IR Sensors- will they attack hot Chiaphuas?

[Rich Kressly]

Originally Posted by Wayne C.

I'm sitting way up in the stands this season...
WC

Don't you every year Wayne?
Get some sleep, you are getting goofy already
Have that 9 ft arm built yet?

[Anthony Kesich]

40khz is the wavelength at which the beacons will be transmitting (I'll find the quote in the manual later). If I ma not completely mistaken, that is on the high end of the infared spectrum, just below the visible spectrum, so for someting to emit infared light from being hot, it would need to be REALLY hot, and i think most CIMs will die before getting that hot.

[briholton]

Originally Posted by Anthony Kesich

40khz is the wavelength at which the beacons will be transmitting (I'll find the quote in the manual later). If I ma not completely mistaken, that is on the high end of the infared spectrum, just below the visible spectrum, so for someting to emit infared light from being hot, it would need to be REALLY hot, and i think most CIMs will die before getting that hot.

yikes! 40 khz is not a wavelength - it's a frequency - and I don't think it is the frequency of the electromagnetic radiation coming from the source either. If that were the frequency, the wavelength would be about 10,000 meters. That's a real long radio wave, not infrared. So, yes, you ARE completely mistaken. Perhaps the infrared source is being pulsed at 40khz, but that's a whole difference animal.

[Gary Bonner]

How many pulses must the sensor receive in order confirm which beacon it is seeing, and therefore, how fast can the sensors sweep the field and still detect the beacons?

And a less important issue, will the beacons appear as bright lights on videos of the matches since many camcorders are sensitive to IR light?

[Rickertsen2]

Originally Posted by Gary Bonner

How many pulses must the sensor receive in order confirm which beacon it is seeing, and therefore, how fast can the sensors sweep the field and still detect the beacons?

You don't really have to worry about how many pulses they need to see. at 40khz, this will not even be an issue. You can basically assume the response is instantaneous.

Originally Posted by Gary Bonner

And a less important issue, will the beacons appear as bright lights on videos of the matches since many camcorders are sensitive to IR light?

yes. Some cameras more than others. It will show up as bluish white. Point a TV remote into your camera to get an idea of what i mean. You can prolly find IR filters if things are really bad.

[ahecht]

Just to make it clear, these sensors use near-IR, which means the light they use has a wavelength somewhere between 0.75 and 1.25 microns. Thermal IR, on the other hand, has wavelengths of between 3 and 30 microns. Heat will not be a problem.

Also, interference will probably not be a problem. The beacons (like most IR devices) emit bursts of light of varying length, which they use to send a coded message, similar to the way morse code can be used to send a text message (I am not certain what coding scheme is being used, but there are many commonly used including NEC, Sony, Toshiba Mincom, RC2000, RC5, RC6, IrDA, etc.). The Robot Controller can be programmed to look for this message, and if it isn't what it is expecting, ignore the signal.

Furthermore, each burst is not actually a solid burst of light, but is chopped up so that when the beacon is bursting, it is actually flashing 40,000 times a second. Inside each detector, a circuit called a bandpass filter removes all signals coming from light that isn't flashing near 40,000 times a second. Since most IR devices don't operate at 40kHz, most signals will never even make it to the Robot Controller.

[ErikJ]

Al pretty much nails it. The whole modulation thing can be tricky for anyone who hasn't seen it before, so that's probably the big cause of confusion (ie, IR frequency != 40KHz).

However, it should be noted that nearly all (I think it's all) TV/Stereo remotes operate at 40KHz (it's actually just a shade off). Most off the shelf commercial IR sensors (ie, the kind you buy at Radio Shak) only look for this 40KHz signal, and filter everything else out. The way your TV, and your FIRST robot, know the difference between the signals is in the coding scheme used on top of that 40KHz carrier. So... if you go off and write your own code, yes, TV remotes could be a source of interference. Or, perhaps cooler, you could probably allow (for demo/debug purposes) the FIRST controller to use a TV remote as an input, provided you knew the coding scheme of the controller (you can find many of these online).

Something I haven't seen made mention of yet is multipath interference*. I worked on an IR project a few years ago and would get strong multipath interefence when I was transmitting near large metal objects (in this case, lockers at the HS we were working with at the time). I'm interested to see if this becomes an issue, especially as one or more robots approaches the beacon. I'm not sure if this will be a major issue (I suspect not, but it could be depending on robot geometry), but the fix should be simple enough if it is.

* multipath interference - in short, when you recieve a signal that has been reflected off a few objects before you receive it. for example, in a car radio you have a direct path (radio tower to you), and a ground bounce path (tower, ground, you) - mutliple paths, hence multipath interference. In our example, this means that robot reflecting IR waves from the beacon appears to be a source, potentially causing problems for your location algorithm.

[Al Skierkiewicz]

Originally Posted by ErikJ

Something I haven't seen made mention of yet is multipath interference*.

* multipath interference - in short, when you recieve a signal that has been reflected off a few objects before you receive it.

Multipath is a degradation of signal due to a reflected signal arriving with the direct signal at a receiver. When the reflected siganl is far enough apart that the reflected signal is 180 degrees out of phase and near the same level as the direct signal, the two cancel out. Obviouosly at the frequency we are interested in (the IR frequency not the modulation 40kHz) there can be several objects on the field that will produce reflections that are 1/2 wavelength or multiples of 1/2 wavelength giving the out of phase condition.

[ErikJ]

I thought about that source of interference as well, but at these wavelengths very slight differences in distance will result in going from constructive to destructive interference (and back). I figured that then on average the chances of a missed detection are probably pretty slim. That brings my main concern back to the stray signal problem.

We have essentially a Direction of Arrival (DoA) problem - for more info just google for more info, or search IEEExplore if you have access. A similar problem has been solved many times over, especially in test ranges for antennas and radars. They have essentially the same problem we do - you can't distinguish a reflection off a wall from a reflection from the object. Now those guys use some pretty fancy techniques to eliminate those issues, but we can take a page from their notes. Since we have a priori knowledge of the source location (something I didn't have in the example I cited previously - in that case my beacon was moving), we can ignore signals coming from regions we don't expect to see on in. Now, this brings up some potential navigational pit falls, but if you used IR search/track in combination line following you'd be in good shape. If you're just randomly searching for a beacon then stray signals *could* become an issue. It's hard to say for sure (especially for me to say since I'm an EM guy not an E/O guy) until somone tries it out. However, since again the potential source of the stray signal is moving (quickly with respect to wavelength), this might go away just like the true multipath problem.

[Wayne C.]

Originally Posted by Rich Kressly

Don't you every year Wayne?
Get some sleep, you are getting goofy already
Have that 9 ft arm built yet?

20 ft.....

[Curtis Williams]

What I meant about last year's sensors is that they sent a specific pulse pattern to prevent picking up sources other than itself. Although my team didn't use them, I played with the sensors and found them pretty reliable. I dont think we have to worry about someone accidentally confusing the robots with a camera or furby.