Linear positioning element with variable setting of rigidity

Abstract

The application relates to a linear positioning element (1, 1') with variable rigidity, comprising: - a first and a second base element (2, 3; 2', 3') which are connected in a manner translationally movable relative to one another in relation to a longitudinal direction (L); - a volume-incompressible, elastic deformation element (11, 11') which is arranged in a lifting cylinder (12, 12') which is coupled to the base elements (2, 3; 2', 3'); - and adjustment device (17, 17', 18, 18') for the controlled variable releasing of a deformation space (VR) in the deformation element (11, 11') in the interior of the lifting cylinder (12, 12'); - a first actuator (8, 8') which has a first thermoelastic actuator element and which is mechanically coupled to the first and the second base element (2, 3; 2', 3') and is designed to, in particular independently of the adjustment by the adjustment device (17, 17', 18, 18'), exert in a controlled manner an adjusting force directed opposite to an elastic deformation of the deformation element (11, 11 ') such that the first and the second base element (2, 3; 2', 3') are moved relative to one another in an adjustment direction.

Description

[0001] Linear actuator with variable stiffness setting

[0002] Technical area

[0003] The invention relates to linear actuators for adjustment in a longitudinal direction with the possibility of variable adjustment of stiffness.

[0004] Technical background

[0005] Linear actuators for translational adjustment are well known in the art. A basic principle is to variably move a control element against a return spring using a controlled actuator that exerts a variable actuating force.

[0006] Furthermore, damping elements with a certain predetermined elasticity can be provided to absorb compressive loads. This allows mechanical vibrations, particularly in the range of a resonant frequency, to be effectively damped, particularly when subjected to vibration. If the resonant frequency shifts and a variation in the resonance amplitude is required, it may be useful to provide a variably adjustable damping element, i.e., a damping element with adjustable stiffness. Such a variably adjustable damping element can be implemented, for example, in the form of a stiffness actuator with adjustable stiffness.

[0007] The document DE 11 2016 007406 A1 discloses a variable stiffness actuator comprising a shape memory element. The shape memory element assumes a low stiffness state in a first phase and a high stiffness state in a second phase, in which the shape memory element has a higher stiffness than in the low stiffness state. The variable stiffness actuator further comprises an excitation element configured to effect a phase transition of the shape memory element between the first phase and the second phase, and a connecting element that elastically connects the shape memory element and the excitation element. The shape memory element, the excitation element, and the connecting element are conductive, and the shape memory element, the excitation element, and the connecting element are electrically connected to one another.

[0008] It is an object of the present invention to provide a variably adjustable linear actuator which has an adjustable stiffness or elasticity with respect to a pressure load for various linear adjustments.

[0009] Disclosure of the invention

[0010] This object is achieved by the linear actuator with variable stiffness according to claim 1 and by the method for operating a linear actuator with variable stiffness according to the independent claim.

[0011] Further embodiments are specified in the dependent claims.

[0012] According to a first aspect, a linear actuator with variable stiffness is provided, comprising: a first and second base element, which are connected so as to be translationally movable relative to one another in a longitudinal direction; a volume-incompressible, elastic deformation element, which is arranged in an interior of a lifting cylinder coupled to the base elements; an adjusting device for the controlled, variable release of a deformation space in the deformation element in the interior of the lifting cylinder; a first thermoelastic actuating actuator, which is mechanically coupled to the first and second base elements and is designed to effect, in particular independently of the adjustment by the adjusting device, a controlled adjusting force counteracting an elastic deformation of the deformation element, so that the first and second base elements are moved relative to one another.The above linear actuator with variable stiffness can be adjusted using a first thermoelastic actuator to achieve linear adjustment in the longitudinal direction. This means that the first thermoelastic actuator causes an actuation in the longitudinal direction, so that the first and second base elements are displaced relative to each other in the longitudinal direction. The adjustment occurs against an elastic spring force of the elastic deformation element, so that when the latter has been elastically deformed, a resulting restoring force counteracts the actuating force of the first actuator.

[0013] It can be provided that the elastic deformation element is formed from an elastic material, in particular an elastomer, wherein the elastic deformation element is formed from a homogeneous elastic material or has several layers of elastic material with at least two different elasticities, arranged in particular in the adjustment direction.

[0014] The elastic deformation element is arranged in a closed cylinder volume of the lifting cylinder between the first and second base elements. The lifting cylinder is designed as a predominantly passive, "hydraulic" cylinder. The cylinder volume is variable and changes with a relative adjustment of the positions of the first and second base elements, i.e., the cylinder chamber and the piston of the lifting cylinder can be firmly connected to each of the base elements.

[0015] In a first end position of the adjustment device, the cylinder volume has no deformation space, so that the elastic deformation element is initially completely limited in its extension and prevented from deformation. By adjusting or moving the adjustment device with respect to the elastic deformation element and the lifting cylinder towards a second end position, increasing deformation space can be released within the cylinder volume, so that the elastic deformation element can deform elastically and, in doing so, completely or partially fill the deformation space. The adjustment or moving of the adjustment device can take place independently of a setting position or a movement or adjustment by the first actuating actuator.

[0016] During the deformation of the elastic deformation element due to the linear adjustment of the base elements relative to each other by the first actuating actuator, a restoring force acting due to the deformation of the elastic deformation element acts against the actuating force, so that when the actuating force and the restoring force are balanced, a certain linear adjustment, i.e. an actuating path, results.

[0017] By adjusting the adjustment device, the elasticity of the elastic deformation element can be adjusted by releasing the deformation space. Since the elastic deformation element acts analogously to a return spring, the corresponding spring constant of the return force can be adjusted by varying the size of the deformation space. The adjustment device can release or reduce the deformation space without changing the relative position of the base elements or the cylinder volume.

[0018] The adjustment device can comprise an adjustment element that is translationally adjustable in an adjustment direction and that has, at least in a section located in the cylinder volume, a preferably undercut-free cross-sectional enlargement (with a monotonic increase of the cross-section in the adjustment direction) in the axial direction or in the adjustment direction. In the first end position, the adjustment element is completely surrounded by the elastic material of the elastic deformation element in the cylinder volume, and there is no free deformation space. If the adjustment element is moved towards the second end position, the deformation space is created between the interface of the elastic material and the surface of the adjustment element at the section with increasing cross-section.In this way, by moving the adjusting element between the first end position and the second end position, the deformation space usable for elastic deformation of the elastic material can be increased or decreased in order to change the coupling between the base elements from a rigid coupling to an elastic coupling.

[0019] The elastic deformation is always limited by the surface (surface) of the adjustment element, so that when the elastic material is deformed, it comes into contact with the surface of the adjustment element by at least partially filling the deformation space, thereby limiting further local deformation. As a result, the elasticity decreases as the volume of the deformation space decreases. In other words, by variably adjusting the adjustment element in the adjustment direction, the deformation space inside the elastic deformation element can be released, thereby increasing the elasticity of the elastic deformation element.The elasticity of the elastic element can be variably adjusted because, during a typical bulging deformation of the elastic material of the elastic deformation element, the elastic material increasingly comes into contact with the surface of the adjustment element across the deformation space, thereby reducing the deformation space. It is therefore possible to achieve less elasticity by slightly adjusting the adjustment element and releasing a small amount of deformation space than by releasing a larger volume of the deformation space.

[0020] It can be provided that the adjusting device has a second actuating actuator with, in particular, a thermoelastic actuator element in order to move an adjusting element in the interior of the deformation space in a first direction of movement and / or to exert a force in the first adjusting direction and / or wherein the adjusting device has a third actuating actuator with, in particular, a thermoelastic actuator element in order to move the adjusting element in the interior of the deformation space in a second direction of movement opposite to the first direction of movement and / or to exert a force in the second adjusting direction.

[0021] The adjusting element can be coupled to an antagonistic actuator device as part of the adjusting device, with which the adjusting element can be moved in the lifting cylinder and the deformation element, i.e., can be pushed in or pulled out. The antagonistic actuator device is preferably designed with a second actuating actuator with a thermoelastic actuator element and a third actuating actuator with a thermoelastic actuator element and is connected to the adjusting element such that its actuating position can be variably adjusted in the adjusting direction. The second actuating actuator and the third actuating actuator are arranged counteracting one another, i.e., antagonistically, in order to move the adjusting element in opposite adjusting directions.

[0022] Adjustment using the actuators can be achieved by a thermoelastic actuator element made of shape memory material (SMA: Shape Memory Alloy), particularly in the form of a wire, rod, or wire bundle. When heated, the thermoelastic actuator element contracts and exerts a tensile force and / or experiences a shortening in length. After / upon cooling, the tensile force decreases, and the thermoelastic actuator element can be stretched to its original length even with a small restoring force / tensile force. Heating is usually achieved by applying current to the generally conductive thermoelastic actuator element, so that the converted electrical power leads to heat generation. Deactivation occurs by cooling to ambient temperature, particularly by removing the electrical power supply.

[0023] Thermoelastic actuator elements are typically made from a thermoelastic material, such as a shape memory alloy, which utilizes a shape memory effect. At room temperature, the thermoelastic material is in martensite form, regardless of mechanical stress. If the thermoelastic material is then heated from the outside, it transforms into an austenitic state at a temperature that depends on its composition and mechanical stress. This state is the initial state in which the thermoelastic material was manufactured. When a thermoelastic material is subjected to tensile stress, its length shortens against the tensile force. If the heat source is removed, the thermoelastic material can cool down again and transforms back into martensite. If the thermoelastic material is not subjected to mechanical stress, its external shape remains virtually unchanged after the transformation from austenite to martensite.When the thermoelastic material is subjected to tensile stress, it is stretched again by the external tensile force.

[0024] The thermoelastic material of the wire-shaped thermoelastic actuator element can contain, for example, a shape memory alloy, such as NiTi, and thus elastically strain or relax through a phase transition, i.e., a transformation of the lattice structure, upon the addition or removal of heat. Typically, in shape memory alloys exposed to heat, a martensitic material structure transforms into an austenitic material structure, and the length of the wire decreases. When the material is cooled, it returns to its original shape due to elastic deformation when subjected to even a slight tensile stress, with the austenitic material structure reverting to the martensitic material structure.

[0025] The thermoelastic actuator element and / or the further thermoelastic actuator element can extend in the longitudinal direction and in particular can be deflected one or more times so that sections of the actuator element run parallel to one another.

[0026] The adjustment element can also be moved toward a decreasing deformation space, even counteracting the elastic deformation of the elastic deformation element, thereby pushing the elastic material toward its original shape (shape when the adjustment element is in the first end position). This enables active deformation of the elastic deformation element back to its original shape, even when an actuating force acts on the elastic deformation element via the base elements and / or the first actuating actuator.

[0027] Restoration occurs through the restoring force resulting from elastic deformation of the elastic deformation element. To increase the restoring force of the elastic deformation element if necessary, the adjustment element can be adjusted using the antagonistic actuator device to reduce the deformation space. Due to the incompressibility of the elastic material, this creates a force that pushes the two base elements apart. This provides additional support for the restoring of the linear adjustment, i.e., the actuating force of an adjustment movement of the base elements can be increased.

[0028] Furthermore, the adjustment element can comprise a translationally movable adjustment pin that extends through the interior of the lifting cylinder, preferably parallel to the movement axis of the lifting cylinder, and has a section with a cross-sectional change with a monotonically increasing cross-sectional area in the axial direction without an undercut, so that an axial displacement of the adjustment pin in the deformation element is permitted in order to enlarge or reduce the deformation space inside the lifting cylinder. In particular, the cross section in the section can correspond to a circular, elliptical, triangular, quadrangular, polygonal, or oval cross-sectional area.

[0029] The adjustment element is thus preferably designed with an adjustment pin that extends through the lifting cylinder and the elastic element located therein and can be moved between the first and second end positions. The section of the cross-sectional enlargement is provided with an inclined profile and can, in particular, be provided as a conical section with a circular, oval, or other cross-section.

[0030] Accordingly, the elastic deformation element has at least one through-opening with a corresponding cross-section, in which the adjustment pin can be received and moved in and out between the first and second end positions. Furthermore, the first base element and the second base element can be designed as hollow profiles which are guided telescopically into one another, wherein a first holding element is arranged inside the first base element and a second holding element is arranged inside the second base element, wherein a plunger is fixedly connected to the second holding element and is movably guided by the first holding element, wherein the first actuating actuator is arranged on the side of the first holding element opposite the second holding element in order to effect an adjustment movement in a first of the adjustment directions by exerting a tensile force between the corresponding end of the plunger and the first holding element.

[0031] Furthermore, a cylinder chamber of the lifting cylinder can be fixedly arranged in or on the first holding element and can have a piston which is fixedly connected to the second holding element and is guided in the cylinder chamber in a manner known per se, which piston is preferably movable in the direction of the adjustment direction.

[0032] According to one embodiment, a deformation sensor can be embedded in the elastic deformation element, which is designed to detect a deformation of the elastic deformation element, wherein the deformation sensor is provided in particular as a capacitive, resistive or piezoresistive sensor or as an electroactive polymer sensor.

[0033] According to a further aspect, a linear actuator system is provided with the above-mentioned linear actuator and with a control unit configured to first control the adjustment device to set a predetermined linear position and a predetermined stiffness, thereby releasing a deformation space. Simultaneously or subsequently, the first actuating actuator is actuated to adjust the first and second base elements relative to one another in accordance with the predetermined linear position. The linear actuator has an adjustment range available to it, which is determined, on the one hand, by the possible contraction of the first actuating actuator and, on the other hand, by the size of the released deformation space.

[0034] The relative movement of the first and second base elements can be limited by both the maximum stroke of the first thermoelastic actuator and a fixed end stop. In addition to the adjustment between the base elements by actuation by the first thermoelastic actuator, the adjustment path can also depend on an external force acting on the base elements. This allows adjustment to occur depending on the external force, as long as the available deformation space allows it. On the other hand, the relative movement of the base elements can be limited by reducing the deformation space when the load on the base elements changes.

[0035] The stiffness can also be adjusted using the adjustment device, so that identical external mechanical loads lead to different relative movements of the base elements to one another depending on the setting of the adjustment device or the stiffness. If there is no longer any contact between the elastic deformation element and the adjustment device, the elasticity is determined purely by the Shore hardness of the elastic deformation element. However, if the elasticity is too stiff, the stiffness can be reduced by activating the first actuating element. The actuation of the adjustment device and the first actuating actuator enables targeted adjustment of the relative movement of the base elements to one another under changing external loads. The stiffness can also be adjusted using the adjustment device, so that identical external mechanical loads lead to different relative movements of the base elements to one another depending on the setting of the adjustment device or the stiffness.The stiffness of the linear actuator results in different relative movements of the base elements to each other.

[0036] Essentially, the linear actuator system enables linear adjustment between the base elements and adjustment of stiffness or elasticity. Furthermore, the available stroke range within which the base elements can move against the action of a force can be specified by the first end position and the second end position. The first end position serves as the first end stop and is defined by a setting position in which the incompressible elastic material has no deformation space, by the limitation of the stroke range of the first actuator, or by a structurally defined second end stop. The second end position can be defined by the limitation of the stroke range of the first actuator or by a structurally defined second end stop.Thus, the minimum achievable adjustment position between the base elements can be adjusted by the degree of deflection of the adjustment element from the first end position toward the second end position, with a previously set linear longitudinal adjustment. The deflection defines the maximum compression between the base elements possible by increasing the force.

[0037] Furthermore, the control unit can be designed to deactivate the first actuating actuator in order to reset the adjustment between the first and the second base element, and to control the adjusting device in order to increase the restoring force, so that the deformation space is reduced due to the compression of the elastic deformation element.

[0038] It can be provided that an electroactive element is embedded in the elastic deformation element, which is designed to sensorily detect a deformation of the elastic deformation element and / or to adapt the elasticity of the elastic deformation element and / or the restoring force of the deformation element to the holding elements by electrical control with an actuator voltage as an electroactive actuator.

[0039] According to a further aspect, a method for operating the above linear actuator is provided, comprising the steps:

[0040] Controlling the adjustment device so that the deformation space is released, simultaneously or subsequently, controlling the first actuating actuator in order to adjust the first and the second base element relative to each other according to a predetermined linear position,

[0041] Controlling the adjustment device to set a specified stiffness.

[0042] According to a further aspect, a linear actuator system is provided with the above linear actuator and with a control unit which is designed to first control the adjustment device in order to set a predetermined linear position and a predetermined stiffness, so that a deformation space is released and simultaneously or subsequently the first actuating actuator is controlled in order to adjust the first and the second base element relative to one another in accordance with the predetermined linear position.

[0043] Furthermore, the control unit can be designed to deactivate the first actuating actuator in order to reset the adjustment between the first and the second base element, and to control the adjusting device in order to increase the restoring force, so that the deformation space is reduced.

[0044] According to a further aspect, an implant for fixing bone fragments with the above linear actuator is provided, wherein the first and the second base element can each be fastened to a respective bone fragment with a fastening device.

[0045] A possible use of the above linear actuator thus consists in an application in an implant for fixation of bone fragments around a fracture site, wherein the base elements are attached to the bone fragments by suitable fastening devices, such as screws, and the linear actuator is adjusted by linear adjustment for a predetermined distance between the bone fragments.

[0046] It is known that higher implant stiffness at the beginning of fracture healing and lower stiffness over time are better for healing. Accordingly, lower movements in the fracture at the beginning of healing and larger movements later on are desirable. It is also known that a narrower fracture gap requires less interfragmentary movement and stiffer fixation than a wider gap. This change also needs to be individually adjusted depending on the healing process. During the healing process, the stiffness of the tissue forming in the fracture gap increases nonlinearly.

[0047] The adjustment device can be controlled in such a way that the adjustment element is deflected from a first end position, in which there is no deformation space, towards a second end position by a maximum distance corresponding to the distance between the bone fragments.

[0048] One concept of the above implant for fixating bone fragments around a fracture site is that the bone fragments are elastically connected via at least one linear actuator. For this purpose, the bone fragments can be fixed to the base elements, between which the linear actuator is arranged, allowing the distance between the bone fragments and the stiffness of the connection between the bone fragments to be variably adjusted. Furthermore, the maximum compression between the bone fragments can be adjusted.

[0049] This allows the patient to exert different adjustable loads on the bone fragments during implant use, resulting in a corresponding relative micromovement between the bone fragments at the fracture site, depending on the stiffness setting of the linear actuator. The stiffness of the implant can be adjusted using the adjustment device, so that identical external mechanical loads lead to different micromovements at the fracture site depending on the setting of the adjustment device or the stiffness of the linear actuator. This allows for targeted adjustment of the mechanical stimulation of the fracture site according to individual needs and the stage of bone fracture healing.

[0050] In addition to adjusting the stiffness of the implant, active massage of the fracture gap via the adjustment device is possible and desirable if the patient has not generated sufficient micromovements through movement. Studies have shown that cyclical massage of the bone fracture with a stroke of approximately 0.4 to 0.5 mm can lead to better and faster healing. This massage should be performed while the patient is at rest.

[0051] If the patient creates micromovements of a defined size in the fracture gap through their own movements or through externally guided movements, the movement of the bone fragments can be controlled by adjusting the elasticity / stiffness. During the healing process, the patient can only control the micromovements very imprecisely, resulting in uneven loading of the fracture. This could result in less compression force being available at the beginning of the healing process and possibly too much at the end. If the micromovements are too small at the beginning of healing (in the first few days after the fracture) because the elastic element is too stiff or the weight force is too small, this can significantly delay the healing process.In contrast, if the compression caused by the micro-movements is too large because the selected deformation space and thus the elasticity of the elastic deformation element is too small, or the patient applies too much weight to the fracture site, the micro-movements are too large, so that the forming callus is damaged with each movement.

[0052] The optimal amount of micromovement depends on the width and configuration of the fracture gap and can be individually calculated based on computed tomography data. It is also possible to choose an optimal movement of 0.4 to 0.5 mm based on data from the literature. Studies in sheep have shown that 0.4 to 0.5 mm usually produces the best results. Brief description of the drawings

[0053] Embodiments are explained in more detail below with reference to the attached drawings. They show:

[0054] Figure 1 is a cross-sectional view of an exemplary embodiment of the

[0055] linear actuator;

[0056] Figures 2a to 2c show possible cross sections of the section of the adjusting element with the change in cross section;

[0057] Figures 3a to 3c show detailed cross-sectional views illustrating the different states inside the lifting cylinder;

[0058] Figure 4 is a cross-sectional view of another exemplary

[0059] Design of the linear actuator;

[0060] Figure 5 shows an illustration of an implant with a linear actuator; and

[0061] Figures 6a, 6b and 6c show various embodiments for the arrangement of electroactive actuator layers in the deformation element.

[0062] Description of embodiments

[0063] Figure 1 shows a cross-sectional view of a linear actuator 1 with variable stiffness. The linear actuator has a first base element 2 and a second base element 3, which are arranged so as to be displaceable relative to one another in the longitudinal direction. In this embodiment, the first and second base elements 2, 3 represent hollow profiles which are telescopically guided one inside the other in a longitudinal direction L, between which a suitable bearing 4 can be provided. For the sake of simplicity, it is assumed below that the elements shown are base elements with a circular, elliptical, or polygonal cylindrical cross-section defined around a cylinder axis A. A first holding element 5 which is firmly connected to the first base element 2 is arranged within the first base element 2 or on the first base element 2.A second holding element 6 which is fixedly connected to the second base element 3 is arranged within the second base element 3 or on the second base element 3.

[0064] The first and second base elements 2, 3 are coupled to one another via one or more plungers 7. For this purpose, the plungers 7 can be rigidly connected to or within the second holding element 6 and slidably guided through a corresponding through-opening of the first holding element 5. On a side of the first holding element 5 opposite the second holding element 6, each of the plungers 7 is coupled to a first actuator 8.

[0065] The first actuating actuator 8 can be formed in a receiving element 10 using one or more rectified thermoelastic actuator elements 9, in particular as a thermoelastic actuator wire, actuator wire bundle, or shape memory actuator wire. The receiving element 10 can be arranged on the first holding element 5 on a side opposite the second holding element 6. The thermoelastic actuator element 9 is connected between a first fastening point B1 on the receiving element 10 supported on the first holding element 5 and a second fastening point B2 on an end section 71 of the plunger 7 protruding from the second holding element 6 and extending through the first holding element 5. By activating the thermoelastic actuator element 9, the second holding element 6 is moved via the plunger 7 in a first adjustment direction V1 toward the first holding element 5.

[0066] Thermoelastic actuator elements are defined herein as actuators that can induce an actuating movement through the action of heat. Thermal actuator arrangements can, for example, be formed from a thermoelastic material (also referred to as an elastocaloric or mechanocaloric material). Such thermoelastic materials change their microstructure upon exposure to a temperature change. As a result, thermoelastic actuator elements can reduce their dimensions in the direction of loading or exert a tensile force upon heating. Upon cooling, the thermoelastic actuator element returns to its original shape with respect to the direction of loading, particularly when a corresponding restoring force is present. One group of common thermoelastic materials is shape memory alloys.The one or more thermoelastic actuator elements 9 of the first actuator 8 can be heated, for example, by applying current, so that they contract against the external restoring force. When the current is removed, the one or more thermoelastic actuator elements 9 cool down and, when an external restoring force is applied, return to their pre-heated / current-applied length.

[0067] An elastic deformation element 11 is arranged in a lifting cylinder 12 between the first and second base elements 2, 3. The lifting cylinder 12 can be rigidly connected to the first base element 2 and preferably integrated into the first holding element 5. A piston 13 is movably guided in the cylinder chamber of the lifting cylinder 12. This piston 13 is rigidly connected to the second holding element 6 and can, in particular, be formed integrally therewith. Thus, a variable cylinder volume is defined between the cylinder chamber and the piston.

[0068] The elastic deformation element 11 is arranged inside the lifting cylinder 12. In a first end position of the base elements 2, 3, which is reached without the actuating force of the first actuating actuator 8, the elastic deformation element 11 is relaxed and completely enclosed by an inner wall of the lifting cylinder 12 and the piston 13.

[0069] When the base elements 2, 3 move relative to one another in the longitudinal direction L, which is effected by the exertion of an actuating force by the first actuating actuator 8, the piston 13 moves into the lifting cylinder 12, thereby compressing the elastic material of the elastic deformation element 11, provided that it has a free space in the form of a deformation space. If the elastic material does not have a free space in the form of a deformation space, it cannot be deformed due to the incompressibility of the elastic material, thus blocking the movement of the two base elements 2, 3 relative to one another.

[0070] Restoring the linear adjustment between the first and second base elements 2, 3 can be achieved by relaxing the first actuator 8 and based on a restoring force resulting from a previously occurring elastic deformation of the elastic material of the elastic deformation element 11. The thermoelastic actuator element 9 of the first actuator 8 can thus be heated, for example, by applying current, so that it contracts and thereby exerts a compressive force on the elastic material of the elastic deformation element 11.When the current supply is removed, the thermoelastic actuator element 9 cools down and is returned to its length before heating by the elastic restoring force of the elastic material, so that the actuating force in the compression direction of the elastic deformation element 11 decreases and the resulting restoring force leads to an adjustment movement in a second adjustment direction V2 of the second base element 3, opposite to the first adjustment direction.

[0071] The elastic material of the elastic deformation element 11, the lifting cylinder 12, and the piston 13 are each provided with corresponding through-openings through which an adjustment pin 14 is guided as the adjustment element. The adjustment pin 14 is preferably further guided in through-openings of the holding element 5 and / or the holding element 6. A guide element 15 is provided which extends through the adjustment pin 14 and is translationally movable relative to the adjustment pin 14, forming the force feedback to a further, essentially sleeve-shaped receiving element 20 supported on the first holding element 5. The guide element 15 is slidably movable in the adjustment pin 14. The adjustment pin 14 and the guide element 15 extend in the axial direction of the through-opening, which preferably coincides with the longitudinal axis of the linear actuator 1 or can at least run parallel to the longitudinal direction of the linear actuator 1.

[0072] With the force feedback between the additional receiving element 20 and the guide element 15, the adjustment pin 14 can be pulled in the second adjustment direction V2. This enables the movement of the adjustment pin 14 and thus also the relaxation of the elastic deformation element 11, regardless of the position of the holding element 6 and the integrated piston 13.

[0073] The adjustment pin 14 has a section QA inside the lifting cylinder 12 with a monotonically increasing cross-sectional change. The cross-sectional change must be free of undercuts to allow adjustment of the adjustment pin 14 along the through-opening in at least one direction. In a first end position of the first actuating actuator 8 and a first end position of the adjustment pin 14, the section QA with the cross-sectional change is preferably completely surrounded by the elastic deformation element 11, and no deformation space is present between the elastic deformation element 11 and the adjustment pin 14.

[0074] The cross-sectional area in the region of the section QA of the cross-sectional change can have circular, ellipsoidal, elongated, or even polygonal cross-sections, as shown by way of example in Figures 2a to 2c. It is essential that the increase in the cross-section along the section QA of the cross-sectional change is monotonically increasing, whereby a region with a constant cross-section can also be provided within the section.

[0075] In particular, the adjusting pin 14 can be provided with conical or otherwise oblique or inclined surfaces to the direction of movement of the pin, so that by moving the adjusting pin 14 in the direction of a second end position, a deformation space within the lifting cylinder 12 can be released or enlarged, and by moving the adjusting pin 14 in the opposite direction of movement, ie in the direction of the first end position, the deformation space is reduced or the deformation space can be completely eliminated.

[0076] When the adjustment pin 14 is adjusted toward the first end position, the elastic material of the elastic deformation element 11 that is still deformed can be pushed back by the section of cross-sectional change into an initial position, as it exists in the first end position and the first end position, thereby increasing the restoring force between the holding elements 5, 6 and thus also between the base elements 2, 3. This can enable or improve the resetting of the linear adjustment of the linear actuator 1.

[0077] The adjustment pin 14 is preferably adjustable with an antagonistic actuator device 16, with which it is possible to actively move the adjustment pin 14 in both adjustment directions V1, V2 relative to the first holding element 5 and thus relative to the lifting cylinder. The antagonistic actuator device 16 is preferably constructed with a second actuator 17 and a third actuator 18 of an adjustment device, which counteract one another and are controlled alternately. The second and / or the third actuator 17, 18 can each be designed with one or more thermoelastic actuator elements, which can be designed in the manner described above. The first, second and third actuators 8, 17, 18 can be controlled selectively and, if necessary, variably using a control unit.The second actuator 17 is arranged between a first fastening point B3 at a first end 141 of the adjustment pin 14, which is opposite the second holding element 6 with respect to the first holding element 5, and a fourth fastening point B4 at the further receiving element 20 supported on the first holding element 5. Thus, by heating / energizing the second actuator 17, it can be contracted, so that the adjustment pin 14 is moved toward the first end position, i.e., in the first adjustment direction V1.

[0078] The thermoelastic actuator element of the third actuating actuator 18 is arranged between a second end 142 of the adjusting pin 14, opposite the first end, and at an end of the guide element 15 facing the second holding element 6, which is supported or fastened to the further receiving element 20 and thus to the first holding element 5. Thus, by heating / energizing the third actuating actuator 18, it can be contracted, so that the adjusting pin 14 is moved toward the second end position, i.e., in the second adjustment direction V2, relative to the first holding element 5.

[0079] If the second actuator 17 is activated, it exerts an actuating force in a first adjustment direction V1 toward the first end position. If the third actuator 18 is activated, it exerts an actuating force in the second adjustment direction V2 toward the second end position. The second actuator 17 and the third actuator 18 are predominantly activated alternately or in opposition to each other, so that the desired adjustment is performed by the adjustment pin 14.

[0080] The linear actuator 1 thus formed enables a linear translational adjustment relative between the first and the second base element 2, 3. This is achieved by moving the adjustment pin 14 starting from the first end position in the lifting cylinder 12 so that a deformation space VR is created between the adjustment pin 14 and the elastic material 11, as shown in Figure 3a.

[0081] If the first actuating actuator 8 is then activated and a relative actuating movement of the second base element 3 in the first adjustment direction V1 is achieved, i.e. the first and second base elements 2, 3 move towards one another, as shown in Figure 3b, so that the deformation element 11 is compressed and the elastic material of the deformation element 11 deforms into the deformation space VR. This is achieved by the elastic displacement of the elastic material by the piston 13 pressing into the lifting cylinder 12, which is coupled to the second holding element 6. When the elastic material is deformed, a restoring force is generated which counteracts the actuating force of the first actuating actuator 8.

[0082] The deformation of the elastic material occurs within the deformation space VR and is terminated due to the incompressibility of the elastic material, or terminated when an equilibrium is reached between the actuating force caused by the first actuating actuator 8 and the restoring force of the elastic material, or when the deformation space VR is completely filled with deformed elastic material and further deformation is no longer possible. Deactivating the first actuating actuator 8 causes it to cool due to the cooler ambient temperature, and the actuating force decreases, so that the restoring force of the elastic material moves the second holding element 6 back into the second adjustment direction V2, or moves the first and second base elements 2, 3 away from each other in the longitudinal direction.

[0083] The volume of the deformation space VR increases again. If the restoring force is not sufficient to bring about a complete return to the starting position of the first and second base elements 2, 3, the deformation space VR can be reduced by actively moving the adjustment pin 14 in the first adjustment direction V1 of the first end position, and additional pressure can be exerted on the deformed elastic material of the elastic deformation element 11, as shown in Figure 3c. This increases the internal pressure in the elastic material and increases the resulting restoring force. Thus, even in cases where the restoring force is not sufficiently high to achieve complete return by stretching the first actuating actuator 8, the return can be additionally supported.

[0084] Furthermore, upon reaching a certain linear adjustment, i.e. a certain relative position between the first and second base elements 2, 3, the stiffness or the damping between the base elements 2, 3 of the thus formed actuating element can be adjusted by adjusting the adjusting pin 14. It is assumed that the elastic material is deformed by adjusting the first and second base elements 2, 3. When the adjusting pin 14, which forms a deformation space VR within the elastic material, is adjusted, the elastic material is then deformed, in particular in a bulbous manner into the deformation space VR, wherein the elastic material can partially come into contact with the outer surface of the adjusting pin 14.By adjusting the adjustment of the adjustment pin 14, the deformation space VR, which is given to the elastic material for deformation, can now be increased or decreased, whereby the stiffness between the first and second base elements 2, 3 can be adjusted with a previously set linear adjustment.

[0085] Thus, a linear actuator can be provided that, on the one hand, provides a linear longitudinal adjustment and, in addition, can also set a range of variable stiffnesses at each position of the longitudinal adjustment. Furthermore, by adjusting the degree of deflection of the adjustment element 14 from the first end position toward the second end position with a previously set linear longitudinal adjustment, the minimum achievable end position between the base elements 2, 3 can also be set. The deflection defines the maximum compression possible between the base elements 2, 3 by increasing the force.

[0086] Figure 4 describes a further embodiment based on the same operating principle as the embodiment in Figure 1. Elements with the same function are designated by the same terms and provided with the same reference numerals preceded by " ' ". Unless expressly described, the elements of the further embodiment have the same function as the embodiment in Figure 1. The advantage of this embodiment is that the assembly of the linear actuator is simpler, since the first to third actuating actuators 8', 17', 18' can all be arranged on one side of the lifting cylinder 12'.

[0087] Figure 4 shows a cross-sectional view of a linear actuator 1' with variable stiffness. The linear actuator has a first base element 2' and a second base element 3' with identical cross-sections, which are arranged to be displaceable relative to one another in the longitudinal direction. A distance is provided between the opposite ends of the first and second base elements 2', 3', which allows a relative adjustment in a longitudinal direction L'. To guide the base elements 2', 3', a suitable sleeve element 21 is provided inside the base elements 2', 3', which is firmly connected to the second base element 3' and on whose outer surface the first base element 2' can be arranged to be slidably movable. For the sake of simplicity, it is assumed below that the elements shown are base elements 2', 3' with a rectangular cylindrical cross-section defined around a cylinder axis A. Other cross-sections, such asRound, oval, or other polygonal cross-sections are also conceivable. A first holding element 5' is fixedly arranged on the first base element 2'. Fastening can be achieved during final assembly by inserting the sleeve element 21 into the first base element 2' and then securing it. A second holding element 6' is fixedly arranged on the sleeve element 21, preferably at an end of the sleeve element 21 facing the first holding element 5'.

[0088] The first and second base elements 2', 3' are coupled to one another via one or more stamps T. For this purpose, the stamps 7' can be firmly connected to or in the first holding element 5' and guided slidably through a corresponding through-opening of the second holding element 6'.

[0089] On a side of the second holding element 6' opposite the first holding element 5', each of the plungers 7' is coupled to first ends of one or more first actuating actuators 8' via a fastening element 22. Second ends of the actuating actuators 8' are fixedly connected to a fastening element 28, which is spaced apart from the fastening element 22 and is fixedly connected to the sleeve element 21 and thus to the second base element 3.

[0090] The first actuating actuator 8' can be formed within the sleeve element 21 using one or more thermoelastic actuator elements 9', in particular as a thermoelastic actuator wire, actuator wire bundle, or shape memory actuator wire. By activating the thermoelastic actuator element 9', the first holding element 5' is moved via the plunger(s) 7' in a second adjustment direction V2 toward the second holding element 6'. Similarly, by activating the thermoelastic actuator element 9', the second holding element 6' is moved via the plunger(s) 7' in a first adjustment direction V1 toward the first holding element 5'.

[0091] An elastic deformation element 11' is arranged in a lifting cylinder 12' between the first and second base elements 2', 3'. The lifting cylinder 12' is firmly connected to the first base element 2' and preferably integrated into the first holding element 5'. A piston 13' is movably guided in the lifting cylinder 12', which is firmly connected to the second holding element 6' and can, in particular, be formed integrally therewith.

[0092] The elastic deformation element 11' is arranged inside the lifting cylinder 12'. In a first end position of the base elements 2', 3', which is reached without the actuating force of the first actuating actuator 8, the elastic deformation element 11' is relaxed and completely enclosed by an inner wall of the lifting cylinder 12 and the piston 13.

[0093] When the base elements 2', 3' move relative to one another in the longitudinal direction, which is effected by the exertion of an actuating force by the first actuating actuator 8', the piston 13' moves into the lifting cylinder 12', thereby compressing the elastic material of the elastic deformation element 1T, provided that it has a free space in the form of a deformation space VR. If the elastic material does not have a free space in the form of a deformation space VR, it cannot be deformed due to the incompressibility of the elastic material and thus blocks the movement of the two base elements 2', 3' relative to one another, even against the action of the first actuating actuator 8'.

[0094] As above, a resetting of the linear adjustment between the first and second base elements 2', 3' can be achieved due to the relaxation of the first actuating actuator 8' and based on a restoring force resulting from a previously occurring elastic deformation of the elastic material of the elastic deformation element 11.

[0095] The elastic material of the elastic deformation element 11', the lifting cylinder 12', and the piston 13' are each provided with corresponding through-openings through which an adjusting pin 14' is guided. The adjusting pin 14' is preferably further slidably guided in through-openings of the first holding element 5' and / or the second holding element 6'. The adjusting pin 14' is fixedly coupled to an adjusting rod 23, which is provided for sliding movement relative to the punches 7 and the base elements 2, 3' and the holding elements 5', 6'. A translational movement of the adjusting rod 23 can thus move the adjusting pin 14' in and out of the lifting cylinder 12'. The adjusting rod 23 and the adjusting pin 14' extend in the axial direction of the through-opening, which preferably coincides with the longitudinal axis of the linear actuator 1' or can at least run parallel to the longitudinal direction of the linear actuator T.

[0096] As in the first embodiment, the adjustment pin 14' has a section QA with a monotonically increasing cross-sectional change inside the lifting cylinder 12'. The cross-sectional change must be free of undercuts to allow adjustment of the adjustment pin 14* along the through-opening in at least one direction. In a first end position of the first actuating actuator 8' and a first end position of the adjustment pin 14*, the section QA with the cross-sectional change is preferably completely surrounded by the elastic deformation element 11', and no deformation space is present between the elastic deformation element 11' and the adjustment pin 14'.

[0097] In particular, the adjusting pin 14' can be provided with conical surfaces or surfaces that extend in some other way at an angle to the direction of movement of the pin, so that a deformation space within the lifting cylinder 12* can be released or enlarged by a movement of the adjusting pin 14' in the direction of a second end position, and upon a movement of the adjusting pin 14* in the opposite direction of movement, ie in the direction of the first end position, the deformation space is reduced or the deformation space can be completely eliminated.

[0098] When the adjustment pin 14' is adjusted toward the first end position, the still-deformed elastic material of the elastic deformation element 11* can be pushed back by the section of cross-sectional change into an initial position, as it exists in the first end position and the first end position, thereby increasing the restoring force between the holding elements 5', 6* and thus also between the base elements 2*, 3'. This can improve the resetting of the linear adjustment of the linear actuator 1.

[0099] The adjustment pin 14* is preferably adjustable with an antagonistic actuator device 16* as an adjustment device, with which it is possible to actively move the adjustment pin 14* in both adjustment directions V1, V2 relative to the first holding element 5* and thus relative to the lifting cylinder. The antagonistic actuator device 16* is preferably constructed with a second actuating actuator 17* and a third actuating actuator 18*, which counteract each other and are predominantly controlled alternately. By simultaneously controlling the second and third actuating actuators 17', 18*, they can be braced against each other so that the setting position of the adjustment pin 14* can be adjusted more precisely. The second and / or third actuating actuators 17', 18* can each be designed with one or more thermoelastic actuator elements, which can be designed in the manner described above.The first, second and third actuators 8', 17', 18* can be controlled selectively and, if necessary, variably using a control unit.

[0100] The second actuator 17* is arranged between a first fastening element 24, which is firmly connected to the plunger(s) 7*, and a second fastening element 25, which is firmly connected to the adjustment rod 23. Thus, by heating / energizing the second actuator 17*, it can be contracted, so that the adjustment pin 14* is moved toward the first end position, i.e., in the first adjustment direction V1.

[0101] The thermoelastic actuator element of the third actuator 18' is arranged between a third fastening element 26, which is firmly connected to the plunger(s) 7*, and a fourth fastening element 27, which is firmly connected to the adjustment rod 23. Thus, by heating / energizing the third actuator 18', it can be contracted, so that the adjustment pin 14* is moved toward the second end position, i.e., in the second adjustment direction V2, relative to the first holding element 5*.

[0102] If the second actuator 17' is activated, it exerts an actuating force in a first adjustment direction V1 toward the first end position. If the third actuator 18' is activated, it exerts an actuating force in the second adjustment direction V2 toward the second end position. The second actuator 1T and the third actuator 18' are activated alternately or in opposition to each other, so that the desired adjustment is performed by the adjustment pin 14*.

[0103] The linear actuator 1 thus formed enables a linear translational adjustment relative between the first and second base elements 2', 3'. This is achieved by moving the adjustment pin 14* from the first end position in the lifting cylinder 12* such that a deformation space VR is created between the adjustment pin 14* and the elastic material 11*. The mode of operation corresponds to that described above with regard to the first embodiment.

[0104] In all the embodiments described above, an electroactive element can be embedded in the elastic deformation element, which element is designed to sense a deformation of the elastic deformation element 11, 11*. By operating the electroactive element as an actuator, the elasticity of the elastic deformation element can be influenced by electrically controlling it with a voltage. This allows for fine adjustment of the stiffness of the deformation element.

[0105] Figure 5 shows an application example for the linear actuators described above as an implant to accelerate the healing of a bone fracture. It can be seen that the linear actuator with the first base element 2, 2* and the second base element 3, 3* are attached to bone fragments K1, K2 with screws, respectively, and cover a fracture gap S. The linear actuator allows for the distance between the bone fragments K1, K2 and the stiffness of the connection between the bone fragments K1, K2 to be variably adjusted. Furthermore, the maximum compression between the bone fragments K1, K2 can be adjusted.

[0106] This allows the patient's movement to exert different adjustable loads on the bone fragments when using the implant, which lead to a corresponding relative micro-movement between the bone fragments at the fracture site, depending on the stiffness setting of the linear actuator.

[0107] The stiffness of the implant is adjustable using the adjustment device, so that the same external mechanical loads lead to different micromovements at the fracture site depending on the setting of the adjustment device or the stiffness of the linear actuator. This allows for targeted adjustment of the mechanical stimulation of the fracture site according to individual needs and the phase of bone fracture healing.

[0108] Figures 6a, 6b, and 6c show various embodiments that can be combined with the previously mentioned embodiments. These provide for embedding electroactive actuator layers 31 into the deformation element 11, 11' in the lifting cylinder 12, 12'. The orientation can be transverse to the axial direction and / or parallel thereto.

[0109] Electroactive actuator layers 31 comprise, in a conventional manner, a thin, elastically deformable layer provided with actuator electrodes on both sides. When an actuator voltage is applied to the actuator electrodes, the resulting electric field creates a force between the actuator electrodes, which leads to a reduction in the thickness of the elastically deformable layer. When the actuator voltage is reduced, the force decreases, and the actuator electrodes are pushed apart again by a restoring force of the deformed, elastically deformable layer.

[0110] In Figure 6a, the actuator layers run transversely to the axial direction of the lifting cylinder 12, 12*. When actuated, they reduce the restoring force exerted by the deformation element 11, 11' on the first actuating actuator 7. In this way, by actuating the actuator layers 31, the adjustment position of the two base elements 2, 2', 3, 3' can be changed, since the restoring force exerted by the deformation element 11 is reduced. In Figure 6b, the actuator layers 31 run parallel to the axial direction of the lifting cylinder 12, 12". When actuated, they increase the restoring force, since the length of the actuator layers 31 increases. Thus, an additional force acts in the direction of the restoring force exerted by the deformed deformation element 11, 11".In this way, by actuating the actuator layers 31, the adjustment position of the two base elements 2, 2", 3, 3" can be changed, since the restoring force exerted by the deformation element 11 increases.

[0111] In both embodiments, an actuation of the actuator layers 31 has a stiffening effect on the deformation element 11, 11", so that in addition to changing the adjustment position, the elasticity between the base elements 2, 2", 3, 3" can also be variably adjusted.

[0112] In the embodiment of Figure 6c, a combination of the transverse and longitudinal arrangements of the actuator layers 31 in the deformation element 11, 11" is shown in order to enable a variable change of the adjustment position in both directions by selectively controlling the actuator layers 31.

[0113] The actuator layers 31 can additionally be used, if they are not actuated, as deformation sensors that can indicate a degree of deformation of the deformation element 11, 11" accommodated in the lifting cylinder. The degree of deformation can be used in a closed-loop or closed-loop control system to regulate / control the actuation of the adjustment device in order to be able to adjust both the restoring force and the elasticity at a specific adjustment position.

[0114] The contacting of the elastic deformation element 11, 11" can be made through the base of the lifting cylinder 12, 12" opposite the piston. During assembly, the contacting lines can be pushed into the lifting cylinder through openings in the base of the lifting cylinder 12, 12". The contacting lines can be routed to a control unit in the implant.

Claims

Claims 1. Linear actuator (1, 1') with variable stiffness, comprising: a first and second base element (2, 3; 2', 3') which are connected so as to be translationally movable relative to one another with respect to a longitudinal direction (L); a volume-incompressible, elastic deformation element (11, 11') which is arranged in a lifting cylinder (12, 12') coupled to the base elements (2, 3; 2', 3'); an adjusting device (17, 17', 18, 18') for the controlled, variable release of a deformation space (VR) in the deformation element (11, 11') in the interior of the lifting cylinder (12, 12'); a first actuating actuator (8, 8') which has a thermoelastic actuator element and which is connected to the first and second base elements (2, 3;2', 3') is mechanically coupled and designed to, in particular independently of the adjustment by the adjustment device (17, 17', 18, 18'), bring about a controlled adjustment force counteracting an elastic deformation of the deformation element (11, 11'), so that the first and the second base element (2, 3; 2', 3') are moved relative to one another in an adjustment direction.; 2. Linear actuator (1, 1') according to claim 1, wherein the adjusting device (17, 17', 18, 18') has a second actuating actuator (17, 17') with in particular a thermoelastic actuator element in order to move an adjusting element (14, 14') in the interior of the deformation space (VR) in a first direction of movement and / or wherein the adjusting device (17, 17', 18, 18') has a third actuating actuator (18, 18') with in particular a thermoelastic actuator element in order to move the adjusting element (14, 14') in the interior of the deformation space (VR) in a second direction of movement opposite to the first direction of movement.

3. Linear actuator (1, 1') according to claim 2, wherein the adjusting element (14, 14') comprises a translationally movable adjusting pin (14, 14') which runs through the interior of the lifting cylinder (12, 12') and has a section with a Cross-sectional change with a monotonically increasing cross-sectional area in the axial direction without undercut, so that an axial displacement of the adjusting pin (14, 14') in the deformation element (11, 11*) is permitted in order to enlarge or reduce the deformation space (VR) in the interior of the lifting cylinder (12, 12').

4. Linear actuator (1, 1') according to claim 3, wherein the cross section in the section corresponds to a circular, elliptical, triangular, quadrangular or polygonal, or oval cross-sectional area.

5. Linear actuator (1, 1') according to one of claims 1 to 4, wherein the first base element (2') and the second base element (3') are designed as hollow profiles which are guided telescopically into one another, wherein a first holding element (5') is arranged in the interior of the first base element (2') and a second holding element (6') is arranged in the interior of the second base element (3'), wherein a plunger (7') is firmly connected to the second holding element and is movably guided by the first holding element (5'), wherein the first actuating actuator (8') is arranged on the side of the first holding element (5') opposite the second holding element (6') in order to effect an adjusting movement in a first of the adjusting directions by exerting a tensile force between the corresponding end of the plunger (7') and the first holding element (5').

6. Linear actuator (1, T) according to claim 5, wherein the lifting cylinder (12') is fixedly arranged in or on the first holding element (5') and has a piston (13') which is fixedly connected to the second holding element (6') and is preferably movable in the direction of the adjustment direction.

7. Linear actuator (1, T) according to one of claims 1 to 6, wherein a deformation sensor is embedded in the elastic deformation element (11, 11*), which is designed to detect a deformation of the elastic deformation element (11, 11'), wherein the deformation sensor is provided in particular as a capacitive, resistive or piezoresistive sensor or as an electroactive polymer sensor.

8. Linear actuator (1, T) according to one of claims 1 to 7, wherein an electroactive element is embedded in the elastic deformation element (11, 11'), which is designed to sensor a deformation of the elastic deformation element and / or to adapt the elasticity of the elastic deformation element (11, 11') and / or the restoring force of the deformation element (11, 11') to the holding elements (5, 5', 6, 6') by electrical control with an actuator voltage as an electroactive actuator.

9. Linear actuator system with a linear actuator (1, 1') according to one of claims 1 to 8 and with a control unit which is designed to first control the adjusting device in order to set a predetermined linear position and a predetermined stiffness, so that a deformation space (VR) is released and simultaneously or subsequently the first actuating actuator (8, 8') is controlled in order to adjust the first and the second base element (2, 3; 2', 3') relative to one another in accordance with the predetermined linear position.

10. Linear actuator system according to claim 9, wherein the control unit is designed to deactivate the first actuator (8, 8') to reset the adjustment between the first and the second base element (2, 3; 2', 3') and to control the adjustment device to increase the restoring force so that the deformation space is reduced.

11. Implant for fixing bone fragments (K1, K2) with a linear actuator (1, T) according to one of claims 1 to 7, wherein the first and the second base element (2, 3; 2', 3') can each be fastened to a respective bone fragment with a fastening device.

12. A method for operating a linear actuator (1, 1*) according to one of claims 1 to 7, comprising the steps: Controlling the adjustment device (17, 17', 18, 18') so that the deformation space (VR) is released, simultaneously or subsequently, controlling the first actuating actuator (8, 8') in order to adjust the first and second base elements (2, 3; 2', 3') relative to one another in accordance with a predetermined linear position, Controlling the adjustment device (17, 17', 18, 18') to set a predetermined stiffness.

13. Use of the linear actuator (1, 1') according to one of claims 1 to 7 in an implant for fixing bone fragments (K1, K2).

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