Closed Loop control systems can handle a wide range of motions with a wide range of loads if the control system and mechanics are properly designed for the task. A couple of the more difficult combinations to design for are high inertial mismatches and backlash with hard gearing. The question is not just how to make the system stable, but rather how to also get the desired performance.

Dual loop control is often used to improve the performance of a motion control system. Although this appears more complex at first look, the overall system cost and complexity needed to reach the desired level of precision may often be significantly reduced. In the example system, mechanical stiffness was improved by a factor of approximately 100 by the use of a secondary feedback device.In the case of a single loop lead screw system, the feedback is usually located on the back of the motor. 

Unlike HVAC applications, servomotors are not normally operated at the same speed or torque for extended periods, thus the overall efficiency of a servomotor is not easily given a single numeric value. The efficiency varies greatly with the application, and thus different applications favor different types of servomotors.

A quick search of the patents issued, as well as a review of recent articles, shows a wide range of what authors call “servo stepper motors,” “closed-loop step motors,” “hybrid servo motors” and the like. Although the names may sound similar, the performance can vary quite substantially. Most of the “closed-loop stepper motors” are not actually servo motors!