[time-nuts] A real-world precision timing need....

Bob Camp lists at rtty.us
Sun Oct 31 15:06:16 UTC 2010


Hi

Been down this road before.

The issue turns out to be the optics in the sky screens. You can only get "just so good" without fancy optics. The normal photo detector behind a slit with the sky as the light source is far from optimum. If you are going to put money into the system, the optics are where it should go. Optical detection down range (over a large region) is tough. The simple sky illuminated optics you normally see are only "good" out to about 12" from the detector, past that they begin to degrade. Also remember that if you drop sub-sonic at 800 yards the shock wave will arrive before the bullet. That can mess up things as well. 

By far the cheapest solution is going to be a 800' spool of mil surplus telephone wire. You may have to hunt a bit, but the stuff is out there dirt cheap. Run the screens normal (in close spaced pairs).  Interface the wire to the serial output of what ever the sky screens are hooked to. Put on any of the normal line extenders to run down the cable. The baud rates are low enough that it should work quite well. If you run the sky screens in pairs the whole time base problem goes away. 

If you want to run the screens in wide spaced pairs, boost the output with an amp that matches the wire you have. That will give you time of flight, but not actual velocity. 

Another nice thing about wire - if you have AC power at the bench, you can feed the down range stuff from it. No honking big car batteries to haul down range and forget to charge. 

Cost wise - anything that's exposed down range is likely to get hit eventually. Adequate protection starts at about 1" of steel plate if it's angled to the incoming projectiles.  

Bob



 
On Oct 31, 2010, at 8:56 AM, Michael Baker wrote:

>   Hello, Time-Nutters--
>   A real-world precision timing need:
>   As a dedicated long-range rifle shooter and
>   ballistics enthusiast, I am in the early stages
>   of a project I am getting started on...
>   The object is to measure the velocity of a
>   rifle bullet both at the muzzle and downrange at
>   various distances up to 800 yards/meters or so.
>   Conventional optical sky-screens will will be
>   used for measuring the velocity at both ends.
>   However, I also need time-of-flight and this
>   requires knowing the timing relationship between
>   the time the bullet crosses the muzzle sky-screen
>   and the downrange sky-screen. Bullet muzzle velocities
>   will be between 1900 to 3200 feet-per-second.
>   Additionally, I will be using the output from an
>   array of 4 ultrasonic sensors located on the
>   corners of a 4-foot PVC pipe square to determine
>   the size of the shot group at the far end and
>   telemeter this info back to a laptop at the
>   shooting bench.
>   I can use a 10-MHz crystal for the sky-screen clocks
>   and the for the 4 ultrasonic bullet shot location
>   sensors.  However, determining the time-of-flight is
>   a more difficult task as this requires syncing clocks
>   together at both ends to a moderate degree of accuracy.
>   Out to 100 yards I can send the time-of-flight
>   far-end pulse back by wire and compare it to the
>   muzzle-end sky-screen pulse but this is not practical
>   to do by wire out at 800 yards.
>   This project is on a tight budget-- namely, MY
>   wallet, so cost is a major concern.  Suggestions
>   will be most welcome!!
>   Thanks!!
>   Mike Baker
>   Gainesville, Florida, USA
>   ---------------------------------
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