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#1
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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... -q |
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#2
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Re: Contact Area and its Relation to Friction?
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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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#3
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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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#4
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Re: Contact Area and its Relation to Friction?
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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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#5
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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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#6
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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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#7
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Re: Contact Area and its Relation to Friction?
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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 Last edited by Woody1458 : 17-01-2008 at 00:48. |
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#8
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Re: Contact Area and its Relation to Friction?
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#9
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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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#10
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Re: Contact Area and its Relation to Friction?
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Last edited by Richard Wallace : 02-11-2007 at 16:37. |
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#11
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Re: Contact Area and its Relation to Friction?
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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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#12
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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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#13
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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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#14
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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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#15
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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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