Method for detecting a faulty measurement channel in a strain wave gearing mechanism

Abstract

The invention relates to a method for detecting a faulty measurement channel on an elastic transmission element in a gearing mechanism, wherein the elastic transmission element has a first strain gauge assembly with multiple first measurement channels. For each measurement channel of the first measurement channels, a plausibility check is carried out using multiple first measurement signals.

Description

[0001]P221273 DE SHF0237(DE) 1 Method for detecting a faulty measuring channel in a stress wave gear The invention relates to a method for detecting a faulty measuring channel on an elastic transmission element in a gear, in particular a stress wave gear. Furthermore, the invention relates to a drive module with a gear, in particular a stress wave gear. Stress wave gears (also known as strain wave gears, sliding wedge gears; English: “strain wave gearing” or “harmonic drive”) enable almost backlash-free power transmission with a high transmission ratio and are therefore particularly suitable for applications that require precise movements and a small space requirement. Since high torques can be generated with relatively small motors due to the high transmission ratio, stress wave gears can be used to realize very compact drive mechanisms, which are used, for example, in robotics.A stress wave transmission consists of a wave generator, a rigid outer ring (circular spline) with internal gearing, and an elastic transmission element (flexspline) with external gearing arranged between them. In contrast to rigid transmissions, the transmission of torque between the wave generator and the outer ring is based on elastic deformation, in which the transmission ring is deformed into an oval by the wave generator such that it engages with the outer ring on two opposite sides of its circumference. As the wave generator rotates, the transmission ring rolls on the outer ring, so that torque is transmitted between the transmission ring and the outer ring via the intermeshing gears. The transmission ratio of the transmission is determined by the difference in the number of teeth on the transmission ring and the outer ring.During operation, excessive torque, for example when the gearbox is working against high resistance, can lead to so-called meshing disturbances. A meshing disturbance is at least a partial loss of engagement between the transmission ring and the outer ring, so that the teeth of the transmission ring jump over the teeth of the outer ring (“ratcheting”). While the rotation of the transmission ring and that of the outer ring are strictly coupled during normal operation, such a meshing disturbance temporarily leads to an uncontrolled relative rotation between the two rings. CONFIDENTIAL P221273 DE SHF0237(DE) 2 This creates an unknown angular offset on the output side compared to the input side, making precise control of the angular position impossible. Torque sensors in the stress wave gearbox can provide information about the condition of the stress wave gearbox during operation and can be used to monitor its operation.The torque sensors are designed as strain gauges and arranged on the elastic transmission element to measure the mechanical stress on the transmission element. The torque acting on the elastic transmission element is then determined from the mechanical stress. Typically, strain gauge arrangements with multiple strain gauges are used, arranged, for example, in a bridge circuit. If a strain gauge arrangement only covers a limited angular range of the transmission element, the measurement signal depends on the angular position of the wave generator. If, on the other hand, the strain gauge arrangement extends over the entire or almost the entire circumference of the transmission element, a measurement signal can be obtained that is not dependent, or only slightly dependent, on the angular position of the wave generator.To detect faulty strain gauges in a strain gauge array, WO 2021 / 148068 A1 discloses a method for checking an array of strain gauges, which allows the detection of a single defect in one of several strain gauges. The method uses a predictive model determined by finite element analysis or machine learning and therefore requires considerable computing power. Against this background, the objective is to provide a method and a system that increases functional safety by detecting a failure or malfunction of a measuring channel in a strain gauge array, while reducing the computing power required to detect such a defect.The object is achieved by a method for detecting a faulty measuring channel on an elastic transmission element in a transmission, in particular a stress wave transmission, wherein the elastic transmission element has a first strain gauge arrangement with a plurality of first measuring channels, and wherein a plausibility check is carried out for each of the first measuring channels using a plurality of first measuring signals. CONFIDENTIAL P221273 DE SHF0237(DE) 3 The method according to the invention makes use of an elastic transmission element which comprises a strain gauge arrangement. The first strain gauge arrangement has a plurality of first measuring channels, each of which provides first measuring signals. By linking these first measuring signals, an output signal can be obtained which is proportional to the torque.This output signal can be highly accurate and used to measure the torque during operation of the transmission element. The method according to the invention makes it possible to detect faulty first measuring channels by performing a plausibility check for each of the first measuring channels using a plurality of first measuring signals. A measuring channel of a strain gauge arrangement can have one or more strain gauges. The strain gauges measure the mechanical stress in the elastic transmission element, with the mechanical stress being proportional to the existing torque. A faulty measuring channel can occur in particular if a strain gauge is damaged. Each first measuring channel preferably comprises a plurality of strain gauges that are connected in a bridge circuit, in particular a Wheatstone bridge.The first strain gauge arrangement can be arranged on a lateral surface (parallel to the rotational axis of the transmission element) or an end face (perpendicular to the rotational axis of the transmission element) of the elastic transmission element. The strain gauges of the plurality of first measuring channels of the first strain gauge arrangement are preferably arranged such that two first measuring signals each deliver a virtually identical value. The redundancy of the measuring signals can further increase the integrity of the transmission, in particular of the stress wave transmission. The torque is determined from the measuring signals and the measured mechanical stresses, wherein the determined torque from the first measuring channels depends on the angular position of the wave generator. The first strain gauge arrangement can, for example, have two, four, or eight measuring channels, wherein the first strain gauge arrangement particularly preferably has four measuring channels.According to a preferred embodiment, the gear is designed as a stress wave gear, wherein a wave generator acts on the elastic transmission element, wherein the first measurement signals of the individual first CONFIDENTIAL P221273 DE SHF0237(DE) 4 measurement channels of the first strain gauge arrangement are dependent on the rotational position of the wave generator. According to an alternative preferred embodiment, the gear is designed as a planetary gear. According to a preferred embodiment, it is provided that, for plausibility checks, a difference is formed between several, in particular two, terms which are dependent on several first measurement signals. By comparing several terms, it can be determined whether a measurement channel is faulty. The difference formation of several terms is not very computationally intensive and thus has a particularly positive effect on the required computing power.The plurality of terms preferably comprise one or more proportionality factors and / or one or more constants, which are preferably determined empirically. A term can be formed from a first measurement signal, a first measurement signal and a proportionality factor, a plurality of first measurement signals, a plurality of first measurement signals with a proportionality factor for the sum of the plurality of first measurement signals, or a plurality of first measurement signals, each with a proportionality factor. Furthermore, it is conceivable that one or more constants are added to the measurement signal or that one or more constants are added to the combination of one or more measurement signals with the proportionality factor. These proportionality factors and constants can be determined from experimental data or from simulation data or by calibration.For plausibility checks, the amount of the difference can be calculated and compared with a tolerance value. This tolerance value can be determined empirically or by simulation, whereby the tolerance value can be selected to be identical for all measuring channels or can be determined individually for each measuring channel. Furthermore, the tolerance value can have a fixed value or can be adjusted as the operating time progresses. According to a preferred embodiment, the elastic transmission element has a second strain gauge arrangement with a second measuring channel, wherein a second measuring signal of the second measuring channel of the second strain gauge arrangement is proportional to the torque and does not depend, or depends only to a small extent, on an angular position of the wave generator, wherein the plausibility check is carried out for each of the first measuring channels using one or more first measuring signals and / or the second measuring signal.The second strain gauge arrangement has a second measuring channel, the second measuring signal of which depends not or only to a small extent on the angular position of the wave generator. The second strain gauge arrangement can extend around the entire or almost the entire circumference of the elastic transmission element. The second strain gauge arrangement can be arranged on a lateral surface (parallel to the axis of rotation of the transmission element) or an end face (perpendicular to the axis of rotation of the transmission element) of the elastic transmission element. The first and the second strain gauge arrangement are preferably arranged together on the lateral surface or together on the end face. Particularly preferably, the second measuring channel each comprises a plurality of strain gauges which are connected in a bridge circuit, in particular a Wheatstone bridge.Advantageously, the integrity of the torque measurement can be further increased by a plausibility check with a second measuring channel. It is conceivable that the plausibility check of the first measuring channels is carried out using one or more first measuring signals in optional combination with the second measuring signal. For example, the second measuring signal can be taken into account for selected first measuring channels when checking the plausibility of the selected first measuring channel and not for other first measuring channels. According to a preferred embodiment, a plausibility check is carried out for the second measuring channel using a plurality of first measuring signals. The plausibility of the second measuring signal can be checked using a plurality of first measuring signals. Checking the second measuring signal increases the integrity of the measured torque value.The second strain gauge arrangement may not depend on the angular position of the wave generator, or may depend only to a small extent on it, which advantageously enables a torque value that does not require any further calculation step. However, such a torque value may have a certain residual ripple. According to a preferred embodiment, for plausibility checks, a difference is formed between a first term dependent on one or more first measurement signals and a second term dependent on the second measurement signal. It is therefore possible to determine whether a measurement channel is faulty by comparing the first and second terms. This simple calculation has a particularly advantageous effect on the required computing power. CONFIDENTIAL P221273 DE SHF0237(DE) 6 The embodiments and the technical effects explained above with regard tothe term and difference formation, also apply analogously to the first and second terms. A further subject matter of the invention is a method for determining a torque acting on an elastic transmission element of a transmission, in particular a stress wave transmission, wherein a faulty first or possibly second measuring channel is detected according to a method according to one of the preceding embodiments, the faulty measuring channel is corrected using one or more first measuring signals, and wherein the torque acting on the transmission element is determined using the corrected measuring channel and / or the remaining measuring channels. The detection of a faulty first or possibly second measuring channel according to the invention makes it possible to correct the first or possibly second measuring channel identified as faulty. To correct the faulty first or possiblysecond measuring channel, a substitute signal can be determined which is dependent on one or more first measuring signals. In addition, the corrected first or, if applicable, second measuring channel can be included in the determination of the torque acting on the transmission element. The correction of a faulty measuring channel can either mean exchanging the measuring signal from a faulty measuring channel with a measuring signal from a non-faulty measuring channel, or the determination of the measuring signal present at the faulty measuring channel via a weighted or unweighted summation of several measuring signals from non-faulty measuring channels. During the summation, it is also conceivable that one or more constants are added, whereby the constant or constants are predefined depending on the faulty measuring channel. The advantage is that a faulty channel can be corrected orcan be replaced so that this channel further increases the integrity of the measured torque at the stress wave gear. Preferably, only exactly a first measurement signal and a second measurement signal are required to correct the measurement channel identified as faulty. This does not require any computationally intensive prediction models. Preferably, a first measurement signal and the second measurement signal are each multiplied by a proportionality factor and / or a constant is added to this. CONFIDENTIAL P221273 DE SHF0237(DE) 7 Preferably, with a corrected second measurement signal, no further measurement channel is required to determine the torque acting on the transmission element. This also has a particularly positive effect on the required computing power, although this results in a reduced quality of the torque output signal.A further subject matter of the invention is a drive module with a gear, in particular a stress wave gear, and with an evaluation unit configured to detect a faulty first or possibly second measuring channel according to a method according to one of the above-mentioned embodiments, to correct the faulty measuring channel based on one or more first measuring signals, and to determine the torque acting on the transmission element based on the corrected measuring channel and / or one or more remaining measuring channels. The drive module can achieve the same advantages and effects as have already been described in connection with the method according to the invention for detecting a faulty measuring channel.A further subject matter of the invention is a robot arm with at least one drivable arm segment, wherein the arm segment can be driven via the drive module according to one of the preceding embodiments of the invention. The same advantages and effects can be achieved with the robot arm that have already been described in connection with the inventive method for detecting a faulty measuring channel. The advantageous embodiments and features described in connection with the inventive method for detecting a faulty measuring channel can also be applied to the robot arm, alone or in combination. Further details and advantages of the invention will be explained below with reference to the exemplary embodiment illustrated in the drawings. These show: Fig. 1 an exemplary embodiment of an elastic transmission element with a first and a second strain gauge arrangement; Fig.2 shows a schematic representation of a preferred connection of strain gauges of a measuring channel; 3 shows a plurality of measurement signals from different first measuring channels of an exemplary embodiment of the method according to the invention; 4 shows a schematic flow diagram of an exemplary embodiment of the method according to the invention; and 5 shows an exemplary embodiment of a robot in a schematic representation. 1 shows an elastic transmission element 1, wherein the elastic transmission element 1 has a second strain gauge arrangement 10 and a first strain gauge arrangement 20. The elastic transmission element 1 can be used as part of a transmission designed as a stress wave transmission. Alternatively, the elastic transmission element 1 can also be used in a planetary transmission.The stress wave transmission can have a wave generator, a rigid outer ring with internal gearing, and the elastic transmission element 1 arranged between them. In the exemplary embodiment, the two strain gauge assemblies are arranged on an end face of the transmission element 1, which is arranged perpendicular to a rotational axis of the transmission element 1. The second strain gauge assembly 10 extends completely over the circumference of the transmission element 1, i.e., it completely covers the angular range around the rotational axis. The first strain gauge assembly 20 comprises several, here four, first measuring channels 22, 23, 24, 25. In the exemplary embodiment, the first strain gauge assembly 20 is arranged radially outside the second strain gauge assembly 10.According to a modification of this exemplary embodiment, the first strain gauge arrangement 20 can be arranged radially inside the second strain gauge arrangement 10. The second measuring channel and the first measuring channels 22, 23, 24, 25 each have a plurality of strain gauges. As a result of mechanical stress, the strain gauges are deformed and their electrical resistance changes. This change in resistance can be detected, and the mechanical stress in the elastic transmission element 1 and the torque acting on the transmission element 1 can be derived from this. Fig. 2 shows an exemplary schematic representation of a preferred circuit 30 of the strain gauges of a measuring channel as a Wheatstone bridge. This circuit is preferably used for the second measuring channel and the first measuring channels 22, 23, 24, 25. CONFIDENTIAL P221273 DE SHF0237(DE) 9 The circuit shown in Fig.The bridge circuit shown in Figure 2 comprises four strain gauges 31, 32, 33, 34. Two of these strain gauges 31, 32, 33, 34 are each connected in series to form a branch of the bridge circuit. The bridge circuit comprises two branches connected in parallel. A supply voltage is applied to the branches between a supply voltage terminal 35 and a ground terminal 36. Between the two strain gauges of each branch, a tap 37, 38 is provided, via which the measurement signal of the respective measurement channel can be taken. Figure 3 shows first measurement signals ST2, ST3, ST4, ST5 of the first measurement channels of an exemplary embodiment. In this exemplary embodiment, the first measurement channels are provided on the elastic transmission element 1 in such a way that each two first measurement channels deliver a virtually identical measurement signal. In the present case, the measurement signals ST2 and ST4 are virtually identical, as are the measurement signals ST3 and ST5.The sum of all four first measuring signals ST2, ST3, ST4, ST5 is proportional to the torque present at the elastic transmission element 1. The torque can be calculated by multiplying the sum of all four first measurement signals ST2, ST3, ST4, ST5 by a predefined proportionality factor C2: T2 = (ST2 + ST3 + ST4 + ST5) * C2 The second measurement channel can also supply a measure of the torque with proportionality factor C1 using its second measurement signal ST1: T1 = ST1 * C1 Further torque values ​​T3, T4, T5, T6 can be determined, each of which is dependent on just three first measurement signals: T3 = (2*ST2 + ST3 + ST5) * C3 T4 = (ST2 + 2*ST3 + ST4) * C4 T5 = (ST3 + 2* ST4 + ST5) * C5 T6 = (ST2 + ST4 + 2*ST5) * C6 The values ​​C3 to C6 are predefined proportionality factors.The torque values ​​T3, T4, T5, T6 shown above each describe possible ways of calculating the torque, wherein one of the four first measurement signals ST2, ST3, ST4 CONFIDENTIAL P221273 DE SHF0237(DE) 10 or ST5 is not included. Instead of the respective first measurement signal ST2, ST3, ST4, ST5 not included, one of the remaining first measurement signals ST2, ST3, ST4, ST5 is included with a double weighting. In an alternative preferred embodiment, each measurement signal ST1, ST2, ST3, ST4, ST5 can have its own proportionality factor and / or a constant. Thus, the torque values ​​are determined, for example, as follows: T3 = (2*ST2*C32 + ST3*C33 + ST5*C35) * C3. For the proportionality factors according to the format Cxy, x describes the index for the torque value Tx to be determined, and y describes the index for the measurement signal STy to be weighted. Figure 4 shows a flowchart of an embodiment of the method 400' according to the invention.In test step 407, the following plausibility signals PS1 to PS7 are formed: ^^1 = ^^^(^^2 – ^^4) ^^2 = ^^^(^^3 – ^^5) ^^3 = ^^^((2⋅^^2+^^3+ ^^5)∙^_3−^^1∙^_1 )=^^^(^_3−^_1) ^^4 = ^^^((^^2+2⋅^^3+ ^^4)∙^_4−^^1∙^_1 )=^^^(^_4−^_1) ^^5 = ^^^((^^3+2⋅^^4+ ^^5)∙^_5−^^1∙^_1 )=^^^(^_5−^_1) ^^6 = ^^^((^^2+^^4+2⋅^^5)∙^_6−^^1∙^_1 )=^^^(^_6−^_1) ^^7=((^^2+^^3+^^4+ ^^5)∙^_2−^^1∙^_1)=^^^(^_2−^_1) Alternatively or additionally, the plausibility signals PS8 to PS17 listed below can be formed, with the aid of which a plausibility check is only possible on the basis of the first measurement signals ST2, ST3, ST4, ST5, which are included in the terms T2 to T6. PS8=abs(T_3-T_2) PS9=abs(T_4-T_2) PS10=abs(T_5-T_2) PS11=abs(T_6-T_2) PS12=abs(T_4-T_3) PS13=abs(T_5-T_3) CONFIDENTIAL P221273 DE SHF0237(DE) 11 PS14=abs(T_6-T_3) PS15=abs(T_5-T_4) PS16=abs(T_6-T_4) PS17=abs(T_6-T_5) The plausibility signals are each compared with specified tolerance values ​​LV1 to LV7.The factors c_1-6 represent proportionality factors, which are preferably determined and set empirically before the stress wave gear is first put into operation. The plausibility signals therefore represent only simple differences between the measurement signals, which require particularly little computing power. On the one hand, a difference can be formed between several, in particular two, terms that depend on several first measurement signals for plausibility checks (see PS1, PS2 and PS8 to PS17). On the other hand, a difference can be formed between a first term dependent on one or more first measurement signals ST2, ST3, ST4, ST5 and a second term dependent on the second measurement signal ST1 (see PS3 to PS7). These differences can be compared with tolerance values ​​and, in predefined conditional equations, enable a statement to be made about the faultiness of a measurement channel.In order to determine which of the signals is faulty, the following conditional equations are checked: 1. (^^1>^^1) ∩ (^^3>^^3),. the measurement signal ST2 of measuring channel 22 is faulty. 2. (^^1>^^1) ∩ (^^5>^^5), ^ the measurement signal ST3 of measuring channel 23 is faulty. 3. (^^2>^^2) ∩ (^^4>^^4), ^ the measurement signal ST4 of measuring channel 24 is faulty. 4. (^^2>^^2) ∩ (^^6>^^6), ^ the measurement signal ST5 of measuring channel 25 is faulty. 5. (^^7>^^7) ∩ (^^1≤^^1) ∩ the measurement signal ST1 of the second measurement channel is faulty. This means that a faulty first or, if applicable, second measurement channel can be clearly detected. Measurement signals from faulty measurement channels can be corrected as follows: CONFIDENTIAL P221273 DE SHF0237(DE) 12 Faulty ST2: The measurement signal ST2 is replaced by the measurement signal from ST4. Faulty ST4: The measurement signal ST4 is replaced by the measurement signal from ST2. Faulty ST3: The measurement signal ST3 is replaced by the measurement signal from ST5. Faulty ST5: The measurement signal ST5 is replaced by the measurement signal from ST3. Faulty ST1: The measurement signal ST1 is replaced by the following formula: ST1 = (ST2+ST3+ST4+ST5) ∙ c_2 / c_1, where c_1 and c_2 are proportionality factors that were preferably determined empirically before the stress wave gear was first put into operation. First, in a method step 408, it is checked whether all the first measuring channels 22, 23, 24, 25 are faulty.If this is the case (arrow 409), a state 414 with four defective first measuring channels 22, 23, 24, 25 exists. In a notification step 415, an error message is generated and, if necessary, transmitted to the engine control unit. Error correction is not possible. If not all of the first measuring channels 22, 23, 24, 25 are defective (arrow 409'), error registers for the respective defective measuring channels 22, 23, 24, 25 are set in step 410. Subsequently, in step 411, a check is carried out to determine whether only a single first measuring channel 22, 23, 24, 25 is defective. If such a single error exists in a first measuring channel 22, 23, 24, 25 (arrow 412), the respective faulty first measuring channel 22, 23, 24, 25 is corrected. The faulty measurement signal ST2, ST3, ST4, ST5 is corrected in step 413 and after the correction the stress wave transmission can continue to operate normally.If there is no faulty measurement signal, the operation of the stress wave gear continues normally (arrow 412). In an alternative preferred embodiment, in the exemplary embodiment described above, the second measurement channel is checked in parallel with the check of the first measurement channels 22, 23, 24, 25. If the first measurement channels 22, 23, 24, 25 are not faulty and only the second measurement channel has a defect, the second measurement signal ST1 can be replaced by one or more first measurement signals ST2, ST3, ST4, ST5, or determined or corrected. CONFIDENTIAL P221273 DE SHF0237(DE) 13 Fig. 5 shows a schematic representation of an exemplary embodiment of a robot designed as an industrial robot 200 with a plurality of arm segments 201, which are each rotatably connected via drive modules 100, in which the invention is implemented.Although the industrial robot 200 shown here has three arm segments 201 and three drive modules 100, configurations of the industrial robot 200 with a different number of arm segments 201 and drive modules 100 are conceivable, for example, four, five, six, or seven. Furthermore, one drive module 100 can be used for any robot joint. Such industrial robots 200 are often used as collaborative robots that work in close cooperation with humans. CONFIDENTIAL.

Claims

P221273 DE SHF0237(DE) 14 claims 1. Method for detecting a faulty measuring channel on an elastic transmission element (1) in a transmission, wherein the elastic transmission element (1) has a first strain gauge arrangement (20) with a plurality of first measuring channels (22, 23, 24, 25), characterized in that for each of the first measuring channels (22, 23, 24, 25) a plausibility check is carried out on the basis of a plurality of first measuring signals (ST2, ST3, ST4, ST5).

2. Method according to claim 1, characterized in that the transmission is designed as a stress wave transmission, with a wave generator acting on the elastic transmission element (1), the first measurement signals (ST2, ST3, ST4, ST5) of the individual first measurement channels (22, 23, 24, 25) of the first strain gauge arrangement (20) being dependent on the rotational position of the wave generator.Method according to one of the preceding claims, characterized in that, for the plausibility check, a difference is formed between several, in particular two, terms that are dependent on several first measurement signals (ST2, ST3, ST4, ST5).

4. Method according to one of the preceding claims, characterized in that the elastic transmission element (1) has a second strain gauge arrangement (10) with a second measurement channel, wherein a second measurement signal (ST1) of the second measurement channel of the second strain gauge arrangement (10) is proportional to the torque and does not depend, or depends only to a small extent, on an angular position of the wave generator, wherein for each of the first measurement channels (22, 23, 24, 25), the plausibility check is carried out using one or more first measurement signals (ST2, ST3, ST4, ST5) and / or the second measurement signal (ST1).Method according to claim 4, characterized in that a plausibility check is carried out for the second measuring channel based on a plurality of first measuring signals (ST2, ST3, ST4, ST5).

6. Method according to claim 4, characterized in that a difference between one of one or more first measuring signals (ST2, ST3, ST4, CONFIDENTIAL) is used for the plausibility check. P221273 DE SHF0237(DE) 15 ST5) dependent first term and a second term dependent on the second measurement signal (ST1).

7. Method for determining a torque acting on an elastic transmission element (1) of a transmission, in particular a stress wave transmission, wherein a faulty first (22, 23, 24, 25) or possibly second measurement channel is detected according to a method according to one of the preceding claims, the faulty measurement channel is corrected using one or more first measurement signals (ST2, ST3, ST4, ST5) and wherein the torque acting on the transmission element (1) is determined using the corrected measurement channel and / or the remaining measurement channels.

8. Drive module (100) with a transmission, in particular a stress wave transmission, and with an evaluation unit which is configured to detect a faulty first or possibly second measurement channel.detecting a second measuring channel (22, 23, 24, 25) according to a method according to one of claims 1 to 6, correcting the faulty measuring channel using one or more first measuring signals (ST2, ST3, ST4, ST5), and determining the torque acting on the transmission element (1) using the corrected measuring channel and / or one or more remaining measuring channels.

9. Robot arm (200) with at least one drivable arm segment (201), wherein the arm segment (201) is drivable via the drive module (100) according to claim 8. CONFIDENTIAL.

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