Chainless Mecanum Drive
 

Our team is looking into trying out mecanum wheels in an off-season competition. Due to time constraints and not wanting to have to mess around with chain, we were wondering if it is possible to have a chain-less mecanum drive. If so, how do we do this? Pictures or diagrams would be great. We are using standard Andy Mark mecanum wheels and the Andy Mark gear boxes. Thanks for your help:)

(On a side note, as I was looking through the forums researching mecanum drive, I saw some posts mentioning nona drive. However, I wasn't able to find an explanation of it, or any pictures. What is it?):confused:

Re: Chainless Mecanum Drive
 

http://www.andymark.com/searchresults.asp?cat=215

That (should be) the link to the "hubs" section of the andymark.biz site. You're going to want to find the size that coincides with the size of your output shaft/key. I wasn't actually around when we got them in and put them on, but unless I'm mistaken, you slide it over the output shaft, and the outer holes correspond to the (long) bolts on the inside of the wheels. There might be another part necessary to keep it on the output shaft, but that would be a good starting point.

(Someone correct me if I'm wrong, I had already left for college when they got everything together.)

Re: Chainless Mecanum Drive
 

Nonadrive (or Neunadrive) is 4 traction wheels, each in a pod with an omni wheel, and a fifth omni wheel in the middle at right angles to the rest. The pods rotate, allowing for traction or agility as needed. Used by 148 and 217 this year.

Re: Chainless Mecanum Drive
 

Chain-less Mecanum:

Use AndyMark Toughboxes or Toughbox Nano's and mount the mecanum wheels on a hub, directly on the axle. Works well. Many teams that I saw did this (most mecanums I saw were direct drive), although I have no specifics. If you look at AndyMark's website, they have a drivetrain kit for 4-wheel direct-drive mecanum with toughboxes.


Nonadrive:
The creation of teams 148 and 217 (Robowranglers and Thunderchickens, respectively), it is a 9-wheel drive system essentially a convertible slide drive. Here is how it works:

There are four omni wheels on the perimeter of the robot, driven in standard tank drive. Each wheel is on a "pod" with a high-traction wheel, and the pods are pneumatically moved so the traction wheels can either provide traction or float above the ground.

In the middle there is a single omni wheel, driven by a single CIM, which is sideways (to cross the bumps, 148 and 217 pneumatically lifted this wheel a few inches to give them the center clearance necessary to cross the bumps).

This provides the "standard" amount of power (4 CIM's, or around 1.2 kw) in the forward/backward direction, while allowing non-pushing motion sideways. Since omni wheels are push-able, they can lower the perimeter wheels to push or avoid being pushed.

Re: Chainless Mecanum Drive
 

The nonadrive is the pretty much the coolest new drivetrain in FIRST. It has the traction of a 4 traction wd (normally not used because it can't really turn) and the manuverability of a slide drive (normally gets pushed around alot in the heat of compitition). However, Its arguably on the level of complexity with a crab drive, so its not for the faint of heart. Definatly an offseason project.

lol, beaten to it.

Re: Chainless Mecanum Drive
 

Direct drive mecanum is trivially easy. You just need the appropriate reduction in your gearbox for the size of wheels you're using. Like this.

Re: Chainless Mecanum Drive
 

We designed a chainless mechanum chassis as a mental exercise during the summer of 2008. Included suspension. We never built this design.

Here's the link:
http://wiki.team1640.com/index.php?title=Mecanum

Cheers,

Re: Chainless Mecanum Drive
 

Thanks! How heavy is a standard nonadrive? Five motors, 9 wheels, and four pneumatic cylinders, that must add up...

Re: Chainless Mecanum Drive
 

We learned this the hard way. During Overdrive we tried a mecanum drive and our robot would not move. At first it was thought to maybe be a code issue but turned out to be that our gear boxes (stackerboxes) were not geared down enough to move our robot. :yikes:

Re: Chainless Mecanum Drive
 

Not to take away from innovation and technical achievement, but I don't really see the Nonadrive as some kind of new Holy Grail in FRC drivetrains, and it is absolutely not something a team without the engineering resources to design a direct drive mecanum drive should do. It's not exactly a cheap, light, or simple drivetrain.

Re: Chainless Mecanum Drive
 

So if we're using the 6 in mecanums, what is the appropriate gear box reduction?

Re: Chainless Mecanum Drive
 

12.75:1 seems to be a reasonable compromise between speed and torque (more or less depending on the game, of course)

Just curious, can you share a bit with us about your team's decision process that led you to your decision to explore mecanum wheels?

Re: Chainless Mecanum Drive
 

Our main reason for trying them now is so that we have experience with them if we decide to use them for the FRC season. We bought them last year but decided not to use them for FRC because we didn't have any experience with them and we didn't think they would work well on the ramp. We want to learn how to use them because they are one of the most manueverable drive trains. From what I saw at the Portland regional last year the teams that used them were able to position themselves very well with the ball and goal and ended up doing pretty good. Basically, we think that they could be advantagous in this years competition and we want to be able to use them if they are.

Re: Chainless Mecanum Drive
 

On a sheet-metal chassis, which can be lighter than a more standard box-tubing chassis.

For further questions on the Nonadrive, you might want to take a look at the following two threads (and yes, there are two, one for each team that used it): 217 and 148--and definitely watch the videos that open the threads. (Warning: watching said videos may cause a sore jaw, due to said jaw hitting computer, desk, or floor.) Unfortunately, I can't seem to find tech specs...

But I do know where CAD files can be found!

To echo what Chris said, you need to know what you're doing to try a nanodrive. They'd have had a hanger on their robots in competition if they hadn't decided to reinforce their drive and make other improvements--and that's another scary part, they improved it from the videos.

Re: Chainless Mecanum Drive
 

The key words in this statement are: "could be" and "if".

Right now is a really good time to do a mecanum drive as an offseason project. You get a chance for rookies to build a robot, you test out new technology, and you gain knowledge and experience.

But the problem is that when all you have is a hammer, every problem looks like a nail. When Kickoff shows up, you will be tempted to use mecanum, even if the game is reminiscent of 2002 (where high traction, great pushing and low maneuverability were great benefits). Trust me, you will be tempted.

So, what you do is you take your offseason robot and a competing drivetrain (say, 6WD drop-center, or a 4WD built from the Kitbot) and run them through a series of tests, either individually or against each other with an offense vs defense. Then you evaluate which is better.

I'll let you in on a little secret: 330 has had the capability to use mecanum drive since 2005; development/small-scale prototyping started around 2003, with a full-scale base during the 2005 build season (thanks to the Kitbot frame). The closest they've ever come was 2009 with a turning 6WD system in the middle of their robot. The only time they've used wheels with rollers in competition was omni wheels in 2010, for added agility on a 4WD. They just haven't found it advantageous to use them...yet.

Re: Chainless Mecanum Drive
 

Actually, Eric, I'd say they're doing exactly the right thing by trying it now. There's really no better of a time to see if you can get it working and learn about the drawbacks of such a system. I mean, one could say the very same thing about a six wheel drop prototype (ignore for a minute that 6 wheel drop has been consistently very successful).

If I'd say anything it's to make sure you guys are very, very critical of your final project and to make sure the advantages are actually called for in your design process, and that the disadvantages are not absolute. I would say a lot of teams that build mecanum drives get a little distracted by the whole coolness factor and build it without regard to the realities of pushing and whether or not strafing is required.

Re: Chainless Mecanum Drive
 

Which is exactly what I said in the second paragraph (right after pointing out the key words).

I also pointed out that just because you have an uber-widget doesn't mean that you should use it, but having it is nice because if you should use it, you can do it very quickly.

OK, so I also suggested a comparison test that should be easy to whip up come build season...

Re: Chainless Mecanum Drive
 

Well, there's a lot of information here on the CD forums about mecanum wheels, both pro and con. If you haven't yet searched the forums and read the links, you might want to consider doing that.

Re: Chainless Mecanum Drive
 

Spot on about the direct-drive Mecanum setups. It's very efficient, though be sure to clean the rollers every couple of matches.

An interesting thing about nonadrive is the penta drive configuration. 5 omni wheels can be driven by 3 motors for a team with limited resources that wants agility.

- Less code complexity than most non-standard (skid) drive systems
- Less expensive than Mecanum or Killough (traditional 4-wheel Omni)
- Arguably more traction than "out-of-the-box" Mecanum, and definitely more traction than Killough
- In recent years, it would leave at least 1 CIM available for other things
- Can be used in 4WD or 6WD skid configurations, though that is highly coupled with need-based strategy

I've toyed with a concept that uses 5 wheels in nonadrive's pentadrive configuration with the middle wheel being a traction crab module that pivots via pneumatic linkage (pneumatic to keep the code simple). This concept gives a mid-grade complexity while also potentially providing some of the agile+tractive advantages of nonadrive without the weight. Of course it doesn't have the natural anti-turn capability of nonadrive, but that's the trade off. The concept was inspired by nonadrive and the limited pictures I've seen of 330's 2009 bot.

Unfortunately I've been overruled for our offseason prototype due to team survivability, so maybe another team will prototype it?

Re: Chainless Mecanum Drive
 

I would suggest that a slide drive config that wants at least 1 CIM for something else use 1 CIM + 1 FP in the drivetrain instead of just 1 CIM. Acceleration is helped a pretty good amount.

Re: Chainless Mecanum Drive
 

Actually, I would make the opposite argument. Mechanically a Nonadrive (one that does not need to cross bumps) is only slightly more complicated than a standard 6wd robot. Remember, the perpendicular wheel does not need to raise or lower if it is a flat field. From a programming standpoint it is much much simpler. The hardest part about mechanum drive machines is controlling them. It involves either lookup tables and interpolating results or using lots of sins and cosines or matrices (Thank you Ether). A nonadrive is simple in that for translation all you need to do put the Y component of your translation stick to the left and right drive motors and the X component to the cross drive motor(s) Rotation is a little trickier but you could just scale the left and right based on your rotation input. Either way, much simpler than mechanum.

It should also come out near the weight of a mechanum drive train. Remember mechanum needs 4 gearboxes, Nonadrive needs 3. Yes Nonadrive requires 4 cylinders but if you were planning on using air anyway that is not a ton of weight.

I will admit that the Nonadrive has, at least after a cursory glance, more failure points.

TLDR, there are benefits and drawbacks to both systems (like any systems) but neither one is inherently more complicated.

EDIT: Appended note about Ether's solution.

Re: Chainless Mecanum Drive
 

Programming mecanum is simple and straightforward.

No trigonometry or lookup tables are required.

You can find theoretical analysis and example programming here.

Re: Chainless Mecanum Drive
 

I would still claim that it is not as simple and straightforward but it is an interesting solution to the problem. Thank you.

Re: Chainless Mecanum Drive
 

For robot-centric steering, it is that simple. For field-centric steering (where pushing away on the joystick moves the robot away from the driver's station regardless of robot orientation) it can get quite complex.

Do you mean for the sideways component? If so, that's a good point. A crab-style middle wheel would put 3 CIMs accelerating forward at a maximum, 2 minimum (which is acceptable under 11fps or so). The sideways movement would only ever have 1 CIM maximum, which may be a problem since should have the same speed as the forward-only wheels -- if the forward component is 12fps or more, then the single CIM moving sideways would cause some sluggishness.

Re: Chainless Mecanum Drive
 

It's a bit more work, but not at all complex.

And it's no more complex for mecanum than it is for nonadrive.

To do field-centric control, you must know the angle of the robot with respect to the field (using a gyro for example).

Use this angle to do a 2D coordinate rotation on the joystick X and Y inputs* . Then use these rotated values (plus the unmodified Z joystick value*) as inputs to your robot-centric code.


*assuming X and Y represent the strafe and fwd/rev commands, and Z represents the spin command

Re: Chainless Mecanum Drive
 

With the Nonadrive, I would say the mechanical complexity in design would be in making the pivot modules rigid enough for FRC applications. In particular, when resisting pushing a moment is applied through the modules (I think?) since the traction wheel is on a module, so the forces are transferred through the chassis via the pivot axle. It seems a lot esaier to make a standard 6wd than to make a Nonadrive. The extra weight assumes a lack of sheet metal capability and having to use thicker sideplates due to a lack of ability to add flanges among other things like that.

I would argue that Nonadrive is simpler to code because it basically takes zero programming, but basic robot centric mecanum code is not a tough challenge either.

@JesseK: I was suggesting 1 CIM + 1 FP in each of the forward wheels and then just a CIM on the side wheel since strafing is secondary to forward motion for a slide drive (if it isn't secondary, you should probably be using a holonomic chassis). With a crab module in the center I would probably suggest 2 CIMs on that and 1 CIM + FP on the outsides (off the top of my head here, not based on math or anything)

Re: Chainless Mecanum Drive
 

I don't think this would work at as well as you'd hope. The crab module would only have so much normal force on it, based on CG location. On top of that, if someone to to try to rotate the robot, it wouldn't be nearly as difficult as trying to rotate a 4wd/6wd, etc... You'd also have to design carefully to ensure that when the module rotates, the module rotates relative to the floor, not the module stays in place and the robot rotates around it.

I'm making the assumption that the crab module would raise under normal operation.

Really, the best way to answer the question without building it is to do a full Free body diagram and look at the forces involved.

Re: Chainless Mecanum Drive
 

The only reason it worked when 330 used it in 2009 was those two extra wheels that all robots had to have. The trailer kept the robot body from being a big old turret.

Re: Chainless Mecanum Drive
 

so robot-centric nonadrive in tank mode looks something like this:

and robot-centric mecanum in tank mode looks something like this:


Frankly, I don't see where nonadrive is simpler.

Re: Chainless Mecanum Drive
 

That's because you're adding things that aren't required. For a basic nonadrive, three joystick axes directly control three motor outputs, and one joystick switch directly controls one solenoid output. Sure, you can make it fancy with toggle functions and motor limiting, but you don't need to.

A basic mecanum drivebase is certainly easy, but the connection between inputs and outputs is not one-to-one, and you have to do some post-processing to scale motor values if you don't want unexpected behavior at high speed.

Re: Chainless Mecanum Drive
 

Just to be clear: For a fair comparison of the code required, you have to compare apples to apples. Mecanum requires no special operator intervention or programming to actuate raising or lowering of wheels. To be functionally similar to this for comparison, nonadrive requires extra operator input(s) and extra code to respond to those extra operator input(s) to raise and lower the traction wheels and control whether or not to power the center wheel. To be functionally equivalent (same driver interface and same robot behavior), nonadrive requires extra logic to react to the normal joystick axis driver commands and decide automatically when to raise or lower the traction wheels (for example, when strafing or tight turning is commanded).

This is not an argument that mecanum is "better". Just a comparison of the code involved. The code for either is fairly straightforward.

It would be interesting to hear from teams that have fielded successful nonadrive robots. Did you add extra input(s) and associated code for the driver to manually raise and lower the traction wheels, or did you add logic to process the "normal" driver commands and let the robot automatically handle this decision in order to keep the driver interface simpler, or did you design a "hybrid" compromise with manual operator input(s) plus some automatic behavior?

Re: Chainless Mecanum Drive
 

One item to consider with the nonadrive is that even on flat ground you will VERY likely want your center wheel to still be actuated by a pneumatic cylinder.

There is a reason stools usually have 3 legs. Having a 4 legged stool is more difficult. From experience (we attempted a solid 6 wheel rectangular drivetrain in '08 with all omnis), having your corner wheels and the middle wheel all touch if they are fixed is nearly impossible. This is why you must have pneumatics - if you rock on the center sideways drive wheel you will end up going in circles. It needs to push against the floor, but not with enough force to actually support the robot. That means some sort of suspension is required.

We used a laser to get our frame and 6 wheels straight, then realized that the floor many of the arenas was imperfect as well. We didn't much want to go back and engineer suspension, so we dropped the system and went standard 6 wheel.

It looked like this:

.......Wheel.........
........................
W....................W
h.....................h
e.....................e
e.....................e
l......................l
........................
W....................W
h.....................h
e.....................e
e.....................e
l......................l
........................
.......Wheel.........

We saw several teams using this same drive train this year, and they all seemed to experience the same problems we did during our prototyping before we threw it out - with no suspension the robot reacts differently depending on which wheels are touching the ground (Obvious when looking at it, perhaps not so obvious when designing it).

Re: Chainless Mecanum Drive
 

This gave us major steering problems back in Overdrive as well. (In Lunacy we went with it.) That whole "3 [noncollinear] points define a plane" thing means extra contact points need to be somewhat forgiving. As pointed out, life isn't perfect even if your chassis somehow is. How you address that is up to you, whether suspension or actuation (or even somehow in driving), etc, but I would recommend addressing it. Not that this would necessarily add an unacceptable (or even particularly significant) level of complexity.

Re: Chainless Mecanum Drive
 

What ratio do the Andy Mark gear boxes come in? We have several assembled from last year so would we actually have to change them?

Also, I am new to mechanical and am not too familiar with gearboxes. What does 12.75:1 actually mean?

Re: Chainless Mecanum Drive
 

They are 12.75:1

It means that for every 12.75 turns of the motor shaft, the gearbox output shaft turns once. You are trading speed for torque.

Re: Chainless Mecanum Drive
 

12.75:1 is the default gear ratio for the AM toughboxes and toughbox-derived gearboxes (Nano, etc.)



12.75:1 is the ratio between the input (in this case the output of a CIM motor) and the output (The output shaft sprocket or direct-driven wheel). With it you can find the output speed and torque of a gearbox if you know the speed and torque of the motors going into it.

If you need the long output shafts (you might not) you would have to get new long output shafts from AndyMark and put them in the existing toughboxes.

Edit: Beaten to it

Re: Chainless Mecanum Drive
 

Thank you guys! It looks like doing the mecanum drive is going to be a lot easier than I originally thought:D

Re: Chainless Mecanum Drive
 

Many teams get away with 1 CIM per side for a 10fps (or less) drive train and do just fine. Calculating the motor load based upon robot weight and gearing inefficiency puts the motor efficiency within a nominal amount of its maximum efficiency while just driving. If sound judgment is used on the drive train with respect to turning (wheel base for traction, or use of omnis, 6WD drop-center, etc), almost the same results would apply. Thus, as long as the team doesn't try to push another traction robot they'll be ok.

Your initial instinct is correct for what many teams want -- 11+ fps.

Good point, I didn't even think about that... It'd almost have to be an omni wheel at that point in order to prevent this completely.

The first iteration of Mecanum is always easy! If you build your drive train in the off season, play around with some extra weight. Specifically, see the effects of pushing a robot when the contact point removes traction from the front (lifts your robot slightly), and see the effects of just driving when your c.g. is too close to one of the wheels. Some interesting behaviors come out.

Re: Chainless Mecanum Drive
 

I disagree, really. I can't think of any teams that enjoyed competitive success this year with a 2 CIM drivetrain, so I would hardly say "many teams" did. Who are some examples?

Even without pushing, running just 2 CIMs in a drivetrain even geared at 9 fps has many negative drawbacks. For one thing, at 9 FPS your CIMs will stall against a wall. Unlike a 4 CIM drivetrain, which just dips deep past the circuit breaker limits to get the wheels to slip, a 2 CIM drivetrain can't ever slip the wheels, so pushing other robots becomes basically not even an option. Turning is also pretty slow since skid steering relies on wheel slipping. Acceleration is also hampered a bit: In terms of actual distance traveled from a dead stop with 0.5 second of acceleration, a 2 CIM drive goes only 2.6 feet, while a 4 CIM drive goes 3.1 feet. (This effect is a lot more noticeable / dramatic in reality than "on paper").

If pushing is of zero concern for whatever reason (though, in my opinion there has not been a single FIRST game other than 2001 where at least being able to resist pushing was vital), it would make more sense to use 2 CIMs with ball casters supporting some of the robot's weight. This lets you draw less current while driving the same speeds, since your wheel force is lowered.

Re: Chainless Mecanum Drive
 

Could you articulate this argument in a bit more detail? It's not entirely clear what you are saying.

Re: Chainless Mecanum Drive
 

By using a drivetrain where roughly half of the robots weight is supported by non driven wheels, the normal force on the driven wheels decreases. This puts the motors under less load at the expense of traction and pushing power. Generally a bad thing, but if pushing isn't called for at all it can be a good idea, since it lets you gear your drivetrain for a higher speed than otherwise possible.

Re: Chainless Mecanum Drive
 

Robot A has 1 CIM+gearbox on each side, which drives the front wheel and the rear wheel via a belt or chain.

Robot B is identical to Robot A in all respects, except that the belt (or chain) drives only the front wheel. (The rear wheels are not driven).

How would your analysis apply to this scenario?

Re: Chainless Mecanum Drive
 

Assuming a perfectly balanced CG:

With only two wheels connected, only 50% of the robot's weight is "over" the driven wheels (the other 50% of the weight is born by the non driven wheels which don't contribute traction since they are essentially just big bearings). For this case, the robot's max pushing force is effectively half (the mass portion of the formula to overcome static friction is halved). Since it requires less force / torque to slip the wheels, the motor connected to the wheel does not need to draw as much current, since the motor "reaches" the necessary torque at a lower amperage. However, the robot can only push with half as much force.

With only 2 CIMs in the drivetrain, a 148 pound robot geared to 9 FPS with roughtop tread (1.3 CoF) has a max pushing force that is torque limited. The motor's stall torque is greater than the torque necessary to slip the wheels. In addition, stalling a CIM draws 133 amps of current which quickly trips the robot circuit breakers. By halving the pushing force, you can make the drivetrain traction limited at some amperage. One can do this by lowering the traction of their wheels or by lowering the robot's traction overall by not driving a set of wheels / casters that support the robot weight.

Re: Chainless Mecanum Drive
 

I'm with Chris, we will never do another 2 CIM drivetrain on a carpet field again.

It's really (strictly from my experience) a mater of drive-ability. Turning acceleration and other things that make for a good, easy to drive robot all suffer without the torque you can get from 4 motors.

Re: Chainless Mecanum Drive
 

We have competed with only 2 CIMs for drive with only 2 CIMs on our drive.

In that time, we've been a regional finalist, regional champion twice over and division finalist at the Championship. We're not a powerhouse by any means, but with the proper application of resources, it's certainly possible to find success with a less powerful drive.

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).