[patrickrd]

Originally posted by Frank(Aflak)
when I say that i will be tough to make the chips and drills to run at the same speed . . . I do not mean at full power. However, as I understand these things, if you were to take a chip and a drill, gear them so their free RPM's are equal, and then increase the voltage from zero to twelve at the same rate, the different motors will have different rates of increasing . . . ness . . . . osity . . . . or something. I mean, that, while you can sync the motors at 12 volts, they won't nescessarily be synced when you are giving them half power. Does this make sense? So, our base will run a straight line at full speed, but will pull to one side at half power and stuff. This isn't that large a problem, but its something to think about.

Hi Frank,

The best options is to gear the motors such that the torque-speed curve lines are parallel for both motors. Then, for the motor with more power (ie the one further away on the torque-speed graph), you want to multiply the voltage applied by a constant less than one to bring the curve inward to the weaker motor. For example, the weaker motors would be used fully (PWM range -127 to 127) while the stronger motors would not be used fully (something like -108 to 108). This does waste power, but believe me, with four powered motors geared to an appropriate gear ratio and with high-traction wheels (see below), you will not have a power problem. Ensuring the same torque-speed curve using my method above will also help a LOT with control problems. There will still be some variance from motor to motor and over time and with varying heat conditions: If I were to do it, I would actually place encoders on each wheel, and use feedback control to ensure the robot is going in the commanded direction... But I don't know if FIRST rules allow purchasing of encoders. I know there are teams that make their own with the optical sensors. But to start with, ignore the encoders idea, and if things don't work well enough, you might consider it.

Now, about wheels, I have a lot of experience designing omniwheels, Here is a design I worked on last year (I've also attached a picture of final product):
Exploded view of wheel
Engineering Drawing
Engineering Drawing of Lexan "Hub"
Engineering Drawing of Roller (small rollers around the wheel)
CAD View of Robot w/ Wheels

The wheels have lexan hubs and delrin or aluminum rollers with rubber Buta-N o-rings wrapped around the outside of the rollers. The coefficient of friction between wheel and ground of this particular design is 1.3 for a felt-like material. I suspect it is at least 1.5 (probably closer to 2.0) on a FIRST carpet.

You can machine very similar wheels with manual mill & lathe, but you will need a rotary table to do the lexan pieces. It is better if you have access to CNC. You will also need to make larger wheels, probably about double the size of these wheels (which are 2" diameter). You can order O-Rings from McMaster or try and find them at a hardware store.

If you are really interested, you can actually read our mechanical design documentation at:
Cornell Robocup 2003 Mechanical Design Documentation

There may be some things of use in the documentation that I did not include here.

Good luck! And let me know if you have any questions.

- Patrick