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'Ideas for Pic-based Motion detector?'
1999\01\06@185508 by adastra

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Greetings, PICMASTERS:

I need to devise a system which will detect a small movement between two
adjacent wires.  The wires would be about 30 feet long and spaced
(non-uniformly) about 1 inch apart. They are normally stationary (fixed to
moveable objects), and I need to detect if they are suddenly moved either
closer together or farther apart, approximately plus-or-minus 1/4 inch, as a
result of an impact on one of the objects.  (For various reasons, simple
contact switching will not work in this application.)

I am thinking of a system where an RF signal(say,100 khz)is applied to one
wire and picked up by the other. The gain of the return signal could be
automatically set to establish an accurate baseline level, and an output
signal generated if the level changed suddenly.

Perhaps the PIC could generate the RF signal, modulate it, and synchronously
detect the return.  (Maybe the synchronous detection is overkill?) I also
need the PIC to generate some specific digital output data when the movement
is detected, but that is less of a mystery to me than the transmit-receive
scheme I am contemplating, as outlined above.

Any comments or links pertaining to this (or any alternative)approach would
be much appreciated.

(I guess this is essentially an analog problem, and I apologize if it is too
far off topic.)

Thanks, Foster

1999\01\06@192914 by Scott Dattalo

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On Wed, 6 Jan 1999, adastra wrote:

> Greetings, PICMASTERS:
>
> I need to devise a system which will detect a small movement between two
> adjacent wires.  The wires would be about 30 feet long and spaced
> (non-uniformly) about 1 inch apart. They are normally stationary (fixed to
> moveable objects), and I need to detect if they are suddenly moved either
> closer together or farther apart, approximately plus-or-minus 1/4 inch, as a
> result of an impact on one of the objects.  (For various reasons, simple
> contact switching will not work in this application.)
>
> I am thinking of a system where an RF signal(say,100 khz)is applied to one
> wire and picked up by the other. The gain of the return signal could be
> automatically set to establish an accurate baseline level, and an output
> signal generated if the level changed suddenly.

The 'theoretical' capacitance of this geometry is:

C /meter = pi * epsilon / arccosh(s/d)

where

pi = 3.1415...
epsilon = 8.854e-12 F/m (unless there's an epsilon sub r too)
arccosh = inverse hyperbolic cosine
s = wire separation (1")
d = wire diameter

If you assume that the wire diameter is 20 mils (0.020 inches) then 30
feet (approximately 10 meters) has about 60 pF of capacitance (if I did
the arithmetic correctly). If the distance of separation is decreased by a
quarter of an inch, the capacitance increases to about 64 pF - or an
increase of about 6%. Similarly, if the distance is increased by a quarter
of an inch the capacitance will decrease to 56pF. There are numerous ways
to measure capacitance differences this small (for example, the
capacitance could be made to be part of an LC resonator - the pic could be
made to monitor the frequency...).

However, if the wires are deformed such that only a small section is
affected then the capacitance change would be much smaller.

Scot

1999\01\06@202656 by Michael J. Ghormley

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Scott Dattalo wrote:

<SNIP all kinds of good capacitance stuff & good advice>

Since the L/C is going to be prone to drift, the PIC could save the last sample
and
look for too big a jump (up or down) as opposed to a hard-coded setpoint.  Thus
temperature changes, plate tectonics <VBG>, etc. would have little effect on the
detection of a bump.

Just my two pence...

Michael

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1999\01\06@214203 by Dwayne Reid

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>I need to devise a system which will detect a small movement between two
>adjacent wires.  The wires would be about 30 feet long and spaced
>(non-uniformly) about 1 inch apart. They are normally stationary (fixed to
>moveable objects), and I need to detect if they are suddenly moved either
>closer together or farther apart, approximately plus-or-minus 1/4 inch, as a
>result of an impact on one of the objects.  (For various reasons, simple
>contact switching will not work in this application.)

Just a silly thought - place a large DC potential between the wires through
a largish resistor and see if you see a small spike when the wires are
impacted.  Note this will respond only to sudden changes in spacing.  I
don't have any idea what kind of sensitivity you can expect, but it may be
worth a couple of hours of experimenting to find out.

dwayne




Dwayne Reid   <spam_OUTdwaynerTakeThisOuTspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(403) 489-3199 voice     (403) 487-6397 fax

1999\01\07@063502 by paulb

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Scott Dattalo wrote:

> There are numerous ways to measure capacitance differences this small
> (for example, the capacitance could be made to be part of an LC
> resonator - the pic could be made to monitor the frequency...).

 Hmm, but that's lots of extra parts!  I would have thought along the
lines of using the "standard" PIC port capacitance measurement circuit
and a megohm or two.  RC for 1M and 60pF is about 60µs, is it not?

 The nice thing about a PIC is the ability to implement an "auto-zero"
of any time constant you like.
--
 Cheers,
       Paul B.

1999\01\07@074744 by Harrison Cooper

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What about using a strain guage on each wire.

1999\01\07@075407 by James R. Hall

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You prolly couldn't do that with just a straight wire but if you make it a
coil,
or something with inductance then you could prolly measure the change in
inductance
when one of the objects move (thus streching or compressing the coil)..
or you could take and put some kind of ferrious material in one object and a
metal
detector in the other :P then just use the pic to measure and act on the
doppler
freq. change.
couple of analog ideas...

    -James, N9XLC
    .....-jrhallKILLspamspam@spam@globalsite.net
    -certified NERD
{Original Message removed}

1999\01\07@082206 by Thomas McGahee

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Foster,
Be aware that unless the two wires are both enclosed in some sort of
Faraday cage (such as a simple grounded metal enclosure) you will
have a nice 30 foot antenna that is going to pick up all kinds of
local RF. If you want to go with a variable capacitance type circuit
I would change from using wires to using two thin foil strips
instead. That will allow a MUCH greater capacitance value as your
starting value, and will make the circuit less prone to errors due
to the proximity of other objects.

Since you are not interested in absolute position information, but
only want to detect rapid CHANGES, you might want to investigate
using a circuit similar to the following:

  O +Vcc
  |
  |
  \         V Vref (adjustable)
  /         |
  \         |    |\
  /         -----| \ comparator
  |              |  >---->  Output pulse
  *--------*-----| /
  |        |     |/
  \       ---
  /       --- 30 foot foil capacitor
  \        |
  /        |
  |        |
  |        |
 ---      ---
  -        -
The comparator must be one with a very high
input impedance and capable of operating with
very small input currents... on the order of
picoamps. Most FET type op amps will do.

The two resistors should be high values, such as
22 Megohms. They set a midpoint voltage of Vcc/2.
Normally the capacitor is just charged to that
level. But if the plates of the capacitor are
moved rapidly, then the voltage at the junction
will change. If the comparator has been properly
adjusted then a pulse will appear at it's output
indicating the change was detected.

If the capacitance is too small to begin with, then
the two resistors will have to be made even larger.
Instead of a comparator you could connect a
buffer op amp to the capacitor and look at the
output on a scope (use AC input selection to
remove the DC bias). Turn the gain up and you
should see a "noise" that represents the system
vibration or movement.

Hope this helps.
Fr. Tom McGahee
Electronics Department
Don Bosco Technical High School


----------
{Quote hidden}

1999\01\07@084516 by Tjaart van der Walt

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Thomas McGahee wrote:

{Quote hidden}

You can also explore using it in a self calibrating setup :

   O +Vcc
   |
   |
   \                  
   / 10k              
   \             R         |\
   /       ---\/\/\/\--*---| \ comparator
   |       |           |   |  >---->  Output pulse
   *-------*-----------(---| /
   |                   |   |/
   \                  ---
   /  10k             --- 30 foot foil capacitor
   \                   |
   /                   |
   |                   |
   |                   |
  ---                 ---
   -                   -

If you choose R slightly larger than the equivalent
leak resistance of the foil cap, the circuit won't
need any further adjustment.

R=2M2 would probably do it.

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1999\01\07@100132 by Gerry Cox

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The comparator input should have a series resistor (1 meg or so) to protect
it from transients picked up by the foil or wires.



{Original Message removed}

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