Re: Dropping marker signals?

[Joyce Poon <poonj@ecf.utoronto.ca>]

It's time for me to seriously flex my holiday brain... 

On Tue, 28 Dec 1999, Tim Vanderhoek wrote: 

> I won't even immediately outlaw something as simple as a directly wired
> connection where we simply trail a cord of string wire behind us that is
> connected to dropped sensors. 

According to the rules, that would involve some sort of tag
transmission. 

Anywho, I am slightly concerned about the structure of this extra
transmitter.  Firstly, its sensors have to be kind of "tall" (at the same
height as the beacon we place on the enemy), unless we angle down the
emitters on the beacon (but that is rather silly as it will unnecessarily
limit our sensor range).  I think we can only place 1 thing on the enemy
and that's the beacon.  We can't put extra emitters on the enemy's wheels
or body (I don't think). 

> 
> You're the one with the catalogue.  What's the average price of a cheap
> IR sensor?  :-) 
> 
> [yes, I know, the only ones listed have sensor ranges of ~5mm, but
>  extrapolate based on that to produce the price for a sensor range
>  of ~5m...  ;-) ]
> 

	This is kind of tricky... You see, the ones with sensor ranges
listed only have a range of circa 5 mm.  But, there are some that don't
have any ranges listed and are supposedly used for remote control systems. 
Detector diodes cost as little as $.50 to $.70 each (ie may require some
extra components), while modules (ie may require less extra components) 
cost about $4 each. 

> I propose that for the purposes of analyzing the efficy of rebroadcast
> transmitters, the following model is suitable: 
>     i)  the playing field can be modelled as an empty box with walls
>          only along the edges and with no obstacles. 
>     ii)  robots can be modelled as travelling in straight lines at
>          constant speed, but bouncing off walls in a random direction. 
>     iii) sensor ranges can be modelled as being equivalent for both
>          robots and rebroadcast transmitters. 
>     iv)  a single rebroadcast transmitter can be modelled as being
> placed
>          in a random location. 

	Alright, we'll play by your rules... ;) 

	However, I don't think it's fair to assume that both robots will
have the same sensor range.  It is better to assume that the enemy has
greater sensor range than our robot, and that our robot and our
rebroadcast transmitters have the same sensor range (since they'll be
built by the same person... ahem...) 

> Evaluate based on the above model: Visualize three dots placed randomly
> in a square.  Draw circles of some radius around each dot.  Choose two
> dots and move them according to the model's rules for robot movement.
> Whenever either the a) the circles around the two moving dots overlap,
> or b) the circles around the two moving dots simultaneously overlap the
> circle around the stationary dot, then there is sensory contact between
> our robot and the enemy robot. 

	Wait a sec, the circles are having different meanings.  The circle
around the enemy (hereby denoted as E) is representing our beacon
transmission range.  The circle around us (hereby denoted as U) represents
our sensory range.  The circle around the rebroadcast transmitter (hereby
denoted as T) represents both transmission and detection range.  This is
really rather confusing, since we don't really care about transmission
range.  All that truly matters is detection range (the detection range IS
the transmission range. ie if we can detect a weak signal then our
detection range or transmission range has increased).

	To unify everything -- may I suggest that all 3 circles represent
reception ranges.  In fact, to simplify the model, remove the circle
around E (ie let us not concern ourselves with the enemy's detection
range).  Hence, sensory contact is made when E is in the circle around U
OR when both E is inside the circle around T and T is inside the circle
around U. 
 
> Now.....I admit that when I visualize this test using the above model,
> the rebroadcast idea isn't quite as useful as my original expectations,
> but it seems to me that it still increases the number of incidences of
> sensory contact enough to be worth serious consideration. 

	Yes, the probability of making sensory contact will definitely
increase with increased number of detection devices.  However, with my
modification to your thought experiment, we can see that the increase in
probability is not that high unless we have a large sensor range to start
with (ie large radii for the circles, so there is a greater chance of E
actually being in a circle).  I don't know what circle sizes you were
thinking of, but suppose we were thinking of the same circles, you
expectation for the detection range would have been twice as large as mine
(assuming you have put the same sized circles around each dot).  This will
probably explain why you feel stronger about these rebroadcasters than
me... :) 
 
> One big thing that doesn't show-up under the model: If the enemy has
> signifantly greater sensor range than us, it's possible that the only
> way we'll ever be able to find them is by artificially extending our
> sensor range (since they can otherwise avoid us while remaining outside
> or sensor range). 

	Agreed. 
  
> The idea would __not__ be to take time to specifically place the dropped
> sensors in the most optimal position. 
> 
> Rather, during the course of pursuit, drop them when certain events
> occur.  Remember...if the evader has a half-decent evasion algorithm, we
> will possibly travel in rouch circles several times before actually
> tagging them.  Drop a beacon, for example, when we run into an obstacle
> (thus we can always find our way back to the obstacle if we are the
> evader and need to hide, or if we want to try going in circles around
> the obstacle, or ...), drop a beacon, for example, when we make a sharp
> turn (since we may soon find ourselves travelling in circles). 

	Hmm... in fact, why can't the computer just mark down where we
made sharp turns or where we encountered obstacles on the map.  I think
that would be easiest. Moreover, dropping a beacon behind an obstacle is
rather silly (esp if we are using IR) since the obstacle will probably
block the signal. 

	Ok, I will assume that extra beacons will only be used for enemy
detection.  Again, how many of these extra beacons can we carry?  (This
goes back to the question I raised about the physical structure of the
beacon.)  Since these transmitters will be kind of "tall", I envision that
it will be difficult to drop them on the fly without something bad
happening to them (like falling over).  For enemy detection, ideally, the
beacon should be placed in an open area to maximize its sensory range
(although one can raise the question of why our robot will be near the
open space to receive the signal emitted from the beacon particularly when
we are in evade mode). 

	Maybe the best mode of transmission for these extra beacons should
be radio signals.  We can have different frequencies of radio waves so
that each frequency corresponds to a certain beacon.  However, I did a
cursory browse on the web today and I couldn't find anything on tracking
down the origin of a radio signal.  However, if the comp can somehow mark
what beacon we dropped at a certain location, we can track it down again. 

	Furthermore, if our beacon (the one on E) also emits radio waves
and the rebroadcaster detects radio waves of a certain intensity (ie
within a certain range), then there is no need to make the rebroadcasters
"tall."  As a side note, such a system will not be ideal for our robot
itself since I have not yet found a feasible way of tracking down sources
of radio waves. 
 
> > However, in theory, even dropping 1 beacon will reduce the desired
> sensor > range in half (if the beacon is at the right place...).  Hmm...
> since the
> 
> Not quite, I believe, but I'll give you marks for recognizing that even
> dropping 1 beacon will reduce the desired sensor range.  :-) 

	Gee thanks. :) 

> It will only cut the desired sensor range in half under certain ratios
> of playing-field size to beacon-sensor size.  Not that I've done a
> formal mathematical analysis, of course, so I could theoretically be
> wrong...  :-) 

	I think on average, it barely improves sensor range (we just
talked about that...).  But certain configurations will result in a
doubling of sensor range (ie when pt U is just at the circumference of
T...) 
 
> Back to my idea of pulsing the IR to help us differentiate different IR
> signals.  Obviously it is possible to transmit considerable information
> via IR --- your TV remove transmits at least 10 distinct numbers, and IR
> keyboards, printers, etc. have far more information to transmit.  What
> does your electronics catalogue have to say about IR communications? 

	Yeah... I thought of that before (I was thinking of all the
different makes of VCRs and TVs; their signals don't get mixed up)... but
the catalog says nothing about IR communications...  I will read the stuff
on that Rice University website tomorrow. 
 
> > Other silly ideas for interference: How monochromatic is IR light?  If
> we > can somehow diffract the light emitting from us, we coud mask
> ourselves > (stealth!).  Or we could drop a diffraction grating and
> stand behind it...  > then there would suddenly be a bunch of IR signals
> -- that should confuse
> 
> LOL! 

	I thought of something else last night before I went to bed.  But
I better see where that thought takes me before I share it with you (for
fear of eternal humiliation ;)). 

> Oi.  Why not make the beacons into little helicopters while we adding
> the rotating glass/plexiglass... 

	Actually, while I was in the shower today, I was thinking why not
make these beacons move.  Instead of dropping them, we could deploy them. 
Of course, we will then have to somehow control them (like remote control
cars?).  Or we could build a really simple (?) circuit that just says, "go
forward 5 steps and stop."  But who knows how the deploying object rule
will apply in this case, since our deployed objects would be like little
robots, and this could mean a lot more work. 

<please excuse the screwy brackets in the next paragraph... I don't know
what went wrong here... the following paragraph is taken from Tim's last
email>

  For now, suffice it to say that having > known stationary
beacons/markers would help to prevent the map from > drifting as small
sensor errors accumulate (imagine trying to follow a > map whose
directions are written in the form "walk X feet in the Y > direction" if
you always accidentally walked only 3/4 the instructed > distance when
travelling in the North-South direction -- the > accumulation of
systematic errors can be most annoying). 

	As long as things are close enough the actual situation, all will
be well. 

> If we're limited to one IR frequency (ugh!), then in the worst case
> multiple lines of communication could be multiplexed over the single
> channel using pulsed IR broadcasts at a different prime frequency for
> each secondary communication line (tricky to get quite right, I
> imagine).  In the more likely and more likeable case (still considering
> one available IR channel), the retransmitters could act as
> retransmitters in the simplest sense --- just amplify the IR signal from
> the beacon ontop of the enemy when they sense it. 
> 
> Make sense?  I know this was confusing in the past...  Ask questions. 
> :) 

	I think I understand... you are saying that we can have different
frequencies of IR pulses for each extra communication line we need. 
Direct signal amplification may confuse our robot (since it will be
possible for the robot to sense 2 IR signals in 2 directions at the same
time). 

> I don't think there's any reason it should complicate the design
> significantly at all. 

	If you say so... 
 
> By comparison, I'm not sure that there's much we can do to "just
> maximize the sensor range" of our robot.  It seems to me that the sensor
> range is a function of which receiver/transmitter IR pair you happen to
> be using, more than a function of careful robot design. 

	I think there is stuff you can tinker with... again, I have to do
more research. 

> Some amount of testing to determine the correct heights and angles for
> sensors and transmitters, (remember too that the transmitter will
> potentially be placed at a varying height depending on the enemy robot's
> constructors), but that would seem to be about all. 

	There is a 5 cm height range for the beacon placement.  We could
stick some mirrors/reflective material around the IR emitter to make the
signal "wider." 

> PS: Happen to know CBC's TV schedule for Sunday night?  :) 

	Hehe. Yup. 8 pm, right? ;)

	This has been too taxing for my holiday brain... 

Joyce :)