Hi,
yes 'potentially' is indeed the keyword.
Take an AC Step/Direction servo. A stream of pulses, let say 10,000 pulses per second are applied to a servo with a
10,000 count per revolution encoder. The servo will turn one revolution per second. Should the servo be loaded
it will slow somewhat, but the servo drive recognizes that its slipping behind and drives the servo to catch up.
Modern servos can maintain angular position and/or angular speed to within very small fractions of 1%, in the case of the
servo I described above to within 2 parts in 10,000 or so, or 0.02%. More modern servos with 20 bit encoders are very
much better again.
There are limits of course, if the load exceeds the torque the servo can produce it will slip behind and cannot catch up,
a servo would fault 'following error'.
Excluding the situation where the servo is overloaded the ability of the servo to accurately follow its commanded position
(or speed) is determined by the frequency at which it can correct itself. Modern servos (and VFDs) have typically 'refresh
rates' of 5-10kHz. This results in a closed loop servo bandwidth of 500Hz to 1kHz. Such a servo can respond and correct
a fluctuation in position (or speed) with 1/500th-1/1000th of a second, pretty damn good.
The ESS PID refresh rate is approximately 1kHz for a servo bandwidth of 100Hz. About 10th thats available by
an AC servo but still INFINITELY better than open loop. That is what ESS spindle PID does, it makes a feedback servo
of an ordinary spindle motor, albeit with somewhat reduced bandwidth.
You will certainly recall conversations on the forum where the concept of Mach enacting feedback has been discussed.
The communication delays preclude Mach itself from doing a good job. External motion controllers like the Hicon and CSMIO/A
can do so by adding a PID controlled analogue voltage to an analogue servo drive. They add that ability at considerable
cost. The ESS has enacted a similar feature but with one motor only ( the spindle) at no extra cost.
Thus the 'potential' is in fact 'ACTUAL' within the exception of overload, for which PID feedback cant fix, nor any other means either.
Craig