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 stepper motor's characteristic of having only periodic errors and no cumulative errors, makes stepper motor control in speed and position control applications very simple. Although stepper motors are widely used, they cannot be used under normal conditions like ordinary DC or AC motors. They must be used in a control system composed of dual-ring pulse signals and power drive circuits.
Stepper Motor Dynamic Specifications and Terminology
1 step angle accuracy
The error between the actual value and the theoretical value of each step angle rotated by the stepper motor. Expressed as a percentage: Error / Step Angle * 100%. The value varies depending on the number of operating cycles; it should be within 5% for four-cycle operation and within 15% for eight-cycle operation.
2 steps lost
The number of steps a motor actually takes while running is not equal to the theoretical number of steps. This is called a missed step.
3 misalignment angle
The angle by which the rotor tooth axis deviates from the stator tooth axis will inevitably result in an offset angle during motor operation. The error caused by the offset angle cannot be solved by microstepping drive.
4 Maximum no-load starting frequency
The maximum frequency at which a motor can be directly started without load, under certain drive conditions, voltage, and rated current.
5. Maximum no-load operating frequency
The highest speed frequency of a motor without load under certain drive conditions, voltage, and rated current.
6. Operating torque-frequency characteristics
The curve showing the relationship between the output torque and frequency of a motor under certain test conditions is called the torque-frequency characteristic. This is the most important of the many dynamic curves of a motor and the fundamental basis for motor selection. Other characteristics include inertial frequency characteristics and starting frequency characteristics. To maximize the average current, the drive voltage should be increased as much as possible, and a motor with a small inductance and high current should be used.
7. Motor forward and reverse rotation control
When the motor windings are energized in the sequence AB-BC-CD-DA or (), the rotation is forward; when the sequence is DA-CA-BC-AB, the rotation is reverse.
Drive control system composition
The use and control of stepper motors must be achieved through a control system composed of a circular pulse generator, power amplifier, etc.
Generation of pulse signals
Pulse signals are generally generated by microcontrollers or CPUs. The duty cycle of pulse signals is usually around 0.3-0.4. The higher the motor speed, the larger the duty cycle.
Signal distribution
Two-phase four-beat motors have a step angle of 1.8 degrees; two-phase eight-beat motors have a step angle of 0.9 degrees; four-phase motors also have two operating modes. The four-phase four-beat mode is AB-BC-CD-DA-AB with a step angle of 1.8 degrees, and the four-phase eight-beat mode is AB-B-BC-C-CD-D-AB with a step angle of 0.9 degrees.
Power amplifier
Power amplification is the most crucial part of the drive system. The torque of a stepper motor at a given speed depends on its dynamic average current, not its static current. Currently, common driving methods include constant voltage, constant voltage with series resistance, high/low voltage drive, constant current, and microstepping. The higher the stepper motor's speed and torque, the greater the required motor current and the higher the voltage of the drive power supply.
Subdivision driver
When the step angle of a stepper motor cannot meet the requirements, a microstepping driver can be used to drive the stepper motor. The principle of a microstepping driver is to control the operation of the stepper motor by changing the magnitude of the adjacent -AB- currents to change the angle of the combined magnetic field.
Stepper motor selection
A stepper motor consists of three main elements: step angle, number of phases, static torque, and current. Once these three elements are determined, the model of the stepper motor is determined.
Step angle selection
The step angle of a motor depends on the accuracy requirements of the load. It involves converting the minimum resolution of the load onto the motor shaft, determining the angle the motor should travel for each equivalent load, including deceleration. The step angle should be equal to or less than this angle. Currently, stepper motors on the market typically have step angles of 0.36 degrees/0.72 degrees, five-phase motors 0.9 degrees/1.8 degrees, and two- and four-phase motors 1.5 degrees/3 degrees.
Selection of static moment
The dynamic torque of a stepper motor is difficult to determine immediately; we usually determine the static torque first. The static torque is selected based on the load on the motor, which can be divided into two types: inertial load and frictional load. Once the static torque is determined, the motor's frame and length can be determined.
Current selection
Motors with the same static torque can have very different operating characteristics due to different current parameters. The motor current can be determined by referring to the torque-frequency characteristic curve, along with the driving power supply and driving voltage.
Torque to Power Conversion
Stepper motors are generally used with adjustable speed over a wide range, and their power varies. They are usually measured only by torque. The conversion between torque and power is as follows:
P=Ω·M
Ω=2π·n/60
P=2πnM/60
P represents power in watts; Ω represents angular velocity in radians per second; n represents rotational speed per minute; M represents torque in Newton-meters; P = 2πfM/400 (half-step operation, where f is the number of pulses per second, abbreviated as PPS).
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