Robert Nansel wrote:
>
> Some thoughts on the analog side:
>
> Use a transformer to match impedances, for sure. Also, what I've seen of
> ultrasonic transducers, especially the piezo variety, is that they are such
> high Q resonators that you don't even really need to drive them with an
> accurate frequency. A friend of mine in the Seattle Robotics Society built
> a system where just hitting the transducer with a pulse was enough to
> elicit an ultrasonic pulse (much like ringing a bell with a hammer tap).
> He used three op amps (actually, a TL082 dual op amp and an LM311
> comparator) for the receive function, if I recall correctly. It didn't use
> AGC, but had a very simple timed gain control, so that the longer you
> waited for the pulse to return (and therefor the weaker the pulse you are
> looking for), the more gain. I can dig up the schematics if anyone is
> interested.
>
> One thing that always intrigued me about Keith's design was how simple it
> was: he used just a transistor switch (TIP122, darlington configuration), a
> few resistors & caps, and a 1:5 impedance matching transformer (a TK2002, I
> think) to drive the transducer. There was a 1500 pF cap in parallel with
> the transmit transducer to make it behave like a resonant tank circuit.
>
> I remember Keith told me that he could watch the transducer ring decay on
> his scope; the decay period was long enough that it was this that limited
> the close-in range. It occurred to me that, since this design was entirely
> software driven (with some artful analog electronics where needed), that if
> you paid very careful attention to the frequency and phase of the forcing
> function that it should be possible to get both a good loud chirp _and_ a
> quick decay time. It would go something like this:
>
>         1) Bang on the transducer for N pulses, where N < 10, with a pulse
>            frequency selected to match the 40 KHz center frequency.
>
>         2) Wait an integral number of cycles (however long you want the
>            chirp to be) plus 1/2 cycle.
>
>         3) Bang on the transducer again for M pulses (M<N), but now you
>            are 180 degrees out of phase with the transducer oscillation,
>            so this has the effect of _damping_ the oscillation, thus
>            giving a cleaner and quicker pulse.
>
> Now, I don't know of anybody who's tried this, but it sure sounds worth a
> try. Thoughts?
>
> --BN
>
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Very interested in schematics!  Please send.

                Perry

pburdick@bcn.net