Title: Method for producing a key which is to be actuated by way of a pressure force directioned and dimensioned in a certain manner

Description: The invention relates to a method for producing a key to be actuated with a pressure force directioned and dimensioned in a predetermined manner.

The key comprises a piezoelectric transducer and an electronic circuit having an amplifier for the triggering operation. The key is provided with a transducer which comprises a directionally and permanently polarized piezoelectric material, in the form of a substrate, which carries electrodes thereupon for the transfer of electrical voltage and current. The body of the transducer comprises an approximately constant cross section in each plane perpendicular to the direction with which the force affects the body and has a thickness d which is small compared with a further dimension of the body. The body is polarized in the direction of the thickness d and the electrodes are positioned on opposite surfaces of the body at a spacing of the thickness d therebetween. The bodies are arranged in the key in such a way that the thickness d is essentially directed perpendicular to the direction of the course which affects the body. Each body is polarized in a volume d . h.sub.2 . l, where the dimension l is a length dimension which is essentially perpendicular to the thickness d which is perpendicular to the direction of the force, the dimension l being at least a multiple larger than the dimension d and the dimension h.sub.2 being a dimension of a portion of the height of the body and perpendicular to the thickness d and the length l. The dimension h.sub.2 is larger than approximately ten times the thickness d. A low-pass filter, including a serially connected capacitor and resistor, is provided in the electronic circuit, parallel to the electrodes of the body of the converter, the resistance of the low-pass filter being constituted by a resistance layer applied onto the body of the converter.

SUMMARY OF THE INVENTION

It is a primary object of the invention to provide an inexpensive and automated method for the production of a piezo-ceramic pressure key.

This object is achieved, in accordance with the invention, in such a way that a continuous metal layer is applied onto the rear of a ceramic member having a length L which is at least a multiple of the length l of the individual keys to be produced. Two mutually separate metal layers are applied onto the front of the ceramic substrate and a resistance layer, for each key, is applied onto the ceramic substrate connecting the front metal layers at a spacing corresponding to the distance of the individual keys. The rear metal layer and one of the front metal layers have electrodes applied thereto at a spacing corresponding to the spacing of the individual keys, and an auxiliary electrode is connected to the other front metal layer. The ceramic substrate is subsequently encased with, in particular, a thixotropic resin by submerging the element in resin, and the resin is hardened. At least one of the electrode pairs is short-circuited and that part of the ceramic substrate which is between the rear metal layer and a front metal layer is polarized by applying a polarization voltage between the short-circuited electrode pair and the auxiliary electrode. The short circuited electrode pair is then interrupted and the individual keys are obtained by severing the elements so produced at distances l.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the invention, its organization, construction and mode of operation will be best understood from the following detailed description taken in conjunction with the accompanying drawings, on which:

FIG. 1 is an elevational view of a plurality of keys during construction in accordance with the method of the present invention;

FIG. 2 is a left-hand end view of the structure illustrated in FIG. 1; and

FIG. 3 is a flow chart illustrating the method of production of the individual keys according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings, a ceramic substrate 1 is illustrated having a length L. The length L is a multiple of a length l of individual pressure keys, plus a small section as will be apparent from the description below. The ceramic substrate 1 has a height H and a thickness d which correspond to the dimensions of the individual pressure keys. On the rear surface of the ceramic substrate 1, a continuous metal layer 2 is applied and on the front surface of the substrate 1, a pair of mutually separate metal layers 3 and 4 are applied, having the heights h.sub.1 and h.sub.2. The metal layers 2, 3 and 4 are applied onto the ceramic substrate in a manner which is well-known in the prior art, and preferably consists of silver.

After completion of the metalization steps, the front metal layers 3 and 4 are interconnected by way of printed resistance layers 5. The number of the printed resistances 5 corresponds to the number of the pressure keys to be formed from the ceramic substrate 1. The rear metal layer 2 is provided with a plurality of electrodes 6 and the front metal layer 3 is provided with a corresponding plurality of electrodes 7. The number of the electrodes pairs (6,7) again corresponds to the number of pressure keys to be formed. The front metal layer 4 is provided with an auxiliary electrode 8 which is used for subsequent polarization of the ceramic substrate 1.

After the ceramic substrate has been provided in the described manner with the metal layers, the resistances 5 and the electrodes 6, 7 and 8, it is encased within an artificial resin in order to obtain sufficient mechanical stability, the casing being advantageously obtained by submerging the ceramic substrate and the elements carried thereby into a thixotropic resin. The resin, if required, may then be hardened.

Since the ceramic substrate 1 is subjected to high temperatures, both during the application of the resistances 5 and during the application of the casing, a previous polarization would become ineffective, and due to this, such a polarization can only be carried out after the hardening of the casing. In order to avoid that the capacitively effective part between the metal layer 3 and the rear metal layer 2 is polarized in the completed pressure key, an electrode pair 6 and 7 is short-circuited, as indicated by the broken lines in FIG. 2, and the polarization voltage is applied between the short-circuited electrode pair, indicated by an electrode 9, and the auxiliary electrode 8, so that only that part of the ceramic substrate 1 is polarized which is positioned between the front metal layer 4 and the rear metal layer 2. After the polarization has been carried out, the individual pressure keys are obtained by severing the sections l from the ceramic substrate 1, whereby the part carrying the auxiliary electrode 8 is discarded.

The thickness d of the ceramic substrate 1 may be between 0.05 and 0.15 mm. The length l of the individual pressure key may vary between 3 and 2mm, and the height h.sub.2 of the metal layer 5 may be between 5 and 25 mm, whereby the ratio between the height h.sub.2 of the metal layer 4 and the entire height H of the ceramic substrate 1 is between 0.6 and 0.8. The length L of the ceramic member 1 depends on the condition of the ceramic and is generally dimensioned in such a way that approximately eight individual pressure keys may be obtained from one ceramic substrate 1.

All together, the method of the invention is characterized in that it permits a rational production of piezoelectric pressure keys, since a large number of such pressure keys can be produced and polarized simultaneously.

Although I have described may invention by reference to a particular illustrative embodiment thereof, many changes and modifications of the invention may become apparent to those skilled in the art without departing from the spirit and scope of the invention. I therefore intend to include within the patent warranted hereon all such changes and modifications as may reasonably and properly be included within the scope of my contribution to the art.