Planar torsion springs for series elastic actuators.

Abstract

Planar torsion spring for a series elastic actuator. The invention relates in particular to a planar torsion spring for a series elastic actuator, which has an inner clamping point, at least two outer clamping points and at least two spring arms, each of which elastically connects the inner clamping point to one of the outer clamping points, the spring arms having a similar contour and extending symmetrically, preferably point-symmetrically, with respect to the inner clamping point. According to the invention, the spring arms of the planar torsion spring have an S-shaped profile. Furthermore, the invention relates to a series elastic actuator with such a planar torsion spring and to a method for manufacturing the planar torsion spring.

Description

[Technical field]

[0001] The invention relates to a planar torsion spring, in particular for a series elastic actuator, which has an inner clamping point, at least two outer clamping points and at least two spring arms, each of which resiliently connects the inner clamping point to one of the outer clamping points, the spring arms having a similar contour and extending symmetrically, preferably point-symmetrically, with respect to the inner clamping point, and also to a series elastic actuator comprising such a planar torsion spring and to a method for manufacturing the planar torsion spring.

[0002] Series elastic actuators with planar torsion springs are used, for example, in robotic applications to elastically connect a robot arm to a base body and drive the robot arm. Robotic applications in industry accomplish a variety of tasks, including the movement and positioning of different objects. A typical industrial robot for automated production may have one or more arms equipped with grippers to pick up, transport and position objects. A key mechanical requirement of such industrial robots is to generate large and precise forces and torques while keeping the control stable. These torques and forces are generated by electromechanical actuators, usually electric motors, which apply high torques in response to electrical control signals that are transferred directly to rotary motion or to linear forces via linear conversion elements.

[0003] Series elastic actuators with planar torsion springs are also used, for example, in exoskeletons and walking robots. There are many other applications in which the torque generated by the actuator is transmitted. Planar torsion springs provide the elasticity of the transmission device, which makes it possible to prevent or at least greatly reduce strong shocks to the actuator mechanism and to the environment.

[0004] Simple rigid actuators can generate large forces in robot joints from small joint displacements, allowing a high range of force transmission and accurate position control. However, the rigidity of conventional robot joints makes it difficult to control the forces. Due to the importance of force control in robot joint drives, high stiffness is avoided to achieve better force control. A widely used option for this is to connect an elastic element in series with the actuator. Such series elastic actuators have improved force control compared to rigid actuators, since a series-connected elastic element requires a larger deformation to generate the required force. Moreover, such elastic elements allow the detection of the applied force via the position, rather than directly identifying the force via the control of the actuator, improving the precision and stability of the drive and reducing noise at the same time.

[0005] When developing elastic elements for series elastic actuators of robot joints, the boundary conditions of the installation space of the constraints as well as the requirement to withstand a large number of movements and large load torques without slippage or wobble, as well as economical production, are required. The dimensions of the actuators and the robot joints and the size restrictions due to the working environment, together with the mechanical requirements, must be considered as major challenges. Technically, essentially flat bending springs are known for this purpose, which are often also called torsion springs and are used as elastic elements in the small available installation space of robot joint drives. For example, DE 10 2009 056 671 A1 shows such a robot joint with a flat torsion spring. The spring brackets disclosed therein are arranged in one rotational direction. For each spring bracket, a first section is attached to the inner mounting segment at a circumferential position different from the position at which the second section is attached to the outer mounting segment. In addition to conventional spring steels and especially titanium for springs, composite materials with carbon fiber and glass fiber are also used for such flat torsion springs. In addition to simple spring designs, specially optimized springs with special shapes and holes in the less stressed parts are also used. Optimized designs of planar torsion springs are known, for example, from EP 3 121 472 A1. Depending on the material used, they are manufactured by bending, milling or by erosion and grinding. The costs of these manufacturing processes can be very high, especially for materials with high stiffness, especially when subtractive manufacturing methods are used. Additive manufacturing of spring steel and spring titanium using sintering or printing processes requires expensive post-treatment of the surface for smoothing and hardening. In addition, additively manufactured torsion springs have problems, especially at the connection points for force transmission, due for example to twisting of pins or seizing of mounts, which gives rise to additional hysteresis in the torque and changes in the torsion angle curve.

[0006] SUMMARY OF THE PRESENT EMBODIMENTS It is therefore an object of the present invention to provide an improved planar torsion spring for a series elastic actuator that provides repeatable deflection with increased strength at low cost.

[0007] This problem is solved in typical planar torsion springs in that the spring arms of the planar torsion spring have an S-shaped profile.

[0008] The special integral shape of the planar torsion spring according to the invention includes at least two S-shaped spring arms with identical shape and a symmetrical arrangement with respect to the inner clamping point, i.e. an arrangement of at least two S-shaped spring arms offset by 180° (or 360° / number of S-shaped spring arms) at the inner clamping point, or a symmetrical, preferably point-symmetrical design of the planar torsion spring with respect to the center of the inner clamping point. The S-shaped spring arms elastically connect one of the outer clamping points with the inner clamping point, and the S-shaped spring arms have a similar contour. The S-shaped spring arms produce optimal spring characteristics for use in a series elastic actuator. By shaping the planar torsion spring according to the invention, the strength and spring elasticity can be adjusted to the values ​​required for use in a series elastic actuator. The essentially linear spring characteristic curve and the resulting reproducible deflection of the planar torsion spring are very high. This allows the torque of the series elastic actuator to be measured based on the deflection of the planar torsion spring. Due to the S-shape of the spring arm, the mechanical stresses arising during the transmission of force to the torsion spring are evenly and optimally distributed over the entire spring arm, so that no parts or sections of the spring arm are undesirably overloaded or underloaded.

[0009] In a preferred embodiment, the torsion spring has two spring arms, even if the number of spring arms can be varied. The more the number of spring arms, the greater the spring constant and therefore the less the torsional flexibility of the planar torsion spring. Since the spring arms should be accommodated in the smallest possible volume, two spring arms have proven to be the optimal balance between spring elasticity and required installation space. The symmetrical S-shaped structure of the planar torsion spring, with two S-shaped spring arms running anti-parallel to the inner fastening point, allows the maximum spring length in the smallest space, with an essentially linear spring characteristic, without the torsion spring colliding with itself.

[0010] In a useful embodiment, a torsion spring centerline is provided that runs as a straight line through the outer clamping point and the center point of the torsion spring, and each S-shaped spring arm has an inner arc adjacent to the inner clamping point and an outer arc adjacent to the outer clamping point, and the length of the distance of the inner arc from the outer contour of the torsion spring centerline and the length of the distance of the outer arc from the outer contour of the torsion spring centerline are less than 20%, preferably less than 10%, more preferably the length of the two distances are equal. In addition to optimizing the spring characteristics, the special design of the S-shaped spring arm provided at the inner clamping point can minimize the radial movement of the spring that occurs when a tangential force is introduced, preferably less than 20% of the introduced tangential movement. The symmetrical design of the S-shaped spring arm creates a particularly linear characteristic of the spring stiffness over a large torsional range of the planar torsion spring. Due to the S-shape, the mechanical stresses are evenly distributed in the spring arm, and stress peaks or stress changes in the spring arm are almost nonexistent or at least reduced.

[0011] In a preferred embodiment, the S-shaped spring arm has a first spring section with a gradually decreasing width starting from a wide connection adjacent to the inner fastening point, followed by a second spring section with an increasing width, followed by a third spring section with a gradually decreasing width, the third spring section being adjacent to the outer fastening point. This special shape of the S-shaped spring arm located at the inner fastening point allows the torsion spring to be small in diameter and thin at the maximum spring length, in addition to the linear spring characteristics due to the spring arm extending symmetrically to the inner fastening point, so that the spring constant and maximum continuous alternating torque of the torsion spring can be optimized. The optimal approach here is to generate a relatively high degree of softness with a small diameter torsion spring.

[0012] In a preferred manner, the extension of the center line of the wide connection part of the S-shaped spring arm can have an angle in the range of 35° to 55° at the intersection with the torsion spring center line passing through the outer clamping point. Furthermore, the extension of the center line of the third tapered spring section adjacent to the outer clamping point can have an angle in the range of 80° to 100° at the intersection with the torsion spring center line passing through the outer clamping point. Here again the center line is the bisector of the contour line of the S-shaped spring arm with the same distance to the outer contour line. According to the angular course of the center line, the S-shaped spring arm has a closed S-shape with an absolute angular change of the center line between the inner and outer clamping points of more than 360°, preferably at least 370°, in particular at least 385°. This arrangement and this angle of the two S-shaped spring arms improves the geometry of the spring and leads to improved spring constants and spring characteristics.

[0013] In a further preferred embodiment, the spring sections having a similar radial distance to the spring element center point as the outer clamping point have a smaller width, and preferably the area with the smallest width is at the smallest radial distance to the outer clamping point. Optimal spring stiffness can be achieved by widening the spring sections that are a little further radially away from the center of the planar torsion spring and tapering the width of the spring sections that are a lot further radially away from the center of the planar torsion spring. Spring sections that are a larger radial distance from the center of the planar torsion spring than the outer clamping point should have a wider spring section to achieve optimal spring stiffness.

[0014] In a special embodiment, the planar torsion spring is manufactured using an injection molding process, in which a liquid material is injected under high pressure into a sealable mold, producing molded parts with high precision and reproducibility. Due to the very smooth surface of the injection mold, a very good surface quality of the injection molded planar torsion spring can be achieved, so that additional polishing of the surface can be omitted, thus simultaneously saving work time and production costs.

[0015] In another preferred embodiment, the planar torsion spring is made from an amorphous metal. This allows the planar torsion spring to be manufactured in one piece. Such amorphous metals, also called metallic glasses, have an abnormal amorphous atomic arrangement and are generally harder, more corrosion resistant, and stronger than normal metals, but their deformability is usually significantly lower. Therefore, the torsion spring using the amorphous metal can produce an optimally designed torsion spring that is 10 times or more lighter and smaller than conventional titanium spring alloys (Titanium Grade 5) in spring stiffness and bending energy density.

[0016] According to the technological properties of amorphous metals, such integral planar torsion springs for series elastic actuators have very small hysteresis in the torque or torsion diagrams, thus allowing more accurate force measurement and controllability of the series elastic actuator or the associated drive system. Furthermore, the lower weight and smaller dimensions of planar torsion springs made of amorphous metals allow lower inertia, smaller vibrations and smaller dimensions of the entire system. Metallic glasses or amorphous metals have so far only been produced in thin layers or thin ribbons, for example by centrifugal melting, since the change from the natural crystallization to the amorphous crystallization structure requires rapid cooling of the initial melt in order to freeze the atomic mobility before the atoms can assume their regular crystalline arrangement. In most metal alloys, this requires very high cooling rates. When cooling a metal alloy, a large amount of heat needs to be transported from the interior of the material to the outer surface, so that only a certain material thickness can be achieved depending on the required cooling rate and thermal conductivity, regardless of the ambient temperature. The technological use of amorphous metals has therefore hitherto been limited to products that can be produced using simple mechanical processing steps from thin strip and wire that can be produced using centrifugal casting.

[0017] The injection molding process for amorphous metals is similar to that for plastics: after the injection process is complete, the injection mold can be cooled very quickly using integrated cooling tubes and systems, and the high thermal conductivity of the material used in the mold, so the achievable thickness of a torsion spring made from an amorphous metal is essentially limited by the cooling rate of the material required to form the amorphous atomic structure, and the thermal conductivity of the material to remove heat from the interior of the spring within the injection mold.

[0018] In practical variants, the S-shaped spring arms have different thicknesses between the inner and outer clamping points. In particular, in injection molding processes, the spring sections can have material thicknesses that vary not only in width but also in thickness. This means that the spring properties of the planar torsion springs can be varied in three dimensions of the material thickness, opening up further possibilities in the design of planar torsion springs and the achievement of technical spring properties.

[0019] In another preferred embodiment, the planar torsion spring has a constant thickness. A constant thickness facilitates better and more uniform cooling of the melt, especially in the injection molding process, and thus allows the formation of an amorphous atomic arrangement in the planar torsion spring. The term "constant thickness" refers to the undisturbed surface of the S-shaped spring arm, measured in the region of the centerline of the spring arm, i.e., the contour bisector of the spring arm equidistant to the outer edge, without taking into account the reduction in thickness at the edge of the spring arm due to injection molding. Preferably, the inner clamping point can also have the same thickness as the S-shaped spring arm.

[0020] In a preferred embodiment, the upper and / or lower sides of the S-shaped spring arms have recesses. The recesses allow material to be saved. This reduces the weight of the planar torsion spring and therefore the inertia. Furthermore, saving material also reduces material costs. Furthermore, the planar torsion spring can be hardened better and faster when produced as a cast part, especially a core, or can achieve a sufficiently high cooling rate in an injection molded part.

[0021] In a further preferred embodiment, one side of the S-shaped spring arm has a bulge, which allows the S-shaped spring arm to be easily removed from the injection mold.

[0022] The internal fastening point can have a cornered receptacle, preferably a rectangular mount, for a secure torque input. One or more additional holes or receptacles for anti-torsion fixing of the planar torsion spring via the internal fastening point can thus be omitted. The rectangular mount can have rounded corners. The rectangular mount allows for a safe introduction of the transmitted torques and forces and therefore allows for low hysteresis in the torque or torsion diagram.

[0023] For a safe installation and problem-free transmission of forces, the external fastening point is preferably rotatably pivotable by means of a needle bearing. Due to the rotatable bearing of the external fastening point, the forces are optimally transmitted to the planar torsion spring without causing significant friction losses in the circular mounting eyelet. Due to its long and flat design, needle roller bearings are particularly suitable for absorbing large bearing forces in a small installation space. The circular mounting eyelet can have a larger thickness than an S-shaped spring arm, one end face of the mounting eyelet being flush with one surface of the spring arm and the other end face protruding from the second surface of the spring arm. The circular mounting eyelet of the external fastening point allows the coupling with the driven component via a shaft mounted on the needle bearing. In addition to decoupling the spring constant from the connection with the driven component, this leads to low friction values, low heating and low influence on the hysteresis in the torque or torsion diagram, so that an overall better measurement precision can be achieved.

[0024] The expedient design provides that the planar torsion springs are produced essentially without post-processing. In this context, "essentially without post-processing" means that no surface contouring and extensive polishing is required, saving additional work steps, time and costs. However, during the production of planar torsion springs in the injection molding process, sprue residues, mold seams and sharp edges may occur, which require corresponding smoothing, deburring and rounding. Such work steps should not be included in the term "essentially without post-processing" but should be assigned to the injection molding process. When the planar torsion springs are produced by the injection molding process, even complex shapes do not require erosion or milling and only few additional injection molding-related work steps, so that they can be produced in series at low cost.

[0025] The present invention further relates to a series elastic actuator, preferably an actuator for a robot joint, comprising an electric drive and a planar torsion spring according to the above-mentioned embodiment. The series elastic actuator according to the present invention also allows the actuator itself to be miniaturized, and the dimensions of the planar torsion spring to be reduced, thereby realizing low inertia, low vibration, high precision force measurement, and improved controllability of the system. At the same time, the overall manufacturing costs can be significantly reduced by eliminating the post-processing steps in the production of the planar torsion spring made from amorphous metal and by reducing the dimensions of the actuator itself. The electric drive can consist of an electric motor, preferably an ironless electric motor or an iron-core internal rotor motor, and a gearbox, preferably a planetary gearbox, an eccentric gearbox or a shaft gearbox.

[0026] In a preferred manner, the inner clamping point of the planar torsion spring can be fixedly connected to the electric drive, and the outer clamping point of the planar torsion spring can be rotatably connected to the actuating element. In particular when used as an actuator for a robot joint, it is advantageous to design the spring end so that it can rotate relative to the driven component via the outer clamping point, in order to allow the lowest possible hysteresis in the torque or torsion diagram and thus high measurement accuracy and precise control.

[0027] Furthermore, the present invention relates to a method for manufacturing a planar torsion spring, in particular a method for manufacturing a series elastic actuator according to one of the above-mentioned embodiments, comprising the steps of providing an injection mold for the planar torsion spring, injecting a material suitable for injection molding, preferably an amorphous metal alloy or a plastic material, into the injection mold, cooling the injection mold and the injection molded planar torsion spring, removing the planar torsion spring made of amorphous metal from the injection mold, and, if necessary, machining the planar torsion spring to remove sprue residues and injection molded seams, remove burrs, and round edges. Furthermore, the surface of the planar torsion spring that undergoes the demolding angle can be optionally made straight by machining the planar torsion spring, so that the surface can function as a seat for a rectangular mount or bearing. The demolding angle helps to simplify the demolding of the planar torsion spring from the injection mold. The demolding angle prevents the planar torsion spring from tilting during demolding from the injection mold. In the areas that serve as bearings or rectangular seats, the surfaces can also be made sufficiently straight after demolding by precision machining, precision milling or reaming so that they can serve as a seat for a rectangular mount or bearing, for example as a seat for a circular mounting eyelet in the outer fastening section.

[0028] Such a manufacturing process provides planar torsion springs with small dimensions, light weight, high spring constants and high stiffness. The use of the planar torsion springs manufactured in this way in series elastic actuators allows lower vibrations, more accurate force measurements and improved controllability of the actuators while maintaining small dimensions and lower weight. The amorphous metal alloy is preferably composed of at least four elements with radically different atomic sizes. The resulting crystal structure is a well-formulated eutectic that allows cooling times of a few seconds without inducing recrystallization processes, making it possible to produce planar torsion springs from amorphous metals in an injection molding process.

[0029] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. [Brief description of the drawings]

[0030] [Figure 1] FIG. 1 is a block diagram of a series elastic actuator according to the present invention. [Diagram 2] FIG. 2 is a plan view of a planar torsion spring according to the present invention for the series elastic actuator of FIG. [Diagram 3] FIG. 3 is a perspective view of the planar torsion spring of FIG. [Figure 4] FIG. 4 is a side perspective view of the planar torsion spring of FIG. [Diagram 5] FIG. 5 is a top view of the planar torsion spring of FIG. 2 with the centerline of the torsion spring and the centerline of the spring arm. [Figure 6] FIG. 6 is a perspective view of a planar torsion spring with varying thickness of the spring arms. [Figure 7a] FIG. 7a is a top view of another embodiment of a planar torsion spring. [Figure 7b] FIG. 7b is a top view of another embodiment of a planar torsion spring. [Figure 8a] FIG. 8a is a cross-sectional view of an S-shaped spring arm. [Figure 8b] FIG. 8b is a cross-sectional view of an S-shaped spring arm. [Figure 9] FIG. 9 is a perspective view of a portion of an S-shaped spring arm of varying cross section. [Figure 10] FIG. 10 is a further cross-sectional view and a perspective view of a portion of an S-shaped spring arm. [Figure 11] FIG. 11 is a further cross-sectional view of an S-shaped spring arm.

[0031] Detailed Description of the Invention

[0032] The block diagram in FIG. 1 shows the essential components of a series-elastic actuator 1, which has a torque generating unit, usually an electric motor 2, connected via a gearbox 3 and a spring element 4 to an actuating element 5, e.g. a robotic arm, an exoskeleton or a walking robot. The gearbox 3 allows the use of a smaller electric motor 2, which operates at a higher speed. The gearbox 3 may be an integral part of the electric motor 2 or may be separate from the electric motor 2. The electric drive may comprise the electric motor 2 and the gearbox 3. The gearbox 3 often reduces the speed of the electric motor 3, which increases the torque of the electric drive in the same ratio. As the electric motor 2, an ironless or slotless electric motor is often used due to its high dynamic properties. For example, a planetary gear, an eccentric gear or a shaft gear can be used as the gearbox 3. In this series-elastic actuator 1, the spring element 4 is coupled in series with the output of the gearbox 3 or, if a separate gearbox 3 is not used, is directly connected to the electric motor 2. The element 5 actuated by the series-elastic actuator 1 is connected in series with the second side of the spring element 4. The spring element 4 introduces series elasticity at the interface between the series elastic actuator 1 and the actuated element 5, allowing precise control of the force applied to the actuated element 5. Such series elastic actuators 1 are used in large numbers in industrial robots as robot joint drives, where large forces and torques must be transmitted accurately and durably to the actuated element 5, i.e. the robot arm.

[0033] When used in industrial robots, the axial distance between the series elastic actuator 1 and the driven element 5, and thus also the maximum length of the series elastic actuator 1, can be narrowly limited, which in particular affects the maximum length of the spring elements 4 used. Similarly, the radial size of the series elastic actuator 1 can be severely limited, thereby limiting the maximum diameter of the spring elements 4. It is therefore important to accommodate the required stiffness of the spring element 4 in a small installation space and at the same time ensure a sufficiently reliable coupling of the spring element 4 with the gearbox 3 and the actuating element 5 to avoid slippage and thus hysteresis in the torque or torsion angle curves.

[0034] In the series elastic actuator 1 shown in FIG. 2, a planar torsion spring 6 is used as the spring element 4, the spring arms 9 having an S-shape. As can be seen in FIG. 2, the planar torsion spring 6 comprises an inner fastening point 7 with a rectangular mount 8 with rounded corners to receive a rectangular profile for coupling to the gearbox 3, two S-shaped spring arms 9 each having an identical profile and extending point-symmetrically with a 180° offset to the inner fastening point 7, and two outer fastening points 10 connected to the outer ends of the S-shaped spring arms 9. The rectangular mounts 8 of the inner fastening section 7 allow a slip-free coupling of the planar torsion spring 6 with the associated torque drive by the electric motor 2 or the gearbox 3. The outer fastening points 10 are designed as circular mounting eyelets 11. Such mounting eyelets 11 allow a rotatable mounting, for example by arranging a mounting shaft with needle bearings (not shown) for a low-friction coupling with the actuating element 5 to minimize heating due to friction and prevent hysteresis effects improving the accuracy of the torque measurement.

[0035] The spring element 4 of the actuator 1 can be two planar torsion springs 6 that preferably extend in opposite directions. This can prevent the deflection of the spring element 4 from becoming asymmetrical depending on the rotation direction of the actuator 1. In addition, a preload of the planar torsion springs 6 can be generated between the outer fastening points of the adjacent planar torsion springs 6 and the rectangular mount 8.

[0036] The perspective views of the planar torsion spring 6 in Fig. 3 and Fig. 4 show that the thickness of the two S-shaped spring arms 9 and the inner clamping section 7 of the planar torsion spring 6 is constant, and the planar torsion spring 6 is manufactured from an amorphous metal, for example, by an injection molding process. At the outer edges of the S-shaped spring arms 9, the inner clamping point 7, and the outer clamping point 10, a circumferential mold seam 12 is visible, which is created by the two-part mold used in the injection molding process. In contrast to the inner clamping point 7 and the S-shaped spring arms 9, the outer clamping point 10 has a much larger thickness, and the mounting eyelet 11 protrudes only on the upper side of the planar torsion spring 6. As can be seen in the perspective side view of Fig. 4, the lower mounting eyelet 11 of the planar torsion spring 6 is in the same plane as the S-shaped spring arms 9 and the inner mounting section 7. This makes the two lower or rear sides of the two planar torsion springs 6 flat against each other.

[0037] The inner and outer fastening points 10 and 7 are more centrally located or point-shaped with respect to other parts of the S-shaped spring arm, which are called sections, but are referred to as inner and outer fastening points because the inner and outer fastening points 10 and 7 can also be understood as structural sections.

[0038] As is evident from figures 2 and 3, the planar torsion spring 6 can have two S-shaped spring arms 9 formed with a special profile. The two S-shaped spring arms 9 of the planar torsion spring 6 according to the invention extend in opposite directions offset by 180° from the inner clamping point 7. In this case, the S-shaped spring arm 9 extends from a wide connection section 13 at the inner clamping point 7 into a first spring section 14, which narrows in width and extends into the region of a first inner arc 15 of the S-shaped spring arm 9. This first spring section 14 of gradually decreasing width is followed in the region of the inner arc 15 by a second spring section 16, which widens in width and extends into a second outer arc 17 of the S-shaped spring arm 9, where it merges into a third spring section 18 of gradually decreasing width. The third spring section 18 extends to a connection section 19 to the mounting eyelet 11 .

[0039] In the top view shown in FIG. 5 of the planar torsion spring 6 shown in FIG. 2, in addition to the curved contour of the S-shaped spring arm 9, its center line m is also drawn, as well as the contour bisector on the surface of the S-shaped spring arm 9, which continues equidistantly from the outer contour of the S-shaped spring arm 9 between the inner clamping point 7 and the outer clamping point 10. Here too, the special contour of the S-shaped spring arm 9 is clearly visible, which extends from the inner clamping point 7 and the adjacent wide connection section 13, the first tapered spring section 14, the inner arc 15, the second extended spring section 16, the outer arc 17 and the third tapered spring section 18, as well as via the connection section 19 to the outer clamping point 10. The extension of the center line m of the wide connection section 13 at the intersection with the torsion spring center line DM extending through the center point M of the inner fastening point 7 of the mounting eyelet 11 and the outer fastening point 10 has an angle α in the range of 35° to 55° with respect to the torsion spring center line DM. Also, the end of the S-shaped spring arm 9 in the third spring section 18 has an angle β between the extension of the center line m and the torsion spring center line DM in the range of 80° to 100°. Thus, the profile of the S-shaped spring arm 9 forms a perfect S-shape, and the center line m has an absolute angle change of more than 360° over its course on the S-shaped spring arm 9.

[0040] In the exemplary form of a planar torsion spring 6 according to the present invention shown in FIG. 5, the S-shaped spring arms 9 have an absolute angular change of over 400°.

[0041] The torsion spring centerline DM is a straight line that runs through the center point M and at least one external fastening point 10. Depending on the arrangement of the S-shaped spring arms, the torsion spring centerline DM may run through the center point M and two external fastening points 10.

[0042] In addition to being completely S-shaped, the S-shaped spring arm 9 of the planar torsion spring 6 according to the invention also has an unusual progression of the width of the S-shaped spring arm 9 between the wide connection section 13 and the tapered third spring section 18. This profile of the S-shaped spring arm 9, which can be characterized as thick-thin-thick-thin in relation to the distance from the torsion spring centerline DM to the inner arc 15 and the outer arc 17, allows for a high linearity of the planar torsion spring 6 according to the invention. Here, it is preferred that the length of the distance a of the outer contour of the inner arc 15 from the torsion spring centerline DM is comparable to the length of the distance b of the outer contour of the outer arc 17 from the torsion spring centerline DM. As can be seen in FIG. 5, in the embodiment of the planar torsion spring 6 shown here, the distance b of the outer contour of the outer arc 17 to the torsion spring centerline DM is less than 20%, preferably less than 10%, than the distance a of the outer contour of the inner arc 15 to the torsion spring centerline DM.

[0043] From the center point M of the planar torsion spring 6 to the circular mounting eyelet 11, the S-shaped spring arm 9 consists of several sections. A wide connecting section 13 is followed by a first spring section 14, an inner arc 15, a second spring section 16, an outer arc 17 and finally a third spring section 18. A circle can be drawn around the planar torsion spring 6, centered on the center point M of the planar torsion spring 6 and having a radius corresponding to the distance r between the center point M and the outer fastening point 10. The sections of the S-shaped spring arm 9 close to the circle are narrower than those further away from the circle. This applies to the area of ​​the S-shaped spring arm section that is inside the circle, as well as to the area of ​​the S-shaped spring arm section that is outside the circle.

[0044] FIG. 6 shows a planar torsion spring 6 with an S-shaped spring arm 9, which has a different thickness between the inner clamping point 7 and the outer clamping point 10. Due to the varying thickness of the different sections of the S-shaped spring arm 6, the spring elastic properties of the S-shaped spring arm 6 can be adjusted not only by changing the width, but also by changing the thickness. This allows additional options in the design of the planar torsion spring 6. In particular in the case of parts produced by injection molding, it is possible to define in three dimensions the shape of the injection mold and thus the shape of the component produced therein. If two planar torsion springs 6 are arranged in the actuator 1 as described above, the varying thickness should be on the side facing away from the second planar torsion spring 6. For example, it is also possible to provide spacer cams for the mutual interference of the two planar torsion springs 6.

[0045] Figures 7a and 7b show further embodiments of a planar torsion spring 6. The planar torsion spring 6 of Figure 7a has three S-shaped spring arms 9, and the planar torsion spring 6 of Figure 7b has four S-shaped spring arms 9. Planar torsion springs 6 with more than four S-shaped spring arms 9 are also possible. It should be noted, however, that the higher the number of S-shaped spring arms 9, the higher the risk of the S-shaped spring arms 9 colliding with each other when twisting the planar torsion spring 6.

[0046] 8a and 8b show different cross-sectional views of the S-shaped spring arm 9 of the planar torsion spring 6. The S-shaped spring arm 9 has an upper side 20 and a lower side 21. The other two surfaces connecting the upper side and the lower side 21 form the side surfaces 22. As shown in FIG. 6, the width and thickness, and therefore the cross-section, of the S-shaped spring arm 9 can vary. In FIG. 8a, the corners of the approximately rectangular cross-section of the S-shaped spring arm 9 are slightly rounded. In FIG. 8b, the corners of the approximately rectangular cross-section of the S-shaped spring arm 9 are significantly rounded. Rounding the corners can, for example, reduce fatigue failure of the S-shaped spring arm 9.

[0047] FIG. 9 shows perspective views of a portion of an S-shaped spring arm 9 having various cross sections. The cross sections taper toward the lower right end, and therefore toward the proximal end of the perspective view of FIG. 9, and increase toward the upper right or end of the perspective view of the section of the S-shaped spring arm 9. Additional auxiliary lines are drawn in the center of the outer surface of the S-shaped spring arm 9 to indicate the expansion of the cross sections. The auxiliary lines are drawn on the upper side 20 and the side 22 of the S-shaped spring arm 9. The corners of the generally rectangular cross sections of the S-shaped spring arm 9 are rounded to various degrees depending on the cross sections. The rounding of the corners varies depending on the bending load requirements.

[0048] FIG. 10 shows a perspective view of a portion of a section of the S-shaped spring arm 9 with yet another cross-section. The cross-section of the S-shaped spring arm 9 is optimized for the spring elastic load and the weight of the S-shaped spring arm 9. The upper side 20 of the S-shaped spring arm 9 and the lower side 21 of the S-shaped spring arm 9 are concave or bent towards the center 23 of the cross-sectional area of ​​the S-shaped spring arm 9. This results in the upper side 20 having an M-shaped cross-section, while the lower side is slightly reminiscent of a W-shape. The height of the cross-section is greatest at the side 22 of the S-shaped spring arm 9, while the height of the cross-section, and therefore the thickness of the S-shaped spring arm 9, is smallest towards the center. The cross-section may vaguely resemble a double-T beam. The cross section prevents premature fatigue and prevents fatigue failure of the S-shaped spring arm 9. Also, by reducing the height of the cross section and the thickness of the S-shaped spring arm 9 toward the center, material is saved and the weight of the planar torsion spring 9 can be reduced.

[0049] FIG. 11 shows the cross-section of FIG. 10 with a pronounced bulge on the side 22 of the S-shaped spring arm 9. The bulge on the side 22 slightly changes the width of the S-shaped spring arm 9. The bulge occurs due to the release angle that the surface of the injection molded component must make with respect to the release direction. The release angle, which can be between 0.1° and 10°, allows the injection molded component to be removed from the injection mold more easily and without tilting. In an injection mold with similar half shells with the same release angle, the maximum bulge on the side 22 is in the center of the side 22. The surface with the release angle can be straightened by finishing. As shown in FIG. 10, the upper side 20 and the lower side 21 have a depression that continues to a rounded shape. The depression is relatively flat with a slight slope in the center. The gradient increases towards the side 22.

[0050] In contrast to conventional products made of amorphous metals, which are first produced as thin layers or strips and then further processed, the planar torsion spring 6 according to the invention is produced using a special injection molding process that allows high precision and reproducibility. Such an injection molding process allows the complex shape of the planar torsion spring 6 according to the invention to be realized and can be produced cost-effectively in series production without the need for machining or polishing of the surface contours. Depending on the design of the injection mold, such an injection molding process only requires deburring and rounding of sprue residues, mold seams and edges. In order to carry out such an injection molding process, it is necessary to provide a suitable injection mold for the planar torsion spring 6, which not only has a cavity for molding the planar torsion spring 6, but is made of a material with good thermal conductivity and has corresponding cooling channels and cooling devices to achieve the high cooling rates necessary for the amorphous solidification of the alloy. After the amorphous metal alloy is injected into the injection mold heated to the temperature of the metal alloy, the next step is to quench the injection mold into which the planar torsion spring 6 was injected before removing the planar torsion spring 6 made of solidified amorphous metal from the injection mold. Generally, an injection mold consists of at least two halves that are pressed together for the injection molding process, but allows for easy removal of the solidified planar torsion spring 6. The interface between the two halves of the injection molding mold can usually be identified by a mold seam 12 on the injection molded planar torsion spring 6.

[0051] Instead of amorphous metal, the planar torsion spring 6 can also be made of another material suitable for injection moulding, such as plastic or ceramic. Plastic has the advantage that it is light and can be procured and processed at low cost. Advantageously, the injection moulded material has good spring elasticity properties, whether by a base material with high elasticity or by the addition of suitable components such as glass or carbon fibres. [Explanation of symbols]

[0052] 1...Actuator, 2...electric motor, 3…Gearbox, 4…Spring element, 5...actuated element, 6... Planar torsion spring, 7…Inner tightening point, 8...Rectangular mount, 9...S-shaped spring arm, 10…External fastening point, 11...Circular mounting eyelet, 12...Molded seam, 13…Wide range of connection sections, 14…first spring section, 15…Inner bend, 16…second spring section, 17...outward bending part, 18…Third spring section, 19…Connection section, 20...Upper part of S-shaped spring arm, 21...lower part of S-shaped spring arm, 22...Side of S-shaped spring arm, 23...Corner portion of cross-sectional area; a…distance, b...distance, m...center line, DM: Torsion spring center line, M...center line, α...Angle, β...angle, r...Radius.

Claims

1. A planar torsion spring (6), in particular for a series elastic actuator (1), comprising: A planar torsion spring (6) having an internal fastening point (7), at least two external fastening points (10) and at least two spring arms (9), each of which resiliently connects said internal fastening point (7) to one of said external fastening points (10), said spring arms (9) having a similar contour and extending symmetrically, preferably point-symmetrically, with respect to said internal fastening point (7), characterized in that said spring arms (9) have an S-shaped profile.

2. A planar torsion spring (6) according to claim 1, characterized in that the torsion spring (6) has two spring arms (9).

3. 3. A planar torsion spring (6) according to claim 1 or 2, characterized in that the torsion spring (6) has a torsion spring center line (DM) that extends linearly from the external fastening point (10) to a center point (M), the S-shaped spring arms (9) each have an inner circular arc (15) adjacent to the inner fastening point (7) and an outer circular arc (17) adjacent to the external fastening point (10), the difference between the length of the distance (a) of the inner circular arc (15) to the torsion spring center line (DM) of the external contour and the length of the distance (b) of the outer circular arc (17) to the torsion spring center line (DM) of the external contour is less than 20%, preferably less than 10%, and particularly preferably the two lengths of the distance (a) and the distance (b) are equal.

4. 4. A planar torsion spring (6) according to any one of claims 1 to 3, characterized in that the S-shaped spring arm (9) starts from a widened connection section (13) adjacent to the inner fastening point (7) and has a widened first spring section (14), a widened second spring section (16) and a widened third spring section (18), the third spring section (18) being adjacent to the outer fastening point (10).

5. 5. The planar torsion spring (6) according to claim 4, characterized in that the extension of the centre line (m) of the widened connection section (13) of the S-shaped spring arm (9) has an angle (α) in the range of 35° to 55° at the intersection with the torsion spring centre line (DM) via the external fastening point (10).

6. A planar torsion spring (6) according to claim 4 or 5, characterized in that the extension of the centre line (m) of the wider third spring section (18) adjacent to the external fastening point (10) has an angle (β) in the range of 80° to 100° at the intersection of the torsion spring centre line (DM) with the external fastening point (10).

7. A flat torsion spring (6) according to any one of claims 1 to 6, characterized in that the spring sections (14, 16, 18) having a radial distance from the centre (M) of the torsion spring (6) equivalent to the external fastening point (10) are narrow, the narrowest region being the region having the smallest radial distance from the external fastening point (10).

8. A planar torsion spring (6) according to any one of claims 1 to 7, characterized in that it is produced by an injection moulding process.

9. A planar torsion spring (6) according to claim 8, characterized in that the planar torsion spring (6) is made from an amorphous metal.

10. A planar torsion spring (6) according to any one of claims 1 to 9, characterized in that the S-shaped spring arm (9) has a different thickness between the inner fastening point (7) and the outer fastening point (10).

11. A planar torsion spring (6) according to any one of the preceding claims, characterised in that the S-shaped spring arm (9) has an indentation on its upper side (20) and / or on its lower side (21).

12. A planar torsion spring (6) according to any one of claims 1 to 11, characterized in that the side surface (22) of the S-shaped spring arm (9) has a bulge.

13. A planar torsion spring (6) according to any one of the preceding claims, characterized in that the internal fastening point (7) has an angular receptacle with rounded corners, preferably a square mounting part (8).

14. A planar torsion spring (6) according to any one of the preceding claims, characterized in that the external fastening point (10) is rotatably pivoted, preferably by means of a needle bearing.

15. A series elastic actuator (1), preferably an actuator for a robot joint, comprising an electric drive and a planar torsion spring (6) according to any one of claims 1 to 14.

16. 16. The series elastic actuator (1) according to claim 15, characterized in that the inner fastening point (7) of the planar torsion spring (6) is fixedly connected to the electric drive and the outer fastening point (10) of the planar torsion spring (6) is rotatably connected to the actuated element (5).

17. A method for manufacturing a planar torsion spring (6), in particular a series elastic actuator (1) according to any one of claims 1 to 14, comprising the steps of: providing an injection mould for said planar torsion spring (6), injecting a material suitable for injection moulding, preferably an amorphous metal alloy or a plastic material, into said injection mould; Cooling the injection mold and the injection molded planar torsion spring (6); removing said planar torsion spring (6) made of injection moldable material from said injection mold; and optionally machining said planar torsion spring (6) to remove sprue residues and mould seams and to deburr and round edges; Machining the flat torsion spring (6) to straighten the surface so that the surface receiving the release angle is straight enough to function as a seat for an angular holder (8) or bearing; A method comprising:

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