Awhile back, I saw team 868’s Totebot, a robot in a tote that drives at 50mph. I was inspired by their video to design a robot that can go as fast as possible using only FRC legal parts.
I have named the design: Sanic
It has 6 cims, geared at 34-50 at low, and 50-34 at high. achieving 35mph and 75mph respectively.
Amazingly, the math all works out, for the amperage at least. By limiting the amperage for each cim at startup to 40, and then bumping it up to around 50 when shifting to high, it can reach 75mph without the 40 amp snap action cim breaker, or 120 amp main breaker popping. It comes very darn near to dying, but it doesn’t.
I’m not a electronics guy, so I’m not sure if there is anything else that will go wrong (there probably is) except for the roboRIO browning out, if that is the correct term.
Of course, this isn’t something that I will ever build. It was a fun thought experiment, and a challenge, to see how far I can push the equipment that we have.
And before anyone asks, no. I don’t have a way to stop it. :yikes:
Thoughts? Questions? I would love to hear what the community thinks of a crazy design such as this.
Can you explain how running 6 CIMs at 40 amps each will not trip the 120amp breaker switch after a few seconds?
Did you use the JVN Design calculator to figure your expected motor currents with that configuration? Although I don’t have the other specs that I need of your design (wheel diameter, weight on wheels), I did some guessing and the resulting “pushing match” current (in this case accelerating from 0) runs up to 152 amps per motor.
6 CIMs at 40 amps draws 240 amps. AndyMark’s data sheet for the 120 amp breaker states that at 200% of the rated current (lets say a bit more because of other electronics) it can last a bit less then 10 seconds. It takes less then 10 seconds for me to reach 75mph. And the 50 amps only happen for like half a second when switching, and the cim snap breakers can draw that for a couple seconds.
wheel diameter is 4in
weight is around 50lbs (the 10lbs for electronics is added in, but not shown)
the pushing match current is if you are pressed up against a wall. in which case, this drivetrain is more then likely power limited which is why the current drawn is so high.
Im a perfect world with no opposing force, it would take a 75m runway to reach 75mph. Im guessing it would be around 100 irl.
I think you may be going a little overkill with the motors there.
If you want something to go fast (as in, high speed, not high acceleration) then you just need enough power to counteract the drag you would see at that speed. Getting rid of drag is just as good as adding power. I would suggest focusing more on a small profile robot with a gearing system that lets you go through a wide range of ratios. With the size of the components, I don’t think you’d need more than 1 CIM to power it if it was in an aerodynamic shell.
Also, you should get rid of that 6wd tank style steering system… It works good when you are driving a robot around small spaces on an FRC field, but you won’t be able to control a fast moving robot with it very well. Cars use Ackermann style steering because it’s easy to control when you’re driving down the road.
Aaron, you’ve inspired me to think more about this concept… make as fast a robot as possible with ‘ordinary’ FRC parts. I think this’ll be a design project for me for a while.
Having thought about it some, I think a good design would probably look a fair bit more different from the typical FRC robot than your design does.
I agree that you have too many motors and not enough shifting speeds. 4 CIMs are probably about all your battery and breakers can handle for the acceleration period, I’d think. If I were designing this, I’d have 3 speeds (or more) based on a custom ballshifting setup… if you make a custom ballshifting shaft and plunger you can easily get a sequential 3-speed gearbox with two .5" throw pancake cylinders. 4 speeds would be nice, but in that case you might want to start looking at a more elegant (but still highly fast!) shifting setup. 3 speeds should be able to keep you in a ‘happy band’ of 1800-4200rpm for most of your acceleration time where each motor is always producing 220-300+W and drawing 30-90A.
I’d definitely second using anything but a 6wd Tank setup… Tank is good for pushing with decent maneuverability in tight spaces. It’ll start to be intensely difficult to control above ~30fps or so, I suspect. Ackermann Steering would be ideal, but is substantially less easy. Turntable Steering might be a reasonable compromise, if you’re looking for something that has ‘adequate’ steering. A tank setup would be OK if you’re only looking for a drag racer (not an RC car), in which case I’d recommend making the rocker very small.
I’m calculating that it’d (sort of) realistically take no less than 15 seconds… I’ll share my calculations, but I’d be interested in knowing what you’re doing to get 10s or less. I’m guessing when you factor in air resistance, a realistic battery, and battery depletion that it might take 10-15s to get to 50mph and another 10-15s to get to 75mph from there. Internal Combustion engines typically peak in power and torque at high rpms… unfortunately for a design like this, to get from 50mph to 75mph not only do you need to be able to push against twice the drag force and double your kinetic energy, but you’re doing it as your motor starts putting out less and less power and torque.
I’d generally recommend using larger wheels so you need lower rpms to get to 50-75mph. A 4" wheel needs to spin at 6000rpm or so to get to 75mph… not only is this potentially unsafe for many of the 4" wheels we use in FRC, but it means you need to have your CIMs above a 1:1 in high gear. Use an 8" or larger wheel… you should be able to do your reduction in 1.5-2 stages and it’ll be much safer. Additionally, I’d recommend using a pneumatic wheel or something that has some compliance. Driving little wheels with minimal compliance and no suspension on exterior surfaces at 50+mph could easily shock or vibrate apart a frame or damage the electronics or battery.
Your weight seems a little low for including electronics and battery… are you including both?
Because I think it’ll take a substantial percentage of the time to cover the final 25mph, I suspect the 75m may be quite low. This will definitely not accelerate linearly.
I’d use one of those fancy internal hub shifting things that are used on bikes to get tons of different gear ratios. I’d also use two batteries with two main breakers.
I know this configuration isn’t optimal, but I wanted to design something that my team has the resources to make. Something like, for example, a round aerodynamic shroud that covers the entire robot, would be beyond what we can build.
Yes, looking back, more shifting speeds would definitely be more optimal than my solution of more motors. Less stress on all the other electronics while being lighter. However, the reason for all the motors is that it has to start and stop within a 200m runway, as that is the longest we have at school. An artificial restraint, of course, as we won’t build it, but was definitely a fun, additional challenge for me. Would more stages compensate for lower power? I don’t know how to design shifters, but I’m thinking of having two or even three shifters next to each other.
The idea behind the tank drive was that the track at school has a turn in the beginning, so for the first, perhaps 70m I will be turning while accelerating. I was thinking of sticking my swerve design on the front, and taking out the CIMs, (is that what turntable is?) but the extra weight stopped me. Ackerman steering sounds quite difficult to design as well as build. I was thinking a simple caster, but traction may become a factor.
The reason for the smaller wheel was all because of weight. not only is the wheel larger, but now I need a way to mount the wheel above the drivetrain as I would think that keeping the robot as low as possible would be the best. But perhaps I am wrong on that point.
The weight is that low because everything is made from 1/16. That is also how I got an 18t sprocket to work. We have custom weights on most COTS parts, and solidworks estimated everything to be around 40lbs (with battery). I’m guessing that the electronics and chain would add another 10. Thus, 50
Wow! That just tore my design completely to pieces. :eek: My calculations were all done in an ideal world. No voltage drop, no air resistance, etc. I did calculate air resistance, but only at 75mph, and with a lot of guessing, since that is the extent of my knowledge in physics. So I guess that this design is no longer an “it might work in theory” but rather an “it will probably crash and burn in theory”. Though I won’t be working on this for quite a while. It has already taken up enough of my time that I was dedicating to college apps.
My calculations are as follows:
Amperage and pushing strength at taken from JVNcalc
Pushing strength at low gear, with motors limited to 40amps is 27lbs or 120N
F=MA
120=22.68A
A=5.29m/s^2
Pushing strength at low gear, with motors limited to 40 amps is 13lbs or 57.8N
F=MA
57.8=22.68A
A=2.55m/s^2
Vf=Vo+At
15.64=0+5.29t
t=2.96s
33.5=15.64+2.55t
T=7s
7s+2.55s=9.55s
120 amp breaker pops in 10s when under 200% load,
Of course, this is all in a ideal world. So irl, it probably won’t be as nice as this. Acceleration drops over time, but amperage does as well. Is that enough to stop parts from dying? Im not sure.
I’d use one of those fancy internal hub shifting things that are used on bikes to get tons of different gear ratios. I’d also use two batteries with two main breakers.
Shifting chain would save a lot of weight, but I wouldn’t necessarily trust it while going that fast. As for more batteries and breakers. that would make it FRC illegal. but I don’t recall any rule that limits robot speed
This is the sort of shifting transmission I was thinking about. Somebody made a cool battery powered tricycle using an 8 speed one that could get going pretty quickly.
If you were going to use a bicycle hub transmission, get something like a shimano alfine 11.. Just make sure to cut the current during the shift, because the won’t shift under full torque.
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…)
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.
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)
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.
(We have a similarly “creole” unit system here)
