Measuring fast moving objects

Steb

6

Measuring fast moving objects

« on: 02 / January / 2010, 03:16:00 »

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Hi - I hope I'm in the correct forum. I'd like to share some thoughts I have over using CHDK to measure certain parameters of the propulsion of a golf ball and welcome critique or suggestions. I must say I'm flabbergasted by CHDK--I'd been researching ways to measure the golf ball, on the verge of giving up--then this whole new world appears.

Essentially what I'm after is two horizontal photographs of the golf club arriving before impact, and two horizontal photographs of the ball after impact. From those photographs a wealth of information can be derived such as club velocity, club approach angle, ball launch angle, ball velocity and ball spin rates. There are quite a few devices on the market that do just this, however well out of my budget.

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Steb

6

Re: Measuring fast moving objects

« Reply #1 on: 02 / January / 2010, 03:16:17 »

To simplify, a ball would be propelled at max say 3000 inches per second. Let's say we want ball to move 3 inches between frames, so 1 msec between shots.

A multiple exposure image with 1kHz strobe would be just perfect, except I'm thinking this would only work when the environment is reasonably dark to start with. So outdoors it would be useless.

But am I right in thinking this? If I set a minimum aperture, minimum iso and set shutter speed just long enough to capture all four strobes, could this be enough to eliminate ambient sunlight and let the flash singly expose? Maybe a ND filter too? Can flashes overwhelm sunlight at close distances, say 12 inches away?

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Steb

6

Re: Measuring fast moving objects

« Reply #2 on: 02 / January / 2010, 03:16:37 »

Or might I need four cameras, usb'ed together with a 1ms delay between triggering? That's not really a route I wish to take.

A single camera (I have an IXUS40/SD300 but could get something more powerful that supports CHDK) is not going to be able to take four images 1ms apart correct, even if read from the cameras buffer? (if that is possible?)

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Steb

6

Re: Measuring fast moving objects

« Reply #3 on: 02 / January / 2010, 03:17:46 »

Of course the trigger mechanism is going to be tricky as well. I had a quick play with Fudgey's motion detection but the 100 odd milliseconds reaction time is just too long. 100ms before impact the club is roughly still pointing vertically upwards.

Even a 35ms reaction time as it appears some cameras can handle with special builds will have the clubhead still outside the legs. The field of view would have to be too wide and leg movement would trigger incorrectly.

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Steb

6

Re: Measuring fast moving objects

« Reply #4 on: 02 / January / 2010, 03:23:57 »

I had a look at the delays for USB remote cables - although better than motion detection it still seems like triggering the camera before impact via a light sensor of something is just not going to work. It just takes too long for the camera to take the shot. I can't even see four separate cameras doing the job now. Maybe it can only be strobe?

Here's one device on the market that seems to strobe. This device also has integrated lasers to more accurately measure speed, but I'm pretty sure that is 8 flashes surrounding the camera (the brochure also says "flash number: 8"). Plus this device works outside in sunlight. Apparently the user can change the trigger delay between 1 - 5msec, the exposure from 2usec - 4msec and the interval time from 2usec - 4msec.

Any thoughts?

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kuku

14

Re: Measuring fast moving objects

« Reply #5 on: 02 / January / 2010, 08:54:21 »

I think you can do it with a more expensive flash that has strobe option you can meansure the ball speed without problems
look at the manual: http://eosdoc.com/manuals/?q=580EX*

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Steb

6

Re: Measuring fast moving objects

« Reply #6 on: 02 / January / 2010, 16:57:20 »

Hi kuku, the 580EX seems to have a maximum strobe rate of 199Hz, the ball would have moved about 15 inches between flashes. But thanks for the suggestion.

There seem to be hand held strobe flashes out there that can fire up to 10000Hz or more, but they're quite big and mains powered. I think I'm understanding why the machine I posted above has 8 separate flashes now instead of one strobing one.

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