The following sounds sarcastic, but the question is unavoidable...

WHY does the flow have to be so precisely regulated? Are you worried about
bursting the lungs of the person on the respirator?

It would be much easier to have the turbine run at a constant speed and use
a simple valve at the patient's end to bleed off excess flow. (Of course, if
respirators were made that way, we wouldn't be discussing how to regulate
the turbine speed.)