Re: crab drive vs. mecanum drive system
 

I'll be honest, I have nothing quantitative to share with you, but...A really good 6 wheel skid is hard to be beat. It's lighter than a swerve and if you look at the teams that do it really well, their performance really isn't that much worse if worse at all from a good swerve. Keep the driver in mind too, different drivers prefer different systems. Skid steer requires basic code, successful swerves require much more complex code, especially if you want it to be intuitive.

Re: crab drive vs. mecanum drive system
 

I shudder at the thought of Mecanum with only 2 CIMs. You might as well disable trying to rotate while moving in any transverse direction. You also might as well disable the diagonal movements. The wheels simple eat up too much torque due to the 45-degree roller offset.

Simply put, either drive train gives a good driver very similar capabilities. Crab gives the driver an edge in finely tuned movements, yet Mecanum may give the driver more practice since it's much simpler to build.

With Mecanum, square up on another robot before pushing it. Programming is relatively simple. The Toughbox Nano's make a Mecanum drive train easily constructable since 4 nano's with cantilevered Mecanum wheels should be able to support the entire weight of the robot without issue. Seriously, If AM redesigned the toughbox with a steel box extrusion like what the nano has, they would probably be strong enough for military-grade and/or volatile environment (think chernoble cleanup) robots.

With Crab, your driver needs to dictate the driving style, which then dictates how the 4 wheels are steered (2-2 sides, 2-2 front/back, or all together) as well as how the robot is programmed. This driver would definitely be more focused on the robot during a match and less focused on what's going on around him/her.

It's all about how you use the drive train. A 6-wheel drive train has shortcomings when attempting maneuvers, especially in autonomous. For great examples of what I'm talking about, look at Wildstang's 2007 and 2009 robots. Both autonomous routines used maneuvers with swerve that would have probably been much less accurate (statistically) than had the maneuvers been performed with skid -- especially in 2009.

Re: crab drive vs. mecanum drive system
 

I'll just give my real quick 2 cents because I am compiling a post which contains all of the information from my senior design project at Northeastern University.

My senior design project was the investigation of the most effective drive system one could use in FIRST, followed by the optimization of such a drive and then constructing a prototype. I went through a lot of decision making processes to determine what was most effective. When it came down to it, the argument was exactly what is being argued right now...swerve vs. 6wd. Some of the decisions I made to determine between the two were based on opinion, but this is inherent with this kind of decision. I ended up designing, analyzing, optimizing, building, assembling and testing a swerve system.

The argument between the two was settled for me based on the fact that I feel the only reason people widely consider 6wd more reliable is because they are simply done much more frequently than swerve/crab systems. Yes there are more moving parts in a swerve system, inherently making the system more prone to failure. However, with more evolution of swerve design the systems can become just as reliable as 6wd systems today. Which is why I chose to optimize that system. A new post will be up in a few days which contains my report (>100 pages) and my presentation (~40 detailed slides) which explains what I did and the findings I had.


Brando

Re: crab drive vs. mecanum drive system
 

I believe it should depend on the game. Personally, I dont much care for mecanum drive trains however they can be useful in some games. For example, in 2006 I cant remember the team number but their was a team that climbed onto the ramp and then using the mecanum drive would move all the way to one end of the ramp to allow room for their partners to get on the ramp as well.

-Sam

Re: crab drive vs. mecanum drive system
 

When basing your decision, remember that they best option for your team may not be the "best" drive train. I know that for our team, in the end we generally choose to go with a 6-wheel bot, not because we lack the capacity to make a more complex holonomic or omnidirectional (I personally dislike mecanum, too little tourque) drive train, but because we know that we can throw our 6-wheeled drive train together in three days, and maintain the full six weeks for developing our manipulation and coding scheme. Part of engineering is not only trying to build the best, but also being pragmatic with time and resource management.

So in short, remember to not only look at the benefits of each drive, but also the costs (from a time and maintence perspective).

Re: crab drive vs. mecanum drive system
 

I think the bigger issue with 2 CIM mecanum isn't torque but that you can't independently control 4 wheels with 2 motors. Well, with any amount of reasonability.

Re: crab drive vs. mecanum drive system
 

I have a better example: I could easily argue that what we were able to do in 2003 in autonomous was both, extremely advantageous, and almost impossible to do without Crab steering.

Re: crab drive vs. mecanum drive system
 

Team 40 has tried most of the drive combinations mentioned here over the last few years.

2005 - 6wd Solid Center, front/rear omnis
Robot was good a could maneuver well, be wasn't very repeatable in in autonomous. Our implementation caused the robot to rock on the center wheel. I was also difficult to drive, the robot wouldn't drive strait to save it's life.

2006 - Mecanum w/3 Speed Dewalts
Robot could go in any direction, the drivers did seem to complain about difficulty driving it. It worked great for auto tracking and shooting in autonomous. On the other hand the robot had zero pushing power, and could be easily knocked off coarse in auto or shoved in a corner during operator control. It would take a lot of convincing for us to go back to this drive train.

2007 - 6wd Drop center, 2 speed servo shift by AndyMark
The robot drove well, it was easy to program, and we used a turret in place of maneuverability. While the turret was nice, we missed the ease of navigation. Also it rocked on the center wheel which would cause the drivers to readjust the tube.

2008 - 4wd Swerve, Motors In Pods, Pods Grouped F/R for Steering
Based on the 2008 Control System, this was extremely difficult to program and still have enough compute power to control all the other devices. Autonomous was very easy to program with "robocoach" and allowed the robot to strafe at will. Driving the robot was like driving car and was extremely natural, and through software the robot could enter crab mode and strafe in any direction. Other then a few broken POTS this drive was extremely reliable. We are definitely sold on this drive system, if we have 6 motors to dedicate to the cause.

2009 - 3wd Crab Drive
We liked the swerve so much in 2008 we did it again in 2009. Though we ran out of motors and space so we did a tri-cycle drive and linked the steering together and used a single motor to steer instead of two. Programming in the 2009 system with C++ was very simple with the included PID functions. With the surface being so slick the robot could still rotate through software tricks. This allowed us to have a extremely successful autonomous. The drivers did struggle a bit more this year, as driving directionally and slow rotation caused some issues.

Re: crab drive vs. mecanum drive system
 

My teams have used 4wd, 6wd, mecanum and crab drive. About the only mode we have not used yet is holomonic or kiwi. Many of the advantages and disadvantages have already been covered so I will not repeat them now.

AndyMark is now selling off-the-shelf Swerve drive modules. Not cheap, (cheaper than paying a shop for custom parts) but it gets around most of the machining issues. AM products we have used in the past, traction wheels, mecanum wheels and shifting gearboxes have all been solid.

My personal favorite for general use is the 6wd with center axis slightly lowered. It is quite maneuverable with great ability to push.

I would not lock he team into any specific drive model until you determine how you want to play the game. We are exploring several options so we can pick the type that suits our chosen strategy once the game is announced.

Re: crab drive vs. mecanum drive system
 

Leet,

I'm curious...any preference/pros/cons between the two 6wd versions? We've done 6wd with dual omnis in the corners twice before, never a dropped center.

- Steve

Re: crab drive vs. mecanum drive system
 

6WD flat has stability on its side. 6WD drop acts like a 4WD most of the time, but which end is the 4WD at any given time varies with robot motion. During a spin, or if the CG is somehow over the center wheels, it's common for it to act like a 2WD on the center wheels. The typical West Coast drivetrain is a 6WD drop.

The toughest part of designing a drop-center system is figuring out how far to drop the center. Depending on wheel choice, anywhere between 1/16" and about 1/4" has been used effectively.

Re: crab drive vs. mecanum drive system
 

I was introduced to Holonomic drive not long ago, which might explains why I am asking you this question: How come efficiency becomes only 70% if the wheels are placed at the corners instead of being placed at 90 degrees from the center which will result in 100% efficiency?

Re: crab drive vs. mecanum drive system
 

Neither results in 100% efficiency.

Re: crab drive vs. mecanum drive system
 

I think you misunderstood. You get the equivalent of 100% efficiency of a two-motor system, as only two of the four motors are in use when traveling forward or backward.

Re: crab drive vs. mecanum drive system
 

ohhhhhh, but if the wheels were placed at the corners, then the tradeoff would 100 % efficient going diagonally, correct?