That is exactly my point: "IF you can measure the strength of ONLY the
return signal you can make this work." But how do you know that what
the receiver is picking up is the RETURN signal and not the
transmitter's DIRECT signal or, most likely, some combination of the
two.
The problem is that the received signal has no *signature* to identify
it as the received signal. There is not time shift, as there is in
normal echo location. There is no frequency shift as in a "chirped"
sonar. Just a steady *carrier* where the transmitted signal is perhaps
2X to 100X the reflected signal's strength. (we're talkin' off the
concrete here. Remember a reflection off an angled hard surface will
look like absorption because little or no energy will return. If you
don't believe me try it!). At best we might have to look for the
difference between a 90% signal (transmitted signal + a 5% return from
grass) and a 100% signal (transmitted signal + 50% return from flat
concrete normal to the incoming ultrasound. Typically, the signal will
be between those to value and varying all over the map.
I'm sure that there are work arounds using multiple sensors, additional
signal processing and other techniques that could make this concept work
but we're setting out to design and build a mowbot, not to develop a new
ground detector.
It's not as black and white as everyone seems to think.
Tom