Is a shifting transmission really necessary?

[Andrew]

I agree with most that you need to first work out your functional requirements before making a decision. There are a lot of possibilities that are not really considered by the topic.

2 motors versus 2 motors + shift?

2 motors + shift versus 4 motors?

4 motors + shift versus 6 motors?

6 motors + shift versus ... ?

Many people have been going down the 4 motors in a gear box + shifting for a while. This seems to be a pretty wide area in functional space for the general constraints that FIRST has been imposing.

Having a shifting gear box design in your tool box is probably a pretty good idea. In 2004, as an example, if you used a hanger to pick your robot up, you would probably want a shifting gearbox. Hanger Up = FAST, low torque, Hanger Down = SLOW, high torque. In other words, having a shifting gearbox in your tool box does not mean you will only use it on your drive system.

What appears to be another wide area in functional space is the six motor, independently driven wheel design.

This design gives both low end torque and high speed, without a transition point that would come in a transmission. One thing which has not been talked about is getting up to speed. Unless you have a shift-on-the-fly transmission, you have to tolerate low acceleration to get to high speed. This may account for 1-2 seconds delay for a 12 fps relative to a multi-driven-wheel design.

Therefore, the multi-driven-wheel design has an attribute which I will call "quickness." This translates into an ability to maneuver (which can also be accomplished by an omni-wheel design).

The other advantage to a multi-drive-wheel design is "failure tolerance." If one motor blows out, freezes, over-heats, you still preserve the majority of your function. As an example, in 2004, one of our drive motors burned out. We noticed that the robot "pulled" to that side, but it took two matches before we realized that we had lost a motor.

If you only have one drive axle per side, a single failure can eliminate half your drive system. In our case in 2004, with three driven axles, it would take a triple failure on one side to take us completely out of commission.

My experience in FIRST has taught me that, whatever you do, make sure that no single failure can eliminate half your drive system. Whether this is a resetable breaker, drive shaft, drive chain, speed controller, pneumatic pump, etc., make sure that there is no single path to failure. The worst feeling in the world is having your robot driving around in circles when there is critical business to be done on the playing field.