Fast braking method for brushless direct current motor

Abstract

The invention relates to the technical field of motor control, in particular to a rapid braking method for a brushless direct current motor, the braking speed is high, the problems of abnormal large current, power source impact and the like cannot be caused during braking, and a microcontroller, a pre-driving circuit, a three-phase inverter bridge, a comparator, a position detection module and the brushless direct current motor are included; when the current at the lower bridge arm switching tubes of the currently conducted corresponding three-phase inverter bridge exceeds a preset value, the microcontroller receives an output signal of the comparator, enters a high-resistance state, closes all the lower arm switching tubes, starts the timer and rapidly attenuates the current, and when the timer reaches a preset value, the timer stops working; entering the next control period and conducting the lower bridge arm switch tube again, and continuing to increase the brake current until the comparator is turned over again; and the braking current is maintained near the level corresponding to the preset value by utilizing the circular connection and disconnection of the lower bridge arm switch tube, so that the rotating speed of the motor is rapidly reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of motor control, and in particular to a fast braking method for a brushless DC motor. Background Art

[0002] Brushless DC motors (BLDCs) are widely used in industrial automation, electric vehicles, and other fields due to their high power density and long lifespan. Their braking performance is directly related to system safety and dynamic response quality. Currently, common braking methods include mechanical braking, dynamic braking, short-circuit braking, reverse braking, and regenerative braking, but each has its own drawbacks.

[0003] Mechanical braking, which applies friction through mechanical structures, is the most direct method. However, its disadvantages include cost, wear, and maintenance, as well as slow response, especially when rapid braking is required. Dynamic braking, during braking, switches the motor to a resistive load, dissipating kinetic energy into heat. This method suffers from significant heat generation, requiring a heat sink, energy waste, and low efficiency. Short-circuit braking, which short-circuits the motor windings and utilizes the motor's back EMF to generate braking torque, is simple, but the braking effect may diminish as the speed decreases. Furthermore, at high speeds, high currents may be generated, potentially damaging the motor or drive. Reverse braking involves changing the direction or phase of the current, causing the motor to generate reverse torque. This generates high current surges, which can damage the motor and drive, and can also make precise braking force control difficult. Regenerative braking uses the motor as a generator during braking, converting kinetic energy into electrical energy and feeding it back to a power source or energy storage device. However, this requires the drive to support energy regeneration and the power system to be able to accept the regenerated energy. Otherwise, additional circuitry is required, increasing cost and complexity. Summary of the Invention

[0004] In order to solve the above technical problems, the present invention provides a brushless DC motor fast braking method, which has a fast braking speed and does not cause problems such as abnormally large current and power supply impact during braking.

[0005] The technical solution is as follows: a brushless DC motor fast braking method, characterized in that it includes a microcontroller, a pre-drive circuit, a three-phase inverter bridge, a comparator, a position detection module and a brushless DC motor. During braking, when the current at the lower arm switch tube of the corresponding three-phase inverter bridge that is currently turned on exceeds a preset value, the microcontroller receives the output signal of the comparator, enters a high-resistance state, turns off all lower arm switch tubes and starts a timer, and the current decays rapidly. When the timer reaches a preset value, it enters the next control cycle and turns on the lower arm switch tube again. The braking current continues to increase until the comparator flips again; the braking current is maintained near the level corresponding to the preset value by cyclically turning on and off the lower arm switch tube, and the motor speed drops rapidly.

[0006] After adopting the present invention, when a braking command is received, the braking current is monitored in real time to see if it exceeds the preset value. When the current exceeds the preset value, a high-resistance state is triggered, and after a fixed period of time, the circuit is turned on again to enter the next control cycle. By alternating between the conduction of the lower bridge arm switch tube and the high-resistance cycle, the braking current is maintained near the level corresponding to the preset value, thereby achieving rapid braking. During the braking process, problems such as abnormally large current and impact on the power supply will not be caused. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1 This is a schematic diagram of the system of the present invention; Figure 2 Schematic diagram of current changes during braking. DETAILED DESCRIPTION

[0008] See Figure 1 and Figure 2 As shown, a method for rapid braking of a brushless DC motor includes a microcontroller, a pre-driver circuit, a three-phase inverter bridge, a comparator, a position detection module, and a brushless DC motor. The microcontroller 10 is an integrated circuit and may also include additional circuits related to or unrelated to motor control. For example, the microcontroller 10 may include a microprocessor, a microcomputer, a system-on-chip (SOC), etc. Based on feedback from the position detection module 70, the microcontroller 10 calculates the rotor position and, based on the commutation logic for driving the brushless DC motor 50, performs corresponding commutation operations and outputs six PWM signals. The PWM signals are coupled to the pre-driver module 20, which amplifies the lower voltage PWM control signals to the voltage level required to drive switching elements (such as IGBTs, MOSFETs, or transistors), ensuring sufficient drive capability to drive the three-phase inverter bridge 30. The three-phase inverter bridge 30 controls the on and off states of power switching devices (such as IGBTs and MOSFETs) and, according to a specific pattern, divides the DC power supply into a series of pulse sequences. This converts the input DC power into three-phase AC power with adjustable frequency and amplitude, providing appropriate power input to the brushless motor 50 and generating a rotating magnetic field, thereby driving the rotor. Furthermore, to limit the current, the system continuously monitors the phase current using current mirrors located in the lower-arm switches S1, S3, and S5. This current is converted into a corresponding voltage V_cur, which is then fed into the comparator 60 and fed back to the microcontroller for appropriate operation. For simplicity, Figure 1 The current mirror is not shown.

[0009] The position detection device 70 is used to detect the real-time position signal of the rotor, and is typically a circuit based on back-EMF zero-crossing detection. The back-EMF zero-crossing detection circuit is based on the fact that the back-EMF in the stator winding changes periodically with the position of the rotor. In the back-EMF variation curve of each phase winding, there will be zero-crossing points, that is, the moments when the back-EMF changes from positive to negative or from negative to positive. The back-EMF detection circuit detects these back-EMF zero-crossing points and then delays for a certain period of time to achieve phase switching. Similarly, as the rotor rotates through one electrical cycle, a zero-crossing signal appears every 60°, and the rotor's position information can be determined based on this signal.

[0010] During normal motor rotation, the microcontroller 10 periodically applies energizing signals to the coils in a specific sequence (e.g., AB, AC, BC, BA, CA, CB during forward rotation) based on the rotor's real-time position, causing the motor to continuously rotate. For example, during the PWM ON period, only S0 and S3 are conducting, resulting in phase A coupled to the power supply, phase B coupled to ground, and phase C floating. At this point, phases AB in the motor are conducting, generating current in the coils, which interacts with the motor's rotor permanent magnets to drive the motor. During the PWM OFF period, S3 remains conducting, S0 is disconnected, and the corresponding lower arm switch S1 is conducting. At this point, the current enters its decay phase. Due to the coil's inductance, the current cannot change suddenly, and the coil current still interacts with the rotor's permanent magnets to generate a forward driving force.

[0011] Taking the AB phase conduction as an example, when the brake command is received, no matter what state the PWM is in, S0 will be immediately disconnected, and the corresponding lower bridge arm switch tube S1 will be turned on, and S3 will remain on. The coil current will circulate between the two lower bridge arm switches, that is, between S1 and S3; due to the high speed of the rotor, the large back electromotive force continues to act on the AB coil, causing the current to continue to increase in the negative direction, as shown. Figure 2The lower transistor in the circuit is on; once the current mirror at S3 detects that the current exceeds the preset value V_ref, the microcontroller 10 receives this overcurrent feedback and triggers a high-impedance state when the current exceeds the limit. At this time, all lower-arm switches are turned off and a timer is started. Since all switches are off, the current path presents a high impedance, and the current flows through the diodes of the inverter bridge, causing the current to decay rapidly. When the timer reaches a preset value t1, such as a few milliseconds, the next control cycle begins and S1 and S3 are turned on again. At this time, although the speed has dropped slightly, it is still high, and the back EMF will cause the braking current to continue to increase until the comparator flips again. According to the above-mentioned on and off cycle of S1 and S3, the braking current can be maintained near the level corresponding to the preset value V_ref, and the motor speed can be rapidly reduced. During the AB phase braking period, although the speed drops, the rotor will continue to rotate forward. When the position detection module 70 detects the zero crossing of phase C and delays for 30 degrees, the braking phase is switched to the AC phase, and the lower bridge arm switch tube S1 of phase A and the lower bridge arm switch tube S5 of phase C are turned on. The other switch tubes are still disconnected, and the coil current will flow between S1 and S5. At this time, the back electromotive force continues to act on the AC coil, causing the current to continue to increase. Once the current mirror at S5 detects that the current exceeds the preset value V_ref, the microcontroller 10 receives the overcurrent feedback, immediately disconnects S1 and S5, and starts the timer. Since all switches enter the high-resistance state, the current will decay rapidly at this time. After the timing ends, the lower tubes of the original phase, that is, S1 and S5, are turned on again. At this time, the rotor speed further decreases.

[0012] According to the above steps, when the rotor continues to move forward (for example, the sequence is AB, AC, BC, BA, CA, CB when rotating forward), the braking phase combination is updated in real time with the rotor position, and the braking current circulates and continuously acts on the motor braking phase. The motor speed will decrease rapidly until the speed drops to a threshold, such as tens of revolutions per minute. At this time, the corresponding back electromotive force is also very small. The microcontroller 10 disconnects all switches, and the residual current will flow into the power supply. Since the residual current is very small, the impact on the power supply voltage is also small, which effectively protects the power supply system.

[0013] The fast braking logic of the present invention: after receiving the braking command, the braking phase is dynamically selected based on the rotor position, and graded braking control is implemented in the selected phase, that is, the lower bridge arm switch tube of the target phase is closed to form a current loop, and the real-time braking current is monitored to see whether it exceeds the preset value V_ref. When the current exceeds the preset value, a high-resistance state is triggered, and after a fixed period of time, it is turned on again to enter the next control cycle; by alternating between the conduction of the lower bridge arm switch tube and the high-resistance cycle, the braking current is maintained near the level corresponding to the preset value V_ref, thereby achieving fast braking; the braking phase is switched in real time following the rotor position, forming full-cycle dynamic braking.

[0014] The advantages of the present invention are: (1) dynamic phase following mechanism: the braking phase combination is updated in real time with the rotor position, avoiding torque fluctuations caused by traditional fixed-phase braking; (2) graded energy consumption control: the conduction-high resistance state operates alternately, accelerating energy dissipation while suppressing voltage spikes, protecting the power supply system; (3) adjustable braking torque: the braking torque is adjusted by V_ref, and braking is rapid.

Claims

1. A brushless DC motor rapid braking method, characterized in that: It includes a microcontroller, a pre-drive circuit, a three-phase inverter bridge, a comparator, a position detection module and a brushless DC motor. During braking, when the current at the lower arm switch tube of the corresponding three-phase inverter bridge that is currently turned on exceeds a preset value, the microcontroller receives the output signal of the comparator, enters a high-resistance state, turns off all lower arm switch tubes and starts a timer, and the current decays rapidly. When the timer reaches a preset value, it enters the next control cycle and turns on the lower arm switch tube again. The braking current continues to increase until the comparator flips again; the braking current is maintained near the level corresponding to the preset value by cyclically turning on and off the lower arm switch tube, and the motor speed drops rapidly.

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