Road surface friction coefficient estimation method and road surface friction coefficient estimation device

Abstract

To improve estimation accuracy of a road surface friction coefficient based on drive current of a motor steering a wheel.SOLUTION: A road surface friction coefficient estimation method includes: acquiring a current measurement value of drive current of a steering motor steering a steering wheel (S2); calculating a change rate measurement value which is a change rate of a current measurement value to a steering angle change at a predetermined steering angle (S5); and estimating a first friction coefficient of a road surface on the basis of a relation between the previously acquired change rate of the drive current of the steering motor to the steering angle change at the predetermined steering angle and a friction coefficient of the road surface, and the change rate measurement value (S6).SELECTED DRAWING: Figure 8

Description

Technical Field

[0001] The present invention relates to a road surface friction coefficient estimation method and a road surface friction coefficient estimation device.

Background Art

[0002] The road surface friction coefficient estimation device described in Patent Document 1 estimates the road surface friction coefficient based on the motor drive current when the left and right rear wheels are steered by a predetermined angle by a motor while the vehicle is stopped.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In a tire in a fixed-steering state where the steering angle does not change, a moment due to tire twist or the like may remain. When the motor that steers the steering wheel starts steering, the residual moment may affect the drive current of the motor, resulting in a decrease in the estimation accuracy of the road surface friction coefficient. An object of the present invention is to improve the estimation accuracy of the road surface friction coefficient based on the drive current of a motor that steers a wheel.

Means for Solving the Problems

[0005] In a road surface friction coefficient estimation method according to an aspect of the present invention, a current measurement value of the drive current of a steering motor that steers a steering wheel is acquired, a rate-of-change measurement value that is a rate of change of the current measurement value with respect to a steering angle change at a predetermined steering angle is calculated, and based on a relationship between a rate of change of the drive current of the steering motor with respect to a steering angle change at a predetermined steering angle obtained in advance and the friction coefficient of the road surface, and the rate-of-change measurement value, a first friction coefficient of the road surface is estimated.

Effects of the Invention

[0006] According to the present invention, it is possible to improve the estimation accuracy of the road surface friction coefficient based on the drive current of the motor that steers the wheels.

Brief Description of the Drawings

[0007]

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Embodiment for Carrying Out the Invention

[0008] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that each drawing is schematic and may be different from the actual one. In addition, the embodiments of the present invention shown below are examples of devices and methods for embodying the technical idea of the present invention, and the technical idea of the present invention does not specify the structure, arrangement, etc. of the components as follows. The technical idea of the present invention can be variously modified within the technical scope defined by the claims described in the claims.

[0009] (First Embodiment) (Configuration) Referring to FIG. 1. The road surface friction coefficient estimation device 10 of the embodiment includes a front wheel steering motor 11F, a rear wheel steering motor 11R, a front wheel steering angle measuring device 12F, a rear wheel steering angle measuring device 12R, and a controller 13. In the following description, the left front wheel 2FL and the right front wheel 2FR may be collectively referred to as "front wheels 2F", and the left rear wheel 2RL and the right rear wheel 2RR may be collectively referred to as "rear wheels 2R".

[0010] The front wheel steering motor 11F and the rear wheel steering motor 11R receive a steering angle command value and a steering angular velocity command value from the controller 13, and realize the steering angle command value and the steering angular velocity command value of the front wheels 2F and the rear wheels 2R with the motor drive current calculated to realize them. In addition, the motor drive current required for steering is measured and output to the controller 13. For example, the front wheels 2F are steered by a steering wheel operated by a driver, a steering shaft connected thereto, and a steering mechanism capable of changing the angle of the front wheels 2F with respect to the vehicle 1. The front wheel steering motor 11F applies a steering assist torque for assisting the steering by the driver to the steering mechanism.

[0011] For example, the rear wheels 2R do not have a mechanical mechanism with the steering wheel, and are steered by a steer-by-wire system that electrically connects the two and controls steering. The rear-wheel steering motor 11R generates a steering torque for steering the rear wheels 2R. As long as the configuration can realize the steering angle command value and the steering angular velocity command value received from the controller 13, regardless of the presence or absence of a mechanical mechanism between the steering wheel and the front wheels 2F or the rear wheels 2R, each wheel may be independently steered.

[0012] The front-wheel steering angle measuring device 12F and the rear-wheel steering angle measuring device 12R respectively measure the steering angles of the front wheels 2F and the rear wheels 2R with respect to the traveling direction of the vehicle 1, and output the measured steering angles to the controller 13. Generally, the steering angle and the rotation angle of the steering wheel have a relationship of a ratio (steering gear ratio) determined by the steering mechanism. For example, the steering angle of the front wheels 2F may be calculated by measuring the rotation angle of the steering shaft and using the relationship of the steering gear ratio, and the steering angle of the rear wheels 2R may be calculated based on the rotation angle of the rear-wheel steering motor 11R. However, the means for measuring the steering angle is not limited to this.

[0013] When estimating the road surface friction coefficient, the road surface friction coefficient estimating device 10 steers one or both of the front wheels 2F or the rear wheels 2R at a predetermined steering angle command value or steering angular velocity. In the following description, steering the front wheels 2F or the rear wheels 2R to estimate the road surface friction coefficient may be referred to as "estimation steering". Also, the wheels steered in the estimation steering among the front wheels 2F and the rear wheels 2R may be referred to as "steering wheels". Also, the steering angle of the steering wheel steered in the estimation steering may be referred to as "steering angle θ".

[0014] Also, the front-wheel steering motor 11F and the rear-wheel steering motor 11R may be collectively referred to as "steering motor 11", and the front-wheel steering angle measuring device 12F and the rear-wheel steering angle measuring device 12R may be collectively referred to as "steering angle measuring device 12". When the steering wheel and the steering wheel are connected by a mechanical mechanism, the road surface friction coefficient estimation device 10 executes estimated steering in a state where there is no steering input from the driver. When using a steer-by-wire system, during estimated steering, the estimation may be continued by not reflecting the driver's operation in the steering angle θ.

[0015] The controller 13 is an electronic control unit (ECU: Electronic Control Unit) that executes estimated steering of the steering wheel and estimates the friction coefficient of the road surface on which the vehicle 1 travels based on the motor drive current of the steering motor 11 during estimated steering. The controller 13 determines a steering angle command value and a steering angular velocity command value necessary for estimating the road surface friction coefficient from the steering angle information acquired from the steering angle measurement device 12, and controls the steering of the steering wheel based on these command values. The road surface friction coefficient is estimated based on the steering angle information and the motor drive current information respectively acquired from the steering angle measurement device 12 and the steering motor 11 during estimated steering.

[0016] The controller 13 includes a processor 13a and peripheral components such as a storage device 13b. The functions of the controller 13 described below are realized, for example, when the processor 13a executes a computer program stored in the storage device 13b. The controller 13 may be provided with dedicated hardware for executing each information process described below. For example, the controller 13 may be provided with a functional logic circuit set in a general-purpose semiconductor integrated circuit. For example, the controller 13 may have a programmable logic device (PLD) such as a field-programmable gate array (FPGA).

[0017] FIG. 2 is a characteristic diagram of the tire reaction torque with respect to the road surface friction coefficient and the steering angle θ. The tire reaction torque with respect to the steering angle θ of the steering wheel changes depending on the road surface friction coefficient μ. Therefore, the controller 13 estimates the road surface friction coefficient μ based on the motor drive current of the steering motor 11.

[0018] However, a moment due to tire twist or the like may remain in the tire in the fixed steering state before the estimated steering is executed. Refer to Fig. 3(a). In the following description, a moment Mr generated by the deviation of the current tire direction 2c with respect to the neutral point 2n, which is the steering angle of the steering wheel at which no tire twist occurs, is referred to as a "residual moment". In the example of Fig. 3(a), the direction 2c of the tire in the fixed steering state is deviated from the neutral point 2n by an angle θ. For example, the residual moment Mr occurs when tire twist occurs due to the vehicle state until the steering wheel is fixed.

[0019] If the residual moment Mr is less than the maximum static friction torque of the steering mechanism, the residual moment Mr balances with the static friction torque, so the steering wheel does not move and the residual moment Mr remains in the tire. When the estimated steering is started, the residual moment Mr affects the motor drive current, so there is a risk that the estimation accuracy of the road surface friction coefficient will decrease.

[0020] Figs. 3(b) and 3(c) show the relationship between the torque acting on the steering wheel when the steering direction of the estimated steering is the same as and different from the direction of the residual moment, respectively. In the following description, the case where the steering direction of the estimated steering is the same as the direction of the residual moment may be referred to as "in-phase", and the case where the steering direction of the estimated steering is different from the direction of the residual moment may be referred to as "anti-phase".

[0021] The arrows Tm, Tr, Mr, and Tt schematically represent the mechanical friction torque of the steering mechanism, the road surface reaction torque corresponding to the steering angle, the residual moment, and the steering torque of the estimated steering, respectively. The steering torque Tt balances with the sum of the mechanical friction torque Tm, the road surface reaction torque Tr, and the residual moment Mr. Therefore, depending on the direction of the residual moment existing at the start of the estimated steering, the influence of the residual moment on the steering torque Tt changes during the estimated steering, and there is a risk that the estimation accuracy of the road surface friction coefficient will decrease.

[0022] Refer to FIG. 2. In the linear region R1 where the steering angle θ is relatively small, the tire reaction torque increases linearly as the steering angle θ increases. In the saturation region R2 where the steering angle θ is relatively large, the tire reaction torque hardly changes even when the steering angle θ increases. On the other hand, in the region R3 between the linear region R1 and the saturation region R2, the tire reaction torque increases non-linearly as the steering angle θ increases. In the following description, the region R3 may be referred to as the "non-linear and non-saturated region R3". In the non-linear and non-saturated region R3, the change rate (ΔT / Δθ) of the tire reaction torque changes according to the road surface friction coefficient μ.

[0023] After the mechanical friction of the steering mechanism transitions to dynamic friction, the current change rate (ΔI / Δθ) is uniquely determined with respect to the steering angle and the road surface friction coefficient without being affected by the residual moment. Therefore, if the relationship between the current change rate (ΔI / Δθ) at a predetermined steering angle θ1 and the road surface friction coefficient is known, the road surface friction coefficient can be accurately estimated based on the current change rate (ΔI / Δθ). Therefore, the controller 13 estimates the road surface friction coefficient μ1 based on the current change rate (ΔI / Δθ) of the motor drive current with respect to the steering angle at a steering angle (hereinafter referred to as "predetermined steering angle θ1") preset within the non-linear and non-saturated region R3.

[0024] Note that the residual moment may occur not only when the vehicle 1 is stopped but also when the vehicle 1 is traveling in a held-steering state (for example, turning while maintaining a certain steering angle). Therefore, the road surface friction coefficient estimation method of the present invention is also effective when estimating the road surface friction coefficient based on the motor drive current of the steering motor 11 while the vehicle 1 is traveling.

[0025] FIG. 4 is a block diagram of an example of the functional configuration of the controller 13 according to the first embodiment. The controller 13 includes a steering command calculation unit 20, a measurement value acquisition unit 21, a change rate calculation unit 22, and a friction coefficient estimation unit 23. When executing the estimated steering, the steering command calculation unit 20 determines the steering angle command value and the steering angular velocity command value necessary for estimating the road surface friction coefficient, and controls the steering of the steered wheels based on these command values. While the steering wheel is being steered by estimated steering, the measurement value acquisition unit 21 sequentially acquires the measured steering angle value of the steering angle θ measured by the steering angle measurement device 12 and the measured current value of the motor drive current measured by the steering motor 11.

[0026] The change rate calculation unit 22 extracts data near a predetermined steering angle θ1 from among the measured steering angle value and the measured current value acquired by the measurement value acquisition unit 21. The change rate calculation unit 22 detects a change ΔI in the motor drive current with respect to a change Δθ in the steering angle θ based on the extracted data. The change rate calculation unit 22 calculates the current change rate (ΔI / Δθ) at the predetermined steering angle θ1 by dividing ΔI by Δθ.

[0027] The friction coefficient estimation unit 23 estimates the road surface friction coefficient μ1 based on the current change rate (ΔI / Δθ). If it is within the steering angle range where the reaction moment by the tire does not saturate, the reaction moment by the tire monotonically increases with an increase in the steering angle θ. For this reason, the motor drive current also monotonically increases, and the current change rate (ΔI / Δθ) has a positive value. The reason will be described using the brush model of the tire with reference to FIGS. 5(a) to 5(d).

[0028] In FIGS. 5(a) to 5(d), the symbols “xs” and “xe” indicate the ground contact start point and the ground contact end point of the tire along the longitudinal direction of the tire, respectively. Assuming the tire slip angle is “α”, the lateral elastic coefficient is “Cy”, the tire ground contact width is “w”, the slip ratio is “s”, and the distance from the ground contact start point is “x”, the lateral tire generating force Fyad in the adhesion region is determined by Fyad = Cy × w × x × tan α.

[0029] The boundary point xb between the adhesion region and the slip region is obtained as the position where the lateral tire generating force Fyad in the adhesion region intersects the distribution of the maximum static friction force acting on the ground contact surface. In the adhesion region (xs ≦ x ≦ xb), the lateral tire generating force Fyad in the adhesion region is integrated, and in the slip region (xb ≦ x ≦ xe), the distribution of the maximum sliding friction force acting on the ground contact surface is integrated, and the sum of these determines the tire lateral force Fy.

[0030] Figs. 5(a) and 5(b) show the brush model when the road surface friction coefficient is relatively high. The area of the hatched region in Fig. 5(a) represents the tire lateral force Fy in the case of the tire slip angle α, and the area of the hatched region in Fig. 5(b) represents the tire lateral force Fy in the case of the tire slip angle α+Δα. Comparing the areas of the hatched regions in Figs. 5(a) and 5(b), it can be seen that the tire lateral force Fy increases with the increase Δα of the tire slip angle. Therefore, in the steering angle range where the reaction moment by the tire does not saturate, the reaction moment increases monotonically with the increase of the steering angle θ, and the motor drive current for balancing the reaction moment also increases monotonically.

[0031] Figs. 5(c) and 5(d) show the brush model when the road surface friction coefficient is relatively low. The area of the hatched region in Fig. 5(c) represents the tire lateral force Fy in the case of the tire slip angle α, and the area of the hatched region in Fig. 5(d) represents the tire lateral force Fy in the case of the tire slip angle α+Δα. Comparing Figs. 5(a) and 5(b) with Figs. 5(c) and 5(d), it can be seen that the increase in the tire lateral force Fy decreases as the road surface friction coefficient decreases. From this, it can be seen that when the road surface friction coefficient becomes small, the increase rate (ΔI / Δθ) of the motor drive current also becomes small.

[0032] Therefore, the relationship between the current change rate of the motor drive current with respect to the steering angle change at a predetermined steering angle θ1 and the road surface friction coefficient is acquired in advance and stored in the storage device 13b. The relationship between the previously acquired current change rate and the road surface friction coefficient may be determined in advance by, for example, a calculation formula, or may be stored in the form of a look-up table or a map. Fig. 6 is an explanatory diagram of an example of the relationship between the current change rate of the motor drive current acquired in advance and the road surface friction coefficient acquired in advance.

[0033] In addition, when identifying the relationship between the current change rate and the road surface friction coefficient in advance, it is preferable to acquire the current change rate on three or more road surfaces with different road surface friction coefficients and obtain the relationship between the current change rate and the road surface friction coefficient. The friction coefficient estimation unit 23 obtains the road surface friction coefficient μ1 at the current change rate (ΔI / Δθ) based on the relationship between the previously obtained current change rate and the road surface friction coefficient, and the current change rate (ΔI / Δθ) at the predetermined steering angle θ1 calculated by the change rate calculation unit 22.

[0034] Refer to FIG. 7. Due to the characteristics of the tire, the moment in the steering rotation direction generated by the torsion of the tire saturates at a certain steering angle as the steering angle θ increases. When the steering rotation moment saturates, the current change rate of the motor drive current also saturates, and there is no significant difference with respect to the road surface friction coefficient. Therefore, it is preferable to set the predetermined steering angle θ1 to an angle smaller than the steering angle θsat at which the motor drive current becomes maximum when steering on the road surface with the smallest road surface friction coefficient among the assumable driving road surface environments.

[0035] On the other hand, if the predetermined steering angle θ1 is set to an excessively small angle, the difference in the current change rate of the motor drive current due to the difference in the road surface friction coefficient becomes small. Therefore, the predetermined steering angle θ1 may be set to an angle near the steering angle θsat and smaller than the steering angle θsat. In this way, by setting the predetermined steering angle θ1 to the steering angle at which the most significant difference is obtained with respect to the current change rate, the estimation accuracy can be improved.

[0036] FIG. 8 is a flowchart of an example of the road surface friction coefficient estimation method according to the first embodiment. In step S1, the steering command calculation unit 20 steers the steered wheels at a predetermined steering angular velocity. In step S2, the measurement value acquisition unit 21 acquires the steering angle measurement value of the steering angle θ and the current measurement value of the motor drive current. In step S3, the change rate calculation unit 22 determines whether the steering angle measurement value of the steering angle θ has reached the predetermined steering angle θ1. If the steering angle measurement value has not reached the predetermined steering angle θ1 (step S3: N), the process returns to step S1. If the steering angle measurement value has reached the predetermined steering angle θ1 (step S3: Y), the process proceeds to step S4.

[0037] In step S4, the change rate calculation unit 22 detects a change ΔI in the motor drive current with respect to a change Δθ in the steering angle θ at a predetermined steering angle θ1. In step S5, the change rate calculation unit 22 calculates a current change rate (ΔI / Δθ) at the predetermined steering angle θ1. In step S6, the friction coefficient estimation unit 23 obtains a road surface friction coefficient μ1 at the current change rate (ΔI / Δθ) based on the relationship between the previously obtained current change rate and the road surface friction coefficient and the current change rate (ΔI / Δθ). Then the process ends.

[0038] (Second Embodiment) The controller 13 of the second embodiment estimates a second road surface friction coefficient μ2 by an estimation method different from that of the first embodiment, and is used as a limiter for improving the accuracy of the road surface friction coefficient μ1 (hereinafter sometimes referred to as "the first road surface friction coefficient μ1") calculated by the friction coefficient estimation unit 23 by the estimation method of the first embodiment. The controller 13 of the second embodiment estimates a bias component generated in the motor drive current due to mechanical friction in the steering mechanism and tire twist existing in the steered wheels in the holding steering state before estimated steering. Then, the second road surface friction coefficient μ2 is estimated after removing the bias component from the measured value of the motor drive current.

[0039] FIG. 9 is a block diagram of an example of the functional configuration of the controller 13 of the second embodiment. The controller 13 of the second embodiment includes, in addition to the configuration of the first embodiment, a residual direction determination unit 24, a bias component calculation unit 25, a drive current correction unit 26, a maximum value detection unit 27, and a friction coefficient estimation unit 28. The measurement value acquisition unit 21 sequentially acquires a steering angle measurement value of the steering angle θ measured by the steering angle measurement device 12 and a current measurement value of the motor drive current measured by the steering motor 11 during the period from the start of the estimated steering until the steered wheels are steered to a predetermined steering angle θpre.

[0040] Referring to FIGS. 10(a) to 10(f), the characteristics of the motor drive current will be described. FIGS. 10(a) to 10(c) are characteristic diagrams of the motor drive current corresponding to the mechanical friction torque Tm, the tire reaction force torque, and the steering torque Tt with respect to the steering angle θ in the case of in-phase. FIGS. 10(d) to 10(f) are characteristic diagrams of the motor drive current corresponding to the mechanical friction torque Tm, the tire reaction force torque, and the steering torque Tt with respect to the steering angle θ in the case of reverse-phase.

[0041] Generally, in the region where the absolute value of the steering angle θ is small, as shown in FIGS. 10(b) and 10(e), the tire reaction force torque can be regarded as linear with respect to the steering angle θ of the steered wheel. In the case of in-phase (FIG. 10(b)), there is a negative residual moment (-Mr) at the start of the estimated steering (θ = 0). As the steering angle θ increases, the residual moment decreases and is eliminated at the neutral point, and the tire reaction force torque becomes "0".

[0042] Furthermore, as the steering angle θ further increases, a positive road surface reaction force torque Tr that increases linearly with respect to the steering angle θ is generated. Therefore, the tire reaction force torque has a value obtained by biasing the road surface reaction force torque Tr (dashed-dotted line) that occurs when there is no residual moment at the start of steering with the residual moment (-Mr).

[0043] The mechanical friction torque Tm (FIG. 10(a)) becomes a value (Mr) that balances the residual moment at the start of the estimated steering. For this reason, the torque required for steering at the start of the estimated steering becomes "0", and the motor drive current (FIG. 10(c)) starts from "0". When the steered wheel starts to move, the mechanical friction torque Tm decreases to the dynamic friction torque Tmd. Along with this, since the steering torque Tt decreases, the motor drive current (FIG. 10(c)) temporarily becomes a negative value.

[0044] When the mechanical friction torque Tm switches to the dynamic friction torque Tmd, the mechanical friction torque Tm becomes approximately constant. Then, since the steering torque Tt increases due to the tire reaction torque, the motor drive current changes from decreasing to increasing. For this reason, an extreme point Pext occurs in the characteristics of the motor drive current with respect to the steering angle θ. When steering further in the same direction, in the linear region, the motor drive current increases approximately linearly with an increase in the steering angle θ. In the following description, the steering angle and the motor drive current at the extreme point Pext may be denoted as the "extreme point steering angle θext" and the "extreme point current Iext".

[0045] On the other hand, in the case of reverse phase (Fig. 10(e)), a positive residual moment Mr exists at the start of the estimated steering. For this reason, the tire reaction torque has a value obtained by biasing the road surface reaction torque Tr (dashed line) when there is no residual moment at the start of steering with the residual moment (+Mr).

[0046] The mechanical friction torque Tm becomes a value (-Mr) that balances the residual moment at the start of the estimated steering. For this reason, the torque required for steering becomes "0" at the start of the estimated steering, and the motor drive current (Fig. 10(f)) starts from "0". When the steered wheel starts to move, the mechanical friction torque Tm increases to the positive dynamic friction torque Tmd. The motor drive current (Fig. 10(f)) increases while remaining positive as the mechanical friction torque Tm and the road surface reaction torque increase.

[0047] When the mechanical friction torque Tm switches to the dynamic friction torque Tmd, the mechanical friction torque Tm becomes approximately constant. For this reason, an inflection point Pinf occurs in the characteristics of the motor drive current with respect to the steering angle θ. When steering further in the same direction, in the linear region, the motor drive current increases approximately linearly with an increase in the steering angle θ. In the following description, the steering angle and the motor drive current at the inflection point Pinf may be denoted as the "inflection point steering angle θinf" and the "inflection point current Iinf".

[0048] Refer to FIG. 9. The residual direction determination unit 24 estimates the direction of the residual moment based on the change in the measured current value of the motor drive current during the estimated steering. For example, the direction of the residual moment is estimated based on the measured current value of the motor drive current at the initial stage of the estimated steering (for example, the measured current value immediately after the start of the estimated steering). Generally, it is known that the tire reaction torque with respect to the steering angle θ of the steering wheel is linear and monotonically increasing in a region where the absolute value of the steering angle θ at the initial stage of steering is small. Taking advantage of this characteristic, the direction of the residual moment is estimated based on the sign of the measured current value of the motor drive current at the initial stage of the estimated steering (hereinafter sometimes referred to as the "initial current value Iinit_ave").

[0049] For example, the residual direction determination unit 24 calculates the integrated value of the measured current value of the motor drive current during the steering of the steering wheel from the start of the estimated steering until a predetermined steering angle θ0 at which the tire reaction torque with respect to the steering angle becomes linear as shown in FIG. 10(f) as the initial current value Iinit_ave. However, if the sign of the change rate of the measured current value changes from negative to positive before the steering angle θ reaches the steering angle θ0, the integrated value of the measured current value from the start of the estimated steering until the time when the change rate of the measured current value changes from negative to positive is calculated as the initial current value Iinit_ave.

[0050] The residual direction determination unit 24 determines that it is in-phase when the sign of the initial current value Iinit_ave is negative, and determines that it is out-of-phase when the sign of the initial current value Iinit_ave is positive. At this time, if no residual moment is generated at all, the residual direction is determined to be out-of-phase. However, in this case, the offset Iitr described later becomes 0, so it does not affect the estimation result.

[0051] The bias component calculation unit 25 calculates the bias component generated in the motor drive current due to the mechanical friction of the steering mechanism and the residual moment. Since the steering torque Tt balances the sum of the mechanical friction torque Tm and the tire reaction torque, the motor drive current includes a bias component corresponding to the sum of the mechanical friction torque Tm and the residual moment (-Mr or +Mr) at the start of steering. For example, the bias component calculation unit 25 calculates a bias component corresponding to the sum of the dynamic friction torque Tmd and the residual moment at the start of steering.

[0052] Refer to FIG. 10(c). In the in-phase case, in the range of the extreme point steering angle θext or more, the bias component corresponds to the sum of the residual moment (-Mr) and the dynamic friction torque Tmd. Also, in the range of the extreme point steering angle θext or more, within the linear region, the motor drive current increases almost linearly with an increase in the steering angle θ. Therefore, based on the measured current value of the motor drive current in the range of the steering angle θ of the extreme point steering angle θext or more, an approximate straight line Lapp of the motor drive current with respect to the steering angle θ is calculated, and by obtaining the intercept (-Iitr) at the steering angle θ = 0, a bias component corresponding to the sum of the dynamic friction torque Tmd and the residual moment (-Mr) can be estimated. By obtaining the intercept (-Iitr), even if the mechanical friction changes from static friction to dynamic friction at the start of steering, the sum of the dynamic friction torque Tmd and the residual moment (-Mr) at the start of steering can be estimated.

[0053] For example, as shown in FIG. 11, the bias component calculation unit 25 may detect, as the extreme point steering angle θext, the steering angle at which the change in the measured current value of the motor drive current changes from a decrease to an increase as the steering angle θ increases after the start of steering. Also, the measured current value at the extreme point steering angle θext may be detected as the extreme point current Iext. The bias component calculation unit 25 may acquire the measured current value (Iext + ΔI) of the motor drive current at the steering angle (θext + Δθ) obtained by further steering in the same direction by a predetermined angle Δθ from the extreme point steering angle θext, and calculate the current change rate ΔI / Δθ.

[0054] Then, a straight line passing through the extreme point Pext(θext, Iext) and having a slope (ΔI / Δθ) may be calculated as the approximate straight line Lapp. By calculating the approximate straight line \(L_{app}\) at the extreme point \(P_{ext}(\theta_{ext}, I_{ext})\), it is possible to calculate the approximate straight line \(L_{app}\) at a steering angle \(\theta\) that is as small as possible. Therefore, in a region where the linearity of the motor drive current with respect to the steering angle \(\theta\) is good, the approximate straight line \(L_{app}\) can be calculated.

[0055] Refer to Fig. 10(f). In the case of the reverse phase, in the range of the inflection point steering angle \(\theta_{inf}\) or more, the bias component corresponds to the sum of the residual moment \((+M_r)\) and the dynamic friction torque \(T_{md}\). Also, in the range of the inflection point steering angle \(\theta_{inf}\) or more, if it is within the linear region, the motor drive current increases almost linearly with an increase in the steering angle \(\theta\). Therefore, based on the measured current value of the motor drive current in the range of the steering angle \(\theta\) of the inflection point steering angle \(\theta_{inf}\) or more, an approximate straight line \(L_{app}\) of the motor drive current with respect to the steering angle \(\theta\) is calculated, and by obtaining the intercept \(I_{itr}\) at the steering angle \(\theta = 0\), a bias component corresponding to the sum of the dynamic friction torque \(T_{md}\) and the residual moment \((M_r)\) can be estimated.

[0056] For example, the bias component calculation unit 25 may detect the steering angle at which the first-order time derivative value of the measured value of the motor drive current when steering at a constant steering angular velocity becomes maximum as the inflection point steering angle \(\theta_{inf}\). Figs. 12(a) to 12(d) are time charts of the steering angle, the steering angular velocity, the motor drive current, and the first-order time derivative value of the motor drive current when steering at a constant steering angular velocity. The bias component calculation unit 25 may detect the steering angle at which the first-order time derivative value becomes maximum as the inflection point steering angle \(\theta_{inf}\), and detect the measured current value at the inflection point steering angle \(\theta_{inf}\) as the inflection point current \(I_{inf}\).

[0057] Refer to Fig. 13. The bias component calculation unit 25 may acquire the measured current value \((I_{inf}+\Delta I)\) of the motor drive current at the steering angle \((\theta_{inf}+\Delta\theta)\) obtained by further steering in the same direction by a predetermined angle \(\Delta\theta\) from the inflection point steering angle \(\theta_{inf}\), and calculate the current change rate \(\Delta I / \Delta\theta\). Then, a straight line passing through the inflection point \(P_{inf}(\theta_{inf}, I_{inf})\) and having a slope \((\Delta I / \Delta\theta)\) may be calculated as the approximate straight line \(L_{app}\).

[0058] Refer to FIG. 9. The drive current correction unit 26 calculates a corrected measured value Icrt by correcting the current measured value acquired by the measurement value acquisition unit 21 with the bias component (i.e., the intercept Iitr or the intercept (-Iitr)) calculated by the bias component calculation unit 25. Specifically, the corrected measured value Icrt is calculated by subtracting the bias component from the current command value. As shown in FIGS. 11 and 13, the maximum value detection unit 27 detects the maximum value Icmx of the corrected measured value Icrt. By the correction process of the bias component calculation unit 25 and the drive current correction unit 26, even if the residual moment directions are different, if the road surface friction coefficients are the same, the maximum value Icmx can be made to be approximately the same value.

[0059] Based on the relationship between the maximum value of the motor drive current value acquired in advance and the road surface friction coefficient, the friction coefficient estimation unit 28 obtains the road surface friction coefficient at the maximum value Icmx of the corrected measured value Icrt as the second road surface friction coefficient μ2. FIG. 14 is an explanatory diagram of an example of the relationship between the maximum value of the motor drive current acquired in advance and the road surface friction coefficient. The relationship between the maximum value of the motor drive current and the road surface friction coefficient may be determined in advance by, for example, a calculation formula, or may be stored in the storage device 13b in the form of a look-up table or a map. Note that when executing the estimated steering, it is preferable for the steering command calculation unit 20 to set a predetermined steering angular velocity to the same speed as the steering angular velocity at which the steering wheel was steered by the steering motor 11 when the relationship between the maximum value of the motor drive current and the road surface friction coefficient shown in FIG. 14 was identified in advance.

[0060] Refer to FIG. 9. The guarantee range setting unit 29 sets the guarantee range (upper and lower limit values) of the estimation result of the first road surface friction coefficient μ1 based on the variation range of the second road surface friction coefficient μ2 estimated by the friction coefficient estimation unit 28. For example, the guarantee range setting unit 29 may set the guarantee range based on the variance (standard deviation) of the second road surface friction coefficient μ2. For example, the guarantee range setting unit 29 calculates the standard deviation σ of the second road surface friction coefficient μ2 and sets ±3σ as the upper and lower limit values of the first road surface friction coefficient μ1. The friction coefficient estimation unit 23 restricts the first road surface friction coefficient μ1 with the upper and lower limit values set by the guarantee range setting unit 29, and outputs the final estimated result of the road surface friction coefficient.

[0061] (Application example of the embodiment) An application example of the estimated road surface friction coefficient is shown. The road surface friction coefficient corresponds to the division result of dividing the maximum acceleration that the vehicle 1 can generate by the gravitational acceleration. Therefore, for example, when the target driving trajectory of the future vehicle 1 is given, by estimating the maximum acceleration that the vehicle 1 can generate based on the estimated road surface friction coefficient, the longitudinal acceleration and the lateral acceleration for traveling along the target driving trajectory can be calculated by calculation.

[0062] (Effect of the embodiment) (1) In the road surface friction coefficient estimation method of the embodiment, a current measurement value of the drive current of the steering motor that steers the steered wheels is acquired, a change rate measurement value that is the change rate of the current measurement value with respect to the change in the steering angle at a predetermined steering angle is calculated, and based on the relationship between the change rate of the drive current of the steering motor with respect to the change in the steering angle at a predetermined steering angle obtained in advance and the friction coefficient of the road surface, and the change rate measurement value, the first friction coefficient of the road surface is estimated.

[0063] Generally, when a mechanism with large mechanical friction is adopted, a residual moment may occur due to tire twisting or the like when the vehicle stops. However, the influence of the residual moment is mainly that the static friction force due to the mechanical friction that balanced the residual moment becomes the sliding friction force when starting to slide and instantaneously decreases, which affects the motor drive current value at the initial stage of steering when trying to steer at a constant steering angle. On the other hand, in the current change rate of the drive current of the steering motor with respect to the change in the steering angle, after the transient state from static friction to sliding friction ends, it is not affected by the residual moment and is uniquely determined with respect to the road surface friction coefficient at a predetermined steering angle. Therefore, by referring to the correlation, the road surface friction coefficient can be accurately estimated. Thereby, the estimation accuracy of the road surface friction coefficient based on the drive current of the motor that steers the wheels can be improved.

[0064] (2) The predetermined steering angle may be set to an angle smaller than a first steering angle at which the drive current of the steering motor becomes maximum when steering on a road surface with the smallest road surface friction coefficient among the assumable driving road surface environments. Thereby, the maximum steering angle at which the road surface friction coefficient does not saturate can be determined. Note that the steering angle at which the moment saturates varies depending on the road surface friction coefficient, and the smaller the road surface friction coefficient, the smaller the saturating steering angle. Therefore, based on the steering angle at which the motor drive current becomes maximum (the change rate saturates) on the road surface with the smallest road surface friction coefficient among the assumable driving road surface environments, it can be determined.

[0065] (3) The predetermined steering angle may be set to a second steering angle in the vicinity of the first steering angle and smaller than the first steering angle. Thereby, within the steering angle range set in (2) above, the steering angle at which the most significant difference in the current change rate can be obtained with respect to the road surface friction coefficient can be determined. (4) When preliminarily identifying the relationship between the change rate of the drive current of the steering motor and the friction coefficient, the change rate may be obtained on three or more road surfaces with different road surface friction coefficients. Although the relationship between the road surface friction coefficient and the change rate of the motor drive current has non-linearity, when obtaining the correlation between the change rate of the motor drive current and the road surface friction coefficient in advance, if it is carried out on two or fewer road surfaces with different road surface friction coefficients, the non-linearity cannot be grasped. By obtaining the change rate on three or more road surfaces with different road surface friction coefficients, the non-linear relationship between the road surface friction coefficient and the change rate of the motor drive current can be grasped.

[0066] (5) Based on the relationship between the maximum value of the drive current of the steering motor that changes according to the previously obtained steering angle and the road surface friction coefficient, and the current measurement value, the second friction coefficient may be estimated, and the estimation result of the first friction coefficient may be restricted based on the variation range of the second friction coefficient. By limiting with the upper and lower limit values determined based on the estimation results using the estimation methods in different steering ranges, the steering angle region of the current value used for estimation can be expanded, and it is possible to suppress the estimation error due to noise or the like without depending on the current value in a narrow range of the steering angle region.

Explanation of symbols

[0067] 1…Vehicle, 2F…Front wheel, 2R…Rear wheel, 10…Road surface friction coefficient estimation device, 11…Steering motor, 12…Steering angle measurement device, 13…Controller, 13a…Processor, 13b…Storage device, 20…Steering command calculation unit, 21…Measurement value acquisition unit, 22…Rate of change calculation unit, 23…Friction coefficient estimation unit, 24…Residual direction determination unit, 25…Bias component calculation unit, 26…Drive current correction unit, 27…Maximum value detection unit, 28…Friction coefficient estimation unit, 29…Guarantee range setting unit

Claims

1. Obtain a current measurement value of the drive current of a steering motor that steers a steering wheel, Calculate a rate-of-change measurement value that is the rate of change of the current measurement value with respect to a steering angle change at a predetermined steering angle, Estimate a first friction coefficient of the road surface based on a relationship between a rate of change of the drive current of the steering motor with respect to a steering angle change at the predetermined steering angle obtained in advance and a coefficient of friction of the road surface, and the rate-of-change measurement value. A road surface friction coefficient estimation method characterized by the above.

2. The method for estimating a road surface friction coefficient according to claim 1, characterized in that the predetermined steering angle is set to an angle smaller than a first steering angle at which the drive current of the steering motor becomes maximum when steering on a road surface with the smallest coefficient of friction among possible driving road surface environments.

3. The method for estimating a road surface friction coefficient according to claim 2, characterized in that the predetermined steering angle is set to a vicinity of the first steering angle and a second steering angle smaller than the first steering angle.

4. When preliminarily identifying the relationship between the rate of change of the drive current of the steering motor and the coefficient of friction, the rate of change is obtained on three or more road surfaces with different coefficients of friction. The road surface friction coefficient estimation method according to any one of claims 1 to 3, characterized by the above.

5. Estimate a second friction coefficient based on a relationship between a maximum value of the drive current of the steering motor that changes according to a steering angle obtained in advance and a coefficient of friction of the road surface, and the current measurement value, Limit the estimation result of the first friction coefficient based on the variation range of the second friction coefficient. The method for estimating a road surface friction coefficient according to any one of claims 1 to 3, characterized by the above.

6. A process of obtaining a current measurement value of the drive current of a steering motor that steers a steering wheel, A process of calculating a rate-of-change measurement value that is the rate of change of the current measurement value with respect to a steering angle change at a predetermined steering angle, A process of estimating a first friction coefficient of the road surface based on a relationship between a rate of change of the drive current of the steering motor with respect to a steering angle change at the predetermined steering angle obtained in advance and a coefficient of friction of the road surface, and the rate-of-change measurement value. A road surface friction coefficient estimation device characterized by the above.

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