I. Working Principle of Stepper Motor
The stepper motor driver, based on external control pulses and direction signals, uses its internal logic circuitry to control the stepper motor windings to be energized in a specific timing sequence in the forward or reverse direction, causing the motor to rotate in the forward/reverse direction or lock.
Taking a 1.8-degree two-phase stepper motor as an example: When both phase windings are energized, the motor output shaft will remain stationary and locked in position. The maximum torque required to keep the motor locked under rated current is the holding torque. If the current in one phase winding changes direction, the motor will rotate one step (1.8 degrees) in a predetermined direction.
Similarly, if the current in another winding changes direction, the motor will rotate one step (1.8 degrees) in the opposite direction. When the current through the coil windings is sequentially energized by changing direction, the motor will continuously rotate in a predetermined direction with very high precision. A 1.8-degree two-phase stepper motor requires 200 steps to complete one revolution.
Two-phase stepper motors have two winding configurations: bipolar and unipolar.
A bipolar motor has only one winding coil per phase. When the motor rotates continuously, the current needs to be energized by changing direction sequentially within the same coil. The drive circuit design requires eight electronic switches for sequential switching. A unipolar motor has two winding coils with opposite polarities per phase. When the motor rotates continuously, the two winding coils on the same phase are energized alternately.
The drive circuit design requires only four electronic switches. In bipolar drive mode, because the winding coil of each phase is 100% energized, the output torque of the motor in bipolar drive mode is increased by about 40% compared to unipolar drive mode.
II. How to classify stepper motors according to their structure
A stepper motor is a type of electric motor that converts electrical pulse signals into corresponding angular or linear displacement. For each input pulse signal, the rotor rotates by an angle or moves forward one step. The output angular or linear displacement is proportional to the number of input pulses, and the rotational speed is proportional to the pulse frequency. Therefore, a stepper motor is also called a pulse motor.
Stepper motors can be classified into various types based on their structure, including variable reluctance (VR) stepper motors, permanent magnet (PM) stepper motors, hybrid stepper motors (HS) stepper motors, single-phase stepper motors, and planar stepper motors. In my country, variable reluctance stepper motors are the most commonly used type.
The operating performance of a stepper motor is closely related to its control method. From the perspective of control method, stepper motor control systems can be divided into three categories: open-loop control systems, closed-loop control systems, and semi-closed-loop control systems. In practical applications, semi-closed-loop control systems are generally classified as either open-loop or closed-loop systems.
Reactive type: The stator has windings and the rotor is made of soft magnetic material. It has a simple structure, low cost, and small step angle (down to 1.2°), but poor dynamic performance, low efficiency, high heat generation, and difficulty in ensuring reliability.
Permanent magnet type: The rotor of a permanent magnet stepper motor is made of permanent magnet material, and the number of poles of the rotor is the same as the number of poles of the stator. Its characteristics are good dynamic performance and large output torque, but this type of motor has poor precision and a large step angle (usually 7.5° or 15°).
Hybrid type: Hybrid stepper motors combine the advantages of reactive and permanent magnet types. Their stator has multi-phase windings, and the rotor uses permanent magnet materials. Both the rotor and stator have multiple small teeth to improve step accuracy. They are characterized by high output torque, good dynamic performance, and small step angle, but their structure is complex and their cost is relatively high.
Based on the stator windings, stepper motors are categorized into two-phase, three-phase, and five-phase series. The most popular is the two-phase hybrid stepper motor, accounting for over 97% of the market share, due to its high cost-effectiveness and excellent performance when paired with microstepping drivers. This type of motor has a basic step angle of 1.8°/step; with a half-step driver, the step angle is reduced to 0.9°, and with microstepping drivers, the step angle can be subdivided up to 256 times (0.007°/microstep). Due to friction and manufacturing precision limitations, the actual control accuracy is slightly lower. The same stepper motor can be equipped with drivers of different microstepping levels to change the accuracy and performance.
In my country, reactive stepper motors are the most common type of stepper motor. The performance of a stepper motor is closely related to its control method. From the perspective of control method, stepper motor control systems can be divided into three categories: open-loop control systems, closed-loop control systems, and semi-closed-loop control systems. Semi-closed-loop control systems are generally classified as either open-loop or closed-loop systems in practical applications.
