110ft/s (75mph) robot design

[JesseK]

Picture!

Caveat - doesn't account for battery & wiring resistance, which affects available power. Also doesn't account for wind resistance, which would be a big deal at higher speeds. Also, you'll want ROUND SHAFTS so your bearings can be ABEC rated for maximum efficiency.

Edit - added the 2nd picture, but it doesn't take into account a 2nd gear. 'Current from friction' is 38 amps and is total, with the above caveats and is after full acceleration is reached. So you're well above 38 amps for 16 seconds. Because physics will most definitely account for what I don't account for, I don't know if it's totally plausible.

On the surface, if you could shave another 10lbs off of it (no shifting, carbon fiber frame, only 4 wheels so it's a straight line, belts) then your time & current are MUCH lower.

(Not sure why we want to combine MPH with meters as metrics for a design ... but ok...)

[GeeTwo]

Originally Posted by Aaron Ng

Ackerman steering sounds quite difficult to design as well as build.

As you have limited your acceleration by only driving two wheels anyway, steering two unpowered wheels is not terribly complex. If you're only driving two wheels, drive them together (perhaps with a solid shaft), and have one shifting gearbox that drives it. Hopefully you can get away without a differential. Then, make a parallelogram with hinges, with one edge being the frame chassis and the two adjacent sides mounting idle wheels. Use a motor geared really low (I'm thinking an AM PG188, but the may be a better answer) to steer it. As I read your last post, it seems that you need to steer most early in the run, while you are significantly accelerating. In this case, it may make sense to use front-wheel drive and rear-wheel steer.

Also as for making it aerodynamic, it wouldn't be optimal, but you could do a lot better than having open equipment by simply putting a fairly sheet of plastic over the top and bottom of the robot, a half-cylinder on the front (perhaps a PVC pipe cut lengthwise), and a bit of wedged tail.

Finally, I agree that more shifting range would be better than more CIMs in trying to get an intentionally light robot from zero to 75 using a single FRC battery. I would use cascaded shifters, with one at around 4:1 and the other around 2.5:1 (gearboxes near these ratios are available off the shelf), then you can get composite ratios of 1, 2.5, 4, and 10. If custom designing shifters, I would aim to make one of them the square of the other (e.g. 2.5:1 and 6.25:1, giving ratios of 1, 2.5, 6.25, and 15.625). While I have never tried to shift directly from the CIM, I can certainly see how it would be a problem; moving the shifting as close to the wheel speeds as possible makes sense.

Caveat: I have not built any of these things; just a geek with a couple of physics degrees and three+ years of experience in FRC.

[MrForbes]

Originally Posted by Aaron Ng

Ackerman steering sounds quite difficult to design as well as build.

uh...no, it's very easy to design an build. We whipped up some steering knuckles real quick in 2008 to play with, connected a window motor and a very simple tie rod to these, they pivot on a bolt, and a wheel fits over the spindle. I don't have any pics of the assembled thing, unfortunately.



we had it working in a couple hours.

(s_forbes might be kind enough to make a sketch of how all this gets assembled?)

(or just google ackerman steering and look at all the drawings, it should be obvious)

[Aaron Ng]

Originally Posted by nuclearnerd

They work with friction wheels that turn in a toroidal cylinder (some car CVTs work in a similar method). They're also really wasteful

loosing so much efficiency for a shifting mechanism doesn't seem like a worthy trade off, but i will look into it.

On the surface, if you could shave another 10lbs off of it (no shifting, carbon fiber frame, only 4 wheels so it's a straight line, belts) then your time & current are MUCH lower.

hmmm. getting rid of 4 cims would shave off a little over 11lbs. so if i can design a lightweight shifter...

i was thinking of using carbon fiber, but was hesitant because our shop doesnt have the capability to work with material like that. and i wanted the design to be feasible irl.

so it takes 16 seconds to accelerate? lets say it draws 300 amps. that means, at best, the breaker lasts 17 seconds before popping. it can still work! though it is pretty clear to me at this point that this design definitely needs a few major changes to have any feasibility.

on another note, how did you create those graphs and spreadsheet?! they are amazing. did you calculate by hand and make a nice spreadsheet, or is there an actual program that generates it?

As you have limited your acceleration by only driving two wheels anyway

wait... 4 wheels are on the ground at once, and all 6 are powered. shouldn't i not loose acceleration because all the wheels are powered? or has my life been a lie for the past two years?

If you're only driving two wheels, drive them together (perhaps with a solid shaft), and have one shifting gearbox that drives it. Hopefully you can get away without a differential.

wouldn't that mean that the outside wheel will scrub a lot when you turn?

steering two unpowered wheels is not terribly complex.

uh...no, it's very easy to design an build.

i was thinking of powered ackermann as being difficult, because my gut tells me that unless most of the weight is in the front, when you turn the wheels, they will just slide and create friction, acting like a brake instead of actually turning. of course, cars do this, and they are fine, and this is my gut, and my gut is often wrong.

Finally, I agree that more shifting range would be better than more CIMs in trying to get an intentionally light robot from zero to 75 using a single FRC battery. I would use cascaded shifters

cascaded shifters? is that a shifter driving another shifter? (that was my idea for a easy solution) im afraid i cant picture what you are saying.

Could I get a picture of the gearboxes (and cims) only. I'm very intrigued.

i dont have a render of that, but i can describe it.
its three cims that are chained together, and that drives the output of a ball shifter. and the ball shifter gears drive the hex shaft for the center wheel. and the center wheel is chained to the wheels on both ends.

[philso]

Originally Posted by Aaron Ng

loosing so much efficiency for a shifting mechanism doesn't seem like a worthy trade off, but i will look into it.

Often, the "lost energy" becomes heat in one way or another. Be ready to deal with it if you do choose to go this route.

[Chak]

Originally Posted by Chak

Hey Aaron, carpevdav000 has a rather appropriate signature for this thread:

I've gotten 3 reputations thingies for this post, so I think I should clear the air and explain to everyone else who had the same negative reaction but did not directly contact me about it.
I am on the same team as the OP, and have been playing devil's advocate for this idea. This post was made just to tease the OP, and he knows. I made the post all in good fun, probably the same way carpevdav000 made that signature. So I'm not being mean on the internet, or discouraging some young student, or whatever.
I now realize the post would only be amusing to the OP and myself, so I should've used PM. So I guess my lesson here is to use Private Message to message people privately. Sorry for wasting everybody's time, won't happen again.

On another note, I love how CD took Aaron's wild idea and turned it into a plausible design. Maybe we'll try building an 80mph robot one day; after there's a brake on it. Disc brake?

[Travis Schuh]

Originally Posted by nuclearnerd

They work with friction wheels that turn in a toroidal cylinder (some car CVTs work in a similar method). They're also really wasteful, < 85% efficient.

I couldn't find a statement in your reference to wikipedia that the efficiency is <85% for the NuVinci transmission, but found 85% used in a hypothetical analysis in a discussion of the tradeoffs between powerplant efficiency and transmission efficiency. There is a comparison in the wikipedia article to the tilting-ball class of transmissions, where the "efficiency of "Tilting-ball drive" type CVTs is typically in the range of 70% to 89%." The article goes on to clarify that the NuVinci geometry "does differ significantly from the Kopp type of tilting ball variator in the reference in that the NuVinci has its torque transfer contacts on the outside diameter rather than the inside diameter," so while this gives a likely comparison, it is hard to say what this means for the NuVinci. Where are you getting your <85% number from?

The best I can find from a quick google search about the NuVinci efficiency is that the company policy is to not publish an efficiency number. That said, there is internet agreement that it is less efficient than the geared hubs. There are some numbers and conjectures in the comments in this thread, but no clear sources.

[asid61]

Originally Posted by Travis Schuh

I couldn't find a statement in your reference to wikipedia that the efficiency is <85% for the NuVinci transmission, but found 85% used in a hypothetical analysis in a discussion of the tradeoffs between powerplant efficiency and transmission efficiency. There is a comparison in the wikipedia article to the tilting-ball class of transmissions, where the "efficiency of "Tilting-ball drive" type CVTs is typically in the range of 70% to 89%." The article goes on to clarify that the NuVinci geometry "does differ significantly from the Kopp type of tilting ball variator in the reference in that the NuVinci has its torque transfer contacts on the outside diameter rather than the inside diameter," so while this gives a likely comparison, it is hard to say what this means for the NuVinci. Where are you getting your <85% number from?

The best I can find from a quick google search about the NuVinci efficiency is that the company policy is to not publish an efficiency number. That said, there is internet agreement that it is less efficient than the geared hubs. There are some numbers and conjectures in the comments in this thread, but no clear sources.

Company policy not to release efficiency numbers is a good business move but is infuriating lol.
That being said I definitely can't see it being as good as gears, given that it's a friction-based transmission.

[GeeTwo]

Originally Posted by philso

Often, the "lost energy" becomes heat in one way or another. Be ready to deal with it if you do choose to go this route.

I would say that "in general' the lost energy becomes heat. OBTW, for a physicist (e.g. me), "in general" is a code word for "always".

[julianpowell]

Originally Posted by Aaron Ng

i was thinking of powered ackermann as being difficult, because my gut tells me that unless most of the weight is in the front, when you turn the wheels, they will just slide and create friction, acting like a brake instead of actually turning. of course, cars do this, and they are fine, and this is my gut, and my gut is often wrong.

That would only happen if you break traction on your front wheels, which mainly depends on:

-Your slip angle (i.e how much you turn the wheels relative to your overall velocity vector)
-normal load on the wheels (as long friction remains linear more normal load = more traction. Also don't forget that normal load can change due to weight transfer from acceleration)
-characteristics of your wheels (you want something with a lot of lateral traction capability, i.e definitely not omni wheels)

If you put less weight on your front wheels by shifting your CG rearward you will break traction at smaller slip angles, but you will also get more yaw moment at a given slip angle! Assuming your tractive capacity is entirely linear with normal force (this depends on your wheel characteristics and the magnitude of the normal force) it all cancels out and the position of your CG has no effect on your lateral acceleration!