Patent Application
20100199952
The invention relates to a piezoelectric actuator module having a protection layer system and to a method for its production, as generically defined by the preambles to the main claim and to the coordinate method claim.
A piezoelectric actuator module of this kind can be used for instance in a piezoelectric injector for precise metering of fuel in terms of timing and quantity in an internal combustion engine. The piezoelectric injector essentially comprises a holder body for the piezoelectric actuator module and at least one piezoelectric actuator, disposed in the piezoelectric actuator module, having piezoelectric elements stacked one above the other and disposed between an actuator head and an actuator base, each of which comprises piezoelectric layers enclosed by inner electrodes.
With the use of a material of a suitable crystalline structure (piezoelectric ceramic) for the piezoelectric layers, the piezoelectric elements of the piezoelectric actuator are constructed in such a way that when an external voltage is applied to the inner electrodes, a mechanical reaction of the piezoelectric elements ensues, which as a function of the crystalline structure and the contact regions of the electrical voltage represents a compression or tension in a predeterminable direction. In the case of a piezoelectric injector, the piezoelectric actuator is connected a nozzle needle, so that by application of a voltage to the piezoelectric elements, a nozzle opening is uncovered.
These common rail injectors can be constructed in such a way that a nozzle needle controlled indirectly by the piezoelectric actuator is present, and the piezoelectric actuator is surrounded directly or indirectly by the pressure of the fuel, and only a hydraulic coupling chamber is provided between the nozzle needle and the piezoelectric actuator. If the piezoelectric actuators are located in the low-pressure range of the fuel (approximately 60 bar), as is the case with indirectly controlled needle motions, then a metal sleeve seal of the piezoelectric actuator can certainly be achieved with a membrane as a stroke-compensating and/or temperature-compensating element.
In Diesel kinks, in what is called direct nozzle needle control, the piezoelectric actuators are operated directly in the Diesel fuel at high pressure (approximately 2000 bar); the aforementioned relatively rigid sleeve/diaphragm seal would not withstand the high system pressures and the greater reciprocating motions of the piezoelectric actuator, given the space available.
To protect the piezoelectric elements of the piezoelectric actuator, for instance in view of their electrical insulation as well, it is therefore also known to sheathe the piezoelectric actuator with an elastomer. Since the presence of fuel, especially with regard to its conductive components, on the surface of the piezoelectric actuator can lead to an electric sparkover between the electrodes and thus to the failure of the piezoelectric actuator, the piezoelectric actuator must be protected against direct contact with the fuel, including by means of suitable tightness at the seam or connection locations.
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 piezoelectric elements of the piezoelectric actuator are disposed in a sheath provided with a metal bellows.
From International Patent Disclosure WO 2001 48834 A2, it is known that the electrical insulation of a piezoelectric actuator is provided by means of a protective jacket of plastic, which is applied for instance by injection molding. From U.S. Pat. No. 7,036,198 B2, it is also known that this protection layer system is formed of a shrink-on hose, which by means of a shrinkage process brings about the sealing relative to the surrounding medium.
The critical points of the seal are the attachments of the ends of the sheath or of the protection layer system, particularly to the actuator head and/or to the actuator base, so that good tightness is durably assured there as well.
The invention is based on a piezoelectric actuator module as described at the outset, having one or more piezoelectric actuators, which is or are fastened between an actuator head and an actuator base, and in which at least the piezoelectric actuator is provided with a protection layer system. According to the invention, a surrounding element for fastening the protection layer system to the actuator head, the actuator base, and/or the piezoelectric actuator, is provided which advantageously comprises a material having a shape memory and, after widening that is required for assembly, can be pressed onto the protection layer system. Preferably, to that end, the at least one surrounding element for contact with the protection layer system comprises a shape memory material, preferably an NiTi alloy (such as an NiTi=nickel-titanium alloy in which the proportions are 50/50) or an NiTiCu alloy.
The protection layer system of the piezoelectric actuator module of the invention, as a result of the proposed use of what are known as shape memory materials (shape memory alloys) is attached by simple means, durably and solidly, to the actuator head and/or the actuator base of the piezoelectric actuator module and is thus sealed off from the surrounding media, such as fuel.
From German Patent Disclosure DE 10 2005 045 230 A1, it is known per se that to insulate a conductor, a ring of shape memory material is provided, which is cold-widened and then, by being subjected to heat, is restored to the shape it had before it was widened.
The special advantage of using the materials according to the invention is thus that in certain regions, the shape memory materials exhibit what is called superelasticity or pseudoelasticity. In this region, they can be elongated mechanically by up to approximately 8% without deforming plastically. That is, the proposed surrounding elements, for instance comprising the corresponding NiTi alloys, in a version as a spring or ring, can be widened greatly, unlike the springs or rings made from conventional materials, and can thus be slipped over onto the actuator head or the actuator base without losing their later tensing or clamping force.
This allows greater degrees of freedom in the structural design of the actuator head or actuator base, so that larger diameters are possible, and a two-part design may possibly not even be necessary. On the other hand, the assembly of the components of the piezoelectric actuator module is simplified considerably, since mechanical widening of the surrounding element for simple assembly is possible within relatively wide limits, and furthermore, the tensing or clamping force can be adjusted in advance by way of the degree of widening.
In an advantageous method for producing a piezoelectric actuator module, in a first method step, the piezoelectric actuator and at least some portions of the actuator head and/or of the actuator base are coated with the protection layer system; in a second method step, the surrounding element of a material having a shape memory is widened within predetermined limits and is slipped onto the protection layer system in the region of the actuator head and/or of the actuator base; and/or in a further method step, preferably by a thermal process, the surrounding element is pressed onto the protection layer system at the actuator head and/or at the actuator base.
The assembly of a sufficiently large surrounding element is even possible without prior widening; in that case, the pressing is accomplished only by the shape memory effect, by means of a reduction in size by heating.
The piezoelectric actuator module according to the invention can advantageously be formed with a protection layer system of one or more layers of a polymer or metal material. This kind of proposed attachment of the protection layer system is especially advantageous whenever welding or soldering of the protection layer system is not possible, particularly in the case where a polymeric protection system, such as a shrink-on hose or shaped hose, is used.
The invention will be described below in terms of the exemplary embodiment shown in the drawings. In the drawings:
In
The piezoelectric actuator module 1 has a piezoelectric actuator 2 or a series of a plurality of such piezoelectric actuators 2, which are constructed with the piezoelectric elements described in the prior art section for effecting a mechanical stroke in the length of the piezoelectric actuator module 1. An actuator head 3 is shown here on the left face end of the piezoelectric actuator 2, and an actuator base 4 is shown on the right face end of the piezoelectric actuator 2. The piezoelectric actuator 2 is sheathed by a protection layer system 5 comprising one or more layers, which is sealingly attached mechanically to the actuator head 3 and the actuator base 4 by means of the surrounding elements 6.
For that purpose, the surrounding elements 6 for contact with the protection layer system 5 are preferably made from a shape memory material, preferably an NiTi alloy or an NiTiCu alloy. These shape memory materials can be cold-widened and then, by being subjected to heat, be returned to the shape they had before they were widened, without plastic deformation.
In the production of the piezoelectric actuator module 1, the surrounding elements 5 can be widened within predeterminable limits after the protection layer system 5 has been attached and can be slipped onto the protection layer system 5 in the region of the actuator head 3 and/or the actuator base 4 and past them. By reverse expansion in the elastic range or by subjection to heat, the surrounding elements 6 can then be pressed onto the protection layer system 5 at the actuator head 3 and/or the actuator base 4.
The protection layer system 5 of the piezoelectric actuator module 1 of the invention, by the proposed use of shape memory materials (shape memory alloys) for the surrounding elements 6, can be attached by simple means, durably and solidly, to the actuator head 3 and/or the actuator base 4 of the piezoelectric actuator module 1, and thus the piezoelectric actuator module is sealed off from the surrounding media, such as fuel.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2007 042 400.2 | Sep 2007 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP08/59572 | 7/22/2008 | WO | 00 | 3/2/2010 |
