Working Principle and Driving Method of stepper motor

From: http://www.eepw.com.cn/article/73889.htm

Stepper motor is an electromechanical component that converts an electrical pulse signal into an angle displacement or linear displacement. The input value of the stepper motor is a pulse sequence, and the output value is the corresponding incremental displacement or step motion. Under normal motion, it has a fixed number of steps each week. When performing Continuous walking, its rotation speed is strictly in line with the frequency of the input pulse, it is not affected by voltage fluctuations and load changes. Because the stepper motor can directly accept the control of the number, it is particularly suitable to use a microcomputer for control.

1. Type of stepper motor

Currently, three stepping motors are commonly used:

(1) Reactive stepper motor (VR ). The reactive stepping motor has simple structure, low production cost, and small step angle, but poor dynamic performance.

(2) permanent magnet stepping motor (PM ). The permanent magnet stepping motor has a large output and good dynamic performance, but its stride angle is large.

(3) Hybrid Stepping Motor (HB ). Hybrid Stepping Motor integrates the advantages of reactive and permanent magnet stepping motor. It has a small step angle, large output, and good dynamic performance. It is the Stepping Motor with the highest performance. It is also called Permanent Magnet induction substepping motor.

2. Working Principle of stepper motor

Figure 1 structure of three-phase Reactive Stepping Motor

1 -- stator 2 -- rotor 3 -- Stator Winding {pagination }}

Figure 1 is the section of the most common three-phase Reactive stepping motor. The stator of the motor has six evenly distributed magnetic poles with an angle of 60 °. A coil is mounted on each pole, and A, B, and C three-phase winding is connected according to Figure 1. The rotor is equipped with 40 small teeth. Therefore, the tooth distance of each tooth is θ E = 360 °/40 = 9 °, and the pole arc of each pole of the stator also has 5 small teeth, the pitch and width of the stator and rotor are the same. Because the number of teeth in the stator and rotor is 30 and 40 respectively, the ratio is a fraction, which leads to the so-called tooth dislocation. If the tooth is aligned with the tooth of the-phase pole and the tooth of the rotor, the tooth distance between the tooth of the B-phase and the C-phase pole is 3 °. Therefore, the reluctance of the B and C poles is larger than that of the pole. If B is connected to electricity, the B-phase winding generates the stator magnetic field, and its magnetic line crosses the B-phase magnetic pole, and tries to close it according to the path with the smallest reluctance, which causes the rotor to receive the reactive torque (Reluctance torque) until the teeth on the B pole are aligned with the rotor teeth, the rotor turns 3 °. At this time, the teeth under the and C pole are staggered with the rotor teeth by 1/3. Then, the B-phase winding is powered on, and the C-phase winding is powered on. Similarly, the rotor turns 3 ° clockwise due to the reaction torque. And so on, when the three-phase winding cyclically powers up in the order of A → B → C → A, the rotor will rotate in a clockwise direction with a 3 ° cycle of each power-on pulse. If the order of power-on is changed and the power-on is cyclically powered on in the order of A → C → B → A, the rotor rotates 3 ° in a clockwise direction with each power-on pulse. Because each moment only one phase winding is powered on, and the cycle is powered on by three power-on states, it is called a single three-shot operation mode. The step moment angle θ B is 30 ° during Single-three-shot operation. There are two power-on modes for the three-phase stepper motor, namely, the power-on by AB → BC → ca → AB sequential cycle, and the single and double six-shot operation, that is, A → AB → B → BC → C → ca → A is used for cyclic power-on. The moment angle of the six beats will be reduced by half. The step angle of the reactive stepping motor can be calculated as follows:

θ B = 360 °/NER (1)

Type er-Number of rotor teeth;

N -- number of running beats, n = km, M is the number of winding phases of the stepping motor, k = 1 or 2.

3. Driving Method of stepper motor

The stepper motor cannot directly connect to the Power Frequency AC or DC power supply, but must use a dedicated stepper motor driver, as shown in figure 2, it consists of a pulse generation control unit, a power driving unit, and a protection unit. The two units enclosed by the dot strip in the figure can be controlled by a microcomputer. The driving unit is directly coupled with the stepper motor. It can also be understood as the power interface of the stepper motor microcomputer controller.

Figure 2 stepper motor driver Controller

1. Single-voltage power drive interface

Shows circuit 3. In the motor winding loop, the resistance Rs is stringed to reduce the time constant of the motor loop. at high frequencies, the motor can produce large electromagnetic torque and relieve the low-frequency resonance of the motor, but it causes additional losses. In general, RS is indispensable in a simple single-Voltage Drive Line. RS improves the single-step response of stepper motor 3 (B ). {Pagination }}

Figure 3 single-voltage power driving interface and one-step response curve

Figure 4 dual-voltage power driving Interface

2. Dual-voltage power drive interface

The dual-voltage driving power interface 4 is shown. The basic idea of Dual-voltage driving is to use a lower voltage ul to drive at a lower (low frequency segment), while a higher voltage uh to drive at a high speed (high frequency segment. This power interface requires two control signals, namely, the high-voltage effective control signal, and the U is the pulse width-adjusted driving control signal. In the figure, Power Transistor th and diode DL constitute a power conversion circuit. When the uh low level, th off, DL positive offset, low voltage ul power supply to the winding. On the contrary, the High Level, th turn-on, DL reverse bias, and high voltage er power supply to the winding. This type of circuit enables the motor to have greater output in high frequency bands, while the timing power consumption of static locks is reduced.

3. High and low voltage power driving Interface

Figure 5 high/low voltage power driving Interface

The high and low voltage power driving interfaces are shown in figure 5. The design idea of high-voltage and low-voltage drives is that, regardless of the operating frequency of the motor, high-voltage uh power supply is used to improve the current frontier of the On-phase winding, use a low voltage ul to maintain the current of the winding. This function also improves the high-frequency performance of the drive and eliminates the need to concatenate the RS resistance to eliminate additional losses. The high-voltage and low-voltage drive power interfaces also have two input control signals, namely, uh and ul. They must be synchronized and the front-end switches at the same time, as shown in Figure 5. In the figure, the turn-on time TL of the high-voltage tube Vth cannot be too large or too small. When it is too large, the motor current is overloaded. When it is too small, the dynamic performance is not significantly improved. Generally, 1 ~ is recommended ~ 3 ms. (This value is appropriate when it is consistent with the electrical time constant of the motor ). {Pagination }}

4. Chopper Constant Current Power Driving Interface

The design idea of constant current driving is to keep the current of the conduction phase winding in a fixed value regardless of locking, low frequency, and high frequency. Enables the motor to have constant torque output. This is a kind of power interface that is widely used and has better performance. Figure 6 shows the schematic diagram of the chopper Constant Current Power interface. In the figure, R is a small resistance value used for current sampling. It is called a sampling resistance. When the current is not large, both vt1 and VT2 are controlled by the step pulse. When the current exceeds the constant current value, VT2 is blocked and the power U is removed. Because the motor winding has a large inductance, at this time, it relies on the diode VD to continue the current, to maintain the winding current, the motor relies on the magnetic field energy consumed in the inductance to generate output. In this case, the current will decrease according to the exponential curve, and the same current sample value will decrease. When the current is less than the value given by the constant current, VT2 is turned on and the power supply is switched on again. So repeatedly, the motor winding current is stable on the value determined by the given level, forming a small jagged wave, as shown in figure 6.

Figure 6 chopper Constant Current Power Driving Interface

The chopper Constant Current Power Driving interface also has two input control signals, in which U1 is a digital pulse and U2 is a analog signal. The features of this power interface are: high-frequency response is greatly improved, close to the constant torque output characteristics, and the resonance phenomenon is eliminated, but the line is complicated. Currently, corresponding integrated power modules are available.

5. Boost Power Driver Interface

In order to further improve the high-frequency response of the driver system, you can use the boost-Boost Power Driver Interface. This interface is linearly related to the voltage provided by the winding and the operating frequency of the motor. Its main circuit is actually a switching regulated power supply. The frequency-voltage converter is used to convert the frequency of the drive pulse to a DC level and use this level to control the input of the switching regulated power supply, this constitutes a power driving interface with frequency feedback.

6. Integrated Power Driver Interface

Currently, multiple integrated power driving Interface Circuits for Low-Power stepper motors are available.

The L298 chip is an H-bridge driver designed to receive standard TTL logic-level signals and can be used to drive inductive loads. The H-bridge can withstand a voltage of 46v and the phase current is up to 2.5a. The logic circuit of L298 (or xq298, sgs298) uses 5 V power supply, and the power amplifier uses 5 ~ The 46v voltage and the lower bridge emitter pole are extracted separately, so that the current sampling resistance can be connected. L298 (etc.) adopts a 15-pin double-row direct-insertion small wattage package and industrial product grade. Its internal structure is 7. The main feature of the H-bridge drive is that it can power the motor winding in both positive and reverse directions. The L298 is particularly suitable for the two-phase or four-phase stepper motor. {Pagination }}

Figure 7 L298 schematic diagram

Similar to L298, there is also TER's 3717 circuit, which is a single H-bridge circuit. SGS sg3635 is a single-Bridge Arm circuit, IR IR2130 is a three-phase bridge circuit, Allegro has a2916, a3953 and other small power drive modules.

Figure 8 is a stepper motor driving system with a constant current chopper function, which consists of L297 (dedicated ring distributor chip) and L298.

Figure 8 stepping electric driving system composed of dedicated chips