Detailed explanation of stepper motor knowledge, no longer afraid of not understanding stepper motors!
Stepper motors, as executive components, are one of the key products of mechatronics integration and are widely used in various automation control systems. With the development of microelectronics and computer technology, the demand for stepper motors is increasing day by day, and they have applications in various fields of the national economy. As an electrician, one should not only have a limited understanding of stepper motors, but also have a deep understanding of their structure, basic principles, and applications.

What is a stepper motor

A stepper motor is an electromechanical device that directly converts electrical pulses into mechanical motion. By controlling the sequence, frequency, and quantity of electrical pulses applied to the motor coils, the steering, speed, and rotation angle of the stepper motor can be controlled. Without relying on a closed-loop feedback control system with position sensing, a simple and low-cost open-loop control system composed of a stepper motor and its accompanying driver can achieve precise position and speed control.

working principle:
The stepper motor driver, based on external control pulses and direction signals, controls the winding of the stepper motor to be powered forward or backward at a certain timing through its internal logic circuit, causing the motor to rotate forward/backward or lock.

Taking a 1.8 degree two-phase stepper motor as an example: when both windings are energized and excited, the motor output shaft will come to a standstill and lock its position. The maximum torque required to keep the motor locked at rated current is the holding torque. If the current of one phase winding changes direction, the motor will rotate one step (1.8 degrees) in a predetermined direction.

Similarly, if the current of another winding changes direction, the motor will rotate one step (1.8 degrees) in the opposite direction to the former. When the current through the coil winding changes direction in sequence towards excitation, the motor will achieve continuous rotation and stepping in the predetermined direction, with very high operating accuracy. For a 1.8 degree two-phase stepper motor, it takes 200 steps to rotate once.

Two phase stepper motors have two winding forms: bipolar and unipolar. Bipolar motors have only one winding coil on each phase, and when the motor rotates continuously, the current needs to change direction and excite in the same coil in sequence. The design of the drive circuit requires eight electronic switches for sequential switching.

 
A unipolar motor has two winding coils of opposite polarity on each phase. When the motor rotates continuously, it only needs to alternately energize and excite the two winding coils on the same phase. Only four electronic switches are required in the design of the driving circuit. In bipolar drive mode, because the winding coils of each phase are 100% excited, the output torque of the motor in bipolar drive mode is increased by about 40% compared to unipolar drive mode.

Acceleration/deceleration motion control
When the operating frequency of the motor is within the continuous operating range of the speed torque curve, it is crucial to shorten the acceleration or deceleration time when the motor starts or stops, so that the motor can run at its optimal speed for a longer period of time, thereby improving the effective operating time of the motor.

 
The dynamic torque characteristic curve of the stepper motor is in a horizontal straight line state during low-speed operation; During high-speed operation, the curve experiences an exponential decrease due to the influence of inductance.

VIBRATION AND NOISE
Generally speaking, when a stepper motor operates under no-load conditions, resonance occurs when the operating frequency of the motor is close to or equal to the natural frequency of the motor rotor, and in severe cases, the phenomenon of out of step may occur.

 
Several solutions for resonance:
 

A. Avoid vibration zone: Ensure that the operating frequency of the motor does not fall within the vibration range

B. Adopting a segmented driving mode: Using a micro step driving mode, the original step is segmented into multiple steps to improve the resolution of each step of the motor and reduce vibration. This can be achieved by adjusting the phase current ratio of the motor. Microstep does not increase the accuracy of step angle, but it can make the motor run more smoothly and with less noise. When a general motor runs in half a step, the torque will be 15% smaller than when it runs in the whole step, while when using sine wave current control, the torque will be reduced by 30%.