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What is servo control of stepper motor?

The most advanced closed-loop stepper control method is to operate the motor as a two-phase brushless (BLDC) motor. (Note that many stepper motors have two phases offset by 90° whereas brushless dc motors have three phases offset by 120°.) This method is referred to as servo stepper or closed-loop stepper control.
With servo control for stepper motors, instead of the drive providing full current to the motor to produce movement, feedback from an encoder detects the shaft position, and a control loop (typically a PID loop) determines the exact torque needed for the shaft to follow the move profile. In other words, the motor is driven by a torque command rather than by pulses of current.
What is servo control of stepper motor?
Because it delivers only the current required to achieve the move profile, the servo control method yields greater efficiency than other stepper control methods. And greater efficiency means less heat and longer motor life. Servo control also eliminates resonance issues that plague other control methods, provides high torque at low speeds, and allows the motor to use its full rated torque without the limitations of pullout torque. And although more complex than an open-loop system, implementing servo control for a stepper motor is still less demanding than implementing a full servo system.
Note that there is another method of closed-loop stepper control, often referred to as sensorless control. In this method, there is no feedback device; instead, the controller uses an indirect means, such as measuring back-EMF, to determine the rotor position. Sensorless control, however, is difficult to implement and doesn’t provide a high level of certainty regarding the motor’s estimated position. 
Stepper motor systems using closed-loop control represent a small percentage of stepper motor applications, but if loss of position could be catastrophic to the application, yet the system requires high torque at low speed, relatively simple architecture, and relatively low cost (compared to a true servo motor system) a closed loop stepper motor might be the most appropriate solution.

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