Life estimation device

Abstract

To provide a lifetime estimation device capable of highly precisely estimating the lifetime of a ball screw of an electric power steering while reducing the number of reducing man-hours and cost.SOLUTION: A lifetime estimation device 40 includes: a damage ratio storage section 32 which stores a damage ratio (a first damage ratio) of a ball screw 12 relative to a steering angle and a damage ratio (a second damage ratio) of the ball screw 12 relative to a vehicle total weight; a first damage number calculation section 35 which calculates the total damage number of the ball screw 12 corresponding to the frequency of a steering angle based on steering angle information concerning the frequency of the steering angle and the first damage ratio; a second damage number calculation section 36 which calculates the total damage number of the ball screw 12 corresponding to the frequency of a vehicle total weight based on vehicle total weight information concerning the frequency of the vehicle total weight and the second damage ratio; and a final damage number calculation section 37 which calculates the total damage number of the final ball screw 12 based on the total damage number of each ball screw 12.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a life estimation device.

Background Art

[0002] For example, Patent Document 1 describes an electric power steering. The electric power steering described in Patent Document 1 includes a steering wheel, a steering shaft connected to the steering wheel, and a steering assist motor that applies an assist force for assisting a steering operation by rotationally driving the steering shaft.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] By the way, as one type of electric power steering (EPS), there is a rack parallel type. In a rack parallel type EPS, the rotation of the steering assist motor is transmitted to a ball screw via a belt, and the rotational motion is converted into a linear motion by the ball screw. Such a ball screw of EPS is likely to generate rolling fatigue damage. Therefore, it is required to estimate the life of the ball screw. In order to estimate the life of the ball screw, it is conceivable to simultaneously measure data corresponding to the tie rod thrust, the tie rod angle, and the moment arm during the operation of the vehicle, and calculate the rolling fatigue damage generated in the ball screw. However, in this case, development and installation of sensors for measuring the tie rod thrust and the tie rod angle, modification of communication devices of the vehicle, etc. are required, which consequently leads to an increase in man-hours and costs.

[0005] The objective of the present invention is to provide a life estimation device that can accurately estimate the lifespan of a ball screw in an electric power steering system while reducing man-hours and costs. [Means for solving the problem]

[0006] The inventors conducted extensive research on the lifespan of ball screws in electric power steering systems and discovered that steering angle, steering rotation frequency, and vehicle weight are all factors used in calculating the ball screw's lifespan. Further investigation revealed that the rolling fatigue damage to the ball screw differs significantly depending on the steering angle and vehicle weight, leading to the completion of the present invention.

[0007] (1) That is, one aspect of the present invention is a life estimation device for estimating the life of a ball screw that converts the rotational motion of an assist motor into linear motion of a shaft connected to a wheel in an electric power steering system, comprising: a damage ratio storage unit that stores the damage ratio of the ball screw to the steering angle as a first damage ratio and the damage ratio of the ball screw to the gross vehicle weight as a second damage ratio; an information acquisition unit that acquires steering angle information relating to the frequency of the steering angle and gross vehicle weight information relating to the frequency of the gross vehicle weight; and the steering angle information acquired by the information acquisition unit and the damage ratio stored in the damage ratio storage unit The system includes: a first damage calculation unit that calculates the total number of ball screws damaged according to the frequency of steering angles based on a damage ratio; a second damage calculation unit that calculates the total number of ball screws damaged according to the frequency of vehicle gross weight based on gross vehicle weight information acquired by an information acquisition unit and a second damage ratio stored in a damage ratio storage unit; and a final damage calculation unit that calculates the final total number of ball screws damaged according to the frequency of steering angles calculated by the first damage calculation unit and the total number of ball screws damaged according to the frequency of vehicle gross weight calculated by the second damage calculation unit.

[0008] In this type of life estimation device, the damage ratio of the ball screw to the steering angle (first damage ratio) and the damage ratio of the ball screw to the gross vehicle weight (second damage ratio) are pre-stored. Then, steering angle information related to the frequency of steering angle changes and gross vehicle weight information related to the frequency of gross vehicle weight changes are acquired. Based on the steering angle information and the first damage ratio, the total number of damaged ball screws corresponding to the steering angle frequency is calculated, and based on the gross vehicle weight information and the second damage ratio, the total number of damaged ball screws corresponding to the gross vehicle weight frequency is calculated. Finally, the total number of damaged ball screws corresponding to the steering angle frequency and the total number of damaged ball screws corresponding to the gross vehicle weight frequency are used to calculate the final total number of damaged ball screws. Here, the steering angle information and gross vehicle weight information are acquired as vehicle operation information. Therefore, it is not necessary to simultaneously measure data corresponding to tie rod thrust, tie rod angle, and moment arm when the vehicle is in operation. Consequently, dedicated sensors and other equipment for estimating the life of the ball screws are not required. This reduces labor and costs. Furthermore, by using the first damage ratio, the total number of ball screws damaged according to the frequency of steering angle changes will approach the actual damage. By using the second damage ratio, the total number of ball screws damaged according to the frequency of vehicle gross weight changes will approach the actual damage. As a result, the lifespan of the ball screws in electric power steering can be estimated with high accuracy.

[0009] (2) In (1) above, the first damage calculation unit may calculate the total damage to the ball screw according to the frequency of steering angles by multiplying the ball transfer distance of the ball screw by the number of steering angles and the first damage ratio for each range of steering angles, and adding the multiplier values ​​for each range of steering angles. In such a configuration, the total damage to the ball screw according to the frequency of steering angles will come even closer to the actual damage. Therefore, the lifespan of the ball screw of the electric power steering can be estimated with even greater accuracy.

[0010] (3) In (1) or (2) above, the second damage calculation unit may calculate the average value of the gross vehicle weight and multiply the average value of the gross vehicle weight by the second damage ratio corresponding to the average value of the gross vehicle weight to calculate the total number of ball screws damaged according to the frequency of the gross vehicle weight. In such a configuration, the process of calculating the total number of ball screws damaged according to the frequency of the gross vehicle weight is simplified. Therefore, the man-hours and costs are further reduced.

[0011] (4) In any of (1) to (3) above, the life estimation device may further include a deterioration prediction unit that predicts the deterioration time of the ball screw based on the final total number of ball screws calculated by the final damage number calculation unit, and provides notification according to the predicted value of the ball screw deterioration time. In such a configuration, when the deterioration time of the ball screw approaches, a notification is given to that effect, allowing the ball screw to be replaced at an appropriate time. [Effects of the Invention]

[0012] According to the present invention, the lifespan of the ball screw in an electric power steering system can be accurately estimated while reducing man-hours and costs. [Brief explanation of the drawing]

[0013] [Figure 1] This is a schematic diagram showing an electric power steering system to which a life estimation device according to one embodiment of the present invention is applied. [Figure 2] Figure 1 is a plan view showing the main components of the electric power steering system. [Figure 3] This is a block diagram showing the configuration of a vehicle monitoring system equipped with a life estimation device according to one embodiment of the present invention. [Figure 4] Figure 3 shows a graph illustrating an example of the first and second damage ratios of a ball screw stored in the damage ratio memory unit. [Figure 5] Figure 3 is a flowchart showing the procedure for the damage calculation process performed by the first damage calculation unit. [Figure 6]It is a flowchart showing the procedure of damage number calculation processing executed by the second damage number calculation unit shown in FIG. 3. [Figure 7] It is a flowchart showing the procedure of damage number calculation processing executed by the final damage number calculation unit shown in FIG. 3. [Figure 8] It is a graph showing an example of the total damage number integrated value of the ball screw. [Figure 9] It is a flowchart showing the procedure of deterioration prediction processing executed by the deterioration prediction unit shown in FIG. 3.

Embodiment for Carrying Out the Invention

[0014] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0015] FIG. 1 is a schematic configuration diagram showing an electric power steering to which a life estimation device according to an embodiment of the present invention is applied. FIG. 2 is a plan view showing a main part of the electric power steering shown in FIG. 1.

[0016] In FIGS. 1 and 2, an electric power steering 1 (EPS) is mounted on a vehicle 2 such as a truck. The vehicle 2 is an electric vehicle (EV vehicle) here. The electric power steering 1 is a rack parallel type EPS.

[0017] The electric power steering 1 includes a steering wheel 3 (steering), a steering shaft 4 connected to the steering wheel 3, a shaft portion 5 extending in the left - right direction of the vehicle 2, a pair of left - right tie rods 6 connected to both ends of the shaft portion 5, a pair of left - right knuckle arms 8 connected to each tie rod 6 and holding the left - and - right front wheels 7 (wheels), and a rack & pinion type gearbox 9 connecting the steering shaft 4 and the shaft portion 5. The shaft portion 5 is connected to the front wheels 7 via the tie rods 6 and the knuckle arms 8.

[0018] When the driver D of the vehicle 2 operates the steering wheel 3 (steering operation), the rotational motion of the steering shaft 4 is converted into the linear motion of the shaft portion 5 by the gearbox 9. Then, the linear motion of the shaft portion 5 is transmitted to the front wheels 7 via the tie rod 6 and the knuckle arm 8, and the front wheels 7 are steered.

[0019] Also, the electric power steering 1 includes an assist motor 10 that applies an assist force for assisting the steering operation by the driver D, a pulley 11 fixed to the output shaft 10a of the assist motor 10, a ball screw 12 provided on the shaft portion 5, and a belt 13 that transmits the rotation of the assist motor 10 to the ball screw 12. In FIG. 1, the pulley 11 and the belt 13 are omitted for the sake of convenience.

[0020] The ball screw 12 has a screw shaft 14 and a nut 15 that is axially movable relative to the screw shaft 14. The ball screw 12 converts the rotational motion of the assist motor 10 into the linear motion of the shaft portion 5 by rolling a plurality of balls (not shown) between the screw shaft 14 and the nut 15. The belt 13 is wound around the pulley 11 and the nut 15.

[0021] Also, the electric power steering 1 includes a torque sensor 16 attached to the steering shaft 4, a vehicle speed sensor 17, and a steering ECU 18.

[0022] The torque sensor 16 is a sensor that detects the steering torque (steering operation force) generated during the steering operation by the driver D. The vehicle speed sensor 17 is a sensor that detects the vehicle speed of the vehicle 2.

[0023] The steering ECU 18 is an electronic control unit (ECU) for the EPS (Electric Power Steering). The steering ECU 18 controls the assist motor 10 to generate output torque according to the detected values ​​of the torque sensor 16 and the vehicle speed sensor 17. Specifically, the steering ECU 18 controls the assist motor 10 so that the output torque increases as the steering torque increases. Also, the steering ECU 18 controls the assist motor 10 so that the output torque increases as the vehicle speed decreases.

[0024] Figure 3 is a block diagram showing the configuration of a vehicle monitoring system equipped with a life estimation device according to one embodiment of the present invention. In Figure 3, the vehicle monitoring system 20 is a system that monitors the status of vehicle 2.

[0025] The vehicle monitoring system 20 includes a steering angle sensor 21, a torque sensor 22, a status monitoring ECU 23, a transmitting / receiving unit 24, a display unit 25, a transmitting / receiving unit 26, and a processing unit 27. The steering angle sensor 21, torque sensor 22, status monitoring ECU 23, transmitting / receiving unit 24, and display unit 25 are mounted on the vehicle 2. The transmitting / receiving unit 26 and processing unit 27 are provided in the external monitoring device 28.

[0026] The steering angle sensor 21 is a sensor that detects the steering direction and steering angle of the steering wheel 3. The steering angle sensor 21 detects the peak value (maximum value) of the steering angle during a single steering operation as the steering angle of the steering wheel 3. The detected value of the steering angle sensor 21 is output at predetermined intervals (for example, several seconds).

[0027] The torque sensor 22 is a sensor that detects the output torque of the drive motor 29 that rotates the front wheels 7 of the vehicle 2. The output torque of the drive motor 29 corresponds to the load on the drive system of the vehicle 2. The detected value of the torque sensor 22 is output at predetermined intervals (for example, every few seconds).

[0028] The status monitoring ECU 23 is an electronic control unit (ECU) for monitoring the vehicle's condition. The status monitoring ECU 23 includes a steering angle frequency counter 30 and a gross vehicle weight frequency estimation unit 31.

[0029] The steering angle frequency counting unit 30 counts the frequency of steering angles of the steering wheel 3 based on the values ​​detected by the steering angle sensor 21. Specifically, the steering angle frequency counting unit 30 counts the steering angles of the steering wheel 3 detected by the steering angle sensor 21 in predetermined angle increments (e.g., 50 degrees) and stores the steering angle and its count in the data logger. For example, the data logger stores 50 instances of steering at 500 degrees, 100 instances of steering at 700 degrees, and so on. The fractional part of 50 degrees may be rounded up, rounded down, or the nearest value may be used.

[0030] The gross vehicle weight frequency estimation unit 31 estimates the frequency of the gross vehicle weight of the vehicle 2 based on the values ​​detected by the torque sensor 22. Specifically, the gross vehicle weight frequency estimation unit 31 estimates the gross vehicle weight in predetermined weight increments (e.g., 200 kg) from the output torque of the drive motor 29, and stores the gross vehicle weight and the time it occurred in the data logger. For example, the data logger stores that the state in which the gross vehicle weight is 3000 kg occurred for 600 hours. The fractional part of 200 kg may be rounded up, rounded down, or the nearest value may be used.

[0031] The transmitting / receiving unit 24 communicates wirelessly with the transmitting / receiving unit 26 of the external monitoring device 28. The transmitting / receiving unit 24 periodically transmits data accumulated by the steering angle frequency counting unit 30 and the gross vehicle weight frequency estimation unit 31 to the external monitoring device 28. The transmitting / receiving unit 24 transmits this data to the external monitoring device 28, for example, at the same time every day or when the ignition switch is turned ON. The transmitting / receiving unit 24 also receives notification signals (described later) from the external monitoring device 28.

[0032] When the display unit 25 receives a notification signal from the external monitoring device 28 via the transmitting / receiving unit 24, it displays that the ball screw 12 is nearing the time for replacement. At this time, the display unit 25 may display a screen message or it may display a warning message along with an audible warning.

[0033] The transmitting / receiving unit 26 communicates wirelessly with the transmitting / receiving unit 24 of the vehicle 2. The transmitting / receiving unit 26 receives vehicle operation information (described later) from the vehicle 2. The transmitting / receiving unit 26 also transmits notification signals (described later) to the vehicle 2.

[0034] The processing unit 27 consists of a damage ratio storage unit 32 and a server 33. The server 33 includes an information acquisition unit 34, a first damage calculation unit 35, a second damage calculation unit 36, a final damage calculation unit 37, and a degradation prediction unit 38.

[0035] The damage ratio storage unit 32, information acquisition unit 34, first damage calculation unit 35, second damage calculation unit 36, final damage calculation unit 37, and deterioration prediction unit 38 constitute the lifespan estimation device 40. The lifespan estimation device 40 is a device for estimating the lifespan of the ball screw 12 of the electric power steering 1.

[0036] Here, we will explain the method for estimating the lifespan of the ball screw 12. When the ball screw 12 is used over a long period of time, damage occurs to the ball screw 12, for example, when the balls themselves detach or when the grooves of the screw shaft 14 in which the balls are housed are worn down.

[0037] The damage life of the ball screw 12 is calculated based on the surface pressure perpendicular to the ball screw 12 during steering of the vehicle 2. Specifically, the rolling fatigue damage applied to the ball screw 12 is calculated by multiplying the ball screw axial load by the ball transfer distance. The ball screw axial load L is calculated by the following formula, as shown in Figure 2. The ball screw axial load L is the load applied to the nut 15 of the ball screw 12 in a direction perpendicular to the axis of the ball screw 12 (direction of the ball screw axis).

[0038] Ball screw shaft load L = T × sinθ × M T: Tie rod thrust θ: Tie rod angle M: Moment Arm

[0039] The tie rod thrust T is the force that transmits the output of the shaft 5 to the front wheel 7, as shown in Figure 2. The reaction force of the tie rod thrust T is absorbed by the ball screw 12. The greater the tie rod thrust T, the greater the damage to the ball screw 12. The tie rod thrust T varies depending on the vehicle speed of the vehicle 2, the steering angle (steering amount) of the steering wheel 3, and the load of the vehicle 2 (front load). Specifically, the lower the vehicle speed, the greater the tie rod thrust T. The greater the steering angle, the greater the tie rod thrust T. The greater the load, the greater the tie rod thrust T.

[0040] The tie rod angle θ is the angle between the shaft 5 and the tie rod 6, as shown in Figure 2. The component of the tie rod thrust T perpendicular to the ball screw axis depends on the tie rod angle θ. The larger the tie rod angle θ, the larger the component of the tie rod thrust T perpendicular to the ball screw axis. The tie rod angle θ varies depending on the steering angle of the steering wheel 3 and the load of the vehicle 2. Specifically, the larger the steering angle, the larger the tie rod angle θ. The larger the load, the larger the tie rod angle θ.

[0041] As shown in Figure 2, the moment arm M is the length from the connection point between the tie rod 6 and the shaft portion 5 to the axial center of the nut 15 of the ball screw 12. The longer the moment arm M, the greater the input perpendicular to the ball screw axis. The moment arm M varies depending on the steering angle of the steering wheel 3. Specifically, the larger the steering angle, the longer the moment arm M.

[0042] Based on the above, the tie rod thrust T and tie rod angle θ are related to the steering angle and the gross vehicle weight. The moment arm M is related to the steering angle. In addition, the ball transfer distance of the ball screw 12 is related to the steering angle and the number of times it is moved.

[0043] Therefore, in this embodiment, as described above, the data accumulated by the steering angle frequency counter 30 and the gross vehicle weight frequency estimation unit 31 is sent to the life estimation device 40 as ICT information. The data of steering angle and its count accumulated by the steering angle frequency counter 30 is steering angle information relating to the frequency of steering angle. The data of gross vehicle weight and its time accumulated by the gross vehicle weight frequency estimation unit 31 is gross vehicle weight information relating to the frequency of gross vehicle weight. The steering angle information and gross vehicle weight information are vehicle operation information. The life estimation device 40 then uses the steering angle information and gross vehicle weight information to estimate the life of the ball screw 12.

[0044] Returning to Figure 3, the damage ratio storage unit 32 stores the damage ratio of the ball screw 12 to the steering angle of the steering wheel 3 as the first damage ratio, and the damage ratio of the ball screw 12 to the total weight of the vehicle 2 (gross vehicle weight) as the second damage ratio.

[0045] The first damage ratio is set for each range of steering angle of the steering wheel 3, as shown in Figure 4(a). The steering angle range corresponds to the predetermined angles (e.g., 50 degrees) and is converted into percentages in 10% increments. The first damage ratio is predetermined by experiment or calculation. The first damage ratio increases non-linearly (e.g., exponentially) as the percentage of the steering angle increases.

[0046] The second damage ratio, as shown in Figure 4(b), replaces the total vehicle weight with the front axle load of vehicle 2. The second damage ratio is also predetermined by experiment or calculation. The second damage ratio increases non-linearly or linearly as the front axle load increases.

[0047] The information acquisition unit 34 acquires steering angle information and gross vehicle weight information transmitted from the vehicle 2 via the transmission / reception unit 26. The information acquisition unit 34 stores the acquired steering angle information and gross vehicle weight information in the data logger.

[0048] The first damage calculation unit 35 calculates the total damage to the ball screw 12 according to the frequency of steering angles, based on the steering angle information acquired by the information acquisition unit 34 and the first damage ratio stored in the damage ratio storage unit 32. The first damage calculation unit 35 calculates the total damage to the ball screw 12 according to the frequency of steering angles by multiplying the ball transfer distance of the ball screw 12 by the number of steering angles and the first damage ratio for each steering angle range, and adding the multiplied values ​​for each steering angle range.

[0049] Figure 5 is a flowchart showing the procedure for the damage calculation process performed by the first damage calculation unit 35. This process is performed using steering angle information for the most recent week.

[0050] In Figure 5, the first damage calculation unit 35 first acquires steering angle information for the past week stored in the information acquisition unit 34 (procedure S101). Next, the first damage calculation unit 35 converts the steering angle of the steering wheel 3 into a percentage (procedure S102). As a result, the steering angle is expressed in steering angle ranges of 10% increments.

[0051] Next, the first damage calculation unit 35 sums up the number of steering inputs for each steering angle range (procedure S103). For example, if the number of 500-degree steering inputs is 50 (as mentioned above), then 50 is added up for the steering angle range corresponding to 500 degrees.

[0052] Next, the first damage calculation unit 35 calculates the ball transfer distance for each steering angle range by multiplying the unit ball transfer distance corresponding to the steering angle by the number of steering operations (procedure S104). The unit ball transfer distance is the ball transfer distance when one steering operation is performed. For example, if the unit ball transfer distance for a 500-degree steering operation is X, then when the number of steering operations is 50, the ball transfer distance will be 50X.

[0053] Next, the first damage calculation unit 35 calculates the number of damages to the ball screw 12 for each steering angle range by multiplying the ball transfer distance by the first damage ratio for each steering angle range (procedure S105). For example, in a steering angle range corresponding to 500 degrees, 50X is multiplied by the first damage ratio.

[0054] Next, the first damage calculation unit 35 calculates the total damage to the ball screws 12 according to the frequency of steering angles in the past week by adding up the damage to the ball screws 12 in each steering angle range (10% to 100%) (procedure S106).

[0055] Returning to Figure 3, the second damage calculation unit 36 ​​calculates the total damage to the ball screws 12 according to the frequency of the vehicle's total weight, based on the vehicle's total weight information acquired by the information acquisition unit 34 and the second damage ratio stored in the damage ratio storage unit 32. The second damage calculation unit 36 ​​calculates the average value of the vehicle's total weight and calculates the total damage to the ball screws 12 according to the frequency of the vehicle's total weight by multiplying the average value of the vehicle's total weight by the second damage ratio corresponding to the average value of the vehicle's total weight.

[0056] Figure 6 is a flowchart showing the procedure for the damage calculation process performed by the second damage calculation unit 36. This process is performed using the total vehicle weight information for the most recent week.

[0057] In Figure 6, the second damage calculation unit 36 ​​first acquires the vehicle total weight information for the most recent week stored in the information acquisition unit 34 (procedure S111). Next, the second damage calculation unit 36 ​​calculates the average value of the vehicle total weight from the vehicle total weight and the time (procedure S112).

[0058] Next, the second damage calculation unit 36 ​​obtains a second damage ratio corresponding to the average value of the total vehicle weight as the average damage ratio (procedure S113). For example, as shown in Figure 4(b), the average damage ratio Da corresponding to the average value Fa of the total vehicle weight (front axle load) is obtained.

[0059] Then, the second damage calculation unit 36 ​​calculates the total damage to the ball screws 12 according to the frequency of the vehicle's total weight in the past week by multiplying the average value of the vehicle's total weight by the average damage ratio (procedure S114).

[0060] Returning to Figure 3, the final damage calculation unit 37 calculates the final total damage to the ball screws 12 based on the total damage to the ball screws 12 according to the frequency of steering angles calculated by the first damage calculation unit 35, and the total damage to the ball screws 12 according to the frequency of the total vehicle weight calculated by the second damage calculation unit 36.

[0061] Figure 7 is a flowchart showing the procedure for the damage calculation process performed by the final damage calculation unit 37.

[0062] In Figure 7, the final damage calculation unit 37 calculates the final total damage to the ball screws 12 over the past week by multiplying the total damage to the ball screws 12 according to the frequency of steering angle by the total damage to the ball screws 12 according to the frequency of total vehicle weight (procedure S121).

[0063] Next, the final damage calculation unit 37 updates the cumulative total damage value of the ball screws 12 by adding the final total damage of the ball screws 12 over the past week to the current cumulative total damage value of the ball screws 12 (procedure S122). As a result, the cumulative total damage value of the ball screws 12 increases every week, as shown in Figure 8.

[0064] Returning to Figure 3, the deterioration prediction unit 38 predicts the deterioration timing of the ball screw 12 based on the final total number of damages of the ball screw 12 calculated by the final damage calculation unit 37, and notifies the vehicle 2 according to the predicted deterioration timing of the ball screw 12.

[0065] Figure 9 is a flowchart showing the procedure for the degradation prediction process performed by the degradation prediction unit 38.

[0066] In Figure 9, the deterioration prediction unit 38 first obtains the cumulative total damage value of the ball screw 12 as shown in Figure 8 (procedure S131). Next, the deterioration prediction unit 38 calculates the slope A of the cumulative total damage value of the ball screw 12 over the most recent predetermined period (for example, 10 weeks) (see Figure 8) (procedure S132).

[0067] Next, the deterioration prediction unit 38 determines, based on the slope A of the cumulative total damage value of the ball screw 12, whether the cumulative total damage value of the ball screw 12 after a specified number of days (for example, 90 days) is equal to or greater than a predetermined threshold S (see Figure 8) (procedure S133).

[0068] When the deterioration prediction unit 38 determines that the cumulative total damage value of the ball screw 12 after a specified number of days is equal to or greater than the threshold S, it determines that the ball screw 12 is nearing deterioration and sends a notification signal to the vehicle 2 via the transmitting / receiving unit 26 to instruct the replacement of the ball screw 12 (procedure S134). As a result, the notification signal is sent to the display unit 25 of the vehicle 2 via the transmitting / receiving unit 24, and the display unit 25 displays a message prompting the replacement of the ball screw 12.

[0069] If the deterioration prediction unit 38 determines that the cumulative total damage value of the ball screw 12 after a specified number of days is less than the threshold S, it determines that the deterioration time of the ball screw 12 is not near and does not execute procedure S134.

[0070] Incidentally, the frequency of steering angle and gross vehicle weight are values ​​that are accumulated at regular intervals. However, the steering angle and gross vehicle weight are not necessarily measured at the same time. Therefore, if the lifespan of the ball screw 12 is estimated using only the frequency of steering angle and gross vehicle weight, a discrepancy may occur between this estimate and the actual damage to the ball screw 12.

[0071] For example, the ball transfer distance of the ball screw 12 will be the same whether there are 10 steering inputs at 10% or 1 steering input at 100%, but the actual damage dealt by the ball screw 12 will be significantly different. Specifically, the actual damage dealt when there are 1 steering inputs at 100% will be considerably greater than the actual damage dealt when there are 10 steering inputs at 100%.

[0072] To address these challenges, in this embodiment, the damage ratio of the ball screw 12 to the steering angle of the steering wheel 3 (first damage ratio) and the damage ratio of the ball screw 12 to the gross vehicle weight (second damage ratio) are stored in advance. Then, steering angle information related to the frequency of steering angle of the steering wheel 3 and gross vehicle weight information related to the frequency of gross vehicle weight are acquired. Based on the steering angle information and the first damage ratio, the total number of ball screws 12 damaged according to the frequency of steering angle is calculated, and based on the gross vehicle weight information and the second damage ratio, the total number of ball screws 12 damaged according to the frequency of gross vehicle weight is calculated. Finally, the total number of ball screws 12 damaged according to the frequency of steering angle and the total number of ball screws 12 damaged according to the frequency of gross vehicle weight are used to calculate the final total number of ball screws 12 damaged. Here, the steering angle information and gross vehicle weight information are acquired as operating information of the vehicle 2. Therefore, it is not necessary to simultaneously measure data corresponding to tie rod thrust, tie rod angle, and moment arm when the vehicle 2 is in operation. Therefore, dedicated sensors and other equipment for estimating the lifespan of the ball screw 12 become unnecessary. This reduces man-hours and costs. Furthermore, by using the first damage ratio, the total number of damages to the ball screw 12 corresponding to the frequency of steering angle changes approaches the actual damage. By using the second damage ratio, the total number of damages to the ball screw 12 corresponding to the frequency of gross vehicle weight changes approaches the actual damage. As a result, the lifespan of the ball screw 12 of the electric power steering 1 can be estimated with high accuracy.

[0073] In an actual demonstration experiment using a test vehicle, it was confirmed that the lifespan of the ball screw 12 could be estimated with comparable accuracy to when data corresponding to tie rod thrust, tie rod angle, and moment arm were measured simultaneously.

[0074] Furthermore, in this embodiment, the total number of damages to the ball screw 12 corresponding to the frequency of steering angles is calculated by multiplying the ball transfer distance of the ball screw 12 by the number of steering angles and the first damage ratio for each steering angle range, and adding the multiplier values ​​for each steering angle range. With this configuration, the total number of damages to the ball screw 12 corresponding to the frequency of steering angles becomes even closer to the actual damage. Therefore, the lifespan of the ball screw 12 of the electric power steering 1 can be estimated with even greater accuracy.

[0075] Furthermore, in this embodiment, the average value of the gross vehicle weight is calculated, and the total number of damages to the ball screws 12 according to the frequency of the gross vehicle weight is calculated by multiplying the average value of the gross vehicle weight by a second damage ratio corresponding to the average value of the gross vehicle weight. With this configuration, the process of calculating the total number of damages to the ball screws 12 according to the frequency of the gross vehicle weight is simplified. Therefore, the man-hours and costs are further reduced.

[0076] Furthermore, in this embodiment, the deterioration time of the ball screw 12 is predicted based on the total number of damages to the ball screw 12, and a notification is given according to the predicted deterioration time of the ball screw 12. With this configuration, when the deterioration time of the ball screw 12 approaches, a notification is given to that effect, allowing the ball screw 12 to be replaced at an appropriate time.

[0077] It should be noted that the present invention is not limited to the embodiments described above. For example, in the above embodiments, the steering angle of the steering wheel 3 is converted to a percentage, but the invention is not limited to this form, and the steering angle of the steering wheel 3 may be used as is.

[0078] Furthermore, in the above embodiment, a first damage ratio corresponding to each range of steering angle of the steering wheel 3 is used, but a first damage ratio corresponding to the steering angle of the steering wheel 3 may be used without specifically adopting a range of steering angles.

[0079] Furthermore, in the above embodiment, the output torque of the drive motor 29 is detected as the load of the drive system, and the total vehicle weight is replaced with the front axle load of the vehicle 2 for calculation. However, the embodiment is not limited to this configuration, and the total vehicle weight may be detected as the front axle load of the vehicle 2, or the front axle load of the vehicle 2 may be detected directly.

[0080] Furthermore, in the above embodiment, a second damage ratio corresponding to the average value of the gross vehicle weight is used as the average damage ratio, and the total number of damages to the ball screws 12 according to the frequency of the gross vehicle weight is calculated by multiplying the average value of the gross vehicle weight by the average damage ratio, but the embodiment is not limited to this form. For example, a second damage ratio corresponding to each range of gross vehicle weight may be used for each range of gross vehicle weight, or a second damage ratio corresponding to the gross vehicle weight may be used without adopting a range of gross vehicle weight.

[0081] Furthermore, in the above embodiment, the total number of ball screws 12 damaged in the most recent week is calculated using steering angle information and gross vehicle weight information for the most recent week, but the period is not limited to the most recent week. For example, the total number of ball screws 12 damaged in the most recent month may be calculated using steering angle information and gross vehicle weight information for the most recent month. Alternatively, the cumulative total number of ball screws 12 damaged may be directly calculated using all past steering angle information and gross vehicle weight information.

[0082] Furthermore, in the above embodiment, the final total number of damages to the ball screws 12 is calculated by multiplying the total number of damages to the ball screws 12 according to the frequency of steering angle and the total number of damages to the ball screws 12 according to the frequency of gross vehicle weight, but the embodiment is not limited to this form. For example, the final total number of damages to the ball screws 12 may be calculated by multiplying the total number of damages to the ball screws 12 according to the frequency of steering angle and the total number of damages to the ball screws 12 according to the frequency of gross vehicle weight by a weighting coefficient (ratio).

[0083] Furthermore, in the above embodiment, the life estimation device 40 for estimating the life of the ball screw 12 is provided in the external monitoring device 28, but the form is not particularly limited, and the life estimation device 40 may be mounted on the vehicle 2.

[0084] Furthermore, in the above embodiment, vehicle 2 is an electric vehicle, but vehicle 2 is not limited to an electric vehicle; it may be a hybrid vehicle (HV vehicle) or an engine-powered vehicle. In any case, the load of the drive system of vehicle 2 may be detected, and the frequency of the gross vehicle weight may be estimated based on the load of the drive system.

[0085] Furthermore, in the above embodiment, the vehicle 2 to which the life estimation device 40 is applied is a truck, but the vehicle 2 is not limited to a truck and may be a passenger car, bus, or the like. [Explanation of symbols]

[0086] 1...Electric power steering, 2...Vehicle, 3...Steering wheel (steering), 5...Shaft section, 7...Front wheel (wheel), 10...Assist motor, 12...Ball screw, 32...Damage ratio memory unit, 34...Information acquisition unit, 35...First damage calculation unit, 36...Second damage calculation unit, 37...Final damage calculation unit, 38...Deterioration prediction unit, 40...Lifespan estimation device.

Claims

1. A life estimation device for estimating the lifespan of a ball screw that converts the rotational motion of an assist motor into the linear motion of a shaft connected to a wheel in an electric power steering system, A damage ratio storage unit stores the damage ratio of the ball screw to the steering angle as a first damage ratio, and the damage ratio of the ball screw to the total vehicle weight as a second damage ratio. An information acquisition unit that acquires steering angle information relating to the frequency of the steering angle and gross vehicle weight information relating to the frequency of the gross vehicle weight, A first damage calculation unit calculates the total number of damages to the ball screw according to the frequency of the steering angle, based on the steering angle information acquired by the information acquisition unit and the first damage ratio stored in the damage ratio storage unit. A second damage calculation unit calculates the total number of damages to the ball screws according to the frequency of the total vehicle weight, based on the total vehicle weight information acquired by the information acquisition unit and the second damage ratio stored in the damage ratio storage unit. A life estimation device comprising a final damage calculation unit that calculates the final total damage number of the ball screws based on the total damage number of the ball screws according to the frequency of the steering angle calculated by the first damage calculation unit and the total damage number of the ball screws according to the frequency of the total vehicle weight calculated by the second damage calculation unit.

2. The life estimation device according to claim 1, wherein the first damage calculation unit multiplies the ball transfer distance of the ball screw by the number of steering angles and the first damage ratio for each range of the steering angle, and adds the multiplied values ​​for each range of the steering angle to calculate the total number of damages of the ball screw according to the frequency of the steering angle.

3. The life estimation device according to claim 1, wherein the second damage calculation unit calculates the average value of the total vehicle weight, and multiplies the average value of the total vehicle weight by the second damage ratio corresponding to the average value of the total vehicle weight to calculate the total number of damages of the ball screw according to the frequency of the total vehicle weight.

4. The life estimation device according to claim 1, further comprising a deterioration prediction unit that predicts the deterioration time of the ball screw based on the final total number of damages of the ball screw calculated by the final damage calculation unit, and provides notification according to the predicted value of the deterioration time of the ball screw.

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