Improved piezoelectric switch

The invention relates to a piezoelectric switch, comprising at least a substantially flat first piezoelectric element, a substantially flat second piezoelectric element an operating unit that can move in the direction transverse to the principal plane of the first piezoelectric element for exerting a force on the first and the second piezoelectric element during operation, said force deforming the first and the second piezoelectric element, electrical connecting means connected to opposite sides of the first piezoelectric element for conducting signals generated by the first piezoelectric element during operation and electrical connecting means connected to opposite sides of the second piezoelectric element for conducting signals generated by the second piezoelectric element during operation.

Such switches are known from DE-A-21 25 068.

These switches belonging to the prior art are increasingly applied on account of their durability and resistance to vandalism. This makes these switches attractive for applications that place high requirements on the reliability of the switch, for example in applications whereby safety depends on the switch. Such applications are usually subject to legal requirements for the switches to be provided with two independent switching elements. In the prior art switch mentioned above this has been reached by placing two piezoelectric elements one above the other, whereby the top element can be operated by an operating element and by transferring the movement of the top element to the bottom element. In this switch the connection means are of one of the poles are connected within the switch as the crystals are located on the same support. The connections means of the other poles are mutually connected adjacent to the switch. Thus the independence of the both electrical circuits of both crystals is not ensured completely.

The aim of the present invention is to provide such a switch wherein the circuits of both crystals are implemented independently as far as possible.

This aim is reached in that the electrical connecting means of each piezoelectric element are at least adjacent to the switch galvanically separated from each other.

It turns out that the deformation of both elements is not the same, so that different magnitudes of piezoelectric voltage are generated, whereby the switch-on time also differs. There is also a risk of only one of the elements responding and generating a voltage when the operating element is pressed. The present invention attempts to avoid these problems.

This object is achieved in that at least a second piezoelectric element is positioned in the housing, said element being mechanically linked to the operating unit in such a way that when the operating unit is operated, a force is exerted on at least the first and second piezoelectric elements which mechanically deforms the piezoelectric elements, in that the at least second piezoelectric element is provided with electrical connecting means on its opposite sides for conducting signals generated by the second piezoelectric element during operation, and in that the electrical connecting means of each piezoelectric element are at least partially galvanically separated from each other. As a result of these measures, an equal mechanical load is exerted on the piezoelectric elements.

According to a first embodiment, the mechanical link between the operating unit and the second piezoelectric element is arranged for causing a deformation in the second piezoelectric element when the operating unit is pressed, said deformation being substantially equal to that of the first piezoelectric element. By means of these measures, the mechanical load of the elements is made as equal as possible for each element, so that the electrical signals generated by the elements are made as equal as possible to each other.

The mechanical structure of the switch is simplified if the first and second piezoelectric element are extended in parallel to each other and in a concentric fashion.

According to a further preferred embodiment, each of the piezoelectric elements comprises a piezoelectric crystal positioned on a carrier, a film is applied to the side of the element facing away from the carrier and at least the carrier of the element that is positioned closest to the operating element is suspended in the housing so that it can move in the direction in which the operating element moves. By means of this structure,

the movement of the top element is easily transferred to the bottom element, so that the deformations of the elements are as equal as possible.

The similarity of the movements and deformations of either element is further increased, if the first and second piezoelectric elements are positioned with their carriers facing each other in the housing and if both carriers are suspended in the housing so that they can move in the direction in which the operating element moves.

A more specific preferred embodiment provides for the measure that the carriers are attached to a film that can move in the direction in which the operating unit moves at least at the edge of the carrier and that an electrically insulating film is applied between the carriers extending over at least a part of the surface of the carriers, with electrically conductive tracks being mounted on either side of the film, said tracks being arranged for functioning as part of the electrical connecting means. This measure makes it possible to meet the requirements with respect to separating the electrical circuits while maintaining a structurally attractive solution.

The measure of incorporating openings in the film simplifies the movement of the carriers and the piezoelectric elements positioned thereon.

The electrically insulating film is preferably clamped into position at its edges between two layers of the side wall of a housing in which the components of the switch are positioned. The structure is thus further simplified.

Yet another preferred embodiment provides for the measure that the film applied on the side of the element facing away from the carrier is produced out of electrically insulating material and that electrically conductive tracks are mounted on this film, which are arranged for functioning as part of the electrical connecting elements. In this way it is possible to provide the electrical connections in a relatively simple fashion while maintaining the galvanically separated circuits.

The voltage generated by a piezoelectric element is largely dependent on the temperature, which can lead to problems in a number of applications. To avoid these problems, an embodiment proposes providing at least a third piezoelectric element that

is connected with opposite polarity and in series to the first piezoelectric element and is thermally linked at least to the first piezoelectric crystal. This thermal link ensures that the piezoelectric elements each have the same temperature, so that the temperature- dependent part of the voltage generated by the piezoelectric elements is the same as far as possible.

For the same reason, another preferred embodiment provides measures providing for at least a fourth piezoelectric element that is connected with opposite polarity and in series to the second piezoelectric element and is thermally linked at least to the second piezoelectric crystal.

In this respect it should be noted that it is in principle possible to also apply this thermal compensation switch in simple switches, in other words in switches where only one piezoelectric crystal is used.

To keep the temperature of the elements contained in a thermal compensation switch as constant as possible, it is attractive if at least one of the pairs of the first and third piezoelectric element or respectively second and fourth piezoelectric element are positioned on a carrier that can conduct heat effectively.

For the function referred to above to be effective, whereby the temperature is compensated, it is important for the second element only to have to fulfill its compensating function. This is possible if the third or respectively fourth piezoelectric element cannot be operated.

It is however also possible for the third and fourth piezoelectric elements to be joined to a structure corresponding to that of the first and second piezoelectric elements, for the first, second, third and fourth piezoelectric elements to each be connected to a circuit and for the circuit to only be effective if the combination of the first and second elements or the combination of the third and fourth elements is operated.

A piezoelectric switch comprises a piezoelectric element, the electrical connections of which are connected to an electronic circuit that is arranged to respond to the signal generated by the piezoelectric element. The electrical connections of the piezoelectric

element are connected by a resistance. When used as a pulse generator, this resistance acts as a leakage resistance, which causes the charge generated as a result of the deformation of the piezoelectric element to drain away, in turn causing the voltage over the piezoelectric element to decrease. As a result of this, the circuit will produce a pulse starting at the point at which the voltage generated by the piezoelectric element exceeds a threshold value and ending when the voltage drops below this threshold value. A piezoelectric element continues to retain the voltage generated during the process of deformation even after it has been deformed. This means that each of the elements is provided with a resistance to allow the charge generated during the process of deformation to drain away.

It is however also possible to apply the piezoelectric elements in a switch having a permanent function. The configuration used to this end corresponds to that of the switch with pulse function described above, provided that the electronic circuit is arranged for switching off the resistance as soon as the threshold value referred to above is exceeded. The voltage generated by the piezoelectric element is then indeed retained. Particularly in the last case, it is important for the temperature to be compensated to avoid any undesired operation of the switches.

Further features of the present invention will emerge from the accompanying drawings, in which the following are shown:

Figure 1 : a cross-sectional view of a first embodiment of a switch according to the invention in rest position;

Figure 2: a cross-sectional view of a variant of the switch shown in Figure 1 in actuated position;

Figure 3: a cross-sectional view of another variant of the switch shown in Figure 1 in actuated position;

Figure 4: a cross-sectional view of a second embodiment of a switch according to the invention in rest position; Figure 5: a cross-sectional view of a third embodiment of a switch according to the invention in rest position; and

Figure 6: a cross-sectional view of a third embodiment of a switch according to the invention in rest position.

The piezoelectric switch shown in Figure 1 comprises a housing that is formed by a base plate 1 and a flexible operating plate 2 extending in parallel thereto, which are separated at their edges by two rings or spacers 3, 4. Two piezoelectric elements 5, 6 are positioned in the housing, each formed by a carrier 7, or respectively 8, produced out of an electrically conductive material, a piezoelectric crystal 9, 10 and a conductive layer 11, 12 applied thereto. Such piezoelectric elements are known per se. With the present invention, they are positioned back to back, in other words with their carriers 7, 8 facing each other.

The carriers 7, 8 are each connected to a film 13 of electrically insulating material. The edges of this film 13 are clamped into position between the spacers 3, 4. Conductive tracks 15, 16 are mounted on either side of the film 13 and are used to connect the corresponding side of the piezoelectric element 5, 6.

As a result of the difference in thickness between piezoelectric elements and spacers, the side of the elements 9, 10 on which the film layer 17, 18 is present is pressed against the electrically insulating film 17, 18 that is provided with a conductive track 19, 20. These tracks 19, 20 are also used to connect the corresponding side of the piezoelectric element 5, 6. The films 17, 18 are retained between the respective spacers 3, 4. A pressure piece 23, or respectively 24, is affixed to the center of the base plate 1 , or respectively operating plate 2.

This piezoelectric switch functions as follows: The switch is operated by pressing the operating plate 2. This causes the center of the plate to move downwards, with the pressure piece 23 being included in this movement. This vertical movement is distributed equally over both piezo elements as a result of the symmetrical structure. Both piezoelectric elements 5, 6 are thus deformed and both crystals 9, 10 generate substantially the same voltage at substantially the same time. In this respect, the elastic suspension of the piezoelectric elements is important to enable the piezoelectric elements to move in the direction of operation. This elastic suspension is achieved by the presence of the elastic films 13, 17 and 18. Although in this exemplary embodiment, the piezoelectric elements are positioned as mirror images, it is also perfectly possible to position the elements one under the other without rotating either.

Figure 2 shows a slightly changed configuration, this time in the actuated position. In this configuration - apart from the dimensioning - an electrically conductive film 37, or respectively 38, is applied instead of an insulating film 17, or respectively 18 on which conductive tracks are mounted. In addition, an electrically conductive film 35, or respectively 36, is applied to either side of the insulating film 13, fulfilling the function of tracks 15, 16 from an electrical perspective. An extra spacer is also added with a ring-shaped air chamber, freeing up space between the piezoelectric elements to bend toward each other.

A further slightly changed configuration is shown in Figure 3. In this configuration, film 13 with a round recess between the carriers 7, 8 is replaced by a spacer 33 without any opening, thus obtaining better electrical insulation between the electrical circuits connected to the various piezoelectric crystals.

Figure 4 shows a combination of two switches according to the embodiment of Figure 3. In this configuration, one of the switches is used to compensate the thermal drift of the piezoelectric crystals. To this end, a crystal in the left switch is connected with opposite polarity and in series to one of the crystals in the right switch. The conductors are therefore connected through to the corresponding films.

Such a switch is suitable for switching functions on and off, whereby one of the switches is used for switching on the function and the other switch is used for switching off the function in question. From a functional perspective, this prevents the left and right switch from being operated at the same time. However, if such a switching operation does happen, it will not have any consequences.

It should be noted in this respect that a non-operated piezoelectric crystal displays thermal drift in the same way as an operated piezoelectric crystal. If both are connected with opposite polarity and in series, the thermal drift voltages will compensate each other, so that the resultant voltage is independent of the temperature.

In the configuration according to Figure 4, it is possible to refer to a duplicate arrangement of switches creating the redundancy required for safety-critical applications. However, it is also possible to use simple switches with temperature

compensation, as shown in Figure 5, whereby the base plate and bottom film with conductor are replaced with a printed circuit board having conductors on its upper side to conduct the current away.

The switch in question comprises a printed circuit board 41, a flexible operating plate 42 and a spacer 43 positioned between the films 46 and 47. Two piezoelectric elements 44 and 45 are arranged inside the switch.

The upper side of the piezoelectric elements is connected by conductor 46 and the bottom side of the first piezoelectric element 44 is connected to the electronic circuit via a conductive film 47 on the printed circuit board 41. The bottom side of the second piezoelectric element 45 is connected to the circuit via a conductor 48 on the printed circuit board. As a result of this configuration, both piezoelectric crystals are connected in series. This embodiment also includes the provision that only one of the piezoelectric elements is operated at any one time.

Finally, Figure 6 shows an embodiment in which only one of the piezoelectric elements, namely element 44, can be operated. In comparison with the configuration shown in Figure 5, a spacer body 50 is positioned extending above the non-operable piezoelectric element 45 and a pressure piece 51 is positioned above the operable element. The overall structure is covered by a cover plate 52 that is flexible in one direction. This means that the element 45 cannot be operated, so it is always only used as a compensator for the temperature.

It should be noted that there are numerous ways of deviating from the configurations shown above, without departing from the scope of the invention.