Using LED PWM and The Rolling Shutter Effect to calculate RPM

D2000

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Hi guys and girls,


Not sure if this is the right place for it but it seems like it. Basically what the title says - I have a new spinning top that can spin really REALLY fast, especially if spun up to speed with an air compressor. During the speed up I notice that even to the naked eye I can see the rolling shutter effect whilst my LED light is over it. (The rolling shutter effect is the effect you see when a car wheel appears stationary or even rolling backwards, whilst in motion under a street lamp, or on camera)


I notice the cycles of the rolling shutter effect occur about 10 times during the process of it ramping up. I figure that when the spinning top appears stationary in the light, its reached a multiple of the frequency of the PWM of the LED. (100, 200, 300, 400, 500 etc..)


For example:
The LED PWM is 100Hz. During the speed up I see the spinning top as a static image (rolling shutter effect) 5 times. That means the 5th time I see the spinning top slow down to a still image that frequency is the 5th multiple of 100(Hz), making the spin calculated to 500 times a second, (which would be 30000 RPM)

Is this process of deduction or logic correct?

I will edit this post with a link to a YouTube video of it, once it finishes uploading.


EDIT:https://youtu.be/cjmQUdcnVu4

Thanks,
Dave
 
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Lexel

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Sep 15, 2016
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Germany
Many flashlights run on 4500Hz PWM
you can check with a camera and use the fastest shutter you can use with flash on
for most models this if 1/200 more expensive dslr 1/400

then go to your shower and light up the drops and make a picture
count the points on every drop lit by the pwm and multiple by the shutter time

it is important not to use a faster shutter value than the camera is taking the photo in one piece

IMG_8481.jpg
 
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D2000

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Joined
Apr 29, 2013
Messages
352
Location
Sydney Australia
Many flashlights run on 4500Hz PWM
you can check with a camera and use the fastest shutter you can use with flash on
for most models this if 1/200 more expensive dslr 1/400

then go to your shower and light up the drops and make a picture
count the points on every drop lit by the pwm and multiple by the shutter time

it is important not to use a faster shutter value than the camera is taking the photo in one piece

IMG_8481.jpg
This is am amazing answer. Thank you. This is why I stick around on this site.

This should help with my calculations.
 

reppans

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Mar 25, 2007
Messages
4,873
Many flashlights run on 4500Hz PWM
you can check with a camera and use the fastest shutter you can use with flash on
for most models this if 1/200 more expensive dslr 1/400

then go to your shower and light up the drops and make a picture
count the points on every drop lit by the pwm and multiple by the shutter time

it is important not to use a faster shutter value than the camera is taking the photo in one piece

IMG_8481.jpg

I don't claim to be an expert, but my understanding of photography and camera flashes was that the flash burst duration is nearly always shorter than shutter speed. In the method you describe above, you are counting the dots illuminated by the flash duration, while the hertz calculation is performed on what may be an unrelated shutter speed duration?


...But flash is not affected by shutter speed. Flash is a near-instantaneous pulse, possibly 1/1000 second duration, or speedlights are perhaps much shorter, but much faster than our shutter speed duration. Therefore, regardless if our shutter speed is 1/200 second, 1/60 second, or 1 full second, the flash does all it can do in perhaps 1/1000 second, or even much less. The shutter merely must be open when the instantaneous flash pulse happens. It simply does not matter to the flash exposure how much longer the shutter might stay open after the flash stops - the flash already finished long ago. The significance of this is that shutter speed is simply NOT a factor for normal flash exposure.

From here:
http://www.scantips.com/lights/flashbasics2.html


EDIT: That said, it seems like you could always illuminate the shower drops with a constant Incan light from the side, and photograph the droplets against a black background and do this same calculation.
 
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Lexel

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Sep 15, 2016
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Location
Germany
It is important not to choose a shutter speed that is too fast that is related how the shutter of a cameara works at speeds higher than the flash synchronisation
Simple flash works the way it waits that the 1. shutter openes 100%, then flashes, and 2. shutter closes
the time of a fully opened shutter is mostly 1/200 or 1/400s thats physically limitated

But that camera is able to get shutter times of 1/4000 to 1/8000
therefore a highspeed flash synchronisation on 300-600$ flashes flash multiple times to get the whole picture lit

I added this picture you can see that the shutter opening gets smaller at higher speeds
that will not be able to detect all the flashlights pulses if the droplets are not traveling from top of the picture to the bottom if you have the camera horizontal
If you have the camera vertical it wont work at all
screenshot3.jpg

focalplane.jpg
 
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ssanasisredna

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Joined
Oct 19, 2016
Messages
457
Outstanding post!

Many flashlights run on 4500Hz PWM
you can check with a camera and use the fastest shutter you can use with flash on
for most models this if 1/200 more expensive dslr 1/400

then go to your shower and light up the drops and make a picture
count the points on every drop lit by the pwm and multiple by the shutter time

it is important not to use a faster shutter value than the camera is taking the photo in one piece

IMG_8481.jpg
 
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