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Controlling Motor Speed

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jwatte:
A potentiometer is only barely good enough for a hobby servo used for little RC cars. I don't know of one that covers the entire range of movement, even -- you'll have a "dead zone" where there is no output. It is also usually noisy, because it relies on a mechanical wiper moving over a resistive material, and it also will wear out over time.

An optical encoder is a LOT better. It may have some sensitivity to dirt and grime if you don't get an enclosed one, and it still needs mechanical connections, but optical encoders are used in many industrial control and "pro" servo solutions.

If you can get away with it, absolute magnetic encoders can be fully contactless, and can give as good resolution as optical, if not better. From what I hear, they are generally less plug-and-play, though, requiring calibration after set-up before they can perform right. I have not used one myself (other than as integrated in servos like the Dynamixel MX-64T.)

waltr:
A method to get more resolution from an optical encoder to have it attached to the motor shaft before the reduction gears. So that it is turned at a fast rate than the gear reduction output shaft.

Shaft speed is then measured by either counting time periods (hardware timer in the processor) between encoder ticks or by counting the number of encoder tick within a set time period.

For absolute shaft position the total number of encode ticks are counted and keep track of. If a Quadrature output encoder is used then the direction of the shaft's rotation is also know.

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