The invention relates to a piezoelectric actuator, for instance for actuating a mechanical component, as defined by the generic characteristics of the main claim.
It is known, for instance from German Patent Disclosure DE 199 28 189 A1, that by using the so-called piezoelectric effect, a piezoelectric element for controlling the needle stroke of a valve or the like can be constructed from a material having a suitable crystalline structure. Upon application of an external electrical voltage, a mechanical reaction of the piezoelectric element occurs, and this reaction, depending on the crystalline structure and on the regions to which the electrical voltage is applied, represents a pressure or tension in a predeterminable direction.
Because of the extremely fast and precisely regulatable stroke effect, such piezoelectric actuators can be provided for constructing final control elements, for instance for driving switching valves in fuel injection systems in motor vehicles. The voltage- or charge-controlled deflection of the piezoelectric actuator is utilized in that case for positioning a control valve, which in turn regulates the stroke of a nozzle needle.
Since the required electrical field intensities for actuating the piezoelectric actuator are in the range of several kV/mm, and since as a rule, moderate electrical voltages for triggering are desired, the construction of this piezoelectric actuator is done here in a plurality of layers of metallized piezoelectric ceramics, stacked one above the other, to make a so-called multilayer actuator. For this purpose, between the various layers, there are inner electrodes which are applied for instance by a printing process, and there are also outer electrodes, by way of which the electrical voltage is applied. A typical method for producing such layers is the foil casting technique. The various layers are metallized and stacked one above the other for producing the inner electrodes, and then the piezoelectric effect develops between two layers with inner electrodes of different polarity.
Typically, such piezoelectric actuators are provided with inner electrodes which over the full broad side have a recessed or in other words buried inner electrode on one of four sides. On all the other sides, the inner electrode extends outward as far as the edge of the ceramic. Piezoelectric actuators are also known whose inner electrodes are recessed on all sides, with the exception of the bonding strip on the outside of the piezoelectric actuator. These arrangements can have various bonding zones, for instance over a surface or at corners.
As a rule, for installation in corresponding actuator arrangements, these known piezoelectric actuators have passive head and foot parts, which are not provided with inner electrodes. In other words, special guidance or fitting zones must be provided, and a covering or insulation layer is necessary, which could easily become damaged.
The piezoelectric actuator known from the prior art is provided with a multilayer construction comprising piezoelectric layers that each comprise a passive region with or without inner electrodes and a piezoelectrically active region with bonded inner electrodes located between the layers. In the active region, the piezoelectric actuator has bonding of the inner electrodes that changes from one layer to the next, for subjection to an electrical voltage. In the invention, conversely, the piezoelectric actuator, including a head and/or foot region, is advantageously constructed actively with bonded inner electrodes, but the inner electrodes in the head and/or foot region are fully buried, except for a minimal contact zone for the respective external bond, unlike the central active region.
For many applications, piezoelectric actuators whose active region extends from the head to the foot over the entire piezoelectric actuator are advantageous, so that guidance or fitting zones in the foot region and optionally in the head region are omitted. Active regions with inner electrodes conventionally extended to the outside, however, must intrinsically be covered in an electrically insulated manner, to prevent surface conduction or creepage. As a rule, covering compositions are used, such as silicone resins or silicone elastomers or other insulating materials, insulating paints, or insulation layers, which are highly vulnerable to mechanical damage in production and further processing.
The embodiment according to the invention of a fully active piezoelectric actuator does not require any covering with an insulation layer in the fitting or joining region at the head or foot. Here an advantageous combination of different burials of the inner electrodes in the layered construction of the piezoelectric actuator is proposed, in which the only partly buried inner electrodes make an unhindered expansion possible in the central, free region, while in the joining region at the head and/or foot, additional layers with virtually fully-buried inner electrodes are activatable.
The slight sacrifices that occur here from the slightly lesser expansion capacity of the virtually fully-buried inner electrode design in the head and/or foot region are easily tolerated. The remaining contact zone in each case makes up a relatively small proportion of the total width of the external bond.
One exemplary embodiment of the piezoelectric actuator of the invention will be described in conjunction with the drawing figures. Shown are:
In
The piezoelectric actuator 1 is constructed entirely of such piezoelectrically active regions, specifically a central, free region 2 and a head region 3 as well as a foot region 4; the head and/or foot region 3, 4 serves the purpose in particular of mechanically retaining the piezoelectric actuator 1.
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Number | Date | Country | Kind |
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102004031596.5 | Jun 2004 | DE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP05/51832 | 4/25/2005 | WO | 12/6/2006 |