Method, evaluation unit and system for detecting engagement faults in a stress wave transmission using strain sensors

Abstract

The invention relates to a method, an evaluation unit and a system for detecting meshing disturbances in a stress wave transmission, wherein the stress wave transmission has an elastic transmission element with an external toothing and a wave generator and a rigid outer ring with an internal toothing, wherein the elastic transmission element has an elastic transmission element with a strain sensor arrangement comprising a first strain sensor and a second strain sensor, wherein the strain sensor arrangement is formed in the undeformed elastic transmission element substantially in the circumferential direction of a first circle with a first diameter, wherein the first strain sensor is arranged on the first circle substantially diametrically opposite to the second strain sensor, wherein the first strain sensor generates a first sensor signal and the second strain sensor generates a second sensor signal,wherein a time derivative of the first sensor signal and a time derivative of the second sensor signal are formed, wherein an intervention disturbance is detected when first a dedoidal state of the transmission element is detected and subsequently a non-dedoidal state of the transmission element is detected.,

Description

[0001] The invention relates to a method for detecting meshing faults in a stress wave transmission. The invention also relates to an evaluation unit configured to execute the method for detecting meshing faults in the stress wave transmission. The invention further relates to a system comprising an elastic transmission element of the stress wave transmission and the evaluation unit. Background of the invention

[0002] Strain wave gears (also known as wave gears or sliding wedge gears) enable virtually backlash-free power transmission with a high gear ratio and are therefore particularly suitable for applications requiring precise movements and a small footprint. Because the high gear ratio allows high torques to be generated with relatively small motors, strain wave gears can be used to create very compact drive mechanisms, which are used in robotics, for example.

[0003] The main components of a stress wave transmission are a wave generator, a rigid outer ring (circular spline) with internal gearing, and an elastic transmission element (flexspline) with external gearing arranged between them. The transmission of torque between the wave generator and the outer ring is based on elastic deformation of the transmission element. The wave generator deforms the elastic transmission element into an ellipse, so that the elastic transmission element engages with the outer ring on two opposite sides of its circumference. The elastic transmission element rolls on the outer ring due to the rotation of the wave generator, and torque is transmitted between the elastic transmission element and the outer ring through the toothing between the elastic transmission element and the outer ring.The gear ratio of the transmission is determined by the difference in the number of teeth on the transmission ring and the outer ring.

[0004] If excessive torque occurs during operation, for example when the gearbox is operating under heavy load, this can lead to a meshing fault between the transmission element and the outer ring, whereby the teeth of the transmission element jump over the teeth of the outer ring, a condition known as ratcheting. In normal operation, the transmission element rotates at a uniform tooth feed. In the case of a meshing fault, however, there is a temporary, uncontrolled relative rotation between the outer ring and the transmission element. This results in an unknown angular offset compared to the drive side, making precise control of the angular position impossible. It is not possible to determine from the condition of the gearbox whether such a meshing fault has occurred. This makes it difficult to determine the cause of the fault.

[0005] In this context, a method is known from JP7394551B2 in which an array of strain gauges is used to measure the torque acting on the output side of a stress wave transmission. This makes it possible to detect meshing disturbances and the onset of a dedoidal state in which the flexible transmission element is not concentric with the rigid outer ring. However, the method described therein requires an additional sensor and a more complex detection process, which leads to downtime of the stress wave transmission. Furthermore, a count of recurring tooth jumps caused by meshing disturbances is not described in the cited document.

[0006] DE 10 2022 101 977 A1 describes a method for detecting an engagement fault in a stress wave transmission. A temporal decrease in the measured torque is determined and compared with a decrease threshold. If the decrease threshold is exceeded, a warning signal is triggered.

[0007] DE 10 2022 128 423 B3 also describes a method for detecting a meshing fault of a stress wave transmission and comprises the steps of measuring a temporal torque curve on the deformable transmission ring, detecting a meshing fault based on the measured torque curve, determining a number of teeth skipped during the detected meshing fault based on the torque measurement, and calculating an angular offset between the transmission ring and the outer ring. This corresponds to the determined number of skipped teeth.

[0008] US 2022 / 0 273 378 A1 describes a robot manipulator with an isolation mechanism for a force / torque sensor. The isolation mechanism is coupled to the output of the gearbox driving a robot joint. The sensor has a stationary part and a part coupled to and movable relative to the stationary part. The sensor is coupled to the isolation mechanism, which deforms in response to forces induced by the gearbox to mechanically isolate the force / torque sensor from the forces induced by the gearbox. Disclosure of the invention

[0009] Against this background, the task arises to provide a method, an evaluation unit for carrying out the method and a system comprising the evaluation unit and an elastic transmission element, which enables the detection of meshing disturbances and dedoidal states, as well as the counting of repeatedly occurring tooth jumps in the stress wave gear with less effort.

[0010] The object is achieved by a method for detecting meshing disturbances in a stress wave transmission, wherein the stress wave transmission has an elastic transmission element with external toothing and a wave generator and a rigid outer ring with internal toothing, wherein the elastic transmission element is an elastic transmission element with a strain sensor arrangement comprising a first strain sensor and a second strain sensor, wherein the strain sensor arrangement is formed in the undeformed elastic transmission element substantially in the circumferential direction of a first circle with a first diameter, wherein the first strain sensor is arranged on the first circle substantially diametrically opposite to the second strain sensor, wherein the first strain sensor generates a first sensor signal and the second strain sensor generates a second sensor signal,A time derivative of the first sensor signal and a time derivative of the second sensor signal are formed, wherein an engagement fault is detected when a dedoidal state of the elastic transmission element is first detected and subsequently a non-dedoidal state of the elastic transmission element is detected. The object is further achieved by an evaluation unit configured to carry out the method for detecting engagement faults and by a system comprising the evaluation unit and the elastic transmission element.

[0011] A dedoidal state is understood to be a state in which the flexible transmission element is not arranged concentrically with the rigid outer ring. A non-dedoidal state, on the other hand, is understood to be a state in which the flexible transmission element is arranged concentrically with the rigid outer ring.

[0012] According to the invention, the method for detecting engagement disturbances in a stress wave transmission is characterized in that the dedoidal state is detected if a test shows that - at a first point in time - the magnitude of a first instantaneous value of the time derivative of the first sensor signal and the magnitude of a second instantaneous value of the time derivative of the second sensor signal is greater than a predetermined threshold value and that a product of the first instantaneous value and the second instantaneous value is negative, and that the non-dedoidal state is detected if a test shows that - at a point in time following the first point in time,second time - the magnitude of a third instantaneous value of the time derivative of the first sensor signal and the magnitude of a fourth instantaneous value of the time derivative of the second sensor signal is greater than the predetermined threshold value and that a product of the third instantaneous value and the fourth instantaneous value is negative, and that a product of the first instantaneous value and the third instantaneous value or a product of the second instantaneous value and the fourth instantaneous value is negative.,

[0013] This results in the advantage of the method according to the invention, the evaluation unit according to the invention configured to carry out the method according to the invention and the system according to the invention that by comparing the absolute values ​​of instantaneous values ​​of the derivatives of the sensor signals with a threshold value and by considering the signs of products of the instantaneous values, the operating states associated with intervention disturbances - the dedoidal state and the non-dedoidal state - can be detected with less effort.

[0014] According to an advantageous embodiment of the invention, the strain sensor arrangement comprises a third strain sensor and a fourth strain sensor, wherein the third strain sensor is arranged on the first circle substantially diametrically opposite to the fourth strain sensor, wherein the third strain sensor generates a third sensor signal and the fourth strain sensor generates a fourth sensor signal, wherein a time derivative of the third sensor signal and a time derivative of the fourth sensor signal are formed, wherein the dedoidal state is detected if a test additionally shows that - at the first time - the magnitude of a fifth instantaneous value of the time derivative of the third sensor signal and the magnitude of a sixth instantaneous value of the time derivative of the fourth sensor signal is greater than the predetermined threshold value, wherein the non-dedoidal state is detected if a test additionally showsthat – at the second time – the magnitude of the fifth instantaneous value of the time derivative of the third sensor signal and the magnitude of the sixth instantaneous value of the time derivative of the fourth sensor signal are greater than the predetermined threshold value. Such a configuration can increase the precision of detecting the dedoidal state and the non-dedoidal state.

[0015] According to a further advantageous embodiment of the invention, it is provided that the first sensor signal is low-pass filtered before the time derivative of the first sensor signal is formed, and / or that the second sensor signal is low-pass filtered before the time derivative of the second sensor signal is formed, and / or that the third sensor signal is low-pass filtered before the time derivative of the third sensor signal is formed, and / or that the fourth sensor signal is low-pass filtered before the time derivative of the fourth sensor signal is formed. Low-pass filtering allows high-frequency interference signal components to be suppressed, thus reducing the risk of incorrect evaluation of the sensor signals due to high-frequency interference.

[0016] According to a further advantageous embodiment of the invention, the number of detected meshing disturbances is counted. This makes it possible to more accurately estimate the relative position of the elements of the stress wave transmission. For example, if it is assumed that with each meshing disturbance, the flexible transmission element shifts by exactly one tooth relative to the rigid outer ring, the tooth position of the stress wave transmission can be corrected by one tooth for each detected meshing disturbance.

[0017] According to a further advantageous embodiment of the invention, it is provided that the number of detected intervention faults is displayed in a display device or transmitted to a control unit.

[0018] A further subject of the invention is an evaluation unit configured to carry out the method according to the invention. The evaluation unit according to the invention can achieve the same technical effects, advantages, and configurations as those already described in connection with the method according to the invention.

[0019] Furthermore, a further subject matter of the invention is a system comprising the evaluation unit and an elastic transmission element with external teeth, wherein the elastic transmission element has an elastic transmission element with a strain sensor arrangement comprising a first strain sensor and a second strain sensor, wherein the first strain sensor arrangement is formed in the undeformed elastic transmission element substantially in the circumferential direction of a first circle with a first diameter, wherein the first strain sensor is arranged on the first circle substantially diametrically opposite to the second strain sensor, wherein the first strain sensor generates a first sensor signal, wherein the second strain sensor generates a second sensor signal.The system according to the invention can achieve the same technical effects, advantages and configurations as have already been described in connection with the method according to the invention.

[0020] Further details and advantages of the invention will be explained below with reference to the exemplary embodiments illustrated in the drawings. Herein: Fig. 1 an elastic transmission element with a strain sensor arrangement with four strain sensors in a schematic sectional view; Fig. 2 the dedoidal state due to an engagement disturbance, as it occurs in elastic transmission elements, in a schematic sectional view; Fig. 3 an outer ring, an elastic transmission element and a wave generator in a schematic sectional view as well as an enlarged section of a region of the tooth meshing to illustrate meshing disturbances; Fig. 4 photographic images (first row) and corresponding schematic representations (second row) of the three phases of an interventional disturbance Fig. 5 experimentally determined signal curves of sensor signals and their time derivatives of four strain sensors and pairs of two temporally successive instantaneous values ​​of the time derivative of the sensor signals, as used in the method according to the invention Fig. 6 Enlarged representation of a section of the experimentally determined signal curves of sensor signals and their time derivative of four strain sensors and pairs of two temporally successive instantaneous values ​​of the time derivative of the sensor signals, as used in the method according to the invention Fig. 7 a first embodiment of the method according to the invention in program-based form represented by a program flow chart

[0021] In the various figures, identical parts are always provided with the same reference symbols and are therefore usually named or mentioned only once.

[0022] Fig. Figure 1 shows a schematic sectional view of an elastic transmission element 1 with a strain sensor arrangement 2 with four strain sensors 2.1, 2.2, 2.3, and 2.4. Such elastic transmission elements 1 are used on the drive side of stress wave drives 10 (see Fig. 3) is used for torque transmission. The strain sensor arrangement 2 has a first strain sensor 2.1, a second strain sensor 2.2, a third strain sensor 2.3, and a fourth strain sensor 2.4, wherein the first strain sensor 2.1 is arranged diametrically opposite the second strain sensor 2.2 and the third strain sensor 2.3 is arranged diametrically opposite the fourth strain sensor 2.4. Such strain sensors can be designed as strain gauges, which change their electrical resistance upon stretching or compressive deformation. The strain gauges can be attached to elastic transmission elements by means of an adhesive connection. Alternatively, it is possible for conductor tracks of the strain gauge to be applied to a base body of the flexible transmission element. Bridge circuits of strain gauges in the form of a quarter, half, and / or full bridge are preferably used for the measurement.

[0023] Fig. 2 shows a first schematic representation of a meshing disturbance, or ratcheting, between the external toothing 1.1 of an elastic transmission element 1 and the internal toothing 6.1 of a rigid outer ring 6 of a stress wave transmission 10 (see Fig. 3). One of the contact points along the main axis of a wave generator 5 (see Fig. 3) has shifted by one tooth in the direction of the shorter axis. This state is also known as the dedoidal state D (see Fig. 7). During operation of the stress wave transmission, after a dedoidal state D, the non-dedoidal state N (see Fig. 7) with normal tooth engagement, in which case a jump of one tooth remains.

[0024] Fig. Figure 3 shows a second schematic representation of a meshing disturbance between the external toothing 1.1 of an elastic transmission element 1 and the internal toothing 6.1 of a rigid outer ring 6 of a stress wave transmission 10, as well as an enlarged section of a tooth meshing area to illustrate the meshing disturbances. The enlarged section shows the result of meshing disturbances P that occurred in the stress wave transmission. After a single meshing disturbance P.1, there is a jump of one tooth, while after the three meshing disturbances shown here as examples, there is a jump of three teeth.

[0025] Fig. Figure 4 shows photographic images (first row) and, below each one, a corresponding schematic representation (second row) of the three phases of a meshing disturbance. The first sub-image of the second row (second row left) shows how - for example, due to excessive input torque - the external teeth of an elastic transmission element detach from the internal teeth of a rigid outer ring, known as the pre-ratcheting phase. The second sub-image of the second row (second row center) shows how a displacement of one tooth occurs as a result of the wave generator 5 continuing to rotate, known as the mid-ratcheting phase (1st slip). At this moment, a dedoidal state of the flexible transmission element exists. The third sub-image of the second row (second row right) shows how the previous offset of the wave generator is reversed by a jump back of one tooth, known as the mid-ratcheting phase (2nd slip).At this moment, the flexible transmission element is in a non-dedoidal state.

[0026] Fig. 5 shows four experimentally determined signal curves of four sensor signals ST1, ST2, ST3 and ST4 and their time derivatives DST1, DST2, DST3 and DST4, which are generated by the four strain sensors 2.1, 2.2, 2.3 and 2.4 during operation of the stress wave drive 10 as a function of time t. A uniform rotation of the wave generator 5 between the start time of the recording and time 138 causes a cyclically increasing and decreasing strain of the strain sensors 2.1, 2.2, 2.3 and 2.4 due to an oval cross-section of the wave generator 5. The uniform rotation of the wave generator 5, i.e. at a constant angular velocity, initially leads to sinusoidal sensor signals (harmonic oscillations) in the period between the start time of the recording and time 138.The sensor signals of two diametrically opposed strain sensors (ST1 and ST2 as well as ST3 and ST4) are in phase, since the main axis (the longer axis) of the oval-shaped cross-section of the wave generator 5 stretches the strain sensors simultaneously and equally.

[0027] The occurrence of an engagement disturbance in the stress wave transmission 10 is manifested by discontinuities in the sensor signals ST1, ST2, ST3, and ST4 and by signal peaks in the time derivatives of the sensor signals DST1, DST2, DST3, and DST4, such as those that occur between times 139 and 140. During a transition from a non-dedoidal state N (see Fig. 7) into a dedoidal state D (see Fig. 7), the elastic transmission element 1 shifts along the minor axis (the shorter axis) of the oval-shaped cross-section. This rapidly occurring movement causes, depending on the positions of the strain sensors 2.1, 2.2, 2.3, and 2.4 at the time the interference disturbance occurs, a rapid increase or decrease in the sensor signals ST1, ST2, ST3, and ST4 of the strain sensors 2.1, 2.2, 2.3, and 2.4. Two diametrically opposed strain sensors 2.1 and 2.2 or 2.3 and 2.4 exhibit different signs of the signal peaks in their time derivatives DST1 and DST2, or DST3 and DST4, respectively. If the elastic transmission element 1 shifts again at a later time such that a non-dedoidal state N is present, this shift is accompanied by signal peaks of the time derivatives DST1 and DST2 or DST3 and DST4 of the opposite sign.The presence of the dedoidal state D or the non-dedoidal state N, as well as the number of intervention disturbances that have occurred, can therefore be determined by detecting these signal peaks. A threshold value G serves to distinguish the relevant signal peaks from those that occur, for example, as a result of unavoidable noise that is generally superimposed on the sensor signals ST1, ST2, ST3, and ST4.

[0028] Fig. Figure 6 shows an enlarged view of a section of the experimentally determined signal curves from Fig. 5 of sensor signals ST1, ST2, ST3, and ST4 and their time derivatives DST1, DST2, DST3, and DST4 in a period between time 139 and time 140, during which a first intervention disturbance occurs. Also shown are the signs of a first instantaneous value DST1.1 of the time derivative DST1 of the first sensor signal ST1 and a second instantaneous value DST2.1 of the time derivative DST2 of the second sensor signal ST2 at a first time t1. In addition, the signs of a third instantaneous value DST1.2 of the time derivative DST1 of the first sensor signal ST1 and a fourth instantaneous value DST2.2 of the time derivative DST2 of the second sensor signal ST2 at a second time t2 are shown. As can be seen in the illustration, the signs of the first pair of values ​​occurring at the first time t1 consist of the first and second instantaneous values ​​DST1.1 and DST2.1 is opposite to the sign of the second pair of values ​​occurring at the second time t2, consisting of the third and fourth instantaneous values ​​DST1.2 and DST2.2.

[0029] Fig.7 shows an embodiment of the method according to the invention in program-based form, represented by a program flow chart. At the beginning of the program flow, a state variable n is set to zero and a counter variable m is set to zero. The program flow also provides a loop within which the occurrence of signal peaks exceeding a threshold value G in the derivatives DST1, DST2, DST3 and DST4 of sensor signals ST1, ST2, ST3 and ST4 filtered by low-pass filters T1, T2, T3 and T4 is detected. The derivatives DST1, DST2, DST3 and DST4 are formed by differentiators D1, D2, D3 and D4. Instantaneous values ​​of the derivatives DST1, DST2, DST3 and DST4 thus formed are checked in a first query to determine whether their magnitudes exceed a threshold value G. If the instantaneous values ​​do not exceed the threshold value G, the query is repeated.If the instantaneous values ​​exceed the threshold, a query is made as to whether one of the products of two instantaneous values ​​from derivatives of opposing strain sensors 2.1 and 2.2 or 2.3 and 2.4 is negative or not. If the product is non-negative, the query is repeated after the absolute values ​​of the new instantaneous values ​​have been exceeded. If the product is negative, the state variable is incremented by one. If the value of the state variable n is then one, the last queried instantaneous values ​​are stored as the first instantaneous value DST1.1, the second instantaneous value DST2.1, the fifth instantaneous value DST3.1, and the sixth instantaneous value DST4.1. If, however, the value of the state variable n is two, the last queried instantaneous values ​​are stored as the third instantaneous value DST1.2, the fourth instantaneous value DST2.2, the seventh instantaneous value DST3.2 and the eighth instantaneous value DST4.2.If the state variable n then has a value less than two, a dedoidal state D exists. A display of a current state Z is then generated, and the query is repeated at a subsequent time after the threshold value G has been exceeded by the absolute values ​​of new instantaneous values. If the state variable n has a value greater than or equal to two, a query is made as to whether the product of the first instantaneous value DST1.1 and the third instantaneous value DST1.2, or the product of the second instantaneous value DST2.1 and the fourth instantaneous value DST2.2, or the product of the fifth instantaneous value DST3.1 and the seventh instantaneous value DST3.2, or the product of the sixth instantaneous value DST4.1 and the eighth instantaneous value DST4.2 is negative.If the product is negative, the transition from the dedoidal state D to the non-dedoidal state N is detected, the state variable n is set to zero, and the counter variable m is incremented by 1, so that the number of tooth jumps that have occurred so far is stored in the counter variable m. The number of tooth jumps that have occurred so far E is then displayed, and a display of a current state Z is generated. If, on the other hand, the product described above is non-negative, a faulty state F is detected, and a display of a current state Z is generated. The query is then repeated at a subsequent time to determine whether the threshold value G has been exceeded by the absolute values ​​of new instantaneous values.

[0030] In the described embodiment of the method, an interference fault is thus detected when a dedoidal state D of the transmission element 1 is first detected and subsequently a non-dedoidal state N of the transmission element 1 is detected. The dedoidal state D is detected when a test shows - that - at a first time t1 - the magnitude of the first instantaneous value DST1.1 of the time derivative DST1 of the first sensor signal ST1 and the magnitude of the second instantaneous value DST2.1 of the time derivative DST2 of the second sensor signal ST2 are greater than the predetermined threshold value G and - that the product of the first instantaneous value DST1.1 and the second instantaneous value DST2.1 is negative.

[0031] The non-dedoidal condition is recognized when a test shows - that - at a second time t2 following the first time t1 - the amount of the third instantaneous value DST1.2 of the time derivative DST1 of the first sensor signal ST1 and the amount of the fourth instantaneous value DST2.2 of the time derivative DST2 of the second sensor signal ST2 is greater than the predetermined threshold value G and - that the product of the third instantaneous value DST1.2 and the fourth instantaneous value DST2.2 is negative, and - that the product of the first instantaneous value DST1.1 and the third instantaneous value DST1.2 or a product of the second instantaneous value DST2.1 and the fourth instantaneous value DST2.2 is negative. List of reference symbols 1 elastic transmission element 1.1 External gearing 2 Strain sensor arrangement 2.1 first strain sensor 2.2 second strain sensor 2.3 third strain sensor 2.4 fourth strain sensor ST1 first sensor signal ST2 second sensor signal ST3 third sensor signal ST4 fourth sensor signal DST1 time derivative of the first sensor signal DST2 time derivative of the second sensor signal DST3 time derivative of the third sensor signal DST4 time derivative of the fourth sensor signal 5 wave generator 6 rigid outer ring 6.1 Internal gearing T1 first low-pass filter for the first sensor signal T2 second low-pass filter for the second sensor signal T3 third low-pass filter for the third sensor signal T4 fourth low-pass filter for the fourth sensor signal D1 first differentiator for the first sensor signal D2 second differentiator for the second sensor signal D3 third differentiator for the third sensor signal D4 fourth differentiator for the fourth sensor signal D Dedoidal state E Number of tooth jumps that have occurred so far F Faulty condition N Non-dedoidal state Z Display of a current state U circumferential direction G threshold n state variables m counter variable t time t1 First time point t2 Second time point P Intervention disturbances P.1 Displacement of one tooth after a surgical intervention P.2 Displacement of three teeth after three interventions

Claims

[1] Method for detecting meshing disturbances in a stress wave transmission, wherein the stress wave transmission has an elastic transmission element (1) with an external toothing (1.1) and a wave generator (5) and a rigid outer ring (6) with an internal toothing (6.1), wherein the elastic transmission element (1) has an elastic transmission element (1) with a strain sensor arrangement (2) comprising a first strain sensor (2.1) and a second strain sensor (2.2), wherein the strain sensor arrangement (2) is formed in the undeformed elastic transmission element (1) substantially in the circumferential direction (U) of a first circle with a first diameter, wherein the first strain sensor (2.1) is arranged on the first circle substantially diametrically opposite to the second strain sensor (2.2), wherein the first strain sensor (2.1) generates a first sensor signal (ST1) and the second strain sensor (2.2) generates a second sensor signal (ST2), wherein a time derivative (DST1) of the first sensor signal (ST1) and a time derivative (DST2) of the second sensor signal (ST2) are formed, wherein an intervention fault is detected when first a dedoidal state (D) of the transmission element (1) is detected and subsequently a non-dedoidal state (N) of the transmission element (1) is detected, wherein the dedoidal state (D) is detected when a test shows - that - at a first time (t1) - the amount of a first instantaneous value (DST1.1) of the time derivative (DST1) of the first sensor signal (ST1) and the amount of a second instantaneous value (DST2.1) of the time derivative (DST2) of the second sensor signal (ST2) is greater than a predetermined threshold value (G) and - that a product of the first instantaneous value (DST1.1) and the second instantaneous value (DST2.1) is negative, whereby the non-dedoidal state (N) is detected if a test shows, - that - at a second time (t2) following the first time (t1) - the amount of a third instantaneous value (DST1.2) of the time derivative (DST1) of the first sensor signal (ST1) and the amount of a fourth instantaneous value (DST2.2) of the time derivative (DST2) of the second sensor signal (ST2) is greater than the predetermined threshold value (G) and - that a product of the third instantaneous value (DST1.2) and the fourth instantaneous value (DST2.2) is negative, and - that a product of the first instantaneous value (DST1.1) and the third instantaneous value (DST1.2) or a product of the second instantaneous value (DST2.1) and the fourth instantaneous value (DST2.2) is negative. [2] Method according to claim 1, characterized byin that the strain sensor arrangement (2) has a third strain sensor (2.3) and a fourth strain sensor (2.4), wherein the third strain sensor (2.3) is arranged on the first circle substantially diametrically opposite to the fourth strain sensor (2.4), wherein the third strain sensor (2.3) generates a third sensor signal (ST3) and the fourth strain sensor (2.4) generates a fourth sensor signal (ST4), wherein a time derivative (DST3) of the third sensor signal (ST3) and a time derivative (DST4) of the fourth sensor signal (ST4) are formed, wherein the dedoidal state (D) is detected if a test additionally shows that - at the first time (t1) - the absolute value of a fifth instantaneous value (DST3.1) of the time derivative (DST3) of the third sensor signal (ST3) and the absolute value of a sixth instantaneous value (DST4.1) the time derivative (DST4) of the fourth sensor signal (ST4) is greater than the predetermined threshold value (G), wherein the non-dedoidal state (N) is detected if a test additionally shows that - at the second time (t2) - the absolute value of the fifth instantaneous value (DST3.1) of the time derivative (DST3) of the third sensor signal (ST3) and the absolute value of the sixth instantaneous value (DST4.1) of the time derivative (DST4) of the fourth sensor signal (ST4) is greater than the predetermined threshold value (G). [3] Method according to one of the preceding claims, characterized bythat the first sensor signal (ST1) is low-pass filtered before the formation of the time derivative (DST1) of the first sensor signal (ST1) and / or that the second sensor signal (ST2) is low-pass filtered before the formation of the time derivative (DST2) of the second sensor signal (ST2) and / or that the third sensor signal (ST3) is low-pass filtered before the formation of the time derivative (DST3) of the third sensor signal (ST3) and / or that the fourth sensor signal (ST4) is low-pass filtered before the formation of the time derivative (DST4) of the fourth sensor signal (ST4). [4] Method according to one of the preceding claims, characterized by that the number of detected intervention faults is counted. [5] Method according to claim 4, characterized by that the number of detected intervention faults is displayed in a display device or transmitted to a control unit. [6] Method according to one of the preceding claims, characterized bythat a currently detected dedoidal state (D) or non-dedoidal state (N) is displayed in a display device or transmitted to a control unit. [7] Evaluation unit configured to carry out a method according to one of claims 1 to 6. [8] System comprising an evaluation unit according to claim 7 and an elastic transmission element (1) with external teeth (1.1), wherein the elastic transmission element (1) is an elastic transmission element (1) with a strain sensor arrangement (2) comprising a first strain sensor (2.1) and a second strain sensor (2.2), wherein the first strain sensor arrangement (2) is formed in the undeformed elastic transmission element (1) substantially in the circumferential direction (U) of a first circle with a first diameter, wherein the first strain sensor (2.1) is arranged on the first circle substantially diametrically opposite to the second strain sensor (2.2), wherein the first strain sensor (2.1) generates a first sensor signal (ST1), wherein the second strain sensor (2.2) generates a second sensor signal (ST2).

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