It will be possible to build electronically controlled headlights using LED-Arrays. The key problem to generate a smooth light distribution from discrete LED pixels with gaps while keeping a sharp cut off can be solved. To keep technical effort and costs in a reasonable range, several new developments in electronics and optic had been necessary. On the way to a product is the remaining key issue the best compromise between pixel count and required function. The optical system for a LED-array headlight is much more complicated and challenging than a simple projection lens. In technical terms, one has to do the step from a paraxial illumination system to a corrected flat field imaging system: because we do not move the module, it is not sufficient to produce a sharp shutter image in the center as a HID projection lens does. A potential vertical cut-off line must be sharp at least up to the road shoulder. A LED array is made from individual LED chips. The electronic control unit (ECU) can be divided in two parts: first, a front end module which drives the LED. This uses a DC-DC-converter to generate a controlled current and pulse width modulation (PWM) to adjust the LED brightness. The standard concept is to use one DC-DC-converter for a chain of 8-12 LEDs. Each LED is bypassed by a transistor. If the transistor is OFF, the LED is ON and vice versa. The base of the transistor is connected to the PWM generator. The second module of the ECU calculates the required light distributions and processes the vehicle signals (for bending light, vertical cut-off line, spotlight function). As mentioned, commonly used is one DC-DC-converter for a chain of LED. The maximum number of LED is defined by the sum of the forward voltages and the maximum voltage of the converter. Additionally, the efficiency is considerably influenced by the required control range (minimal & maximum voltage). Together with suppliers, we have equipped two cars with LED-array-headlights. In the first car we are using a densely packed 4 x 20 array without primary optic, in the second car, we are using a dilute array of 96 LEDs (which are part of a 4 x 30 grid) together with a primary optic. With both approaches it is possible to generate an illuminance in excess of 100lx in 25m distance. However, the best compromise between sharpness and homogeneity still has to be found.
Adaptive headlights utilizing LED-Arrays
2009
10 Seiten, 11 Bilder, 5 Quellen
Conference paper
English
SAE Technical Papers | 1925
|Tema Archive | 1976
|Engineering Index Backfile | 1921
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