Abstract:
A piezoelectric expansion actuator for d33 piezoelements, which allows vibrations in structures to be suppressed. An expansion actuator ( 1 ) includes a piezoelectric stack ( 2 ), which consists of d33 piezoelectric elements and is arranged between output elements ( 4 ), which are attached to the surface of the structure ( 7 ). The invention applies to a piezoelectric extension actuator, which is used to control vibrations in structures. Alternatively, in order to damp vibrations between the main gearbox of a helicopter rotor and the cellular structure of the cockpit, a power application point of the output element ( 18, 19, 180, 190; 35, 36 ) is arranged at a distance from the corresponding end plate of the piezoelectric stack ( 22, 220; 31, 32, 33 ) in the axial direction (X).

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
   This application claims the priority of German Application Nos. 100 49 176.6, filed Oct. 5, 2000 and 101 39 686.4, filed Aug. 11, 2001 and International Application No. PCT/EP01/11360, filed Oct. 2, 2001, the disclosures of which are expressly incorporated by reference herein. 
   The invention relates to a piezoelectric expansion actuator. 
   The use of d31 piezoplates or d31 piezosegments is known for the purpose of vibration control and to influence vibrations in structures. d31 piezoplates take advantage of the elastic transverse contraction of the piezoelectric material. Several piezoplates or piezosegments will be described as piezoelectric stacks. A piezoelectric stack consists of several, but at least 2 piezoelements. With the above d31 piezoelements, for example, expansions are introduced into carrier structures for helicopter transmissions so as to suppress the transmission of body sound onto the helicopter cell. In doing so, the d31 piezoelements are integrated in accordance with their expansion direction, which acts parallel to the surface of the d31 elements, into the surface of the carrier structure across a large surface, e.g. through an adhesion technique. 
   By contrast, the expansion in the familiar d33 piezoelements acts perpendicular to the surface of the elements because d33 piezoplates take advantage of the expansion of the piezoelectric material in the direction of the applied field. 
   German Patent No. 198 13 959 A1 describes a device for body sound suppression that more effectively reduces the transmission of equipment vibrations and oscillations through a carrier structure onto a cellular structure of a cockpit in a simple construction and at relatively low integration complexity. German Patent 198 13 959 A1 provides that the sound suppression device includes at least one piezoactuator, which introduces the oscillations into the carrier structure in order to block the body sound transmission path onto the insulating structure substantially and to compensate acoustic excitation by use of the existing and excited system dimensions of the sound generator more effectively. This technical idea is not limited to use in helicopter manufacturing. It can be employed in all areas of mechanical engineering where a device for body sound suppression becomes necessary. 
   Contrary to other familiar expansion actuators, the piezoactuator does not implement the application of power onto the carrier structure at only points, but rather across a relatively large surface of the carrier structure. The carrier structure can be arranged for example between the main gearbox of a rotor and a cellular structure of the cockpit of a helicopter. In this case, the carrier structure would be one or more struts (also called gear struts). The piezoactuator is largely arranged along the entire circumference of the strut and exhibits a defined expansion in the axial direction of the strut. Forces are introduced by the piezoactuator pursuant to German Patent DE 198 13 959 A1 via its surface. 
   The efficiency of power application is limited by the effective surface of the strut that is to be covered. 
   The invention is based on the development of a piezoelectric expansion actuator for d33 piezoelements, with which vibrations can be suppressed in structures, and furthermore of considerably increasing the efficiency of power application of a piezoactuator despite the contrary tendency of decreasing construction volume of the piezoactuator. 
   A solution pursuant to the invention is based on the fact that a d33 piezoelement in the form of a stack is clamped into a mechanical frame, which is fastened to the surface of the structure. Apart from a highly specific, mechanical power, the expansion actuator also achieves good efficiency. Also beneficial is the application of mechanical pre-stress that is integrated in the actuator which allows critical tension strain to be avoided for the piezoelements. Optionally, devices can be integrated in the frame with which stroke speed transformations or stiffness transformations can be beneficially achieved. 
   In another solution pursuant to the invention, the efficiency of power application for the piezo actuator can be improved by considerably increasing the distance between the resting areas of two output elements of a piezoactuator and a corresponding end plate of the piezoelectric stack in the axial direction towards the strut end. The output elements of the mechanical frame form the power transmission device from the piezoactuator to the strut. The thus considerably enlarged strut distance between the resting areas of the two output elements exhibits less stiffness, consequently leading to an expansion of this strut section with less force required than in a comparable configuration of a piezoactuator where the distance of the resting surfaces of the output elements largely corresponds to the length of the piezoelectric stack. The piezoelement also uses the d33 piezoelectric elements. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Based on the drawing, exemplary designs of the invention are explained in more detail in the following: 
       FIG. 1  is a section view of an expansion actuator, 
       FIG. 2  is an expansion actuator with stroke speed transformation, 
       FIG. 3  shows an alternative version of an expansion actuator with stroke speed transformation, 
       FIG. 4  shows a diagram of a strut with axially spaced output elements of a piezoactuator, 
       FIG. 5  illustrates a sectional view of a strut with collar-shaped output elements of a piezo actuator, and 
       FIG. 6  shows an alternative design of the output element with recesses. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The expansion actuator  1  shown in  FIG. 1  is rigidly fastened to the surface of a structure  7  and consists of a d33 piezoelectric stack  2 , two end plates  3 , two output elements  4  and a prestress element  6 . 
   The d33 piezoelectric stack  2  is arranged in its mechanical frame such that its expansion direction runs parallel to the surface of the structure  6  in which the expansion actuator  1  transmits its piezoelectrically generated expansions. The d33 piezoelectric stack takes up ⅓ of the material volume of a d31 piezoelectric stack for equivalent active expansions. 
   In the design in  FIG. 1  the mechanical frame is formed by the two output elements  4 , which are rigidly attached to the structure  6 . To accomplish this, the output elements  4  are attached in such a way to the surface of the structure  7  that their output surface  5  is aligned parallel to the respective end plate  3  of the piezoelectric stack  2 . The output elements  4  can be fastened to the structure  7  by means of familiar attachment techniques, for example by gluing. 
   The output elements  4  can be adjusted on their attachment surface to variously bent or plane structural surfaces. In the design shown the structure is a pipe with a circular concave surface. The length of the piezoactuator corresponds to the length of the strut section that is supposed to be expanded. 
   The piezoelectric stack  2  is seated between its two end plates  3  and held in place with a prestress element  6  at mechanical precompression stress. Possible damaging tensile loads acting upon the expansion actuator  1  are compensated with this precompression stress and can thus have no effect on the piezoelectric stack  2 . 
   The prestress element  6  can be implemented for example with one or more mechanically acting tension springs—as indicated symbolically in the design in  FIG. 1 . It is also possible, however, to design the end plates  3  as elastic plates and to insert the piezoelectric stack  2  with compressed end plates  3  into the mechanical frame at precompression stress. 
   The expansion actuator  1  with stroke speed transformation shown in  FIG. 2  corresponds to the previously described design except for the following deviations. The end plates  3  of the piezoelectric stack  2  are not connected directly with the output surfaces  5  of the output elements  4 , but rather by means of an elastic pressure web  8 , and the output elements  4  comprise an inwardly one-sided open slot  9  that runs parallel to the surface of the structure  7 . Furthermore, the two output elements  4  are rigidly connected with each other by means of a non-expanding support bar  11 , which engages in the free end  10  of the output elements  4 . In place of a support bar  11  two parallel support bars  11  that are arranged on either side of the piezoelectric stack  2  can be used, as is revealed in  FIG. 2  with a support bar  11  shown in the drawing. 
   On each output element  4 , the elastic pressure webs  8 , slots  9  and support bars  11  form three joints “a”, “b” and “c” about which the lever sections of the output elements  4  can rotate and generate a stroke speed transformation in the expansion actuator  1 . 
     FIG. 3  depicts an expansion actuator  1  with stroke speed transformation in an alternative design version compared to  FIG. 2  of the output elements  4  and joints “a”, “b” and “c”. The effect of lever sections about the joints “a”, “b” and “c” in principle corresponds to the previously described design in  FIG. 2 . 
   The output elements  4  here are designed with a lever  12  that is seated in joint “a”. The joint “b”, in which the piezoelectric stack  2  engages with an output web  14 , is arranged on the lever  12  with a first lever section  13  at a distance to joint “a”. 
   Joint “c” is arranged on the lever  12  with a second lever section  15  at a distance to joint “b”. Joint “c” engages in the support bar  11 . 
     FIG. 4  shows a diagrammatic image of a strut  16 . The strut can be, for example, a steel pipe with a fastening loop that is welded onto each end. Such a strut is used, for example, in a quadruple setting in order to connect the main gearbox of the rotor of the helicopter with the cellular structure of the cockpit of the helicopter. The main gearbox is hereby located above the ceiling of the cellular structure of the cockpit. The two components are connected in 4 locations by a strut  16 , respectively. The main gearbox of the rotor is one of the main sources of noise generated in the cockpit. Since the strut  16  is seated on the interface between the main gearbox and the cellular structure, it is useful if elastic dimensional changes are generated on the strut, which can largely compensate the forces introduced via the strut. This is effected, for example, through a controlled dimensional change (expansion or contraction) of the strut  16  in the axial direction X. The controlled elastic dimension change is implemented with the piezoactuator  17 , which initiates a dimensional change, particularly a change in length in the axial direction X in a certain section D of the strut. In the strut  16  pursuant to  FIG. 4  additionally there are two output elements  18 ,  19  arranged per piezoelectric stack. The output elements  18 ,  19 , however, are not connected directly behind the end plate  20 ,  21  of the piezoelectric stack  22  with the surface of the strut  16 , but the resting surfaces  23 ,  24  of the output elements  18 ,  19  are arranged at a distance to the end plate  20 ,  21  of the piezoelectric stack  22  in the axial direction X. The piezoelectric stack  22  does not have to rest directly on the surface of the strut. The force that is generated by the piezoelectric stack  22  is introduced into the strut  16  on the resting surface  23 ,  24 . This force effects an elastic dimensional change in a section D of the strut  16  between the two resting surfaces  23 ,  24 . 
   The section D along the strut circumference includes the corresponding sectional spatial structure of the strut. The elastic dimensional change compensates the vibration force in the strut  16 , specifically in the area of the interface of strut and cellular structure. 
   The piezoelectric expansion actuator  17  is formed by d33 piezoelectric elements, which are arranged in a piezoelectric stack  22 . The two ends of the piezoelectric stack  22  are limited by the end plates  20 ,  21 . The output elements  18 ,  19  are arranged on the end plates  20 ,  21 . The power application point of an output element  18 ,  19  on the strut  16  is arranged at a distance from the end plate  20 ,  21  in the axial direction X towards the fastening loop  160 ,  161 . Gaining such a distance is associated with gaining a lever arm that engages on both sides of the end plates of the piezoelectric stack. One lever arm E, F each is formed by an output element  18 ,  19 . The lever arms E, F increase the section D by their length since originally section D corresponded only to the length of the piezoelectric stack. 
   The tensile force, for example, that is generated in a selection of the piezoelectric expansion actuator is introduced into the strut via the output surface of the output element. The section D located between the output surfaces  23 ,  24  of the strut  16  is thus exposed to a controlled dimensional change in an axial direction X. This change represents an elastic dimensional change. Compared to the previously described solution, this alternative solution takes advantage of the lower rigidity of an enlarged strut section. This increases the efficiency of power application of a piezoactuator  17  considerably. It is, therefore, possible to use a substantially smaller piezoelectric stack without having to accept an efficiency loss. 
   Multi-axis influencing of the dimensional change of the described section D of the strut  16  can be controlled as a function of the piezoelectric stack&#39;s configuration along the circumference of the strut. 
   The arrangement shown in  FIG. 4  can be also designed in the inside of a tubular strut. 
   As  FIG. 4  also shows, this elastic dimensional change of the section D is controlled with a control unit  25 . In the area of each output element  18 ,  19 , preferably in the vicinity of the resting surface  23 ,  24 , the control unit comprises a sensor  26 , which determines a signal from the quantitative value of the existing vibrational forces and supplies it to the control device. The control device  25  regulates the piezoelectric stack  22  such that a force is generated by the piezoelectric stack and is introduced into the strut  16  via the output elements  18 ,  19  so as to affect an elastic dimensional change of the strut section D. 
   The distance D between the resting surfaces  23 ,  24  can be implemented in a variably adjustable manner by designing at least one lever arm E, F such that it can be enlarged and reduced. 
   The above explanations apply similarly to the piezoactuator  170  in  FIG. 4  with the output elements  180 ,  190  and the piezoelectric stack  220  with the end plates  200 ,  210 . 
     FIG. 5  shows a sectional view of a design for a strut  30 , wherein the two fastening loops on the ends of the strut  30  are not depicted. 
     FIG. 5  depicts three piezoelectric stacks  31 ,  32 ,  33 , which form a piezoactuator  34  along with the output elements  35 ,  36 . These piezoelectric stacks  31 ,  32 ,  33  are offset from each other, for example, by 120°. Each piezoelectric stack can be arranged at a distance from the surface of the strut. The stacks, however, can also be arranged on the surface of the strut  30 . The first version is shown in the example. 
   The axial axis of the piezoelectric stack is aligned in the direction of the axial axis X of the strut  30 . Each piezoelectric stack is arranged between two output elements  35 ,  36 . The two output elements  35 ,  36  engaging one piezoelectric stack  31 ,  32 ,  33 , respectively, contain each an annular socket  37 ,  38 , which encloses the strut  30  in an interlocking and non-positive manner along its circumferential surface. Extending from the annular socket  37 ,  38  the output element  35 ,  36  opens up in a bell-shaped manner like a collar, which is arranged at a distance from the strut starting from the edge of the annular socket to its annular edge. This shape is described as an annular collar  39 ,  40 . On the edge  41  of the collar  39  rests one end of the piezoelectric stack  31 ,  32 ,  33 , respectively. The other end of the three piezoelectric stack  31 ,  32 ,  33  is respectively located on the edge  42  of the collar  40 . 
   The annular socket  37 ,  38  of the collar  39 ,  40  exhibits sufficient rigidity and firmness that corresponds to a resting surface  370 ,  380  which is connected with the surface of the strut  30  in the circumferential direction in an interlocking and non-positive manner. The forces generated by the piezoelectric stacks  31 ,  32 ,  33  are introduced via the resting surfaces  370 ,  380 . Such a design for an output element  35 ,  36  permits action in a variety of spatial axes. Hence more variable design possibilities exist for introducing power into the strut  30 . The forces and bending moments that are introduced into the strut  30  can be used to effect an excursion in the longitudinal (axial) direction, a lateral bending excursion in any random direction and also torsion of the strut  30 . 
   This elastic dimensional change also affects a structural region in the strut  30  along section D. 
   By using at least two piezoelectric stacks that are arranged around the strut, the strut can be displaced in the longitudinal and lateral directions through appropriate selection of the individual piezoelectric stacks. A suitable control or regulating device is not shown in  FIG. 5 . 
   It is also possible to introduce torsional forces by inserting the piezoelectric stack at an angle, i.e. a configuration of at least one piezoelectric stack that is tilted in relation to the longitudinal axis X of the strut  30 . 
     FIG. 6  depicts another possible design of an output element. It is shown as a single output element  360  without strut and without piezoelectric stack. The output element  360  is guided in the direction of the axial axis X of a strut and fastened to the surface of the strut by means of its annular socket  390 . A collar  400  is incorporated on the annular socket. This collar  400  contains recesses  401  so that weight of the output element can be saved. The collar  400  includes for example three arms, which can be arranged at an angle of 120° in relation to one another. These three arms of the collar  400  are connected and limited at their ends by a ring  420 . This ring  420  forms the edge of the collar  400 . One end of a piezoelectric stack is arranged on the edge of the collar, respectively. 
   Pursuant to another design (not shown), it is also possible to divide an output element  360  partially into sectional output elements. To accomplish this, sectional output elements are arranged in a non-positive manner with each other in one direction along the circumference of a strut in segments and are connected. In the viewing direction of the X-axis, an output element can be divided into individual (wedge-shaped) segments, which are arranged around the X-axis. The output element is thus composed in segments of sectional output elements. The output element pursuant to  FIG. 5 , for example, could be composed of three sectional output elements in the case of three piezoelectric stacks. This configuration of sectional output elements is an easy option for retrofitting a strut on the helicopter that has already been installed. 
   Such a configuration makes it possible to reduce the vibrations generated by the main gearbox in relation of the cellular structure of the cockpit efficiently for the pilot and noticeably for the passengers.