Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
1999-07-15
2001-11-13
Budd, Mark O. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
Reexamination Certificate
active
06316863
ABSTRACT:
BACKGROUND OF THE INVENTION
Piezoactuators usually consist of several piezoelements arranged in a stack. Each of these elements in turn consists of a piezoceramic layer which is provided with metal electrodes on both sides. If a voltage is applied at these electrodes, the piezoceramic layer responds with a grid deformation which extends along a main axis to a useful longitudinal extent. Since this in turn amounts to less than 0.2% of the layer density along the main axis, a correspondingly higher layer thickness of active piezoceramic must be made available in order to achieve a desired absolute longitudinal extent. However, the voltage required for the actuation of the piezoelement rises with increasing layer thickness of the piezoceramic layer within a piezoelement. To keep this voltage within manageable limits, multilayer actuators are produced wherein the thickness of individual piezoelements is usually between 20 and 200 &mgr;m.
Known piezoactuators of multilayer construction thus consist of up to a few hundred individual layers altogether. For their production, piezoceramic green foils are arranged in a stack in alternation with electrode material, and these are laminated and sintered together into a monolithic compound up to about 5 mm high. Larger actuators with larger absolute excursion can be obtained by gluing together several such stacks, for example. Only piezoactuators of fully monolithic multilayer construction have sufficiently high rigidities, particularly when large forces must be transmitted with the piezoactuator.
For the electrical contacting of such piezoactuators of multilayer construction, metallization strips are attached to the exterior of the piezoactuator, for example, or in a borehole in the middle of the surface of the individual actuator. To connect every second electrode layer with one of the metallization strips, for example, this must be insulated against the intervening electrode layers. This occurs easily in that every other electrode layer comprises a recess in the region of the one metallization strip, in which recess said electrode layer is not led to the metallization strip. The remaining electrode layers then comprise the recesses in the region of the second metallization strip, in order to enable a contacting with alternating polarity. Wires for the electrical connection are soldered to the metallization strips.
Piezoactuators whose alternating contacting occurs via recesses of the electrode layers are piezoelectrically inactive in the contacting zone, since an electrical field cannot build up there due to the one electrode that is missing, respectively. As a result, in the polarization as well as in the operation of the piezoactuator, mechanical tensions build in this piezoelectrically inactive contacting zone, which tensions can lead to tears in the inactive regions and thus at the metallization strips parallel to the electrode layers as well. This can lead to the complete splitting of the metallization strip and produces the result that, given punctate voltage supply to the metallization strips from outside, a part of the piezoactuator becomes dependent on the power supply and thus becomes inactive. The number of tears depends on the total height of the actuator and on the stability of the boundary surface between the inner electrode and the piezoceramic and can rise in continuous operation given alternating load conditions. Since, in the dynamic operation, a dynamic changing of the tears, or respectively, the tear openings also derives, the metallization strips are thereby further damaged during the operation of the actuator.
SUMMARY OF THE INVENTION
It is the object of the present invention to propose a ceramic actuator, along with a method for production, which has electrical contacting that can be handled securely and easily and which demonstrates an increased stability with respect to a tear formation.
The inventive actuator can have a conventional and preferably a monolithic construction. Piezoelectric ceramic layers and electrode layers are arranged upon one another in alternation in the manner of a stack and are preferably sintered together. For alternating contacting of the electrode layers, at least two electrically conductive contact lugs are inventively provided at the stack on the outside. These are connected to the electrode layers via an edge and extend over the entire height of the electrically active region of the stack. To the side of the connected edge, they comprise a protruding region, and in the region of the outer edge that is averted from the stack, they comprise an electrical terminal element which projects laterally or which projects beyond the stack in height.
With the contact lug, it is possible to bridge, in an electrically conductive manner, tears in the metallizations that may arise in the operation of the actuator. If the projecting region is selected so as to be sufficiently wide, then the tears end inside the contact lug, or respectively, inside the projecting region. All individual elements of the actuator thus remain electrically functional, even if tears arise at the metallizations. The inventive actuator thus does not demonstrate any power losses whatsoever in operation.
The terminal element at the outer edge enables the simple connection of the contact lug to an external current or voltage supply. It protrudes beyond the contact lug laterally or beyond the stack in height and is thus still easily accessible given the installation of the stack in a housing and enables a simple current connection.
In the simplest embodiment, the terminal element is produced from the material of the contact lug, or respectively, is an integral component of the contact lug. This comprises at least one electrically conductive layer. It preferably consists of a compound material with at least one plastic film and at least one metallic film or layer, however. This type of compound material comprises a high flexibility and a high elasticity and resistance to tearing, at the same time. Geometrically, the terminal element represents an extension of the outer edge, which is averted from the stack, of the contact lug in an upward direction, or an extension of the upper edge to the outside, or respectively, to the side. Besides the thus simplified connection, the terminal element also serves for easier handling of the contact lug, or respectively, of the actuator that is provided with the contact lug, particularly during the installation into a housing. The terminal elements can serve as a guide here.
The terminal element is preferably constructed such that it represents an additional mechanical reinforcement of the outer edge of the contact lug. In an advantageous development, the terminal element is constructed as a metallic terminal pin. This can be soldered or otherwise electrically conductively fastened on the contact lug, or respectively, on its metallic layer. The terminal pin can extend over the entire outer edge or can be connected only to a part of the edge. The guidance, or respectively, handling of the contact lug that is fastened at the stack is simplified with a mechanically reinforcing terminal element such as this.
The terminal element, which is constructed as a terminal pin, can also be part of an electrical plug connection. This enables an extremely simple connection to an electrical voltage source.
The actuator preferably comprises a pressure plate at a face of the stack. This has openings through which the terminal elements are inserted or guided. In mechanically reinforced terminal elements, the openings are designed such that a guidance and retaining of the terminal elements is guaranteed.
The pressure plate preferably has a recess for fixing the stack. This enables a secure installation of the stack, including the contact lug, in an actuator housing, whereby the stack is securely oriented and centered.
In the pressure plate, other elements of the actuator can be integrated, such as force sensors, temperature sensors, other sensors (e.g. Hall sensors) or a second actuator as an adjusting element for r
Hamann Christoph
Hekele Wilhelm
Scherer Clemens
Schuh Carsten
Budd Mark O.
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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