Anyone know how the self-leveling mechanism operates on Xenon headlights and how quickly they are supposed to react?
The reason that I ask is that a car following me tonight (Audi A4 or Passat, I think) produced two distictive 'flashes' of his lights whenever he hit a bump in the road. It looked like the control mechanism was over-compensating for a brief period.
Kevin...
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Nearly every xenon equiped car I have seen at night does that. Fiats appear to be worse.
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HID have self levellers but just Xenon's do not. Are you sure it was not just the front wheels then the back whells hitting the bump. The HID type levellers are quite damped to avoid nodding on uneven roads Regards Peter
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Stupid question alert. What's the difference between HID and Xenon. Is one where it looks like a normal headlamp but at certain angles looks blue - and the other the brilliant whitish blue light such as on new Lagunas, Focus ST170s etc?
Cheers
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Adam
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I agree that this is a problem ("characteristic" in dealer-speak) with HID's and have posted on here about it before.
I am not sure whether it is over-compensation, underdamping or slow repsonse speed, but whatever it is I think its a problem on some cars.
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Find out all about the subject at:
www.sylvania.com/LearnLighting/LightAndColor/
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What\'s for you won\'t pass you by
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On my Vectra, there are two sensors fitted to the suspension. One monitors the suspension level on the front offside, the other monitors the level of the rear offside suspension.
This method of beam auto-alignment (as opposed to a tilt sensor) does not alter the beam height when going up or down hills, for instance. The system does not appear to over-compensate for undulations in the road. When applying the handbrake at standstill on a downhill slope, the rear of the car lowers markedly, but the headlights stay fosused at exactly the same height on the vehicle in front. No over/undershoot on beam height can be seen.
I suspect that what you are seeing is the transition point between the *very* sharp cut-off and the actual beam, where most glare will occur.
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On my Vectra, there are two sensors fitted to the suspension. One monitors the suspension level on the front offside, the other monitors the level of the rear offside suspension. This method of beam auto-alignment (as opposed to a tilt sensor) does not alter the beam height when going up or down hills, for instance. The system does not appear to over-compensate for undulations in the road. When applying the handbrake at standstill on a downhill slope, the rear of the car lowers markedly, but the headlights stay fosused at exactly the same height on the vehicle in front. No over/undershoot on beam height can be seen. I suspect that what you are seeing is the transition point between the *very* sharp cut-off and the actual beam, where most glare will occur.
The car I had a problem with was actually a new Vectra. Driving on an undulating road at night between two local villages it was very disconcerting to see the light pattern bobbing up and down. I read a road test of the Signum where this problem was also highlighted.
I suspect when driving at speed on an undulating road the servo mechanism in the headlight simply cannot keep up.
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I think much *though not all* of the problems with HIDs dazzling is because your eyes seem to get instinctively drawn to them. There is something about those bright actinic rays, some animal instinct seems to take over and before you know it, you are temporarily blinded. I have to make a concious effort to look away.
The HIDs on my Omega are a bit slow to respond if you stop behind another vehicle and apply the handbrake, then blip the throttle (automatic trans) the car pitches, but you can see the lights trying to catch up!
the worst HID cars I have had have been behind and lighting up the inside of my car are 4x4s and Japanese or far-eastern cars.
At the end of the day, there are only several HID systems by a few companies, and these are fitted in custom shaped lamp hosuings.
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The HIDs on my Omega are a bit slow to respond if you stop behind another vehicle and apply the handbrake, then blip the throttle (automatic trans) the car pitches, but you can see the lights trying to catch up!
Well, that's over at least a second or two, isn't it?
Imagine driving over an undulating road and the lights would be forever playing 'catch up' - this is where the problem lies I think.
Personally, having driven behind various types of light, I am not overly impressed by the HID's. Apart from the levelling problem they are horribly expensive and if one fails you stand the chance of the car being of the road for a couple of days whilst new part is ordered, fitted etc etc plus minimum £200 bill!
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Adski. Soory If I confused you but High Intensity Discharge lights are self leveling and are Xenon technology with high voltage arc discharge lamps. but you can also get, as most cars have Xenon filament bulbs H2, H4 H7's etc etc. These, in some models, have manual height adjustment. Hope that clears the issue. The Passat Estate with HID uses a sensor in the rear of the car and as the front rises and compresses the suspension the rear sensor does not detect too much of the ftonts movement and the delay before the back wheels encounter the bump the HID have bounced. Not ideal, and disconcerting on rural bumpy roads.
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I also have a Passat estate with HID. I agree with Peter D that the self levelling is not particulary effective. I understood that it only responds initially to heavy loads etc. in the back when the lights/ignition are first switched on and that it doesn't actively respond on bumpy roads. It's not fully active self levelling as I think, used to be fitted to some Citroens. In passing, VW Passats (and probably other VW's) with HID do not have a manual headlight height control on the dashboard so that's another way that these lights can be identified (apart from the initial extra cost of about £600!).
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Fords fitted with Xenons are certainly self levelling, as are most others I have come across. I.e on a Mondeo fitted with Xenons the manual electric headlight adjustment is missing.
Personally I like to be able to manually adjust the level of the lights from the drivers seat, Xenons or not.
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>HID have self levellers but just Xenon's do not.
Point taken Peter, but I don't consider normal halogen lights doped with Xenon to be real 'Xenon lights'.
>Are you sure it was not just the front wheels then the back whells hitting the bump.
They weren't really 'bumps', a better description would be undulations as NabLane described so I don't think it was that.
I've seen quite a few oncoming cars with HID produce a quick flash before the self-levelling reacts but the car following me was quite different. Until I worked out what was happening, it looked like he was deliberately flashing me.
Maybe he was carrying something extra heavy in the boot.
Kevin...
PS. I've done a bit of googling but still can't find anything that describes the technology used for self-levelling. Particularly the actuators used to regulate beam height.
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I thought the xeon part of the headlight unit was mounted on a simple dampened gimball (spelling) as in ships compass.
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Although I don't know much about these systems, there is a fairly obvious problem to consider when designing the controller.
You need to make sure that the lights aren't going to dazzle people, so you need to make the control loop fast enough to deal with changes in the car's attitude, possibly caused by variations in road gradient. However, you also need to make sure that the lights don't respond to every bump and jolt - I can imagine that might make for nauseating progress!, not to mention the annoyance caused for other drivers. So, the problem is, how to define and implement the cut off.
My first guess at implementing this would be to use a low pass filter with a bandwidth some factor below the pitch frequency of the car on its springs. This would allow headlamp aim corrections slower than the pitch frequency, and block those faster inputs which excite the car in pitch vibration. To approximate this in a cheap and automotive way, you could just specifiy a headlamp aim motor which will run slowly!
number_cruncher
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