Potential Omni Drive System

[vps]

Hey guys. Someone had an idea for a drive train on our team that seems a little funky but I can't put my finger on why. What do you guys think are the advantages and disadvantages of this drivetrain:

Two Large wheels in the back and two perpendicular omni wheels in the front for a type of swerve drive.

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Kind of like that.

[Munchskull]

Are the back wheels swerve?

[T3_1565]

Originally Posted by vps

Hey guys. Someone had an idea for a drive train on our team that seems a little funky but I can't put my finger on why. What do you guys think are the advantages and disadvantages of this drivetrain:

Two Large wheels in the back and two perpendicular omni wheels in the front for a type of swerve drive.

-- --
| |

Kind of like that.

I don't think its an omni system you are describing.

It seems like it probably drive like a shopping cart? I feel that would be the case. Of course I'm just guessing, but I feel like the front of your robot would float a lot.

It would be very hard to drive straight unless everything on your back wheels run perfect (proper weight distribution, power distribution, etc.)

Might be too finicky for its advantages.

Just my 2 cents though

[Ether]

Originally Posted by vps

What do you guys think are the advantages and disadvantages of this drivetrain:

Two Large wheels in the back and two perpendicular omni wheels in the front for a type of swerve drive.

-- --
| |

Kind of like that.

• It will be difficult to drive in a straight line
  
  
• It will not do any swerve maneuvers.
  
  
• It will have less traction than a chained (or belted) drop-center 6WD
  
  
• Any usable maneuvers it could do, a well-built drop-center 6WD could likely do much better

[vps]

Thanks T3_1565. I hadn't thought of that.

At Ether, I thought that it's a weak system that can at least swerve; why do you think it can't swerve.

[Ben Wolsieffer]

Originally Posted by vps

At Ether, I thought that it's a weak system that can at least swerve; why do you think it can't swerve.

I believe you and Ether are referring to different definitions of swerve. The standard definition of swerve in FRC is a drivetrain where each wheel can swivel to allow the robot to move in any direction (example). I think you mean that the robot can pivot around one end, which it probably can do to some extent.

[Ether]

Originally Posted by vps

At Ether, I thought that it's a weak system that can at least swerve; why do you think it can't swerve.

It can't drive in a straight line in any arbitrary robot-centric direction like a swerve can.

It can't rotate while driving in a straight line like a swerve can.

It can't pivot around any arbitrary point like a swerve can.

[vps]

Originally Posted by Ether

It can't rotate while driving in a straight line like a swerve can.

It can't pivot around any arbitrary point like a swerve can.

Can you describe why it cannot do those?
If the omni wheels are moving orthogonal to the two wheels in the back, won't it be able to rotate while driving and pivot an arbitrary point?

Thanks.

[cgmv123]

Originally Posted by vps

Can you describe why it cannot do those?
If the omni wheels are moving orthogonal to the two wheels in the back, won't it be able to rotate while driving and pivot an arbitrary point?

Thanks.

Rotating the robot while driving requires rotating the back wheels. Rotating the back wheels changes the robot's direction of travel.

[Ether]

Originally Posted by vps

If the omni wheels are moving orthogonal to the two wheels in the back, won't it be able to rotate while driving

Note that I didn't say it couldn't rotate while driving. I said it couldn't rotate while driving in a straight line (like a swerve, mec, or omni can do).

Of course it can turn while driving, but so can a drop-center 6WD skid steer.

and pivot an arbitrary point?

It can't pivot around either of the front wheels, for example. If you think it can, describe what you think the necessary speed of each of the four wheels would need to be to accomplish that.

[Jared Russell]

Switching to a drive configuration that looks something like:

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(all four wheels are omniwheels)

...would remedy these problems while being roughly as easy to implement as your proposed configuration. Team 341 did this in 2007 and 2008, and Team 1218 did it in 2010 (I think?).

[Ether]

Originally Posted by Jared Russell

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(all four wheels are omniwheels)

Inverse kinematics:

S1 = STR + (L/2)*Omega
S2 = FWD + (W/2)*Omega
S3 = STR - (L/2)*Omega
S4 = FWD - (W/2)*Omega

... then normalize if any wheel exceeds max speed

L = wheelbase, ft
W = trackwidth, ft

FWD = forward, fps
STR = strafe right, fps
Omega = clockwise rotation rate around center, rad/sec

S1 front right wheel linear speed, fps
S2 front left wheel linear speed, fps
S3 rear left wheel linear speed, fps
S4 rear right wheel linear speed, fps

[Spoam]

Originally Posted by Jared Russell

|   -

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(all four wheels are omniwheels)

Ether, since this a relatively novel drive that lacks the wealth of information available for other drives, do you think you could list some disadvantages of a drive like this (especially compared to comparable drive trains like slide, mecanum and skid-steer)? I'm specifically interested also in the speed and rotational limitations/capabilities of a drive like this. Also ability to drive straight and the effect of CoMon driving.

Originally Posted by Jared Russell

1218 did it in 2010 (I think?).

Based on CMP footage, it looks like the they used some sort of more standard 4WD in 2010. May have been a different year or a pre-CMP design.

[Ether]

Originally Posted by Spoam

Ether, since this a relatively novel drive that lacks the wealth of information available for other drives, do you think you could list some disadvantages of a drive like this?

I've never built, driven, or even seen a drive like this in action, so take the following speculations with a grain of salt.


Disadvantages

It's not going to be as easy to drive in a straight line as skidsteer or swerve.

It may require closed-loop speed control of the wheels to get stable and predictable performance.

Traction is poor compared to swerve and skidsteer.

Available force is not equal in all directions, and is less than swerve.

Advantages

It has (theoretically) all the degrees of freedom that a mec or swerve has. (Slide drive does not).

It seems to be rather simple to build and code.

It's probably lighter that mec and swerve.

It's probably less expensive than mec and swerve.

(especially compared to comparable drive trains like slide, mecanum and skid-steer)?

I don't consider slide drive to be comparable to mec and swerve. It does not have 3 degrees of freedom.

and the effect of CoMon driving

What is "CoMon" driving?

[Spoam]

Originally Posted by Ether

What is "CoMon" driving?

Whoops, that should say CoM as in center of mass. Ie, theoretically, would weight distribution affect this robot's ability to turn or drive properly?