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    Open-loop vs. Closed-loop Stepper Motor

    One of the main advantages of stepper motors is that they are suitable for open-loop control. Under open-loop control, the step motor is controlled by a pulse sequence with definite time interval, and the feedback to the sensor and the corresponding electronic circuit are unneeded in the control system. This kind of circuit is simple with low cost, so that the open-loop control system of stepper motor can be widely used.
    However, the open-loop control of the stepper motor cannot avoid the inherent disadvantages of the stepper motor itself, that is, resonance, oscillation, step loss and difficult to achieve high speed. On the other hand, it is difficult for the accuracy of open-loop control stepper motor system to be higher than the classification and its positioning accuracy is relatively low. Therefore, in the system with high requirement of accuracy and stability standards, the closed loop control system should be adopted.
    The closed-loop control of stepper motor can determine the phase transition with the rotor position by using position feedback and (or) speed feedback, which can greatly improve the performance of stepper motor.
    In the closed-loop control system of the stepper motor, or the tracking and feedback with specific accuracy, we should expand the range of working speed, improve the tracking and positioning accuracy at a given speed, or get the speed limit and accuracy index limit. Compared with the open-loop control performance, the closed-loop control performance of the stepper motor has the following advantages:

    1. With the increase of output torque, the speed of the two decreases in nonlinear form, but the closed-loop control improves the torque frequency characteristic.
    2. In the closed-loop control, the output power / torque curve can be improved. The reason is that the motor excitation conversion is based on the rotor position information in the closed loop condition. The current value depends on the motor load. Therefore, even in low speed range, the current can also be fully converted to torque.
    3. Under closed loop control, the efficiency torque curve is improved。
    4. The higher running speed can be obtained in closed-loop control with more stable and smooth rotation speed.
    5. The stepper motor can be automatically and effectively accelerated and decelerated in the close-loop control.
    6. The efficiency can be increased to 7.8 times and the speed can be increased to 3.6 times in the close-loop drive. The performance of the closed loop stepper motor is better than that of the open-loop stepper motor in all aspects. Stepper motor closed-loop drive has the advantages of open-loop drive and brushless DC servo motor.
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