If you take a video of a spinning wheel, you’ll probably notice that the spokes appear to turn more slowly than the wheel is actually rotating, and sometimes in the wrong direction. This is caused by a near match in the frame rate of the camera and the rate of rotation of the wheel – each time the camera captures a frame, the wheel has rotated a spoke into nearly the same position as in the last frame. If you time the exposures carefully, as [Excessive Overkill] did in his latest video, this effect can seemingly freeze moving objects, such as a fan or saw blade.
Most cameras only allow relatively coarse, fixed adjustments to frame rate, making it difficult to synchronize the shutter to an object’s motion. To get around this, [Excessive Overkill] used an industrial camera (previously used in this aimbot), which has fine frame rate control and external triggering. He connected the external trigger to a laser sensor, which detects a piece of retroreflective tape every time it passes by (for example, on one blade of a fan). When the laser sensor sends a signal, it also triggers a powerful LED flash. The flash is so powerful that dark materials create a hum when exposed to it, as pulses quickly heat the material, but each pulse is also so brief that the flash board doesn’t require any cooling.
Even to the naked eye, these stroboscopic pulses make rotating objects seem to stand still – an effect which made [Excessive Overkill] extra cautious when working around a lathe. When using a suitably long exposure time to avoid rolling-shutter distortion, the effect worked even using a normal camera without frame-rate matching. [Excessive Overkill] took videos of debris flying away from a seemingly motionless bandsaw, milling machine, chop saw, and jigsaw, though it was harder to freeze the rotation of a weed trimmer and a drone.
We’ve seen this effect used to freeze motion a few times before, both for art and for entertainment. If you’d like to recreate it, check out this high-speed LED flash.
Thanks to [Keith Olson] for the tip!
That’s always been considered a bad idea…. you know, for safety.
Neat idea. You could even use it to measure the phase angle of the part. Maybe even something like measuring when in the rotation cycle a spark plug fires in an engine!
Couldn’t you just take a video normally with no special hardware required and get your video editing software to only keep 1 frame out of x frames and discard all other ones?
Say, you record a 120FPS video of something that rotates at 3037RPM. You won’t be able to freeze the movement as you would get a frame every 151,85 degrees. Only one frame per minute of recording would be viable.
On related note there is the matter of shutter speed, depth of field and motion blur. To avoid motion blur you need high shutter speed. To get a decent depth of field you need a small aperture. This limits the light that gets to the sensor, and you can’t really crank up the ISO value if you want a decent quality and lots of details. That’s why strobe lights were invented.
So we’ve reached a point where no one remembers putting a chalkmark on a timing belt with a strobe gun to tune the engine?
Or “Doc” Edgarton aka “Papa Flash”?
No critique of the project but man it seems like we’re forgetting more than we’re learning.
I may be old, but I still remember last week! This effect is also why I’ve got high frequency tubes in the garage lights, and will only consider fully-rectified led lamps when I catch up with the 21st century.
The other day I was looking for a way to add text-to-speech to Cardputer ADV app, and the only suggestion from a search engine was to use a small AI model. This was done in software in 1990’s on ordinary 486 PCs using software called Syntalk that was developed in Poland. My blind friend used it at school with DOS and Norton Commander. This kind of software could run on ESP32-S3 in the background as another standard library with little penalty to its performance. Yet today this is completely forgotten technology, and only AI things get promoted by search engines.
Maybe Mr. Overkill does remember this and that’s what help inspire this video?
I have a stroboscopic tachometer and use use it often for finding out of balance fans and the like. If you have an out of balance fan you can put a mark on one blade and by speeding or slowing the strobe you can determine the out of balance blade. Works great. Just be careful of using it around people who suffer siezures.
I remember talk about florescent tubes not being allowed as illumination for machine tools, (because it’s can make the parts/cutting tool appear stationary. ) I guess this this must have been the anglepoise lamps commonly fitted to old lathes, etc.