Re: measuring distance between two cars using infrared circuits

From: keith (krw_at_att.bizzzz)
Date: 01/24/05 

Date: Sun, 23 Jan 2005 22:59:46 -0500

On Mon, 24 Jan 2005 00:09:10 +0000, Mac wrote:

> On Sun, 23 Jan 2005 14:07:55 -0500, keith wrote:
>
>> On Sun, 23 Jan 2005 17:51:45 +0000, Mac wrote:
>>
>>> On Sat, 22 Jan 2005 23:41:45 -0500, keith wrote:
>>>
>>>> On Sat, 22 Jan 2005 07:32:19 +0000, Mac wrote:
>>>>
>>>>> On Fri, 21 Jan 2005 04:24:38 -0800, Andrew Holme wrote:
>>>>>
>>>>>>
>>>>>> gaurav.patil@gmail.com wrote:
>>>>>>> hello,
>>>>>>> i want to construct a circuit which will be able to measure distance
>>>>>>> between two moving cars.The two cars are needed to maintain a fixed
>>>>>>> distance between each other.
>>>>>>> if the distance between them increases or decreases the circuit
>>>>>>> should be able to detect this change and notify the amount of change
>>>>>>> in the distance to both cars.
>>>>>>> i want to construct this circuit using infrared LEDS ,so if any one
>>>>>>> can help me out with this(circuit idea) please mail it to me .
>>>>>>
>>>>>> It might be easier with microwaves than infra-red:
>>>>>>
>>>>>> Measure relative velocity using the Doppler effect. By integrating
>>>>>> this, you get a running estimate of the change in distance. Weird
>>>>>> things might happen when you go around corners!
>>>>>>
>>>>>> Unfortunately, absolute measurement of short distances using
>>>>>> electromagnetic waves is difficult / impossible due to the speed of
>>>>>> light.
>>>>>
>>>>> It is not difficult at all. It just requires bandwidth. Before anybody
>>>>> jumps on my case about detecting short CW pulses, let me point out that
>>>>> short CW pulses have a LOT of bandwidth, and the shorter they are the more
>>>>> bandwidth they have.
>>>>
>>>> Ok, what's the bandwidth of a kHz modulated ~2GHz carrier (wherever there
>>>> is some free bandwidth). It should be trivial to measure the round-trip
>>>> delay to withing a nS, which is about six inches. At a kHz,
>>>> that gives us a distance measuremnt every millisecond, which should be
>>>> enough for distance and differentiate to give a relative velocity
>>>> number.
>>>
>>> Are you talking about on/off modulation of a 2GHz carrier at a 1KHz
>>> rate? How long is the "on" time?
>>
>> Yes, pick your poision.
>
> It looks like it doesn't really matter, anyway. The Fourier transform is
> just a sum of two sinc() functions, one shifted right and one shifted left
> by the carrier frequency. The pulse duration controls the magnitude of the
> FT.

Sure. I'm looking at launching a ~2GHz (wherever the FCC allows) CW pulse
and measuring its time in flight. At a ns/ft that's 6"/ns round-trip.
Some tricks should be able to get this down significantly less than this.
A ns is a long time these days.

> I believe the total bandwidth is infinite, but any finite signal
> has infinite bandwidth, so that doesn't really help us.

Sure. I don't see a few kHz on either side of 2GHz to be a big deal
though. It might be a challenge to gate an uwave tranmsitter on in a
millisecond, but...

> Unfortunately, I'm not sure I know how to answer the question myself.
>
> I'll try to remember to ask some people who might know tomorrow and get
> back to you. (It also might pay to ask in the radar/sonar newsgroup.)

RADAR was my primary interest here. Measuring ns delays is rather trivial
these days. ...and that gets us to 6" distance resolution. Put enough of
these together with a (very) little computation and we get velocity. I
don't see how the mechanics of a couple of cars will exceed the physics or
computational needs.

> But the more you constrain the bandwidth, the more difficult it will be
> to identify exactly where the pulse starts or stops. So for precise
> ranging, you need more BW, regardless of pulse duration.

Ok. We can measure more points of the envelope. The question is where is
the bandwidth limitation. I suspect it will be in the transmitter,
though I don't know. Again, a few kHz isn't a lot of bandwidth.
>
>>>>> The same rules would apply to a modulated IR signal. There is no way
>>>>> the OP is going to get any kind of high resolution ranging using IR
>>>>> alone because there is just not enough bandwidth. (Some laser diodes
>>>>> have more than enough bandwidth to do this, but I don't think they
>>>>> put out enough power)
>
> [snip]
>
>>>>> Ultrasound might work well. You could have a transponder on the back
>>>>> of the car in front and a range-finder on the front of the car in
>>>>> back.
>>>>
>>>> It's *is* done without any transponder, which would make the idea
>>>> useless.
>>>
>>> I admit that the transponder is not essential. It just makes it easier
>>> to detect the signal, and increases the range over which the system
>>> would work.
>>
>> It also adds an unknown and significant delay into the path.
>
> Depending on exactly how the system is set up, the delay could be
> completely neutralized by using a PLL.

How? The PLL has to capture the signal and then re-launch the "answer".
That's time. If we're measuring the round-trip delay of two cars ten
meters apart on the Autobahn, the capture/retransmit time is an error I'd
rather not make. 

-- 
  Keith
> --Mac

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