Abstract:
A piezoelectric actuator and an injector for an internal combustion engine are proposed, the electromagnetic compatibility of which is significantly improved in comparison with conventional injectors. The actuator head and/or actuator foot is manufactured out of a ceramic material, thus reducing the capacitance of the electric capacitor that is formed between the electrical ground and the piezoelectric actuator. An advantage of the piezoelectric actuator according to the invention is that no additional components are required. The change from metallic materials to ceramic materials also increases the rigidity of the piezoelectric actuator since ceramic materials have a significantly higher modulus of elasticity than metals.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a 35 USC 371 application of PCT/EP 2007/053748 filed on Apr. 18, 2007. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention is based on a piezoelectric actuator. 
     There are already known piezoelectric actuators, e.g. from DE 198 38 862 A1, which have piezoelectric ceramic layers that are placed one on top of another to form a multilayered stack, with a respective electrode layer provided between each pair of the piezoelectric ceramic layers. The electrode layers are connected to a power supply by means of a first external electrode and a second external electrode. 
     Piezoelectric actuators are advantageously triggered cyclically at a high frequency by a control unit. The rapid switching actions produce significant electromagnetic interference that must be reduced through complex measures in the control unit or in the electric supply lines of the piezoelectric actuators. 
     OBJECT AND SUMMARY OF THE INVENTION 
     The object of the present invention is to create a piezoelectric actuator and an injector equipped with a piezoelectric actuator, both of which have improved electromagnetic compatibility (EMC). 
     In a piezoelectric actuator, which includes a stack of a plurality of piezoelectric ceramic layers with a respective electrode layer provided between each pair of piezoelectric ceramic layers and has a foot part and a head part, this object is attained according to the invention in that the foot part and/or the head part is composed of a ceramic material. 
     The invention counteracts the interference signals coming from the piezoelectric actuator at their point of origin. Because the actuator head and/or actuator foot is manufactured out of a ceramic material, this reduces the capacitance of the electric capacitor that is formed between the electrical ground and the piezoelectric actuator. This sharply reduces the intensity of the interference signals generated with the switching of the piezoelectric actuator and therefore improves the electromagnetic compatibility of the piezoelectric actuator according to the invention. 
     Preferably, the actuator head and the actuator foot are composed of a ceramic material that is selected to have the lowest possible dielectric constant. Aluminum oxide, for example, has turned out to be a particularly advantageous material. Another significant advantage of the piezoelectric actuator according to the invention lies in the fact that no additional components are required so that the number of components does not increase. The change from metallic materials to ceramic materials also increases the rigidity of the piezoelectric actuator since ceramic materials have a significantly higher modulus of elasticity than metals. 
     Since the piezoelectric actuator itself is also composed of a ceramic material, the manufacture of the actuator head and actuator foot can be integrated with relative ease, from a production engineering standpoint, into the manufacturing process of the piezoelectric actuator. This simplifies the manufacture of the actuator according to the invention and results in no additional assembly steps in the overall manufacturing chain. 
     According to a particularly advantageous embodiment of the invention, the piezoelectrode layers of the piezoelectric actuator are contacted by means of two external electrodes and the external electrodes are routed between the foot part and an end surface of the actuator. As a result, it is easily possible in particular to protect the ends of the external electrodes from mechanical damage. 
     According to a particularly advantageous modification of this embodiment, at least one cable conduit is provided in the foot part and the electrical lines extend in the cable conduit or conduits. This makes it possible to easily connect the piezoelectric actuator to the power supply. This type of power supply and contacting of the external electrodes is extremely rugged and therefore also very reliable. 
     The piezoelectric actuator according to the invention can be further improved with regard to its electromagnetic compatibility if the actuator is enclosed by an elastic, fluid-tight jacket; this jacket is connected in a sealed fashion to the head part and the foot part. It is thus possible to dispense with a closed metal housing and there is a further reduction in the interference signals coming from the piezoelectric actuator. This simultaneously simplifies the structural complexity required for sealing the piezoelectric actuator in relation to fuel. 
     According to advantageous embodiments of the invention, the head part and/or the foot part has one or more circumferential grooves and the jacket is attached to the circumferential groove or grooves in a form-locked fashion. This results in a very durable, long-lasting, fluid-tight attachment of the jacket to the actuator head and actuator foot. 
     The jacket can be manufactured by extrusion coating the actuator, the head part, and the foot part with rubber or plastic. 
     Alternatively, it is also possible for the jacket to be manufactured out of a heat-shrinkable sleeve. To this end, the heat-shrinkable sleeve is drawn over the actuator foot, the piezoelectric actuator, and the actuator head and is then heated. In reaction to the heat, the heat-shrinkable sleeve contracts and forms a fluid-tight jacket. 
     In an injector, which is for a fuel injection system of an internal combustion engine and has a housing and in which the housing contains a piezoelectric actuator, a hydraulic control valve, and a nozzle module equipped with a nozzle needle; at one end, the piezoelectric actuator rests at least indirectly against the housing; at the other end, the piezoelectric actuator rests at least indirectly against the control valve; and an adjusting plate is provided between the piezoelectric actuator and the control valve, the object mentioned at the beginning is attained in that the adjusting plate is composed of a ceramic material. 
     The adjusting plate serves to compensate for length tolerances in the injector and to adjust the stroke of the piezoelectric actuator. Here, too, the change of the material from a metallic material to a ceramic material can increase the electromagnetic compatibility of the injector without increasing the number of components. At the same time, the transmission of the stroke of the piezoelectric actuator to the control valve is improved due to the greater rigidity of the ceramic adjusting plate. This also improves the operating behavior of the injector according to the invention. 
     Because as a rule, ceramic materials also have a lower specific gravity than metal, in particular steel, the change of the material also reduces the mass inertia of the moving parts of the injector, which has an advantageous effect on the dynamics of the injector. 
     The electromagnetic compatibility of the injector according to the invention is further improved if a spacer composed of a ceramic material is provided between the piezoelectric actuator and the housing. 
     This also increases the rigidity inside the injector according to the invention and simultaneously improves the electromagnetic compatibility. 
     According to a particularly advantageous embodiment of the injector according to the invention, it is thus possible to combine the measures according to the invention and for their effects to be at least partially added to each other. This yields an injector with a very favorable electromagnetic compatibility. This makes it possible to eliminate complex measures for suppressing the interference signals of the interference signal coming from the piezoelectric actuator or else makes it possible to at least replace these measures with very much simpler and less expensive alternative measures. 
     All of the measures proposed according to the invention share the feature of not increasing the number of components so that they entail no additional manufacturing and assembly complexity. 
     In addition, the measures according to the invention require no additional space, which constitutes another very important advantage. As a result, it is frequently also possible to completely or partially retrofit series produced injectors without significant structural changes. For example, the adjusting plates can be replaced with ceramic adjusting plates, without structural changes. The same is true for the spacers between the actuator and the housing. Naturally, it is also possible to install the piezoelectric actuator according to the invention in conventional piezoelectric injectors, Here, too, no structural changes to the housing of the injector, the control valve, or the nozzle module are required. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in greater detail below in conjunction with the drawings. 
         FIG. 1  shows a longitudinal section through a first exemplary embodiment of an injector according to the invention and 
         FIG. 2  shows a partial longitudinal section through an exemplary embodiment of a piezoelectric actuator according to the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a very simplified longitudinal section through a piezoelectric actuator according to the invention. The piezoelectric actuator according to the invention has a housing  1 . The housing  1  contains a piezoelectric actuator  3 , a hydraulic coupler  5 , a control valve  7 , and a nozzle module  9  equipped with a nozzle needle  11  and has a plurality of injection ports  13 . The housing  1  of the injector is also provided with a high pressure connection  15 , a leakage outlet  17 , and a power supply  19 . 
     Via the power supply  19 , the injector is connected to a control unit that is not shown. When the piezoelectric actuator  3  is to be activated, an engine control unit (not shown) applies an electrical voltage to the power supply  19 . The power supply  19  is connected to the piezoelectric actuator  3  via electrical lines  21 . It is thus possible to activate the piezoelectric actuator  3  by triggering the power supply  19 .  FIG. 1  does not show the details of the electrical contacting of the piezoelectric actuator. 
     With its upper end in  FIG. 1 , the piezoelectric actuator  3  rests via a foot part  23  against the housing  1  of the piezoelectric actuator. According to the invention, this foot part  23  is composed of a ceramic material with the lowest possible dielectric constant. Aluminum oxide, for example, is a suitable ceramic material. 
     Beneath the piezoelectric actuator  3 , there is a head part  25 , which transmits the actuator stroke to the hydraulic coupler  5  via an adjusting plate  27 . According to the invention, the head part and/or the adjusting plate  25  is composed of a ceramic material with a low dielectric constant. The replacement of the foot part  23 , the head part  25 , and the adjusting plate  27 , which are usually manufactured of steel, with parts composed of a ceramic material that has a low dielectric constant drastically reduces the capacitance of the capacitor formed by the piezoelectric actuator  3  and the electrical ground of the vehicle in which the injector according to the invention is installed. This also drastically reduces the interference signals that the high-frequency triggering of the piezoelectric actuator introduces into the vehicle via the electrical ground. This consequently improves the electromagnetic compatibility of the piezoelectric actuator according to the invention. This embodiment has the particular advantage that the cause of the interference signals is effectively counteracted and as a result it is possible to completely or partially dispense with complex measures, whether in the wiring harness or in the control unit of the internal combustion engine. 
     Another significant advantage of the invention is that the measures according to the invention require no additional space and also do not increase the number of components. Finally, it should also be noted that the ceramic materials have a very much greater modulus of elasticity than steel or other metallic materials, thus increasing the rigidity of the apparatus according to the invention, which is composed of the foot part  23 , the head part  25 , the piezoelectric actuator  3 , and the adjusting plate  27 . This improves the dynamics of the piezoelectric actuator according to the invention and of the injector equipped with this actuator, thus increasing the effective stroke of the piezoelectric actuator and of the actuating element (unnumbered) in the control valve  7 . 
     As an additional measure, it is also possible for a spacer  29  to be provided above the head part  25 . The spacer  29  is situated between the housing  1  of the injector and the foot part  23  of the injector and is likewise composed of a ceramic material with a low dielectric constant. 
       FIG. 2  shows a partial section through a particularly advantageous embodiment of the piezoelectric actuator  3  according to the invention. 
     In this significantly enlarged depiction of the piezoelectric actuator  3 , it is evident that the actual piezoelectric actuator is composed of a plurality of ceramic layers  31 , which for the sake of visibility, have not all been provided with reference numerals. Electrode layers  33  and  35  are situated between the ceramic layers  31 . Here, too, for the sake of visibility, not all of the electrode layers  33  and  35  have been provided with reference numerals. 
     The electrode layers  33  constitute a first group of electrode layers and are connected in an electrically conductive fashion to a first external electrode  37 . 
     The electrode layers  35  constitute a second group of electrode layers and are connected in an electrically conductive fashion to a second external electrode  39 . 
     Above the uppermost electrode layer  35  in  FIG. 2 , a first cover layer  41  is depicted in sectional fashion. This first cover layer  41  is thicker than the ceramic layers  31 , as is the second cover layer  43  situated beneath the lowermost electrode layer  33 . The two cover layers are likewise composed of a ceramic material. Usually, they are composed of the same ceramic material as the ceramic layers  31 . The cover layers  41  and  43 , however, make no contribution to the stroke of the piezoelectric actuator  3  since no electrical field between an electrode layer  33  and an electrode layer  35  is produced in them when the piezoelectric actuator  3  is supplied with current. 
     The first cover layer  41  has recesses  45  let into it, in which the first external electrode  37  and the second external electrode  39  are routed. This means that the external electrodes  37  and  39  end at one end surface of the piezoelectric actuator  3 . 
     Two cable conduits  47  are provided in the foot part  23  of the piezoelectric actuator  3 . The electrical lines  21  extend in the cable conduits  47 . The electrical lines  21  are connected in an electrically conductive fashion to the respective external electrodes  37  and  39  in the region of the end surface of the piezoelectric actuator  3 . This type of contact is extremely rugged since the contact point is enclosed on all sides, whether by the first cover layer  41  of the piezoelectric actuator  3  or by the foot part  23  of the piezoelectric actuator  3 . 
     Because the foot part  23  is also composed of ceramic material, it is possible to manufacture the electrical connection between the electrical lines  21  and the external electrodes  37  and  39  in the same manufacturing process in which the electrical connection is also produced between the electrode layers  33  and  35  and the external electrodes  37  and  39 . This results in a simplification of the manufacturing process and a higher throughput. 
       FIG. 2  clearly shows another essential defining characteristic of the piezoelectric actuator  3  according to the invention: Usually, piezoelectric actuators must be sealed in a fluid-tight fashion in relation to their surroundings. In conventional piezoelectric actuators, this is achieved by placing a metal housing around the piezoelectric actuator  3 , the bead part  25 , and the foot part  23 . Naturally, this metal housing must be elastic enough to permit it to transmit the stroke of the piezoelectric actuator  3  without breaking. This metal housing is very cost-intensive and has a negative impact on the electromagnetic compatibility. 
     In the piezoelectric actuator  3  according to the invention, the piezoelectric actuator  3 , the head part  25 , and the foot part  23  are encompassed by a jacket  49 . This jacket  49  completely encloses the piezoelectric actuator  3  and partially encloses the foot part  23  and the head part  25 . The jacket  49  can be produced, for example, by extrusion coating the foot part  23 , the head part  25 , and the piezoelectric actuator  3  with rubber or an elastic plastic. Alternatively, it is also possible for the jacket  49  to be composed of a heat-shrinkable sleeve, which, in its stretched state, is slid over the foot part  23 , the piezoelectric actuator  3 , and the head part  25 . Then, the heat-shrinkable sleeve is heated and contracts as a result. This produces a seal of the piezoelectric actuator  3  that is fluid-tight and sufficiently elastic. 
     In order to even further increase the ruggedness of the connection between the jacket  49  on the one hand and the foot part  23  and head part  25  on the other, a plurality of circumferential grooves  51  are provided in both the foot part  23  and the head part  25 . These circumferential grooves  51  permit a form-locked connection between the jacket  49  and the foot part  23  and head part  25 . 
     The foregoing relates to the preferred exemplary embodiment of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.