A method and circuit for detecting zero-crossing point of back electromotive force of a brushless motor

Abstract

The present invention discloses a method and circuit for detecting the zero-crossing point of the back electromotive force of a brushless motor. The detection method includes: a controller sampling at set time intervals, collecting the two conducting phase voltages and one suspended back electromotive force of the brushless motor in real time; and in the PWM-ON phase, when the suspended back electromotive force is on an upward trend, determining whether the suspended back electromotive force meets a first zero-crossing condition; when it is on a downward trend, determining whether it meets a second zero-crossing condition, to determine whether to enter a zero-crossing interrupt processing program; in the PWM-OFF phase, when it is on an upward trend, determining whether the suspended back electromotive force is not less than a preset range of the upper bridge conducting phase voltage; when it is on a downward trend, determining whether the suspended back electromotive force is not greater than a preset range of the upper bridge conducting phase voltage, to determine whether to enter a zero-crossing interrupt processing program. The present invention achieves high accuracy by collecting data in the PWM-ON and PWM-OFF phases and comparing them with the upper bridge conducting phase voltage.

Description

Technical field

[0001] The present invention relates to the technical field of brushless and inductorless motors, and in particular to a method and circuit for detecting a zero-crossing point of back electromotive force of a brushless motor. [Background Technology]

[0002] Existing brushless motors obtain the position of the rotor by using a position sensor or a position sensorless drive control. When using position sensorless drive control, the rotor position information is obtained by collecting phase voltage, and the commutation information is obtained by relying on the back electromotive force zero crossing point. The motor is usually driven to rotate by a PWM drive signal, and the back electromotive force voltage is collected during the PWM-ON phase to determine whether it has crossed zero. However, for a motor with constant speed control, when the load suddenly changes from heavy load to light load, its inertia is large. Due to the existence of PID, the PWM duty cycle changes from large to small. The entire PWM-ON time width is very small and the motor speed is very fast, which causes the back electromotive force detection to easily lag in commutation and lose step. Therefore, the zero crossing point cannot be accurately collected.

[0003] Please refer to Chinese Invention Patent No. CN113131805A, published on July 16, 2021, which discloses that when a PWM-ON modulation method is used, the zero-crossing detection scheme is to detect Vdc / 2 on the leading edge of the terminal voltage during PWM-ON and Vdc during PWM-OFF, and to detect Vdc / 2 on the trailing edge of the terminal voltage during PWM-ON and 0 during PWM-OFF. These are used as the back EMF zero-crossing points to determine the rotor position, and then perform commutation in advance. However, when PWM is OFF, the electrical energy in the winding will continue to flow through the diode, clamping the winding voltage to a low level. At this point, the voltage is very small and cannot be compared with Vdc, resulting in low accuracy.

[0004] Therefore, it is necessary to design a high-precision brushless motor back electromotive force zero-crossing detection method and circuit to solve the above problems. [Summary of the invention]

[0005] In view of the deficiencies in the prior art, the present invention aims to provide a method and circuit for detecting the zero-crossing point of the back electromotive force of a brushless motor with high precision.

[0006] The present invention solves the problems of the prior art by adopting the following technical solutions: a method for detecting the zero-crossing point of the back electromotive force of a brushless motor, wherein the brushless motor adopts a two-two conduction three-phase six-state working mode, and the detection method comprises: S1: a controller performs sampling at a set time interval, collects the voltage of the two conduction phases and the back electromotive force of a suspended phase of the brushless motor in real time, and judges whether it is in the PWM-ON stage or the PWM-OFF stage. If it is in the PWM-ON stage, it judges whether the suspended back electromotive force is in an upward trend or a downward trend and selects to enter S2 or S3. If it is in the PWM-OFF stage, it judges whether the suspended back electromotive force is in an upward trend or a downward trend and selects to enter S4 or S5. S2: in the PWM-ON stage, when the suspended back electromotive force is in an upward trend, it judges whether the suspended back electromotive force is in a downward trend. Whether the potential meets the first zero-crossing condition, if so, the zero-crossing interrupt processing procedure is performed, otherwise it returns to S1; S3: In the PWM-ON stage, when the suspended back electromotive force is in a downward trend, it is judged whether the suspended back electromotive force meets the second zero-crossing condition, if so, the zero-crossing interrupt processing procedure is performed, otherwise it returns to S1; S4: If in the PWM-OFF stage, when the suspended back electromotive force is in an upward trend, it is judged whether the suspended back electromotive force is not less than the preset range of the upper bridge conduction phase voltage, if so, the zero-crossing interrupt processing procedure is entered, otherwise it returns to S1; S5: If in the PWM-OFF stage, when the suspended back electromotive force is in a downward trend, it is judged whether the suspended back electromotive force is not greater than the preset range of the upper bridge conduction phase voltage, if so, the zero-crossing interrupt processing procedure is performed, otherwise it returns to S1.

[0007] A further improvement scheme is: the brushless motor includes a three-phase winding, the controller is connected to the brushless motor through an inverter circuit, the inverter circuit includes an upper bridge arm and a lower bridge arm connected in series, the connection points of the upper bridge arm and the lower bridge arm are respectively connected to the three-phase winding, and the two-phase winding is controlled to be conductive, the two-phase winding is divided into an upper bridge conduction phase and a lower bridge conduction phase, the voltage values ​​of the upper bridge conduction phase and the lower bridge conduction phase are respectively recorded as VH and VL, and the suspended back electromotive force is recorded as VE.

[0008] A further improvement is as follows: the first zero-crossing condition is: VE≥b%VH and VE>VEback+Voffset, the second zero-crossing condition is: VE≤d%VH and VE<VEback-Voffset, where VEback is the VE value collected last time, Voffset is the voltage offset value, and the preset range is e%VH.

[0009] A further improvement is as follows: the value range of b is 45-55, the value range of d is 45-55, and the value range of e is 45-55.

[0010] A further improvement is as follows: the voltage offset value Voffset is taken as the ad value set by the controller and the value of the voltage offset value Voffset is less than the value of the change rate of the suspended back electromotive force VE, wherein the value of the change rate of the suspended back electromotive force VE is the change value of the voltage per unit time of the back electromotive force within one electrical commutation cycle; the electrical commutation cycle is: the time between the start of the current commutation and the start of the next commutation.

[0011] A further improvement scheme is: the zero-crossing interrupt processing program performs commutation operation for the three phases of the brushless motor, that is, after detecting that the suspended phase electromotive force passes through the zero point, the corresponding delay time is set according to the preset electrical angle of the commutation point lagging behind the zero point. When the delay time is reached, the brushless motor enters the next commutation working state for commutation.

[0012] A further improvement scheme is: in the PWM-OFF stage, the two-phase voltage of the brushless motor and the opposite electromotive force of the suspended phase are collected in real time, and it is determined whether the two-phase voltage of the brushless motor and the opposite electromotive force of the suspended phase are less than the preset voltage. If so, the collection is continued; otherwise, the two-phase voltage of the brushless motor and the opposite electromotive force of the suspended phase are stored and it is determined whether to enter the zero-crossing interrupt processing program.

[0013] The present invention can also adopt the following technical solution to solve the existing technical problems: a brushless motor back electromotive force zero-crossing point detection circuit, including: positive and negative terminals of a power supply, the positive and negative terminals of the power supply are connected to the brushless motor through an inverter circuit, the inverter circuit is connected to the controller, the controller is connected to the brushless motor through a back electromotive force detection circuit, the controller generates a PWM signal to the inverter circuit to drive the brushless motor, the controller detects the two conducting phase voltages and a suspended back electromotive force of the brushless motor in the PWM-ON stage and the PWM-OFF stage through the back electromotive force detection circuit, and compares the suspended back electromotive force with the conducting phase voltage of the upper bridge to determine whether to enter the zero-crossing interrupt processing program.

[0014] A further improved solution is: the controller includes an operation module, a timing unit connected to the operation module, a storage unit and a PWM control module, and an AD module connected to the timing unit and the storage unit, and the AD module is connected to the back electromotive force detection circuit.

[0015] A further improved solution is: the brushless motor includes a three-phase winding, and the back electromotive force detection circuit includes a voltage divider circuit respectively connected to the three-phase winding.

[0016] Compared with the prior art, the present invention has the following beneficial effects: the controller detects the two conducting phase voltages and one suspended back electromotive force of the brushless motor through the back electromotive force detection circuit in the PWM-ON stage and the PWM-OFF stage, compares the suspended back electromotive force with the conducting phase voltage of the upper bridge to determine whether to enter the zero-crossing interrupt processing program, and performs zero-crossing judgment based on the real-time motor winding voltage, which can meet the normal zero-crossing detection of the motor with high accuracy.

Brief Description of the Drawings

[0017] The specific embodiments of the present invention are further described in detail below with reference to the accompanying drawings:

[0018] Figure 1 This is a circuit diagram for detecting the zero-crossing point of back electromotive force of a brushless motor according to the present invention;

[0019] Figure 2 This is a waveform diagram of the suspended back electromotive force of the brushless motor of the present invention;

[0020] Figure 3 It is a partial waveform diagram of the reverse electromotive force of the upper bridge conduction phase and the suspended phase of the brushless motor of the present invention;

[0021] Figure 4 This is a flow chart of the zero-crossing detection of the back electromotive force of the brushless motor of the present invention.

[0022] The meaning of the reference numerals in the figures:

[0023] 1. Controller 11, Operation Module 12, Timing Unit 13, Storage Unit 14, PWM Control Module 15, AD Module 2. Inverter Circuit 3, Brushless Motor 4, Back Electromotive Force Detection Circuit 5, Drive Module [Specific implementation method]

[0024] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0025] like Figures 1 to 3 A method for detecting the zero-crossing point of the back electromotive force of a brushless motor 3 is shown. The brushless motor 3 adopts a two-two conduction three-phase six-state working mode, including the following steps:

[0026] S1: The timing unit 12 interrupts at a set time interval, and the controller 1 performs sampling at a set time interval, collecting the two conducting phase voltages and one suspended back electromotive force of the brushless motor 3 in real time, and judging whether it is in the PWM-ON stage or the PWM-OFF stage. If it is in the PWM-ON stage, it is judged whether the suspended back electromotive force is in an upward trend or a downward trend and selects to enter S2 or S4. If it is in the PWM-OFF stage, it is judged whether the suspended back electromotive force is in an upward trend or a downward trend and selects to enter S6 or S7. In this step, the initial state is interrupted by synchronizing the PWM cycle to ensure that when the controller 1 collects the voltage, it starts from the PWM-ON stage;

[0027] S2: In the PWM-ON stage, when the suspended back electromotive force is on an upward trend, determine whether the suspended back electromotive force meets the high voltage preset condition. If so, return to S1, otherwise enter S3;

[0028] S3: Determine whether the suspended back electromotive force meets the first zero-crossing condition. If so, execute the zero-crossing interrupt processing procedure, otherwise return to S1;

[0029] S4: In the PWM-ON stage, when the suspended back electromotive force is in a downward trend, it is determined whether the suspended back electromotive force meets the low voltage preset condition. If so, it returns to S1, otherwise it enters S5;

[0030] S5: Determine whether the suspended back electromotive force meets the second zero-crossing condition. If so, execute the zero-crossing interrupt processing procedure, otherwise return to S1;

[0031] S6: If in the PWM-OFF stage, when the suspended back electromotive force is on an upward trend, determine whether the suspended back electromotive force is not less than the preset range of the upper bridge conduction phase voltage. If so, enter the zero-crossing interrupt processing program, otherwise return to S1;

[0032] S7: If in the PWM-OFF stage, when the suspended back electromotive force is in a downward trend, determine whether the suspended back electromotive force is not greater than the preset range of the upper bridge conduction phase voltage. If so, perform the zero-crossing interrupt processing procedure, otherwise return to S1.

[0033] In this embodiment, the brushless motor 3 includes a three-phase winding. The controller 1 is connected to the brushless motor 3 via an inverter circuit 2. The inverter circuit 2 includes an upper bridge arm and a lower bridge arm connected in series. The connection point of the upper bridge arm and the lower bridge arm is respectively connected to the three-phase winding and controls the conduction of two phase windings. The two phase windings are divided into an upper bridge conducting phase and a lower bridge conducting phase. The voltage values ​​of the upper bridge conducting phase and the lower bridge conducting phase are respectively denoted as VH and VL. The suspended back electromotive force is denoted as VE. The high voltage preset condition is: VE> a%VH, the first zero-crossing condition is: VE≥b%VH and VE>VEback+Voffset, the low voltage preset condition is: VE<c%VH, and the second zero-crossing condition is: VE≤d%VH and VE<VEback-Voffset, where VEback is the last collected VE value and Voffset is the voltage offset value. The preset range is e%VH. The value range of a is 70-90, further values ​​are 70-75, 75-80, 80-85, 85-90, the value range of b is 45-55, further values ​​are 45-50, 50-55, the value range of c is 10-30, further values ​​are 10-15, 15-20, 20-25, 25-30, the value range of d is 45-55, further values ​​are 45-50, 50-55, the value range of e is 45-55, further values ​​are 45-50, 50-55, preferably, a value is 85, b value is 50, c value is 15, d value is 50, and e value is 50. The Voffset value is the ad value set by the controller 1 and the Voffset value is less than the VE change rate value, wherein the VE change rate value is the change value of the back electromotive force voltage per unit time within one electrical commutation cycle, wherein the ad value is the reference voltage value corresponding to the accuracy of a single peripheral of the controller 1, and the electrical commutation cycle is: the period between the start of the current commutation and the start of the next commutation.

[0034] See also Figure 1 As shown, a circuit diagram of a brushless motor back electromotive force zero-crossing detection circuit according to the present invention is shown. The circuit includes positive and negative power supply terminals, which are connected to a brushless motor 3 via an inverter circuit 2. The inverter circuit 2 is connected to the controller 1. The controller 1 is connected to the brushless motor 3 via a back electromotive force detection circuit 4. The controller 1 generates a PWM signal to the inverter circuit 2 to drive the brushless motor 3. The controller 1 detects two conducting phase voltages and a suspended back electromotive force of the brushless motor 3 in the PWM-ON stage and the PWM-OFF stage through the back electromotive force detection circuit 4, and compares the suspended back electromotive force with the conducting phase voltage of the upper bridge to determine whether to enter the zero-crossing interrupt processing program.

[0035] In this embodiment, the brushless motor 3 takes the three-phase winding star connection as an example, the inverter circuit 2 includes an upper bridge arm and a lower bridge arm connected in series, the connection points of the upper bridge arm and the lower bridge arm are respectively connected to the three-phase winding, the upper bridge arm includes a first upper bridge arm, a second upper bridge arm, and a third upper bridge arm, the lower bridge arm includes a first lower bridge arm, a second lower bridge arm, and a third lower bridge arm, the upper bridge arm includes an upper switch tube Q1~Q3 respectively connected to each upper bridge arm, the lower bridge arm includes a lower switch tube Q4~Q6 respectively connected to each lower bridge arm, the switch tube Q1~Q6 is respectively connected in parallel with a diode D1~D6 for freewheeling, the three-phase windings U, V, and W of the brushless motor 3 are respectively connected to the connection points of the upper bridge arm and the lower bridge arm, the brushless motor 3 adopts a two-two conduction three-phase six-state working mode, and the control The device 1 sends a PWM signal to the inverter circuit 2, so that the upper switch tubes of one upper bridge arm and the lower switch tubes of any other lower bridge arm are turned on in pairs, and the upper and lower switch tubes of the same bridge arm cannot be turned on at the same time, so as to control the conduction of the two-phase windings each time. Therefore, the inverter circuit 2 can control the conduction of the switch tubes in the following six commutation working states: Q1Q5 is turned on, Q1Q6 is turned on, Q2Q4 is turned on, Q2Q6 is turned on, Q3Q4 is turned on, and Q3Q5 is turned on. The corresponding windings UV, UW, VU, VW, WU, and WV are energized in sequence. The commutation of the brushless motor 3 is achieved by switching between the six commutation working states, so that current flows through each coil in a sequential manner. The magnetic field generated by the current flow in the coil interacts with the permanent magnet of the rotor, and the magnetic poles of the permanent magnet rotor change, thereby rotating. The back electromotive force detection circuit 4 detects the three-phase voltage. The two-phase winding is divided into an upper bridge conduction phase and a lower bridge conduction phase. The voltage values ​​of the upper bridge conduction phase and the lower bridge conduction phase are respectively recorded as VH and VL. The suspended back electromotive force is recorded as VE.

[0036] The controller 1 is connected to the inverter circuit 2 via a drive module 5. The controller 1 includes a calculation module 11, a timing unit 12 connected to the calculation module 11, a storage unit 13, and a PWM control module 14, and an AD module 15 connected to the timing unit 12 and the storage unit 13. The AD module 15 is connected to the back electromotive force detection circuit 4. The PWM control module 14 transmits a PWM signal through the drive module 5 to control the switching tube of the inverter circuit 2 to conduct. The timing unit 12 controls the AD module 15 to sample the two conducting phase voltages and the suspended back electromotive force of the brushless motor 3 at set time intervals and store them. The calculation module 11 determines whether the two conducting phase voltages and the suspended back electromotive force are zero-crossing based on the stored conditions. That is, the zero-crossing interrupt processing procedure is to control the brushless motor 3 to enter the next commutation working state and commutate after detecting that the suspended back electromotive force crosses the zero point, according to the preset electrical angle after the commutation point crosses the zero point, and according to the corresponding stored delay time. When the delay time expires, the brushless motor 3 is controlled to enter the next commutation working state and commutate.

[0037] The back electromotive force detection circuit 4 includes a voltage divider circuit respectively connected to the three-phase windings of the brushless motor 3, including voltage divider resistors R1 to R6, the voltage divider resistor R1 is connected in series with the voltage divider resistor R2, and the upper end of the voltage divider resistor R1 is electrically connected to the U-phase winding, and the lower end of the voltage divider resistor R2 is grounded; the voltage divider resistor R3 is connected in series with the voltage divider resistor R4, and the upper end of the voltage divider resistor R3 is electrically connected to the V-phase winding, and the lower end of the voltage divider resistor R4 is grounded; the voltage divider resistor R5 is connected in series with the voltage divider resistor R6, and the upper end of the voltage divider resistor R5 is electrically connected to the W-phase winding, and the lower end of the voltage divider resistor R6 is grounded; the connection point of the voltage divider resistors R1 and R2, the connection point of R3 and R4, and the connection point of R5 and R6 are all electrically connected to the AD module 15.

[0038] The value of the VE change rate mentioned above can be: assuming that the operating voltage of the brushless motor 3 is 20V, the brushless motor 3 has two pairs of poles, and the speed is 25000r / min, then one electrical commutation cycle is 60*106 / (12*25000)=200us, and the suspended back electromotive force rises from 0V to 20V or drops from 20V to 0V in one electrical beat (200us), and the VE change rate value of 1us is 100mV; among them, 12 is 6 electrical beats*2 pairs of poles, and 60*106 is 1 minute=60*106us.

[0039] The Voffset value mentioned above can be: if R1=R3=R5=30k, R2=R4=R6=4.3k, Voffset takes 6 AD values, for example, the sampling accuracy of the AD module 15 is 12 bits (ie the total number of AD is 4096), the sampling time interval is 10us, and the operating voltage of the controller 1 is 3.3v, then the reference voltage value corresponding to the single peripheral accuracy (ie each AD) is 3.3*34.3 / (4096*4.3)=6.43mv, then the Voffset value is 6.43*6=38.58mv, for The corresponding VE change rate within 10us is 1000mv, that is, the Voffset value is less than the VE change rate value. In this way, when the AD module 15 collects the suspended opposite electromotive force, it can filter out the voltage value collected on the unstable noise, thereby improving the accuracy of voltage collection. At the same time, it ensures that when the suspended opposite electromotive force is in an upward trend, the AD module 15 judges zero crossing in the rising edge stage of the suspended opposite electromotive force, and when the suspended opposite electromotive force is in a downward trend, the AD module 15 judges zero crossing in the falling edge stage of the suspended opposite electromotive force, thereby reducing the error rate of zero crossing detection of the suspended opposite electromotive force.

[0040] In this embodiment, whether it is in the PWM-ON stage or the PWM-OFF stage is determined by the number of sampling times. If the PWM frequency is 10KHZ, the period is 100us, the PWM duty cycle is 25%, the PWM-ON time is 25us, the PWM-OFF time is 75us, the sampling time interval is 10us, and the timing start time is synchronized with the PWM-ON start time. Therefore, the operation module 11 is in the PWM-ON stage in the first and second sampling time intervals, and in the PWM-OFF stage in the third to tenth sampling time intervals.

[0041] Combined with attachment Figure 4 As shown, the preferred back electromotive force zero-crossing detection method of the present invention is as follows:

[0042] S1: the timing unit 12 enters the time interval interruption;

[0043] S2: Real-time acquisition of the two conducting phase voltages VH and VL of the brushless motor 3 and a suspended back electromotive force VE, and determines whether it is in the PWM-ON stage or the PWM-OFF stage. If it is in the PWM-ON stage, it is determined whether the suspended back electromotive force is on an upward or downward trend and the process is selected to enter S3 or S8. If it is in the PWM-OFF stage, it is determined whether the suspended back electromotive force is on an upward or downward trend and the process is selected to enter S11 or S15. In this step, the initial state is interrupted by synchronizing the PWM cycle to ensure that the controller 1 starts from the PWM-ON stage when acquiring the voltage;

[0044] S3: Determine whether the time interval interrupt is in the PWM-ON stage. If so, the AD module 15 collects the two conducting phase voltages VH and VL and the floating phase voltage VE. At the same time, the calculation module 11 accumulates the number of times the AD module 15 collects the three-phase phase voltages Sampcount++ and enters S4;

[0045] S4: When Sampcount=1, the process proceeds to S16, where the storage unit 13 stores the suspended phase voltage VE currently collected by the AD module 15, and uses it to update the suspended phase voltage VEback collected last time. When Sampcount≥2, if the suspended phase back electromotive force is on an upward trend, the process proceeds to S5; if the suspended phase back electromotive force is on a downward trend, the process proceeds to S8.

[0046] S5: The calculation module 11 determines whether the suspended phase voltage VE is greater than 85% VH. If so, the process proceeds to S16; otherwise, the process proceeds to S6;

[0047] S6: The calculation module 11 determines whether VE is greater than or equal to 50% VH. If so, it goes to S7; otherwise, it goes to S16;

[0048] S7: The operation module 11 determines whether VE is greater than the sum of the previous VE value VEback and the voltage offset value Voffset. If so, it enters the zero-crossing interrupt handling program, and at the same time, Sampcount is cleared; otherwise, it enters S16;

[0049] S8: The operation module 11 determines whether VE is less than 15% of VH. If so, it enters S16; otherwise, it enters S9;

[0050] S9: The operation module 11 determines whether VE is less than or equal to 50% of VH. If so, it enters S10; otherwise, it enters S16;

[0051] S10: The operation module 11 determines whether VE is less than the difference between the previous VE value VEback and the voltage offset value Voffset. If so, it enters the zero-crossing interrupt handling program, and at the same time, Sampcount is cleared; otherwise, it enters S16;

[0052] S11: Determine whether the voltage of two conducting phases and the back electromotive force of a floating phase of the brushless motor 3 is less than a preset voltage, such as 2V. If so, return to S1; otherwise, enter S12;

[0053] S12: The operation module 11 increments the count Sampcount of the number of times the AD module 15 samples the voltage of two conducting phases and the back electromotive force of a floating phase, and enters S13;

[0054] S13: When Sampcount = 1, it enters S16, and the storage unit 13 saves the floating-phase voltage VE currently sampled by the AD module 15 to update the previous floating-phase voltage VEback sampled; when Sampcount ≥ 2, if the back electromotive force of the floating phase is on the rising trend, it enters S14, and if the back electromotive force of the floating phase is on the falling trend, it enters S15;

[0055] S14: The operation module 11 determines whether VE is greater than or equal to 50% of VH and VE > VEback. If so, it enters the zero-crossing interrupt handling program, and at the same time, Sampcount is cleared; otherwise, it enters S16;

[0056] S15: The operation module 11 determines whether VE is less than or equal to 50% of VH and VE < VEback. If so, it enters the zero-crossing interrupt handling program, and at the same time, Sampcount is cleared; otherwise, it enters S16;

[0057] S16: The storage unit 13 saves the floating-phase voltage VE currently sampled by the AD module 15 to update the previous floating-phase voltage VEback sampled by the AD module 15, and returns to S1.

[0058] The above back electromotive force zero-crossing detection method is combined with the attached Figure 2As shown, the PWM control module 14 outputs a PWM signal, and the AD module 15 samples at the PWM-ON position, for example, at a, and samples at the PWM-OFF position, for example, at b. When switching, since the voltage on the winding cannot change suddenly, the diode in parallel with the switch tube continues to flow, and the suspended back electromotive force is on an upward trend, generating P1, P2, P3, and P4 segment waveforms. The transition period of the P1 segment waveform is 1us to 2us, which is relatively short. When the AD module 15 collects voltage, the P1 segment has passed, and it is directly collected at the P2 segment. The voltage collection interval of the AD module 15 is 10us. First, when VE>85%VH, the waveform with the suspended back electromotive force higher than 85%VH is filtered out, that is, The circuit then filters out waveforms with the back EMF falling below 50% VH, i.e., segments P3, P4, and P5, when 50% VH ≤ VE ≤ 85% VH. Finally, when VE > VEback + Voffset, the circuit enters a zero-crossing interrupt handler. Furthermore, when the back EMF is on a downward trend, waveforms with the back EMF below 15% VH are filtered out, i.e., segment P7. Then, when 15% VH ≤ VE ≤ 50% VH, waveforms with the back EMF rising above 50% VH are filtered out, i.e., segments P8, P9, and P10. Finally, when VE < VEback - Voffset, the circuit enters a zero-crossing interrupt handler. Furthermore, during the PWM-OFF phase, the two conducting phase voltages and one suspended back EMF of the brushless motor 3 experience demagnetization time. Therefore, the portions with low conducting phase voltages and back EMF voltages, such as those less than 2V, should be filtered out. It can solve the problem that due to the freewheeling energy existing in the inductive characteristics of the winding, the suspended back electromotive force waveform suddenly increases or decreases, causing false back electromotive force detection, resulting in false zero crossings, causing errors in commutation information, affecting motor control performance, and even abnormal shutdown.

[0059] like Figure 1 and Figure 3 As shown, if the windings U and V are turned on, the PWM control module 14 outputs the PWM-ON stage, the switch tubes Q1 and Q5 are turned on, and the upper bridge conduction phase voltage c is close to the power supply voltage. If in the PWM-OFF stage, the switch tubes Q1 and Q5 are turned off, and the diodes D2 and D4 are used for freewheeling. At this time, the voltage at c is clamped to the ground, as shown at d. At this time, the voltage is very small. When the freewheeling ends, due to the inertia of the brushless motor 3 itself, the voltage gradually rises. At this time, zero crossing may exist at e and f, that is, VE = 50% VH.

[0060] The controller 1 of the present invention detects the two conducting phase voltages and one suspended back electromotive force of the brushless motor 3 through the back electromotive force detection circuit 4 in the PWM-ON stage and the PWM-OFF stage, and compares the suspended back electromotive force with the conducting phase voltage of the upper bridge to determine whether to enter the zero-crossing interrupt processing program. The zero-crossing judgment is performed based on the real-time motor winding voltage, which can meet the normal zero-crossing detection of the motor with high accuracy.

[0061] The present invention is not limited to the specific embodiments described above. Those skilled in the art will readily appreciate that numerous alternatives exist to the brushless motor back EMF zero-crossing detection method and circuit described herein without departing from the principles and scope of the present invention. The scope of protection of the present invention shall be determined by the claims.

Claims

1. A method for detecting the zero-crossing point of back electromotive force of a brushless motor, wherein the brushless motor adopts a two-two conduction three-phase six-state operation mode; characterized in that: The detection method includes: S1: the controller samples at a set time interval, collects the two conducting phase voltages and the suspended back electromotive force of the brushless motor in real time, and determines whether it is in the PWM-ON stage or the PWM-OFF stage. If it is in the PWM-ON stage, it determines whether the suspended back electromotive force is in an upward trend or a downward trend and selects to enter S2 or S3. If it is in the PWM-OFF stage, it determines whether the suspended back electromotive force is in an upward trend or a downward trend and selects to enter S4 or S5. S2: in the PWM-ON stage, when the suspended back electromotive force is in an upward trend, it determines whether the suspended back electromotive force meets the first zero-crossing condition. If it meets the condition, the zero-crossing interrupt processing procedure is performed, otherwise it returns to the zero-crossing interrupt processing procedure. Return to S1; S3: In the PWM-ON stage, when the suspended back electromotive force is in a downward trend, determine whether the suspended back electromotive force meets the second zero-crossing condition. If so, perform the zero-crossing interrupt processing procedure, otherwise return to S1; S4: If in the PWM-OFF stage, when the suspended back electromotive force is in an upward trend, determine whether the suspended back electromotive force is not less than the preset range of the upper bridge conduction phase voltage. If so, enter the zero-crossing interrupt processing procedure, otherwise return to S1; S5: If in the PWM-OFF stage, when the suspended back electromotive force is in a downward trend, determine whether the suspended back electromotive force is not greater than the preset range of the upper bridge conduction phase voltage. If so, perform the zero-crossing interrupt processing procedure, otherwise return to S1; The brushless motor includes a three-phase winding, the controller is connected to the brushless motor via an inverter circuit, the inverter circuit includes an upper bridge arm and a lower bridge arm connected in series, the connection points of the upper bridge arm and the lower bridge arm are respectively connected to the three-phase winding, and the two-phase winding is controlled to be conductive, the two-phase winding is divided into an upper bridge conductive phase and a lower bridge conductive phase, the voltage values ​​of the upper bridge conductive phase and the lower bridge conductive phase are respectively recorded as VH and VL, and the suspended back electromotive force is recorded as VE; The first zero-crossing condition is: VE≥b%VH and VE>VEback+Voffset, and the second zero-crossing condition is: VE≤d%VH and VE<VEback-Voffset, where VEback is the VE value collected last time, Voffset is the voltage offset value, and the preset range is e%VH.

2. The method for detecting the zero-crossing point of back electromotive force of a brushless motor according to claim 1, wherein: The value range of b is 45-55, the value range of d is 45-55, and the value range of e is 45-55.

3. The method for detecting zero-crossing point of back electromotive force of a brushless motor according to claim 2, wherein: The voltage offset value Voffset is the ad value set by the controller and the value of the voltage offset value Voffset is less than the value of the change rate of the suspended back electromotive force VE, wherein the value of the change rate of the suspended back electromotive force VE is the change value of the voltage per unit time of the back electromotive force within an electrical commutation cycle; the electrical commutation cycle is: the time between the start of the current commutation and the start of the next commutation.

4. The method for detecting the zero-crossing point of back electromotive force of a brushless motor according to claim 1, wherein: The zero-crossing interrupt processing program performs commutation operation on the three phases of the brushless motor, that is, after detecting that the suspended phase electromotive force crosses the zero point, the corresponding delay time is set according to the preset electrical angle of the commutation point lagging behind the zero point. When the delay time is reached, the brushless motor enters the next commutation working state for commutation.

5. The method for detecting the zero-crossing point of back electromotive force of a brushless motor according to claim 1, wherein: In the PWM-OFF stage, the two-phase voltage and the suspended phase electromotive force of the brushless motor are collected in real time, and it is determined whether the two-phase voltage and the suspended phase electromotive force of the brushless motor are less than the preset voltage. If so, the collection continues; otherwise, the two-phase voltage and the suspended phase electromotive force are stored and it is determined whether to enter the zero-crossing interrupt processing program.

6. A brushless motor back electromotive force zero-crossing detection circuit for implementing the detection method according to any one of claims 1 to 5, comprising: Positive and negative power supply terminals, the positive and negative power supply terminals are connected to the brushless motor through an inverter circuit, the inverter circuit is connected to the controller, and the controller is connected to the brushless motor through a back-electromotive force detection circuit; characterized in that: the controller generates a PWM signal to the inverter circuit to drive the brushless motor, the controller detects the two conducting phase voltages and a suspended back-electromotive force of the brushless motor through the back-electromotive force detection circuit in the PWM-ON stage and the PWM-OFF stage, and compares the suspended back-electromotive force with the conducting phase voltage of the upper bridge to determine whether to enter the zero-crossing interrupt processing program.

7. The brushless motor back electromotive force zero-crossing detection circuit according to claim 6, characterized in that: The controller includes a calculation module, a timing unit connected to the calculation module, a storage unit and a PWM control module, and an AD module connected to the timing unit and the storage unit. The AD module is connected to the back electromotive force detection circuit.

8. The brushless motor back electromotive force zero-crossing detection circuit according to claim 6, characterized in that: The brushless motor includes a three-phase winding, and the back electromotive force detection circuit includes a voltage divider circuit respectively connected to the three-phase winding.

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