Hall-less BLDC motor control

5.2 PWM modulation method

After the motor enters the closed loop, the speed can be adjusted by adjusting the PWM duty cycle. The power-on and judgment of each motor winding are controlled by the PWM port. When the PWM duty cycle is large, the current flowing through the motor winding is large, the stator magnetic field is strong, and the speed is high; conversely, when the PWM duty cycle is small, the motor speed is low.

There are two types of PWM modulation: full-bridge modulation and half-bridge modulation. During the 120° conduction period, both the upper and lower bridges of the power inverter bridge are driven by PWM, which is called "full-bridge modulation"; during the 120° conduction period, only the upper bridge (or lower bridge) of the power inverter bridge is driven by PWM, and the lower bridge (or upper bridge) is always on, which is called "half-bridge modulation".

The switching frequency of the MOS tube under full-bridge modulation is about twice that of the half-bridge modulation method, and the loss is relatively large, so it is rarely used. The half-bridge modulation method includes H-PWM-L-ON (within the 120° conduction interval (the same below), the upper bridge arm MOS tube uses PWM modulation, and the lower bridge arm MOS tube is always on), H-ON-L-PWM (the upper bridge arm MOS tube is always on, and the lower bridge arm MOS tube uses PWM modulation), PWM-ON (60° PWM in the front, 60° always on in the back), ON-PWM (60° always on in the front, 60° PWM in the back), etc., each with its own characteristics, can be based on

The specific modulation method is selected according to the specific circuit and application occasion, which is not described in detail in this article. The first two are the most commonly used, which are relatively simple to implement and can meet general applications.

6. Starting method of BLDC motor

The biggest difficulty in BLDC motor control is not position detection and commutation, but the starting method. Since the back electromotive force of the motor winding is positively correlated with the speed, when the speed is very low, the BEMF is also very small and difficult to detect accurately. Therefore, when the motor starts from zero speed, the back electromotive force method is often not applicable. It is necessary to use other methods to pull the motor to a certain speed so that the BEMF reaches a level that can be detected before switching to the back electromotive force method for control.

6.1 Positioning

Only by determining the position of the rotor when it is stationary can we decide which two switching tubes should be triggered for the first time when starting. We call the process of determining the initial position of the rotor positioning.

6.1.1 Establishment of closed loop

The simplest and most commonly used method is to energize any two phases and control the motor current so that it is not too large. After a period of power-on, the rotor will move to the predicted position corresponding to the power-on state, completing the positioning of the rotor.

9 as an example, if the AB phase is energized, the position of the stator magnetic potential Fa is as shown in the figure. At this time, if the rotor magnetic potential Ff

In the position shown in the figure, the rotor will rotate clockwise through 120° electrical angle and align with the direction of the stator magnetic field.

Figure 9 Rotor positioning

However, there is a problem with this method. If the rotor is in the opposite direction of Fa before AB is energized, the direction of the magnetic field force will be 180° with the rotor after power is applied, which will cause a deadlock and the rotor will not rotate, resulting in positioning errors.

To avoid this problem, you can first energize AC and BC to form a magnetic field that is perpendicular to Fa. Then the rotor will rotate to a position that is perpendicular to Fa (even if there is a deadlock at this time, the rotor will rotate to a position that is 180 degrees from the specified direction).

° position, which is still perpendicular to Fa), and then energize AB to ensure that the rotor rotates to the direction of Fa.

One disadvantage of the two-phase energization method is that it is difficult to determine the length of the energization time. If it is too short, positioning cannot be completed. If it is too long, it will cause overcurrent. The appropriate energization time must be determined through repeated tests. Moreover, if the load changes during startup, the energization time also needs to be re-determined.

6.1.2 Positioning by Sensitivity Detection

A more effective method is to use the change in the inductance of the motor winding to detect the initial position of the rotor. This method does not rely on any characteristics of the motor, so it is applicable to any motor, and even changing the starting load of the motor can still effectively achieve positioning. This method is based on the following principle: a voltage is applied to the coil in the magnetic field of a permanent magnet. Depending on the direction of the magnetic field, the current generated will increase or decrease the strength of the magnetic field, thereby reducing or increasing the inductance of the coil.

Figure 10 Positioning by sensitivity detection method

The specific implementation method is shown in Figure 10. First, connect one phase winding to a high level and the other two phases to ground. The direction of the stator magnetic field generated at this time is shown in the figure. Then change the grounded two-phase winding to a high level, and the winding originally connected to the high level to ground, generating a magnetic field in the opposite direction. The power-on time in both cases is very short, the rotor does not rotate, and a current pulse is generated in the winding. By comparing the size of the current pulse in these two cases, the size of the two winding inductances can be compared, so that the rotor can be positioned within a range of 180°. Then change a phase motor winding and repeat the previous process to position the rotor within another 180° range. Each of the three phase windings is tested once, and the overlap of the three ranges can determine the 60° range where the rotor is located.

Since the winding is energized for a very short time each time in this method, there is no need to worry about overcurrent. In addition, since the rotor position will not be changed, this method can also be used to detect the rotor position in the gap between rotor operations.

6.2 Acceleration

After clarifying the initial position of the rotor, we can decide which switches should be turned on for the first time, which two phases should be energized, and control the rotor to rotate forward or reverse to the next position, that is, the first phase change. If the back electromotive force generated in the disconnected phase winding during the first phase change is sufficient to detect the zero crossing, we can directly enter the closed-loop control. However, the actual situation is often not so ideal. At the speed of the motor when it changes phase for the first time from a stationary state, it is often not enough to generate enough back electromotive force to realize zero crossing detection. Therefore, we can only accelerate the motor open loop to a certain speed first, so that the back electromotive force reaches a level that can detect the zero crossing, and then switch to closed-loop speed regulation.

The so-called open-loop acceleration means ignoring the current position of the rotor, forcing the phase change according to the set order and frequency, and pulling the rotor to rotate. And gradually increase the phase change frequency to accelerate the rotation of the rotor. In closed-loop control, we know the current position of the rotor by detecting the zero crossing point, and then decide which two phases to energize in the next step, so that the stator generates a magnetic field with a phase leading the direction of the rotor magnetic field by 120° electrical angle, and pulls the rotor to the next 60° with the optimal torque. In open-loop control, because we cannot detect the back electromotive force and there is no position sensor signal, we have no way of knowing the current position of the rotor, so we cannot control the angle between the stator magnetic field and the rotor magnetic field, which is easy to produce overcurrent or loss of step. If the rotor has reached a position aligned with the direction of the stator magnetic field and has not changed phase, the current in the stator winding will be very large, resulting in overcurrent. If the rotor has not turned to the specified position and the phase change occurs, a loss of step will occur, which may cause the direction of the stator magnetic field to lag behind the rotor magnetic field, causing the rotor to reverse, and then switch between forward and reverse rotation, causing the rotor to swing repeatedly, and even deadlock in the end.

Since open-loop acceleration is very unstable, a reasonable acceleration curve must be designed in advance. One method is to first determine 3 to 4 key points on the acceleration curve through experiments, and then fit the expression of the entire curve.

The successful implementation of this method is affected by many factors such as motor load torque, external voltage, acceleration curve and moment of inertia. By optimizing the acceleration curve, this method can ensure the smooth start of the motor, but the corresponding optimized acceleration curves are different for different motors and different loads, resulting in low universality.

Another acceleration method is to use the variable induction detection method introduced in the previous "positioning" section. After each acceleration period, use this method to detect the rotor position, and then adjust the phase sequence to be energized according to the rotor position to continue accelerating. Repeat detection - acceleration - detection - acceleration... until the motor runs at the required speed.