IR Sensing

[Sparks333]

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

[Mike Betts]

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.

[Sparks333]

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

[Billfred]

<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. )

[Sparks333]

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

[Mike Betts]

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.

[Sparks333]

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

Sparks

[Mike Betts]

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.

[Sparks333]

Okay! i'll get on it.

Sparks

[Al Skierkiewicz]

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.

[Sparks333]

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

Sparks

[SoftwareBug2.0]

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.

[Al Skierkiewicz]

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.

[Mike Betts]

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...