So I've been thinking about trying to make an archery chronograph. My plan is to use 2 magnetic strips on the arrow and a hall sensor mounted on the bow's shelf. Then I can just measure the time between readings and a bit of simple math and I've got my FPS.
I'm expecting the arrow will be traveling about 200 feet per second so the arduino should be able to handle it without a problem. The only thing I'm not sure of is the hall/magnet. I'm going to want as small a magnet as possible so it doesn't interfere with the flight of the arrow (or slow it down), and even without a magnet attached an arrow can move some on the shelf. Will the sensor pick up a fairly small magnet if it's not right next to it? I know there are different types, is there one that would be better suited? Anything else I've not thought of?
Arrows flex a lot during the launch phase so you may need to use multiple hall sensors in a vertical line or in a donut shape through which the arrow passes.
Is your goal to make something cheaper than a $200 optical chronograph?
Right, especially since this is a long bow. I like the idea of using more than 1 hall sensor.
Yeah, I was thinking it would be a really cheap way of doing it although it's not so much about the cost, as a fun project. I think I have everything I need already in fact, unless there's a better option for the sensor.
I.R. reflective sensor similar to what is used in the line following bots; paint rings on the arrow a known distance apart. you might want to take into consideration that the speed of the arrow increases as it leaves the bow. so an accurate measurement would be taken 4-6 feet from the bow, rather than on the bow itself.
Hmm an I.R. reflective sensor might be worth a try, a reflective strip would be less of a problem to attach to the arrow. Although I found that home depot sell "Magnetic Tape" that seems to be just a little thicker than electrical tape. Still, always good to have a plan B right
I had thought what point to measure the top speed. I could be wrong but I'd have thought that top speed would be the moment the arrow leaves the string as after that there is no longer any force accelerating it. As such if I have the sensors in front of the shelf and markers on the arrow at the rear it should be at or near the fastest shouldn't it?
Maybe there is a way to make a small bow mounted one...
But, we may be able to "assume" that the first time the "beam" is broken is when the arrowhead passes through, then the beam is "unbroken" when the flights/knock pass through...so maybe just the one sensor just after the shelf thingy.
I just started archery last week! So I am unsure of all the terms.
The problem with a light beam is it would be hard to have it consistently broken because the arrows turning. I thought about a bar laser, but how would you pick up a break? It'd have to be further away if you're measuring the whole arrow too (not that I hard set on having it bow mounted).
Archery's a fun sport! I'm going to an event called the Tennessee Classic in a couple of weeks, it'd be great to have a prototype to take along. I'll be sure to post anything I come up with.
Sorry, the maximum speed is NOT 'just' after the arrow leaves the bow (nock is disengaged from string).
The acceleration curve has a steep rise from t0 (release of the arrow) to a peek at t+something (some point beyond the bow, miliseconds after the arrow is released from the bowstring). Then the curve starts a long slope down to terminis (hits a target or gravity drags it to the ground...)
The above assuming that you plot time on the x axis and arrow speed on the y axis.
Take a look at some of these, I'd poke around but I have to run.
(there's duplication with variations in quantities and package type used for shipping)
Click Active, In Stock in the top section, then Unit Price in the bottom section.
A method we used in school many years ago was to have the projectile break 2 thin aluminium foil strips a known distance apart on it's way through, and time the breaks .
Assuming the tip is metal...a metal detector like system.
A coil that has a specific frequency could detect the eddy currents if a metal tip passes trough it?
allanhurst:
A method we used in school many years ago was to have the projectile break 2 thin aluminium foil strips a known distance apart on it's way through, and time the breaks .
Allan
I think the Mythbusters used that strategy in one of their projects, because the object they were measuring the speed, or trigger point, of was not flying a consistent path and they needed reliable triggering.
A pair of strips or sheets separated by a few mm, that shorted when penetrated.
Tom...
allanhurst:
Surely that only gives you one signal - you need two to time......
Allan
Two sets, one pair to start timer, another set to stop.
Although they may only have needed to start a count down timer, zero triggered something else.
Tom..
123Splat I tried searching for that and found a few things by Hatcher, but couldn't find the study of external ballistics. Is it contained within one of his books? I'd be interested to read it. From a physics standpoint I can see why a bullet would keep accelerating since there's a huge amount of pressure behind it in the barrel, but that wouldn't be the case with an arrow.
I dug out some hall sensors I already had last night and got it to show when a regular magnet was present, but I tried it with a flexible tape magnet my wife had for art projects and nothing. I think it must be because they don't have a regular north and south pole. A little disappointing, but I did find some tiny magnets on ebay for dirt cheap. They may still be large enough to get in the way so I might end up going with one of the other suggestions. I'll keep you posted
