vicinity of the film element, and an elastic wave absorbing means intermediate thereto. Either the film element, the oscillator or the intermediate member is adapted to move into and out of a position whereby the oscillator, the film element and the absorbing means are in contact, with a contact pressure so as to produce an oscillating piezoelectric field between the electrodes in accordance with changes in the contact pressure.

FIGS. 1 and 2 are schematic views showing first and second embodiments, respectively, of the piezoelectric switch according to this invention.

FIG. 1 shows a piezoelectric thermoplastic film element 1 which includes a polyvinylidene fluoride film 2 having a piezoelectric constant d31 on the order of, for example, 10.sup.-8 to 10.sup.-6 (Cgs esu). Attached to both surfaces of the film 2 are thin film electrodes 3,3' of gold, silver, aluminum, tin, nickel, or other suitable metals. The thin film electrodes are attached to the film 2 by means of any suitable process, such as, metal plating, sputtering, adhering, printing, or the like. The reference numeral 4 indicates an oscillator for producing mechanical oscillations such as, for example, a piezoelectric vibrator such as a bimorph piezoelectric inorganic element, an electromagnetically vibrated magnetic element, or any other well known vibrator which normally oscillates with a constant wave length and a constant strength.

The elastically absorbing means 5 is made of a material such as rubber or plastics and serves to protect the thin film thermoplastic piezoelectric element as well as the electrodes attached thereto from breaking and also serves to prevent electrical noise due to air vibrations. The material used will not completely absorb the elastic waves produced by the mechanical oscillator 4. If it were to do so, the device would not be operative. On the other hand, some of the elastic waves produced by the mechanical oscillator 4 are absorbed for the purpose of protecting against breakage as described above. Additionally, when the mechanical oscillator 4 is oscillating, even though it is not in contact with the thin film thermoplastic piezoelectric element, it will nonetheless produce vibrations in the air. The thin film thermoplastic piezoelectric elements are extremely sensitive and can respond to the vibrations in the air caused by the oscillator 4 to provide an output, which will be classified as electrical noise. It has been found by applicants that the use of an absorbing means 5 as shown can avoid the occurrence of electrical noise due to air vibrations by effectively shielding the air vibrations caused by the oscillator. The piezoelectric film element 1 and the absorbing member 5 are held between two frames 6,6'. The member 5 may be attached or coated on the surface of the electrode attached to the piezoelectric film element. An amplifier 7, such as a field effect transistor (FET), is connected to electrode 3, and the electrode 3' is grounded.

When the piezoelectric film element spanned between the frames moves, together with the frame, into contact with the oscillator 4, the oscillation from the oscillator 4 is applied to the piezoelectric film element 1 to oscillate it so as to induce piezoelectric fluctuations on the surface thereof in accordance with the oscillations therebetween. Accordingly, an analog signal can be produced in the electric circuit electrically coupled to the electrodes in accordance with the strength of the contact pressure. Instead of moving the piezoelectric film element 1 and the frames 6,6' towards the oscillator as mentioned above, the oscillator may be moved towards the film element 1 to accomplish the same result.

FIG. 2 shows another embodiment of this invention in which the piezoelectric thermoplastic film element 1 is attached to one face of the frame 6, and the frame 6 and the oscillator 4 are fixed relative to one another. The reference numeral 8 indicates a contact element which may be the elastic wave absorbing member or may simply be a non-elastic contact element. It will be noted that in the latter case the elastic wave absorbing means 5, as shown in FIG. 1, would have to be provided also in FIG. 2. However, according to the illustration of FIG. 2, the contacting element 8 is also the elastic wave absorbing means inasmuch as it is constructed of an elastic material such as rubber or plastic. When the contact element 8 moves downwards into contact with both the film element 1 and the oscillator 4 with a contact pressure, the oscillation from the oscillator 4 is transmitted to the film element 1 so as to produce an oscillating piezoelectric field between the electrodes 3 and 3' in accordance with the strength of the contact pressure.

Also in the case shown in FIG. 2, it is possible to prevent a direct transmission of sound wave generated by the oscillator 4 to the electrode 3 from occurring by making the contact element 8 to always exist between the oscillator 4 and the film element 1 so as to act as a shelter for sound wave. This effect may be improved by coating the electrode 3 and/or the oscillator 4 with an elastic wave absorbing material (not shown) which is different from the material forming the electrode 3 and/or the oscillator.

Since the piezoelectric field produced by the switch of this invention oscillates with the same periodic time as the oscillation from the oscillator, the respective switch positions in a multi-switch apparatus can be separately indicated by arranging each switch to have a different oscillating frequency. Further, since the piezoelectric field continues during the time when the oscillator is held in a direct or indirect contact with the film element, the switch produces a continuous signal and is useful as a switch for producing an electric signal for a prolonged or short time.