Abstract:
The invention ris an actuating drive for deflecting an actuating element by a solid body actuator ( 2 ) which, when an electrical voltage or an alternating magnetic field is applied, experiences a length change. A housing ( 1 ) encloses the solid body actuator to define an interspace ( 13 ) in a fluid-tight manner. Furthermore, the solid body actuator includes a hollow duct ( 6 ) having one end connected to a first hollow conduit ( 20 ) passing through the housing and the other end opening into the interspace. The interspace is additionally connected in a fluid-tight manner to a second hollow conduit ( 21 ) passing through the housing.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    Reference is hereby made to German Patent Application No. DE10 2010014 892.5, filed on Jul. 27, 2013 and PCT/EP2013/002192, filed Jul. 23, 2014, which applications are incorporated herein by reference in their entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to an actuating drive for uniaxial, bi-directional deflection of an actuating element, which can be actuated directly or indirectly by a solid body actuator, which has at least one solid body actuator end braced on a frontal face in a housing that undergoes a change in length in a longitudinal extension that is assignable to the solid body actuator in response to a change in an electrical voltage that may be applied to the solid body actuator or a magnetic field acting on the solid body actuator, depending on the actuator type, and which may be transferred directly or indirectly to the actuating element via a second frontal face opposite the first frontal face in the longitudinal extension of the solid body actuator, displacing the actuating element and method for removing heat from a solid body actuator housed in an actuating drive with an actuating element. 
         [0004]    2. Description of the Prior Art 
         [0005]    Actuating drives in which solid body actuators—preferably based on piezoelectric or magnetostrictive materials which are integrated for the purpose of travel deflection, are prone to heating particularly during dynamic power operation due to the hysteresis inherent in the transducer material and the enclosure of the solid body actuator required by the system. If no measures are implement to cool the solid body actuator, not only is there a danger that the solid body actuator may lose geometric dimensional stability due to thermal expansion of the material, but if the material-specific Curie temperature is exceeded, the energy transducing properties acquired by the material are lost. Moreover, operationally induced self-heating of the solid body actuator results mostly in an increased electrical power requirement, which has a very detrimental effect on the activation of the solid body actuator. 
         [0006]    Heat dissipation is typically assured with an elastomer-like medium, most often cast with additional heat dissipating particles around the solid body actuator inside the actuating drive housing. Accordingly, the most prevalent cooling principle employed is to dissipate the heat from the solid body surface to the surrounding housing via the elastomeric medium. 
         [0007]    The activation of control valves or injection valves in motor vehicles represents a typical application of an actuating drive equipped for example with a piezoelectric actuator. Such a piezoelectric actuator is described in DE 199 14 411 A1 and has a multilayer laminate of stacked layers of piezoelectric material with interposed metal or electrically conductive, layers functioning as electrodes. The actuator body, also known as a stack actuator, is integrated inside a hollow cylindrical travel path housing and one side of the body rests on a baseplate fixed inside the housing. The opposite end of the stack actuator is in contact with a valve stem, for the purpose of deflecting it in the lengthwise direction of the stack actuator. Also resting on the opposite end of the stack actuator inside the actuator housing is a spring that generates a restoring force. For purposes of cooling the stack actuator, an electrically insulating elastomer sleeve is provided which fits tightly against the inner wall of the hollow cylindrical actuating drive housing and against the stack actuator. 
         [0008]    A similar fuel injector having a piezoelectric element is disclosed in DE11 2009 001 571 T5, in which a piezoelectric element is introduced into an injector housing, so that a cavity is delimited between the housing and the piezoelectric element. The cavity is at least partially filled with a thermally conductive material to transfer heat from the piezoelectric element to the housing. 
         [0009]    German Patent No. DE 103 28 373 A1 discloses a piezoelectric assembly having an integrated temperature control device that is made from a composite material having an elastomeric polymer and serves as a support matrix for carbon fibers, which are preferably used in the form of carbon nanotubes for particularly effective cooling of the piezoelectric device. 
         [0010]    The object of German Patent No. DE 10 2004 050 880 B4 discloses a fuel injector having an encased actuator, wherein the surface of the enclosure facing away from the actuator has microstructures that increase the surface arear thereof for improved heat dissipation. 
         [0011]    Similar actuating drives, each equipped with piezoelectric actuators that provide electrically activatable cooling elements in the form of Peltier elements for actively cooling the piezoelectric actuator are also disclosed in DE 10 2009 026 533 A1 and DE 198 26 339 A1. 
         [0012]    Japanese Patent JP 07131085 A describes a piezoelectric actuator having an actuator stack composed of a plurality of individual piezoelectric perforated discs, in which a double-tube like cooling structure is created in the interior hollow duct for cooling purposes. 
         [0013]    JP 7094797 A discloses a piezoelectric actuator stack composed of a plurality of individual piezoelectric perforated disks, having an interior cavity and also encloses an interspace with a housing that surrounds the piezoelectric actuator stack. For cooling purposes, a coolant is circulated through the inner cavity in a closed cooling circuit and through the radially outer interspace of the actuator. The circulation of the coolant is controlled by valves which are alternately opened and closed. 
       SUMMARY OF THE INVENTION 
       [0014]    The invention is an actuating drive for uniaxial, bi-directional deflection of an actuating element, which can be actuated directly or indirectly by a solid body actuator. At least one end of the solid body actuator is braced on a frontal face in a housing that undergoes at least a change in length in a longitudinal extension of the solid body actuator in response to a change in an electrical voltage that may be applied to the solid body actuator or a magnetic field acting on the solid body actuator, which longitudinal extension may be transferred directly or indirectly to the actuating element via a second frontal face opposite the first frontal face of the longitudinal extension of the solid body actuator. The actuating element is displaced, in such manner that operational heating of the solid body actuator is counteracted, particularly in a dynamic operating mode, by more effective heat dissipation than is the case with the previously known cooling measures. In particular, measures for cooling the solid body actuator, should neither impede nor limit scope of application of the actuator, and furthermore should be representable with the simplest, most inexpensive design engineering interventions possible. 
         [0015]    According to the invention, an actuating drive is configured such that the housing at least partly surrounds the solid body actuator and serves together with the solid body actuator to delimit at least one interspace in a fluid-tight manner. The solid body actuator also has a hollow duct that passes through the solid body actuator at least in sections, and preferably completely. One end of the hollow duct is connected to a first hollow conduit that passes through the housing. The other end of the hollow duct that at least partially protrudes through the solid body actuator hollow duct opens directly or indirectly into the at least one interspace that is delimited in fluid-tight manner by the solid body actuator and the housing. In addition, the at least one interspace is connected in fluid-tight manner with a second hollow duct that passes through the housing. 
         [0016]    The cooling according to the invention cools the solid body actuator which heats up in normal operation by using the thermal coupling of a gas-phase or liquid coolant to the immediate surface of the solid body actuator enclosed inside the actuating drive. To ensure effective cooling, the at least one solid body actuator provides at least one inner hollow duct through which the coolant is supplied via a suitable supply line. The coolant thus initially flows through the inner hollow duct of the solid body actuator, and undergoes effective heat extraction from the inside. After passing through the hollow duct, the coolant is diverted suitably so that it comes into thermal contact with the outer solid body actuator surface in the opposite overflow direction compared to the throughflow direction along the inner hollow duct. In this step, the solid body actuator undergoes heat extraction from the outside. After the coolant has preferably flowed over the entire outer surface of the solid body actuator, it is transported away from the actuator and also out of the actuating drive via a corresponding drain arrangement. Of course, it is possible to charge the actuating drive with the cooling in a reverse flow direction, that is, to cool the outer side of the solid body actuator before the coolant is passed through the at least one inner hollow duct and then transported away. 
         [0017]    Appropriate coolants may be selected from gas-phase and liquid substances depending on the structural design of the actuating drive, and also on the intended use and purpose thereof. In an advantageous variation of the cooling according to the invention for solid body actuators integrated in actuating drives, the cooling medium transported away from the solid body actuator, which itself undergoes warming as a result of thermal coupling due to the extraction of heat from the solid body actuator, undergoes a cooling process that is external, that is spatially separate from actuating drive, and after corresponding cooling is returned to the solid body actuator in a closed circuit. In this way, loss of coolant is avoided. 
         [0018]    The cooling according to the invention is usable for all solid body actuators fitted inside an actuating drives, and so preferably for solid body actuators made from piezoelectric or magnetostrictive material. For the purposes of explaining the cooling concept according to the invention, in the following reference is made to an actuating drive equipped with a piezoelectric actuator, which however is not limiting of the general concept for cooling a solid body actuator integrated in an actuating drive. 
         [0019]    BRIEF DESCRIPTION OF THE INVENTION 
         [0020]    The invention will be described in the following without limitation of the general inventive thought on the basis of an exemplary embodiment thereof and with reference to the drawing. In the drawing: 
         [0021]    Fig. is a longitudinal section through an actuating drive with a cooling concept according to the invention; 
         [0022]      FIGS. 2   a  and  b  show a cross section and a longitudinal section through a connecting element for connecting two actuator elements inside the actuating drive; 
         [0023]      FIGS. 3   a - d  are cross sectional views of further variants for the formation of connecting elements; and 
         [0024]      FIG. 4  is a longitudinal section through a highly schematic actuating drive having four hollow cylindrical actuator elements connected axially in series 
     
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]      FIG. 1  shows a longitudinal section through an actuating drive configured according to the invention, which provides a solid body actuator  2  made from piezoelectric material inside a housing  1 . The actuator is constructed in the embodiment shown from two hollow cylindrical piezoelectric solid body elements  3  and  4 . The hollow cylindrical piezoelectric solid body elements  3  and  4  are joined by a connecting element  5 , preferably made of plastic and in the form of a sleeve, to form a unitary hollow cylindrical body enclosing a hollow inner duct  6 . 
         [0026]    The individual piezoelectric solid body elements  3  and  4  are each shaped from a monolithic piezoelectric material into a hollow cylinder, and electrodes are attached to both hollow cylindrical end faces thereof, via which an electrical voltage is applied to cause a controlled longitudinal expansion of piezoelectric solid body elements  3  and  4 . Alternatively, it is also possible to combine the individual, hollow cylindrical piezoelectric elements  3 ,  4  from a plurality of piezoelectric elements constructed from layers or perforated discs to create an actuator stack, wherein the individual layered or disc-like piezoelectric elements are tuned to each other by a comb-like electrode structure and charged with electrical voltage to activate the actuators. 
         [0027]    Through the serial, coaxial coupling of the two individual piezoelectric elements  3  and  4  to create a solid body actuator  2  with the form of a hollow cylinder, appropriate electrical activation additive superposes the deflection travel paths in both individual piezoelectric elements  3  and  4 . Of course, it is possible to combine additional hollow cylindrical piezoelectric elements with each other using suitably designed connecting elements  5 , thereby increasing the total deflection travel path of solid body actuator  2 . 
         [0028]    Connecting piece  5  is a cylindrical or sleeve-like plastic component that serves to connect the two piezoelectric elements  3  and  4  and has an outer sleeve diameter equal to the inner diameter of each of hollow cylindrical piezoelectric elements  3  and  4 . In order to connect both hollow cylindrical piezoelectric elements  3  and  4 , the end faces of the two elements are abutted flush with one another, wherein the two piezoelectric elements  3  and  4  are joined permanently by sticking them to the inner wall of sleeve-like connecting element  5  with an adhesive. In this way, it is ensured that the axial positioning forces acting between the two piezoelectric elements  3  and  4  arising from the controllably producible longitudinal changes in the two piezoelectric elements  3  and  4  are transmitted without loss from one element to the other. 
         [0029]    The solid body actuator  2  produced as described in the preceding from two individual hollow cylindrical piezoelectric elements  3  and  4  thus has a first frontal face  7 ′ and a second frontal face  7 ″ opposite thereto. Both frontal ends  7 ′ and  7 ″ of solid body actuator  2  are also attached to sleeve elements  8  and  9 , which are permanently adhesively attached to the inner wall of the hollow cylinder of solid body actuator  2  and project beyond solid body actuator  2  to the outside. As will be explained in the subsequent description, the portions of sleeve elements  8  and  9  that protrude beyond frontal ends  7 ′ and  7 ″ of solid body actuator  2  serve to enable precise centering of solid body actuator  2  inside housing  1 . Thus, the portion of sleeve element  8  that protrudes beyond first frontal end  7 ′ of solid body actuator  2  fits snugly and in fluid-tight manner in a recess  11  created in a valve block  10 , by which the solid body actuator  2  is joined in fixed manner relative to valve block  10 . In turn, valve block  10  engages in the frontal face end of hollow cylindrical housing  1  by an extension  12  that matches the inner cross section of housing  1 , and connects the housing to valve block  10  in fluid-tight manner. 
         [0030]    The dimensions of the outer diameter of cylindrical solid body actuator  2  and the inner diameter of the hollow cylindrical housing  1  that surrounds solid body actuator  2  are matched with each other in such manner that an interspace is formed between the inner wall of housing  1  and the cylindrical outer surface of solid body actuator  2 . Interspace  13  extends over the entire length of solid body actuator  2 , which is centered inside housing  1  by the second frontal face  7 ″ thereof. To this end, the portion of sleeve member  9  that protrudes beyond second frontal face  7 ″ engages in a corresponding stepped recess  14  of an axially movably mounted actuating element  15 . Actuating element  15  is in turn centered relative to housing  1  via a centering element  16 , on the radially inwardly facing sliding surface of which, actuating element  15  is in axial sliding contact with an annular extension  15 ′. The radially outer side of centering element  16  is attached to the inner contour of housing  1  in fixed manner by a press fitting, so that the connection between centering element  16  and housing  1  is sealed in fluid-tight manner. In the same way, the sliding contact also forms a fluid-tight join between annular projection  15  of the actuating element  15  and the inner wall of centering element  16 . 
         [0031]    In order to exert a mechanical bias on solid body actuator  2 , which is essential particularly for the actuator compression after the actuator has undergone linear expansion, a spring element  17  is provided that radially partially encloses the part of actuating element  15  that projects beyond housing  1  and is located axially adjacent one side of a mounting plate  18 , which is preferably connected to the right frontal face of housing  1  against the spring force of spring element  17  via fasteners  19  that preferably are screw connectors. Spring element  17  is supported by the side of the spring opposite mounting plate  18  directly on an annular shoulder  15 ″ on actuator  15 , and thus drives the actuating element  15  by the application of the spring force axially towards the solid body actuator  2  that is mounted in centred fashion inside housing  1 . 
         [0032]    If solid body actuator  2  is activated by a corresponding charging with an electric voltage, solid body actuator  2  expands against the spring force of spring element  17  and moves actuating element  15  axially, that is, when solid body actuator expands longitudinally, actuating element  15  is deflected to the right. In the absence of electrical voltage, spring element  17  helps to hasten the reduction in length of solid body actuator  2 . Such an actuating driver is capable of carrying out highly dynamic actuator deflections, such as are used particularly when conducting tests on sample. However, such highly dynamic processes cause substantial heating of solid body actuator  2 , and the actuator needs to be cooled effectively with the aid of the illustrated construction according to the invention. 
         [0033]    In order to actively cool the solid body actuator, a first hollow conduit  20  is provided in valve block  10 , which conduit is connected in fluid-tight manner to hollow duct  6 , which passes completely through solid body actuator  2 . In the embodiment shown, two conduit feeds  20  are created in valve block  10 , and these engage in hollow duct  6  of solid body actuator  2  by means of sleeve-like element  8 . 
         [0034]    In the area of its second frontal face  7 ″, hollow duct  6  is in fluid communication with interspace  13 , which both solid body actuator  2  and housing  1  delimit radially. For this purpose, at least one radially fitted passthrough opening  15 ′″ is provided in the region of actuating element  15 , and is disposed inside a blind hole-like recess in actuating element  15 . The blind-hole-like recess formed inside actuating element  15  is connected in fluid-tight manner to hollow duct  6  via the sleeve-like element  9 . Finally, interspace  13  is connected via at least one second hollow conduit  21  to housing  1  in the area of the first frontal face  6  of solid body actuator  2 . 
         [0035]    For cooling the actuator, liquid or gas-phase coolant is passed through first hollow conduit  20  and into hollow duct  6  of solid body actuator  2 , so that in the case illustrated the coolant flows from left to right through hollow duct  6 . The coolant enters interspace  13  via connecting opening  15 ′″ and here it flows over the outer surface of solid body actuator  2  in the opposite flow direction, that is to say in the case shown from right to left, and so is able to cool said surface. Finally, the coolant flows to the outside through second hollow duct  21 . 
         [0036]    Of course, it is possible to cause the coolant to flow in the reverse flow direction through the actuating drive for cooling purposes, i.e., the coolant is introduced via second hollow conduit  21  so that the coolant first flows through interspace  13 , and is thus able to cool the outside of solid body actuator  2 . Then, the coolant passes through the at least one connection opening  15 ′″ and into hollow duct  6  of solid body actuator  2 , and finally escapes to the outside through first hollow conduit  20 . 
         [0037]    Suitable conduit pressure, with which the coolant is propelled through the corresponding hollow conduits inside the actuating drive, is provided by a feed pump, not shown, which is connected to a coolant reservoir, which is also not shown. 
         [0038]    To avoid unnecessary loss of coolant, it is advisable to cool the coolant after it exits the actuating drive, i.e., to feed it to a corresponding cooling unit, which is connected in a closed circuit with a coolant reservoir, from which the supply of coolant to the actuating drive is assured. 
         [0039]    The actuating drive as shown is mounted in a test frame P. For this purpose, the actuating drive is mounted permanently on a correspondingly constructed mounting plate  22  via valve block  10 . The protruding area of actuating element  15  opens axially into a load cell  23 , which in turn is also mounted on test frame P via a mounting plate  24 . 
         [0040]    As was noted previously, in a serial coaxial connection of two piezoelectric elements  3 ,  4  to form a unitary, solid body actuator  2  with the appearance of an elongated hollow cylinder, it is helpful and required to join the two elements  3 ,  4  to each other permanently with a cylindrical or sleeve-like connector in the area where the end faces thereof are closest to each other. In this context, the end faces of both piezoelectric elements  3 ,  4  come into direct contact, while inner wall areas of both elements close to the end faces are stuck to each other permanently by means of adhesive applied to the sleeve-like connection member. 
         [0041]    In an effort to increase the size of the solid body actuator  2  as a whole, and so to create larger travel paths and/or greater displacement forces, it is logical to combine more than two hollow cylindrically shaped single actuators, whether in the form of piezoelectric or magnetostrictive hollow cylindrical elements or actuator stacks, of which each is constructed from a plurality of individual, layered or disc-like piezoelectric or magnetostrictive components. However, this requires a connection element that ensures optimized, mutual stable support and exact centring of the axially assembled actuator elements not only relative to each other but also with regard to the housing surrounding the entire solid body actuator. 
         [0042]    A preferred embodiment for constructing such a connector  5 ′ is shown in longitudinal section in  FIG. 2   a , and in a cross sectional view through an actuating drive in  FIG. 2   b . Connecting element  5  consists substantially of a cylindrical or sleeve-like base body  51 , the outer sleeve diameter of which matches the respective inner diameter of hollow cylindrical actuator elements  3 ′ and  4 ′. In the example illustrated, it is assumed that hollow cylindrical actuator elements  3 ′ and  4 ′ have the same geometry, so that hollow cylindrical base body  51  has a uniform outer sleeve diameter. Of course, variations of the hollow cylindrical base body  51  are conceivable in which the outer diameter is adapted individually to match the inner diameters of differently dimensioned, hollow cylindrical actuator elements  3 ′ and  4 ′. 
         [0043]    For axial support of the end faces of the two hollow cylindrical actuator elements  3 ′ and  4 ′, connection element  5 ′ provides a sleeve collar  52  surrounding the hollow cylindrical base body, which collar has two support surfaces  53  and  54  aligned orthogonally with outer surface M of hollow cylindrical base body  51  and facing axially away from each other, on which the front faces of hollow cylindrical actuator elements  3 ′ and  4 ′ are supported directly. Sleeve collar  52  extends around the entire outer surface M of hollow cylindrical base body  51 , so that the end faces of actuator elements  3 ′ and  4 ′supported over the full area of each end face  53  and  54  thereof. 
         [0044]    Hollow cylindrical actuator elements  3 ′ and  4 ′ are preferably joined to connecting element  5 ′ loosely, that is to say without any adhesive. In this way, any effects of linear expansion along hollow cylindrical actuator elements  3 ′ and  4 ′ are not impeded by a fixed material connection to outer surface M of connecting element  5 ′. 
         [0045]    The dimensioning and arrangement of connecting element  5 ′ is selected between the respective axially joined hollow cylindrical actuator elements  3 ′ and  4 ′ such that there is little or no effect on coolant flow K. Sleeve collar  52  is arranged in each case at a radial distance from housing  1 , so that coolant flow Kflowing in the example of  FIG. 2   a  from the bottom up along hollow duct  6  and from the top down inside interspace  13  remains unaffected. 
         [0046]    For centering the hollow cylindrical actuator elements  3 ′ and  4 ′ inside housing  1 , centring means  25  that are preferably in the form of pin-like or peg-like spring elements, and each in the form of a spring hanger clamped on two-sides, are joined by the radially inwardly oriented end thereof to the connecting element  5 ′ and by the radially outwardly facing end thereof to housing  1 . To ensure a defined axial fixing of centring elements  25 , a mechanical stop  26  is provided on the inner side of housing  1 , and the respective radially outer ends of centring elements  25  bear on said stop. A fixing sleeve  27  is mounted on the inner wall of housing  1  and is used for axial securing and fixing of centring elements  25 . 
         [0047]    As shown in the cross-sectional view in  FIG. 2   b , centring elements  25  fix connecting sleeve  5 ′ centrically with housing  1  at four local points, and are able to deform axially when the length of hollow cylindrical actuator elements  3 ′ and  4 ′ changes, thus ensuring that centring elements  25  cannot obstruct the change in length of actuator elements  3 ′ and  4 ′. Of course, fewer or more than four centring elements  25  may be provided for the purpose of centring connecting sleeve  5 ″ inside the housing, as evenly distributed as possible around the circumference of sleeve-like connecting piece  5 ′. In order to avoid hindering the axial position change of actuator elements  3 ′ and  4 ′, it is particularly advantageous to support the radially inward-facing ends of the centring elements so as to be radially slidably disposed in connecting element  5 ′. 
         [0048]    Connecting element  5 ′ may be used to create actuating drives in which the solid body actuators are assembled from two, but particularly three or more individual, hollow cylindrical actuator elements. The connectors secure both the axial joining of two actuator elements in each case, an in particular they also help to avoid radial tilting or position shifting, especially in solid body actuators, which are assembled from many axially coupled actuator elements and so have a considerable total actuator length. 
         [0049]      FIG. 3   a  shows a further embodiment in longitudinal section and in perspective view as a single part for producing a connecting element  5 ″ which, similarly to the embodiment of  FIGS. 2   a  and  2   b , has a hollow cylindrical base body  51 ′ with an outer diameter that matches the inner diameters of the otherwise identically shaped hollow cylindrical actuator elements  3 ″ and  4 ″. The hollow cylindrical connecting element  5 ″ also has a sleeve collar  52 ′ that completely surrounds hollow cylindrical base body  51 ′ of connecting element  5 ″ in the form of a ring, and two supporting surfaces  53 ′ and  54 ′, on which the end faces of the two actuator elements  3 ″ and  4 ″ are supported. 
         [0050]    In addition, sleeve collar  52 ′ comprises a plurality of radially oriented through ducts  28 , through which coolant may flow from inner hollow duct  6  into interspace  13  or in the other direction. Housing  1  is shown only in outline. 
         [0051]    The plurality of through ducts  28  is positioned in azimuthal uniform distribution inside sleeve collar  52 ′, so that a flow and/or exchange of coolant between hollow duct  6  and interspace  13  may be effected as symmetrically as possible through connecting element  5 ′. 
         [0052]    In a variation of the supply and removal of coolant into and out of hollow duct  6 , as described in connection with the embodiment shown in  FIG. 1 , it is helpful in application of connecting element  5 ′ illustrated in  FIG. 3   a  to provide the coolant supply through the radially aligned through ducts  28  inside connecting element  5 ′. In this way, a division of the direction of flow into two axially opposite flow directions inside hollow duct  6  emerges, as is shown by the flow arrows in  FIG. 3   a . With regard to coolant supply along through ducts  28 , it is expedient to provide connecting ducts  28 ′, which for example extend through housing  1  and ensure an external coolant supply, and are in direct or indirect fluid communication with the through ducts. In this way, the plurality of connecting ducts  28 ′ connected with through ducts  28  ensures that connecting element  5 ″ is securely centered inside housing  1 . 
         [0053]    To ensure the axial flow of coolant to radially adjacent interspace  13  of the solid body actuator constructed through the coaxial combination of two or more hollow cylindrical actuator elements  3 ″ and  4 ″, it is important to provide flow diversions at the axially opposite (not illustrated in  FIG. 3   a ) ends of solid body actuator  2 ′, through which the respective coolant streams K flow axially inside hollow duct  6  and into the radially outward interspace  13 . Diversion of the flow at the two axially opposite ends of solid body actuator  2 ′ causes two coolant flows that converge in the region of connecting element  5 ′, as is indicated by the flow arrows in  FIG. 3   a . In order to transport the converging subflows K away, it is important to provide at least one outgoing hollow conduit  21  in housing  1 , via which the warmed coolant K may be forwarded to a corresponding cooling facility. 
         [0054]      FIG. 3   b  shows another variant for forming a connecting element  5 ″. In this case, it is assumed that lower actuating element  4 ′″ is constructed as a solid cylinder with the upper end face abutting directly with a punch shaped connecting element  5 ′″. The upper actuating element  3 ′″ is a hollow cylinder and has a hollow cylindrical interior, into which a hollow cylindrical shaped section  55  of the punch shaped connecting element  5 ′″ fits precisely. The hollow cavity surrounded by hollow cylindrical shaped section  55  is in fluid communication with a plurality of radially oriented through ducts  28 ′ which open radially into interspace  13 . One possible coolant flow scenario is identified by the coolant flow arrows K in  FIG. 3   b , wherein coolant is introduced along hollow duct  6 , so that the coolant flow after passing through the radially oriented through ducts  28  flows bidirectionally both axially upwardly and downwardly through interspace  13 . In this case, it is important to provide corresponding dissipating hollow ducts at the respective axially opposite ends of solid body actuator  2  for dissipating the heated coolant. 
         [0055]      FIG. 3   c  shows an embodiment of a connecting element  5 ″″, with which it is possible to combine at least three hollow cylindrical actuating elements with each other. For example, two hollow cylindrical actuating elements  3 A and  3 B are arranged coaxially with one inside the other and actuating elements  3   a  and  3   b  being dimensioned so that outer hollow cylindrical actuating element  3 A surrounds inner hollow cylindrical actuating element  3 B with a radial clearance gap  29 . Both actuating elements  3 A and  3 B are able to increase the overall force to be generated due to their coaxial parallel arrangement. Both hollow cylindrical actuating elements  3 A and  3 B are supported on a shared support surface  53 ′″ of connecting element  5 ″″. A hollow cylindrical actuating element  4 ″″ abuts lower support surface  54 ″″ with its end face. Just as in  FIGS. 2   a  and b and  3   a , connecting element  5 ″″ has a hollow cylindrical base body  51  with the outer diameter matching the inner diameter of hollow cylindrical actuating elements  3 B and  4 ″″. 
         [0056]    For cooling actuating elements  3 A and  3 B and  4 ″″, through ducts are created inside sleeve collar  52 ″″. At least one first through duct  28 A provides a fluidic connection between the outer interspace  13  and the radial clearance gap  29 . At least one second through duct  28   b  ensures an exchange of coolant between inner hollow duct  6  and interspace  13 . 
         [0057]    The flow arrows in  FIG. 3   c  reflect the flow of coolant K that establishes itself inside hollow duct  6 , interspace  13  and radial clearance gap  29 . 
         [0058]      FIG. 3   d  shows a further embodiment of the axial combination of two hollow cylindrical actuating elements  3 ″″ and  4 ″″, in which the two actuating elements differ significantly with regard to the thickness of their hollow cylinder wall. In the case as shown, the wall is thicker in upper hollow cylindrical actuating element  3 ′″″ than in lower actuating element  4 ′″″. In this case, connecting element  5 ′″″ between the two actuating elements has no hollow cylindrical base body. Instead, connecting element  5 ′″″ is a connecting plug having an outer contour conforming precisely and in a fluid-tight manner with the inside diameters of the respective hollow cylindrical actuating elements  3 ′″″ and  4 ′″″. Connecting element  5 ′″″ has through ducts  28 C and  28 D, which are dimensioned and arranged such that first through duct  28 C provides a fluid connection between hollow duct  6 B, which is delimited by lower actuating element  4 ′″″, with which interspace  13  is created, whereas second through duct  28 D connects hollow duct section  6 A, which is bounded by upper hollow cylindrical actuator element  3 ′″″, with the radially outward interspace  13 . Both through ducts  28 B and  28 C open into a uniform axial Exit plane A, which passes through sleeve collar  52 ′″″. 
         [0059]    All connecting elements illustrated in  FIGS. 1 to 3  ensure the reliable axial combination of each of two hollow cylindrical actuating elements, or as in  FIG. 3   b , a solid cylindrical actuating element with a hollow cylindrical actuator element. Of course, all of the connecting elements as shown may be used for the axial combination of any number of individual hollow cylindrical actuating elements with each other to create a large, elongated solid body actuator in any combination. 
         [0060]      FIG. 4  illustrates one of many possible ways to produce an actuating drive having more than two coaxially combined, individual hollow cylindrical actuating elements. The actuating drive  1 , which is illustrated extremely diagrammatically in  FIG. 4 , comprises within housing  1  four hollow cylindrical single actuators  30 ,  31 ,  32  and  33  axially combined in series. The total serial arrangement of the actuator forms the solid body actuator with the lower end being supported on a mechanically fixed counter bearing  34  on one side of housing  1 , and the upper end of which comprises actuating element  15 , which extends through housing  1 .  FIG. 4  shows in particular that several different connecting elements  5 ″,  5 ′ and  5 ″ may be used to construct a solid body actuator  2 . Thus, hollow cylindrical actuating elements  30  and  31  as well as  32  and  33  are joined axially by connecting element  5 ″, as is shown in  FIG. 3   a . The connecting element  5 ″ in question are furnished with radially oriented through ducts  28 ′, which in the illustrated embodiment of  FIG. 4  are connected in fluid-tight manner with housing wall  1  and in this way allow the radial inflow or outflow of a coolant K from the outside to the interior of solid body actuator  2 , or in the other direction. Connecting ducts  28 ′ of connecting element  5 ″, which connect hollow cylindrical actuating elements  32  and  33  coaxially with each other, serve in the present example as outflow ducts, through each of which the coolant can drain from inner duct  6  to the outside, where it is forwarded to a cooling device (not shown), from which the coolant K may be returned again through passthrough ducts  28 ′ of the connecting element  5 ″ that connects actuating elements  30  and  31  for feeding into the solid body actuator in the circuit. 
         [0061]    In order not to restrict the axial expansion of the solid body, it must be ensured that passthrough ducts  28 ′ are designed as flexibly as possible, so that solid body actuator  2  is able to expand in the longitudinal direction thereof without obstruction. 
         [0062]    For the coaxial connection of hollow cylindrical actuating elements  31  and  32 , the connecting element  5 ′ shown in  FIGS. 2   a  and b is used with the radially orientated centering elements  25  enabling centering solid body actuator  2  centrally and coaxially inside housing  1 . 
         [0063]    Of course, any other combinations of the connecting elements described in the preceding, and particularly with reference to the connecting elements pictured in  FIGS. 3   a  to d may be used in the configuration of an actuating drive constructed in accordance with the solution having a solid body actuator with substantial actuator lengths. 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           1  Housing 
           2  Solid body actuator 
           3 ,  4  Piezoelectric element 
           3 ′,  3 ″,  3 ′″,  3 ″″, and  3 ′″″ Hollow cylindrical actuating element 
           4 ′,  4 ″,  4 ′″,  4 ″″, and  4 ′″″ 
           4 ′″ Solid cylindrical actuating element 
           5  Sleeve element 
           51  Sleeve body 
           52  Sleeve collar 
           53  Support surface 
           54  Support surface 
           55  Hollow cylindrical section 
           6  Hollow duct 
           6 A and  6 B Hollow duct sections 
           7 ′ and  7 ″ First and second frontal face of the solid body actuator 
           8  Sleeve element 
           9  Sleeve element 
           10  Valve block 
           11  Recess 
           12  Extension 
           13  Interspace 
           14  Stepped recess 
           15  Actuating element 
           15 ′ Annular extension 
           15 ″ Annular stop surface  15 ″′ Connection opening 
           16  Centering element 
           17  Spring element 
           18  Mounting plate 
           19  Fasteners 
           20  First hollow conduit 
           21  Second hollow conduit 
           22  Mounting plate 
           23  Load cell 
           24  Mounting plate 
           25  Centring element 
           26  Mechanical stop 
           27  Fixing sleeve 
           28 , 28 A, 28 B, 28 C and  28 D Through duct 
           28 ′ Connecting duct 
           29  Radial clearance gap 
           30 - 33  Hollow cylindrical actuator 
           34  Mechanical stop 
         A Exit plane 
         P Test frame