Keywords

1 Introduction

In recent years, various ADAS have been developed to ensure road traffic safety, and to further promote these effects, the effect of autonomous driving is expected in the next stage. Currently, various sensing systems and algorithms for safety management have been improved one after another from the viewpoint of reducing accidents, reducing the burden on drivers, and improving reliability, compared to when ADAS was first introduced, and it is believed that the effectiveness of safety support has been greatly improved. For these various safety systems to function effectively, it is important to fully understand the friction characteristics of the road surface, which is a prerequisite for the control systems to function effectively. Road surface friction characteristics generally vary greatly depending on road conditions, weather, temperature, region, etc., and the friction coefficient of road surfaces that ordinary vehicles face varies greatly from approximately 0.15 to approximately 1.2. On roads with large differences in the friction coefficients, emergency braking distances can vary by several times or even ten times more depending on changing road conditions, which poses a major problem from the perspective of safety management. When driving on snowy or icy roads in snowy or mountainous regions in winter, building a road friction estimation system is one of the extremely important research subjects from the perspective of safety management. However, to build such a system, it is important to construct a database related to road friction, but such a system has not yet been constructed. Given this situation, our research purposes are to establish a method for continuous measurement of road surface friction, to construct a broad database of road surface friction, and to construct an environmental database that is highly correlated with the friction characteristics. This paper presents the results of continuous measurement results of the road friction coefficient for various paved road surfaces that have been developed through past activities, as well as the results of continuous measurements of road friction on snowy and icy roads.

2 Measurement Device on Road Friction

Road friction measuring devices have been developed for various purposes and are now widely used for various purposes. Roughly speaking, these can be divided into two types: one is a system for evaluating the road surface itself, and the other is a system for measuring the characteristics of tires to understand the vehicle behaviors. The required friction characteristics need to be considered separately for the longitudinal and lateral directions, but since the characteristics of normal tires can be roughly considered in terms of a friction circle, we will focus on the braking characteristics especially for μ-s characteristics, which are important from the perspective of safety. Figure 1 shows various road friction measurement systems.

Fig. 1.
figure 1

Differences in road friction measurement equipment

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These two groups of road friction characteristic measuring devices have different characteristics due to their different purposes. Road surface evaluation instruments are designed to measure road surface friction at measurement points on the road surface, because the relationship between the measurement position and the road surface characteristics is important. Currently, instruments used on highways and airport runways can measure the surface friction continuously. These devices are used for continuous measurements to grasp changes over time and identify areas of damage, but they cannot measure the detailed lateral force characteristics and/or braking /driving characteristics required for vehicle dynamics analysis. On the other hand, dynamic vehicle tire characteristic measuring devices are designed to measure tire characteristics under various road surface conditions, and measure tire characteristics by continuously changing brake torque and/or sideslip angle under the assumption that the friction coefficient in the measurement section is constant. Therefore, although it is suitable for measurements on maintained road conditions such as proving grounds, it is not suitable for measurements on road surfaces such as ordinary roads where friction characteristics change. To build a road friction database, it is important to measure both characteristics, especially the ability to continuously measure road friction characteristics as well as the ability to measure μ-s characteristics at each point. To simultaneously satisfy both requirements, we developed a new road friction measurement system. The basic idea behind the construction of this system is to measure a finite set of μ and s, and use these results to estimate μ-s characteristics. An important element in the construction of this system is the introduction of a function that represents the μ-s characteristics well. The magic formula proposed by Pacejka describes very well the characteristics of the tire, including not only lateral force characteristics but also braking force and torque characteristics1). A simplified description of this formula can be written as in Eq. 1.

(1)

The three constants a, b, and c in the formula are used to identify the μ-s characteristic from as few experimental results as possible. Figure 2 shows the estimated peak μ and locked μ when three sets of μ and s are measured, using the data on the μ-s characteristics measured so far and the characteristics of snow-and-ice roads extracted from literature. In this identification, the friction coefficients of slip ratios of 2%, 12%, and 22% are assumed to be known. These results show that sufficient braking characteristics can be obtained by using this method.

Fig. 2.
figure 2

Peak μ and lock μ on μ-s characteristic estimated from three sets of μ and s (s = 2%, 12%, 22%).

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3 Constructed Measurement Device

A road friction characteristic measuring device with the above-mentioned structure was constructed. Figure 3 shows a trailer-type measuring device having three measuring tires, which are driven by chains from the trailer tires using different teeth of sprocket.

The basic idea behind the construction of this system is to measure a finite set of μ and s, and use these results to estimate μ-s characteristics. An important element in the construction of this system is the introduction of a function that represents the μ-s characteristics well.

For this reason, various control methods have been adopted to improve safety and comfort, and their effectiveness has been demonstrated. Road friction characteristics are extremely important for the various systems for these safety measures, and ensuring these friction characteristics is a prerequisite for the control.

The coefficient of road friction on an actual road surface varies greatly depending on the road surface material, road surface condition, tire structure, and other factors. Furthermore, the reduction of the coefficient of friction on snowy and icy roads in wintertime in snowy regions poses a significant threat to road traffic safety. In the past, road surface friction measurements have used sliding friction such as BPN, or truck/bus/trailer type road surface friction measurement systems to measure μ-s characteristics by gradually applying braking. In particular, the measurement of peak μ at each location on the road is important for estimating road friction characteristics since the widespread use of ABS in recent years has enabled braking near peak μ and automatic braking system is based on such control. However, the road friction measurement systems required the assumption that the friction characteristics of the measured section are constant. However, it can be inferred that the friction characteristics may always change between a road surface where frequent braking is applied just before an intersection and a section where most vehicles pass at a constant speed, and that the friction characteristics always change on partially frozen or partially snow-covered road surfaces. In such measurements related to road surface friction, it is important to understand the continuous change in peak μ, but until now, there has been no system capable of continuously measuring this characteristic. In recent years, the authors have proposed a new road friction measurement system to overcome this problem and have shown its measurement results.

In this paper, we present an overview of the system we have constructed and the measurement results, followed by the results of continuous measurement of peak μ under various conditions, including snow and ice on actual roads. Furthermore, we summarize the identification of these measurement problems and future issues.

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