Tachometer mode vs. voltage mode vs. EMC

I am converting a Bridgeport Series II Interact 2 CNC mill to EMC.

I am currently using Advanced Motion 30A8 drives for all axes.

The drives give me a selection of tachometer mode (with feedback from motor's tachometer) or voltage mode (which means output the asked voltage without any regard for motor feedback).

The question is what mode to use. Voltage mode seems to be safer, in that if a tachometer breaks or stops sending signals, it would not go into a runaway mode.

Am I losing anything such as accuracy, by not using tachometer feedback?

Now, if I use tachometer mode with EMC and a tachometer stops working: Does EMC have any kind of safety feature that would detect (via encoders) that the actual speed differs from asked speed (as would happen with missing tachometer feedback) and error out?

Thanks guys.

Velocity mode with tachometers would require separate tuning of the drives velocity loop and EMCs position loop
but will probably give better performance than pure voltage mode.

Unless the drives have specific circuitry to detect a disconnected tachometer I would expect a run-away should the tachometer become disconnected

EMCs following error will detect run-aways (Following error is distance between commanded position and actual position)
(but don't bet life or limb on any software fault detection system)

(my 2 pence)

PCW, when you refer to "better performance", what does it mean? Would there be an increase in accuracy, if so what is the extent of it?

Two things tend to favor velocity mode servos over torque or voltage mode:

1. Tachometers are usually better velocity feedback devices than encoders especially at low speeds (less grainy)
2. The velocity loop in the drive typically has higher bandwidth than provided by EMCs 1 KHz default sample interval (~300 Hz)
(though the sample interval can be changed)

The actual increase in accuracy is really machine and tuning dependent and may not be important.