Re: How will FIRST measure muzzle velocity?
 

The question is now like last year with seeing how many tetras a team could raise or how much weight it could lift. What speed can you get your Poof balls to go upon exiting the muzzle?

Re: How will FIRST measure muzzle velocity?
 

geez how big of wheels and motor r u using to get the ball to go that fast we still are trying to figure it out.

Re: How will FIRST measure muzzle velocity?
 

Who said anything about a motor? There are many other methods available and easy to prototype.

Re: How will FIRST measure muzzle velocity?
 

wow! did you use a differential equation for the air resistance in the spread sheet? I'm lucky if I can set up Excel to sum a column of numbers and get the right results!

Like most things in the real world projectile motion in air is nonlinear. Even with differential equations the best we can do is approximate it.

Re: How will FIRST measure muzzle velocity?
 

...robot moving forward at... Gee, the robot speed isn't limited*!! Add a few meters per second to that velocity.

*By rules. Physics is another matter.

Don

Re: How will FIRST measure muzzle velocity?
 

They will probably know how fast 12 m/s looks. If it looks faster, they would probably be able to know.

Re: How will FIRST measure muzzle velocity?
 

Yeah, yeah. It's a terribly grungy approximation. I'm assuming it errs on the short side. It seems like the coefficient of drag could be rather low while the ball was at 12 m/s. At any rate, I don't have enough info on the ball's roughness to figure out what weird curve relating Reynold's Number to Cd I should be using. And I was just too lazy to pull out my numerical analysis book and use something better than a first order approximation.

Re: How will FIRST measure muzzle velocity?
 

It seems trivial to me that you could use an optical sensor (like the Allen-Bradley...) and measure how long it takes for the ball to pass. If it is an 18 cm ball (about) and the speed is 1200 cm/sec, then 18/1200 = 0.015 or 15 milliseconds. less than that, it's going too fast.

You should be able to build something like that into a 10" diameter tube perhaps 6" long and just hold it at the end of the launcher, with the launched ball going into a net for safety.

If you want to build one: Note that the A-B sensor has a 1.0 mS 'on' delay and a 1.5 mS 'off' delay, which needs to me measured to see if it is consistent. Also, the RC probably cannot measure time accurately enough. Easiest would be to use an oscilloscope - even a cheap one would be OK.

Have fun. Post photos.

Don

Re: How will FIRST measure muzzle velocity?
 

sounds like coming up with a device to measure the muzzle velocity of the balls could be an engineering design contest of all its own!

Re: How will FIRST measure muzzle velocity?
 

we either just shoot our balls up in the air and measure the distance and use v=sqrt(2gh) or use a photogate borrowed from the physics department. We tried untrasonic probes, but they have a narrow beam angle and are kind of a hassle

Re: How will FIRST measure muzzle velocity?
 

A problem with radar guns is that they only measure the componen of the velocity that is aligned to the line of sight of the radar gun. If the radar gun is perpendicular to the ball trajectory (the extreme case), the radar gun will see zero velocity. This would make using them for on field measurements problematical IMHO.

Re: How will FIRST measure muzzle velocity?
 

Put two photogates right after your launcher assembly. divide distance by time till trigger.

Done.

But that's not FIRST measureing it, as for them, they'll probably check it the way they check weights.

Re: How will FIRST measure muzzle velocity?
 

my guess is that during inspection they will ask you to fire a ball at full speed, and measure it there with a radar gun. I have never been to the inspection though, so i dont really know how feasable it would be to test that.

Re: How will FIRST measure muzzle velocity?
 

The judge's "weighing" and "size-checking" station is typically located near the pit area. It would be very easy to set-up a 40 foot bullpen firing range in that same space. If your shooting mechanism clears 40', you're way over the maximum muzzle velocity (at a standstill) as in <S02> in section 4.3.1

Now, isn't that easier for the judges than a radar gun? Always think outside the box.....

Re: How will FIRST measure muzzle velocity?
 

Judges, i think you mean inspectors. Also many venues have limited space so having a 40 foot long area to test fire in may be feasible at say 2006 UTC but not so feasible at say Annapolis.