A method for compensating sixth-order electromagnetic torque fluctuation using dead time

Abstract

The present invention relates to the technical field of permanent magnet synchronous motors, and more specifically, to a method for compensating for sixth-order electromagnetic torque fluctuations using dead time. Compared with the prior art, the present invention simultaneously compensates for the dead time and the harmonics of the main magnetic flux of the motor rotor, two factors that contribute more to the sixth-order electromagnetic torque fluctuations, so that the adverse effects they produce offset each other, and fully compensate for the sixth-order electromagnetic torque fluctuations. The present invention only needs to modify the original dead time in the control system, without changing other modules or increasing the complexity of the algorithm. Compared with the online compensation method, no additional torque sensor is required, saving hardware costs.

Description

Technical Field

[0001] The invention relates to the technical field of permanent magnet synchronous motors, in particular to a method for compensating sixth-order electromagnetic torque fluctuations by utilizing dead time. Background Art

[0002] The permanent magnet synchronous motor drive unit of EPS can be regarded as a torque amplification and tracking system. When the vehicle driver turns the steering wheel, the torque sensor installed on the steering mechanism sends a torque signal to the EPS controller, and the EPS controller receives the signal and transmits it to the torque command algorithm. The torque command algorithm processes the input torque signal and other signals such as vehicle speed and motor speed to generate a torque command for the permanent magnet synchronous motor drive unit. The drive unit generates output torque according to the torque command.

[0003] Customers' demand for smooth steering feel means that the motor and controller must produce small torque fluctuations. High torque fluctuations will cause poor steering feel, cause resonance in the mechanical structure, and thus cause noise, affecting the comfort of the electric power steering system.

[0004] The algorithm of three-phase permanent magnet synchronous motor is based on the torque equation, which is derived from the ideal motor model. In the ideal motor model, the main magnetic flux generated by the permanent magnet is distributed in the air gap between the stator and the rotor according to the sine law. However, in the actual permanent magnet synchronous motor, the perfect sine distribution does not exist, and the high-order harmonic components in the main magnetic flux are an important factor causing the 6th-order torque fluctuation.

[0005] On the other hand, in order to avoid the power supply short circuit caused by the simultaneous opening of the upper half-bridge MOSFET and the lower half-bridge MOSFET of the three-phase bridge drive circuit, a sufficiently large dead time must be added to the space vector modulation of the permanent magnet synchronous motor control algorithm. The dead time ensures the normal operation of the inverter, but causes output voltage distortion, which will cause the three-phase motor to generate two disturbances autonomously within one cycle of each phase current change, and passively receive two disturbances from each adjacent half bridge, ultimately leading to electromagnetic 6th-order torque fluctuations.

[0006] The existing compensation method for 6th-order electromagnetic torque fluctuation is to compensate for the dead time of the inverter, but the dead time is not the only factor that produces 6th-order electromagnetic torque fluctuation. The rotor main magnetic flux of the permanent magnet synchronous motor actually produced is not a perfect sinusoidal distribution due to process errors. The 5th and 7th harmonic components of the main magnetic flux will also cause 6th-order electromagnetic torque fluctuation in the motor output torque. This makes it impossible to eliminate the 6th-order electromagnetic torque fluctuation caused by the motor itself under the existing compensation method.

[0007] Therefore, it is necessary to design a method to compensate for the sixth-order electromagnetic torque fluctuation by using the dead time, and simultaneously compensate for the sixth-order electromagnetic torque fluctuation caused by the inverter dead time and the permanent magnet flux harmonics of the permanent magnet synchronous motor rotor. Summary of the invention

[0008] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a method for compensating sixth-order electromagnetic torque fluctuations by utilizing dead time, while compensating for the sixth-order electromagnetic torque fluctuations caused by inverter dead time and permanent magnet flux harmonics of a permanent magnet synchronous motor rotor.

[0009] In order to achieve the above object, the present invention provides a method for compensating sixth-order electromagnetic torque fluctuation by using dead time, which is characterized by comprising the following steps: Step 1, an electronic control unit receives a torque command sent by a torque sensor, and a torque controller calculates a direct-axis current reference value I according to the torque command. dref , quadrature axis current reference value I qref ; Step 2, the current sensing module collects the three-phase current of the motor and obtains the measured value of the direct-axis current I d , the measured value of the cross-axis current I q ; Step 3, set the direct axis current reference value I dref , quadrature axis current reference value I qref , measured value of direct axis current I d , the measured value of the cross-axis current I q As the input of the current controller, the direct axis voltage V is calculated. d , quadrature axis voltage V q ; Step 4, direct axis voltage V d The quadrature axis voltage Vq is transformed by Park inverse transformation to obtain the direct axis control voltage V α , quadrature axis control voltage V β ; Step 5, set the direct axis control voltage V α , quadrature axis control voltage V β As the input of the space vector pulse width modulation module, the dead time is used as the compensation of the space vector pulse width modulation module, and the three-phase voltage is converted by Clarke inverse transformation; step 6, the three-phase voltage is converted with the frequency f PMW The triangular wave is compared to generate a PWM wave signal for controlling the inverter MOSFET. The inverter outputs three-phase current to drive the permanent magnet synchronous motor to output torque.

[0010] The calculation formula of the dead time is: T offset is the dead time offset, amp is the amplitude of the dead time sinusoidal component, It is 6 times the electrical angle, and phase is the phase of the sinusoidal component of the dead time.

[0011] The direct axis current reference value I dref=0, quadrature axis current reference value T ref is the torque command reference value, N p is the number of motor pole pairs, ψ pm is the main magnetic flux of the motor rotor.

[0012] The measured value of the direct axis current I d , the measured value of the cross-axis current I q It is obtained by the three-phase current of the motor through Clarke transformation and Park coordinate transformation.

[0013] The direct axis voltage V d The calculation formula is Quadrature axis voltage V q The calculation formula is L d is the direct axis inductance of the motor, L q is the motor quadrature axis inductance, R is the motor phase resistance, ψ pm is the main magnetic flux of the motor rotor, 1 / T is the bandwidth of the current controller, s is the Laplace operator, W el is the electrical angular velocity.

[0014] The Park inverse transform calculation formula is: It is the motor rotor angle signal.

[0015] The Clarke inverse transform calculation formula is:

[0016] The output torque of the permanent magnet synchronous motor is Among them, ψ d =L d I d +ψ pm ,Ψ q =L q I q , L d is the direct axis inductance of the motor, L q is the motor quadrature axis inductance, N p is the number of motor pole pairs, ψ pm is the main magnetic flux of the motor rotor.

[0017] Compared with the prior art, the present invention simultaneously compensates for the dead time and the motor rotor main flux harmonics, two factors that contribute greatly to the sixth-order electromagnetic torque fluctuation, so that the adverse effects they produce offset each other. For complete compensation of the sixth-order electromagnetic torque fluctuation, the present invention only needs to modify the original dead time in the control system without changing other modules or increasing the complexity of the algorithm. Compared with the online compensation method, no additional torque sensor is required, saving hardware costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is the permanent magnet synchronous motor control system architecture and flow chart of the present invention.

[0019] Figure 2 This is the Simulink model diagram of the permanent magnet synchronous motor control of the present invention.

[0020] Figure 3 This is a time domain signal diagram of the output torque of the ideal motor control model in an embodiment of the present invention.

[0021] Figure 4 This is a frequency domain signal diagram of the torque output by the ideal motor control model in an embodiment of the present invention.

[0022] Figure 5 This is a frequency domain signal diagram of the output torque of the motor model in an embodiment of the present invention, in which the main magnetic flux of the rotor has harmonic components.

[0023] Figure 6 This is a frequency domain signal diagram of the output torque of the motor model in an embodiment of the present invention, in which the main magnetic flux of the rotor has harmonic components.

[0024] Figure 7 This is a time domain signal diagram of the output torque after inserting the dead time in an embodiment of the present invention.

[0025] Figure 8 This is a frequency domain signal diagram of the output torque after the dead time is inserted in an embodiment of the present invention.

[0026] Fig. 9 This is a basic principle diagram of torque compensation in an embodiment of the present invention. DETAILED DESCRIPTION

[0027] The present invention will now be further described with reference to the accompanying drawings.

[0028] See also Figure 1 , Figure 2 In the Simulink model, i is used d =0A control strategy, the specific method of this embodiment is as follows:

[0029] Step 1: The electronic control unit receives the torque command sent by the torque sensor, and the torque controller calculates the direct-axis current reference value I according to the torque command. dref , quadrature axis current reference value I qref .

[0030] Direct axis current reference value I dref =0A, quadrature axis current reference value T ref is the torque command reference value, N p is the number of motor pole pairs, ψ pmis the main magnetic flux of the motor rotor.

[0031] Step 2: The current sensing module collects the three-phase current of the motor and obtains the measured value of the direct-axis current I d , the measured value of the cross-axis current I q , the measured value of direct axis current I d , the measured value of the cross-axis current I q It is obtained by the three-phase current of the motor through Clarke transformation and Park coordinate transformation.

[0032] Step 3: Set the direct axis current reference value I dref , quadrature axis current reference value I qref , measured value of direct axis current I d , the measured value of the cross-axis current I q As the input of the current controller, the direct axis voltage V is calculated. d , quadrature axis voltage V q .

[0033] Direct axis voltage V d The calculation formula is Quadrature axis voltage V q The calculation formula is L d is the direct axis inductance of the motor, L q is the motor quadrature axis inductance, R is the motor phase resistance, ψ pm is the main magnetic flux of the motor rotor, 1 / T is the bandwidth of the current controller, s is the Laplace operator, W el is the electrical angular velocity.

[0034] Step 4: Direct axis voltage V in the rotating coordinate system d The quadrature axis voltage Vq is transformed by Park inverse transformation to obtain the direct axis control voltage V in the stator coordinate system. α , quadrature axis control voltage V β .

[0035] The calculation formula of Park inverse transform is: It is the motor rotor angle signal.

[0036] Step 5: Set the direct axis control voltage V α , quadrature axis control voltage V β As the input of the space vector pulse width modulation module, the dead time is used as the compensation of the space vector pulse width modulation module, and the three-phase voltage is converted by Clarke inverse transformation.

[0037] The dead time calculation formula is: T offset is the dead time offset, amp is the amplitude of the dead time sinusoidal component, is 6 times the electrical angle, and phase is the phase of the sinusoidal component of the dead time. amp and phase can be used to adjust the amplitude and phase of the torque fluctuation caused by the dead time. offset To ensure the dead time T d Greater than the switching time of the MOSFET to avoid short circuits.

[0038] The Clarke inverse transform calculation formula is:

[0039] Step 6: Compare the three-phase voltage with the frequency f PMW The triangular wave is compared to generate a PWM wave signal for controlling the inverter 7MOSFET. The inverter outputs a three-phase current to drive the permanent magnet synchronous motor to output torque.

[0040] The output torque of the permanent magnet synchronous motor is Among them, d =L d l d +ψ pm ,Ψ q =L q I q , L d is the direct axis inductance of the motor, Lq is the quadrature axis inductance of the motor, N p is the number of motor pole pairs, ψ pm is the main magnetic flux of the motor rotor.

[0041] The dead time of the present invention is a constant plus a sinusoidal component that changes periodically with the rotor position to change the amplitude and phase of the torque fluctuation caused by the dead time. When the torque fluctuation caused by the dead time and the torque fluctuation caused by the rotor main magnetic flux harmonic are of equal amplitude and opposite direction, the torque fluctuation caused by the two factors is completely offset, thereby achieving the purpose of torque fluctuation compensation.

[0042] The beneficial effects of the present invention will be confirmed below through the simulation results of Simulink.

[0043] The relevant parameters of the Simulink control model in this example are: stator phase resistance R is 12mΩ, the bandwidth 1 / T of the current controller is 200πHz, and the motor direct axis inductance value L d is 24uH, the motor quadrature axis inductance value is L q 30uH, motor pole number N p There are 4 pairs, the motor rotor main magnetic ψpm is 0.007Wb, the PWM signal frequency is 16kHz, the reference torque is 4Nm, and the motor speed is 500rpm.

[0044] If the permanent magnet synchronous motor in the simulation model is an ideal model, that is, the motor rotor main magnetic flux does not have any high-order harmonics and the dead time of the SVPWM module input is zero. The time domain signal and frequency domain signal of the output torque of the permanent magnet synchronous motor are as follows: Figure 3 and Figure 4 For a motor with 4 pole pairs, the 6th order electromagnetic torque fluctuation is reflected as the 24th order torque fluctuation. The Fourier transform of the output torque signal shows that the amplitude corresponding to the 24th order (200 Hz) of the torque signal is 4.26×10 -5 Nm.

[0045] The fifth and seventh order harmonic components of the rotor main flux are added to the ideal permanent magnet synchronous motor model, with component amplitude ψ5 = 10 -5 Wb,ψ7=10 -7 Wb, other parameters remain unchanged, at this time the motor output torque time domain and frequency domain waveforms are as follows Figure 5 and Figure 6 As shown in the figure, the corresponding amplitude of the torque signal at the 24th order (200Hz) is 0.1285Nm. It can be confirmed that the rotor main flux harmonic will indeed cause the 6th order electromagnetic torque fluctuation of the motor to increase.

[0046] Now remove the harmonic component of the rotor main flux added to the motor model, set the amp of the sinusoidal component added to the dead time to 0, and the dead time T d is 2us, at this time the amplitude of the motor 24th order torque fluctuation is 0.0215Nm, the simulation results are as follows Figure 7 and Figure 8 shown.

[0047] According to the simulation results, the dead time inserted in the pulse modulation process and the harmonics of the rotor permanent magnet main flux can both cause the sixth-order electromagnetic torque fluctuation of the motor. The difference is that the rotor permanent magnet flux harmonics are determined by the physical structure of the motor and are unavoidable inherent factors, while the dead time is a parameter that is artificially changed in the control algorithm.

[0048] The basic principle of torque compensation is usually as follows Fig. 9 According to this principle, the change of dead time can be approximately regarded as the change of torque command, because the dead time will change the current waveform provided to the motor. Similarly, the ultimate purpose of the change of torque command is to change the current waveform provided to the motor. The present invention utilizes this feature and uses the current change caused by the dead time to compensate for the torque fluctuation caused by the harmonics of the motor main magnetic flux.

Claims

1. A method for compensating sixth-order electromagnetic torque fluctuation using dead time, characterized in that: The steps include: Step 1: the electronic control unit receives the torque command sent by the torque sensor, and the torque controller calculates the direct axis current reference value I according to the torque command. dref , quadrature axis current reference value I qref ; Step 2, the current sensing module collects the three-phase current of the motor and obtains the measured value of the direct-axis current I d , the measured value of the cross-axis current I q ; Step 3, set the direct axis current reference value I dref , quadrature axis current reference value I qref , measured value of direct axis current I d , the measured value of the cross-axis current I q As the input of the current controller, the direct axis voltage V is calculated. d , quadrature axis voltage V q ; Step 4, direct axis voltage V d The quadrature axis voltage Vq is transformed by Park inverse transformation to obtain the direct axis control voltage V α , quadrature axis control voltage V β ; Step 5, set the direct axis control voltage V α , quadrature axis control voltage V β As the input of the space vector pulse width modulation module, the dead time is used as the compensation of the space vector pulse width modulation module, and the three-phase voltage is converted by Clarke inverse transformation; step 6, the three-phase voltage is converted with the frequency f PMW The triangle wave is compared to generate a PWM wave signal for controlling the inverter MOSFET. The inverter outputs a three-phase current to drive the permanent magnet synchronous motor to output torque. The calculation formula of the dead time is: T offset is the dead time offset, amp is the amplitude of the dead time sinusoidal component, is 6 times the electrical angle, phase is the phase of the sinusoidal component of the dead time, The direct axis current reference value I dref =0, quadrature axis current reference value T ref is the torque command reference value, N p is the number of motor pole pairs, ψ pm is the main magnetic flux of the motor rotor, The direct axis voltage V d The calculation formula is Quadrature axis voltage V q The calculation formula is L d is the direct axis inductance of the motor, L q is the motor quadrature axis inductance, R is the motor phase resistance, ψ pm is the main magnetic flux of the motor rotor, 1 / T is the bandwidth of the current controller, s is the Laplace operator, W el is the electrical angular velocity, The Clarke inverse transform calculation formula is:

2. The method for compensating sixth-order electromagnetic torque fluctuation by using dead time according to claim 1, characterized in that: The measured value of the direct axis current I d , the measured value of the cross-axis current I q It is obtained by the three-phase current of the motor through Clarke transformation and Park coordinate transformation.

3. The method for compensating sixth-order electromagnetic torque fluctuation by using dead time according to claim 1, characterized in that: The Park inverse transform calculation formula is: It is the motor rotor angle signal.

4. The method for compensating sixth-order electromagnetic torque fluctuation by using dead time according to claim 1, characterized in that: The output torque of the permanent magnet synchronous motor is Among them, d =L d I d +ψ pm ,Ψ q =L q I q , L d is the direct axis inductance of the motor, L q is the motor quadrature axis inductance, N p is the number of motor pole pairs, ψ pm is the main magnetic flux of the motor rotor.

Images