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Originally Posted by Nathan Streeter
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.
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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.
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.
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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.
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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
