What wheels do you use on your drivetrain? Do you use all omni-wheels, mecanums or combinations of wheels? Also, do you use a tank drive or a holonomic drivetrain? What are the advantages/disadvantages of each design?
Our team uses a six wheel drive. It is a tank drive, with the center wheels being rubber (non-Omni) in order to mitigate being pushed sideways by other bots. Only the back two wheels are directly driven, with those two wheels being chained to the center wheels. The two frontal wheels are free rolling, and not driven.
Our team uses a regular tank drive with 4" omni-wheels. In Nothing but Net our aim is to be able to be fast and precise in driving. Holonomic drivetrains are generally slower, but you are able to trade in for the ability to move sideways without turining. I do not advise using 3" omni-wheels if you are using a tank drive. Maximize the circumference of your wheels so that it travels faster. I also do not advise using chain for the drive, for it causes more friction in your wheel rotation. Use gears if you can. Please note that mecanum wheels are EXTREMELY HEAVY. According to the specs, they weight almost half a pound. You have four of those, you get two pounds just from your wheels. Their precision is questionable.
We use a 6 wheel tank drive. The 4 wheels on the corners of the robot are omni wheels, and the 2 wheels in the middle are traction. All wheels are 3.25" because we can run a higher gear ratio than on 4" wheels such that the drive is slightly faster. Also, the 3.25" traction wheels work better with omni wheels (as in they actually have traction). None of the wheels are direct driven; all of them are driven speed motors with a 2:1 gear ratio (using sprockets). It is powered by 6 motors.
We decided against a holonomic drive very early in the season. We didn’t see much point in strafing, seeing as the intake is probably not mounted on a turret and therefore requires the robot to turn to intake stacks anyway. Also, an X-drive or mecanum drive requires the number of motors to be divisible by 4, which means we would be unable to use our 6 motor drive setup, and plus and H drives a) waste motors when not strafing and b) require a lot of space that would otherwise go to the intake.
In response to the previous post, the reduced circumference of the wheels can be compensated for with a higher gear ratio because wheel diameter is inversely proportional to torque. And yes, chain can add more friction to a drive, but it also lets you move motors to the middle of the robot, leaving room for a wider intake. We leave about a single link of slack in our chain to reduce the added friction, and just ignore the loud snapping sound when our driver does wheelies. We haven’t had issues with the chain slipping ever because we avoid pushing matches most of the time.
We have the same except 3:1 using gears with an internal gearing of speed (1.6) and it is powered with 8 motors.
With smaller wheels, you can safely gear them up without stalling.
It’s counter intuitive but holonomic drives go faster than tank drives and turn slower. Explanation
I’m currently using a high speed tank drive, but I’m planning to use some sort of holonomic drive next year.
Probably an X drive depending on the game.
Also, there are no 3" wheels.
But I agree with everything else you said.
I meant the 2.75" double rollers.
I get what you are saying about safely gearing the wheels, but directly driving 4" wheels will have the same speed as gearing up the 2.75" wheels. Right?
At some point, I want to try only powering the traction wheels and connecting all of the motors with gears. I feel like that might cause the robot to do a burnout without slew rate control, but I still want to try it. Actually, now I really want to try it. I also want to try a differential-based steering system like that of a car as a side project, so I will probably make them overlap. I will post the project’s progress on Vex Forum once it gets underway, which will probably be about the second week of May, after my AP Euro exam on May 6.
Also, direct driving 4" wheels can be the same speed as gearing up 2.75" or 3.25" wheels, or it can be faster or slower. The angular velocity of the wheel (how fast the robot moves, assuming zero wheel slippage) can be calculated using this formula:
Velocity = motor_RPM * gear_ratio * wheel_diameter * PI Note: gear_ratio is written input_teeth:output_teeth for this; otherwise, divide by gear_ratio.
If you make the gear ratio high enough, you can make smaller wheels faster. Also, speed motors on 4" wheels aren’t as fast as our speed 2:1 drive on 3.25" wheels, and we had issues with turbo motors stalling on 4" wheels.
It depends on how you gear them.
We used a high speed tank drive on 4 inch omni wheels
You actually don’t want large wheels if you end up being pushed. 3" wheels geared turbo can actually push a speed geared 4" wheel drive because the torque is better applied at the edge of the wheels. Chain can also be tensioned in a way thar doesn’t cause too much friction.
I use 6 wheels chained together with one spacer tensioner which rolles freely and one sprocket tensioner that drives encoders on each side. 3" omnis in front and back and 3" traction in the center. No robot at my school can push me farther than an inch and it weighs around 14 lbs. My drive is also on pillow bearings so I have .75" of ground clearance for climbing.