As an important safety system, the braking system of automobile has a direct impact on the travel safety of drivers and passengers.Therefore, in the vehicle development stage, a lot of performance tests are conducted to verify whether the braking performance is up to standard.Car end users, however, is a person, the vehicle braking performance is perceived by the human body, from this perspective, the subjective evaluation results more close to the actual user experience of customers, therefore, the research and the subjective evaluation method of user requirements and laminating, establish a subjective evaluation and objective testing associated test evaluation system, which can effectively evaluate vehicle braking system performance and to improve the user experience of vehicle braking performance, has important practical significance.

1. Braking performance evaluation index of the vehicle

Whether it is a traditional fuel car or a new energy car representing the future development direction, the braking system works by converting the kinetic energy of the car into heat energy or through the energy recovery system.Automobile braking system is mainly composed of power supply device, control device, transmission device, brake and other parts.In addition to the traditional car braking system, those with energy recovery system are usually equipped with electric motor, ebooster and other electrical devices.

From the perspective of subjective evaluation, the vehicle braking performance includes static evaluation and dynamic evaluation.From the static point of view, including the position sense and operation sense of the control device when the driver operates the brake device;From the dynamic point of view, it includes the feedback of vehicle brake pedal force and travel when the driver operates the brake device, the speed of vehicle deceleration and establishment, and the controllable direction of vehicle during braking.

Therefore, the subjective evaluation factors of vehicle braking performance mainly include the following aspects:

(1) braking system ergonomics;

(2) brake pedal sense;

(3) braking efficiency;

(4) braking direction stability;

(5) braking thermal stability;

(6) brake vibration and noise.

This paper mainly starts from the user’s actual car, studies the driver’s subjective feeling to the vehicle braking performance, and studies the vehicle braking performance subjective evaluation method from the user’s actual car.

2. Subjective evaluation method of vehicle braking performance

Research on the development of the whole vehicle braking performance, need from braking sense of man-machine engineering, the brake pedal, brake performance, brake direction stability, thermal stability, the noise of brake vibration, subjective and objective model is feasible and effective, the user language into engineering language, with objectivity for design goal, plug in the vehicle parameters, based on the theory of the braking system design match experience formula and parameters, and parameters of each brake department for the simulation calculation, finally choose the theoretical performance of the optimal solution, and make a sample, carry on the test and then vehicles for subjective evaluation and objective test, verify the subjective and objective target,Under the guidance of the subjective target, the sub-performance of the vehicle is improved and measured to achieve the set target.

2.1 braking system ergonomics

For the braking system, the first degree of perception of the driver in the actual use process consists of the braking system ergonomics.The driver’s subjective feelings towards the braking system mainly include the height of the brake pedal, the Angle of the brake pedal, the distance between the brake pedal and the accelerator pedal, and the difference between the height of the brake pedal and the accelerator pedal.

Figure 1 brake and accelerator pedal measurement

Parking in simulated user conditions to normal driving posture, respectively between trample brake pedal, the brake and accelerator pedal switch, and the seat, the steering wheel to the different attitude, evaluate the brake pedal height and Angle under different posture, braking distance and height difference between the accelerator pedal and user friendliness, evaluate and measure distance and height difference of the brake pedal and the accelerator pedal, brake pedal height, etc.Statistics scoring records and measurement data, in high grade cars, fitting out Δ L, Δ H, H data range method (see figure 1), and then based on the positioning of models set design goals.

2.2 brake pedal sensation

Brake pedal sense, refers to the driver in the process of stepping on the brake pedal, the vehicle speed reduction, brake pedal force, pedal travel, etc., is an important evaluation index to reflect the braking performance.The evaluation method is mainly to simulate the relationship among pedal travel, pedal force and pedal speed reduction under the driving mode of different vehicle speed and different pedal speed.In the power field, different car systems have different brake pedal style, the typical Japanese car style is linear and comfortable, the European and American car style is compact and slightly heavy.

FIG. 2 establishment of a model of subjective and objective connection of brake pedal perception

On the brake pedal feel style set, need from the perspective of the user, for different vehicle speed, deceleration, condition evaluation and objective data measuring, analysis of the correlation between subjective and objective, subjective and objective evaluation system of associated user language into engineering language, set the input to the vehicle development system and the brake pedal sense of style, working condition of the commonly used evaluation and assessment key points as shown in table 1:

Table 1 brake pedal sensory evaluation method

2.3 braking efficiency

Braking efficiency is to simulate the braking ability of the vehicle when the user encounters a sudden obstacle in the process of driving and takes quick step braking or emergency braking.The main evaluation indexes are braking distance and braking reduction speed. The main evaluation method is the evaluation of braking reduction speed, braking response, and braking process confidence and peace of mind of the driver after the driver conducts fast braking and emergency braking operations at low, medium and high speeds respectively.In order to better cover the driving habits of different users, different evaluation methods, such as pedal pedaling at different speeds, constant stepping force, light braking, heavy braking, heavy braking, light braking and so on, can be used to evaluate braking efficiency, security and responsiveness respectively.The commonly used evaluation conditions and evaluation points are shown in table 2:

Table 2 braking efficiency evaluation method

2.4 braking directional stability

Braking directional stability is to simulate the ability of the vehicle to slow down while maintaining the lane after the user conducts braking operations in different degrees in different roads.In linear and curved driving, the evaluation of slow braking, medium braking and emergency braking of vehicles, whether the vehicle can keep the lane running, the main evaluation index is the vehicle’s left and right braking force, braking response speed is the same, the evaluation process, do not allow vehicles to run off, tail swing and other unsafe vehicle dynamic.The commonly used evaluation conditions and evaluation points are shown in table 3:

Table 3 braking direction stability evaluation method

2.5 braking thermal stability

Brake thermal stability is to simulate user mountain road, high-speed driving conditions, such as the vehicle for continuous multiple deceleration, brake lining and brake disc temperature, tread braking force and the change of the schedule and will be able to meet the demand of actual driving, braking performance recession are not allowed to appear in the process of evaluation of the brake pedal stroke hit bottom, a serious shortage of vehicle deceleration, and so on and so forth.The commonly used evaluation conditions and evaluation points are shown in table 4:

Table 4 brake thermal stability evaluation method

2.6 brake vibration

Brake vibration noise mainly refers to the friction plate, brake disc, brake calipers and brake system other related components to produce vibration or noise from vibration, common brake vibration noise include the noise sound (Squarel), peristalsis (Groan), moo note (Moan), sounds (Clonk), brush plate collision (Wire brush), jitter (Judder), etc.Its influence factors is more complicated, such as the wear of brake itself, environmental temperature, humidity, weather, etc all can produce great influence on the braking noise, so evaluation needs to run on the different range and different working conditions is simulated users of different driving conditions, such as let stand at night, wash the car, rainy day, etc. Under the condition of simulated users since morning, morning and evening peak, urban driving, evaluate the working condition of mountain roads and high speed.

Table 5 brake vibration and noise evaluation method

Reference at home, major manufacturers in the United States and Spain on brake noise evaluation method of vibration and use the anhui huangshan mountain climate characteristics of high humidity, precipitation, to establish a set of relatively mature from 0 km to 16000 km of brake vibration noise evaluation methods, including 8000 km highway, 8000 km highway, and set a fixed in the fixed route evaluation points, according to different noise by using different evaluation methods, including common evaluation method as shown in table 5.

3. Practical application

Based on the research on the subjective evaluation method of vehicle braking performance from 2.1 to 2.6, combined with the feedback information from market users, this paper established A set of subjective and objective evaluation system through A lot of subjective evaluation and objective tests, and applied it in the development of model A.Held in quantity antenatal the vehicle braking performance evaluation activity, user evaluation an evaluation to the participants before the project shows that evaluation using the evaluation method, finally the car evaluation score of 7.24 points in the user evaluation, selected for the car in the upper second, shows that the evaluation method for the improvement of the whole vehicle braking performance has a certain guiding significance, the subjective evaluation method has certain adaptability, effectiveness, to promote the competitiveness of the whole vehicle performance.

FIG. 3 subjective evaluation of vehicle braking performance

4, summarize

This paper enumerates the vehicle braking performance each performance index in the subjective evaluation role in the development of braking performance and the corresponding evaluation method, based on the most automakers to subjective assessment – an objective test – the development process of subjective evaluation, subjective evaluation method on the braking performance of the research and reference to establish a set of perfect effective vehicle braking performance subjective and objective validation system, which can effectively evaluate the pros and cons of the braking system, effectively guide the development of the braking system and improve the direction, to improve the user experience and vehicle performance, shorten the development cycle, at the same time, for a long time for different subjective evaluation of the project,It can form an effective and perfect subjective evaluation system, and the continuously expanding evaluation system database can provide subjective evaluation basis for the development of new models.