Siemens make magnetic sensors, KMZ10? that sense a car through a wall,
maybe this could help.

George

----- Original Message -----
From: "Russell McMahon" <apptech@PARADISE.NET.NZ>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Tuesday, December 16, 2003 3:41 AM
Subject: Re: [OT]: Farm Gate/Grid/Road Monitoring, 6km range, no LOS


> > Grid 3  3km away        Site 1, great   Site 2, lousy
> > Site 1 to Site 2, 4km away, okay.
> >
> > A future expansion is to add "oncoming traffic in section"
signalling,
> > because the road is windy and narrow.
>
> Several optical beams (say 3) in closish proximity at lowish level
would
> allow you to monitor period of occlusion and time between sensors.
While the
> odd roo travelling at speed may trigger 2 sensors its unlikely to hit
all 3.
> (Low-high-low-high .... :-) ).A vehicle will occlude all sensors, and
each
> sensor for a significant period compared to a roo at the same speed.
Speed
> can be deduced both by time of occlusion and the time between sensors.
With
> even two sensors the two speed estimates can be compared and with 3
the
> chances of not being able to distinguish sheep/cow/roo/car is minimal.
> Without too much effort you can get both vehicle speed and length and
MAY be
> able to make some useful guesses about type eg car versus cattle
truck. Even
> sheep versus roo!
>
> dS1_S2  / (tS2on - tS1on )= velocity
>
> Velocity x (tS1off - tS1on) = vehicle length.
>
> ________
>
> Radio propagation is not too important as long as SOME messages get
through.
> If you have 2 way coms you can repeat until you get an ACK. At 300
baud (or
> even 1200) you don't need a very good channel to get good enough
quality for
> async data.
>
> _______________
>
> Inductive loops can be let into a thin deep slit well below road
surface -
> virtually just a saw cut. (Maybe a dead chain saw blade deep :-) .  I
would
> imagine that they could be made bulletproof enough for this
application.
>
> Side of road capacitive may also be viable.
>
> Laser pointer should give good range with minimal development effort.
>
>
>         Russell McMahon
>
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