pic: Prototype Drivetrain

[548swimmer]

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[Hawiian Cadder]

looks good, however you will probably want a couple more sideways bracing to attach things to

[Chris is me]

Why the tiny, centered wheelbase?

[RMS11]

The wider it is the easier it will be to turn, I believe.

[ttldomination]

Originally Posted by RMS11

The wider it is the easier it will be to turn, I believe.

I do believe that is correct. Someone with a more sound physics base please feel free to correct me, but it all has to do with torque.

On a wide base, when the center of rotation is assumed to be between the wheels, the wheels turn on a larger radius than in this CAD. Therefore, the large radius gives the driving wheels more torque.

Although I can understand the concept. I do believe that with this kind of drive, you'd be able to turn quicker, but I think that with a 120-pound robot, you're not going to get the performance with a basic 6WD.

- Sunny

[548swimmer]

Originally Posted by Chris is me

Why the tiny, centered wheelbase?

Over the summer, team 548 is planning to come up with a plethora of designs to prepare our CAD team for kickoff. Also, we will be creating a binder of "crazy ideas" to reference once we get the challenge. Anything we can come up with now will help give us a jumping off point for our final design. This drivetrain is designed for maneuverability.

This wheelbase will allow our robot to not only have a zero turn radius, but also turn much more quickly than wheels on the outside, due to the smaller arc length traveled for the same change in angle. I am anticipating extreme difficulty in the driving of this machine. If the game requires maneuverability, we'll be in great shape though.

[548swimmer]

Originally Posted by ttldomination

On a wide base, when the center of rotation is assumed to be between the wheels, the wheels turn on a larger radius than in this CAD. Therefore, the large radius gives the driving wheels more torque.

- Sunny

A wider wheelbase does give it more torque due to the increased radius. This will not be needed to turn though, because with 2 cims powering 1 wheel, we will have unlimited power (not actually, but for FIRST applications). Also, because there are omni's in the front and rear, there will be a negligible force of friction counteracting the torque.

[DonRotolo]

Originally Posted by 548swimmer

because with 2 cims powering 1 wheel, we will have unlimited power

...until the wheel slips, at least.

Because of the moment of inertia of a relatively heavy mass swinging around a central axis with little friction (those omnis) you will find such a design difficult to control - even with a wider wheelbase.

Commenting on the sheet metal: Cut the thickness by 30% and put a 1" flange (bent inwards for both) at the top & bottom. Saves weight, adds significant rigidity. Some of the flange can be lost or reduced by the omni wheels for clearance if necessary.

To keep the assembly from racking (parallelogram), consider very thin braces from outside corners, in an X pattern. Even 1/8" wire rope would be fine, since it is in tension.

Your hole pattern could be improved. The solid pieces should all meet at stress points, not tangentially to them. See the small round 'pin' in front, 1/4 way from the left side of the image.

Nice CAD work.

[ttldomination]

For some reason I can't figure out how to delete this post, but Don Rotolo pretty much summed it up.

[548swimmer]

Originally Posted by Don Rotolo

...until the wheel slips, at least.

Because of the moment of inertia of a relatively heavy mass swinging around a central axis with little friction (those omnis) you will find such a design difficult to control - even with a wider wheelbase.

Commenting on the sheet metal: Cut the thickness by 30% and put a 1" flange (bent inwards for both) at the top & bottom. Saves weight, adds significant rigidity. Some of the flange can be lost or reduced by the omni wheels for clearance if necessary.

To keep the assembly from racking (parallelogram), consider very thin braces from outside corners, in an X pattern. Even 1/8" wire rope would be fine, since it is in tension.

Your hole pattern could be improved. The solid pieces should all meet at stress points, not tangentially to them. See the small round 'pin' in front, 1/4 way from the left side of the image.

Nice CAD work.

I am expecting this to be hard to control, and thats why I pulled the wheels toward the center. I figured it'd be spinning out of control untill you got the hang of it, so I should just design it for experienced drivers (hence the fast turn speed).

As for the metal, putting such a large flange on both pieces will leave a 1/2 inch gap between the side plates (only 2.5 inch spacing). I thought about just downgauging and flanging one piece (outside), but wanted to leave it simple for the first prototype CAD.

Supports: We have always been fine without any angle braces since the tubes go through a square hole in the inside plate. Would just a simple angle bracket suffice?

Lightening

Does it really matter since the piece will never be under tension/compression in the direction that the lightening will help?

As far as the CAD goes, I enjoy it, so I've gotten really good with basically teaching myself.

[apalrd]

Originally Posted by 548swimmer

I am expecting this to be hard to control, and thats why I pulled the wheels toward the center.

That makes it theoretically worse.
Because the diameter of the circle between the wheels is smaller, it takes less imbalance to cause it to stray to one direction.

We had a 6 wheel drive in 2008 that was flat, with 4 corner omnis and 2 center plaction wheels. It was too hard to keep it driving straight, for a second year driver, so we replaced the outer rear wheel with a kit wheel to add a little more friction sideways.

If it doesn't veer that much, you can probably fix it in software using a gyro. It has to be somewhat controllable for the software to work.

[sanddrag]

Nice CAD model, but physics says this design will not work too well. Also, it looks like your design is based around BaneBots gearboxes, which I would recommend against. You may consider placing those center wheels closer to the outside of the vehicle, and powering the omni wheels as well.

[Akash Rastogi]

Pull the wheels out to the edges. Even if you didn't realize the physics behind this, just think "if it was an advantage to pull the wheels closer together, more teams would be doing it by now." Common sense goes a long way. Also I hope you plan to drive those Omni wheels.

Other than that I can't say much since I'm not one to judge about lightening patterns and structural integrity of sheetmetal parts just yet.

[LLogan]

I would advise you to drive the four omnis on the corners. Your tractive force is based off of the sum of the forces of friction on all of your wheels. With the design you have right now, you are essentially "wasting" the normal force on the corner wheels. Whatever weight you will have on those sections of the robot will not contribute to your pushing force, giving you less than what you would generally want.

[548swimmer]

Originally Posted by apalrd

That makes it theoretically worse.
Because the diameter of the circle between the wheels is smaller, it takes less imbalance to cause it to stray to one direction.

You must not have understood what I meant. With this drivetrain design and current wheel placement, it will never be terribly easy to control. With this in mind, I figured it would not make it too much worse to pull the wheels in. We also would be able to use a gyro to help keep it straight (we used one for autonomous this year).

As for the banebots, we used them this year and never had any issues. In the past we have used toughboxes, and they kept breaking. I'm not sure exactly how, but I was told not to use them, and I'm fine with that.

I have no intention of powering the omni's because with this design, there is no good way to get power out to them, short of flipping where the wheels are, which would defeat the purpose of this drive.