Micron-level accuracy over a meter?

[RJSakowski]

My micrometers travel .025" or 635 microns per revolution. Making sub micron adjustments would be touchy. One micron of travel would require a rotation of 0.5º. Not to say a micrometer couldn't be used. We did it that way 50 years ago.

Not to mention that a micrometer would need to be cannibalized for the mechanism. A stepper driven screw would be inexpensive. Stepper motors from computer hard drives are readily available at no cost. Simple step and direction control is easily accomplished. Microstepping controllers are available for a few bucks from eBay or Banggood. You still need a a lead screw and nut of some sort but the pitch wouldn't have to be as fine with a microstepper drive.

 

[homebrewed]

An interesting discussion, but any approach that uses mechanical components will be subject to thermal effects that will be difficult to nail down.

 

[RJSakowski]

An interesting discussion, but any approach that uses mechanical components will be subject to thermal effects that will be difficult to nail down.

Since absolute position would be made via the glass or magnetic scale, mechanical components wouldn't come into play. Of consideration would be the thermal expansion coefficients of the bar mounting the knife edges and the glass or magnetic scale.

A lot depends upon the particular application.

 

[zondar]

A cheap donor micrometer can be had for about $25. A micrometer would not be for measuring, just positioning, but I take your point about how difficult it might be to operate manually.

I priced out a 1200 mm lead-screw, etc., for stepper drive. Not too bad, but it's a rather rapid screw that would require fairly high micro-stepping, so I might have to look for a tighter one. I happen to have an unused stepper and driver at present.

The cost of a 1+ meter unsegmented "1um" glass scale is very high - about $400 delivered from a reputable source (dropros), and that's for a bare scale. If anyone has a positive experience with a less expensive source, please let me know.

If it's accepted that micrometer precision isn't going to be achieved anyway, it's possible that the "5um" scales will do - their accuracy over a meter is quoted as almost the same as the 1um scales. But that wouldn't feel right!

I also priced an Invar rod. Yikes, that stuff is expensive. About $100 for a 3 foot 3/8" rod from McMaster-Carr. I happen to have some 4 ft. carbon fiber tubes, which are purported to have a low coefficient of expansion, but it's not documented, and I'd be pretty certain it's worse than Invar.

I'm sure that mechanical issues will be important. Backlash or other movement (and there's always some) between the microscope and the scale could easily be in the 10's of micrometers range. I mentioned that Invar is actually less prone to temperature effects than the glass scale would be. So how the scale is mounted, even, could be important. For example, perhaps it should be mounted in its center, and only loosely held at the ends.

To do this right means having an error budget: so much for the scale, so much for this and for that. I think in the end getting 10 micrometer accuracy over a meter by mechanical means would be very, very difficult.

It's an interesting project. If anyone can guess what it's ultimately for, have at it.

 

[RJSakowski]

A cheap donor micrometer can be had for about $25. A micrometer would not be for measuring, just positioning, but I take your point about how difficult it might be to operate manually.

I priced out a 1200 mm lead-screw, etc., for stepper drive. Not too bad, but it's a rather rapid screw that would require fairly high micro-stepping, so I might have to look for a tighter one. I happen to have an unused stepper and driver at present.

The cost of a 1+ meter unsegmented "1um" glass scale is very high - about $400 delivered from a reputable source (dropros), and that's for a bare scale. If anyone has a positive experience with a less expensive source, please let me know.

If it's accepted that micrometer precision isn't going to be achieved anyway, it's possible that the "5um" scales will do - their accuracy over a meter is quoted as almost the same as the 1um scales. But that wouldn't feel right!

I also priced an Invar rod. Yikes, that stuff is expensive. About $100 for a 3 foot 3/8" rod from McMaster-Carr. I happen to have some 4 ft. carbon fiber tubes, which are purported to have a low coefficient of expansion, but it's not documented, and I'd be pretty certain it's worse than Invar.

I'm sure that mechanical issues will be important. Backlash or other movement (and there's always some) between the microscope and the scale could easily be in the 10's of micrometers range. I mentioned that Invar is actually less prone to temperature effects than the glass scale would be. So how the scale is mounted, even, could be important. For example, perhaps it should be mounted in its center, and only loosely held at the ends.

To do this right means having an error budget: so much for the scale, so much for this and for that. I think in the end getting 10 micrometer accuracy over a meter by mechanical means would be very, very difficult.

It's an interesting project. If anyone can guess what it's ultimately for, have at it.

You won't need a meter long lead screw, only a short one on the sub stage. It only need be long enough for your precision adjustment. Move the stage to an approximate position an lock it. Then make final adjustments with the sub stage.

The microscope would have to be rigidly mounted to the stage. Backlash isn't important since you would be measuring position with the DRO. That's the beauty of a DRO. You would probably want to calibrate the scale. Borrowing some gage blocks would be helpful. I would want to calibrate as close to the final length as possible to eliminate any extrapolation errors.

 

[homebrewed]

A cheap donor micrometer can be had for about $25. A micrometer would not be for measuring, just positioning, but I take your point about how difficult it might be to operate manually.

I priced out a 1200 mm lead-screw, etc., for stepper drive. Not too bad, but it's a rather rapid screw that would require fairly high micro-stepping, so I might have to look for a tighter one. I happen to have an unused stepper and driver at present.

The cost of a 1+ meter unsegmented "1um" glass scale is very high - about $400 delivered from a reputable source (dropros), and that's for a bare scale. If anyone has a positive experience with a less expensive source, please let me know.

If it's accepted that micrometer precision isn't going to be achieved anyway, it's possible that the "5um" scales will do - their accuracy over a meter is quoted as almost the same as the 1um scales. But that wouldn't feel right!

I also priced an Invar rod. Yikes, that stuff is expensive. About $100 for a 3 foot 3/8" rod from McMaster-Carr. I happen to have some 4 ft. carbon fiber tubes, which are purported to have a low coefficient of expansion, but it's not documented, and I'd be pretty certain it's worse than Invar.

I'm sure that mechanical issues will be important. Backlash or other movement (and there's always some) between the microscope and the scale could easily be in the 10's of micrometers range. I mentioned that Invar is actually less prone to temperature effects than the glass scale would be. So how the scale is mounted, even, could be important. For example, perhaps it should be mounted in its center, and only loosely held at the ends.

To do this right means having an error budget: so much for the scale, so much for this and for that. I think in the end getting 10 micrometer accuracy over a meter by mechanical means would be very, very difficult.

It's an interesting project. If anyone can guess what it's ultimately for, have at it.

In your OP you said

The distance between the knife edges is to be measured.

Therefore I assume that the invar/carbon rod is used as a reference for something the knife edges are mounted on. You're not looking at fast changes if a DRO solution is acceptable, so it could be something like creep -- ground movement -- or slow changes in length due to applied stress, aging, curing etc. I don't think you are trying to detect gravity waves .

It appears that you want an accurate measurement in terms of PPM so you want to know the distance between the knife edges, along with the relatively small displacement of the material the knife edges are attached to. You haven't mentioned any kind of automated measurement scheme, nor discounted the use of a microscope, so perhaps you want to measure an effect due to an applied stress (internal or external) of some kind, over a relatively short range of time.

I still think that environmental effects could well swamp out the variation you are trying to measure, unless the time frame is short enough that your environment changes less over time than the stress you're applying or creating.

A microwave interferometer setup might work. 300MHz in free space has a wavelength of 1 meter. A relatively simple transmitter/receiver setup using the same master oscillator and a high-resolution (24-bit) DVM would provide enough resolution. The output of the receiver would be proportional to the phase shift between the transmitter and receiver, which depends on the distance between them. Since the wavelength is the same as your measurement baseline there wouldn't be any ambiguity in the position. To avoid multipath effects you'd want the T/R antennae to be highly directional.

In this context you wouldn't need an invar rod, sorry about that The speed of light is the equivalent to your invar rod....

 

[zondar]

You won't need a meter long lead screw, only a short one on the sub stage. It only need be long enough for your precision adjustment. Move the stage to an approximate position an lock it. Then make final adjustments with the sub stage.

The microscope would have to be rigidly mounted to the stage. Backlash isn't important since you would be measuring position with the DRO. That's the beauty of a DRO. You would probably want to calibrate the scale. Borrowing some gage blocks would be helpful. I would want to calibrate as close to the final length as possible to eliminate any extrapolation errors.

The "meter long" lead-screw, if used, would just be to move the stage over the full length at a steady pace via a stepper rather than by hand, out of fear of missing steps in the scale. But as you say a small lead screw could be used as long as it can be slid into position, hence my comments about maybe just using a micrometer for those last few mm.

By "backlash" (probably not the most appropriate term), I meant any potential movement in the mechanism between the microscope and the scale, like if a force is applied during use. Some is inevitable no matter how rigid (and I don't expect to be able to put in the time and expense to achieve lathe-like rigidity), and almost any would be in the micrometer range.

I can't calibrate the scale without some other scale that is also about a meter long. Ringing together a whole bunch of gauge blocks would probably accumulate small errors, but it's a thought. I don't know what other physical reference could be used. NIST probably has an actual, physical, meter-long meter in a vault, if only for historical reasons, but I doubt they will let me borrow it. ;-) Calibration of a scale is presumably done at the factory via optical means (e.g. laser interferometry), or at least one hopes, but with cheap scales, probably not?

 

[zondar]

I don't think you are trying to detect gravity waves .

It appears that you want an accurate measurement in terms of PPM so you want to know the distance between the knife edges, along with the relatively small displacement of the material the knife edges are attached to. You haven't mentioned any kind of automated measurement scheme, nor discounted the use of a microscope, so perhaps you want to measure an effect due to an applied stress (internal or external) of some kind, over a relatively short range of time.

I still think that environmental effects could well swamp out the variation you are trying to measure, unless the time frame is short enough that your environment changes less over time than the stress you're applying or creating.

A microwave interferometer setup might work. 300MHz in free space has a wavelength of 1 meter. A relatively simple transmitter/receiver setup using the same master oscillator and a high-resolution (24-bit) DVM would provide enough resolution. The output of the receiver would be proportional to the phase shift between the transmitter and receiver, which depends on the distance between them. Since the wavelength is the same as your measurement baseline there wouldn't be any ambiguity in the position. To avoid multipath effects you'd want the T/R antennae to be highly directional.

In this context you wouldn't need an invar rod, sorry about that The speed of light is the equivalent to your invar rod....

The Invar rod would be to hold and space-apart two knife edges at each end.

As you say, interferometry is the real solution, but I don't have the equipment for that.

The reason I can't borrow someone's huge DRO'd lathe or mill table for a one-time measurement is exactly because of the environmental issues. I'd preferably be able to measure it in-situ at current temperatures, etc.

But as I mused above, the scales are almost certainly more sensitive to temperature than a good Invar rod is. So now I'm thinking that in-situ measurements might not be as useful as I thought. Maybe borrowing the use of someone's big DRO'ed lathe or mill, with a microscope mounted to it, is actually the most all-around practical solution after all.

And yes, I want to measure gravity waves. Well not really, lol, but that would be super cool!

 

[rwm]

Gravity is a particle.

 

[zondar]

Gravity is a particle.

Particles are actually perturbations in fields.

Or maybe the resolution of probabilities in fields once observed. Something like that!