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Robot ``Easter Egg Hunt'' discussion digest

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Date: 26 Aug 1999
DJ: Darrell Johnson
BH: Ben Hitchcock
KH: Ken Huntington
SB: Steven Bolt
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IR black/white floor detection

If no objections are received before September 20, this proposal will be copied to the Contest definition:

SB> Gathering area: matt black surface. Paint or paper ensuring 
SB> maximum contrast with the matt white Playground floor, to 
SB> normal and IR-sensitive eyes.
SB> Always this size (radius 20cm) and in a corner. The border of
SB> this area is also painted matt black.
SB> For a contest, the gathering corner will thus be required to show
SB> contrast in IR light, as checked using Ken's method:

KH> A generic IR photo-transistor was used with a 22K collector load;
KH> the IR source, a 60W incandescent bulb about 50cm above the
KH> surface(s) to be tested. Using a 5V power supply, with the
KH> photo-transistor masked to permit light to enter only through the
KH> lens of the device, point it at the surface at about 60deg at 1
KH> to 2cm above it. The collector voltage should read 3V or better
KH> while pointed at a low reflective surface and 0.2V or less with a
KH> high reflective surface and about 4.5V with the light turned off.
KH> It seems pretty rinky dinky but works well.

The procedure is indeed not critical, and neither are the materials:

SB> I tried it using the detector-half of a CNY37 IR optocoupler
SB> (open type, intended to see the passing of a vane between emitter
SB> and detector), with a 22K bias. The results obtained fit Ken's
SB> values; my low and high reflective surfaces were ordinary black and
SB> white paper.

KH> I have a feeling that two coats of any matt black paint would do
KH> the job quite nicely.

SB> So do I, and at least some black papers appear to be equally
SB> acceptable. 

So why bother bringing up this point? Well, An IR emitter and detector in the middle of a Hunter's underside allows very simple circuitry (like just two resistors) to give a clean one/zero output, so the `bot has no difficulty knowing whether it's on the white Playground floor or in the black gathering area. I originally thought that all matt whites and blacks would be just about equally visible to normal and infrared `eyes', but Ken discovered that aint so:

KH> I may have run into a problem concerning the definition of the
KH> gathering area

KH> Gathering area: matt black surface (piece of black paper). Always
KH> this size (radius 20cm) and in a corner. The border of this area
KH> is also matt black.

KH> At first glance, it all looks good but I had a problem when
KH> building the commutators for Ameba. I tried to use a pattern that
KH> I printed. It looked beautiful but the black ink is invisible to
KH> the Infrared TXRX assembly and I ended up having to paint my
KH> wheels matt black using modeling spray paint and applying the
KH> pattern by hand. My problem and possibly yours is that I use the
KH> same type of IR TXRX device to detect the gathering area and
KH> while my playground will be no problem as I will be using the
KH> same paint, there is no guarantee that the same will be true for
KH> any other playground.

KH> Although I could replace my current circuit with another using a
KH> red LED and photo detector, the IR circuits are most common, come
KH> in 99 flavours and are dirt cheap. (especially if you can find
KH> them your bit bucket :)

SB> Your arguments in favour of facilitating IR black/white floor
SB> detection are fair enough, imho. Just one question:
SB> Black paper doesn't equal black ink. Are we sure that an IR
SB> optocoupler has difficulty seeing the difference between matt white
SB> wood and ordinary matt black paper?
SB> I admit that I didn't actually try to confirm that - should have,
SB> sorry. (Jaws is supposed to use a simple form of distance
SB> measurement relative to the IR beacon, to determine whether he's in
SB> the corner.)

KH> I've now done some investigation and found:

KH> Canon bubble jet printer              invisible to IR.
KH> Black construction paper              invisible to IR.
KH> Laser printer toner                   visible to IR.
KH> Data books printing                   visible to IR.
KH> Model paint                           visible to IR.

KH> It's not a big list and it may not be true for every laser
KH> printer, model paint or construction paper.

SB> To make this a practical part of the standard Playground, we need
SB> to specify either the black surface treatment, or a standard test.

At this point, Ken came up with his test involving a generic IR photo-transistor. As determining IR contrast is easy enough, a contest organizer will be required to `qualify' the matt black of the contest Playground in this way, if doubts arise - and assuming no one objects to this demand before September 20. Ben commented:

BH> I am planning to use the visible spectrum to determine where my
BH> bot is. A simple LDR should do the job nicely, I think. I have
BH> no objections to making the corner dark to IR, so long as it is
BH> still reasonably black to human viewers.

Which it will be; as black as matt black can be :)

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