Abstract:
A piezoelectric actuator has at least one monolithic piezoelement of a piezoelectric material, having a tubular form and a penetrating bore extending along a longitudinal direction surrounded by a tubular wall. An inner electrode is provided on the inside of the tubular wall of each piezoelement. Correspondingly, an outer electrode is provided on the outside of the tubular wall of each piezoelement. When an electric voltage is applied between the inner electrode and the outer electrode, an electrical field is formed that is essentially aligned perpendicularly to the longitudinal direction of the tubular piezoelement, the electrical field effecting a deformation of the piezoelement in its longitudinal direction in order to exert an actuation force in the longitudinal direction of the piezoelement.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a piezoelectric actuator, which can be used, in particular, for actuating a fuel injector. 
     BACKGROUND INFORMATION 
     In a fuel injector known from German Patent No. DE 35 33 085 A1, a piezoelectric actuator is provided for actuating a valve-closure member to seal off a spray orifice, the valve-closure member being operatively connected with force-locking by way of a transfer bolt to a valve needle coupled to the valve-closure member. The piezoelectric actuator is comprised of a plurality of piezoelements arranged one behind the other in a stack. The piezoelements have a disk shape and, on both disk surfaces, have electrodes capable of receiving an electric voltage. When voltage is applied, each of the disk-shaped piezoelements, arranged one behind the other in a stack, expands in the direction of the electrical field being formed between the electrodes. 
     Since the expansion of the piezoelements used for the valve lift is in the direction of the electrical field, the electrodes are arranged, of necessity, on surfaces extending normal to the expansion direction. To achieve a high enough electrical field strength in each piezoelement, the layer thickness of the piezoelements must not be too great. Therefore, to achieve sufficient valve lift, it is necessary for a large number of thin piezoelements to be disposed in a stack, one behind the other, and to be pressed against one another with a suitable mechanical prestressing in the expansion direction. Since the electrodes run parallel to the contact surface where the individual piezoelements lie together contiguously, a monolithic manufacturing of the piezoelectric actuator is not possible. 
     Fuel injectors having a hydraulic displacement transformer are described in German Patent No. DE 43 06 073 C1 and German Patent No. DE 195 00 706 A1, where a relatively small displacement of a working piston coupled to the piezoelectric actuator is transformed into a substantially larger displacement of a reciprocating piston coupled to the valve-closure member. In this case, the piezoelectric actuators are comprised of a number of stacked piezoelements. 
     A drawback of the known piezoelectric actuators is that a costly manufacturing process is required to produce them. An additional drawback is that tensile stress may not be applied to the piezoelectric actuators, since this would entail the risk of the electrodes that are vapor- or sputter-deposited onto the piezoelectric crystals becoming detached. To counteract this, a mechanical compressive prestressing of the known piezo-actuators is necessary, requiring additional components. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an actuator having piezoelements that are inexpensive and uncomplicated to produce. An extrusion molding method, e.g., which would enable the piezoelements to be produced with a relatively substantial structural length as a monolith, presents itself here. 
     The outer electrode and the inner electrode of the piezoelement can be easily applied inside and outside of the tubular wall of the tubular piezoelement over a large surface, e.g., by means of sputtering, vapor deposition, a CVD process, or another known coating process. Since the electrodes are disposed parallel to the actuating direction of the actuator, and there is no need for electrodes disposed transversely to the actuating direction, it is possible to produce the actuator from a monolithic, piezoelectric crystal that is not interrupted by electrodes. 
     In contrast to related-art actuators, the actuator of the present invention can also receive a tensile stress, since when working with the electrode configuration of the present invention, there is no risk of the electrodes becoming detached from the piezoelectric crystal in response to a tensile stress. Therefore, in the case of the actuator of the present invention, one can eliminate the special components required in the related art to mechanically prestress the actuator, making it possible to further reduce manufacturing and assembly costs. 
     When a plurality of piezoelements designed in accordance with the present invention are used, e.g., two piezoelements mechanically connected in series, the lift of the piezoelectric actuator can be increased at a given structural length, i.e., the length of the piezoelectric actuator can be shortened at a given lift. The piezoelements designed in accordance with the present invention utilize the so-called d 31  effect (described later in greater detail), while the piezoelements known from the related art utilize the so-called d 33  effect (likewise described later in greater detail). Given the same mechanical length of the piezoelectric actuator, the lift attainable by utilizing the d 31  effect is only roughly half of the lift attainable by utilizing the d 33  effect. However, by using two piezoelements designed in accordance with the present invention, nested one inside the other, and mechanically connected in series, a total lift comparable to the d 33  effect is attained. Also, when two or more piezoelements, which are nested one inside the other, are used, the total costs for manufacturing a piezoelectric actuator of the present invention are less than the costs that need to be expended for a related-art piezoelectric actuator, since, in comparison, the individual piezoelements can be produced extremely cost-effectively, e.g., using an extrusion molding process. 
     According to the present invention, an especially space saving arrangement of two or more piezoelements that are mechanically connected, one behind the other, is achieved when an outer piezoelement is concentrically disposed, in each case, around an inner piezoelement, and when a connecting element joins one end of the inner piezoelement to the opposite end of the outer piezoelement. The connecting element can be integrated quite advantageously in an annular space between the inner piezoelement and the outer piezoelement. This results in an especially compact type of construction for the piezoelectric actuator of the present invention. 
     The electrodes of the individual piezoelements can be electrically connected in parallel or independently of one another to a voltage source. In the latter case, a stepped driving of the piezoelectric actuator is possible, the result being a graduated actuator lift, depending on how many piezoelements receive the electric voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an axial section through an exemplary piezoelectric actuator according to the present invention. 
     FIG. 2 shows a schematic representation of an exemplary mode of operation according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows an axial section through an exemplary piezoelectric actuator 1 of the present invention. Piezoelectric actuator 1 can be used, for example, to actuate a valve-closure member of a fuel injector, not shown in greater detail, in particular for direct-fuel injection. However, a multiplicity of other application possibilities are also conceivable, e.g., the actuation of hydraulic valves, the driving of micropumps, or the actuation of electrical relays. 
     In the exemplary embodiment that is only shown schematically, piezoelectric actuator 1 has two piezoelements 2, 3, nested one inside the other, namely an inner piezoelement 2 and an outer piezoelement 3. Both inner piezoelement 2 and outer piezoelement 3 are produced from a piezoelectric material as a monolith, i.e., as a monocrystal. Suitable materials are, for example, quartz, tourmaline, barium titanate (BaTiO 3 ) or special piezoceramics, e.g. of Ba- and Ti-salts. Organic salts, such as NaK-tartrate or many other known piezoelectric materials are also suited. 
     In accordance with the present invention, piezoelements 2, 3 have a tubular, i.e., sleeve-shaped design, and have in each case penetrating bores 5, 6, which extend along a longitudinal direction illustrated by arrow 4. In one preferred exemplary embodiment, piezoelements 2, 3 are designed in radial symmetry as hollow cylinders of different diameters, which are arranged concentrically to a longitudinal axis 7. Penetrating bores 5 or 6 are each surrounded by a tubular wall 8 or 9. On the inside of tubular wall 8 or 9, provision is made for an inner electrode 10 and 11, respectively, while on the outside of tubular wall 8 or 9, provision is made for an outer electrode 12 and 13, respectively. Inner electrodes 10 and 11, and outer electrodes 12 and 13 are made of a suitable metal, which is applied using a suitable coating process, e.g., by means of sputter or vapor deposition, or a CVD process, to the inner or outer periphery of tubular walls 8 and 9. Inner electrodes 10 and 11, as well as outer electrodes 12 and 13 are connected to electrical lines 14 through 17, which are shown schematically. 
     When an electric voltage is applied to electrical lines 14 and 15, on the one hand, and to electrical lines 16 and 17, on the other hand, within tubular walls 8 or 9, an electrical field is formed that is aligned radially to longitudinal axis 7. Depending on the direction of the field strength, the electric field causes piezoelement 2 or 3 in question to contract or expand in longitudinal direction 4, thus normal to the direction of the electric field. The deformation in the direction normal to the direction of the electric field is more or less half as great as the deformation in the direction of the electric field utilized in conventional actuators. Given the same structural length of piezoelectric actuator 1, this leads to a corresponding reduction in the actuation lift. This can be compensated, as shown in FIG. 1, by using two piezoelements 2 and 3, which are mechanically connected in series and nested one inside the other. In this context, outer piezoelement 3 concentrically surrounds inner piezoelement 2. 
     A first end 20 of inner piezoelement 2 is braced against a support member, e.g., against housing 31 of a fuel injector. A second end 21 of inner piezoelement 2 is joined to a connecting element 22, which mechanically couples inner piezoelement 2 and outer piezoelement 3 to one another. Connecting element 22 can have a pot-shaped design, for example, with a cup-type base 23, against which second end 21 of inner piezoelement 2 is braced, with a cylinder section 25 running in an annular space 24 formed between inner piezoelement 2 and outer piezoelement 3, and with a peripheral collar 26 integrally formed on the end facing away from cup-shaped base 23 on cylinder section 25, a first end 27 of outer piezoelement 3 being braced against said collar 26. In this context, first end 27 of outer piezoelement 3 is disposed so as to face the first end 20 of inner piezoelement 2 and so as to oppose second end 21 of inner piezoelement 2. A second end 28 of outer piezoelement 3 is joined to a reciprocating piston 29, which transfers the actuating force of actuator 1 of the present invention to corresponding actuating elements, e.g., to a valve needle or a hydraulic displacement transformer of a fuel injector. 
     An extremely compact type of construction results from the nesting arrangement of two piezoelements 2 and 3 shown in FIG. 1, with an actuating lift comparable to that of a conventional actuator. Of course, other piezoelements can also be configured in the same kind of nesting arrangement, a third piezoelement (not shown) concentrically surrounding piezoelement 3 being connected to piezoelement 3 with a corresponding further connecting element. A fourth piezoelement and other piezoelements can be arranged in a corresponding manner, with each of the piezoelements being connected to the next inner piezoelement via a connecting element in such a way that the ends of the inner piezoelement in question oppositely disposed in the longitudinal direction and of the surrounding outer piezoelement in question are operatively connected to one another with force-locking. 
     Electrical lines 14 through 17 can be so connected that inner electrodes 10 and 11, on the one hand, and outer electrodes 12 and 13, on the other hand, are each able to be linked to a pole of a shared voltage source. It is also conceivable, however, to connect electrodes 10, 12 of inner piezoelement 2 and electrodes 11, 13 of outer piezoelement 3 independently of one another to the voltage source, so that piezoelements 2, 3 can receive a supply voltage independently of one another. In this manner, a stepped actuation lift of actuator 1 of the present invention can be achieved, a first actuation lift stage being achieved when only one of the two piezoelements 2 or 3 receive the supply voltage, and a second actuation lift stage being achieved, when both piezoelements 2 and 3 receive the supply voltage. When piezoelectric actuator 1 of the present invention is used to actuate a valve-closure member of a fuel injector, a stepped valve opening lift results, which is advantageous for certain applications. 
     Piezoelements 2 and 3 are able to be produced quite advantageously as a monolith using an extrusion molding process. In this context, particularly low manufacturing costs are achieved for piezoelements 2, 3. 
     To enable a better understanding of the present invention, on the basis of FIG. 2, the following will elucidate the d 31  effect utilized in the present invention, as contrasted to the d 33  effect usually used in known piezoactuators. 
     For the sake of simplicity, in FIG. 2, piezoelectric crystal 40, shown in a cube shape, is sketched in a coordinate system, whose coordinates are denoted by 1, 2 and 3. When an electric field E i  acting in coordinate direction I is applied to piezoelectric crystal 40, the crystal 40 undergoes a deformation, transforming it into piezoelectric crystal 40&#39; shown in FIG. 2 with a dashed line, for the case that I=3. Between the relative deformation: ##EQU1## in coordinate direction I and an electric field E j  in coordinate direction j, the three-dimensional relation ##EQU2## generally applies. In this context, d ji  denotes the piezoelectric coefficient, which is also described as piezoelectric charge constant. 
     When, as in FIG. 2, electrical field E 3  acts in coordinate direction 3, piezoelectric coefficient d 33  is an index for the field-dependent deformation in the direction of electric field E 3 , while piezoelectric coefficients d 3 , and d 32  are an index for the field-dependent deformation normal to the direction of electrical field E 3 , thus in the direction of coordinates 1 or of coordinates 2. When piezoelectric crystal 40 undergoes an expansion in coordinate direction 3, as shown in FIG. 2, then it undergoes a corresponding contraction normal to the direction of electrical field E 3 . Therefore, in this Application, the deformation in the direction of electrical field E 3  is denoted as the d 33  effect, while the deformation in coordinate direction I normal to the direction of electrical field E 3  is designated as the d 3 , effect. While conventional actuators utilize, as a rule, the d 33  effect, actuator 1 of the present invention makes use of the d 3 , effect or the d 32  effect, thus a deformation that is transverse to the direction of electrical field E 3 . When working with an isotropic, piezoelectric crystal 40, the contraction in coordinate direction 1 or in coordinate direction 2, is about half as great as the expansion in coordinate direction 3.