Technical Knowledge

Stepper motor working principle and instructions for use
Release Date : 2019-03-08 10:34:03

first. Introduction

A stepper motor is an open-loop control element that converts an electrical pulse signal into an angular displacement or line displacement. In the case of non-overloading, the speed and stop position of the motor depend only on the frequency of the pulse signal and the number of pulses, and are not affected by the load change, that is, a pulse signal is applied to the motor, and the motor rotates through a step angle. The existence of this linear relationship, plus the stepper motor only periodic error without cumulative error. It is very simple to control the change with a stepping motor in the control field such as speed and position.

Although stepper motors have been widely used, stepper motors are not as conventional DC motors, and AC motors are used under normal conditions. It must be composed of a dual ring pulse signal, a power drive circuit, etc. to form a control system. Therefore, the use of stepper motors is not an easy task. It involves many professional knowledge such as machinery, motors, electronics and computers. At present, there are many manufacturers of stepping motors, but they have professional technicians who can develop them themselves. There are very few manufacturers. Most of them are only one or twenty people, even the most basic equipment. Just in a blind copying stage. This causes the user to cause a lot of trouble in product selection and use. In the above situation, we decided to take a wide range of inductive stepper motors as an example. Describe the basic working principle. Hope to help the majority of users in the selection, use, and machine improvement.

Second, the working principle of the induction stepper motor

(1) Principle of reactive stepping motor

The working principle of the reactive stepping motor is relatively simple. The principle of a three-phase reactive stepping motor will be described below.

1. Structure:

The rotor of the motor is evenly distributed with many small teeth. The stator teeth have three excitation windings, and their geometric axes are respectively shifted from the rotor tooth axis. 0, 1/3 て, 2/3 て, (the distance between the axes of the adjacent two rotor teeth is the pitch of the tooth), that is, A is aligned with the tooth 1, and B and the tooth 2 are 1/3 turn to the right. C and tooth 3 are shifted to the right by 2/3て, A' is aligned with tooth 5 (A' is A, tooth 5 is tooth 1)

2, rotation:

If the phase A is energized and the phases B and C are not energized, the teeth 1 are aligned with A due to the action of the magnetic field (the rotor is not subject to any force and below).
If phase B is energized, when phase A and phase C are not energized, tooth 2 should be aligned with B. At this time, the rotor moves to the right by 1/3て, at which time the teeth 3 and C are offset by 1/3て, and teeth 4 and A Offset (て-1/3て)=2/3て.

If the C phase is energized, the A and B phases are not energized, and the tooth 3 should be aligned with C. At this time, the rotor is shifted to the right by 1/3 て, and the tooth 4 is aligned with the A offset by 1/3 .
If the phase A is energized, the B and C phases are not energized, the teeth 4 are aligned with A, and the rotor is shifted to the right by 1/3.

Thus, after A, B, C, and A are respectively energized, the tooth 4 (ie, the first tooth of the tooth 1) is moved to the A phase, and the motor rotor is rotated to the right by a pitch, if continuously pressed A, B, C, A. ..... energized, the motor rotates 1/3 turn per step (per pulse) and rotates to the right. If A, C, B, A... is energized, the motor will reverse.

It can be seen that the position and speed of the motor are in a one-to-one correspondence between the number of times of conduction (number of pulses) and the frequency. The direction is determined by the order of conduction.

However, due to the consideration of torque, stability, noise and angle reduction. The conductive state of A-AB-B-BC-C-CA-A is often used, so that 1/3 每 of each step is changed to 1/6 て. Even with a different combination of two-phase currents, the 1/3 turns become 1/12 て, 1/24 て, which is the basic theoretical basis for the motor subdivision drive.

It is not difficult to introduce: there is m-phase excitation winding on the stator of the motor, and its axis is offset from the rotor tooth axis by 1/m, 2/m...(m-1)/m,1 respectively. And the conduction of the motor in a certain phase sequence can be controlled by forward and reverse - this is the physical condition of the stepper motor rotation. As long as this condition is met, we can theoretically manufacture stepper motors of any phase. For cost and other considerations, the market generally has two, three, four, and five phases.

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 staggered at a certain angle, force is generated.
F is proportional to (dФ/dθ) S
Its magnetic flux Ф=Br*S
Br is magnetically dense and S is magnetically conductive area
F is proportional to L*D*Br
L is the effective length of the core, and D is the diameter of the rotor.
N•I is the number of excitation windings (current multipliers) R is the reluctance.
Torque = force * radius
Torque is proportional to the effective volume of the motor * ampoule * magnetic density (only linear state is considered)
Therefore, the larger the effective volume of the motor, the larger the number of excitation amps, the smaller the air gap between the stator and rotor, and the greater the motor torque, and vice versa.

(2) Inductive stepper motor

1. Features:

Compared with the traditional reactive stepping motor, the inductive stepper motor has a permanent magnet on the structure to provide the working point of the soft magnetic material, and the stator excitation only needs to provide a changing magnetic field without providing the working point of the magnetic material. The energy consumption is high, the motor has high efficiency, low current and low heat generation. Due to the existence of permanent magnets, the motor has a strong back EMF, and its own damping effect is better, which makes it relatively stable during operation, low noise, and low frequency vibration.
The inductive stepper motor can be regarded as a low-speed synchronous motor to some extent. A four-phase motor can be operated in four phases or in two phases. (Double-pole voltage drive must be used), while reactive motors cannot. For example: four-phase, eight-phase operation (A-AB-B-BC-C-CD-D-DA-A) can completely adopt the two-phase eight-beat operation mode. It is not difficult to find that the condition is C=, D= .
The internal winding of a two-phase motor is exactly the same as that of the four-phase motor. The low-power motor is usually directly connected to two-phase, and the motor with larger power is used to flexibly change the dynamic characteristics of the motor for convenient use. Lead wire (four-phase), when used, can be used as a four-phase motor, and can be used as a two-phase motor winding in series or in parallel.

2, classification

Inductive stepper motors can be divided into two phases: two-phase motor, three-phase motor, four-phase motor, five-phase motor, and so on. The frame number (motor outer diameter) can be divided into: 42BYG (BYG is the induction stepper motor code), 57BYG, 86BYG, 110BYG, (international standard), and like 70BYG, 90BYG, 130BYG are domestic standards.

3. Static index terminology of stepper motor

Phase number: The number of excitation coil pairs that produce different pairs of N and S magnetic fields. Commonly used m.
Number of beats: The number of pulses required to complete a periodic change of the magnetic field or the conductive state is represented by n, or the number of pulses required for the motor to rotate through a pitch angle. Taking a four-phase motor as an example, there is a four-phase four-shot operation mode, that is, AB. -BC-CD-DA-AB, four-phase eight-shot operation mode is A-AB-B-BC-C-CD-D-DA-A.
Step angle: Corresponding to a pulse signal, the angular displacement of the rotor of the motor is indicated by θ. θ=360 degrees (number of rotor teeth J* running beats), taking the conventional two-phase and four-phase, and the rotor teeth as a 50-tooth motor as an example. The step angle of the four-shot operation is θ=360 degrees/(50*4)=1.8 degrees (commonly known as the whole step), and the step angle of the eight-shot operation is θ=360 degrees/(50*8)=0.9 degrees (commonly known as Half step).
Positioning torque: The locking torque of the motor rotor itself when the motor is not energized (caused by the harmonics of the magnetic field tooth shape and mechanical errors)
Static torque: The locking torque of the motor shaft when the motor is not rotating due to the rated static electricity. This torque is a measure of the motor's volume (geometry) and is independent of the drive voltage and drive power.
Although the static torque is proportional to the electromagnetic ampere amperage and is related to the air gap between the fixed tooth rotors, it is not advisable to reduce the air gap excessively and increase the static ampere to increase the static torque, which will cause the motor to Heat and mechanical noise.

4, stepper motor dynamic indicators and terminology:

1, step angle accuracy:

The error between the actual value of the stepping motor and the theoretical value per revolution of the step angle. Expressed as a percentage: error / step angle * 100%. The values of different running beats are different. The four-shot operation should be within 5%, and the eight-shot operation should be within 15%.

2, out of step:
The number of steps in the motor running is not equal to the theoretical number of steps. Call it out of step.

3, misalignment angle:
The axis of the rotor tooth is offset from the axis of the stator tooth. There must be an offset angle in the motor running. The error caused by the misalignment angle cannot be solved by subdivision driving.

4, the maximum no-load starting frequency:

The maximum frequency at which the motor can be started directly without load under certain driving forms, voltages and rated currents.

5, the maximum no-load operating frequency:

The maximum speed of the motor without load under certain driving forms, voltage and rated current.

6, running torque frequency characteristics:

        The curve of the relationship between the output torque and the frequency of the motor under certain test conditions is called the running torque frequency characteristic. This is the most important of the many dynamic curves of the motor and the fundamental basis for the motor selection.
Other characteristics include inertial frequency characteristics, starting frequency characteristics, and the like.
Once the motor is selected, the static torque of the motor is determined, but the dynamic torque is not. The dynamic torque of the motor depends on the average current (not the quiescent current) when the motor is running. The larger the average current, the larger the output torque of the motor, that is, the motor. The harder the frequency characteristics.
Among them, curve 3 has the largest current or the highest voltage; curve 1 has the smallest current or the lowest voltage, and the intersection of the curve and the load is the maximum speed point of the load. In order to make the average current large, the driving voltage is increased as much as possible, so that a motor with a small inductance and a large current is used.

7, the resonance point of the motor:
       The stepping motor has a fixed resonant region, and the resonant region of the two- and four-phase induction substepping motor is generally between 180-250 pps (step angle of 1.8 degrees) or around 400 pps (step angle of 0.9 degrees). The higher the motor drive voltage is, the larger the motor current is, the lighter the load is, the smaller the motor volume is, the smaller the resonance area is, and vice versa, so that the motor output electric moment is large, the step noise and the noise of the whole system are reduced. The working point should be offset from the resonance area.

8, motor forward and reverse control:
When the motor winding energization timing is AB-BC-CD-DA or (), it is forward rotation, and when the energization timing is DA-CA-BC-AB or (), it is reverse rotation.

Third, the drive control system consists of
The control system that uses and controls the stepping motor must consist of a ring pulse, power amplification, etc., and its block diagram is as follows: the control system that uses and controls the stepping motor must consist of a ring pulse, power amplification, etc.

1. Generation of pulse signals.

The pulse signal is generally generated by a single chip microcomputer or a CPU. The duty ratio of a general pulse signal is about 0.3-0.4. The higher the motor speed, the larger the duty ratio.

2, signal distribution
The induction stepper motor is mainly composed of two-phase and four-phase motors. The two-phase motor works in two-phase four-shot and two-phase eight-shot. The specific distribution is as follows: two-phase four-shot, the step angle is 1.8 degrees; Two-phase eight beats, the step angle is 0.9 degrees. There are also two modes of operation for the four-phase motor. The four-phase four-shot is AB-BC-CD-DA-AB, the step angle is 1.8 degrees, and the four-phase eight-shot is AB-B-BC-C-CD-D-AB. (The step angle is 0.9 degrees).

3, power amplification

Power amplification is the most important part of the drive system. The torque of a stepper motor at a certain speed depends on its dynamic average current rather than the quiescent current (while the current on the sample is quiescent current). The larger the average current, the larger the motor torque. To achieve the average current, this requires driving.
The system tries to overcome the back EMF of the motor. Therefore, different driving modes are adopted in different occasions. Up to now, the driving methods generally have the following types: constant voltage, constant voltage string resistance, high and low voltage driving, constant current, and subdivision number.

Once the stepper motor is finalized, its performance depends on the drive power of the motor. The higher the speed of the stepping motor, the larger the force distance is, the higher the current of the motor is required, and the higher the voltage of the driving power source.

4, subdivision driver
Under the condition that the stepping motor step angle can not be used, the subdivision driver can be used to drive the stepping motor. The principle of the subdivision driver is to change the size of the adjacent (A, B) current to change the clip of the synthetic magnetic field. The angle controls the operation of the stepper motor. Fourth, the application of stepper motor

(1) Selection of stepper motor
Stepper motors are composed of three major elements: step angle (involving phase number), static torque, and current. Once the three major factors are determined, the model of the stepper motor is determined.

1, the choice of step angle

The step angle of the motor depends on the accuracy of the load. The minimum resolution (equivalent) of the load is converted to the motor shaft, and the angle of each equivalent motor should be taken (including deceleration). The step angle of the motor should be equal to or less than this angle. At present, the step angle of stepping motors on the market generally ranges from 0.36 degrees/0.72 degrees (five-phase motors), 0.9 degrees/1.8 degrees (two-phase and four-phase motors), and 1.5 degrees/three degrees (three-phase motors).

2, the choice of static torque
The dynamic torque of the stepper motor is difficult to determine at once. We often determine the static torque of the motor first. The static torque selection is based on the load on which the motor operates, and the load can be divided into two types: inertial load and friction load. A single inertia load and a single friction load are not present. When starting directly (generally from low speed), both loads should be considered. When starting acceleration, the inertia load is mainly considered. When the constant speed is running, only the friction load is considered. Under normal circumstances, the static torque should be 2-3 times of the friction load. Once the static torque is selected, the base and length of the motor can be determined (geometry).

3, the choice of current
Motors with the same static torque have different operating characteristics due to different current parameters. The current of the motor can be judged according to the torque frequency characteristic curve (reference drive power and drive voltage).
In summary, the following steps should be followed to select the motor:

4, torque and power conversion
Stepper motors are generally used in a wide range of speeds, and their power is variable. Generally, they are only measured by torque. The torque and power are converted as follows:
P= Ω•M
P is the unit of power in watts, Ω is the angular velocity per second, the unit is radians, n is the speed per minute, M is the unit of torque for Newton meters, P = 2πfM/400 (half-step operation) where f is the number of pulses per second ( Referred to as PPS)

(B), the point of attention in the application


1. Stepper motor is used in low-speed occasions---the speed per minute does not exceed 1000 rpm, (6666PPS at 0.9 degrees), preferably between 1000-3000PPS (0.9 degrees), which can be used to work here through the reduction gear. At this time, the motor has high working efficiency and low noise.
2. It is better not to use the whole step state of the stepping motor, and the vibration is large in the whole step state.
3. For historical reasons, only the motor with the nominal 12V voltage uses 12V. The voltage value of other motors is not the driving voltage volt value. The driving voltage can be selected according to the driver (recommended: 57BYG uses DC 24V-36V, 86BYG uses DC 50V, 110BYG uses 80V higher than DC. Of course, 12V voltage can be used in addition to 12V constant voltage drive. Other drive power can be used, but temperature rise should be considered.
4. The load with large moment of inertia should choose the motor with large frame size.
5. When the motor is at a high speed or a large inertia load, it is generally not started at the working speed, but the gradual up frequency is used to speed up, one motor does not lose the step, and the second can reduce the noise and improve the positioning accuracy of the stop.
6. When it is high precision, it should be solved by mechanical deceleration, increasing the speed of the motor, or by using a driver with high subdivision number. It is also possible to use a 5-phase motor, but the whole system is more expensive, fewer manufacturers, and it is eliminated. The argument is a layman.
7. The motor should not work in the vibration zone. If it must be solved by changing the voltage, current or adding some damping.
8. The motor works below 600PPS (0.9 degrees) and should be driven by small current, large inductance and low voltage.
9. The principle of selecting the motor after selecting the motor should be followed.

Fourth, other instructions
        Low frequency vibration, lifting speed, mechanical resonance, error of working reciprocating motion, plane arc X, Y interpolation error and other problems. The specific solution is not convenient here. The users of our factory can call us and can solve it according to the specific situation. Different manufacturers of motors differ greatly in design, materials and processing techniques. Stepper motors should be used with reliability, light performance, heavy quality and light price.