In industrial automation, stepper motors are widely used. For example, industrial robots, 3D printers, computer hard drives, etc. all have stepper motors. The particularity of the stepper motor is that it can control the angular position of the rotor without the need for sensors to control the position, so it is an open-loop control system. Similar to the DC brushless motor, they all rotate by controlling the energization sequence of the coil to generate an interactive force between the magnets.

According to the structure of the stepping motor, it can be divided into three types: permanent magnet type, reactive type and hybrid type. At present, the hybrid stepping motor is the most commonly used, because it combines the advantages of the permanent magnet type and the reactive type. We first understand some of its structure and control principles through permanent magnet stepping motors, and then further understand the structure and control principles of reactive and hybrid stepping motors.

In order to increase the magnetic induction intensity and reduce the turbine loss, the rotor and stator frame of the stepping motor are made of silicon steel sheets with high magnetic permeability. There are 4 stator teeth on the stator frame. After two sets of coils are wound around them and energized, according to the principle of electromagnetic induction, the coils become electromagnets at this time.

According to the right-hand rule, one end will become the N pole and the other end will become the S pole. The change of this polarity can be realized by changing the positive and negative poles of the power supply. If the positive and negative poles of the power supply are converted with a certain regularity, the stator part will form a rotating magnetic field, so that the rotor will rotate with the rotating magnetic field. The rotation of the rotor has a very important parameter, that is: the step angle. The step angle is the angle rotated by a pulse signal rotor, and its calculation formula is: 360 degrees divided by the number of teeth of the bracket multiplied by the number of running beats (the number of teeth of the permanent magnet can be considered as 1). The step angle of this stepper motor is 90 degrees.

The stepper motor cannot be directly connected to the power supply, because the stepper motor is controlled by the pulse signal, so the work of the stepper motor needs to be controlled by the driver. The number and frequency of pulses and the power sequence of each phase winding of the motor are controlled by PLC programming to control the rotation of the stepper motor. There are three driving modes of stepper motor, namely full step, half step and microstep drive. According to the number of energized phases, full-step drive is divided into single-phase full-step drive and two-phase full-step drive. We can easily understand this by looking at the picture.

Half-step drive is to drive only half a step at a time, which is realized by alternately energizing single and double power. When single-phase is energized, one coil produces a magnetic field, and the rotor points toward the energized coil, while when dual-phase is energized, both coils produce a magnetic field, and the combined magnetic field keeps the rotor in an intermediate position. You can see the picture to understand.

Microstepping is actually not difficult to understand. As the size of the current changes, the strength of the coil magnetic field will also change, and the balance position of the rotor will also change, which is the principle of microstep driving. The A coil starts to have maximum current, the B coil current is 0, and the rotor points to the A coil. If the current of the A coil decreases slowly and the current of the B coil increases slowly, the rotor will slowly turn to the B coil. From a macro point of view, the current changes of the A and B coils are close to the trigonometric function curve. The size of the current change in each step determines the size of the micro-step movement. Although the more steps, the smoother the movement, but the torque will be reduced accordingly.

The rotor of the reactive stepper motor is made of soft material, and there are many salient poles on the rotor, so that when the coil is energized, it will attract the rotor to rotate. This type of motor has a simple structure and low cost, but has poor dynamic performance and poor reliability, so it is rarely used.

At present, the most commonly used is the hybrid stepping motor, which combines the advantages of the above two motors. The step angle is small, the moment is large, and the dynamic performance is good. The rotor of the hybrid stepping motor is special. It consists of two toothed steel cups mounted on the central shaft of the permanent magnet. One end of the rotor is the N pole and the other end is the S pole, and the teeth of the N and S poles are dislocated. The commonly used hybrid stepping motor has a step angle of 1.8 degrees, the rotor has 50 teeth, and the stator has only 48 teeth, which are divided into 8 main poles. If the 8 main poles are divided into 4 pairs, we will find that the teeth of one pair of stators and rotors are completely aligned, the teeth of one pair of stators and rotors are misaligned, and the teeth of the other two groups of stators and rotors are completely aligned. The teeth are half aligned. When a pulse signal is given to the stator, the magnetic field of the stator changes, and the rotor moves slightly to align with the stator teeth. In the case of full drive, each movement is a concave tooth or half of a convex tooth, and the angle is 1.8 degrees . If it is a half-step drive, this angle will be further reduced, which is 0.9 degrees.