Very precise measurement of XY position

[Ecir Hana]

Hello,

please, does any one knows how positioning of a plotter head works? I mean very precise measurement, say 0.001 mm and the moving area of around 100 x 100 mm...

I'm asking what principles are used, I thought one way of doing this are two distance sensors - here is a link how a distance sensor works but I'm not sure this would yield the precision I'm asking about:

http://sites.google.com/site/todddanko/home/webcam_laser_ranger

[Artificer]

The accuracy standard you specified is .00003937" , or 1 micron. Plotters are not nearly that accurate, they use stepper motors and are open loop (they have no position feedback), I would doubt they are .01" accurate.

To measure to a micron on a positioning system you would use a high precision linear scale.  You would also have to have an environmentally controlled room, and the system would have to be on a concrete pad that went to bedrock. If you breathe on a micron positioning system you can cause positioning error.

[Ecir Hana]

This is all very true, the thing I'm interested in is how do you compensate for the errors if one breathes on such systems? What kind of "high precision linear scale" are you referring to?

Even if a stepper could be as precise as 0.1 mm there would have to be gears with 1:100 ratio which could cause some errors in itself.

I thing one has to use feedback for the position measurement but what kind? What does it mean if one says "laser calibrated", i.e. how does it work?

[Soeren]

Hi,

Look here

[Ecir Hana]

Interferometry, ...,  uses interference phenomena to measure distances based on the wavelength of light. In visible light, this is a distance of about 590 nm, or 1/43,000th of an inch.

Albert A. Michelson`s early experiments were affected by street-traffic vibrations up to 1000 feet away.

"Interferometry is the most sensitive, practical distance-measuring technique available"

Wow! Thank you for the link!

[Artificer]

A laser interferometer is used for testing a micron accurate system, their price makes them impractical for most applications. When you buy a micron accurate system you will get a calibration report where they graph the system using an interferometer. If you need micron accuracy you need to specify a system twice as accurate as you require, preferable four times more accurate.

How you compensate depends on what you are doing. An industrial motion system will return to the specified location, but if you are machining you will have created an imperfection. You have to hold the temperature and humidity constant, and you have to be isolated from vibration. If you are machining you have to let your material temperature stabilize to the working environment.

Here is a link to Sony's scales:

http://www.sonysms.com/prod_main.asp?division_id=1&products_id=1

Out of curiosity what are you trying to do?

[Ecir Hana]

I just saw this article:
http://www.livescience.com/technology/091229-3d-bioprinter.html
and was wondering how it is possible to achieve such incredible precision.

Btw, a red blood cell is roughly 8 microns small:
http://learn.genetics.utah.edu/content/begin/cells/scale/

[Ecir Hana]

Btw, those Sony scales - are they working like an optical mouse?

[Artificer]

The literature links on the Sony link above explain it better than I can. In a manner of speaking all encoders have equally spaced graduations that the system counts. Comparing a sub micron accurate linear scale to a optical mouse is like comparing a skate board to a Corvette.

The bio printer in the article is not like like normal plotter. It looks like a linear servo (gears and screws are bad when dealing with micron precision) with a scale on it and it was calibrated with a laser interferometer.

Micron accurate systems tend to be used for research and precision optics.