Chronographs?

[mikegideon]

[quote name='sL1k' timestamp='1352089574' post='839276']
I'm hoping that question mark was accidental because I'm not sure who here will check your math to confirm this... I bow before Lester lol
[/quote]

Yeah. I'm glad he made his brain hurt . I took another pass at all the Chrony's today. I'm sticking with my CE. The only other one I would consider is the Oehler.

[Guest Lester Weevils]

[quote name='mikegideon' timestamp='1352048165' post='838899']
Lester!!! I have been putting off going thru the math. I love it when you go full geek on us . Fact is, precision in a chrony should be a non-issue. I may actually pick up one of the M2's, even though it has a fairly low rating (3.1) on the Midway site. It does have some sexy features.
[/quote]
[quote name='sL1k' timestamp='1352089574' post='839276']
I'm hoping that question mark was accidental because I'm not sure who here will check your math to confirm this... I bow before Lester lol
[/quote]

Yep am a geek but ain't an engineer. I don't know about the innards of the shooting chrony except-- I know the crystal frequency and there is the right kind of guts in the box that it could be using my imagined method, but it may be doing something entirely different. Neither got time or interest to reverse-engineer it.

Was using a "possibly naive" definition of accuracy. It is possibly proper to figure accuracy some other way. Ferinstance at that 12 MHz clock example, with the 1 foot sensor spacing, there are 12 million clock ticks in a second, so if the tick counter happens to count 1714, then twelve million ticks in one second, divided by 1714 ticks counted for the duration of bullet travel past the one foot sensor, would give 7001.17 fps. If we gradually reduce the velocity of the bullet, the count will stay at 1714 and the chrony would keep reading 7001.17 fps until it finally gets slow enough to count 1715 ticks. And then the chrono display would jump from 7001.17 fps down to 6997.08 fps. At the high velocity part of the chrono's range, that imagined design can't read-out any answers between those two numbers. The shooting chrony doesn't display 6 digits, am listing higher accuracy example numbers than the gadget would display.

So I was trying to calculate the "worst case" accuracy at that velocity range. If the bullet happened to be JUST BARELY faster than 6997.08 it would be measured as 7001.17 fps and have an error of about 4 fps. And then if the bullet is just slightly slower, at 6997.08 or slightly slower, then the chony would be measuring "near perfect" again. Was calculating "the worst that accuracy could be" missing the mark by up to 4 fps at the 7000 fps high range of the chrono.

But maybe such accuracies in quantized instruments should be calculated considering statistics theory. Dunno. If there were many shots at many velocities between 6997 and 7001 fps, then some of them would be bang-on, and some of them would be off by about 1 fps, and some of them would be off by about 2 fps, etc. So the odds favor that the majority of shots will have less error than the worst-case error, and only a few of the shots will be near the worst-case error.

[Guest Lester Weevils]

The arduino has been around a long time but I'm so out of touch just now noticed it. Was in a Radio Shack and they had lots of arduino stuff on the shelf, and things have to get pretty dang "mainstream" before it gets onto a shelf in a Radio Shack.

 

http://www.arduino.cc/

 

http://en.wikipedia.org/wiki/Arduino

 

Wikipedia mentions that one of the recent arduino's (Arduino Due) is based on a 32 bit ARM microcontroller with an 84 MHz clock rate, but some of the earlier risc-based, 32 byte-sized register models will run up to 20 MHz. Just scanning the specs, looks like a fella could build a nice little chrono with an arduino. It wouldn't save any money compared to buying a chrono, but there would be the fun and flexibility of knowing how the completed gadget works, and knowing how well it works and why.

 

It looks like an arduino chrono wouldn't require lots of soldering, and the programming language looks like a straightforward subset of C. Hopefully it would be easy to drop into asm for time-critical code sections, but that would seem near-mandatory for folks writing driver library functions for the thing.

[mikegideon]

You could do it with a plain ole 8051 if the clock was fast enough to minimize the timing error. I'm thinking the key to accuracy in a chrony is sensor conditioning. No sense in rolling your own electronics when you can buy one for a little over 100 bucks.

[Guest Lester Weevils]

You could do it with a plain ole 8051 if the clock was fast enough to minimize the timing error. I'm thinking the key to accuracy in a chrony is sensor conditioning. No sense in rolling your own electronics when you can buy one for a little over 100 bucks.

 

Yep an 8051 would work fine. I haven't made any 8051 embedded system for a long time. Maybe nowadays one could find proto boards and free compiler/assemblers and such, and roll up an 8051 gadget with alpha numeric LCD display without too much actual designing and soldering. In the old days it was either time- and/or money-intensive do develop one-off 8051 gadgets. The arduino just looked like less work for a one-off.