Strategic Uses of Swerve Drive

[Jared Russell]

We used a holonomic omniwheel based drive in both 2007 and 2008, and I cannot foresee us doing it again unless defense is explicitly forbidden ala 2008 (even though we managed to win a regional in both of those years). Simply put, the gain in mobility did not offset the loss in traction. Determined defense from a skid steer bot can still shut you down.

As far as omni vs mechanum, the dynamics are identical. The only benefits of mechanum are that it is often easier to mount the wheels and motors parallel to the frame. Also, it is somewhat easier to make/buy mechanum wheels with larger floor contact patches and better tread material (though I have seen omniwheel with roughtop tread rollers before).

I should also emphasize that using omni/mechanum drives effectively is still a large challenge. Wheel speed must be closely controlled, a suspension is highly desirable, and weight must be distributed carefully.

[JVN]

JVN's Swerve Drive manifesto...
I would only build a Swerve Drive if all of the following are true:

1. The game is such that a swerve drive would provide a DISTINCT advantage.
2. The team can execute the design, fabrication, and execution (programming) of the swerve drive without removing resources from other areas of the team/robot which would BETTER benefit the team.
3. The driver is capable of handling the swerve such that he/she can achieve a DISTINCT advantage in match play.

Some teams also may just decide to build one for "cool factor". More power to them, but that's now how 148 rolls.

-John

[Rich Kressly]

Originally Posted by JVN

JVN's Swerve Drive manifesto...
I would only build a Swerve Drive if all of the following are true:

1. The game is such that a swerve drive would provide a DISTINCT advantage.
2. The team can execute the design, fabrication, and execution (programming) of the swerve drive without removing resources from other areas of the team/robot which would BETTER benefit the team.
3. The driver is capable of handling the swerve such that he/she can achieve a DISTINCT advantage in match play.

Some teams also may just decide to build one for "cool factor". More power to them, but that's now how 148 rolls.
-John

Swerve Drive Manifesto, my foot! Seems a lot like a strong Life Manifesto. "Never over complicate things and know thine own skills and resources well." I'm big on trying to make sure the machines and people get along well; in a perfect world enhancing each others abilities.

Thanks for including the higher level thinking here JVN. I'm a fan of the way you guys roll.

[Alan Anderson]

Originally Posted by Jared341

As far as omni vs mechanum, the dynamics are identical. The only benefits of mechanum are that it is often easier to mount the wheels and motors parallel to the frame.

With perfectly free-spinning rollers, this is true. However, with suitable attention to the tightness of the roller axles, one can in theory "tune" a mecanum system for better forward traction at the expense of sideways traction.

[Jared Russell]

Originally Posted by Alan Anderson

With perfectly free-spinning rollers, this is true. However, with suitable attention to the tightness of the roller axles, one can in theory "tune" a mecanum system for better forward traction at the expense of sideways traction.

That is absolutely true. For that matter one could mount omni wheels at angles other than 45 degrees to accomplish something similar.

[IKE]

Originally Posted by Chris is me

What thought process do you guys go through to determine if a swerve chassis has strategic benefit for a particular year?

Why don't you build a swerve every year? What are the drawbacks?

Have you ever regretted the choice to build a swerving chassis?

Any answers would be appreciated.

For our team we do a PRO/Con Analysis. In general we only attempt swerve if we really need aditional maneuverability. We thought we needed it in 2005, but ended up changing it to Fixed during the afternoon at our first competition due to stability issues (Swerve often has shorter wheelbase and/or track because the wheels have to fit inside the frame.

The maneuverability usually comes at a pretty significant weight, development time hit. It also usually means single speed which can cause a lack of pushing or top speed (due to singel gear). This year we new that traction limited the pushing issue, but it was difficult to add additional speed at the end of the year when we wanted it.

Incidentally 2005 and then this year were the only years I witnessed competitions where we did not get picked Saturday afternoon (GLR in 2005 and IRI in 2009).

I would highly recommend doing a crab as an off-season project to get some hands on lessons learned. You will gain a whole new level of admiration for those that successfully implement swerve drives (111, 71, 1625....).

Looking back, would we do it again? It all depends on the game.

BTW, the sheet metal construction technique was easy for the students to do, but required a lot of tweaking.

[Nikhil Bajaj]

Originally Posted by Akash Rastogi

Swerve > Mecanum > Omni wheels/ Kiwi drive

I think this dependent on your needs. Mechanum/omni-wheels/kiwi drive can in certain situations switch directions faster as they are closer to true holonomic drive systems than swerve drives. I would say Mechanum/etc beat swerves in maneuverability, but whether you need that much over a swerve is a different issue. If the time that you need to rotate your wheels enough in a swerve is too much, you might consider mechanum.

I wouldn't go so far as to say that if you need a maneuverable drive system and you have the capability to build a swerve drive easily that you should do it. There exist reasons not to do it even from a functionality point of view (let alone weight, actuation, sensing, etc.)

[Al Skierkiewicz]

Chris you should have come and talked to me at IRI. We do not choose crab drive before the game is announced and we proto-play the game. In our tests, we took one of our old crab drive robots and attached a trailer to it. It was clear after our trials that no one was considering the shift in center of rotation that the weight of the trailer introduced. Crab allowed a more controlled turn with the trailer attached which was deemed better for defense and ball pick up. We later found with practice that crab could be used effectively in scoring by wrapping the robot/trailer around an opponent while our alliance partner scored. It was easy to block two quadrants of movement on an opponent.
Let me say again...We do not choose crab or any function for the robot until we play the game and see what is needed. The Whole team (except for field build) brainstorms from the minute the satellite transmission goes to the First logo and we spend days at it. We have groups of people play the game as a robot to see what the interaction is and what affects scoring.
There are disadvantages to crab. The modules take a lot of space within the base. We have managed to get ours as small as can be so that they can be placed near the edges of the robot. Still the wheels will never be within three inches of the outside edge due to the rotation. Crab uses more motors, we have in the past used CIM in all four wheels, or two CIM and two FP or two FP and two drill motors. We usually use two Globe for steering but software needs to know where the steering is at so a special mount is required for feedback and position sensing. We never allow rotation much beyond 360 degrees so mechanical stops are needed as a backup to position sensing. All the hardware for construction of the crab eats into the weight budget. Crab only puts four wheels on the floor when more might be better.
And above all it requires practice, practice, practice. The first step is the realization for the driver that all of their life experience is out the window. They know how to control a bike, a big wheels or a car by steering. Crab now gives them the ability to move like they do in a hallway at school. Translating that motion takes practice, and a fair amount of software help to integrate the hand controller with robot function.