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[Jeffrafa]

Originally Posted by gpetilli

How did this work out for you? We are building something very similar as our off season project. This style chassis is extremely stiff and has a very low center of gravity so I doubt you had the predicted stability problems nor needed a suspension.

Although its an old thread, I thought I would chime in with a little info and our teams experience with a similar kiwi drive this past year.

A 4-wheel Holonomic, as 2587 did in 2011 has similar issues encountered by mecanum when it comes to maintaining contact and similar weight distribution between all 4 wheels. Without equal contact throughout a movement, motion will become unpredictable - this can be mitigated with a decent closed-loop control system using a gyro sensor or other feedback, but you are just trying to overcome an inherent mechanical issue.

The 3 basic factors are the floor flatness, frame stiffness, and (optionally) the use of suspension. obviously 4-wheel mecanum drivetrains are much more common than 4-wheel holonomic, so it is easier to look there for successful platforms. Essentially, most successful mecanum bases have either a flexible frame (like a kitbot C-Base), or they have a stiff frame and some sort of suspension. On very flat fields without any 1/4" lexan to drive on some teams may have mild success with a stiff frame and no suspension, but this is not ideal.

An interesting early 4-wheel holonomic used in FIRST competition is 116's 2005 robot: http://team116.org/our-team/robots/2005-robot/ They did a few unique things, including 82-degree cambered omni-wheels (this accomplished 2 primary things - pushed their contact point right to the edge of the frame for stability, and put more rollers in contact with the ground at a time for a much smoother ride on the older-style AM trick wheels). With dual roller omni's available today the second benefit isn't important, but the 1st one is still pretty cool. They had a pretty stiff welded frame, and I can't tell if they had suspension - but it was a very flat field and I think they at least had an adjustment to make sure all 4 wheels were planar.

Last year we exercised the flexible frame perimiter rules to our advantage, chose to do an omni-directional drive, but side-stepped many of these potential issues by going with a 3-wheel holonomic (sometimes called a kiwi drive). Because you only have 3 points of contact, they are all always touching the ground (remember, 3 points make a plane, and a 3-legged stool will never teter). It was the first time we had built something like that (had experimented with mecanum before), but it was a huge success for us. We attended 2 regionals and championships, ranked 4th in our division and made it to semi-finals (our best CMP showing in our 10-year history). In 44 matches throughout the season, plus another ~25 in off-season events we never once changed wheels, or performed any transmission maintenance - the drive was completely trouble-free.

We definitely didn't play defense much, but with a skilled driver we were pretty good at quickly maneuvering around the field and outrunning/avoiding defense. We used robot-relative control (not field-relative, but our driver had RC experience and a fair amount of drive practice). We only used one gyro, which greatly improved our rotational control, and was pretty simple to implement. If the frame perimeter remains flexible, we have a flat field, and we value maneuverability over pushing power I would say we might likely re-use this drivetrain, but in other situations it might not be ideal (fitting it in 28"x38" may be a pretty big stability sacrifice).