DIYbanter - View Single Post - Precision Electronic Levels

Joseph Gwinn

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On Apr 6, 4:29 am, Joseph Gwinn wrote:
In article
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On Apr 5, 6:39 pm, Joseph Gwinn wrote:
In article ,

Wes wrote:
Joseph Gwinn wrote:

As for manufacture by a HSM, either approach is practical, but the
Wyler
approach is far simpler mechanically. In either case, the
electronics
part is dead simple by present-day standards, but was a big deal
back in
the 1960s and 1970s.

Are you planning to try it? Btw, thanks for the list of patents.

I am thinking about making a level. Don't know if I'll really do it.
Probably depends on how practical an approach I can dream up.

The big issue to be figured out is how to handle temperature
variations.

The original Wyler unit (described in 4,023,413) may be made of fused
quartz, which would be easy for a big company like Wyler to do, but a
problem for me. I know how, but it would be far too much work. The
patent mentions quartz as a suitable material. So, the question is if
quartz is really necessary. Symmetry may suffice.

The subsequent patent (5,022,264) mentions that the diaphram mass is
made of 0.003" brass foil, which is easy to get and to photoetch. One
can also use stainless steel, but it must be non-magnetic so the
Earth's
magnetic field doesn't cause false tilt readings.

Joe Gwinn

PS. The German book came, and I was able to read it well enough to see
that it wasn't all that useful. Most or all circuit diagrams in the
book were clearly wrong (in the sense of being impossible), so I assume
that the authors had no idea how these things actually work. JMG

fused quartz is a good choice. The quartz you can get from burnt out
halogen bulbs. Procedures in Experimental Physics by Strong has info
on fused quartz. In another life I made coil springs out of fused
quartz.

I have read Strong's book, which is quite good.

One can buy fused quartz from glass suppliers, and work it like glass.
It is possible for an amateur to make lenses and mirrors, and many
people made their own telescopes. Willmann-Bell publishes books on how
to do this. I can see how to wet grind the fused quartz to make the
spherical depressions in the flat plates of the sensor assembly, and to
drill the holes for the connections to the electrodes. However, vacuumn
depositing the electrodes would have to be sent out. Wet silvering
could be used, but that's another whole process.

For the record, the usual way to grind the depressions would be to use a
machined cast iron tool charged with carborundum grit (or diamond grit)
held in a machiine the moves the tool such that its surface describes a
sphere. Mor modern would be a diamond cup curve-generation tool in a
similar machine.

But I don't have the equipment to handle this, and don't want to get it
for a one-off project. So, I'm thinking of alternatives like two glass
sheets, two brass electrodes, a brass shim diaphram, and a pair of
machined ceramic spacers, assembled with epoxy. Or, two circuit boards
with metal faces inward, two brass spacers, and a brass sheet, assembled
with epoxy. And so on. The temperature and humidity coefficients of
the dielectrics are the big issues, as metals are far simpler.

Joe Gwinn

Hi Joe,
forgive my ignorant rantings here, but I see there would be a much
more simple way of doing this....
If you don't want to worry about absolute accuracy, ie when leveling a
lathe bed where you are only concerned with differences, then what
about...

Use the capacitance method, ie to each side of centre - a bit of
double sided circuit board would suffice.

I have been considering circuit board for the outer electrodes, two
glass spacers with the brass shim stock diaphragm between, all held
together with epoxy. (In Europe, the standard would be Araldite.)

Wyler apparently uses fused quartz and invar, but the patent does cover
a design with a stack of sheets glued with epoxy.

The glass sheets can easily be drilled on a drill press with abrasives,
so it will be easy to make the 1" diameter hole for the diaphragm.

Suspension for the pendulum
could be out of one of those $1 battery clocks in the junk shops -

That won't really work. The key innovation in the patent is the use of
the perforated diaphragm as the proof mass. The diaphragm is made by
photoetching, which isn't difficult.

use 2 capacitance meters, one each side, set balance. (or use a
comparator, depends on how fancy you want to go) You could do this
every time you use it so drift/temp. compensation would not be a
problem.

The design in the Wyler patent (4,023,413) uses a differential
capacitor, which is optimum, and is far easier than trying to measure
the two capacitances independently. The classic capacitance-bridge
circuit uses a centertapped transformer to generate the drives to the
outer plates. The moving center plate will have zero voltage when that
plate is centered. The balanced configuration causes the non-varying
part of the capacitance to cancel out, allowing the varying part to come
out loud and clear.

I have built such circuits before, and they are very stable and
well-behaved. The transformer is easily built using a ferrite cup core,
which can also be part of the drive oscillator. The exact frequency is
not important.

The subsequent digital sensor from Wyler does measure the two
capacitances independently, and uses their ratio. There is a patent,
but I don't have the number at hand.

So, set to balance/zero/whatever figures it shows at one end of the
lathe, this will establish a datum point, move to the other end, note
difference (in picofarads, whatever) - then use this as a measure of
how far out it is, and which way to go in correcting.

Initial balance of the sensor would be mechanical, being a set of
leveling screws. Given the difficulty of coming up with a reference
surface that's exactly horizontal (to less than 0.1 arc seconds) in a
home shop, the approach will be to swap ends and adjust for equal
deviations from zero. Ultimately, one can use the sensor to adjust the
surface plate to exact horizontal and the sensor to exact adjustment, in
a converging cyclic process.

Resolution - no idea. Probably someone smarter than me could do it all
in a PIC micro or similar.

Transformer-driven capacitance bridges with synchronous detectors are
*very* good, and the PIC can do no better. I was imagining a purely
analog approach.

Now, I suspect (sigh...) you could drive a truck through my approach,
but would appreciate constructive comments...

Andrew VK3BFA.

PS - will be away from the PC for at least a week (minor surgery) so
will be looking forward to your reply.

Good luck with the surgery.

Joe Gwinn