The Subdivision Control of The Stepper Motor

　　The stepper motor is a widely used actuator, which is used in the various automatic control system. Recently, I participate in the project of grating splicing device for laser compression, whose accuracy requirements reaches up to nanoscale, which has a high requirement for the stability of the system.

　　The traditional motor control technology can not meet the design requirements, and the subdivision inspires me. I will share these with you.

　　1. The introduction of the stepper motor

　　The stepper motor is an actuator that converts electric pulse into angular displacement. When the stepper motor diver receives a pulse signal, it will drive the stepper motor to rotate in a set direction at a fixed angle. And its rotation runs step by step at a fixed angle. Therefore, we can control the angle of rotation by control the number of the pulse. Besides, by controlling pulse frequency, the speed and acceleration can be controlled.

　　The stepper motor has three types: variable reluctance type, permanent magnet type, and hybrid type. The variable reluctance type consists of stator and rotor. The rotor has teeth or grooves, which always turns to a minimum reluctance position in the stator magnetic fields and produces medium torque. The step angle is from 0.9°to 15°. The rotor of the permanent magnet stepper motor is composed of a cylindrical permanent magnet, which has teeth or grooves. The torque is relatively low and has holding torque. The step angle is from 7.5°to 90°, which is widely used in instruments. The hybrid stepper motor has the advantages of permanent magnet and variable reluctance, which has large torque and step angle is from 0.9°to 15°.

　　2. Why subdivision?

　　Many peoples say the subdivision is to improve the position accuracy. However, it is not the main factor. The subdivision is to improve the operational performance of the motor. Taking a two-phase stepper motor as an example, if the rated current of the motor is 5A, when uses conventional drive mode, the motor runs, the phase current is from 0 to 5A or from 5A to 0. The rapid change of current will cause the vibration of the motor. If the subdivision technology is used and subdivision is 50, when the motor runs, the change of current is only 0.1A, which can significantly improve the vibration. After subdivision, the output torque of the motor is added, the torque improves 30%-40% compared with no subdivision. This improves the resolution of the motor, reduces step angle, and increases the uniformity of the step.

　　3. The method of subdivision control?

　　Taking the two-phase reactive stepper motor as an example, if the number of beats is N. when inputting f pulses per second, the rotor speed is:

　　When f is constant, then the change of N will change the speed. After subdivision, when the N increases, the number of beats will reduce the step angle. In this way, the operation speed of the motor will decreases. To ensure the normal operation speed, the number of pulses should correspondingly increase.

　　If the input pulse switches, the change is only the rated current in windings, the rotation of the rotor is only the part of the step angle. The rotor rotates an original step angle with certain steps, which bases on the level of rated current. By controlling the phase current of the stepper motor, the motor operates with a smaller step angle and can reduce the step size and vibration in low frequency. In short, the subdivision drive converts the control of the rotor current into the change of the size and direction of each phase winding current, which gradually changes the composite magnetic field.

　　In the actual control, the current change of coil A and coil B is trap cut. Due to the high frequency of the chopper, the current fluctuation at each stage can be ignored, which can be regarded as DC output. Under the combined action of coil A and coil B, a circular rotating magnetic field will form, the stepper motor can operate at a single step.