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New While we are on interview questions
Here's one I got awhile back. I'm not sure I did that well on it. Not because I didn't solve it. But because I'm more of an intuitive than an analytic, I don't think he liked my explanation.

You have a disk painted half black and half white, mounted at the end of a shaft so it spins like a platter.

At the 12 o'clock position, you have a sensor that you can test that returns true if it see's white, false if it see's black. You want to determine the direction the disk is spinning. How many more sensors do you need? Where do you put them? Why? How fast must you sample relative to RPM for your data to be valid?

I reckon tonytib knows this one cold - but having no mechanical actuator experience, I had to solve it on the spot from first principles.



[link|http://www.blackbagops.net|Black Bag Operations Log]

[link|http://www.objectiveclips.com|Artificial Intelligence]

[link|http://www.badpage.info/seaside/html|Scrutinizer]
New Simple 1 line incremental optical encoder
so just use two sensors with a 90 degree phase angle between them - that gives direction, and if everything is equally spaced, four positions (1 line x 4 counts/line).

Sampling rate should be governed by the Nyqist theorem - you need to sample at least twice the frequency of interest.

Real world is buy some hardware - encoders from US Digital, Avago, and hundreds of others, and quadrature decoder chips (or implement in FPGA, etc). Actual quadrature decoders are based on the edge, not sampling, but of course the circuitry has to be fast enough to detect the edges.

[link|http://zone.ni.com/devzone/cda/tut/p/id/4763|Brief NI summary]
[link|http://www.fpga4fun.com/QuadratureDecoder.html|FPGA implementation of a quadrature decoder]

--Tony
     While we are on interview questions - (tuberculosis) - (1)
         Simple 1 line incremental optical encoder - (tonytib)

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