Abstract:
A force generator is configured for attachment to a structure in order to controllably introduce vibrational forces into the structure in order to influence the vibration thereof. The force generator encompasses a flexural arm that is fastenable at least at one end to the structure; and an inertial mass that is coupled to the flexural arm remotely from the fastening end of the flexural arm; the flexural arm being equipped with at least one electromagnetic transducer, and a driving system being provided for the transducer, which system is set up such that by driving the transducer, it warps the flexural arm with the inertial mass and the transducer, and thereby displaces the inertial mass, in such a way that vibrational forces of variable amplitude, phase, and frequency are introducible into the structure.

Description:
CROSS REFERENCE TO PRIOR APPLICATIONS 
       [0001]    This application is a U.S. National Phase application under 35 U.S.C. § 371 of International Application No. PCT/EP2006/011569, filed on Dec. 1, 2006 and claims benefit to German Patent Application No. DE 10 2005 060 779.9, filed on Dec. 16, 2005. The International Application was published in German on Jul. 5, 2007 as WO 2007/073820 under PCT Article 21 (2). 
         [0002]    The present invention relates to a force generator and to a method for operating the force generator. The force generator serves in particular to influence the vibration of structures, counter-vibrations being deliberately introduced into a structure in order to reduce the overall vibration level in the structure. The invention further relates to an apparatus for influencing vibration. The invention is applicable in particular to vibration control in helicopters and aircraft. 
     
    
     BACKGROUND 
       [0003]    Force generators serve to generate a desired force by means of a predetermined inertial mass. The forces always result in this context from the inertia of the inertial mass, moved in whatever fashion. In order to generate the greatest possible force, on the one hand the inertial mass can be moved with a maximum possible acceleration (or displacement). Alternatively or in addition thereto, a large force of this kind can also be generated by way of an inertial mass that is as large as possible. 
         [0004]    Force generators based on the electrodynamic principle, in which the interaction between two moving electric charges is utilized, are already known. For this, an electrical conductor wound into a coil and provided with a current pulse is immersed in a magnetic field. The charges in the conductor thereupon experience a force impulse, with the result that the coil is caused to move. One disadvantage in this context is that the coil possesses a large mass, and can generate only relatively small accelerations and therefore small forces. The ratio between mass used and force generated is relatively high. In addition, an unfavorable energy balance exists with electrodynamic principles because of the ohmic resistance of the coil. 
         [0005]    Force generators of this kind are used, for example, for controlled introduction of forces into vibrating structures (e.g. aircraft, motor vehicles, or machine components), in order to counteract high vibration levels and cancel them out. Problems occur in this context especially when the frequency of the structure to be regulated varies to a greater or lesser extent, as is the case, for example, in different operating states of the vibrating structure. Different operating states of this kind occur, for example, in aircraft in the different phases of flight, in particular on takeoff and on landing. With the known arrangements, vibration usually can be reduced only in a very narrow frequency range, which for many applications is disadvantageous. 
       SUMMARY OF THE INVENTION 
       [0006]    An object of the present invention is to provide a force generator that, with a predefined inertial mass, generates large accelerations and therefore forces, and at the same time has a favorable ratio between the inertial mass and the force generated therewith. The force generator according to the present invention is further intended to exhibit high quality, i.e. to have low self-damping and a low energy consumption. A further object is to provide a force generator that is universally and variably usable, i.e. with which, in particular, vibrations can be effectively reduced over the widest possible frequency range. A further object is that of providing a method with which such a force generator can be operated. 
         [0007]    The present invention provides a force generator as described herein. 
         [0008]    The force generator according to one aspect of the present invention is configured for attachment to a structure in order to controllably introduce vibrational forces into the structure in order to influence the vibration thereof, and encompasses a flexural arm that is fastenable at least at its one end to the structure, as well as an inertial mass that is coupled to the flexural arm remotely from the fastening end of the flexural arm. The flexural arm is equipped with an electromagnetic transducer, and a driving system is provided for the electromagnetic transducer, which system is set up such that by driving the electromagnetic transducer, it warps the flexural arm with the inertial mass and the transducer, and thereby displaces the inertial mass, in such a way that vibrational forces of variable amplitude, phase, and frequency can be generated in the structure, and are introducible via the fastening end into the structure. 
         [0009]    It is particularly advantageous in this context that the driving system is set up to cause the inertial mass, the flexural arm, and the electromagnetic transducer to vibrate at adjustable amplitude, phase, and frequency. Different vibrational forces can thereby be deliberately generated, in particular over a wide frequency range, and introduced into a structure that is to be influenced. It is possible in this context either to excite the inertial mass and the flexural arm including the transducer less strongly, so that a lower vibration amplitude and thus a lower acceleration and lower force are achieved, or else to excite them strongly, so that a high vibration amplitude and thus a large acceleration and large force are achieved. In addition to adaptation of the vibration amplitude, the phase as well as the frequency are also variably adjustable. 
         [0010]    A further advantage of the present invention is that the electromagnetic transducer can also be driven in such a way that introduction of vibrational forces at two or more frequencies simultaneously is possible. Driving occurs here at multiple frequencies or over a predetermined frequency range. 
         [0011]    If the force generator is operated at resonant frequency (or in the vicinity of its resonant frequency/ies), the dynamic exaggeration of the displacement of the inertial mass can thereby advantageously be utilized in order to generate particularly large forces. Excitation in the region of the resonant frequency allows a large vibration amplitude for the inertial mass to be achieved for a predetermined inertial mass. This is accompanied by high acceleration, so that relatively large forces can be generated by the inertial mass. 
         [0012]    Usefully, the inertial mass constitutes a multiple of the mass of the flexural arm including the transducer, so that force generator possesses a relatively small total mass and achieves high efficiency. 
         [0013]    The transducer is preferably a piezoelectric actuator. An actuator of this kind possesses a very rapid response characteristic and can be precisely regulated in terms of both its displacement travel amplitudes and its frequencies. Accurately predetermined excitation frequencies can thus be established for the force generator. A piezoelectric actuator operates with long displacement travels and high resolution even with large counterforces, so that vibrational forces can be reliably generated even with a large inertial mass. 
         [0014]    Particularly preferably, the piezoelectric actuator is a stacked piezoelement (i.e. a so-called “piezostack”) having a d33 effect. With the d33 effect, which as is known is also referred to as a longitudinal effect, the change in the length of the piezoelectric element occurs in the direction of the applied electric field, i.e. along the stack direction or longitudinal direction of the piezoelement. The change in length produced in this context is known to be greater than the change in length in the context of the d31 effect, in which the change in length occurs transversely to the direction of the applied electric field. 
         [0015]    According to a preferred embodiment, the transducer is drivable in such a way that it effects a change in length in the longitudinal direction of the flexural arm. This results in a warping of the flexural arm, with the result that in turn the inertial mass is displaced, so that vibrations of the flexural arm with the inertial mass and the transducer are triggered in order to generate corresponding vibrational forces. If the transducer is arranged parallel to a neutral ply that extends, in the context of a symmetrically constructed flexural arm, along the center line of the flexural arm, the length of a ply provided parallel to the neutral ply can thus be changed as compared with the neutral ply. The ply having the greater length induces a deflection in the direction toward the ply having the shorter length. If the change in length is repeated at periodic intervals, the result is a flexural vibration of the flexural arm including the transducer and the inertial mass. With an excitation in the resonant frequency range, the system oscillates to large amplitudes. 
         [0016]    Particularly preferably, at least one transducer is arranged respectively on mutually oppositely located sides of the neutral ply, so that a deflection to both mutually oppositely located sides of the neutral ply is generated, with the result that, advantageously, the displacement of the inertial mass can be increased. 
         [0017]    Preferably, the transducer is non-positively and/or positively connected to the neutral ply. This on the one hand ensures that the transducer is positioned in stationary fashion and can effect an accurately repeatable warping of the flexural arm. On the other hand, because the transducer is positioned in the vicinity of the neutral ply, the transducer is deflected relatively little at very high vibration amplitudes. This is a feature to protect the transducer from mechanical deformation resulting from bending. The protection can be enhanced if the at least one transducer is arranged inside the flexural arm or embedded thereinto. Damage to a mechanically sensitive transducer from outside is thus possible only with difficulty. In addition, an encapsulation of the transducer can be achieved by arranging the transducer inside the flexural arm, so that the force generator is also usable, for example, in a wet or chemically aggressive environment. 
         [0018]    According to a further preferred embodiment of the invention, a spacing element is arranged between the inertial mass and one end of the transducer. The spacing element allows the transducer to be positioned even more securely in its location. The spacing element preferably has a low density, in order to increase the ratio between the inertial mass and the mass of the flexural arm including the transducer. In particular, the resonant frequency of the assembly made up of the flexural arm, transducer, and inertial mass can be deliberately influenced by appropriate selection of the material for the spacing element. 
         [0019]    In addition, a protective outer ply of the flexural arm, which ply is arranged at a lateral distance from the neutral ply, can be non-positively and/or positively connected to the transducer. The use of an outer ply results in a layered design for the flexural arm, and thus provides simple protection from external influences on the transducer. Non-positive connection, for example by adhesive bonding, and positive connection, for example by bolting, ensure accurate positioning of the parts with respect to one another. 
         [0020]    Particularly preferably, the flexural arm is embodied as a fiber composite design with an integrated transducer. The flexural arm is manufactured in layered fashion using fiber composite materials, in particular glass fiber-reinforced (GFR) plastic, the layered construction being, in a last working step, infiltrated or injected with a resin system e.g. by means of a known resin transfer molding (RTM) method, and then cured. A particularly long service life for the force generator may be achieved by way of a fiber composite design of this kind. 
         [0021]    The transducer is preferably under a compressive preload. The result of this is that even with a high vibration amplitude (e.g. with resonance exaggeration) of the flexural arm, it is always compressive forces, and not tensile forces that are hazardous to the transducer, that act on the transducer. This is of particular importance for a transducer that comprises piezoceramic layers. The transducer that is under compressive preload on the transducer can better withstand large vibration amplitudes. The compressive preload can be impressed mechanically. The transducer can, however, also be thermally preloaded. This can be achieved, for example, by introducing it into a matrix that possesses a coefficient of thermal expansion different from that of the transducer. A compressive preload can then be achieved upon thermal curing of the matrix. Another possibility is to apply an electrical offset voltage to the transducer. The transducer is thus always exposed to compression, and is protected from tensile loading even at large vibration amplitudes. 
         [0022]    The force generator according to the present invention typically has a length of 3 to 60 centimeters. With suitable dimensioning of all the components, the inertial mass can then have imparted to it a vibration that exhibits a maximum vibration amplitude in the range from 0.1 to 3 centimeters. 
         [0023]    Another aspect of the present invention is provided as a method for operating the force generator as described above, such that by suitable driving of the electromagnetic transducer, the flexural arm with the inertial mass and the transducer is warped, and the inertial mass thereby displaced, in such a way that vibrational forces of variable amplitude, phase, and frequency are generated. 
         [0024]    Another aspect of the present invention is an apparatus for influencing vibration that is embodied for attachment to at least one structure in order to controllably introduce vibrational forces into the structure, two force generators of the kind described above being arranged in such a way that the flexural arm of the first force generator is arranged along the extension of the flexural arm of the second force generator. 
         [0025]    The force generator according to the present invention can thus also be used in a symmetrical design, two individual force generators of the above-described kind being used in such a way that they are each fastened, not with the ends of the flexural arms coupled to the inertial mass, to a structure to be influenced in terms of vibration, or are connected to one another in such a way that they form a flexural arm having inertial masses arranged on either side, i.e. at both ends of a flexural arm. The inertial masses should have the same offset from the structure, i.e. the lever arms of the flexural arms are preferably identical. The arrangement can be driven in such a way that the inertial masses are displaced in parallel fashion, i.e. in the same direction, or in antiparallel fashion, i.e. in opposite directions. In the latter case, not only forces but also moments can be introduced into the structure. 
         [0026]    A further symmetrical use of the force generator according to the present invention is the arrangement, hereinafter also referred to as a “swing oscillator,” in which the flexural arm of a first force generator is lengthened, so to speak, out beyond the inertial mass, and the free end of the lengthened flexural arm is likewise attached to the structure but at a different point. In other words, a flexural arm is provided whose opposite ends are fastenable to a structure, at least one inertial mass being provided at the center of the flexural arm. With an arrangement of this kind, the introduction of forces occurs in moment-free fashion. 
         [0027]    The force generator according to the present invention, and its symmetrical application, are used in particular for active vibration control of structures (aircraft, motor vehicles, machine components, etc.). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    Further features and advantages of the invention are evident from the description below of various exemplifying embodiments according to the present invention in conjunction with the accompanying drawings, in which: 
           [0029]      FIG. 1  schematically depicts a first embodiment of the force generator according to the present invention in a rest position; 
           [0030]      FIG. 2  schematically depicts the first embodiment of the force generator of  FIG. 1  in a deflected position; 
           [0031]      FIG. 3  schematically depicts a second embodiment of the force generator according to the present invention in a rest position; 
           [0032]      FIG. 4  schematically depicts the second embodiment of the force generator of  FIG. 3  in a deflected position; 
           [0033]      FIG. 5  schematically depicts a third embodiment of the force generator according to the present invention in a rest position; 
           [0034]      FIG. 6  schematically depicts the third embodiment of the force generator of  FIG. 5  in a deflected position; 
           [0035]      FIG. 7  schematically depicts a fourth embodiment of the force generator according to the present invention in a rest position; and 
           [0036]      FIG. 8  schematically depicts a further embodiment according to the present invention that encompasses two symmetrically arranged force generators; and 
           [0037]      FIG. 9  shows a symmetrical arrangement, alternative to  FIG. 8 , of two force generators. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]      FIG. 1  schematically depicts a first embodiment of force generator  1  according to the present invention. It comprises a flexural arm  2  that is attached at one end  10  to a structure  3 , and comprises an inertial mass  4  at the other end. Structure  3  is, for example, an aircraft, a motor vehicle, a machine component, or any other component; structure  3  vibrates in an undesired fashion. To reduce these vibrations, force generator  1  is connected to structure  3  so that counter-vibrations can be deliberately introduced into structure  3  in order to reduce the overall level of the vibrations in structure  3 , as explained below in greater detail. 
         [0039]    Mounted on flexural arm  2  is an electromagnetic transducer  5 , in particular a piezoelectric actuator, that is electrically connected to a driving system  6 . The position of driving system  6  is arranged at a distance from flexural arm  2  and from transducer  5  such that it does not impede the movement of flexural arm  2  including transducer  5  and inertial mass  4 . In the position depicted in  FIG. 1 , flexural arm  2 , together with inertial mass  4  and electromagnetic transducer  5 , is located in a rest position such that center line  7  of flexural arm  2  extends horizontally. 
         [0040]    Electromagnetic transducer  5  is driven in such a way that it experiences a positive change in length Δl in the longitudinal direction of flexural arm  2 . Transducer  5  is connected to upper edge fiber  8  of flexural arm  2  in such a way that change in length Δl of transducer  5  is transferred into upper edge fiber  8  so that its length  1  is extended by an amount Δl. Because no change in length is exerted on lower edge fiber  9 , a length difference of Δl is therefore produced between upper edge fiber  8  and lower edge fiber  9 . As is evident from  FIG. 2 , this length difference Δl leads to a warping of flexural arm  2  in the negative y direction. Inertial mass  4 , connected rigidly to flexural arm  2 , is shifted in this context, by an amount Δy, from its rest position depicted with a dashed line into a deflected position depicted by a solid line. As a consequence of the length increase, by an amount Δl, of upper edge fiber  8 , center line  7  of flexural arm  2  thus changes its horizontal orientation into the deflected position depicted by the dot-dash line  12 . As a result of an at least non-positive connection between transducer  5  and flexural arm  2 , transducer  5  follows the curvature of upper edge fiber  8 . 
         [0041]    By appropriate driving of transducer  5 , flexural arm  2  including transducer  5  and inertial mass  4  can consequently be excited to vibrate, such that inertial mass  4  and flexural arm  2  with transducer  5  vibrate up and down about center line  7  extending horizontally, as indicated by arrow  11  in  FIG. 1 . The amplitude, phase, and frequency of the vibration are adjustable by suitable driving (e.g. U(ω) or U (Δω)) of transducer  5 , so that vibrational forces are deliberately introducible via attachment point  10  into structure  3  in order to bring about, by superposition of introduced vibrations and structural vibrations, a reduction, ideally a cancellation, of the vibrations over a wide frequency range and/or at multiple frequencies simultaneously. To regulate the driving system, at least one sensor is provided which senses the vibrations of structure  3  in order to regulate driving system  6  on the basis of the acquired sensor signals. 
         [0042]    If transducer  5  is driven, or the change in length Δl is accomplished, at a frequency that is in the region of the resonant frequency of the system made up of flexural arm  2 , inertial mass  4 , and transducer  5 , inertial mass  4  can be displaced in the y direction by an amount that, as a result of resonance exaggeration, is several times greater than the amount Δy. Inertial mass  4  experiences a greater acceleration as a result of the greater vibration amplitude, so that substantially larger forces or greater vibration amplitudes are generated. 
         [0043]    In the embodiment depicted in  FIGS. 1 and 2 , electromagnetic transducer  5  is preferably a stacked piezoelement having a d33 effect. The stack direction extends substantially in the longitudinal direction of flexural arm  2 , i.e. in a horizontal direction, in order to bring about the above-described change in length Δl in the longitudinal direction of flexural arm  2 . Transducer  5  is non-positively connected to flexural arm  2 , e.g. by adhesive bonding. Alternatively, a recess can be provided in flexural arm  2 , into which recess transducer  5  is fitted in such a way that horizontal shifting or sliding of transducer  5  is not possible. To protect transducer  5 , the arrangement of flexural arm  2  and transducer  5  can additionally be equipped with a protective layer or embedded into a fiber composite material arrangement, the latter being explained in additional detail in connection with the description of  FIG. 7 . 
         [0044]      FIG. 3  depicts a second embodiment of the force generator according to the invention. Flexural arm  2  is constructed in a layered design. It has a neutral ply  19  that extends along center line  7  of flexural arm  2 . Parallel thereto, flexural arm  2  has an upper outer ply  14  and a lower outer ply  18 . Arranged between upper outer ply  14  and neutral ply  19  are a first actuator constituting electromagnetic transducer  5 , and an additional element  13  that is hereinafter also referred to as a spacing element, which occupies the distance between actuator  5  and inertial mass  4  as well as the distance between neutral ply  19  and upper outer ply  14 . A second actuator  15 , and a spacing element  17  adjoining it, are located in the same fashion between neutral ply  19  and lower outer ply  18 . First actuator  5  is coupled to a driving system  6 , and second actuator  15  to a driving system  16 , which systems are respectively regulated as a function of sensor signals that are received from corresponding sensors for sensing the vibration of structure  3 . The driving signals for driving systems  6 ,  16  can be identical or different (e.g. U(ω 1 ) and U(ω 2 )); each individual transducer  5 ,  15 , can also be excited simultaneously at multiple frequencies. 
         [0045]    In the embodiment depicted in  FIG. 3 , transducers  5 ,  15  are once again embodied as piezoelectric actuators, in particular as stacked piezoelements having a d33 effect. The stacking or longitudinal direction of the piezoelement extends horizontally, so that upon application of an electric field in the stacking direction of piezoelement  5 , a change in length occurs in the longitudinal direction of flexural arm  2 . The rest position of force generator  1 , as depicted in  FIG. 3 , can be shifted into a deflected position by driving first piezoelectric actuator  5 . If first actuator  5  experiences a change in length Δl 1  (cf. front end  20  of first actuator  5 ), this change in length Al 1  is transferred, because of the coupling with spacing element  13  and with upper outer ply  13 , to inertial mass  4 . At the same time, second actuator  15  arranged parallel thereto experiences no change in length (cf. front end  21  of second actuator  15 ), so that the length of lower outer ply  18  is not modified. As in the case of the first embodiment depicted in  FIG. 2 , the flexural arm is in this fashion warped in the negative y direction (see  FIG. 4 ). The function and the manner of operation of force generator  1  are otherwise analogous to those of the first embodiment. 
         [0046]    Even more efficient vibration of inertial mass  4  is achieved with the inertial force generator  1  depicted in  FIGS. 5 and 6 . This third embodiment is largely identical to the second embodiment. One difference is that already in the rest position of flexural arm  2 , both transducers  5 ,  15  are driven so that they are displaced by an amount equal to a change in length Δl 2 , i.e. a preload is applied to transducers  5 ,  15 . First actuator  5  is then lengthened by an additional change in length Δl 2 , while second actuator  15  is shortened by that change in length Δl 2  (see  FIG. 6 ). The first actuator therefore effects a change in length equal to Δl 2 +Δl 2 , while the second actuator exhibits no further change in length. This design takes into account the circumstance that starting from its baseline length at which no electric field is applied, a piezoceramic material can only be lengthened. 
         [0047]      FIG. 7  depicts a particularly preferred embodiment of the invention. Flexural arm  2  is embodied as a fiber composite design. Neutral ply  19  and outer plies  14 ,  18  are made of fiber composite material, in particular of glass fiber-reinforced (GFR) plastic. Spacing elements  13 ,  22  and  17 ,  23  arranged respectively on either side of transducers  5 ,  15  can be made of fiber composite materials, other lightweight materials (e.g. foamed material), or metal. In the manufacture of flexural arm  2 , firstly transducers  5 ,  15  are mounted on either side of neutral ply  19 , if applicable by immobilization by adhesive bonding. The regions on the sides of transducers  5 ,  15  are then filled up with corresponding spacing elements  13 ,  22  and  18 ,  23 , respectively, which can be made up of multiple fiber composite material plies. Outer plies  14 ,  18  are put in place to protect piezoelectric actuators  5 ,  15 , and lastly the layered fiber composite material arrangement is injected in known fashion with a resin system and cured, if applicable with the application of heat, typically by means of a known resin injection method such as, for example, the RTM method. Outer plies  14  and  18  protect the sensitive piezoceramic materials of actuators  5 ,  15  from moisture and from the penetration of foreign objects. By appropriate selection of the materials of spacing elements  13 ,  17 ,  22 , and  23 , the resonant frequency of flexural arm  2  with transducers  5 ,  15  and inertial mass  4  can be set to a desired value. A particularly lightweight arrangement, in which the mass of flexural arm  2  with transducers  5 ,  15  is much less than inertial mass  4 , can also be created by suitable selection of materials. 
         [0048]    The force generator described above can also be used in a symmetrical arrangement in order to create an apparatus for influencing vibration.  FIG. 8  schematically depicts a first embodiment having two force generators, the flexural arms of the two force generators being arranged along one another&#39;s extensions. As is apparent from  FIG. 8 , flexural arms  2 ′,  2 ″ of the respective force generators  1  are arranged in such a way, on structure  3  that is to be influenced in terms of vibration, that inertial masses  4 ′,  4 ″ are at identical distances from the respective attachment points  10 ′ and  10 ″. Flexural arms  2 ,  2 ″ are preferably embodied integrally, so that the apparatus for influencing vibration substantially comprises one flexural arm at whose outer ends the respective inertial masses  4 ′ and  4 ″ are arranged. The integral flexural arm is then preferably arranged at the center on structure  3 . The arrangement depicted in  FIG. 8  can be driven, by transducers arranged on flexural arms  2 ,  2 ″, in such a way that inertial masses  4 ′,  4 ″ are displaced in either parallel fashion, i.e. in the same direction (e.g. in the positive y direction), or in anti-parallel fashion, i.e. in opposite directions. In the case of a parallel displacement of inertial masses  4 ′,  4 ″, forces as well as moments can be introduced into structure  3 . With an anti-parallel displacement, force is introduced into structure  3  in moment-free fashion. 
         [0049]      FIG. 9  depicts a further symmetrical arrangement of force generators according to the present invention that shows a so-called “swing oscillator” arrangement. Looking at the left portion of  FIG. 9 , this depicts a force generator as described in conjunction with  FIGS. 1 to 7 , except that flexural arm  2 ′ is, so to speak, lengthened by inertial mass  4 , i.e. to the right in  FIG. 9 , the lengthened end being attached to a further structure  3 ″ or to another point  3 ″ on the structure. In other words, the arrangement according to  FIG. 9  substantially encompasses a flexural arm whose outer ends, i.e. the left and the right end in  FIG. 9 , are attached at different points  3 ′ and  3 ″. Inertial mass  4  is arranged at the center of the flexural arm and is displaced, by analogy with the description above, in a direction perpendicular to the plane of the flexural arm, i.e. in a positive and negative y direction. This introduction of vibrational forces at points  3 ′ and  3 ″ occurs in moment-free fashion.