Quote:
Originally Posted by Bjenks548
A few more questions for cd!
What is a good gear ratio from a duel cim transmission for
1) A high speed robot (reasonable I know you could go 1:1000 and just stall)
2) A high torque robot (again reasonable I know its possible to go 9001:1)
Thanks for the help again!
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"It depends". Say for case 2, your number one priority is pushing. All you need to do is push all day and you don't care about speed or anything else, just pushing.
The limiting factor will be the traction of your wheels. Once your wheels spin in place when your robot pushes against an immovable object, you're done. Gearing lower won't make you push harder, so you might as well gear your robot to right at that point in order to preserve whatever speed you have. However, if that takes more than about 40 amps of current per motor to happen, you will trip your robot's thermal circuit breakers. You can mathematically calculate that gear ratio based on your robot's wheel size, the motors you're allowed, the maximum current you want to draw, and whether or not you've reached your maximum pushing force ("traction limited"). The number's somewhere in the ballpark of 4.5fps for roughtop tread (CoF 1.3), but I'd encourage you to figure that out for yourself.
However, gaining that ability leaves you with a very low top speed. Not many FRC events could be easily won with a robot that only goes 4.5 feet per second! If you want to go faster, either you lose the ability to push against anything forever, or the traction of your wheels is going down reducing your pushing force. That, or you use a shifting transmission and go for both.
Now, the first question: The vast majority of drivetrains require skidding wheels (the exceptions are generally too complex for someone like me to have anything to do with), so your drive will need to be limited by traction at some amperage. This doesn't necessarily need to be 40 amps, since the breakers do not immediately trip and you rarely need to turn for more than a few seconds, but you're obviously constrained by the stall current of the motors, etc. I doubt you'll be gearing your robots close enough to the stall current to matter though because of the concern of acceleration.
You also want to weigh how fast you need to accelerate and how quickly you need to get to something x feet away. At lower speeds (under 8-9 fps or so) you'll probably get to something 5 feet away relatively quickly. Acceleration becomes more of an issue when you get to speeds in excess of 11-12 fps, roughly. How much of an issue depends on the game, how far and fast you need to go in that game, etc. so you'll have to decide for yourself on that one. Luckily someone from team 1640 posted a nice whitepaper on the topic.
In short, there's a lot to consider and no stock answer; that's why more teams don't build identical drivetrains every year. One more thing I want to add, is that how fast you can achieve a game objective is often less affected by the gearing of your drivetrain than you might think.