Gday Greg
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So the high precision encoders do a constant PEC type of thing?
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They dont "do PEC" per se, they simply allow you to bypass it
( and backlash on reversing, and geartooth spacing errors)
They cant help with mechanical misalignments or flexure etc.
In std setups, the encoders are on the motor and hence the vagaries of the geartrain have to be "guessed", so that the motor control loops can provide an "estimated" constant speed "at the final axis"
With a high precision encoder, it is normally fitted directly to the RA or DEC axle itself, hence ( by using a high speed close coupled feedback loop ), the motor simply drives to give the required theoretical encoder rate, and this is guaranteed to be the axis rate, as the encoder is on the axis.
Thus, any PE etc in the geartrain is irrelevant as the system no longer needs to know about it.
Andrew