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In recent years, there have been many significant advances in the production technology and flow of automotive headlamps, which has greatly improved the performance of automotive headlamps. The most significant of these is the advancement in new materials and production processes that make the design of automotive headlamps more flexible.
Reflectors, for example, are no longer limited to simple parabolic reflectors. More sophisticated reflectors (such as free-form surface reflectors) have also begun to be applied to automotive headlamp systems. However, the design cycle required for a complex system also becomes longer, which is not allowed in modern production.
Therefore, how to design a car headlight with superior performance on the premise of minimizing the design cycle becomes an urgent problem to be solved. Computer-aided design software is an effective tool to solve this problem.
The output of computer-aided design software for general automotive headlamps is the iso-illuminance curve on a 25m test screen. Although the designer can obtain the information on the distribution of the headlight type through the iso-illumination curve, it does not answer the question: What is the effect of this light pattern on the real road scene? To this end, people developed a road lamp lighting simulation program for automotive lamps, which can convert the headlight's light pattern into a simulated road scene. Through the simulation of the headlight's light pattern, on the one hand, for designers and manufacturers, it is possible to check and modify the design plan as soon as possible, without having to wait for the prototype to be modified before, thereby greatly reducing the design time and cost; On the other hand, for the average customer, it is intuitive to know the lighting effect of the desired headlamp without the need for a manufacturing model.
2 two methods
Two different methods are commonly used in automotive lamp road lighting simulation programs: simulation based on reflection coefficients and simulation based on real scenes.
2.1 Simulation method based on reflection coefficient
The simulation method based on the reflection coefficient is based on the calculation of the light distribution of the headlamp, and then the simulation effect of the headlamp is obtained based on the reflection coefficient of various objects on the road surface and on the road. Flow of simulation method based on reflection coefficient.
First, establish a geometric model of the road in the computer. The geometric model of the road can be in a database of pre-existing programs. The program can search the database according to the road parameters (the width of the road, the reflection characteristics of the road, the surrounding objects such as trees, signal signs, curves, etc.) requested by the user and find the closest model as the basic model of the road. The geometric model of the road can also be defined by the user himself. First, the user can transfer the basic model from the database according to the pre-determined road types (country roads, city roads, highways, etc.), and then can arrange any objects on the road surface, including trees, pedestrians, traffic signal signs, and bends. Road signs and overhead signal signs. The user can also place an oncoming vehicle or a preceding vehicle on a virtual road surface to simulate the situation when the vehicle is passing or passing.
The user can then determine the condition of the road (whether it is wet or dry) or the weather (sunny, cloudy, rainy or foggy). Based on these conditions, the characteristics of the objects arranged above (including the pavement itself) include the reflectance, color, Can be found from the database.
Finally, the user can determine the angle of observation (bird's-eye view, driver's perspective, or user-defined angle). In this way, we build a complete road model in the computer, which is the basis for simulation.
Then define the car light source model. The light source model can be calculated by the principle of blackbody radiation itself, or it can be the output result of other programs. At this stage, the user can determine the position of the lights and the position of the observer. The program can also calculate the light pattern distribution on the 25m test screen based on the light source model and the type of reflector and lens, as a reference for professional designers. The last is brightness simulation.
After defining the various parameters of the road and the light distribution of the lights, the brightness simulation can be performed. The traditional method of brightness simulation is to use Monte Carlo ray tracing. Nowadays, many programs also use light transfer methods based on energy transfer for brightness simulation.
However, both methods must know the reflection coefficient of each point on the road surface and the object at the incident angle of light. The result of the brightness simulation is the exact brightness value of each point on the road surface and the object. These brightness values ​​can be converted into grayscale on the computer display. In the end, the lighting effect of the defined car light type distributed on the virtual road can be obtained on the display screen.
From the above simulation process we can see that the simulation method based on reflection coefficient has the following advantages:
A1 As required, the location of roads and their affiliated objects can be arranged arbitrarily and the conditions of the roads and weather can be determined, thus greatly enhancing the flexibility of the simulation.
B1 It does not need to do any experimental work. It is entirely a computer's own simulation, so it will not be affected by the weather, measurement equipment and other factors.
The disadvantages of this method are also obvious:
A1 The premise of using this method is to know the reflection coefficient of each point on the road surface and the object in the incident direction of the light, and this reflection coefficient is generally difficult to obtain.
B1 Because this method involves a large number of operations to find the database, and it needs to simulate every point in the field of view, the workload of calculation is very large, which determines that the simulation time will be longer.
2.2 Simulation Methods Based on Real Scenes
The simulation method based on the real scene is a kind of computer aided simulation method formed in recent years with the development of computer graphics and digital technology. Its biggest feature is to make full use of the role of real scenes to bypass the thorny issues such as reflection coefficients. The key to this method is to obtain a database of proportional relations. This database actually includes complex optical processes such as absorption, reflection, and scattering. Therefore, according to this ratio database, the lighting effect simulation map of the headlamp to be measured can be obtained. The program structure using this method is shown in Figure 2.
The advantages of the simulation method based on the real scene are as follows:
a. The principle is very simple and does not involve complex optical conversion process, so the structure of the program is clear and the design is also more convenient.
b. It can simulate various road types (straight, curved, intersection) under different conditions (sunny, cloudy, foggy days).
The disadvantages of this method are:
a. Need to select the standard road in advance and shoot with a digital camera, and once the shooting is completed, the position and characteristics of the entire road and its attached objects are fixed, thus greatly limiting the flexibility of the simulation.
b. Because this method uses relative brightness, it can only be displayed in the final simulation image when the two headlamps have very different light pattern distributions, so the accuracy is relatively low.
3 Application---AFS System
AFS system (AdvancedFrONtlightSystem), also known as intelligent front lighting system, is the development trend of automotive lighting, and generally emphasizes that it can improve the safety of driving. At present, there are two different standards in the standard of automotive headlamps: European standards and American standards, both of which have major differences in low-beam light distribution.
The European standard pays special attention to the control of glare, so there is a cut-off line of 15 degrees on its light distribution, and the illumination is also relatively low. In the United States, the roads are mostly wide, so there is no special requirement for glare, as long as the headlights can shine farther and brighter. However, with the acceleration of economic globalization, people are eager to eliminate the differences between the two standards and establish a unified international standard.
One way is to integrate the reasonable parts of the two standards into a mutually acceptable standard. This is the so-called international coordination. Another method is to establish a headlamp system that includes multiple functions. It can use different lighting methods according to different road conditions and weather. This is the AFS system. The current AFS system includes the following functions: AL (lighting in adverse weather), BL (curve lighting), CL (country road lighting), TL (town road lighting), ML (highway lighting), and OVL (over-top) Signal lighting. After researching, the most favorable and the most needed are AL, followed by BL, and finally OVL. Therefore, when it is impossible to fully implement the AFS function, there is a preference. Using the automotive lamp lighting simulation program to simulate the lighting effects of a variety of automotive headlamps, we found that most headlights can meet the requirements of glare, but we also found that in the illumination, more distant illumination energy Satisfy the requirement, but there is insufficient illumination at close range.
The defects of the automobile headlamps in this respect are not much problem under the conditions of good road conditions and few vehicles. However, in China, especially in some major cities like Shanghai, the roads are narrow and intricate, and there are bicycle paths and sidewalks on both sides of the road. Therefore, there is a relatively high demand for close-up illumination. This is why many drivers in Shanghai always turn on headlamps, position lights, and fog lights at night.
Using the automotive lighting road lighting simulation program, we can simulate the lighting effects of many functions of the AFS system, and compare whether there is a function or a combination of several functions that can meet the requirements of high illumination at close range. It is not necessary to modify the existing AFS system. If not, it is necessary to add a function to the existing AFS system so that it can improve close-range illumination. Then such an improved AFS system can be applied to special situations in China.
1 Introduction