Stress wave gearbox with measuring grids and system for torque measurement on a stress wave gearbox

Abstract

Stress wave transmission (2) comprising a flex spline (6), a circular spline (10) and a wave generator (14) which has a plug (18) and a rolling element bearing (22) surrounding the plug (18), wherein a plurality of measuring grids (28) for strain measurement are arranged on the flex spline (6), wherein the wave generator (14) is designed such that three engagement areas are formed between the teeth of the flex spline (6) and the circular spline (10).

Description

[0001] The invention relates to a stress wave drive comprising a flex spline, a circular spline, and a wave generator, which has a plug and a rolling element bearing surrounding the plug, wherein a plurality of measuring grids for strain measurement are arranged on the flex spline. It further relates to a system with such a stress wave drive.

[0002] Conventional stress wave gears feature a circular spline with internal teeth and a flexible flex spline with external teeth located within the circular spline. A wave generator, also located within the flex spline, deforms the flex spline radially. This deformation creates a positive-locking, torque-transmitting connection between the circular spline and the flex spline at four points on either side of the contact with the circular spline. Such stress wave gears thus comprise three main components: the wave generator, the flex spline, and the circular spline.

[0003] In the reduction mode of the wave gear, i.e., during speed reduction, the elliptically shaped wave generator serves as the drive element. Via a thin-section roller bearing, the wave generator (WG) deforms the flexspline (FS), which engages in the internally toothed ring gear, the circular spline (CS). As the wave generator rotates, the major axis of the ellipse shifts, and thus the tooth engagement area changes. Since the flexspline has fewer teeth than the circular spline, specifically two fewer, it rotates relative to the circular spline by an angle of one tooth pitch during half a revolution of the wave generator and by an angle of two tooth pitches during a full revolution. When the circular spline is stationary, the flexspline rotates in the opposite direction to the rotation of the wave generator.

[0004] The wave generator typically consists of an elliptical steel disc or plug and a thin-section roller bearing mounted on it. This component is used as a drive element in reduction operation. The design of the flexspline allows for large elastic deformations in the radial direction. It is shaped into an elliptical form by the wave generator. In the region of the major axis of the ellipse, the external teeth of the flexspline engage with the internal teeth of the circular spline.

[0005] If a torque sensor is to be implemented, it seems logical in the case of wave gears to use the flexspline directly as the deformation body, i.e., to mount the sensor directly onto the flexspline. The problem here is that the strains measured in this way are partially caused by the rotating wave generator. Consequently, the measurement signal exhibits significant ripple. Therefore, effectively compensating for this ripple is a major challenge when using the flexspline as a torque-measuring component.

[0006] The invention is therefore based on the objective of improving a voltage wave transmission with measuring grids in such a way that it enables more precise torque measurement.

[0007] Furthermore, a system for torque measurement on a voltage wave gear will be provided.

[0008] With regard to the stress wave drive, the aforementioned problem is solved according to the invention by a stress wave drive with the features of claim 1. The stress wave drive comprises a flex spline, a circular spline, and a wave generator, which has a plug and a rolling element bearing surrounding the plug. A plurality of measuring grids for strain measurement are arranged on the flex spline. The wave generator is designed such that three engagement areas are formed between the toothing of the flex spline and the circular spline.

[0009] Advantageous embodiments of the invention are the subject of the dependent claims.

[0010] The invention is based on the consideration that applying sensors or measuring grids directly to the flexspline of a wave generator presents challenges. The resulting and consequently measured strains are not only caused by the torque being measured, but also partly by the basic deformation caused by the rotating wave generator. This creates a ripple in the signal. Previous approaches attempt to reduce this ripple by cleverly arranging individual measuring grids for strain measurement in the form of strain gauges at dedicated angular positions and mathematically processing the individual signals or by averaging over many strain gauges.

[0011] Strain gauges can be advantageously evaluated using a Wheatstone bridge. For this purpose, four resistance strands are typically formed from the strain gauges, which is why a number of strain gauges divisible by four is desirable. Conventional wave gears generally have two diametrically opposed (or offset by 180°) engagement areas and therefore a corresponding symmetry in the deformation. This symmetry reduces the mean-value effect when the number of strain gauges is divisible by four and evenly distributed around the circumference.

[0012] As has now been recognized, an advantageous solution for precise torque measurement can be achieved by designing the wave generator such that three engagement zones, offset by 120°, are formed between the toothing of the flex spline and circular spline. The combination of three engagement zones with 4 x n uniformly distributed strain gauges results in very good averaging even for small values ​​of n, where n = 1, 2, 3, ...

[0013] In a preferred embodiment, the plug and / or the rolling element bearing of the wave generator has a threefold rotational symmetry in cross-section; i.e., the cross-section does not change during a third of a rotation.

[0014] Advantageously, the Wave Generator plug has three cams, each offset by 120°. These cams create a threefold symmetry. The cams are preferably located on areas of the plug that protrude relative to an imaginary inner circumference.

[0015] The three engagement areas between the teeth of the flexspline and circular spline can also be realized by other embodiments of the wave generator. For example, the rolling bearing can have threefold symmetry. The rolling bearing can, for instance, consist of three small bearings arranged circumferentially on the plug at angular offsets of 120° each. Another preferred embodiment comprises electromagnets or piezoelectric elements which, when energized, deform the rolling bearing such that three engagement areas are formed between the teeth of the flexspline and circular spline.

[0016] In a preferred embodiment, the measuring grids are designed as individual strain gauges. This embodiment is, in a sense, a minimal variant, as it uses individual strain gauges, resulting in the fewest strain gauges and the fewest contacts, which are preferably arranged alternately at angles of +45° and -45° to the radial direction (V-arrangement). This keeps costs low and achieves high reliability, since only a few components that can fail are used.

[0017] In an alternative preferred embodiment, the measuring grids are designed as double strain gauges with grids in a V-arrangement. Thus, two measuring grids (each with angles of +45° and -45° to the radial direction) are used per strain gauge (or the film that carries the strain gauges). This results in a higher contacting effort, but simultaneously allows for a higher signal quality.

[0018] In further preferred embodiments, multiple strain gauges are used as a ring or as a quarter-ring, i.e., as a 90° segment, with several grids on a single film. Part of the contacting can then be carried out directly on the carrier film and does not need to be done separately. The two strain gauges of the respective double strain gauge are preferably arranged at an angle of +45° / -45° to the radial direction of the flex spline. In this way, their respective strains are equal in magnitude.

[0019] The number of measuring grids is advantageously a multiple of four. In such a configuration, the elimination of harmonics that interfere with the measurement result is particularly effective, thus enabling a precise measurement of the torque.

[0020] In a preferred embodiment, exactly eight measuring grids are provided.

[0021] In a preferred embodiment, several measuring grids are arranged together on an annular or annular segment-shaped carrier film. This enables a cost-effective and easy-to-handle application of the measuring grids to the flexspline.

[0022] In a first preferred embodiment, the flexspline is designed in a silk-hat construction with a thin-walled collar, wherein the respective measuring grid is arranged on the thin-walled collar.

[0023] In an alternative preferred embodiment, the flexspline is designed in a pot construction with a thin-walled pot base, wherein the respective measuring grid is arranged on the thin-walled pot base.

[0024] With regard to the system for measuring torque on a tension wave gear, the above-mentioned problem is solved according to the invention by a system with the features of claim 12. The system for measuring torque on a tension wave gear comprises a tension wave gear as described above and a measuring unit connected on the signal input side to at least one pair of measuring grids.

[0025] Advantageously, the measuring unit comprises at least one, preferably symmetrical, Wheatstone measuring bridge consisting of four strands, each strand comprising a number of measuring grids and / or supplementary elements in series and / or parallel connection.

[0026] In a first preferred embodiment, the system for measuring torque on a stress wave gear comprises a stress wave gear in which the measuring grids are designed as individual strain gauges and has a number of individual strain gauges that is a multiple of four, in particular eight.

[0027] In a second preferred embodiment, the system for measuring torque on a stress wave gear comprises measuring grids designed as double strain gauges with grids in a V-arrangement, wherein in particular the two strain gauges of the respective double strain gauge are arranged at an angle of +45° / -45° to the radial direction of the flex spline, and a number of double strain gauges with grids in a V-arrangement that is a multiple of four, in particular eight.

[0028] Advantageously, the system for measuring torque on a voltage wave gear has at least one supplementary element, which is designed as a passive electronic component, in particular as a resistor.

[0029] The advantages of the invention lie particularly in the fact that, in an embodiment of the stress wave drive with three engagement areas between flex spline and circular spline, comprehensive ripple compensation is achieved with comparatively few strain gauges, which allows for a more cost-effective and reliable solution.

[0030] Torque measurement can be used purely for measurement purposes (e.g., weighing or condition monitoring), for safety aspects (e.g., safety functions), for control tasks (e.g., force feedback, such as impedance control), for compensation (e.g., active vibration damping), or for similar purposes. Applications are possible in industries such as robotics / cobots, machine tools, aerospace, medical technology, semiconductor technology, handling technology, and automation technology.

[0031] An embodiment of the invention is explained in more detail with reference to the drawings. All features described and / or illustrated, whether individually or in any meaningful combination, constitute the subject matter of the present invention, even independently of their compilation in the claims or their cross-reference.

[0032] Some of these show schematically: Fig.1 a stress wave gear in a preferred embodiment, Fig. 2. a state-of-the-art wave generator, Fig. 3 a wave generator in a preferred embodiment, Fig. 4 a flexspline with strain gauges, Fig. 5 a system for torque measurement on a tension shaft gearbox, Fig. 6 a Wheatstone bridge in a first representation, and Fig. 7 a Wheatstone bridge in a second illustration.

[0033] Identical or equivalent components are identified by reference numerals in the following figures of the drawing, based on one embodiment, to improve readability.

[0034] A in Fig.The tension wave transmission 2 shown in Figure 1 comprises a flex spline 6, a coaxially arranged circular spline 10, and a wave generator 14. The circular spline 10 is designed as an internally toothed, cylindrical ring gear. The flex spline 6 has, in some areas, the form of a thin-walled hollow cylinder with external teeth. The wave generator 14 is located within the flex spline 6 and consists of a centrally located disk, the so-called plug 18, and a rolling bearing 22 mounted on the outer surface of the plug 18.

[0035] The cylindrical, thin-walled rings of the rolling bearing 22 and the flexspline 6 are elastically deformed by the plug 18. This deformation causes the external teeth of the flexspline 6 to engage with the internal teeth of the circular spline 10 in several areas. Using the plug 18 as the drive element and the circular spline 10 as the output element results in a gearbox with a high reduction ratio in a single stage.

[0036] A state-of-the-art Wave Generator 14 is in Fig.Figure 2 shows the plug 18 having an elliptical cross-section, through which the rolling bearing 22 and the flexspline 6 are elastically deformed. In a stress wave gear 2 with such a wave generator 14, the external teeth of the flexspline 6 have two fewer teeth than the internal teeth of the circular spline 10. When the plug 18 is rotated about the gear axis, the tooth engagement areas shift circumferentially along the major axis of the ellipse. Due to the different number of teeth of the flexspline 6 and the circular spline 10, the components rotate relative to each other by an angle of two tooth pitches with each revolution of the plug 18. As a result of the deformation, the external teeth of the flexspline 6 engage with the internal teeth of the circular spline 4 in two areas, or engagement areas, on both sides of the major axis of the ellipse.The plug 18 of the wave generator 14 has a twofold symmetry with two cams 26 offset by 180°.

[0037] The one in Fig. In contrast, the voltage wave transmission 2 shown in Figure 1 according to the invention has a wave generator 14, which is in Fig. Figure 3 shows the plug 18 of the wave generator 14 exhibits threefold symmetry with three cams 26 offset by 120° each. The eccentricities of the plug 18 are shown in the Fig. 2 and Fig. 3 exaggerated representation. The wave generator 14 imprints its shape on the cylindrical part of the flexspline 6. The resulting strains also extend to the membrane-shaped part of the flexspline 6 or the thin-walled collar 30 (see Fig.4) These strains are superimposed on the strains caused by the torque load. The actually measurable strain therefore represents a superposition of the torque-induced (useful signal) and wave generator-induced (disturbance) components.

[0038] In the embodiment chosen here, the external teeth of the flexspline 6 have three fewer teeth than the internal teeth of the circular spline 10. When the plug 18 is rotated about the gear axis, the tooth engagement areas shift circumferentially following the three cams 26. Due to the different number of teeth of the flexspline 6 and the circular spline 10, the components rotate relative to each other by an angle of three tooth pitches with each revolution of the plug 18.

[0039] The strain distribution in the membrane of the Flexspline 6 is modulated in the circumferential direction. In the standard case with a Wave Generator 14 according to Fig.2 these are two periods per revolution, in the variant according to the invention according to Fig. Three periods. This waveform is not purely sinusoidal. Particularly under additional torque loading, significant distortions occur. This can be mathematically described precisely as a superposition of the aforementioned elementary order with its harmonics.

[0040] The following table lists a standard type of Wave Generator 14, which is in Fig. 2 is shown and leads to two intervention areas between Flexspline 6 and Circular Spline 10, and for the triangle type of the Wave Generator 14, which is in Fig. 3 is shown and leads to three intervention areas between Flexspline 6 and Circular Spline 10, the orders of the correspondingly occurring harmonics are listed. type elementary order Harmonics standard 2 4, 6, 8, 10, 12, ... triangle 3 6, 9, 12, ...

[0041] The temporal evolution of the strain at an observed point corresponds to the local evolution of the strain in the circumferential direction at constant moment and rotating Wave Generator 14.

[0042] To measure the strain of the flexspline 6 and thus to measure the applied torque, double strain gauges 34 are used (see Fig. 4) is used. While the torque-induced strain component is the same for all individual strain gauges 32 arranged on a specific radius, the wave generator 14's induced component changes depending on its phase relationship to the rotating input wave. By cleverly arranging the double strain gauges 34, the useful signal can be amplified and the noise signal eliminated.

[0043] In Fig. 4 is the flexspline 6 of the tension wave drive 2 designed in silk-hat construction according to Fig.Figure 1 shows a perspective view. A plurality of measuring grids 28, designed as double strain gauges 34, are attached to a thin-walled collar 30 of the flexspline 6. For clarity, only a few of these are labeled. The double strain gauges 34 each have grids arranged in a V-shape. In this case, 16 double strain gauges 34, a multiple of four, are arranged on the inner collar region 30 of the flexspline 6.

[0044] In Fig. Figure 5 schematically depicts a system 40 for torque measurement on a tension wave gear. The system 40 for torque measurement on a tension wave gear comprises the tension wave gear 2 according to... Fig. 1 and a measuring unit 48, which is connected on the signal input side to at least one pair of measuring grids 34.

[0045] The strain-induced change in resistance of a strain gauge is difficult to measure directly. Therefore, a Wheatstone bridge 50 consisting of two pairs of oppositely detuned strain gauges 32, 34 is almost always used, which generates a signal voltage due to the detuning. The measuring unit 48 of the system 40 for torque measurement on a tension wave gear has at least one Wheatstone bridge 50, whereby a voltage measurement across the Wheatstone bridge 50 is performed to determine the torque. This measured voltage is a function of the strain of the respective strain gauge 32, 34.

[0046] The approach known from the literature, which involves arranging a few strain gauges at precisely defined locations and recording them individually in order to determine harmonics of the expected waveform, fails in practice for a number of reasons: • The waveform is load-dependent and therefore not predictably stable; • The individual DMS can only be placed with finite accuracy; • Signal acquisition for multiple individual strain gauges is very complex compared to a measuring bridge.

[0047] The preferred approach is therefore to use and sum many DMS 32, 34 evenly distributed across the circumference, as exemplified in Fig. Figure 4 shows that the efficiency in eliminating waves depends on the order of the wave and the number of strain gauges 32, 34.

[0048] The following table lists, for a stress wave gearbox 2 with a standard wave generator 14 with twofold symmetry, the orders that are included in the strain curve in ascending order of amplitude, provided they are elementary orders or their harmonics for the gearbox type under consideration. Depending on the number of strain gauges, a "-" indicates that elimination is possible, and an "X" indicates that elimination fails. A configuration is considered successful if all harmonics are compensated in ascending order. Order No. OberWelle Number of DMS 3 4 8 12 16 2 0 - - - - - 4 1 - X - - - 6 2 X - - - - 8 3 - X X - - 10 4 - - - - - 12 5 X X - X - 14 6 - - - - - 16 7 - X X - X 18 8 X - - - - 20 9 X - - - -

[0049] The case of "three strain gauges for standard gearboxes" does not fulfill the rule of "number of strain gauges divisible by 4," but it is found in the literature as the "smallest possible" variant. Compensation fails from the 6th order onward. For the other configurations of the standard gearbox (4, 8, 12, 16 strain gauges), failure occurs at the order corresponding to the number of strain gauges.

[0050] The following table lists the orders present in the strain curve of a stress wave gearbox 2 with a wave generator 14 with threefold symmetry, in order of decreasing amplitude, provided they are elementary orders or their harmonics for the gearbox type under consideration. Depending on the number of strain gauges, a "-" indicates that elimination is possible, and an "X" indicates that elimination fails. A configuration is considered successful if all harmonics are compensated in ascending order. Order No. OberWelle Number of DMS 4 8 12 16 3 0 - - - - 6 1 - - - - 9 2 - - - - 12 3 X - X - 15 4 - - - - 18 5 - - - - 21 6 - - - -

[0051] As can be seen from this table, ripple and harmonic compensation is significantly better with a number of strain gauges 32, 34 divisible by 4 in a stress wave gear 2 according to the invention with three engagement areas than in a standard stress wave gear 2. With the exception of the configuration "12 strain gauges" (elementary order 3 is a divisor of 12), the configurations only fail at an order that corresponds to three times the number of strain gauges 32, 34. With, for example, only 8 strain gauges 32, 34, all orders up to and including the 21st order can be compensated. In the conventional stress wave gear 2 with a wave generator 14 with twofold symmetry, this is only possible up to the 6th order.

[0052] In the Fig. 6 and Fig. Figure 7 shows Wheatstone bridges 50. In a half-bridge circuit of the Wheatstone bridge 50 according to Fig.In the 6-phase circuit, resistors R1 and R2 are active resistors, namely individual strain gauges 32. The half-bridge circuit is completed by two passive resistors R3 and R4. The strains of R1 and R2 must be similar in magnitude but opposite in sign, i.e., R1 and R2 are detuned relative to each other.

[0053] In an alternative and preferred embodiment, the Wheatstone bridge is operated as a full bridge circuit. In this case, all four resistors R1, R2, R3, R4 are strain gauges 32. The strains of the strain gauges 32 R1, R2, R3, R4 are equal in magnitude, with the strains of R1 and R3 being opposite to the strains of R2 and R4. The full bridge circuit provides a larger voltage output signal compared to the half bridge circuit.

[0054] In Fig.Figure 7 shows a Wheatstone bridge 50 for 8 double strain gauges 34. Each of the four resistance strands R1, R2, R3, R4 comprises four measuring grids, with each resistance strand containing two pairs of measuring grids connected in series, and these pairs being connected in parallel. The circuit configuration is as follows: G1L and G1R are the two measuring grids on a first double strain gauge No. 1. The two measuring grids G1L and G1R are each arranged at an angle of + / - degrees to the radial direction (L=left=+45° / R=right=-45°). The 54-strand of resistor R1 is formed from measuring grids G1L to G4L. The 54-strand of resistor R2 is formed similarly from measuring grids G1R to G4R. The 54-strand of resistor R3 is formed from measuring grids G5L to G8L. The 54-strand of resistor R4 is formed similarly from measuring grids G5R to G8R. Thus, a total of 8 pairs of measuring grids are installed. Reference symbol list 2 voltage wave gears 6 Flexspline 10 Circular Spline 14 Wave Generator 18 Plug 22 rolling bearings 26 cam 28 measuring grids 30 thin-walled collars 32 individual strain gauges 34 double strain gauges 40 System for torque measurement on a voltage shaft gearbox 48 units of measurement 50 Wheatstone measuring bridge 54 strand GL1L, GL1 R measuring grid GL2L, GL2R measuring grid GL3L, GL3R measuring grid GL4L, GL4R measuring grid GL5L, GL5R measuring grid GL6L, GL6R measuring grid GL7L, GL7R measuring grid GL8L, GL8R measuring grid R1 resistor R2 resistance R3 resistor R4 resistor

Claims

[1] Stress wave transmission (2) comprising a flex spline (6), a circular spline (10) and a wave generator (14) which has a plug (18) and a rolling element bearing (22) surrounding the plug (18), wherein a plurality of measuring grids (28) for strain measurement are arranged on the flex spline (6), characterized by , that the Wave Generator (14) is designed such that three engagement areas are formed between the toothings of Flexspline (6) and Circular Spline (10). [2] Voltage wave gear (2) according to claim 1, characterized by , that the plug (18) and / or the rolling element bearing (22) of the wave generator (14) has a threefold rotational symmetry in cross-section. [3] Voltage wave gear (2) according to claim 2, characterized by , that the plug (18) of the wave generator (14) has three cams (26) offset by 120° each. [4] Voltage wave gear (2) according to any one of claims 1 to 3, characterized by, that the measuring grids (28) are designed as individual strain gauges (32). [5] Voltage wave gear (2) according to any one of claims 1 to 3, characterized by , that the measuring grids (28) are designed as double strain gauges (34) with grids in a V-arrangement. [6] Voltage wave gear (2) according to claim 5, characterized by , that the two strain gauges of the respective double strain gauge (34) are arranged at +45° / -45° angles to the radial direction of the flexspline (6). [7] Voltage wave gear (2) according to any one of claims 4 to 6, characterized by , that the number of measuring grids (28) is a multiple of the number four. [8] Voltage wave transmission (2) according to claim 7 with exactly eight measuring grids (28). [9] Voltage wave gear (2) according to any one of the preceding claims, characterized by , that several measuring grids (28) are arranged together on an annular or annular segment-shaped carrier film. [10] Voltage wave gear (2) according to any one of claims 1 to 9, characterized by , that the flexspline (6) is designed in silk-hat construction with a thin-walled collar (30), wherein the respective measuring grid (28) is arranged on the thin-walled collar (30). [11] Voltage wave gear (2) according to any one of claims 1 to 9, characterized by , that the flexspline (6) is designed in a pot construction with a thin-walled pot base, wherein the respective measuring grid (28) is arranged on the thin-walled pot base. [12] System (40) for measuring torque on a tension wave gear (2), comprising a tension wave gear (2) according to one of the preceding claims and a measuring unit (48) connected on the signal input side with at least one pair of measuring grids (28). [13] System (40) according to claim 12, characterized by, that the measuring unit (48) comprises at least one, preferably symmetrical, Wheatstone measuring bridge (50) consisting of four strands (54), wherein each strand (54) comprises a number of measuring grids (28) and / or supplementary elements in series and / or parallel connection. [14] System (40) according to claim 13 comprising a stress wave drive (2) according to claim 4 and comprising a number of individual strain gauges (32) which is a multiple of four, in particular eight. [15] System (40) according to claim 13 comprising a stress wave transmission (2) according to claim 5 or 6 and comprising a number of double strain gauges (34) with grids in a V-arrangement, the number being a multiple of four, in particular eight. [16] System (40) according to one of claims 13 to 15, comprising at least one supplementary element which is designed as a passive electronic component, in particular as a resistor.

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