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-   -   Contact Area and its Relation to Friction? (http://www.chiefdelphi.com/forums/showthread.php?t=59372)

JesseK 03-11-2007 22:19

Re: Contact Area and its Relation to Friction?
 
Quote:

When turned on, the base with 2 wheels per axle could overcome and outpush the other base every time. We did this 12 times, each time changing to a new, fresh battery.
When you mean "overcome and outpush", what exactly happened? The single-wide drive train's wheels started spinning? Or both robots maintained traction and one won out over the other?

CraigHickman 03-11-2007 22:22

Re: Contact Area and its Relation to Friction?
 
Quote:

Originally Posted by sumadin (Post 649472)
Are you sure it wasn't just different gear ratios? If both are driven by the same motors, the tracks having a lower gear-ratio in their gearbox would make a big difference.

I watched outback tracks (which used to run on an andymark shifter) push other bots running on andymark shifters. Not sure of the details.


Quote:

Originally Posted by JesseK (Post 649482)
When you mean "overcome and outpush", what exactly happened? The single-wide drive train's wheels started spinning? Or both robots maintained traction and one won out over the other?

There was a second or so of nothing, then the single wheel slipped a tiny bit, and the two wheel pushed it forward steadily until I turned the drive switch off.

=Martin=Taylor= 04-11-2007 01:04

Re: Contact Area and its Relation to Friction?
 
Meet team 1516:



Builders of one of the most enigmatic drivetrains in FIRST.

In the last few years this team has won:

-Highest Rookie Seed 2005
-Cal Games Champion 2006
-5th Seed? Silicon Valley Champion 2007
-8th Seed Archimedes division 2007

Most of these years they have not scored any points, and have just played defense. Most people will tell you that a kit-bot running four skyway wheels off a couple banebots cannot play defense. But how can those people explain team 1516? They certainly didn't win all those competitions scoring points.

So here’s my point: the age old question of "who pushed who" cannot be solved by merely looking at traction, gear reductions, or motors. Its a combination of infinitely many things.

Cory 04-11-2007 03:12

Re: Contact Area and its Relation to Friction?
 
Smart defense doesn't even require your robot to push another.

eugenebrooks 04-11-2007 19:09

Re: Contact Area and its Relation to Friction?
 
Friction, as taught in introductory physics courses, is a macroscopic empirical model. The model does surprisingly well, much better than an empirical model should do, but one should be careful in how far one takes it. One should not assume that the dogma of the empirical model is true for every possible situation.

Your approach of actually measuring the effect of doubling up on the wheel surface area for your given situation is the right one to take. Don't be surprised when the data shows that the empirical model is a little out of step with reality.

There will be those that say that there is a lot more going on than the simple friction when a wedgetop, or roughtop, tread is forced to slide across an industrial carpet, and this is true. There is also something much more complicated than the simple macroscopic empirical friction model going on when two "hard surfaces" are sliding across each other, when you actually look at the microscopic details of what is happening at the atomic level, which you must do to understand the physical process in a predictive manner.

Eugene



Quote:

Originally Posted by 114ManualLabor (Post 649451)
This is an interesting thread.

Short disclaimer: I have yet to take physics, so anything I write is a result of observation.

Anywhoo. I think I understand what people here are talking about, yet I'm still confused. If everyone says that the contact area doesn't matter, then can someone explain how this makes sense: I built two robots. Both were with the kitbot chassis. Both were driven off of Banebots transmissions. Both had two driven wheels, and two casters. One bot had two andymark kit wheels per axle for driving, and the other had only one per axle. I added extra weight on each in order to make the weight exactly equal. I wired both motors to a single battery and switch. Then I put the two drive bases head to head, set so they would drive directly into each other, on an area of FIRST carpet. When turned on, the base with 2 wheels per axle could overcome and outpush the other base every time. We did this 12 times, each time changing to a new, fresh battery.

Anyone care to help me out here? I guess that physics and math and stuff say this shouldn't work, but it did. So I'm confused.


JesseK 07-11-2007 09:44

Re: Contact Area and its Relation to Friction?
 
Quote:

Both had two driven wheels, and two casters. One bot had two andymark kit wheels per axle for driving, and the other had only one per axle. I added extra weight on each in order to make the weight exactly equal. I wired both motors to a single battery and switch. Then I put the two drive bases head to head, set so they would drive directly into each other, on an area of FIRST carpet. When turned on, the base with 2 wheels per axle could overcome and outpush the other base every time. We did this 12 times, each time changing to a new, fresh battery.
After a bit more thinking, I do not have the answer to why this particular scenario happened. However, I do have a solution for how to avoid it -- move to 4 driven wheels on your drive train.

Since friction for a given wheel is dependant upon the force of gravity exerted by the mass of your robot, a 2-driven-wheel system needs most, if not all, of its weight centered directly over the drive axle for maximum traction.

Changing to a 4-driven-wheel system spreads out where in your system the center of mass can be and still practically contribute to all of the wheels' traction. Come to think of it, I think someone previously in this thread mentioned it, and most of the drive train guides I've come across have the underlying assumption Center of Mass rests somewhere in between 4 driven wheels.

AdamC 07-11-2007 10:15

Re: Contact Area and its Relation to Friction?
 
I'd like to pose a different question related to contact area v. friction. What benefit is given by adding additional wheels to your drive train? In 2005, we ran a 16 wheel drive train. In 2006, we went to 8x8, and in 2007 did 6x6. However, according to the coefficient of friction*normal force, we should maintain the same amount of grip regardless of presenting additional surface area by adding extra wheels. In all our robots, our CG is usually towards the rear. So, once again, I present the question. Is it worth making massive drive trains (besides the fact that you add many more fail-points, reducing chances of a crippling blow), or should we just stick with 4 wheels?

Richard McClellan 07-11-2007 13:52

Re: Contact Area and its Relation to Friction?
 
Quote:

Originally Posted by Kevin Sevcik (Post 649053)
PID traction control is pretty simple, just not anything that's typically done in FIRST. The whole point is to keep your wheels from slipping. Wheels slip when applied force exceeds the static friction force. Applied force is proportional to applied torque which is (mostly) proportional to motor torque which is proportional to motor current. So your goal would be to PID control the current being supplied to (or sourced from) the motors. Current-mode motor drivers and amplifiers are awesome for this, but we don't have any, sooo the idea would be to use a solid state current sensor on your motor leads, and PID control this.

This thread has spawned a lot of good discussion. While we're on the topic of traction control, I wanted to ask another question, how exactly does an anti-lock brake system on a car work? This would seem to require some sort of a traction control system, where you would PID control the current of a motor (or for a car, the gas being injected into the engine). Theoretically, this control system would work when the wheel material and the ground were made of the same material, as is the case in a FIRST competition.

But for cars, the problem is much more challenging, because road conditions are always different. ABS systems allow fairly large deceleration on dry roads, but when the road is wet or icy, this is somehow compensated for and maximum deceleration is much smaller. Can anyone explain this? This concept seems like it would be extremely useful if it could be applied to a FIRST robot.

JesseK 07-11-2007 14:45

Re: Contact Area and its Relation to Friction?
 
This used to be a good guide with an animated gif showing how the internal cyllinders work in the brake pump -- but it looks like they've changed it a bit in the couple of years since I've looked at it. I too had this question a few years ago:
http://auto.howstuffworks.com/anti-lock-brake.htm

It's "sorta" how it works. Instead of a speed sensor, some cars used to use a "slip" sensor that was a combination of a shaft encoder and accelerometer. Since then it's been proven to be easier and faster to use speed differentials to control the brake fluid pressure.

Richard McClellan 07-11-2007 15:14

Re: Contact Area and its Relation to Friction?
 
Quote:

Originally Posted by JesseK (Post 650175)
This used to be a good guide with an animated gif showing how the internal cyllinders work in the brake pump -- but it looks like they've changed it a bit in the couple of years since I've looked at it. I too had this question a few years ago:
http://auto.howstuffworks.com/anti-lock-brake.htm

It's "sorta" how it works. Instead of a speed sensor, some cars used to use a "slip" sensor that was a combination of a shaft encoder and accelerometer. Since then it's been proven to be easier and faster to use speed differentials to control the brake fluid pressure.

Ok, so according to that article, the control system operates with a predetermined constant for the "maximum allowed deceleration" to prevent the wheels from slipping. But this "constant" is extremely dependent upon the coefficient of friction between the wheel and the ground, which changes depending on the surface of the road.

Based on my experience with ABS, this "constant" is smaller when driving on ice, than when driving on a dry road. But how does the control system know this?

Theoretically, this could be done with an accelerometer and encoders on the wheels. Is this the way it's done? The problem that might happen is that by the time the controller realizes the wheel speed is different than the car speed to start removing brake pressure, the wheels will already be slipping. Doesn't ABS prevent this situation completely?

mikeleslie 07-11-2007 17:05

Re: Contact Area and its Relation to Friction?
 
Not to want to carry on the ABS thing too long, but the article is a bit misleading. (and this part is actually valueable in a FIRST situation)
The wheel sensors look at rotational speed at each axle (i.e. an encoder at each wheel) The system then can look at the realtive speed of each wheel. When you mash on the brakes, it monitors each wheel to see that they are moving (more or less) at the same speed. When one stops turning, (no changes at the wheel sensor), the system sees the difference and dumps the pressure to that brake line until it starts turning again, whn it's turning again, the dump valve is closed and pressure returns. now since the systems lost pressure, the pump pushes some extra back in so your foot does not hit the floor. (this is a very generic description here, so some license is taken with when the valve closes etc)

Now looking at vehicle stability systems is where you find the big use of the acceleramoters. They know what is supposed to be happening (accel, brake, turn etc,) and look at the body response. If it's out of bounds (accel a direction not intended), they use the ability of the ABS to apply and release the brakes in combination with changes in engine timing to reduce power to try and get things back within a safe zone. (again, this doesn't describe everything and there's much variation in the specific systems)

So how can you use this on a robot?....well, if you can look at each of your wheels independently (one channel per independently rotational wheel ) to see what they are doing, and you can look at the operator input to see wht you want to be happening, and you can look at the net affect on the body (2d acceleramoter) then you can use this info to do things like pulse the motors (to change from the dynamic friction during wheel rotation) or stop the wheels from turning, or trun away......of course, a really good driver just does this without even thinking about it. sort of a hardware vs software trade

mikeleslie 07-11-2007 17:20

Re: Contact Area and its Relation to Friction?
 
Paul is right. It's not the surface area.

and since it's not what is it? (and this is just my thoughts here)

The surface we are dealing with is not normal. It's complex, it has threads, and some of thes threads are in contact with the wheel. so looking at the thread to wheel interaction is a start. I would look at the shape of the thread under your wheel when they are in contact. How can you use the fact that the thread is glued to the carpet mat as an advantage? The carpet is 3 dimensional. can projecting into the carpet be an advantage, can there even be a "best shape" for these projections relative to the carpet fibers? Don't think at the 1 square inch level, think at the .1sq mm level. How do I get the fibers to do more for me than the other guy? How can I trap them, bend them and make them do my bidding? Don't think about pushing down, think about pulling across. Find the exact right combination of shape, projection and force direction and you will solve this puzzle.

114Klutz 30-11-2007 22:00

Re: Contact Area and its Relation to Friction?
 
Quote:

Originally Posted by 114ManualLabor (Post 649451)
This is an interesting thread.

Short disclaimer: I have yet to take physics, so anything I write is a result of observation.

Anywhoo. I think I understand what people here are talking about, yet I'm still confused. If everyone says that the contact area doesn't matter, then can someone explain how this makes sense: I built two robots. Both were with the kitbot chassis. Both were driven off of Banebots transmissions. Both had two driven wheels, and two casters. One bot had two andymark kit wheels per axle for driving, and the other had only one per axle. I added extra weight on each in order to make the weight exactly equal. I wired both motors to a single battery and switch. Then I put the two drive bases head to head, set so they would drive directly into each other, on an area of FIRST carpet. When turned on, the base with 2 wheels per axle could overcome and outpush the other base every time. We did this 12 times, each time changing to a new, fresh battery.

Anyone care to help me out here? I guess that physics and math and stuff say this shouldn't work, but it did. So I'm confused.

I can explain. The physics supports this.

Lets take bot A and bot B, both bots have X amount of weight. The coefficient of friction between the wheels and the ground is Us

Bot A has 6 wheels, 4 of which are powered, so, 2/3rdsX * Us is the max tractional force provided.

Bot B has 4 wheels, 2 of which are powered, so 1/2X*Us is the max tractional force.

This is, assuming, of course, that the distribution of weight and the wheel spacing is perfectly even.

Richard McClellan 01-12-2007 02:49

Re: Contact Area and its Relation to Friction?
 
Quote:

Originally Posted by 114Klutz (Post 654680)
I can explain. The physics supports this.

Lets take bot A and bot B, both bots have X amount of weight. The coefficient of friction between the wheels and the ground is Us

Bot A has 6 wheels, 4 of which are powered, so, 2/3rdsX * Us is the max tractional force provided.

Bot B has 4 wheels, 2 of which are powered, so 1/2X*Us is the max tractional force.

This is, assuming, of course, that the distribution of weight and the wheel spacing is perfectly even.


Hmm....I think 114Klutz is on the right track, but slightly off in the explanation.

Bot A (1 wheel per axle)
____________________________
|back---------x------------front|
|___________________________|
/---\---------|----------/---\
(----)--------|---------(caster)
\ _ /---------|----------\ _ /
--------------|-----------------
/\----------|-----------/\
|----------|-----------|
|----------\/----------|
F1---------W----------F2
|<----L1---->|<----L2--->|

Sum of forces = zero: F1+F2=W
Sum of moments about point x = zero: F1*L1 = F2*L2
So, F1 = (W-F1)*L2/L1 = W*L2/L1 - F1*L2/L1
This simplifies to: F1 = W*L2/(L1+L2)
The max tractional force can be calculated by using: Ff=mu*F1

Or, Ff = mu*W*L2/(L1+L2)


Bot A (2 wheels per axle)
__________________________
|back---x----------------front|
|_________________________|
/--\----|---------------/--\
(----)---|-------------(caster)
\ _ /---|---------------\ _ /
--------|---------------------
---/\---|----------------/\
---|----|----------------|
---|----\/---------------|
--F1---W---------------F2
|<-L1->|<------L2----->|

When the extra rear wheels are added, the center of mass (the W) shifts more towards the back (the picture is slightly exaggerated).

So, looking at the formula for traction force again:
Ff = mu*W*L2/(L1+L2)
L1+L2 is still the same but L2 has gotten bigger, which increases the traction force (Ff).

So, it makes sense that the robot with more wheels would push the robot with less wheels even though the weights are the same because the weight distributions aren't perfectly equal.

114ManualLabor - maybe you could try powering all of the wheels - that way the distribution of weight doesn't matter as long as it is somewhere in between the wheel axles. I'd be really curious to see if the 2/axle bot could still beat the 1/axle bot. Then this would mean that mu does in fact change with surface area (to a very slight degree).

Jay TenBrink 07-12-2007 21:40

Re: Contact Area and its Relation to Friction?
 
1 Attachment(s)
Recently we did a tire traction experiment to try to answer some of these same questions. Attached you will find our results. I present these only for your enjoyment and not as an endorsement for any particular tire. Since these were tires we had on hand, some were well worn, some were new.

The results were not what I expected. I was surprised at the performance of the large knobby tire. Perhaps this could be attributed to oxidation on the surface of the tire that made it less sticky.

We have not paid too much attention to the air pressure in our tires in the past. We will now.

Jay


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