DE102009020238B4 - Piezoelectric actuator with electrical contact pins and method for contacting a piezoelectric actuator - Google Patents

Abstract

Piezoelectric actuator (10) in multilayer construction, in which piezoelectric layers and electrode layers (20) are arranged alternately stacked to one another, wherein the electrode layers (20) on opposite sides of the stack out of this and electrically via an outer electrode (18) with a contact pin ( 14), wherein an electrical connection between the outer electrode (18) and the contact pin (14) via a conductive intermediate element is made, wherein the conductive intermediate element is a wire spiral (16), wherein the wire spiral (16) a Pin receptacle (22), in which the contact pin (14) is received, wherein the pin receptacle (22) has a plurality of rings which firmly surround the contact pin (14) and hold stable, the diameter of the rings are smaller than the diameter of the wire spiral (16).

Description

A multilayer-type piezoactuator is described in which piezoelectric layers and electrode layers are arranged alternately one above the other to form a stack, wherein the electrode layers lead out of the stack on opposite sides of the stack and are electrically connected to a contact pin via an outer electrode. When an electrical voltage is applied to the internal electrodes, the stack expands perpendicular to the electric field resulting from the applied voltage.

By means of such a piezoelectric actuator, for example, a valve piston of a control valve is actuated, which serves as an injection valve in a motor vehicle.

A piezoelectric actuator is for example in the WO 2005/035971 A1 , of the DE 196 48 545 A1 and the DE 199 45 933 C1 described. Furthermore, from the publication WO 2005/047 689 A1 a piezoelectric actuator with a Kontaktierungsmatte forth, having a plurality of juxtaposed wires. From the publication DE 102 36 986 A1 goes out a piezoelectric actuator with a helical outer electrode to which a contact pin is welded.

It is an object to be solved to provide a piezoelectric actuator, which has a high reliability and can be produced with the simplest possible manufacturing process.

This object is solved by the subject matters of the independent claims.

Advantageous embodiments and modifications of the invention will become apparent from the dependent claims.

It is a piezoelectric actuator in multilayer construction specified, in which piezoelectric layers and electrode layers are arranged alternately stacked, the electrode layers on opposite sides of the stack lead out of this and electrically connected via an outer electrode to a contact pin, wherein an electrical connection between a the outer electrodes and a contact pin is made via a conductive intermediate element. The second outer electrode can be connected to a contact pin in the same way.

It is a piezoelectric actuator specified, which has a plurality of thin films of piezoceramic, which are arranged to form a stack. These films are made, for example, from lead zirconate titanate. Between the layers of piezoceramic lie electrode layers. However, it is not absolutely necessary to apply an electrode layer to each film of piezoceramic. Depending on the desired voltage-dependent behavior of the piezo stack, for example, only every second layer of piezoceramic can be provided with an electrode layer. In order to form these electrode layers, a metal paste, for example, a silver palladium paste or a copper-containing paste may be applied to the films by a screen printing method.

Furthermore, it must be ensured in the production of a piezo stack that the electrode layers do not extend over the entire surface of a film of piezoceramic in a piezo stack in which the electrode layers alternately lead out of the piezo stack. The respective free surfaces on a film of piezoceramics, in which adjacent electrode layers do not overlap in the stacking direction, are called inactive zones. The areas provided with an electrode layer which have an overlap with electrode layers adjacent in the stacking direction are referred to as the active zone.

A construction of a piezo stack, in which active and inactive zones are formed, allows, for example, a common connection of all electrode layers with the same polarity.

A disadvantage of a piezo stack construction, in which active and inactive zones are formed, is that tensions and breakages often occur at the transitional areas between active and inactive zones.

After a stack has been formed from the films with the applied electrode layers, the outer electrodes are applied to two opposite outer surfaces of the piezo stack. An outer electrode is made, for example, by means of a stoving paste applied to the outside of the stack by a screen printing process. Then it is sintered with the piezo stack. In order to conductively connect these external electrodes to a contact pin and via this to an external voltage source, it is possible, for example, to use what is known as a wire harp.

Such a wire harp connects the electrode layers via the outer electrode to the contact pin by means of a thin wire. This electrically conductive wire is wound in a plurality of turns around the piezo stack and the terminal pins. Subsequently, this is soldered to the outer electrode and the contact pins. In a further process section, the existing wire connections between the two outer electrodes are severed and the connection is complete to completely isolate the two external electrodes.

After the two outer electrodes are completely isolated from each other, a passivation can be applied to protect against external influences. In addition, a film may be applied to the edges of the piezo stack as edge protection. Subsequently, this body can be put in sleeves and shed with plastic such as silicone.

Since this lengthy process can only be automated to a limited extent, instead of a wire harp, a conductive intermediate element is used which consists of as few individual parts as possible, preferably of a single part, and at the same time is stable against stresses such as occur in a piezo stack described above.

Preferably, the intermediate element extends over the entire length of the outer electrode.

The fact that the outer electrode is extended over the entire length of the piezo stack, all electrode layers of the same polarity are contacted together. In a subsequent step, the conductive intermediate element is attached to the outer electrode. The intermediate element has the task of ensuring a stable connection between the outer electrode and a contact pin.

For example, when using a cylindrical wire spiral as an intermediate element, each turn of the wire spiral can be soldered to the outer electrode. If, for example, a contact point between the intermediate element and the outer electrode tears off in the operation of the piezoelectric actuator, the piezoelectric actuator remains operational, since the voltage supply can be maintained over the other turns.

The conductive intermediate element has a cylindrical shape.

According to the invention, the conductive intermediate element is a wire spiral.

By attaching a cylindrical wire spiral as an intermediate member, it is possible to make an electrical connection at a plurality of locations on the outer electrode.

Preferably, the wire spiral used is elastic. This contributes to a transmission of vibrations is reduced to the contact pin, because the wire spiral due to their elastic properties can dampen the strains and vibrations of the piezo stack. The contact pin may, for example, be soldered to this wire spiral and thereby remains firmly connected to the outer electrode, and thus the stack, and is thus held in position.

An elastic intermediate element, which can expand and contract together with the piezo stack, is particularly advantageous if, for example, an AC voltage is applied to the piezo stack and the stack then expands or contracts with each change of voltage. As a result of this dynamic load, cracks may form on the outside of the piezo stack, which, if an inelastic intermediate element is used, may in places tear off individual contact points or damage the outer electrode. In the worst case, this would lead to a complete failure of the piezoelectric actuator. For example, if the piezoactuator actuates a valve piston of a control valve which serves as an injection valve in a motor vehicle, then the piezoactuator must be exchanged immediately in order to ensure further operation of the injection valve and thus of the engine.

In a further advantageous embodiment, the number of turns per unit length of the spiral and the number of electrode layers per unit length of the piezo stack should not differ from each other by more than 20%.

A spiral has a certain number of turns. These turns of the spiral are connected to the electrode layers via the outer electrode. So that the wire spiral can optimally adapt to the length of the stack, it must be stretchable between the contact points and have a spring constant adapted to the required load capacity.

Since the expansion of the stack when applying a voltage is proportional to the number of layers of piezoceramic, it is necessary to adapt the spring constant of the wire spiral to this situation. This can be achieved, for example, by adjusting the turns per unit length, by using another material for the spiral, or by changing the shape of the spiral.

Should it happen when using the piezoelectric actuator and the dynamic load occurring during operation to cracks or to release a contact on a winding, the electrical connection still exists over a contact point on another turn. In this way, a long life of such a piezoelectric actuator can be achieved because a permanent and reliable connection to an external electrical connection is made.

According to the invention, a pin receptacle for receiving the contact pin is provided on the intermediate element.

By means of this pin holder, the contact pin can be firmly connected to the intermediate element, and thus to the piezo stack. Likewise, the contact pin is always conductively connected to the piezo stack and is located at the same distance from the stack.

Usefully, it may be provided that the pin receptacle comprises a plurality of elements.

The advantage is that the arrangement of the contact pins with respect to the piezo stack is more stable and the pins can not move in the course. The more points the contact pin is connected to the intermediate element, the less likely it is that the electrical contact between the contact pin and the intermediate element is interrupted by the stresses occurring in the piezoelectric stack. Since the piezo stack with the attached intermediate element and contact pin is preferably potted in the further course of production, it is important that the contact pins are located at a fixed distance from the piezo stack and that they have a fixed orientation with respect to the piezo stack.

This alignment with respect to the piezo stack can be achieved by using an intermediate element which has the necessary rigidity and strength to prevent a rotation or displacement of the contact pins with respect to the piezo stack.

According to the invention, the pin receptacle comprises a plurality of rings.

The rings of the pin holder tightly enclose the contact pin. It is also possible that the contact pin closely enclosing rings are soldered to this. Thus, the contact pin is mechanically held in the immediate vicinity of the intermediate element. The rings of the pin holder can be attached to both the intermediate element, as well as to the contact pins.

Furthermore, it can also be provided that the pin receptacle comprises a plurality of elements, and these are arranged at a distance from each other.

Should the contact pin be fastened at one or more locations directly adjacent, then a torque applied to the contact pin could cause the contact pin in question to no longer have the proper orientation with respect to the piezostack used for later connection to the voltage source is necessary. If, however, the contact pin is fastened at several points which are spaced apart from each other, a rotation of the contact pin with respect to the piezo stack can be avoided.

Furthermore, it can be provided that the piezo stack, the intermediate element and the contact pin are arranged parallel to each other.

It is particularly preferred that a contact pin is held by means of an intermediate element at a constant distance from the piezo stack.

The arrangement in this position and at a certain distance is advantageous for the subsequent electrical contacting. In order to use the finished piezoelectric actuator, it is advantageous if the pins fit into predetermined connections. This can be achieved by the contact pins are arranged at a fixed distance from each other and have a fixed orientation to each other and to the piezo stack. This can be achieved for example by means of a cylindrical wire spiral.

In order to contact a piezo stack, a contact pin is attached to an intermediate element in a first step. This intermediate element is then brought, with the contact pin attached thereto, to the piezo stack and soldered there to the outer electrode of the piezo stack.

By means of a suitable intermediate element, it is possible to automate the contacting process. The contact pins and the intermediate elements can be produced in advance and then only have to be attached to the outer electrode of the piezo stack. The invention will now be explained by way of example with reference to the accompanying drawings by means of particularly preferred embodiments.

It shows:

1a a piezoelectric actuator with contact pins connected to the electrode layers via an intermediate element,

1b a top view of a piezoelectric actuator with a via an intermediate element connected to the electrode layers pins,

2a a wire spiral with attached pin holder and a contact pin,

2 B a wire spiral with attached pin holder and a contact pin mounted in the pin holder,

2c How to connect two wire spirals with a pin attached to the pin holder.

1a schematically shows a side view of a piezoelectric actuator 10 , The electrode layers are clear 20 to recognize which mutually from the piezo stack 12 lead out. On the right and left side of the piezostack 12 is the outer electrode 18 applied, wherein in the representation used here only the right outer electrode 18 is visible.

A cylindrical wire spiral 16 is conductive via several contact points with the outer electrode 18 connected, so that on this particular side of the piezostack 12 leading out electrode layers 20 electrically connected to the outside can be connected. Such a connection is made via the on the cylindrical wire spiral 16 attached contact pins 14 produced.

These pins 14 are each surrounded by two spaced-mounted rings which serve as a pin recording. By means of these contact pins 14 can apply a voltage to the piezo stack 12 be created. Because the rings do not match the contact pins 14 are soldered, but only close tightly with them, the entire construction is stable but still flexible.

In 1b is a top view of the in 1a described Piezostacks 12 to see. However, this is in addition of a potting compound 26 wrapped for stability serves as well as protection against contamination. In the middle of the drawing is the piezo stack 12 to see. On the side surfaces of the stack is the outer electrode 18 , from a stoving paste, which is screen printed on the piezo stack 12 was applied, shown.

Also are the cylindrical wire spirals 16 to see which ones with the contact pins 14 are connected. The diametrically opposite the contact pins part of the cylindrical wire spiral 16 is with the outer electrode 18 via different contact points 24 connected. The whole construction is with a potting compound 26 , such as silicone, surrounded.

2a shows a first step to a connection between an outer electrode 18 and an external applied electrical power source. This will be a cylindrical wire spiral 16 which has a pin holder 22 features, with a contact pin 14 connected.

2 B shows a cylindrical wire spiral 16 which is connected to a spring pin. The pin receptacle is designed in the illustrated case of two spaced-apart rings, which firmly surround the contact pin and hold it so stable. This cylindrical wire spiral 16 with the attached contact pin 14 It is very easy to produce, cheap to produce and can be prefabricated to be attached to the piezo stack in a single step.

2c then shows a piezo stack with mutually out of the piezo stack leading out electrode layers and applied on opposite sides outer electrodes 18 , To these outer electrodes 18 Now each is an already prefabricated cylindrical wire spiral 16 , which with a contact pin 14 connected, introduced. Subsequently, the contact pins 14 diametrically opposite part of the cylindrical wire spiral 16 over the entire length of the piezostack 12 and thus the outer electrode 18 by vapor phase soldering on the outer electrode 18 attached. This piezostack 12 with the two attached cylindrical wire spirals 16 and the contact pins 14 , then can still with a potting compound 26 , such as silicone, shed.

LIST OF REFERENCE NUMBERS

10

piezo actuator

12

piezostack

14

pin

16

cylindrical wire spiral

18

outer electrode

20

electrode layer

22

pin receptacle

24

contact points

26

sealing compound

Claims (9)

Piezoelectric actuator ( 10 ) in multilayer construction, in which piezoelectric layers and electrode layers ( 20 ) are arranged alternately one above the other to form a stack, wherein the electrode layers ( 20 ) on opposite sides of the stack lead out of this and via an outer electrode ( 18 ) electrically with a contact pin ( 14 ), wherein an electrical connection between the outer electrode ( 18 ) and the contact pin ( 14 ) is made via a conductive intermediate element, wherein the conductive intermediate element is a wire spiral ( 16 ), wherein the wire spiral ( 16 ) a pen recording ( 22 ), in which the contact pin ( 14 ), wherein the pen holder ( 22 ) has a plurality of rings which the contact pin ( 14 ) and firmly hold, wherein the diameter of the rings are smaller than the diameter of the wire spiral ( 16 ). Piezoelectric actuator ( 10 ) according to claim 1, wherein the intermediate element extends over the entire length of the outer electrode ( 18 ). Piezoelectric actuator ( 10 ) according to claim 1 or 2, wherein the conductive intermediate member has a cylindrical shape. Piezoelectric actuator ( 10 ) according to claim 3, wherein the number of turns of the wire spiral ( 16 ) per unit length and the number of electrode layers ( 20 ) do not differ by more than 20% per unit of length of the stack. Piezoelectric actuator ( 10 ) according to one of claims 1 to 4, wherein the pin receptacle ( 22 ) has a plurality of elements which are arranged spaced from each other. Piezoelectric actuator ( 10 ) according to one of the preceding claims, wherein the stack, the intermediate element and the contact pin ( 14 ) are arranged parallel to each other. Piezoelectric actuator ( 10 ) according to one of the preceding claims, wherein the intermediate element is adapted to the contact pin ( 14 ) at a constant distance from the stack. Method for contacting a piezoactuator ( 10 ), which comprises the following steps: - fixing a contact pin ( 14 ) on an intermediate element, wherein the intermediate element is a wire spiral ( 16 ) and wherein the contact pin ( 14 ) in a pen holder ( 22 ) the wire spiral ( 16 ), the pen recording ( 22 ) has a plurality of rings, which the contact pin ( 14 ) and firmly hold, wherein the diameter of the rings are smaller than the diameter of the wire spiral ( 16 ), - bringing the intermediate element with the contact pin attached thereto ( 14 ) to the piezo stack ( 12 ) - soldering the intermediate element including the contact pin ( 14 ) on the outer electrode ( 18 ) of the piezostack ( 12 ). Method according to claim 8, wherein the soldering of the intermediate element to the outer electrode ( 18 ) by vapor phase soldering.

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