Hi. I am doing research on swerve modules, but I am not able to differentiate from coaxial swerve modules, and non-coaxial ones.
Could someone explain the difference, and the advantages of one over the other?
Coaxial implies two axles sharing the same center. Basically this means you’re driving a shaft that goes down the pivot shaft of the module, and that shaft drives the wheel to create motion.
Non-coaxial would imply a motor mounted on the module, so the module pivot isn’t coaxial.
Coaxial solves many problems with non, but the cost is higher.
Coaxial swerve has the motor fixed mounted above the pivoting wheel module and the drive shaft is coaxial with the turning axis.
Non-coaxial usually have the drive motor mounted within the module.
The motor wires are usually mounted through the center, but do not have to be. This limits the number of turns you can have on the wheel module.
The advantage of coaxial is that the wheel has infinite turning capability without worrying about wire entanglement. This comes at the cost of some complexity (bevel gears) and weight.
The non-coaxial is usually lighter and more compact, though your mileage may vary. with this design, you should programmatically make sure you limit the number of spins of the module.
This is a great question and definitely one that does not have a clear answer anywhere online. I am no expert on the subject, but from my experience a coaxial swerve has the drive shaft from the motor/gearbox going directly through the rotation axis of the module. This allows for full 360 degree (and more) movement of the module. In other words, the module can spin indefinitely without wires getting in the way, or any need for a slip ring.
Coaxial Example: 1640 has an awesome wiki on their application with awesome diagrams and other drawings. (Keep in mind, they did not go for swerve, but they show the different options for this type of drive).
http://team1640.com/wiki/index.php?title=4-Wheel_Pivot_Drive
A non-coaxial swerve does not have this capability. It can either spin until the wires get wrapped around the module, or you need a slip ring.
Non-coaxial example:
http://www.team221.com/viewproduct.php?id=51
Notice how on this system the CIM rotates with the module. This is indicative of a non-coaxial swerve. It has limited rotations before the wires get tangled.
So if the definition of coaxial is that the power for the wheels goes through the axis of rotation, then this would not be coaxial, but it still is able to turn indefinitely without slip rings.
How would it be classified?
This is an example of a Coaxial Swerve:
http://i.imgur.com/PogljdA.png
From the outside, we can see that both the steering and the driving motors are mounted off of the module.
Here’s a section view that should help explain how the power is transmitted:
http://i.imgur.com/fdTiJ4Q.png
This shows how the pivot shaft is actually two axles. The outermost one is the pivot shaft (our team calls it the Top Hat). This shaft anchors the module to the frame, and rides inside a bushing for lowering friction while steering. The Top Hat is retained by a large circlip.
The next shaft, running down through the Top Hat, is the drive shaft. It’s supported by two bearings, and drives a miter gear to transition from vertical rotation to horizontal rotation. This lets the module rotate infinitely, which carries several advantages in both coding, sensor use, and so on.
That module is coaxial, as the CIM motor does not rotate with the module.
This is in fact a coaxial swerve. The axis of rotation of the module is the same axis that the motor is on.
Just because the rotation motor is offset doesn’t mean it is not a coaxial system.
The picture I linked wasn’t a very good one, this is a better one. While the CIM does not rotate with the module, the axis of power is offset from the axis of rotation, going through a bearing in the rotation gear.
This would still be a coaxial module as the power for the wheel is still transferred through the axis of rotation.
Coaxial swerve tends to have two different methods in today’s FRC. There are those with the bevel gears done directly off the axis of rotation, and there are those with the bevel gears offset from the axis of rotation with a separate stage of gearing. Both are coaxial modules, just packaged differently. The offset bevels tend to be a little harder to design or machine parts for, but tends to be a little more compact then the other method (certainly shorter).
A non-coaxial swerve cannot rotate infinitely without a slip ring. That’s the plain and simple way of looking at it.
As Nick already stated, the drive power for this module is still coming through the axis of rotation whether the bevel gear is offset or not.
The rule of thumb is: Can you rotate it indefinitely? If yes, Coaxial. If no, non-coaxial.
Thanks for explaining! Now I understand that just because the axle is offset the power is not.