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); estimating the drive current of the steering motor required for generating a steering torque corresponding to reaction force generated by mechanical friction in a steering mechanism steering the steering wheel and tire torsion existing in the steering wheel in a steering holding state, as a bias component (S7 and S10); and estimating a first friction coefficient of a road surface, on the basis of a difference obtained by subtracting the bias component from the current measurement value (S11 to S13).SELECTED DRAWING: Figure 10

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, an electric current measurement value of a drive current of a steering motor that steers a steering wheel is acquired, and a steering torque corresponding to a reaction force generated by mechanical friction in a steering mechanism that steers the steering wheel and tire twist existing in the steering wheel in a fixed-steering state is estimated as a bias component of the drive current of the steering motor required to generate the reaction force, and a first friction coefficient of the road surface is estimated based on a difference obtained by subtracting the bias component from the electric current measurement value.

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]

Figure 1

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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) Refer 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 between them and 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. Note that 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 steered independently.

[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 estimates the friction coefficient of the road surface on which the vehicle 1 travels based on the estimated steering of the steering wheel and 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 steered wheels varies 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 when the steered wheels are steered up to a predetermined steering angle θpre.

[0018] However, there may be a moment remaining in the tire in the holding steering state before the estimated steering is executed due to tire twist or the like. 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 steered wheels where 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 holding steering state is deviated by an angle θ with respect to the neutral point 2n. For example, the residual moment Mr occurs when tire twist occurs due to the vehicle state until the steered wheels are held in the steering position.

[0019] If the residual moment Mr is less than the maximum static friction torque of the steering mechanism, since the residual moment Mr balances the static friction torque, the steered wheels do not move and the residual moment Mr remains in the tire. When the estimated steering is started, since the residual moment Mr affects the motor drive current, 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 torques acting on the steered wheels 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". The arrows Tm, Tr, Mr, and Tt schematically represent the mechanical friction torque of the steering mechanism, the road surface reaction force torque corresponding to the steering angle, the residual moment, and the steering torque of the estimated steering, respectively. The steering torque Tt balances the sum of the mechanical friction torque Tm, the road surface reaction force torque Tr, and the residual moment Mr. Therefore, depending on the direction of the residual moment present 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.

[0021] Therefore, the controller 13 estimates the bias component generated in the motor drive current due to the mechanical friction in the steering mechanism and the tire twist present in the steered wheels in the holding steering state before the estimated steering. Then, after removing the bias component from the measured value of the motor drive current, the road surface friction coefficient is estimated. 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 holding steering state (for example, turning at a constant 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.

[0022] 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 residual direction determination unit 22, a bias component calculation unit 23, a drive current correction unit 24, a maximum value detection unit 25, and a friction coefficient estimation unit 26. 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. The measurement value acquisition unit 21 sequentially acquires the steering angle measurement value of the steering angle θ measured by the steering angle measurement device 12 and the current measurement value of the motor drive current measured by the steering motor 11 from the start of the estimated steering until the steered wheels are steered to a predetermined steering angle θpre.

[0023] Referring to FIGS. 5(a) to 5(f), the characteristics of the motor drive current will be described. FIGS. 5(a) to 5(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. 5(d) to 5(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.

[0024] Generally, in the region where the absolute value of the steering angle θ is small, as shown in FIGS. 5(b) and 5(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. 5(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".

[0025] 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).

[0026] The mechanical friction torque Tm (FIG. 5(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. 5(c)) starts from "0". When the steered wheel starts to move, the mechanical friction torque Tm decreases to the dynamic friction torque Tmd. Accordingly, since the steering torque Tt decreases, the motor drive current (FIG. 5(c)) temporarily becomes a negative value.

[0027] 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".

[0028] On the other hand, in the case of reverse phase (Fig. 5(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 by the residual moment (+Mr).

[0029] 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 at the start of the estimated steering becomes "0", and the motor drive current (Fig. 5(f)) starts from "0". When the steering wheel starts to move, the mechanical friction torque Tm increases to a positive dynamic friction torque Tmd. The motor drive current (Fig. 5(f)) increases while remaining positive as the mechanical friction torque Tm and the road surface reaction torque increase.

[0030] 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".

[0031] Refer to FIG. 4. The residual direction determination unit 22 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").

[0032] For example, the residual direction determination unit 22 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 to a predetermined steering angle θ0 at which the tire reaction torque with respect to the steering angle becomes linear as shown in FIG. 5(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 to the point when the change rate of the measured current value changes from negative to positive is calculated as the initial current value Iinit_ave.

[0033] The residual direction determination unit 22 determines that it is in-phase when the sign of the initial current value Iinit_ave is negative, and determines that it is in anti-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 in anti-phase, but in this case, the offset Iitr described later becomes 0 and thus does not affect the estimation result.

[0034] The bias component calculation unit 23 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 is balanced with 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 23 calculates a bias component corresponding to the sum of the dynamic friction torque Tmd and the residual moment at the start of steering.

[0035] Refer to Fig. 5(c). In the case of in-phase, 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, if it is within the linear region, the motor drive current increases almost linearly with the 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, the 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.

[0036] For example, as shown in Fig. 6, the bias component calculation unit 23 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 23 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.

[0037] 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 \(P_{ext}(\theta_{ext}, I_{ext})\), the approximate straight line \(L_{app}\) at the smallest possible steering angle \(\theta\) can be calculated. Therefore, the approximate straight line \(L_{app}\) can be calculated in a region where the linearity of the motor drive current with respect to the steering angle \(\theta\) is good.

[0038] Refer to Fig. 5(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 the increase of 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, the 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 \(\theta = 0\), the bias component corresponding to the sum of the dynamic friction torque \(T_{md}\) and the residual moment \((M_r)\) can be estimated.

[0039] For example, the bias component calculation unit 23 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. 7(a) to 7(d) are time charts of the steering angle, steering angular velocity, motor drive current, and first-order time derivative value of the motor drive current when steering at a constant steering angular velocity. The bias component calculation unit 23 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 \(I_{inf}\).

[0040] Refer to Fig. 8. The bias component calculation unit 23 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 \(L_{app}\).

[0041] Refer to FIG. 4. The drive current correction unit 24 calculates a corrected measurement value Icrt by correcting the current measurement 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 23. Specifically, the corrected measurement value Icrt is calculated by subtracting the bias component from the current command value. As shown in FIGS. 6 and 8, the maximum value detection unit 25 detects the maximum value Icmx of the corrected measurement value Icrt. By the correction process of the bias component calculation unit 23 and the drive current correction unit 24, 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.

[0042] 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 26 obtains the road surface friction coefficient μ1 at the maximum value Icmx of the corrected measurement value Icrt. FIG. 9 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 the steering command calculation unit 20 preferably sets a predetermined steering angular velocity when executing the estimated steering 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. 9 was identified in advance.

[0043] FIG. 10 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 steering wheel at a predetermined steering angular velocity. In step S2, the measurement value acquisition unit 21 acquires the measured value of the steering angle θ and the measured value of the motor drive current. In step S3, the steering command calculation unit 20 determines whether or not the steering has reached a predetermined steering angle θpre. If the steering has not reached the predetermined steering angle θpre (step S3: N), the process returns to step S1. If the steering has reached the predetermined steering angle θpre (step S3: Y), the process proceeds to step S4.

[0044] In step S4, the residual direction determination unit 22 estimates the direction of the residual moment based on the initial current value Iinit_ave of the motor drive current at the start of the estimated steering. When the initial current value Iinit_ave is negative, the residual direction determination unit 22 determines that it is in-phase (step S4: Y), and the process proceeds to step S5. When the initial current value Iinit_ave is positive, it is determined that it is in the reverse phase (step S4: N), and the process proceeds to step S8.

[0045] In step S5, the bias component calculation unit 23 detects the extreme point (θext, Iext). In step S6, the bias component calculation unit 23 calculates the current change rate ΔI / Δθ from the extreme point steering angle θext to the steering angle (θext + Δθ). In step S7, the bias component calculation unit 23 calculates the intercept (-Iitr) of the approximate straight line Lapp passing through the extreme point (θext, Iext) with the slope (ΔI / Δθ) as the bias component. Then the process proceeds to step S11.

[0046] In step S8, the bias component calculation unit 23 detects the inflection point (θinf, Iinf). In step S9, the bias component calculation unit 23 calculates the current change rate ΔI / Δθ from the inflection point steering angle θinf to the steering angle (θinf + Δθ). In step S10, the bias component calculation unit 23 calculates the intercept (Iitr) of the approximate straight line Lapp passing through the inflection point (θinf, Iinf) with the slope (ΔI / Δθ) as the bias component. Then the process proceeds to step S11.

[0047] In step S11, the drive current correction unit 24 calculates a corrected measurement value Icrt by correcting the current measurement value acquired by the measurement value acquisition unit 21 with a bias component. In step S12, the maximum value detection unit 25 calculates the maximum value Icmx of the corrected measurement value Icrt. In step S13, the friction coefficient estimation unit 26 obtains the road surface friction coefficient μ1 at the maximum value Icmx based on the relationship between the maximum value of the motor drive current value acquired in advance and the road surface friction coefficient. Then the process ends.

[0048] (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 "first road surface friction coefficient μ1") calculated by the friction coefficient estimation unit 26 by the estimation method of the first embodiment. FIG. 11 is a characteristic diagram of the tire reaction torque with respect to the road surface friction coefficient and the steering angle θ. 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 a "non-linear / non-saturation region R3". In the non-linear / non-saturation region R3, the change rate (ΔT / Δθ) of the tire reaction torque changes according to the road surface friction coefficient μ. The controller 13 of the second embodiment estimates the second road surface friction coefficient μ2 based on the current change rate (ΔI / Δθ) of the motor drive current with respect to the steering angle θ at a preset steering angle (hereinafter referred to as "predetermined steering angle θ1") within the non-linear / non-saturation region R3.

[0049] When estimating the road surface friction coefficient using the maximum value of the motor drive current, the residual moment affects the current maximum value. On the other hand, 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) and the road surface friction coefficient at a predetermined steering angle θ1 is known, the second road surface friction coefficient μ2 can be accurately estimated based on the current change rate (ΔI / Δθ).

[0050] FIG. 12 is a block diagram of an example of the functional configuration of the controller 13 according to the second embodiment. The controller 13 according to the second embodiment includes, in addition to the configuration of the first embodiment, a change rate calculation unit 27, a friction coefficient estimation unit 28, and a guarantee range setting unit 29. The change rate calculation unit 27 extracts data near a predetermined steering angle θ1 from among the steering angle measurement value and the current measurement value acquired by the measurement value acquisition unit 21. The change rate calculation unit 27 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 27 calculates the current change rate (ΔI / Δθ) at the predetermined steering angle θ1 by dividing ΔI by Δθ.

[0051] The friction coefficient estimation unit 28 estimates the second road surface friction coefficient μ2 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 increases monotonically with an increase in the steering angle θ. For this reason, the motor drive current also increases monotonically, 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. 13(a) to 13(d).

[0052] In FIGS. 13(a) to 13(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 that the tire slip angle is "α", the lateral elastic coefficient is "Cy", the tire contact width is "w", the slip ratio is "s", and the distance from the contact start point is "x", the lateral tire generating force Fyad in the adhesion region is determined by Fyad = Cy × w × x × tanα.

[0053] 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 with the distribution of the maximum static friction force acting on the 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 contact surface is integrated, and the sum of these gives the tire lateral force Fy.

[0054] Figures 13(a) and 13(b) show the brush model when the road surface friction coefficient is relatively high. The area of the hatched region in Fig. 13(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. 13(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. 13(a) and 13(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.

[0055] Figures 13(c) and 13(d) show the brush model when the road surface friction coefficient is relatively low. The area of the hatched region in Fig. 13(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. 13(d) represents the tire lateral force Fy in the case of the tire slip angle α + Δα. Comparing Figs. 13(a) and 13(b) with Figs. 13(c) and 13(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 understood that the increase rate (ΔI / Δθ) of the motor drive current also decreases as the road surface friction coefficient becomes smaller.

[0056] 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 obtained in advance and stored in the storage device 13b. The relationship between the previously obtained 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. 14 is an explanatory diagram of an example of the relationship between the current change rate of the motor drive current obtained in advance and the road surface friction coefficient obtained in advance.

[0057] When identifying the relationship between the current change rate and the road surface friction coefficient in advance, it is preferable to obtain the current change rate on three or more road surfaces with different road surface friction coefficients and determine the relationship between the current change rate and the road surface friction coefficient. The friction coefficient estimation unit 28 obtains the second road surface friction coefficient μ2 in the current change rate (ΔI / Δθ) based on the relationship between the current change rate of the motor drive current obtained in advance 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 27.

[0058] Referring to FIG. 15. 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 no significant difference occurs with respect to the road surface friction coefficient. For this reason, 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 assumed driving road surface environments.

[0059] 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. For this reason, the predetermined steering angle θ1 may be set to a steering 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.

[0060] Refer to FIG. 12. 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 26 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 estimation 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 the maximum acceleration that the vehicle 1 can generate by the gravitational acceleration. Therefore, for example, when the target travel 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 travel trajectory can be calculated by calculation.

[0062] (Effect of the Embodiment) (1) In the road surface friction coefficient estimation method of the embodiment, the current measurement value of the drive current of the steering motor that steers the steered wheels is acquired, and the drive current of the steering motor required to generate the steering torque corresponding to the reaction force generated by the mechanical friction in the steering mechanism that steers the steered wheels and the tire twist existing in the steered wheels in the holding steering state is estimated as the bias component, and the first friction coefficient of the road surface is estimated based on the difference obtained by subtracting the bias component from the current measurement value. Thereby, even when a mechanism with large mechanical friction is adopted, its influence can be considered, so that the estimation accuracy of the road surface friction coefficient based on the drive current of the motor that steers the wheels can be improved.

[0063] (2) The steering angle measurement value of the steering angle of the steering wheel may be obtained, and a bias component may be estimated based on the steering angle measurement value and the current measurement value. Thereby, since the bias component can be estimated only from the steering angle measurement value and the current value measurement value, it is possible to obtain, by a steering operation, a steering torque corresponding to a reaction force that is not uniquely determined by driving conditions such as the state of the road surface, the previous steering, and the manner of acceleration and deceleration. (3) The direction of the residual moment of the tire of the steering wheel due to tire twist may be estimated based on the current measurement value during the steering of the steering wheel by the steering motor. Thereby, since the direction of the residual moment can be known from the steering angle measurement value and the current measurement value, even when the manner of influence is different depending on the direction of the residual moment, it is possible to select a process for eliminating the influence of the residual moment corresponding to the residual direction, and the estimation accuracy can be improved. (4) The direction of the residual moment may be estimated based on the sign of the current measurement value at the initial stage of the start of the steering of the steering wheel by the steering motor. Thereby, since the method for estimating the bias component can be selected during the steering operation, the estimation can be performed in real time during the steering operation, and the responsiveness of the estimation is improved.

[0064] (5) When the direction of the residual moment and the steering direction of the steering wheel by the steering motor are the same, a first steering angle at which the current measurement value changes from decreasing to increasing during the steering of the steering wheel by the steering motor is detected, and the current measurement value at the first steering angle is detected as a first current value. The current measurement value at a second steering angle at which the steering wheel is further steered in the same direction from the first steering angle by the steering motor is detected as a second current value. An approximate straight line between the steering angle of the steering wheel and the drive current of the steering motor is calculated based on the first steering angle and the first current value, and the second steering angle and the second current value, and the intercept of the approximate straight line may be estimated as the bias component. Thereby, the bias component can be estimated.

[0065] (6) When the direction of the residual moment is different from the steering direction of the steered wheels by the steering motor, detect a first steering angle at which the first-order time derivative value of the current measurement value is maximized when the steered wheels are steered by the steering motor at a constant steering angular velocity, detect the current measurement value at the first steering angle as a first current value, detect the current measurement value at a second steering angle at which the steered wheels are further steered in the same direction from the first steering angle by the steering motor as a second current value, calculate an approximate straight line between the steering angle of the steered wheels and the drive current of the steering motor based on the first steering angle and the first current value and the second steering angle and the second current value, and estimate the intercept of the approximate straight line as a bias component. Thereby, the bias component can be estimated.

[0066] (7) A first friction coefficient may be estimated based on the maximum value of the difference when the steered wheels are steered to a predetermined steering angle. Thereby, the road surface friction coefficient can be estimated from the motor drive current. (8) A first friction coefficient may be estimated based on the relationship between the maximum value of the drive current of the steering motor acquired in advance and the road surface friction coefficient and the maximum value of the difference. Thereby, the road surface friction coefficient can be estimated by identifying the characteristics in advance.

[0067] (9) When identifying the relationship between the maximum value of the drive current of the steering motor and the friction coefficient in advance, the steering angular velocity at which the steered wheels are steered by the steering motor may be made to coincide with the steering angular velocity when estimating the first friction coefficient. Thereby, by unifying the drive conditions of the steering motor, highly accurate estimation incorporating motor characteristics becomes possible. (10) Estimate a second friction coefficient 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 acquired in advance and the road surface friction coefficient and the current measurement value, and limit the estimation result of the first friction coefficient based on the variation range of the second friction coefficient. Thereby, by combining the estimation methods in two different steering ranges, the estimation error due to the variation of the state quantity can be suppressed.

Explanation of Signs

[0068] 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…Residual direction determination unit, 23…Bias component calculation unit, 24…Drive current correction unit, 25…Maximum value detection unit, 26…Friction coefficient estimation unit, 27…Change rate calculation 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, Estimate the drive current of the steering motor required to generate a steering torque corresponding to the reaction force generated by mechanical friction in the steering mechanism that steers the steering wheel and tire twist existing in the steering wheel in the holding steering state as a bias component, Estimate a first friction coefficient of the road surface based on the difference obtained by subtracting the bias component from the current measurement value. A road surface friction coefficient estimation method characterized by the above.

2. Obtain a steering angle measurement value of the steering angle of the steering wheel, Estimate the bias component based on the steering angle measurement value and the current measurement value. The road surface friction coefficient estimation method according to claim 1, characterized by the above.

3. Estimate the direction of the residual moment of the tire of the steering wheel due to the tire twist based on the current measurement value during the steering of the steering wheel by the steering motor. The road surface friction coefficient estimation method according to claim 2, characterized by the above.

4. Estimate the direction of the residual moment based on the positive or negative of the current measurement value at the initial stage of the start of the steering of the steering wheel by the steering motor. The road surface friction coefficient estimation method according to claim 3, characterized by the above.

5. When the direction of the residual moment and the steering direction of the steering wheel by the steering motor are the same, detect a first steering angle at which the current measurement value changes from decreasing to increasing during the steering of the steering wheel by the steering motor, and detect the current measurement value at the first steering angle as a first current value. Detect the current measurement value at a second steering angle at which the steering wheel is further steered in the same direction from the first steering angle by the steering motor as a second current value. Calculate an approximate straight line between the steering angle of the steering wheel and the drive current of the steering motor based on the first steering angle and the first current value, and the second steering angle and the second current value. Estimate the intercept of the approximate straight line as the bias component. The road surface friction coefficient estimation method according to claim 3 or 4, characterized by the above.

6. When the direction of the residual moment and the steering direction of the steering wheel by the steering motor are different, detect a first steering angle at which the first-order time derivative value of the current measurement value becomes maximum when the steering wheel is steered at a constant steering angular velocity by the steering motor, and detect the current measurement value at the first steering angle as a first current value. Detect the measured current value at a second steering angle at which the steering wheel is further steered in the same direction from the first steering angle by the steering motor as a second current value. Calculate an approximate straight line between the steering angle of the steering wheel and the drive current of the steering motor based on the first steering angle and the first current value, and the second steering angle and the second current value. Estimate the intercept of the approximate straight line as the bias component. The method for estimating the road surface friction coefficient according to claim 3 or 4, characterized in that.

7. The method for estimating the road surface friction coefficient according to any one of claims 1 to 4, characterized in that the first friction coefficient is estimated based on the maximum value of the difference when the steering wheel is steered to a predetermined steering angle.

8. The method for estimating the road surface friction coefficient according to claim 7, characterized in that the first friction coefficient is estimated based on the relationship between the maximum value of the drive current of the steering motor acquired in advance and the road surface friction coefficient and the maximum value of the difference.

9. The method for estimating the road surface friction coefficient according to claim 8, characterized in that when identifying the relationship between the maximum value of the drive current of the steering motor and the friction coefficient in advance, the steering angular velocity at which the steering wheel is steered by the steering motor is made to coincide with the steering angular velocity when estimating the first friction coefficient.

10. Estimate a second friction coefficient based on the relationship between the rate of change of the drive current of the steering motor with respect to the change in the steering angle at a predetermined steering angle acquired in advance and the road surface friction coefficient, and the measured current value. Limit the estimation result of the first friction coefficient based on the variation range of the second friction coefficient. The method for estimating the road surface friction coefficient according to any one of claims 1 to 4, characterized in that.

11. A process of acquiring a measured current value of the drive current of a steering motor that steers a steering wheel. A process of estimating, as a bias component, the drive current of the steering motor required to generate a steering torque corresponding to the reaction force generated by mechanical friction in the steering mechanism that steers the steering wheel and tire twist existing in the steering wheel in the holding state. A process of estimating the first friction coefficient of the road surface based on the difference obtained by subtracting the bias component from the measured current value. A road surface friction coefficient estimation device comprising a controller that executes the above.

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