I know there is already a bumper thread out there, but I think this deserves a separate discussion.
<R08> Section M: The entire length of the BUMPER backing must be supported by the structure/frame of the ROBOT (i.e. the backing material must not be in “free space” between or beyond attachment points) (see Figure 8 – 3).
Many teams design their robots with the wheels being outside the frame using support posts to attach bumpers to (which is plenty strong), but according to this rule that would be illegal. I have asked the question in the forums and I’m waiting for an answer, but I’m not optimistic about the answer.
Am I missing something in the bumper rules or will we have to modify our chassis design?
Thanks for posting a question, Craig. Many teams (mine included) will be interested in the response.
I agree 3/4 x 5 inch plywood supported at two or more points per segment is plenty strong, and many teams used that approach in conjuction with cantilevered axles in previous years. <R08-M> appears to prohibit that approach this year, but hopefully the GDC will clarify that. My guess is that some kind of structure/frame, itself securely mounted in a fixed location w.r.t. the chassis at approximately mid-bumper height, will be required to contact the entire length of each segment of the BUMPER backing plywood.
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?
I highly doubt that rule will be changed. As they said in the kickoff video, they are expecting a lot of high speed impacts this year. Because of that, I believe the GDC feels that the bumpers need to have plenty of support - support that won’t risk cracking and breaking after a dozen matches.
High speed in this game won’t be more that 7 or 8 fps. You need traction to gain speed quickly. With 6 robots and trailers out there on a slippery surface, no one will be able to gain high speed for a collision. Collisions in the past 2 or 3 years were much higher. Also, it will take an awful lot of force to break 3/4" in plywood with padding in front - considering it will be hit with another padded bumper, I don’t believe there is any way that a reasonbly supported bumper would break.
I am not anti-bumper here; in fact, I like being able to use bumpers. We’ve used bumpers every year since our rookie year in 2006; even when they were optional. They save a lot of robot wear-and-tear. What I do object to are bumper requirements that limit creativity with robot design due to the requirements. Take a look at the length of the bumper portion of the manual compared to other sections. In 2006 and 2007 the bumper requirements were two pool noodles covered by fabric, backed by 3/4" plywood securely attached to the robot. Last year they became a little more detailed due to the fact that everyone was required to use them. This year, they seem to have gone crazy with the bumper regulations.
Oh well, we’ll deal with them whatever the rules are - some of the requirements just seem very unnecessary.
During ‘Rack and roll’ our robot, with a maximum speed of 6.5 ft/sec broke our front bumper (38" long, only supported on the ends) 3 times. I’ll grant you that we played a lot of defense (who doesn’t up here in New England) but we never received any penalties for ramming / playing too aggressive defense.
Having an unsupported bumper this year will invite broken bumpers.
There’s a difference between a 38" long bumper segment only supported at the ends and a 38" long bumper segment supported every 12". The latter is never going to break if you’re using high quality plywood.
The example I gave is what I believe Craig is referring to.
I hate to “break” it to you, but that is not quite the case. During early testing, I was driving a robot with bumpers attached in exactly the manner you describe (rugged stiff fasteners, robust stanchions about every 12 inches, bumper proud of the robot frame by about three inches). In the very first drive-from-one-end-of-the-field-to-the-other-as-fast-as-you-can test, the bumper broke and splintered upon impact with another robot at the far end. Within just a few minutes, I did it again while pushing the robot around manually, just to see how fast I could push it against wheel slip.
/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.
I’m not counting on the rule changing and we’ll just go back to our previous chassis design - we just thought it might be nice to try something new (or at least new to us). That’s not a real big deal. I am a little suspect of being able to break 3/4" plywood supported every 12". If you point the grain correctly and you use “plywood” (as opposed to OSB or something similar) it should be near impossible to splinter. 1/2" plywood is commonly used on roofs with 24" inch centers between trusses - again, if the grain is pointed correctly it is very strong. Keep in mind that roofs in MI need to hold a lot of snow at times. I don’t know what your set up was and it’s quite possible that I’m wrong - it wouldn’t be the first time. I may have to set up and run some tests just to satisfy my curiosity and skepticism.
But that’s not my main argument here. Every team knows (or at least learns quickly - sometimes the hard way) that you need to build a robot robustly to compete in a FIRST competition; especially in elimination rounds. If a robot is not robust enough you will spend your time fixing it as opposed to competing. If a team tries to cut corners to save weight or $$$, they do so at there own risk knowing that the robot needs to hold up to the rigors of competition. I think that the same should go for bumpers - if you choose not to follow the GDC’s recommendations for installation you do so at your own risk knowing that you can’t compete with broken bumpers.
But, like I said; life goes on - we’ll just adjust our designs accordingly and move on.
The language of the bumper rules in the 2009 manual
has been clear from the start, although there is no specific
specific specification for the thickness of the support
for the bumper, only that it must be there.
Additionally, the manual has indicated that collisions are expected in
the game, Bill has advised us to put the velcro on the bottom
of the driver stations in his blog, and now Dave has posted his
60 G estimate for worst case collisions.
I guess the word to the wise is to design the
electronics and battery mounting carefully.
There are many grades and types of 3/4" plywood. From experience most plywood that teams buy at a typical home center is not the best for high impact and structural strength. A high grade Baltic or Finnish birch plywood would survive the impacts. How ever it is not normally locally available to all teams and is expensive. The average plywood teams use needs the reinforcement . I would agree with the GDC’s assessment.
I decided to do some more math to see exactly how it would be possible to have robots carry out these super high speed colisions.
Using f=m•a and f=μ•f[sub]n[/sub], where f[sub]n[/sub] = m•g on a level surface, solving into each other we get a=μ•g. Since gravity is 9.81 m/s2, and the given coefficient μ of static friction in the game manual is 0.06, we get a maximum robot acceleration this year of 0.5886 m/s2. It would take a robot ~4.66 seconds of maximum acceleration to reach 9ft/sec or ~2.74 m/s. During this time, the robots would each need ~6.39m to accelerate to this final velocity, or about 20ft. If their wheels were slipping the entire way, resolving for dynamic friction would give us about ~7.66m or ~25.1 ft covered while accelerating to top speed.
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?
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.
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!
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.