A stepper motor is an open-loop control unit 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. When the stepper driver receives a pulse signal, it drives the stepper motor to rotate a fixed angle in a set direction, called the "step angle." Its rotation occurs step by step at fixed angles. The angular displacement can be controlled by controlling the number of pulses, thus achieving accurate positioning; simultaneously, the motor's speed and acceleration can be controlled by controlling the pulse frequency, thus achieving speed regulation.
I. Working principle of stepper motor
This stepper motor is a four-phase stepper motor powered by a unipolar DC power supply. By energizing each phase winding of the stepper motor in the appropriate timing sequence, the stepper motor can rotate in steps. Figure 1 is a schematic diagram of the working principle of this four-phase reactive stepper motor.
Initially, when switch SB is connected to the power supply and SA, SC, and SD are disconnected, the B-phase magnetic pole aligns with rotor teeth 0 and 3. Simultaneously, rotor teeth 1 and 4 misalign with the C and D phase winding poles, and teeth 2 and 5 misalign with the D and A phase winding poles. When switch SC is connected to the power supply and SB, SA, and SD are disconnected, the rotor rotates due to the interaction between the magnetic lines of force of the C-phase winding and the magnetic lines of force of teeth 1 and 4. Teeth 1 and 4 align with the C-phase winding poles. Teeth 0 and 3 misalign with the A and B phase windings, and teeth 2 and 5 misalign with the A and D phase winding poles. This process continues, with the four phase windings (A, B, C, and D) receiving power in turn, causing the rotor to rotate along the A, B, C, and D directions.
Four-phase stepper motors can be classified into three operating modes based on their energizing sequence: single-four-step, double-four-step, and eight-step. The step angles of single-four-step and double-four-step motors are equal, but the torque of single-four-step motors is smaller. The step angle of the eight-step operating mode is half that of both single-four-step and double-four-step motors; therefore, the eight-step operating mode can maintain higher torque while improving control accuracy.
The power-on timing and waveforms for single four-step, double four-step, and eight-step operating modes are shown in Figures 2.a, b, and c, respectively:
a. Single four-beat b. Double four-beat c. Eight-beat
Methods for driving a stepper motor with a 51 microcontroller:
The driving voltage is 12V, and the step angle is 7.5 degrees. One revolution of 360 degrees requires 48 pulses to complete!
This stepper motor has 6 leads, arranged in the following order: 1: red, 2: red, 3: orange, 4: brown, 5: yellow, 6: black. It is driven using the 51 microcontroller with a ULN2003 microcontroller.
The ULN2003 is driven directly using the 5V voltage of the microcontroller system, which may not provide very high torque. Users can increase the drive voltage to 12V themselves.
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, 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 only having periodic errors and no cumulative errors, makes stepper motor control in speed and position control very simple. 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. Therefore, using stepper motors effectively is not easy, involving expertise in mechanics, electrical engineering, electronics, and computers. Currently, while there are many manufacturers producing stepper motors, very few possess the professional technical personnel capable of independent development and research. Most manufacturers have only one or two dozen employees and lack even basic equipment, merely engaging in blind imitation. This causes many problems for users in product selection and use. Given the above, we have decided to use the widely used inductor stepper motor as an example to describe its basic working principle. We hope this will be helpful to users in selecting, using, and improving their machines.
II. Working Principle of Induction Stepper Motor
(a) Reactive stepper motor
Since the working principle of a reactive stepper motor is relatively simple, the principle of a three-phase reactive stepper motor will be described first below.
1. Structure: The motor rotor has many small teeth evenly distributed, and the stator teeth have three excitation windings, whose geometric axes are successively offset from the rotor tooth axes. 0, 1/3て, 2/3て (the distance between two adjacent rotor tooth axes is the tooth pitch, denoted by て), that is, A is aligned with tooth 1, B is offset to the right by 1/3て from tooth 2, C is offset to the right by 2/3て from tooth 3, and A' is aligned with tooth 5 (A' is A, and tooth 5 is tooth 1). Below is the unfolded diagram of the stator and rotor:
2. Rotation: When phase A is energized and phases B and C are not energized, due to the magnetic field, tooth 1 aligns with A (the rotor is not subjected to any force, and the same applies below). When phase B is energized and phases A and C are not energized, tooth 2 should align with B. At this time, the rotor moves 1/3 of a step to the right. At this time, tooth 3 is offset from C by 1/3 of a step, and tooth 4 is offset from A by (1/3 of a step) = 2/3 of a step. When phase C is energized and phases A and B are not energized, tooth 3 should align with C. At this time, the rotor moves 1/3 of a step to the right again. At this time, tooth 4 is offset from A by 1/3 of a step and aligns. If phase A is energized while phases B and C are de-energized, tooth 4 aligns with phase A, and the rotor moves 1/3 of a pitch to the right. After energizing phases A, B, C, and A respectively, tooth 4 (the tooth preceding tooth 1) moves to phase A, and the motor rotor rotates one tooth pitch to the right. If the energizing sequence A, B, C, A… is continuously pressed, the motor rotates to the right by 1/3 of a pitch per step (per pulse). If the energizing sequence A, C, B, A… is pressed, the motor rotates in the opposite direction. Therefore, the motor's position and speed are directly related to the number of conduction pulses and the frequency. The direction is determined by the conduction sequence. However, for considerations of torque, smoothness, noise, and reducing angle, a conduction sequence of A-AB-B-BC-C-CA-A is often used, changing the original 1/3 of a pitch per step to 1/6 of a pitch. Furthermore, by combining different phase currents, 1/3 can be transformed into 1/12, 1/24, etc. This is the basic theoretical basis for microstepping drive of a motor. It's easy to deduce that the motor stator has m-phase excitation windings, whose axes are offset from the rotor tooth axis by 1/m, 2/m, ..., (m-1)/m, 1 respectively. And with the conductors conducting in a specific phase sequence, the motor can be controlled to rotate in both directions—this is the physical condition for the rotation of a stepper motor. Theoretically, as long as this condition is met, we can manufacture stepper motors with any number of phases. However, due to cost and other considerations, two-, three-, four-, and five-phase motors are most common in the market.
3. Torque: Once the motor is energized, a magnetic field (magnetic flux Ф) will be generated between the stator and rotor. When the rotor and stator are misaligned at a certain angle, the force F generated is proportional to (dФ/dθ).
Its magnetic flux Ф=Br*S, where Br is the magnetic flux density, S is the magnetic field area, F is proportional to L*D*Br, L is the effective length of the iron core, D is the rotor diameter, Br=N·I/RN·I is the ampere-turns of the excitation winding (current multiplied by the number of turns), and R is the magnetic reluctance. Torque = force * radius. Torque is proportional to the effective volume of the motor * ampere-turns * magnetic flux density (considering only the linear state). Therefore, the larger the effective volume of the motor, the larger the excitation ampere-turns, the smaller the air gap between the stator and rotor, and the greater the motor torque, and vice versa.
(ii) Induction type stepper motor
1. Characteristics: Compared to traditional reactive stepper motors, inductor stepper motors have a rotor with a permanent magnet to provide the operating point for the soft magnetic material. The stator excitation only needs to provide a changing magnetic field without the energy required to supply the operating point of the magnetic material. Therefore, this motor has high efficiency, low current, and low heat generation. Due to the presence of the permanent magnet, the motor has a strong back EMF and good self-damping, resulting in smoother operation, lower noise, and less low-frequency vibration. Inductor stepper motors can be considered, to some extent, as low-speed synchronous motors. A four-phase motor can operate in four-phase or two-phase mode (it must use bipolar voltage drive), while reactive motors cannot. For example, four-phase or eight-phase operation (A-AB-B-BC-C-CD-D-DA-A) can be completely achieved using a two-phase eight-step operation mode. It is not difficult to find that the conditions are C= and D=. The internal windings of a two-phase motor are exactly the same as those of a four-phase motor. Small-power motors are generally directly connected as two-phase, while larger-power motors, for ease of use and to flexibly change the dynamic characteristics of the motor, are often externally wired with eight leads (four-phase). In this way, it can be used as a four-phase motor or as a two-phase motor with windings connected in series or parallel.
2. Classification: Induction stepper motors can be classified by the number of phases: two-phase motors, three-phase motors, four-phase motors, five-phase motors, etc. They can also be classified by frame size (motor outer diameter): 42BYG (BYG is the code for induction stepper motors), 57BYG, 86BYG, 110BYG (international standard), while 70BYG, 90BYG, 130BYG, etc., are domestic standards.
3. Static Specifications of Stepper Motors: Phase Number: The number of excitation coil pairs that generate different polarity N and S magnetic fields. Commonly represented by m. Step Count: The number of pulses or conductive states required to complete one periodic change in the magnetic field, represented by n, or the number of pulses required for the motor to rotate through one tooth pitch angle. Taking a four-phase motor as an example, there are four-phase four-step operation modes, namely AB-BC-CD-DA-AB, and four-phase eight-step operation modes, namely A-AB-B-BC-C-CD-D-DA-A. Step Angle: The angular displacement of the motor rotor corresponding to one pulse signal, represented by θ. θ = 360 degrees (number of rotor teeth J * number of operation steps). Taking a conventional two- or four-phase motor with 50 rotor teeth as an example, the step angle during four-step operation is θ = 360 degrees / (50 * 4) = 1.8 degrees (commonly known as a full step), and the step angle during eight-step operation is θ = 360 degrees / (50 * 8) = 0.9 degrees (commonly known as a half step). Positioning torque: The locking torque of the motor rotor itself when the motor is not powered on (caused by harmonics of the magnetic field teeth and mechanical errors). Static torque: The locking torque of the motor shaft when the motor is not rotating under rated static electric action. This torque is a standard for measuring the size (geometric dimensions) of the motor and is independent of the drive voltage and drive power supply. Although static torque is proportional to the electromagnetic excitation ampere-turns and related to the air gap between the stator and rotor, it is not advisable to excessively reduce the air gap and increase the excitation ampere-turns to improve static torque, as this will cause the motor to overheat and produce mechanical noise.
4. Stepper motor dynamic indicators 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 steps; it should be within 5% for four-step operation and within 15% for eight-step operation.
2. Missed Step: The number of steps the motor actually moves during operation is not equal to the theoretical number of steps. This is called a missed step.
3. Misalignment angle: The angle at which the rotor tooth axis deviates from the stator tooth axis. There will always be a misalignment angle when the motor is running. The error caused by the misalignment angle cannot be solved by microstepping drive.
4. Maximum no-load starting frequency: The maximum frequency at which the motor can start directly without load under certain drive conditions, voltage and rated current.
5. Maximum no-load operating frequency: The highest speed frequency of the motor without load under a certain drive mode, 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 operating torque-frequency characteristic. This is the most important of the many dynamic curves of a motor and the fundamental basis for motor selection. (See the figure below.)
Other characteristics include inertia frequency characteristics and starting frequency characteristics. Once a motor is selected, its static torque is determined, but its dynamic torque is not. The dynamic torque of a motor depends on the average current (not the static current) during operation. The higher the average current, the greater the motor's output torque, meaning the stiffer the motor's frequency characteristics. See the diagram below:
Curve 3 indicates the highest current or voltage; curve 1 indicates the lowest current or voltage. The intersection of the curves with the load represents the load's maximum speed. 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.
