| The automotive supplier's new
series of head-up displays overcomes problems of package size, optics
and adjustment for different users. And it is intended to bring the technology
out of the luxury segment.
Take your eyes off the road for one second when cruising at 100 km/h and by the time you have read your speedometer in the instrument cluster or tuned your radio your car will have moved on 28 metres to the point where you focus on the road again. This “blackout” is one of the key drivers behind head-up display (HUD) technology.
BMW believes it will be safer to read an HUD without taking your eyes off the road even though you may only be watching the traffic situation via peripheral vision. With the HUD the driver needs to make no downward head movement to read the instruments – hence the HUD’s name.
Such superior ergonomics led BMW to offer the first full-colour HUD, developed and manufactured by Siemens VDO, in its 5-series from 2003. With practically no eye movement and very little eye accommodation work the driver can read the information in a “virtual” display that seems to hover just over the front end of the hood. The HUD, now also available in the 6-series and strongly marketed by BMW, is believed to make driving less tiring, especially at night when the eyes have to work hard anyway.
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Siemens will offer height adjustment in a smaller package |
All this is done by using the windscreen as part of a mirror system that allows the driver to read a TFT display buried deep down in the panel behind the instrument cluster.
And that is where the trouble starts. For the windscreen makes a poor mirror. Over 80 per cent of the display backlighting is lost along the optical path. In effect just 1 per cent of the light reaches the driver’s eye. It takes high power LEDs and a lot of them to make sure that the driver can actually see the HUD.
Also the windscreen is pretty much a free form from the optical point of view. It takes tricky mathematics, very precise mirror manufacturing and system assembly so the mirrors can compensate for the windscreen’s irregular shape. Even then the HUD will only really work with a windscreen that has been optimised for the purpose.
Finally the HUD needs space behind the instrument cluster. This may not be such a problem in a large premium car where you can spend some money on designing a pancake cluster instrument but it is a different matter in a compact car.
So Siemens VDO has taken on the challenge to turn this active safety system into a potential family car feature. The two top targets were to reduce the cost and the system size roughly by half. To that end the supplier has broken down the complex system of the first generation HUD and has partitioned it into a new, modular system that offers various mirror and lens configurations, different TFT display types and backlighting units.
Adding a lens as part of the optical path brings about a bigger visible “screen”, better focus and less distortion. At the same time the number of mirrors is reduced. By increasing the visible display to a wide 9 x 20 cm image format, the second generation HUD is also well suited for night vision applications, claims Dr. Heinz-Bernhard Abel, Siemens VDO director for advanced development and design.
Varying the complexity of the optical system and choosing between full colour and monochrome display has a tremendous effect on the system costs, Abel explains: “Between the basic configuration with just one mirror and the high-end versions with either more mirrors or two mirrors plus lens you are looking at a wide two-digit percentage span of cost difference for the optical system alone.”
At the same time the supplier has halved the dimensions of the HUD box that needs to be integrated into the panel. And while they were at it, the engineers added another option to the new HUD generation: the virtual display’s position can now be adjusted on the vertical axis according to the driver’s body height.
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