Abstract:
A temperature compensated actuator device comprises a piezoelectric stack having first and second ends along a central axis and having a first thermal expansion coefficient; and a compensator arranged on one end of the piezoelectric stack. The compensator comprises a first member in form of a cylinder; a second member in form of a piston plate wherein the first member and the second member are arranged movably along the axis with respect to each other and define a hollow space between them; and a compensation member having a thermal expansion coefficient higher than the first thermal expansion coefficient for filling the hollow space.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention relates to a piezoelectric actuators and their application within fuel injector valves. Such a piezoelectric actuator may be to actuate a control valve such as a servovalve within fuel injection valve for controlling flow of a medium. These kind of valves can be used for the direct control of an injection needle within an fuel injection valve or indirect control of a needle by means of a control valve. These kind of valves usually comprise a housing having an inlet duct and an outlet duct and, a movable body which can reciprocate in order to control communication between the inlet and the outlet ducts, and drive means for the reciprocating movement of the body.  
         [0002]     A known valve of this kind includes a piezoelectric actuating member supported in a valve housing, the housing having an inlet duct and an outlet duct and a movable body which can be displaced by the actuating member in order to control in operation a flow of pressurized fluid through a connection between the inlet and outlet ducts, the surfaces of the movable body on which pressurized fluid can produced hydrostatic force being arranged to minimize any resulting hydrostatic force in a direction tending to oppose movement of the actuating member.  
         [0003]     The possible extension of the piezoelectric element of such a valve and thus the potential stroke of the valve is usually small, for example, of the order to 10 or 20 micrometers. It is therefore essential to minimize the effect of thermal expansion on the valve parts so that the flow through the valve will vary only slightly with changes of ambient temperature. One solution to the problem is to choose a material for the valve housing which material has a similar thermal expansion coefficient to that of the piezoelectric element. An example of such a material is a nickel-iron alloy having a low coefficient of thermal expansion. A possible disadvantage to the use of this material is the high cost of the alloy.  
         [0004]     U.S. Pat. No. 4,284,263 discloses a control valve having an actuating member supported within and between a closed end and a seat end of a housing, the actuating member has a fixed end connected to the close housing end and a free end connected to a movable valve body. The actuating member includes an elongated piezoelectric element which has a relatively low thermal coefficient of expansion. The housing is made from common, inexpensive materials having moderately higher thermal coefficient of expansion, and the actuating member includes a spacer having a high thermal coefficient of expansion, arranged between the closed housing end and the piezoelectric element. By proper dimensioning of the spacer and housing with respect to the piezoelectric element, temperature compensation may be achieved without the use of exotic materials. However, as shown in  FIG. 2  of U.S. Pat. No. 4,284,263, the compensating structure elongates the device significantly.  
       SUMMARY OF THE INVENTION  
       [0005]     It is, therefore, the object of the invention to provide an alternative construction for a control valve which can enable the effects of thermal expansion to be reduced allowing a compact design of the device while maintaining low production costs.  
         [0006]     A temperature compensated actuator device may comprise, for example, a piezoelectric stack having first and second ends along a central axis and having a first thermal expansion coefficient; and a compensator arranged on one end of the piezoelectric stack. The compensator may comprise a first member in form of a cylinder; a second member in form of a piston plate wherein the first member and the second member are arranged movably along the axis with respect to each other and define a hollow space between them; and a compensation member having a thermal expansion coefficient higher that the first thermal expansion coefficient for filling the hollow space.  
         [0007]     A fuel injector valve may comprise a body having an inner cavity for receiving a piezoelectric actuator, wherein the cavity comprises an opening which forms a control valve by means of a valve member which can be actuated by the piezoelectric actuator, wherein the piezoelectric actuator device comprises a piezoelectric stack having first and second ends along a central axis and having a first thermal expansion coefficient; and a compensator arranged on one end of the piezoelectric stack. The compensator may comprise a first member in form of a cylinder; a second member in form of a piston plate wherein the first member and the second member are arranged movably along the axis with respect to each other and define a hollow space between them; and a compensation member having a thermal expansion coefficient higher that the first thermal expansion coefficient for filling the hollow space.  
         [0008]     The actuator device may further comprise a top plate and a bottom plate in between which the piezoelectric stack and the compensator are arranged. The top plate may comprise at least one opening through which the piezoelectric stack can be electrically contacted. The piezoelectric stack may comprise a plurality of piezoelectric elements. The first member can be a cup shaped cylinder having an opening and the second member can be a plate having an elevated section which fits within the opening. The actuator device may further comprise a tube spring coupling the top and bottom plate for preloading the compensator. The tube spring can be made of metal and may have a thermal coefficient of about 11,5×10 −6 /K. The first member may comprise an inner cavity and an opening, wherein a piston plate of the second member is movably arranged within the cavity through the opening to define the hollow space. The actuator device may further comprise a spring arranged within the cavity between the piston plate and the opening. The first member may comprise two parts which can be coupled via a connecting thread. The second member may comprise two parts which can be coupled via a connecting thread. The compensation member can be made of plastic having a high thermal expansion coefficient, such as, about 100×10 −6 /K. The first and second member can be made of metal having a thermal coefficient of about 11,5×10 −6 /K.  
         [0009]     One advantage of such a compensator for a piezoelectric actuator is that it can be designed as a very compact unit because the compensation member is substantially prevented from expanding in a radial direction by a respective housing. This suppression of radial expansion causes an even stronger expansion in an axial direction. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     By way of example, a particular embodiment of the invention will now be described with reference to the accompanying drawings, in which:  
         [0011]      FIG. 1  shows a cross sectional view of a piezoelectric actuator according to the present invention;  
         [0012]      FIG. 2  shows exemplary dimensions of the piezoelectric actuator as shown in  FIG. 1 ,  
         [0013]      FIGS. 3 and 4  show the compensator in a biased and non-biased state,  
         [0014]      FIG. 5  shows another embodiment of the compensator device, and  
         [0015]      FIG. 6  shows a partial sectional view of a fuel injector valve using the actuator according to the second embodiment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]      FIG. 1  shows a sectional view of piezoelectric actuator for use in, for example, a fuel injection valve. Axis A indicates the expansion axis along which the piezoelectric actuator expands to engage with, for example, a control valve member for opening or closing a valve (not shown). The actuator comprises a bottom end plate  2  and top end plate  3  between which a piezoelectric stack  4  is arranged. Top and bottom end plates  3  and  2  can be made from metal such as normal steel or any other suitable material. The piezoelectric stack consists of a plurality of piezoelectric elements and thus comprises a plurality of connecting wires  7  which are combined to two single wires  6  which are led through respective openings in the top end plate  3 . The piezoelectric stack may have an exemplary expansion coefficient of 2×10 −6 /K. On top of the piezoelectric stack between the top end plate  3  and the bottom plate  2  is a compensator device arranged in a suitable way to compensate thermal induced length changes of the piezoelectric stack  4  as will be explained in more detail below.  
         [0017]     This compensator comprises a sandwich arrangement of a piston plate  9 , a compensator cylinder  10  and a cylinder  8 . The piston plate  9  comprises an elevated piston section on top of which the compensator cylinder  10  is arranged. Cylinder  8  is shaped like a cup and completely covers the compensator cylinder  10  and parts of the elevated piston in such a way that the assembly of the three elements is able to expand along the expansion axis of the piezoelectric stack  4 . Cylinder  8  and piston plate  9  are manufactured from a suitable metal, preferably form the same material as the housing of the injector valve. For example normal steel can be used for these parts. The expansion of the compensator is basically controlled and thus dominated by the compensator cylinder  10 . The bottom end plate  2  and the top end plate  3  are coupled and preloaded by a tube spring  1  connecting the top and bottom plate  3  and  2 .  
         [0018]      FIG. 2  shows the same piezoelectric actuator including particular designated expansion areas. The overall length of the actuator is designated by x 1  and will expand to the same extent as a not shown housing of the injection valve. The length of the piezoelectric stack is designated as x 2  and comprises a relatively small thermal expansion when compared to the expansion of the housing of the injection valve. This small expansion is compensated by the relatively large expansion of the compensator whose length is designated by x 3 .  
         [0019]      FIG. 3  shows the compensator when assembled within the piezoelectric actuator. Without the compensator cylinder  10  inserted, cylinder  8  can move along the expansion axis A of the piezoelectric actuator  4  with respect to the piston plate  9  wherein the elevated piston of the piston plate  9  extends into the opening of the cup shaped cylinder  8 . The opening of cavity defined by piston plate  9  and cylinder  8  is however filled with the solid compensation cylinder  10 . This compensation cylinder  10  can be preferably made of a plastic with a high expansion coefficient. Due to the preload of the tube spring  1 , the plastic cylinder  10  completely fills out the cavity defined by the cylinder  8  and piston plate  9 .  FIG. 4  shows the compensator without the preload of the tube spring  1 . The size of the plastic cylinder  10  is chosen in such a way that the gap defined between the side wall  11  of the plastic cylinder  10  and the inner side wall  12  of the cylinder  8  disappears when the preload of the tube spring compresses the sandwich arrangement. Thus, when the preload of the tube spring is engaged, the whole sandwich arrangement becomes stiff or rigid. The deformation of the plastic cylinder  10  is, thus, supposed to take place within an elastic range.  
         [0020]     In case of an increase in temperature, the plastic cylinder will expand at a rate of approximately 100·10 −6 /K. However, other appropriate materials for the compensation cylinder can be used as long the thermal expansion coefficient is high enough to allow for a reasonable size of the compensator. The metallic cylinder  8  prevents almost all radial expansion of the plastic cylinder  10 . Thus, the expansion in axial direction will be multiplied by a factor of 3. In a first approximation, the plastic cylinder must therefore have a length which is defined by the following equation:  
         x1   -       (     x3   +   x2     )     ×   11.5   ×       10     -   6       K       +     x2   ×   2   ×       10     -   6       K       +     x3   ×   300   ×       10     -   6       K         =       x1   ×   11.5   ×       10     -   6       K     ⁢   x3     =         9.5   288.5     ⁢   x2     ≈       1   30     ⁢   x2             
 
         [0021]     Thus, depending on the expansion factor of the plastic cylinder, its length x 3  need only be approximately up to {fraction (1/30)} of the length x 2  of the piezoelectric stack  4 . However, using different materials for the compensation cylinder  10 , different ratios may be achieved. Thus, a much more compact design of an actuator and, therefore, a fuel injection valve is possible while cost efficient materials can be used.  
         [0022]      FIG. 5  shows a second embodiment of a compensator  20 . This type of compensator comprises a “built-in preload” function. Thus, the actuator does not need a tube spring to preload the compensator. The compensator  20  comprises only a few parts which allow an easy assembly. The compensator body is formed by two parts  15  and  16 . The top cover  16  has the shape of a cup and comprises an outside thread on the lower part of the side wall. The bottom cup  15  comprises an opening  22  in its bottom plate the inside of the upper wall is provided with an inner thread which engages with the outer thread  19  of the top cover  16 . When the bottom cup  15  is screwed on the top cover, both parts form a can with an opening in the bottom plate. A stamp plate  14  can be arranged in the inside of this can assembly. The stamp plate  14  comprises an extension cylinder which extends outside the can assembly. The end of the extension cylinder may comprise another outside thread  21 . A connection plate  13  comprises a blind hole with an inner thread. Thus, the connection plate can be screwed on the stamp plate  14 . However, elements  13  and  14  can also be manufactured in a single piece if the design allows positioning of the piston plate  14  within the compensator body  15  and  16 . Inside the compensator  20 , the same plastic cylinder  17  is provided between the top cover and the stamp plate  14 . For providing a preload, a spring  18  is arranged between the stamp plate  14  and the bottom cup  15 . Thus, a very compact compensator is provided without the need of a tube spring to preload the piezoelectric stack-compensator assembly because the compensator  20  has an integrated preload function.  
         [0023]     A variety of other embodiment within the same concept is possible. For example, the compensator body might be manufactured from a single part having an opening that allows insertion of the respective elements  14 ,  17 , and  18 . Snap in technology can be used instead of threads and different embodiments of the spring  18  can be used to generate the preload. For example, the bottom part  15  may have an integrated spring element.  
         [0024]     The compensator  20  as shown in  FIG. 5  can be assembled as follows: The spring is inserted into the bottom cup  15 . Then, the piston plate is inserted whereby the extension comprising the thread  21  reaches and extends through the opening  22 . Next, the connection plate  13  is screwed onto this extension. Finally, the plastic cylinder is placed on top of the piston plate  14  and the top cover is screwed onto the bottom cup  15  to form a rigid compensator arrangement  20 .  
         [0025]      FIG. 6  shows a partial sectional view of an exemplary arrangement of a fuel injection valve using a compensated piezoelectric stack with a compensator as shown in  FIG. 5 . Like elements are designated with like numerals as used in the previous figures. The compensator  20  is fixedly mounted on top of the piezoelectric stack  4  by means of the mounting plate  13  which is coupled with the piston plate  14 . An end plate  24  is provided on the other end of the piezoelectric stack  4  which connects to a actuator member  25  for engaging with the control member  26 . Control member  26  is preloaded by a spring (not shown) to engage with the edge  27  to form a control valve. The control valve, thus, opens or closes a connection between a first chamber  28  and a second chamber formed below the control member  26  (not shown). Respective inlets and outlets (not shown) for these chambers are used to control the pressure within the injection valve to open or close a respective injection valve (not shown). These mechanism are well known in the art and not further explained. The whole arrangement can be placed within a housing  23 . The housing can be of an inexpensive material such as normal steel or any other suitable metal. The larger thermal expansion of this housing in relation to the smaller thermal expansion of the piezoelectric stack will be compensated by the large expansion of the plastic cylinder  17  within the compensator  20  as explained above. Thus, a compact design of a fuel injector is possible which at the same time can be manufactured in an inexpensive manner.