A stepper motor is an open-loop control element that converts electrical pulse signals into angular or linear displacement. Under non-overload conditions, the motor's speed and stopping position depend only on the frequency and number of pulse signals, and are unaffected by load changes. That is, applying a pulse signal to the motor results in it rotating one step angle. This linear relationship, coupled with the fact that stepper motors only have periodic errors and no cumulative errors, makes controlling speed and position using stepper motors very simple.
A stepper motor is a type of induction motor. Its working principle is to use electronic circuits to convert direct current into multi-phase timing control current that is supplied in a time-sharing manner. Only when this current is used to power the stepper motor can it work normally. The driver is a multi-phase timing controller that supplies power to the stepper motor in a time-sharing manner.
Although stepper motors are widely used, they cannot be used like ordinary DC or AC motors under normal conditions. They require a control system composed of dual-ring pulse signals and power drive circuits to function. Therefore, using stepper motors effectively is not easy; it involves expertise in mechanics, electrical engineering, electronics, and computers. As an actuator, the stepper motor is a key product in mechatronics and is widely used in various automated control systems. With the development of microelectronics and computer technology, the demand for stepper motors is increasing daily, and they are used in various sectors of the national economy.
Stepper motors can be classified into various types based on their structure, including Variable Reluctance (VR) stepper motors , Permanent Magnet (PM) stepper motors , Hybrid Stepping ( HS ) stepper motors, single-phase stepper motors, and planar stepper motors. Variable Reluctance stepper motors are the most commonly used in China. The operating performance of a stepper motor is closely related to its control method. Stepper motor control systems can be categorized into three types based on their control methods: 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.
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.
Interpretation of Stepper Motor Parameters and Characteristics
1. Step error
It refers to the difference between the measured step angle and the theoretical step angle under no-load conditions. It reflects the accuracy of the stepper motor's angular displacement.
The step error of domestically produced stepper motors is generally in the range of ± 10 ′ to ± 30 ′, while the step error of high-precision stepper motors can reach ± 2 ′ to ± 5 ′.
2. Maximum static torque
This refers to the maximum applied torque that a stepper motor can withstand when one phase is always energized and the motor is stationary; that is, the maximum electromagnetic torque it can output. It reflects the stepper motor's braking capability and load capacity during low-speed stepping operation.
3. Starting torque-frequency characteristics
This refers to the relationship between the maximum step input pulse frequency (also known as the starting frequency) that a stepper motor can accept to start normally without losing steps when there is an external load torque, and the load torque.
4. Start-up inertial frequency characteristics
This refers to the relationship between the starting frequency and the moment of inertia when a stepper motor drives a purely inertial load to start.
5. Operating torque-frequency characteristics
This refers to the relationship between the output torque and the input pulse frequency when a stepper motor is running. When selecting a stepper motor, the operating frequency and the operating point corresponding to the load torque should be below the operating torque-frequency characteristic to ensure that the stepper motor operates normally without losing steps.
6. Stepping motion and low-frequency oscillation
When the input pulse frequency is very low, if the pulse period is greater than the transient process time of the stepper motor, the stepper motor will operate in a step-and-stop state, which is called stepping operation. Stepper motors have a relatively low natural frequency. When the stepping frequency or low-speed operating frequency is equal to or close to this natural frequency, resonance will occur, causing the stepper motor to oscillate and stop moving. This phenomenon is called low-frequency oscillation.
Methods to avoid low-frequency oscillations:
One approach is to make the operating frequency avoid the natural frequency. Another approach, if the former is not feasible, is to change the natural frequency by adjusting the damper on the stepper motor.
7. Maximum phase voltage and maximum phase current
These refer to the maximum power supply voltage and the maximum current that can be applied to each phase winding of the stepper motor, respectively.
