Re: How will FIRST measure muzzle velocity?
 

Does that mean that when a pitcher throws a curveball it registers as a higher speed than a ball thrown staight?

Re: How will FIRST measure muzzle velocity?
 

Absolutely correct.
RADAR is an acronym for Radio Detection And Ranging. Radar return signal delay with respect to the transmitted signal indicates the two-way distance travelled by the radio wave; i.e., range to the target. Two successive range values can be subtracted and the difference divided by the time elapsed between them, giving a 'divided difference' estimate of speed. Several of these divided differences are then averaged to calculate speed.

So the radar gun's speed indication is based on several range points and is therefore not sensitive to spin.

Re: How will FIRST measure muzzle velocity?
 

Actually, RADAR guns use the Doppler effect to measure the speed of an object rather than distance readings divided by time. I wouldn't expect spin to affect the reading though.

"A radio-locating police radar gun radiates electromagnetic signal, that reflects from metal objects. Reflected signal is again received by the radar gun. If the object moves than frequencies of radiated and reflected signals differ. The difference shows the object speed." Taken from http://www.simicon.com/eng/faq/

Re: How will FIRST measure muzzle velocity?
 

I think Richard described Vascar? an older system used by the police?

the thing with doppler radar is, you cant point it at any one object precisely. The signal reflects off everything it hits (the moving target and anything behind it or around it)

a spectrum of frequencies is returned, and the detector assumes the highest frequency difference is the targeted object.

if you point it at a spinning stationary fan, that is pointing upwards, it will read the speed of the fan blades, not the fan as a whole.

To answer someone elses question, if you throw a curveball then yes, I think the radar will read higher (how much higher? I dont know). You would have to test this by measuring the speed of the ball with some other system, and compair the radar reading for curve balls and normal pitches.

bottom line is, radar guns were designed for one purpose: to clock speeding motor vehicles. Very few cars come down the highway spinning at 600 RPM.

Re: How will FIRST measure muzzle velocity?
 

Does a radar gun even work on a foam ball? Last I checked, radio goes right through foam.

Re: How will FIRST measure muzzle velocity?
 

Bingo.

I expect that a radar gun, which you can buy from radio shack for less then a hundred dollars would be perfectly adequate to test muzzle velocity. It does mean someone has to stand right down the firing line of bots, but I'm sure there is a ref in every bunch who feels like getting shot at.

What, like you don't?

A cronograph could also easily do it (two beam interuption gates and the time between them). As to what FIRST will do I have no idea. Perhaps nothing at all but the ref's guess as to what is over the limit. In anycase, teams should figure out how fast the balls are traveling before

-Andy A.

Re: How will FIRST measure muzzle velocity?
 

Alot of airsoft guns have a mechanism that grabs the top of the BB as it exits the barrel, causing it to spin really fast. If you set it so that it grabs tightly, it "falls" up before it falls down. If you adjust the pressure just right, the BB will take a very straight path by countecting gravity until the spin slows down.

I have no idea what the exact number would be to acheive this is a FRIST robot or if it is possible.

Re: How will FIRST measure muzzle velocity?
 

Hmm an on-field check wouldnt work.. if you're bot is moving at say 12m/s forward toward the goal, and the fire speed is 12m/s, you'd get a composite 24m/s second from the stationary frame. but is that breaking any rules? no. the muzzle v is still the same.. I'd think they'd measure at stations

Re: How will FIRST measure muzzle velocity?
 

There is a quick way to do this. A basics physics program. If they can take a distance of how far the ball travels and then the angle of the shooter, than its a simple physic equation. Basically FIRST has a computer program where you would put in the shooting angle and the distance the ball went, and then the program can give you a number in which is the velocity. Now, the only thing that may be wrong is if it goes a little farther as your shooter is not starting aat zero (ground) but probably 3-4 feet up. So, it would have to have a little bit of leeway.

Re: How will FIRST measure muzzle velocity?
 

That sounds reasonable Eric. I remember from my last year in 2004, the refs and inspectors were very concerned about our hook exceeding the maximum velocity. I can't remember who the inspectors were, but it was a father and son team at the So-Cal regional in 2004 that made us show them a calculation of the theoretical max velocity to approve it.

Talk about taking out a pair of calipers etc...

Re: How will FIRST measure muzzle velocity?
 

IMHO FIRST won't measure it at all. If they treat it like they treated my team's hook in 2004, the inspectors will question the design and operation. If they beleive there is a possiblity of it being over 12 m/s, it will be up to the team to prove that it is under either via demonstration (not feasible in my mind with a shooter), video/photographic proof or calculations. If it is limited speed-wise via code, they may want to see that.

So I guess what I'm saying is that teams should be coming up with thier own ways to measure and document what you come up with.

Re: How will FIRST measure muzzle velocity?
 

I agree with the idea that it would be despicable for a team to intentionally change something after passing inspection. The honor system is there for that type of thing, and there's no getting around it. What I'm recalling are the many times that my team has shown up at competitions wondering exactly how much weight we have to shave off. We don't know until we put the thing on the scale that's verifying our compliance. Of course we weighed it before we put it in the crate, but scales have differences, and robots can sometimes get fatter when you're not looking. Of course, if there was no official scale at the competition, we would go by what our shop scale said, which might be 2 pounds heavier or lighter than a different team's scale.

None of us wants to cheat, but in the absence of an official measured velocity, everyone will pick their own way of measuring, and some will get more than others.

Oh the other hand, since this is a safety rule, and not a robot rule (like weight and size) - maybe they will just look at it shoot balls and say, "ok, that looks safe, you're good".

muzzle velocity and a moving robot
 

This has been brought up already a few times during this thread, I am just looking for some (unofficial) verification....

Am I correct in saying, that the consensus of informed veteran FIRST'ers :) interpret <S02> to mean that FIRST defines "muzzle velocity" as ball exit velocity relative to the robot that's launching it, as opposed to absolute ball velocity (or ball velocity relative to the floor/field).

It may seem obvious to most, but many have interpreted a variety of FIRST's previous rules one way, only to be "enlightened" late in the season...And in this case, it makes a bit of a difference.

Thanks for your input
-Bill

Re: max ball speed
 

The head ref should have a radar gun on hand, but he should NOT be dedicated to doing it. You can't dedicate the head ref to this responsibility and he needs to be alert to all situations so he can call certain calls at a competition. The last thing we need is a ref not being able to make a good decision on a "problem" on the field when they were forced into this one responsibility.

Re: muzzle velocity and a moving robot
 

I'll venture an unofficial opinion: <S02> is a safety rule. It is therefore aimed at preventing injuries, not at establishing a standard of 'fairness' for the competition. I think it should be policed by the referees and other field staff, with a view to promoting safety.

So my unofficial opinion is that the question of whether safety is compromised by shooting while your robot is moving at high speed (say 10 ft/sec) toward the goal is one that each head referee must ultimately decide. In practice such a tactic might enable an accurate-shooting robot to score from about 40 ft. out; i.e., well into the back court. It seems to me that if the robot shooting from that distance hits the target, there is no safety hazard.

On the other hand, a poorly aimed robot using the same tactic from 15 ft. out could send a ball 25 ft. past the player station wall. Depending on what is back there (say queued teams, VIPs, volunteers, etc.) the head ref might think safety was compromised by such a shot, and penalize the shooting team according to <S01>.