A new method for rotor position detection of the h

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A new method for detecting the rotor position of Brushless DC motor

Abstract: a new method for detecting the rotor position of Brushless DC motor without position sensor is introduced. In this method, a pulse train is obtained by using the logic level of non conducting opposite potential after logic processing. The pulse train is frequency doubled by PLL phase-locked technology. The rotor position can be accurately detected by the count value of the frequency doubling circuit counter. By using digital comparison technology to compare the count value with the preset value in the latch, the best commutation time of winding current can be accurately controlled. The commutation angle can be easily adjusted by adjusting the preset value in the latch, which is very suitable for various control algorithms of Brushless DC motor. At the same time, this method overcomes the oscillation and out of step phenomena that are easy to occur in the process of external synchronous starting. Experiments show that this method is correct and effective

key words: brushless motor; No position sensor; Detection

1 introduction

when the brushless DC motor is running, it is necessary to use the position sensor to detect the rotor magnetic field position signal, so as to control the commutation of the inverter power tube and realize the self synchronous operation of the motor. Traditional position sensors use electronic or electromechanical sensors to measure directly, such as Hall effect devices (HED), optical encoders, resolvers and so on. However, these sensors have the following disadvantages:

① low resolution or poor operating characteristics, and some are very sensitive to environmental conditions, such as vibration, humidity and temperature changes, which will degrade the performance

② the number of electrical connections is increased, which brings some difficulties to the anti-interference design

③ occupy the motor structure space, which limits the miniaturization of the motor

therefore, the sensorless technology of Brushless DC motor has attracted more and more attention in recent years. Researchers at home and abroad have made active research in this area and put forward many methods, mainly including back EMF method, inductance method, flux linkage method, rotating coordinate system method, observer method, Kalman filter method, etc. [1 ~ 4]. Back EMF method is simple and reliable, and has been widely used. Other methods are less used because of complex calculation and poor robustness to parameters. However, the disadvantages of the back EMF method are:

① the back EMF is small at low speed, so it is difficult to obtain effective rotor position signals, and the low-speed performance of the system is poor

② a low-pass filter is needed to remove the high-frequency noise in the terminal voltage and phase shift by 30 ° to meet the commutation requirements. The requirements for the filter are high. At the same time, the filter is prone to phase shift error, and the magnitude of phase shift error is related to speed, which is difficult to compensate [5]

③ it is difficult to adjust the commutation angle, and the commutation angle cannot be controlled γ (leading or lagging) size

④ if the external synchronous pulse is used for starting, when the drive signal is switched from the external synchronous pulse drive to the internal synchronous pulse drive, it is easy to produce oscillation or even out of step due to the phase error of the switching point [6]

in view of the above problems, this paper proposes a new method of rotor position detection. Taking a three-phase 6-beat brushless DC motor as an example, this method uses the zero crossing detection signal of non conducting opposite potential, and after logical processing, a pulse train with a period of 60 ° electrical angle is obtained. The frequency of the pulse train is doubled by using PLL phase-locked technology. Through the comparison of the count value of the synchronous counter and the preset value of the latch, The ideal commutation point can be obtained, so that the commutation time can be accurately controlled without time delay. When starting, rely on the best switching time of the motor and put into self synchronous operation in the best triggering mode, so that the starting process is stable and reliable. Using the digital comparison technology, depending on the different assignments in the latch, the commutation angle can be flexibly and accurately controlled in advance or behind. It is very convenient to realize the vector control, field weakening control and other control strategies of Brushless DC motor. The experiment proves that this method is correct and effective

2 principle

the system principle block diagram of Brushless DC motor controlled by position sensor is shown in Figure 1. The motor works in three-phase 6-beat mode, adopts double closed-loop control of voltage outer loop and current inner loop, the inverter power tube works in PWM modulation mode, and the controller takes mc8031 as the core

when the motor is running, detect the zero crossing point of the back EMF of the non conducting phase, and use the zero crossing comparator to convert the back EMF signals EA, EB, EC into logic level signals VA, VB, VC respectively, as shown in Figure 2. The pulse frequency multiplier composed of CMOS gate circuit is shown in Figure 3. After frequency multiplication of VA, VB and VC, a pulse signal h is obtained, as shown in Figure 2. At this time, the period of H pulse train is 60 ° electrical angle. It can be seen from Figure 2 that at time T0, the opposite potential at time t0 crosses zero upward, which should be delayed by 30 ° electrical angle, that is, at the midpoint of time t0 and T1, at time T4, turn on ta+ and turn off tc+; Similarly, tc- should be turned on and tb- should be turned off at T5; At time T6, tb+ should be turned on and ta+ should be turned off. It can be seen that accurately determining the time of T4, T5, T6... Is the key to realize accurate commutation. This paper introduces PLL technology, and uses phase-locked loop to multiply the frequency of H pulse signal n. when the count value of counter in the frequency multiplication circuit is equal to N/2, it is the best commutation point. A digital comparator is used to compare the counter count value with the preset value in the latch, and the value n/2 can be preset in the latch, so that the best commutation time can be detected, and then the commutation subroutine can be called to realize the commutation process, with an error of ± 0.5lsb and ± 30 °/n angle. It can be seen that this method controls the commutation point accurately

in Brushless DC, the operation is more intuitive and simple; In the vector control and field weakening control of the lengthened stretch space motor, it is required to replace the above components of the motor, which are relatively easy to damage the phase angle γ Carry out lead or lag control, and the commutation angle is a time variable that changes with the operating conditions, so the control of commutation angle should be fast and accurate. If the commutation angle calculated by the control algorithm is γ 0 (lead is positive and lag is negative), then the count value m of the counter corresponding to the commutation point should be:

write the value m into the latch immediately, and the next commutation point will be in advance γ Commutation at 0 angle. It can be seen that the commutation angle of this method λ The control is flexible, convenient and fast, which meets the control requirements of various control strategies of Brushless DC motor

when the motor starts, in the commutation subroutine, first use the software to detect the command signal of the external synchronous drive switching to the internal synchronous drive, then read out the states of VA, VB and VC, judge the trigger state of the inverter power tube based on this, and send out the trigger pulse; Otherwise, the interrupt returns. Since the commutation interruption program is triggered at the best commutation time, the switching point must be the best commutation time, so as to avoid oscillation or out of step in the switching process

at the same time, using the frequency doubling signal SP of the H pulse as the position signal, with the help of the constant frequency clock signal as the clock, using the M/T method, the speed of the motor can be easily obtained

3 realization of Brushless DC motor position detection

it can be seen from Figure 2 that after the back EMF signals EA, EB, EC pass through the zero comparator, the back EMF logic level signals VA, VB, VC are obtained, and the three logic level signals VA, VB, VC are respectively passed through the back phase or of the pulse frequency multiplier shown in Figure 3 to obtain the H pulse train. Such a back EMF cycle corresponds to 6 h pulses. The midpoint of the jump edge on the adjacent h pulse is the ideal commutation point. Figure 4 shows the position detection circuit without position sensor. In the figure, the phase-locked loop CD4046 and the binary serial counter/frequency divider cc4024 constitute a 128 frequency doubling circuit, and SP is the frequency doubling signal. The frequency between the upper skip edges of two adjacent h pulses is 128 times, so when the counter cc4024 counts 64, it corresponds to the best commutation time. The 8-bit digital comparator composed of two parallel digital comparators cc14585 always compares the count value of the counter with the output value of the latch. When there are too many oxide scales, the latch is a peripheral port extended by the 8031 system, which mainly stores the commutation angle γ For the corresponding count value, if the value 64 is stored in the latch, whenever the count value of counter cc4024 is 64, the 3 pin of digital comparator cc14585 (1) will send a pulse, which is used as the commutation interrupt request signal to request interrupt from CPU, and call the commutation subroutine to realize the commutation of electric machine winding current

in the process of vector control and field weakening control of Brushless DC motor, CPU calculates the commutation angle of motor winding in time γ, Every time the commutation angle is completed γ Immediately write the corresponding count value m, count value m and commutation angle to the latch γ The relationship is:

then the next commutation point is at the commutation angle γ The commutation error is ± 0.5lsb and ± 0.234 ° electrical angle, which fully meets the accuracy requirements

4 implementation of speed measuring circuit

Figure 5 shows the speed measuring circuit. Counter 0 and counter 2 of 8253 work in mode 2. Among them, counter 0 counts the frequency doubling pulse SP from CD4046, and counter 2 counts the high-frequency clock pulse FC from 8031ale pin. D trigger 74ls74 is used to synchronize the counting of high-frequency clock pulses with the frequency doubling pulse sp. Since 8253 is a negative edge count, inverter 74LS04 is added. P1.6 terminal of 8031 gives start and speed measurement signals. CPU uses M/T method to calculate the speed of the motor in time

5 commutation subroutine

the main function of the commutation subroutine is to determine the corresponding power tube switching state according to the forward and reverse commands and the back EMF logic level state. Its block diagram is shown in Figure 6. In the interrupt subroutine, the CPU first reads the three-phase back EMF logic signals of a, B and C, then judges the forward and reverse states of the motor (according to the positive and reverse signs in the main program), and determines the switching state of the inverter according to the forward and reverse States and the three-phase back EMF logic state signals. In the forward and reverse States, the corresponding relationship between the back EMF logic state signal and the switching state is shown in Table 1 and table 2, which are stored in EPROM in the form of tables

6 experimental results

this paper takes a 750W, 4-pole brushless DC motor as the prototype, which is controlled by 8031 single chip microcomputer. The starting process adopts three-stage starting, and the preset value of the latch is 64. Through experiments, it is proved that using the above position sensorless technology, the motor system starts smoothly without vibration and out of step, and the adjustment range is 150 ~ 1500 R/min. the measured phase relationship between the back EMF and the power tube drive signal is shown in Figure 7. It can be seen that the above method is correct and feasible

7 conclusion

(1) the commutation angle can be accurately controlled by using PLL PLL technology and parallel digital comparison technology γ It is flexible, convenient and fast to control. It is suitable for various control strategies of Brushless DC motor

(2) in the starting process, this method can accurately realize the switching from external synchronous drive to internal synchronous drive, and avoid the oscillation and out of step phenomenon in the starting process

(3) in this method, the motor speed can be obtained by using the frequency doubling signal


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