Working principle of stepping motor

1. Stepping motor is an open-loop control element that converts electric pulse signal into angular displacement or linear displacement. In the case of non-overload, the rotation speed and stop position of the motor only depend on the frequency and number of pulses of the pulse signal, which is not affected by the load change, that is, the motor turns a step angle by adding a pulse signal. The existence of this linear relationship, plus the characteristics of stepping motor, such as only periodic error and no accumulated error. It is very simple to control speed and position with stepping motor. Although stepping motors have been widely used, they cannot be used in daily work like ordinary DC motors and AC motors. It must be used by a control system consisting of a double-loop pulse signal and a power drive circuit. Therefore, it is not easy to use stepping motor well, which involves many professional knowledge such as machinery, motor, electronics and computer. At present, there are indeed many manufacturers of stepping motors, but few manufacturers with professional and technical personnel can independently develop them. Most of them have only ten or twenty people, and they don't even have the most basic equipment. Just in the stage of blind imitation. This has caused many troubles for users in product selection and use. In view of the above situation, we decided to take the widely used induction stepping motor as an example. Describe its basic working principle. I hope it can be helpful to the majority of users in the selection, use and improvement of the whole machine.

Second, the working principle of induction stepping motor

(1) principle of reactive stepping motor Because the working principle of reactive stepping motor is relatively simple. The principle of three-phase reactive stepping motor is introduced below.

1. structure: there are many small teeth evenly distributed on the motor rotor, and the stator teeth have three excitation windings, whose geometric axes are staggered with the rotor tooth axes in turn. 0, 1/3て, 2/3て (the distance between two adjacent rotor gear shafts is tooth pitch, denoted by て), that is, A is aligned with the tooth 1, B is staggered to the right 1/3て, and C is staggered to the right by 2/.

2. Rotation: When phase A is energized and phase B and phase C are de-energized, tooth 1 is aligned with a due to the magnetic field (also applicable to rotors without any force). If phase B is energized and phases A and C are de-energized, tooth 2 should be aligned with B.. At this time, the rotor moves to the right through 1/3て. At this time, teeth 3 and C are offset by 1/3て, and teeth 4 and A are offset (て- 1/3 て). If phase C is energized, phase A and phase B are not. At this time, the rotor moves to the right by 1/3て, and the offset of teeth 4 and A is 1/3て. If phase A is energized, phase B and phase C are de-energized, teeth 4 are aligned with A, and the rotor moves to the right by 1/3て. In this way, a, b, If you press a, b, c continuously, if you press a, c, b, a ... in order to power on, the motor will reverse. Through the conduction times (pulse number) and frequency, we can see that the position and speed of the motor are in one-to-one correspondence. And the direction is determined by the conduction order. However, in order to reduce torque, stability, noise and angle. The conductive state of A-AB-B-BC-C-CA-A is often adopted, and the original step size is changed from 1/3 to 1/6. Even through different combinations of two-phase currents, it changes from 1/3て to112 て and 1/24て, which is the basic theoretical basis of motor subdivision drive. It is not difficult to deduce that there is an M-phase excitation winding on the motor stator, and the offset between its axis and the rotor tooth axis is 1/m, 2/m … (m- 1)/m, 1 respectively. And the conduction can be controlled by the positive and negative rotation of the motor according to a certain phase sequence-this is the physical condition for the rotation of the stepping motor. As long as this condition is met, it is theoretically possible to manufacture stepping motors of any phase. Considering the cost, there are generally two phases, three phases, four phases and five phases in the market.

3. Torque: Once the motor is energized, a magnetic field (magnetic flux Ф) will be generated between the stator. When the rotor and stator are staggered by a certain angle, the force f is proportional to (dФ/dθ).

Magnetic flux Ф = br * sbr is magnetic density, s is magnetic permeability area f is proportional to L*D*Br, l is effective length of iron core, d is rotor diameter br = n I/rn I is ampere turns of excitation winding (current multiplied by turns), and r is reluctance. Torque = Force * Radius Torque is proportional to the effective volume of the motor * ampere turns * magnetic density (only linear state is considered). Therefore, the greater the effective volume of the motor, the greater the excitation ampere-turns, the smaller the air gap between stator and rotor, and the greater the motor torque, and vice versa.

(2) Induction stepping motor

1. Features: Compared with the traditional reactive stepping motor, the rotor of the induction stepping motor is structurally provided with permanent magnets to provide the working point of soft magnetic materials, while the stator excitation only needs to provide the changing magnetic field without providing the energy consumption of the working point of magnetic materials, so the motor has high efficiency, low current and low heat generation. Because of the existence of permanent magnets, the motor has a strong back electromotive force, better damping effect, more stable operation, low noise and small low-frequency vibration. Inductor stepping motor can be regarded as a low-speed synchronous motor to some extent. A four-phase motor can run in four or two phases. (it must be driven by bipolar voltage), but reactive motor can't. For example, four-phase and eight-phase operation (A-AB-B-BC-C-CD-D-DA-A) can completely adopt two-phase and eight-beat operation mode. It is not difficult to find that its conditions are C= and D=. The internal winding of a two-phase motor is exactly the same as that of a four-phase motor. Generally, a low-power motor is directly connected to two phases, while a high-power motor can flexibly change the motor dynamics and is convenient to use.

2. Classification Induction stepping motors can be divided into two-phase motors, three-phase motors, four-phase motors and five-phase motors. According to the number of frames (motor outer diameter), it is divided into: 42BYG(BYG is the code name of induction stepping motor), 57BYG, 86BYG, 1 10BYG (international standard), while 70BYG, 90BYG and 130BYG are all domestic standards.

3. Phase number of the static index item of the stepping motor: the number of excitation coils that generate N and S magnetic fields with different poles. Often expressed in m, beats: The number of pulses required to complete the periodic change of a magnetic field or conductive state is expressed in n, or refers to the number of pulses required for the motor to rotate by a pitch angle. Take a four-phase motor as an example, there are four-phase four-beat operation modes, namely AB-BC-CD-DA-AB, and four-phase eight-beat operation modes, namely A-AB-B-BC-C-CD-D-DA-A, with one pitch angle. θ=360 degrees (the number of rotor teeth J* the number of running beats), taking the conventional two-phase and four-phase motors with 50 rotor teeth as an example. The step angle θ of four beats = 360 degrees /(50*4)= 1.8 degrees (commonly known as the whole step), and the step angle θ of eight beats = 360 degrees /(50*8)=0.9 degrees (commonly known as the half step). Positioning torque: the locking torque of the motor rotor itself when the motor is not energized (caused by magnetic field tooth profile harmonics and mechanical errors); Static torque: the locking torque of the motor shaft when the motor does not rotate under rated static electricity. This torque is a standard to measure the volume (geometric size) of the motor, which has nothing to do with the driving voltage and power supply. Although the static torque is proportional to the ampere-turns of electromagnetic excitation and related to the air gap between stator and rotor, it is not advisable to excessively reduce the air gap and increase the ampere-turns of excitation to improve the static torque, which will cause motor heating and mechanical noise.

4, stepper motor dynamic indicator and terminology:

1, step angle accuracy: the error between the actual value and the theoretical value of each step angle of stepping motor. Expressed as a percentage: error/step angle * 100%. Different running beats have different values, with four beats within 5% and eight beats within 15%.

2. Out-of-step: The number of steps when the motor is running is not equal to the theoretical number. Call it out of step.

3. Misalignment angle: The angle at which the rotor tooth axis deviates from the stator tooth axis, there must be a misalignment angle in the operation of the motor, and the error caused by the misalignment angle cannot be solved by subdivision driving.

4. Maximum no-load starting frequency: the maximum frequency at which the motor can be started directly without load under a certain driving mode, voltage and rated current.

5. Maximum no-load operating frequency: the highest speed frequency of the motor under a certain driving form, voltage and rated current.

6. Operating torque-frequency characteristics: The relationship curve between output torque and frequency measured by the motor under certain test conditions is called operating torque-frequency characteristics, which is the most important among many dynamic curves of the motor and the fundamental basis for motor selection. As shown in the following figure

Other characteristics include inertia frequency characteristics, starting frequency characteristics and so on. 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 static current) when the motor is running. The greater the average current, the greater the output torque of the motor, that is, the harder the frequency characteristics of the motor. As shown in the figure below:

Among them, curve 3 has the highest current or voltage; The current or voltage of curve 1 is the lowest, and the intersection of curve and load is the maximum speed point of load. To make the average current large and the driving voltage as high as possible, use a motor with small inductance and large current.

7, motor * * * vibration point:

The stepping motor has a fixed vibration area. Generally, the * * vibration region of two-phase and four-phase induction stepping motors is between 180-250pps (step angle 1.8 degrees) or about 400pps (step angle 0.9 degrees). The higher the motor driving voltage, the greater the motor current, the lighter the load and the smaller the motor volume. 8. Motor forward and reverse rotation control: the motor winding rotates forward when energized with AB-BC-CD-DA or (), and reverses when the energizing sequence is DA-CA-BC-AB or (). Three. Drive control system The control system for using and controlling the stepping motor must be composed of annular pulse and power amplification, and its block diagram is as follows:

1, generation of pulse signal.

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

2. Signal distributor (also called pulse distributor)

Induction stepping motor is mainly composed of two-phase and four-phase motors. The two-phase motor has two working modes: two-phase four-beat and two-phase eight-beat, and the specific distribution is as follows: two-phase four-beat, with a step angle of 65438 0.8 degrees; Two-phase eight beats, with a step angle of 0.9 degrees. Four-phase motor also has two working modes. Four phases and four beats are AB-BC-CD-DA-AB, and the step angle is 1.8 degrees. The four-phase eight beat 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 stepping motor at a certain speed depends on its dynamic average current rather than static current (and the current on the sample is static current). The greater the average current, the greater the motor torque. In order to achieve the average current, the drive system needs to overcome the back electromotive force of the motor as much as possible. Therefore, different driving modes are adopted in different occasions. Up to now, there are generally the following driving modes: constant voltage, constant voltage series resistance, high and low voltage driving, constant current, subdivision number and so on. In order to improve the dynamic performance of the motor as much as possible, signal distribution and power amplification constitute the driving power supply of the stepping motor. The wiring diagram of SH series two-phase constant current chopper drive power supply, single chip microcomputer and motor produced by our factory is as follows:

Description:

CP is connected to CPU pulse signal (negative signal, active low)

Optoelectronic connection to CPU+5V.

Free off-line, connected to CPU ground wire, drive power supply does not work DIR direction control, connected to CPU ground wire, motor reversal.

Positive terminal of VCC DC power supply

Gndc power supply negative terminal

A. Connect the red wire of the motor lead to the green wire of the motor lead.

B connect the yellow wire of the motor lead to the blue wire of the motor lead.

Once the stepping motor is finalized, its performance depends on the driving power supply of the motor. The higher the speed and torque of the stepping motor, the greater the current of the motor and the higher the voltage of the driving power supply. The influence of voltage on torque is as follows: