8 wheel Kilough

[riverdrake250]

Has anyone ever seen/made an 8-wheel Kilough drive? If not, why not? What are the advantages/disadvantages relative to a regular Kilough drive or a mecanum drive?

[Nate Laverdure]

I have some recent experience in the world of killough drives. Here's some of what I learned:

Less wheels are better than more
Each wheel needs to be individually powered and controlled. You can't couple any of the wheels together and still retain holonomic motion. If you're going to spend resources (time, money, anguish) on an 8-motor omnidirectional drivetrain, why not make a fully independent swerve instead?

For number of wheels N > 3, you must design your chassis to be flexible enough to load the wheels approximately evenly. For N = 3, the chassis can be as rigid as you wish.

(N = 2 is a bicycle, N = 1 is a unicycle)

Odd values of N are better than even values of N
What's interesting about omni and mecanum wheels is that the rollers resist turning. This small but significant amount of friction seems to be worsened when there is an axial thrust on the roller.

When you arrange 4 omni wheels in a rectangle, you get a drivetrain that prefers to travel in the cardinal directions and dislikes traveling in arbitrary, off-cardinal directions. This is because when the robot travels in the cardinal directions, the rollers on the pair of parallel wheels that are pointed in the direction of travel are not spinning. Turning to an arbitrary, off-cardinal direction means overcoming the nonnegligible friction on these rollers. If the robot is accelerating along a cardinal direction, then the addition of axial thrust on the rollers will effectively lock the robot's orientation until the acceleration stops! I have video of this phenomenon that I can share.

This issue is minimized when N = [3, 5, 7, ...] because when the robot is moving in an arbitrary direction, there can never be more than one wheel with motionless rollers.

[ajlapp]

I'm note sure I'd refer to anything with more than three wheels as a Killough Platform.

This subject always reminds me of our original Killough prototype from years back...before we understood how to make omni-wheels. We never got it motorized, but the rolling carcass was really cool.

[riverdrake250]

Okay, maybe I am using the wrong terminology. I am talking about a diamond shape with 2 omniwheels on each side, not an octagon. It would use a motor for each side and (theoretically) could run on mecanum code.

[GeeTwo]

Originally Posted by riverdrake

Okay, maybe I am using the wrong terminology. I am talking about a diamond shape with 2 omniwheels on each side, not an octagon. It would use a motor for each side and (theoretically) could run on mecanum code.

If the wheels are in the same line, it should still work. If you trimmed the chassis to what is normally a diamond on Killough, you'd effectively have two Omni-tank drives at right angles to each other. My initial thought is that it would help reduce the effects of uneven loading due to acceleration. It would have trouble with uneven surfaces; you may be able to reduce this by moving the wheels as close to the corners as you can. If you do this, please post your results. If not, we may make one off-season.

[gpetilli]

Originally Posted by Nate Laverdure

Less wheels are better than more
Each wheel needs to be individually powered and controlled. You can't couple any of the wheels together and still retain holonomic motion. If you're going to spend resources (time, money, anguish) on an 8-motor omnidirectional drivetrain, why not make a fully independent swerve instead?

For number of wheels N > 3, you must design your chassis to be flexible enough to load the wheels approximately evenly. For N = 3, the chassis can be as rigid as you wish.

I agree that more wheels (and motors) is unnecessary complication.

We competed with what I believe is a unique Kilough variant last year. We used four wheels with 30deg toe-in (thin diamond) instead of 45deg. This gave us better traction (closer to tank) in the fwd/rev direction and worse in left/right. The expected fwd traction improvement vs. Mechanum is cos(30)/cos(45) or 22% (and -30% sideways). We used two undriven Vex 2.75" omniwheels with encoders to measure actual movement instead of driven wheel speed. The system performed very well. We did use a stiff frame, but the closed loop control detected and corrected for any wheel slip.

This year, there is no apparent preferred direction of travel so we are designing a more traditional Mechanum system. Given the potentially slippery platforms and wheels loosing contact on the ramps, we will again use instrumented following wheels to correct for wheel slip.

[Christopher149]

Originally Posted by ajlapp

I'm note sure I'd refer to anything with more than three wheels as a Killough Platform.

This subject always reminds me of our original Killough prototype from years back...before we understood how to make omni-wheels. We never got it motorized, but the rolling carcass was really cool.

Funny thing is, that prototype is still intact in 857's storage space (being the most intact kiwi the team has at the moment).

[GeeTwo]

Originally Posted by Nate Laverdure

I have some recent experience
[/b]Each wheel needs to be individually powered and controlled. You can't couple any of the wheels together and still retain holonomic motion.

If the wheels which are powered together drive in the same direction along the same line, you should still have holonomic motion - with the understanding that by virtue of the chain or timing belt, the two (or more) wheels are not independent degrees of freedom, but are effectively the same actuator. I say should because I have not actually tried this yet. I would like to play with a 6-wheel kiwi; it might be a decent platform for a hexagonal or circular 'bot.

[gpetilli]

Originally Posted by GeeTwo

If the wheels which are powered together drive in the same direction along the same line, you should still have holonomic motion - with the understanding that by virtue of the chain or timing belt, the two (or more) wheels are not independent degrees of freedom, but are effectively the same actuator. I say should because I have not actually tried this yet. I would like to play with a 6-wheel kiwi; it might be a decent platform for a hexagonal or circular 'bot.

I agree that independent gearboxes, not wheels, determines the control system degrees of freedom. More wheels per gearbox does little to improve performance given that the lb/in2 goes down proportionally to the increase in wheel surface area. So more than one wheel per gearbox takes more space and weights more.