John,
The RC servos have a potentiometer connected from the housing to the
output shaft. The wiper voltage is compared against the PWM % and the
resulting error signal is used to correct the position by turning the
geared motor in the apropriate direction.

Thus error is not a function of time operating or repititions, but of
potentiometer linearity, PWM input signal and acuracy of converting
input PWM signal to a voltage level. Also there is a small deadband to
allow motor to turn completely off when shaft is close to correct
position and corrective power diminishes as shaft comes close to this
position.

Stepper motors are different and are typically provided with a position
zero reference at one end of travel, unless the driving signal becomes
out of sync' with the motor's response then position is known by
direction and counters from driving software relative to last stop at
the reference end.

regards,
Graham Daniel.



John Walker wrote:
>
> I am looking for information relating to the accuracy of RC servos. That
> is, I want to know if I repeatedly tell the servo to go to a 45 degree
> position and then back to the 0 degree position, what will the error be
> after 50 repetitions, 100 repetitions, etc. So basically if I have my PIC
> send the same pulse stream each repetition what will my angle error be
> after the repetitions are completed. I have been searching for some
> manufacturers specs but have been unsuccessful so far. I can run the test
> myself but I doubt the results will hold for all the types of servos
> available. I want the information in order to determine which servo to purchase.