Thread: Calibrating a Surface Plate

Originally Posted by RayG

That agrees with Josh's 2.5 uinch calculation, and no you can't scale from a larger length, ( that's what I did using 8" per mile, gives you crazy numbers.. )

Anyway, the reason for the question, is related to a little side project I'm working on when I have time... the brief concept is that MEMS sensors. ( microscopic silicon structures) have got differential inclinometers down to around the 0.001 degree level, ( 3.6 arc-seconds) which works out to 0.02 mm/meter order of sensitivity, these sensors cost around the $50-$100, but with oversampling and a bit of fancy signal processing footwork, I think I can get under the 1 arc-second sensitivity... so a differential level setup which is sensitive enough to calibrate surface plates could be an affordable project...

The chip I've been looking at is this one SCA103T Inclinometers | Murata Electronics Oy
data sheet is here http://www.muratamems.fi/sites/defau...261700a3_0.pdf

Regards
Ray

When you next visit take my Taylor Hobson Talyvel and reverse engineer it. Or modernise it

Greg

Originally Posted by RayG

That agrees with Josh's 2.5 uinch calculation, and no you can't scale from a larger length, ( that's what I did using 8" per mile, gives you crazy numbers.. )

Anyway, the reason for the question, is related to a little side project I'm working on when I have time... the brief concept is that MEMS sensors. ( microscopic silicon structures) have got differential inclinometers down to around the 0.001 degree level, ( 3.6 arc-seconds) which works out to 0.02 mm/meter order of sensitivity, these sensors cost around the $50-$100, but with oversampling and a bit of fancy signal processing footwork, I think I can get under the 1 arc-second sensitivity... so a differential level setup which is sensitive enough to calibrate surface plates could be an affordable project...

The chip I've been looking at is this one SCA103T Inclinometers | Murata Electronics Oy
data sheet is here http://www.muratamems.fi/sites/defau...261700a3_0.pdf

Regards
Ray

Interesting idea Ray. I'd been chewing over the idea of using a conventional laser on target mirror reflecting back to closely spaced receiver diodes in a differential configuration to measure angular displacement of the mirror. I doubt it would ever get built however, and I'll stick with optical, but I would think the principle at least would be sound and relatively easy to build. In practice possibly little more than novelty value, as unlike conventional autocollimation, the error would be dependent on distance.

Pete

Originally Posted by Pete F

Interesting idea Ray. I'd been chewing over the idea of using a conventional laser on target mirror reflecting back to closely spaced receiver diodes in a differential configuration to measure angular displacement of the mirror. I doubt it would ever get built however, and I'll stick with optical, but I would think the principle at least would be sound and relatively easy to build. In practice possibly little more than novelty value, as unlike conventional autocollimation, the error would be dependent on distance.

Pete

Nice concept, one problem that would need to be overcome with that idea, is that the beam size is pretty large. And you'd be looking for tiny movements, (depending on how far away you are).. of a largish laser dot, instead of discrete diodes, you could perhaps use a line scan camera sensor, they are available as ccd or photodiode array. I used line scan sensors (4096 pixels if I remember correctly ) in a product design many moons ago. Nice and easy to drive, and you just clock out the pixels and digitize as you go. Then you need an algorithm to accurately determine the center of the reflected beam.

Regards
Ray

PS IT-P1-4096 Linear CCD - Product Detail - Teledyne DALSA

Yeah I figured by setting up the receivers in a differential arrangement, you wouldn't need to hit just one of the receivers. As far as I'm aware, the beam isn't perfect and there is a fringe around the outside of it if that makes sense. By setting up a group of say 4 receivers, you'd just compare the change as one got "more" of the laser on it compared to the one next to it. That way the size of the beam isn't really a limiting factor as it's relatively huge diameter in terms of the measurements being discussed here. You could possibly do a very similar thing with collimated visible light, but that involves lenses etc, and I figured laser would be easier.

A more simplistic example would be with just 2 receivers mounted vertically. As the mirror is tilted back the lower receiver would receive less of the beam and the upper receiver relatively more. You'd just differentially compare the two. A linear CCD like you posted would be perfect as you could use the same principle but adjacent pixels. Exchange receiver for pixel, with each one just 10um, and over the length of the measuring distance I think it would be possible to detect absurdly small changes in angle. Whether you could ever calibrate it to actually quantify and measure something is another matter, but I think the theory would work anyway.

Pete

Hi Ray and Josh,
I thought i'd move this conversation about the ROM to here out of the ebay thread. I found this page online Benchtest.Com - Workshop - DoAll D624-8 Surface Grinder not only does he mention he makes his own ROM but he is doing some granite lapping to. It may be worthwhile emailing him.

Thanks for the tip Ewan, I have emailed him, to see if he has any detail on the repeat-o-meter construction.

Regards
Ray

Jim is a nice guy and I've spoken with him on a number of occasions. I'm sure he'll be most helpful with information.

As far as the Repeat-o-Meter, I'd suggest there's actually nothing special about it, and it simply serves as a convenient way to handle an appropriately sized gauge block while supporting a gauge above it. Starrett also do one, model #81815.

Pete

Originally Posted by Pete F

Jim is a nice guy and I've spoken with him on a number of occasions. I'm sure he'll be most helpful with information.

As far as the Repeat-o-Meter, I'd suggest there's actually nothing special about it, and it simply serves as a convenient way to handle an appropriately sized gauge block while supporting a gauge above it. Starrett also do one, model #81815.

Pete

Thanks Pete, yes, Jim replied with lots of useful information and some construction tips, so the next step is to make one!...

Joe Hovel kindly gave us some nice blocks of cast iron (used to be tractor weights) when he was here ( surface grinding his tool and cutter table) the other day, Josh has already earmarked a likely looking piece for the repeat-o-meter project.

One again a public thanks to Jim for the vital details. , and thanks Ewan for spotting the link!

Regards
Ray