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Originally Posted by Anthony Kesich
As has been posted, you can use current draw,
[EDIT] You could also use a condenser(I think thats what the potentiometers that can spin all the way around over and over all called) and just poll in your programming whenever they loop back to zero.
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Guys,
There are too many variables involved in using current to determine RPM. Remember that there are frictional changes, weight changes (if the robot picks up stuff) load changes when pushing or pulling and the possibility that the monitored wheel will not be in contact with the floor. Add to that, changing resistance in components as they change temperature, and internal resistance in the battery all of which affect the current delivered to a motor. With that said, those teams that have used current sensing in the past have a much better understanding of motor interaction and robot function once they look at current draw vs. match play and monitoring battery terminal voltage at the same time is also very helpful.
Anthony, a condensor is another name for capacitor. It goes back into the previous century. A pot that has continuous rotation is simply know as a continuous pot, a no limit pot or a 360 degree pot. What you may be thinking of is a shaft encoder which will output a variety of signals depending on design.
We used two banner sensors aimed at the wheel on one corner of our robot. Any two sensors (separated by real world conditions) triggered by the same stimulus will output a pulse that is time different from the other. By observing (in software or hardware) which pulse occurs before the other you can determine not only RPM but direction. Depending on how many triggers you get in one rotation of the wheel will determine your accuracy. Ours is accurate down to about one inch of travel.