Abstract:
Actuator device having an expansion unit, which includes a magnetic shape memory alloy material, and a spring unit which interacts therewith in a restoring manner, wherein at least one spring of the spring unit is assigned to the expansion unit, which is designed to perform an expansion movement along an expansion direction, in such a way that the spring can exert a restoring spring force counter to the expansion direction on the expansion unit, and wherein the spring is set up and/or predetermined in its spring characteristic curve properties in such a way that a spring force profile of the spring unit along a stroke range, determined by an expansion force profile of the expansion unit and a restoring spring force profile, of the expansion movement does not form a continuously rising curve, and/or the spring force profile, with respect to a continuously rising curve, extends and/or increases the stroke range.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an actuator device. Furthermore, the present invention relates to a process for producing an actuator device. 
     It is known from the prior art to use magnetic shape memory (MSM) alloy materials for actuator engineering. For this purpose, as shown schematically in  FIG. 19  relating to the supposed prior art, an MSM crystal body (as a representative of a multiplicity of MSM-based materials, for instance foams, polycrystals, composites, etc. and bodies to be manufactured therefrom), typically realized on the basis of an NiMnGa alloy, is exposed to a magnetic field. In the schematic illustration in  FIG. 19 , in this respect the MSM crystal body  10  shown in elongated form is suitably held between a pair of solenoids  12 , which, as illustrated by the line illustration  14 , generate a horizontally running field and act on the body  10  therewith. As a reaction to the exposure to a magnetic field, the crystal body  10  performs an expansion movement in the arrow direction  16 , and, in a specific application, may interact with an appropriately coupled actuating partner. 
     Therefore, magnetic shape memory alloy materials of this type and actuator devices realized therewith offer an interesting possibility for replacing or for complementing common actuator principles (such as electromagnetic actuators); in addition to basic simplicity of mechanical design in realizing such devices (no armature moves as a whole, merely an expansion of a body takes place), an advantage of the magnetic shape memory alloy principle used is primarily a potentially quick reaction time of the expansion to the application of a magnetic field of the required strength; in addition, depending on the configuration, actuating forces which are already sufficient for the current state of the art technology for many application purposes can be generated. 
     Nevertheless, such actuator devices, which are assumed to be known in principle, also entail disadvantages (owing to the principles and construction involved) which to date have restricted a truly universal applicability of such actuators. Thus, for example, a utilizable stroke of the expansion movement (i.e. a degree of elongation of an elongation movement performed by the actuator crystal) is typically limited to approximately 3 to 6% of a corresponding axial extent of the crystal, such that particularly large-stroke movements can only be realized with difficulty by means of shape memory alloy actuators, and therefore there is a need to extend this expansion stroke or to optimize it as far as possible. 
     In addition, typical shape memory alloy materials have the property that the intended expansion movement takes place as a reaction to a magnetic field (of an appropriate minimum field strength), but after a decline in the magnetic field below this threshold, compression does not then automatically take place back into the original compressed state of the expansion unit realized by means of the shape memory alloy crystal. Instead, the crystal remains in the expanded position even in the event of a decline to below an expansion threshold or in the event of complete deactivation. It has therefore been discussed in the case of devices known from the prior art to realize the restoring (i.e. the resetting of the expansion to the non-expanded starting position) with restoring means which either themselves have a shape memory alloy actuating element (expanding in a correspondingly opposite direction), or alternatively such a restoring device exerting a restoring force in the restoring direction and therefore counter to the expansion direction. If the spring force of such a restoring spring is set, with respect to an (unstressed) expansion force of the shape memory alloy crystal, in such a way that the expansion force exceeds the spring force when the shape memory alloy material is subjected to a magnetic field, the intended expansion movement takes place. In the event that the magnetic field declines, and the expansion force accordingly reduces, the spring force is then above the expansion force, however, and accordingly resets the crystal into the contracted starting position thereof. 
     This mechanism of action is illustrated on the basis of  FIG. 20  relating to the supposed prior art. A shape memory alloy expansion crystal, for instance of a type shown in accordance with  FIG. 19 , shows, in the force-travel(-stroke) graph shown in  FIG. 20 , a stroke profile (expansion force profile) as is illustrated in the top curve  20 . It can be seen that, with an approximately constant negative pitch, this expansion force profile extends up to approximately 0.9 mm stroke, and then declines steeply. The lower characteristic curve, as a restoring characteristic curve  22 , shows the input of force which is required for restoration (resetting) counter to the stroke travel in a state of the crystal in which it is not subjected to a magnetic field. 
     The shape memory alloy crystal actuator shown in  FIG. 20  with reference to the characteristic curves  20 ,  22  is combined with a restoring spring, which, realized in the form of a typical helical spring, and in a manner which is not shown in the figures, interacts at the face with the crystal  10  and, counter to the arrow direction  16  ( FIG. 19 ), exerts a restoring force on the crystal which is set by the restoring characteristic curve  24 , in accordance with a Hooke&#39;s straight line  24  which is to be assumed to be potentially idealized. 
     From the points of intersection between said spring characteristic curve (a typical pitch to be applied in the present stroke range is approximately 5 N/mm) and the stroke characteristic curve  20  or the restoring characteristic curve  22  leads to the effective movement or stroke range, which is limited between a lower stroke limit  26  and an upper stroke limit  28 , of the shape memory alloy actuator shown by way of example. Within this range, sufficiently subjecting the crystal to a magnetic field firstly ensures the provision of an actuating force which exceeds the spring force and is therefore sufficient for driving an actuating partner, and at the same time the restoring force of the spring makes it possible, after deactivation of or a decline in the magnetic field, to reset (compress) the expansion body into the starting position thereof within the stroke range slightly above 0. The maximum stroke which thus arises is therefore approximately 0.8 mm in the example. 
     The graph of  FIG. 20  additionally shows, by means of the shaded area  30 , the effective expansion work which results from the interaction of the expansion unit (when subjected to a magnetic field) and a restoring force (acting counter to the latter), this being described as the integral of the difference in force of both partners over the effective expansion stroke (i.e. the region between the portions  26  and  28 ). It not only becomes apparent that this difference in force used for an actuation behavior decreases continuously in the extended (right-hand-side) expansion range, and in this respect provides an actuating partner with increasingly less drive force in the direction towards the expansion direction, but in addition for instance a comparison of the effective expansion work (area  30 ) relative for instance to the hysteresis difference between the stroke characteristic curve  20  and the restoring characteristic curve  22  in the expansion range (and beyond) shows that only a fraction of the expansion work made possible by the shape memory alloy body can be used, as a result of the interaction with the restoring spring. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention, therefore, to improve an actuator device of the generic type having an expansion unit, which comprises a magnetic shape memory alloy material, and a spring unit which interacts therewith in a restoring manner, in such a way that firstly an effective expansion stroke (i.e. a stroke or expansion distance and also a difference in length of the expansion unit in the compressed state and in the expanded state compared with a spring characteristic curve providing a linear-elastic force profile in interaction with the expansion crystal) is increased, and in addition it is possible to increase an effective expansion work, defined as the area between the expansion force profile of the expansion unit and the spring force profile in the effective expansion stroke range, as compared to interaction of the expansion unit with a spring which provides the linearly rising force profile. 
     The object is achieved by the actuator device having the features disclosed herein, and furthermore by the process for producing an actuator device disclosed herein. Advantageous developments of the invention are also described herein. 
     In this case, the “expansion unit” according to the invention means any embodiment of such a shape memory alloy material realized by means of an MSM material, where although an MSM crystal body is a favorable and conventional form of realization, the present invention is not limited thereto. Thus, the invention could be realized by means of any other desired MSM-based materials, for instance composites, foams and polycrystals, if these can be used for actuators in the manner according to the invention. 
     In a manner which is advantageous according to the invention, the spring unit is set, adapted and selected in a targeted manner by means of at least one spring of said spring unit, both in the actuator device according to the invention and in the process according to the invention for producing the actuator device, in such a way that the spring force profile of the spring unit no longer runs in a manner rising continuously linearly in the stroke range of the expansion unit (i.e. between the compressed state and the expanded state, upon loading by the spring unit), but instead experiences another curve profile, specifically in particular runs horizontally at least in certain portions in the stroke range (i.e. has a pitch 0), or even has a negative pitch, to the extent that it then follows an expansion force profile of the expansion unit which declines with an increasing stroke, and in a particular embodiment follows this profile in a parallel or approximately parallel manner. Spring force profiles which are provided according to the invention and are preferred according to the invention by virtue of a particular configuration or adaptation and preselection of the spring or the spring characteristic curve properties thereof are a degressive curve, i.e. a curve with a pitch (declining) which increases with a rising stroke, and in addition or alternatively a curve which has an extreme value in the stroke range, typically a maximum, after which (with a growing stroke) the spring force declines. 
     These variants of the invention are shown in the graph in  FIG. 1 , which, analogously to the illustration in  FIG. 20 , as a force-travel characteristic curve shows in turn the stroke characteristic curve  20  or the restoring characteristic curve  22  of a shape memory alloy actuator crystal;  FIG. 1  also shows the Hooke&#39;s linear-elastic spring force characteristic curve  24 , which, for instance represented by a helical spring sitting on the crystal on the engagement side, can be described by the linearly rising spring force profile over the stroke, and is applicable as a scale or comparison for the improvements according to the invention: thus, for instance, a constant spring force profile (i.e. pitch=0) symbolized by the horizontal characteristic curve  31  shows how not only, in particular in the right-hand (expanded) stroke range, the point of intersection with the expansion characteristic curve  20  (expansion force profile) can be shifted to the right to extend the effective (utilizable) expansion stroke (the area  30  representing the utilizable expansion work would also be increased by the area growth made possible thereby). The degressively running spring characteristic curve  32 , here with a relatively large pitch in the region on the left-hand side, similarly shows a possible way of increasing the effective movement stroke of the overall arrangement (with a further increased stroke work), such as a fundamentally possible curve profile  34  which runs more progressively and therefore non-linearly in certain portions. Therefore, although in general a positive characteristic curve ( 34 ) would not lead to an increase in stroke compared to the linear characteristic curve, it would make a relatively large resulting net force possible. 
     A common feature of all these principles is that, relative to a constant-linear spring force curve, they make it possible, advantageously according to the invention, in particular in the region on the right-hand side (expanded region) of the force-travel characteristic curve, to increase the utilizable movement stroke and/or to increase the area between the expansion force profile and the spring force profile in the effective stroke range (and therefore the stroke work). 
     For realizing the invention in a preferred manner, numerous procedures lend themselves, where in particular flat springs or springs in the form of leaf springs, disk springs or meandering springs, for instance, have proven to be particularly suitable partners for the expansion unit, for interaction therewith in a manner which is advantageous according to the invention, to affording a non-linear, further preferably declining spring force characteristic curve and in this respect to increasing the stroke range according to the invention. 
     In this case, it is possible both to realize the desired characteristic curve properties and/or non-linearities of the (at least one) spring by suitable material selection, where for instance rubber materials or plastics materials have proved to be expedient as preferred means for realizing a non-linear characteristic curve profile in a manner according to the invention, and nevertheless it is also encompassed by the invention, according to a further development, to combine a plurality of (individual) springs with one another, where here in particular it is possible to provide these individual springs with in each case different spring characteristic curve properties and then to set or to control the desired behavior of the (overall) spring unit by combination (for instance then addition of the respective characteristic curves). 
     This applies in particular to those springs or spring combinations in which, according to a further development, an extreme value (i.e. for example a force maximum) is present in the spring force profile within the stroke range. It is therefore possible, for example in the manner of a so-called clicker spring, for such an extreme value (alternatively also a characteristic curve turning point and/or a position of a strong rise or fall) to be set or controlled in accordance with a respective intended actuating task. 
     Therefore, provision is also made according to a further development to configure a spring (also within a spring combination) at least in certain portions in such a way that it exerts a spring force directed counter to the restoring direction (typically within a part or portion of the stroke range). In this respect, it would then also be possible for this action to be utilized in order to promote, for instance, the expansion of the expansion crystal in a targeted manner and/or to neutralize undesirably strong restoring forces of a neighboring, interacting spring in this region. 
     These effects of a targeted, potentially region-wise or pointwise non-linearity and/or of an extreme profile within the spring characteristic curve become particularly interesting when two shape memory alloy bodies, typically counter to one another and directed towards one another, interact with one another counter to such spring(s): it is thus possible, for instance, to set multistable positions (i.e. positions which are stable along the stroke range at any desired position), the arrangement of the expansion unit being stable in each position (with the magnetic field deactivated). 
     As well as the actuator device claimed according to the invention, the process claimed according to the invention for producing an actuator device solves the problem addressed by the invention, i.e. that of predetermining and potentially increasing the (effective) expansion stroke or increasing the achievable expansion work of the actuator device. 
     As a result, the present invention makes it possible to noticeably improve the range of application and therefore the usability of known, generic actuator devices comprising a shape memory alloy material, where various possible ways of influencing or setting the spring characteristic curve of the interacting restoring spring make it possible to adapt to a multiplicity of applications of actuating partners. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages, features and details of the invention become apparent from the following description of preferred exemplary embodiments and also with reference to the figures; these show in  FIG. 1  is a schematic graph in the form of a force-travel characteristic curve for elucidating the principle of function of the present invention; 
         FIG. 2  and  FIG. 3  are possible schematic configurations of the assignment of the spring according to the invention to an MSM crystal body as an expansion unit in the exemplary embodiments shown; 
         FIG. 4  and  FIG. 5  illustrate the schematic use of form springs for achieving a spring characteristic curve profile which is approximately horizontal in the stroke range; 
         FIG. 6  shows an alternative spring characteristic curve profile, which forms an extreme value and, in the exemplary embodiment which is shown of a typical clicker spring action, combines a degressive characteristic curve profile with a subsequent progressive profile; 
         FIG. 7  to  FIG. 10  show various configurations of flat springs and the combinations thereof; 
         FIG. 11  shows three views for elucidating a resulting spring force characteristic curve, composed of a linear characteristic curve profile and also a characteristic curve profile which declines degressively in certain portions; 
         FIG. 12  is a schematic illustration of the interaction of an actuator crystal with a flat spring in various positions and, associated therewith, inversed directions of an input of spring force onto the MSM crystal body; 
         FIG. 13  show two spring characteristic curve illustrations for a spring unit formed from coupled springs; 
         FIG. 14  shows two spring characteristic curve illustrations for a further exemplary embodiment of coupled springs; 
         FIG. 15  and  FIG. 16  show variants for further influencing the characteristic curves by the provision of various, varied spring types; 
         FIG. 17  is a schematic illustration of a further embodiment with two crystal bodies, which are directed towards one another along a common expansion axis and in the expansion directions thereof, with a spring unit arranged lying therebetween; 
         FIG. 18  shows illustrations for elucidating the force-movement behavior of the arrangement shown in  FIG. 17  as a multistable push-push arrangement with individual illustrations of the individual actuators and the superpositions thereof, and 
         FIG. 19  and  FIG. 20  show illustrations for elucidating the prior art consulted as being of the generic type. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 2 and 3  show possible mechanical design variants in order to allow the spring unit  40  ( FIG. 2 ) as a tension spring or a flat spring unit  42  ( FIG. 3 ) as a compression spring to interact with the MSM crystal body  10 . 
     It becomes apparent that (beyond the fundamental advantages relating to the spring characteristic curves discussed above), for instance compared to a helical spring, an arrangement which optimizes installation space is made possible, either in terms of a minimized height or length ( FIG. 2 ) or a minimized width ( FIG. 3 ). Analogous to the illustration in  FIG. 19 , the arrangements in  FIG. 2  and  FIG. 3  would be subjected to magnetic fields running horizontally in the plane of the illustration. 
       FIG. 6  shows how an optimized spring characteristic curve profile can be used within an effective (predetermined) stroke range x to solve the problem addressed by the invention. The graph in  FIG. 6  shows, with the characteristic curve  46 , a typical profile of a so-called clicker diaphragm spring, which within the stroke range (x) initially behaves degressively and then in the last part of the stroke behaves progressively. It can be seen that, (cf. in this respect  FIG. 1 ) with respect to a linear-rising Hooke&#39;s spring characteristic curve profile, not only the effective stroke is effectively extended, but in addition the characteristic curve  46  follows the upper expansion force profile of the MSM crystal in a virtually parallel manner, to this extent generating a force which is largely constant over the stroke range, with an in turn increased stroke work. Such a spring could be achieved typically by the configuration of the disk spring shown in  FIG. 9 , in which the individual free spring portions are appropriately mounted and prestressed for producing the turn-over, snap or return effect shown in  FIG. 6 . 
     Alternatives of such a flat spring configuration are shown by the suitably meandering or flat-helical spring variants in  FIGS. 7 and 8 , and a possible combination (by axial stratification) is shown in  FIG. 10 . 
     With reference to  FIG. 11 , and the individual illustrations (a) to (c) therein, it is shown that a spring combination, specifically a spring force addition of a conventional-linear spring ( 11   a ) with the profile  48  in connection with a non-linear, typically returning profile form  50  ( 11   b ) described above in the addition, yields the curve profile  52 . Recorded in the expansion situation in  FIG. 11 c   , this resulting spring force profile  52  in turn shows considerable improvements compared to the continuous-linear comparative profile in  FIG. 11 a   , both in view of an extended effective expansion stroke and also concerning the achievable expansion work. 
       FIG. 12  shows, in schematic illustrations (a) to (c), the ways in which an elongated shape memory alloy crystal body can interact, as an actuator, with a flat spring shown in idealized form; thus, for instance, in the illustration in  FIG. 12 a    a spring bearing against the crystal body  10  on the end face would initially exert a counterforce directed to the left on the crystal body expanding to the right, and move via a middle position (b) right up to a spring position in  FIG. 12 c   , in which the counterforce in turn reaches the compressive force of the crystal body in this expansion position. This behavior is advantageous for exploiting the relatively large development of force of the crystal body in the case of small expansions, for temporarily storing the work performed thereby in part as spring energy and, for relatively large expansions where there is possibly a lack of force of the crystal body, in turn for releasing it and feeding it to an external actuating task. 
     The exemplary embodiments in  FIGS. 13 to 16 , too, show how spring force characteristic curves which result according to a development within the scope of the invention can be set and provided in a targeted manner by superposition or addition of individual profiles. Thus, for instance,  FIG. 13 a    shows, with a spring provided with tension (force profile  54 ) and a spring provided with pressure (force profile  52 ) each with a linear characteristic curve, that a resulting spring force characteristic curve  56  having a preferred spring profile within the scope of the invention can be achieved from such a combination of two individual springs within the scope of the spring unit according to the invention. The tensile spring here loses force with an increasing expansion of the assigned shape memory alloy crystal body and thus with compression of the tensile spring, whereas the compressive spring increases in force at the same time. 
     In this case, it is possible, and not detrimental to the action achieved, that for instance the spring which provides the characteristic curve  52  even exerts a negative force, i.e. a tensile force in the expansion direction, in the left-hand (slightly expanded) stroke range; in the resulting characteristic curve  56 , this effect has a balancing action in this respect. 
     In general terms, the present invention makes it possible to influence the force profile or the force development of the expansion unit (which is subjected to loading by the spring unit); thus, for instance, the comparison of  FIG. 14 a    (insofar as it corresponds to  FIG. 6 ) with  FIG. 14 b   , in which in the graph the profiles of stroke force  20  or resetting force  22  have been revised about the spring force profile  58  by differential generation, shows how the resulting force profiles  20 ′ and  22 ′ are approximately constant virtually over the entire stroke range, to the extent that they can provide a downstream actuating partner with a constant actuating force in expansion operation over the stroke range. 
     A variant in this respect is shown in  FIG. 15  and  FIG. 16 : here, diaphragm springs were taken as a basis, with which in turn a force excess achieved at the start of the stroke is utilized for conversion into spring energy, which, at the end of the characteristic curve in the direction of the expansion position, can increase the force available. 
     This can also be realized, for example, by a combination of two helical or meandering springs, which work counter to one another in the respective direction of action thereof. 
     An embodiment of the invention which is suitable for realizing a multistable push-push arrangement is shown in  FIGS. 17 and 18 : here, two actuators ( 10 ,  10   a ) are directed counter to one another and act along a common expansion axis counter to springs provided in the middle region,  FIG. 18  showing a stroke work and respective force profiles. It is of interest that it is possible to achieve a plurality of positions which vary along the expansion direction and are stable owing to the snapping and folding behavior of the springs used. 
     In principle, in the case of such a system with two actuators directed towards one another as shown in  FIG. 17  (without the provision of springs), one of these MSM actuators serves as a resetter, in the manner of a resetting spring, for the respective other MSM actuator. 
     If springs are then integrated in such a system, for instance springs for bringing about an above-described clicker effect (see the preceding description), the individual and combined curve profiles shown in  FIG. 18  arise: the “force excess” with small elongations and the “force deficiency” with relatively large elongations of a pushing, expanding element can be compensated for in that the energy at first goes proportionally into the overriding of the assigned spring (for instance typically as far as the middle position), and this spring from there snaps over, i.e. it releases the energy again in the case of relatively large strokes and supports the expanding actuator. Thus, the force-travel behavior of the MSM element and of the assigned spring can be added in the consideration of the characteristic curves. If a second spring is then used for the second, opposing MSM element, this principle applies separately for initially both partners. The added characteristic curves acting counter to one another can then, depending on the spring characteristic curve(s), in turn ensure a greater net stroke and/or a greater net work, as the following illustrations in  FIG. 18  show in collaboration. 
     Thus, for instance, (a) in  FIG. 18  describes the stroke and also the resetting characteristic curve for the first, left-hand actuator A in  FIG. 17 , including the force contributions of the two springs (action force, solid line and resetting force, dashed line), just as graph (b) shows analogously said stroke and also resetting characteristic curve for the right-hand actuator B ( FIG. 17 ) in isolation. Graph c) superposes said characteristic curves over the common movement stroke in the horizontal direction. 
     Graphs (d) and (e) for the actuator A describe the movement of said actuator in cooperation with partner B in such a way that the work which has arisen between the from the stroke force A and the (coupled) resetting force of the partner B as the shaded region, the arrow direction in (e) describing the expansion of actuator A. Contrarily and analogously, graphs (f) and (g) show action in the opposite direction, specifically stroke by actuator B with compression of actuator A and work described accordingly between these curves (see arrow direction in (g)). Graph (h) then superposes these respective work areas. 
     In practical operation of an embodiment according to  FIG. 17 , this firstly means that movement of a first actuator in a positive direction, for instance elongation, brings about compression of the respective other partner, and vice versa, assisted in each case by the assigned springs. Along the common movement axis, this principle is therefore effective right up to that position at which a generated force of the pushing, expanding element falls below the required resetting or compression force of the correspondingly compressed partner.