Basic structure and working principle
1. Basic structure of a stepper motor (as shown in Figure 1)
Figure 1. Basic structure diagram of a stepper motor
2. Working principle
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 the other phase winding changes direction, the motor will rotate one step (1.8 degrees) in the opposite direction. When the current through the coil windings is energized in sequence with changing directions, 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. During continuous rotation, the current must sequentially change direction within the same coil for excitation, requiring eight electronic switches for sequential switching in the drive circuit. A unipolar motor has two winding coils with opposite polarities per phase. During continuous rotation, the two winding coils on the same phase are energized alternately. The drive circuit requires only four electronic switches. In bipolar drive mode, because each phase's winding coil is 100% energized, the output torque of the motor is approximately 40% higher than in unipolar drive mode.
Acceleration /Deceleration Motion Control
2-phase (bipolar) stepper motor 2-phase (unipolar) stepper motor
Figure 2. Working principle diagram of a stepper motor
Characteristics of stepper motors
• Precise position control
The angle of shaft rotation is determined based on the number of input pulses. The position error is very small (less than 1/10 degree) and does not accumulate.
• Precise rotation speed
The speed of a stepper motor depends on the frequency of the input electrical pulses, allowing for precise control and convenient adjustment. Therefore, it is widely used in various motion control fields.
• Forward/reverse rotation, emergency stop and locking function
Effective control of motor torque and position, including static torque, can be achieved across the entire speed range. Even in the motor locked state (where current exists in the motor windings but there is no external pulse command input for rotation), a certain torque output is still maintained.
Precise position control under low speed conditions
Stepper motors can operate smoothly at very low speeds without the need for gearbox adjustments, while outputting large torque, avoiding power loss and angular position deviation, and reducing costs and saving space.
• Longer service life
The brushless design of stepper motors ensures a long service life. The lifespan of a stepper motor typically depends on its bearings.
Vibration and noise
Generally speaking, when a stepper motor is running under no-load conditions, resonance will occur when the motor's operating frequency is close to or equal to the natural frequency of the motor rotor, and in severe cases, step loss will occur.
The following are some solutions to the resonance problem:
A. Avoid the vibration zone
This ensures that the motor's operating frequency does not fall within the vibration range.
B. Adopt a segmented driving mode
Using a microstepping drive mode, the original one step is subdivided into multiple steps, improving the resolution of each step of the motor and thus reducing vibration. This can be achieved by adjusting the phase current ratio of the motor. Microstepping does not increase the accuracy of the step angle, but it makes the motor run more smoothly and with less noise. Generally, the torque of a motor is 15% less when running in half-step mode than when running in full-step mode, while the torque will be reduced by 30% when using sinusoidal current control.
Mingzhi Stepper Motor Technical Parameters and Terminology
1. Load
2. Velocity-torque curve
3. Acceleration/Deceleration Motion Control
summary
Stepper motors are frequently used in mechanical design. For example, they can drive synchronous belt shafts to achieve linear motion; or they can drive ball screw shafts to convert rotary motion into linear motion. Because they don't require a feedback system, the biggest advantage of stepper motors is their cost-effectiveness and ability to achieve good precision. In fact, besides motion platforms in machines, stepper motors can also be found in everyday life, such as in printers, scanners, cameras, ATMs, and 3D printers.
