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.
Quote:
Originally Posted by s_forbes
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. |
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.
Quote:
Originally Posted by s_forbes
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.
|
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.
Quote:
Originally Posted by Aaron Ng
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.
|
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.
Quote:
Originally Posted by Aaron Ng
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.
|
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?
Quote:
Originally Posted by Aaron Ng
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'll post all of my calculations when i get home.
|
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.