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# You need to determine if a spinning disc is rotating clockwise or counterclockwise. The disc has been painted with a black and white pattern. You have a sensor that can sample the disc and determine i

Posted on 2011-02-22
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You need to determine if a spinning disc is rotating clockwise or counterclockwise. The disc has been painted with a black and white pattern. You have a sensor that can sample the disc and determine if the disc at that instance in time is black or white.
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Question by:dshrenik
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relliott66 earned 444 total points
ID: 34954881
Is this the question you were trying to ask:

Imagine a disk spinning like a record player turn table. Half of the disk is black and the other is white. Assume you have an unlimited number of color sensors. How many sensors would you have to place around the disk to determine the direction the disk is spinning? Where would they be placed?
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☠ MASQ ☠ earned 444 total points
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You can't if you only have one sensor, you need two reference points to determine direction
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TommySzalapski earned 664 total points
ID: 34955311
It doesn't matter how many sensors you have. If you don't know how fast the disc is spinning, you can't figure it out. You can never be positive how fast it is spinning, because you can always find a speed (in both directions) that will give you exactly what you are seeing no matter how many samples you take.
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TommySzalapski earned 664 total points
ID: 34955337
If you know the speed and have a perfect clock, then you could sample in one location and move the sensor a quarter turn clockwise and sample again offsetting the sample period by 1/4 and then again by 3/4. If they match, then start over. If the 1/4 matches the original then it's spinning clockwise. If the 3/4 matches it's going anti-clockwise (counter clockwise). If neither match, either the speed was wrong or your clock was messed up.
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ID: 34956432
So was that a 'yes' or 'no' to two sensors?
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relliott66 earned 444 total points
ID: 34957202
if the question I formulated is correct, then I think 2 is the correct answer.  Actually, they have sensors now with two sensors built in so maybe even one could do it.
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Saqib Husain, Syed earned 224 total points
ID: 34958786
It depends on the pattern of black and white. If it is unsymmetrical/regular then one sensor can be sufficient.
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ID: 34958787
sorry ---  unsymmetrical/irregular
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☠ MASQ ☠ earned 444 total points
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All we know is the disk has a pattern it is not necessarily like the one relliot66 has found.  I guess we have to work with the possibility of a random mix of black and white.
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andyalder earned 224 total points
ID: 34987925
I think it's possible with a single sensor so long as you can move it. Assuming evenly spaced black/white then move sensor in the same direction as spin then frequency would go down similarly to dopler shift.
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TommySzalapski earned 664 total points
ID: 34988999
If you allow for unbounded speed, then you can't solve it no matter how many sensors you have unless the sensors sense continually. Since the original question said 'sample' I assume we are talking about discrete sensing.
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