Re: Make your own linear scales

Hugh,

No time frame; I'm not building it. My focus is very narrow and it's
on the drives. I did look at my notebook to see what I had done.

The tube was aluminum 3' long, 1/4" ID. It sort of fit an electret
microphone I had, .2" OD. I super-glued a 40kHz transducer I had
scavenged from a Polaroid camera rangefinder to the end of the tube.

I used a function generator that resonated the thing at 40.2kHz. I
hooked up channel 1 of the scope to the function generator and
triggered off of it. The other channel went to the mike output.

I taped a 1/16" square 36" long balsawood stick to the microphone
that I used to shove the mike back and forth in the tube while
watching the really cool signal on scope channel 2 slide back and
forth in phase cycles relative to the triggered channel.

The things I noted was, The signal level on the mike was large and
didn't change much in amplitude as I moved it from one end to the
other, and the noise from the mike scraping on the inner walls of the
tube did not interfere with easy viewing of the 40kHz signal. I was
surprised that something that crude could work so well.

I then wrote up the speed of sound compensation idea along with some
signal processing ideas and just left it at that. I was designing the
G201 at the time.

Mariss

--- In CAD_CAM_EDM_DRO@y..., "netcom" <smithh@i...> wrote:
> Marris,
>
> Good idea. I like the self compensating/ calibrating aspect of the
device.
>
> 1 - How many off the shelf components could be used, or is totally
a custom
> job?
> 2 - What size tubes are we talking about, 1/4, 1/2", 2", etc?
> 3 - If I only needed .001 resolution, would that mean a 20khz
signal? Would
> it end less expensive?
> 4 - What time frame till 1st article?
> 5- What effects would dents in the tube(s) have?
> 6 - What wall thickness tubes? Thick enough and 5 is eliminated.
>
> Enough for know
> Hugh
> ----- Original Message -----
> From: "mariss92705" <mariss92705@y...>
> To: <CAD_CAM_EDM_DRO@y...>
> Sent: Friday, March 01, 2002 4:52 PM
> Subject: [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