Supershifters and Mechanum wheels

[bs7280]

I will admit that our loaded could have been much better for sure. it was about as wide as a frisbee, not very helpful. We made some changes at competition that allowed us to have a "flapper" that would allow us more surface area to collect frisbees, but anyways, lesson learned there (practice more).

We could try making those changes (as well as checking which need to be fixed) once we meet again. I definatly will push for the gearbox change. If it helps, it could turn its self way better than any human could (on carpet) without us lifting it up.

I like that philosophy. Next year we are going to drive and make some competition pieces so we can get a better idea of what problems we are having.

EDIT:
Almost forgot to mention. Our center of gravity is centered very well (climber is in the middle) and the main problem from driving is that the front and back wheels of the tank drive where having a lot of lateral resistance on the carpet. enough so that no person could push it. So when it tried to turn, that was what it was fighting.

[Anupam Goli]

Could you possible provide some pictures of the drivetrain? Ideally, there should be a lot of resistance in the direction perpendicular to the forward-backward motion. The key to being able to turn a long robot is to have that drop in the center wheels. This way, the robot is on the front 2 and mid 2 or the back 2 and mid 2 wheels, never on all 6. This reduces the length of the drivebase effectively in half, requiring less torque for the robot to turn in place.

Also, make sure you look at all of the other issues before putting in a new transmission in the drivetrain.

[MichaelBick]

Originally Posted by ablatner

Shifters are great, but I'd have to recommend against Mecanum wheels for the usual reasons. You can find many threads on the subject. Many of the top teams use 6 or 8 performance wheels. I know 254 almost always uses the same drive train with eight 4" wheels (this year maybe six because of reduced size). Many other teams have the same setup too, unless they have a good swerve drive like 1717 last year.

254 almost always uses 6wd, not 8wd because you have a lot less parts to make. They also usually vary their wheel size based on the game, but usually stay very small. If I remember correctly in the past few years they've been using wheels that range from 3.5" - 4.5" in diameter.

The kitbot on steroids is a very good drivetrain. It's very easy to make and will give you a drive that is better than 90% of teams. Having a reliable, easy to make drivetrain will give you more time to build a reliable manipulator, tune your programming, and get drive practice.

I find that omnidirectional movement is usually overrated. Mecanum and holonomic drives tend to have a lot of problems, especially when you want to try to get around or play defense. Swerve drives take up a lot of resources. First of all, they are complex, hard to design right, and take a lot of time to machine. Also, code is very complex, and if you don't do your code right you will be a sitting duck.

One of my favorite examples of the complexities of doing swerve right is 973's 2012 robot, Encore. 973, being a powerhouse and the 2011 World Champions, they had the resources to build a very good independent swerve drive. However this took up a lot of their resources, and ended up detracting from the other parts of the robot. This year, 973 decided to revert back to a west coast drive. In my opinion, this decision benefitted them tremendously. With more time and resources to divert to their manipulator, 973 was able to spend more time tuning and practicing with their robot. I feel 973 had by far the best robot at the LA regional, leading them to their 2nd regional win.

That said, I think you should work with building the kitbot on steroids during the offseason. With so many good new products from many new vendors, you may eventually want to spend time improving the kitbot on steroids(i.e. add shifters) and even creating your own custom drive. However the kitbot on steroids will put you on the right path.

[Bruceb]

I dont get it. If you used the kitbot chassis then you have the same chassis we did. Long orientation, 6 kitbot traction wheels, 4 CIM's on toughbox minis as provided in the KOP. Perhaps you didnt notice that the middle hole on the chassis material is off center and you assembled it with the middle axle up? We used, for the first time the entire kitbot chassis and it was strong, reasonably fast and turned on a dime. We seeded 11 in both Wisconsin and Buckeye and made it to finals in Buckeye after being first pick by the first seed. This was, with the belt drive, the MOST BULLETPROOF chassis we have ever used. I am never going back to chains and using onmi's on the ends is just wasting traction.
We had no trouble pushing people around most of the time and having 6 traction wheels made it almost impossible to push us around. The only thing I would change in the future would be to go back to super shifters to give us more speed and power when needed.
Check out you chassis assembly, I bet you have the center axle raised instead of lowered.

[FrankJ]

Since mechanum needs four gear boxes, shifters add bit of weight & complexity. you would want to gear them fast & faster since you are not going to win pushing matches against well built pushers anyway. I don't think shifters would be worth it. They really don't do as bad as most think against the average drive train. Once again I am not talking about winning pushing matches as much as avoiding them. A well driven mechanum with good drivers & a plan can drive around pushing matches. In this game, lining up on the feeder & aiming is a big plus for mechanum.

[Ether]

Originally Posted by FrankJ

Since mechanum needs...

A well driven mechanum...

...big plus for mechanum.

Hey Frank, did you do that on purpose ? :-)

[bs7280]

Originally Posted by Wing

Could you possible provide some pictures of the drivetrain? Ideally, there should be a lot of resistance in the direction perpendicular to the forward-backward motion. The key to being able to turn a long robot is to have that drop in the center wheels. This way, the robot is on the front 2 and mid 2 or the back 2 and mid 2 wheels, never on all 6. This reduces the length of the drivebase effectively in half, requiring less torque for the robot to turn in place.

Also, make sure you look at all of the other issues before putting in a new transmission in the drivetrain.

Thank you for the advice! I am pretty sure that you are right, in that our center wheel is not lowered. we are meeting tonight and I will be sure to check it out and grab some pictures.

Also keep in mind that we are only going to make minor changes to our competition bot if any (no major drive train changes) like flipping the frame. In the off-season we intended to experiment with frames (among other things) that are not intended to compete. We have good programmers, tools (mills,cnc,lathe) etc... that if we were optimistic we might attempt to build one of the complex drive trains as a prototype.

[bs7280]

Originally Posted by FrankJ

Since mecanum needs four gear boxes, shifters add bit of weight & complexity. you would want to gear them fast & faster since you are not going to win pushing matches against well built pushers anyway. I don't think shifters would be worth it. They really don't do as bad as most think against the average drive train. Once again I am not talking about winning pushing matches as much as avoiding them. A well driven mecanum with good drivers & a plan can drive around pushing matches. In this game, lining up on the feeder & aiming is a big plus for mecanum.

Thank you for your advice! this answered a lot of questions/concerns that I was having. We are most likely not going to go the shifter/mecanum route in the future as it is expensive,heavy, and not worth it. The reason that I was concerned about being fast and able to deal with pushers was that in our elim rounds we had to defend our teams long range shooter from some of the robots with blockers.

When would you say mecanum is worth it?

[FrankJ]

Ether: A big reason that I am an engineer is that mom knew I would fail miserably as an English major & starve.

For all of you that plan to major in engineering for the same reason.... Being able to communicate is really really important.

[jbsmithtx]

I think several people her have fallacies with mecanum. Our team decided week one to use them this year, and we did spend a little too much time perfecting it, but in the end it turned out for the better. We were a feeder station/shooter and we found the mecanum great for lining up. This was the problem we had last year. Even with all the summer practice I got with last years bot (hours upon hours) it still was not all that great. So our team debated week one and saw the omindirectional system as very advantageous, versus the challenge last year. Our original intent was to dive under the pyramids and such, but we never got to end up doing that because of the slight variations in pyramids. However, during competition when our shooter failed, we were plenty good at defense. (Youtube video. Not sure how much defense is shown.. http://www.youtube.com/watch?v=3M_YGV9kKP8). We actually pushed several teams with the kitbot around. We did find it easier to drive aroudn them though. We discovered (all too late), that speed would have definitely helped. I think a lot of teams underestimate speed. When are robot can drive around you, or simply quickly out maneuver you with a mecanum, it really makes a lot of things easier. Our teams has nothing bad to say about our mecanum, just make sure that you use a gyro to help with all the subtle problems. We used 6" AM mecs with TB minis

Also forgot to include that when we decided to add a climber, rather than messing with other systems, we were able to mount our climbers on the side and strafe onto the bar

[bs7280]

Ok, say a team decides to go with a drive base (nothing specific for the sake of helping other teams who view this) for each base, how does one select the "best" gear ratio to be chosen on a bot? Looking at different gearboxes for different drive base types, there are alot of choices and it is quite intimidating to choose considering how expensive they are.

[Anupam Goli]

There is no single blanket "best ratio" to use. It all depends on your strategy and what you want to do. Do you want to be quick? Do you want to have pushing power, but don't want to be necessarily quick? If you are using a single speed gearbox, you can look between 4.67:1 to 13:1. We ran an 8.46:1 ratio in a Toughbox mini. We got about 9 fps and we had decent pushing power. The toughbox might be a good starting point. The good thing about the toughbox is that you can just buy the gears and change the ratio based on your results. You can also adjust the ratio in the sprockets. Remember, the higher the ratio, the less speed you have, but the more torque you will have behind those wheels. Striking the balance is something you and your team have to figure out. If you search in the CD papers, you can find JVN's drivetrain calculator, which allows you to learn everything you want to about a drivetrain before you build it.

[Jeffy]

Originally Posted by bs7280

Ok, say a team decides to go with a drive base (nothing specific for the sake of helping other teams who view this) for each base, how does one select the "best" gear ratio to be chosen on a bot? Looking at different gearboxes for different drive base types, there are alot of choices and it is quite intimidating to choose considering how expensive they are.

We use a two speed and mostly because:
We can set a gear used for acceleration and pushing matches to be traction limited (Torque created at ~40 amps times wheel radius > force of static friction of wheels on carpet)
We can set a gear to get us where we want to go as fast as possible. (balance of acceleration from top speed of gear 1 with top speed)

If you choose a single speed, a balance of top speed and pushing force is ideal. Usually teams set there bot to be right on the edge of traction limited. This is not inherently ideal, but is simple and can give a successful result.

JVN design calc as suggested above is the way to go.

[jbsmithtx]

As above said, there is no "best ratio". It should be fit into your strategy. So, with a wide open field like this year, you might want a higher ratio if you were going to be running cycles to and from the pyramid, so you can cross the court swiftly. Whereas if you were a team that wished to block a cycler, you might have a much lower ratio, so that you could at least push them around and slow them down. However, with a divided field like last year, you would be looking at a totally different setup, with a lower "speed" ratio (because of acceleration up to your top speed). It all depends on strategy and how you think matches will end up being played out.

[EricH]

Originally Posted by bs7280

When would you say mecanum is worth it?

I would say that mecanum is worth it if and only if:
1) The game requires an extremely high degree of maneuverability.
2) The game does not require the movement of heavy objects, or allows for said heavy objects to be carried in the middle of the robot (not other robots).
3) The team has a good programmer.
4) The team does not have the resources (pick any or all of time, personnel, offseason, money) to pursue swerve, or wants to use a KOP-type frame.

Otherwise, swerve adds extra push to the maneuverability.

Oh, and:
5) The team has already prototyped the system, at some point.