JP7533332B2 - Drive shaft fatigue damage estimation device - Google Patents

Description

The present invention relates to a drive shaft fatigue damage estimation device that estimates the degree of fatigue damage of a drive shaft that transmits torque generated by a drive source of a vehicle to the steering drive wheels.

The device described in Patent Document 1 is known as a device that notifies the user when it is time to repair or replace a power transmission system component of a vehicle. The device described in this document estimates the degree of fatigue damage to the power transmission system components based on vehicle driving information. The driving information used to estimate the degree of fatigue damage includes information on the torque generated by the vehicle's drive source. When the degree of fatigue damage reaches a certain value, the device notifies the vehicle user that the power transmission system component needs to be repaired or replaced by displaying it on the instrument panel, etc.

JP 2019-132358 A

Note that the power transmission system components of a vehicle include a drive shaft that transmits torque generated by the drive source to the drive wheels. In a front-wheel drive vehicle, the drive wheels to which the drive shaft transmits torque are steered wheels that change direction in response to steering operations. Here, wheels that are both drive wheels and steered wheels are referred to as steered drive wheels.

In drive shafts connected to steered drive wheels, the progression of fatigue damage also changes depending on the steering operation conditions. However, the estimation of the degree of fatigue damage using the device described in the above-mentioned document 1 does not take into account the unique circumstances of drive shafts, namely, that the steering operation conditions affect the progression of fatigue damage. Therefore, there is still room for improvement in the accuracy of estimating the degree of fatigue damage in drive shafts.

The estimation device that solves the above problem estimates the degree of fatigue damage of a drive shaft that transmits torque generated by a drive source to the steered drive wheels. The estimation device calculates the joint angle of the drive shaft relative to the steered drive wheels based on the steering angle of the steering wheel, and calculates a fatigue load value that indicates the degree of progress of fatigue damage of the drive shaft based on the calculated joint angle value and the shaft torque and rotation speed of the drive shaft, repeatedly performing the process at a predetermined calculation cycle. The estimation device then calculates an estimated value of the degree of fatigue damage by integrating the calculated fatigue load values for each calculation cycle.

The stress amplitude of the repeated stress applied to the drive shaft while the vehicle is running is calculated as a function of the joint angle and the axial torque of the drive shaft. Furthermore, the number of repetitions of the repeated stress in a calculation cycle is a value proportional to the rotation speed of the drive shaft. Therefore, the fatigue load value as described above is calculated as a function of the joint angle and the axial torque and rotation speed of the drive shaft. The integrated value of the fatigue load calculated for each calculation cycle is a value indicating the degree of fatigue damage to the drive shaft. Therefore, the above estimation device can estimate the degree of fatigue damage to the drive shaft in a form that reflects the effect of the joint angle on the progression of fatigue damage. This can improve the accuracy of estimating the degree of fatigue damage to the drive shaft.

FIG. 1 is a diagram illustrating a configuration of an embodiment of an estimation device. 3 is a flowchart of an estimation routine performed by the estimation device. 6 is a graph showing the relationship between the stress amplitude of a drive shaft and the joint angle when the shaft torque is a constant value. 5 is a graph showing the relationship between stress amplitude and the number of cycles at which a drive shaft reaches its life.

Below, one embodiment of an estimation device for the degree of fatigue damage in a drive shaft will be described in detail with reference to Figures 1 to 4. The estimation device 30 of this embodiment is installed in a hybrid vehicle 10.

<Power transmission system configuration>
First, the configuration of a power transmission system of a hybrid vehicle 10 in which an estimation device 30 of this embodiment is installed will be described with reference to Fig. 1. The hybrid vehicle 10 is equipped with three drive sources, namely, an engine 11, a generator 12, and a motor 13. The engine 11 and the generator 12 are connected to a power split mechanism 14. The power split mechanism 14 is a planetary gear mechanism having three rotating elements, namely, a sun gear, a ring gear, and a planetary carrier. One of the rotating elements of the power split mechanism 14 is connected to the crankshaft of the engine 11, another is connected to the rotating shaft of the generator 12, and the remaining is connected to a reduction gear mechanism 15. The reduction gear mechanism 15 is also connected to the rotating shaft of the motor 13.

The reduction gear mechanism 15 is connected to left and right drive shafts 17 via a differential gear 16. The differential gear 16 is a differential device that allows the left and right drive shafts 17 to rotate differentially. Left and right steering wheels 18 are connected to the left and right drive shafts 17, respectively. An inner joint 17A is provided at the connection between the left and right drive shafts 17 and the differential gear 16. An outer joint 17B is provided at the connection between the left and right drive shafts 17 and the steering wheels 18, respectively. In this hybrid vehicle 10, a sliding constant velocity joint is used for the inner joint 17A, and a fixed constant velocity joint is used for the outer joint 17B.

<Configuration of steering mechanism>
Next, the configuration of the steering mechanism of the hybrid vehicle 10 will be described. The steering mechanism includes a link mechanism made up of a tie rod 20 extending in the left-right direction of the vehicle body and a knuckle arm 21 that holds the steered drive wheels 18. The steering mechanism also includes a steering gear mechanism 23. The steering gear mechanism 23 converts the rotation of a steering wheel 22 into left-right movement of the tie rod 20. By transmitting the movement of this tie rod 20 to the knuckle arm 21, the steering angle of the steered drive wheels 18 is changed.

<Configuration of Estimation Device>
The configuration of the estimation device 30 installed in the hybrid vehicle 10 configured as above will be described. The estimation device 30 includes an arithmetic processing device 31, a main storage device 32, and a non-volatile memory 33. The main storage device 32 stores a program and data for estimating the fatigue damage degree of the drive shaft 17. The estimation device 30 performs the process of estimating the fatigue damage degree of the drive shaft 17 by having the arithmetic processing device 31 read and execute the program stored in the main storage device 32. In addition, a current sensor 34 that detects the current value of the motor 13, a resolver 35 that detects the rotation speed of the motor 13, and a steering angle sensor 36 that detects the steering angle of the steering wheel 22 are connected to the estimation device 30. Note that, in reality, an electronic control unit for controlling the hybrid vehicle 10 plays the role of the estimation device 30.

<Fatigue Damage Degree Estimation Process>
Next, the process of the estimation device 30 for estimating the degree of fatigue damage of the drive shaft 17 will be described with reference to Fig. 2. The estimation device 30 repeatedly executes the process of the fatigue damage degree estimation routine shown in Fig. 2 at every predetermined calculation period P while the hybrid vehicle 10 is running. Note that the process of this routine is executed separately for each of the left and right drive shafts 17. Furthermore, each numerical value calculated in this routine is a different value for each of the left and right drive shafts 17.

In the drive shaft 17 for which the estimation device 30 of this embodiment estimates the degree of fatigue damage, the surface layer of the ball groove of the outer joint 17B is the area where fatigue damage progresses most quickly. Strictly speaking, the fatigue damage estimation routine estimates the degree of fatigue damage in the surface layer of the ball groove of the outer joint 17B.

When this routine starts, the estimation device 30 first calculates the joint angle θ of the outer joint 17B from the steering angle of the steering wheel 22 in step S100, i.e., the joint angle θ of the drive shaft 17 relative to the steered drive wheels 18. In the following step S110, the estimation device 30 calculates the rotation speed D of the drive shaft 17 from the rotation speed of the motor 13. In the same step S110, the estimation device 30 also calculates the shaft torque T of the drive shaft 17 from the current value of the motor 13. Note that the rotation speed D indicates the number of rotations of the drive shaft 17 per unit time.

The left and right drive shafts 17 are connected to the motor 13 via the differential gear 16 and the reduction gear mechanism 15. Therefore, the rotation speed D of the left and right drive shafts 17 can be calculated from the rotation speed of the motor 13 and the gear ratio of the reduction gear mechanism 15 and the differential gear 16. The shaft torque of the motor 13 can be calculated from the current value of the motor 13. The input torque of the differential gear 16 can be calculated from the shaft torque of the motor 13. The input torque of the differential gear 16 is distributed to the left and right drive shafts 17. Therefore, the shaft torque T of the left and right drive shafts 17 can be calculated from the current value of the motor 13.

Then, in step S120, the estimation device 30 calculates a value that satisfies the relationship in equation (1) based on the joint angle θ as the value of the corrected shaft torque Tc. "θmax" in equation (1) is the maximum joint angle. The maximum joint angle θmax indicates the maximum value of the joint angle θ within the steering angle change range of the steered drive wheels 18.

Furthermore, in step S140, the estimation device 30 calculates a value that satisfies the relationship of equation (2) as the value of fatigue load F, based on the corrected shaft torque Tc and the rotation speed D of the drive shaft 17. "k" in equation (2) represents a torque constant that is a preset constant.

In the following step S150, the estimation device 30 updates the value of the cumulative fatigue load AF so that the updated value is the sum of the value of the fatigue load F calculated in step S140 plus the value before the update. The value of the cumulative fatigue load AF is recorded in the non-volatile memory 33. The value recorded in the non-volatile memory 33 is retained even when the system of the hybrid vehicle 10 is stopped. Meanwhile, the calculation of the fatigue load F in step S130 and the update of the value of the cumulative fatigue load AF in step S140 are performed each time this routine is executed. In addition, the estimation device 30 executes this routine for each calculation period P. Therefore, the value of the cumulative fatigue load AF is a value obtained by accumulating the calculated values of the fatigue load F for each calculation period P.

In the next step S150, the estimation device 30 calculates the damage level of the drive shaft 17 based on the accumulated fatigue load AF, and then ends the processing of this routine. The damage level is calculated as a value indicating the degree of fatigue damage to the drive shaft 17. In this embodiment, ten levels are set as the damage levels, from level 0 to level 9. Then, in step S150, the damage level value is calculated so that when the value of the accumulated fatigue load AF is gradually increased from 0, the number of levels increases stepwise from level 0 to level 9.

In the hybrid vehicle 10, when the damage level calculated by the estimation device 30 in this manner exceeds a certain value, a warning light provided on the instrument panel or the like is turned on. In addition, in the hybrid vehicle 10, when the damage level exceeds a certain value, output restrictions are imposed on the engine 11 and the motor 13.

If the hybrid vehicle 10 is equipped with a data communication device for communication with a data center for vehicle management, the damage level can be used as follows. The data center for vehicle management collects the damage level and mileage from each vehicle under its management. The data center then predicts when to replace the drive shaft 17 of each vehicle from the collected data. Based on the prediction results, the data center then notifies the user of the date the vehicle will be brought into the maintenance workshop. The data center also predicts the future trend in the number of drive shaft 17 replacements from the predicted replacement times for each vehicle, and places an order with the supplier so that the required number of drive shafts 17 are delivered at the required time.

<Actions and Effects of the Embodiment>
The operation and effects of this embodiment will be described.
While the hybrid vehicle 10 is traveling, repeated stress is generated in each portion of the drive shaft 17. The repeated stress causes fatigue damage to progress in each portion of the drive shaft 17. The stress amplitude S of the repeated stress in each portion of the drive shaft 17 is determined as a function of the axial torque T and the joint angle θ. The stress amplitude S in the surface layer of the ball groove of the outer joint 17B is proportional to the axial torque T and to the tangent value of "θ/2".

Figure 3 shows the relationship between the stress amplitude S of the surface layer of the ball groove of the outer joint 17B and the joint angle θ when the axial torque T is constant. Here, the stress amplitude S when the joint angle θ is the maximum joint angle θmax is "Smax". In this case, the stress amplitude S1 when the joint angle θ is "θ1" is the value shown in formula (3). "Smax" when the joint angle θ is the maximum joint angle θmax is a value proportional to the axial torque T. Therefore, the corrected axial torque Tc calculated as a value that satisfies the relationship of formula (2) above is a value proportional to the stress amplitude S of the surface layer of the ball groove of the outer joint 17B.

FIG. 4 shows the relationship between the stress amplitude S and the number of replacement cycles Nf. The number of replacement cycles Nf indicates the number of repeated stress cycles N until fatigue damage requiring part replacement occurs when repeated stress with a constant stress amplitude S is applied to the surface layer of the ball groove of the outer joint 17B. Such a relationship between the stress amplitude S and the number of replacement cycles Nf can be obtained by a fatigue test. In the case of this embodiment, the relationship shown in formula (4) has been confirmed between the stress amplitude S and the number of replacement cycles Nf by a fatigue test. Note that the above-mentioned torque coefficient k is set to the value of "k" in the relational expression (4).

Here, as shown in FIG. 4, the number of replacement repetitions Nf when the stress amplitude S is "S2" is "N2". The period from the start of use of the drive shaft 17 until fatigue damage requiring part replacement occurs is defined as the total life of the drive shaft 17. In this case, the remaining life of the drive shaft 17 when a repeated stress with a stress amplitude S of "S2" is applied once is reduced by the value obtained by multiplying the total life by the reciprocal of "N2". This is the reduction in the remaining life, that is, the progression of fatigue damage. Therefore, as shown in formula (5), the progression ΔF of fatigue damage per one rotation of the drive shaft 17 is a value obtained by exponentiating the stress amplitude S with the torque coefficient k as the exponent.

On the other hand, as described above, the stress amplitude S is a value proportional to the corrected shaft torque Tc. Moreover, the number of repetitions of the repeated stress in the calculation cycle P is a value proportional to the rotation speed D of the drive shaft 17. Therefore, the value of the fatigue load F calculated as a value satisfying the relationship of formula (2) is a value proportional to the progress of fatigue damage of the drive shaft 17 in the calculation cycle P. The accumulated fatigue load AF, which is the sum of the calculated values of the fatigue load F for each calculation cycle P, is a value indicating the degree of fatigue damage of the drive shaft 17 from the start of use to the present. That is, in this embodiment, such accumulated fatigue load AF is obtained as an estimate of the degree of fatigue damage of the drive shaft 17.

According to the above-described device 30 for estimating the fatigue damage degree of the drive shaft 17 of the present embodiment, the following effects can be achieved.
(1) In this embodiment, the fatigue load F is calculated for each predetermined calculation cycle P based on the joint angle θ and the shaft torque T and rotation speed D of the drive shaft. The calculated values of the fatigue load F for each calculation cycle P are then integrated to calculate the cumulative fatigue load AF, which is an estimate of the fatigue damage level of the drive shaft 17. Therefore, the fatigue damage level of the drive shaft 17 can be estimated in a form that reflects the effect of the joint angle θ on the progression of fatigue damage. This improves the accuracy of estimating the fatigue damage level of the drive shaft 17.

(2) Because it is possible to estimate the degree of fatigue damage to the drive shaft 17 with high accuracy, it is also possible to predict with high accuracy when to replace the drive shaft 17. This allows appropriate preparations for the replacement of the drive shaft 17 to be made, such as informing the user of the date of bringing the drive shaft 17 into the repair shop and ordering replacement parts.

(3) If fatigue damage to the drive shaft 17 progresses and causes problems such as abnormal noise and increased vehicle body vibration, the user's trust in the vehicle will be damaged. In contrast, in this embodiment, it is possible to accurately predict when to replace the drive shaft 17, and to encourage part replacement at the appropriate time before a problem occurs. This improves the user's evaluation of the vehicle.

This embodiment can be modified as follows: This embodiment and the following modifications can be combined with each other to the extent that there is no technical contradiction.
The estimation device 30 of the above embodiment may be implemented as a device that estimates the degree of fatigue damage of a drive shaft in a vehicle other than the hybrid vehicle 10 .

- In the above embodiment, the shaft torque T and rotation speed D of the drive shaft 17 are calculated from the current value and rotation speed of the motor 13, but they may be calculated from other parameters. For example, in the case of a conventional vehicle equipped with an engine as the only driving source, it is possible to calculate the shaft torque T and rotation speed D of the drive shaft 17 from the engine torque, rotation speed, gear position of the transmission, etc.

In the above embodiment, the degree of fatigue damage in the surface layer of the ball groove of the outer joint 17B was estimated as the degree of fatigue damage in the drive shaft 17. The degree of fatigue damage in other parts of the drive shaft 17 may also be estimated.

- In the above embodiment, the corrected shaft torque Tc, which is a value proportional to the stress amplitude S of the repeated stress, was calculated to be a value that satisfies the relationship of formula (1). The relationship between the shaft torque T, joint angle θ, and stress amplitude S varies depending on the configuration and location of the drive shaft. Even in such cases, the stress amplitude S can be found as a function of the shaft torque T and joint angle θ. Therefore, it is advisable to adopt an appropriate function of the shaft torque T and joint angle θ used to calculate the corrected shaft torque Tc according to the configuration of the drive shaft and the location for which the fatigue damage degree is to be estimated.

- In the above embodiment, the value of the fatigue load F, which indicates the degree of progression of fatigue damage for each calculation cycle P, was calculated as a value that satisfies the relationship of formula (2) derived from the relationship between the stress amplitude S and the number of replacement cycles Nf. The relationship between the stress amplitude S and the number of replacement cycles Nf also differs depending on the configuration and location of the drive shaft. Even in such cases, the fatigue load F can be found as a function of the joint angle θ, the shaft torque T, and the rotation speed D. Therefore, it is advisable to adopt appropriate functions for the joint angle θ, the shaft torque T, and the rotation speed D used to calculate the fatigue load F according to the configuration of the drive shaft and the location for which the degree of fatigue damage is to be estimated.

REFERENCE SIGNS LIST 10 hybrid vehicle 11 engine 12 generator 13 motor 14 power split mechanism 15 reduction gear mechanism 16 differential 17 drive shaft 17A inner joint 17B outer joint 18 steered drive wheel 20 tie rod 21 knuckle arm 22 steering 23 steering gear mechanism 30 estimation device 31 calculation processing device 32 main memory device 33 non-volatile memory 34 current sensor 35 resolver 36 steering angle sensor

Claims (1)

A device for estimating a degree of fatigue damage of a drive shaft that transmits torque generated by a drive source to steering drive wheels, comprising:
a joint angle of the drive shaft relative to the steered drive wheels is calculated based on a steering angle of a steering wheel, and a fatigue load value indicating a degree of progress of fatigue damage of the drive shaft is calculated based on the calculated joint angle value and the shaft torque and rotation speed of the drive shaft, the process being repeatedly performed at predetermined calculation intervals;
and calculating an estimated value of the fatigue damage degree by integrating the computed values of the fatigue load for each computation cycle.