Sensorless direct-current brushless motor driving system and method for detecting dynamic compensation by using back electromotive force

Abstract

The invention discloses a sensorless direct-current brushless motor driving system and method for detecting dynamic compensation by using back electromotive force. The system comprises an MCU (Microprogrammed Control Unit), a gate driver, an MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor), a voltage dividing network and a direct-current brushless motor, the MCU is integrated with a comparator, a DAC (Digital-to-Analog Converter) and a PWM (Pulse Width Modulation) controller and is provided with a PID (Proportion Integration Differentiation), the comparator is used for BEMF zero crossing point detection, the DAC outputs a reference voltage Vref and a compensation voltage Vcomp, the PID controls the size of the Vcomp, and the Vcomp and the Vref are superposed and then are input to an inverted input end of the comparator; three paths of phase voltages of the direct-current brushless motor generate three paths of comparison voltages through the voltage dividing network, and then the three paths of comparison voltages are input into the in-phase input end of the comparator through the multi-path selection switch. The MCU controls three pairs of complementary PWM commutation, the PWM generates driving voltage for controlling the MOSFET after passing through the gate driver, the MOSFET generates driving voltage to drive the brushless motor, commutation precision is improved, and torque fluctuation is reduced.

Description

Technical Field

[0001] The invention relates to a DC brushless motor driving technology, in particular to a sensorless DC brushless motor driving system and method utilizing back electromotive force detection and dynamic compensation. Background Art

[0002] Brushless DC Motor (BLDCM) has the advantages of high efficiency, high reliability, low maintenance cost and simple drive, and has been widely used. The three-phase stator winding of the brushless DC motor is usually connected in star shape. Assuming that the stator of the motor is evenly distributed and has the same parameters, the equivalent circuit of the motor drive system composed of the brushless DC motor and the three-phase inverter is as follows: Figure 1 As shown in Figure 1, the ideal BLDCM has a trapezoidal BEMF waveform with a flat top width of 120°, the phase current is a standard square wave, and there is a 120° phase difference between each phase. The relative relationship between the BEMF and the phase current is as follows: Figure 2 As shown, the electromagnetic torque generated by BLDCM can be expressed as:

[0003]

[0004] Where, T e is the electromagnetic torque, ω is the angular velocity of the rotor of the BLDCM, E a is the back electromotive force, i a is the phase current.

[0005] Ideally, the commutation period of each sector is equal. When the rotor is at any position within an electrical cycle, it can be seen from the electromagnetic torque expression that the electromagnetic torque remains constant and no torque fluctuation occurs. At the same time, since the BEMF waveform is in phase with the phase current waveform, the driving efficiency can be maximized. The commutation timing of BLDCM is obtained by detecting the BEMF of the non-driven phase, and the terminal voltage of the non-driven phase is compared with half the power supply voltage to obtain the moment when the BEMF crosses zero, and further obtain the accurate time of commutation.

[0006] However, under non-ideal conditions, various reasons may cause uneven commutation time and torque fluctuation, which will lead to reduced drive efficiency and performance degradation, mainly in three aspects: (a) Due to the uneven distribution of the motor's stator structure or the permanent magnet of the rotor, as well as inconsistent parameters of the three-phase stator winding, the BEMF is not an ideal trapezoidal wave; (b) Due to the inductance characteristics of the stator winding, the phase current cannot be of any shape, so the ideal rectangular current waveform does not exist; (c) Due to errors in the voltage divider resistors used to collect BEMF and zero point offsets in the ADC or comparator, the BEMF zero-crossing position detection is inaccurate. Regardless of the reason, the commutation timing will eventually deviate, making the six commutation cycles inconsistent, resulting in torque fluctuation and reduced efficiency.

[0007] In order to match the BEMF waveform of the BLDCM's back electromagnetic force (BEMF) to achieve optimal driving efficiency and reduce torque fluctuations, the switching timing of the three-phase inverter switch combination of the drive system is crucial. Usually, the switching timing is given by a position sensor installed on the motor stator; however, due to the motor structure and actual working environment, system cost and reliability requirements, position sensors are often inconvenient to install. Therefore, the drive solution without position sensors has gradually become the mainstream of BLDCM drive.

[0008] The traditional position sensorless driving scheme obtains the position information of the motor rotor by detecting the BEMF zero-crossing point of the non-driving phase. The BEMF zero-crossing point detection is usually implemented through a comparator or ADC (Analog to Digital Converter). Since the three phases of the BLDCM are not completely symmetrical, and the comparator or ADC has the influence of factors such as zero-point offset, the detected BEMF zero-crossing time has deviations, resulting in inconsistent driving steps and increased torque fluctuations, which not only brings mechanical vibration and noise problems to the motor drive system, but also reduces the performance and efficiency of the system. Therefore, suppressing torque fluctuations has always been a problem that needs to be solved urgently.

[0009] At present, two methods are commonly used to solve the torque fluctuation problem. One method is to reduce torque fluctuation and improve driving efficiency by balancing the neutral point voltage. However, this method has high requirements for the reconstruction of the neutral point voltage and is difficult to implement. The other method is to improve the commutation accuracy and drive performance through deep learning network model prediction. However, this method relies on the training of the network model with prior parameters such as the known motor's equivalent resistance and equivalent inductance. However, in actual situations, the motor parameters change with factors such as temperature and aging, resulting in inaccurate model prediction, poor control effect on the motor, and unsatisfactory torque fluctuation suppression effect, which limits its application in actual production. Summary of the invention

[0010] In view of the deficiencies in the prior art, the object of the present invention is to provide a sensorless DC brushless motor drive system and method using back electromotive force detection dynamic compensation, which improves commutation accuracy and reduces torque fluctuations.

[0011] In order to achieve the above object, the present invention adopts the following technical solutions:

[0012] A sensorless DC brushless motor drive system using back electromotive force detection dynamic compensation, comprising an MCU, a gate driver, a MOSFET, a voltage divider network and a DC brushless motor;

[0013] The MCU integrates a comparator, a DAC and a PWM controller, and deploys a PID algorithm, wherein: the comparator is used for BEMF zero-crossing detection; the DAC is used to output a reference voltage V ref Stack and compensation voltage V comp ;PID algorithm is used to control the compensation voltage V comp The size of the compensation voltage V comp and reference voltage V ref After superposition, it is used as the input quantity of the inverting input terminal of the comparator;

[0014] The three-way phase voltage of the brushless DC motor generates three-way comparison voltage through a voltage divider network, and the three-way comparison voltage is used as the input quantity of the comparator's non-phase input terminal after passing through a multi-way selection switch;

[0015] The MCU controls the three pairs of complementary PWM commutations output by the PWM controller according to the output signal of the comparator. After the PWM is level-shifted by the gate driver, a driving voltage for controlling the on and off of six MOSFETs in the three-phase inverter is generated;

[0016] The MOSFET generates a driving voltage to drive the brushless DC motor.

[0017] A sensorless DC brushless motor driving method using back electromotive force detection dynamic compensation comprises the following steps:

[0018] Step 1: By recording the time when the output of the comparator jumps when the back electromotive force crosses the zero point, the commutation time T of each sector of the brushless DC motor is obtained. n , then the average value of the commutation period is T avg It is expressed as:

[0019]

[0020] Where n = 1, 2, ... 6;

[0021] Step 2: Tn With T avg The difference is taken as the commutation error, and the commutation error is taken as the input of the PID algorithm. The PID algorithm controls the compensation reference voltage V according to the input. ref The compensation voltage V comp The size of which:

[0022] During PWM ON, the reference voltage V ref It is expressed as:

[0023]

[0024] Where V DC is the power supply voltage;

[0025] During the PWM OFF period, the reference voltage V ref =0;

[0026] Step 3: The three phase voltages of the brushless DC motor are passed through a voltage divider network to generate three comparison voltages V a 、V b and V c The compensation voltage V comp and reference voltage V ref After superposition, the signals are input to the inverting input terminal of the comparator;

[0027] Step 4. The output end of the comparator generates a high-level or low-level output signal according to the magnitude relationship between the voltages at the in-phase input end and the inverting input end. The MCU controls the three pairs of complementary PWM commutations according to the output signal at the output end of the comparator. After the PWM is level-shifted by the gate driver, a driving voltage is generated to control the on-off of the six MOSFETs in the three-phase inverter. The driving voltage generated by the MOSFET is used to drive the brushless DC motor.

[0028] Furthermore, in step 2, when the output of the PID algorithm tends to a stable state, the commutation error tends to 0, that is, T n ≈T avg .

[0029] Compared with the prior art, the present invention has the following technical effects:

[0030] 1) The present invention uses the DAC of the microcontroller to output the compensation voltage, uses PID to control the size of the compensation voltage, dynamically adds the compensation voltage in the BEMF detection link, and dynamically adjusts the reference voltage of the comparator without relying on motor parameters, thereby eliminating the BEMF detection error, making the commutation period of the brushless DC motor more uniform, effectively suppressing torque fluctuations, making the output torque more stable, reducing the vibration of mechanical parts caused by torque changes, reducing noise, and improving efficiency and commutation accuracy.

[0031] 2) The present invention uses the expected commutation period as a reference target for regulation, is effective for BEMF zero-crossing point detection errors caused by different reasons, and has a wider range of applications.

[0032] 3) The present invention reduces the wear caused by periodic stress changes by suppressing torque fluctuations, reduces the wear of components such as bearings, extends the service life of mechanical equipment, and reduces maintenance costs.

[0033] 4) The present invention outputs a compensation voltage through the built-in DAC of the MCU, thereby suppressing the torque fluctuation of the motor, without the need for additional components, with low cost, strong versatility, and easy promotion and application. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 : Equivalent circuit diagram of brushless DC motor drive system;

[0035] Figure 2 : Phase current and BEMF waveform of brushless DC motor;

[0036] Figure 3 : A structural diagram of the BEMF detection automatic compensation system of the present invention;

[0037] Figure 4 : Schematic diagram of the structure of the brushless DC motor drive system of the present invention. DETAILED DESCRIPTION

[0038] The specific contents of the present invention are further explained in detail below in conjunction with embodiments.

[0039] like Figure 3 and Figure 4 As shown, a sensorless DC brushless motor drive system using back electromotive force detection dynamic compensation includes a microcontroller unit MCU, a gate driver Gate Driver, a MOSFET, a voltage divider network and a DC brushless motor;

[0040] The MCU integrates a comparator, a DAC and a PWM controller, and deploys a PID algorithm, wherein: the comparator is used for back electromotive force zero-crossing detection, that is, BEMF zero-crossing detection; the DAC is used to output a reference voltage V ref and compensation voltage V comp ;PID algorithm is used to control the compensation voltage V comp The size of the compensation voltage V comp and reference voltage V ref After superposition, the signals are input to the inverting input terminal of the comparator;

[0041] In order to solve the commutation error problem caused by the comparator, it is necessary to compare the reference voltage V of each sector of the 6-step commutation of the brushless DC motor. ref However, there are many reasons for the comparison error, such as phase voltage acquisition voltage divider resistor error, comparator zero offset and motor parameter inconsistency, which will cause comparison error, and the compensation amount of each sector is different, so it cannot be compensated with a fixed compensation amount. Therefore, this embodiment proposes the following Figure 3 The BEMF detection automatic compensation system shown includes a comparator and a PID algorithm. The PID can automatically adjust the control amount according to the deviation between the expected value and the measured value to achieve the purpose of the expected value and the measured value tending to be consistent. Therefore, the PID control principle is introduced into the BEMF zero-crossing point detection process to achieve the purpose of the commutation time of each sector tending to be consistent;

[0042] The three-phase voltage of the brushless DC motor generates three comparison voltages V through a voltage divider network. div , three-way comparison voltage V div Input to the comparator non-inverting input terminal through the multi-way selection switch S;

[0043] The MCU controls the three pairs of complementary PWM commutations output by the PWM controller according to the output signal of the comparator. The gate driver has a corresponding current driving capability. After the 6-way PWM is level-shifted by the gate driver, a driving voltage for controlling the on and off of the 6 MOSFETs in the three-phase inverter is generated.

[0044] The MOSFET generates a driving voltage to drive the brushless DC motor.

[0045] A sensorless DC brushless motor driving method using back electromotive force detection dynamic compensation comprises the following steps:

[0046] Step 1: By recording the time when the output of the comparator jumps when the back electromotive force BEMF crosses zero, the commutation time T of each sector of the brushless DC motor is obtained. n , then the average value of the commutation period is T avg It is expressed as:

[0047]

[0048] Where n = 1, 2, ... 6, T n With T avg The size of the difference reflects the BEMF zero-crossing detection error;

[0049] Step 2: T n With T avg The difference is taken as the commutation error, and the commutation error is taken as the input of the PID algorithm. The PID algorithm controls the compensation voltage V output by the DAC according to the input.comp The size of the compensation voltage V comp As the reference voltage V of the DAC output ref The compensation amount is:

[0050] During PWM ON, the reference voltage V ref It is expressed as:

[0051]

[0052] Where V DC is the power supply voltage;

[0053] During the PWM OFF period, the reference voltage V ref =0;

[0054] When the PID algorithm output tends to a stable state, the commutation error tends to 0, that is, T n ≈T avg ;

[0055] Step 3: The three phase voltages of the brushless DC motor are passed through a voltage divider network to generate three comparison voltages V a 、V b and V c The compensation voltage V comp and reference voltage V ref After superposition, the signals are input to the inverting input terminal of the comparator;

[0056] Step 4. The output end of the comparator generates a high-level or low-level output signal according to the magnitude relationship between the voltages at the in-phase input end and the inverting input end. The MCU outputs three pairs of complementary PWMs according to the output signals at the output end of the comparator. After the PWM is level-shifted by the gate driver, a driving voltage is generated to control the on-off of the six MOSFETs in the three-phase inverter. The driving voltage generated by the MOSFET is used to drive the brushless DC motor.

[0057] The sensorless DC brushless motor driving method using back-electromotive force detection dynamic compensation proposed in this embodiment does not rely on motor parameters, uses the DAC of the microcontroller to output the compensation voltage, and uses PID to control the size of the compensation voltage, thereby automatically compensating for BEMF detection errors, reducing torque fluctuations, and improving commutation accuracy.

Claims

1. A sensorless DC brushless motor drive system using back electromotive force detection dynamic compensation, characterized in that: Includes MCU, gate driver, MOSFET, voltage divider network and brushless DC motor; The MCU integrates a comparator, a DAC and a PWM controller, and deploys a PID algorithm, wherein: the comparator is used for BEMF zero-crossing detection; the DAC is used to output a reference voltage V ref and compensation voltage V comp ;PID algorithm is used to control the compensation voltage V comp The size of the compensation voltage V comp and reference voltage V ref After superposition, it is used as the input quantity of the inverting input terminal of the comparator; The three-way phase voltage of the brushless DC motor generates three-way comparison voltage through a voltage divider network, and the three-way comparison voltage is used as the input quantity of the comparator's non-phase input terminal after passing through a multi-way selection switch; The MCU controls the three pairs of complementary PWM commutations output by the PWM controller according to the output signal of the comparator. After the PWM is level-shifted by the gate driver, a driving voltage for controlling the on and off of six MOSFETs in the three-phase inverter is generated; The MOSFET generates a driving voltage to drive the brushless DC motor.

2. A sensorless DC brushless motor driving method using back electromotive force detection dynamic compensation based on the driving system of claim 1, characterized in that: The steps include: Step 1: By recording the time when the output of the comparator jumps when the back electromotive force crosses the zero point, the commutation time T of each sector of the brushless DC motor is obtained. n , then the average value of the commutation period is T avg It is expressed as: Where n = 1, 2, ... 6; Step 2: T n With T avg The difference is taken as the commutation error, and the commutation error is taken as the input of the PID algorithm. The PID algorithm controls the compensation reference voltage V according to the input. ref The compensation voltage V comp The size of which: During PWM ON, the reference voltage V ref It is expressed as: Where V DC is the power supply voltage; During the PWM OFF period, the reference voltage V ref =0; Step 3: The three phase voltages of the brushless DC motor are passed through a voltage divider network to generate three comparison voltages V a 、V b and V c The compensation voltage V comp and reference voltage V ref After superposition, the signals are input to the inverting input terminal of the comparator; Step 4. The output end of the comparator generates a high-level or low-level output signal according to the magnitude relationship between the voltages at the in-phase input end and the inverting input end. The MCU controls the three pairs of complementary PWM commutations according to the output signal at the output end of the comparator. After the PWM is level-shifted by the gate driver, a driving voltage is generated to control the on-off of the six MOSFETs in the three-phase inverter. The driving voltage generated by the MOSFET is used to drive the brushless DC motor.

3. The sensorless DC brushless motor driving method using back electromotive force detection dynamic compensation according to claim 2, characterized in that: In step 2, when the output of the PID algorithm tends to a stable state, the commutation error tends to 0, that is, T n ≈T avg .

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