Friction, consistency, statistics, eddy currents, & more

Discussion in 'The Projects Forum' started by tracecom, Apr 26, 2013.

  1. tracecom

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    Apr 16, 2010
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    I have been experimenting with using eddy currents to differentiate between copper and zinc pennies. Here is a link to the history of penny composition. http://en.wikipedia.org/wiki/Penny_(United_States_coin)

    I built a little jig and circuit to attempt to measure the time it takes a penny to slide the length of its diameter while passing through a magnetic field. That setup is shown in the attached photos.

    I selected three pennies: a 1958 "wheat" penny, a 1977 "copper" penny, and a 2006 "zinc" penny. I cleaned and polished each of the pennies. (I know that I ruined the collector value, but that's a cost I can bear :).)

    Then, I placed each penny into the slide, and it moved down the slide until it was stopped by the paper clip. Then, I pulled the paper clip up to release the penny and allow it to slide under the magets and between the LED and the phototransistor mounted in the slide. The time was measure by the PIC. I repeated the process 20 times with each penny with the "tail" down, and 20 times with each penny with the tail up.

    Attached is the data that I recorded. I am no statistician, so I am not sure how much credibility to place in it, or exactly what to make of it. I would appreciate any input.

    Specifically, I am concerned with the variance between the results of the 20 trials for a specific penny, and how I can reduce the variance as much as practical.

    Thanks.
     
  2. wayneh

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    Sep 9, 2010
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    To do anything with the data it'd be great if you could please attach the data in a file so it could be used without re-entering by hand.

    Two means, with lots of reps, separated by 1 standard deviation is likely a significant result. Excel can do T-tests for you, and that's how you can compare two samples for a significant difference, at whatever significance level you want. Be sure you understand the parameters.

    Why do you think the 1958 penny is so different?

    You should definitely get yourself some 1943 "silver" (aluminum) pennies to add to your experiments. They're only 2-3¢ apiece I think.

    Oh, and I think you should try to minimize friction as much as possible by making your chute vertical instead of sloped. You don't want gravity and friction to be the main variables if you're trying to look at eddy currents. This should reduce variation due to friction AND reduce the average effect of friction. Less friction noise, more relative eddy signal.
     
  3. Kermit2

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    Friction and surface roughness being so influencial in your chosen method of testing, it may not be possible to detect the influence of the eddy current effects vs. the other elements of sliding friction.

    A sensitive balance would be a better choice, since the variations in weight between copper and zinc is considerable with the size of a penny being very tightly controlled in the minting process
     
  4. tracecom

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    An txt file is attached; you will need to change the extension to csv. Whatever analysis you can do would be appreciated, but you will probably have to explain it to me. I had a semester of stat as an undergrad and another semester in graduate school, and made an A in both, but it was memorization instead of comprehension.

    The 1958 penny has a slightly different metal content (a little tin mixed with the zinc), but why it differs so much from the 1977 penny is a mystery to me. The reverse side is a different design ("wheat") but the obverse is essentially identical to the 1977 and the 2006.

    I also tried a vertical drop, but the inconsistency was even greater than with the slide. I'll try it again, and post the data.

    ETA: 1943 pennies are steel with a zinc plating and stick tight to the magnet. :eek:
     
    Last edited: Apr 26, 2013
  5. tracecom

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    Heres a csv file with the vertical drop data added. Change the extension from txt to csv for use in Excel.
     
  6. tracecom

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    It can be done that way; the copper penny's mass is 3.11 grams and the zinc penny's mass is 2.5 grams. If I can't make the eddy current method work, I may give mass measurement a try. Generally, it's slower, however. Thanks.
     
  7. wayneh

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    Oops about the 1943 penny - I should have known that. I've got a few hundred of them.

    I'll take a look at your data soon.

    Here's an idea: Wouldn't the eddy current "braking" effect be more pronounced if the penny was moving faster as it passes the magnet? If the penny were in essentially a free fall (guided to be on edge) and you measure the pre-magent velocity (probably a good idea for ANY configuration) and then the post-magnet velocity, you should get a measure of eddy braking.
     
  8. THE_RB

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    You probably have mechanical issues. Mainly friction. You need to mount the penny starting position in something very slippery like Delrin plastic.

    Then your release mechanism needs to be very repeatable, and release the coin very fast and in a way that does not impart any energy into the coin.
     
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  9. GopherT

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    I am having trouble seeing where your sensor is relative to the magnet. It will be best if the sensor is one penny width (3/4") down stream of the magnet so the full braking force is transferred to the penny before the speed trap (otherwise, the penny is still decelerating).

    Faster penny will see more difference (mentioned above)

    Pre-1962 pennies have a 5% non-copper that contains (tin and zinc). 1962 to 1983 pennies also have 5% non-copper (all zinc, no tin).
     
  10. shortbus

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    What brand of paper clip were you using? :p
     
  11. tracecom

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    There are two curved hard drive magnets glued to the "ceiling" of the track. The phototransistor is in the area between the magnets, and the LED is directly below in the "floor" of the track. The first magnet is above the penny before it breaks the light and the second magnet is above the penny after it clears the light. Maybe I need a third magnet just above the first.
     
  12. wayneh

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    I've taken your data and added a bunch t-tests in Excel to compare the 6 data sets.

    You're seeing statistically different results between the years, and whether the coin is place face-up or down.

    The result of the t-test is the probability of randomly getting the same means by drawing from a single population. We generally want to see that probability below 0.05 (5%) to claim significance, but that level is an arbitrary choice and depends on what you're doing.

    Statistical significance, by itself, doesn't tell you much about the physics causing the differences in the data.

    View attachment Penny Data.zip
     
  13. wayneh

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    What's the point of a magnet after you've taken the measurement?
     
  14. tracecom

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    The timing is for the duration of the period that the penny is between the LED and the phototransistor. The leading edge of the penny is over the lower magnet well before the trailing edge of the penny clears the light, thus the eddy currents (hopefully) are still affecting the movement of the penny before the measurement is completed.

    Here's a picture inside the slide that shows the magnets surrounding the phototransistor.
     
    Last edited: Apr 26, 2013
  15. shortbus

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    Just a dumb thought, don't you need an alternating magnetism to make an eddy current?
    Was under the impression that stationary permanent magnets and DC electro-magnets didn't generate eddy currents.
     
  16. tracecom

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    That would be true without the movement of the penny; it is the movement of the conductor through the magnetic field (or the movement of the magnet through the conductor) that produces the eddy currents which resist the movement. At least, that's the way I understand it. One of the most dramatic examples I have seen is when a neodymium magnet is dropped through a heavy copper tube. If you haven't seen it, it's worth a look on you-tube.
     
  17. wayneh

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    Or take a strong magnet and try to move it quickly over an aluminum slab. The pushback is an eerie sensation.

    I still contend that you need two speed measurements to reduce variation due to friction, the paper clip gate and so on; one before and one after braking. Ratio the times.

    Also, does your magnetic field have a return path? Ideally you'd pass the coin thru a narrow slit in a "C" shaped field. Two opposing magnets with their back ends connected by a piece - any shape - of iron would help intensify the field.
     
  18. shortbus

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    OK, I get that the penny is moving and then can make an eddy current. But both magnetic poles have to be within the diameter of the penny at the same time. And like Wayne said they have to have a return path to each other. You can't have a north pole of one magnet and a south pole of another. Can you?
     
  19. tracecom

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    I am just a tinkerer, and don't really know much about what I am doing. However, the way I understand the construction of these neodymium magnets is that they are through pole magnets. Instead of one end being north and the other south, they are polarized on the sides.

    For example, I have some small disc shaped magnets. They will stack perfectly as long as they are all oriented with their poles in the same direction, but if one is turned the opposite way, it repels. It's the same with the hard drive magnets that I am using. The poles are on the sides instead of on the ends.
     
  20. wayneh

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    I can't follow. Are these disk magnets polarized along the central axis? That's the "normal" arrangement and you can stack them as you say without concern for their rotation about that axis. If you turn one pancake over, then it repels. Is that right? (It doesn't matter if they're not round disks. I just mean that the field is perpendicular to the large flat face, not the edge.)

    Anyway, you want the field to have only a narrow air gap thru which the penny drops. The rest of the circuit needs to maintain the field as best as possible. A gapped iron toroid, for instance, with a magnet on one side of the gap, will lose very little of the field as it bends the field lines around to the other side of the gap. The field "only" has to jump the air gap, which is essentially an insulator when it comes to magnetic fields. Less gap = less insulation.
     
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