pic: Prototype Drivetrain



This is a new drivetrain I came up with a few nights ago. I’d love any feed back you can give. Here are the specs:
Weight~30. lbs
Motors: 4 cims
Tranny: 2 banebot p80’s (not sure about gear ratio, feedback desired)
Suspension: rear omni only to keep traction during pushing matches
Driven wheels: 2 custom 6 inch with the black tread (what’s it called)
Sheet metal: 1/8 aluminum
tube braces: 1in x 1in 2with 1/8 wall, pressfit threaded inserts

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

Why the tiny, centered wheelbase?

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

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.

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.

…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.

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

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 :smiley:

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.

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.

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.

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. :o

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.

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.

With this in mind, what are you essentially gaining? Just trying to find the reasoning for this since the wider stance of the wheels would be better for turning and control. What is the purpose you are trying to get out of this?

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Though the force is “wasted” the majority of the weight will be placed on the drive wheels due to where the CG is. Our current build leader, whom I have discussed this design with, guesstimates that roughly 75% of the weight will be put on the center wheels. Also, I believe that in previous posts I addressed the design goal of maneuverability. The omni’s are there as casters, meant to prevent the robot from scratching the floors. There is also the suspension, that will transfer weight during a pushing match onto the center wheel.

You gain the ability to have a zero turn radius, and turn really quickly. Anticipating lack of control even with a “normal” wheel configuration, I opted for a tighter center system. Programming can keep us straight. I have faith in them :D. Additionally, we can just turn down the sensitivity of the joysticks until they reach 50% of their total travel, allowing finesse, while still maintaining your ability to react quickly.

So you saw there might be a problem, and decided to make it worse… why?

Because it CAN be done. This is just a concept. Teams do just fine with mecanums or all omni’s. Neither of these designs provide much pushing power. I do not want this robot to be driven just to push stuff around. My goal is to inspire those below me who saw our robot from this year push someone accross a gym floor while they were standing up, or tow 3 people at once. I want this robot to be driven with a unique drive philosophy, and the best way to do that is make it a unique robot.

If this isn’t used, but gives one of the design team a new idea to help accomplish whatever we will be challenged with, it will be a success in my mind.