Re: [CAD_CAM_EDM_DRO] Re: Make your own linear scales

> Hi,
>
> Here's a crazy idea I've thought about but never pursued beyond a few
> experiments to see if it is feasable:
>
> Take two long cylinders (like a double-barreled shotgun). In one
> cylinder place fixed ultrasonic transmitter (speaker) at one end,
> have a movable ultrasonic receiver (microphone) that can move the
> lenght of the tube, linked to an axis.
>
> Drive the transmitter with a 40 kHz signal; this will launch sound
> waves down the tube. The tube acts as waveguide so there is no
> attenuation of the sound down its lenght.
>
> Pick up the received signal from the microphone and compare it
> against the transmitted signal. At 40kHz, the wave-lenght is
> about .025". As you move the microphone, a phase detector (receiver
> vs. transmitter) would show a 360 degrees phase shifht for every .025"
> you move. Resolve the phase detector output to about 1.5 degrees (not
> hard to do) and you can read a 0.0001" movement.
>
> Now for the other tube. Have the speaker and microphone fixed at the
> ends a known distance apart. Do the same with the signals as for the
> first tube. This is the reference channel.
>
> The speed of sound changes with temperature, humidity and barometric
> pressure; probably other things as well. These changes will produce a
> phase shift in the reference channel. Servo the transmitter frequency
> (up or down) to keep the reference phase equal to zero (give or take
> a degree). This cancels the change in the speed of sound.
>
> The "encoder" reads 40,000 times a second. Signal averaging and
> bandwidth techinques can cancel noise very effectively.
> This "encoder" would be perfectly linear and have an accuracy equal
> to the resolution and independent of its lenght. You could also
> change its "lines per inch" by simply shifting its operating
> frequency or the phase detector resolution.
>
> So, what do you think. Crazy idea?
>
> Mariss
>
> --- In CAD_CAM_EDM_DRO@y..., beer@s... wrote:
> > > I've always thought the "spherosyn" could be "home-shop-able"
> > > (precision ball bearings in a tube, read capacitively sin/cos)
> > >
> > > Also have thought about the old "wire" tape recorders...
> > > What about recording a sine wave (signal generator created), and
> > > reading it with a std. recorder head. Then looking for a way to
> have
> > > TRUE metal tape. Iron oxide?
> >
> > I've thought about both of these as well, and every time I get
> excited
> > about it, I go out to the shop, grab something or other, and shake
> off
> > a whole pile of magnetized chips.
> >
> > Frankly, I don't understand just what it is in the cutting or
> milling
> > process that magnetizes previously non-magnetized steel.
> >
> > Anyway, the sight of all those filings clinging to things puts me
> off
> > any magnetic solution.
> >
> >
> > On the rotary front, though, I heard from guy a few years ago who
> told
> > me of his setup. I'm still not completely clear in my own mind
> that it
> > works, lacking the geometric skills to prove it one way or the
> other,
> > but the solution is more than a little intriguing.
> >
> > There are basically two problems with a pulley system. The first is
> > slippage. While this can be reduced with a enough spring tension,
> it is
> > hard to eliminate. Also, there is some chance of wear over time as
> a
> > result of that tension.
> >
> > The second is the problem of getting the pulley exactly the right
> size,
> > that pesky PI entering into the pulley diameter equation.
> >
> > This fellow's solution ( and I apologize for not giving him the
> credit
> > he is due, as I cannot find the original message ) is to stretch a
> > length of shim stock. He used .008" stock. A pulley close to the
> > right size then rides along this length of shim stock and the
> friction
> > between the two turns the pulley.
> >
> > The width of the stock gives a wide bearing area, and so no wear is
> > likely.
> >
> > The pulley is tensioned against the "tape" ( shim stock ) by a pair
> of
> > bearings to either side.
> >
> > -- crude ascii art --
> >
> >
> > ----/.\--\*/--/.\ ---
> >
> > where
> >
> > --- is the shim stock
> > . is a bearing, about 3/4" OD, 1/4" ID, and the width of the
> tape.
> > * is the pulley
> >
> > Things are aligned so that the tape rides OVER the first bearing,
> > slightly above the nominal centerline of the tape, UNDER the pulley
> > then OVER the last bearing.
> >
> > The OVER distance is slight, perhaps even zero ( I'll have to think
> > about that as I can't remember )
> >
> > The UNDER distance of the pulley is similarly small, but not zero,
> and
> > is adjustable ( as is one end of the tape, so that as the UNDER
> distance
> > is adjusted, suitable tension can be reapplied )
> >
> > The pulley is machined slightly smaller than what common sense would
> > suggest. ( On a perfectly flat tape, then, the readings would be
> high.)
> >
> > Here's the magic and the important point. As a result of the small
> > "loop" formed by the two bearings and the UNDER distance, the actual
> > distance the pulley travels is greater than the distance between
> the
> > ends of the tape.
> >
> > By adjusting the magnitude of the UNDER distance, one can zero out
> the
> > error in the pulley diameter.
> >
> > All in all, a clever solution, it seems, assuming the geometry
> actually
> > works.
> >
> > Alan
> >
> > --
> >
> > Alan Rothenbush | The Spartans do not ask the number
> of the
> > Academic Computing Services | enemy, only where they are.
> > Simon Fraser University |
> > Burnaby, B.C., Canada | Agix of
> Sparta
>
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