Abstract:
An actuator module including a piezoactuator, for example for a piezoinjector for metering fuel in an internal combustion engine, is proposed. The piezoactuator has piezoelements stacked one above another between an actuator head and an actuator foot and is provided with at least one elastomer layer enclosing the piezoelements. The elastomer layer is enveloped by a corrugated bellows or by telescopic tubes, composed of a material that is diffusion-impermeable with respect to a fuel to be metered by the piezoinjector. The corrugated bellows or the telescopic tubes have on their periphery grooves which can absorb expansions of the elastomer layer or of the piezoactuator.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a 35 USC 371 application of PCT/EP 2007/059613 filed on Sep. 13, 2007. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an actuator module with a piezoelectric actuator having a sheath, which can be used for instance in a piezoelectric injector for chronologically and quantitatively precise metering o fuel in an internal combustion engine. 
     2. Description of the Prior Art 
     One such piezoelectric injector essentially comprises a holder body and the piezoelectric actuator disposed in the holder body; the piezoelectric actuator has elements, stacked one above the other between an actuator head and an actuator foot, and each of the elements have piezoelectric layers enclosed by inner electrodes. The elements are constructed, using a material with a suitable crystalline structure (piezoelectric ceramic) for the piezoelectric layers, in such a way that upon application of an external voltage to the inner electrodes, a mechanical reaction of the elements ensues, which as a function of the crystalline structure and the regions of contact with the electrical voltage represents a compression or tension in a predeterminable direction. In a piezoelectric injector, the piezoelectric actuator is connected to a nozzle needle, so that by application of a voltage to the elements, a nozzle opening is uncovered. 
     In diesel piezoelectric injectors, in so-called direct nozzle needle control, the piezoelectric actuators operate directly in the diesel fuel at high pressure. For protecting the elements, for example with a view to insulating them electrically, it is known to sheathe the piezoelectric actuator with a diesel-proof elastomer. A disadvantage of this is that elastomers used for the purpose are not diffusion-proof to diesel fuel, water, and other media in the fuel. 
     From German Patent Disclosure DE 101 39 871 A1, a piezoelectric injector with an actuator module, acting on a valve member via a hydraulic pressure booster, is known in which the elements of the piezoelectric actuator are disposed in a sleeve provided with a corrugated bellows. 
     ADVANTAGES AND SUMMARY OF THE INVENTION 
     The invention is based on an actuator module, described at the outset, with a piezoelectric actuator which has elements, disposed between an actuator head and an actuator foot, and a fluid-tight sheath, surrounding at least the elements, of the piezoelectric actuator. According to the invention, an sheath lying over an elastomer layer advantageously, at least in some regions, has grooves formed by undulations or has telescoping tubes, with which by their orientation on the circumference of the sheath, radial and/or axial expansion motions of the piezoelectric actuator and/or of the elastomer layer can be absorbed. 
     In addition to the requisite diffusion-proof sheathing of the piezoelectric actuator, the refinement according to the invention of the actuator module is advantageous in particular because the elastomer layer, lying beneath the sheath, can expand unhindered upon heating, without mechanically destroying the sheath from the internal pressure that occurs, for instance from cracks. Moreover, air bubbles and voids in the elastomer layer can be compacted by means of the invention without tearing of the sheath. 
     In a first embodiment of the invention, the actuator module is provided with a corrugated bellows as the sheath; the corrugated bellows advantageously has the grooves, formed by undulations, in some regions on its circumference, with which grooves, by their orientation with the circumference of the sheath, radial and/or axial expansion motions of the piezoelectric actuator and/or of the elastomer layer can be absorbed. This is not attainable with the corrugated bellows known from the prior art, since that corrugated bellows remains radially rigid. 
     In a second embodiment of the actuator module of the invention, the sheath is formed of telescoping tubes guided inside one another, of which preferably at least one has the grooves formed by undulations on its circumference in some regions, with which grooves, by their orientation with the circumference of the sheath, radial and/or axial expansion motions of the piezoelectric actuator and/or of the elastomer layer can be absorbed. 
     In this last proposed embodiment, the interstices between the telescoping tubes, joined one inside the other, can also be filled with elastomer, and as a result, advantageously, the diffusion length of the surrounding fluid is lengthened, and the diffusion area is reduced, in comparison with the elastomer layer lying on the piezoelectric actuator. 
     If smooth telescoping tubes are used as sheaths here, then compensation takes place of axial expansion between the telescoping tubes filled with elastomer; because of the relatively narrow gap between the telescoping tubes, the elastomer is subjected to strong shear stress, but it is almost impossible to compensate for radial expansion, for instance upon thermal expansion. With the advantageous disposition of grooves in accordance with the invention, compensation for both radial and axial expansion is thus made substantially easier. 
     The proposed grooves can advantageously also be axially extending longitudinal grooves, which then facilitate an axial thermal expansion of the piezoelectric actuator without the risk of damage to the sheath. On the other hand, the proposed longitudinal grooves may instead be radially extending transverse grooves, which absorb radial expansion of the piezoelectric actuator, thereby reducing the thrust stresses in the elastomer. However, the grooves can also be embodied as single- or multi-thread and clockwise and/or counterclockwise grooves extending at a predetermined angle to the longitudinal or transverse axis of the piezoelectric actuator, or a combination of all the embodiments of grooves proposed can be provided. 
     Particularly from the axial component of the orientation of the grooves, it can thus be assured in a simple way that the internal pressure of the elastomer layer can decrease upon a thermal expansion and will not damage the actuator module or the sheath. If there are remaining air bubbles or other remaining voids between the elastomer layer and the sheath, the longitudinal grooves can easily yield radially and compress the air, again without damaging the rest of the sheath. 
     The threadlike or helical grooves can be provided particularly to facilitate production, since they are easy to produce using a rolling tool. To preclude torsional motions, both a clockwise and counterclockwise helical groove region may advantageously be provided. 
     The grooves or various combinations of the grooves can be disposed for instance in the middle region of the sheath or in regions on at least one lateral end in the longitudinal extent, or over the entire circumference. The arrangements of longitudinal grooves or helical or orthogonal transverse grooves can be distributed, to suit the demand of the longitudinal or transverse expansion to be absorbed, over the circumference in the longitudinal extent of the corrugated bellows, or of a telescoping or other kind of sheath, and the lengths and angles in the helical circumferential groove can also be varied and combined, for instance within a range from 0° to 45°. 
     The sheath as a corrugated bellows or as a telescope in the actuator module of the invention can preferably be produced from a metal material such as steel. The corrugated bellows or the telescoping tubes can be welded in a simple way to the actuator head and/or foot, as a rule also of steel, of the piezoelectric actuator. 
     A preferred application of the actuator module of the invention is obtained if, as already mentioned in the background section, the piezoelectric actuator is a component of a piezoelectric injector for an injection system for fuel in an internal combustion engine, in which the fuel bathes the sheath. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described below in conjunction with drawings. In the drawings: 
         FIG. 1  is a schematic illustration of a piezoelectric injector with an actuator module of the prior art in longitudinal section; 
         FIG. 2  is a schematic illustration of an actuator module of the invention, having a corrugated bellows, as a first exemplary embodiment of a sheath of a piezoelectric actuator in the actuator module with an expandable sheath; 
         FIG. 3  is an illustration of an actuator module in longitudinal section through  FIG. 2 , with a corrugated bellows with longitudinal grooves in a central radially elastic zone; 
         FIG. 4  is a cross section through the actuator module of  FIG. 3  along the section line A-A; 
         FIG. 5  is a view of a corrugated bellows with longitudinal grooves in the side regions; and 
         FIG. 6  is a view of a corrugated bellows with helical transverse grooves; 
         FIG. 7  is a schematic illustration of an actuator module of the invention with telescoping tubes, as a second exemplary embodiment of a sheath of the piezoelectric actuator; 
         FIG. 8  is a view of an actuator module with transverse grooves on the order of a corrugated bellows in a lateral, axially elastic zone of a telescoping tube; 
         FIG. 9  is a view of an actuator module with longitudinal grooves in a lateral, radially elastic zone, in this case of an outer telescoping tube; 
         FIG. 10  is a section through the actuator module of  FIG. 9  along the section line A-A; and 
         FIG. 11  is an illustration of an actuator module with longitudinal grooves on both the inner and the outer telescoping tube, over virtually the entire length. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A piezoelectric injector  1  in accordance with the prior art, shown in  FIG. 1  to explain the actuator module of the invention, essentially includes a holder body  2  and a piezoelectric actuator  3 , which is located in the holder body  2  and has an actuator head  4  and an actuator foot  5 . Between the actuator head  4  and the actuator foot  5 , there is a plurality of elements  6 , stacked one above the other, which each comprise piezoelectric layers of piezoelectric ceramic and inner electrodes  7  and  8  enclosing the piezoelectric ceramic. 
     The inner electrodes  7  and  8  of the elements  6  are contacted electrically via a plug part  9  with lead lines to outer electrodes  10  and  11 . The piezoelectric actuator  3 , which with other components, not shown in detail here, represents a so-called actuator module, is connected to a nozzle needle  13  via a coupler  12 . By application of a voltage to the elements  6  via the inner electrodes  7  and  8  and by the resultant mechanical reaction, a nozzle opening  14  is uncovered, as explained in the background section. The actuator module with the piezoelectric actuator  3 , in the application shown in  FIG. 1  as a piezoelectric injector, is bathed by the fuel to be metered, in a chamber  15 . 
     In  FIG. 2 , an actuator module  20  according to the invention is shown, with a piezoelectric actuator  21  that is surrounded here, from the actuator head  22  to the actuator foot  23 , including the elements  24 , by an electrically insulating elastomer layer  25 . Around the elastomer layer  25  is a metal corrugated bellows  26 , as a fluid-tight sheath. The corrugated bellows  26  is joined at both ends to the actuator head  22  and the actuator foot  23 , by a welded connection  27  in each case. The corrugated bellows  26  serves both to seal the elements  24  off in a diffusion-tight manner from plastic surrounding a piezoelectric injector and to absorb thermal and mechanical longitudinal expansions of the piezoelectric actuator  21  elastically without generating major longitudinal force and without absorbing thermal or otherwise-caused changes in volume of the elastomer layer  25 . 
     Since when used in a piezoelectric injector, the relatively high fuel pressure prevails at the corrugated bellows  26  on all sides, and the corrugated bellows  26  for space reasons is relatively thin, the elastomer layer  25  need not fill the interior between the piezoelectric actuator  21  and the corrugated bellows  26  completely. As a result, the corrugated bellows  26  comes under pressure on all sides, without additional tensile or compressive stresses. In  FIG. 2 , regions L 1  and L 3  of the corrugated bellows  26  can be seen, and there is a middle region L 2 , which is intended here to absorb radial expansions. 
     To facilitate radial thermal expansion of the elastomer layer  25  substantially, without generating radial tensile stress in the corrugated bellows  26 , longitudinal grooves  28  are disposed in a middle region in the exemplary embodiment of  FIG. 3 . In the lateral regions of the corrugated bellows  26 , instead of the radial circumferential grooves of the corrugated bellows  26  as in  FIG. 2 , in  FIG. 3  threaded or helical circumferential grooves  29  are provided, with a pitch angle α to the radial circumferential groove. To facilitate torsional motions without putting a load on the elements  24 , different clockwise or counterclockwise helical circumferential grooves  29  are disposed on the two sides of the longitudinal grooves  28 . 
       FIG. 4  shows a section through the actuator module  20  of  FIG. 3  along the section line A-A; here, both the longitudinal grooves  28  and the corrugated bellows  26  and the elastomer layer  25  surrounding the piezoelectric actuator  21  can be seen. 
     In  FIG. 5 , an embodiment of the invention can be seen, in which the longitudinal grooves  28  are disposed on both sides of a middle region of the corrugated bellows  26 . 
       FIG. 6  shows different clockwise or counterclockwise helical circumferential grooves  30  disposed on both lateral regions of the sheath on the corrugated bellows  26 , which here have a pitch angle α to the longitudinal axis of the piezoelectric actuator. In  FIG. 6 , the incident stroke Δx of the piezoelectric actuator is also shown, which leads to a torsion Δα between the lateral regions and the different clockwise or counterclockwise helical circumferential grooves  30 . By optimizing the pitch angle α and the number of circumferential grooves  30 , the torsion Δα can be minimized, so that the rigidity in the x-direction (stroke direction of the piezoelectric actuator) remains low. 
     A second exemplary embodiment of the invention is shown in  FIG. 7 , with an actuator module  40  having a piezoelectric actuator  41 , which here is again surrounded by an electrically insulating elastomer layer  45 , from the actuator head  42  to the actuator foot  43 , including the elements  44 . As a fluid-tight sheath around the elastomer layer  45 , here there are an inner telescoping tube  46  and outer telescoping tube  47 . The inner telescoping tube  46  is welded to the actuator head  42  and the outer telescoping tube  47  is welded to the actuator foot  43 , in each case by means of a weld connection  48 . 
     In the exemplary embodiment of  FIG. 7 , the interstices between the telescoping tubes  46  and  47  can also be filled with the aforementioned elastomer of the elastomer layer  45 ; because of the relatively narrow gap, upon mechanical longitudinal expansion, these elastomer regions are subjected to strong shear stress. It is almost impossible to compensate for radial expansion, for instance upon thermal expansion of the elastomer layer  45 ; only thrust expansion between the two telescoping tubes  46  and  47  is possible. 
     In  FIG. 8 , an embodiment for improving the arrangement of  FIG. 7  is shown, in which, to avoid the great axial expansion of the elastomer layer  45  between the two telescoping tubes  46  and  47 , a region L 0  with transverse grooves  49  extending all the way around the circumference of the outer telescoping tube  47  is provided. The length of the region L 0  is adaptable to the required conditions for expansion compensation. 
     In an embodiment of  FIG. 9 , longitudinal grooves  50  are provided in a lateral region, in a region L 1  on the outer telescoping tube  47 , and these can compensate especially well for the radial thermal expansion of the elastomer layer  45  in particular. From  FIG. 10 , a section can be seen through the actuator module of  FIG. 9  along the section line A-A in the region of the outer telescoping tube  47  having the longitudinal grooves  50 . 
       FIG. 11  shows a view of the actuator module  40  with the piezoelectric actuator  41 , with longitudinal grooves  51  on the outer telescoping tube  47  in the entire longitudinal expansion L 2 ; once again, corresponding longitudinal grooves can be provided on the inner telescoping tube  46 , in the same way, with a length L 3 . A combination of the transverse grooves  49  of  FIG. 8  and the longitudinal grooves  51  of  FIG. 11  is also possible, in a manner not shown here. 
     The foregoing relates to the preferred exemplary embodiments 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.