A braking speed control system for a brushless DC motor and a low-speed electric vehicle

Abstract

The present invention relates to a braking speed control system for a brushless DC motor and a low-speed electric vehicle. The system adjusts the braking control duty ratio according to the real-time speed and the preset speed of the motor. The PWM modulation module performs PWM modulation on the three-phase bridge circuit according to the braking control duty ratio, so as to control the real-time speed or the real-time acceleration of the motor during the braking process of the brushless DC motor, and feedback the energy of the motor during the braking process to the storage battery; the system also performs protection control on the DC voltage and the DC charging current of the storage battery during the braking process, suppresses the overshoot of the voltage and the current, and has overvoltage protection, overcurrent protection and their warning functions. The present invention improves the braking driving experience and the cruising range of the low-speed electric vehicle while ensuring a relatively fast braking response speed; it can effectively protect the safety of the driver and the low-speed electric vehicle.

Description

Technical Field

[0001] The present invention relates to the technical field of motor control, and more particularly, to a braking speed control system for a brushless DC motor and a low-speed electric vehicle. Background Art

[0002] Low-speed electric vehicles generally refer to electric vehicles with a maximum speed of less than 70 km / h, relatively small vehicle body volume and weight, and relatively short running distance; mainly including electric golf carts, electric lawn mowers, electric floor scrubbers, electric motorcycles, electric bicycles, electric scooters, electric shuttle buses, electric patrol cars, electric tricycles, electric tourist sightseeing vehicles, electric elderly mobility vehicles, electric beach vehicles, etc. Compared with fuel vehicles, electric vehicles have the advantages of low noise, high efficiency, low cost, and more environmentally friendly and energy-saving. Low-speed electric vehicles mainly use brushless DC motors, permanent magnet synchronous motors, etc. as traveling motors, and are driven by a battery and a three-phase bridge circuit. Low-speed electric vehicles have a wide range of applications, various types, and a huge market, and are the main development direction of current low-speed vehicles.

[0003] Low-speed electric vehicles start / stop the whole vehicle by stepping on / releasing the accelerator, and only use mechanical brakes in cases where emergency stops are required. When the accelerator is released, the low-speed electric vehicle enters the braking control system of the brushless DC motor. Currently, this braking control system mainly uses two methods: short-circuit braking and regenerative braking. Short-circuit braking means gradually short-circuiting the three-phase windings at a constant speed until they are completely short-circuited or directly short-circuiting them completely; this braking control form is simple, but since it is an open-loop control, it cannot be adjusted according to the working conditions, and the braking experience is poor. Regenerative braking uses the method of energy regeneration (converting the mechanical energy of the motor into electrical energy and feeding it back to the battery) to achieve the braking control of the motor. Currently, the regenerative braking of brushless DC motors mainly has methods such as maximum regeneration power control, maximum regeneration efficiency control, maximum braking current control, and constant braking current control. The doctoral thesis "Research on Key Technologies of High-Power Brushless DC Motor Control for Electric Vehicles" and the master's thesis "Research and Design of Brushless DC Motor Regenerative Braking Control System" explain the current control status of the above regenerative braking. The above control methods respectively make the regeneration power in the energy regeneration process the largest, the regeneration efficiency the highest, the braking time the shortest, and the braking torque constant, etc. Regenerative braking is mainly designed from the perspective of energy regeneration, which helps to improve the cruising range of the whole vehicle, but does not consider enough the driving experience during the braking process, and when the regenerated energy is too large and too fast, it is easy to cause damage to the battery and the three-phase bridge circuit, etc., posing a hidden danger to the safety of the driver and equipment.

[0004] In summary, the current research on the braking control of brushless DC motors mainly focuses on aspects such as energy feedback and reduction of system losses, and insufficient attention is paid to issues such as driving experience and safety stability during the braking process. To improve the driving experience during the braking process, it is necessary to control the motor speed or acceleration during braking; protection control should also be carried out for problems such as overcurrent and overvoltage caused by excessive and rapid feedback energy. In view of this, a braking speed control system for a brushless DC motor of a low-speed electric vehicle is proposed. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a braking speed control system for a brushless DC motor and a low-speed electric vehicle.

[0006] The technical solution adopted by the present invention to solve its technical problems is: to construct a braking speed control system for a brushless DC motor, including a braking control module, a PWM modulation module, a rotor position detection module, and a three-phase bridge circuit. The DC input terminal of the three-phase bridge circuit is connected to a storage battery, and the three-phase bridge arm output terminals of the three-phase bridge circuit are connected to a brushless DC motor. The braking control module is connected to the three-phase bridge circuit through the PWM modulation module, and the braking control module is connected to the rotor position detection module. The rotor position detection module is used to detect the rotor position of the brushless DC motor; after receiving a braking signal, the braking control module calculates the real-time speed according to the rotor position information obtained by the rotor position detection module, and obtains a braking control duty cycle D according to the real-time speed and a preset speed b ; the PWM modulation module performs PWM modulation on the switching tubes of the three-phase bridge circuit according to the braking control duty cycle D b to control the real-time speed or real-time acceleration of the brushless DC motor during braking, and feedback the energy of the brushless DC motor to the storage battery through the three-phase bridge circuit.

[0007] Further, in the braking speed control system for a brushless DC motor of the present invention, the three-phase bridge circuit includes a first switching tube Q11, a second switching tube Q12, a third switching tube Q13, a fourth switching tube Q14, a fifth switching tube Q15, and a sixth switching tube Q16; the first switching tube Q11, the third switching tube Q13, and the fifth switching tube Q15 are located at the upper ends of the three-phase bridge arms of the three-phase bridge circuit, and the second switching tube Q12, the fourth switching tube Q14, and the sixth switching tube Q16 are located at the lower ends of the three-phase bridge arms of the three-phase bridge circuit;

[0008] The PWM modulation module controls the first switching tube Q11, the third switching tube Q13, and the fifth switching tube Q15 to be turned off, and with the braking control duty cycle D bPerform PWM modulation on the second switching transistor Q12, the fourth switching transistor Q14, and the sixth switching transistor Q16.

[0009] Furthermore, in the braking speed control system of the brushless DC motor according to the present invention, the braking control module includes a feedforward control unit and a braking speed control unit; the feedforward control unit obtains a feedforward control duty ratio D according to the real-time speed at the initial braking moment. f , and the braking speed control unit obtains a braking speed control duty ratio D according to the real-time speed and a preset speed. s , and the braking control duty ratio D b is the sum of the feedforward control duty ratio D f and the braking speed control duty ratio D s .

[0010] Furthermore, in the braking speed control system of the brushless DC motor according to the present invention, the feedforward control duty ratio D output by the feedforward control unit f is inversely correlated with the real-time speed at the initial braking moment.

[0011] Furthermore, in the braking speed control system of the brushless DC motor according to the present invention, if the preset speed is 0, then the braking speed control unit obtains a braking speed control duty ratio D according to the real-time speed and the preset speed. s , including:

[0012] The braking speed control unit obtains a braking speed control duty ratio D according to the real-time speed. s , and the increasing speed of the braking speed control duty ratio D s is positively correlated with the real-time speed.

[0013] Furthermore, in the braking speed control system of the brushless DC motor according to the present invention, the increasing speed of the braking speed control duty ratio D s being positively correlated with the real-time speed includes:

[0014] The step size of each increase in the braking speed control duty ratio D s remains unchanged, and the time interval of each increase is inversely correlated with the real-time speed; or

[0015] The time interval of each increase in the braking speed control duty ratio D s remains unchanged, and the step size of each increase is positively correlated with the real-time speed; or

[0016] The time interval of each increase in the braking speed control duty ratio D s is inversely correlated with the real-time speed, and the step size of each increase is positively correlated with the real-time speed; or

[0017] Take the real-time rotational speed as the input of the PI controller, and obtain the braking rotational speed control duty ratio D after adjustment by the PI controller s .

[0018] Further, in the braking rotational speed control system of the brushless DC motor according to the present invention, the braking rotational speed control unit includes a PI controller. If the preset rotational speed is a straight line that changes with time, the straight line decreases as time increases, and the value at the initial moment of the straight line is the real-time rotational speed at the braking initial moment. The absolute value of the slope of the straight line corresponding to the traveling speed straight line of the low-speed electric vehicle is less than 10 m / s 2 ; then the braking rotational speed control unit obtains the braking rotational speed control duty ratio D according to the real-time rotational speed and the preset rotational speed s , including:

[0019] Obtain the preset rotational speed value corresponding to the straight line and time, take the difference between the real-time rotational speed and the preset rotational speed value as the input value of the PI controller, and obtain the braking rotational speed control duty ratio D after adjustment by the PI controller s ; or

[0020] Design the slope of the straight line, the slope of the straight line is the preset acceleration, obtain the real-time acceleration from the real-time rotational speed, take the difference between the real-time acceleration and the preset acceleration as the input value of the PI controller, and obtain the braking rotational speed control duty ratio D after adjustment by the PI controller s .

[0021] Further, in the braking rotational speed control system of the brushless DC motor according to the present invention, the braking rotational speed control unit includes a PI controller. If the preset rotational speed is a curve that changes with time, the curve decreases as time increases, and the value at the initial moment of the curve is the real-time rotational speed at the braking initial moment. The absolute value of the slope of the curve corresponding to the traveling speed curve of the low-speed electric vehicle is less than 10 m / s 2 , and the absolute value of the derivative of the slope of the curve corresponding to the traveling speed curve of the low-speed electric vehicle is less than 10 m / s 3 ; then the braking rotational speed control unit obtains the braking rotational speed control duty ratio D according to the real-time rotational speed and the preset rotational speed s , including:

[0022] Obtain the preset rotational speed value corresponding to the curve and time, take the difference between the real-time rotational speed and the preset rotational speed value as the input value of the PI controller, and obtain the braking rotational speed control duty ratio D after adjustment by the PI controller s ; or

[0023] Design the slope of the curve. The slope of the curve is a preset acceleration. Obtain the real-time acceleration from the real-time rotational speed. Use the difference between the real-time acceleration and the preset acceleration as the input value of the PI controller. After being adjusted by the PI controller, obtain the braking rotational speed control duty cycle D s .

[0024] Furthermore, in the braking rotational speed control system of the brushless DC motor according to the present invention, the system further includes a voltage detection module for detecting the DC voltage of the three-phase bridge circuit during braking. The braking control module is connected to the voltage detection module; the braking control module further includes an overvoltage protection control unit; when the overvoltage protection control unit monitors that the DC voltage is greater than a first preset overvoltage value, reduce the braking control duty cycle D b to reduce the DC voltage; when the overvoltage protection control unit monitors that the DC voltage is greater than a second preset overvoltage value, control all the switching tubes of the three-phase bridge circuit to turn off and send an alarm message to prompt the user that the electronic braking fails and manual mechanical braking is required; the second preset overvoltage value is greater than the first preset overvoltage value.

[0025] Furthermore, in the braking rotational speed control system of the brushless DC motor according to the present invention, the system further includes a current detection module for detecting the DC charging current of the battery during braking. The braking control module is connected to the current detection module; the braking control module further includes an overcurrent protection control unit; when the overcurrent protection control unit monitors that the DC charging current is greater than a first preset overcurrent value, reduce the braking control duty cycle D b to reduce the DC charging current; when the overcurrent protection control unit monitors that the DC charging current is greater than a second preset overcurrent value, control all the switching tubes of the three-phase bridge circuit to turn off and send an alarm message to prompt the user that the electronic braking fails and manual mechanical braking is required; the second preset overcurrent value is greater than the first preset overcurrent value.

[0026] Furthermore, in the braking rotational speed control system of the brushless DC motor according to the present invention, after braking the motor to a standstill by using this braking rotational speed control system, the PWM modulation module will increase the braking control duty cycle D b and maintain it at a full duty cycle, so that the three-phase windings of the brushless DC motor are completely short-circuited.

[0027] In addition, the present invention also provides a low-speed electric vehicle, including the braking rotational speed control system of the brushless DC motor as described above.

[0028] The present invention has the following beneficial effects:

[0029] 1. The rotational speed or acceleration during the braking process of a brushless DC motor is controlled by feedback braking, which improves the braking driving experience and the cruising range of a low-speed electric vehicle while ensuring a fast braking response speed.

[0030] 2. The DC voltage or the DC charging current of the battery during the braking process can be protected and controlled, suppressing the overshoot of the voltage and the current, and having overvoltage protection, overcurrent protection, and their alarm functions, which can effectively protect the safety of the driver and the low-speed electric vehicle. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The present invention will be further described below in conjunction with the drawings and embodiments. In the drawings:

[0032] Figure 1 is the structural diagram of the braking rotational speed control system of the brushless DC motor provided by the embodiment of the present invention;

[0033] Figure 2 is the drive circuit of the brushless DC motor provided by the embodiment of the present invention;

[0034] Figure 3 is the internal structural diagram of the braking rotational speed control unit provided by the embodiment of the present invention;

[0035] Figure 4 is the structural diagram of another braking rotational speed control system of the brushless DC motor provided by the embodiment of the present invention;

[0036] Figure 5 is the software operation flow chart of the system when the preset rotational speed is a curve provided by the embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0037] In order to have a clearer understanding of the technical features, objectives, and effects of the present invention, the specific embodiments of the present invention will now be described in detail with reference to the drawings.

[0038] In one embodiment, the structural diagram of the braking rotational speed control system of the brushless DC motor is as shown in Figure 1 and includes a braking control module 10, a PWM modulation module 20, a rotor position detection module 30, and a three-phase bridge circuit 40. Among them, the braking control module 10 includes a feedforward control unit 101 and a braking rotational speed control unit 102. The DC input terminal of the three-phase bridge circuit 40 is connected to the battery 60, and the three-phase bridge arm output terminal of the three-phase bridge circuit 40 is connected to the brushless DC motor 50.

[0039] In one embodiment, the drive circuit of the brushless DC motor is as shown in Figure 2As shown, it includes a storage battery 60, a three-phase bridge circuit 40, and a brushless DC motor 50. The three-phase bridge circuit 40 includes a first switching tube Q11, a second switching tube Q12, a third switching tube Q13, a fourth switching tube Q14, a fifth switching tube Q15, and a sixth switching tube Q16, and their specific connections are as Figure 2 shown. The three-phase bridge circuit 40 includes an A-phase output terminal, a B-phase output terminal, and a C-phase output terminal. The connection point of the first switching tube Q11 and the fourth switching tube Q14 serves as the A-phase output terminal of the three-phase bridge circuit 40. The connection point of the third switching tube Q13 and the sixth switching tube Q16 serves as the B-phase output terminal of the three-phase bridge circuit 40. The connection point of the second switching tube Q12 and the fifth switching tube Q15 serves as the C-phase output terminal of the three-phase bridge circuit 40; the three-phase drive terminals of the brushless DC motor 50 are A1, B1, and C1 respectively, where the A-phase output terminal is connected to A1, the B-phase output terminal is connected to B1, and the C-phase output terminal is connected to C1. The DC capacitor C dc is connected in parallel across the two ends of the storage battery 60; the resistor R is a sampling resistor for the DC charging current of the storage battery 60 and is connected in series between the storage battery 60 and the three-phase bridge circuit 40.

[0040] The PWM modulation module 20 is used to perform PWM modulation on the switching tubes of the three-phase bridge circuit 40 to achieve its power electronic conversion. The PWM modulation module 20 disconnects all three upper tubes (the first switching tube Q11, the third switching tube Q13, and the fifth switching tube Q15) of the three-phase bridge circuit 40 to brake and control the duty cycle D b to perform PWM modulation on the three lower tubes (the second switching tube Q12, the fourth switching tube Q14, and the sixth switching tube Q16). At this time, the three-phase bridge circuit 40 is essentially a three-phase parallel Boost circuit, and its voltage ratio M is:

[0041] M = 1 / (1 - D b ) = V H / V L (1)

[0042] Where: V H is the high-voltage side voltage (specifically as Figure 2 shown), and the high-voltage side is connected to the storage battery 60; V L is the low-voltage side voltage (specifically as Figure 2 shown), and the low-voltage side is connected to the brushless current motor 50. In one embodiment, the PWM modulation duty cycle can also change following the braking control duty cycle D b to vary.

[0043] The rotor position detection module 30 is used to detect the rotor position of the brushless DC motor 50, and sensors such as Hall position sensors, optical encoders, or resolvers can be selected. In one embodiment, the rotor position detection module 30 adopts a Hall position sensor and its signal processing circuit, and the Hall position sensor is installed in a 60-degree manner. The braking control module 10 divides the rotor position into 6 sectors according to the Hall signal values (1-6) detected by the Hall position sensor, and each sector corresponds to an electrical angle of 60 degrees. The braking control module 10 calculates the real-time mechanical speed of the brushless DC motor 50, that is, the real-time speed, according to the change period of the Hall signal value and the number of pole pairs.

[0044] After receiving the braking signal, the braking control module 10 starts to perform braking speed control. The feedforward control unit 101 obtains the feedforward control duty ratio D according to the real-time speed at the initial moment of braking f , and the braking speed control unit 102 obtains the braking speed control duty ratio D according to the real-time speed and the preset speed s , and the braking control duty ratio D b is the sum of the feedforward control duty ratio D f and the braking speed control duty ratio D s . After the braking control module 10 obtains the braking control duty ratio D b , the PWM modulation module 20 performs PWM modulation on the three-phase bridge circuit 40 according to D b , and then controls the real-time speed or real-time acceleration of the brushless DC motor 50 during braking, and returns the energy of the brushless DC motor 50 to the battery 60 through the three-phase bridge circuit 40.

[0045] The feedforward control unit 101 is used to provide an initial value for the braking control duty ratio D b (that is, the feedforward control duty ratio D f ), so as to return the energy of the brushless DC motor 50 to the battery 60 in time, so that there is a large braking force at the initial moment of braking and the response speed of the braking speed control is improved. When the three-phase bridge circuit 40 performs PWM modulation of the present invention, it is essentially a Boost circuit, and its voltage ratio M = 1 / (1 - D b ) = V H / V L ; only when the DC voltage V H generated by the Boost circuit is higher than the current voltage of the battery 60, can the energy be returned to the battery 60. Therefore, at the initial moment of braking, D f needs to be calculated according to the real-time speed; at other moments of braking, D f remains unchanged at the value at the initial moment of braking. When the real-time speed is lower, that is, the back electromotive force of the motor is lower (that is, V L is lower), D is requiredf The larger it is, that is, the larger the voltage ratio M is, so as to generate a sufficiently large V H , ensuring that the mechanical energy of the brushless DC motor 50 can be converted into electrical energy as soon as possible and fed to the storage battery 60, and improving the response speed of the braking speed control. Therefore, the feedforward control duty ratio D f is inversely related to the real-time speed. In a specific embodiment, D f The specific calculation formula is as follows:

[0046]

[0047] In the above formula, n is the real-time speed, and D f is linearly inversely proportional to the real-time speed, and the rated speed of the motor is 3000 rpm. When designing the calculation formula of D f , it is mainly necessary to consider the back electromotive force coefficient of the brushless DC motor 50 (one of its definitions is: the effective value of the motor's counter electromotive force / the electrical angular frequency), the rated DC voltage and the maximum charging current of the storage battery 60, the rated power of the system, etc.

[0048] In one embodiment, the internal structure diagram of the braking speed control unit 102 is as Figure 3 shown. In the figure, the preset speed forms include three forms: the preset speed is 0, the preset speed is a straight line, and the preset speed is a curve. The braking speed control modes include the control mode where the braking speed control duty ratio D s is positively related to the real-time speed, the braking speed closed-loop control mode with the preset speed of n * , and the braking acceleration closed-loop control mode with the preset acceleration of a * ; among them, the control mode where D s is positively related to the real-time speed includes four implementation methods. The user can select and design the above preset speed forms and braking speed control modes according to performance requirements.

[0049] When the preset speed is 0, the braking speed control unit 102 obtains the braking speed control duty ratio D s according to the real-time speed, specifically: adopting the control mode where the increasing speed of D s is positively related to the real-time speed to obtain D s . It includes the following four positive correlation implementation methods:

[0050] The first positive correlation method: the step size of each increase in the braking speed control duty ratio D s remains unchanged, and the time interval of each increase is inversely related to the real-time speed;

[0051] The second positive correlation method: the time interval of each increase in the braking speed control duty ratio D s remains unchanged, and the step size of each increase is positively related to the real-time speed;

[0052] The third positive correlation method: the braking speed controls the duty cycle D s The time interval increased each time is inversely correlated with the real-time speed, and the step size increased each time is positively correlated with the real-time speed;

[0053] The fourth positive correlation method: using the real-time speed as the input of the PI controller, and obtaining the braking speed control duty cycle D after being adjusted by the PI controller s .

[0054] In the embodiment of the first positive correlation method, the rotor position detection module 30 uses a Hall position sensor to detect the current rotor position of the brushless DC motor 50. As the motor rotates, the Hall signal value circulates between 1 and 6. When it is detected that the Hall signal value changes, the braking speed control duty cycle D s is increased by 0.5%. When the real-time speed is higher, the Hall signal value changes faster, that is, the time interval between two adjacent Hall signals is shorter. In the same time, the number of times D s increases is more, so that D s increases faster, that is, the increase speed of D s is positively correlated with the real-time speed.

[0055] When the preset speed is 0, using the control mode in which the increase speed of D s is positively correlated with the real-time speed can make the real-time speed gradually decrease. And as D s increases, the decreasing speed of the real-time speed becomes faster and faster. At the same time, the braking force gradually increases and its increasing speed becomes slower and slower. When the real-time speed is relatively high, D s is small but increases quickly, that is, the braking force is small but increases quickly; this can effectively reduce the sense of jerk caused by applying a large braking force when the real-time speed is relatively high. When the real-time speed is relatively low, D s is large but increases slowly, that is, the braking force is large but increases slowly; this can give a large braking force when the real-time speed is relatively low. Since the real-time speed is already very low, the large braking force will not cause the deterioration of the braking driving experience, but can brake the motor to a standstill relatively quickly. This braking speed control mode can improve the braking driving experience while ensuring a relatively fast braking response speed.

[0056] Figure 3 In, when the preset speed is a straight line or a curve, the braking speed closed-loop control mode with the preset speed of n * or the braking acceleration closed-loop control mode with the preset acceleration of a * can be adopted, and the above two modes are selected by the control mode selection unit.

[0057] When the preset rotational speed is selected as a straight line, the design of this straight line is carried out first. Specifically: this straight line should decrease as time increases, that is, its slope (i.e., the preset acceleration) should be less than 0; the initial moment value of this straight line is the real-time rotational speed at the initial moment of braking; in order to make the braking process not too violent, the absolute value of the slope of this straight line corresponding to the traveling speed straight line of the low-speed electric vehicle should be less than 10 m / s 2 .

[0058] When the preset rotational speed is a straight line and the braking rotational speed closed-loop control mode with the preset rotational speed of n * is adopted, the expression of the preset rotational speed n * is:

[0059] n * = a0 * t + n0 (3)

[0060] In the formula: a0 is the slope of this straight line, and n0 is the real-time rotational speed at the initial moment of braking; the difference between the real-time rotational speed n and the preset rotational speed value n * is used as the input value of the PI controller. After being adjusted by the PI controller, D s .

[0061] When the preset rotational speed is a straight line and the braking acceleration closed-loop control mode with the preset acceleration of a * is adopted, the expression of the preset acceleration a * is:

[0062] a * = a0 (4)

[0063] In the formula: a0 is the slope of this straight line; the real-time acceleration a of the motor is calculated according to the real-time rotational speed n. The difference between the real-time acceleration a and the preset acceleration a * is used as the input value of the PI controller. After being adjusted by the PI controller, D s .

[0064] In an embodiment of the braking acceleration closed-loop control mode where the preset rotational speed is a straight line and the preset acceleration is a * , the execution period of the braking acceleration closed-loop control and the period of calculating the real-time acceleration are both 5 ms. The slope of the traveling speed of the low-speed electric vehicle during braking is set to -3 m / s 2 , the reduction ratio of the gearbox of the low-speed electric vehicle is 1:20, and the wheel radius is 0.2 m. Then the slope of the corresponding preset rotational speed straight line (i.e., the preset acceleration) a * is -2866 rpm / s. The real-time acceleration a of the motor is calculated according to the real-time rotational speed n. The difference between the real-time acceleration a and the preset acceleration a * is used as the input value of the PI controller. After being adjusted by the PI, the duty ratio D of the braking rotational speed control is obtained sWhen the real-time acceleration is greater than the preset acceleration, after PI regulation, D s increases, so that D b increases, the braking force increases, causing the real-time speed to decrease faster and decrease according to the preset acceleration; when the real-time acceleration is less than the preset acceleration, after PI regulation, D s decreases, so that D b decreases, the braking force decreases, causing the decrease of the real-time speed to slow down and decrease according to the preset acceleration. Therefore, when the preset speed is a straight line and the braking acceleration closed-loop control mode with the preset acceleration of a * is adopted, the real-time speed always decreases according to the slope of the preset speed straight line; similarly, when the preset speed is a straight line and the braking speed closed-loop control mode with the preset speed of n * is adopted, the real-time speed always follows the preset speed straight line to decrease.

[0065] In summary, when the braking speed control mode with the preset speed being a straight line is adopted, the real-time acceleration of the whole vehicle remains constant, that is, the force on the driver remains constant. Therefore, by reasonably designing the preset speed straight line (i.e., adjusting the force on the driver), the whole vehicle can have a good braking driving experience while ensuring a fast braking response speed.

[0066] When the preset speed is selected as a curve, first, the curve is designed. Specifically: the curve should decrease as time increases, that is, its slope (i.e., the preset acceleration) should be less than 0; the initial moment value of the curve is the real-time speed at the initial moment of braking; in order to make the braking process not too violent, the absolute value of the slope of the curve corresponding to the traveling speed curve of the low-speed electric vehicle should be less than 10 m / s 2 ; in order to obtain a good braking driving experience, the absolute value of the derivative of the slope of the curve corresponding to the traveling speed curve of the low-speed electric vehicle should be less than 10 m / s 3 . The derivative of the curve slope (i.e., the derivative of the preset acceleration with respect to time) is called jerk. Jerk reflects the change rate of the force on the driver during the driving process and is a standard to measure the quality of the driving experience. To obtain a good driving experience, the absolute value of the jerk of the whole vehicle needs to be small.

[0067] When the preset speed is a curve and the braking speed closed-loop control mode with the preset speed of n * is adopted, the expression of the preset speed n * can be:

[0068] n * =(a0 + 0.5 * j * t) * t + n0 (5)

[0069] Where: a0 is the slope at the initial moment of the curve, j is the derivative of the slope of the curve (i.e., jerk), n0 is the real-time speed at the initial moment of braking; comparing the real-time speed n with the preset speed value n* The difference is used as the input value of the PI controller, and after being adjusted by the PI controller, D is obtained s .

[0070] When the preset rotational speed is a curve and the preset acceleration is a * in the closed-loop control mode of braking acceleration * , the expression of the preset acceleration a

[0071] a * = a0 + j * t (6)

[0072] Where: a0 is the slope at the initial moment of the curve, and j is the derivative of the slope of the curve (i.e., jerk); the real-time acceleration a of the motor is calculated according to the real-time rotational speed n, and the difference between the real-time acceleration a and the preset acceleration a * is used as the input value of the PI controller, and after being adjusted by the PI controller, D is obtained s .

[0073] In an embodiment of the closed-loop control mode of braking rotational speed where the preset rotational speed is a curve and the preset rotational speed is n * , the execution period of the closed-loop control of braking rotational speed is 1 ms, the reduction ratio of the gearbox of the low-speed electric vehicle is 1:20, the wheel radius is 0.2 m, and the initial moment value n0 of the preset rotational speed curve is the real-time rotational speed at the initial moment of braking; the slope of the traveling speed of the low-speed electric vehicle at the initial moment of braking is set to -3 m / s 2 , and the jerk of the traveling speed of the low-speed electric vehicle during braking is -0.1 m / s 3 , then the slope at the initial moment of the corresponding preset rotational speed curve (i.e., the initial preset acceleration) a0 = -2866 rpm / s, and the jerk j of the preset rotational speed curve = -95.54 rpm / s 2 ; then the expression of the preset rotational speed curve is:

[0074] n * = (-2866 - 47.77 * t) * t + n0 (7)

[0075] Extract the preset rotational speed values n of the curve at different braking times * , and use the difference between the real-time rotational speed n and the preset rotational speed value n * as the input value of the PI controller, and after PI adjustment, the duty ratio D of braking rotational speed control is obtained s . When the real-time rotational speed n is greater than the preset rotational speed value n * , after PI adjustment, D s increases, so that D b increases, the braking force increases, and the real-time rotational speed decreases faster and decreases according to the curve; when the real-time rotational speed n is less than the preset rotational speed value n * , after PI adjustment, Ds decreases, thus causing D b to decrease, the braking force decreases, slowing down the decrease of the real-time rotational speed and decreasing according to this curve. Therefore, when the preset rotational speed is a curve and the braking rotational speed closed-loop control mode with the preset rotational speed of n * is adopted, the real-time rotational speed always decreases according to the preset rotational speed curve; similarly, when the preset rotational speed is a curve and the braking acceleration closed-loop control mode with the preset acceleration of a * is adopted, the real-time acceleration is consistent with the slope of the preset rotational speed curve throughout the braking process.

[0076] To sum up, when the braking rotational speed control mode with the preset rotational speed as a curve is adopted, the real-time acceleration of the whole vehicle is always consistent with the slope of the preset rotational speed curve. Therefore, by reasonably designing the preset rotational speed curve (including the design of acceleration, jerk, etc.), the force on the driver can change slowly and evenly, enabling the whole vehicle to have a good braking driving experience while ensuring a fast braking response speed.

[0077] When the preset rotational speed is a straight line, the preset acceleration a * is a constant value, and its expression is simpler than that of the preset rotational speed n * . At this time, the braking acceleration closed-loop control mode with the preset acceleration of a * is simpler than the braking rotational speed closed-loop control mode with the preset rotational speed of n * . However, when calculating the acceleration, it is necessary to differentiate the rotational speed, which is likely to introduce noise. Therefore, appropriate filtering is required. The braking acceleration closed-loop control mode with the preset acceleration of a * only performs closed-loop control on the acceleration. Therefore, there is a certain deviation between the real-time rotational speed n and the preset rotational speed n * (when the execution frequency and PI parameters of the PI controller are large enough, this deviation can be very small). When the preset rotational speed curve is relatively complex (such as designing a preset rotational speed table and obtaining the preset rotational speed curve by looking up the table), it is difficult to obtain the expression of the slope of this curve (i.e., the preset acceleration). At this time, only the braking rotational speed closed-loop control mode with the preset rotational speed of n * can be adopted.

[0078] When the preset rotational speed is 0, only the step size increased each time for D s and the time interval increased each time, etc. need to be designed, and the design is relatively simple; but D sThe control mode in which the increase speed is positively correlated with the real-time rotational speed only roughly controls the real-time rotational speed. Therefore, its braking driving experience is slightly worse than that of the braking control in the latter two preset rotational speed forms, and it is relatively difficult to balance the braking response speed and the braking driving experience. When the preset rotational speed is a straight line, the design of the preset rotational speed straight line and the PI parameters is required, and the design is relatively complex; however, by designing this straight line, the braking response speed and the braking driving experience can be balanced simultaneously, and the performance is relatively good. When the preset rotational speed is a curve, the design of the preset rotational speed curve (including acceleration, jerk, etc.) and the PI parameters is required, and the design is relatively complex; however, by designing this curve, the braking response speed and the braking driving experience can reach a better combination, and the performance is the best among the three.

[0079] In one embodiment, the structural diagram of another braking rotational speed control system for a brushless DC motor is as Figure 4 shown. In addition to the Figure 1 various modules and units shown, the system further includes a voltage detection module 70 and a current detection module 80. The braking control module 10 further includes an overvoltage protection control unit 103 and an overcurrent protection control unit 104.

[0080] In one embodiment, the voltage detection module 70 detects the DC voltage by means of resistor voltage division. The overvoltage protection control unit 103 is used to suppress the overshoot of the DC voltage that may be brought about by the PWM modulation of the present invention and control the DC voltage within a safe range. When the PWM modulation of the present invention is adopted, the three-phase bridge circuit 40 is a three-phase parallel Boost circuit, and the energy of the brushless DC motor 50 is fed back to the storage battery 60 for braking. When the real-time rotational speed is relatively high or the duty ratio D of the braking control b increases relatively fast, it may cause the energy fed back to the storage battery 60 to be too large and too fast, resulting in overshoot of the DC voltage. When the overvoltage protection control unit 103 monitors that the DC voltage is greater than the first preset overvoltage value, by reducing D b , the voltage ratio M of the three-phase bridge circuit 40 is made to be M = 1 / (1 - D b ) = V H / V L to decrease, thereby reducing the high-voltage side voltage V H and achieving the purpose of protecting the storage battery 60 and the three-phase bridge circuit 40; if the overshoot of the DC voltage still cannot be suppressed by reducing D b , and the DC voltage overshoots to be greater than the second preset overvoltage value, then all the switching tubes of the three-phase bridge circuit 40 are disconnected and an alarm message is sent to prompt the user that the electronic braking fails and manual mechanical braking is required. In one embodiment, the execution frequencies of the voltage detection module 70 and the overvoltage protection control unit 103 are both equal to the PWM frequency (10 kHz), and the protection control change step duty ratio D pis 1%. The first preset overvoltage value and the second preset overvoltage value can be designed with reference to the maximum operating voltage of the battery 60, the maximum voltage of the DC capacitor C dc and the maximum overvoltage value of the switching tube. In a specific embodiment, the first preset overvoltage value is set to 75V, and the second preset overvoltage value is set to 85V. When the DC voltage is greater than 75V, each time the overvoltage protection control unit 103 executes, it reduces the braking control duty cycle D b at the previous moment p repeatedly until the DC voltage is less than 75V; when the DC voltage is greater than 85V, the overvoltage protection control unit 103 disconnects all the switching tubes of the three-phase bridge circuit 40 and issues an alarm message to prompt the user that the electronic braking fails and manual mechanical braking is required. When the DC voltage returns to the normal operating range, the braking control duty cycle D b = the braking speed control duty cycle D s + the feedforward control duty cycle D f .

[0081] In one embodiment, the current detection module 80 uses Figure 2 the sampling resistor R in Figure 2 to cooperate with the sampling conditioning circuit to detect the DC charging current of the battery 60, and it is stipulated that the positive direction of the current flowing through b the sampling resistor R in b is from the negative terminal of the battery 60 into the three-phase bridge circuit 40 (i.e., taking the charging current of the battery 60 as the positive current). The overcurrent protection control unit 104 is used to suppress the overshoot of the DC charging current that may be brought about by the PWM modulation of the present invention and control the DC charging current within a safe range. When using the PWM modulation of the present invention, the three-phase bridge circuit 40 is a three-phase parallel Boost circuit, and the energy of the brushless DC motor 50 is fed back to the battery 60 for braking. When the real-time speed is relatively high, the load is relatively large, or the braking control duty cycle D b increases relatively fast, it may cause the energy fed back to the battery 60 to be too large and too fast, resulting in overshoot of the DC charging current. When the overcurrent protection control unit 104 monitors that the DC charging current is greater than the first preset overcurrent value, by reducing D H , the voltage ratio M of the three-phase bridge circuit 40 = 1 / (1 - D L ) = V H / V H is reduced, and the high-side voltage V bIt is still impossible to suppress the overshoot of the DC charging current. When the DC charging current overshoots to be greater than the second preset overcurrent value, all the switching tubes of the three-phase bridge circuit 40 are disconnected and an alarm message is sent to prompt the user that the electronic braking fails and manual mechanical braking is required. In one embodiment, the execution frequencies of the current detection module 80 and the overcurrent protection control unit 104 are equal to the PWM frequency (10 kHz), and the protection control change step duty ratio D p is 1%. The first preset overcurrent value and the second preset overcurrent value can be designed with reference to the maximum charging current of the storage battery 60 and the maximum overcurrent value of the switching tubes; in a specific embodiment, the first preset overcurrent value is set to 25 A and the second preset overcurrent value is set to 32 A. When the DC charging current is greater than 25 A, each time the overcurrent protection control unit 104 is executed, the braking control duty ratio D b at the previous moment is decreased D p , and it is repeatedly executed until the DC charging current is less than 25 A; when the DC charging current is greater than 32 A, the overcurrent protection control unit 104 disconnects all the switching tubes of the three-phase bridge circuit 40 and sends an alarm message to prompt the user that the electronic braking fails and manual mechanical braking is required; when the DC charging current returns to the normal operation range, the braking control duty ratio D b = the braking speed control duty ratio D s + the feedforward control duty ratio D f .

[0082] In some embodiments, the present invention can also be used to implement the anti-theft function of low-speed electric vehicles. After the system brakes the motor to a standstill, the PWM modulation module 20 will increase the braking control duty ratio D b and maintain it at the full duty ratio to completely short-circuit the three-phase windings of the brushless DC motor 50. When an external force pushes the low-speed electric vehicle, the short-circuited three-phase windings will generate a braking current, which hinders the rotation of the motor. The greater the external force pushing, the greater the braking current generated by the short-circuited three-phase windings, that is, the greater the braking torque generated, so as to minimize the movement of the low-speed electric vehicle and play an anti-theft role. In one embodiment, when D b reaches the full duty ratio and after a period of time (such as 1 s), if the real-time speed still does not drop to 0, it means that after the system operates with the maximum braking torque, the motor still cannot be stationary, then D b should continue to maintain the full duty ratio to maintain the complete short-circuit of the three-phase windings and send an alarm of electronic braking failure to prompt the user to perform manual mechanical braking, etc.

[0083] In one embodiment, a low-speed electric vehicle includes the braking speed control system of the above-mentioned brushless DC motor.

[0084] In an embodiment of a brushless DC motor braking speed control system for a low-speed electric vehicle, when the preset speed is a curve that varies with time, the system software operation flowchart is as follows Figure 5 shown. It includes the following steps:

[0085] Step S501: Detect the position of the motor rotor through a Hall position sensor and calculate the real-time speed n0 at the initial braking moment.

[0086] Step S502: Calculate the feedforward control duty ratio D according to n0 f , and the feedforward control duty ratio D f is inversely related to n0, and the specific calculation formula is shown in Equation (2).

[0087] Step S503: Set the jerk j of the preset speed curve and the slope at the initial braking moment (i.e., the initial preset acceleration) a0; in this embodiment, j = -95.54 rpm / s 2 and a0 = -2866 rpm / s.

[0088] Step S504: Calculate the expression of the preset speed curve:

[0089] n * = (a0 + 0.5×j×t)×t + n0 = (-2866 - 47.77×t)×t + n0.

[0090] Step S505: Detect the position of the motor rotor through a Hall position sensor, and detect it once every 1 ms.

[0091] Step S506: Determine whether the Hall signal has not changed for 1 s continuously; if so, execute Step S520, that is, if the Hall signal has not changed within 1 s, it is considered that the motor is in a stationary state, otherwise execute Step S507.

[0092] Step S507: Determine whether the Hall signal value has changed; if so, execute Step S508, otherwise execute Step S511.

[0093] Step S508: Determine whether the braking control duty ratio D b has maintained the full duty ratio for more than 1 s; if so, execute Step S510, otherwise execute Step S509.

[0094] Step S509: Calculate the real-time speed n(t k ) through the time interval between two adjacent Hall signals.

[0095] Step S510: If the three-phase windings of the motor are completely short-circuited for more than 1 s and the motor has not reached the stationary state, an alarm for braking failure is given, the three-phase windings are kept completely short-circuited, and the driver is prompted to perform manual mechanical braking.

[0096] Step S511: If the Hall signal detected at the current moment is the same as the Hall signal detected 1 ms ago, it is considered that the real-time speed n remains unchanged, and the real-time speed 1 ms ago is assigned as the real-time speed at the current moment, that is, n(t k ) = n(t k-1 ).

[0097] Step S512: Detect the DC voltage V dc and the DC charging current I dc , and it is specified that when the battery 60 is charging, the direction of I dc is positive.

[0098] Step S513: Obtain the speed preset value n(t k ) of the preset speed curve at the current moment t k .

[0099] Step S514: Calculate the speed difference err = n(t k ) - n * (t k ).

[0100] Step S515: err is input into the PI controller, and after PI adjustment, the braking speed control duty ratio D s (t k ) is obtained.

[0101] Step S516: Calculate the braking control duty ratio D b (t k ) = D s (t k ) + D f .

[0102] Step S517: Determine whether V dc is greater than the first preset overvoltage value. Here, the first preset overvoltage value is set to 75 V; if so, execute Step S519, otherwise execute Step S518.

[0103] Step S518: Determine whether I dc is greater than the first preset overcurrent value. Here, the first preset overcurrent value is set to 25 A; if so, execute Step S519, otherwise execute Step S524.

[0104] Step S519: The braking control duty ratio D b (t k ) = D b (t k-1 ) - D p ; (Protect the control change step duty ratio D p = 1%).

[0105] Step S520: The brushless DC motor 50 is in a stationary state.

[0106] Step S521: Determine whether the braking control duty cycle D b reaches the full duty cycle; if so, execute Step S523, otherwise execute Step S522.

[0107] Step S522: Increase the braking control duty cycle D b by 2%, i.e., D b (t k ) = D b (t k-1 ) + 2%.

[0108] Step S523: When the braking control duty cycle D b has reached the full duty cycle, maintain the full duty cycle unchanged.

[0109] Step S524: Determine whether V dc is greater than the second preset overvoltage value, here, the second preset overvoltage value is set to 85V; if so, execute Step S527, otherwise execute Step S525.

[0110] Step S525: Determine whether I dc is greater than the second preset overcurrent value, here, the second preset overcurrent value is set to 32A; if so, execute Step S527, otherwise execute Step S526.

[0111] Step S526: The PWM modulation module 20 performs PWM modulation with D b (t k ) and drives the three-phase bridge circuit 40 to control the braking speed of the brushless DC motor 50. And return to Step S505.

[0112] Step S527: Disconnect all the switching tubes of the three-phase bridge circuit, and prompt the user that the electronic braking fails and manual mechanical braking is required.

[0113] The above embodiments are only for illustrating the technical concept and features of the present invention, and the purpose is to enable those who are familiar with this technology to understand the content of the present invention and implement it accordingly, and cannot limit the protection scope of the present invention. All equivalent changes and modifications made to the scope of the claims of the present invention shall fall within the scope covered by the claims of the present invention.

Claims

1. A braking speed control system for a brushless DC motor, characterized in that, It includes a brake control module, a PWM modulation module, a rotor position detection module, and a three-phase bridge circuit. The DC input terminal of the three-phase bridge circuit is connected to a storage battery, and the three-phase bridge arm output terminals of the three-phase bridge circuit are connected to a brushless DC motor. The brake control module is connected to the three-phase bridge circuit through the PWM modulation module. The brake control module is connected to the rotor position detection module, and the rotor position detection module is used to detect the rotor position of the brushless DC motor. After receiving a brake signal, the brake control module calculates a real-time speed according to the rotor position information obtained by the rotor position detection module, and obtains a brake control duty ratio according to the real-time speed and a preset speed. D b ; The PWM modulation module D b performs PWM modulation on the switching tubes of the three-phase bridge circuit, controls the real-time speed or real-time acceleration of the brushless DC motor during the braking process, and feeds back the energy of the brushless DC motor to the storage battery through the three-phase bridge circuit; The braking control module includes a feedforward control unit and a braking speed control unit; the feedforward control unit obtains a feedforward control duty ratio according to the real-time speed at the initial braking moment D f , and the braking speed control unit obtains a braking speed control duty ratio according to the real-time speed and a preset speed D s , and the braking control duty ratio D b is the sum of the feedforward control duty ratio D f and the braking speed control duty ratio D s .

2. The braking speed control system of the brushless DC motor according to claim 1, wherein The three-phase bridge circuit includes a first switching tube Q11, a second switching tube Q12, a third switching tube Q13, a fourth switching tube Q14, a fifth switching tube Q15, and a sixth switching tube Q16; the first switching tube Q11, the third switching tube Q13, and the fifth switching tube Q15 are located at the upper ends of the three-phase bridge arms of the three-phase bridge circuit, and the second switching tube Q12, the fourth switching tube Q14, and the sixth switching tube Q16 are located at the lower ends of the three-phase bridge arms of the three-phase bridge circuit; The PWM modulation module controls the first switch Q11, the third switch Q13, and the fifth switch Q15 to turn off, with the braking control duty cycle D b perform PWM modulation on the second switch Q12, the fourth switch Q14, and the sixth switch Q16.

3. The braking speed control system of the brushless DC motor according to claim 1, characterized in that, The duty cycle of the feedforward control output by the feedforward control unit D f is inversely correlated with the real-time rotational speed at the initial moment of braking.

4. The braking speed control system of the brushless DC motor according to claim 1, characterized in that, If the preset rotational speed is 0, the braking rotational speed control unit obtains the braking rotational speed control duty ratio according to the real-time rotational speed and the preset rotational speed D s , including: The braking speed control unit obtains the braking speed control duty ratio according to the real-time speed D s , and the increasing speed of the braking speed control duty ratio D s is positively correlated with the real-time speed.

5. The braking speed control system of the brushless DC motor according to claim 4, wherein The duty ratio of the braking speed control D s The increasing speed is positively correlated with the real-time speed and includes: Brake Rotation Speed Control Duty Cycle D s The step size increased each time remains unchanged, and the time interval increased each time is inversely correlated with the real-time rotation speed; or Brake rotation speed control duty cycle D s The time interval increased each time remains unchanged, and the step size increased each time is positively correlated with the real-time rotation speed; or Brake Rotation Speed Control Duty Cycle D s The time interval increased each time is inversely correlated with the real-time rotation speed, and the step size increased each time is positively correlated with the real-time rotation speed; or Take the real-time rotational speed as the input of the PI controller, and obtain the duty cycle of the braking rotational speed control after being adjusted by the PI controller D s 。 6. The braking speed control system of the brushless DC motor according to claim 1, characterized in that The braking speed control unit includes a PI controller. If the preset speed is a straight line that changes with time, the straight line decreases as time increases, the value at the initial moment of the straight line is the real-time speed at the initial braking moment, and the absolute value of the slope of the straight line corresponding to the traveling speed straight line of the low-speed electric vehicle is less than 10 m / s 2 ; then the braking speed control unit obtains the braking speed control duty ratio according to the real-time speed and the preset speed D s , including: Obtain the preset rotational speed value corresponding to the straight line and time, use the difference between the real-time rotational speed and the preset rotational speed value as the input value of the PI controller, and obtain the braking rotational speed control duty ratio after the adjustment of the PI controller D s ; or Design the slope of the straight line, where the slope of the straight line is a preset acceleration. Obtain the real-time acceleration from the real-time rotational speed, and use the difference between the real-time acceleration and the preset acceleration as the input value of the PI controller. After being adjusted by the PI controller, the braking rotational speed control duty cycle is obtained. D s 。 7. The braking speed control system of the brushless DC motor according to claim 1, characterized in that, The braking speed control unit includes a PI controller. If the preset speed is a curve that changes with time, the curve decreases as time increases. The initial value of the curve is the real-time speed at the initial braking moment. The absolute value of the slope of the curve corresponding to the traveling speed curve of the low-speed electric vehicle is less than 10 m / s 2 , and the absolute value of the derivative of the slope of the curve corresponding to the traveling speed curve of the low-speed electric vehicle is less than 10 m / s 3 ; then the braking speed control unit obtains the braking speed control duty ratio according to the real-time speed and the preset speed D s , including: Obtain the preset rotational speed value corresponding to the curve and time, use the difference between the real-time rotational speed and the preset rotational speed value as the input value of the PI controller, and obtain the braking rotational speed control duty ratio after adjustment by the PI controller D s ; or Design the slope of the curve, where the slope of the curve is a preset acceleration. Obtain the real-time acceleration from the real-time rotational speed, and use the difference between the real-time acceleration and the preset acceleration as the input value of the PI controller. After being adjusted by the PI controller, the duty ratio of the braking rotational speed control is obtained. D s 。 8. The braking speed control system of the brushless DC motor according to claim 1, characterized in that The system further includes a voltage detection module for detecting the DC voltage of the three-phase bridge circuit during braking, and the braking control module is connected to the voltage detection module; the braking control module further includes an overvoltage protection control unit; when the overvoltage protection control unit monitors that the DC voltage is greater than a first preset overvoltage value, the duty ratio of the braking control is reduced D b to reduce the DC voltage; when the overvoltage protection control unit monitors that the DC voltage is greater than a second preset overvoltage value, all the switching tubes of the three-phase bridge circuit are controlled to be turned off and an alarm message is sent to prompt the user that the electronic braking fails and manual mechanical braking is required; the second preset overvoltage value is greater than the first preset overvoltage value.

9. The braking speed control system of the brushless DC motor according to claim 1, wherein The system further includes a current detection module for detecting the DC charging current of the storage battery during braking, and the braking control module is connected to the current detection module; the braking control module further includes an overcurrent protection control unit; when the overcurrent protection control unit monitors that the DC charging current is greater than a first preset overcurrent value, the braking control duty ratio is reduced D b to reduce the DC charging current; when the overcurrent protection control unit monitors that the DC charging current is greater than a second preset overcurrent value, all the switching tubes of the three-phase bridge circuit are controlled to be turned off and an alarm message is sent to prompt the user that the electronic braking fails and manual mechanical braking is required; the second preset overcurrent value is greater than the first preset overcurrent value.

10. The braking speed control system of the brushless DC motor according to claim 1, characterized in that, After braking the motor to a standstill using this braking speed control system, the PWM modulation module will increase and maintain the braking control duty cycle D b at the full duty cycle, causing the three-phase windings of the brushless DC motor to be completely short-circuited.

11. A low-speed electric vehicle, characterized in that, A braking speed control system for a brushless DC motor as claimed in any one of claims 1 to 10.

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