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.
The biggest difference between a stepper motor and other motors used for control applications is that it receives digital control signals (electrical pulse signals) and converts them into corresponding angular or linear displacement. It is itself an actuator that completes digital mode conversion. Furthermore, it allows for open-loop position control; inputting a single pulse signal yields a specified position increment. This incremental position control system significantly reduces costs compared to traditional DC control systems, requiring almost no system adjustments. The angular displacement of a stepper motor is strictly proportional to the number of input pulses and synchronized with them in time. Therefore, by controlling the number, frequency, and phase sequence of the pulses and the motor windings, the desired angle, speed, and direction can be obtained.
As a special type of motor used for control, stepper motors cannot be directly connected to DC or AC power supplies and must use a dedicated drive power supply (stepper motor driver). Before the development of microelectronics technology, especially computer technology, the controller (pulse signal generator) was entirely implemented in hardware. The control system used individual components or integrated circuits to form the control loop. This was not only complex to debug and install, consuming a large number of components, but also required a complete redesign of the circuit if the control scheme was changed after it was finalized. This necessitated the development of different drivers for different motors, resulting in high development difficulty and cost, and significant control challenges, thus limiting the widespread adoption of stepper motors.
Stepper motors have various structural forms and classification methods. Generally, they are classified into three types according to their excitation method: reluctance, permanent magnet, and mixed magnet; and according to the number of phases: single-phase, two-phase, three-phase, and multi-phase. In my country, the reactive stepper motor is the most common 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. Semi-closed-loop control systems are generally classified as either open-loop or closed-loop systems in practical applications.
(1) Permanent magnet stepper motor.
Its rotor has permanent magnet poles that generate an alternating magnetic field in the air gap, and the stator consists of four-phase windings. When phase A winding is energized, the rotor will turn in the direction of the magnetic field determined by that phase winding. When phase A is de-energized and phase B winding is energized, a new magnetic field direction is generated. At this time, the rotor rotates by an angle and is located in the new magnetic field direction. The order of the energized phases determines the direction of rotor rotation.
(2) Reluctance stepper motor.
The inner and outer surfaces of the stator and rotor cores are provided with similar toothed grooves distributed in a certain pattern. The change in the relative position of the toothed grooves of the stator and rotor cores causes a change in the magnetic reluctance of the magnetic circuit, thereby generating torque.
(3) Hybrid stepper motor.
Its stator and rotor core structure is similar to that of a reluctance stepper motor. The rotor has permanent magnets that generate a unipolar magnetic field in the air gap, which is also modulated by the slots and teeth of the soft magnetic material on the rotor. Its stator is no different from that of a four-phase reluctance stepper motor, but the rotor structure is more complex (the rotor contains cylindrical permanent magnets with soft magnetic material at both ends, and small teeth and slots around the perimeter). Most are two-phase or four-phase; require positive and negative pulse signals; have a larger output torque than permanent magnet stepper motors; have a smaller step angle than permanent magnet stepper motors; have no positioning torque when power is off; have a higher starting and running frequency; and are a rapidly developing type of stepper motor.
1. The accuracy of a stepper motor is 3-5% of the step angle, and it does not accumulate. (Stepper motors only have periodic errors and no cumulative errors.) 2. The maximum allowable temperature for the stepper motor's exterior. It has the characteristic of no cumulative error (100% accuracy). 3. The torque of a stepper motor decreases as the speed increases. 4. A stepper motor can operate normally at low speeds, but it will fail to start above a certain speed and will produce a whistling sound.
When selecting a stepper motor, it is necessary to comprehensively consider factors such as application requirements and environmental conditions, and to choose a motor with high reliability, good applicability, and a good brand.
