Analysis of the technical route of intelligent vehicle lighting source
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Intelligent control technology for automotive lighting is formed by the close combination and mutual penetration of light source technology, electrical technology, microelectronics technology, detection technology, microcomputer technology, automatic control technology, sensor technology, and communication technology. With the extensive application of human high-tech in the field of automotive lighting, automotive lighting control technology will continue to develop in the direction of energy saving, intelligence, informationization, humanization, artistry, and personalization. Intelligent control occupies an important position in automotive lighting and is also an indispensable part, and will be more and more popular.
0 1 Development trend of smart headlights
The concept of headlights that change light patterns according to different road conditions was first proposed as early as 1958, but it was technically difficult to achieve in the era when halogen lamps were the main light source for headlights.
And with the continuous improvement of automobile lighting technology, there have been a lot of technological innovations in the areas of LED light source promotion, sensor and algorithm processing. Now the more advanced headlight system has realized the function of adjusting diversified light patterns under various complex road conditions, and implemented intelligent lighting actions, such as multi-road mode switching, intelligent follow-up steering, automatic recognition of oncoming vehicles, glare-free high beam, road sign recognition, and pedestrian warning.
Figure 1. Market share trend assessment of ADB (Adaptive Driving Beam)
Regardless of the type of intelligent lighting action implemented, the core of the light source technology is to divide the high and low beam light types into multiple areas of varying numbers, and to control the switch or brightness of each area based on the data input from the camera or sensor and the preset algorithm.
The larger the area divided, the more light types can be combined, and more complex intelligent lighting actions can be achieved. As the number of divided areas gradually increases and the area of individual areas decreases, the industry has begun to use the concept of display technology "pixel" to refer to such areas.
0 2 Analysis of light source technology route
Based on the characteristics of LEDs, such as small size, easy driving and fast response, the basic solution for multi-pixel design of entry-level smart headlights is to use multiple LEDs to form a row or matrix layout. Compared with ordinary LED headlights, LED matrix headlights require more drivers, greater heat dissipation capacity and a more complex secondary optical system for each LED to be divided into independent pixels.
1. Headlamp Adaptive Control Technology
Headlight adjustment
1. The light intensity sensor senses the ambient brightness outside the vehicle.
2. The turning angle sensor measures the turning angle of the car.
3. The yaw rate sensor measures the yaw rate of the vehicle.
4. The vehicle speed sensor measures the current speed of the vehicle.
5. The accelerometer sensor obtains the vehicle height (vehicle center of mass).
6. The headlight angle sensor obtains information on changes in the headlight angle.
7. Multi-sensor information interaction optimizes the light distribution of car headlights.
2. Matrix LED beam adjustment
Matrix LED headlights can achieve precise control of the lighting area, that is, within the coverage of the light source, the system can select specific areas for lighting, or select some areas for shielding. Based on the characteristics of LEDs being small in size, easy to drive, and fast in response, using multiple LEDs to form a row or matrix layout is the basic solution for entry-level smart headlight multi-pixel design. After meeting the car, the LED beads that were previously turned off due to shielding will automatically turn on to resume normal lighting.
Comparison of Xenon Headlights and LED Matrix Headlights
Regardless of whether all single-chip LED particles are used or a mixture of multi-chip particles are used, due to the limitation of LED package size, the final pixel level can basically reach hundreds, which is the limit.
At the same time, as the number of LEDs increases, the difficulty of regulating the consistency of parameters such as brightness, color and voltage between LEDs increases proportionally. The difficulty of calibrating the secondary optical system and LEDs during processing will also increase proportionally with the number of LEDs. The above factors restrict the application of this solution in high-pixel intelligent headlights.
3. Anti-glare technology
The matrix LED headlight control system can sense and track other vehicles within a distance of 800 meters through the vehicle detection system; when it detects vehicles and pedestrians in front of the car or in the opposite lane, the headlight control system will detect and track the target vehicle in real time and turn off the LED monomers at the corresponding position, while other LED monomers continue to light up. This can avoid dazzling the other party and ensure the normal lighting needs of the driver.
No Matrix headlight working condition
Matrix headlight anti-glare function
Figure 2: Example of LED matrix smart headlight
As the number of pixels increases, the intelligent headlight lighting function gradually has the display function. LCD (Liquid Crystal Display) is the current mainstream display technology and is naturally an option in the intelligent headlight light source system.
Except for the RGB Color Filter that is not needed for headlights, LCD headlights, like ordinary LCD displays, require basic components such as backlight, polarizer and liquid crystal panel.
In addition, since the power density is much higher than that of ordinary displays, the LED matrix backlight generates a lot of heat, making it impossible to place the LCD panel directly on the backlight like an LCD display. Some secondary optical devices such as reflectors need to be added to form a light path at a certain distance.
Even so, because of the relatively high brightness, the loss of light absorbed by the polarizer and LCD panel is much higher than that of ordinary LCD displays. In addition, it has to pass stringent automotive-grade certification. This type of equipment, to be used as an LCD panel, needs to be specially customized by the manufacturer.
The number of pixels in current LCD headlights has reached tens of thousands. Given that the current LCD technology used for displays can achieve much higher pixel levels, there is reason to believe that LCD headlights will be able to break through the 100,000 or even higher pixel level in the near future.
Compared with the DLP headlight light source system based on projection technology introduced below, the LCD type has the advantages of relatively low cost, relatively small size, wider light shape stretching angle, and higher light-dark contrast.
What is more important to note is that the LCD panels that can meet the needs of headlights must be specially customized in order to be customized together with the LCD panel manufacturers. Only a fairly large-scale lighting factory can do this. Now it is estimated that only a few panel factories can manufacture panels that meet the requirements, so there are certain difficulties in promoting this technology.
Figure 3: Example of LCD smart headlight
Similar to the reason for developing LCD-style smart headlights, DLP technology (Digital Light Processing) based on DMD devices (Digital Micromirror Device), which is the current mainstream technology of projection equipment, has naturally become an optional technical route for multi-pixel smart headlight light source systems.
The DLP headlight light source system can be understood as using only white pixels, and the basic principle is essentially the same as that of a projector. Of course, in order to meet the requirements of vehicle regulations and certification, especially the harsh operating environment requirements inside the headlight, both the DMD device and the optical-mechanical system that matches it must be designed and optimized. In addition, the projection surface of the headlight is a horizontal road surface, and the longer the projection distance, the more significant the impact on the trapezoidal distortion of the projected image, so relevant image algorithm correction is also required.
In terms of light source, similar to current projection technology, both LED and laser can be used as light sources for DLP systems. Since RGB three-primary color laser light mixing technology is not suitable for headlight systems that only need white light, the laser light source is mainly a solution of blue laser + phosphor conversion to white light.
The advantage of LED+DMD is that the technology is relatively mature, and the main parameters such as brightness and efficiency are also good enough. The advantage of laser+DMD is that, thanks to the strong directionality of laser, even if phosphor is needed to convert white light, the light outlet of the optical machine can still be made very small, which can reduce the system volume on the one hand, and the small light outlet itself is also a unique headlight design language.
In terms of efficiency, lasers can theoretically reach a higher level than LEDs, but from the perspective of the current automotive-grade blue laser technology level and phosphor conversion efficiency, the overall gap is not obvious. In addition, using lasers as light sources also requires solving common problems that automotive laser light sources face, such as the life of automotive lasers, light decay at high temperatures, and the potential safety hazards of direct exposure to human eyes caused by phosphor shedding (for example, after a collision).
In terms of the overall solution, the biggest advantage of DLP over other current multi-pixel technologies is the large number of pixels. The first DLP-based smart headlight has already exceeded one million pixels, far ahead of other technologies, and there is still room for further improvement in the future. In addition, although the technical threshold of projection optical machines is relatively high, automobile OEMs or lighting manufacturers can leverage their design advantages of being familiar with automotive industry specifications to cooperate with traditional projection optical machine manufacturers to achieve relevant technology transfer and technology upgrades.
In addition, the current automotive-grade DMD devices have limited projection angles, and a single DMD is only suitable for near-field small-range projection, unless there are wide-angle DMD devices specially customized for headlights in the future.
When a car encounters a variety of traffic conditions, the DMD (Digital Micromirror Device) technology used can provide the most suitable lighting solution. From a technical perspective, DMD technology allows matrix laser headlights to have infinite possible control methods.
In order to greatly expand the image range of the DLP system (for example, using projection to perform direct steering), it may only be possible to add additional DMD devices or re-add mechanical rotation structures. The former will significantly increase the cost, while the latter runs counter to the trend of digitalization of smart headlights, once again increasing system complexity and reducing reliability.
Figure 4: Example of DLP-based smart headlight
AFS is the industry's abbreviation for Addressable LED Pixel Array, an LED technology specifically developed for multi-pixel smart headlight systems.
In the traditional LED process in the past, each chip had only a single positive electrode and a single negative electrode (multi-chip LEDs simply integrate several independent LED chips into a single LED package). After the external driver provides power, the entire chip lights up at the same time.
The AFS integrates a matrix CMOS control circuit in the silicon substrate of the chip in advance. Combined with the chip that has also been processed with a matrix microstructure, it realizes the functions of turning on and off and adjusting the current of each independent microstructure area on the chip, making each microstructure area directly become an independently controllable pixel in the headlight type.
Therefore, although AFS still uses LED as the light source, the difference between it and LCD and DLP headlight light source systems that also use LED as the light source lies in the formation of pixels: AFS forms pixels directly at the LED chip level; LCD forms pixels through liquid crystal panels, and DLP forms pixels through DMD devices.
Compared with LCD and DLP, the main limitation of AFS is the number of pixels. The number of pixels of AFS currently available is in the thousands, and it is expected to increase to tens of thousands in the next few years. Products with more than 100,000 pixels are in the planning stage in the long term.
Figure 5 Example of EVIYOS-AFS smart headlight
Laser scanning projection technology has begun to be applied in consumer and industrial fields. Its basic principle is to use a high-precision scanning mirror based on MEMS technology (Micro-Electro-Mechanical System) to periodically reflect the laser light path from different angles in sequence, forming a fast-refreshing image on the projection surface that is much faster than the human eye's reaction rate.
If this technology can pass the automotive certification and be applied to the smart headlight system, it will be the most efficient and smallest solution. Its pixel order of magnitude can also be close to that of DLP.
Figure 6 Schematic diagram of laser scanning projection technology
However, this technology is still quite far from passing automotive certification. Especially in the high temperature and strong vibration working environment of headlights, the current MEMS scanning mirror technology is far from meeting application requirements.
In addition, the scanning projection image may form a frequency superposition with the vibration of the vehicle in real road conditions, causing image jitter or flicker that is perceptible to the human eye, which may cause discomfort to the driver in severe cases.
450 nanometer light is radiated onto the 3 mm lens, which is as precise as minimally invasive surgery.
From this light source irradiation distance diagram, it can be seen that the irradiation distance of the laser light source is almost twice that of the LED high beam, with the farthest distance being 500 meters. The diameter of the laser light diode is only a few microns, which is much smaller than the diode that makes up the LED light.
OLED car lighting technology
Advantages of OLED (Organic Light-Emitting Diode):
1. High visibility and brightness, and more accurate colors.
2. The thickness of a monolithic diode is less than 1 mm, and no optical components such as reflectors and light guides are required, so the weight is naturally reduced.
3. The driving voltage is low and the power saving efficiency is high. Under the same brightness, the working life can be increased by 10 times, and it can work normally at minus 40°.
Excluding the laser scanning headlights, which are not yet mature, the four high-pixel technologies of LED+LCD, LED+DMD, Laser+DMD and AFS, which are relatively similar in technology and have their own strengths, are compared with the main parameters of the entry-level low-pixel LED matrix. The comparison chart is as follows:
Figure 7 Comprehensive comparison of various technologies
LED+LCD is generally balanced in all aspects, with efficiency being the bottleneck; LED/LASER+DMD stands out in terms of the number of pixels; and AFS has considerable advantages in terms of efficiency, contrast, operating temperature range, etc.
Typical example: using ultra-high pixel LCD or DLP to form high-definition patterns (such as pedestrian instructions, bicycle safety zone signs, etc.) or concise intelligent actions in the near field without causing too much interference to the driver (such as navigation arrows projected on the road); at the same time, AFS large-area lighting is used in the far field and main lighting area to achieve functional intelligent actions (such as glare-free high beam); and discrete LEDs are used as a supplement to the light type (such as the extension of the light type with dynamic steering, etc.). As shown in Figure 8 below:
Figure 8 Organic combination of several technologies
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