BACK ELECTROMOTIVE FORCE SENSING CIRCUIT, BACK ELECTROMOTIVE FORCE SENSING METHOD AND DRIVING MODULE OF THREE-PHASE PERMANENT MAGNET MOTOR

A back electromotive force sensing circuit can include: a sampling circuit configured to acquire a sampling signal representing a phase current of one of three phases of a three-phase permanent magnet motor, where the three-phase permanent magnet motor adopts sine wave control; and a signal processing circuit configured to receive the sampling signal, and to obtain a back electromotive force of the one phase according to a difference between a phase voltage of the one phase and a sum of a voltage across a phase resistor and a voltage across a phase inductor of the one phase.

RELATED APPLICATIONS

This application claims the benefit of Chinese Patent Application No. 202210184110.2, filed on Feb. 24, 2022, which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention generally relates to the field of controlling the rotation of a rotating body, and more particularly to back electromotive force sensing circuits and methods, and an associated driving module of a three-phase permanent magnet motor.

BACKGROUND

In order to reduce control costs and save installation space of position sensors, three-phase permanent magnet motors can be controlled without position sensors. The most common approach to finding the position of the motor without position sensor is to observe the back electromotive force of the motor. A back electromotive force observation approach with single-phase current detection can be used to control of three-phase permanent magnet motor without a position sensor, and the back electromotive force of the motor can be directly obtained through the electrical relationship of the motor.

DETAILED DESCRIPTION

Referring now toFIG.1, shown is a schematic diagram of an example back electromotive force observation method of a brushless direct current motor (BLDC). The back electromotive force of phase c can be obtained by the motor electrical relationship BemfC=(VC−VCT)−Rc*Ic, and then term Rc*Ic may further be ignored when the C phase is suspended. Thus BemfC=VC−VCT, where VC is the terminal voltage of the C phase and VCT is the voltage at the motor center point. This approach can be used in BLDC motor with square wave control. When sine wave control is used, the three phases of the permanent magnet motor may not be suspended, so there can be a relatively large error when using this approach to observe the back electromotive force.

Referring now toFIG.2, shown is a schematic diagram of an example three-phase permanent magnet motor system, in accordance with embodiments of the present invention. In this particular example, the three-phase permanent magnet motor system can include three-phase permanent magnet motor21, and a three-phase permanent magnet motor driving module including back electromotive force sensing circuit22, control circuit23, and driving circuit24. Here, three-phase permanent magnet motor21is described by taking a brushless DC motor as an example. The brushless DC motor can be equivalent to a structure including three stator windings a, b and c. The brushless DC motor may have three symmetrical phases, and each phase can include corresponding a phase resistor, a phase inductor, and a back electromotive force. Here, set phase resistors Ra=Rb=Rc and phase inductors La=Lb=Lc, and the back electromotive forces of three phases are respectively Ea, Eb and Ec.

In the example three-phase permanent magnet motor driving module, back electromotive force sensing circuit22can obtain the back electromotive force of the current phase according to DC bus voltage Udc at the current sampling time, pulse-width modulation (PWM) chopping duty ratios PWMa, PWMb and PWMc of the three-phase bridge arms in driving circuit24, and the sampling signals of the phase current of one of the three phases at the current sampling time and the last sampling time. For example, back electromotive force sensing circuit22of the three-phase permanent magnet motor can include sampling circuit221and signal processing circuit222. Sampling circuit221can obtain a sampling signal representing a phase current of one phase. Signal processing circuit222may receive the sampling signal to generate the back electromotive force of the phase according to the difference between the phase voltage of the phase and the sum of the voltage on the phase resistor and the voltage on the phase inductor.

Signal processing circuit22may include a phase inductor voltage processing circuit, a phase voltage processing circuit, and a phase resistor voltage processing circuit. For example, the phase inductor voltage processing circuit can discretize the product of the phase inductor and the differential of the sampling signal obtained by sampling circuit221to obtain the voltage across the phase inductor. Alternatively, the phase inductor voltage processing circuit can discretize the product of the phase inductor and the differential of the sampling signal obtained by sampling circuit221, and then may perform digital filtering to obtain the voltage across the phase inductor. The phase voltage processing circuit can discretize the phase voltage signal to obtain the phase voltage. The phase resistor voltage processing circuit may discretize the product of the sampling signal obtained by sampling circuit221and the phase resistor to obtain the voltage across the phase resistor.

Control circuit23can obtain a zero-crossing point of the back electromotive force of one phase and the motor speed according to the back electromotive force of the corresponding phase, in order to calculate the motor rotor position, and then may generate PWM chopping duty ratios PWMa, PWMb and PWMc of the three phase bridge arms according to the motor rotor position and the DC bus voltage.

Driving circuit24can be controlled by PWM chopping duty ratios PWMa, PWMb and PWMc of the three phase bridge arms, such that the driving current of the three-phase permanent magnet motor is sinusoidal. For example, driving circuit24may be formed by three-phase inverter circuit. Transistors Q1and Q2can be used as switches of the upper arm bridge and the lower arm bridge of the a-phase stator winding, transistors Q3and Q4can be used as switches of the upper arm bridge and the lower arm bridge of the b-phase stator winding, and transistors Q5and Q6can be used as switches of the upper arm bridge and the lower arm bridge of the c-phase stator winding. For example, transistors Q1-Q6may be controlled to be turned on and off according to PWM chopping duty ratios PWMa, PWMb and PWMc, such that one of a, b, and c phase stator windings can connect to the positive terminal of DC bus voltage Udc, another one can connect to the negative terminal of DC bus voltage Udc, and the third one can be in a power-off state. Each of transistors Q1to Q6can connect in parallel with a freewheeling diode. Therefore, control circuit23can control the drive signals output by drive circuit24to the brushless DC motor by controlling the value of the PWM duty ratio of each transistor.

In particular embodiments, when back electromotive force sensing circuit22of the three-phase permanent magnet motor obtains the back electromotive force of a certain phase, it may first obtain the expression between the back electromotive force of one phase and the phase voltage, phase current, phase resistor, and phase inductor of that phase according to the motor voltage equation. Then, the expression can be discretized. Subsequently, digital filtering may be carried out on the part corresponding to the differential term in the expression. Finally, the back electromotive force of the corresponding phase can be obtained according to the expression.

When the three-phase permanent magnet motor is controlled, the back electromotive force of the motor can be calculated only by sampling the phase current of one phase and the DC bus voltage, and then the position information of the stator rotor can be obtained. In addition, the expression of the back electromotive force can be directly obtained through the motor voltage equation, and the differential term in the expression is filtered, such that the fluctuation of the calculated back electromotive force is reduced.

Referring to the circuit structure diagram of the three-phase permanent magnet motor system inFIG.2, on the premise that the motor is symmetrical in three phases, the phase resistors Ra=Rb=Rc, the phase inductors La=Lb=Lc, and the back electromotive forces of three phases are respectively Ea, Eb and Ec. The relationship between back electromotive force and the phase voltage, phase current, phase resistor and phase inductor can be obtained according to the motor voltage equation can be as shown in Formula (1):

In Formula (1), Ea is the back electromotive force of phase a, Ua is the phase voltage signal of phase a, Ra is the phase resistor of phase a, Ia is the sampling signal of the phase current of phase a, and La is the phase inductor of phase a. Back electromotive force Ea of phase a equals to phase voltage signal Ua of phase a minus the voltage across phase resistor Ra and the the voltage across phase inductor La of phase a. As long as phase voltage signal Ua of phase a and sampling signal Ia of the phase current of phase a at each moment can be calculated or detected, back electromotive force Ea of phase a can be calculated.

The phase voltage of phase a can be calculated according to PWM chopping duty ratios PWMa, PWMb, and PWMc of the three-phase bridge arms given by control circuit23and DC bus voltage Udc detected by the ADC, and the calculation formula is as follows:

In Formula (2), Ua is the phase voltage signal of phase a, Udc is DC bus voltage, PWMa, PWMb, PWMc are respectively PWM chopping duty ratios of three-phase bridge arms. For example, sampling signal Ia of the phase current of phase a can be obtained by a sampling resistor or other suitable approaches, phase resistor Ra of phase a and phase inductor La of phase a can be measured by instruments, and the voltage across phase inductor La of phase a involves the differentiation of the current. If the part corresponding to the differential term is represented as La×Didt, the processing of the part of Didt can be obtained by the following formula in the discrete control system:

In Formula (3), Ts is the system sampling period, Ia[kTs] is the sampling signal of the phase current of phase a obtained at sampling moment kTs, and Ia[(k−1)Ts] is the sampling signal of the phase current of phase a obtained at sampling moment (k−1)Ts, in order to calculate the current differential. By bringing Formulas (2) and (3) into Formula (1), the discretized back electromotive force calculation formula can be obtained as follows:

In Formula (4), Udc[kTs], PWMa[kTs], PWMb[kTs], PWMc[kTs] and Ia[kTs] are the sampling signals of DC bus voltage, PWM chopping duty ratios of three-phase bridge arms, and the phase current of phase a obtained at sampling moment kTs, respectively, and Ia[(k−1)Ts is the sampling signal of the phase current of phase a obtained at sampling moment (k−1)Ts.

Particular embodiments may provide a back electromotive force calculation method based on Formula (4), which can be effectively and stably used for the sine wave control of a three-phase permanent magnet motor. As compared with the back electromotive force observation approach that ignores the product term of the phase inductor and the differential of the phase current, although omitting the product term of inductor and current differential reduces the complexity of the algorithm, this can lead to great deviation in back electromotive force observation when the motor speed is high and the current is high, and even lead to out-of-step phenomenon.

However, the introduction of the current differential term into the back electromotive force observation formula may cause the following problems. Since the current differential is obtained by directly dividing the difference between the phase currents sampled at sampling moment kTs and sampling moment (k−1)Ts by sampling period Ts, the accuracy of the phase current sampling may be limited to the accuracy of ADC, which can result in errors. However, sampling period Ts is typically relatively small, and the sampling error of ADC can be amplified after dividing the difference between the sampled phase currents by the sampling period. If the back electromotive force is calculated directly according to Formula (4), the calculated back electromotive force may fluctuate greatly when the motor speed is high and the motor inductor is relatively large.

Referring now toFIG.3, shown is a waveform diagram of the back electromotive force and current differential without filtering by using the back electromotive force calculation method, in accordance with embodiments of the present invention. From the waveforms of back electromotive force Ea (as CH1shown inFIG.3) and current differential term Didt (as CH2shown inFIG.3), the fluctuation of the current differential term leads to the fluctuation of the back electromotive force observed by Formula (4). Based on the above, Formula (4) is further processed. When the part corresponding to the differential term is expressed as La×Didt, the result obtained after digital filtering processing of Didt can be expressed by:

In Formula (5), f is the digital filter coefficient, and f is a positive number less than 1, the initial value of Didtfilteris 0. Further, Formula (7) can be obtained by replacing the current differential term in Formula (4) with Formula (5), and Formula (7) is the calculation formula for back electromotive force finally used in the invention:

Referring now toFIG.4, shown is the waveform of back electromotive force Ea calculated by Formula (7) and current differential term Didtfilterafter digital filtering. In this particular example, the waveform fluctuation of the back electromotive force and current differential term obtained by the back electromotive force calculation can clearly be reduced.

After the back electromotive force of one phase is obtained, the zero-crossing point of the back electromotive force can be obtained by judging whether the back electromotive force changes from negative to positive or from positive to negative. Then, the motor speed can be calculated by the time between two zero-crossing points of the back electromotive force of the same phase, and the motor rotor position can be further calculated. Further, the PWM chopping duty ratios PWMa, PWMb, and PWMc of the three-phase bridge arms can be generated according to the motor rotor position and the DC bus voltage, in order to achieve the purpose of sine wave current control of the permanent magnet motor.

As described above, particular embodiments may provide a method for calculating the back electromotive force of a three-phase permanent magnet motor by sampling the phase current of one phase and the bus voltage. In certain embodiments, the expression of the back electromotive force can directly be obtained through the motor voltage equation, and after the expression is discretized, the differential term in the expression may be filtered to reduce the fluctuation of the calculated back electromotive force. The back electromotive force calculation formula of certain embodiments can be used to estimate the back electromotive force of the permanent magnet motor in sine wave control. Thus, the example control method is relatively simple and stable, and the cost relatively low since the phase current of one phase needs to be sampled.