JP2009096229A - Road surface friction coefficient estimation device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a road surface friction coefficient estimation device which can accurately estimates road surface friction coefficients in a wide driving area and is excellent in versatility. <P>SOLUTION: The road surface friction coefficient estimation device is configured to calculate a generated driving/braking force Fd generated by a wheel based on the longitudinal acceleration of a vehicle, and to calculate an estimated driving/braking force Fm to be added to the wheel based on the output torque of the driving source of the vehicle and the braking force of a brake, and to calculate a generated driving/braking force difference value ΔFd between this time value and past value of the generated driving/braking force Fd and an estimated driving/braking force difference value ΔFm between this time value and past value of the estimated driving/braking force Fm, and to estimate a driving stiffness coefficient Q by using a parameter identification method based on the generated driving/braking force difference value ΔFd and the estimated driving/braking force difference value ΔFm, and to, on the basis of a speed V and the driving stiffness coefficient Q, set a road surface friction coefficient estimated value μE by referring to a characteristic map showing a relationship between prestored driving stiffness coefficient Q and speed V and the road surface friction coefficient estimated value μE. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a road surface friction coefficient estimation device that can be applied not only when traveling straight but also when turning, and that estimates a road surface friction coefficient with high accuracy and early even during acceleration.

  In recent years, various control techniques for traction control, braking force control, torque distribution control, and the like have been proposed and put into practical use in vehicles. Many of these techniques use a road surface friction coefficient for calculation or correction of necessary control parameters, and in order to appropriately execute the control, it is necessary to estimate an accurate road surface friction coefficient.

For example, in Japanese Patent Application Laid-Open No. 2003-237558, the average wheel speed of four wheels is obtained as a vehicle body speed, the vehicle body speed is differentiated and calculated as the longitudinal acceleration of the vehicle, and the hydraulic multi-plate of the power distribution control device during main brake braking Engage the clutch in the disengagement direction, calculate the slip speed difference variable by dividing the difference between the wheel speed of the rear wheel and the wheel speed of the front wheel by the wheel speed of the front wheel, and based on the longitudinal acceleration and slip speed difference variables of the vehicle A technique for estimating a road surface state based on a preset map is disclosed.
JP-A-5-338457

  However, in the technique of the above-described Patent Document 1, for example, in a four-wheel drive vehicle having a front-rear driving force distribution mechanism, the control is generally well-known in feedforward control according to drive torque and wheel differential rotation. When executing by the corresponding feedback control, the driving force (and slip ratio) of the front and rear wheels changes from moment to moment because all wheels are in a driving state during acceleration, making it difficult to estimate the vehicle speed, especially during acceleration. There is a problem that it is difficult to estimate the road surface friction coefficient. Moreover, since the technique of patent document 1 is a technique which estimates the road surface friction coefficient using the difference between the front and rear slip speeds generated according to the road surface friction coefficient, it cannot be accurately estimated during turning, and erroneous determination is made. There is a fear. That is, during turning, a difference in slip ratio between the front and rear wheels according to the turning radius occurs, so external factors other than the difference in slip ratio between the front and rear wheels generated according to the road surface friction coefficient are added, resulting in road surface friction. There is a risk of misjudging the coefficient. As a result, there is also a problem that the principle of estimation can be applied only near the straight line.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a road surface friction coefficient estimating device that can be applied not only when going straight but also when turning, and capable of estimating the road surface friction coefficient accurately and quickly even during acceleration. To do.

  The present invention is based on the first braking / driving force calculating means for calculating the first braking / driving force generated by the wheels based on the longitudinal acceleration of the vehicle, and the output torque of the vehicle driving source or the braking force of the brake. A second braking / driving force calculating means for calculating a second braking / driving force to be applied to the wheel, and a difference value between the current value of the first braking / driving force and a past value is defined as a first braking / driving force difference. A first braking / driving force difference value calculating means for calculating a value, and a second value for calculating a difference value between the current value and the past value of the second braking / driving force as a second braking / driving force difference value. A braking / driving force difference value calculating means; and a storage means for presetting and storing a relationship between a driving stiffness coefficient, a vehicle speed, and a road surface friction coefficient at a predetermined slip ratio based on tire characteristics indicating a relationship between a slip ratio and a road surface friction coefficient. The first braking / driving force difference value and the second braking / driving force Driving stiffness coefficient calculating means for calculating the driving stiffness coefficient based on the fraction value, and the driving stiffness coefficient, vehicle speed, and road surface friction stored in the storage means based on the vehicle speed and the driving stiffness coefficient calculated by the driving stiffness coefficient calculating means. A road surface friction coefficient estimating means for estimating a road surface friction coefficient from the relationship with the coefficient is provided.

  The road surface friction coefficient estimating apparatus according to the present invention can be applied not only when traveling straight but also when turning, and has an excellent effect that the road surface friction coefficient can be estimated accurately and quickly even during acceleration.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
1 to 5 show an embodiment of the present invention, FIG. 1 is a functional block diagram showing the configuration of a road surface friction coefficient estimation device, FIG. 2 is a flowchart of a road surface friction coefficient estimation program, and FIG. 3 is a driving stiffness coefficient. FIG. 4 is an explanatory diagram showing an example of a characteristic map of vehicle speed and road surface friction coefficient estimated value, FIG. 4 is an explanatory diagram showing the relationship between driving stiffness coefficient, vehicle speed and slip ratio, and FIG. 5 is an explanation of tire characteristic curves of road surface friction coefficient and slip ratio. FIG.

  In FIG. 1, reference numeral 1 denotes a road surface friction coefficient estimating device that is mounted on a vehicle and estimates a road surface friction coefficient. The road surface friction coefficient estimating device 1 includes a control unit 2, a four-wheel wheel speed sensor 3, an engine. The control unit 4, the transmission control unit 5, the brake fluid pressure sensor 6, and the longitudinal acceleration sensor 7 are connected, and the four-wheel wheel speed ωfl (left front wheel speed), ωfr (right front wheel speed), ωrl (left rear wheel speed). ), Ωrr (right rear wheel speed), engine speed Ne, throttle opening θth, turbine speed Nt, transmission gear ratio i, brake hydraulic pressure PB, and longitudinal acceleration Gx.

  Then, the control unit 2 of the road surface friction coefficient estimation device 1 executes a road surface friction coefficient estimation program, which will be described later, based on each of the input signals described above, and estimates and outputs the road surface friction coefficient (the road surface friction coefficient estimated value μE). Output). That is, as shown in FIG. 1, the control unit 2 includes a vehicle speed calculation unit 2a, a generated braking / driving force calculation unit 2b, an estimated braking / driving force calculation unit 2c, a generated braking / driving force difference value calculation unit 2d, and an estimated braking / driving force difference. A value calculation unit 2e, a driving stiffness coefficient calculation unit 2f, and a road surface friction coefficient estimated value setting unit 2g are mainly configured.

  The vehicle speed calculation unit 2a receives the wheel speeds ωfl, ωfr, ωrl, and ωrr of each wheel from the four-wheel wheel speed sensor 3, and calculates the vehicle speed V (= (ωfl + ωfr + ωrl + ωrr) / 4) by calculating the average of these. And output to the road surface friction coefficient estimated value setting unit 2g.

The generated braking / driving force calculation unit 2b receives the longitudinal acceleration Gx from the longitudinal acceleration sensor 7, calculates the generated braking / driving force Fd as the first braking / driving force by, for example, the following expression (1), and It outputs to the driving force difference value calculating part 2d. That is, the generated braking / driving force calculation unit 2b is provided as first braking / driving force calculation means.
Fd = ((M · r ² + Jw) / r) · Gx (1)
Here, M is the vehicle weight, r is the tire radius, and Jw is the moment of inertia of the rotating part that combines the wheel and the axle.

The estimated braking / driving force calculation unit 2c receives the engine speed Ne and the throttle opening θth from the engine control unit 4, receives the turbine speed Nt and the transmission gear ratio i from the transmission control unit 5, and receives the brake hydraulic pressure sensor 6 From the brake fluid pressure PB. Then, for example, the estimated braking / driving force Fm as the second braking / driving force is calculated by the following equation (2), and is output to the estimated braking / driving force difference value calculating unit 2e. That is, the estimated braking / driving force calculation unit 2c is provided as second braking / driving force calculation means.
Fm = (Tm + KB · PB) / r (2)
Here, Tm is the driving torque, KB is the brake hydraulic pressure gain, and Tm is calculated by the following equation (3), for example.
Tm = Te · i · if · tconv (3)
Here, Te is the engine output torque calculated based on the preset engine speed Ne and throttle opening θth characteristic map, if is the gear ratio of the final reduction gear, and tconv is a torque converter (not shown). The torque converter ratio tconv is obtained from a map set in advance based on the speed ratio e (= Nt / Ne) of the torque converter. The engine output torque Te may be a value directly input from the engine control unit 4 or the like, and the torque converter ratio tconv may be a value directly input from the transmission control unit 5 or the like.

The generated braking / driving force difference value calculation unit 2d receives the generated braking / driving force Fd from the generated braking / driving force calculation unit 2b. The difference value (generated braking / driving force difference value) ΔFd between the current value Fd (k) of the generated braking / driving force Fd and the previous value (previous value: Fd (k-1) in the present embodiment). Is output to the driving stiffness coefficient calculation unit 2f. That is,
ΔFd = Fd (k) −Fd (k−1) (4)
Thus, the generated braking / driving force difference value calculation unit 2d is provided as first braking / driving force difference value calculation means for calculating the generated braking / driving force difference value ΔFd as the first braking / driving force difference value. . In this embodiment, the generated braking / driving force difference value ΔFd is calculated from the current value Fd (k) and the previous value Fd (k−1), but it is not the previous value but a few samplings. The previous value may be used as the past value.

The estimated braking / driving force difference value calculation unit 2e receives the estimated braking / driving force Fm from the estimated braking / driving force calculation unit 2c. Then, a difference value (estimated braking / driving force difference value) ΔFm between the current value Fm (k) of the estimated braking / driving force Fm and a past value (previous value: Fm (k-1) in the present embodiment). Is output to the driving stiffness coefficient calculation unit 2f. That is,
ΔFm = Fm (k) −Fm (k−1) (5)
As described above, the estimated braking / driving force difference value calculation unit 2e is provided as second braking / driving force difference value calculation means for calculating the estimated braking / driving force difference value ΔFm as the second braking / driving force difference value. . In this embodiment, the estimated braking / driving force difference value ΔFm is calculated from the current value Fm (k) and the previous value Fm (k-1). The previous value may be used as the past value.

The driving stiffness coefficient calculation unit 2f receives the generated braking / driving force difference value ΔFd from the generated braking / driving force difference value calculation unit 2d, and receives the estimated braking / driving force difference value ΔFm from the estimated braking / driving force difference value calculation unit 2e. . Then, by using the generated braking / driving force difference value ΔFd and the estimated braking / driving force difference value ΔFm, a tire stiffness driving stiffness coefficient Q indicating the relationship between the slip ratio λ and the road surface friction coefficient μ, which will be described later, is calculated to estimate the road surface friction coefficient. Output to the value setting unit 2g. In the present embodiment, as a method for estimating the driving stiffness coefficient Q from time series data, a fixed trace method which is one of general parameter identification methods is used. That is, according to the fixed trace method, an equation for obtaining φe, which is an estimated value of φ, is given by the following equation (6). Note that the subscript (k) in the expression indicates the current value, and (k-1) indicates the previous value.
φe (k) = φe (k−1) − (F (k−1) · p (k)) / (ζ + p (k) ^T · F (k−1) · p (k))
・ (P (k) ^T・ φe (k-1) -y (k)) (6)
Here, ζ is given by the following equation (7).
ζ = 1 / (1 + F (k−1) ² · p (k) ² ) (7)

  That is, the driving stiffness coefficient Q is given by Q = ΔFd / ΔFm, as shown in the following equation (14). Since the generated braking / driving force difference value ΔFd includes pitching motion and suspension system fluctuation that change from moment to moment, a stable and accurate driving stiffness coefficient Q can be obtained by using a parameter identification method. It has become.

Then, parameters are substituted into the above equations (6) and (7) as follows to estimate the driving stiffness coefficient Q.
φe (k) = Q, p (k) = p (k) ^T = ΔFm, y (k) = ΔFd
Note that the trace gain F (k−1) is, for example, 0.0001.

  Thus, the driving stiffness coefficient calculation unit 2f is provided as a driving stiffness coefficient calculation means.

  The road surface friction coefficient estimated value setting unit 2g receives the vehicle speed V from the vehicle speed calculation unit 2a, and receives the driving stiffness coefficient Q from the driving stiffness coefficient calculation unit 2f. The road surface friction coefficient is stored with reference to a characteristic map (FIG. 3) showing the relationship between the driving stiffness coefficient Q, the vehicle speed V, and the road surface friction coefficient estimated value μE stored in advance based on the vehicle speed V and the driving stiffness coefficient Q. Estimated value μE is set and output. That is, the road surface friction coefficient estimated value setting unit 2g is provided as a storage unit and a road surface friction coefficient estimation unit.

Here, the map showing the relationship among the driving stiffness coefficient Q, the vehicle speed V, and the road surface friction coefficient μ in FIG. 3 will be described. When M is the vehicle weight, Mw is the rotating part weight, V is the vehicle speed, Vw is the average vehicle speed of four wheels, ω is the driving wheel rotational speed, N is the vertical load, and μ is the road friction coefficient. The equation is the following equation (8).
Mw · (dVw / dt) = Fm−Fd (8)

  The equation of motion of the vehicle body is given by the following equation (9).

M · (dV / dt) = Fd (9)
However, the wheel motion is Mw = Jw / r ² , Fm = (Tm + KB · PB) / r, and Vw = r · ω.

A characteristic between the road surface and the tire (tire characteristic) is described by a road surface friction function (see FIG. 5) of a slip ratio and a road surface friction coefficient, and μ is defined by the following equation.
μ = Fd / N (10)
Here, the slip ratio λ during driving is expressed by the following equation.

λ = (Vw−V) / Vw (11)
In the following, only driving will be considered, and the tire / road surface system will be handled by approximation of operating points. Here, the slope of the road surface friction coefficient μ at a certain slip ratio λ0 is defined as a. When the perturbation system of the equations (8) to (11) is made and the slip ratio λ is eliminated, the following equations (12) and (13) are obtained.

ここで、Ｖ０、Ｖw0は、それぞれ動作点における車体速度と駆動輪速度である。 (Dx / dt) = A · x + B · ΔFm (12)
ΔFd = C · x (13)
However,

Here, V0 and Vw0 are the vehicle body speed and the driving wheel speed at the operating point, respectively.

When the transfer function from the torque applied to the tire to the driving force is calculated as described above, the following equation (14) for the driving stiffness coefficient Q is obtained.
Q = ΔFd / ΔFm = K / (1 + τ0 · s) (14)
here,
K = (M · (1−λ0)) / (Mw + M · (1−λ0)) (15)
τ0 = ((Mw · Vw0) / (a · N))
(M / (Mw + M. (1-λ0)) (16)

  When the relationship between the driving stiffness coefficient Q and the slip ratio λ is shown on the basis of the above equation (14), the characteristic diagram shown in FIG. 4 is obtained.

  Then, based on the tire characteristic curve of the road surface friction coefficient and the slip ratio shown in FIG. 5, the road surface friction coefficient estimated value with the target slip ratio (for example, 7%) before the road surface friction coefficient reaches the peak. When μE is set to 0.3, for example, and the relationship between the vehicle speed V, the driving stiffness coefficient Q, and the road surface friction coefficient estimated value μE is set, the driving stiffness coefficient Q, the vehicle speed V, and the road surface friction coefficient are estimated as shown in FIG. A characteristic map of value μE is obtained.

Next, a road surface friction coefficient estimation program executed by the control unit 2 of the above-described road surface friction coefficient estimation device 1 will be described with reference to the flowchart of FIG.
First, in step (hereinafter abbreviated as “S”) 101, necessary parameters, that is, four-wheel wheel speeds ωfl, ωfr, ωrl, ωrr, engine speed Ne, throttle opening θth, turbine speed Nt, transmission gear ratio. i, the brake fluid pressure PB, and the longitudinal acceleration Gx are read.

  Next, it progresses to S102 and calculates the vehicle speed V in the vehicle speed calculating part 2a.

  Next, the process proceeds to S103, where the generated braking / driving force calculation unit 2b calculates the generated braking / driving force Fd by the above-described equation (1), and the estimated braking / driving force calculation unit 2c estimates by the above-described equation (2). The braking / driving force Fm is calculated.

  Next, the process proceeds to S104, where the generated braking / driving force difference value calculating unit 2d calculates the generated braking / driving force difference value ΔFd by the above-described equation (4), and the estimated braking / driving force difference value calculating unit 2e The estimated braking / driving force difference value ΔFm is calculated by the equation (5).

  Next, the process proceeds to S105, where the driving stiffness coefficient calculation unit 2f uses the fixed trace method, which is one of the parameter identification methods, based on the generated braking / driving force difference value ΔFd and the estimated braking / driving force difference value ΔFm, that is, ( 6) The driving stiffness coefficient Q is estimated using the equation.

  In S106, the road surface friction coefficient estimated value setting unit 2g determines the relationship between the driving stiffness coefficient Q, the vehicle speed V, and the road surface friction coefficient estimated value μE stored in advance based on the vehicle speed V and the driving stiffness coefficient Q. The road surface friction coefficient estimated value μE is set and output with reference to the characteristic map shown (FIG. 3).

  Thus, according to the present embodiment, the generated braking / driving force Fd generated by the wheel is calculated based on the longitudinal acceleration of the vehicle, and the wheel is applied based on the output torque of the vehicle driving source and the braking force of the brake. The estimated braking / driving force Fm to be added is calculated, and the difference value (generated braking / driving force difference value) ΔFd between the current value of the generated braking / driving force Fd and the past value and the current value of the estimated braking / driving force Fm and the past A difference value (estimated braking / driving force difference value) ΔFm is calculated, and a driving stiffness coefficient Q is estimated by using a parameter identification method from the generated braking / driving force difference value ΔFd and the estimated braking / driving force difference value ΔFm. On the basis of the driving stiffness coefficient Q, the road surface friction coefficient estimated value μE is set with reference to a characteristic map indicating the relationship between the driving stiffness coefficient Q, the vehicle speed V, and the road surface friction coefficient estimated value μE stored in advance. It has become. In this embodiment, since the difference between the wheel speed of the front wheel and the wheel speed of the rear wheel is not used for estimating the road surface friction coefficient, it is not affected by the wheel speed difference between the front wheel and the rear wheel that occurs along with turning. It is possible to estimate the road friction coefficient regardless of turning or going straight. For this reason, it is possible to estimate the road surface friction coefficient with high accuracy in a wide driving range of acceleration, deceleration, and steering, and there is an excellent effect of excellent versatility.

  In addition, the characteristic map indicating the relationship between the driving stiffness coefficient Q, the vehicle speed V, and the road surface friction coefficient estimated value μE stored in advance is a slip ratio before the road surface friction coefficient peaks (for example, 7% ) Is set in advance as a target, so that determination with a small slip rate is possible and responsiveness is good.

  Further, the estimation of the driving stiffness coefficient Q is performed using the parameter identification method based on the generated braking / driving force difference value ΔFd and the estimated braking / driving force difference value ΔFm, so that an error due to a road gradient or the like can be eliminated by each difference value, It is possible to estimate the road surface friction coefficient with high accuracy.

  In this embodiment, the road surface friction coefficient is estimated in all the operation regions, but the operation region in which the road surface friction coefficient is estimated by this method is limited to, for example, the acceleration region only. Also good.

  Further, in the present embodiment, the road surface friction coefficient estimated value setting unit 2g includes the driving stiffness coefficient Q, the vehicle speed V, and the road surface friction coefficient estimated value μE stored in advance based on the vehicle speed V and the driving stiffness coefficient Q. The road surface friction coefficient estimated value μE is set with reference to the characteristic map showing the relationship and output as it is. The road surface friction coefficient estimated value μE (k) set this time and the road surface friction coefficient estimated value set last time The smaller value may be output by comparing with μE (k−1).

  The road surface friction coefficient estimated value μE set as described above is output to an external display device (not shown), for example, and used to call the driver's attention by displaying on the instrument panel or the like. Alternatively, it is output to the engine control unit, the transmission control unit, the driving force distribution control unit between the front and rear shafts or between the left and right wheels, the brake control unit (none of which are shown), and contributes to the setting of the control amount in each control unit Is done.

  For example, when used for front-rear driving force distribution control of a four-wheel drive vehicle in which the front shaft: rear shaft torque distribution can be varied by a transfer clutch between 100: 0 and 50:50, fuel efficiency improvement and front-rear driving force distribution control In order to achieve both, it is necessary to reduce the unnecessary rear wheel transmission torque on the high μ road and reduce the internal circulation torque, and on the low μ road, it is necessary to generate a predetermined rear wheel transmission torque with an emphasis on stability. Therefore, the engagement force of the transfer clutch torque is controlled according to the estimated road surface friction coefficient estimated value μE, and the rear wheel torque distribution ratio is continuously changed.

  In the present embodiment, the driving stiffness coefficient computing unit 2f obtains the driving stiffness coefficient Q using a parameter identification method. However, the generated braking / driving force difference value ΔFd and the estimated braking / driving force difference value ΔFm If the value can be obtained stably, the driving stiffness coefficient Q may be simply estimated based on the ratio between the generated braking / driving force difference value ΔFd and the estimated braking / driving force difference value ΔFm.

Functional block diagram showing the configuration of the road surface friction coefficient estimation device Flowchart of road friction coefficient estimation program Explanatory drawing showing an example of a characteristic map of driving stiffness coefficient, vehicle speed, and road surface friction coefficient estimated value Explanatory diagram showing the relationship between driving stiffness coefficient, vehicle speed and slip ratio Explanatory diagram of tire characteristic curves for road friction coefficient and slip ratio

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Road surface friction coefficient estimation apparatus 2 Control part 2a Vehicle speed calculating part 2b Generated braking / driving force calculating part (1st braking / driving force calculating means)
2c Estimated braking / driving force calculating section (second braking / driving force calculating means)
2d Generated braking / driving force difference value calculation unit (first braking / driving force difference value calculation means)
2e Estimated braking / driving force difference value calculation unit (second braking / driving force difference value calculation means)
2f Driving stiffness coefficient calculation unit (driving stiffness coefficient calculation means)
2g Road surface friction coefficient estimated value setting unit (storage means, road surface friction coefficient estimation means)
3 4 wheel speed sensor 4 engine control unit 5 transmission control unit 6 brake fluid pressure sensor 7 longitudinal acceleration sensor

Claims (3)

First braking / driving force calculating means for calculating the first braking / driving force generated by the wheels based on the longitudinal acceleration of the vehicle;
Second braking / driving force calculating means for calculating a second braking / driving force applied to the wheel based on an output torque of a vehicle driving source or a braking force of a brake;
First braking / driving force difference value calculating means for calculating a difference value between the current value of the first braking / driving force and a past value as a first braking / driving force difference value;
Second braking / driving force difference value calculating means for calculating a difference value between the current value and the past value of the second braking / driving force as a second braking / driving force difference value;
Storage means for presetting and storing a relationship between a driving stiffness coefficient, a vehicle speed, and a road surface friction coefficient at a predetermined slip ratio based on tire characteristics indicating a relationship between a slip ratio and a road surface friction coefficient;
Driving stiffness coefficient calculating means for calculating the driving stiffness coefficient based on the first braking / driving force difference value and the second braking / driving force difference value;
Road surface friction coefficient estimating means for estimating the road surface friction coefficient from the relationship between the driving stiffness coefficient stored in the storage means, the vehicle speed and the road surface friction coefficient based on the vehicle speed and the driving stiffness coefficient calculated by the driving stiffness coefficient calculating means; ,
A road surface friction coefficient estimating device comprising:   2. The road friction coefficient estimating device according to claim 1, wherein the driving stiffness coefficient calculating means obtains a driving stiffness coefficient from a ratio between the first braking / driving force difference value and the second braking / driving force difference value. .   2. The driving stiffness coefficient computing means computes the driving stiffness coefficient from the first braking / driving force difference value and the second braking / driving force difference value using parameter identification. Road friction coefficient estimation device.

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