What is Closed Loop Stepper Motor?

The control modes of the stepper motor are divided into two types: open-loop control and closed-loop control.

The simplest control method of the open-loop stepper motor is a ring control system. in this method, the input of pulse controlled by the stepper motor doesn’t depend on the position of the rotor but sends out its control pulse in a fixed law. For the uniqueness of the stepper motor, this control method is suitable to control the stepper motor.

The Introduction of Closed-loop Stepper Motor:

Close-loop control is a basic concept of cybernetics.it refers to a relationship in which the controlled output return to a control input in a certain way, and exerts control influence. The closed-loop control of the stepper motor uses position feedback and(or) speed feedback to determine the phase conversion which is suitable to the rotor position, which can greatly improve the performance of the stepper motor.

In the system of the closed-loop motor, it can expand the performance of stepper motor when tracking and feedback in a given accuracy, or can improve tracking and positioning accuracy in a given speed, or can get the limit speed index and limit accuracy index.

The Working Principle of Closed-loop Stepper Motor:

This control method is directly or indirectly to detect the position or speed of the transfer or load, and then through feedback and appropriate processing, automatically give the driving pulse sequence of the stepper motor. The change of driving pulse sequence is based on the change of position of load or rotor, which has many methods to achieve.

The Control Mode of Closed-loop Stepper Motor:

(1) Keep the phase relationship between exciting magnetic flux and current consistent, so it can generate the electromagnetic torque that can drive the load torque. This control method is the same as the brushless DC motor. So it is called a brushless drive mode or current closed-loop control method.

(2) The motor current is kept constant, the method of controlling exciting magnetic flux and phase angle is the power angle closed-loop control. The power angle is the phase angle formed by the mutual attraction between the rotor pole and stator excitation phase. The power angle is relatively small at low speed or light load, and relatively high at high speed or high load.

The Advantage of Closed-loop Stepper Motor:

(1) With the output torque increases, the speed decreases in a non-linear form. But the torque frequency characteristic is improved.

(2) Under the closed-loop control, the output power/ torque curve is improved. Because the motor excitation conversion is based on the position information of the rotor and the current value is determined by the motor load. So, the current can be fully converted into torque at low speed.

(3) Under closed-loop control, the efficiency-torque curve improves.

(4) Compared with open-loop control, in the mode of closed-loop mode, you can get a higher running speed, more stable speed.

(5) Under closed-loop control, the stepper motor can accelerate and decelerate automatically and efficiently.

(6) In terms of quantitative evaluation of the improvement of closed-loop control relative to open-loop control in speed, it can be obtained in the IV step comparing the time through a certain path interval.

(7) Using closed-loop drive, the efficiency can increase into 7.8 times, the output power can increase into 3.3 times.

Close-loop drive has the advantage of the open-loop drive of step-by-step motor and brushless DC servo motor. So in the position control system which requires high reliability. Stepper motor with close-loop control is widely be used.

The Difference between Open-loop and Closed-loop Stepper Motor:

(1) The open-loop allows the other party to execute. There is no feedback. Close-loop control needs to be executed by another party and reported to you. There is feedback.

(2) Open-loop control generally completes the control activity in an instant. while close-loop will last for a certain period of time.