Swerve with a twist

[Harshizzle]

So after a discussion with my mentor about swerve drive, we thought of a potentially new drive system that is swerve like but a bit different.

What we pictured was a swerve module where the wheel was extended out from the center of rotation.

(top view- The o represents an axle going through the computer screen).
_
| |
| |---o
|_|

When the wheel is powered, and the o axle is prevented from spinning (perhaps using a brake system), the robot would move in the direction the wheel is spinning.

When the wheel is powered and the o axle is allowed to freely rotate, the wheel would turn in a circle, and could end up pointing in a direction you want it to.

With both those combined, the robot could have holonomic drive capabilities.

The biggest problem I see with this is the weight distribution, as the force would be on the o axle, which would not be touching the ground (or maybe it could hmm...). I also dont see this as very practical for an FRC game, because swerve would probably be much faster at orienting itself.

But this drive system would make every swerve module require only one motor and one braking system, which in some industrial applications, could be better.

Anyway, does this already exist somewhere? If not, thoughts?

[Ether]

Originally Posted by Harshizzle

What we pictured was a swerve module where the wheel was extended out from the center of rotation.

(top view- The o represents an axle going through the computer screen).
_
| |
| |---o
|_|

Could you please post a hand sketch showing top, front, and side views? I cannot form a confident mental image from that ASCII art.

[Harshizzle]

Originally Posted by Ether

Could you please post a hand sketch showing top, front, and side views? I cannot form a confident mental image from that ASCII art.

Apologies for the darkness, but this is what I had in mind

[Ether]

Originally Posted by Harshizzle

Error 403

You can attach the sketch to your post. No need to upload it elsewhere.

[Ryan_Todd]

^^^ no image


Also, here's a quick Inkscape job:

[Peyton Yeung]

I guess I still don't understand how this works.

[Joe G.]

I feel like this would introduce a lot of variables which hamper reliability, which are not seen in a normal swerve. Your module pivot becomes a lot more sensitive to variations in terrain, weight distribution, and small variations between the modules, for example. The steering code would be no simpler, and in fact may take on new layers of complexity since your motors are performing double-duty. Some maneuvers, such as spinning while translating, would likely become more difficult to control.

It may seem simpler on paper, but I worry that you're in fact introducing things that will generate a whole lot more trouble for you. In my never-ending quest to learn how to effectively and quantitatively assess design simplicity (ironically, a very complex problem), I've become increasingly convinced that "minimize actuator count at all costs" is a crude and shortsighted way of approaching the problem.

[Harshizzle]

Originally Posted by Skywalkar

^^^ no image


Also, here's a quick Inkscape job:

Yeah, this is exactly what I was attempting to describe.

There are two states, one where the o axle is allowed to coast, and one where it is not.

I agree, for FRC purposes this would create far more problems than it would solve. But I wanted to bring it up for discussion anyway.

This design cannot do simultaneous steering and moving with any sort of accuracy, but in cases when that is not needed (I'm picturing something like an amazon warehouse robot), this could be an option.

[Ryan_Todd]

Originally Posted by Harshizzle

Yeah, this is exactly what I was attempting to describe.
[...]
This design cannot do simultaneous steering and moving with any sort of accuracy, but in cases when that is not needed (I'm picturing something like an amazon warehouse robot), this could be an option.

Thanks for confirming my interpretation.

Personally, I would be less interested in this concept as a "Plan A" design than as a case study in control systems design. As discussed above, this still has the same total number of outputs as a traditional independently-steered swerve drive, but it nonetheless manages to be an under-actuated system; this would therefore save relatively little in cost and complexity, while giving up a fair amount of controllability in the process. If you're looking to save on cost and complexity, after all, it's hard to beat a good old-fashioned skid-steer setup.

Don't get me wrong, however, because I strongly believe that this project would still be completely worthwhile-- and indeed, quite valuable in the real world! The key is to shift gears a bit, and instead view this as a research project.

As a software engineer in the automotive industry, I always need to be mindful of the fact that the real world isn't perfect; things go wrong all the time, and my work needs to be able to take that in stride. As a result, my job is not simply to get my component to work right, but rather to ensure that even if the rest of the system is compromised to the point where my component cannot work right, it will still never work wrong. If that means slowing down the engine to prevent it from overheating (or stopping the engine altogether to prevent it from exploding), then so be it!
This kind of thinking is essential whenever the end user's safety is at stake, and strongly advisable in many other applications.

With this in mind, can you see a way that it might be helpful to have an understanding of how to control a swerve drive platform, even if one or more axes of control are disabled, modified, or behaving abnormally?

(It may, for example, be advantageous to design a swerve drive specifically to allow for the possibility of falling back to an alternative control scheme like the one described above.)

[lovelj]

What would be better is a powered caster. By offsetting the steering axis but having it rotated 90 deg from your design you can achieve true Holonomic omnidirectionality. The steering axis imparts a velocity that is orthogonal to your rolling axis. No matter what the configuration of the pods, you can instantaneously move in any direction. There is a nice closed form solution for the inverse kinematics. We built a munition loader for the navy using this approach. I've debated about trying it on a FIRST robot with 3824. Good summer project. I've got a publication on it if anyone is interested

[Kevin Leonard]

Originally Posted by Skywalkar

(It may, for example, be advantageous to design a swerve drive specifically to allow for the possibility of falling back to an alternative control scheme like the one described above.)

Dude that would be so cool.
A steering motor burns out, so somehow the robot shifts into using the drive motors for both drive and steering.
I'd love to see that implemented by someone (although probably not on an FRC robot, that sounds far too complex and unnecessary. If a motor burns out in a match, its probably better to just wait two minutes until the match is over and repair it.)

[lovelj]

If you want holonomic omnidirectionality (swerve is by definition non holonomic, the system discussed here is likewise non holonomic), here is a link to a fact sheet on a system we developed that uses powered casters which provided holonomic omnidirectionality using conventional tires.

http://web.ornl.gov/sci/ees/eesrd/res/OCILOW.pdf

Its a slight twist on the swerve (e.g. in-line orientable wheels) and the conventional car steering topology discussed here. It's the third option for a powered steering/rolling system. OCILOW - Off-Center In-Line Orientable wheels (e.g. a fancy way of saying powered caster). By having the steering axis offset from the rolling axis and behind the wheel rather than next to it, the steering axis causes a velocity vector that is orthogonal to the rolling axis. Coordinated control of the steering and rolling enable instantaneous velocity vectors in any direction (e.g. holonomic) using conventional tires. You get very cool catenary type coordinated motions of the wheels, no skid while transitioning instantaneously from forward to sideway motion. The system is very robust and is highly redundant. You can still have full holonomic control with only two wheel pods active, the one or two being passive casters. It had a very cool double enveloping worm gear to drive the rolling axis. The challenge is the controls. If you have 4 active pods, you have 8 motors to control 3 degrees of freedom (x, y, theta). However, there is a closed for solution for the inverse kinematics, solving for the instantaneous velocities for all rolling and steering motors giving a desired Cartesian velocity vector (x_dot, y_dot, theta_dot). The cool part is that, no matter what the wheel pod configuration (if you look at the picture, we intentionally got the steering axes all different), it can still instantaneously move in any direction.

My group actually developed the split hub spherical wheel in the mid 1990s which was similar to the mecanum. We built a weapon loading system for the Air Force but the tires would wear out fast. It had a 7 DOF force reflecting arm on a holonomic omnidirectional platform. One person could load a 2000 lb weapon on the wing of an aircraft feeling the wedging and jamming when loading the weapon on the wing pylon. The holonomic motion was necessary for navigating under the wing. Unfortunately, like the mecanum, you have point, rather than surface, contact with the ground so your traction is low and tire stresses are high (not good when handling a bomb). If you really want/need holonomic omnidirectionality and you want to use conventional tires, it's the only way.

[Ether]

Originally Posted by lovelj

swerve is by definition non holonomic

Please post a link to the definition referred to above.

Originally Posted by lovelj

I've got a publication on it if anyone is interested

Please provide a link to your paper, or post it here.

[lovelj]

The best paper I've seen was by Wada and Morit, "Holonomic and Omnidirectional Vehicle with Conventional Tires" in 1996 IEEE Int. Conf on Robotics and Automation. Unfortunately, they didn't provide the closed for solution for the inverse kinemtatics . However, figure 1 in the paper says it all. Just take a minute and look at it and it will explain why you will never, by definition, get holonomic omnidirectionality with a swerve.

http://ftp.imp.fu-berlin.de/pub/Roja...l/00509272.pdf

I'll dig up our work on OCILOW.

[Ether]

Originally Posted by lovelj

Just take a minute and look at it and it will explain why you will never, by definition, get holonomic omnidirectionality with a swerve.

http://ftp.imp.fu-berlin.de/pub/Roja...l/00509272.pdf

The word "swerve" appears nowhere in that paper. Please define what you mean by swerve in the context of your previous post ("swerve is by definition non holonomic").