Hi Gary,
yes you are right Mach can't close the loop, but it doesn't have to if your servos/spindle is up to it.
If you Gcode a coordinated helical move, Mach will issue to the controller the position/velocity/time data and the machine
will follow that. If the spindle has the necessary torque it will perform the helical move very accurately. If it has insufficent
torque it will lag behid. IF WE HAD a closed loop controller the controller would try to compenste by speeding up the spindle
to catch up to where it should be. Just because our hypothetical closed loop controller says 'speed up' the spindle can't,
I mean if it didn't have enough torque to keep up before a closed loop controller doesn't magically increase its torque.
Its very similar to the argument about open loop steppers and closed loop steppers. People imagine that closed loop steppers are
going to work better, they aren't, they are no more powerful than open loop steppers. Either the stepper has the grunt to keep
up or it doesn't. Sure a closed loop stepper might try to correct itself but usually fails because it doesn't have the power and
so fault out 'following error'.
I have as I stated in this thread bought a second hand Allen Bradley AC servo, 1.8kW. I have now become an absolute convert,
why bother to have Mach close the loop when the manufacturers servo drive closes it SO SO SO much better!
Craig