Labview and Length Encoders

[AlignMe]

This post relates to the use of Length encoders (typically glass-scale type), not Rotary Encoders with "linear" response characteristics. I should also point out that I am a newbie w.r.t. Labview!

In an effort to increase precision, I would like to use two glass-scale encoders arranged to produce simultaneous outputs for the same change in linear position.

The increased precision would result from combining the digital signals "in quadrature".

Is there a way to do this directly within Labview? Or, do I have to take the outputs from Labview, and manipulate them elsewhere?

Thanks in advance for your comments and suggestions.

[Michael_78]

Hi, the easiest way would probably be to use an NI DAQ device which supports quadrature decoding. I found this link which is likely to be helpful.

Best of luck,

Michael. 

[AlignMe]

Not sure we're on the same page, so to speak!

I am interested in using two encoders rigidly linked to the same linear motion, in this fashion:

1. Zero both counters
2. Move both linear encoders the same amount
3. processing the pulse count from each, resulting in a length estimate from each of the two channels
4. Combine the pulse count / length estimate from the two devices "in quadrature" direclty within LabView (that is, mathematical quadrature ......i.e., as an average with an associated Sigma RMS value)?

[cbutcher]

If you can express a formula that describes the result you want, it should be easy to implement in LabVIEW. Primitives exist for calculating an RMS value, but if you only have two points (I'm not certain I understood the setup - I originally thought you meant to set up your two encoders to form a kind of manual quadrature encoder, which has pulse trains offset by 90deg, in order to constrain the position within a single pulse high or low) then it might be simpler to just use the math primitives (which also operate on arrays of numbers).

[Bob_Schor]

I'm puzzled by what you are proposing.  If you are "building a quadrature encoder" yourself from two linear encoders, I would venture to guess that you'd get better accuracy and precision from using a single encoder.  A quadrature encoder is generally constructed by the manufacturer to fairly exacting specs, probably better than combining two single encoders yourself.

Bob Schor

[cbutcher]

I think (although am eagerly awaiting the authoritative explanation from the OP) that perhaps the idea is that the counts aren't accurate? And that by counting two encoders (s)he can get a 'more accurate' reading.

Presumably if the counts are accurate, the results will be the same, and RMS is fairly meaningless on two identical points.

[AlignMe]

My objective has nothing to do with building a "quadrature encoder". I am not interested in ascertaining the direction of movement, only an accurate (and precise) absolute position.

The problem is in fact one of accuracy. The grating of the glass scale is such that, for an incremental movement of about 0.0001 inch, a pulse may not be detected from a single scale. It is in the "half-digit" realm, if you like. But by combining two scales (with the gratings simply located at random--not aligned in any fashion)  and averaging the result from each, it should be possible to effectively double the pulses per increment (= increase the resolution by 2x).

[AlignMe]

Understood: the standard deviation (sigma) for two readings is of course fairly meaningless. But it can form a useful "internal check" and one could set a flag if it exceeds some threshold value.

M