Basically what the title says… what does it take to make a great 6 or 8 wheel tank drive?
Driver Practice.
Any robot takes driver practice to be successful… some more than others…
I was asking in reference to the the construction of such a drive, and not just any 6 or 8 wheel drive, one that really gets the job done.
Specifically what type of 6 or 8wd? Drop center? Omni + traction? direct driven?
Its a bit too vague of a question and not really one that has a real answer to it.
I’ve seen teams with kit drives that “get the job done.” As long as it doesn’t fall apart on the field you’re basically good.
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Sorry for the ambiguity…
I’ve seen teems with tank drives that are relatively slow and don’t turn well. With direct reference to a 6WD/8WD, I meant one without omni’s that still turns quickly, and that has a 2-speed transmission to be able to push well in low gear and move quickly in high gear.
Is drop center better? Would Omni’s in front and back be better? What gear ratios would work best? Is it worth it to use FP motors?
What is “the job” that needs to be done?
What is your definition of successful?
Not much. You want a dropped center, a good transmission (shifting usually so you can get 9fps safely, more or less depends on the game), and practice.
Dropped center is better than omnis because when you get pushed, you’ll “rock” onto that side and won’t be spun nearly as easily as an omni based drivetrain. I personally like 8wd because the rock has less of an effect on driving, but it’s probably not worth the added weight and complexity for most flat games.
The KOP Frame, KOP Transmissions. Read the instructions. No, I am not kidding. You can save a bit of weight by doing it all custom but since you already have an effective solution why not use it?
Build it week 1 and get people practicing. Work your kinks out early.
Fail Faster
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Focus and Execute
It’s mostly based on the game for the year.
There is also this thread too.
which thread?
A 6 WD with a dropped center does its job. We made a 6 WD with a dropped center using a standard toughbox, and that thing was crazy. It was quick, zippy, and the torque was crazy. As per a 8WD, I think that the were dominant this year because teams didn’t want to bottom out on top of the bump. But usually, you’ll see 6 WD.
As for gear ratios, read this post before you think of something crazy. I personally agree with this logic. One can always find a nice balance.
This method worked really well for my team this year. The kit base is highly underrated. It’s not perfect, but it’s a lot better than people seem to think. Having a working chassis with almost zero design time was huge for us. There was only one major thing I felt was missing, and that was chain tension. Maybe we were doing something wrong, but we never could seem to make the chains happy; they were always too loose or too tight. One of our freshmen got really good with the chain breaker, because he had to use it daily :rolleyes:. If only the kit chassis had an integrated tensioner system, I’d try to use it every year.
Edit: The kit chassis does have a tensioner system, but I feel it’s not implemented well. The mount bracket for the Toughboxes has slotted mounting holes, so you can pull it one way or another to tension chain. The problem is that if you do everything by the manual (standard 4WD), you’ll have two chain runs coming from the Toughbox output shaft in opposite directions, so when you tighten one, you’ll be loosening the other. You can get around that with some creative chain routing, but my point is that there isn’t a really good tensioning system already integrated.
If I was trying to design the ultimate skid-steer drive, I’d make it somewhat like 254/968/27 style (West-coast drive), like 221’s Universal Chassis. If my aim was anything less than “the ultimate skid-steer drive” (if I had limited resources, etc.), I’d go with the kit chassis, 6WD, dropped center (there’s already a lowered hole for this in the kit chassis’s side rails), and spend my design time creating a great tensioner system.
My $0.02
A few items to consider for a good 6 or 8 wheel tank drive (or any other chassis, for that matter):
- For better turning, keep the track width (side/side spacing) large relative to the wheelbase (front/back spacing). For a long aspect ratio chassis, this requires either lowering the center wheels or using omni wheels at one (6WD) or both (8WD) ends.
- For better stability, keep the wheelbase long relative to the track width.
- Ideally you want a good balance between turning ability and stability. A wide aspect with wheels at the corners, or a 6WD with dropped center wheels is about right. If you can fit more traction wheels between the other rigid (non-omni) wheels, that is good and can be done without lowering them.
- Keep the wheels as close to the frame perimeter as possible.
- You will spend a lot more time turning, lining up on a game piece, accelerating, and pushing than you will going fast. A “slow/strong” single-speed is usually better suited than a “fast” single-speed, and simpler (to build and drive) than a two-speed.
- Develop and test chassis designs during the fall/off-season. Try to develop a “system” (chassis structural modules, gearbox mounting, chain tensioners, axle mounts, maintenance & repair accomodation, etc.) that can be adapted to a wide variey of final configurations (wide/narrow, # drive wheels, wheel
size & type, ground clearance, etc.). After kickoff, it is relatively quick to finalize the design and get started on building the “bottom end” while you are still figuring out the “top end” (manipulator). We build our own chassis’s and are usually driving by the end of week 2.
Step 1: Figure out what you want it to do.
Step 2: Figure out what it takes to get it to do what you want.
Step 3: Figure out how to make what it takes.
Step 4: Adjust your strategy if you can’t make what it takes.
Most successful designs follow this process. One of the most successful strategies/designs ever was drive fast, drive straight, have amazing pushing ability, hold position.
So Tee? What would you like your 6x6 or 8x8 to do?
I would give you drastically different advice depending on the game. While our chassis from 2006-2008 look very similar, they have very specific attributes that were very specific to the games those years.
Its more than just hardware… it’s also in the software.
Put a gyro on your robot and you can make sure that it drives straight, even when you’ve got unequal drag on each side… or when you’re being bumped. A gyro can respond more quickly and accurately than a driver.
We’ve also found that using seperate control sticks for Forward/Reverse and Left/Right can help tracking. That way, when using “arcade” style driving (rather than tank style, where you control each side or the robot with a seperate stick) when you want to go straight forward you just lift your thumb off the “turning” stick, and you will always get a perfectly straight forward/reverse input to the robot. When you want to stay still and turn, you take your thumb off the forward/reverse, and all you get is turn. Sure… you can tune your deadbands and all that, but this is what we’ve found to work best.
Jason
Jason,
I’m looking for a little more detail on what you mean by this. I presume when you refer to “taking your thumb off” you’re using a gamepad for driver control, rather than the traditional kit joysticks.
Most teams use the two kit-of-part joysticks to control the robot in “tank drive” style where only front/back on the sticks controls anything. In this way, the left stick controls front / back on the left side of the robot, and the right stick controls front / back on the right side.
The other common alternative is to have “arcade drive” where one stick controls the robot via front/back and right/left stick movements.
What I understand you to be suggesting is to instead use the two joysticks so that one of the joysticks controls front/back of the whole robot (like front/back in “arcade style”) and the other joystick controls right/left turning (like right/left in “arcade style”) but using 2 joysticks to provide this input rather than just one input. Am I correctly understanding what you are suggesting? Presumably, also rather than actually using two separate joysticks, you simply use a gamepad to provide the above control?
Our team has explored a lot of different drive approaches, but I don’t think we’ve ever tried the above. Sounds like something our team should give a go during the off-season… Thanks for sharing!
–ken
I thought this was actually pretty common, driving with an arcade mode, but with separated joysticks. It’s something I’ve certainly wanted to try for awhile, but I’m stuck on teams that love their joysticks and tank drives. 
Maybe if my team enters a mentor match I’ll bring my gamepad…
Good point, Ken… yes, I was talking about using a PS2/Xbox 360/etc. style game pad. We’ve been using them for three years now and find that they offer more flexibility for the driver than the joysticks did.
The primary benefit is that the driver is able to move around freely while driving… an advantage when something is blocking his or her view, or… as in Overdrive, when reflections make it difficult to see parts of the floor. It is also useful for testing in the shop as it is easy to pass control from one driver to the next and to position the control board on surfaces of differing heights, or even on the floor, while allowing drivers of different heights to drive comfortably. It also makes it easier to design a compact control board, as the game pads don’t need to be fixed in place and can easily stow inside the control board (we build ours as a wooden box with a hinged lid) as opposed to being exposed on the outside of the board. Minor benefits, perhaps, and ones that could be duplicated with joysticks with some effort, but benefits nonetheless.
We’ve experimented with tank drive, and while it definitely offers some advantages when it comes to turning about one side of the robot or the other, our drivers have preferred “arcade” style with the turning and forward/reverse functions seperated. I wouldn’t say that it could be proven to be, objectively, the very best way for everyone to drive their robot, but it has worked well for us.
I guess the main point that I was making was that once a team gets a drive train that is mechanically capable of moving and turning that there are still optimizations to be had through sensors, software and driver interface in order to maximize the “successfulness” of the drivetrain, even for the relatively simple 6wd/8wd layout.
Jason
I am just going to add that while hardware is important, we had an instance at the DC regional this past year where our driver was complaining that he couldnt drive straight, but is able to overcome it. After the season, I read somewhere that the library for Jaguars has an output from -.9, 1.0] which caused the robot ot turn a little as it was directed to drive “straight.” We were using joysticks, no gyro, and the default WPILib for anyone who wanted to know.
I’d like to add that the frame cannot be warped**, regardless of “6WD”, “8WD”, “Rocking 8WD”, “4 traction + 2 omni”, etc. This is one of many key elements of any successful drive train. Our team defines “success” as a drive train that moves at a speed we’ve set forth in our strategies and with the [strategically] desired pushing power while never breaking down.
Oh yea, reliability. The drive train can be creative or simple, yet if the drive train doesn’t move then the design of the entire robot is for naught. Not only do teams need testing to find/fix failure points, they need a maintenance plan that quickly exposes any failures that occurred during previous match play. If you have a choice to fix a kicker or your drive train 30 minutes before a match – choose the drive train.
**Teams that do custom welded frames need to ensure the frame is flat and 4 wheels touch the ground when 4 wheels are supposed to touch the ground. A good aluminum welder or welding mentor is a MUST for this reason. If only 3 wheels are touching the ground when there are supposed to be 4, then the bot will skew one way or the other when put on carpet. Not only does this make a perfect autonomous nearly impossible, it makes driving the robot a bit of a hassle in some situations.