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6WD vs 8WD
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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. |
Re: 6WD vs 8WD
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. |
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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. |
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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 |
Re: 6WD vs 8WD
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.
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First off, There is alot of past discussion on this and similar topics. Even So..
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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. Quote:
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) Quote:
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. Good Luck, Bryan |
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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.
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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?
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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. |
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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... |
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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. Sorry if I had misspellings :) |
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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. |
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I've seem teams have beastly drive trains, 1771, 1114, etc. and they use the non WCD style of the drive train. - Sunny |
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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"... -Brando |
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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 Ff=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. |
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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. |
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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. |
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Another point is that radial wheel [drivetrain] acceleration and linear robot acceleration are two different matters, though often linked together in FRC scenarios. |
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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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-Brando |
Re: 6WD vs 8WD
Which was the point I was making, the gearing, not the friction is what limits the force of acceleration and thus the acceleration of the robot. We can come to this conclusion because the acceleration of 25.74 ft/s2 is so unreasonable that it is obviously not the limiting factor. So if it were geared as such, a lighter robot could in theory accelerate faster than other heavier robots.
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The ride height was controlled by the claw, and is automated (more drop when turning, less when not). The driver must request a bump-cross, but when not bump-crossing, it handles ride height on its own. From Danial Ernst's pictures from the State Championship: ![]() The rear chassis (the part we attach the bot-bottoms to) is mobile, pivoting around the rear axle (omni wheels), and containing the drive motors (4 CIM + AM shifters), and most of the control system. The gearbox is chained directly to the middle and rear wheels, and the rear wheels are chained to the front wheels via two jack shafts per side (over the top above where the chassis floats). In software, the normal claw movement (including the bump-crossing and flat centered positions) is handled with the claw axis pot, and center dropping is handled by a string pot in the chassis which measures drop. |
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So, I was very inspired by some of the physics guys up above, and have a challenge:
Assume 16m dash (approx. 54 feet- robot length), and COF of 1.0. Also assume a 4 cim power-train at 300 Watts/CIM. What weight is your bot so that you are just barely traction limited the moment you strike the other wall? Is this less than 4 cims & a battery? |
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He implied an acceleration of 25.74 ft/s^2 over a time interval of one second would result in a velocity of 25.74 ft/s. He then commented based on experience/anecdotal evidence that teams are not doing this, and therefore are not traction limited. |
Re: 6WD vs 8WD
Can someone tell me what a "West coast drive" is?
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Gearing was never mentioned, now was it? I'm sure it's possible for a 90-lb robot to be geared to move at 50 fps. How long it takes to get up there, though, might take longer than the length of the field.
Chris, the implication was that traction limiting happens at that acceleration for a 90-lb robot, not that the robot was geared high. It's extremely rare to see a robot geared that high, or traveling that fast, but it could happen. Some of the lapbots from 2008 had pretty fast speeds and good drag-racing potential, IIRC, though the field in 2 seconds is pretty fast for an FRC robot. Being traction limited may give a higher maximum top speed than gearing allows. However, top speed is always limited by gearing. |
Re: 6WD vs 8WD
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PT = 4PCIM P = 1200 watts (~80 - 95 % efficient depending on number of gears, transmission, and other variables) Ff = mgµ = m*9.8*1 = 9.8m Work = Force * Distance = 9.8m * 16 = 156.8*m time = Work / Power = 156.8*m / (between 960 and 1140) watts = between .138*m and .163*m seconds so to solve for mass (in kg) it would work out to between 6.135 * t to 7.246 * t depending on the efficiency. Of course, this all assumes that the robot would be constantly accelerating at the maximum possible value (no slipping/static friction) and that the robot would be able to keep getting faster and faster, but I'm pretty sure it would be impossible to gear 4 cims without a transmission such that they provided exactly maximum static frictional force and still could reach rotational speeds that would allow it to continue accelerating...would probably need really really small wheels, but not too sure about that one. |
Re: 6WD vs 8WD
i personally like 6 wheel it is lighter and well tested. as a matter of fact we built a 6wd drivetrain this off season and it preforms excellent, very maneuverable and fast. video form testing this afternoon: http://www.youtube.com/watch?v=KwOr-Mpl8DA
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Now that I think about it, the size of the wheel really wouldn't matter...
Although I would usually love to sit down and work out the rest if this problem, at this point I feel like that would be selfish of me because I'm sure my team would rather have me well rested for kickoff which outweighs my own personal satisfaction of solving the problem |
Re: 6WD vs 8WD
The final speed, as well as the acceleration are functions of the angular velocity and acceleration of the drive shaft (respectively), as well as the wheel radius.
Think about a wheel with radius = r and a driveshaft with some torque T. We then sum the torques about the center of the wheel and get Fa*r < Fsfmax*r (theta is ninety so it goes away sin(90)=1). Cancel the r, plug in uFn for Fsfmax, and you have your max Fa to not slip. Use T = F * rsin(theta) and you get the max torque from the driveshaft to be traction limited. From there it's just some basic ratios back to the torque output of the motor and that ratio is your desired gear ratio. Now, this assumes that you have a perfectly efficient gearing system, and that your wheel has no mass and therefore no moment of inertia to slow the acceleration, but it gets you close. For speed based, do the same basic thing only with rpm instead of torque. |
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@Adam: I misinterpreted that post as saying that in order to be traction limited you needed to have a free speed of 27 feet per second or something. My bad.
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1) How long does it take to go 16m when you are traction-limited the whole way? Ffriction = 1.0 (CoF) * Wrobot (because Ff = mu * Fnormal) a = Ffriction / Mrobot (because F=m*a, so a=F/m) a = G (because Wrobot/Mrobot = G, gravity) D = .5 * a * t^2 t = sqrt(16 m * 2 / G ) t = 1.81 seconds 2) How fast are you going just before you hit the wall? v = a * t v = G * 1.8 seconds v = 17.7 m/s 3) At this ultimate speed, we are barely traction limited, so: P = F * speed (1 Watt = 1 N * m/s) F = Wrobot speed = v = 17.7 m/s 1200W = Wrobot * 17.7 m/s Wrobot = 67.8 N = 15.2 lbs So yes, less than 4 CIMs and one battery. |
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"You must spread some reputation around before giving it to Jared again".
Now if you use the same formulas, for the 2009 game with an assumption of CoF around 0.1, you will get a very interesting result (as demonstrated in the 2009 Kick-off video). |
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I think sometimes we rely to much on the math. An engineer is trained that way and it's a good thing for an engineer to do. But a designer has to also rely on their gut. They need to use their experience and instincts more than an engineer does.
What I'm saying is don't be afraid to let that right brain out to play a bit when your designing you drive. |
Re: 6WD vs 8WD
Bumping this thread to discuss the pros and cons of 6 and 8 wheel drives in Logomotion.
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i think that 6wd is a more agile drivtrain, it turns a bit easier, and is also a bit faster accelerating, due to less stuff needing to be accelerated. however an 8wd is generally beefier, more precise. i think they tend to be a bit slower turning, but also harder to turn. i noticed this year that we had little difficulty spinning some top teams with 6 wheel tank. however any 8wd teams we attempted to defend were much more difficult to knock of course, notably 610 and 691. if it were a game like overdrive, or this year, where speed kills, 6wd would probably perform slightly better. if it were a game like 2010 or aim high, i might choose 8wd due to the way it is less likely to be pushed.
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Look carefully at how an 8wd with the center two wheels dropped is built. Specifically their wheel base. Then look at 6wd. Then apply that thought to this white paper. Does the math support your hypothesis? If not what other factors could be coming into play? |
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well, as far as i understand, the goal of having the outer wheels raised is to make them act more like omni wheels. for the purpose of the argument imagine all we care about is drive systems with omni wheels on the corners. in a 6 wheel configuration, 1/3 of the weight is on the traction wheels, and there are 2 of them, in an 8 wheel configuration 1/2 of the weight is on the traction wheels and there are 4 of them. so if the goal of having the outer wheels raised is to make them behave more like omni wheels, in that they can scrub sideways with less resistance, then logic supports that an 8 wheel drive-train will be harder to turn, because there are 4 wheels, and they scrub sideways a little bit, and because there is more of the robots weight on those wheels at any given time.
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As you raise the wheel (or increase the center wheel drop) you decrease the load on those wheels. Yes, it decreases the sideways slipping force on the wheel, but it also decreases the tractive force in the same proportion. The first point is that it never really behaves like a omni wheel, just as a wheel with less traction. The second point is that the load drops off across the distance of your wheel dropped (teams range from 1/16-1/4") which makes it hard to tune. I worked with an adjustable drop 6WD for a few years and we discovered that 2-3mm drop was what worked for us as optimum. Outside that sweet spot we began to get significantly different driving performance. We also discovered that a 1/2 day of matches was enough wear on gum rubber treads to get us outside this range. We eventually decided that it was not worth the effort and decided to let the wheel off the ground and allow the robot to rock between the two 4 wheel systems. It was much simpler in the long run and allowed us to apply our resources elsewhere. I know the 2011 kitbot configuration has enough lift to accomplish this. You may want to spot check with teams to see if all of their wheels are actually on the ground or just really close. How might having one set of corners off the ground affect the logic you used above? |
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For Logomotion, we had an 8WD chassis with 6" wheels. The four at the corners were "kit" wheels. The four in the center were closely spaced pneumatics with adjustable drop. (I noticed in St. Louis that 330 used the same arrangement). We adjusted the tire pressure and center wheel drop so that the corners were about 1/16" off the floor on a hard surface, and had light contact on the carpet.
We were quite happy with the arrangement. It turned very well, since the pneumatic tires had better traction and higher loading than the corner wheels. It didn't rock or bounce during accelearation or turning since the corner tires were hard. During acceleration or in a pushing situation, all four pneumatics and one set of corner tires were in contact with the carpet. It was very difficult for others to push or turn us. It was a good balance between the agility of a 6WD with center drop or omnis, and the pushing power of an 8WD with pneumatic tires. |
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eight
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From my experience, the 8 gives you more stability at higher speeds and better predictability of how the robot will handle. We also decreased the diameter of the outside wheels instead of offseting the axle height.
mike d |
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no change of ratio, just let it ride.
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Personally, I like 8WD, although my opinion might be a bit biased because I haven't driven a 6WD robot anywhere near as much as I've driven an 8 wheeled one.
This year my team used eight 4" diameter wheels geared 7:1 and I loved it. Here is a video of us playing with the chassis a week into build season: http://www.youtube.com/watch?v=inI40uMKuEE If I remember correctly, the center two wheels are dropped 1/16". We later added another two motors to the drive train which gave us a nice balance of speed and power. |
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Key Features are: -Cantilever Wheels -Direct Drive -Inline system -CAMS -Sliding Bearing Blocks -Small Wheels (We goofed, big wheels are ugly :P) There are a few other tidbits, but that should sum it up. -RC |
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-2 speed -Custom gearboxes -Custom wheels |
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