Drivetrain Help

[Ken Leung]

A lot of the presentation in the white paper talks about the science behind motors. While it is great to understand those science, sometime it is more important to experiment with the actual motors itself.

Experiences tell us that the drill motors are great for drive train, and the science tells us that drill motors are the most powerful motors among the collection of motors in the kit. You should at least have a pair of drill motors in the drive train to achieve a good combination of speed and pushing force, and it is up to you to gear the drive train to the speed you see best fit for the purpose of competition.

Take last year's competition for example. If your robot isn't pushing the goals up the bridge, chances are, you want speed more than pushing force to get across the playing field quickly (although you still need certain pushing force to get up the bridge or across the center barrier).

A lot of teams shift gears or use alternate wheels because they don't have enough power from the drive train motors for both high speed and high pushing force. Which is a pretty good solution, since you probably won't need both speed and force at the same time. But there are chances when the gear shifting fail... Well, let's just say it won't be 100% successful rate. And, you will need some percise machining to make a good shifter.

As Andy said, the drill gearbox gear switcher is pretty good at shifting with servos controlling it. You just have to mount the servo the right place to keep gear switcher in place.

But I've seen HOT bot's shifter (67) last year, which switch outside of the gearbox with pneumatics, and it works really well. Basically, imagine 4 gears on two shaft, A B C D with A and B on shaft 1, and C and D on shaft 2. Shaft 1 is the one powered by drill motors, and shaft 2 connect to the wheel. So, by sliding C D on 2, you can only engage C onto A or D onto B at a time (because AB and CD have different distance apart). And AC have one ratio, BD have another.

1-------A-----------B-------- 1-------A-----------B--------
2-------C------D------------ 2------------C------D--------

But my understanding is that the HOT bot didn't need to shift gear that often.

And, it is pretty tough building a great shifter... even tougher for the "holy grail" for changing speed.

MY feeling is that more motors on the drive train with more power is easier to build. Although it will use up some of the weight and space, you won't worry about stoping the robot half way to shift gear, and being pushed while shifting... With 4 motors, you can build up quite some momentum AND push other robots with full force if needed

I thought a really good 2 powered wheel with two idle wheel system worth mentioning is 131's 2000 system. Their drive train was a 4 motors system, with two drill and two fisher price (second most powerful motor), and another pair of idle wheel.

-------------------------------------Idle--O------------fisher---O
------------------------------------------I---------------------I
-----------------------------------------o---------------------o
----------------------------------------I----------------------I
--------Drill powered--O----fisher price O-----Drill--O----idle--O

Their 4 back wheels are mounted on two bar on a pivot, so they can either rotate the two fisher price powered wheel on the ground for more power, or two idle for greater manuveriability.