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Thread: The speed of a disc?

  1. #1 The speed of a disc? 
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    Hi,

    I need to set a disc spinning at 16&2/3 revolutions per second but I don't know how to determine this speed. I have taken digital stills photos of the disc (approx 7.5" / 20cm diameter) at 1/15, 1/20 & 1/25th of a second and they all show a tendency to freeze the motion, which would suggest I am near to the desired speed, but still unable to fix it.

    It is the edge of the disc that needs to travel at 16.67 revolutions per second.

    There are 'strobe wheels' (patterns) which will allow you to determine revolutions per minute but I haven't seen anything like this for revolutions per second.

    Advice appreciated.

    Thanks,

    Ric


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  3. #2  
    Genius Duck Dywyddyr's Avatar
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    Quote Originally Posted by phaeton View Post
    It is the edge of the disc that needs to travel at 16.67 revolutions per second.
    The entire disc rotates at the same speed.

    There are 'strobe wheels' (patterns) which will allow you to determine revolutions per minute but I haven't seen anything like this for revolutions per second.
    16.66.. rps = (16.66.. x 60 rpm) = 1000 rpm.


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  4. #3  
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    Quote Originally Posted by Dywyddyr View Post
    Quote Originally Posted by phaeton View Post
    It is the edge of the disc that needs to travel at 16.67 revolutions per second.
    The entire disc rotates at the same speed.

    There are 'strobe wheels' (patterns) which will allow you to determine revolutions per minute but I haven't seen anything like this for revolutions per second.
    16.66.. rps = (16.66.. x 60 rpm) = 1000 rpm.
    Thanks bach

    But how do I determine that my disc is spinning at 1000rpm - is there a test for this please?

    Ric
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  5. #4  
    Genius Duck Dywyddyr's Avatar
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    Uh, you just told us that there are strobe wheels for that!

    Or, depending on what equipment you have, you could work out the peripheral speed and put some sort of speedometer against the edge of the disc...
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  6. #5  
    Brassica oleracea Strange's Avatar
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    How are you driving the wheel? Are you looking for a electronic or mechanical solution? Is it motor driven? How are you controlling the speed? Perhaps you could use an optical or hall effect sensor to measure the speed? Or use an adjustable strobe light?
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  7. #6  
    Genius Duck Dywyddyr's Avatar
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    Gaaah!
    I must be tired.
    The rpm/ rps is determined by the motor/ gearing speed.
    Can't you work it up from the motor rpm?
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  8. #7  
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    Hi,

    The wheel or disc is driven by a small electric motor 1:4 gear ratio. The motor is powered by a DC adapter 3-12v. The speed is controlled by a speed regulator. The load (disc) will affect the unladen speed estimate.

    I tried to work out where I was by taking photographs. Here is a photograph taken with a 1/15th of a second shutter speed. I need a speed of 16.67 revolutions per second so 1/15 is close. You will notice that a white dot has moved about one tenth along the circumference.

    What does this mean? Does it mean the disc needs to turn ten times faster, so that the dot blurs all the way around the edge of the disc, in this way telling me that the disc is revolving 15 times per second please?

    15_zps0f60cc90.jpg

    I sent off for a laser tachometer over a month ago, it never showed up. I ordered another one two weeks ago - nothing yet. This is the reason I am looking for other ways to determine the speed of the disc.

    Many thanks,

    Ric
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  9. #8  
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    What you need is a tachometer.
    Amazon.com: Neiko 20713A Professional Digital Laser Photo Non-Contact Tachometer with Accuracy 99,999 RPM Measurement: Home Improvement

    Edit: Never mind, I see you already ordered a tachometer.

    If your shutter speed is 1/15 of a second, then I would say you need to adjust the speed so that there is a continuous blur all the way around.
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  10. #9  
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    Yes, thanks but "I sent off for a laser tachometer over a month ago, it never showed up. I ordered another one two weeks ago - nothing yet. This is the reason I am looking for other ways to determine the speed of the disc."

    We have a slight problem over here at Amazon UK. More and more sellers advertise themselves as UK companies, then when you order items from them they either don't turn up, or they show months later with a postmark half the world away. This is blatantly misleading the public but there is a trading loop hole which allows them to basically give out a false impression as to where their company is based.

    Moan over
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  11. #10  
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    Quote Originally Posted by Harold14370 View Post

    Edit: Never mind, I see you already ordered a tachometer.

    If your shutter speed is 1/15 of a second, then I would say you need to adjust the speed so that there is a continuous blur all the way around.
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  12. #11  
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    Here we go, what do you think? It looks like motor speeds 8 & 9 get me up to 15 revs per second. That's almost top speed for the motor & practically rattles my meccano to pieces

    edgesequence12vat15secenhancedcopy_zpse94a104d.jpg

    http://i1295.photobucket.com/albums/...pse94a104d.jpg
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  13. #12  
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    error
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  14. #13  
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    You could also measure the gyroscopic effect. But getting a meter on it is probably your best shot. Option 2: draw a dot, preferrably whit on a black disc. Take a picture of the disc standing still. (with a long shutter time. And measure the brightness (digitally) on the computer. Then measure the disc without dot. You know no the difference in brightness between dot and no dot. Then take a picture while rotating. The amount of rotations passed during a picture is a good measure of its brightness.
    In the information age ignorance is a choice.
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  15. #14  
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    Thanks Kerling - that's more or less what I am doing at message #11. When the dot blurs right round the edge of the disc it means that the disc has turned 15 times in one second, because the camera was set to 1/15th of a second, shutter speed.
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