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Unread 02-11-2007, 11:33
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Re: Contact Area and its Relation to Friction?

So... the consensus is that contact area has little or nothing to do with friction? But it does?

Ok... i'll go get empirical data sometime...

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Unread 07-11-2007, 13:52
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Re: Contact Area and its Relation to Friction?

Quote:
Originally Posted by Kevin Sevcik View Post
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.
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Unread 07-11-2007, 14:45
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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.
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Unread 07-11-2007, 15:14
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Re: Contact Area and its Relation to Friction?

Quote:
Originally Posted by JesseK View Post
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?
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Unread 07-11-2007, 17:05
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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
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Unread 07-11-2007, 17:20
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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.
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Unread 17-01-2008, 00:42
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Re: Contact Area and its Relation to Friction?

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Originally Posted by squirrel View Post
Friction is weird, so yeah, you probably do have to derive a formula from empirical data.

An example of friction being weird: Most material interfaces have a higher coefficient of static friction, than of dynamic friction. But aluminum to aluminum has a higher coefficient of dynamic friction than static friction.

And when you consider automobile tires, think about what might be happening when the situation in my avatar occurs....

Good luck!
Although I'm only a high school student I must question your comment on aluminum on aluminum friction. If the static friction is lower then kinetic, this means as soo as you start movement with minimum force it will stop going back to static then starting again then moving imediatly back to static. Thus never moving at all, thus always staying as static friction. Am I missing something?

Also as an answer to the question mu the mathmatical ratio of the force of friction/normal force is unique not only for every material but for every object. Thus, the contact area is calculated in when stating mu. I couldn't give you a mu of rubber on asphalt, but i could give you a mu of a specific tire with a specific contact area and texture.

just as an explination of your source, I take Honors Physics and got a 20/20 on my friction quiz (no tests in this class) =P
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Unread 17-01-2008, 09:31
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Re: Contact Area and its Relation to Friction?

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Originally Posted by Woody1458 View Post
Also as an answer to the question mu the mathmatical ratio of the force of friction/normal force is unique not only for every material but for every object. Thus, the contact area is calculated in when stating mu. I couldn't give you a mu of rubber on asphalt, but i could give you a mu of a specific tire with a specific contact area and texture.
Sort of. In classical friction you ignore the effect of surface area and the mu doesn't take it into account, which provides a very good approximation with hard materials. On the other hand, with interlocking materials (which I am still not convinced applies to FIRST carpet), there is something of an effect from surface area, but it is probably (I have never tried) a bit hard to predict well enough to warrant including in calculations. While the best results for traction tests would be between the same two objects (manufacturing irregularities would get taken into account) and testing them, similar objects can work pretty well. For example, if I wanted a very accurate number for how much traction a robot has, I would take it to a competition field with a force gauge. Unfortunately, that is unrealistic, so one can just test with a sample of the material on similar (ideally the same) carpet, and still get pretty decent numbers.
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Unread 02-11-2007, 16:24
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Re: Contact Area and its Relation to Friction?

QBranch (aka Alex) wants a definitive answer and he will get it right now.

Static friction force does not depend upon surface area. Static friction force does not depend upon surface area. Static friction force does not depend upon surface area.

This assumes one major thing: The surface pressure between the two items is low enough to not cause material failure at either surface (wheel/tread or carpet).

You must make your wheel width wide enough to not rip up the carpet and not yield your rubber (at least, too much). You should design your wheel width to not fail either material. Once you have done that, the width doesn't mean squat.

Alan is correct about dynamic friction: surface area plays a bigger role.

I have posted numerous times on this and the width does not matter.

I will not argue with any of you about this. I am as certain as can be on this issue ... believe me.
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Unread 02-11-2007, 16:34
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Re: Contact Area and its Relation to Friction?

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Originally Posted by Paul Copioli View Post
QBranch (aka Alex) wants a definitive answer and he will get it right now....

You must make your wheel width wide enough to not rip up the carpet and not yield your rubber (at least, too much). You should design your wheel width to not fail either material. Once you have done that, the width doesn't mean squat. ...

I will not argue with any of you about this. I am as certain as can be on this issue ... believe me.
Word. Thanks, Paul. Definitive answers are a good thing. (emphasis mine)
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Unread 02-11-2007, 20:13
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Re: Contact Area and its Relation to Friction?

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Originally Posted by Paul Copioli View Post
Static friction force does not depend upon surface area. Static friction force does not depend upon surface area. Static friction force does not depend upon surface area.
You can change this slightly and avoid certain potential counterarguments:

Static friction force depends only on the coefficient of friction and the "normal force" (weight, for horizontal surfaces).



There are boundary conditions for some combinations of materials where the coefficient of friction can change based on pressure, and since pressure depends on area and force, changing the area can affect the friction force, but the static friction force still depends only on the coefficient of friction and the normal force.
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Unread 02-11-2007, 23:15
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Re: Contact Area and its Relation to Friction?

So if width doesnt mean squat quick question. in st louis us (1625) had to face 217 and 148 at times 217 had 6 wheel drive with im guessing 1inch wide tires and 148 had 6 wheel drive with 2 inch wide ifi traction wheels. both with what i believe to be identical tread. yet we could push 217 easily and 148 we tied head on. we had a 3 speed 4 wheel swerve drive with 1.75" wheels covered in lower cof blue nitrile roughtop from mcmaster. any explanation? my next years plans already inclue 2.5"wide wheels at the moment
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Unread 03-11-2007, 01:35
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Re: Contact Area and its Relation to Friction?

Gear ratios maybe? If you had a three-speed transmission, there's a good chance your lower gear was lower than theirs. That would result in more torque being delivered to your wheels, and hence a greater pushing power.
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Unread 03-11-2007, 07:17
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Re: Contact Area and its Relation to Friction?

Head on, 1625 did not push 217 easily. I can show you the video evidence. However, we were easily pushed sie to side for reasons completely separate from surface area and frictional force. Our side bumpers were located such that we (inadvertantly) were giving our opponents the ability to transfer our weight to them in a side pushing match, which lowered our normal force and increased our opponents normal force.

Besides, who says our robot was optimized for max pushing force last year? We had a single speed transmission that was not optimized for pushing.

The fact that your team could push the T-Chickens last year has nothing to do with the fact that surface area has nothing to do with static friction.
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Unread 03-11-2007, 08:30
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Re: Contact Area and its Relation to Friction?

Please keep in mind that what is written into fomulae is often NOT directly transferrable to robot drive systems in a way that can be accurately and completely calculated.

Yes, you can make general assumptions regarding friction and the effects of one material vs. another, etc. - but, some arguements relative to traction, best drive system configuration, best wheel type and material, treads vs wheels, and other drive system decisions - are best left to experimenting and lessons learned in the real world.

Therefore, overly concerning yourself about static friction will only address one element of the problem. Robots are usually not designed for static friction.

Something I learned a long time ago relative to contact area and friction. This applies more to mechanisms that are designed to slide, not grip.
If the opposing materials are too smooth (maximizing the contact area) they will react opposite of what you would expect and want them to do. They have more difficulty sliding over each other. Sliding is accomplished easier when the contact surface is a little rough, giving up a little contact surface is productive in some cases.

As to my own experiences in drive systems relative to this question, I would have to say that our robots with more contact area produced better traction against the carpet. When comparing the robots using wheel chair wheel (smooth) vs. treaded pnumatic tires - the treaded pnuematic tires won hands down. The differences could be attributed to a combination of both different material and more surface contact as the pnuematic tire actually increases in contact area as they are pushed against due to the forces subjected to.

Will you get to a point where increasing contact area no longer makes a significant difference? Yes and No - it all depends on what you are attempting to do with it. Andy explained it well in his response. But at the same time, would the Beatty Beast have been such an immovable object without all of those file cards??

Just my thoughts - good topic,
Mike Aubry
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