Abstract:
A piezoelectric actuator with piezoelectric elements, fastened between an actuator head and an actuator foot, and with a plastic sleeve surrounding at least the piezoelectric elements is proposed, which is set in a clamping ring on the actuator head and/or the actuator foot of the piezoelectric actuator. The respective clamping ring may be a metal part or a plastic part, which is equipped with clamping lugs suitable for setting the plastic sleeve and is thrust, pressed, or shrunk onto the respective actuator head and/or actuator foot of the steel piezoelectric actuator.

Description:
REFERENCE TO FOREIGN PATENT APPLICATION 
       [0001]    This application is based on German Patent Application No. 10 2006 045 663.7 filed 27 Sep. 2006, upon which priority is claimed. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a piezoelectric actuator sheathed with a sleeve and bathed for instance by liquid media, which can be inserted into a piezoelectric injector for chronologically and quantitatively precise metering of fuel for an internal combustion engine. A piezoelectric injector of this kind substantially comprises a holder body and the piezoelectric actuator disposed in the holder body, the piezoelectric actuator having piezoelectric elements stacked one above the other and disposed in an actuator head and an actuator foot. 
         [0004]    2. Description of the Prior Art 
         [0005]    It is known per se that for constructing a piezoelectric actuator of the type described above, the piezoelectric elements can be inserted in such a way that by utilizing the so-called piezoelectric effect, a control of the needle stroke of a valve or the like can be accomplished. Piezoelectric layers of the piezoelectric elements are constructed from a material with a suitable crystalline structure, in such a way that when an external electrical voltage is applied, a mechanical reaction of the piezoelectric element ensues that, depending on the crystalline structure and the contact regions of the electrical voltage represents compression or tension in a predeterminable direction. Such piezoelectric actuators are suitable for instance for applications in which reciprocating motions take place under high actuation forces and at high cycle frequencies. 
         [0006]    For example, one such piezoelectric actuator is known as a component of a piezoelectric injector in so-called common rail injection systems (CR injector), from German Patent Disclosure DE 100 26 005 A1. In this piezoelectric actuator, a stack of a plurality of electrically and mechanically coupled-together piezoelectric elements is present, and this stack is retained between two stops under initial tension via an actuator foot and an actuator head. Each piezoelectric layer is set between two inner electrodes, by way of which an electrical voltage can be applied from outside. Because of this electrical voltage, the piezoelectric elements then each execute short reciprocating motions in the direction of the potential drop, and these add up to make the total stroke of the piezoelectric actuator. This total stroke can be varied by way of the magnitude of the applied voltage and can be transmitted to a mechanical final control element. 
         [0007]    In these known CR injectors, there is a nozzle needle controlled indirectly by the piezoelectric actuator; via a sleeve and diaphragm arrangement as a sheath, which compensates for stroke and temperature, the piezoelectric actuator is surrounded directly or indirectly by the pressure of the fuel, which in such low-pressure systems is approximately 60 bar. 
         [0008]    In CR injectors with direct control of the nozzle needle, however, this known concept cannot readily be adopted, since in this case the piezoelectric actuator is operated in the high-pressure region of the fuel, and a rigid sleeve and diaphragm arrangement as a sheath would not withstand the high system pressures and the longer reciprocating motions of the piezoelectric actuator, under the space conditions available in the piezoelectric injector. 
         [0009]    To achieve an electrical and mechanical insulation of the piezoelectric actuator, a flexible sheath of the piezoelectric actuator is often also proposed. From German Patent Disclosure DE 102 30 032 A1, to avoid the above-described disadvantages, an arrangement with a piezoelectric actuator in bathing media is known in which the piezoelectric elements are cast in an amorphous insulation compound, which in turn is placed in a housing jacket that is solidly closed off from the medium laterally and on the upper and lower ends. 
       OBJECT AND SUMMARY OF THE INVENTION 
       [0010]    The invention is based on a piezoelectric actuator with piezoelectric elements, described at the outset, that is fastened between an actuator head and an actuator foot and in which a sleeve surrounding at least the piezoelectric elements is present as a component of the sheath. According to the invention, the sleeve is a plastic sleeve, which is set on the cylindrical actuator head and/or on the actuator foot with a cylindrical clamping ring. Advantageously, the respective clamping ring may be a metal part or a plastic part, which is equipped with clamping lugs that are suitable for setting the plastic sleeve and that in particular are also deformable, and is thrust, pressed, or shrunk onto the respective actuator head and/or actuator foot and thus firmly held. 
         [0011]    The proposed CR injector may advantageously be constructed in such a way that a nozzle needle controlled directly by the piezoelectric actuator is present, and the control can be accomplished in one or two stages. Between the piezoelectric actuator and the nozzle needle, there is only a coupler with a hydraulic booster, so that the needle motion can be stepped up directly, and the coupler can follow the motion brought about by the piezoelectric actuator. 
         [0012]    With the application of the plastic sleeve according to the invention, the piezoelectric actuator, which is located in the interior of the holder body, is protected in a simple way against harmful liquids, such as diesel fuel, water, and RME as conventional types of biodiesel fuel or other conductive substances, as well as particles, floating material, and the prevailing high pressures resulting from these substances and their pressure and temperature fluctuations. 
         [0013]    In particular flexible plastic sleeves may be used here, which can be shrunk on or adapted in some other way to the component contour. The plastic sleeves intrinsically achieve the necessary sealing action from the piezoelectric actuators and assure an additional stroke and temperature compensation in the range required here, which is from −40 to +160° C. To attain secure sealing of the component combination, the plastic sleeve is sealed off or closed against an entry of media by the clamping rings of the invention on the open ends. 
         [0014]    Advantageously, the respective clamping ring can be precentered or preoriented and additionally welded or glued to the actuator head and/or the actuator foot. Particularly when shrink-type hoses are used as the plastic sleeve, the contour of the actuator head and/or of the actuator foot can be tapered in the direction of the piezoelectric elements in such a way that the shrunk-on plastic sleeve easily conforms to them. 
         [0015]    The actuator head and/or the actuator foot can also at least partially have a self-locking surface in the region or vicinity of the clamping with the clamping ring, and the self-locking surface can for instance comprise a fused glass inclusion or annular offsets or indentations. It is also possible for the respective clamping ring to have a soft coating, particularly of gold, on the side toward the actuator head and/or the actuator foot. 
         [0016]    In a further embodiment of the invention, the respective clamping ring may rest on a conical actuator head and/or actuator foot that is chamfered on the side toward the piezoelectric elements. It is also especially advantageous if at least one clamping lug of the respective clamping ring has barbs or bite or sealing edges, which prevent the plastic sleeve from slipping out of the clamping ring. 
         [0017]    Secure fastening of the plastic sleeve is also attainable by providing that the at least one outer clamping lug, before the introduction of the plastic sleeve, is bent outward and pressed, after the introduction of the plastic sleeve, against the plastic sleeve by calking. A type of fastening in which the plastic sleeve, in the region of the enclosure by means of the respective clamping ring, has a thickened portion that is prefabricated or made later and that comes to rest in a suitably shaped hollow chamber of the clamping ring, is also secure. 
         [0018]    In summary, it can be stated that with the invention, an economical, simple, and durable sealing off of the ends of a plastic sleeve for preventing the entry of fuel or of actuator-damaging substances, in a piezoelectric actuator that operates in the high-pressure region of a liquid, is achieved over a long service life. By the shape or the resilient or other proposed properties of the clamping lugs of the respective clamping ring, or by the calking of the clamping ring lugs to the plastic sleeve, pressing of the plastic sleeve against the actuator head and/or the actuator foot of metal, and hence a good sealing action, are attained. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of preferred embodiments, taken in conjunction with the drawings, in which: 
           [0020]      FIG. 1  is a section through a piezoelectric injector having a piezoelectric actuator in accordance with the prior art; 
           [0021]      FIGS. 2   a  and  2   b  show a first version according to the invention of a clamping ring as a calking ring for securing a plastic sleeve to a piezoelectric actuator; 
           [0022]      FIG. 3  shows a modification of the arrangement of the clamping ring of  FIGS. 2   a  and  2   b , with mounting of the clamping ring on a chamfer of an actuator foot; 
           [0023]      FIG. 4  shows a modification of the arrangement of the clamping ring of  FIGS. 2   a  and  2   b , with an additional weld seam of soldered seam; 
           [0024]      FIGS. 5   a  through  5   c  show exemplary embodiments with a clamping ring with a hollow chamber for receiving a thickened portion of the plastic sleeve; 
           [0025]      FIGS. 6   a  through  6   c  show further exemplary embodiments, with a clamping ring that has two clamping lugs, and also show a plastic sleeve, thrust into the resultant groove as a precentering means, in the unshrunk-on and in the shrunk-on state; 
           [0026]      FIG. 7  shows exemplary embodiments of the clamping ring with barbs or beads; and 
           [0027]      FIG. 8  shows exemplary embodiments of the clamping ring with various bite or sealing edges. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0028]    In  FIG. 1 , a basic construction of a piezoelectric injector  1  with a piezoelectric actuator  2  of the prior art is shown, which can be used for instance for needle stroke control in an injection system for fuel in an internal combustion engine. In the upper part, there is a holder body  3 , which can be adapted in its geometric dimensions essentially to the particular intended application and the specific location where it will be used. By means of the holder body  3 , with a specifically adapted plug embodiment, the electrical voltage is carried for triggering piezoelectric elements  5 , which are stacked one above the other in an inner chamber  4  of the holder body  3  and which together with an actuator head  6  and an actuator foot  7  form the piezoelectric actuator  2 . An insulating sleeve  8  shown here only schematically is placed as a sheath around the piezoelectric actuator  2 . 
         [0029]    Upon an actuation, as mentioned in the background section in terms of a piezoelectric injector, the piezoelectric elements  5  act on a nozzle needle  10 , via a mechanical arrangement with a coupler  9  that in this case is located vertically below, in such a way that an opening of a nozzle opening  11  can be effected. Fuel carried in the interior of the piezoelectric injector  1  through the inner chamber  4  in the holder body  3  can thus be injected into the combustion chamber of an internal combustion engine, not shown here. 
         [0030]    Below, exemplary embodiments of connections according to the invention of an insulating sleeve to suitably designed versions of a piezoelectric actuator will be explained. 
         [0031]    In  FIG. 2   a , a first exemplary embodiment can be seen, in which, as in the following exemplary embodiments, only the region where clamping rings are connected to a plastic sleeve, as an insulating sleeve, is shown.  FIG. 2   a  shows a plastic sleeve  20  comprising a material capable of shrinkage under the influence of temperature, in the state in which it is shrunk onto the contour of the piezoelectric actuator, and also shows a clamping ring  21  which by means of a calkable clamping lug  22  receives the inserted plastic sleeve  20 . The clamping ring  21  is thrust onto and fixed on a metal actuator foot  23  of the piezoelectric actuator (compare the actuator foot  7  of the piezoelectric actuator  2  in  FIG. 1 ) by means of surface pairing, for instance via an O-ring, not shown here. 
         [0032]      FIG. 2   b  shows the exemplary embodiment described above, with an already calked clamping lug  22 , which penetrates the plastic sleeve  20  and thereby forms a sealed face  24 . 
         [0033]    In  FIG. 3 , a modification of the embodiment described above can be seen, in which the clamping ring  21  is seated in self-locking fashion on a chamfer  25 , which is conical with a shallow angle, of the actuator foot  23 , rather than on a cylindrical shape. However, still other geometric designs that have a self-locking effect are also conceivable. Because of this self-locking effect, any additional welded connection or other fastening provisions are superfluous, and a fused glass inclusion can additionally be attached here.  FIG. 4  furthermore shows the additional placement, which may become necessary, of a welded or soldered seam  26  between the clamping ring  21  and the foot part  23 . 
         [0034]    In  FIG. 5   a , an exemplary embodiment is shown in which the clamping ring  21  has a hollow chamber for receiving a thickened portion  27  of the plastic sleeve  20 , as a result of which, in addition to the effect of the sealing face  24 , an even more secure hold of the plastic sleeve  20  in the clamping lug  21  is attainable.  FIG. 5   b  shows a modification with a pressure equalization bore  28  leading to the hollow chamber of the clamping ring  21 , and  FIG. 5   c  shows further possible designs of the thickened portion  27  as examples. 
         [0035]    If the end of the plastic sleeve  20  is made thick in a shape similar to an O-ring, as described in conjunction with  FIGS. 5   a  through  5   c , then by means of the shape of the hollow chamber in the clamping ring  21 , compartmentalization of the plastic sleeve  20  can be attained. If the plastic sleeve  20  is intended to have a swelling behavior, then as a result of the aforementioned compartmentalization, an additional positive sealing action is achieved. The swelling behavior and the resilience of bite edges of the clamping lugs  22  (see  FIGS. 2   a ,  2   b ,  3 ,  4 ,  5   a , and  5   b ) can compensate for possible fatigue phenomena, such as flowing, reduced elasticity, and so forth, of the material comprising the plastic sleeve  20 . The thickening and prefabrication (see  FIGS. 5   a  through  5   c ) can be attained in a simple and economical way in preceding work steps; in addition to the thickening, shaping elements for forming the thickened portions  27  can be vulcanized onto the plastic sleeve  20 . 
         [0036]    The remaining air volume between the end of the plastic sleeve  20  and the clamping ring  21  should be kept as small as possible, in order to minimize forcing out of the plastic sleeve  20  from the bite edge upon a pressure buildup in the volume. However, it must be expected that at no time can a higher pressure occur in the volume than in the surrounding system. If that should nevertheless be the case, then the volume can for instance be adapted to the ambient pressure and thus connected in a pressure-equalized way, for instance via the pressure equalization bore  28  described in conjunction with  FIG. 5   b.    
         [0037]    Further options in embodying the connection between the clamping ring  21  and the plastic sleeve  20  are shown in  FIGS. 6   a ,  6   b , and  6   c . Here, the plastic sleeve  20  is shown first in the non-shrunk-on state in  FIGS. 6   a  and  6   b  and is press-fitted between two clamping lugs  22  of the clamping ring  21 .  FIG. 6   b  shows one option for sinking the clamping ring  21  in a space-saving way in an offset  29  of the actuator foot  23 , and  FIG. 6   c  also shows the shrunk-on plastic sleeve  20  and the upper clamping lug  22  calked onto it. 
         [0038]      FIG. 7  shows four exemplary embodiments of the clamping lugs  22  of the clamping ring  21  (compares  FIGS. 6   a  through  6   c ) with beads  30  located on the inside, or with barbs  31 , which are meant to prevent the plastic sleeve  20  from slipping out. The groove or space between the clamping lugs  22  in  FIG. 7  acts as a guide and precentering means for the plastic sleeve  20  in the case of a single-step assembly concept of the plastic sleeve  20  in the clamping ring  21 . 
         [0039]    In  FIG. 8 , a further four modifications of the clamping lugs  22  can be seen, which are designed on the front end in such a way as to create in this case bite and sealing edges  32  or a spring element  33 . The clamping lugs  22  with the bite and sealing edges  32  and spring  33  shown in  FIG. 8  prevent the plastic sleeve  20  from being pulled out of the clamping ring  21  in the shrinkage operation. Depending on the design of the surface pairing between the clamping ring  21  and the metal on the actuator foot or the actuator head, in this case the actuator foot  23 , the clamping ring is displaced longitudinally upon shrinkage, and pulling out of the plastic sleeve  20  from the clamping ring  21  is prevented. 
         [0040]    The actual sealing action is generated, in the exemplary embodiments shown, at the sealing faces  24  (see  FIG. 2   b ), pressed against by the respective bite and sealing edge (see  32  and  33  in  FIG. 8 ), between the plastic sleeve  20  and the metal actuator head or the actuator foot  23 . In addition, the bite and sealing edge  32  or spring  33 , which after the clamping ring  21  has been pressed on are pressed into the material of the plastic sleeve  20  in a subsequent work step and create a high-pressure-proof positive engagement, assure further sealing locations, which contribute to increasing the security against the entry of media. At the same time, in this region the plastic sleeve  20  is held in its position by the bite and sealing edges  32 ,  33  and the clamping lugs  22 . 
         [0041]    The clamping rings  21  shown in the drawings along with their connection to the actuator foot  23  and the plastic sleeve  20  can also be provided with additional characteristics, not always explicitly shown here. It is advantageous here if a partner in the connection, preferably the clamping ring  21 , is given a soft, easily deformable surface, for instance a gold-plated surface, on an elastic metal substrate. This soft surface provides for a strong sealing action because of the plastic positive engagement in the surface structures of the joined partners, that is, the actuator foot or actuator head and the clamping ring  21 . 
         [0042]    To save space, the metal actuator head or the actuator foot may be provided with offsets or indentations, to minimize the radial structure. This is helpful above all in the case of the preassembled, thrust-on variant of the clamping rings  21  in  FIGS. 2   a  through  6   c , since here the plastic sleeve  20  can be joined to the clamping ring  21  in a previous work step and positioned exactly above the indentation. Especially in large-scale mass production, this simplifies assembly considerably. 
         [0043]    The foregoing relates to 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.