JP3022038B2 - Road friction coefficient detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for detecting a coefficient of friction of a road on which a vehicle is traveling.

[0002]

2. Description of the Related Art Several systems for detecting the coefficient of friction of a road surface (hereinafter referred to as "road surface μ") have already been proposed. For example, as described in Japanese Utility Model Laid-Open Publication No. 58-134758, a method of detecting a road surface μ based on a wheel speed difference between a driving wheel and an idle wheel, or a method in which a driver receives from a steering wheel during steering. Road surface μ based on steering reaction force
Or a method of detecting the road surface μ based on the wheel acceleration of a wheel near the lock limit.

[0003]

However, all of these conventional systems have a problem that the timing restrictions when detecting the road surface μ are severe. For example, the method of detecting the road surface μ based on the wheel speed difference between the driving wheels and the idle wheels has a restriction that the road surface μ cannot be detected unless the vehicle is driven, and further, based on the steering reaction force. In the method of detecting the road surface μ, there is a restriction that the road surface μ cannot be detected unless the steering is being performed, and the method of detecting the road surface μ based on the wheel acceleration of the wheel near the lock limit is not available. However, there is a restriction that the road surface μ cannot be detected unless strong braking is performed and the wheels are near the lock limit.

As described above, the conventional system has a problem that the road surface μ is severely restricted and the road surface μ cannot be easily detected.

[0005] The present invention has been made to solve this problem.

[0006]

According to the present invention, there is provided a road friction coefficient detecting apparatus which comprises: (a) a method in which a lateral slip angle is not zero during running of a vehicle, as shown in FIG. Among the vibrations of the wheels rotating in the state, at least the frequency range in which the specific relationship that the vibration level changes in one direction corresponding to the friction coefficient of the road surface in contact with the wheels changes in one direction (B) a road surface friction coefficient estimating unit for estimating the road surface friction coefficient based on the detected vibration level in a specific frequency range. Features.

[0007]

As is well known, each portion of a tire rotating in a state where the lateral slip angle is not zero (hereinafter referred to as "each tread portion") in the circumferential direction of the tread contact portion of the tire. Repeats lateral displacement. Specifically, when each part of the tread starts to come into contact with the road surface, it is pulled by the frictional force with the road surface and gradually moves away from the center of the tire. Later, each part of the tread overcomes the elastic force of the tire trying to pull back to the center of the tire, and the lateral displacement increases, but when the elastic force eventually becomes larger than the frictional force, it slides sideways on the road surface and the tire center I will return to.

The applicant of the present invention has determined that the lateral displacement of the tread and the road surface μ
We studied the relationship with and found the following phenomena. That is, if the vibration level of a wheel rotating while the lateral slip angle is not 0 while the vehicle is running is detected, within a specific frequency range of the detected vibration level, the road surface with which the wheel is in contact is detected. They discovered a phenomenon in which a specific relationship holds that the vibration level changes in one direction in response to the friction coefficient changing in one direction. The mechanism of occurrence of this phenomenon will be described in detail in Examples.

Therefore, if this phenomenon is used, if only the vibration level in the frequency range is known, the road surface μ can be detected based on a predetermined correlation between the vibration level and the road surface μ. Can be. The lateral displacement of the tire tread naturally occurs during turning of the vehicle, but also occurs during straight traveling of the vehicle. That is, in the vehicle, in order to improve the straight running stability of the vehicle, etc., toe angles are given to the left and right front wheels that are the steered wheels,
Generally, a camber angle is set, and as a result, a lateral displacement occurs in the tire tread even when the vehicle is traveling straight. As described above, the tread lateral displacement, which is a phenomenon necessary for detecting the road surface μ, is a phenomenon that is almost always occurring.

Based on such knowledge, in the road friction coefficient detecting apparatus according to the present invention, the wheel vibration level detecting means 1 detects the vibration of the wheels rotating with the lateral slip angle being not zero while the vehicle is running. Of these, at least a vibration level in a frequency range where a specific relationship in which the vibration level changes in one direction corresponding to a change in the friction coefficient of the road surface in contact with the wheel in one direction is detected, and the road surface friction coefficient is estimated. Means 2 estimates the road surface friction coefficient based on the detected vibration level in the specific frequency range.

[0011]

As described above, according to the present invention, since the road surface μ can be detected by utilizing the tread lateral displacement which is a phenomenon which is almost always occurring, the road surface μ can be relatively easily detected. The effect of being able to detect is obtained.

However, when the vehicle is traveling straight, the lateral slip angle is zero.
Therefore, it is possible to implement the present invention by detecting the vibration level during the turning of the vehicle and detecting the road surface μ for a wheel in which lateral displacement does not occur.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below in detail with reference to the drawings.

In FIG. 2, reference numeral 10 typically represents one of the left and right front wheels which are the steered wheels. The front wheel 10 is rotatably supported by a carrier 12 via a bearing (not shown), and the carrier 12 is connected to a vehicle body 18 via an upper arm 14 and a lower arm 16.
The upper arm 14 connects the vehicle body 18 and the carrier 12, and the lower arm 16 connects the vehicle body 18 and the carrier 12 so that relative displacement in a vertical plane is possible.
The left and right front wheels 10 are provided with a toe angle so that a lateral displacement occurs in the tread contact portion of the tire of the front wheels 10 even when the vehicle is traveling straight.

An acceleration sensor 20 is attached to the lower arm 16. The acceleration sensor 20 mainly includes a weight and a piezoelectric element, and indirectly detects a lateral vibration of the front wheel 10 as an acceleration G of the lower arm 16.

The acceleration sensor 20 is connected to a road surface friction coefficient estimating device 24 mounted on the vehicle body 18. As shown in FIG. 3, the road friction coefficient estimating device 24 includes a CPU 30, a ROM 32, a RAM 34, an input interface 38, and an output interface 4.
The main component of the computer is a computer including 0, and the acceleration sensor 20 is connected to the input interface 38 of the computer. The input interface 38 also includes a steering angle sensor 42 for detecting the steering angle θ of the steering wheel, and a temperature sensor 4 for detecting the outside air temperature t.
4 are connected. On the other hand, the output interface 40
Is connected to a road surface friction coefficient indicator 46. The road surface friction coefficient display 46 is mounted at a position in the vehicle compartment that is easily seen by a driver.

The road friction coefficient estimating device 24 includes a ROM 32
The road friction coefficient estimating routine stored in advance in FIG.
Is executed by the CPU 30 while using the RAM 34 and the like, whereby the road surface μ is detected based on the output signal from the acceleration sensor 20 and the like, and the result is displayed on the road surface friction coefficient display 46. Notify the driver via

Here, the contents of the road friction coefficient estimation routine of FIG. 4 will be described in detail. First, the principle of road surface μ detection will be described.

Prior to the present invention, the present applicant conducted the following experiment using the vehicle. That is, when the outside air temperature t is 2
Under the condition of 0 ° C., the road surface μ is 0.8, 0.5,
The vehicle travels straight ahead on four different road surfaces, such as 0.2 and 0.1, respectively.
This is an experiment in which the acceleration G is measured with 0. In addition,
Here, a road surface having a road surface μ of 0.8 is a dry asphalt road, and a road surface having a road surface μ of 0.5 is a dry low μ asphalt road and a road surface having a road surface μ of 0.2. Is a wet low μ asphalt road, and a road surface having a road surface μ of 0.1 is a wet porcelain tile road.

FIG. 5 is a graph showing an example of the result of frequency analysis of the measurement result of the acceleration G. This graph is
The horizontal axis represents the frequency f of the acceleration G, and the vertical axis represents the PSD value (power spectrum density) of the acceleration G. That is, in the present embodiment, the PSD value of the acceleration G is one aspect of the “vibration level” in the present invention. The following facts can be seen from this graph. That is, the frequency f
In the vicinity of about 6 kHz, the tire tends to resonate,
Moreover, the level of the resonance, that is, the resonance frequency f ₀
(This is one aspect of the present invention, which is a “specific relationship in which the vibration level changes in one direction in response to the friction coefficient of the road changing in one direction” in the present invention.) There is a fact that there is).

The mechanism of the phenomenon that the PSD value at the resonance frequency f ₀ decreases as the road surface μ increases is estimated as follows.

As shown in FIG. 6, on a high μ road, when a portion of the tread contact portion in the circumferential direction starts to contact the road surface, the frictional force with the road surface is large, so that the elastic portion moves away from the center of the tire. It deforms and does not displace laterally relative to the road surface, that is, no relative lateral slip occurs. After that, until the tread immediately before leaving the road surface, there is no relative lateral slip in each part of the tread,
Just before the tread leaves the road surface, each part of the tread is returned toward the center of the tire by its elasticity,
Only then does a relative lateral slip occur. On the other hand, on a low μ road, when each part of the tread starts to contact the road surface, the frictional force with the road surface is small, so that the tread elastically deforms away from the center of the tire, but immediately returns to the center of the tire, and thereafter, the tread The vehicle continues to generate a lateral slip relative to the road surface until the vehicle leaves the road surface. Thus, on a high μ road, the adhesive area of the tread is long and the lateral slip area is short, whereas on a low μ road, the adhesive area is short,
The lateral slip area becomes longer.

On the other hand, when the tread slips laterally with respect to the road surface, as shown schematically in FIG. 7, the unevenness of the road surface (this is substantially independent of the size of the road surface μ, (Meaning microscopic irregularities that are present at the same height and at the same interval), and can be vibrated both vertically and horizontally. In the drawing, the spring and the damper indicate that the tire can be approximated by a model in which a myriad of elastic bodies composed of the spring and the damper are arranged independently of each other in the circumferential direction.

Therefore, when the tread slips laterally with respect to the road surface, the tread is vibrated by using the unevenness of the road surface as a vibration source, and its resonance point is estimated to be the resonance frequency f ₀ of the acceleration G. And the longer the lateral slip area of the tread, the stronger the tread is subjected to vibration due to uneven road surface,
As a result, it is estimated that the front wheel 10 is vibrated strongly. That is, as the road surface μ is smaller, the lateral slip region of the tread becomes longer, the tread is more vibrated by the unevenness of the road surface, and the front wheel 10 is vibrated more strongly. As a result, the PSD value of the acceleration G at the resonance frequency f ₀ is obtained. Is estimated to have increased.

The resonance frequency f ₀ of the tire varies depending on the physical properties, size, surface temperature, etc. of the tire. Among them, the physical properties, size, etc. are almost unchanged during running of the vehicle, but the surface temperature is relatively remarkable. Is expected to change. In general, the tire resonance frequency f ₀ is generally expected to increase as the tire surface temperature increases. Therefore,
In this embodiment, the surface temperature of the tire is indirectly detected as the outside air temperature t, and the road surface μ is detected based on the detected temperature.

Next, a routine for estimating a road surface friction coefficient will be described in detail with reference to FIG. This routine is executed at regular intervals. At the time of each execution, first, step S1
(Hereinafter referred to as “S1”; the same applies to other steps), the current steering angle θ is read from the steering angle sensor 42, and whether or not this steering angle θ is substantially 0, that is, It is determined whether the vehicle is currently in a substantially straight running state. If the vehicle is not traveling straight, the determination is NO
And one execution of this routine ends immediately,
If the vehicle is in the straight traveling state, the determination is YES, and the process proceeds to S2 and subsequent steps.

In S2, the current outside air temperature t is read from the temperature sensor 44. Subsequently, in S3,
The resonance frequency f of the tire corresponding to the current outside air temperature t
₀ is determined. The relationship between the outside air temperature t and the resonance frequency f ₀ , for example, represented by a graph in FIG. 8 is stored in the ROM 32 in advance, and by using the relationship, the relationship between the outside air temperature t and the current outside air temperature t is obtained. The resonance frequency f ₀ is determined. Note that the relationship was obtained experimentally.

Next, in S4, the acceleration G is read from the acceleration sensor 20 for a fixed short period of time (for example, about 5 seconds), and a frequency analysis is performed on the plurality of accelerations G input during that time. The relationship between f and the PSD value is obtained. Further, in this step, PS
Among the D values, the one at the frequency f that matches the resonance frequency f ₀ is searched, and this is set as the current PSD value.

Then, in S5, a road surface μ corresponding to the current resonance frequency f ₀ and the PSD value is determined. The relationship between the resonance frequency f ₀ , the PSD value, and the road surface μ, for example, a graph shown in FIG. 9 is stored in the ROM 32 in advance, and the current road surface μ is determined by using this relationship. Because This relationship is also obtained experimentally.

Subsequently, in S6, the determined road surface μ
Are displayed on the road surface friction coefficient display 46 by numerical display, pictorial display, graph display, or the like. This completes one execution of this routine.

As is clear from the above description, in this embodiment, the current road surface μ is detected by using the relationship between the vibration level at the resonance frequency f ₀ of the tire and the road surface μ, and the vibration of the tire is This is based on the lateral displacement of the tire while the vehicle is traveling straight, and the lateral displacement occurs regardless of whether the vehicle is being driven and whether the tire is near the lock limit. It is a phenomenon. Therefore, in this embodiment, if the vehicle is substantially in a straight running state, regardless of whether the vehicle is being driven or not, and regardless of whether the tires are near the lock limit, Can be detected.

As is clear from the above description, in the present embodiment, the acceleration sensor 20 constitutes one mode of the "wheel vibration level detecting means 1" of the present invention, and the road surface friction coefficient estimating device 24 comprises the steering angle sensor. 42 and temperature sensor 4
4 together with one embodiment of the "road surface friction coefficient estimating means 2" of the present invention.

While the embodiment of the present invention has been described in detail with reference to the drawings, the present invention can be implemented in other embodiments.

For example, in the above-described embodiment, the resonance frequency f ₀ is set variably in accordance with the outside air temperature t, but the present invention can be implemented with a fixed value.

In the above embodiment, the PSD value of the acceleration G is used as one mode of the "vibration level" in the present invention. However, for example, the amplitude of the acceleration G can be used.

In the above-described embodiment, the road surface μ is detected only when the vehicle is in a substantially straight running state. It is based on the estimation that the vibration level of the front wheel 10 is different between a value situation and a situation larger than the value. Therefore, for example, for each of the plurality of values of the lateral slip angle, that is, the plurality of values of the steering angle θ, the relationship between the vibration level and the road surface μ is experimentally acquired, and the acquired relationship can be stored in the ROM 32. For example, the present invention can be implemented such that the road surface μ can be detected even during turning of the vehicle.

Further, in the above embodiment, the frequency analysis is performed on the acceleration G detected by the acceleration sensor 20, and the PSD values at a plurality of frequencies f obtained thereby correspond to the current resonance frequency f ₀ . However, for example, the output signal of the acceleration sensor 20 is supplied to the road surface friction coefficient estimating device 24 via a band-pass filter, and the band-pass filter is The present invention can be embodied as allowing only the acceleration G having the frequency f that matches the resonance frequency f ₀ to pass.

In the above-described embodiment, the detected road surface μ is output to the driver. However, antilock control, rear wheel steering angle control, The present invention can also be implemented by performing vehicle motion control such as driving / braking force distribution control.

The present invention can be carried out in various modified and improved forms based on the knowledge of those skilled in the art without departing from the scope of the claims.

[Brief description of the drawings]

FIG. 1 is a diagram conceptually showing a configuration of the present invention.

FIG. 2 is a front view showing the vicinity of front wheels of a vehicle on which the road friction coefficient detecting device according to the embodiment of the present invention is mounted.

FIG. 3 is a diagram showing a configuration of the road surface friction coefficient detecting device.

FIG. 4 is a flowchart showing a road friction coefficient estimation routine stored in a ROM in FIG. 3;

FIG. 5 is a graph showing experimental data for explaining the principle of detecting a road surface friction coefficient by the routine of FIG. 4;

FIG. 6 shows a power spectrum density PSD of tire vibration.
FIG. 4 is a diagram for explaining a mechanism by which a value at a resonance frequency f ₀ changes according to a friction coefficient of a road surface.

FIG. 7 is a power spectrum density PSD of tire vibration.
FIG. 4 is a diagram for explaining a mechanism by which a value at a resonance frequency f ₀ changes according to a friction coefficient of a road surface.

8 is a graph showing the relationship between the outside air temperature t and the resonance frequency f ₀ in the routine of FIG.

9 is a graph showing a relationship between a resonance frequency f ₀ , a power spectrum density PSD of tire vibration, and a road surface friction coefficient in the routine of FIG. 4;

[Explanation of symbols]

 Reference Signs List 10 front wheel 20 acceleration sensor 24 road friction coefficient estimating device 42 steering angle sensor 44 temperature sensor 46 road friction coefficient display

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int. Cl. ⁷ , DB name) G01M 17/00-17/06 G01N 19/02 B60T 8/00 B60T 8/32-8/96

Claims (1)

(57) [Claims] 1. Among the vibrations of a wheel rotating with a lateral slip angle not being zero during running of a vehicle, the vibration corresponding to at least a change in the friction coefficient of a road surface in contact with the wheel in one direction. Wheel vibration level detecting means for detecting a vibration level in a frequency range in which a specific relationship in which the level changes in one direction is established; road surface friction estimating the friction coefficient of the road surface based on the detected vibration level in the specific frequency range A road friction coefficient detecting device, comprising: coefficient estimating means.