directory
1, the concept of stepping motor
2. Characteristics of stepper motor
3. Classification of stepper motors
4. Working principle of stepper motor
5. Operation mode of stepper motor
6, motor control drive
1.Step motor concept
Stepper motor is an open loop control motor that converts electric pulse signal into angular displacement or linear displacement, also known as pulse motor. In the case of non-overload, the motor speed, stop position only depends on the pulse signal frequency and pulse number, and is not affected by load changes. When the stepper driver receives a pulse signal, it can drive the stepper motor to turn a fixed Angle in a set direction, called the “step Angle.”
The rotation of the stepper motor is operated step by step at a fixed Angle. The angular displacement can be controlled by controlling the number of pulses, so as to achieve the purpose of accurate positioning. At the same time, the speed and acceleration of the motor can be controlled by controlling the pulse frequency, so as to achieve the purpose of speed regulation. Stepper motors are mostly used in the external devices of digital computers, as well as printers, plotters and disks. For specific use, please refer to the following demo video:
2,Characteristics of stepper motor
Stepper motor working position and speed signal does not feedback to the control system, if the motor working position and speed signal feedback to the control system, then it belongs to the servo motor. Compared with servo motor, stepper motor control is relatively simple, but not suitable for high precision occasions.
The advantages and disadvantages of the stepper motor are very prominent, the advantages focus on simple control, high precision, the disadvantages are noise, vibration and efficiency, it has no cumulative error, simple structure, convenient maintenance, low manufacturing cost. Stepper motor drive load inertia capacity is large, suitable for small and medium-sized machine tools and speed accuracy requirements are not high, the disadvantage is low efficiency, high fever, sometimes “out of step”. The advantages and disadvantages are shown below.
Advantages:
1. Motor operation is easy to control through pulse signal input to motor;
2. There is no need for feedback circuit to return the position and speed information of rotation axis (open loop control);
3. Greater reliability achieved by not touching the brush.
Disadvantages:
1. Need pulse signal output circuit;
2. If the control is inappropriate, the synchronization may be lost.
3. Heat is generated because the current is still present after the rotation axis stops.
3. Classification of stepper motors
Under the condition of the same current and the same torque output, the unipolar stepper motor has twice more coils than the bipolar stepper motor, the cost is higher, and the structure of the control circuit is different. At present, the popular market is mostly bipolar stepper motor.
Stepper motors are usually classified in accordance with rotor characteristics and stator windings. The following two types of classification will be introduced in detail.
According to rotor classification, there are three main types: reactive type (VR), permanent magnet type (PM), hybrid type (HB).
reactive
The stator has windings made of soft magnetic materials. Its structure is simple, the cost is low, the step Angle is small, up to 1.2 degrees, but the dynamic performance is poor, the efficiency is low, the heat is high, the reliability is difficult to guarantee.
Permanent magnet
The rotor of permanent magnet stepper motor is made of permanent magnet material, and the number of poles of the rotor is the same as that of the stator. It is characterized by good dynamic performance and large output torque, but the motor degree is poor and the step Angle is large (generally 7.5 degrees or 15 degrees).
The hybrid
The hybrid stepper motor combines the advantages of reactive type and permanent magnet type. There are many phase windings on the stator and permanent magnet material on the rotor. Both the rotor and stator have several small teeth to improve the step accuracy. It is characterized by large output torque, good dynamic performance and small step Angle, but complex structure and relatively high cost.
Step motor according to the stator windings to classification, a total of two phase, three phase and five equal series. At present, the most popular is the two-phase hybrid stepper motor, accounting for more than 97% of the market share, its reason is cost-effective, with a good effect after the subdivision of the drive.
The basic step Angle of the motor is 1.8 degrees/step, and the step Angle is reduced to 0.9 degrees with the half-step driver, and the subdivision driver. The step Angle can be subdivided up to 256 times (0.007 degrees/micro step). Due to friction and manufacturing accuracy, the actual control accuracy is slightly lower. The same step motor can be equipped with different subdivision of the driver to change the accuracy of the effect.
4. Working principle of stepper motor
Stepper motor is controlled by pulse signal, each input a pulse signal, the stepper motor further. Step Angle of stepper motor rotation is produced by proportional control on the basis of motor structure. If the subdivision control of control circuit remains unchanged, the step Angle of stepper motor rotation is a fixed Angle in theory. In practical work, the step Angle of motor rotation will be slightly different, mainly due to the fixed error of motor structure, and this error will not accumulate.
The greater the total number of poles of the stepping motor, the higher the machining accuracy will be required. Generally, the step Angle of industrial hybrid stepper motor is 1.8 degrees, that is, 200 poles.
Phase current and magnetic field of stepper motor, follow ampere right hand helix law, generate magnetic field energy by electric energy, control motor phase current, can make the magnetic pole direction of motor stator reverse, the change of two-phase magnetic field matches, and then produce the rotation of motor.
If the direction of the current changes, the direction of the magnetic poles will also change, the current of the stepper motor through the stator to generate a magnetic field process is called excitation.
Usually referred to as a two-phase stepper motor, the rotation of the rotor of the motor, containing the magnetic field of different magnetic poles repulsion and suction to achieve. As shown in the figure below, phase A generates an N-pole magnetic field to attract the s-pole of the rotor, while phase B generates an S-pole magnetic field to attract the N-pole of the rotor, making the stator generate A power to rotate. If you change the direction of current in the A – and B-phase stator coils, another rotation will occur. Continuous rotation will be produced by changing the current direction of the stator coils of phase A and phase B.
The motion of the motor is achieved by changing the flow of current through the motor. The electronic rotor repels the stator of the B phase magnetic pole and attracts the stator of the A phase magnetic pole. This produces another step operation, as shown in the figure below:
To perform another step operation, the stator magnetic poles of the motor are reversed, and the rotor repels the stator with B phase magnetic poles and attracts the stator with A phase magnetic poles, as shown in the figure below:
Whenever the direction of the current in the stator coil changes, the magnetic poles will reverse and the rotor repeats the step operation. Toshiba stepper motor drive control circuit to the motor magnetic field excitation control, is through the pulse width modulation to achieve high efficiency, stable operation of the motor. As shown below:
5,Operation mode of stepper motor
The basic operation mode of stepper motor is called “excitation mode”, which can make the stepper motor work in full-step mode, half-step mode and micro-step mode. The micro-step mode can effectively reduce the noise of the phase current of the stepper motor and improve the inherent noise vibration problem of the stepper motor. Three excitation modes are described below.
All step model
The so-called full step mode is based on the inherent structure of the motor design fixed step Angle work, an electric pulse, the stepping motor forward a step Angle. This step Angle is determined by the design structure of the motor, which can also be understood as the motor rotates at the maximum step Angle.
Half step mode
The half-step mode is a step rotation at half of the step Angle determined by the inherent structure of the motor. As shown in the figure below, the total number of poles of the stepping motor is 4, and the corresponding step Angle is 90 degrees. Then, in the half-step mode, each pulse of the stepping motor rotates by 45 degrees.
Micro step model
The micro step mode is similar to the half-step mode, with smaller step Angle, i.e. 1/4 step, 1/8 step, 1/16 step, which can be very high subdivision. The corresponding step Angle is the full step Angle multiplied by the micro step coefficient.
The smaller the step Angle of the stepper motor is, the higher the machining accuracy will be required, and the greater the error of the corresponding stepping Angle will be.
6, motor control drive
Stepper motor can not be directly connected to the industrial frequency AC or DC power supply work, and must use a special stepper motor driver, it has pulse generation control unit, power drive unit, protection unit and so on. As shown below:
The drive unit is directly coupled with the stepper motor, which can also be understood as the power interface of the stepper motor microcomputer controller. Examples of systems using MCU and separated components are shown below. MCU is equivalent to the brain controlling the motor. It sends the step Angle time, rotation direction and repetition times of the motor to discrete devices. Discrete devices amplify voltage and current and send them to the motor according to the signal sent by MCU, so as to drive the motor to rotate.
As shown in the figure below, the system uses MCU and motor control driver IC. From the input control signal to distinguish, stepper motor controller IC can be divided into phase force type and clock force type. The input force type means that the current direction of each excitation phase of the motor is controlled by the input signal, while the clock input force type means that the drive of the motor is controlled by the pulse signal.
Phase type into force
Input force motor drivers require A and B two-phase control signals, only clock signals, need to control the signal MCU to do more transport work.
Clock into force type
The control interface of clock-in force motor driver needs clock signal (single pulse signal) input, its control signal is relatively simple, and the RESOURCE occupation of MCU is less.
Power-on reset function (POR)
The power-on reset function will monitor the motor drive and the power supply to the motor drive controller. To prevent motor operation failure, it will force off the output signal until the supply voltage remains stable. As shown below:
Overcurrent detection function (ISD)
Overcurrent off function will monitor the current of the output unit, if the current exceeds the specified value, will force off the output, the purpose of this function is to temporarily stop the IC output in case of short circuit. As shown below:
Thermal turn-off function (TSD)
The thermal turn-off function is to turn off the output when the temperature of the motor control driver chip exceeds the specified value and keep this state until the temperature drops.
Source of this article: Toshiba Semiconductor
Extended learning: The difference between brushless motor and brushless motor