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Originally Posted by patrickrd
1) Stiction. This is a made-up term for how much force is required to get an object to transition from static (not moving) to dynamic (moving). All wheels tend to dig into the ground and at a microscopic level, interweave with the abnormal properties of the ground material. This creates a bond between the ground and wheel. A certain amount of force is needed to break this bond, or overcome the "stiction." Stiction comes from many places in a mechanical system other than the wheels, too: ball bearings, grease in gearboxes, and many others. The more steps you have in your transmission, the more stiction you probably will have. Stiction -- to my knowledge -- is not a real scientific term, but it is commonly used.
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"Stiction" is actually a legitimate term. It is a contraction of "stick-slip friction" and is characterized as the force required to overcome static adhesion/bonding forces that may occur when two objects are at rest with respect to one another. It is also sometimes called "static friction" although this is a slightly less accurate definition of the term.
Salik - to get to the heart of your question, there is not a simple, practical way for the vast majority of FIRST teams to accurately calculate the answer you seek. While classic physics equations will give a first order approximation (see "f = m * a"), drag on the system must also be factored in. Mechanical drag can come from many sources such stiction, dynamic friction, gear train efficiency losses (all outlined by Patrick), but also from binding due to bearing misalignments, bearing housings clamped a smidge too tight, rough finishes on bearing surfaces, burrs on gear teeth, metal chips falling into gears, misalignment of gear teeth, binding in chain linkages, wheels being slightly under inflated, wheel camber slightly off spec, adhesion characteristics of the wheel material, and about a zillion other causes.
What all this means is that rather than spend way too much time trying to characterize and something that cannot be completely characterized, just cheat and go straight to the answer. Do this by either measuring or estimating. If you are doing the analysis on a pre-existing robot or mechanism, the easiest solution is to just measure the mechanical drag. Connect a spring scale to the front of the robot, pull it over a known distance in a measured time increment, and calculate the force needed to do the task (disconnect and remove the motors so you don't have any back EMF concerns). The difference between the measured force and the theoretical force will be the measured drag.
If you are trying to do this for a future build, and are not characterizing an existing machine, then the problem is slightly harder. The best suggestion is to still measure one of your existing robots, as that will give you a ballpark estimate of the drag on a robot built with your production capabilities. Then add 10-15% to give a conservative estimate.
Note that a drag number that seems very high is not all that unusual. Don't be surprised if your drag coefficient is in the range of 40% for a typical FIRST robot. Once you have determined the drag on the robot, the fun part is trying to figure out where it is coming from, and how to reduce it. Here is one hint to get started: a friend who is a professional bicycle builder once told me that over 50% of the mechanical drag on a typical high-end racing bicycle comes from the interaction between the chain and the sprockets, and the chain itself. Get rid of the chain, and your mechanical drag coefficient goes WAY down.
-dave
p.s. I really, desperately want Dr. Joe or Andy Baker to come along and tell me that I have forgotten about some really simple, accurate way to predict mechanical drag - I'll even buy a case of Dew if they do! But I don't think it exists.