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Combination of stepper motor and drive circuit
- Aug 08, 2018 -

Stepper motor overview

The stepping motor is a discrete value control motor that converts the electric pulse excitation signal into a corresponding angular displacement or line displacement. This motor is called a pulse motor when it inputs one electric pulse and moves one step.

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The driving power of the stepping motor is composed of a variable frequency pulse signal source, a pulse distributor and a pulse amplifier, whereby the driving power source supplies a pulse current to the motor winding. The running performance of the stepper motor is determined by the good fit between the motor and the drive power.


The advantage of stepping motor is that there is no accumulated error, the structure is simple, the use and maintenance are convenient, the manufacturing cost is low, the stepping motor has the ability to drive the load inertia, and is suitable for small and medium-sized machine tools and places where the speed precision is not high. The disadvantage is that the efficiency is low. It is very hot and sometimes "out of step".


Why does a stepper motor need a drive circuit to work?


Classification of stepper motors

1. Electromechanical stepping motor

The electromechanical stepping motor is composed of a core, a coil, a gear mechanism and the like. When the solenoid coil is energized, a magnetic force is generated to push the core to move, the output shaft is rotated by an angle by a gear mechanism, and the output shaft is maintained at a new working position by the anti-rotation gear; the coil is re-energized, and the shaft is rotated by an angle. Step motion is performed in sequence.


2, magnetic electric stepper motor

The magnetoelectric stepping motor has a simple structure, high reliability, low price and wide application. There are mainly permanent magnet, magnetoresistive and hybrid.


(1) Permanent magnet stepping motor. The rotor has a magnetic pole of a permanent magnet, which generates an alternating magnetic field in the air gap, and the stator is composed of a four-phase winding (see figure). When the Phase A winding is energized, the rotor will turn to the direction of the magnetic field determined by the phase winding. When the A phase is de-energized and the B-phase winding is energized, a new magnetic field direction is generated. At this time, the rotor rotates at an angle and is in the direction of the new magnetic field. The order of the excited phases determines the direction of rotation of the rotor. If the stator excitation changes too fast, the rotor will not be consistent with the change in the direction of the stator field, and the rotor will lose its step. The low starting frequency and operating frequency are a disadvantage of permanent magnet stepping motors. However, the permanent magnet stepping motor consumes less power and has higher efficiency. In the early 1980s, the rotor was a disc-type permanent disk stepping motor, which made the step angle and operating frequency reach the level of the reluctance stepping motor.


(2) Magnetoresistive stepping motor. The inner and outer surfaces of the rotor core are provided with similar tooth grooves distributed according to a certain regularity, and the magnetic pole resistance changes caused by the relative position changes of the fixed and rotor core slots, thereby generating torque. The rotor core is made of silicon steel sheet or soft magnetic material. When a certain phase of the stator is excited, the rotor will be turned to the position where the magnetic resistance of the magnetic circuit is minimized. When the other phase is excited and the rotor is turned to another position to minimize the magnetic reluctance, the motor stops rotating. At this time, the rotor is rotated by a step angle θb, that is, N is the number of running shots of the rotor rotated by one pitch; ZR is the number of teeth of the rotor.


Magnetoresistive stepping motors have many structural forms. The stator core has a single-stage and multi-stage type; the magnetic circuit has radial and axial directions; the number of winding phases has three phases, four phases and five phases. The step resistance angle of the reluctance stepping motor can be 1 ° ~ 15 °, or even smaller, the accuracy is easy to guarantee, the starting and running frequency is higher, but the power consumption is larger and the efficiency is lower.


(3) Hybrid stepping motor. Its fixed and rotor core structure is similar to the reluctance stepping motor. The rotor has permanent magnets that create a unipolar magnetic field in the air gap that is also modulated by the cogging of the soft magnetic material on the rotor.


The hybrid stepping motor has the advantages of both the permanent magnet stepping motor and the reluctance stepping motor. The motor has a small step angle, high precision, high operating frequency, low power consumption and high efficiency.


3, linear stepper motor

There are two types of reaction and Sawyer. The Sawyer linear stepping motor consists of a stationary part (called a reaction plate) and a moving part (called a mover). The reaction plate is made of a soft magnetic material on which teeth and grooves are uniformly formed. The mover of the motor consists of a permanent magnet and two magnetic poles A and B with coils. The mover is supported by an air cushion to eliminate mechanical friction during movement, to make the motor run smoothly and improve positioning accuracy. The motor has a maximum moving speed of 1.5 m/s, an acceleration of 2 g and a positioning accuracy of more than 20 μm. A planar motor is constructed by assembling two Sawyer linear stepping motors perpendicularly to each other. By giving the two motors in the x and y directions (Fig. 3) a different combination of control currents, the motor can be moved in any plane in any geometric path. Large automatic plotters are new devices that combine computers and flat motors. Planar motors can also be used in laser trimming systems with control accuracy and resolution up to tens of microns.


Stepper motor working principle

We use Figure 11.20 to illustrate how this motor works.

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Why does a stepper motor need a drive circuit to work?


The multi-phase excitation winding is mounted on the stator of the reluctance stepping motor, and the schematic diagram of the most commonly used three-phase winding stepping motor is shown in Figure 11.20. The three-phase winding forms six magnetic poles. The rotor is made of soft magnetic material with 4 teeth on it. When the A-phase winding is energized and the B and C-phase windings are not energized, the magnetic path resistance is minimized due to the minimum path of the magnetic flux resisting reluctance, so that the reluctance torque is generated to make the axis of the teeth 1, 3 and the stator A The phase poles are aligned. Powering phase B at the next moment, disconnecting phase A power will align the axes of rotor teeth 2, 4 with the phase B poles, and the rotor thus rotates counterclockwise by 30° overall. Therefore, circulating the three winding wheels in the order of A-B-C-A... will cause the rotor to continuously rotate counterclockwise. If energized in the order of A-C-B-A..., the rotor will rotate clockwise. The following judgment can be obtained:


(1) The direction of rotation of the stepper motor depends on the order in which the windings are energized;


(2) The speed of the motor depends on the frequency of winding on and off;


(3) Each time the winding is switched on, the distance of the corner step is l/m of the angular distance between the rotor teeth, that is, the step angle is l/m of the pitch.


In the above stepping motor model, the step angle of each step is 30°, which is difficult to meet the requirements of fine control. The actual motor is constructed as shown in Figure 11.21. In this structure, there are some small teeth uniformly distributed on the pole arc of the stator pole, and small teeth are evenly distributed on the surface of the rotor. The pitch of the teeth between the small teeth of the rotor is exactly equal to the pitch of the stator. The so-called pitch is the angle between the center lines of two adjacent teeth, also known as the pitch angle DT = 360 ° / Zr DT - pitch; Zr - the number of teeth of the rotor.


Since these small teeth are opened, the rotation of the rotor during winding switching can find a position with the smallest reluctance within a range smaller than DT, which greatly reduces the step angle and improves the resolution of motion.


It is noted from the analysis of Fig. 11.20 that when the teeth of the rotor completely coincide with the teeth of a certain magnetic pole, for the m-phase motor, the teeth of the rotor and the teeth of the other two-phase magnetic poles must be sequentially shifted by 1/m pitch. For a three-phase motor, when the A phase is energized, the small teeth of the rotor and the small teeth on the magnetic poles of the B and C phases must be shifted by DT/3. Under this constraint, the number of teeth of the rotor cannot be any value, but the following conditions must be met:


Zr/2p=K±1/m ie Zr=2p(K±1/m)=2pK±2


Where K-positive integer; p-pole logarithm; m-phase number, p=m.


Hybrid stepper motor works

The motor stator in the figure has four circumferentially evenly distributed teeth which are wound on the teeth and connected in pairs. Two-stage rotors with different polarities each have three teeth. In the figure, the S segment is indicated by a solid line, the N segment is indicated by a broken line, and the two segments are staggered by a half pitch.


When there is no current in the windings, because the permanent magnets in the rotor always try to reduce the reluctance in the magnetic circuit, the rotor will tend to a limited number of positions until one of the N and S pole rotors is aligned with the stator poles. For the motor in the figure, there are 12 such positions. The torque to hold the rotor in these positions is usually not large and is referred to as the holding torque.


Why does a stepper motor need a drive circuit to work?

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If there is current through a phase winding as shown in Figure 11.28(a), the N and S poles produced on the stator will attract the teeth on the opposite rotor segment, in this case only the same number of teeth as the rotor. For a stable position, the torque to pull the rotor away from the positioning position is much greater, called the locking torque.


Switch the power-on mode from (a) to (b) and the stator field to 90. And will attract another pair of teeth, and as a result the rotor rotates by 30. Is equivalent to a whole step. In Figure (b) to Figure (c), the excitation returns to the previous winding, but the current is reversed, allowing the rotor to advance a further step. Reversing the current of the second phase winding in Figure (d) can be advanced. This way the rotor has passed a pitch. The steps from Figure (d) to Figure (a), and so on, form the rotational motion of the motor, requiring 12 steps per revolution. Obviously, the stator windings are excited in reverse order and the motor will reverse.


Usually, the small teeth of the stator are evenly distributed at a different tooth pitch than the rotor. In a motor with a large number of teeth (Fig. 11.27), the pitch of the stator and the rotor are arranged such that only two teeth opposite the rotor are spaced apart from each other by 180. . The stator teeth are perfectly aligned. At the same time, the distance is 90. The stator teeth at the mechanical corner are completely staggered. For a hybrid motor of such a structure, the number of steps per revolution can be calculated by the following formula: N=┃NrNs/(Ns-Nr)┃


Where N is the number of steps per revolution; Nr and Ns are the number of teeth of the rotor and the stator, respectively. For the example in Figure 11.27, where Nr and Ns are 8 and 10, respectively, the motor can be calculated for 40 steps per revolution with a step angle of 9.


Why does a stepper motor need a drive circuit to work?

The stepping motor is designed for precise displacement. In order to achieve higher precision, the efficiency is inevitably lower than that of the DC motor. And the stepping motor is to generate the pulse by a single chip to control the torque. The driving current of the single chip itself is small, the motor winding cannot be driven, and the driving circuit is used to generate a large current, and the direct driving will burn the single chip.



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