<R08> Section M

[Matt C]

Originally Posted by artdutra04

In order for this to happen, the two robots would have to be at opposite ends of the playing field, and simultaneously floor it and accelerate as fast as possible without wheel slip until then both hit each other head on. You know, kind of like what's going to happen in autonomous? ;-)

Are you implying that with the way the robots are positioned at the beginning of a match, that autonomous is going to be like some . . Robot Demolition Derby?

[Richard Wallace]

The 2003 game (Stack Attack) had four robots start the game by simulatenously charging up a ramp, trying to be first to hit a wall of bins and knock/plow as many as possible into their own scoring zones. The frequent result was high-speed collisions, mitigated (i.e., damped) in most cases by bins interposed between the colliding robots. Bumpers were not required back then so most robots didn't have them. Fortunately, many robots also lacked sufficiently powerful drivetrains to develop significant kinetic energy at the moment of impact; however, in a few cases the crashes were spectacular.

Lunacy will provide much more frequent crash opportunities. Bumpers designed to mitigate the effects of those crashes are not just a good idea, they are the law.

[Jon Stratis]

Originally Posted by Matt C

Are you implying that with the way the robots are positioned at the beginning of a match, that autonomous is going to be like some . . Robot Demolition Derby?

yeah... it should be fun to watch! Everyone's autonomous mode is going to be "get away from the guy right behind me chucking balls into my trailer"... and on top of that, everyone starts out pointed straight at the center point!

[johnr]

So the gdc probably isn't going to come out with a minimum requirement for bumper backing. So i say leave it up to the teams to decide how to protect their robot,but have a test at inspection. Maybe a 120 pound weight with a six inch bumper on it. Pull it back(to a set distance) and let it fly. If your bot 's bumper survives your good to go. If not ,at least your in the pits.

[Karthik]

Originally Posted by Richard

What I'd like is for the GDC to provide some guidance for teams, and for robot inspectors, on what qualifies as structure/frame. Is a strip of 0.062" thick aluminum too flimsy to qualify? What about angle with a similar thickness? Or maybe 0.125" thickness is needed? What about other materials, such as plastic and wood -- can they qualify as structure/frame?

The following Q&A entry addresses some of your concerns, in regards to one specific example.

http://forums.usfirst.org/showthread.php?t=11349

[Richard Wallace]

Thanks, Karthik.

I am glad that the GDC addressed this issue with a specific example. To me, their response makes it clear that "structure/frame" is intended to mean "the strong part of your robot at bumper level"; i.e., the part that carries the load, not something you added in an attempt to satisfy <R08-M>.

[eugenebrooks]

Perhaps it is time to think inside the box.

Cantilever wheels pointed in from the frame, instead of out?

Eugene

[BJT]

So now we know that 1/8 inch flat aluminum isn't enough to support bumpers. It would seem difficult for the GDC to make that decision without knowing a few more things about the setup in question. How far does it span between other support, how wide is it?
We have been building a west coast style frame with 2 standoffs between each wheel leaving about 7 inches between each. I'm not sure what to do with it now. Is 3/16 plate between them enough to satisfy the rule?

[eugenebrooks]

One could ask the GDC how thick an aluminum plate needs to be,
mounted on standoffs shown, to satisfy the letter and intent of the
rules. Do that and carry a copy of the QandA to the inspectors at
the regional just in case.

One could also make the backing plate from stacked aluminum tubing,
stacked 4130 tubing that is available as small as 3/8 square, or laminated
carbon fiber panels. The tubing would certainly be considered structural,
and the carbon fiber would also be if it were laminated thick enough.

Eugene

[dtengineering]

Originally Posted by dlavery

/edit/I just ran a few numbers out of curiosity. In a "perfect collision" situation (two full weight 151 pound robots hitting head-on at 9 fps, with one of the robots skewed so it impacts the other "corner first") the impact forces get pretty impressive. As the robots collide, they compress the pool noodles down to 20% of their original thickness in about 0.009259 seconds. At a closure velocity of 18 fps, this is a peak change in velocity of 1944 ft/sec/sec, or a 60.75-G impact. Since I said the robot impacted "corner first" I will posit an impact area of 1.5 square inches. Assuming the pool noodles absorb about 18% of the impact energy during compression (not too bad for material of this type), that still means that the localized impact pressure is right around 10,000 pounds per square inch. I haven't looked at the bending moment of 3/4-inch plywood on 12-inch support centers yet. But I am now really not surprised by what happened to the bumpers. /edit/

There is a reason for that rule. Don't count on it changing.

-dave




.

Okay... seeing as how my other car is still on this planet, I'm a bit hesitant to question these numbers, but I was doing some calculations with our programmers this evening to figure out peak velocities and such and have to question the assumed closure velocity cited here.

We used the published value of static coefficient (.06) of friction to determine that a 150 lb (68kg) robot would have a normal force of 668n and a peak forward force of 40N. The mass of the robot, plus trailer, is 186lb, or 84kg, giving a peak accelleration of 0.47 m/s/s

Next we assumed that the effective length of the playing field was 15m. Although 54 feet works out to be 16.5m, or thereabouts, the length of the robot and trailer, as well as the driver station bumpers must be subtracted from the space available for picking up speed.

Assuming constant acelleration, of .47m/s/s over 15m, it should take a minimum of 8 seconds to cross the playing field from one end to the other, with a peak impact velocity of 3.76 m/s or... 12.3 feet per second.

Now this is the peak velocity of a robot hitting the end... but it is also the maximum impact velocity that any two robots could sustain. If each started out at one end of the playing field, they would meet in the middle, and would each only have reached 6.15 fps each, for a closing velocity of 12.3 fps, which is just 2/3 of the assumed 18 fps velocity impact. (Actually it would be lower than 12.3fps, as the effective length of the playing field would again be diminished by the length of the second robot/trailer combo unit.)

That isn't to say that some robots might not exceed the published coefficient of friction as the playing field wears, or that a 12 fps impact is something to be laughed off without concern... we'll be building a solid robot and strapping solid bumpers on it... we agree with the point of the post and if this were anything but FRC would probably just say "close enough, good enough" on the calculations, but the peak closure speed and resulting extreme G-forces didn't mesh with our calculations and we were wondering if we had somehow missed something.

Or, perhaps, if the 18fps impact velocity is based on actual testing of robots on regolith, then the published coefficients of friction don't provide an accurate estimation of robot performance. I know a few teams have posted suggesting that their experimental results for coefficients of friction are much higher than the published values.

Any suggestions?

Jason

<Edit> first assumption... that is not quite right. We assumed all of the weight of the trailer would be over the trailer wheels. Some of it will contribute to the normal force of the robot and thus improve traction and accelleration. Even assuming 100% of the trailer weight does so, however, peak accelleration is just .6 m/s/s and it takes 7 seconds to make the trip with a peak velocity of 14 fps. We're getting closer...

second assumption... we were assuming a straight line path from one end to the other... it may be possible to achieve a slightly higher peak velocity by taking a curved path along the playing surface... </edit>

<edit 2> third assumption in these calculations is that accelleration will take place on the regolith. Maybe, just maybe, if everything is right and teams are driving at least partly on the carpet, an 18 fps impact speed is a theoretically possible event </edit>

[eugenebrooks]

Use the measured figures for the coefficient of friction and you will be closer to reality with your estimates. Several teams have posted measured figures on CD, and their measurements are roughly twice the inline published values.

I would also like to say that a driver who accelerates all the way down the field and then crashes into the back wall, or into another robot near the back wall, is not engaging in an accident. A reasonable expectation is that drivers will be required to attempt to maintain control of their robots, and will be expected to plan their acceleration and braking so that they arrive at their destination without a high speed crash. I would at least hope that this will be the case, although I have not yet run across this expectation spelled out as I read the rules. I will have to read the rules a little more closely, I guess...

Eugene

Originally Posted by dtengineering

Okay... seeing as how my other car is still on this planet, I'm a bit hesitant to question these numbers, but I was doing some calculations with our programmers this evening to figure out peak velocities and such and have to question the assumed closure velocity cited here.

We used the published value of static coefficient (.06) of friction to determine that a 150 lb (68kg) robot would have a normal force of 668n and a peak forward force of 40N. The mass of the robot, plus trailer, is 186lb, or 84kg, giving a peak accelleration of 0.47 m/s/s

Next we assumed that the effective length of the playing field was 15m. Although 54 feet works out to be 16.5m, or thereabouts, the length of the robot and trailer, as well as the driver station bumpers must be subtracted from the space available for picking up speed.

Assuming constant acelleration, of .47m/s/s over 15m, it should take a minimum of 8 seconds to cross the playing field from one end to the other, with a peak impact velocity of 3.76 m/s or... 12.3 feet per second.

Now this is the peak velocity of a robot hitting the end... but it is also the maximum impact velocity that any two robots could sustain. If each started out at one end of the playing field, they would meet in the middle, and would each only have reached 6.15 fps each, for a closing velocity of 12.3 fps, which is just 2/3 of the assumed 18 fps velocity impact. (Actually it would be lower than 12.3fps, as the effective length of the playing field would again be diminished by the length of the second robot/trailer combo unit.)

That isn't to say that some robots might not exceed the published coefficient of friction as the playing field wears, or that a 12 fps impact is something to be laughed off without concern... we'll be building a solid robot and strapping solid bumpers on it... but the peak closure speed and resulting extreme G-forces didn't mesh with our calculations and we were wondering if we had somehow missed something.

Or, perhaps, if the 18fps impact velocity is based on actual testing of robots on regolith, then the published coefficients of friction don't provide an accurate estimation of robot performance. I know a few teams have posted suggesting that their experimental results for coefficients of friction are much higher than the published values.

Any suggestions?

Jason

<Edit> first assumption... that is not quite right. We assumed all of the weight of the trailer would be over the trailer wheels. Some of it will contribute to the normal force of the robot and thus improve traction and accelleration. Even assuming 100% of the trailer weight does so, however, peak accelleration is just .6 m/s/s and it takes 7 seconds to make the trip with a peak velocity of 14 fps. We're getting closer...

second assumption... we were assuming a straight line path from one end to the other... it may be possible to achieve a slightly higher peak velocity by taking a curved path along the playing surface... </edit>

[dtengineering]

I'm not counting on drivers being expected to do anything... in fact I would suggest that from the laissez-faire attitude of the head ref at kickoff to the repeated "build for impact" rules and recommendations that we should be ready for a full contact, full-bore, game.

Jason

[Daniel_LaFleur]

Originally Posted by dtengineering

I'm not counting on drivers being expected to do anything... in fact I would suggest that from the laissez-faire attitude of the head ref at kickoff to the repeated "build for impact" rules and recommendations that we should be ready for a full contact, full-bore, game.

Jason

I agree with Jason here. I fully expect very high speed collisions this year. FIRST seems to be taking off the gloves this year and actually encouraging vigorous robot to robot interaction. Build it to withstand this contact or play at your own peril.

And, Jason, I expect impacts that will be far higher than both you and Dave have calculated

[johnr]

Where and when is this perfect collision going to happen. Everyone seems to agree that every robot should at least move forward at the beginning. So in auto five bots would have to be dead and one runs across the field and hits the one in the oppisite corner. Even then it would be a head on hit. That might cause the dead bot to jack knife or worse case to form an upside down v and snap trailer hitch. During regular play you would need another bot dead in a coner,sideways to fueling station,and all the other bots out of the way. This collision would be head-to-side, maybe causing bot to tip over that bar. Never mind ,this game is starting to sound like fun.

[dlavery]

Originally Posted by dtengineering

Okay... seeing as how my other car is still on this planet, I'm a bit hesitant to question these numbers, but I was doing some calculations with our programmers this evening to figure out peak velocities and such and have to question the assumed closure velocity cited here.

We used the published value of static coefficient (.06) of friction to determine that a 150 lb (68kg) robot would have a normal force of 668n and a peak forward force of 40N. The mass of the robot, plus trailer, is 186lb, or 84kg, giving a peak accelleration of 0.47 m/s/s...

Jason-

Some of my numbers are based on empirical observations rather than theoretical values. During early development, we used standard kit-bots with prototype trailers for concept testing. We were regularly able to make the robots accelerate from the end of the field to reach top speed (9-11 fps, depending on the gearing installed) well before reaching the mid-field line. I understand that these observations may not agree with the theoretical performance calculated with the given COF. But the observed results were consistent and repeatable. So I am going to go with those.

There is one mistake in my calculations. I forgot to add the mass of the trailer into the impact calculations. With the trailer included, the localized instantaneous impact pressure is in the range of 12,300 psi.

-dave



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