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Unread 06-01-2011, 18:43
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Re: 6WD vs 8WD

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.
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Unread 06-01-2011, 22:17
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Re: 6WD vs 8WD

Quote:
Originally Posted by IKE View Post
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?
First of all THANK YOU for putting it in meters to spare me the conversions necessary to use watts and meters. However, there would be a missing variable in the necessary calculations if I am not mistaken, because we would need to know a time that it would take to travel said 16m. But I can solve for this with time as a variable anyway.

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.
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  #33   Spotlight this post!  
Unread 06-01-2011, 22:29
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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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Unread 06-01-2011, 23:01
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Re: 6WD vs 8WD

Quote:
Originally Posted by KrazyCarl92 View Post
First of all THANK YOU for putting it in meters to spare me the conversions necessary to use watts and meters. However, there would be a missing variable in the necessary calculations if I am not mistaken, because we would need to know a time that it would take to travel said 16m. But I can solve for this with time as a variable anyway.

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.
Assuming constant acceleration at said traction limit, D=1/2*a*t^2.
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Unread 06-01-2011, 23:22
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Re: 6WD vs 8WD

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
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  #36   Spotlight this post!  
Unread 06-01-2011, 23:44
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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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Unread 07-01-2011, 00:58
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Re: 6WD vs 8WD

@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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  #38   Spotlight this post!  
Unread 07-01-2011, 08:47
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Re: 6WD vs 8WD

Quote:
Originally Posted by IKE View Post
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?

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.
  #39   Spotlight this post!  
Unread 07-01-2011, 09:43
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Re: 6WD vs 8WD

"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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Unread 07-01-2011, 11:03
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Re: 6WD vs 8WD

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.
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Unread 09-01-2011, 13:58
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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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Unread 19-05-2011, 23:59
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Re: 6WD vs 8WD

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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Unread 20-05-2011, 03:17
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Re: 6WD vs 8WD

Quote:
Originally Posted by Hawiian Cadder View Post
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.

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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Unread 20-05-2011, 16:14
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Re: 6WD vs 8WD

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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Unread 20-05-2011, 16:42
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Re: 6WD vs 8WD

Quote:
Originally Posted by Hawiian Cadder View Post
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.
In six- and eight-wheel drive systems with lowered wheels, we examine the behavior of each as two or three four-wheel drive systems, respectively. The raised wheel isn't treated as an omniwheel, it's treated as if it doesn't exist.
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