Patent Application
20060113870
The invention relates to an actuator unit comprised of a piezoelectric actuator. Actuator units of this kind are use among other things in fuel injection systems and in particular in fuel injection valves since the switching times of such actuator units are very short. The short switching times permit a more exact metering of the injected fuel quantity and permit an improved shape of the injection curve over time. In connection with the present invention, the generic term “fuel injection valve” is understood to mean all types of fuel injection valve, for example, the injectors for common rail injection systems or injection nozzles of conventional fuel injection systems. A fuel injection valve with a piezoelectric actuator is triggered by applying an electric voltage to the piezoelectric actuator; this causes a rapid expansion of the piezoelectric actuator due to known physical effects of piezoelectric ceramic and causes a valve-closure member to lift away from a valve seat. The piezoelectric actuator has a certain mass that is accelerated in the course of this. If the voltage applied to the actuator is reduced, this causes the actuator to contract. As a function of the triggering speed, the inertia of the previously accelerated mass of the actuator generates damaging tensile forces in the piezoelectric actuator, in particular causing fractures in the soldered connections between the individual layers of the piezoelectric actuator. In order to prevent this kind of damage, it has become common practice to use a cylindrical hollow body in the form of a spring to prestress the piezoelectric actuator in the axial direction. A device of this kind is known, for example, from WO 00/08353 (Siemens). This hollow body is bent from a flat metal sheet and welded at the first joint thus produced. The first joint extends parallel to the longitudinal axis of the hollow body.
Among other things, the welding of the first joint has the following disadvantages: the welding causes a generally undesirable structural change to the hollow body in the immediate vicinity of the welding seam. A second problem is the spatters produced during the welding, which can lead to difficulties in assembly of the actuator unit or can even lead to functional failures of the fuel injection valve when one or more spatters come loose during operation. A third problem is a sinking-in of the welding seam (seam sinkage) at the beginning and end of the welding seam and the resulting notch effect and voltage spikes.
The actuator unit according to the present invention has a hollow body and a piezoelectric actuator. The hollow body is elastically embodied, prestresses the actuator, is provided with recesses, has a joint extending parallel to the longitudinal axis, has a bridge piece between each pair of recesses, and has a first end and a second end. According to the present invention, the recesses adjacent to the joint are smaller than the rest of the recesses.
Alternatively, it is also possible according to the present invention for the bridge piece between a recess adjacent to the joint and a recess adjacent to said recess to be wider than the bridge pieces between the rest of the recesses.
A disadvantage in actuator units with hollow bodies whose joints are not closed is that the spring rigidity in the axial direction is not constant over the entire circumference. As a rule, the spring rigidity of the hollow body is reduced in the region of the joint. This causes the hollow body to act on the piezoelectric actuator with forces in the axial and radial direction in addition to bending moments. This results in the uneven exertion of forces and bending moments on the piezoelectric actuator, which is undesirable.
The embodiments according to the present invention, which can be comprised in embodying the recesses adjacent to the joint as smaller than the rest of the recesses of the blank and/or embodying the bridge pieces in the region of the joint as wider than in the rest of the blank, serve to intentionally reinforce the hollow body in the regions in the immediate vicinity of the joint so as to compensate for the reduction in the spring rate in the region of the joint. It is therefore possible to achieve a spring rate of the hollow body that is constant and/or rotationally symmetrical over its entire circumference so that the piezoelectric actuator that the spring force of the hollow body acts on is loaded with forces exclusively the axial direction and not with lateral forces or bending moments. This can significantly extend the service life of actuator units equipped with a hollow body according to the present invention. It has turned out to be advantageous if the ratio of the width of a bridge piece between a recess adjacent to the joint to the width of the remaining bridge pieces of the blank has a value between 1.3 and 1.9, preferably 1.6. This means that the bridge pieces in the immediate vicinity of the joint are wider, for example by a factor of 1.6, than the rest of the bridge pieces of the blank.
In special cases, it can also be helpful to embody the width of the bridge pieces as a function of the load; the widths of the bridge pieces can differ from one another by up to a factor of 3.
The recesses in the blank are advantageously disposed so that when the blank is formed into a hollow body, they are disposed in planes and the planes extend parallel to one another. This improves the behavior of the hollow body and makes it easier to manufacture.
It is particularly advantageous if the recesses are disposed in an odd number of planes in the axial direction. In exemplary embodiments that were tested in practice, 15 or 17 turned out to be an advantageous number of planes. Providing an odd number of planes in the blank assures that the uppermost and lowermost planes are the same so that the behavior of the hollow body at its upper end is the same as the behavior of the hollow body at its lower end. This measure also improves the behavior of the hollow body in that at its end faces, the hollow body only transmits axially oriented spring forces to the piezoelectric actuator, a booster piston of a hydraulic coupler, or another component of the injector.
It has also turned out to be advantageous if a number of recesses are disposed one after another in a plane and this plane forms a right angle with the longitudinal axis of the hollow body. It is particularly advantageous if there is an even number of recesses in a plane. This arrangement results in the fact that the spring rate is constant over the circumference of the hollow body and consequently, no lateral forces are transmitted to the actuator.
For reasons involving the manufacture and durability of the hollow body, it has turned out to be advantageous if the recesses are embodied as bone-shaped and extend lateral to a longitudinal axis of the hollow body.
The “bone-shaped” geometry of the recesses can be described in that the recesses are comprised of a middle piece and two head pieces; the head pieces have at least a first radius, the middle piece has a second radius, and the recesses have a length. Various trials have shown various proportions to be favorable among the principle measurements of the first radius (R1), the second radius (R2), and the length (L), as well as the width of the bridge piece at the joint in relation to the width of the rest of the bridge pieces:
In a favorable embodiment form, the radius R1 of a recess adjacent to the joint is smaller by a factor of 0.867 than the radius R1 of the rest of the recesses. In addition, the second radius R2 of a recess adjacent to the joint is larger by a factor of 1.317 than the radius R2 of the rest of the recesses of the blank. Moreover, the length of a recess adjacent to the joint is shorter by a factor of 0.984 than the length of the rest of the recesses. The width of the bridge piece at the joint is expressed by the equation b>a/2; in particular b=1.4·a/2. A detailed description of the related values, in particular the values “a” and “b,” is given below in conjunction with the figures.
In another exemplary embodiment, it has also turned out to be advantageous if the recesses adjacent to the joint have the following dimensions:
In another embodiment form, the recesses adjacent to the joint have the following dimensions:
For the rest of the recesses that are not adjacent to the joint, the following dimensions have turned out to be favorable:
In another advantageous exemplary embodiment, the recesses that are not adjacent to the joint have the following dimensions:
It has also turned out to be advantageous if the first radii of the head pieces of a recess adjacent to the joint are different from each other, which will be explained by way of example below in conjunction with
It is also advantageous if the recesses of two adjacent planes are offset from one another. It is particularly advantageous if the offset of the recesses of two adjacent planes is equal to half the repeat pattern of the recesses of a plane. The term “repeat pattern” will be explained in greater detail below in conjunction with
According to the present invention, the hollow body can also have a region that is not perforated by recesses at its first end and/or at its second end. As a result of this, the spring force transmitted by the hollow body to a cover plate or another component of the injector is comparatively, since the hollow body is intentionally reinforced in the region of its ends. This translates into a reduction in the maxima of the spring force over the circumference of the hollow body and further alleviates the problem of lateral forces introduced into the piezoelectric actuator by the hollow body.
The hollow body according to the present invention can be used in actuator units in which the piezoelectric actuator is disposed inside the hollow body and in which the prestressed hollow body acts on the piezoelectric actuator with compression. This means that the hollow body itself is loaded with tension.
The hollow body according to the present invention can, however, also be used in actuator units in which the piezoelectric actuator is disposed outside the hollow body and the prestressed hollow body acts on the piezoelectric actuator with compression. In this case, the hollow body is usually loaded with compression.
In order to be able to transmit the prestressing force of the hollow body to the piezoelectric actuator in the best possible way, it is advisable for the first end of the hollow body to be connected to an upper cover plate or an adjusting disk and for its second end to be connected to a lower cover plate or a coupler housing. These connections can be produced, for example, by means of welding or crimping.
If only a radial fixing of the hollow body is required, then this can occur by means of an annular groove or a shoulder in the upper and/or lower cover plate or in the adjusting disk and coupler housing. This can be sufficient, for example, if the hollow body is loaded not with tension but with compression. In these embodiment variants, it is particularly advantageous that the annular groove and the shoulder center the hollow body in relation to the piezoelectric actuator or to the hydraulic coupler. This effect can be further improved if the annular groove and shoulder are dimensioned so that they cause the hollow body to flare slightly during assembly.
Other advantages and advantageous embodiments of the present invention can be inferred from the following drawings, their description, and the claims.
The hollow body 4 and the cover plates 5, 6 act on the piezoelectric actuator 1 with compression by means of a prestressing force. This means that before being welded to the upper and lower cover plate 5, 6, the hollow body 4 is prestressed and then welded.
The hollow body 4 is preferably made of spring steel. The hollow body 4 is provided with a multitude of recesses 7 in order to be able to set a desired spring rate with a predetermined wall thickness “s.” For the sake of clarity, not all of the recesses in
Since the present invention essentially relates to an actuator unit and a hollow body 4 associated with it, not all of the details the injector 71 are explained; instead, essentially only the connection of the actuator unit to the injector 71 is described. The remaining functionalities of the injector 71 are already known to those skilled in the art in the field of injection technology and therefore require no further explanation.
The injector 71 has a high-pressure connection 73. Highly pressurized fuel (not shown) is supplied to the injector 71 via the high-pressure connection 73. If an injection into the combustion chamber, not shown, of an internal combustion engine is to take place, a nozzle needle 75 lifts away from its seat, not shown, and unblocks injection orifices that are also not shown. A piezoelectric actuator 79 actuates a control valve 77, which controls the nozzle needle 75. Between the piezoelectric actuator 79 and the control valve 77, a hydraulic coupler 81 is provided, an enlargement of which is depicted on the right side of
The hydraulic coupler 81 is essentially comprised of a valve piston 83 and a booster piston 85 that are guided in a coupler housing 86. Between the valve piston 83 and the booster piston 85, there is a coupler gap 87, which is filled with fuel (not shown). This coupler gap 87 is required, among other reasons, because the thermal expansion coefficients of the piezoelectric actuator 79 and the metallic components of the injector 71 differ greatly from each other.
By means of its valve piston 83, the hydraulic coupler 81 actuates the control valve 77, while a projection 89 of the booster piston 85 rests against the piezoelectric actuator 79. A hollow body 4 according to the present invention, which is prestressed with compression, presses the booster piston 85 against the piezoelectric actuator 79, thus acting on it with a compressive prestressing force. The first end 15 of the hollow body 4 rests against a shoulder 91 of the coupler housing 86. The second end 17 of the hollow body 4 rests against an adjusting disk 93. The adjusting disk 93 transmits the spring force of the hollow body 4 to the projection 89 of the booster piston 85 and therefore to the piezoelectric actuator 79.
In order to position the hollow body 4 concentric to the hydraulic coupler 81 and therefore also concentric to the piezoelectric actuator 79, the diameter D, of the shoulder 91 is matched to the inner diameter of the hollow body 4 so that the hollow body 4 flares slightly when it is slid onto the shoulder 91. Since the hollow body 4 according to the present invention has a first joint 31 (not shown) extending over the entire length of the hollow body 4, this allows the hollow body 4 to flare relatively easily so that it fits onto the shoulder 91.
If, as in the exemplary embodiment according to
The blank 9 is curved or bent to form a cylindrical or polygonal hollow body so that the edges 15 and 17 constitute the first end 15 and the second end 17 of the hollow body 4 (see
When the blank 9 is bent in the above-mentioned fashion to form a cylinder or polygon, the edges 11 and 13 touch each other and form a first joint 31 (see
The blank 9 contains groups of recesses 7, each of which comprises a number of recesses in a row. The recesses are separated from one another by bridge pieces 19. Here, too, for the sake of clarity, not all of the bridge pieces 19 of the blank 9 have been provided with reference numerals. When the blank 9 is bent to form a hollow body in the manner described above, the recesses 7 disposed in a row lie in a plane. By way of example, a line 20 in
As is clear from
When the blank 9 is rolled to form a hollow body 4 (see
It is clear that wherever a bridge piece 19 “supports” the edge 15, a large force F can be transmitted, as indicated by the maxima 27 of the line 25. The sole exception is where the edges 11 and 13 abut each other. The “cut” recess 7 there, with its parts 7′ and 7″, weakens the structure of the blank 9 so that the force F transmitted between the upper cover plate 5 and the hollow body 4 is weaker at this point. This fact is indicated in
The edge 17 behaves similarly. As is clear from
As is clear from the lower F/j graph in
As a result of this circumferentially uneven transmission of force between the upper cover plate 6 and the edge 15 on the one hand and between the lower cover plate 5 and the edge 17 on the other hand, the hollow body 4 produces a bending moment that acts on the upper cover plate 6 and the lower cover plate 5 when the hollow body 4 is attached with a prestressing force to the upper and lower cover plates 6, 5. This bending moment is naturally also transmitted to the piezoelectric actuator 1, which has an unfavorable effect on its operational reliability and lifespan. This bending moment is also undesirable in hydraulic valve elements actuated by the actuator unit.
According to the present invention, in the blank 9, the recesses 7a and 7b adjacent to the edges 11 and 13 have a geometry that has been altered in comparison to the rest of the recesses 7, not all of which have been provided with reference lines. The different geometries of the recesses 7, 7a, and 7b will be explained in greater detail below in conjunction with the detail A from the blank 9. In this exemplary embodiment, the recesses 7a and 7b have the same geometry. As is clear from
There is a bridge piece 19 between each pair of recesses 7. The first row of recesses 7 that are disposed in the immediate vicinity of the edge 17 is comprised of six recesses 7. The six recesses 7 of the first row are disposed so that one recess is split. This recess 7 is divided into two symmetrical halves by the edges 11 and 13.
The second row contains four recesses 7 and one each of recesses 7a and 7b. The recesses 7a and 7b are disposed so that they are in the immediate vicinity of the edges 11 and 13. Since the recesses 7a and 7b are smaller than the recesses 7, the hollow body 4 is reinforced at a circumference angle j of 30° and a circumference angle j of 330°, namely in those places where the recesses 7a and 7b influence the spring rate of the hollow body 4. This reinforcing in the region of the circumference angles of j=30° and 330° compensates for the weakening of the hollow body 4 by the joint 31 disposed between the edges 11 and 13 (see
With regard to the width of the bridge pieces 19, which is labeled with “a” in detail A, and the width “b” of the half bridge pieces 41 between a recess 7a and the edge 11 and between a recess 7b and the edge 17, respectively, the following quantitative relationship has turned out to be advantageous. The width b of the half bridge piece 41 should be greater than a/2, in particular, should reflect the equation b=1.4·a/2.
a,
8
b, and 8c show blanks 9 or details of blanks 9, with a dimensional depiction of the recesses 7a and 7b adjacent to the joint 31. These exemplary embodiments are also able to attain the object underlying the present invention, which is essentially comprised in achieving a prestressing of the piezoelectric actuator 1 and 79 in the axial direction without exerting any lateral forces.
The embodiment forms whose details are depicted in
However, reference is made to the lower detail B in
The blank 9 is not perforated in the region of the edges 15 and 17 that correspond to a second end and a first end of the hollow body 4. This reinforces the hollow body 7 in the region of its first end 17 and in the region of its second end 15, which reduces the value of the maxima 27 (see
A second essential measure for improving the hollow body 4 is comprised in individually adapting the width a of the bridge pieces 19 to the loads that occur. The bridge piece 19.1 in the first row of recesses 7 that are disposed in the immediate vicinity of the edges 11 and 13 is thus wider than a bridge piece 19.2 that is disposed in the blank, farther away from the edges 11 and 13. In the exemplary embodiment shown, the width a, of the bridge piece 19.1 adjacent to the edges 11 and 13 is 1.2 mm, whereas the other bridge pieces 19.2 have a width a2 of only 0.75 mm. Depending of the dimensioning of the bridge piece widths a1 and a2, there can even be an overcompensation for the weakening of the hollow body 4 due to the presence of the joint 31. This effect is demonstrated in
In conjunction with
As a matter of course, each of the characteristics described in the specification, the drawings, or the claims can be essential to the present invention either individually or in combination with other characteristics.
| Number | Date | Country | Kind |
|---|---|---|---|
| 103 19 600.5 | May 2003 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/DE04/00565 | 3/19/2004 | WO | 6/8/2005 |
