Method for monitoring the wear of a component of a mechanical chain rotated by a motor

Abstract

This method for monitoring the wear of a component in a mechanical chain rotated by a motor comprises the following steps: - Retrieval (step 9) of the torque values ​​applied to the component; - Calculation (step 11) of the shear stresses applied to the component; - Identification and counting (step 13) of stress cycles; - Determination (step 15) of a damage value for each identified stress cycle; - Summation (step 17) of all the damage values ​​determined for each identified stress cycle into a total damage value; and - Comparison (step 19) of the total damage value to a threshold damage value at which the component presents a risk of failure. Figure for the abstract: Fig. 2

Description

Title of the invention: Method for monitoring the wear of an element of a mechanical chain rotated by a motor Technical field

[0001] The present invention relates to methods for predicting the wear of components of a mechanical chain.

[0002] In particular, the present invention relates to monitoring the wear of a safety pin in an actuation and force compensation system in an aircraft, such as a trim actuator of a helicopter, and in particular an active trim actuator.

[0003] Generally speaking, the invention applies to any element of a mechanical chain whose wear or lifespan must be monitored for maintenance or safety purposes. Previous techniques

[0004] The main function of an aircraft force compensation system is to return to the pilot of the aircraft a resistive force on the flight controls in the form of force feedback. This force feedback can be generated by a spring, a friction device or a motor connected to the control stick of the aircraft by the mechanical chain of the force compensation system.

[0005] Current architectures of force compensation systems, such as helicopter trim actuators, comprise shafts forming a force path along the mechanical chain. Said shafts are connected by mechanical elements making it possible to transmit a movement from one shaft to another.

[0006] One of the mechanical elements of a force compensation system is a pin positioned at the interface between a mechanical chain connected to the control stick and the mechanical chain of the force compensation system.

[0007] The pin allows the pilot, if the force compensation system seizes, to break said pin so that the force compensation system does not hinder the movements of the control stick.

[0008] [Fig.l] shows a sectional view of a first and a second pin 1 and 3 positioned at the interface between a shaft 5 connected to the control stick and a connecting element 7 of the mechanical chain of a force compensation system. The connecting element 7 is in annular contact with the shaft 5.

[0009] The first pin 1 is tubular and comprises an annular shear section whose shear torque varies as a function of the internal diameter of said first pin 1.

[0010] The second pin 3 is solid and of circular shear section.

[0011] The first and second pins 1 and 3 each extend along their long axis horizontal so as to secure the shaft 5 connected to the control stick and the connecting element 7 of the mechanical chain of the force compensation system when said pins 1 and 3 are not broken.

[0012] Each pin 1 and 3 is configured to be broken by the pilot applying a torque to the control stick, said torque exceeding a shear torque of the pin 1 or 3.

[0013] Each pin 1 and 3 is thus designed to break only by voluntary action of the pilot when the latter detects the seizure of the force compensation system. However, a pin is subjected to repeated force cycles which can damage it. The pilot then risks feeling discomfort in his piloting, for example because the pin takes up play. It can even go as far as suffering premature breakage. This latter risk is all the more important for so-called active force compensation systems, or also called "active trims", which include a motor acting as a force feedback generator. Active trims include electronics integrated into the motor, the latter having high load requirements, and providing significant torques contributing to the deterioration of a pin.

[0014] There is therefore added value in knowing the residual life of such a pin in order to adapt its maintenance to the exact need. Statement of the invention

[0015] The present invention therefore aims to overcome the aforementioned drawbacks and to provide a method for monitoring the wear of elements of a mechanical chain set in rotation by a motor.

[0016] The present invention relates to a method for monitoring the wear of an element of a mechanical chain rotated by a motor, the method comprising the following steps: - Recovery of the torque values ​​applied to the element during rotation of the mechanical chain; - Calculation of the shear stresses applied to the element based on the recovered torque values; - Identification and counting of stress cycles by applying a counting method for analysis of the fatigue of the element; - Determination, for each identified stress cycle, of a damage value from an equivalence table; - Sum of all damage values ​​determined for each cycle of constraint identified in a total damage value; and - Comparison of the total damage value to a threshold damage value for which the element presents a risk of rupture.

[0017] Thus, the so-called active architecture of the mechanical chain, because it includes a motor, makes it possible to determine the shear stresses applied to the element, and to follow these stresses over time in order to monitor the wear of said element and to be able to carry out predictive maintenance if necessary.

[0018] Advantageously, the torque data recovery step is performed by measuring the intensity of the motor output current.

[0019] In one embodiment, the cycle counting method comprises associating increasing minima and decreasing maxima of the shear stresses calculated on a time scale, identifying a cycle comprising associating a cycle type with the amplitude of said cycle between the minimum shear stress and the maximum shear stress of said cycle.

[0020] Advantageously, the counting method is a Rainflow type method.

[0021] Advantageously, the step of determining the damage value is carried out using a Basquin type method.

[0022] In a particular embodiment, the damage value determined for each identified stress cycle of a certain type corresponds to the inverse of the number of cycles of said type before rupture of the element, the equivalence table giving the number of cycles before rupture for a given type of cycle.

[0023] Advantageously, the step of identifying and counting the stress cycles is carried out only for each cycle whose amplitude between the minimum and maximum shear stresses of said cycle is greater than a predetermined reference amplitude.

[0024] In one embodiment, the method further comprises a step of generating an alert when the total damage value is greater than 80% of the threshold damage value.

[0025] In one embodiment, the element comprises a pin positioned in the diameter of a shaft and a connecting element in annular contact with said shaft, the step of calculating the shear stress being carried out such that:

[0026] _ ^RK,ini L / j \2 ~ nDd1 V 1 k DJ

[0027] with rs the shear stress, R the total reduction ratio of the mechanical chain, q the cumulative efficiency of reduction stages of the mechanical chain, lm the intensity of the current at the output of the motor, Kt the torque constant of the motor, d the diameter of the element and D the diameter of the shaft.

[0028] Advantageously, the method is a computer-implemented method. Brief description of the drawings

[0029] Other aims, characteristics and advantages of the invention will appear on reading the following description, given solely by way of non-limiting example, and made with reference to the appended drawings in which:

[0030] [Fig.l] which has already been mentioned, is a schematic sectional view of a first and a second pin positioned at the interface between a shaft connected to a control stick and a connecting element of the mechanical chain of a force compensation system;

[0031] [Fig.2] is a schematic representation of the different stages of the method for monitoring the wear of an element of a mechanical chain according to the invention; and

[0032] [Fig.3] is a schematic sectional view of an element whose wear is monitored by the method according to the invention.

[0033] Detailed description of at least one embodiment

[0034] [Fig.2] shows schematically the different stages of a method for predicting the wear of an element 3 of a mechanical chain set in rotation by a motor.

[0035] The element 3 comprises for example a pin 3 as defined above and shown in [Fig. 1]. The pin 3 is for example alone at the interface between a shaft 5 connected to a control stick and a connecting element 7 of a mechanical chain of a force compensation system.

[0036] To implement the method for predicting the wear of the element 3, a step 9 is first carried out for recovering the torque values ​​applied to the element 3 during rotation of the mechanical chain.

[0037] In the mechanical chain, the stress applied to element 3 is directly linked to the torque produced by the motor, this torque itself being linked to the current leaving the motor.

[0038] Thus, step 9 of torque data recovery can be carried out by measuring the intensity lm of the motor output current. By analyzing the output currents, the stresses applied to element 3 can be represented as a function of time. The motor is therefore used as a torque sensor.

[0039] The recovery step 9 is carried out regularly over time in order to obtain, during the following step 11, the value of the shear stress on a time scale. The recovery step 9 is for example carried out at a frequency

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[0057] between 500 Hz and 5 kHz. We then carry out a step 11 of calculating the shear stresses applied to element 3 as a function of the recovered torque values. This calculation step 11 depends on the position and size of element 3 in the mechanical chain. [Fig. 3] shows a schematic section of element 3 previously illustrated in [Fig. 1]. The parameters essential for calculating shear stresses are also shown. Calculation step 11 includes the calculation of the torque Cm generated by the motor from the intensity of the current Im at the output of said motor, with Kt the torque constant of the motor in Nm / A: Cm = KtxIm Then, if the mechanical chain includes reduction stages, we deduce the torque applied to the pin Cg such that: C8 = fi x ü x Cm , with R the total reduction ratio of the mechanical chain and q the cumulative efficiency of the reduction stages. In order to calculate the shear stress applied to the element, we have on the one hand: - — 4F> ~lP , with Fs the shear force of element 3 along the shear section •Sg of said element, d being the diameter of element 3, element 3 here being a solid pin 3 as illustrated in [Fig.l]. On the other hand, we have: F, = Ftxcos / ? = x ^l-(sin / ?)2 = x ^1- , with Ft the tangential shear force, the angle between the shear force Fs and the tangential shear force Ft, and D the diameter of a shaft, here the shaft as illustrated in [Fig.l] and connected to the control stick. Thus, the calculation step makes it possible to calculate the shear stresses applied to the element as a function of the recovered torque values, and in particular as a function of the intensity Im. Ts - V1 ' VD / “ rrDcP V 1 ' \ O / Shear stress is usually expressed in N / m2 or Mpa. A step 13 of identification and counting of stress cycles is then carried out by applying a counting method for fatigue analysis of element 3.

[0058] In one embodiment, step 13 of identification and counting is carried out by a Rainflow type method.

[0059] In order to identify a stress cycle, the identification and counting step 13 comprises the association of increasing minima and decreasing maxima of the shear stresses previously calculated on a time scale. A stress cycle therefore corresponds to a temporal grouping of increasing minima and decreasing maxima.

[0060] During this step 13, the minimum shear stress, the maximum shear stress and the average shear stress are identified. The value amplitude between the minimum shear stress and the maximum shear stress of a cycle makes it possible to define and assign a stress cycle type to said stress cycle. Thus, the identification of a stress cycle is carried out when a stress cycle type is assigned to said stress cycle, namely when the value amplitude between the minimum shear stress and the maximum shear stress of said cycle is determined.

[0061] Optionally, when the value amplitude between the minimum shear stress and the maximum shear stress of a cycle is less than a given reference amplitude, said cycle is not taken into account for the rest of the steps. It is considered not to be counted and not identified as a stress cycle.

[0062] A step 15 is then carried out to determine, for each identified stress cycle, a damage value from an equivalence table.

[0063] In a particular embodiment, step 15 of determining the damage value of each identified cycle is carried out using a Basquin-type method. This Basquin-type method is a model in which each identifiable cycle type corresponds to a maximum number of authorized cycles beyond which there is rupture of the element 3, also called the number of cycles before rupture.

[0064] The Basquin model generally takes the form of an equivalence table, for example an abacus specific to a material of a given shape, which for a cycle of a given amplitude, gives the number of cycles before rupture of said material.

[0065] In a particular mode of implementation, the damage value Vw of an identified cycle n, therefore of a certain type having a certain amplitude) corresponds to the inverse of the number Nn of cycles of this type before rupture:

[0066] V = ' ' has

[0067] A step 17 is then carried out to sum each of the damage values ​​determined for each identified stress cycle into a total early damage value. Thus, for i identified stress cycles, we have:

[0068] v j_ h>t

[0069] Then, a step 19 is carried out for comparing the total damage value Vtot with a threshold damage value beyond which the element 3 presents a risk of rupture. The threshold damage value is predetermined, for example using the equivalence table described above.

[0070] In a particular implementation mode, a step 21 of generating an alert is carried out when the total damage value is greater than 80% of the threshold damage value.

Claims

Claims

1. Method for monitoring the wear of an element (3) of a mechanical chain rotated by a motor, characterized in that it comprises the following steps: - Recovery (step 9) of the torque values ​​applied to the element (3) during rotation of the mechanical chain; - Calculation (step 11) of the shear stresses applied to the element (3) as a function of the recovered torque values; - Identification and counting (step 13) of stress cycles by applying a counting method for analyzing the fatigue of the element (3); - Determination (step 15), for each identified stress cycle, of a damage value from an equivalence table; - Summation (step 17) of all the damage values ​​determined for each identified stress cycle into a total damage value;and - Comparison (step 19) of the total damage value with a threshold damage value for which the element (3) presents a risk of rupture.;

2. The method of claim 1, wherein the step (9) of retrieving torque data is performed by measuring the intensity of the output current of the motor.

3. A method according to either of claims 1 and 2, wherein the cycle counting method comprises associating increasing minima and decreasing maxima of the shear stresses calculated over a time scale, the identification of a cycle comprising associating a cycle type with the amplitude of said cycle between the minimum shear stress and the maximum shear stress of said cycle.

4. A method according to any one of claims 1 to 3, wherein the counting method is a Rainflow type method.

5. A method according to any one of claims 1 to 4, wherein the step (15) of determining the damage value is carried out with a Basquin type method.

6. A method according to any one of claims 1 to 5, wherein the damage value determined for each identified stress cycle of a certain type corresponds to the inverse of the number of cycles of said type before rupture of the element, the equivalence table giving the number of cycles before rupture for a given cycle type.

7. A method according to any one of claims 1 to 6, wherein the step (13) of identifying and counting stress cycles is performed only for each cycle whose amplitude between the minimum and maximum shear stresses of said cycle is greater than a predetermined reference amplitude.

8. A method according to any one of claims 1 to 7, further comprising a step (21) of generating an alert when the total damage value is greater than 80% of the threshold damage value.

9. Method according to any one of claims 1 to 8, the element (3) comprising a pin (3) positioned in the diameter of a shaft (5) and a connecting element (7) in annular contact with said shaft (5), the step (11) of calculating the shear stress being carried out such that _ I fd}2 “ -DcP y1" \ D / with the shear stress, R the total reduction ratio of the mechanical chain, q the cumulative efficiency of reduction stages of the mechanical chain, Im the intensity of the current at the output of the motor, Kt the torque constant of the motor, d the diameter of the element (3) and D the diameter of the shaft (5).

10. Method according to any one of claims 1 to 9, characterized in that it is implemented by computer.