LIDAR

[Zseli Szabolcs]

A trafiban alkalmazott felvezetolezernek  is vannak teljesitmenykorlatai,
es a trafi is csak akkora teljesitmennyel adhat
hogy az akar direkt a szembe iranyitva se okozhasson serulest. Ebbol a nem
tul eros fenybol a reflexio szerintem nagysagrendel alacsonyabb szintu mint
pl 25-30 IR lede. Ha ezt a gyenge reflexiot erzekelni akarjak nagyon
erzekeny vevot kell alkalmazniuk AGC aramkorrel, hogy a jel a kesobbi AD
konverziora alkalmas legyen. Amenyiben a vevot tulvezerlem ir fennyel es a
jel/zaj nagyon leromlik nem lessz mit elemeznie a trafi jelfeldolgozo
egysegenek mivel az erzekelorol az AD  allandoan kozel maximum jelet kap.

Talaltam + infot:
LIDAR in general
Radar is starting to show it's age. The new kid on the block is, and has
been for some time, LIDAR. This stands for Light Detection and Ranging.
These are usually infra-red laser-based devices and are far more accurate
and reliable than radar-based devices, not to mention a lot lighter. This
makes them ideal to be built in to handheld devices such as the LTI20.20.
Laser-based LIDAR devices can be used either manned or unmanned, and with or
without cameras attached to them. Pretty flexible then. Their basic method
of operation is different from the doppler-method described above.
LIDAR-base devices work on a time-of-flight mode. ie. they measure the time
it takes light to travel from the gun's transmitter to the object, and back
again.
LIDAR devices typically pulse at between 45Hz and 72Hz - i.e. 45 to 72 times
a second. The speed of light is 300million metres/second. Let's take two
pulses from a LIDAR device as an example.
Pulse 1 emitted from the device and detected by the receiver 0.000001333
seconds later. Well, speed equals distance over time. So distance = speed x
time. The speed of light is 300,000,000m/s and the time is 0.00000133s so
the distance is
300,000,000 x 0.000001333 = 399metres. That's the distance the light
travelled from the gun to the target and back again, so the distance to the
object is half that, or in this case 199.5metres
Pulse 2 is emitted 1/45th of a second later, and detected 0.000001325
seconds after that. The same calculation again gives the distance of
300,000,000 x 0.000001325 = 397.5metres round-trip, or a device-to-target
distance of 198.75metres.
Now it's another calculation using the above figures. The difference in
distance between the two readings is 0.75metres. The time between pulses was
1/45th of a second so the object travelled 0.75 metres in that time, or
0.75 x 45 = 33.75metres/second. It's now even simpler maths to scale that up
from metres/second to km/h -
33.75m/s x 3600 = 121500metres in an hour which in turn is 121.5km/h. Now
bear in mind that all that those calculations happen 45 times a second (or
more). The device needs two or three consecutive readings all with similar
results to give a finite speed which is why LIDAR devices take 0.3 seconds
to lock-on and read your speed.
Bear this in mind before you buy a laser-detector then - if you can react in
0.3 seconds and reduce your speed, then you're lying.
This is important so read it carefully. The ACPO guidelines state that
"operators should bear in mind that the device confirms and corroborates
prior personal observations." What this means is that officers who stand at
the side of the road indiscriminately targetting every vehicle that comes
into their line of sight are in breach of their own guidelines. Remember
this if you get stopped because it is important!
ACPO guidelines also prohibit the use of handheld laser or optical speed
measuring devices from within a vehicle unless the window is down. ie. the
beam will not be projected through a window or off a mirror.




----- Original Message -----
From: <ide.ne.irj at freemail.hu>
To: <elektro at tesla.hu>
Sent: Friday, April 08, 2005 10:43 PM
Subject: Re: LIDAR


> Thus spake Zseli Szabolcs:
>
> > Az eddig osszegyujtott informaciok:     A hazai és europai trafik
> > fenyforrasa 904nm  hullamhosszu infra lezerdioda.  A trafiban talalhato
> > erzekelo az aldozatrol visszaverodo infra tartomanyba eso fenyt meri. Az
> > adas es vetel kozotti idorest meri és eltarolja, az ismetelt meresek
> > idoadatainak elemzese utan az auto sebessege kiszamithato. Mivel az
autokon
>
> Gondoljuk vegig elvileg. Ha folyamatos jellel zavarod a vevojet, akkor a
> DC-t levagja, nem erdekli, siman mukodik. Ha meg akarod vakitani, akkora
> amplitudo kell, hogy a rendorbacsira raeg a sapka, te meg az egesz
hatralevo
> eleted a sitten toltod. Ha eleg ugyes a kutyu, impulzussal is kb
lehetetlen
> meghulyiteni.
> A masik oldalrol gondolkodva: en ugy csinalnam a meroket, hogy rengeteget
> mernek, teljesen veletlenszeru idokozonkent. Ezzel nem tud interferalni
> semmilyen zavaro impulzusgenerator. A visszaerkezo impulzusok kesleteteset
> abrazolva a zavaras egy fuvet alkotna (kiveve ha a veletlenszam-generator
> torheto), melybol a hasznos jel szepen kilogna. Akarmit csinalsz, ez igy
> marad, nem lehet megzavarni. Az impulzusok gyakorisagat novelve a fu
> magassaga none, elobb utobb a jel/zaj viszony annyira leromlana, hogy nem
> lehetne hitelesen merni. De lehet hogy ehhez mar egy BFG9000-et kozelito
> atlagos lezerteljesitmeny kell.
> Nyilvan egy vadaszgep elektronikaja tud ilyen trukkoket, talan egy utszeli
> Fama nem. Ha szabalyos idokozonkent mer, es a mukodese ismert, akkor talan
> lehet valamit tenni. Pl a beerkezo jelet erzekelve es ismerve a kovetkezo
> jel kibocsatasanak idopontjat, az impulzus erkezese elott ki lehet adni a
> zavarjelet, igy a kocsit kozelebbinek fogja merni a kutyu. Ha sokat mer,
es
> a nagyon elteroeket (hibas meresek) eldobja, ez egy megoldas.
> Persze ha osszesen kettot mer villamgyorsan egymas utan, nincs semmi
eselye
> az elektronikanak, logikailag sem! Ha olyan ugyes is, hogy az elso
impulzust
> is garantaltan erzekeli es reagalni tud ra, a sebessegmerest csak folfele
> lehet hamisitani, ami nem jo :) Illetve irrealis sebesseget merve
> hiteltelenne teheto a meres, de igy a zavaras tenye is nyilvanvalova
valik.
>
> > Az egyszeru kivitel:    A sugarzok allandoan mukodnenek.
>
> Ezt lezeres mero zavaro detektor kutyuvel kiszurjak, sitt.
>
> > A legbonyolultabb kivitel:    A sugarzokat a kritikus meresi pontok
kurul
> > elhelyezett infra erzekelok inditanak, a trafi meresi frekije korul a
trafi
> > sugarzott jele utan veletlenszeru idoressel sugarozna. Ezzel talan
> > megzavarna a kiertekelest.
>
> Talan, ha eleg hulye a muszer. Ha en csinalnam, nem lehetne igy
megzavarni.
>
> > Hat roviden ennyi, akinek hasznalhato velemenye, otlete vagy
tapasztalata
> > van ne tartsa vissza, ha valamiben tevedtem javitson ki.
>
> En ugy erzem, ilyen high-tech kutyukkel a meresben a yardnal az elony.
> A merest nehez zavarni, befolyasolni. Mashogy talan konnyebb elintezni...
>
> > ZssZ
>
> --
> Valenta Ferenc <vf at elte.hu>   Visit me at http://ludens.elte.h u/~vf/
> "A kulturalt ember nem szemetel. A tobbieknek meg tilos!"
>
> -----------------------------------
>  Szponzorunk: http://tonerbolt.hu/