Motor control circuit and control method

Abstract

The present invention discloses a motor control circuit and control method, comprising: a power supply, a drive circuit, a PWM signal generating circuit, a controller, and a back-electromotive force (BEMF) detection circuit. The power supply is connected to the motor via the drive circuit, the drive circuit is connected to the controller via the PWM signal generating circuit, the controller controls the drive circuit by changing the PWM signal frequency, and the controller is connected to the motor via the BEMF detection circuit to obtain BEMF parameters. The controller stores a correspondence between the BEMF parameters and the PWM signal frequency. The controller receives the BEMF parameters and sends a corresponding PWM signal to the drive circuit to drive the motor. The present invention directly obtains the motor parameters to change the signal driving the motor, and can meet the normal operation of the motor under high speed and heavy load, with high accuracy and low power consumption.

Description

Technical field

[0001] The present invention relates to the technical field of motor control, and in particular to a control circuit and a control method for a motor PWM signal. [Background Technology]

[0002] Existing brushless motors generally use PWM drive. When the motor speed is relatively low, a lower PWM frequency is used, while when the speed increases, a higher PWM frequency is used. This is because, for sensorless control of some brushless motors, there are requirements for the minimum effective pulse width and minimum startup duty cycle of the PWM. For example, the startup duty cycle cannot exceed 10%, and the minimum pulse width is 10us. Therefore, the startup PWM frequency needs to be 10kHz. However, if such a motor operates at very high speeds during normal operation, such as exceeding 200,000 RPM, a 10kHz frequency will not meet these requirements. To ensure normal operation, the PWM frequency needs to be increased to 20kHz. In other words, the higher the speed, the higher the PWM frequency. However, for tools that achieve constant speed control, if a high PWM frequency is still used at high speeds and high torque, and the PWM frequency is not fully duty cycled, the power devices driving the brushless motor will switch at high frequencies, resulting in significant losses and high temperature rise, which will affect the life of the power devices.

[0003] Please refer to Chinese Invention Patent No. CN111657787B, published on May 10, 2022, which discloses that a control unit sets the PWM frequency to a relatively low level when the motor power (current value) detected by the current detection unit is relatively high, and sets the PWM frequency to a relatively high level when the detected motor power (current value) is relatively low. Setting the PWM frequency based on a comparison with the power threshold (current threshold) can suppress switching losses in switching element SW1. However, obtaining motor power indirectly reflects the motor's operating condition and cannot accurately reflect the actual motor operating condition, resulting in low accuracy.

[0004] Therefore, it is necessary to design a motor control circuit and control method with high precision and low power consumption to solve the above problems. [Summary of the invention]

[0005] In view of the shortcomings of the prior art, an object of the present invention is to provide a motor control circuit and a control method with high precision and low power consumption.

[0006] The present invention solves the existing technical problems by adopting the following technical solutions: a motor control circuit, comprising: positive and negative terminals of a power supply, a drive circuit, a PWM signal generating circuit, a controller and a back electromotive force detection circuit, wherein the positive and negative terminals are connected to the motor through the drive circuit, the drive circuit is connected to the controller through the PWM signal generating circuit, the controller controls the drive circuit by changing the PWM signal frequency, the controller is connected to the motor through the back electromotive force detection circuit to obtain back electromotive force parameters, the controller stores the correspondence between the back electromotive force parameters and the PWM signal frequency, the controller receives the back electromotive force parameters and sends the corresponding PWM signal frequency to the drive circuit to drive the motor.

[0007] A further improvement scheme is: the motor includes a three-phase winding, the controller controls the conduction of two-phase windings, the back electromotive force detection circuit detects the back electromotive force of the suspended phase winding, the back electromotive force parameters include the back electromotive force voltage change, and the back electromotive force voltage change is the difference between the back electromotive force voltages collected at each adjacent two switching times.

[0008] A further improved solution is: the correspondence between the back electromotive force parameter and the PWM signal frequency includes multiple voltage change thresholds stored in the controller and the PWM signal frequencies corresponding to the multiple voltage change thresholds, the multiple voltage change thresholds include a first voltage change threshold and a second voltage change threshold, a first PWM signal frequency corresponding to the first voltage change threshold, and a second PWM signal frequency corresponding to the second voltage change threshold, the first voltage change threshold is greater than the second voltage change threshold, and the first PWM signal frequency is less than the second PWM signal frequency, when the back electromotive force voltage change is greater than the first voltage change threshold, the controller outputs the first PWM signal frequency to the drive circuit; when the back electromotive force voltage change is greater than the second back electromotive force threshold, the controller outputs the second PWM signal frequency to the drive circuit.

[0009] A further improvement is that the correspondence between the back electromotive force parameter and the PWM signal frequency includes a corresponding functional relationship between the back electromotive force voltage change and the PWM signal frequency, wherein the functional relationship is Fpwm=f(ΔV), and the PWM signal frequency is inversely proportional to the back electromotive force voltage change.

[0010] A further improvement is that the corresponding relationship between the back electromotive force parameter and the PWM signal frequency includes the corresponding relationship between the set speed, the back electromotive force voltage change and the PWM signal frequency.

[0011] A further improvement scheme is: the motor includes a three-phase winding, the controller controls the conduction of two-phase windings, the back electromotive force detection circuit detects the back electromotive force of the suspended phase winding, and the back electromotive force parameters include the back electromotive force freewheeling time, which is the freewheeling time generated each time the phase is changed.

[0012] A further improved solution is: the correspondence between the back electromotive force parameter and the PWM signal frequency includes multiple freewheeling time thresholds stored in the controller and the PWM signal frequencies corresponding to the multiple freewheeling time thresholds, the multiple freewheeling time thresholds include a first freewheeling time threshold and a second freewheeling time threshold, a first PWM signal frequency corresponding to the first freewheeling time threshold, and a second PWM signal frequency corresponding to the second freewheeling time threshold, the first freewheeling time threshold is less than the second freewheeling time threshold, and the first PWM signal frequency is greater than the second PWM signal frequency, when the freewheeling time is greater than the first freewheeling time threshold, the controller outputs the first PWM signal frequency to the drive circuit; when the freewheeling time is greater than the second freewheeling time threshold, the controller outputs the second PWM signal frequency to the drive circuit.

[0013] A further improvement is that the correspondence between the back electromotive force parameter and the PWM signal frequency includes a corresponding functional relationship between the freewheeling time and the PWM signal frequency. The functional relationship is Fpwm=f(Δt), and the PWM signal frequency is inversely proportional to the freewheeling time.

[0014] A further improvement scheme is: the corresponding relationship between the back electromotive force parameter and the PWM signal frequency includes the corresponding relationship between the set speed, the freewheeling time and the PWM signal frequency.

[0015] The present invention can also adopt the following technical solutions to solve the existing technical problems: a motor control method, the control method comprising: step 1, selecting a set speed of the tool; step 2, the controller obtains back electromotive force parameters through a back electromotive force detection circuit; step 3, the controller compares the back electromotive force parameters with the corresponding relationship between the internally stored back electromotive force parameters and the PWM signal frequency to select the corresponding PWM signal frequency to the drive circuit to drive the motor.

[0016] Compared with the prior art, the present invention has the following beneficial effects: the controller is connected to the motor through the back electromotive force detection circuit to obtain the back electromotive force parameters, the controller stores the corresponding relationship between the back electromotive force parameters and the PWM signal frequency, the controller receives the back electromotive force parameters and sends the corresponding PWM signal to the drive circuit to drive the motor, directly obtains the motor parameters to change the signal driving the motor, and can meet the normal operation of the motor at high speed and heavy load, with high accuracy and low power consumption.

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 is a circuit diagram of a motor control circuit of the present invention;

[0019] Figure 2 1 is a waveform diagram of the back electromotive force and bus current of the motor of the present invention;

[0020] Figure 3 is a control flow chart of the first embodiment of the present invention;

[0021] Figure 4 It is a control flow chart of the second embodiment of the present invention.

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

[0023] 1. Drive circuit 2. Motor 3. PWM signal generation circuit 4. Controller 5. Speed ​​setting module 6. Back electromotive force detection circuit [Specific implementation method]

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

[0025] See also Figure 1 As shown, a motor control circuit involved in the present invention includes positive and negative terminals of a power supply, and the positive and negative terminals are connected to the motor 2 through a drive circuit 1. The drive circuit 1 is connected to a controller 4 through a PWM signal generating circuit 3. A speed setting module 5 is connected to the controller 4 so that the user can select a suitable motor gear. The controller 4 is connected to the motor 2 through a back electromotive force detection circuit 6 to obtain back electromotive force parameters. The controller 4 stores the corresponding relationship between the back electromotive force parameters and the PWM signal frequency. The controller 4 receives the back electromotive force parameters and sends a corresponding PWM signal to the drive circuit 1 to drive the motor 2.

[0026] The motor 2 is a brushless motor. In this embodiment, taking the three-phase winding star connection as an example, the drive circuit 1 includes a first bridge arm, a second bridge arm, and a third bridge arm. Each bridge arm is connected between the positive and negative terminals of the power supply. The first bridge arm is composed of a first switch tube Q1 (also called an upper switch) and a fourth switch tube Q4 (also called a lower switch) connected in series; the second bridge arm is composed of a second switch tube Q2 (also called an upper switch) and a fifth switch tube Q5 (also called a lower switch) connected in series; the third bridge arm is composed of a third switch tube Q3 (also called an upper switch) and a sixth switch tube Q6 (also called a lower switch) connected in series. The first switch tube Q1 to the sixth switch are connected in series. Each transistor Q6 is connected in parallel with a diode for freewheeling. The three-phase windings U, V, and W of the motor 2 are connected to the midpoint of each bridge arm, respectively. The controller 4 sends a PWM signal to the PWM signal generating circuit 3. The PWM signal generating circuit 3 controls the upper switching transistors of one bridge arm of the drive circuit 1 and the lower switching transistors of any other bridge arm to turn on in pairs. The upper and lower switching transistors of the same bridge arm cannot be turned on at the same time to control the conduction of two phase windings at a time. Therefore, the drive circuit 1 can control the switching transistors to turn on in the following six states: Q1Q5 is on, Q1Q6 is on, Q2Q4 is on, Q2Q6 is on, Q3Q4 is on, and Q3Q5 is on. The corresponding windings UV, UW, VU, VW, WU, and WV are energized in sequence. The phase change of the motor 2 is achieved by switching between the six 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 magnets of the rotor, changing the magnetic poles of the permanent magnet rotor and causing rotation. The back-EMF detection circuit 6 detects the back-EMF of the suspended phase winding. Because energy is present in the winding during commutation, freewheeling current is required through the corresponding diode to dissipate this energy. The freewheeling time varies depending on the load condition. A PWM signal consists of an ON phase and an OFF phase. The duration of each ON phase and each OFF phase is the PWM signal period, and the reciprocal of the period is the frequency. The controller 4 changes the PWM signal frequency based on the back-EMF parameter.

[0027] like Figure 2 and Figure 3The figure shows the back EMF and bus current waveforms of the motor C-phase winding and the control flow chart of the first embodiment. In this embodiment, as the load increases, the instantaneous current increases, and the voltage drop across the switch tube increases, that is, the back EMF voltage changes. The back EMF parameters include the back EMF voltage variation, which is the difference between the back EMF voltages collected at each two adjacent commutation times. The correspondence between the back EMF parameters and the PWM signal frequency includes the controller 4 storing multiple voltage variation thresholds and corresponding PWM signal frequencies. The multiple voltage variation thresholds include a first voltage variation threshold and a second voltage variation threshold, a first PWM signal frequency corresponding to the first voltage variation threshold, and a second PWM signal frequency corresponding to the second voltage variation threshold, wherein the first voltage variation threshold is greater than the second voltage variation threshold, and the first PWM signal frequency is less than the second PWM signal frequency. When the back EMF voltage variation is greater than the first voltage variation threshold, the controller 4 outputs the first PWM signal frequency to the drive circuit 1; when the back EMF voltage variation is greater than the second back EMF threshold, the controller 4 outputs the second PWM signal frequency to the drive circuit 1. The specific control process includes: step 1, setting the speed through the speed setting module 5, and the motor starts with the initial PWM signal; step 2, entering the PWM adjustment entrance to adjust the PWM signal frequency; step 3, judging whether the sampling time has arrived, if so, entering step 4, collecting the back electromotive force voltage change, such as the back electromotive force voltage difference between the commutation time a and the commutation time b; step 5, gradually comparing the back electromotive force voltage change with the voltage change threshold, if the back electromotive force voltage change is greater than the first voltage change threshold ΔV1, such as 3V, then judging whether it meets the current constraint condition, which is: (1) 1 / Fpwm=Tpwm;

[0028] (2)(60 / (N*p)) / 6=Tm;

[0029] (3)Tpwm≤Tm*duty;

[0030] in:

[0031] Fpwm is the PWM frequency

[0032] Tpwm is the PWM period

[0033] Duty is the PWM duty cycle

[0034] N is the current mechanical speed of the motor

[0035] p is the number of motor pole pairs

[0036] Tm is the current sector time of the motor; if it meets the requirements, the corresponding PWM signal frequency PWM1 is set, such as 12500Hz, to the drive circuit 1, and then return to step 2, enter the PWM adjustment entrance to continue collecting, if it does not meet the current constraints, directly return to step 2; if the back electromotive force voltage change ΔV is not greater than the first voltage change threshold ΔV1, then determine whether it is greater than the second voltage change threshold ΔV2, such as 2V, if it is greater than the second voltage change threshold ΔV2, then determine whether it meets the current constraints, if it meets, then set the corresponding PWM signal frequency PWM2, such as 16000Hz, and then return to step 2; if it is not greater than the second voltage change threshold ΔV2, then continue to determine whether it is greater than the third voltage change threshold ΔV3, and so on. If all conditions are not met, that is, when the back electromotive force voltage change ΔV is less than the minimum voltage change threshold, then it is set to the initial PWM signal.

[0037] In another embodiment, the correspondence between the back electromotive force parameters and the PWM signal frequency may include a corresponding functional relationship between the back electromotive force voltage change and the PWM signal frequency. The functional relationship is determined to be Fpwm=f(ΔV) through testing or simulation methods. The PWM signal frequency is inversely proportional to the back electromotive force voltage change, that is, the greater the back electromotive force voltage change, the smaller the PWM signal frequency.

[0038] In another approach, the correspondence between the back-EMF parameter and the PWM signal frequency includes a corresponding table relationship between the set speed, the back-EMF voltage change, and the PWM signal frequency. That is, at the set speed, the PWM signal frequency can be selected according to different back-EMF voltage changes. Of course, the values ​​in the table can be more refined. If the back-EMF voltage change is less than the minimum voltage change threshold, it is set as the initial PWM signal. The table is shown below:

[0039] ΔV1=3V ΔV2=2V ΔV3=1V ...... 80000RPM 12500Hz 16000Hz 20000Hz ...... 67500RPM 11500Hz 14500Hz 17500Hz ...... 55000RPM 10000Hz 12500Hz 16000Hz ......

[0040] like Figure 2 and Figure 4As shown, there is a back electromotive force and bus current waveform diagram of the motor C phase winding and a control flow chart of the second embodiment. In this embodiment, the back electromotive force parameters include the controller 4 storing multiple freewheeling time thresholds and corresponding PWM signal frequencies, the multiple freewheeling time thresholds include a first freewheeling time threshold and a second freewheeling time threshold, a first PWM signal frequency corresponding to the first freewheeling time threshold, and a second PWM signal frequency corresponding to the second freewheeling time threshold. The first freewheeling time threshold is greater than the second freewheeling time threshold, and the first PWM signal frequency is less than the second PWM signal frequency. When the freewheeling time is greater than the first freewheeling time threshold, the controller 4 outputs the first PWM signal frequency to the drive circuit 1; when the freewheeling time is greater than the second freewheeling time threshold, the controller 4 outputs the second PWM signal frequency to the drive circuit 1. The specific control process includes: step 1, setting the speed through the speed setting module 5, and the motor 2 starts with the initial PWM; step 2, entering the PWM adjustment entrance to adjust the PWM signal frequency; step 3, judging whether the sampling time has arrived, if so, entering step 4 to collect the continuous flow time; step 5, gradually comparing the continuous flow time Δt with the continuous flow time threshold. If the continuous flow time Δt is greater than the first continuous flow time threshold Δt1, such as 40us, then judging whether it meets the current constraint condition, which is: (1) 1 / Fpwm=Tpwm;

[0041] (2)(60 / (N*p)) / 6=Tm;

[0042] (3)Tpwm≤Tm*duty;

[0043] in:

[0044] Fpwm is the PWM frequency

[0045] Tpwm is the PWM period

[0046] Duty is the PWM duty cycle

[0047] N is the current mechanical speed of the motor

[0048] p is the number of motor pole pairs

[0049] Tm is the current sector time of the motor; if it meets the requirements, the corresponding PWM signal frequency PWM1 is set, such as 12500Hz, to the drive circuit 1, and then return to step 2, enter the PWM adjustment entry to continue collecting, if it does not meet the current constraints, directly return to step 2; if the freewheeling time Δt is not greater than the first freewheeling time threshold Δt1, then determine whether it is greater than the second freewheeling time threshold Δt2, such as 35us, if it is greater than the second freewheeling time threshold Δt2, then determine whether it meets the current constraints, if it meets, then set the corresponding PWM signal frequency PWM2, such as 16000Hz and then return to step 2; if it is not greater than the second freewheeling time threshold Δt2, then continue to determine whether it is greater than the third freewheeling time threshold Δt3, and so on. If all conditions are not met, that is, when the freewheeling time is less than the minimum freewheeling time threshold, it is set to the initial PWM signal.

[0050] In another embodiment, the correspondence between the back electromotive force parameter and the PWM signal frequency may include a corresponding functional relationship between the freewheeling time and the PWM signal frequency. The functional relationship is determined to be Fpwm=f(Δt) through testing or simulation methods. The PWM signal frequency is inversely proportional to the freewheeling time, that is, the greater the freewheeling time, the smaller the PWM signal frequency.

[0051] In another approach, the correspondence between the back electromotive force parameter and the PWM signal frequency includes a corresponding table relationship between the set speed, the freewheeling time, and the PWM signal frequency. That is, at the set speed, the PWM signal frequency can be selected by different freewheeling times. The values ​​in the table can be more refined. If the freewheeling time is less than the minimum freewheeling time threshold, it is set as the initial PWM signal. The table is shown below:

[0052]

[0053]

[0054] The controller of the present invention is connected to the motor through a back-electromotive force detection circuit to obtain back-electromotive force parameters. The controller stores the corresponding relationship between the back-electromotive force parameters and the PWM signal frequency. The controller receives the back-electromotive force parameters and sends a corresponding PWM signal to the drive circuit to drive the motor. The controller directly obtains the motor parameters to change the signal that drives the motor, and can meet the normal operation of the motor at high speed and heavy load, with high accuracy and low power consumption.

[0055] The present invention is not limited to the specific embodiments described above. Those skilled in the art will readily appreciate that numerous alternatives to the motor control circuit and control method of the present invention exist 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 motor control circuit, comprising: A power supply comprising positive and negative terminals, a drive circuit, a PWM signal generating circuit, a controller, and a back-electromotive force detection circuit, wherein the positive and negative terminals are connected to the motor via the drive circuit, the drive circuit is connected to the controller via the PWM signal generating circuit, and the controller controls the drive circuit by changing the PWM signal frequency. The invention is characterized in that: the controller is connected to the motor via the back-electromotive force detection circuit to obtain back-electromotive force parameters, the controller stores a correspondence between the back-electromotive force parameters and the PWM signal frequency, the controller receives the back-electromotive force parameters and sends the corresponding PWM signal frequency to the drive circuit to drive the motor, the motor comprises a three-phase winding, the controller controls the conduction of two-phase windings, and the back-electromotive force detection circuit detects the back-electromotive force of the suspended phase winding; The back electromotive force parameter includes a back electromotive force voltage variation, and the back electromotive force voltage variation is the difference between the back electromotive force voltages collected at every two adjacent commutation times; or, The back electromotive force parameters include a back electromotive force freewheeling time, and the back electromotive force freewheeling time is a freewheeling time generated during each commutation.

2. The motor control circuit according to claim 1, wherein: The correspondence between the back electromotive force parameter and the PWM signal frequency includes multiple voltage change thresholds stored in the controller and the PWM signal frequencies corresponding to the multiple voltage change thresholds, the multiple voltage change thresholds include a first voltage change threshold and a second voltage change threshold, a first PWM signal frequency corresponding to the first voltage change threshold, and a second PWM signal frequency corresponding to the second voltage change threshold, the first voltage change threshold is greater than the second voltage change threshold, and the first PWM signal frequency is less than the second PWM signal frequency. When the back electromotive force voltage change is greater than the first voltage change threshold, the controller outputs the first PWM signal frequency to the drive circuit; when the back electromotive force voltage change is greater than the second voltage change threshold, the controller outputs the second PWM signal frequency to the drive circuit.

3. The motor control circuit according to claim 2, wherein: The correspondence between the back electromotive force parameters and the PWM signal frequency includes a corresponding functional relationship between the back electromotive force voltage change and the PWM signal frequency, wherein the functional relationship is Fpwm= f(ΔV), and the PWM signal frequency is inversely proportional to the back electromotive force voltage change, wherein Fpwm is the PWM frequency and ΔV is the back electromotive force voltage change.

4. The motor control circuit according to claim 2, wherein: The corresponding relationship between the back electromotive force parameters and the PWM signal frequency includes the corresponding relationship between the set speed, the back electromotive force voltage change and the PWM signal frequency.

5. The motor control circuit according to claim 1, wherein: The correspondence between the back electromotive force parameter and the PWM signal frequency includes multiple freewheeling time thresholds stored in the controller and the PWM signal frequencies corresponding to the multiple freewheeling time thresholds, the multiple freewheeling time thresholds include a first freewheeling time threshold and a second freewheeling time threshold, a first PWM signal frequency corresponding to the first freewheeling time threshold, and a second PWM signal frequency corresponding to the second freewheeling time threshold, the first freewheeling time threshold is greater than the second freewheeling time threshold, and the first PWM signal frequency is less than the second PWM signal frequency. When the freewheeling time is greater than the first freewheeling time threshold, the controller outputs the first PWM signal frequency to the drive circuit; when the freewheeling time is greater than the second freewheeling time threshold, the controller outputs the second PWM signal frequency to the drive circuit.

6. The motor control circuit according to claim 1, wherein: The correspondence between the back electromotive force parameter and the PWM signal frequency includes a corresponding functional relationship between the freewheeling time and the PWM signal frequency, wherein the functional relationship is Fpwm = f(Δt), and the PWM signal frequency is inversely proportional to the freewheeling time, wherein Fpwm is the PWM frequency and Δt is the freewheeling time.

7. The motor control circuit according to claim 1, wherein: The corresponding relationship between the back electromotive force parameters and the PWM signal frequency includes the corresponding relationship between the set speed, the freewheeling time and the PWM signal frequency.

8. A motor control method, characterized in that: The control method is applied to the motor control circuit according to any one of claims 1 to 7, and the control method includes: Step 1, select the set speed of the tool; Step 2: The controller obtains back electromotive force parameters through a back electromotive force detection circuit; Step 3: The controller compares the back electromotive force parameter with the corresponding relationship between the back electromotive force parameter and the PWM signal frequency stored internally to select the corresponding PWM signal frequency for the driving circuit to drive the motor.

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