The bright blue-white light emitted by high intensity discharge (HID) xenon headlamps is a common sight throughout Europe, and the technology is rapidly gaining popularity in North America. Although HID lights have significant advantages over conventional halogen lights, the new lamps are also responsible for a dramatic increase in complaints to government regulators concerning headlight glare.
In an attempt to reduce glare, European authorities require automotive (HID) lighting to be equipped with automatic headlamp leveling systems. These systems compensate for changes in a vehicle's inclination relative to the road surface by making slight vertical adjustments to the headlamp's light beam.
The recent introduction of adaptive front-lighting systems (AFS) take adjustable headlamps a step further by swiveling the light beams in advance of the vehicle's turning. This places light into the turning radius, with the result that the driver's cornering visibility is dramatically improved.
Leveling to Reduce Glare
Headlamp leveling systems keep light parallel to the road surface regardless of the vehicle's tilt. A vehicle may tilt as a result of a relatively slow-changing event, such as the filling of a fuel tank, or by a quick-changing event such as traversing a speed bump. In both cases, the headlamps must be maintained level with the roadway. Most headlamp leveling systems correlate their adjustment angles based on a variety of sensor datain particular suspension compression data from the front and rear axles.
Most slow-changing events occur while the vehicle is stationary and the ignition off; events such as the addition or removal of a trailer, placing heavy loads in the trunk, or the entrance or exit of passengers. Clues that a slow-change event might soon occur also abound; for example, the opening of a trunk or rear passenger doors both give a pre-emptive indication that vehicle tilt may happen.
Some rapid-change events such as hard braking or acceleration also provide early warning clues. Together with suspension and yaw-rate data, these clues help the headlamp leveling system decide the timing and magnitude of adjustment required.
As shown in above, the application of the brake or accelerator pedal provides a clue that the vehicle will soon tilt in response to rapid deceleration or acceleration. The angle of the vehicle's nose can be determined based on the rate of change in velocity along with known information about suspension travel and compression. The leveling control unit uses these clues to intelligently filter the sensor data while calculating the inclination level necessary to keep the lamp stable.
Swiveling Improves Safety
The vehicle's data network also contains real-time sensor data on steering angle and wheel speed. Based on this information, AFS equipped headlamps can match the light distribution with the vehicle's turning angle so that upcoming curves and intersections receive maximum illumination, especially at the driver's gaze point. The significant increase in light helps reduce driver stress and fatigue and improves the ability to see obstacles that fixed-beam headlamps might not illuminate.
On U.S. vehicles, steerable headlights are set-up to function only at speeds above 10 mph. The left-side headlamps, whose beams are focused lower to avoid blinding oncoming drivers, can swivel up to 15° off the straight-ahead position in left turns, bends, or lane changes. Right headlamps only need to swing up to 5° in right-hand turns since they have higher, farther-reaching beams.
Various studies on swivel-beam headlamps have shown up to a 300% increase in the illumination of the driver's gaze point as the vehicle turns into a corner (below). The additional corner illumination results in a 58% increase in the driver's ability to recognize an obstacle.
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