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Chronographs to measure projectile speed?

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LAN_403

Quantum Singularity

Fri Feb 24 2006, 01:31AM

Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282

I was wondering if anyone here uses chronographs for coilguns, railguns, or ring/disk launchers? Since I am a hunter I wanted to buy a chrono, especially for my bow. If I can use one also for an EM launcher than I'll probably buy one for sure. What I wasnt sure of is if a stong magnetic pulse would mess them up if spaced close? For a railgun, I was thinking the muzzle blast might mess it up as well as they are an optical device? Any other suggestions? I saw one plan online for building a chrono using the freq meas of a dmm but it didnt appear to be easily scalable atleast for firearm and bow applications.

...

Fri Feb 24 2006, 05:19AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Chronographs can be used for coil guns (I doubt the emp will be too bod, but if it is just move it further away), but the muzzle flash from a rail gun seems to mess them up. Some people have good luck just putting a piece of cardboard in front of the gun to stop the blast (the projectile just punches a hole thorough)...

The easiest way to build one yourself is to take a a pair of IR led's and a pair of photodiodes, power the LED's, and hook the photo diodes into a flip-flop logic gate, which then feeds an and gate with the output of a 1mhz self contained crystal oscillator which is divided by 10 to give 100khz, which then goes to a counter. Or if you are into programming PIC's there is some code floating around that uses a pic to do all of the hard work. With the PIC model you can change the code to make it work for most anything, and by using different clock speeds you can adjust the range for the discrete one.

Good Luck!

Bjørn

Fri Feb 24 2006, 05:38AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

http://sciencezero.org/electronics/speedtrap.htm

This one has been built by many people without any reported problems. If you look around for cheap parts it can be made almost for free.

FastMHz

Fri Feb 24 2006, 06:26PM

Registered Member #179 Joined: Thu Feb 16 2006, 02:08AM
Location: Hagerstown, Maryland - Close to Prime Outlets
Posts: 287

I use a Shooting Chrony, which can measure up to 7000fps...works for my ring launchers and coilguns and costs about $70.

Yohan

Fri Feb 24 2006, 09:08PM

Registered Member #194 Joined: Fri Feb 17 2006, 07:52PM
Location:
Posts: 19

I also use a Shooting Chrony. (on number 4, only had to pay for 1

)

We've toasted 3.
2 to EMI and 1 to stupidity in interpretation of projected trajectory.

In our design, we experienced interruptions as far as 10 feet from the last coil.
See the archives for the design.

aahz

Fri Feb 24 2006, 11:37PM

Registered Member #186 Joined: Thu Feb 16 2006, 07:22AM
Location:
Posts: 42

Yohan, you mentioned in the archives (in regard to your excellent efficiency) that you did several things atypical of most coilgunners. Would you care to explain some of the 'very significant differences' in your design? Pleeeaaaasse?

So far, I see the following contributing to high efficiency: High-capacitance coupled with variable pulse times (Not depleting the stored energy of the capacitor, keeping 'on' time current high), Slotted armature, half-bridge design, Non-conductive barrel, multiple stages...... I'm sure I missed some obvious things....

...

Sat Feb 25 2006, 12:49AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I believe the main thing that gave them good efficiency is how they used the half bridge to recover the excess energy from the coil and feed it back into the cap bank (instead of dissipating it in a diode)...

aahz

Sat Feb 25 2006, 05:06AM

Registered Member #186 Joined: Thu Feb 16 2006, 07:22AM
Location:
Posts: 42

Not quite.... he reported around 14% efficiency with a non half-bridge design (using all six stages), as well as a 'horrible' 5% efficiency when shooting for higher speeds.

After the half-bridge implementation, he reported 13% efficiency with a single stage. But still... prior to being a half-bridge design, the efficieny was excellent.

Quantum Singularity

Sat Feb 25 2006, 07:09AM

Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282

FastMHZ, Yohan, and others who use comercial shooting chronos, do you have any difficulties using the chronos indoors? I know some I have seen had optional low light/indoor hoods that incorporated a light into the hood. Will they work indoors without these or can you build your own indoor hood?

The only problem I have with DIY is that most use a photo trigger which requires the projectile to break the beam. If I am to use this with my bow also, it would be kinda difficult to be precise enough at distance or even up close for that matter to pass my projectile within 1" of the center of the beam or so.

About $70 you say... is that new/retail? Most new ones I have seen (example-cabelas) are $99.

I think its great to talk about cg efficiency and such but we might want to start a new thread for that so others who arent interested in chronos might notice it.

Bjørn

Sat Feb 25 2006, 07:47AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

If you place aluminium foil on each side of a sheet of paper you get a fairly reliable trigger that can take a lot of shots berfore it needs to be replaced.

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