> From:         Mike Harrison[SMTP:mike@WHITEWING.CO.UK]
> Sent:         Thursday, March 11, 2004 4:40 AM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: [PIC:] Chessboard (sensor)

> On Thu, 11 Mar 2004 02:19:57 +0100, you wrote:

>>>
>>> Permanent magnets can have significantly differing strengths due to the way
>>> they are magnetised. Hall effect sensors have significant  tempco, and of
>>> course there is the earth's magnetic field, and local magnetic fields to
>>> contend with. This would also be somewhat expensive.
>>>
>>
>>zero point adjustment is the magic word... but you're right, this is awful to
>>realize.
>>
>>> My money's still on my earlier suggestion of a simple tuned circuit in each
>>> piece, and coils etched into a PCB under the board - cheap & easy to build.
>>> Even if you couldn't figure out a way to do it in a matrix, the switching
>>> would only need nine 8-1 analogue multiplexers (ultra-cheap 4000 CMOS) ,
>>> and the detection hardware would be simple and cheap - PC generates the
>>> frequencies, and a simple detector generates a DC level for the PIC ADC to
>>> detect the presence of a piece - maybe a handful of passives and an opamp.
>>> Can't see how you could get much cheaper than this.
>>>
>>> You could even make it retro-fittable to existing pieces, as you could usea
>>> PCB coil with a cutout for the capacitor, forming a simple disc shape that
>>> can be stuck to the bottom of the piece.
>>>
>>> Proximity of the piece will not cause false detection (e.g. if piece
>>> hovers) - you are simply looking for a resonance at one of 12 frequencies.
>>> The position of the piece within the square should also not be much of a
>>> problem. The proximity of the board coil to the (pcb) coil in the base of
>>> the piece would be so close (2-3mm max) that cross-coupling to an adjacent
>>> piece should not be an issue.
>>>
>>> Even if PCB coils turn out to not give enough coupling at a suitable
>>> frequency, small surface-mount inductors could be used - still cheap, and
>>> very good coupling - I've recently used these to do an inductive datalink
>>> into a potted product.
>>>
>>
.I've got another Idea... Resonant  frequencies aren't that perfect (lot of
>>tuning and choosing the right frequency for each pice without constructing
>>overcutting resonants).
>>But we can still go forth with coils. We can put a resonant coil (only one
>>frequency for all) in each figure in parallel to a load resistor.
>>If we choose different load resistors, then each figure would draw a different
>>amount of power from the coils under the chessboard, this could be
>>measured...  You have to admit, that this is a little bit easier than using
>>multiple resonant frequencies, that all want to be scanned on each square of
>>the board.

> No it isn't.
> We are back to all the possible variations of signal strength giving false results, in particular
> distance from piece to coil.
> With different resonant frequencies, we only have to detect presence or absence of loading at a
> given frequency for each square, which should give a much more reliable and definite result.

Why make it so complicated? You don't need mutiple driving frequencies. Just excite the coil
under each square with a step function and look for the ringing frequency. Some amplification
boosts the signal to the amplitude that can be detected with an analog comparator and
timer/capture module.

John Power

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads