Drive train questions

[JamesCH95]

Originally Posted by BJC

Actully, I disagree. I think that your robot should be geared to go as fast as the drivers can control it. If you look at teams such as 67, 1114, and 254 (just off the top of my head) they all have extreamly fast drivetrains that they can all control very well. This is generally because their drivers are very experienced from a combination of past seasons and practice robot driving.
Because games in FIRST are unfailingly "who can do more/faster" senerios if you want to win you have to go fast. So if you are really good but not that fast you can never hope to be as good as the robots that are both.

As for low gear make it low enough to push stuff but not low enough that other robots can run away, that aside its all preferance.

my 2 cents

I agree with you in principle, that a team should build a robot to be as fast as can be controlled. However I would assume that a team asking how fast they should gear their robot probably does not have the driver experience to handle a robot that's faster than 10-12 ft/s, which is remarkably fast.

Here's the mandatory "it depends on the game" disclosure: if it's a wide-open field, like 2008, where a robot can get up to speed and maintain that speed for a good portion of the match then a very fast robot would be more useful. If it's a segmented field, much like 2010, where there is not as much room to accelerate and maintain a high speed, a robot might be more effective (i.e. have a higher average speed) if it could reach a lower top speed faster.

[Bjenks548]

Originally Posted by JamesCH95

I agree with you in principle, that a team should build a robot to be as fast as can be controlled. However I would assume that a team asking how fast they should gear their robot probably does not have the driver experience to handle a robot that's faster than 10-12 ft/s, which is remarkably fast.

Assuming no one does better then him next year with the practice bot, we will have a second year driver. Also just because i have no idea how to do the math on this, if anyone one wants to what would be the reduction for 8" wheels if I want it to go 4 fps and lets say 15fps. If anyone wants to do the math thanks, if not I understand, its summer.

[JesseK]

In essence (be sure to keep units in check):

Robot Speed (feet/sec) = Wheel Circumference (feet/rotation) * Wheel Rotation Speed in (rotation/second)
= [Pi * Diameter] * [Motor Rotation Speed / Gear Ratio]

Assuming Diameter = 8" (0.67 feet) and Motor RPM for a CIM is ~= 5400RPM (90 rot / sec)
15fps = Pi * .67 ft * 90 rot/sec / GearRatio

or

Gear Ratio = Pi * .67 * 90 / 15
= 12.65:1 for a 15ft/sec bot with 8" wheels.

Feel free to put in the numbers for 4ft/sec.

[Chris is me]

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!

"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.