Piezoelectric actuator

Abstract

Piezoelectric actuating device (11, 11'), in particular a piezoelectric metering valve, with a piezoelectric stack (17) comprising a plurality of stacked layers (21) of piezoelectric material, each provided with electrodes (19, 20), and a lever (22, 22') mechanically connected to the piezoelectric stack (17) for converting a drive movement of the stack (17) into an actuating movement of an actuating element (15) provided on the lever (22, 22'), in particular a valve plunger. and with a cooling device (30) for removing heat from the piezoelectric stack (17), wherein the piezoelectric stack (17) is divided into at least two substacks (17A, 17B) arranged side by side and each equipped with associated electrodes (19, 20), wherein the substacks (17A, 17B) are separated from each other by a space (34), wherein the cooling device (30) is at least partially arranged in the space (34) and / or fluid present in the space (34) forms part of the cooling device (30), and wherein the cooling device (30) comprises at least one cooling element (37, 39) made of a thermally conductive material, preferably metal, arranged at least partially in the space (34), characterized by the fact that a base cooling fin (37) extending from a base (25) and projecting into the space (34) and / or a lever cooling fin (39) extending from the lever (22, 22') and projecting into the space (34) is / are provided as a heat sink.

Description

[0001] The present invention relates to a piezoelectric actuating device, in particular a piezoelectric metering valve, with a piezoelectric stack comprising a plurality of stacked layers of piezoelectric material, each provided with electrodes, and a lever mechanically connected to the piezoelectric stack for converting a drive movement of the stack into an actuating movement of an actuating element provided on the lever, in particular a valve plunger.

[0002] Piezoelectric actuators are characterized by high positioning accuracy and fast switching behavior. In such devices, the piezoelectric stack can be clamped between a base and a counter-base to provide the necessary preload. The lever can either be attached to the counter-base as a separate component or be integrally formed with it.

[0003] Especially with metering valves in process engineering, the aim is to achieve the highest possible switching frequency. A limiting factor here is the heat energy inevitably generated within the piezoelectric stack during operation, which typically amounts to about 10% to 20% of the supplied energy and can lead to undesirable overheating of the stack. Since piezoelectric ceramic components have a comparatively low thermal conductivity, the problem of potential overheating is particularly acute with high-power piezoelectric stacks and correspondingly large volumes.

[0004] DE 100 42 941 A1 discloses a piezoelectric actuator with two stacks arranged side by side, which are clamped between two clamping plates, wherein the associated clamping element extends through the space formed between the stacks.

[0005] In DE 10 2010 015 171 A1 a piezoelectric actuator with two stacks arranged side by side is disclosed, which are protected by a plastic covering provided with cooling channels.

[0006] DE 101 41 820 A1 discloses a piezoelectric component comprising several ceramic layers.

[0007] In DE 10 2006 026 932 A1 a piezo actuator with a housing and a heat sink attached to it is disclosed.

[0008] WO 00 / 63 980 A1 discloses a piezoelectric actuator with an actuator body in the form of a multilayer laminate.

[0009] It is an object of the invention to extend the application range of piezoelectric actuators of the type mentioned above and in particular to enable higher switching frequencies.

[0010] The problem is solved by a piezoelectric actuator with the features of claim 1.

[0011] According to the invention, a piezoelectric actuator comprises a cooling device for dissipating heat from the piezoelectric stack. Depending on the application, such a cooling device can operate passively, i.e., by transferring heat to the surrounding air, or actively using a transported cooling fluid. A combination of active and passive cooling is also possible. Due to the integrated cooling device, a piezoelectric stack of a given volume can be controlled at a higher frequency, as the risk of heat buildup within it is reduced.

[0012] The cooling device can comprise a layer of thermally conductive material through which at least one outer surface, in particular a side surface, of the piezoelectric stack is in contact with a wall section of a housing of the actuator. Such a layer with increased thermal conductivity allows heat energy to be efficiently dissipated from the stack surface, ultimately contributing to a temperature reduction inside the stack. Preferably, the layer of thermally conductive material extends over the entire surface of the outer surface in question. Particularly in the case of the narrow housing shape desired for many metering valves, connecting two opposite side surfaces of the piezoelectric stack to the housing wall via corresponding separate layers of thermally conductive material can be advantageous.

[0013] The invention provides that the piezoelectric stack is divided into at least two, preferably exactly two, adjacent sub-stacks, each equipped with its own electrodes. This increases the stack surface area while maintaining essentially the same power output, thus facilitating heat dissipation from the stack. If the application requires it, a division into more than two sub-stacks is also possible.

[0014] The piezoelectric stack can be divided, in particular, along a division plane parallel to the stacking direction, preferably into sub-stacks of identical size. In other words, the thickness of the stack is preferably divided. Such a division does not affect the maximum deflection of the stack.

[0015] According to the invention, the sub-stacks are separated from each other by an intermediate space. This counteracts unwanted heat transfer between the sub-stacks.

[0016] Furthermore, it is planned that the cooling device is at least partially located in the space between the stacks and / or that fluid present in the space, such as air, forms part of the cooling device. This not only enables effective heat dissipation from the area between the stacks, but also a particularly space-saving design.

[0017] According to the invention, the cooling device comprises at least one heat sink made of a thermally conductive material, preferably metal, arranged at least partially in the space between the stacks. This enables particularly simple and cost-effective cooling of the stack.

[0018] A base cooling fin extending from the base and projecting into the space between the two parts, and / or a lever cooling fin extending from the lever and projecting into the space between the parts, are provided as heat sinks. Such cooling fins can, for example, be molded directly onto the base or the lever, thus eliminating the need for separate heat sinks. The base and the lever can therefore be used for heat dissipation in addition to their mechanical function.

[0019] The cooling system can further include a thermally conductive material that fills the voids between the surfaces of the sub-stacks and any components located in the space between them. This can further improve heat dissipation from the stack.

[0020] Another embodiment of the invention provides that the piezoelectric stack is clamped by means of a clamping element between a base common to all sub-stacks and a counter-base common to all sub-stacks, the clamping element extending parallel to the stacking direction through the gap. Thus, only a single clamping element is required. In contrast, with an undivided stack, at least two clamping elements positioned laterally next to the stack are generally necessary to avoid excessively uneven clamping force. Utilizing the gap to guide the clamping element through the entire stack therefore contributes to a reduction in manufacturing costs and the required installation space.

[0021] The clamping element can include a shaft section with a flattened cross-section designed for placement in the space between the stacks. The flattened cross-section reduces the width of the clamping element in the area between the stacks, allowing the stacks to be positioned relatively close together. Specifically, the clamping element can be a clamping screw with a threaded end for attaching a clamping nut. Since clamping is achieved via the clamping nut in this case, the clamping screw itself does not need to have a completely round cross-section.

[0022] The invention also relates to an actuating device as described above, comprising a piezoelectric stack which includes a plurality of stacked layers of piezoelectric material, each provided with electrodes, a base mechanically connected to a first end face of the stack, and a lever mechanically connected to a second end face of the stack for converting a drive movement of the stack into an actuating movement of an actuating element provided on the lever, in particular a valve tappet.

[0023] According to one embodiment of the invention, at least one section of a force transmission arrangement comprising the base, the piezoelectric stack, and the lever is designed to be spring-elastic in order to cause the positioning movement to overshoot the drive movement. By utilizing this overshoot effect, it is possible to increase the positioning stroke for a given stack deflection. While it is common practice in the field to design the components of the force transmission arrangement to be as rigid as possible, the invention has shown that by selectively providing one or more spring-elastic sections, an overshoot of the lever and the positioning element provided on it can be achieved, which is accompanied by an increase in stroke.Such an increase in stroke is particularly advantageous because, for a given stroke and frequency, less energy needs to be supplied to the piezoelectric stack, thus generating less heat energy in the stack.

[0024] The lever can have a pivot point that allows a first lever part to pivot resiliently relative to a second lever part, or the lever itself relative to an adjacent component. This enables a particularly simple design. Preferably, the pivot point is located in the vicinity of the stack, i.e., at the end of the lever closest to the stack, in order to utilize the leverage effect during overshoot.

[0025] The joint can be formed by a flexible section of the lever. In this case, no separate joint components are required.

[0026] Specifically, the lever may have a reduced cross-sectional size and / or a modified cross-sectional shape in the area of ​​the joint. Such a "thin section" is particularly advantageous from a manufacturing perspective.

[0027] Furthermore, it is preferred that the resonant frequency of the oscillating force transmission arrangement is matched to a predetermined nominal frequency of the actuating element. The resonant frequency is determined by the mass of the relevant components, i.e., the base, the stack, the lever, and / or the actuating element, as well as by the spring constant of the elastic sections. By appropriately dimensioning these parameters, the oscillating system can thus be designed such that the normal operation of the actuating device essentially takes place at the resonant frequency. The stroke of the actuating element then does not follow the deflection of the stack, as in existing actuating devices, but is considerably greater than it—independent of the direct mechanical leverage effect. In addition, the time course of the stroke is phase-shifted by 90° relative to the time course of the stack deflection.An actuating device according to the invention can therefore be designed in such a way that it is predominantly operated in resonance, resulting in a particularly pronounced increase in stroke and a corresponding reduction in heat.

[0028] To ensure such resonant operation, an actuating device according to the invention can comprise an electronic control unit which is designed to operate the piezoelectric stack essentially at the resonant frequency of the oscillating power transmission arrangement.

[0029] Another embodiment of the invention provides that the stack is designed for a tilting drive movement, which is converted by the lever into a linear positioning movement of the actuating element. Such arrangements are particularly characterized by their small footprint.

[0030] Furthermore, the stack can have a piezoelectrically passive region in which no electric field is generated. The electrical contacts can be arranged in this piezoelectrically passive region to reduce their mechanical stress.

[0031] Further developments of the invention are also specified in the dependent claims, the description and the accompanying drawings.

[0032] The invention is described below by way of example with reference to the drawings. Fig. Figures 1A to 1C show a first embodiment of a piezoelectric actuating device according to the invention in a perspective view, a side view and a top view. Fig. Figure 2 shows a second embodiment of a piezoelectric actuating device according to the invention in a perspective view. Fig. 3A and Fig. Figure 3B shows the time course of the stroke of an actuator and the deflection of a piezoelectric stack of the in Fig. 2. Actuating device shown at different switching frequencies.

[0033] The in the Fig. The piezoelectric actuator 11 shown in Figures 1A to 1C is designed as a metering valve and comprises a housing 13 (only partially shown), a linearly displaceable actuator 15 in the form of a valve plunger, and a piezoelectric stack 17 for actuating the actuator 15. The piezoelectric stack 17 is constructed in a generally known manner from a plurality of stacked layers 21, each provided with electrodes 19, 20 ( Fig. 1C) is composed of piezoelectric material. Furthermore, the piezoelectric stack 17 is designed for a tilting drive movement, as shown in Fig. 1A is indicated by the double arrow. Specifically, the piezoelectric stack 17 operates according to the so-called double-stack principle and comprises, in addition to two piezoelectrically active areas, a piezoelectrically passive area, as disclosed in EP 0 947 002 B1.

[0034] To convert the tilting drive movement of the piezoelectric stack 17 into a linear positioning movement of the actuating element 15, a lever 22 is provided, which is mechanically connected to the piezoelectric stack 17 at one lever end 23 and carries the actuating element 15 at the other lever end 24. As shown in Fig. As can be seen in Figures 1A to 1C, the piezoelectric stack 17 is clamped between a base 25 and a counter-base 27. In the illustrated embodiment, the counter-base 27 forms a separate component to which the lever 22 is attached. In principle, however, the counter-base 27 could also be formed integrally with the lever 22.

[0035] By electrically activating the piezoelectric stack 17, the actuating element 15 can be moved relative to a valve seat (not shown) to control fluid metering. For this purpose, an electronic control unit (also not shown) is provided, which is connected to the electrodes 19, 20.

[0036] Particularly when the actuator 11 is operated at a high actuating frequency, considerable heat is generated in the piezoelectric stack 17. To dissipate this heat, a cooling device 30 is provided, which comprises several components.

[0037] Firstly, outer heat-conducting layers 33 are applied to the two lateral outer surfaces 31 of the piezoelectric stack 17, as shown in the figure. Fig. 1B and Fig. 1C. The heat-conducting layers 33 consist of a material with high thermal conductivity and each, on their flat side facing away from the stack 17, is in contact with a side wall of the housing 13 (not shown). Thus, effective heat dissipation from the lateral outer surfaces 31 of the stack 17 to the preferably metallic housing is possible.

[0038] As a further cooling measure, the piezoelectric stack 17 is divided into two sub-stacks 17A, 17B arranged side by side and each equipped with associated electrodes 19, 20. The two sub-stacks 17A, 17B do not directly adjoin each other, but are separated by a gap 34.

[0039] Due to the division, the surface area to volume ratio, and thus the heat dissipation capacity, of the entire stack 17 is greater than it would be for an undivided stack. Furthermore, heat sinks are arranged in the space 34, namely a base cooling fin 37 extending from the base 25 and a lever cooling fin 39 extending from the lever 22. As shown, the base cooling fin 37 and the lever cooling fin 39 each project approximately halfway into the space 34 and therefore largely fill it. In the remaining space between the base cooling fin 37 and the lever cooling fin 39 on the one hand, and the sub-stacks 17A and 17B on the other, internal heat-conducting layers 35 made of a material with high thermal conductivity are arranged.

[0040] The base 25, like the counter-base 27, is designed as a single unit. A clamping screw 43 serves to clamp the partial stacks 17A and 17B together between the base 25 and the counter-base 27. To generate a uniform clamping pressure, the clamping screw 43 is centrally located with respect to the entire stack 17. It extends parallel to a stacking direction S of the stack 17 through the gap 34. Fig. Figure 1A shows that the clamping screw 43 has a shaft section 45 with a flattened cross-section extending through the gap 34. A nut 47 is provided for tightening the clamping screw 43.

[0041] The in Fig. The actuating device 11' shown in Figure 2, designed according to a further embodiment of the invention, is similarly designed to the actuating device 11 described above, wherein identical or equivalent components are designated with the same reference numerals. A difference from the one shown in Fig. The actuating device 11 shown in Figures 1A-1C consists in the fact that the lever 22' is not designed as a rigid component, but has a flexible section 49. The flexible section 49 is formed by the lever 22' having a reduced cross-sectional area in an end region facing the counterbase 27'. Since the lever 22' is made of an elastic material, at least in the region of the flexible section 49, the flexible section 49 forms a joint 50, which allows the lever 22' to pivot resiliently relative to the stack 17. The counterbase 27' is designed here as part of the lever 22' itself. If required, sections of the base 25, the counterbase 27', and / or the piezoelectric stack 17 could also be designed to be spring-elastic.

[0042] The oscillatory nature of the force transmission arrangement 55, formed by the base 25, the piezoelectric stack 17, the counter-base 27', the lever 22', and the actuating element 15, causes the actuating movement to overshoot the drive movement within a specific frequency range. This overshoot can be used to increase the stroke H of the actuating element 15 at the associated valve seat for a given torsional stroke Δx / -Δx of the piezoelectric stack 17. The overshoot effect is strongest in the case of resonance, that is, when the actuating device 11' is operated at the resonance frequency of the oscillatory force transmission arrangement 55. This is in Fig. 3A and Fig. 3B illustrates this.

[0043] Fig. Figure 3A shows the time course of the stroke H and the Δ Δx during operation of the actuator 11' far from the resonance frequency. Here, the stroke H follows the torsional stroke Δx exactly, and the amplitudes of the two quantities are equal—apart from a proportionality factor dependent on the length of the lever 22', which is not considered here. An identical course of the stroke H with respect to the torsional stroke Δx would result for an actuator without a spring-elastic section of the force transmission arrangement 55. In contrast, [the following] shows Fig. 3B the time course of the stroke H and the torsional stroke Δx during operation of the actuator 11' at the resonance frequency. In this case, the amplitude of the torsional stroke Δx is significantly smaller than the amplitude of the stroke H. In addition, the two curves are after a short transient phase, which is in Fig.3B extends over a single oscillation, phase-shifted by 90°. The fact that, with the same stroke H of the actuating element 15, there is a significantly smaller torsional stroke Δx of the piezoelectric stack 17 means a considerable reduction in the input energy to be supplied to the piezoelectric stack 17 and thus a correspondingly lower heating of the piezoelectric stack 17.

[0044] To make the best possible use of the overshoot effect, the resonant frequency of the force transmission arrangement 55 can be adapted to a predetermined nominal frequency of the actuating element 15. For this purpose, in the example shown, the mass of the lever 22' and the spring constant of the joint 50 must be adjusted.

[0045] Overall, the invention enables a considerable increase in the performance of piezoelectrically operated actuators and, in particular, an increase in the switching frequency compared to conventional arrangements. Reference symbol list 11, 11' piezoelectric actuator 13 cases 15 Actuator 17 piezoelectric stacks 17A, 17B Sub-stack 19 electrode 20 electrode 21 layers of piezoelectric material 22, 22' lever 23 Lever end 24 Lever end 25 base 27, 27' Counterbase 30 Cooling equipment 31 lateral outer surface 33 outer thermal conductivity layer 34 space 35 inner thermal conductivity layer 37 Base cooling fin 39 Lever cooling fin 43 Tensioning screw 45 shaft section 47 Mother 49 flexible section 50 joint 55 Power transmission arrangement S Stacking direction H stroke of the actuator Δx / -Δx Torsional stroke of the piezoelectric stack

Claims

Piezoelectric actuating device (11, 11'), in particular a piezoelectric metering valve, comprising a piezoelectric stack (17) which includes a plurality of stacked layers (21) of piezoelectric material, each provided with electrodes (19, 20), a lever (22, 22') mechanically connected to the piezoelectric stack (17) for converting a drive movement of the stack (17) into an actuating movement of an actuating element (15) provided on the lever (22, 22'), in particular a valve plunger, and a cooling device (30) for dissipating heat from the piezoelectric stack (17), wherein the piezoelectric stack (17) is divided into at least two sub-stacks (17A, 17B) arranged side by side and each provided with associated electrodes (19, 20), wherein the sub-stacks (17A, 17B) are separated from each other by a space (34). are,wherein the cooling device (30) is at least partially arranged in the space (34) and / or fluid present in the space (34) forms part of the cooling device (30), and wherein the cooling device (30) comprises at least one cooling element (37, 39) made of a thermally conductive material, preferably metal, which is at least partially arranged in the space (34), characterized in that a base cooling fin (37) extending from a base (25) and projecting into the space (34) and / or a lever cooling fin (39) extending from the lever (22, 22') and projecting into the space (34) is / are provided as a cooling element. Actuating device according to claim 1, characterized in that the cooling device (30) comprises a layer (33) of thermally conductive material, via which at least one outer surface (31), in particular a side surface, of the piezoelectric stack (17) is in contact with a wall section of a housing (13) of the actuating device (11, 11'). Actuating device according to claim 1 or 2, characterized in that the piezoelectric stack (17) is divided into exactly two sub-stacks (17A, 17B) arranged next to each other and each provided with associated electrodes (19, 20). Actuating device according to claim 3, characterized in that the piezoelectric stack (17) is divided along a division plane parallel to the stacking direction (S), preferably into identically sized sub-stacks (17A, 17B). Actuating device according to one of the preceding claims, characterized in that the cooling device (30) comprises a thermally conductive material with which free spaces are filled that are present in the space (34) between the surfaces of the sub-stacks (17A, 17B) and, if applicable, components present in the space (34). Actuating device according to one of the preceding claims, characterized in that the piezoelectric stack (17) is clamped by means of a clamping element (43) between a base (25) common to all sub-stacks (17A, 17B) and a counter-base (27, 27') common to all sub-stacks (17A, 17B), wherein the clamping element (43) extends parallel to the stacking direction (S) through the space (34). Adjusting device according to claim 6, characterized in that the clamping element (43) comprises a shaft section (45) with a flattened cross-section provided for arrangement in the space (34). Actuating device (11') according to one of the preceding claims, characterized in that the base (25) is mechanically connected to a first end face of the stack (17) and the lever (22') is mechanically connected to a second end face of the stack (17), wherein at least one section of a force transmission arrangement (55) comprising the base (25), the piezoelectric stack (17) and the lever (22') is spring-elastic in order to cause the actuating movement to overshoot the drive movement. Actuating device according to claim 8, characterized in that the lever (22') has a pivot point (50) which enables a resilient pivoting of a first lever part relative to a second lever part or of the lever (22') itself relative to an adjacent component (27). Actuating device according to claim 9, characterized in that the joint point (50) is formed by a flexible section (49) of the lever (22'). Actuating device according to claim 9 or 10, characterized in that the lever (22') has a reduced cross-sectional size and / or a modified cross-sectional shape in the area of ​​the joint point (50). Actuating device according to one of claims 8 to 11, characterized in that the resonance frequency of the oscillating force transmission arrangement (55) is adapted to a predetermined nominal frequency of the actuating element (15). Actuating device according to one of claims 8 to 12, characterized by an electronic control device which is designed to operate the piezoelectric stack (17) substantially at the resonance frequency of the oscillating force transmission arrangement (55).

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