How will FIRST measure muzzle velocity?
 

I was reading inthe rules that there was a max speed of the balls. I think its a great rule because everybody is on the same level when it comes to how fast a ball can travel. Rules are also great if you can police them, how does FIRST plan to police this rule, are they going to have six different radar guns to clock how fast the balls are going? I hope they have a simple way to inforce this rule or I see another "what is entanglemant?" debate we had a couple of years ago.

Re: max ball speed
 

It's a safety and inspection kind of rule. You'll just have to satisfy the inspectors that your robot isn't going to throw balls faster than 12 m/s. That should be good for the whole competition unless you start throwing balls across the entire field or denting the plexiglass or something.

Re: max ball speed
 

I think the inspection that the robot goes through before you can compete will probably include a speed test of your robots throwing speed. And if the judges or an official thinks that you changed your launcher they will make you test it again. thats the most logical thing i can think of since i dont think each ref will have a radar gun. :)

Re: How will FIRST measure muzzle velocity?
 

If FIRST believes there will be an issue with exceeding the rule for "muzzle" velocity of a ball being airborne, it would be very easy for them to set-up a test of some kind at the SAME time & place that they'll be checking your robot for weight and size constraints.

Basically, a robot MUST get to mid-field or "closer" in order to NOT be in violation of the high-speed, long-toss rule. Maybe they'll set-up a test station across the top of the pits (wow.....)

Hopefully, with the official Poof-branded balls, it won;t be a big problem across the board, but wait and see during the build season who starts to "tout" their hypersonic ball tosser first......LOL :yikes:

Re: How will FIRST measure muzzle velocity?
 

knocking holes through the ceiling of the arena with a nerf ball would be a dead give-away :^)

or a switch on the operator interface for the launcher, with two settings:

STUN

KILL

Re: How will FIRST measure muzzle velocity?
 

I fear any team that can get a ball to the ceiling at Reliant Arena. And if a team hits the ceiling in the Georgia Dome? That'd be quite a feat. (No fair smuggling a robot into the nosebleed seats.)

Re: How will FIRST measure muzzle velocity?
 

I don't know if they'll need to measure it, as they already know the max distance possible (which is for the "best" angle, which I believe to be around 30 degrees, at the max muzzle velocity they've specified).

So, if your scoring from further out than that, you've got a lot of explaining to do.

- Scott 358

Re: How will FIRST measure muzzle velocity?
 

tail wind?

positive spin (floater pitch)?

Human player telekinesis?

spacial disturbance in the time/space continuum?

Re: How will FIRST measure muzzle velocity?
 

Heh heh. Check out the special "black box" we will have on our robot. Just don't stand too close. We lose way too many rookie team members that way... :)

-dave

Re: max ball speed
 

Yeah - only the head ref gets the radar gun!

Re: How will FIRST measure muzzle velocity?
 

Am I missing soemthing? The physics I remember says that the "best angle" is 45°. :confused:

Re: How will FIRST measure muzzle velocity?
 

45° gives you the farthest distance with no air (wind) resistance.

With something big and light (like a nerf ball) the best angle will be different (air resistance has a greater effect on the trajectory than gravity does).

Re: How will FIRST measure muzzle velocity?
 

The "best angle" still appears to be near 45 degrees. I've made up a spreadsheet that roughly accounts for air resistance and I'm still getting the best range at around 43 degrees. Definitely not 30 degrees. However, my spreadsheet also doesn't think 35 feet is possible at 30 degrees, so make of that what you will. This is, of course, completely theoretical. We're going to cobble together a prototype in a day or two and see what that says.

Re: How will FIRST measure muzzle velocity?
 

A quick prototype that we created had one of the balls going well over 40 feet at a speed of approx 24 mph, and that was measured with a radar gun that targeted the ball as soon as it left the muzzle.

Re: How will FIRST measure muzzle velocity?
 

I concur with Jon, we built a quicky prototype last night - but I didn't have gun to measure the speed.
My prototype was set at 30 degrees, after firing the ball it landed well beyond 35 feet, so I had to back off on the force being used in order to stay within the 12 m/s rule.

No doubt exceeding the 12 m/s can easily be accomplished - hand held radar guns could be used to measure with if available, other wise during inspection each team could be asked to set up at the 30 degree angle and shoot a ball to see if the travel is in excess of 35 feet.

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.

Re: How will FIRST measure muzzle velocity?
 

Maybe they'll test, like they test many rules; the honor system.
FIRST has said it must be less than 12 m/s, design accordingly.

Re: How will FIRST measure muzzle velocity?
 

exactly! it would be very easy to turn down your launcher speed while the bot is being inspected, then turn it back up on the field.

and it would be very contriversal if someone is able to put a lot of spin a the ball, so a 12M/s ball can float from one end of the field to the other like a frisbee.

Part of GP is that FIRST expects us (the teams) to be professionals, ie. that we know what we are doing.

Re: How will FIRST measure muzzle velocity?
 

Thanks for the clarifications:

In NASCAR as well as SoapBox Derbies, your vehicle may be inspected before and/or after the event...

So, best to simply abide by the rules of 12M/sec and not risk a total DQ....It's just a game...it's just a game....it's just a game.

ps; please explain the aerodynamics of a relatively-hard-surfaced round ball being able to be shaped to fly like a Frisbee or airfoil? ;)

Re: How will FIRST measure muzzle velocity?
 

Someone above mentioned that you have to be on the near side of the field to score legally. This is wrong. There is only a limit on muzzle velocity. You can go much farther if your robot is moving 14fps and applying 500rpm backspin to the ball..

Re: How will FIRST measure muzzle velocity?
 

I think John and Ken (in his follow-up post) are exactly right. This is one of several FIRST rules that are best enforced by the GP ethic that we all aspire to.

That said, as a lead robot inspector I will follow whatever procedure FIRST calls for. My personal opinion is that the head referee's authority under <S01> and <S02> is very broad and gives him/her several options, including a 10 pt. penalty, DQ, and mandatory re-inspection. So I don't think team compliance with the muzzle velocity rule will be an issue.

Re: How will FIRST measure muzzle velocity?
 

any ball that is spinning as it flys through the air will have a curved trajectory. This is what happens when you slice a golf ball, it curves off to one side in flight

this is how a curve ball, sink ball and floater in baseball works.

with enough spin you can actually make a ball curve upwards (instead of downwards) as it flys through the air.

The reason it happens is the spinning surface of the ball moves the air in contact with it in one direction. The front surface is hitting more air than the back surface so it pushes the air to one side. If the ball is spinning in the vertical axis the path will curve sideways.

Doesnt anyone play wiffle-ball anymore?!

Re: How will FIRST measure muzzle velocity?
 

I respectfully disagree - Rules enforced by the honor system have honor-system-y requirements: "Strategies aimed soley at the destruction of other robots won't be tolerated", for example. Rules with measured numbers in them seem like they should require something to be measured.

There ought to be an incentive for a team to take the extra time to specifically tune their shooting mechanism to be below the limit, instead of relying on their design, which may have assumed things (ball mass, coefficients of friction, motor constants, air density, moon phase) that might not be true.

Re: How will FIRST measure muzzle velocity?
 

Suppose a team designs and builds a mechanism driven by an electric motor that can launch a ball at 20 m/sec; to comply with <S01> and <S02> they limit motor voltage by setting an upper bound on PWM command in their software. They could then pass an inspection that includes a muzzle velocity test. The inspector might not realize that the team had some means of increasing that PWM limit later, so the team is on their honor not to do so.

In any case the head referee will be in much better position than the lead robot inspector to ensure that the intent (i.e., safety) of <S02> is not violated.

Re: How will FIRST measure muzzle velocity?
 

I dont know about that. Ive never seen a judge pull out a pair of calipers to make sure your primary battery wire actually was 6 AWG wire?

Ive never seen a judge pull a pressure gauge out of his pocket to make sure our pressure gauges were calibrated?

Re: How will FIRST measure muzzle velocity?
 

With any inspection checklist item, you are on your honor not to modify the robot to violate it after inspection, or to have it re-inspected if you do modify and it might violate a rule. ANY inspection item. I fail to see how this will be different. Like any other inspection item, it will be incumbent on refs, teammates, and other teams to bring it to the attention of officials if they believe some team might (accidentally) have modified their robot to violate this rule.

To Kris: The entire Fix-It Window set of rules is on the honor system. It still has numbers in it.

Re: How will FIRST measure muzzle velocity?
 

I think the intent of the original question was;

"Do you suppose they will have an inspection station with a radar gun"

not

"How will they make sure teams do not change their muzzle velocity after inspection"

It seems to me that a radar gun muzzle velocity check during robot inspection would be simple and probably will happen. I know that is how we intend to tune our robot.

Re: How will FIRST measure muzzle velocity?
 

I say that they will have a gun, or some sort of sensor, maybe even a pressure sensor that they'll have teams shoot at and then measure the velocity there. Then on the field, the Refs will throw a flag if they see a shot that is out of the possible range (isn't it around 34 feet?) or that looks to be firing faster than it should be. They then could use the the same sensor and test it right on the field after the match.

It'll be intresting, expect an update after 2/22 that'll announce it.

Re: How will FIRST measure muzzle velocity?
 

I hope that they have some station where you can test the velocity of the balls. Something similar to a chronograph at a paintball field. We can do all the calculations in the world but its impossible to know exactly how fast the ball is going with out some kind of radar machine. That would allow us to fine tune the speed of the ball launcher and get everyone on the same playing field. Just remember "cheaters never prosper"

Good Luck
~Kirk

Re: How will FIRST measure muzzle velocity?
 

Maybe something similar to what they use on paintball guns...just on a bigger scale. (The pic attached is one kind, another kind rests on a bigger tripod like chrono. They may put a big round chrono machine near the exit point of the ball.)

Re: How will FIRST measure muzzle velocity?
 

If the ball is spinning it can go further than the specified distance at 12m/s

and to confuse things even more, a radar gun would not work if the ball is spinning. I believe a radar gun registers the highest returned reading, which would be the part of the ball that is spinning towards you, including the angular speed along with the linear speed.

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>.

Re: How will FIRST measure muzzle velocity?
 

http://forums.usfirst.org/showthread.php?t=122

Plain and simple even FIRST doesn't know.

Re: How will FIRST measure muzzle velocity?
 

well they know, they just don't want to release it until ship date comes (39 days) so people can't try to design around the method they plan on using. I'm also guessing they may have multiple methods too, that way its twice as hard to beat the system.

Re: How will FIRST measure muzzle velocity?
 

Hate to sidetrack, but that is the offical QA forum? How did you find it and why isnet it linked to from http://www.usfirst.org/robotics/2006/qa.htm?

Just curious if we can point everyone in this thread http://www.chiefdelphi.com/forums/sh...ad.php?t=41762
to that link.

Re: How will FIRST measure muzzle velocity?
 

It is indeed the official Q&A forum, and the link on the FIRST site is just out of date. It was the original link they gave during the first weekend, and that is when they got bombarded with Q's from people, and A's from non GDC people. That is why the new system which your team leader should have access too was set into place. And yes they people in that other thread probably already know whats going on, they just don't seem like it much.

Re: How will FIRST measure muzzle velocity?
 

My main question is this:
if your robots shooter has the ABILITY to go past 12 m/s are you still in trouble? i mean even if you program it to slow down the motors so that it doesn't exceed 12 m/s?

Re: How will FIRST measure muzzle velocity?
 

The judges could use a simple chronograph to measure muzzle velocity.

Re: How will FIRST measure muzzle velocity?
 

It seems to me that measuring muzzle velocity is almost a trivial exercise with two light sensors (in fact the ones in the kit will do it!). To build a portable, wireless clip-on widget about a 20 cm long that mounts to the exit point of the muzzle in line with the exit vector only need work in the millisecond framework - a long time by today's electronics standards. Surely the FIRST Fathers could build a couple of these using only 5% of their brainpower.

Now...not that anyone would...but what prevents anyone from testing at the 80% speed and running up to 100% during play...? (Please don't pelt me with poofs for even thinking this...)

Re: How will FIRST measure muzzle velocity?
 

I think this gets right to the heart of <S02>. As I said earlier in this thread, the muzzle velocity limit is a (game) safety rule, not a robot rule. Compliance cannot be policed by robot inspectors because inspectors cannot be sure that they are witnessing the robot's maximum possible muzzle velocity.

The head referee must make the call when an unsafe shot (as defined in <S02>) occurs on the field, impose the prescribed penalties, and require the team to correct the robot so that it doesn't repeat the unsafe shot.

Re: How will FIRST measure muzzle velocity?
 

1 mile=5280 ft

5280
* 12(inches per foot)
------------------------

= 63360(inches per mile)
* 20(velocity robot shoots(mph))
---------------------------------

=1267200(inches per hour)/3600(seconds in an hour)
--------------------------------------------

= 352 inches per second.

(1/352)*7(diameter of ball)=0.0198863636363

in laymens terms it takes about .02 seconds to pass through the sensor at 20 miles per hour.

using what I did to calculate, you can work backward from the time it takes to pass through the sensor to figure out your velocity.

http://photobucket.com/albums/f327/X...t=100_6574.jpg
http://photobucket.com/albums/f327/X...t=100_6575.jpg