Team 842 has been thinking of future drive trains recently and here are some of our ideas.
6WD
One design is a 6 wheel drive consisting of 2 super shifters in the center of our robot belted up to omnis in the front and back and a center traction wheel. We are also considering replacing the omnis with the same traction wheels used in the center. What are the advantages and disadvantages of doing this?
8WD
Our 8 wheel drive train idea is entirely based off team 67’s drive train. From what I have seen this past year, this drive train seemed to dominate all others. What makes this such a good drive train?
A few questions we have are what are the pros and cons of both of these drive trains and which one would be faster and which one would have more traction. I mean this on flat carpet by the way. I also have one other question, why is it that some wheels get lower? Is it only because there was a bump in last years game? One more question, what are the advantages of using the different size wheels in the 8 wheel drive train idea. If you have any ideas on how to improve these drive trains it would be greatly appreciated.
Speed and traction: Depends mainly on surfaces and gearing. A 6WD can have the same speed as an 8WD, or faster or slower, based solely on gearing.
Why do some wheels get dropped lower? If you’ve ever tried a 4WD non-omni, long configuration on carpet, you may have noticed that they bounce whenever they turn. This is due to the long wheelbase and relatively high traction. 6WD and 8WD have the same problem unless the center wheels are dropped, effectively shortening the wheelbase.
6WD with omnis on the corners vs 6WD with traction on the corners: Use caution. Personally, I’d go with high traction in the center and lower-traction (possibly omni) on the corners. If you’re doing omnis, you can do a flat drive instead of a drop-center drive. If you’re using traction, you’ll want the drop.
8WD: Anything I’ve said about 6WD applies to the 8WD as well, except that the 8WD can be a little more forgiving in handling and dealing with obstacles and a little less forgiving when building, depending on exact type.
The dropped center is not just for the terrain. One of the most-copied drivetrains is the West Coast Drive, a 6WD drop-center cantilevered live-axle pioneered by teams 254 and 60 in 2004. As explained above, the drop is necessary for smooth turning.
Six wheel drive, six traction wheels, the center wheel dropped 1/8" is in my opinion the best “all around” drivetrain in FRC. Unlike an omni-based six wheel drive, it’s relatively hard to spin, but can still spin itself.
8 wheel drive uses the same principles as six wheel drive, just lowering two wheels instead of one. It’s really about the same, albeit heavier.
I do wish to caution that 67’s drive was designed for the very specific requirements of climbing the bumps and that many features in your design would be unnecessary without them.
I believe that 67s drive train appeared to dominate was due to a few things.
-Ability to go over the bump
-Fit under the tunnel
-Had great manipulators accompanying it
-Great drivers and coaches
-I think i heard at Kettering that their entire robot only weighted 90lbs allowing them to accelerate faster then the 120lb bots
I don’t know about other teams, but we chose to do a 8wd instead of our usual 6wd because, through our prototyping tests, we found that the 8wd traversed the bump much more smoothly. If the field was completely flat, we would have used the 6wd over the 8wd.
First off, There is alot of past discussion on this and similar topics. Even So…
6WD: Probably the most commonly used drivetrain (excluding some years with terrain.) This drivetrain is generally liked because it strikes a good balence between everything you want in a drivetrain and is easy to build.
Omnis on the front and back lead to really nice turning. There also is no rock in the chassis because the middle wheel does not need to be lowered. However, most people use all traction wheels. When all the wheels have traction, however, the middle wheels have to be dropped about 1/8". This shortens your wheels base (only four wheels are touching at a time) and allows turning while still being able to hold your own in a pushing match. If well built, a all traction wheel drive can be made to turn just as well or better than its omni counterpart. However, be warned if you’re using the KoP chassis or another semi-flexable chassis you need to watch out for the opposite outer wheels bending down and scrubbing the ground. This is more of a problem when going fast (like Overdrive 2008) but can still make turning a bit of a chore at lower speeds too if your chassis is to flexable.
8wd: This functions similarly to the all traction 6wd above with the 4 inner wheels lowered. It has a shorter wheel base then the above (1/3 the wheel base of a 4wd as opposed to 1/2 the wheel base with the 6wd) and a central axis of turning. It’s much more stable because it doesn’t rock back and forth like a 6wd. It is also much better at climbing (making it the choice of many top notch teams in 2010). However, the advantages come at a price. It is both heaver and more complex.
As to 67 having a dominate drivetrain. I wouldn’t say that. 67 has built a completly different chassis for every year they have been with FIRST; their drivetrain was good, but not the best. The reason they look dominate is… well, they are! They have AMAZING drivers! This is the result lots and lots of practice and drills with their practice robot. Team 67 would be awesome even if they were driving a tin can!
If you need an example of a really well thought out drivetrain that has been boiled down to (imo) near perfection, look up team 254. They have a set chassis design they make better every year. (and they’ve been at it for a long time)
Answering the questions above:
Outer Wheels are raised to prevent the outer wheels from rubbing on the carpet making turning difficult for the robot. Speed is relative, as are torque and acceleration. No wheel base is inhearently faster than any other; it’s motor dependent. Different sizes of wheels have some various pros and cons. Larger wheels go faster than smaller wheels with the same gear ratios. However, smaller wheels are theoretically more efficent. Larger wheels don’t where out as fast as smaller wheels. An 8" wheel wears half as fast as a 4" wheel. However, smaller wheels are lighter in weight. There is endless tradeoffs, one is not a naturally better choice than the other. Again, go look at 254’s drivetrain.
I dont know if anyone saw 610’s 2010 drive. It was a 6 wheel, but the middle wheels were mobile (I dont know the details), so that they could either move the wheel up or down to traverse bumps. It was amazing.
Related question: Other things being the same, does 8wd tUrn easier than 6wd? I would assume so because when movng the wheelbase is decreased to 1/3 of total length, as opposed to 1/2 for a 6wd. Is this correct?
All other things equal, it both turns more easily and is turned more easily. The effect IMO is slight.
However, 6wd “mid rock” when resting between its outer wheels (only on the middle 2) has no resistance to turning. I have no idea if this phenomenon actually happens during a 6wd turn or not.
It does if the weight is distributed equally between the front and the back. We got pretty close to this on our 2010 'bot, which is pretty maneuverable. It’s not a problem in a pushing match, because the pushing robot will tend to throw you off the delicate balance and back onto 4 wheels where you aren’t turned as easily.
From that logic, I would say that drop center 8wd is even harder to be turned (by another robot) because it’s highly unlikely (pretty much impossible) that you would stay on two wheels. On the other hand, it might be harder to turn yourself for the same reason. I think…
from my past experience I can say that if you want your robot to have a good maneuverability the best way to do it is with “west coast drive”.
If you do it with 8WD it would be better than with 6WD.
This phenomenon does in fact happen with some machines.
Our 2008 robot would often get spun around if we were hit while turning at a high speed. It was 6WD with approximately 15" effective wheel base (distance between the wheels that were driven on, either middle to front, or middle to center.)
One of the quirks I’ve noticed about 6WD’s is that the driving wheels can change depending on the game and manipulator placement. Again, our 2008 robot would go from driving on the rear 4 wheels to driving on the front 4 wheels once we picked up a track ball. This wasn’t really an issue because it gave us a point of rotation that was more around the center point of our robot and trackball, but I would imagine that this could get annoying with another game piece depending on the manipulator.
8WD’s rotate a bit differently than 6WD’s. The point of rotation is almost always in the center of the robot, and they often rotate more easily than a 6WD depending on what the center wheel base is. Personally I prefer driving 6WD over 8WD because I find them to be more stable.
Drivetrain performance always comes down to execution (as it does with most other items on the robot). To say one drive train is “best”, is unwise because every team has their own set of requirements to work within. Some teams have the ability to build beautiful sheetmetal sculptures, while others use wood or kit frames because it works best within their team.
The key to a great drivetrain is to design the details in, and learn from yours and others past mistakes. From there, whatever style of drivetrain you choose will work for you, but keep in mind JVN’s addage “Know Thyself”…
Before we get carried away, let’s examine this statement.
Let’s start with the simple equation F=ma. Based on this, when you reduce your weight (and consequently mass) and keep your force the same, your acceleration should increase (which agrees with the bolded statement). However, when examined further, we find that the force does not always remain constant in the FRC world when your mass changes.
The force we’re speaking about in this application is the force exerted by the drive wheels. This is limited by friction between the wheel surface and the playing surface. The frictional force is calculated as F[sub]f[/sub]=μN, where μ is the coefficient of friction and N is the normal force. Subbing this back into the first equation, we now have:
μN=ma
In most situations, the normal force is going to be a function of the mass, usually just N=mg when on a flat surface. Putting this back into the previous equation we have:
μmg=ma
Which simplifies to:
μg=a
In other words, in friction limited drivetrains (ie drive systems that have enough torque to “spin out” their wheels), acceleration is not governed by the mass of the robot, as the mass plays into both sides of the equation.
However, consider that the driving force (i.e. output torque of the motors) decreases with speed. At some speed the motors will lack the torque required to spin the wheels, barring extreme cases. At that point, the situation will become the F=ma situation we all know and love and the lighter robot will be able to accelerate faster.
Another interesting tidbit is that the wheels’ coefficient of friction may not be constant, but rather vary with contact pressure. Assuming that the coefficient of friction is constant the heavy and light robots will accelerate at the same rate in a friction-limited case. However, if the tread/playing surface is sensitive to contact pressure then the lighter robot will have the advantage, all else being equal.
Also note that a drive-train does not require power to break traction, but rather requires torque, specifically torque on the drive wheels.
The equation is actually F[sub]f[/sub] <= μN, which brings us back to the question of traction limitted drive trains. Because 67 used traction wheels that would be driven and in contact with the ground at all times, I highly doubt that they would be fraction limited. Let’s assume that the coefficient of friction of their wheels is .8 (conservative estimate), then their force of friction would be .8 x 90 <= 72 lbs. That means that they would have up to 72 pounds of force moving them forward. This lead to the acceleration of a 90lbs robot up to:
mass = 40.82 kg
force = 320.27 N
acelleration = 320.27N / 40.82 kg = 7.846 m/s2 = 25.74 ft/s2
Given that no robots can get up to 25.74 ft/s in a single second (or at all for that matter), the acceleration is clearly limited by gearing/motors, not the traction of the wheels. So in this case 67 would in fact accelerate about at about 4/3 the rate of the 120 robots that they compete against.
You raise valid points about what is limitting acceleration (and, as you and JamesCH95 alluded to, it’s often elements of the motor/drivetrain itself), but you have a pretty significant flaw in your argument. 25.74 ft/s2 isn’t equal to 25.74 ft/s. Just because you’re accelerating at rate x, doesn’t mean your maximum speed is x.
Another point is that radial wheel [drivetrain] acceleration and linear robot acceleration are two different matters, though often linked together in FRC scenarios.
Sorry I used the equation V = at without mentioning it, using 1 second as t for reference because I figured that we would all agree that no robots would reach a velocity of 25.74 ft/s at one second at that acceleration, indicating that it is other factors, not the friction that limits the acceleration of the robot.
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