How to use bipolar stepper motor to achieve "microstepping"

In today's intelligent era, stepper motors are used in all walks of life due to their unique open-loop position control performance. During the rotation of the stepper motor, each device has certain requirements for the smoothness of its output torque.

For a PTZ camera, the stability of the rotation will affect the quality of the captured image. This performance is not only related to the process of the stepper motor itself, but also closely related to the control method of the stepper motor.

What we are going to introduce today is one of the control methods - segmentation control.

So how can stepper motors be trained to perform micro-control to improve their skills?

PART1 What is a bipolar stepper motor?

A long time ago, there was a motor in the motor industry called a bipolar stepper motor (from the perspective of motor control). In addition, it is also called a two-phase four-wire stepper motor (from the perspective of the motor itself). It is naturally unique and has a lot of secrets.

01 Stator

The eight stators are respectively wound with two-phase bipolar windings (see Figure 1). The A-phase winding is wound starting from stator 1 and then wound around stators 3, 5, and 7 in sequence. It is worth noting that the winding directions of stators 1 and 5 are the same, and the winding directions of stators 3 and 7 are the same, and the two groups of windings have opposite directions (see Figure 2).

The B phase winding is also wound using the same principle. Each stator core has five teeth.

Figure 1: Schematic diagram of bipolar stepper motor structure

Figure 2: Polarity stepper motor winding schematic

02Rotor

Usually, an axially magnetized permanent magnet is attached to the rotor (see Figure 3). The magnetic lines of force of the permanent magnet are closed inside the motor body. Due to the magnetic lines of force and magnetic resistance effect, the stepper motor will have a certain locking torque even when it is not powered (see Figure 4).

The rotor has 50 teeth, and relative to the stator gear, due to the number of teeth and phase structure, it has a step angle of 1.8 degrees (see Figure 5). Step angle: The mechanical angle that the stepper motor rotor advances when the electrical cycle completes 90 degrees.

Figure 3: Schematic diagram of rotor structure

Figure 4: Side cross-sectional view

Figure 5: Schematic diagram of 1.8 degree step angle structure

PART2 Stepping mode of bipolar stepper motor

In order to facilitate the explanation of the subsequent control method, the complex structure diagram is turned into a schematic diagram (see Figure 6). It can be considered that the stator and rotor of the stepper motor have only one tooth. It makes the drive different from other motors and is called a dual full-bridge drive. Its A-phase winding is connected to the first full-bridge drive, and the B-phase winding is connected to the second full-bridge drive (see Figure 7).

Figure 6: Simplified schematic of a bipolar stepper motor

Figure 7: Dual full-bridge drive circuit diagram

It is still unknown in the motor industry as it does not know how to do detailed control. In order to make a name for itself in the motor industry, it practices hard and studies different control methods, hoping that its rotational performance can gain a place in the industry in the future.

So far, it has mastered three control methods, namely single-phase stepping, full-step stepping, and half-step stepping. These three methods make it very powerful. Let's take a closer look.

Table 1: Step mode table

01The first move: single-phase stepping

When phase A and phase B are energized in sequence according to the single-phase stepping mode, the stator magnetic field will change accordingly, and the rotor will rotate due to polar attraction. The AB phase energization sequence and the rotor rotation position can be seen in Table 1. Of course, the best effect is to watch the video!

Briefly describe the following single-phase stepping process:

When A is energized, the driving current flows from Q1 to Q4. At this time, the upper end of stator A is N, the lower end is S, and the rotor rotates to position 8.

Next, phase B is energized. When phase B is energized, the drive current flows from Q5 to Q8. At this time, the left end of stator B is S, the right end is N, and the rotor rotates to position 2.

The principles of the next two states are similar to the above, so I will not elaborate on them. By cycling this power-on sequence, the rotor starts to rotate.

Figure 8: Single-phase stepping AB phase current waveform

02 The second trick: whole step forward

Unlike single-phase stepper, the whole stepper AB winding will be energized at the same time. And there are also four corresponding energization modes and rotor electrical positions, but the position space is different from the single-phase stepper in electrical space. According to the energization sequence of the whole part, the rotor can also rotate. I won't go into details about the specific process. If you want a vivid effect, remember to watch the video!

Figure 9: Full-step AB phase current waveform

03The third move: half-step

Readers who have carefully read Table 1 must have discovered the secret of stepping motor steps!

The final trick is to combine the above two control methods to get half-step control, which has more electrical angle positions, more detailed current waveform and smoother rotation.

However, these three skills are far from enough for the stepper motor to make a name for itself in the motor industry where there are so many experts, so it needs to continue to practice hard.

So, what exactly is segment control?

How does subdivision control control the dual full-bridge drive?

In this issue of Power Supply Class, let us take a look at the following:

Subdivision Control

In the previous episode, we know that the half-stepping mode combines single-phase stepping and full-stepping to obtain more electrical angle positions.

So if we want to get more and finer stepping electrical angles, can we add more angle positions to achieve it? In fact, this is the idea of ​​subdivision control.

Figure 1: Eight-division electrical angle

This is an example of eight subdivisions in subdivision control (see Figure 1). In the figure, we can see that the 90-degree electrical angle of the single-phase step AàB is divided into 8 equal parts, that is, there are 8 current positions. The current at each position is vector-synthesized by the currents of the A-phase winding and the B-phase winding in space, and the synthesized amplitude is unit 1. We project the current at each position onto the A-phase and B-phase respectively to obtain the control value table on the right (see Table 1).

Table 1: Eight-segment control value table

If we need the current at this position, we can control the current of phase A and phase B according to the corresponding values ​​in the numerical table, and then we can achieve the current control at this angle position. The controlled current of phase A and phase B can synthesize the current vector of the corresponding angle.

Under the control of eight subdivisions, the current of the stepper motor will become quasi-sinusoidal (see Figure 2), and the currents of phase A and phase B will move in accordance with the value of each composite current, just like walking on stairs. If the number of subdivisions is greater, the current waveform will be closer to sine. The sinusoidal current waveform will reduce the fluctuation of the motor output torque, form a circular rotating magnetic field in space, and enhance the stability of the stepper motor rotation.

Figure 2: Phase A and Phase B current waveforms under eight-segment control

So how do we adjust the size of each current step to stabilize the current at this value?

PART3 Current Regulation

1Usually, in order to stabilize the current at the corresponding value, slow decay and fast decay control methods are used. Let's take a closer look at one of the "steps" of phase A. Figure 3 is the current regulation waveform of phase A, which is achieved by controlling the on and off of the four tubes of the full-bridge drive of phase A (see Figure 4).

Figure 3: Phase A current regulation waveform

Figure 4: A-phase full-bridge drive slow decay process

When Q1 and Q4 are turned on, the supply voltage U is applied to the A-phase winding, and the current begins to rise. In the corresponding equivalent circuit, R and L are equivalent impedances, and E is the reverse electromotive force generated by the A-phase winding cutting the magnetic flux lines when the rotor rotates. The reverse electromotive force reacts on the winding loop. Once the current value reaches the set "step" value, it is necessary to reduce the current, otherwise the current will continue to increase, which will exceed the value set by this "step". At this time, slow decay is needed.

Turn off Q1 and turn on Q2 (ignore the dead time), which is equivalent to short-circuiting the A-phase winding. Due to the inductive load of the motor, the current direction will not change suddenly. The current forms a loop in the two lower tubes. Only the back electromotive force E acts on the loop. The current is subject to the reverse voltage drop -E and begins to drop. If the voltage drop caused by the resistance is ignored, we can assume that the current drops at a rate of -E/L at this time.

When the current drops for a period of time, turn off Q2 and turn on Q1 to let the current rise again. In this way, the current can be stabilized at the value set by this "step".

2 Then when the current is about to enter the next downward "step", the current needs to drop further. If the current drop speed is not fast enough, it will take a long time, and the slow decay speed may not be enough, so fast decay is needed. (See Figure 5)