Gearing for 20 FPS?
 

As hard as it would be to control, would it be possible to gear a normal 120lb FRC robot to travel at 20 feet per second while still having decent acceleration? It wouldn't be practical but it sure would be a fun driving experience.

Re: Gearing for 20 FPS?
 

Well, some of the lapbots in 2008 were reportedly geared near 25-30 FPS. Admittedly, they weren't 120 lb.

So, yes, it is theoretically possible. Now comes the fun part: calculating the gearing. I would suggest having a 2+ speed gearbox, one for starting motion and one for flying.

Re: Gearing for 20 FPS?
 

I think you also want at least 6 motors in the drive. That should help quite a bit with acceleration in addition to a multiple speed gearbox. 254 routinely gears for about 17.5 fps in their high speed but they are also usually underweight and have their west coast drive down to a science.

As an actual competetive robot it would be pretty crazy for most games, but just as a offseason project it could be pretty cool.

Re: Gearing for 20 FPS?
 

It is definitely possible. I can't quite remember where I read it (it may have been on their website), but I believe Slipstream (Team 254's 2011 robot) had a max speed of 20fps in one of their gears. I believe CD-Media has a few pictures of their gearbox around it. You may be interested in looking around for those and see if you can't get a good look at their gearing. When watching 254, I rarely saw them use it aside from the moments when they had to make quick runs across the field, and with 5 other robots on the field, that was not often. But it it would still be a nice feature to have in case of emergencies.

It is definitely possible.

Hope this helps!
-Leeland

Edit: Okay, 19.8. This robot also weighs only 100lbs, but it should still be possible.

Re: Gearing for 20 FPS?
 

This looks like a job for the JVN calculator!

...which I haven't spent enough time studying to learn what all the numbers mean. :\

Re: Gearing for 20 FPS?
 

Most people find 12-14 feet per second to be the upper limit of what is controllable on a regulation field. (And even that is game-dependent; while it was desirable in a game like FIRST Overdrive or LogoMotion, you wouldn't want to think about that in a game like Breakaway with serious obstacles slowing you.)

Now, COULD you build an FRC-legal drivetrain? It's plausible with the right math (I echo the endorsements of JVN's mechanical design spreadsheet) and from parts available on AndyMark's website. As an exercise, try to figure it out before you hit the spoiler:

Re: Gearing for 20 FPS?
 

It would be extremely difficult for a driver to control, which would certainly take some of the fun out of the driving experience (but it might be a good learning exercise anyhow). We learned our lesson the hard way in 2008. The plan was to have a closed-loop control system which in theory would have mitigated the problem, but our wheel encoders were unreliable and we had to fall back on an open-loop approach with disastrous results. YMMV...approach with caution.

Re: Gearing for 20 FPS?
 

I'm no physics buff, and could be wrong, but wouldn't this have a pretty slow acceleration?

Re: Gearing for 20 FPS?
 

Not if you design it correctly. 973 was geared for 17/7ftps and excelerated faster in high gear. And while we did use 6 motors in drive for half the season, it was designed to work with only the 4 cims. This is why the gearbox calculater is such an essential tool for custom drivetrains because it allows you to know for sure if it will work or not.

Sidenote: 148 in 2008 was originaly designed for 30fps, but later reduced because it was too uncontrolable.

Re: Gearing for 20 FPS?
 

I didn't run it that far through--he just wanted 20 FPS, he never asked if he could reach it on a regulation field! ;)

Someone want to channel their inner Mark Leon and do the math?

Re: Gearing for 20 FPS?
 

Is this actually true? John? Other 148 members? That seems like something even 148 wouldn't attempt.


Our drive this year was geared for 18.5 ft/s in high gear after accounting for losses. Before losses, the theoretical free speed was something like 22+ft/s.

I discussed this in another thread, but controlling the robot at that high rate of speed is not something to take lightly. Here is the other thread- Post #s 8, 10 and 12 were my comments on driving at that speed: http://www.chiefdelphi.com/forums/sh...42#post1085342

Some teams routinely can handle the high speed, but it is certainly not for everyone. My main point is don't assume you can drive that quickly and be able to just control it like you could a slower robot. Make sure you have good reasons to go for it, you've done the math, have a solid design and a good strategy for control and you will be ok.

-Brando

Re: Gearing for 20 FPS?
 

Post Here: http://www.chiefdelphi.com/forums/sh...2&postcount=11

Re: Gearing for 20 FPS?
 

Thanks!

Now as I'm remembering back, I do vaguely recall that. That would have been something to see that little nine-sided robot cranking around the field at 30 ft/s.

-Brando

Re: Gearing for 20 FPS?
 

Something tells me that it may have been underwhelming. (Really fast in a straight line but hard as heck to handle making it painful to watch)

Considering how that swerve was driven (RC Car controller) if any robot was going to do 30FPS well it would've been 148... But with John saying that it would've been foolish, it makes me believe that 30FPS is just too fast to be successful on an FRC field.

I remember seeing a few 25fps-ish robots in 2008 and most of them seemed to be under driven or barely controllable.

(Though, at 30 FPS, they'd have been running laps in what? 5 seconds?)

Re: Gearing for 20 FPS?
 

Yeah I agree with that. If any robot in any game was going to move that quickly, a round swerve drive robot driven with an RC controller definitely seems like the one to do it. I imagine taking non-banked turns at that speed is what started causing some issues (not to mention just the shear reaction time required at that speed).

-Brando

Re: Gearing for 20 FPS?
 

You're dead on Brando. Check out their 2008 video: http://www.youtube.com/watch?v=6fLf71xlVhE

Looks like 148 could've risked rolling over at 30fps, they were getting up on two wheels (Maybe one? Don't know how a 3WD swerve handles) during hard cornering.

Re: Gearing for 20 FPS?
 

You don't need 30 FPS for a slightly longer than 5 second lap without traffic.

Speed Racer, still my favorite robot. A perfect example of KISS. Don't even touch the trackball and score more points than 90% of hurdlers.

Re: Gearing for 20 FPS?
 

Maybe someone's already done this, but here's my quick attempt at the math.

Re: Gearing for 20 FPS?
 

Our 2008 lapbot was geared for 23 fps and had very good acceleration. 4 cims, 70 pounds, 6:1 ratio from CIMs to 6" wheels.
http://www.youtube.com/watch?v=52PxEzpm9Us

Re: Gearing for 20 FPS?
 

I can vouch for that, speedy little bugger

Re: Gearing for 20 FPS?
 

That's 23 fps at CIM free speed. Do you know what was the vehicle's actual top speed? (I'm trying to get some data on realistic numbers for speed de-rating)

Re: Gearing for 20 FPS?
 

6 motors geared for 20 fps will draw a LOT of current at stall (when starting, pushing, or turning in a skid steer arrangement). More than I'd want to design for; you risk resetting the cRIO (if you use the original 8 slot model) or the wireless gaming adapter due to momentary drops in supply voltage when drawing 600+ amps of current. You will then be sitting lame on the field for 20+ seconds while the component reboots.

With 4 motors, 20 fps will result in a woefully slowly accelerating bot.

Both of the above statements assume a bot at or near the weight limit plus bumpers and battery - obviously, a robot that weighs 80 lbs soaking wet can be geared faster and still accelerate at a good clip (although since it is soaking wet, maybe not). Maybe those crazy Cheesy Poof guys who design ridiculously fast welterweight bots are on to something :cool:

Re: Gearing for 20 FPS?
 

Frankly, the easiest way to reach >= 20 FPS is, besides throwing on more motors, NOT approach the 120lb weight limit (which, when adding in the bumpers and battery, goes to the limit of about 150 lbs). Since F=MA, and V=int(A(t),t), Shaving 20 or 30 pounds would be a huge boost in acceleration, and thus make it possible to reach those speeds within the length of the field.
Taking our 2011 robot as an example, it only had sprockets+CIMple boxes as a reduction, while weighing about 110 pounds competition-ready, it accelerated faster to higher speeds than our similar robots in the past that hit the 150 pound mark, but had higher reductions to get similar acceleration up to their lower max speed.

Re: Gearing for 20 FPS?
 

If you open up the question to bots that are geared fps free speed, it's not terribly uncommon. Our 2011 comp bot had a free speed of 20.6 fps, and our 2011 prototype Emperor Swerve was geared for 20.7 fps.

Frictional losses obviously bring these numbers down.

Both these robots have shifting to provide a low gear, and our comp bot was somewhat underweight and had very little friction in drive.

Using the oldschool version of JVN's design calc with the drive modeling tab allows you to graph the position and it's derivatives over time, and study what changing mass, friction, power, gear ratio, etc... has on them. It's very interesting how a *slight* friction difference can change speed appreciably.

Re: Gearing for 20 FPS?
 

Sorry no measurements. With a #25 chain directly from each CIM to each dead axle wheel, and its light weight, the friction loss was very low.

Re: Gearing for 20 FPS?
 

I just looked at this spreadsheet and I must say, I am floored. As a student who knows very little about DC motor physics (or physics in general), I am amazed at the amount of work that went into creating it. I would be interesting in reading an explanation of some of the math/concepts behind it, if someone would know where to find it. Also, if anyone has any tips on using it now that the motor and gearbox scene is slightly different from where it was in 2004, I would definitely appreciate that too.
Well done JVN, it would be cool to see some of these features re-incorporated into a later iteration of the design calculator.

Re: Gearing for 20 FPS?
 

2016's 2008 robot, with an ackerman steering module, there are pictures on CD somewhere of the module.. was set around 21 ft/s, we never got to measure the actual speed. Also I would look at 102's robot from 2008 they had a really fast robot as well.

we were running somewhere between a 6-7:1 gear ratio on two cims with 6" wheels. Pretty controllable for lapping, outside of lapping it was like driving a car, literally. we actually set that robot up with a steering and pedals to make it easy to drive and handle on the field...

heres one of the matches: http://www.youtube.com/watch?v=g58N0...eature=related

Btw, this is a full 120lb machine a little more with the bumpers and battery. i think we recorded 13 laps at chesapeake in one of the matches.

20ft + is not undoable in FIRST, controlling it is the hard part. One of the only reasons we were able to control ours was because of the ackerman steering and running laps being the objective. (steering was favored to help turning left).

Re: Gearing for 20 FPS?
 

btw here is the ackerman module...

http://www.chiefdelphi.com/media/photos/31565