HELP; moving omni robot to move sideways

[CalTran]

Originally Posted by GeeTwo

Another variant of killough that we played with diagrams o a few years ago had four wheels at the corners of the robot, each aligned with a different side. This should drive similarly to regular killough or mecanum, but be more stable due to the larger wheelbase and not require non-square cuts. See attached image. (Again, we just brainstormed the idea; we never tried to build one.)

Wouldn't this be functionally the same as Killough, just a different geometry for the wheelbase?

[ctt956]

Originally Posted by Parrot62

But basically the robot will be able to go sideways whether the frame was square or rectangular, right?

I would say yes, though the size and shape could change the speed. But it should work.

[ctt956]

Originally Posted by GeeTwo

In order to drive in an arbitrary direction with omni wheels, the wheels must be independently driven (four gearboxes for four wheels) and not all pointing the same direction. The angle does not actually have to be 45 degrees; if you will be driving forward/reverse most of the time and just need the "strafe" motion for minor adjustments, you can angle them at (for example) 30 degrees.

To search for various configurations of omni wheels, I suggest:

• omni tank (4 wheels on two drive trains, no sideways driving)
• kiwi (three wheels, most commonly oriented 120 degrees apart)
• killough (3 wheels in Vex and FTC, but more commonly 4 wheels at an angle in FRC)
• slide or H drive (5+ wheels on 3 axes); omni tank plus one or more wheels mounted perpendicularly, most commonly forming the letter H.

Another variant of killough that we played with diagrams o a few years ago had four wheels at the corners of the robot, each aligned with a different side. This should drive similarly to regular killough or mecanum, but be more stable due to the larger wheelbase and not require non-square cuts. See attached image. (Again, we just brainstormed the idea; we never tried to build one.)

Interesting. It looks like it would be a bit slow with only two wheels driving it in each direction while the others are being "dragged" on small rollers.

[Ether]

Originally Posted by CalTran

Wouldn't this be functionally the same as Killough, just a different geometry for the wheelbase?

Depends on what exactly you mean by "functionally the same".

The inverse kinematics would be entirely different.

To go forward you would power only 2 wheels (with possible small corrections to the other 2 wheels to track straight).

To strafe you would power only 2 wheels.

Also when going true forward or true strafe, angular wheel speed times wheel radius would translate directly to forward (or strafe) vehicle speed with GeeTwo's "Killough variant". Not so with "standard" Killough with angled wheels.

[GeeTwo]

Originally Posted by CalTran

Wouldn't this be functionally the same as Killough, just a different geometry for the wheelbase?

Yes, as I noted, the two advantages were that no non-square cuts were required, and that the contact patches of the wheels with the carpet are farther apart for a given chassis size. Unless you designate the front of the robot as one of the corners, you would have to rotate the inverse kinematics 45 degrees. In principle it would be simpler. Designating the wheels:

1 2
3 4

As pictured, an assuming that a positive input to each motor causes a clockwise torque to the robot chassis, and assuming a "strafe plus rotation" joystick setup, the linear inverse kinematics would be:

Wheel 1: R + X
Wheel 2: R - Y
Wheel 3: R + Y
Wheel 4: R - X

Edit2: Also, it would have more usable interior space for controls and other mechanisms.

[The other Gabe]

Originally Posted by Rich Kressly

Search CD for "kiwi drive" which is a term that describes the 4-omni wheels (seen in the previous post) positioned on 45 degree 'slants' in relationship to the front, back, and sides of your robot.

I was under the impression that the 4-wheel set up was called holonomic, while the 3-wheel triangle is called kiwi drive

[GeeTwo]

Originally Posted by The other Gabe

I was under the impression that the 4-wheel set up was called holonomic, while the 3-wheel triangle is called kiwi drive

Holonomic is a fairly general term. Originally it meant that a system is controlled by the same number of degrees of freedom as it is free to move. In FRC robotics, it usually refers to wheels or rollers which apply a force in one direction while allowing free motion in one or more directions at right angles to it. By the first definition, only the kiwi and H/slide drive (three independent gearboxes controlling rotation, forward, and lateral motion) qualify; by the latter, all of the drive systems discussed so far in this thread qualify.

Note that swerve drives are NOT holonomic by either definition - in order to drive in the three degrees of freedom of the carpet, typically 5-8 independent actuators control motion, and the wheels resist motion in the direction they do not apply force.

That said, I have rarely seen kiwi used to refer to anything other than 3 omni wheels.