Patent Publication Number: US-2010109219-A1

Title: Modular interface for damping mechanical vibrations

Description:
RELATED APPLICATIONS/PRIORITY CLAIM 
     This Application is a Divisional Application of pending U.S. patent application Ser. No. 10/565,469 filed Jan. 19, 2006 entitled “Modular Interface for Damping Mechanical Vibrations”, and which is a National Phase Application of PCT Application PCT/EP2004/007986 filed Jul. 16, 2004, which claims priority to German Application No. 103 33 492.0 filed Jul. 22, 2003 and German Application No. 103 61 481.1 filed Dec. 23, 2003; the disclosures of all the above being expressively incorporated herein by reference, and to all of which, priority is hereby claimed in this Application. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an interface for damping or isolating mechanical vibrations by means of a plurality of energy converter systems. Such interfaces are used, for example, for damping vibrations in the field of general machine engineering, the automotive industry, the construction industry or the aerospace industry. 
     Dynamic mechanical interference in the form of vibrations which are excited, for example, by the operation of assemblies (for example power supply assemblies) or by other ambient conditions, are produced in machines, vehicles and similar modules. The frequencies of these vibrations extend into the relatively high frequency acoustic range and bring about undesired dynamic and/or acoustic effects locally at the location where the interference is produced or applied, or further away after transmission over mechanical load paths. This results in losses of comfort, safety problems, damage to components owing to structural fatigue, shortened service life, reduced functionalities etc. 
     2. Description of the Related Art 
     What is referred to as material damping, in which the mechanical energy of the vibration is converted directly into thermal energy, is frequently used to damp or isolate mechanical vibrations. Examples of this are elastic or viscoelastic damping systems. 
     In addition, measures which are based on other energy converter systems are increasingly used. These energy converter systems generally convert mechanical energy into electrical energy and vice versa. Both effects are used to damp mechanical vibrations. The distinction is generally made here between active, semiactive and passive vibration dampers. 
     In the case of passive and semiactive vibration damping, the mechanical energy of the vibrations is firstly converted into electrical energy using an electric/mechanical energy converter (for example a piezoceramic). This electrical energy is then dissipated, i.e. converted into thermal energy, in a passive electrical circuit (e.g. an ohmic resistor) in the case of passive vibration damping, or diverted using an active electric circuit (for example electric damper) in the case of semiactive vibration damping. Such systems are described, for example, in N. W. Hagood and A. von Flotow: Damping of Structural Vibrations with Piezoelectric Materials and Passive Electrical Networks, Journal of Sound and Vibration 146 (2), 243 (1991). 
     In the case of active vibration isolation, at least one actuator system is connected between an interference source (base side) and a connection side. In this context, “actuator” refers to an energy converter which, for example, can convert electrical signals into mechanical movements, for example a piezoactuator or a pneumatic actuator. What is decisive is that the characteristic (for example extent) of the actuators can be varied in a controlled fashion by means of an actuation signal. An example of a system for active vibration isolation using actuator elements is disclosed in U.S. Pat. No. 5,660,255. Actuator elements and a small additional mass are interposed between a base housing and a useful load which is to be isolated. Sensors which record the displacement of the small mass are mounted on said small mass. An actuation signal for the actuator elements is generated from the displacement using an electronic closed-control circuit and an external electrical energy source. The actuator elements are actuated in such a way that the vibration movement at the location of the useful load is largely eliminated. 
       FIG. 1  shows a satellite as an example of active isolation of interference sources and sensitive components which should be protected from mechanical interference. The satellite contains internal interference sources  1 , for example mechanical coolers, motors etc. Mechanical interference from these interference sources  1  is damped by active elements  2 ,  3 ,  4  so that the interference from the interference sources  1  does not act on the sensitive components  5  (cameras, reflectors, etc.) via transmission paths  3 ,  4 . 
     In addition to the use for active, passive and semiactive vibration damping, the electric/mechanical energy converters can often simultaneously be used as actuating elements for mechanical positioning of a useful load. This may be done, for example, by virtue of the fact that an annular arrangement of a plurality of actuators is integrated into a vibration-damping interface which can bring about, for example, selective tilting of a structure with respect to a base. Such a system is disclosed, for example, in DE 195 27 514 C2. 
     For structural reasons, actuator systems are frequently operated in practice with a preload. This is frequently a mechanical preload in the form of compressive loading or tensile loading on the actuator system. For example in the case of piezoactuators in which extension beyond the length at rest (i.e. length of the actuator without voltage applied) would lead to mechanical damage to the actuator, operation without preloading is in practice inappropriate or not possible. However, the structural implementation of a device for exerting a preload presents problems, in particular in the case of the actuator or actuators whose extension direction extends parallel to the force (for example the force of the weight) exerted by the useful load, and has a frequently negative effect on the effectiveness of the actuator. U.S. Pat. No. 5,660,255 does not disclose a satisfactory solution to this problem. 
     DE 195 27 514 C2 discloses an interface for reducing vibrations in structural dynamic systems in which vibration insulation occurs between a structure-side component and a base-side component by means of a plurality of actuators which have a main direction. Pressure pretensioning on the actuators is ensured by anti-fatigue bolts between the base-side component and the structure-side component. However, such a rigid mechanical connection between the base-side component and the structure-side component has the disadvantage that as a result a bridge is provided via which vibrations can propagate from a base-side interference source to the structure-side component. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to disclose an interface for vibration reduction which transmits as little sound as possible and which can be used for active, semiactive and passive vibration isolation as well as for mechanically positioning a load. 
     This object is achieved by means of the invention having the features of the independent claim. Advantageous developments of the invention are characterized in the subclaims. 
     An interface for reducing mechanical vibrations is proposed which has a base connection element, a load connection element and at least one support element. In this context, at least a first energy converter system extends between at least one engagement point located on the base connection element and at least one engagement point located on the load connection element. 
     The energy converter system can be based on various physical principles depending on the application and requirements. In particular piezoactuators have proven to be particularly advantageous. However, actuators which are based on what are known as shape-memory alloys or other materials with a memory effect as well as magnetostrictive or electrostrictive actuators, pneumatic or hydraulic actuators, magnetorheological or electrorheological fluid actuators and damping elements can be advantageously used. Combinations of different energy converter systems are also possible, for example the combination (for example a series or parallel circuit) of a piezoactuator with a “conventional” damping system, for example a spring system or a rubber damper. 
     Spring systems or elastic materials can also be used in combination with piezoactuators in order to generate a preload on the piezoactuator or to increase an existing preload. Vibrations in different frequency ranges can also be compensated by combining different operative principles and energy converter systems, that is to say for example high-frequency vibrations due to active or passive damping by means of piezoactuators, low-frequency vibrations due to conventional damping elements (for example viscoelastic dampers). 
     At least one second energy converter system extends between at least one engagement point located on the support element and at least one engagement point located on the load connection element. The descriptions stated above relating to the first energy converter system apply appropriately for the selection and the composition of this second energy converter system. 
     The base connection element is connected to the at least one support element via at least one pretensioning device in such a way that the pretensioning device can exert a preload on the first energy converter system and on the second energy converter system. This preload may be, for example, mechanical compressive loading or tensile loading. It is optionally also possible to operate with a preload of zero, that is to say an operating mode in which force is not exerted on the energy converter systems. This preload (also the preload of zero) can also be combined, for example, with an initial electrical load. The pretensioning device may be elastic or inelastic. The preload can be exerted directly or indirectly on the energy converter systems, that is to say for example also indirectly by means of an additional spring system. 
     The load connection element is to have a part which is located in an intermediate space between the base connection element and the support element and a part which is located outside the intermediate space between the base connection element and the support element. 
     Intermediate space is to be understood here as not only a closed-off cavity but also any space between the base connection element and the load connection element. 
     This condition ensures the advantage that the load connection element is easily accessible for mounting a load. The vibration insulation is provided, for example, by means of the part located between the base connection element and the support element, as a result of which compressive pretensioning can be exerted on the energy converter systems. On the other hand, a load is mounted on that part of the load element located outside the intermediate space between the base connection element and the support element. Said part is then no longer restricted by the spatial dimensions of the intermediate space, that is to say may be configured as desired in terms of shape and size and as a result, for example, take into account specific requirements of the connection geometry of the load. 
     The base connection element and the load connection element may have, for example, a planar mounting face. This facilitates the installation of the interface in existing structures for isolating a vibration-sensitive load from one or more interference sources. Furthermore, in this way it is possible to easily connect a plurality of interfaces in series. 
     The described interface can be integrated into structures 
     as a bearing element, 
     as a modular transmission element and/or 
     as an actuation element. 
     The proposed arrangement is characterized in particular by the fact that the load connection element is generally connected to the support element or the base connection element only via the first and second energy converter systems. In this way, the number of sound bridges between base-side interference sources and a useful load is reduced to the technically necessary minimum. 
     Despite this reduction in the sound bridges, the rigid or flexible pretensioning device, which produces a connection between the base connection element and the support element, permits a defined setting of a pretensioning of the energy converter systems. This may be done, for example, by virtue of the fact that the pretensioning device used is an elastic element, anti-fatigue screws or similar elements with a variable length. 
     It is possible to use energy converter systems with a common preferred direction or with different preferred directions, the latter option having the purpose, for example, of isolating vibrations in different spatial directions. A separate support element and a separate pretensioning device is then advantageously used for each spatial direction. Preferably in each case at least one energy converter system which extends between the base connection element and the load connection element and at least one energy converter system which extends between the load connection element and the support element is used for each spatial direction. In turn, pretensioning can then be exerted on the energy converter systems without sound bridges being provided between the base element and the load connection element. 
     Furthermore, it is also possible to use more than two energy converter systems for one spatial direction. This may be advantageous in particular if the energy converter systems are actuator systems which are intended to bring about not only pure translation of the load connection element but also, for example, tilting. If, for example, two pairs of actuator systems are arranged in parallel, unequal extension of the two pairs leads to tilting of the load connection element about an axis perpendicular to the preferred direction of the two pairs of actuator systems. In analogous fashion it is possible to use a plurality of pairs of actuator systems to bring about tilting of the load connection element about a plurality of axes. In this way it is possible, for example, also to isolate torsional vibrations in the base connection element from the load connection element. 
     In a further advantageous refinement of the invention, the base connection element and the load connection element each have standardized connection geometries. These connection geometries may be, for example, threads, flanges, screwed bolts etc. This permits rapid and cost-effective exchange or supplementation of existing elements and structures by the described interface for reducing vibration. For example, in satellite engineering it is easily possible to connect the interface between the main body, which contains, for example, interference sources in the form of motors, and a position-sensitive antenna without structural changes being necessary to the entire arrangement. It is possible, for example, to resort to standardized flange geometries. 
     The pretensioning device advantageously has a pipe which surrounds the actuator systems. The pipe may have circular, rectangular or any desired cross-sectional geometry. 
     This is advantageous in particular if all the energy converter systems have a common preferred direction. The enclosed pipe may be of rigid or flexible design and is particular designed in such a way that the tubular axis is oriented approximately parallel to the preferred direction of the actuator systems. The pipe protects the actuator systems against environmental influences such as, for example, moisture, dirt or the like. Furthermore, the pipe stabilizes the energy convert systems against effects of forces perpendicular to the preferred direction (for example shearing forces) which could cause mechanical damage to the energy converter systems. 
     In different methods for damping vibrations it is advantageous to generate information about the actual vibration of the load connection element. For this reason, a sensor system for determining, for example, travel, velocity, acceleration or force can be connected or intermediately connected to an element of the interface. In particular it is advantageous if a sensor is connected to the load connection element. Further sensors systems may be connected, for example, to the base connection element. 
     The sensor systems may be, for example, capacitive or piezoelectric acceleration or force sensors or magnetic, electrostatic or interferometric position or velocity sensors. 
     The information of the at least one sensor system may be used, for example, for active vibration damping. In this context, actuator systems may be used in particular as energy converter systems. The signals of the sensor system are made available to an electronic closed loop control system. The electronic closed loop control system generates control signals (target function) from the sensor signals, said control signals being converted into actuation signals for the actuator systems by means of a power supply. These actuation signals are used to excite the actuator systems to vibrate, said vibrations being, for example, in antiphase with respect to the vibrations to be isolated and eliminating or damping said vibrations at the location of the load. 
     In one development of the invention, at least one energy converter system is embodied entirely or partially as an actuator system. In this context, part of this actuator system will be in turn capable of being used at the same time as an energy converter which can convert mechanical energy into electrical energy. 
     In this development, both energy conversion directions are therefore used simultaneously. Whereas electrical energy is typically converted into mechanical energy in an actuator, in this embodiment of the invention mechanical energy is converted simultaneously into electrical energy at least in part of an actuator. Actuators which are capable of carrying out this reversal of the converter principle are also referred to as multifunctional converter systems. The materials used in this context, which can simultaneously bear mechanical loads and act as an actuator or sensor (see below) are referred to as multifunctional materials. 
     The conversion can be carried out, for example, by utilizing the piezoelectrical effect, for example by means of a piezoceramic. In this context, a pressure on a piezoceramic or fluctuations in pressure in a piezoceramic are converted into electrical signals. Since piezoactuators are frequently composed of stacks of a large number of piezoceramic layers, it is possible, for example, to use a layer from this stack simultaneously for converting mechanical energy into electrical energy. 
     This development has various advantages. On the one hand, it is possible to dispense at least partially with the use of additional sensors. The electrical signals which are generated by the actuator system serve simultaneously as sensor signals and can contain, for example information about the acceleration or velocity of the movement of a useful load. 
     In this way it is possible to determine the system response of the entire system to interference, for example by means of the interface. For example the actuator systems of the interface can have a specific reference structure stimulation applied to them. This reference structure stimulation brings about a structure response by the entire system in the form of mechanical vibrations. By recording the electrical signal of an actuator which acts as an energy converter between mechanical energy and electrical energy it is possible to record the structure response by means of measuring equipment. The measured structure response, e.g. of the transmission properties, between the actuator-induced reference stimulation and sensor or the determination of impedance permits conclusions to be drawn about the current structural state of the entire system, for example by comparing the measured structure response or determining structure characteristic values with reference structure responses or reference structure characteristic values stored in a database. 
     A further advantage of the simultaneous use of at least part of an actuator system as a mechanical/electrical energy converter is the possibility of using it as a passive or semiactive vibration damper. In this context, an electronic circuit is used to dissipate the electrical energy. 
     In the simplest case, this electronic circuit is composed of ohmic resistor in which the electrical energy is converted partially into heat. Even more efficient vibration damping can be achieved by additionally using one or more coils and/or one or more capacitors. For example, the mechanical vibrations of the interface can thus result in periodic fluctuations in the charges on the surfaces of a piezoceramic of a piezoactuator of the interface. This corresponds to periodically fluctuating charges on the plates of a capacitor. If the two plates of the capacitor (that is to say the two surfaces of the piezoceramic) are connected to one another by means, for example, of an ohmic resistor and a coil, the mode of operation of the arrangement corresponds to the effect of a damped electrical oscillatory circuit. 
     A further increase in the efficiency of the vibration damping can be achieved by using what is referred to as a “synthetic inductor” instead of at least one coil. This synthetic inductor is generally composed of a combination of a plurality of ohmic resistors with one or more operational amplifiers. In this way it is possible to achieve higher inductances than with conventional coils. As a result, the damping of the oscillatory circuit is increased further. This technology is described, for example, in D. Mayer, Ch. Linz and V., Krajenski: Synthetic Inductors for Semipassive Damping, 5. Magdeburger Maschinenbautage, 2001. 
     The efficiency of the vibration damping can be further increased by connecting in series a plurality of the interfaces described above in one of the described configurations and wiring arrangements in cascades. In this context, in each case the base connection element of the following interface is connected to the load connection element of the preceding interface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       In the text which follows the invention will be explained in more detail with reference to exemplary embodiments which are illustrated schematically in the figures. However, it is not restricted to the examples. Identical reference numbers in the individual figures relate to elements which are identical or functionally identical or which correspond to one another in terms of their functions. In particular: 
         FIG. 1  shows a satellite with active isolation of interference sources and sensitive components in accordance with the prior art; 
         FIG. 2  shows a structural mechanical interface for vibration damping; 
         FIG. 3  shows a simplified electrical wiring arrangement for active vibration damping of the interface illustrated in  FIG. 2 ; 
         FIG. 4  shows a means of actuating the actuator systems of the interface in  FIG. 2  for the selective tilting of a useful load; 
         FIG. 5  shows the possible tilting axes for an interface with actuators which are arranged with 120.degree. rotational symmetry; 
         FIG. 6  shows an alternative embodiment of a structural mechanical interface for vibration damping; 
         FIG. 7  shows a further alternative embodiment of a structural mechanical interface for vibration damping; 
         FIG. 8  shows a structural mechanical interface for vibration damping in a perspective partial illustration with a cut-out segment; 
         FIG. 9  shows a structural mechanical interface for vibration damping in two spatial directions which are perpendicular to one another; 
         FIG. 10  shows a structural mechanical interface for vibration damping in three spatial directions which are not perpendicular to one another; 
         FIG. 11  shows an arrangement for the partial use of a piezoactuator of a structural mechanical interface as a sensor for a structural analysis; 
         FIG. 12  shows an electrical wiring system of part of a piezoactuator of a structural mechanical interface for passive vibration damping; and 
         FIG. 13  shows an electrical wiring system of part of a piezoactuator of a structural mechanical interface for passive vibration damping which is an alternative to  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  illustrates a preferred embodiment of the described interface for vibration damping. A base connection element  10  is connected to a support element  14  via a pretensioning device  12 . A first energy converter system, which is composed of the piezoactuators  16  and  18 , extends between the engagement points  20  and  22  on the base connection element  10  and the engagement points  24  and  26  on the load connection element  28 . A second energy converter system which is composed of the piezoactuators  30  and  32  extends between the engagement point  34   s  and  36  on the support element  14  and the engagement points  38  and  40  on the load connection element  28 . 
     The illustrated arrangement shows merely a cross section through the structural mechanical interface. The arrangement of this example is symmetrical with the indicated axis  42  of symmetry, with the exception of the piezoactuators  16 ,  30 ,  18 ,  32 . The base connection element  10  is therefore a circular disk and the support element  14  an annular disk. The load connection element  28  is in the shape of a cylindrical cap, with part of the load connection element being located in the intermediate space between the pretensioning device  14  and the base connection element  10  and part being located outside. The pretensioning device  12  is composed of an elastic pipe with a diameter which is identical to the external diameter of the circular disk of the base connection element  10  and to the external diameter of the annular disk of the support element  14 . The pretensioning is carried out by virtue of the fact that the length of the elastic pipe is selected such that the pipe is expanded in the state of rest of the arrangement. As a result compressive pretensioning is exerted simultaneously on all the piezoactuators. 
     Instead of the illustrated four piezoactuators it is also possible to use more than four actuators. These piezoactuators are preferably arranged in a rotationally symmetrical fashion with respect to the axis  42  of symmetry. 
     The base connection element  10  and the load connection element  28  are configured in such a way that simple and rapid mounting of the interface between a base side which is excited to oscillate by interference sources  1  and a load which is to be isolated can take place. For this purpose, the base connection element  10  and the load connection element  28  are provided with threaded bores with standard dimensions. 
     If the piezoactuators  16  and  18  are lengthened by simultaneous electric actuations and the piezoactuators  30  and  32  are shortened to the same degree by suitable electrical actuations, the distance between the load connection element  28  and base connection element  10  is increased. Correspondingly, shortening the piezoactuators  16  and  18  and simultaneously lengthening the piezoactuators  30  and  32  reduces the distance between the load connection element  28  and base connection plate  10 . The electric actuations of the piezoactuators are not illustrated in  FIG. 2 . 
     If the piezoactuators  16  and  30  and  18  and  32  are each in antiphase, for example actuated with sinusoidal alternating voltage of suitable amplitude and frequency, the load connection element  28  swings up and down in relation to the base connection element  10 . This can be used, for example, for active vibration damping. 
       FIG. 3  illustrates an electric wiring system for the interface according to  FIG. 2 . An acceleration sensor  60  which is secured to the load connection element  28  is connected to the input of an electronic closed loop control system  64  via a phase shifter  62 . An output of the electronic closed loop control system  64  is connected to the piezoactuators  30  and  32  via a post-amplifier  66 . Furthermore, the output of the electronic closed loop control system  64  is connected to the piezoactuators  16  and  18  via a 180.degree. phase shifter  68  and a second postamplifier  70 . 
     If vibrations of the base connection element  10  are to be isolated from the load connection element  28 , these vibrations are detected using the acceleration sensor  60 . The sensor signal is then converted into suitable antiphase actuation signals for the piezoactuators  16  and  30  and  18  and  32  using the electronic closed loop control systems  64  and the first phase shifter  62 . The first phase shifter  62  can serve, for example, for compensating phase shifts between the actual movement of the load connection element  28  and the sensor signals. This necessity depends, inter alia, on the method of operation of the sensor  60 . For example in the case of sinusoidal vibrations in which a phase shift of 90.degree. occurs between the acceleration and velocity and between the velocity and position, the signal of a velocity sensor would have to be phase shifted by 90.degree. in order to be able to bring about a suitable change in length of the piezoactuators. Delays in the electronic closed loop control system  64  and resulting phase shifts can also be compensated by the phase shifter  62 . 
     The signals generated in the electronic closed loop control system  64  are amplified further in the postamplifiers  66  and  70  and fed to the actuators  30  and  32  and  16  and  18 . The second phase shifter  68  is necessary since the two actuator systems  16 ,  18  and  30 ,  32  generally have to be actuated in antiphase. 
     The piezoactuators  16  and  30  and  18  and  32  are each excited to undergo antiphase vibrations by which the vibrations are transmitted to the load connection element  28 . In the load connection element  28  the vibrations excited by the piezoactuators are superimposed on the vibrations of the basic connection elements  10  in a destructive fashion if the phase is selected suitably so that the vibrations of the load connection element  28  are damped. 
     In the illustrated arrangement, the piezoactuators  16 ,  18  of the first actuator system and the piezoactuators  30 ,  32  of the second actuator system are each configured in the same way, i.e. identical actuation signals bring about identical changes in length. For this reason, in each case a single postamplifier  66  or  70  can be used for the actuators of an actuator system. If different actuators are used within an actuator system, different postamplifiers would have to be used for each of the actuators. 
     The actuation of the piezoactuators is illustrated in a highly simplified form. As a rule, each piezoactuator has two electrical terminals to which different voltages have to be applied. The difference in voltage between the electric terminals determines the extension of the length of the piezoactuator. 
       FIG. 4  illustrates how tilting oscillations of the base connection element  10  can also be compensated or damped by selective actuation of the piezoactuators of the arrangement in  FIG. 2 . By virtue of the fact that the piezoactuator  30  is set, by a suitable electric actuation signal, to a greater length than the piezoactuator  32 , and the piezoactuator  16  is correspondingly set to a smaller length than the piezoactuator  18 , the load connection element  28  is tilted relative to the plane of the base connection element  10 . For this purpose, the piezoactuators  16 ,  18 ,  30 ,  32  require individual electric actuation means (not shown). 
     If tilting vibrations occur in the base connection element  10 , they can be detected, for example, by comparing the signals of different sensors which are mounted at different locations on the surface of the load connection element  28 . The signals are then converted into suitable actuation signals of the piezoactuators using an electronic closed loop control system  64  So that the load connection element  28  carries out a tilting vibration relative to the base connection element  10 , and said tilting oscillation is superimposed in a destructive fashion on the tilting oscillation of the base connection element  10  and thus damps it in the load connection element  28 . 
     The electronic closed loop control system  64  can, for example, be constructed in such a way that a sum signal and a difference signal are formed from the signals of two sensors which are secured to the load connection element  28  and said sum signal and difference signal are converted in separate controllers to form actuation signals for the piezoactuators. The actuation signal for each piezoactuator is then a superimposition of signals from the two controllers. 
     In this exemplary embodiment in which only two actuator pairs  16  and  30  and  18  and  32  are used, the load connection  28  can only be tilted about an axis perpendicular to the axis  42  of symmetry. If, as described above, more actuator pairs are used, tilting about a plurality of axes perpendicular to the axis  42  of symmetry is possible.  FIG. 5  illustrates in sketch form in a plan view an interface with three actuator pairs  80 ,  82 ,  84  as an example. Only the actuator pairs  80 ,  82 ,  84  and the tilting axes are illustrated. Each of the actuator pairs  80 ,  82  and  84  is respectively composed of an actuator which extends between an engagement point on the base connection element  10  and an engagement point on the load connection element  28 , and an actuator which extends between an engagement point on the support element  14  and an engagement point on the load connection element  28 . The actuators of, in each case, one actuator pair are arranged linearly and perpendicularly with respect to the plane of the drawing in this embodiment and therefore cannot be seen individually. The actuator pairs  80 ,  82 ,  84  are arranged in a rotationally symmetrical fashion through 120.degree. about the axis  42  of symmetry which is perpendicular to the plane of the drawing. 
     The arrangement allows the load connection element  28  to tilt about the three tilting axes  88 ,  90  and  92  which are each arranged perpendicularly to the axis  42  of symmetry. 
     The invention provides the advantage that in addition to tilting vibrations about various axes it is also possible to damp torsional vibrations of the base connection element  10 . This can be done, for example, by cyclically actuating the actuator pairs  80 ,  82  and  84 . 
       FIG. 6  illustrates an example which shows that the engagement points of one of the actuators of an actuator pair do not need to be arranged in a line. The load connection element  28  is embodied in this design in such a way that the sum of the distances between the engagement points  34  and  36  and  38  and  40  and the distances between the engagement points  24  and  26  and  20  and  22  is greater than the distance between the base connection element  10  and the support element  14 . In other words, the load connection element  28  can be configured in such a way that the sum of the lengths of an actuator pair  16 ,  30  and  18 ,  32  does not need to correspond to the distance between the base connection element  10  and support element  14 . Configurations in which the length of an individual actuator exceeds the distance between the base connection element  10  and support element  14  are also possible. 
     As a result, it is possible to make use of the different lengths of actuators without the external design, which is determined essentially by the distance between the base connection element  10  and support element  14 , having to be significantly changed. Since the maximum change in length of a piezoactuator depends on the overall length of the piezo, it is thus possible to lengthen the actuation path of the interface by using relatively long piezoactuators. Furthermore, by using different piezoactuators it is possible to damp vibrations with different vibration frequencies since the resonant frequency of the piezoactuators also depends significantly on the overall length of the piezoceramic. 
       FIG. 7  illustrates an exemplary embodiment which shows that the actuators  100  which extend between the engagement points on the base connection element  10  and the load connection element  28  and the actuators  106  which extend between the engagement points on the support element  14  and on the load connection element  28  do not need to be arranged on the same side of the axis  42  of symmetry. A piezoactuator  100  extends between an engagement point  102  on the base connection element  10  and an engagement point  104  on the load connection element  28 . A further piezoactuator  106  extends between an engagement point  108  on the load connection element  28  and an engagement point  110  on the support element  14 . 
     In many cases, the actuators are arranged in such a way that overall the torques which are exerted on the load connection element  28  cancel one another out. This ensures that all the actuators are always subjected to pressure pretensioning. In the arrangement illustrated in  FIG. 7 , this can occur, for example, by further piezoactuators (not illustrated in this sectional view) being adjacent to the piezoactuator  106 , said further piezoactuators extending between engagement points on the base connection element  10  and engagement points on the load connection element  28  and thus compensating the torque which is exerted on the load connection element  28  by the piezoactuator  106 . For example, the arrangement can have six piezoactuators which are rotationally symmetrical through 120.degree. Said piezoactuators are arranged in such a way that in each case one actuator of the first actuator system (i.e. extending between engagement points on the base connection element  10  and the load connection element  28 ) and one actuator of the second actuator system (i.e. extending between engagement points on the support element  14  and the load connection element  28 ) lie opposite one another relative to the axis  42  of symmetry. Adjacent actuators are associated with different actuator systems. 
       FIG. 8  illustrates an interface for vibration damping in a perspective partial illustration with a cut-out segment. A piezoactuator  130  extends between an engagement point  132  on the base connection element  10  and an engagement point  134  on the load connection element  28 . A further piezoactuator  136  extends between an engagement point  138  on the load connection element  28  and an engagement point  140  on the support element  14 . 
     In this illustration it is apparent that both the base connection element  10  and the surface of the load connection element  28  are freely accessible for mounting purposes. The pretensioning device  12  is embodied as an elastic, cylindrical pipe which completely encloses the actuator systems and thus protects them against undesired loading by shearing forces perpendicular to their preferred direction and against environmental effects. The electrical feedlines to the piezoactuators can be routed to the piezoactuators  130  and  136  through an opening  142  in the base connection element  10 , for example. 
       FIG. 9  illustrates a plan view of an arrangement which shows the use of the invention for vibration damping in various spatial directions. An actuator system which is composed of the piezoactuators  160  and  162  extends between the engagement points  164  and  166  on the base connection element  10  and the engagement points  168  and  170  on the load connection element  28 . An actuator system which is composed of piezoactuators  172  and  174  extends between the engagement points  176  and  178  on a first support element  180  and the engagement points  182  and  184  on the load connection element  28 . The actuators  160 ,  162 ,  172  and  174  have the same spatial direction (referred to below as the X direction) as the preferred direction. 
     An actuator system which is composed of the piezoactuators  190  and  192  extends between the engagement points  194  and  196  on the base connection element  10  and the engagement points  198  and  200  on the load connection element  28 . An actuator system which is composed of the piezoactuators  202  and  204  extends between the engagement points  206  and  208  on a second support element  210  and the engagement points  212  and  214  on the load connection element  28 . The actuators  190 ,  192 ,  202  and  204  have the same spatial direction (referred to below as the Y direction) as the preferred direction, with this spatial direction being perpendicular to the abovementioned preferred direction of the actuators  160 ,  162 ,  172  and  174 . 
     In this exemplary embodiment, the support element  14  is composed of two separate support elements  180  and  210 . They are each connected to the base connection element  10  with a pretensioning device  216  or  218  (for example a rubber cube). 
     The load can be mounted on the load connection element  28  having, in this example, a cross-shaped cross section, by virtue of the fact that the load connection element additionally has a planar mounting plate which is mounted on the cross of the load connection element. 
     The arrangement has various advantages. On the one hand, transverse vibrations of the base connection element  10  in the X and Y directions can be damped by suitably actuating the piezoactuators. In this context it is possible, for example, to use, for each spatial direction, an electronic circuit for active vibration damping in a way which is analogous to the circuit described in  FIG. 3 . In addition, tilting vibrations of the base connection element  10  toward the X axis or Y axis can also be compensated by suitable actuation of the piezoactuators in a way which is analogous to  FIG. 4 . 
     The piezoactuators are pretensioned differently in the two spatial directions by the pretensioning devices  216  and  218 . This may be advantageous for applications in which different types of piezoactuators are to be used in the X and Y directions owing, for example, to different vibrations being expected in these two spatial directions. 
     In addition to the actuators which are illustrated here in the X and Y directions, it is also possible to use additional actuators in an analogous fashion in the spatial direction which is perpendicular to the X and Y directions. A separate support element is also appropriate for this again. This support element is preferably embodied again in such a way that the load connection element  28  is freely accessible for mounting purposes. 
       FIG. 10  illustrates an arrangement for vibration damping in various spatial directions which is an alternative to  FIG. 9 . The arrangement has, like the arrangement in  FIG. 9 , in turn a base connection element  10  and two support elements  180  and  210  which are connected to the base connection element  10  via the pretensioning devices  216  and  218 . A first piezoactuator  230  extends between an engagement point  232  on the base connection element  10  and an engagement point  234  on the load connection element  28 . A second piezoactuator  236  extends between an engagement point  238  on the support element  180  and an engagement point  240  on the load connection element  28 . A third piezoactuator  242  extends between an engagement point  244  on the support element  210  and an engagement point  246  on the load connection element  28 . 
     The arrangement shows that it is not absolutely necessary for in each case an actuator which extends between the base connection element  10  and the load connection element  28  and an actuator which extends between a support element and the load connection element  28  to have the same preferred direction. 
     As an alternative to the arrangement illustrated in  FIG. 10  it is also possible to use further piezoactuators for damping vibrations in further spatial directions. Thus, for example four piezoactuators and three support elements could be arranged in such a way that the piezoactuators each point into the corners of a tetrahedron which is standing on one of its tips. 
       FIG. 11  illustrates how a piezoactuator can be used as a sensor for a structural analysis. Said figure is a detailed view of any piezoactuator from one of the abovementioned exemplary embodiments, that is to say for example the piezoactuator  16  in  FIG. 2 . The piezoactuator is composed in this example of a stack of a plurality of piezoceramic elements. 
     A specific voltage is applied to the piezoactuator  16  by means of a variable voltage source  260 , with the switch  262  being initially closed. If the switch  262  is then suddenly opened, the length of the piezoactuator  16  changes suddenly. The entire system, that is to say also the other elements which are not illustrated here such as, for example, the load connection element  28 , starts to vibrate. This is referred to as the structural response of the entire system to the stimulation by opening the switch  262 . 
     The vibrations of the entire system in turn bring about a periodically changing pressure on the piezoactuator  16 . Owing to the piezo effect, these pressure fluctuations result in fluctuations in the electrical voltage between the electrodes  264  and  266  of a piezoceramic element  268  of the piezoactuator  16 . These voltage fluctuations can be registered and recorded using a measuring device  270 . 
     Instead of simply switching off the voltage which is applied to the piezoactuator  16  it is also possible to stimulate the entire system by means of other voltage profiles. For example, a simple sinusoidal voltage can be used or a voltage pulse. The respective structural response of the entire system to various types of stimulations can be used for a system analysis of the entire system by comparison with simulation values or by comparison with reference structural responses. If, for example, the structure interface is integrated into a carrier arm of a satellite system or into a spring-damper system in the region of the chassis of a motor vehicle, for example defects (for example due to material fatigue, etc.) can be detected and suitable countermeasures taken early by means of regular structural analyses. 
     Furthermore, the piezoceramic element  268  which acts as a sensor in  FIG. 11  can also be used for active vibration damping according to  FIG. 3 . Instead of the signal of the acceleration sensor  60  in  FIG. 3 , the voltage which occurs between the electrodes  264  and  266  (after suitable phase shifting in the phase shifter  62 ) is then used as an input signal for the electronic closed loop control system  64 . In this way it is possible to dispense with additionally providing a sensor in the interface. 
       FIGS. 12 and 13  show possible wiring arrangements of the energy converters for vibration damping. These are again any piezoactuator of the interface, and a plurality of actuators can also be wired simultaneously in this way or a similar way. In the text which follows it is assumed that the actuator is the actuator  16  which extends between the base connection element  10  and the load connection element  28 . The base connection element  10  and the load connection element  28  are illustrated in highly simplified form and the engagement points  20  and  24  and the other components of the interface are not illustrated for reasons of simplification. 
     The piezoactuator  16  in  FIGS. 12 and 13  is, similar to the arrangement illustrated in  FIG. 11 , configured again as a stack of a plurality of piezoceramic elements (sixteen in this case). The piezoceramic elements  7  to  13  (counted from the side where the basic connection element  10  is) are combined to form a unit  280  in such a way that the electrical potential of this unit can be tapped off between a terminal  282 , near to the base connection element  10 , of the unit  280  and a terminal  284 , near to the load connection element  28 , of the unit  280 . 
     In  FIG. 12 , the terminals  282  and  284  are each connected to one end of an ohmic resistor  286 . Furthermore, the terminal  282  is connected to ground potential. In  FIG. 13 , the terminal  282  is connected to an inductor  288 . This inductor  288  is connected to an ohmic resistor  286  which is in turn connected to the terminal  284 . Furthermore, the terminal  282  is connected to ground potential. If the load connection element  28  carries out mechanical vibrations relative to the base connection element  10 , this results in periodically fluctuating pressure on the piezoactuator  16 . Owing to the piezoelectric effect, these pressure fluctuations lead to fluctuations in the charge on the surfaces of the unit  280  lying opposite. These charge fluctuations result in a fluctuation of the voltage between the terminals  282  and  284 , which leads to a periodic flow of current through the electric wiring arrangement. 
     The arrangement in  FIG. 13  acts as a damped series oscillatory circuit composed of a capacitor, an inductor and an ohmic resistor. The terminals  282  and  284  act here like the plates of a capacitor whose charge varies periodically. At each oscillation, some of the electrical energy in the ohmic resistor  286  is converted into thermal energy and the vibration is thus damped. The selection of suitable ohmic resistors and inductors is made in accordance with the method described in N. W. Hagood and A. von Flotow: Damping of Structural Vibrations with Piezoelectric Materials and Passive Electrical Networks, Journal of Sound and Vibration 146 (2), 243 (1991). 
     LIST OF REFERENCE NUMERALS 
     
         
         
           
               1  Internal interference sources 
               2  Active element 
               3  Active element 
               4  Transmission paths 
               5  Sensitive elements 
               10  Base connection element 
               12  Pretensioning device 
               14  Support element 
               16  Piezoactuator of the first actuator system between base connection element  10  and load connection element  28   
               18  Piezoactuator of the first actuator system between base connection element  10  and load connection element  28   
               20  Engagement point of the actuator  16  on the base connection element  10   
               22  Engagement point of the actuator  18  on the base connection element  10   
               24  Engagement point of the actuator  16  on the load connection element  28   
               26  Engagement point of the actuator  18  on the load connection element  28   
               28  Load connection element 
               30  Piezoactuator of the second actuator system between support element  14  and load connection element  28   
               32  Piezoactuator of the second actuator system between support element  14  and load connection element  28   
               34  Engagement point of the piezoactuator  30  on the support element  14   
               36  Engagement point of the piezoactuator  32  on the support element  14   
               38  Engagement point of the piezoactuator  30  on the load element  28   
               40  Engagement point of the piezoactuator  32  on the load element  28   
               42  Axis of symmetry 
               60  Acceleration sensor 
               62  Phase shifter 
               64  Electronic closed loop control system 
               66  First postamplifier 
               68  180.degree. phase shifter 
               70  Second postamplifier 
               80  First actuator pair 
               82  Second actuator pair 
               84  Third actuator pair 
               88  Tilting axis 
               90  Tilting axis 
               92  Tilting axis 
               100  Piezoactuator 
               102  Engagement point of piezoactuator  100  on the base connection element 
               104  Engagement point of the piezoactuator  100  on the load connection element 
               106  Piezoactuator 
               108  Engagement point of the piezoactuator  106  on the load connection element 
               110  Engagement point of the piezoactuator  106  on the support element 
               130  Piezoactuator 
               132  Engagement point of the piezoactuator  130  on the base connection element 
               134  Engagement point of the piezoactuator  130  on the load connection element 
               136  Piezoactuator 
               140  Engagement point of the piezoactuator  136  on the load connection element 
               142  Engagement point of the piezoactuator  136  on the support element Opening in the base connection element for electric feedlines to the piezoactuators 
               160  Piezoactuator 
               162  Piezoactuator 
               164  Engagement point of the piezoactuator  160  on the base connection element 
               166  Engagement point of the piezoactuator  162  on the base connection element 
               168  Engagement point of the piezoactuator  160  on the load connection element 
               170  Engagement point of the piezoactuator  162  on the load connection element 
               172  Piezoactuator 
               174  Piezoactuator 
               176  Engagement point of the piezoactuator  172  on the support element  180   
               178  Engagement point of the piezoactuator  174  on the support element  180   
               180  Support element 
               182  Engagement point of the piezoactuator  172  on the load connection element 
               184  Engagement point of the piezoactuator  174  on the load connection element 
               190  Piezoactuator 
               192  Piezoactuator 
               194  Engagement point of the piezoactuator  190  on the base connection element 
               196  Engagement point of the piezoactuator  192  on the base connection element 
               198  Engagement point of the piezoactuator  190  on the load connection element 
               200  Engagement point of the piezoactuator  192  on the load connection element 
               202  Piezoactuator 
               204  Piezoactuator 
               206  Engagement point of the piezoactuator  202  on the support element  210   
               208  Engagement point of the piezoactuator  204  on the support element  210   
               210  Support element 
               212  Engagement point of the piezoactuator  202  on the load connection element  28   
               214  Engagement point of the piezoactuator  204  on the load connection element  28   
               216  Pretensioning device 
               218  Pretensioning device 
               230  Piezoactuator 
               232  Engagement point of the piezoactuator  230  on the base connection element  10   
               234  Engagement point of the piezoactuator  230  on the load connection element  28   
               236  Piezoactuator 
               238  Engagement point of the piezoactuator  236  on the support element  180   
               240  Engagement point of the piezoactuator  236  on the load connection element  28   
               242  Piezoactuator 
               244  Engagement point of the piezoactuator  242  on the support element  210   
               246  Engagement point of the piezoactuator  242  on the load connection element  28   
               260  Variable voltage source 
               262  Switch 
               264  First electrode of the piezoceramic element  268   
               266  Second electrode of the piezoceramic element  268   
               268  Piezoceramic element 
               270  Measuring device 
               280  Combined unit composed of piezoceramic elements of the piezoactuator  16   
               282  Terminal of the unit  280  near to the base connection element  10   
               284  Terminal of the unit  280  near to the load connection element  28   
               286  Ohmic resistor 
               288  Inductor