CN108574429A - A method for suppressing brushless DC motors with wide speed range and low torque ripple - Google Patents

Abstract

The present invention relates to a kind of brshless DC motor wide speed regulating range low torque ripple suppressing methods, using the control mode of rotating speed and torque two close cycles, the non-commutation moment carries out direct torque, and the commutation moment carries out overlapping commutation and adds the calculating that current forecasting module carries out duty ratio.When the beginning of hall sensor signal saltus step mark commutation, shutdown phase current, which is reduced to zero flag commutation, to be terminated.The method for using overlapping commutation at this time takes shutdown mutually during commutation and non-commutation is synchronised PWM modulation and opens the measure of phase Heng Tong to reflect the electric current of non-commutation phase on the DC bus during be delayed to turn off.The Non-commutation phase current value and motor speed at sample current time simultaneously i.e. k moment, according to the operating status at motor current time, that is, k moment, obtain three-phase windings phase back-emf, Predictive control law D is calculated with predictive current control module, to control inverter circuit output, Torque Ripple Reduction of the brshless DC motor under wide speed regulating range is realized.

Description

A method for suppressing brushless DC motors with wide speed range and low torque ripple

technical field

The invention relates to a brushless direct current motor with wide speed regulation range and low torque ripple suppression, and belongs to the field of permanent magnet brushless direct current motor control.

Background technique

As a kind of permanent magnet motor, brushless DC motor not only has the advantages of simple structure, low manufacturing cost, simple and economical maintenance and repair of AC motor, but also has the advantages of large output, good starting and speed regulation performance of DC motor, and brushless DC motor The use of electronic commutator avoids the disadvantages of traditional mechanical brushes. Therefore, brushless DC motors are widely used in various fields of national production such as aerospace, automotive electronics, industrial production, and household appliances. But the most serious problem of brushless DC motor is the torque ripple caused by the processing technology and motor performance. Torque ripple will bring a series of problems such as vibration, noise, resonance, etc., reduce the safety and reliability of system operation, and limit the application of brushless DC motors in high-precision fields.

In order to solve the problem of torque ripple, many scholars at home and abroad have done a lot of research. Control strategies such as PWM current modulation technology, adaptive control, and fuzzy control have been proposed. These control strategies mainly control the torque indirectly by controlling the current, which belongs to the torque open-loop control, and the torque response is slow. Direct torque control (DTC) belongs to the torque closed-loop control, which has the advantages of fast dynamic torque response, simple structure, strong robustness, and easy implementation. DTC adopts the concept of stator magnetic field orientation and space vector, and directly observes the flux linkage and torque of the motor in the stator coordinate system by detecting the stator voltage and current, and compares the observed value with the given value, and the difference is passed through the hysteresis loop. The controller gets the corresponding control signal, and then synthesizes the current flux state to select the corresponding voltage space vector to realize the direct control of the motor torque. It can be divided into two parts functionally: the observation and control part of the stator flux linkage, the role is to select the appropriate voltage space vector to generate hexagonal flux linkage in the stator; the torque observation and control part is to realize the rotation Instantaneous control of torque. Since the permanent magnet brushless DC motor is equipped with a Hall position sensor, and the flux chain formed by the voltage space vector determined by the position sensor on the motor stator during the operation of the motor is hexagonal, so when DTC is used in the brushless DC motor On the one hand, its flux observation part can be omitted to simplify the structure of the control system; on the other hand, the high dynamics of its torque control can be used to limit the torque fluctuation of the motor within a specified range.

The direct torque control strategy of brushless DC motor usually adopts two-phase conduction mode. Although this method can simplify the structure of the control system, when the motor is running in the high-speed section, it has the same impact on the commutation torque as the traditional pulse width modulation current control. The pulsation loses its inhibitory effect.

Contents of the invention

In order to solve the problem of poor commutation torque ripple suppression effect when the direct torque control of the brushless DC motor operates in a high-speed section, the present invention proposes a low torque ripple suppression method with a wide speed regulation range of the brushless DC motor. The current predictive control is added to the traditional direct torque control, the purpose is to minimize the torque ripple of the motor at low speed, medium speed and high speed.

The technical solution of the present invention is: a method for suppressing brushless DC motors with a wide speed regulation range and low torque ripple. The torque T _e ^* , the difference between the reference torque T _e ^* and the actual torque T _e is input to the two-point torque regulator, and the corresponding winding is turned on according to the output of the torque regulator, and combined with the feedback from the detection device The rotor position angle θ of the motor is added to the current predictive control, which ultimately minimizes the torque ripple of the motor at low, medium and high speeds.

Further, in the adjustment of speed and torque, the jump of the Hall position signal marks the start of commutation, and the reduction of the off-phase current to zero marks the end of commutation. The torque closed-loop control is performed at the non-commutation time, and the The commutation is overlapped, and during the commutation period, the measures of switching off the phase and non-commutating phase synchronous PWM modulation and opening the phase constant on are adopted, and a current prediction module is added to calculate the duty cycle.

Further, the current prediction model includes the following control steps:

Step 1) Sampling the non-commutation phase current value and the motor speed at the moment of k at the moment of commutation;

Step 2) Obtain the opposite electromotive force of the three-phase winding according to the current moment of the motor, that is, the operating state at time k;

Step 3) Calculate the budget based on the terminal voltage of the three-phase winding at the current moment k, the back electromotive force of the three-phase winding, the non-commutating current value at the current moment k and the predicted value of the non-commutating phase current at the next moment k+1 Control law D, so as to control the output of the inverter circuit;

Step 4) Steps 1)-3) are repeated until the commutation ends, and torque closed-loop control is performed.

Further, in step 3), the current prediction model calculates the predictive control law D as:

a) The expression of the current predictive control during commutation in the high-speed section:

b) The expression of the current predictive control during commutation in the low-speed section:

Among them, U _(k) is the voltage applied between the off-phase and non-commutated phase windings at time k, R is the winding resistance, i _(k) is the current of the non-commutated winding at time k, i _{(k+1 )} is the non-commutated winding current at time k+1, L is the winding inductance, T is the sampling period, E _(k) is the three-phase electromotive force amplitude at time k, and U _bus is the DC bus voltage.

Furthermore, the electromagnetic torque needs to be observed in the torque adjustment control. In the actual control system, the detectable quantities include the stator-to-ground terminal voltage, the stator current, and the neutral point voltage (for motors with neutral point lead-out wires, the Direct measurement, for a motor without a neutral point lead wire, the neutral point and motor speed can be simulated by connecting a three-phase Y-shaped symmetrical load in parallel at the stator end. The following torque calculation model is adopted:

T _e ＝(e _a i _a +e _b i _b +e _c i _c )/Ω

Among them, u _a , u _b , _uc are three-phase stator-to-ground terminal voltages, _un is neutral point voltage, e _a , e _b , e _c are three-phase stator back EMF, _ia , i _b , _ic is the three-phase stator current, L is the self-inductance of the stator, and M is the mutual inductance between the stators.

When the motor runs at high speed, the drop rate of the off-phase current is greater than the rise rate of the on-phase current, and the non-commutation current causes a current drop, and the direct torque control strategy cannot compensate for the reduction of the non-commutation current. According to the torque calculation formula, it can be seen that the torque decreases at this time, resulting in torque ripple. In order to compensate the drop of non-commutated phase current, current predictive control is introduced. In the current predictive control, the non-commutated phase current remains constant as the predictive reference trajectory, and the predictive model is established to control the current change rate of the off phase and the on phase consistently.

Due to the need to control the rate of change of the on-phase and off-phase currents at the same time, in order to delay the turn-off of the off-phase, the method of overlapping commutation is adopted. At the same time, during the commutation period, the measure of synchronous PWM modulation of the off phase and the non-commutation phase is adopted, and the open phase is constant, so as to reflect the current of the non-commutation phase on the DC bus during the delayed shutdown period. Taking phase B to phase C as an example when the motor is running at high speed, phase B is the off phase, phase C is on, and phase A is the non-commutation phase. Set the duty cycle of the turn-off phase and the turn-on phase during overlapping commutation as D _B . When the PWM is in the ON state, the current flows from the B phase and the C phase into the A phase; when the PWM is in the OFF state, the B phase current will continue to flow through its lower bridge arm diode, and the A phase current will continue to flow through its upper bridge arm diode. At this time, the three-phase voltage is:

From the three-phase voltage equation can get:

After discretization processing, we get:

Among them, T is the sampling period. Equation (3) is the expression of current predictive control during commutation in the high-speed section. When the voltage shown in formula (3) is applied during the period, the current value of the non-commutated winding at the moment (k+1) can be forced to reach the ideal value i* _(k+1) . The predictive control rule shows that the current on the non-commutated phase winding is kept constant during the commutation period as the control target, so that the on-phase is constant and the off-phase is compensated by applying the duty ratio _DB shown in formula (3) .

For the same reason, the current state during the commutation period when the motor is running at low speed, the expression of predictive control in the low speed section can be obtained:

Its physical meaning is: at the current moment k, when the voltage shown in formula (4) is applied between the open phase C and the non-commutated phase A, the current on the non-commutated winding at (k+1) time can be forced The value reaches the ideal value i* _(k+1) . In the formula, _DC is the duty cycle added to the open phase. It can be seen that when the motor is running in the low-speed area, the predictive control rule is as follows: the current on the non-commutated phase winding is kept constant during the commutation period as the control target, so that the off-phase is disconnected and the on-phase is applied with the formula (4). The duty cycle _DC shown is compensated.

Compared with prior art, the beneficial effect of the present invention is:

(1) The direct torque current predictive control strategy can better suppress the torque ripple of the permanent magnet brushless DC motor, and can compensate the reduction of commutation torque at high speed operation, and can weaken the increase of commutation torque at low speed operation .

(2) The direct torque current predictive control strategy does not need to consider the high-speed and low-speed running states of the motor separately, and a unified torque ripple method can be used in the entire speed range. Avoid adding extra hardware circuit and topological structure.

(3) The direct torque current predictive control strategy not only has the advantages of simple direct torque control structure, fast torque response and strong robustness, but also has the advantages of high control precision and strong controllability of current predictive control.

Description of drawings

Figure 1. Direct torque two-point regulation process

Figure 2. Schematic diagram of three-phase current waveforms during commutation

Figure 3. Schematic diagram of modulation measures during commutation

Figure 4. Schematic diagram of the circuit state during commutation

Figure 5. Schematic diagram of the predicted current compensation method

Figure 6. Block Diagram of Wide Speed Range and Low Torque Ripple Suppression System

Detailed ways

As shown in FIG. 1 , the present invention aims to solve the problem that the torque ripple suppression effect of the brushless DC motor is not satisfactory within a wide speed regulation range. Figure 1 is a diagram of the direct torque two-point regulation process. Among them, T _g is the torque given value, T _f is the torque feedback value, ΔT is the torque error, and T _Q is the torque switch signal. The tolerance of the regulator is ± _εm , and a discrete two-point regulation method is adopted. At time t ₁ , ΔT≤—ε _m , T _Q outputs "1". When T _Q = 1, the non-zero voltage space vector is selected in combination with the position signal, at this time the stator flux linkage rotates forward, T _f rises, and ΔT increases; at time t ₂ , ΔT increases to the upper limit of the tolerance + ε _m , that is, ΔT≥+ε _m , T _Q outputs "0". When T _Q = 0, the zero-voltage space vector is added to the motor, the stator flux linkage is stationary, T _f decreases, and ΔT decreases.

As shown in Figure 2, the schematic diagram of the three-phase current waveform of the motor running at different speeds. When the motor runs in the medium speed section (4E _m = U _bus ), the change rate of the on-phase and off-phase currents is basically the same, and no torque ripple will occur. When the motor is running in the low-speed section (4E _m < U _bus ), the rising rate of the on-phase current is greater than the falling rate of the off-phase current, the non-commutation current increases, and the torque increases. In order to reduce the continuous increase of torque , at this time the zero voltage vector is selected, the on-phase current is chopped, and finally the rate of change of the on-phase and off-phase currents is consistent, and the torque ripple is suppressed. When the motor is running in the high-speed section (4E _m > U _bus ), the rising rate of the on-phase current is less than the falling rate of the off-phase current, and the non-commutated phase current drops, and the off-phase is uncontrollable at this time, the selected The non-zero voltage vector can not compensate the reduction of the non-commutated phase current, and the torque ripple appears.

As shown in FIG. 3 , the present invention needs to simultaneously control the rate of change of the on-phase and off-phase currents, so a method of overlapping commutation is adopted, that is, the off-phase is turned off with a delay. In order to reflect the current of the non-commutated phase on the DC bus during the delayed turn-off period, a measure of synchronous PWM modulation of the off-phase and non-commutated phase is adopted during the commutation period, while the open phase is constant.

As shown in FIG. 4 , a schematic diagram of the circuit state of the modulation measure adopted in the present invention. Take the BA phase to CA phase transformation when the motor is running at high speed as an example, the B phase is the off phase, the C phase is the on phase, and the A phase is the non-commutation phase. Set the duty cycle of the turn-off phase and the turn-on phase during overlapping commutation as D _B . When the PWM is in the ON state, the current flows from the B phase and the C phase into the A phase; when the PWM is in the OFF state, the B phase current will continue to flow through its lower bridge arm diode, and the A phase current will continue to flow through its upper bridge arm diode.

As shown in FIG. 5 , the predictive current compensation method adopted in the present invention. In the actual control system, in the first control cycle of the commutation interval, the duty ratio calculated by the predicted current model has not been applied, and the rate of change of the commutation current cannot be guaranteed to be equal at this time, so the current ripple of the non-commutation phase cannot eliminate. The current needs to be compensated, still taking the commutation from BA to CA as an example, i _a(k-1) is the non-commutation phase current in the previous cycle, |Δi _b | is the change of the off-phase current, |Δi _c | is Turn-on phase current variation. The specific implementation method is to make the difference between the current variation of the on-phase and the off-phase, multiply it by the proportional constant K, and then negatively feed back the obtained value to the predicted current given terminal.

As shown in FIG. 6 , the system block diagram of wide speed regulation range and low torque ripple suppression designed by the present invention. The system adopts the speed and torque double-closed-loop control method. The torque given value is obtained from the speed error through PID controller tuning, and the torque is controlled by a two-point torque regulator, voltage space vector selection table, and inverter control brushless DC. Torque control is performed at non-commutation time, and overlapping commutation is performed at commutation time. At the same time, a current prediction module is added to calculate the duty cycle. The jump of the Hall position signal marks the beginning of the commutation, and the reduction of the off-phase current to zero marks the end of the commutation.

To sum up, the present invention provides a method for suppressing low torque ripple of a brushless DC motor with a wide speed regulation range. The specific method is as follows: the direct torque control strategy of the brushless DC motor usually adopts the two-phase conduction mode. Although this method can simplify the structure of the control system, when the motor is running in the high-speed section, it is the same as the traditional pulse width modulation current control. The inhibition effect on commutation torque ripple will be lost, and the present invention adds current predictive control to traditional direct torque control. The system adopts the speed and torque double-closed-loop control mode, torque control is performed at non-commutation time, overlapping commutation is performed at commutation time, and a current prediction module is added to calculate the duty cycle. When the hall position signal jumps, it marks the start of commutation, and the off-phase current decreases to zero to mark the end of commutation. At this time, the method of overlapping commutation is adopted. In order to reflect the current of the non-commutation phase on the DC bus during the delayed shutdown period, the synchronous PWM modulation of the shutdown phase and the non-commutation phase is adopted during the commutation period, and the open phase is constant. measure. Simultaneously sample the non-commutated current value and the motor speed at the current moment, that is, the k moment, and obtain the opposite electromotive force of the three-phase winding according to the current running state of the motor at the k moment, and finally use the current prediction control module to calculate the predictive control law D, so that The output of the inverter circuit is controlled to realize the torque ripple suppression of the brushless DC motor in a wide speed range.

In the description of this specification, references to the terms "one embodiment," "some embodiments," "exemplary embodiments," "example," "specific examples," or "some examples" are intended to mean that the implementation A specific feature, structure, material, or characteristic described by an embodiment or example is included in at least one embodiment or example of the present invention. In this specification, schematic representations of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

Although the embodiments of the present invention have been shown and described, those skilled in the art can understand that various changes, modifications, substitutions and modifications can be made to these embodiments without departing from the principle and spirit of the present invention. The scope of the invention is defined by the claims and their equivalents.

Claims (5)

1. A brushless DC motor wide speed range low torque ripple suppression method is characterized in that the difference between the reference brushless DC motor speed and the actual speed is input in the regulator of the PID, and the PID regulator outputs the reference torque T _e ^* , the difference between the reference torque T _e ^* and the actual torque T _e is input to the two-point torque regulator, and the corresponding winding is turned on according to the output of the torque regulator, and the motor fed back by the detection device is combined The rotor position angle θ is added to the current predictive control, which ultimately minimizes the torque ripple of the motor at low, medium and high speeds. 2. A method for suppressing brushless DC motors with wide speed regulation range and low torque ripple as claimed in claim 1, characterized in that, in the adjustment of the speed and torque, the jump of the Hall position signal marks the beginning of commutation , the off-phase current decreases to zero to mark the end of the commutation, torque closed-loop control is performed at the non-commutation time, overlapping commutation is performed at the commutation time, and synchronous PWM modulation of the off-phase and non-commutation phases is adopted during the commutation period As for the measure of turning on the phase constant flow, a current prediction module is added to calculate the duty cycle. 3. a kind of brushless DC motor wide speed regulation range low torque ripple suppressing method as claimed in claim 2, is characterized in that, current prediction model comprises following control steps: Step 1) Sampling the non-commutation phase current value and the motor speed at the moment of k at the moment of commutation; Step 2) Obtain the opposite electromotive force of the three-phase winding according to the current moment of the motor, that is, the operating state at time k; Step 3) Calculate the budget based on the terminal voltage of the three-phase winding at the current moment k, the back electromotive force of the three-phase winding, the non-commutating current value at the current moment k and the predicted value of the non-commutating phase current at the next moment k+1 Control law D, so as to control the output of the inverter circuit; Step 4) Steps 1)-3) are repeated until the commutation ends, and torque closed-loop control is performed. 4. a kind of brushless dc motor wide speed regulation range low torque ripple suppressing method as claimed in claim 3, is characterized in that, in step 3), the current prediction model calculation predictive control law D is: a) The expression of the current predictive control during commutation in the high-speed section: b) The expression of the current predictive control during commutation in the low-speed section: Among them, U _(k) is the voltage applied between the off-phase and non-commutated phase windings at time k, R is the winding resistance, i _(k) is the current of the non-commutated winding at time k, i _{(k+1 )} is the non-commutated winding current at time k+1, L is the winding inductance, T is the sampling period, E _(k) is the three-phase electromotive force amplitude at time k, and U _bus is the DC bus voltage. 5. a kind of brushless direct current motor wide speed regulation range low torque ripple suppressing method as claimed in claim 2 is characterized in that, also need to observe electromagnetic torque in torque adjustment control, in actual control system , the detectable quantities include stator-to-ground terminal voltage, stator current, neutral point voltage, and motor speed, using the following torque calculation model: T _e ＝(e _a i _a +e _b i _b +e _c i _c )/Ω Among them, u _a , u _b , _uc are three-phase stator-to-ground terminal voltages, _un is neutral point voltage, e _a , e _b , e _c are three-phase stator back EMF, _ia , i _b , _ic is the three-phase stator current, L is the self-inductance of the stator, and M is the mutual inductance between the stators.