PART１.
01
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 amount of electrical pulses applied to the motor coil, the direction, speed and angle of rotation of the stepper motor can be controlled. Without the use of a closed loop feedback control system with position sensing, precise control of position and speed can be achieved using a simple to control and inexpensive open loop control system consisting of a stepper motor and its corresponding driver.
02
Basic structure and working principle
Basic structure:
How it works:
Based on external control pulses and direction signals, the stepper motor driver controls the stepper motor windings to power forward or reverse in a certain timing sequence through its internal logic circuit, causing rotating the motor forward/backward or locking it.
Take a 1.8 degree two-phase stepper motor as an example: when both phase windings are energized and energized, the motor output shaft will be stationary and locked in position. The maximum torque that keeps the motor locked at rated current is the holding torque. If the current in one of the phase windings changes direction, the motor will rotate one step (1.8 degrees) in a given direction.
Similarly, if the current in another winding changes direction, the motor will rotate one step (1.8 degrees) in the opposite direction to the first. When the current flowing through the coil winding changes direction and is excited in sequence, the motor achieves continuous rotational steps in the predetermined direction with very high operating precision. For a two-phase 1.8 degree stepper motor, it takes 200 steps to rotate once.
Two-phase stepper motors are available in two winding styles: bipolar and unipolar. There is only one winding coil on each phase of the bipolar motor. When the motor runs continuously, the current must change direction and excite sequentially in the same coil. The drive circuit design requires eight electronic switches for sequential switching.
Unipolar motors have two coils of opposite polarity on each phase. When the motor continues to rotate, it only needs to alternately power and excite the two coils of the same phase. The control circuit design requires only four electronic switches. In bipolar drive mode, since the winding coil of each phase is 100% energized, the output torque of the motor in bipolar drive mode is about 40% higher than that in unipolar drive mode.
PART2.
01
load
02
Speed-torque curve
The speed-torque curve is an important expression of the output characteristics of the stepper motor.
A. Working frequency point
The motor speed value at a certain point.
n=q*Hz/(360*D)
n: revolution/second
Hz: frequency value
D: subdivision value of the control circuit
q: walking angle
For example: a stepper motor with a step angle of 1.8°, in 1/2 subdivision drive mode (i.e. 0.9° per step), has a rotation speed of 1.25r/s at an operating frequency of 500Hz.
B. Self-start zone
Area where stepper motors can be started and stopped directly.
C. Continuous operation zone
In this area, the engine cannot be started or stopped directly. The engine running in this zone must first pass through the self-start zone and then accelerate to reach this working zone. Likewise, the motor cannot be braked directly in this area, otherwise it would easily cause the motor to lose synchronization. It must first decelerate to the self-start zone before braking.
D. Maximum start frequency
The maximum pulse frequency to ensure that the motor does not lose steps when the motor is idle.
E. Maximum operating frequency
Highest pulse frequency at which an excited motor can operate without losing steps under no-load conditions.
F. Starting torque/pulling torque
It reaches the maximum load torque when the stepper motor starts and starts working at a certain pulse frequency without losing steps.
G. Operating torque/pull-out torque
It meets the maximum load torque of the stepper motor which can work stably at a certain pulse frequency without losing steps.
03
Acceleration/deceleration motion control
When the motor operating frequency point is in the continuous operation area of the speed-torque curve, how to shorten the acceleration or deceleration time when the motor starts or stops, so that the motor can run at optimal speed for a longer period. of time, thus improving the effective operating time of the engine is very critical.
As shown in the figure below, the dynamic torque characteristic curve of a stepper motor is a horizontal straight line when it operates at low speed; when operating at high speed, the curve drops exponentially due to the influence of inductance;
Noticed:
J represents the moment of inertia of the motor rotor when loaded.
q represents the rotation angle of each step, which refers to the step angle of the motor during the entire drive.
During deceleration operation, simply invert the above acceleration pulse frequency and calculate it.
04
Vibrations and noise
Generally speaking, when a stepper motor operates without load, resonance occurs when the operating frequency of the motor is close to or equal to the natural frequency of the motor rotor, and serious offset phenomena occur.
Several solutions for resonance:
A. Avoid vibration area: keep the motor operating frequency within the vibration range.
B. Adopt subdivided drive mode: use micro-step drive mode to subdivide the original step into multi-step operation to improve the resolution of each motor step, thereby reducing vibration. This can be achieved by adjusting the phase current ratio of the motor. Microstepping does not increase step angle accuracy, but it can make the motor run smoother and make less noise. Generally, when the motor operates in half step, the torque will be 15% lower than that in full step. When using sinusoidal current control, the torque will be reduced by 30%.
PART３.
Daguang focuses on providing solutions such as precision CNC machining services (3-axis, 4-axis, 5-axis machining), CNC milling, 3D printing and rapid prototyping services.
