Hi,
1. The driver will pick up on a missed step and "try again", meaning when you are getting marginal on power you get some backstop; and
The problem with that is that IF a stepper misses a step its because it is marginally overloaded, so any extra step inserted to catch up is likely to be missed as well.
The manufacturers claim that closed loop increases the power, pure BS. Closed loop steppers are no more powerful than their open loop counterparts
of the same spec.
2. Outright failure to reach the commanded position errors out and stops the machine - which is a HUGE safety feature.
This is correct, and is a distinct and clear advantage of closed loop steppers over open loop steppers.
One feature that you did not mention was that closed loop steppers can interpolate between full steps whereas open loop steppers can do
1/2 steps reliably but no finer. Thus closed loop steppers have genuinely better resolution than open loop steppers.
These two features are to my knowledge the only two genuine advantages that closed loop steppers enjoy. My contention is that the premium
paid for closed loop steppers is not justified by these two features alone. Others may have a different opinion.
Genuine AC servos are still quite a bit more than closed loop steppers, say 25% or so, and if you are going to pay a premium then servos are worth
the extra. There again that is my opinion, others may differ with that. The principle advantage that a servo has is overload capacity.
When a stepper (open or closed loop) gets overloaded it misses steps or stalls, no ifs or buts, it just stalls, whereas a servo just 'digs' into
its overload capacity, typically 3-4 times its rated output, and just does the job. The short term overload capability of a servo means that it
seems to have MUCH greater output than the specs suggest whereas steppers never seem to quite live up to their specs.
Craig