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Measure your speed (Read 16729 times)
wanderer
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Re: Measure your speed
Reply #30 - Nov 9th, 2006 at 12:26am
 
Now there's a challenge from Matthias.

I tend to think with all of this stuff getting a 'foot in the door' with the measurements is really important. The first ones may not be very good, but having something real helps us in the right direction for improvements.

The extensions to it could easily spread to PhD level if we let it, but no need to leave the experiments to the academics Wink - no offense to the academics Smiley.
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Matthias
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Re: Measure your speed
Reply #31 - Nov 9th, 2006 at 1:28pm
 
One of the things that you do to design a meaningful test is to focus on the big numbers and not sweat the small ones. In this case, the big sources of error are the distance measurements (we dont' know exactly where the stone is released and the whip cracks, for example) and the maual selection of the time between cracks. We could measure the distance from the target to the throw line down to the nearest mm, but our overall accuracy for the thrown distance is likely to be off by as much as 50cm, so It's a waste. The sound card program allows us to choose timing down to 10000ths of a second, but if ten people will choose ten different points, we might as well just use 100ths - anything else is a false accuracy.

It would be fantastic to get a value down to the nearest m/s, but I'd more than happily cheer any result that can resolve 5 or even 10m/s intervals.

If we are throwing 20m, and anticipating speeds around 45m/s - in a simplified world we need to be able to measure
Speed %error Distance Time
1m/s 1% 10cm 0.002s
5m/s 6% 50m 0.012
10m/s 11% 1m 0.024

The nearest 5 looks pretty doable, actually. Run an average of 10 or 20 throws like that and you'll have a number that means something!

Matthias
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Re: Measure your speed
Reply #32 - Nov 9th, 2006 at 3:33pm
 

I'll try hard to soon post some video after running the numbers using the (frames per second) breakdown from the video (as per Yurek's suggestion way back in Aug/05) and be sure and get an accurate distance.

How's that?


TS
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Re: Measure your speed
Reply #33 - Nov 9th, 2006 at 6:21pm
 
I could never pull enough valid frames out of any of my video to make sense of it. 30fps at vga resolution just doesn't seem to be enough, remembering that the glans is going to be moving between 1.5 and 2 m (call it 6 feet) per frame.

If you can get a sequence that shows two clear images of the glans after it's left the pouch, then we're rolling.

Jurek's american-football-spin-polka-dot-ball photo, might be another better photographic approach. Timing the shot might be tough, but with an appropriately coloured glans (retroreflective tape might be best) one could make some pretty nice "streak" photos. Say 1/60 of a second shutter speed?

Matthias
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Re: Measure your speed
Reply #34 - Nov 9th, 2006 at 8:42pm
 
I went out today towards sundown, and under admittedly low light levels, grabbed a couple quick clips.  Very informal and shot from a bad angle.....but based on the rudimentary data gathered, the results are nearly on par with the Chrony tests I did in the 80s.  In the clip, I am slinging a relatively heavy rock at 4.1oz.....about the average normal weight for ammo I use during accuracy exercise.  The distance measurement is 30 feet from target give or take 6in. at moment of release.  Judging by cord angle at release and stone impact on video, we are looking at 150fps, give or take.  My upper limit for accuracy slinging in 1987 was in the low to mid 200's.  And although never having measured my initial velocity during  distance slinging, I can confidently say it was at or surpassing 300fps with a conventional setup.  A sling nearly twice as long and ammo nearly twice as light were used.


The following video clip(s) were originally taken at VGA res, 29.97fps.....and after closely reviewing the original clip, I am confident that my assessment of 150fps is acceptably accurate and will follow up as time permits with better footage, gathering speed data of other styles as well.  For anyone who wants to analyze the original footage for themselves, let me know and I will grab a full res clip containing the pertinent sequence.


GOLIATH'S PAIN




That's it for today.....


Keep'em Whizzing boys!


TS
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« Last Edit: Nov 10th, 2006 at 12:27am by Yahweh Bless you in Yeshua »  

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Re: Measure your speed
Reply #35 - Nov 9th, 2006 at 9:56pm
 
Wow, excellent response!  A couple of us were worried that that we'd chilled people off with the pedantic technicality bashing, really glad we haven't.  As to scoring over  Goliath, now way, not thinking that way at all.  If we get it right the results form this will be given to Goliath  and the wiki.  It's a hole which needs filling, the modern slinging distance records have helped to dispel the myth that most of the ancient authors exaggerated on range.  If we can get the speeds figured that will help to sort out things like what slings could do against various types of armour.

Before replying to the points above I'm going to be lazy, it's late in the Uk and I've just got home, and quote myself from messages ( yes,  that paranoid )  “...And very nice if it means that any slinger who can make a couple of recordings can use whatever method we come up with for an accurate ballistic analysis. If the pitch of the glans was constant only one recording would be needed, we know the start point, we know the time , Doppler shifts would give us relative motion and that's  it.  I think that the main problem is we need a solution which doesn't depend on constant pitch  because barring throwing some gadget we simply cant have it.  That's why I homed in on the distinctive pointy bits, try to make a solution out ot the problem itself.  If we can use each dpb in the same way as the sheets of paper idea we've cracked it.”.  And “Triangulation of the dpb's using a a timeline and knowing the speed of sound is the way to go. Certainly at first, when we have a few samples and some idea of what a particular glans design does  THEN doppler will be very handy as it will allow some sort of single mic technique.  Initially we will probably need at least two, maybe three ( which will be tricky at this end from a practical point of view ).  That said we shouldn't write it off, the estimates for speed that I've seen knockiong around depend on quite a long section of flight  average and slingers who had problems putting a stone through a 1 meter target at ten meters still got speeds only a little short of a handgun.  As if what we're trying should be capable of plotting the projectile for any section of it's flight I think that we may see a pretty big shift in the initial part of it.  It'll be on the recording anyway, might as well look at it.

I know some of that is a little out of context now but feel better for having pulled it out from under the rock, there is some good stuff I can't quote because it's not mine and it would be out of order.  We honestly thought we were boreing the pants of you.

I  know the sling you mean Matthias I thought it was great when I saw  and especially heard it , the whipsling would be perfect for this.  You're right, a single mic is enouth to get the average speed of glands to target, anywhere on the line.  If by slinger then time from crack to hit minus the time the sound takes to get from the target to the mic is bang on.  We are trying for a bit more detail.  But yes agreed simple and something is better than clever nothing.  I think this may best be done in easy stages,  initially ignore the parabola and imagine straight line for example.

On a practical level I got some plastecine and some straws today and tomorow a mate and I will go and try out some prototype loud glans, ( just sling short range, modeling clay isn't gold but isn't free either, at a square of carpet and hope for loud flights. ) I've got a very low end still camera which has limited video capability, also a  mini disc.  Weather permiting, neither can handle damp, will record the tests..  If we get it right then solid ones  based on what works and then we've got our sound source glans and ready to go.  Trouble is we can only manage Fridays and maybe Sundays so it won't be until nest weekend that we can start producing data.

Unless one has a several tens of thousands of pounds worth of camera, frame rates are the problem when it comes to visual esimates, mine is only 20.020 ( strange eh?)  so based only on that I'm only accurate to around 17 meters for distance estimates, okish for big shots into water but not really this.  It's sound rate is also peculiar at around 10,000.    Oh this is useful for trimming vids and exporting the sound track http://www.virtualdub.org/ ; Mini disc  44.000.  Which is why I'm going for sound.

That's a great vid tech stuff, nice shot.  I can't download and play with it though.  Any chance of uploading it here as a mpg or avi?  Just a thought, maybe we should have a “raw data” thread for this, keep things tidy.  Some day I will learn the art of the short post.

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Re: Measure your speed
Reply #36 - Nov 9th, 2006 at 11:58pm
 
The relevant portion of the original clip is over 30mb, although it's of little value in the way of 'exact velocity determination', Chris is welcome to it if he wants it.  I have a feeling I may have to get the Chronograph out again before this is over.  

Hard data and all that fun stuff.


Wink


TS


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It's not gonna ricochet.
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Re: Measure your speed
Reply #37 - Nov 10th, 2006 at 1:01am
 
I guess I was sleeping or something for the first dozen seconds, 'cause my first thought was "who's house is that?" and "wow! Techis getting confident!". Turns out he moved the Goliath between segments...

How are you going to run the travelling sideshow with a smoten Goliath Techstuf? Nice shot!

Matthias
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Re: Measure your speed
Reply #38 - Nov 10th, 2006 at 1:08am
 

Yeah, we brought him back to the house and propped him up against the garage for a shot of the damage.  After I hastily trimmed the clips and uploaded, we were laughing at how it appeared, which was unintentional.

There were a few other hits with smaller ammo which didn't show well.  I find that I can neither feel nor control the shots as well with the light ammo.  The heavy stuff is where it's at for accuracy. 


TS
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Re: Measure your speed
Reply #39 - Nov 10th, 2006 at 6:03am
 
Now that I've had a sleep and got my mind back I understand what was meant by "beating Goliath", not a section of this site, but a target.  Poor chap.  Higher mass is better but given that small is often quite good.   I have an idea which may help the flight and impact show up better, how about chalking the rock,  choose the best colour contrast against your background for the flight,  it may even be jarred off rhe rock on impact and produce a little puff of dust. 

This modeling clay isn't actully plastecine but a clone, seems ok.  I could only get 500grams of it.  For the first casts I've made eight.  A bantum egg sized lump of it weights around 70 grams, bit light but should be ok for the loud gland prototype tests.  Holds its shape well but takes a shallow impress from the weave of the pocket when whirled around.  We've had a frost so with luck it should firm up a little outside. 

My friend can't get here until two so I should be writing really but am sitting here messing with modeling  clay and straws.  Must admit to being a bit fired up over this.   Love slinging, love experimenting, putting the two together ... well yeah!!. 

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Re: Measure your speed
Reply #40 - Nov 10th, 2006 at 11:22am
 
Wow.. this thread seems to be thrashing aroung like an octopus!

Seems to me there are some different levels of basically this problem that can be attacked - the key to them all, of course is the trick of using the sound recording for precise time measurement.

Level 0, is the 'record the whip crack + target thud' which is basic but the easiest experiment one can imagine. Matthias has tabulated some numbers earlier in this thread,,, got to do this, because if it doesn't work well there isn't too much hope for the stuff that is harder to do.

Level 1: Something like my original suggestion of the two targets. My reason for describing it that way was that two identical kinds of target should give very similar looking signals on the recording, and that would help to align things. I would guess that 0.001s should be 'easy' and maybe even to 0.0001s if the signal is clean, without anything beyond 'eyeballing' the traces.

The number I've tried to keep in my mind is that the slug will travel about 1.5 inches or 3.5cm in the time of 0.001s that I think is the lower end, so the other dimensions need to be organised to give reasonable precision in conjunction with that.

These just give two events, so don't tell us about a lot of stuff that many of us would like to know about (maybe? Smiley ), but.. they are experimentally not that awkward (one mic, one recorder).

For the stuff about how the glans changes velocity rather than us measuring with an assumed constant velocity, strikes me as definitely doable, but a lot harder, and we'd gain a lot of valuable experience from some of the simpler experiments.

It's great that some numbers on this seem to be coming in.

For the more fancy measurements we need the glorified buzzing rock. After being stuck on a coach all day trying to catch up on (a lot) of sleep Smiley, I'm thinking about trying slinging cell phones!
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Re: Measure your speed
Reply #41 - Nov 10th, 2006 at 12:49pm
 
I'll dust off the Chronograph and get some video to prove the data regarding 'muzzle' velocity.....how bout someone volunteer to do the 'trajectory midpoint listener' thing.

Maybe wedge one of those 'firework snappers' with the 'tail' in between the rock and the release side of the pocket or affix it to the extreme upper inside of the release side of pocket such that it simply must be squished upon release.  Plywood or metal targets will provide excellent reports on impact.

This, as Matthias has noted, may prove perfectly adequate for obtaining average velocities.


TS
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« Last Edit: Nov 10th, 2006 at 2:13pm by Yahweh Bless you in Yeshua »  

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Re: Measure your speed
Reply #42 - Nov 10th, 2006 at 8:27pm
 
Well we got soaked but it wasn't a total waste of time.  The rain had driven other humans from the reservoir so we had the luxury of slinging there.  There is an overhang which provides a little shelter on one side of a river leading into it.  So we were able to take some footage but from one slinging spot only.  Unfortunately not a place were we'd ever have got what we slung back so none of the test designs just rocks.  One is especially nice; clear crack, just short of the opposite bank, so not far but as far as one could hope to get a visible splash instead of some impossible to see land impact.  Clear splash sound.  Also has a nice few seconds where the wind had dropped for a sample that can be used for noise reduction.   I've been playing with it, using virtual dub to chop to the frames I want, then exporting wav files.  then using audacity to zoom in and get some precise sound timings. 

Specifically from sound of crack to sound of splash (3.46905 ) , for apparant duration of flight.   Frame before splash to sound of splash (0.276057 ) and the following beginning of splash frame to to the sound of splash (0.159645) a difference of 0.116412.  Then due to slow frame rate the door to errors guesswork bit.  Having seen splashes before, and this one's sound shape in particular, I can guesstimate which quarter of the frame it started in but not much closer. For this I'd say a quarter frame before the one it shows in so another 0.029103.   I make that a difference of 0.188748 between the start of the splash image and the start of the splash sound.

Humidity 100%,   pressure 1014,  Temp 10 C  So according to the web calculator I mentioned before the speed of sound was 337.9 m/sec.  Which with a huge margin of error gives 63.7779492 meters as the distance.  It looked about that.

The shot was a massively high lob,   Even so, for the exercise, it would have taken sound 0.188748 second to travel from the splash to camera.  He was probably between 2 and 3 meters from the camera, which was behind and slightly to one side,  but moving around so please forgive me for ignoring that bit for now.  So deduct the time the sound took from the apparent flight time to get the real flight time and its 3.28032.  Had it been a straight line, which it was an astoundingly long way from being, the average speed for the whole trip would have been 19.442599868....oh and lots more digits meters per second.  For someone who hadn't even picked up a sling until a year ago and only has a couple of hours some weeks to practice he did ok, very short sling too.

I'll post the raw video if anyone wants to check, but not from this dial up monster.  Despite the no go on the prototype glands we learned a bit:-  Know what the margin of error is on bottom of the range single digital camera technique.   Know to take an umbrella next time, in case we're lucky enough to get human repelling rain, so we can shoot up the reservoir. The margin of error would be the same but it's % of the total would shrink dramatically on long stuff.  We also chose the roughest   misshaped rocks we could find in the hope of a buzz and they did, every time.  But though we could hear them for at least half of the flight the camera microphone didn't get them at all.  Didn't try the mini disc which may have, but too windy for success  in a plastic bag, and I couldn't shelter both in my hands.   Useful seeing  what a  cam could do on its own anyway. 

It looks like for the future the Doppler effect will be useful, one on the squeeze  side and one on the stretch side could be calculated  asymetrically and combined, this would create a doppler model which would have the same effect as 40ish m/ps would have on the waves of something moving at only 340ish m/ps, seems like a significamt  %.,  even if the glans HAD  gone in a straight line at such a low average total flight speed.   It also seems that triangulation of dpb's would work very well even with such low tech stuff.  Given the dramatic parabolas of slinging ( long range to keep margins of error % down ) we will have to go 3D.sometime.  The more samples the better but I reckon, as far as method is concerned, the above will do ( please check my logic on this, I'm no physicist).  So 2D is next.  For that we need the loud  glands, so I'll concentrate on those.
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« Last Edit: Nov 10th, 2006 at 10:07pm by Stringman »  

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Re: Measure your speed
Reply #43 - Nov 11th, 2006 at 1:39am
 
My logical faculties have just noticed my mental desires putting bias on a result.  I couldn't have heard the stones buzz for "at least half the flight", towards the top of the arc the stone wouldn't have the pushing power to buzz, well I don't think it would, it'd be asking a lot of forward momentum to maintain that on it's own. Especially on such a high shot.   Yet that's the memory......hummm.  I'd gone up with a strong desire to make buzzes with my modeling clay prototypes.  Didn't get the chance and .....The sub concious can be dangerous to empirical science.  Lesson measure Everything! I shall now go and let my left and right brains  beat each other up, been brewing for ages best let them get on with it, clear the air in the long run.  I'm staying  out of it, just have a cup of tea and watch.
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Re: Measure your speed
Reply #44 - Nov 11th, 2006 at 3:27pm
 

Quote:
The sub concious can be dangerous to empirical science.


Conversely, and sadly enough, empirical science has proven quite dangerous to the subconscious mind....and by established tradition, nurtures the seeds of it's own destruction.


TS
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