This application is based on German Patent Application 10 2004 047 105.3 filed Sep. 29, 2004, upon which priority is claimed.
1. Field of the Invention
The present invention is directed to an improved piezoelectric actuator of the type having stacked layers of piezoelectric material.
2. Description of the Prior Art
Piezoelectric actuators are used in numerous industrial applications, known examples of which include, among others, actuators for actuating a valve closure member of a fuel injection valve, for actuating hydraulic valves, or for driving micropumps.
Typically, the piezoelectric actuators, for example of the kind shown in FIG. 3 of DE-198 02 302 A1, are comprised of a number of stacked layers of a piezoelectric material, so-called piezoelectric elements, and electrodes situated between the piezoelectric layers. The electrodes are arranged in an interdigital comb structure, i.e. a first and a second external electrode contact the respective electrodes inside the piezoelectric stack in an alternating fashion. The electrodes inside the piezoelectric stack are called internal electrodes for short and are oriented perpendicular to the two external electrodes. Both layer surfaces of each piezoelectric layer are thus attached to a respective internal electrode and the first or second external electrode can apply an electrical voltage. When the voltage is applied, each of the stacked wafer-like piezoelectric elements expands in the direction of the electrical field produced between the internal electrodes. The large number of stacked piezoelectric elements makes it possible to achieve a relatively large stroke in the stack direction of the entire arrangement with a simultaneously low triggering voltage.
In these actuators with the interdigital electrode structure, because of the attachment of the stacked piezoelectric elements and internal electrodes with the two external electrodes, the piezoelectrically generated expansion mainly occurs in only the central region where the internal electrodes completely overlap one another. In the edge zones where the internal electrodes do not overlap directly with the respective electrodes closest to them, there is a region with an altered field strength, which also results in the occurrence of tensile stresses. As a result of this mechanical strain, actuators of this kind frequently develop cracks.
In order to prevent the this kind of crack development, the application cited above has proposed providing the actuator with a total of four external electrodes; an external electrode is situated on each side of the piezoelectric stack and the respective pairs of external electrodes are electrically connected to a plus pole and a minus pole. The critical regions in which the internal electrodes contact the external electrodes are thus distributed over more side surfaces of the piezoelectric stack, thus permitting these regions to be spaced further apart from one another in the stacking direction. This measure makes it possible to reduce the tensile stresses in the piezoelectric stack and thus reduce the propensity toward crack development.
One disadvantage of the arrangement for crack prevention known from the prior art is that it requires a total of four external electrodes instead of two. In addition, the structure of the piezoelectric stack is more complex since care must be taken to follow the cyclical sequence when producing the contacts between the internal electrodes and the four different external electrodes. Moreover, it is not possible to respond to the presence of a more powerful mechanical strain by expanding to more than four external electrodes since a piezoelectric stack with square piezoelectric wafers has exactly four side surfaces. The potential for increasing the distance between the contact regions of the internal and external electrodes has therefore been exhausted.
The piezoelectric actuator according to the present invention has the advantage over the prior art of significantly reducing undesirable tensile stresses in the edge regions of the piezoelectric elements of the piezoelectric stack without having to provide an external electrode on all four side surfaces. Simple means achieve a reduction in the mechanical strains inside the piezoelectric actuator and therefore a minimization of damage to the material such as crack development or other performance-reducing failures. The piezoelectric actuator according to the present invention also offers the possibility of intensifying the strain-reducing action as needed. Finally, it should be emphasized here that the device according to the present invention is not intended to work around the effects of crack development, but instead, thanks to the strain relaxation it advantageously offers, cracks do not even develop in the first place.
Advantageous modifications of the piezoelectric actuator are possible by means of the measures disclosed and described in the specification.
The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of preferred embodiments taken in conjunction with the drawings, in which:
a,
1
b show possible contacting schemes for the electrodes,
As has already been mentioned above, piezoelectric actuators are usually comprised of a number of stacked layers of a piezoelectric material. Electrodes are placed between the layers and contact a plus pole and a minus pole in an alternating sequence.
a and 1b show the two possible contacting schemes. So-called internal electrodes 2, which extend parallel to the piezoelectric layers 5, are contacted on alternating sides by a first external electrode 3 or a second external electrode 4. The first external electrode 3 here is connected to a minus pole and the second external electrode 4 is connected to a plus pole. The embodiment depicted in
b shows the arrangement in an interdigital electrode structure. The internal electrodes 2 here are shorter than the full-surface internal electrodes 2 from
For illustrative purposes,
According to the present invention, the piezoelectric actuator is provided with at least one structure that reduces mechanical strains occurring in the piezoelectric stack.
The strain-reducing layer 20 with recesses 25, as depicted in
On the whole, the integration of strain-reducing layers 20 achieves a significant mechanical relaxation, which permits the actuator to function in a generally strain-free state. This has a direct, advantageous effect on the reliability of the actuator.
The foregoing relates to preferred exemplary embodiments of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.
Number | Date | Country | Kind |
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10 2004 047 105.3 | Sep 2004 | DE | national |