Hex Kiwi Drive

[Cash4587]

The programming on this would be very very tricky. Figuring out how much power to give each motor in the "Forward" direction would take some playing with and tuning because two wheels are pointed "Forward" and the other four are angled.

Although this is a cool idea and looks interesting it probably is not very practical. In most cases it would make the most sense to just do a 3 wheel kiwi or an H-Drive. For that matter, even a swerve would probably make more sense.

[Dan_Karol]

Interesting idea,

Something to think about is that it will be really hard to guarantee that all 6 wheels are contacting the floor and transferring force. This is especially true since there are no real specifications on the flatness of the field that get measured during setup

Most groups that use force-vector drive trains (kiwi,mecanum) either rely on flexibility in the chassis to help the wheels touch the floor at all times or they limit themselves to 3 wheels.

It may not matter if there is not uniform contact for this implementation as each wheel will contribute less to the overall robot motion so the loss of any single wheel should have less of an impact. If the robot is sitting on three wheels, two or more of which are adjacent there will still be strange behavior.

A way to get around this could be to use the a suspension. one implementation could be similar to the VersaDrop sold by Vex pro with the pressure regulated down just low enough to not cause the pneumatic cylinders to deploy when pressurized. With this system, when a wheel encounters a dip or sag in the floor it will follow the contour and stay in contact.

That said a simple proof of concept would make all of these concerns unnecessary.

Good luck!

[ThaddeusMaximus]

Originally Posted by caume

To counter that argument, look at like 90% of the robotics this year, 6WD. This is just a fancy 6WD if you think about it

6 wheels? yep. fancy? You betcha. Same thing? Heavens no.

6wd gives you maximum pushing force (for both motor and traction) in the direction of orientation. Omniwheels currently available are less grippy than solid wheels currently available, and having 2/3s of your wheels basically inactive when in a straight line reduces motor power. These are not perks...

Not to mention the normal force on any set of wheels is half of that in a kiwi. I'd think for this reason that a kiwi would beat this in a pushing match, which would proceed to get dominated by a 6wd with good solid wheels.

[ThaddeusMaximus]

Originally Posted by Cash4587

The programming on this would be very very tricky. Figuring out how much power to give each motor in the "Forward" direction would take some playing with and tuning because two wheels are pointed "Forward" and the other four are angled.

Unsure why this would be any more confusing than kiwi drive inverse kinematics. Just do the geometry what with the velocity vectors.

[caume]

Originally Posted by ThaddeusMaximus

6 wheels? yep. fancy? You betcha. Same thing? Heavens no.

6wd gives you maximum pushing force (for both motor and traction) in the direction of orientation. Omniwheels currently available are less grippy than solid wheels currently available, and having 2/3s of your wheels basically inactive when in a straight line reduces motor power. These are not perks...

Not to mention the normal force on any set of wheels is half of that in a kiwi. I'd think for this reason that a kiwi would beat this in a pushing match, which would proceed to get dominated by a 6wd with good solid wheels.

I was mostly joking when I said it's a 6WD. Obviously, the only real similarity is the number of wheels. Where did you get the information about omnis being less grippy? I'm not doubting it at all, I just haven't been able to find info about the CoF when I've looked.

[Cash4587]

Only thing I would be worried about is that it probably will act a bit weird when trying to go forward since the side wheels would not be at the 30 degree angle like the other 4. As long is the math is done out right though it probably would be fine.

[EricH]

Originally Posted by caume

I was mostly joking when I said it's a 6WD. Obviously, the only real similarity is the number of wheels. Where did you get the information about omnis being less grippy? I'm not doubting it at all, I just haven't been able to find info about the CoF when I've looked.

0.8-0.88 for an AM 6" aluminum omni (AM site)
A 6" "normal" wheel (rubber tread, not high grip) is 0.9 (AM site).
"HiGrip" wheels are 0.95.

That's going to be a 10%-20% increase in grippiness when using non-omni wheels instead of omniwheels.

[ctt956]

Very interesting drivetrain! I like the shape. As others have said, the bumper mounts on the frame probably need to be stronger. If you use those custom gearboxes, covering them would be a good idea to keep debris, wires, loose parts, etc. out of the gears. There is a PCM near a CIM motor, but I don't see any pneumatics...is it there just in case some will be added? I think the battery connector mounted on the robot is a good idea, but could be higher to avoid bending the wires too much. Maybe a smaller mount too so connecting/disconnecting would be easier. As for practical use in a game, this would probably only be effective if the robot needed to reach something higher up so the frame wouldn't get in the way of doing so, as in 2011. But just because there hasn't yet been a game where this would be practical doesn't mean there won't be one in the future... If nothing else, this would be an impressive demo bot. I wonder how fast that thing could spin in place with all 6 CIMs running at full speed...

[Ether]

Originally Posted by Cash4587

The programming on this would be very very tricky.

wheel1 =   STR + ωR
wheel2 = - FWDcos30  + STRcos60 + ωR
wheel3 = - FWDcos30  - STRcos60 + ωR
wheel4 = - STR + ωR
wheel5 =   FWDcos30  - STRcos60 + ωR
wheel6 =   FWDcos30  + STRcos60 + ωR

(Then divide all wheel speeds by the maximum one if any one exceeds the maximum achievable wheel speed)

[caume]

Originally Posted by ctt956

There is a PCM near a CIM motor, but I don't see any pneumatics...is it there just in case some will be added?

That is a VRM

[Cash4587]

Originally Posted by Ether

wheel1 =   STR + ωR
wheel2 = - FWDcos30  + STRcos60 + ωR
wheel3 = - FWDcos30  - STRcos60 + ωR
wheel4 = - STR + ωR
wheel5 =   FWDcos30  - STRcos60 + ωR
wheel6 =   FWDcos30  + STRcos60 + ωR

(Then divide all wheel speeds by the maximum one if any one exceeds the maximum achievable wheel speed)

I hadn't thought of doing it this way. I had just assumed that "Forward" would be a point of the hexagon moving forward utilizing all six wheels when going forward, instead of just four using this setup. Even still, this all makes more sense now.

[Ether]

Originally Posted by Cash4587

I hadn't thought of doing it this way. I had just assumed that "Forward" would be a point of the hexagon moving forward utilizing all six wheels when going forward, instead of just four using this setup. Even still, this all makes more sense now.

If you want FWD to be a point on the hexagon, just change the inverse kinematics like so:

wheel1 = - FWDcos60  + STRcos30 + ωR
wheel2 = - FWD + ωR
wheel3 = - FWDcos60  - STRcos30 + ωR
wheel4 =   FWDcos60  - STRcos30 + ωR
wheel5 =   FWD + ωR
wheel6 =   FWDcos60  + STRcos30 + ωR

(Then divide all wheel speeds by the maximum one if any one exceeds the maximum achievable wheel speed)

[caume]

Originally Posted by Cash4587

I hadn't thought of doing it this way. I had just assumed that "Forward" would be a point of the hexagon moving forward utilizing all six wheels when going forward, instead of just four using this setup. Even still, this all makes more sense now.

I should have added the pseudo to the original post, but this guy did it just how I did. The only difference is I would have field orientation, so it could have any part of the robot set as forward.

[ctt956]

Originally Posted by caume

That is a VRM

OK, it was hard to tell from the CAD as the two look similar: VRM PCM

[GeeTwo]

Originally Posted by Cash4587

Although this is a cool idea and looks interesting it probably is not very practical. In most cases it would make the most sense to just do a 3 wheel kiwi..

Yes. The great advantage of 3 wheels is the relative insensitivity to irregularities in the drive surface. Unless you reach really high angles or high center on at least one side, all three wheels will carry a share of the robot weight, enabling the vector drive forces to add up as intended. With six wheels, it is rather easy for some of the wheels to come off the floor, sending the net drive force in unintended directions.

Originally Posted by asid61

That might be a tad fast even for 6 CIMs, because of brownouts.

For brownout/gear ratio purposes, this is not the same as a 6 CIM skid/swerve. If you're driving parallel to two of the wheel axles, two of the CIMs do not contribute to the drive at all, and the other four are reduced about 13%. If you're driving perpendicular to two of the wheel axles, two CIMs are fully engaged and four are limited to about half contribution. Due to different efficiencies at different speeds and that you're drving different motors at different levels, things will get more complicated quickly. With a holonomic drive train, you should be more concerned about avoiding or escaping pushing contests than about winning them.