some friends ask me, I know what it means to open the ring, but what is the meaning of the closed loop? The question is that you ask the right person, then it is made up by small group to talk about it.
In particular, the open-loop circuit of a stepper motor drives at high speeds, so stepper motors are called low speed drive systems for speed control or position control.
So the stepper motor is basically in an open loop circuit for driving, is mainly used to control the position, we have to change the words, other than motor stepper motor especially high precision stepper motor open-loop control and did not do so with the positioning of the motor open loop circuit to drive only step motor, I come to you for example, brushless motor for switching phase, then need to measure the rotor position after position closed-loop circuit needs with position sensor, and if at a certain speed to drive, then the need to measure the rotor speed, and the speed of
Closed loop circuit: so if you want to position control, requires a closed-loop circuit containing a rotor position signal to the encoder sensor, so the open-loop stepper motor to compare it with the high cost of closed circuit sensor, so the stepper motor is called low cost speed control and position control drive system.
If the step angle relative to the closed loop control method, then according to the motion position of stepper motor rotor in the appropriate position to go up to the same excitation current, will not have been current closed-loop control of the advantages, but according to the current closed-loop control system change in load current is different, it is mainly in light load when the shortcomings of low efficiency, so low with current closed-loop control, if it is in high speed, then the step angle to closed-loop control, especially the constant excitation current in stepper motor to generate static, can keep the torque, so you can at any time according to the action conditions, to switch control mode the.
In the open-loop stepper motor driver circuit for high-speed rotation, will be out of step and vibration in high speed operation difficulty, so in order to make up for these shortcomings, the stepper motor installation angle sensor to form a closed-loop control, so that it can be used to detect and to avoid the loss of. So the closed-loop control of stepping motor generally there are several classification.
The first is to maintain a certain motor current, is to control the excitation flux and the current phase angle in this way, a method called power angle closed-loop control, the power angle can be as the magnetic pole of the rotor and the stator excitation phase to phase attraction called angle, the angle at low speed or power less light load, so in high speed or high load will be relatively large.
 Can also make the phase relationship with the current magnetic flux is consistent, it will produce electromagnetic torque can drive the load torque, the method to control the motor current is the same with the control method of Brushless DC motor, we call the brushless drive mode or current closed-loop control mode.