[Dprglist] Swerve Drives

[Karim Virani]

You've known about swerve drives for a long time, just maybe not by that
name. In competition robotics, "swerve" is used to refer to any
omni-directional drive-train where the drive wheels can each be steered
(rotating their own drive axes).

The most obvious example is the mars rovers. This allows efficient
strafing, crabbing, in-place rotation and diff steering. There are a number
of different variations. One variation gangs the steering of the drive
wheels to a single motor. This cuts down on the number of motors required,
but then rotating in-place requires slippage like any tank drive.

Usually there are dedicated motors for steering vs. driving each wheel. One
of those videos talks about coaxial vs. differential as if those are the
only two types of swerve modules. But the ExoMy is also a swerve drive and
doesn't fit those patterns with its simpler transmission.

The DiffySwerve option is interesting in that it uses two similar motors
and mixes them through a differential to output both the steering and
drive. The benefit is that when you are driving without turning - the full
power of both motors is available to forward This summer my varsity team
(Iron Reign) designed their own belt driven DiffySwerve module but it
hasn't passed all the way into production - it's very hard to make these in
a small enough size for our robots.

Separately they are working on a hybrid chassis for this season that's also
confusingly called a DiffySwerve for a completely different reason. It's a
tryke - with the two "forward" wheels set up in a differential drive
configuration. These wheels are fixed to the chassis on a common axis and
can't steer. So for in-place turns, the center of the turn is the
mid-point of their common axis. Standard differential drive config.

But the "rear" wheel is not a caster or omni. It's a swerve module. So for
an in-place turn, it needs to drive along the circumference of the circle
defined by its position as the radius relative to the center of chassis
rotation.

They're starting with a simpler swerve module design. But if they get their
DiffySwerve module built, the whole robot will become a DiffyDiffySwerve.

Best,

Karim


On Mon, Oct 11, 2021 at 1:46 PM Murray Altheim via DPRGlist <
dprglist at lists.dprg.org> wrote:

> I hadn't heard of a "Swerve Drive" before but Kumar posted the first
> video to the Personal Robotics server, thought it might be of general
> interest, then found another that goes into detail:
>
>    FRC team 1690 Orbit 2019 off season swerve drive reveal
>    https://youtu.be/wCakzMfRPKs
>
>    Swerve Drive 101 with Clem McKown from FRC 1640 and Jeremy Zang from
> FRC 694 - Stuy Splash 2020
>    https://youtu.be/ykl5CvG53KA
>
> It reminded me a bit of David's dancing robot.
>
> Cheers,
>
> Murray
>
> ...........................................................................
> Murray Altheim <murray18 at altheim dot com>                       = =  ===
> http://www.altheim.com/murray/                                     ===
> ===
>                                                                     = =
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