EP1932190A1

EP1932190A1 - Piezoactuator with protective resistor

Info

Publication number: EP1932190A1
Application number: EP06778246A
Authority: EP
Inventors: Thomas Pauer; Friedrich Boecking
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2005-09-27
Filing date: 2006-08-15
Publication date: 2008-06-18
Also published as: WO2007036392A1; US20100148628A1; DE102005046118A1; CN101273476A

Abstract

Piezoactuator comprising a multiplicity of piezolayers (3), between each of which a layer electrode (5; 6) is arranged, the layer electrodes (5; 6) being alternately connected to a respective connection electrode (8; 9; 8'; 9'; 8'; 9'). A protective resistor (16; 16'; 16') is provided between at least one connection electrode (8; 9; 8'; 9'; 8'; 9') and the layer electrodes (5; 6) connected to said connection electrode (8; 9; 8'; 9'; 8'; 9').

Description

Piezo actuator with protective resistor

The invention relates to a piezoelectric actuator, as it can be used, for example, as an adjusting device in injection systems for internal combustion engines.

State of the art

Ceramic piezoactuators are known from the prior art, in which the piezoelectric effect is utilized for the movement of components. For example, the patent DE 199 28 177 C2 shows a piezoelectric actuator, which consists of a ceramic piezoelectric body. The piezoelectric body is composed of a plurality of piezoelectric layers, between each of which a layer electrode is arranged. In this case, the layer electrodes are mutually connected to a connection electrode, so that directly adjacent layer electrodes are connected to respectively different connection electrodes. If a direct electrical voltage is applied between the two connection electrodes, an electric field is created between the layer electrodes. This causes the piezo layers to change in thickness, so that overall the length of the piezoelectric actuator changes. As a result, the piezoelectric actuator can be used as an actuator, eg in fuel injection systems, depending on the set voltage. Due to the plurality of layer electrodes, which have only a small distance, a very high electric field can be applied without having to use an excessively high electrical voltage. This allows to effect a large stroke of the piezoelectric actuator with a relatively low control voltage. However, the short distance of the layer electrodes from each other, which is usually in the range of 50 to 100 microns, but also represents a weak point of this piezoelectric actuator concept. The layer electrodes are made of a metal, such as silver or silver palladium, and this metal has a certain diffusion mobility within the ceramic piezolayers. Thus, it can happen over time that a line bridge develops between two adjacent layer electrodes as a result of diffusion of metallic layer electrode material, and thus a short circuit between the two connection electrodes. Since the connection electrodes are usually electrically insulated from each other, now a very high current flows through this line bridge, which means on the one hand a voltage loss and on the other hand to a strong warming in this

Area leads. This heating leads to an increase in the damage and can ultimately lead to a destruction of the piezoelectric actuator.

Advantages of the invention

The piezoelectric actuator according to the invention, on the other hand, has the advantage that it still works even when a short circuit occurs between two layer electrodes. This is achieved by providing protective resistors between at least one of the terminal electrodes and the respective layer electrodes. These are dimensioned so that when a line bridge between two layer electrodes is formed, the now flowing high leakage current can melt through the respective protective resistor. As a result, the electrical connection between the defective layer electrode and the affected connection electrode is interrupted, so that the corresponding layer electrode is no longer connected to the connection electrode and thus the interposed piezoelectric layer is no longer exposed to the electric field, but the remaining layer electrodes continue to function , Since piezo actuators, such as those used in injectors for direct-injection internal combustion engines, have a few hundred piezo layers and thus layer electrodes, this plays virtually no role in the overall stroke of the piezoelectric actuator, so that it also controls the injector without problems even if some layers fail can.

By the dependent claims advantageous developments of the subject invention are possible. In an advantageous embodiment of the respective protective resistor is formed within the layer electrode, so that the connection electrodes and the other geometry of the piezoelectric actuator does not need to be changed. In an advantageous manner the protective resistance is arranged within the layer electrode and is, for example, strip-shaped, wherein the strip is formed relatively close to the connection electrode or at the edge of the layer electrode towards the connection electrode. The strip consists of a material which has a corresponding electrical resistance and thus forms the protective resistor. It can be one or a plurality of

Be provided strip.

Advantageously, the protective resistor can be formed by a granular, piezoactive material, wherein the grains are coated with a metal layer. The electrical conduction comes through within this granular, piezoactive material

Metal coating, wherein over the layer thickness of the metal layer, the size of the resistor is adjustable. If the current through this protective resistor exceeds a certain level, the metal with which the grains are coated melts and becomes flowable so that the electrical resistance is finally interrupted. Here, the metal coating of the grains is preferably made of the same material from which the layer electrodes.

The protective resistor within the layer electrode can also be formed by resistance bridges, so that in one strip, one or more resistance bridges are formed, which form the protective resistance. Over the width and length of these resistance bridges, the protective resistance can also be adjusted. It is also possible to form the protective resistors in that metallic resistance bridges are provided between the connection electrodes and the layer electrode, the width of which is chosen such that the electrical resistance lies in the desired range. Such an arrangement is particularly advantageous in cylindrical piezoelectric actuators, in which the connection electrodes run in the interior. Here, radially extending, bar-shaped connections to the layer electrodes can be provided, which form the protective resistor.

In a further advantageous embodiment, the terminal electrodes are designed as a helically wound wire, which produce a connection with the layer electrodes in the interior of the piezoelectric actuator. In this case, the helically wound wire can have such a punctiform contact with the layer electrodes that a suitable protective resistance is thereby formed. - A -

drawing

In the drawing, various exemplary embodiments of the piezoelectric actuator according to the invention are shown. It shows

FIG. 1 shows a piezoactuator with a rectangular cross section, as known from the prior art,

FIG. 2 shows an equivalent circuit diagram for a piezoactuator according to the invention, which has corresponding protective resistors,

Figure 3a and

FIG. 3b show two adjacent layer electrodes of a piezoelectric actuator according to the invention with a rectangular cross-section,

Figure 4a and

FIG. 4b likewise shows two adjacent layer electrodes of a rectangular piezoelectric actuator, wherein the protective resistors are designed differently here,

FIG. 5 shows an enlarged view of a protective resistor, as may be provided inside the layer electrode,

FIG. 6 shows a further exemplary embodiment in which the piezoactuator has a rectangular cross-section, but cylindrical connecting electrodes,

7 shows a cross section through a piezoelectric actuator according to FIG. 6, FIG.

8 shows a cylindrical piezoelectric actuator with internal connection electrodes, FIG.

Figure 9a and

FIG. 9b show two layer electrodes of the cylindrical piezoactuator according to FIG. 8,

FIG. 10 shows, as a further exemplary embodiment, a layer electrode, as can be used in a cylindrical piezoactuator,

FIG. 11 shows an internal connection electrode, as can be used in a cylindrical piezoactuator,

FIG. 12 shows a cylindrical piezoactuator with two layer electrodes shown by way of example and a helical connection electrode which is used for such a piezoactuator.

FIG. 13 shows the helical connection electrode with a coating,

FIG. 14 shows a section through the connection electrode according to FIG. 13 along the lines A - A and

FIG. 15 shows a further cross section corresponding to the representation according to FIG. 14, in which corresponding protective resistors are provided. Description of the embodiments

FIG. 1 shows a piezoactuator 1, as known from the prior art and having a rectangular cross-section, wherein the edges are chamfered. The piezoelectric actuator 1 has a plurality of piezoelectric layers 3, which are made of a piezoactive ceramic material. Between the piezoelectric layers 3, a metallic layer electrode 5, 6 is provided in each case, wherein the respectively adjacent layer electrodes 5, 6 are electrically isolated from each other by the piezoelectric layers 3 lying between them. One half of the layer electrodes 5 is connected to a first connection electrode 8, while the respectively adjacent connection electrodes 6 are connected to a second connection electrode 9. The connection electrodes 8, 9 are in this case applied to the surface of the piezoelectric actuator 1 and electrically conductively connected to the respective layer electrodes 5, 6. The connection electrodes 8, 9 have such great flexibility that, despite the change in length of the piezoactuator 1, an electrical connection to the respective layer electrodes 5, 6 always remains. The connection electrodes 8, 9 are connected to electrical terminals 11, 12, with which an electrical voltage can be applied between the connection electrodes 8, 9.

The application of an electrical voltage between the connection electrodes 8, 9 produces an electric field between the layer electrodes 5, 6, which passes through the piezoelectric layers 3. Depending on the height of the electrical voltage and thus of the electric field, the thickness of the piezoelectric layers 3 and thus the entire length of the piezoelectric actuator 1 changes. This allows to move a corresponding actuator very quickly and also very precisely with the piezoactuator 1.

The layer electrodes 5, 6 consist of a metal, for example silver or silver palladium, wherein this metal has a certain mobility within the ceramic from which the piezo layers 3 consist. In particular, during prolonged operation of the piezoactuator 1, this can lead to metal from the layer electrodes 5, 6 dissolves and a line bridge 14 between two adjacent layer electrodes 5, 6 forms. Such a line bridge 14 leads to a short circuit between two adjacent layer electrodes 5, 6, so that now a correspondingly high current flows through the line bridge 14. This leads locally to a strong heating of the piezoelectric actuator 1 and thus to a melting of the metallic layer electrodes 5, 6, which ultimately leads to a destruction of the piezoelectric actuator 1. To prevent this destruction of the piezoelectric actuator 1, the invention provides protective resistors 16 between the terminal electrode 5, 6 and the piezoelectric layer 3 to be arranged, which serve as a fuse. FIG. 2 shows an equivalent circuit diagram in which the layer electrodes 5, 6 and the connection electrodes 8, 9 are shown. Between the terminal electrode 9 and the layer electrodes 6, a protective resistor 16 is provided which is dimensioned such that it limits the current on the one hand, if between two layer electrodes 5, 6, a line bridge 14 is formed, on the other hand melted at a high current through the line bridge 14 and so interrupts the electrical contact between the defective layer electrode 6 and the connection electrode 9. As a result, the electrical connection between the line bridge 14 and the connection electrode 9 is interrupted, so that the defective layer electrodes 5, 6 are no longer connected to the power supply. The interposed piezoelectric layer 3 has due to the lack of electric field no change in thickness more, but at typically 200 piezo layers 3 of the failure of individual piezo layers for the overall function of the piezoelectric actuator 1 without significant importance.

FIG. 3 a shows a layer electrode 6 according to the invention, in which the protective resistor 16 is formed as a strip within the layer electrode 6. FIG. 3b shows the adjacent layer electrode 5, which is connected directly to the connection electrode 8 without a protective resistor. Since the protective resistor 16 is integrated here directly into the layer electrode 6, there is no change in the piezoelectric actuator 1 in terms of its geometric dimensions, so that both the terminal electrodes 8, 9 and the installation conditions need not be changed.

FIG. 4 a shows an alternative embodiment of the layer electrode 6 with protective resistors 16, which are formed here by resistance bridges 116. These still consist of the granular piezoactive material, with the advantage that over the width of the resistance bridges 116 there is an additional dimensioning possibility for the protective resistor. FIG. 4b shows, in an alternative embodiment of the layer electrode according to FIG. 3a, a protective resistor 16, which is formed here at the edge of the layer electrode 6, is connected directly to the connection electrode 9. However, the mode of operation of the protective resistor 16 is here identical to the protective resistor as shown in FIG. The protective resistor 16, which is provided as a strip in the layer electrode 6, can be formed, for example, by granular piezoactive material, as shown in FIG. 5 in an enlargement. The individual grains 18 are provided with a metal coating 20, which forms the electrically conductive path within the layer electrode 6. If there is now an excessively high current through this metal coating 20, it melts and the metal moves within the protective resistor 16 so that after a certain time it separates the layer electrode 6 from the connection electrode 9.

FIG. 6 shows a further piezoactuator 1, which likewise has a rectangular cross section. The connection electrodes 8 ', 9' are here designed as metal tubes, which protrude into a semicircular recess in the piezoelectric actuator 1. Figure 7 shows a cross section through the piezoelectric actuator according to Figure 6, wherein a layer electrode 6 is shown. The connection electrode 9 'is connected via protective resistors 16' to the layer electrode 6, which are formed by bar-shaped connections. Depending on

Number and width of the protective resistors 16 'forming ridge-shaped connections results in a more or less large protective resistance between the terminal electrode 9' and the layer electrode 6. The layer electrode 6 is here separated electrically from the second connection electrode 8 ', while the above or below Layer electrode 5 is connected to the connection electrode 8 'in the known manner.

FIG. 8 shows a piezoactuator 1 which has a circular cross section. In the piezoactuator 1, two bores 17, 19 are formed, in each of which a connection electrode 82 ', 9 "is arranged, which are connected in a known manner alternately to the layer electrodes 5, 6, the protective resistor 16" being formed hereby. that the connection electrode 9 "is enveloped by a material with a corresponding electrical resistance, by which ultimately the electrical contact between the connection electrode 9" and the layer electrodes 5 comes about. 9a shows a layer electrode 6, which is connected to the connection electrode 9 ", The layer electrode 6 has two recesses 22, 23, so that no electrical connection between the connection electrode 8" and the layer electrode 6 is formed. By means of the material surrounding the connection electrode 9 ", a protective resistor 16" is formed, via which the layer electrode 6 is connected to the connection electrode 9 "The material which surrounds the connection electrode 9" can also be, for example, a grain of metal-coated Ceramic grains, as shown in Figure 5. It However, other materials are conceivable that have a correspondingly high resistivity.

9b shows the layer electrode 5 located above or below it, which is connected to the connection electrode 8 ", which is not connected to the connection electrode 9" by a correspondingly large-dimensioned recess 23 '.

FIG. 10 shows a further exemplary embodiment of a layer electrode 6, as may be provided in a round piezoelectric actuator. The protective resistor 16 'is formed here by resistance bridges, which are provided within the layer electrode 6 and via which the electrical connection to the connection electrode 9 "is formed, in which case the covering of the connection electrodes 9" is omitted. The correspondingly above or below layer electrode 5 is connected to the connection electrode 8 "and insulated from the connection electrode 9".

11 shows a further exemplary embodiment, in which the protective resistor is integrated into the connection electrodes 9 '' .The connection electrode 9 'here consists of a metallic tube 109 which is surrounded by a metal coating 25 which has radially outwardly pointing web-like protuberances form the protective resistor 16 '. Between the protective resistors 16 'electrically insulating, preferably ceramic material 27 is provided, so that the metallic tube 109 is finally connected via the protective resistors 16' with the respective layer electrode 5, 6. The circular piezoelectric actuator 1 can thus be constructed in the known manner and connected to the connection electrodes 8, 9, wherein only one of the connection electrodes must be replaced by a connection electrode 9 "according to FIG.

FIG. 12 shows a further exemplary embodiment of a piezoactuator 1 with a circular cross section. The piezoelectric actuator 1 has two bores 17, 19, which receive the connection electrodes. By way of example, two layer electrodes 5, 6 are shown here, which in a known manner are alternately pulled through to the wall of the bores 17, 19 and can thus be contacted at this point. In the holes 17, 19, a spring electrode 30 is inserted, which consists of a helically wound wire. In order to form the contact resistors 16, the helical spring electrode 30 is coated with a ceramic layer 32, as shown in FIG. In cross-section, as shown in FIG. 14, one can see the ceramic coating 32, which on all sides forms the helical spring. the electrode 30 surrounds. In order to form a corresponding contact resistance, which serves as a protective resistor 16, the ceramic coating 32 is removed with a planar polished section 34, so that a blank position of the width D is formed, as shown in FIG. As a result, the helical spring electrode 30 contacts the respective layer electrodes 5, 6 in a punctiform manner, which, when appropriately dimensioned, produces a contact resistance which serves as a protective resistor 16.

The protective resistors 16 should preferably be dimensioned so that they heat and melt correspondingly in the event of an excessive current, even before the current flowing in the event of a short circuit between two layer electrodes 5, 6 becomes one

Destruction of the piezoelectric actuator 1 leads. In addition to the execution of the protective resistors 16 within the layer electrodes 5, 6 by a granular ceramic material which is metallically coated, it is also possible, for example, to dope the layer electrode 5, 6 in a region in order to obtain a corresponding electrical resistance there ,

Claims

claims

1. Piezoelectric actuator with a plurality of piezoelectric layers (3), between each of which a layer electrode (5; 6) is arranged, wherein the layer electrodes (5; 6) are alternately provided with in each case one connection electrode (8; 9; 8 '; 9'; 8 "; 9"), characterized in that between at least one connection electrode (8; 9; 8 '; 9'; 8 "; 9") and with the connection electrode (8; 9; 8 '; 9'; 8 "; 9") associated layer electrodes (5; 6) protective resistors (16; 16 '; 16 ") are provided.

2. Piezoelectric actuator according to claim 1, characterized in that the layer electrodes (5;

6) is formed by a metal layer between each two piezo layers (3).

3. Piezoelectric actuator according to claim 1 or 2, characterized in that the protective resistor (16, 16 ', 16 ") is arranged within the layer electrode (3).

4. Piezoelectric actuator according to claim 3, characterized in that the protective resistor (16, 16 ', 16 ") forms at least one strip within the layer electrode (3).

5. Piezoelectric actuator according to claim 3 or 4, characterized in that the protective resistor (16, 16 ', 16 ") is formed by granular, piezoactive material, wherein the grains are coated with a metal coating.

6. Piezoelectric actuator according to claim 5, characterized in that the metal coating of the grains consists of the same material as the metallic layer electrodes

(3).

7. Piezoelectric actuator according to claim 3, characterized in that the protective resistor (16, 16 ', 16 ") is formed by one or more resistance bridges (16').

8. Piezoelectric actuator according to claim 1, characterized in that the metallic connection electrodes (8 ', 9', 8 ", 9") are of rod-shaped design.

9. Piezoelectric actuator according to claim 8, characterized in that at least one connection electrode (8 ', 9', 8 ", 9") is connected to the associated layer electrodes (5, 6) by substantially radially extending, web-shaped connections, wherein the bar-shaped connections form the protective resistors (16, 16 ', 16 ").

10. Piezoelectric actuator according to claim 8, characterized in that at least one connection electrode (8 ', 9', 8 ", 9") runs in the interior of the piezoactuator (1).

11. Piezoelectric actuator according to claim 1, characterized in that the piezoelectric actuator (1) has at least one spring electrode (30) as connection electrode, which is designed as a helically wound wire and in the interior of the piezoactuator (1) in a receiving bore (17; ) are arranged.

12. Piezoelectric actuator according to claim 11, characterized in that the protective resistors (16) by the point-like contact between the spring electrode (30) and the layer electrodes (5; 6) is formed.

13. Piezoelectric actuator according to one of the preceding claims, characterized in that the protective resistors (16, 16 ', 16 ") are heated when a maximum permissible current is exceeded to such an extent that they melt through and thereby interrupt the flow of current.