Stepper motors are inherently open-loop devices. They don’t require feedback because each pulse of current delivered by the drive equals one step of the motor (or a fraction of a step in the case of microstepping). Plus with small step sizes (or step angles) the motor’s position can be determined very precisely without the need for a feedback device and complicated control schemes.
So, if a stepper motor’s position can be determined in an open-loop system, why would you add the cost and complexity of closed-loop control to a stepper motor?
There are generally three types of closed-loop control for stepper motors, each offering a different level of positioning control and complexity.
Step-loss compensation: Reactive position correction at the end of the move
The most common type of closed-loop stepper system is based on step-loss compensation, also referred to as step-loss control or stepper position maintenance. In this setup, the drive operates in microstepping mode, and an encoder tracks the shaft (or load) position. If lost steps are detected, based on the commanded position (number of steps multiplied by step angle) versus the actual position read by the encoder, the controller commands additional steps so the motor (or load) reaches the desired position.
Load position control: Continuous, real-time position correction without complex controls
Load position control, also referred to as closed-loop microstepping, continuously monitors the shaft (or load) position and generates an error signal. The controller uses this error signal to adjust the commands in real-time, during the entire move profile.
Servo control: Complete control of torque and position
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.
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 might be the most appropriate solution.