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-   -   6WD vs 8WD (http://www.chiefdelphi.com/forums/showthread.php?t=88272)

JamesCH95 06-01-2011 11:33

Re: 6WD vs 8WD
 
Quote:

Originally Posted by Lil' Lavery (Post 991929)
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 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 power 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.

Good point, and well worded.

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.

KrazyCarl92 06-01-2011 11:42

Re: 6WD vs 8WD
 
Quote:

Originally Posted by Lil' Lavery (Post 991929)
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

The equation is actually Ff <= 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.

Lil' Lavery 06-01-2011 11:43

Re: 6WD vs 8WD
 
Quote:

Originally Posted by JamesCH95 (Post 991932)
Good point, and well worded.

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.

Good points (especially the last one, I have to be more careful about my technical diction around here ;) ).

Lil' Lavery 06-01-2011 11:49

Re: 6WD vs 8WD
 
Quote:

Originally Posted by KrazyCarl92 (Post 991937)
The equation is actually Ff <= 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.

KrazyCarl92 06-01-2011 11:52

Re: 6WD vs 8WD
 
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.

Brandon Holley 06-01-2011 12:10

Re: 6WD vs 8WD
 
Quote:

Originally Posted by KrazyCarl92 (Post 991942)
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.

What Sean is saying though is that you cannot expect any robot to reach that speed if it mechanically is unable to (ie: geared for a lower speed).

-Brando

KrazyCarl92 06-01-2011 12:16

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.

apalrd 06-01-2011 12:43

Re: 6WD vs 8WD
 
Quote:

Originally Posted by Grim Tuesday (Post 991874)
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.

<image snipped>

We had a similar system.
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.

IKE 06-01-2011 14:55

Re: 6WD vs 8WD
 
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?

AdamHeard 06-01-2011 15:21

Re: 6WD vs 8WD
 
Quote:

Originally Posted by Lil' Lavery (Post 991939)
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.

No, his arguement is correct.

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.

Grim Tuesday 06-01-2011 16:07

Re: 6WD vs 8WD
 
Can someone tell me what a "West coast drive" is?

Alan Anderson 06-01-2011 16:35

Re: 6WD vs 8WD
 
Quote:

Originally Posted by Grim Tuesday (Post 992035)
Can someone tell me what a "West coast drive" is?

Try here if you can't get the Chief Delphi search feature to work for you.

JamesCH95 06-01-2011 16:42

Re: 6WD vs 8WD
 
Quote:

Originally Posted by Alan Anderson (Post 992050)
Try here if you can't get the Chief Delphi search feature to work for you.

:D

Chris is me 06-01-2011 17:27

Re: 6WD vs 8WD
 
Quote:

Originally Posted by AdamHeard (Post 992020)
No, his arguement is correct.

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.

How does being traction limited give you a higher top speed than your gearing would allow?

AdamHeard 06-01-2011 17:59

Re: 6WD vs 8WD
 
Quote:

Originally Posted by Chris is me (Post 992065)
How does being traction limited give you a higher top speed than your gearing would allow?

It doesn't. I'm unsure what your comment has to do with my post.


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