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

Hi Mariss;

It's called dopler shift, and it's what makes the whistle/air horn on a
locomotive sound like it's raising or lowering in frequency depending on whether
the train is coming or going, when it actually stays the same.

A piezo crystal would probably be best for your, "transmitter", but I'd
recommend a slightly different setup with respect to the tube. Mount the xmiter
wherever convienient, and mount the mike in the far end of the tube, aiming it
at the xmiter. Make the tube length such that the tube is resonent at the
frequency of interest, sorta like a single tube taken from a multi-tube shotgun
mike.

Bill

On Fri Mar 1 16:52:36 2002, mariss92705, <mariss92705@...> wrote:

>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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