Never Do This, and Other Good Ideas Gone Bad

Technical Technical Discussion
1

So the scissor lift reminded me that there are a lot of designs experienced teams have tried once, and forever sworn off. This thread is meant to be a compilation of “Ideas gone bad” especially the ones you see year after year. Add your Gone Bad designs, and please explain details:

I will let someone else cover scissor lift. My favorite is the big hopper of balls with a single slide open door. These always work in your head, and they always fail in practice. Typically FIRST picks a compliant ball that wedge together and jam at the exit. This leads to frantic ramming in order to unjam the hopper.

2

No idea what you’re talking about with scissorlifts. Our team’s rookie year was 2010, and we had great success with our scissor lift (Designed to climb the tower).

3

I know my former team, 27, in both '06 and '09 refused to do an Archimedes screw after what happened to them in '02. I don’t think they’ll ever return to that idea. It does have great potential though, it’s made it to Einstein, it’s all about implementation I guess.

4

I agree, it’s more the implementation than the idea. Several years before I joined the team, they created a robot that remains infamous to this day. Its name was Scorpio. In 2009, several ideas were thrown out because they had failed on Scorpio. In competition, though, we saw them done well. The implementation was flawed, not the concept.

5

Historically, Drive-trains have not been our strong suit.

In 2002, 2004, and 2005 we used 4 wheel drives using rubber wheels on all 4 corners in a long base configuration. None of those robots turned very well at all, and one of then had a tendency to leave the ground while attempting to turn…

In 2006 and 2007 we decided we were going to try out Omni Directional wheels coupled with IFI High Traction Wheels. For some reason, still unknown to me, we chose not to power the omni wheels and only powered the traction wheels… These didn’t turn all that well either…

Since then, there have been two unspoken drive-train rules on our team: No long base 4WD and No Un-powered Wheels.

6

Relying on gravity to do anything other than to give your robot weight. If it moves down (an arm, an elevator, a ball in a hopper), make it powered.

Ditto scissor lifts and “many-to-one” hoppers. I’ve seldom (but not never) seen implementations of each that were effective.

Mecanum wheels are on the verge of making the list, too (for the same reason).

7

Our team will not likely use vacuums, or conveyor belts due to flawed execution and/or excess time we spent fixing parts. We will also never make a robot with less than an inch of ground clearance again (got stuck on the field repeatedly last year).
I agree that almost anything can be successful if done correctly, some ideas are just more difficult to implement than others.

8

I think this is true for pretty much any design idea. There are few designs that are universally bad in all cases. A design may be poorly suited to a particular game, or the team may fail at execution, but given the right situation and great implementation, even the much-maligned scissor lift can be done successfully.

That being said, I do have a list of designs I will avoid unless I REALLY am convinced they’re necessary. Among them are: mecanum drive, holonomic drive, scissor lifts, the aforementioned “big hopper of balls” with a single (small) door, 3+ axis arms, and high reduction BaneBots gearboxes on high-torque applications. Most of these I learned firsthand, over my college years.

9

Cantilever shafts, try and avoid. Especially over 1.5"-2", try and support both ends of shafts.

10

I would argue with this one… If designed correctly, you can rely on gravity. The elevator we built in 2008 was powered up, and gravity down, and the only time it failed to work as anticipated was when the motor literally broke off its mountings at a pre-ship event. Other than that, it always went up AND down when we told it to, at a consistent, reliable speed.

11

I accidentally tip your robot over with your gravity-down lift up. Oops, your lift doesn’t go down anymore.

330 always adds a “gravity” cable to lifts for the case where you’re tipped over and you need to unblock the field. It also helps to retract the lift for other cases like hanging on a bar. It’s not that hard to add to the drum, either, if the winch is sized right.

Scissors lifts can be really nasty to get right. If you do get them right, they work OK… but they do have their weaknesses. I’ve seen a single-joint arm clean game pieces off the top of a scissor lift with one sweep, back in 2005. Hit a scissors lift so that it sways and… well… not pretty.

330 does have one team rule: All deployables shall be retractable (exception–we decide it’s not necessary to pull them back in). Reason: We don’t want to have to be dragging something around all match because it deployed too early by accident–and it’s really surprising when we pull up ramps and go defend a spoiler (2007) or do other similar things.

12

What difference does it make that a gravity-lowered lift no longer works when the robot’s been tipped over? Talk about not seeing the forest for the trees.

It’s usually not difficult to power lifts in both directions and it’s sage advice to do that whenever possible, but not doing so is not necessarily a catastrophic decision. There are all sorts of things that can be done to mitigate any problems that might arise from relying solely on gravity.

13

If a gravity-lowered lift is up, and the robot is tipped over, that lift is now blocking the field. Because it’s gravity-powered, it can’t go down (robot-relative) any farther. This can lead to penalties (the 84" cylinder, for example–a gravity-lowered lift that falls over while trying the top peg could easily be over 84" from the floor, causing a cylinder violation) or difficulty in getting the robot upright (longer moment arm, so more motion needed to go the same rotation).

If it could be pulled down relative to the robot (not the floor), then there is less chance for penalties, it’s easier for a partner to get the robot upright, and depending on design, the robot itself may be able to assist the righting process.

There have also been some comments–some time back–that dirt could jam the guides, or some other such things, that would interfere with gravity.

Can you mitigate it? Sure. But Murphy says that the problem will happen when the mitigation isn’t happening right.

14

I think anytime you have balls in an open hopper you are asking for trouble. Look at 254/968 in 2006. While they had a big hopper, they several belts at the bottom to get them moving towards the shooter.

We had fair success keeping them in line (i.e. a tube with a width equal to the diameter of the ball so they couldn’t bunch up), but even this clogged on us a few times in 2006. So in 2009, we always kept a grip on them.

Moral of the story: Keep something you can power on gamepieces at all times. Leaving them “to do their own thing” will only lead to heartbreak.

15

Ah, the number of spectacular failures we’ve had.

In 2006, the Pi’s rookie year, the team had a belting company sponsor us. The team decided on treads, and the company supplied us the softest durometer natural rubber they had. At 30 lbs the robot was pushing people across the carpet. At 120, when it turned, the treads went straight and the robot stepped right out of them. They pulled the treads mid-competition, refit wheels, and never went back. However, they had a single match where they hadn’t finished yet, so they put the bot out on the field without any wheels. No one said they had to move, right?

In '07, we had a spring counterbalancing the arm. The spring was strong enough, but the 1/4 bolt holding it wasn’t really. I wasn’t there for the catastrophic failure of approximately 400 lbs. of spring tension. Thank goodness. Gas shocks are your friend.

'07 banebots. 'Nuff said. Never went back - but we’re giving them another shot this year. We have Andy Mark planetaries already here as a backup, because when you buy from Andy Mark you just know it works.

In '08, we went for a 6 wheel omni wheel setup. 4 oriented front to back, and two sideways. Unfortunately, we didn’t have the foresight to make any spring loaded. Ever tried to make every leg on a 6 legged stool touch the ground at once? Our drivetrain was quickly rebuilt and frankly was never where it should have been that year.

Also in '08, we built a forklift. We ordered Bishop Wisecarver ball bearing linear rails. They strung us along for 4 weeks, telling us the parts were coming, before finally telling us we’d never see them. We built the entire 3 stage lift system out of igus slides, and used them in a manner I’m pretty sure that Igus never intended. To this date, we don’t talk about Bishop Wisecarver without nasty glares.

In '10, we tried a linear kicker. Linear bearings from Mcmaster car on precision ground steel rods. Total stroke of 6 inches. 3 inch wideup with a 3 inch slowdown after hitting the ball. It worked great on paper. In reality, we shot 1/4-20 bolts across the room into a couple 4x8 plate glass windows.

16

from 2006, 1086 has sworn off making another turret. just not our strong suit. and since 2007 we said no more elevators after making ours that year. but then we decided buying a premade system would still be possible.

17

The main problem with the scissor lift is that this year you have to lift 9 and a half feet to get to the top middle peg. To make this scissor lift, you would most likely have to have 2 connected at the joints to eliminate sway. Then when the robot raises this 9 feet, you have an amazingly top-heavy and unwieldy robot.

Also, as our coach likes to point out, more joints=more places for failure.

Archimedes screw is fantastic if implented correctly our team used 1 in 2009 to raise the balls from lunacy from the base of our robot to the shooter. We used a 2.5" piece of PVC that had a garden hose nailed to it in a spiral to provide traction.

18

On 397 the running joke for NEVER DO THIS was giving the Schreiber brothers access to gas shocks. I would like it noted that no one was ever seriously hurt despite dealing with 1000lbs of force…

NEVER make absolutes in an engineering discussion.

(ignoring the first one)

Never rely on gut feeling, check the numbers or don’t build it.

Never trust an engineer to do the numbers right the first time, have a couple people look at the numbers.

“If you need slip rings in FRC you are probably over complicating it” - said by 397’s old lead mentor. Totally true for our team.

19

Never build a permanently top-heavy robot. In '08 we ended up having to drive backwards because the lifter/ball-hitter was too far back, and we’d flip if we drove forward.

20

2003… Never use #25 chain for your drivetrain. It may seem like you will never hit that several hundred lb limit, but when you throw in missalignment and shock loading, well lets just say we went through a lot of chain that year.

2004 - Never use 4wd in the 4 corners with high friction wheels… as somone else mentioned it is very bouncy when you try to turn - you DO drive straight very well though! We ended up wraping zip ties arround one set of tires which sort of helped…

2005 - Ropes don’t belong on robots. We had at least 4 ropes on our robot holding things in place, although we managed to get it down to just the winch by competition, which worked pretty good.

2006 - Trust your numbers calculations (after you have checked them 5 times)… We made a ball shooter, calculated the theoretical speeds using 6" wheels, which fit well within our limits. Somone decided it would be better with 8" wheels - Our shooter ended up shooting too fast and we couldn’t use it at the competition… Another lesson is to make your design adjustable - There was literaly no way to take apart the shooter without taking apart half the robot.