For some strange reason we’ve never built a robot more than 135 pounds (prior to seriously considering weight reduction). Usually the kit material puts you in a standard range for extra robot material to wind up around 130-135 pounds, it’s some kind of natural law. Anyhow, I have faithfully thrown my back out every year carrying these things on and off playing fields. But I still would like to see it go to 135. It has been our magic number.
This year we were really on the ball with weight reduction in the design (120 lbs) but, at the comps we had to beef her up to take the punishment everyone was dishing out, and wound up 131 minus shavings to 129.999.
The thing is, years past without trying too hard we got around 135, then this year we went overboard and got 120. It’s not a big difference. Experience has taught me how to do a lot with 5 lbs - true. But inexperience and 5 extra pounds would make making robots (which is extremely stressful) much easier.
It would be good to see a weight study showing what the kit material puts you at for a typical bot configuration. Then see what kind of weight/mass ratio is left over. I bet it’s not much.
Some tips for Weight Management
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“budget” no more than 90% of the weight limit. ie if the limit is 130 lbs then the total estimated weight of all systems should be no more than 117 lbs.
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Identify and quantify non-negotiable weight first. So know how much your controller, breakers, lights etc. weigh. Subtract this weight from the budget.
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what is left is available for adding non-essential systems. Note: depending on the game, a drive system may be negotiable.
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Assign budgets for all subsystems based on criticality. A really critical system should have a bigger budget than a nice-to have.
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Do a weight estimate for any proposed subsystem BEFORE you build it. Hint we’ve found that Inventor is pretty good at weight estimates if you put in the correct material data.
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Use your estimates to trade off between systems.
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Resist the temptation to say “I think I can get a couple of pounds out” and then use the lower number. Every item in your estimate should have documentation to back it up. If you think you can take weight out, fine. Prove it first, then change the number.
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Track the weight of each subsystem continually. If possible weigh each part before it goes onto the robot.
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Use a spreadsheet for your weight tracking. Constantly compare actual vs estimated weight. Beat on the designers of subsystems that are over weight to get the weight out. The spreadsheet can also be used to identify “heavy hitters”. Items that are extraordinarily heavy (and thus prime targets for speed holes) or items that use a lot of weight for minimal functionality.
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Assign a single person whose whole job is to make sure you meet weight. We call ours the “weight czar” a term that comes straight out of the aircraft biz. In fact, at the “bird factory” I work in, every program has a whole department dedicated to tracking the current estimated and actual weight of the aircraft. We wouldn’t do that if we didn’t think it was important, there too many other things we need to throw bodies at to waste them on something unimportant.
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Remember that nothing ever gets done within the budget. That’s why you only allowed 90% at the beginning, to account for unknowns and overruns. If that doesn’t work this year, then next year cut the budget to 85%
Chris is ABSOLUTELY correct. Regardless of what the numerical limit is, it would benefit us all to use these weight management ideas.
Here at Rolls Royce we also have several people dedicated to tracking the weight of our engines. Any change that is proposed for any flight engine is heavily scrutinized for weight impact. We pay a hefty financial penalty if our engines exceed contractual weight limits.
Have we considered the possibility that the teams that run up against weight problems each year just aren’t taking 130 as a serious limitation until much too late in the process. They tend to do this because they are unconsciously thinking “130 pounds - that’s a lot. We don’t have anything to worry about - if we run into problems, then we will just cut a bunch of holes at the end.” As a result, they don’t plan their robot weight budget properly, and have to resort to hacking off entire subsystems or drilling 1482 lightening holes at the last minute.
I think we need to be going the other way. Rather than promote the belief that 130 pounds is a rather generous number, why not reduce the weight restriction to 120 pounds (or less)? I theorize that at 120 pounds, including the battery, nearly all teams will recognize that the weight restriction is a hard problem right up front and will begin to plan accordingly. As a result of the earlier (and arguably better) planning, I would predict that teams will have more weight-conscious designs and the number of last minute “slash-and-hack” weight reduction efforts will be reduced.
So, rather than increasing the weight restriction, we need to decrease it by 10 pounds or so (or just increase the mass of the battery or other non-negotiable parts by 10 pounds while keeping the restriction where it is, which would have the same effect). And then have FIRST throw a copy of the Atkins diet book in with each kit…
-dave
i think i am going to take this discussion in another direction for a second. in this past year it seemed like it was a huge scandal about adding stuff to the robot during the competition and not getting re-weighted. while part of me would love to see the weight increased (make my hair stay in) part of me would like more of a challenge of decreasing the weight limit to 12o (giving a new challenge for the upcoming year) i think that as long as people still tend to bend the rules at the competition adding up to 5 lbs over weight and still competing i think that the weight limit should remain the same as it is still a great ch allege to some teams.
i personally think we should go back to alot of old rules like only using whats in the kit + 200 from small parts. that would be a challenge!
Two points:
1.) We used every motor except for a seat motor at the UTC Regional and still made weight with a 59 inch tall robot. This said the weight goal is not out reach. With a tighter electronics package to reduce wire we could have been under weight, but then we would have added more functions because we could. Most teams seem to feel if the goal is 130 you want to be no less that 129.
2.) I feel the weight is already too high to be safely handled by two people. I belive the OSHA policy is a 35 lb max without lift assist devices. As a guideline the company I work for likes to keep lifts even lower. I would actually like to see a weight reduction to challenge the veteran/powerhouse teams. Having to trade off weight durabillity and functionallity more in a game like last years would make an even better contest.
My $0.02
Our weight budgeting always seems to start something like this:
battery: 12 lbs
electronics / wiring: 10 lbs
chassis: 10 lbs
gearboxes/drivetrain: 100 lbs
remaining functionality: -2 lbs
I probably see it that way because I’m always in charge of remaining functionality, and it never seems like enough. I think 130 pounds is a good, aggressive weight limit, but truthfully we’ll work to whatever the requirement is (higher or lower) and budget according to what’s most important. We feel our drive train is the most important so we make it robust, but we have to make choices - is having two speeds worth not picking up a widget in this game?
Weights and cost work this way: start with a top down budget to all the components from the requirement (with margin) to see where you need to be, then do a bottoms up detail roll-up early on to make sure you can get there. We use goals and requirements - requirements add up to 130 and goals usually about 110. Update it frequently to make sure you’re on track.
I totally agree with you about the weight being a little too much for 2 people to handle. It’s good we have carts and don’t actually carry the robot everywhere.
I also think that 130 is just about right to include max.functionallity and still let two people carry it with moderate difficulty. I like the weight limit as of now because it is just enough to let us build what our wild imaginations tell us, yet safe. Reducing might limit us to a lot things, and increasing is a safety hazard.
How many carbs in aluminum?? :eek: 
:yikes:
I agree with Lavery, I think that 130 is pretty generous (even though I’ve seen us go over it, but usually by just a little). I think 120 would be a better idea then 135 or 140. In real life you can’t just hope they’ll bump up the weight limit. For example, shooting a payload into orbit (and especially if going out of orbit) you have to be very cautious with how you use your resources, because it costs much much more to launch something that weighs more.
One thing I would like to throw into the mix though as a thought, over the past 3 years I have noticed every year that the scales vary so much from competition to competition. In 2002, for example… at the local regional we weighed 129 pounds, yet when we got to Epcot for nationals… we were weighing in at 134. I think many others have run into this issue as well, and I personally wish that the scales were all calibrated equally… but at the same time… it makes you want to try to keep as far under the 130 pounds as possible, in case you happen to run into a scale that overweighs you by 5 pounds. The teams who don’t look into that possibility could run into this problem, even if you design right around 130… so I’d suggest working around 125 because of this issue. Trust me, it’ll save you tons of frustration.
At nationals in 2002 I remember that there were two scales that varied almost four pounds I think, so this is a very good point.
i remember a speech by Dave Lavery talking about the mars mission, and how he thought it was incredible how so many of the same aspects that go into FIRST also went into the building of the mars robots. size, weight, and material limits, objective tasks, required functions, etc. he said that the game really helps you with possibe real-life objectives. did he call up the people building the rocket and say “Hey guys, it’s Dave. Uh, listen, could you maybe improve the rocket a little bit so I can make my bot 5 pounds heavier?” No, he accomplished the mission as it was, and did better than anybody ever imagined. doing the game every year within the 130lb weight limit is all part of the challenge, and i think it should stay the same.
I’ve noticed that too - it’s frustrating but, unfortunately, it’s another real-world thing we’ve got to deal with.
I agree with keeping the weight limit as it is. Having a weight limit of 130 lbs is something teams have been able to count on year after year for a long time. While it would add to the challenge of next year’s game, I think lowering the weight limit would cause a lot of frustration among teams especially if it’s lowered by as much as 10 lbs.
Upping the weight limit wouldn’t help at all. Like Cory said earlier, if you upped it 5 lbs, people would find 10 lbs of stuff to put on their robot. If you upped it 15 lbs, they’d find 20 more lbs to add on. If you want to play that game, why have a weight limit at all? Oh yeah, because it’s part of the challenge. It’s a constraint teams have to deal with. If you don’t want to worry about weight restrictions, go join Battlebots.
I think Dave hit it on the head (as usual). Even after 8 seasons in FIRST our team still doesn’t place enough emphasis on the weight limitation. We discuss it. We think about it in the backs of our minds. But in the end we haul out the Rotozip and go to town. While it’s only a few pounds it’s still a glitch in the process.
I agree with everyone else in that 130 pounds is a part of the challenge. It is just one of the many design constraints that needs to be considered when developing a solution. Adding more weight doesn’t make the challenge any easier, it just makes the robots heavier.
Thanks to ChrisH for the weight management tips. You can be sure we’ll put those to work for us this coming season! 
Seah
This is the best reason yet to change (reduce) the weight restriction on FIRST robots for next year. Veteran teams have become complacent about this issue. Many of them start designing some of their subsystems in the fall, working under the assumption that the weight limit will be 130 pounds. They then have (yet another) leg up on the rookie teams and others that wait until the kick-off to really start planning out their machine.
Imagine the utter chaos that would ensue if every team really had to start from the same baseline of knowledge - or lack thereof! Imagine NOT being able to count on certain things to stay the same from year to year. It would do more to “level the playing field” between veteran and rookie teams than almost anything else FIRST could do.
It makes you wonder what sort of sick, twisted mind would do such a thing to all those innocent teams out there…
-dave
Uhhh…that would be your mind Dave!!
Dropping the weight really would turn the veteran teams on their ear. We’ll have to keep that in mind when considering new ideas for this coming season!
Thanks Dave!
Sean
I don’t wonder at all Dave . But I think it might have been more of a shock if you hadn’t tipped your hand … :yikes:
Although I’m sure it would still shake things up - I suspect anyone reading this would likely expect that it couldn’t happen.
I do, however, think one change might be in order. I agree with most that the weight limit should NOT be increased, but since we are changing out batteries throughout the competitions, and we are subject to “spot” weight checks (which I think are a good thing) I think the official weight should be spec’d W/O battery. I would like to see the limit at 120 (or whatever makes sense) without battery.
This would certainly remove all possibility of a team using a battery with the innards removed for weigh in, as some have been rumored to do in the past. Note: I have no personal knowledge of such an occurrence, but I have heard rumors about it for years. I am not picking on any particular team as I can’t remember any that those particular rumors have been directed at. I think it is an urban legend myself, but it could happen …
As an inspector, I have tried to always make sure that the robot could actually operate with the weight-in battery to minimize the chance this could occur. As a team member, we’ve always weighed both our batteries and marked them so we knew which one to use for weigh-in. Some years it has made a difference and others it hasn’t.
Yes the scales have varied from event to event in the past. In Houston I think the difference between the scales that were there was like four pounds. I know there was one scale in particular that we directed robots that were just barely overweight to. The inspectors had no clue as to which one (if any) was “right”. This year seemed to be better and I noticed that they had calibration weights in the inspection area at the events I attended. So let’s not beat on a problem from past years that has been fixed.
Personally I think that robots should be weighed every time they enter the competition area. Just put a scale at the robot entrance to the arena. Before you can enter, one of the crew must verify that you meet weight. It would certainly stop complaints that “so and so put an entire new arm on after they weighed in”. While it would make things a little bit more difficult for the queuing crew, it might also prompt teams to be a little more prompt for their matches. If teams knew they were going to need to weigh-in and there might be a line then maybe they would leave their pit before the very last second.
I know in LA we inspectors tried to watch out for the “morphing robots”. If we recognized that a robot was sporting a new appendage, then we would send the team back to the scale. But it would probably have been pretty easy to slip several pounds of ballast past us, if it wasn’t too obvious.
While it wasn’t taken to this extent, at the West Michigan regional this year, each robot that was chosen for the elimination rounds had to weigh in again. A sticker was placed on the battery they used for this weigh-in, and that battery was the one that had to be used for their first match in the elims. This helped with the issue of different battery weights in some ways (you could choose which battery to weigh with), but it also hurt you if you tried to use a lighter battery that you didn’t normally use for competition.
Our team has had the same experiences, as most if not every team has. In 2001, for one example, at Great Lakes we weighed 118.5, at West Michigan we weighed 117, and at EPCOT we weighed 123.5! That’s a huge difference to try to plan for. I’m in agreement with all the people who think we should weigh in without the battery and drop the weight limit to another number.
Here’s a suggestion…what if, for a year or two, we changed the robot specs back to what they were in 1992? The robots were small but could still be presented with challenges that they could complete successfully. I’m not familiar with the rules and the specs from the early years, but I’m sure that parts exist that we could use. It would provide a very different challenge than what we’re used to now, with the 10-foot tall bars, different zones in which to score, and ramps. Just a thought.
In my first year (associated with FIRST), the weight was 85 pounds, the current limit was 40 amps (total) and the base constraint was a 30" diameter circle. These constraints (and others) varied from year to year until recently.
We truly anticipated the kickoff because it defined not only the game but the design constraints as well.
This is real world. Being handed “impossible” constraints forces the engineering/”sales”/manufacturing team to consider trade offs.
A few years ago, I had posted on a few threads here where the issue was the size of the main battery current limit (Then a 60 amp fuse… If it blew, you were dead.). I had stated that the Bobcat had never blown a fuse and the teams that had done so had not paid attention to their power budget.
Anyways, the total power was raised (which caused a slew of new problems) and, in retrospect, I’m still not sure that FIRST did themselves or us a favor…
Tackling “impossible” design challenges led to an amazing and unprecedented era of technological developments in the late 20th century. We put men on the moon, telescopes in orbit and robotic rovers on distant planets. The outcome of which can be symbolically represented by the enabling technologies inside the laptops most of you are using to read this message.
Another benefit of fluctuating constraints is that it levels the playing field. Designing a robot (or robot subassemblies) in pre-season becomes less profitable to veteran teams in terms of something which will be used this year. Pre-season activities become slanted more toward educating the workforce (mentor and student alike) to be able to respond to whatever twisted and devious challenges Messrs. Flowers, Lavery and Kamen (et al) might throw at us.
Also, fluctuating constraints in a very tight timetable forces a discipline on the team which is beneficial to everyone involved. We learn (or relearn) the importance of a structured timeline, (sub)system requirements and intelligent trade off analysis. Moreover, it forces the entire team to actually read the rules and approach the design as a team exercise!
Therefore, a proposal for varying constraints:
Weight: 80 to 140 lbs.
Base size: 20 to 30 inches by 30 to 48 inches.
Height: 20 to 60 inches.
Geometry: Approximately every other year, use cylinders, pyramids, trapezoids, etc.
Power: 80 to 120 amps at 12 VDC.
Cost: Between 3K and 5K (with no subsystem constraints).
Components: More variation (example: Skip using CIM motors for a year or vary the number of Drill motors from 2 to 4 for a year).
Bottom line: Some people will complain no matter what the design constraints are… I say: Let ‘em and bring it on!