Re: Chainless Mecanum Drive
 

I agree that casters may help in non-pushing situations, but lower-CoF wheels would help it situations where a robot is pushing a wall (which happens more than we think since drivers sometimes get confused about which way is 'forward').

Many rookie teams used KOP drive trains and could easily push into goals in 2010. That pretty much comprised the DC Regional elims. CMP elims even had KOP bots. Though, I did assume a flat field for my conclusions.

And the only anecdotal evidence I have is my own team from 2007 since we've used 4 CIMs since then. Yet in 2007 we were just fine for our strategy (which didn't include pushing or shoving) & design (6WD drop center). Other robots could beat us to places, but in our matches we were able to figure out how to make that not matter. That year we had some really weird radio problems, which was why we didn't go too far in an event.

It really boils down to strategy, and optimizing the robot for that strategy rather than trying to get the robot to account for all of the unknowns.

I'll see if I can get a picture up of the analysis I did a couple of months ago for 'drag race' situations. The differences are nominal for positioning, yet they do not take into account turning, pushing, or angled field elements.

Re: Chainless Mecanum Drive
 

Perfectly clear. Thank you.

Re: Chainless Mecanum Drive
 

Moreover, a 2WD robot with casters doesn't need anything to slip in order to turn (all contact points can remain in static/rolling friction with the ground while turning). This allows the 2WD robot to be geared to a higher speed than a comparable 4WD robot without compromising agility (though if at least one pair of omniwheels is used with the 4WD robot, the difference may be trivial).

Re: Chainless Mecanum Drive
 

I would say 973 will always do 4 motor over 2, but that doesn't mean 2 isn't capable of winning. I do think it's fair to point for your anecdote that most of those datapoints are from 2009, with a different floor, and where 95+% of the teams only ran two motors.

Re: Chainless Mecanum Drive
 

That is a fair point to make. Our success in 2007 was entirely due to a strategy that did not require we interact vigorously with our opponents. In fact, we used two CIMs that year only because it saved valuable weight. 2008 was unremarkable.

2009 is an anomaly, in a sense, and I didn't think to consider that. In that case, the value of using only 2 CIMs was probably overstated.

In 2010, we again used only 2, but with an 8WD skid-steer robot. It is the most 'traditional' robot we've made to use the arrangement.

If 2011 allows a fifth CIM motor again, we'll seriously reevaluate whether we used 2 or 4 CIMs on drive. I'd like to give 4 another shot again and see where that takes us.

Re: Chainless Mecanum Drive
 

2 Cims is fine, 4 just allows for a totally different style of play. It allows you to gear ~16fps in high with decent acceleration, and have a pretty quick low, usually 5-6 fps for us, that is still traction limited. It's nice knowing that when compared to the average team with a single speed in 7-10 fps, we can push harder and go faster.

It really depends on the style of play though; my love for 4 Cims with shifters came from competing against 330 in 07; they had a real rough and aggressive style of play, and used both high and low very well. If they had a run a dual CIM setup, with the amount of defense they experienced, I doubt they would have even half the success they did that season.

Re: Chainless Mecanum Drive
 

This 2 CIM v. 4 CIM thing is kind off topic, but I'll contribute because I think its a good discussion!


I think a 2 CIM drive system can be done successfully in FIRST, but you need to ensure your strategy is extremely conducive to that arrangement. Meaning: You better have a good reason for doing so.

FRC has changed a bit since '03, but I'll bring it up anyway for the folks who were around then.

Our team (Team 11), completely misjudged the game and built a pretty weak drive system with an elaborate bin collector in addition to several other underwhelming subsystems. We were basically pushed all over the field, even with the games still being 2 v. 2. I knew I never wanted to be in that position again so we made a team decision in '04 to build a very powerful drive system.

We used a combo CIM/drill system with 4 - 12" pneumatic skyways. The bot did NOT want to turn, but it could push like hell. Our robot did 2 things in '04, push and hang from the bar. It was an extremely successful season with 2 semi final losses, a regional victory and a 4th seed in Atlanta.

The point of the story is that a strong drivetrain gives you options. If for some reason you need to play defense, or if you are facing a strong defender, having that extra power in reserve will reap its rewards.

Like I said, FRC has changed a bit since those days, and a strong drivetrain does not get you as far, but you have more options with a strong drive base. Whether its single speed, shifting, or something else thats fairly creative, using 4 CIMs open up more choices for your drive system (and inherently less in your other systems, but thats what engineering is all about! tradeoffs).

-Brando

Re: Chainless Mecanum Drive
 

I have three simple rules for our drive-train

#1 No casters
#2 We will use 4 cim's (or whatever is equivalent.)
#3 It has to be built in two weeks (at least for the practice robot.)


These are not set in stone but the team will have to work real hard to sell me otherwise or if the game "screams" for something else.

Remember Mr Bill's rule for the three most important things for a successful FIRST robot.

#1 Drive-base
#2 Drive-base
#3 Drive-base

Re: Chainless Mecanum Drive
 

Remember KISS!!!

My team used 6WD last year, but could have easily gotten away with 4wd, if all were driven. If you want, you can always do 4wd, but you lose the omnidirectional capabilities. That said, mecanum has a large loss of efficiency because it has the rollers on the wheels, which is why my team is prototyping a swerve system this fall. To answer your origional question, I would lean to getting 4 toughbox nanos and attacing your wheels to that with hubs.

Good luck

Re: Chainless Mecanum Drive
 

I hope you mean with at least one set of omnis on there for the 4wd. Otherwise it is incredibly likely you won't turn.

Re: Chainless Mecanum Drive
 

Mecanum has little or no loss of efficiency (compared to a standard wheel vehicle using the same tread material) in the fwd/rev direction; only less traction.

Mecanum does have loss of efficiency (compared to 100%) in the strafe direction, due to friction in the roller bearings. This can be substantially mitigated by proper design and lubrication of the roller bearings. Of course, a standard non-pivoting wheel vehicle does not have this degree of freedom at all.

Mecanum is actually more efficient than a standard non-pivoting wheel vehicle in the rotation degree of freedom. The standard non-pivoting wheel "scrubs" the floor during turns whereas the mecanum does not.

The main drawbacks of mecanum are less traction, high cost (especially for a well-designed wheel), and greater susceptibility to damage.

The above assumes the word "efficiency" means (power out)/(power in).