LVDT construction

[rhb]

How long is the core in a commercial LVDT relative to the length of the coil?

For those not familiar  with these, they are transformers with dual secondaries and a single primary which have a movable core.  Movement of the core changes the relative voltages in the secondaries.  These are standard kit for millionths measurement and the key sensor in the Talyvel inclinonmeter.

[Conrad Hoffman]

Don't know, but if you can find the dimensions in the catalogs, you can work it out. http://www.althensensors.com/public/media/PDF_Datenblatt/2a_Wegmesstechnik/en/HR-LVDT-en.pdf

[rhb]

Thanks, Conrad.  They don't say, but there is a clue in the ones with reduced stroke.  A DIY Talyvel is looking more and more viable.

A watch with the balance wheel spring removed and a pendulum attached ought to be sufficiently low friction, albeit probably rather fragile.  Three sapphire balls and I should be set to check surface plates for flatness.

I found in some of my telescope making books a procedure for checking a flat using a small spherical mirror,  so it might be that a light film of oil on the plate or polishing with a felt tool would give enough reflection to use that method.

Not important, but I hate having references I can't check. Hence the quest for a gauge block interferometer.

[Conrad Hoffman]

Had to look up the Talyvel as I wasn't familiar with it. Probably not cheap! I was once thinking of checking my lathe bed for wear using a precision level and moving down the bed recording values. I think the math got me, but the technique certainly works. For a granite surface plate, the surface may not be as smooth as you think. A polished steel flat will move over it fine, but anything with small contact points (sapphire balls?) will probably think it's on pavement due to all the tiny pock marks.

I've got a somewhat unusual optical level from surplus. It's basically a hanging mirror, a pendulum. The mirror was polished perfectly parallel when made, checked by using it as an etalon. Then it was silvered. To zero the pendulum, it gets rotated 180 degrees (viewed with an autocollimator) and adjusted (a small moving weight) until there's no difference in angle. Since the mirror is dead parallel, we know it's now vertical. The thing is simple, a mirror with knife edges resting on flats, like a balance. The lower part is in an oil bath to provide damping.

There are probably 50 ways to do what you want, but I'd be thinking a pendulum like my mirror assembly, knife edges not watch jewels. There's a reason chemical balances have been made with knife edges forever. Near zero friction, but the edges do have to be knife sharp. LVDTs are non contact and I've used them with good success in the past. You can certainly DIY one, or buy surplus. I'd probably go for a capacitance sensor on the side of the lower part of the pendulum. Reasonably easy to do. Or, just mount a ferrite rod on the side of the pendulum, inserted halfway into a coil on the frame. Make it part of an oscillator circuit and count the output with a counter. The optical guy in me says use a laser pointer to a PSD, but I don't know if the baseline is long enough to do as well as the other schemes. The resolution of any of those systems can far exceed what you need.

[rhb]

A Talyvel is definitely pricey.  The point of using the sapphire balls is to have 3 point contact that's not prone to wear. I have in mind 5 mm balls.

As for the jewel bearings.  the point of contact between a 0.2 mm wire and a 0.34 mm hole is going to be very close the the radiused edge of an agate equilateral triangle "knife" edge.  For a test, a reamed hole in steel should suffice.

The autocollimater method was the traditional way to evaluate surface plates until lasers came along. It has the problem of requiring an autocollimater, though I've been considering build one from a rifle scope.  It also will not detect a level shift that does not tilt the mirror.

I't also possible to setup a spherical mirror and use a Foucalt test, but a mirror of sufficient size is expensive.  It needs to be 1/3 or larger relative to the surface being evaluated.

A laser pointer has the problem of having sufficient base line, but it occurs to me in writing this that a pair of mirrors which were perpendicular with the laser shining through an aperture might be sensitive enough by multiple reflection.  I picked up 10 leaded LED lasers for such projects, but haven't tried any of them yet.  It might be that a pinhole and a circuit to detect the effect of the beam being reflected into the laser would give accuracy equal to the diameter of the pin hole.  But the angular accuracy would be baseline dependent.

A differential pressure gauge (e.g. MPVX5004 from NXP)  in the 0-4 KPa range with water filled arms should be very sensitive.  With a 24 bit strain gauge ADC one would get resolution to a few microinches over a range of 1 ft.  Whether one could calibrate them to better accuracy is the big question.

As you say there must be 50 ways to do this.  I have a 10 second machinist's level.  Placed on the carriage, bed wear is very obvious when traversing the length of the bed.  In practice I adjust the lathe to give the least deviation at the ends.  For a precision job, I'd tune the bed for the job. 

I recently came across an article where a guy reground a large (~15") lathe bed using a grinder mounted so he could run along the bed and grind the high spots.  My Clausing has hardened ways, so scraping is not an option.  I plan to study that article more when I find it.

[Conrad Hoffman]

I just grabbed a 6 mm ball from my desk and ran it over a couple of our good Starrett pink granite surface plates. You can definitely feel the surface roughness of the plate. I'd probably do something with 3 pads, and lap it.

# of balances made with round bearings- few to none.
# of balances made with knife edges- most of 'em.

The real question is if your jewel bearing slides or just rolls. IMO, if it rolls it will be about as good as a knife edge. If it slides, friction will be way too high. You might also consider a flexure pivot made from thin stainless steel shim stock- the time nuts probably have a lot to say about pendulum suspensions, but I can't look. If it did, there's a good chance I'd become a time nut, along with a volt nut and nut in general.

The thing I described with a simple coil and ferrite plug is really easy. Maybe run about 1 MHz. Somewhere I've got a circuit diagram if you want. Very simple LC oscillator. If you have a frequency counter, job done. I've used the smallest LVDTs and remember the circuitry was surprisingly complex for good results. There was a single chip solution, but I don't remember it as being very good. Today I see the PGA970 that looks far more capable. I used to use the little Schaevitz bare signal conditioning boards and they were quite expensive for what they were. Still are.

Do you frequent the Practical Machinist forum?

[jmelson]

How long is the core in a commercial LVDT relative to the length of the coil?

For those not familiar  with these, they are transformers with dual secondaries and a single primary which have a movable core.  Movement of the core changes the relative voltages in the secondaries.  These are standard kit for millionths measurement and the key sensor in the Talyvel inclinonmeter.

Well, at least not in MY Talyvel.  It has two 2-wire objects placed at either end of the suspended pendulum.  There is maybe 25 thousandths of an inch gap at either end.  I'm pretty sure there is a steel piece that is inserted in the pendulum next to where the sensor is.  I assume these 2-wire objects are solenoid coils, possibly with a central core - but maybe not.

An LVDT COULD be used in a similar manner, however, to detect movement of the pendulum.

LVDTs come in ALL sizes.  Some are used in aircraft to measure movement of rudders and ailerons, and measure a travel of at least several inches.  Some are used in various instruments, and are designed to measure TINY movements.

Jon

[rhb]

The reference  I cited is:

Fundamentals of Dimensional Metrology 4th ed
Dotson, Harlow & Thompson
2003 Thompson-Delmar Learning

It's a trade school textbook, so not the greatest authority.  It does present a wide range of devices for every task, so it seems overall pretty good even if some details may not be correct.  But it's hard to find good books on metrology in my experience.  Any suggestions would be most welcome.

I'd love to know what the principle of operation of a modern Talyvel is.  From your description it sounds as if it were an inductance bridge.  I take it from your description that there are only 4 wires for the sensing.

I'd never heard of the Practical Machinist forum.  Many thanks!

[Conrad Hoffman]

I've been collecting books on all types of metrology for many years, but have near zero with anything concerning basic mechanical measurements. Most of it is related to machine tools and stability, not making the actual measurements. There's some info in a few electrical instrumentation books, some aimed at physics students, but it's way easier to find the electrical books. Stout, Farmer, Laws and Harris come to mind. There may be a few things if you Google NIST dimensional metrology. I also sent you a PM with a link.