Team 5550 - Fall Project

[Lil' Lavery]

Originally Posted by nickbrickmaster

...Yeah :|

Alright, that'll teach me to repeat information without a source and critical thinking. Drawing the vectors makes them look like the same.

Then, why does one never see a 4-omni holonomic drive used in FRC?

Back in FIRST antiquity the early/mid 2000s, you did see more holonomic drives than mecanums. Prior to the 2007 season, there weren't really any COTS options for mecanum wheels, so the few teams that used them had them custom made (namely 357). In the fall of 2006, AndyMark released the first FRC-specific mecanum wheel, and it became a bit of a cultural phenomena within FIRST. At the time, the COTS omni-wheels (also primarily sold by AndyMark) were still several generations behind what they are today, and weren't the greatest option, so when combined with the hype of the mecanum wheels and the relative ease of using a mecanum drive with a "standard" rectangular frame made them the preferred option. Mecanum drives never really looked back from there in terms of being the default choice for teams looking to move sideways.

Additionally, there is one major benefit of mecanum wheels when compared to omni set-ups when it comes to uneven flooring. Unless you're robot has a suspension or is doing some really fancy sensing and drive automation, generally speaking both mecanum and omni wheeled set-ups are reliant on each of the wheels being constantly in contact with the floor AND having a similar load on each wheel. If one wheel has a higher load on it, it will have more traction and exert more force than the other wheels, which can cause some unpredictable driving. Naturally, even worse examples of this occur when a wheel completely loses contact with the floor. Mecnaum wheels have a benefit that they can "climb" obstacles on the floor if approached dead-on such that both leading wheels make contact (more or less) identically. Think of a mecanum wheel rolling up a ramp. That type of maneuver is much more difficult to pull of in an omni-wheel set-up with the wheels spaced every 90 degrees. Thus why you saw mecanum wheeled robots climbing the bumps in 2010 and 2012, for instance.

And to answer your implicit question regarding 3 vs 4 wheeled (or even 5+) omni drives, think back to that previous paragraph. The reason many teams, such as 1114 in 2015, opt for a 3-wheel omni drive ("kiwi drive") is simple. It takes 3 points to make a plane. With only 3 wheels, it guarantees that all 3 of your wheels are in contact with the ground at all times. With 4 or more wheels, even small obstacles in the field can cause a wheel to lose contact with the floor and for erratic driving. I found this out the hard way in 2005, when my team at the time (116) built a 4-wheeled omni drive system, and even the small plastic loading zones (0.5" thick?) were enough for our wheels to sometimes lose contact with the carpet, which caused driving issues. Mecanum drives will also suffer similar consequences if they don't approach obstacles so that all 4 wheels remain in contact with the ground (plenty of teams had issues with the scoring platforms in 2015 if they contacted with only 1 wheel). Conversely, the biggest advantage today of a 4-wheeled omni drive is the simplicity of the frame construction (don't have to worry about a triangular drive base). Depending on the frame perimeter and sizing rules, there have also been points in time where a triangular frame would be space inefficient (or result in significant portions of your robot overhanging beyond your drive base, causing instability issues). Also, there was a time where the limitations on CIM motor quantities were capped at 4, meaning teams would opt to find ways to include all 4 in their drivetrains (difficult with 3 wheels).