Tony's two cent$
 If you can predict the expected G forces in a turn of given radius at a
given speed.Can you not detect easily the absence of acceleration and
assume you're skidding.What do you do then ?
Warren F. Davis wrote:
>
> > Date:          Sat, 11 Jan 1997 19:17:03 +0100
> > Hello PICLIST
> >
> > My suggestion to solve this problem would be to use an ultrasonic
> > transducer, or preferably 2, pointed at an angle to the road, and together
> > with a pair of ultrasonic receivers utilize the Doppler-effect.
> > If it is made as a bridge, it should be possible to get a signal for even
> > very small movements.
> >
> Poul Bundgaard,
>
> If I understand your suggestion correctly, Poul, it will not work.
> At least, it will not work to the extent that the road can be
> modelled as being smooth.  The reason is that, in order to produce
> Doppler shift, the distance between the source, reflection point and
> receiver in some direction must change with time.  As I understand
> the geometry, the distance between the source, reflection point and
> receiver in all (or any) directions would be constant, even though, from the
> perspective of the source/receiver, the road would appear to be
> moving.  Specifically, the road would appear to be displaced along
> itself as the vehicle moves.  That is, if you draw a straight line to
 represent
>  the
> surface of the road, the line (road) will slide along itself; there
> will be no displacement normal to the line (road).  Hence, along any
> reflection path in a given direction from source to receiver, there
> will be no change of path length over time and, hence, no Doppler
> shift.  I am assuming that both the source and receiver are mounted
> on the vehicle.
>
> On the other hand, if the road is not smooth and there are
> projections sticking upward from the road, they will result in
> reflection path lengths along a given direction that will change with
> time as the vehicle moves and will produce Doppler shift.  Obviously,
> the "rougher" the surface the greater the effect.  How rough is
> enough?  As a rough (no pun intended) rule of thumb, the projections
> must be at least on the order of a wavelength in size before their
> Doppler shift can be "seen" reliably.  If you were considering that
> the vehicle might be skidding because it was on smooth ice, it would
> be rather difficult to satisfy this wavelength requirement.
>
> --- Warren Davis
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