pic: Prototype Drivetrain
 

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

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Why the tiny, centered wheelbase?

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

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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

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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.

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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.

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...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.

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

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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 :D

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.

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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.

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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.

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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

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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.

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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.

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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.

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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.

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

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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.

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Physics is not a religion.

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Hahahah.

To help elaborate on what Chris, apalrd, and I were talking about, I urge you to read "Drive Train Basics" by Chris Hibner and the accompanying drivetrain calculator that goes along with it created by Mark Kramarczyk. Use your CAD to plug in your distances into the calculator.

This may prevent you from making somewhat of a very silly and obvious mistake.
Drive Train Basics: How to be sure your robot will turn
Turning Force Analysis

Please at least read through Chris Hibner's paper as it includes the physics that you need to look over. Pay close attention to changes in LsubTW

For some reason I have a feeling you're confusing the difference between the advantage of a "short wheel base" (wheel to wheel distance) and the distance between parallel wheels. Once you go through the pdf you'll see what I mean. :) Have fun.

PS- Non driven wheels are wasted weight and wasted force. Just fyi. I'm still not understanding the rationale you're giving but good luck! Nice CAD.

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No, though it does apply to robots. With the simple push of a button, a gyro sensor could be enabled(wrong word) allowing only forward and backward movement. Or you could go with arcade drive, and as long as the joystick stays 1 degree from the y-axis, you only allow straight movement. That number could be tweaked untill it's optimized.

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I read through the paper, and I discussed all if that today with our design leader. I understand what makes/prevents a robot from turning. No matter where these wheels are, the robot WILL turn. This design may not always be useful, but both he and I agree that it is worth investigating. If you approach the driving of this robot as a robot to be handled with a gentle touch instead of intense fast paced and borderline reckless driving that occurs in some games, you might see where I'm coming from.

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Just to clarify, you are engineering for maneuverability?

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At the cost of 25% of your tractive force, you gain a theoretical slight advantage in turning. This should raise a very large red-flag. I don't think that losing 25% of your pushing force has ever been a good thing. The turn radius for a dropped center 6WD isn't zero, but it can be pretty close. Driving the omni wheels will not cost you much maneuverability at all. The suspension will not magically "transfer" weight to the center. In fact, a suspension is useful when you want to keep all of your wheels on the ground. A suspension makes sense in a mecanum drive, where you want to make sure that the normal force is distributed as evenly as possible, otherwise the perpendicular vectors don't cancel each other out and you cannot drive straight. The springs in your suspension will actually cause a reactive force and increase the normal force on your omnis, which is the exact opposite of what you want to happen.

I apologize if I'm making the assumption that your omni wheels are on an active suspension. But... I would be even more puzzled if you were to use a floating suspension.

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Don't be so quick to assume everything in code will be worked out flawlessly.

When you design mechanically unstable systems, you better have absolute 100% reliable software, otherwise you're a sitting duck.

Putting the main drive wheels right next to each other isn't a smart idea unless you want to implement real-time, closed-loop PD velocity controllers on your drive train. Regardless of physics, putting the drive wheels right next to each other is a bad idea geometrically. Let's assume that since not all motors have identical RPMs (whether due to motor bias or manufacturing tolerances), that one side of your drive train normally operates 5% faster than the other one.

To minimize risk of unpredictably turning with slightly different wheel speeds, you should keep the wheels as far apart as possible. This way, the ICR (instantaneous center of rotation) will be moved much further away from the center of your robot as opposed to a smaller wheelbase. As it is, most existing 6WD drivetrains already turn way too fast at full power to be accurately controllable, with many teams already resorting to smoothing, ramping, or square/cubic response curves.

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I may not be a mechanical/physics person so I have no idea what's being discussed here, but I do know that relying on the programmers to fix something that's wrong with mechanics is never a good idea, especially if the issue can be avoided ahead of time. As Mr. Dutra said, you'd better have flawless programming to back up the design downfalls.

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The others were more polite about this, I am not. Wheels in the center like this are dumb. In January we build a DT like this, just with the wheels in the logical positions and all of them powered. Surprise! It has 0 turning radius as well! It also was almost imposible to have any fine control of the movement. After about 15 hours of testing I was still the only one who could drive the thing. Currently we are modifying our DT with a lower gear ration to gain more precision, and a shifter to maintain speed.

If you are set on this idea, build it during the off season and try it out, I am all for learning. But please, don't build this during those 6 tense weeks, you team will murder you.


Bluntly, Edoc'sil

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Just to add to this, it seems that a main reason behind this drivetrain is a zero turn radius. Take a look at some traditional 6 wheel skids in video, they all have zero turn radii as well, except without the loss of control.

08 was the last year we had basic drivetrain (bumps changed things this year and regolith did last year), watch teams like 968/254, 1114, 217, 1717, 330. These bots were incredibly manuverable and they all had a zero turn radius.

Edit: Check out this thread. http://www.chiefdelphi.com/forums/sh...threadid=85710, watch the video...I know its an 8 wheel, but the concept is the same. All of the wheels are on the outside and they have no problem with manueverability.

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This is actually incredibly helpful, thankyou four your bluntness. I was toying with the idea of putting them on the outside, but thought that the advantages gained in turning speed would make this worth it because of the initial difficulty of control. If you guys had a hard time even with the wheels in a normal spot, I shudder to think how bad this will actually be. In the past years we have always had all sticky wheels, and this year with our drop-omni's was the first where we had omni's as far as I know. Your personal experience with this layout helps a ton, and I'll revise it hopefully this week, though A.P. Chem is looming...

sgreco, With the exception of the 8 wheels, is it still 2 omni's on the outside for that drive?

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OK... This is really starting to annoy me...

Seriously, if you want a zero turn radius robot, you don't need to put 2 wheels in the mid and hope this will work...
you just need that all the wheels will get the same amount of torque and speed, and that all of them will be in the same distance from the center of the turning radius ( Even if it is zero turning radius there is still a center-point) . This will apply for 4WD.
For 6WD you need that the 4 outer wheels will be just like the 4WD logic i said above and that the 2 center wheels will be exactly in the center between the two other wheels at each side.

Another thing, when you design a drive-train take to consideration the specific game, for most teams this drive-train will be a disaster for LUNACY and BREAKAWAY since in those games the space in the center of the robot was important for the mission. you want that your drive-train will take the least space as it can on the robot, and weigh as low as possible so all the other resources can go to the other robot mechanisms. But don't forget that the drive system should be as much maneuverability as you need to the certain game, easy to control for the driver, and that will almost need none program correction as possible (unless you MUST).

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No, they're all 6-inch plaction. The point I was making is that you don't "lose" maneuverability by putting all of your wheels on the outside. The two center wheels are lowered 1/8 of an inch (I think it's an 1/8, Chris can correct me if I'm off). I was basically just pointing out how the traditional style of 6/8 wheel skids, all wheels in line with each other and middle wheel dropped, works really well and has no issues with maneuverability when built well.

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You may not understand how difficult it is to fix drivetrain problems in software. We programmers can fix many things. A squirly drive is not one that is easy to fix (depending on how bad it is). Take our Lunacy bot. 4 wheel swerve, wide, geared fairly fast. It was really hard to drive, because it always wanted to go one way or another. Even with the trailer, it was still hard to keep it going straight. We attempted to fix it in software (and Jim wrote the code, he's a Chrysler programmer) and it did not work. There was too much lag between input and motion with the slippery wheels, and it was too hard to correct. For you to fix your drive in software, you would need to do PD or at least P on both sides to get them to exactly the same speed, tune it very well, and use the gyro to correct for drift. I don't think it would be possible with the wheels where they are.

Move the centers to the outside. It will work better, as many others have said.

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apalrd, trust me i know programing can be annoying...
I was in the programing team last year (LUNACY) and also the driver, and we also had trouble with our drivesystem (we used 2 regular drive on the read and 2 swerve in front that can go about 50 degrees each side to total of 100 degrees) and we couldn't fix all the problems in software so eventually i had to fix it with my driving...

For sure tank drive is the easiest to program and control so if you want to make the life of the program team and driver easier you should aspire to normal tank drive... Though if you aspire for maneuverability you should go for a swerve drive or mecanum (or even nonadrive like 148 and 217 had this year), there are some more configurations that have high maneuverability but they are less common in FRC (like KIWI drive)...

If you cant build a certain drivetrain that you haven't ever used yet before the season start, then don't try to do it within the 6 weeks.... our team tried to do swerve drive this year and we tried to build and program it within the six weeks and it was a total disaster... we eventually went to mecanum...
So my suggestion is that if you can build and test it before January go ahead, but as i stated before, there are many drive systems out there that have lots of maneuverability (from what i understand that is what you aspire for) so try using them instead of making a new system that you not sure that will work.

Generally i suggest that you will build it so all of us will learn a lesson. Many people including me don't think that it will gain what you aspire for but go ahead and prove that we are wrong... If not then what is FIRST about?

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Yeah, I hope to come up with 5 or 6 different drivetrains between now and September. Once we're back in school, we'll most likely build the most promising one or two.

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yeah, I'm the programmer this year.

We didn't have to correct for drive while driving, or autonomous. Since we have a 6wd articulated center, we just removed pressure on the center wheel so it touched under its own weight and that essentially made it a 6 wheel flat, so it didn't really want to turn. We had P control on speed for both sides, so it didn't drift at all (it never went faster than 4 ft/sec in auto, but only 1.2 when doing the kick portion).

I worked on our four-wheel swerve last year. 4 drive and 4 swerve motors, all independent, plus the annoyances of regolith. I only wrote the steering code.

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I have a feeling this idea will be shot down, but go ahead and try your design anyway. All this talk is very theoretical, and stuff sometimes works differentlly in theory that it does in reality. I realize that this is physics, and theoretical physics is rarely wrong, but there is always that chance that it might work better than expected with the right programming ("right" is not the same as "perfect")

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While trying this idea wont hurt anything, you really should take the advice of everyone on here and not waste your time building a sub-par drivetrain. Sure it will move but not with any form of control. The suspension is added complexity and points of failure for no added benefit as well.