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Unread 20-11-2004, 16:32
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IR Sensing

Hello!
I have just had a really screwy idea. Is it at all possible, or even feasible, to calculate how far one is from an IR beacon from attenuation? I am thinking that the answer is no, because the attenuation from less than 20 feet from a relatively low frequency is minimal, and the controller may not have the sensitivity for detection, but I wonder if it's possible anyway?

Thanks for humoring me!

Sparks
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Unread 20-11-2004, 19:06
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Mike Betts Mike Betts is offline
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Re: IR Sensing

Sparks,

The number of photons is reduced as a square of the distance. Choose/design the proper sensor and, yes, it is very doable. Using the RC controller should not be a problem insofar as sensitivity is concerned, The trick is going to be noise, granularity and linearizing the sensor input to get a usable measurement.
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Unread 20-11-2004, 21:35
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Re: IR Sensing

Hmmm...
I may try that if I have some spare time and my teammates are willing to indulge me. There was a lot of hinting to the use of IR in last year's kickoff in this years game. The only other problem I can think of is calibration: as the room gets darker or lighter, it may cause the sensor some grief.

Sparks
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ICs do weird things when voltage is run out of spec.

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Unread 20-11-2004, 21:52
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Re: IR Sensing

<theory type="from a non-programming guy on a team whose robot was sensor-free">
Perhaps you could mount another sensor, pointed somewhere that you know won't experience IR, and use that as a baseline?
</theory>

Just the only thing I could think of--if I'm wrong, don't shoot me. (If I'm right, the tip jar is at the door. )
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Unread 20-11-2004, 21:59
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Re: IR Sensing

Billfred,
No tip, but good thought. I should really write this down before it goes into my bin of forgotten/stupid/really-quite-credible-but-never-had-the-time designs. It will go nicely with my leaf-blower hovercraft drawings and articulation design notes.

Sparks
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ICs do weird things when voltage is run out of spec.

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http://www.ravenblack.net/random/surreal.html

Last edited by Sparks333 : 20-11-2004 at 22:05.
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Unread 20-11-2004, 23:05
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Re: IR Sensing

Quote:
Originally Posted by Sparks333
The only other problem I can think of is calibration: as the room gets darker or lighter, it may cause the sensor some grief.
Sparks,

Depends on the photodetector at the heart of your sensor. Many are very wavelength specific.
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Unread 21-11-2004, 01:39
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Re: IR Sensing

Quite true. Do you happen to know the wavelength of the IR beacon?

Sparks
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ICs do weird things when voltage is run out of spec.

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http://www.ravenblack.net/random/surreal.html
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Unread 21-11-2004, 07:38
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Re: IR Sensing

Quote:
Originally Posted by Sparks333
Quite true. Do you happen to know the wavelength of the IR beacon?
According to figure 11 of http://www2.usfirst.org/2004comp/Spe...n_receiver.pdf it looks to be about 950 nanometers.
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As easy as 355/113...
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Unread 21-11-2004, 12:05
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Re: IR Sensing

Okay! i'll get on it.

Sparks
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ICs do weird things when voltage is run out of spec.

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http://www.ravenblack.net/random/surreal.html
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Unread 21-11-2004, 23:03
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Re: IR Sensing

Guys,
As was said earlier in this post, attenuation is a function of the inverse of the distance (for non-coherent light sources) but that only holds true if the emitter and sensor have no other variables that will affect the system. In reality, neither have that great a production control over the directivity of the emissions. Small directional changes produce great variations in received power. Add to that the transmission attenuation, dispersion and refraction variable of the medium (i.e. air) as it varies with density, temperature and pollutants. A slight breeze of air will bring in a variety of particles of varying sizes that will affect the path loss. When you add multiple emitters as we had last year, then there is interference as the multiple sources add in and out of phase, at the receiver. (Think back to wave theory and interference patterns in a ripple tank.) This not only results in varying levels but in distortions as well. Finally add in the variables involved in the emitter for relative output vs. input current, junction temperatures, chages in overall dimensions, aging, and distortions in the molded case of the LED.
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Unread 22-11-2004, 00:14
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Re: IR Sensing

But could you get a ballpark estimate? With sophisticated enough calibration, it should be okay.

Sparks
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http://www.ravenblack.net/random/surreal.html
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Unread 22-11-2004, 01:26
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Re: IR Sensing

I agree that this would be interesting. As for a baseline, it might be important to take an average of what the ambient is after the match starts, that way you will get a more acurate value.
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Unread 22-11-2004, 07:26
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Re: IR Sensing

Quote:
Originally Posted by Sparks333
But could you get a ballpark estimate? With sophisticated enough calibration, it should be okay.

Sparks
We ran some tests last year with the emitter built to spec. Looking at the output with a scope, you could see reflections from virtually everything on the field especially the player stations. You would think that reflections would be fairly well attenuated but they were not. Range finding on just signal level would be near impossible with the emitter assy from last year. I think that the only thing that would reliably give you data would be some type of time measurement. Since the emitters are not synchronized nor identical in output, the wave interference does not allow measurable data in that realm either. The distances are too short for any of our hardware to accurately make calculations based on time measurements even if the other problems were not present.
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Unread 22-11-2004, 09:06
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Re: IR Sensing

Al,

I never said that it would be easy. The question which started this thread asked if it were possible...

For the record, the Bobcats did not use IR last year for many of the reasons that you have pointed out. It was easier to use other methods...
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Unread 22-11-2004, 16:47
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Lightbulb Re: IR Sensing

Quote:
Originally Posted by Mike Betts
...

For the record, the Bobcats did not use IR last year for many of the reasons that you have pointed out. It was easier to use other methods...
We did not jump into the IR realm last year and had enough on our hands with just line following. We are novices at this with FRC bots. I am interested to know who did have success with IR last season and what issues you over-came and how. The immortal words from last season's kickoff seemed to overtly emphasis IR in coming years.

Thanks,


APS


PS I am going to do a search of the archives now that I have posted.

I found this thread to be very imformative. Anybody have any more to add?
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Last edited by Andrew Schuetze : 22-11-2004 at 17:00. Reason: did a search
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