Keywords

1 Introduction

In addition to meeting the basic conditions of traffic, the road should have good safety. Among them, the anti-skid performance of the road surface can provide good adhesion for high-speed vehicles and reduce the hidden danger of traffic accidents. The safety performance of pavement is expressed by the anti-skid performance of pavement, and the friction coefficient of pavement is the most important index to evaluate its anti-skid performance, so it is necessary to detect and evaluate the friction coefficient of pavement accurately and efficiently.

Due to the limitation of detection range, detection speed, sampling number, etc., the friction coefficient vehicle test method is often used for detection on high-grade roads. The SCRIM, for example, not only represents the braking distance of the vehicle, but also reflects the ability of the road surface to prevent the vehicle from sideslip [1,2,3]. The equipment is installed on the chassis of a large truck and equipped with a large-capacity water tank, which can be continuously tested for long distances without affecting traffic. It has obvious advantages in technical characteristics, working efficiency and equipment working stability, so it is widely used. SCRIM method is used to measure the transverse force coefficient of pavement under the most unfavorable conditions [4,5,5]. Since the test wheel is equipped with a fixed counterweight and is set at an Angle of 20° with the driving direction, when the test wheel touches the ground, the measuring mechanism is subject to the axial transverse force, which is more in line with the condition of the car when it is driving on the road [5,6,7].

The research of this project aims to address the limitations and shortcomings of existing pavement anti slip performance testing, enhance the evaluation ability of pavement friction characteristics, and provide scientific basis for road safety and traffic operation [8].

The current method of quantity traceability does not require key measurement technical parameters such as indication error of friction coefficient. In addition, the calibration technology is based on the dynamic repeated measurement method of coefficient of variation, using the road section method. Due to the limitations of the site, it cannot meet the efficiency requirements of the measurement standard implementation, which increases the difficulty of tracing the value of lateral force friction coefficient, making it difficult to grasp the accuracy of lateral force coefficient testing [9].

Research on the traceability method of lateral force friction coefficient can form technical specifications for ensuring the accuracy of pavement anti slip performance measurement and provide important data support for decision-making [10].

2 Study on the Measurement Source Method of Single-Wheel Friction Coefficient Tester for Lateral Force

2.1 Principle Analysis

The transverse force coefficient (SFC) is assumed that the standard test tire has a certain deflection Angle (20°) with the forward direction of the car traveling at a certain speed, and the contact with the wet road causes it to produce a lateral friction resistance perpendicular to the plane of the test wheel. The ratio of this force to the constant vertical load borne by the test wheel is the SFC value of the transverse force coefficient, as shown in Fig. 1. The lateral friction resistance can be obtained from the force sensor, the vertical load can be obtained according to the known value, the transverse force system value can be obtained, and the road friction coefficient is positively correlated. It can reflect the vertical and lateral friction coefficient of the comprehensive index, is characterized by the ability to measure the emergency braking or sharp turns provided by the road to the vehicle handling performance.

Fig. 1.
figure 1

Schematic diagram of transverse force coefficient testing principle

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The vertical load of the tire has little influence on the transverse force when the side deflection Angle is small, but when the side deflection Angle of the tire is large to almost reach the transverse force saturation state, the vertical load will play a dominant role. This is because the effect of vertical load only occurs in the area where there is relative slide between the tire and the ground, when there is more of this relative slide in the ground, that is, when the side Angle is large, the effect of vertical load will be obvious.

In the pavement lateral force coefficient measurement system, the static vertical load is a certain value (China's standard is 2kN), and the measured value is close to the maximum transverse force under the condition that the side deflection Angle is 20°. In some measuring systems, a constant static vertical load is used to calculate the transverse force coefficient, while in some measuring systems, the static vertical load is only a reference standard for equipment calibration, and the actual vertical load measured in real time is used to calculate the transverse force coefficient.

The horizontal load is also involved in the calculation of SFC, so the specified value of its error is relatively strict, especially when the static SFC parameter is calibrated, the calibration is extremely complex.

According to the requirements of technical indicators in JJG (Traffic) 113–2014 “Single-wheel lateral Force Coefficient Tester”, the technical indicators are measured and calibrated. Through mastering the measurement methods, some influencing factors in the test are understood and relevant problems are found. At present, the test parameters mainly include the deviation Angle of the test wheel, the vertical load of the test wheel, the horizontal load of the test wheel, the distance sensor, the temperature sensor and the repeatability index of the transverse force coefficient output by the dynamic test. None of these indicators is a static influence factor, so the accuracy of the evaluation of skid resistance under dynamic conditions does not meet the conditions.

In order to eliminate the influence factors on the accuracy of SFC parameter calibration, the static SFC value and dynamic SFC value are compared and tested, and the leading parameter of SFC value is horizontal load, and the axial horizontal load is obtained through the six-component force sensor.

The safe driving performance of road vehicles depends on whether the longitudinal motion (braking, traction control), lateral motion (steering control) and vertical motion (road fluctuation) are normal. If these loads and torque are not normally loaded on the car body, it will cause some of its components to produce additional loads, and in bad circumstances, it may damage the parts and even the whole vehicle. The performance of the above various vehicle motion loads is the most concerned by various supervisory departments and enterprises. The six component test system can provide test and data support for the above concerned projects.

At the same time, the six-component test system can provide accurate road spectrum acquisition function, and provide comprehensive road spectrum signal for vehicle testing in the laboratory. At the same time, since the load collected by the six-component test system is the external load of the vehicle, the load of each component in the vehicle body model can be calculated through the external load, so as to obtain the fatigue life of all components. These calculation data have important guiding significance when the body is still in the modeling stage, and problems can be found in the design stage, rather than slowly improving after the whole vehicle is built. The six-component force test method is different from the previous method of measuring the load inside the vehicle. Because the internal load is not able to calculate the load of other components.

Six component force sensor measurement system, the test index meets the requirements of SAE (Fig. 2) coordinate system. Can quickly measure the three-direction force and three-direction torque of the wheel. The sensor is mounted between the hub adapter and the wheel adapter, the tire is mounted on the modified wheel, and the hub adapter is mounted on the axle. In this way, when the wheel is forced on the transmission hub, it will pass through the sensor first. Six independent Bridges measure force and torque, with low dryness between channels, free from temperature changes and electromagnetic interference. Because the sensor measures force and torque in a rotating coordinate system, the output of the radial channel changes as the wheel rotates. Before applying the acquired data, they must first be converted into the vehicle coordinate system. Slip rings with integrated optical encoders transmit the signal to the stator, and the encoder provides the angular position and wheel rotation speed.

Fig. 2.
figure 2

Wheel six component SAE coordinate system

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3 Accuracy of Lateral Force Coefficient

The overall test scheme is to convert the SFC value of the evaluation index into the most original force value, and compare the error value of the standard SFC value obtained through the conversion of the force value with the SFC collected by the single wheel lateral force friction coefficient tester.

3.1 Test Verification

At present, the general calibration method for horizontal load error is as follows:

  1. 1)

    Park the bearing vehicle on the horizontal hard test platform site and turn on the system power supply;

  2. 2)

    Place the afterforce plate of the vehicle horizontally under the test wheel, buckle the reaction device and lower the test wheel to the fully unloaded state, and connect the standard dynamometer with the horizontal loading device in series;

  3. 3)

    After the tester is preheated for 10 min, start the verification state, and start to apply force with the afterburner;

  4. 4)

    When the standard dynamometer is 0, 200N, 400N, 600, 800N, 1000N, 1200N, 1400N, 1600N, 1800N and 2000N, the corresponding response measurement value of the tester is read, the corresponding force value is obtained, and the difference between the two is calculated respectively, and the maximum difference is taken.

For the test of horizontal load error, the six-component force sensor is used to compare the truth value of the error (Fig. 3). At static state, the force standard is applied, and the pressure is 0, 200N, 400N, 600, 800N, 1000N, 1200N, 1400N, 1600N, 1800N, 2000N. The data of single-wheel lateral force friction coefficient tester and force standard sensor were recorded respectively, and the data of six-component force sensor and Mark signal were collected.

Fig. 3.
figure 3

eDAQ collects data

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Fig. 4.
figure 4

Schematic diagram of ten loading data

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It can be seen from the Fig. 4 that the SFC value collected by the single-wheel lateral force friction coefficient tester and the force value data collected by the six-component force sensor are basically in the same linear trend after conversion.

3.2 SFC Repeatability Check

The friction coefficient of the single wheel transverse force tester is measured several times at different speeds on the standard 500 m test road, and the data of the single wheel lateral force friction coefficient tester and the six-component force sensor are recorded respectively (Fig. 5).

Fig. 5.
figure 5

SFC repeatability verification

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Fig. 6.
figure 6figure 6

Dynamic test analysis

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It can be seen from the Fig. 6 that the dynamic SFC values collected by the single wheel lateral force friction coefficient tester at different speeds and the dynamic SFC values converted by the force value data collected by the six-component force sensor are basically the same correlation coefficient.

4 Conclusions

The lateral force coefficient is used to test the anti-skid performance of the pavement. The influencing factors affecting the accuracy of SFC are the main reference factors for the establishment of measurement technology. The external factors in the testing process of the lateral force coefficient are different, so the test results are also related to the pressure of the test wheel, the thickness of the water film and the paving direction of the asphalt surface layer. The tire pressure is less than the standard tire pressure, and the measured result is too large and needs to be corrected. Therefore, the tire pressure should be checked before the test. When the tire tread of the test wheel is in contact with the mixture of the surface layer, under different water film thickness, the anti-skid performance of the tire tread and the road surface will also be affected to a certain extent. The research shows that the thicker the water film thickness, the smaller the measured SFC value. Especially after the rain, the road is in a wet state, the measured data is small.

Internal factors such as temperature, test speed, etc. The influence degree of temperature test value is larger, the temperature is higher and lower, the measured SFC value is different from the actual anti-sliding ability reflected, and this difference will be more obvious on the old road; When the test speed is less than the standard speed (50 km/h), the measured SFC value is larger, reflecting the anti-skid performance preference. When the speed is greater than the standard speed, the SFC value decays significantly.

The six-component test is used to eliminate the influence factors, especially the influence of tire wear state on the test results during the test.

Improve the accuracy of pavement detection results, provide reliable data for detection and maintenance personnel, and improve the quality of engineering construction. Reduce due to inaccurate road anti-skid test data, resulting in detection, maintenance personnel to make wrong remedial judgments to reduce unnecessary traffic accidents caused by road slip, damage to personal and property safety. The independent research of calibration devices has realized the replication of multiple sets of calibration devices in various provincial metrological technical institutions nationwide at the same time, and promoted the establishment of metrological service network.

This experiment starts from the macro perspective of road friction, without studying the microstructure of the road surface. Looking at other industries, most have already entered the micro research of friction coefficient. The micro characterization of road friction coefficient has the advantages of good accuracy and high stability. In the future, relevant research will be carried out on the micro level definition of friction reference materials and road friction coefficient standards.