Brushless direct current motor starting method, control system and electric tool

Abstract

The invention provides a brushless direct current motor starting method, a control system and an electric tool. The starting method comprises the following steps: injecting voltage pulses in the central angle directions of a plurality of sectors to obtain a plurality of current response values; determining a sector where an initial rotor angle is located when the motor is started based on the current response value; determining a continuous acceleration direction to which a rotor acceleration pulse is to be applied according to the sector; applying a pulse in the continuous acceleration direction until a back electromotive force zero crossing point of the non-conducting phase corresponding to the continuous acceleration direction is detected; and performing commutation starting based on the detected back electromotive force zero crossing point. According to the invention, after the sector where the initial position of the rotor is located is determined, especially after the left and right half area positions of the sector where the rotor is specifically located are determined, the acceleration pulse is continuously applied in the selected direction, so that the rotor is rapidly accelerated until the next effective counter electromotive force zero crossing point can be detected, and thus the low-speed forced commutation stage is skipped; zero crossing point commutation is directly used, and rapid and reliable starting of the motor is achieved.

Description

Technical Field

[0001] The present disclosure relates to the field of electric motors, and more particularly to a method for starting a brushless DC motor, a control system, and a power tool equipped with a brushless DC motor. Background Art

[0002] An electric motor converts electrical energy into rotational kinetic energy by utilizing the attraction and repulsion of magnetic poles between permanent magnets and electromagnets. In recent years, due to higher efficiency and lower maintenance requirements, brushless DC motors have replaced brushed DC motors as the mainstream motors used in daily tools or complex equipment.

[0003] Brushless DC motors usually achieve position recognition by detecting back electromotive force. However, since the back electromotive force cannot be detected at low speeds, other methods are required to achieve position recognition during motor startup.

[0004] Existing startups are usually achieved by methods such as rotor pre-positioning combined with forced commutation or initial position recognition combined with forced commutation. However, the starting load of power tools (such as electric drills) varies greatly. In the case of heavy loads, the reliability of forced commutation is low, often resulting in startup failures.

[0005] Therefore, an improved method for starting a brushless DC motor is needed.

[0006] Disclosure

[0007] One technical problem to be solved by the present disclosure is to provide an improved method for starting a brushless DC motor. After determining the sector where the initial position of the rotor is located, by continuously applying acceleration pulses in one direction, the rotor is quickly accelerated to a point where the next effective back electromotive force zero crossing can be detected, skipping the low-speed forced commutation stage, and directly using zero-crossing commutation, thereby achieving a fast and reliable startup of the motor.

[0008] According to a first aspect of the present disclosure, a method for starting a brushless DC motor is provided, including: injecting a plurality of voltage pulses in a direction corresponding to the central angles of a plurality of sectors, and obtaining a plurality of current response values; determining the sector where the initial rotor angle is located when the brushless DC motor starts based on the current response values; determining the continuous acceleration direction for applying rotor acceleration pulses according to the sector where the initial rotor angle is located; applying rotor acceleration pulses in the continuous acceleration direction until the zero crossing of the back electromotive force of the non-conducting phase corresponding to the continuous acceleration direction is detected; and performing commutation startup based on the detected zero crossing of the back electromotive force.

[0009] Optionally, determining the sector where the initial rotor angle is located when the brushless DC motor starts based on the current response value includes: determining whether the initial rotor angle is before or after the central angle of the sector where it is located, and determining the continuous acceleration direction for applying the rotor acceleration pulse according to the sector where the initial rotor angle is located, including: when the initial rotor angle is before the central angle of the sector where it is located, determining the direction 180° ahead of the central angle of the sector as the continuous acceleration direction; and when the initial rotor angle is after the central angle of the sector where it is located, determining the direction 120° ahead of the central angle of the sector as the continuous acceleration direction.

[0010] Optionally, determining the sector where the initial rotor angle is located when the brushless DC motor starts based on the current response value includes: determining the sector where the initial rotor angle is located based on the angle corresponding to the maximum current response value among the six current response values; when the difference between the maximum current response value and the current response value of the previous angle is less than a predetermined threshold, determining that the initial rotor angle is before the central angle of the sector where it is located; and when the difference between the current response value and the current response value of the subsequent angle is less than a predetermined threshold, determining that the initial rotor angle is after the central angle of the sector where it is located.

[0011] Optionally, the method further includes: when the initial rotor angle is before the central angle of the sector where it is located, determining the direction 120° ahead of the central angle of the sector as the auxiliary acceleration direction; and before applying the rotor acceleration pulse in the continuous acceleration direction, applying the rotor acceleration pulse in the auxiliary acceleration direction.

[0012] Optionally, applying the rotor acceleration pulse in the auxiliary acceleration direction before applying the rotor acceleration pulse in the continuous acceleration direction includes: determining the duration and duty cycle of applying the rotor acceleration pulse in the auxiliary acceleration direction according to the current load.

[0013] Optionally, applying the rotor acceleration pulse in the continuous acceleration direction until the back electromotive force zero crossing of the non-conducting phase corresponding to the continuous acceleration direction is detected includes: setting the duty cycle of the rotor acceleration pulse according to the current load so that the rotor accelerates to be able to detect the effective back electromotive force zero crossing of the non-conducting phase.

[0014] Optionally, applying the rotor acceleration pulse in the continuous acceleration direction until the back electromotive force zero crossing of the non-conducting phase corresponding to the continuous acceleration direction is detected includes: continuously applying the rotor acceleration pulse in the continuous acceleration direction without performing an operation to change the rotor acceleration pulse addition direction based on commutation, and directly detecting the back electromotive force zero crossing of the non-conducting phase corresponding to the continuous acceleration direction.

[0015] Optionally, the brushless DC motor includes a three-phase winding, and the continuous acceleration direction is to conduct two of the three-phase windings, where the third phase is the non-conducting phase.

[0016] According to a second aspect of the present disclosure, a brushless DC motor control system is provided, including: a motor; a power supply; a microcontroller; a driver; an inverter; and a bus current acquisition device. Wherein, the pulse sent by the microcontroller is converted by the driver into a voltage pulse to act on the inverter to complete the injection of a plurality of voltage pulses in a direction corresponding to the central angle of a plurality of sectors, and the bus current acquisition device obtains a plurality of current response values; the microcontroller determines the sector where the initial rotor angle is located when the brushless DC motor starts according to the current response values, and determines the continuous acceleration direction for applying the rotor acceleration pulse according to the sector where the initial rotor angle is located; and the pulse sent by the microcontroller is converted by the driver into a rotor acceleration pulse applied in the continuous acceleration direction to act on the inverter until the zero-crossing point of the back electromotive force of the non-conducting phase corresponding to the continuous acceleration direction is detected, and commutation start is performed based on the detected zero-crossing point of the back electromotive force.

[0017] According to a third aspect of the present disclosure, a power tool is provided, including a brushless DC motor started by using the method described in the first aspect, where the starting load of the brushless DC motor is determined by the torque required by the power tool.

[0018] The brushless DC motor starting method of the present disclosure determines the sector where the rotor is located, especially the left and right half-region positions of the specific sector where the rotor is located, reasonably selects the main application direction of the acceleration pulse, immediately detects the zero-crossing point of the back electromotive force closely following, skips the low-speed forced commutation stage, and directly uses zero-crossing commutation, thereby realizing the fast and reliable starting of the motor. Description of the Drawings

[0019] By describing the exemplary embodiments of the present disclosure in more detail in conjunction with the drawings, the above and other objects, features, and advantages of the present disclosure will become more obvious. Among them, in the exemplary embodiments of the present disclosure, the same reference numerals generally represent the same components.

[0020] Figure 1 Shows the main structure diagram of a three-phase PWM inverter BLDC load.

[0021] Figure 2 Shows an example of commutation of a brushless DC motor.

[0022] Figure 3 Shows the distribution diagram of the magnetic potential of the stator winding.

[0023] Figure 4Shows a schematic flowchart of a method for starting a brushless DC motor according to an embodiment of the present disclosure.

[0024] Figure 5 Shows the saturation salient pole effect of the self-inductance of the A-phase winding under the action of current.

[0025] Figure 6A -B gives examples of six current response values obtained when the rotor is in different positions.

[0026] Figure 7 Gives examples where the d-axis of the rotor is on the right side of the AB vector, facing the AB vector, and on the left side of the AB vector.

[0027] Figure 8 Shows a schematic flowchart of a motor starting process based on IPD detection according to an embodiment of the present disclosure.

[0028] Figure 9 Shows the terminal voltage waveform when starting the motor according to an embodiment of the present disclosure.

[0029] Figure 10 Shows a schematic diagram of the composition of a brushless DC motor according to an embodiment of the present disclosure. Detailed implementation manners

[0030] The preferred embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although the preferred embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided so that the present disclosure will be more thorough and complete, and can fully convey the scope of the present disclosure to those skilled in the art.

[0031] The motor uses the attraction and repulsion of the magnetic poles of permanent magnets and electromagnets to convert electrical energy into rotational kinetic energy. A brushless DC motor (BLDC) refers to a motor without a brush and a commutator and driven by direct current. The brushless motor uses a permanent magnet as the rotor and an electromagnet as the stator. The stator windings are mostly three-phase symmetric star windings and receive three-phase voltages converted from a DC power supply by a controller. By controlling the magnitude, direction, and timing of the current applied to the three-phase windings of ABC (performing PWM control, that is, pulse width modulation), the rotor can be made to rotate continuously at a desired speed.

[0032] Figure 1 Shows the main structure diagram of a three-phase PWM inverter BLDC load. Figure 1 The left structure is the inverter, and the right structure is the motor. In a BLDC speed control system, in order to apply PWM, a converter that converts DC electrical energy into AC electrical energy is required using an inverter. Figure 1In the shown three-phase PWM inverter with a BLDC load, power transistors VT1 and VT4 are the upper and lower arm switching transistors of phase A respectively, and D1 and D4 are used as diodes for freewheeling of i a during the dead time; power transistors VT2 and VT5 are the upper and lower arm switching transistors of phase B respectively, and D2 and D5 are used as diodes for freewheeling of i b during the dead time; power transistors VT3 and VT6 are the upper and lower arm switching transistors of phase C respectively, and D3 and D6 are used as diodes for freewheeling of i c during the dead time. A single resistor R on the bus is also shown in the figure for bus current sampling.

[0033] A classic driving method for a brushless DC motor is the three-phase six-step commutation method. The control of a BLDC needs to set different conduction sequences of the three-phase windings A / B / C according to the change of the rotor position. Different winding power-on switches need to be carried out when the rotor rotates to the corresponding position, that is, commutation must be carried out accurately and in a timely manner to provide the maximum torque for the rotor. Commutation control is the basis for ensuring the normal rotation of a BLDC. During the operation of a BLDC, the power-on state of the windings changes every 60°. In order to ensure that the motor can obtain the maximum torque during rotation, it is necessary to ensure that during the rotation of the motor, the angle between the motor rotor and the direction of the resultant magnetic field changes between 60° and 120°. At this time, the average voltage vector is 90°, and the maximum driving torque can be reached.

[0034] Figure 2 An example of the commutation of a brushless DC motor is shown. Capital letters A / B / C represent the respective positions of the three-phase windings, and lowercase letters a / b / c combined with the common ground (com) together with the arrows indicate the current flow direction.

[0035] At the same time, for easy understanding, Figure 3 a distribution diagram of the stator winding magnetomotive force is shown. F A+C- , F B+C- , F A-B+ , F A-C+ , F B-C+ , F A+B- are the stator magnetomotive forces formed under the conduction modes of A+C-, B+C-, A-B+, A-C+, B-C+, A+B- respectively. It should be understood that the stator magnetomotive forces F A+C- , F B+C- , F A-B+ , F A-C+ , F B-C+ , F A+B- and Figure 1It corresponds to the stator magnetomotive force that can be added in the six-step commutation operation shown. Therefore, the "+" sign indicates that the current flows out of this phase, and the "-" indicates that the current flows into it. In the following text, B+C- can be abbreviated as BC conduction, and B-C+ can be abbreviated as CB conduction, that is, the former position indicates that the current flows into this phase, and the latter position indicates that the current flows out of this phase.

[0036] Here, it is assumed that the rotor rotates in the CCW direction, that is, the motor rotates forward. As Figure 2 Shown on the left side BC_1, when it is detected that the N pole of the rotor is facing 330°, it is set that BC conducts (that is, the current flows into phase B, and at the same time the current flows out of phase C). Since the voltage vector of BC conduction is 90°, the voltage vector is then at 120° to the rotor at this time. The rotor then continues to rotate under the action of the BC voltage vector. Figure 2 The middle part BC_2 shows the situation when the N pole of the rotor is facing 0°. At this time, the BC voltage vector is still used to apply torque. As the rotor rotates, the angle between the rotor and the BC voltage vector gradually decreases. And when the rotor continues to rotate to face 30° (that is, when the angle between the rotor and the BC voltage vector decreases to 60°), as Figure 2 Shown on the right side BA_1, it commutes to BA conduction (that is, the current flows into phase B, and at the same time the current flows out of phase A). At this time, the conduction voltage vector is at 120° to the rotor again. Commutation is cycled in this way to continuously ensure maximum torque to drive the motor.

[0037] As can be seen from the above, driving a brushless DC motor requires knowledge of the rotor position. Usually, position identification can be achieved by means of back electromotive force detection. However, since the back electromotive force cannot be detected at low speeds, other methods need to be used to achieve position identification when the motor starts.

[0038] Existing startups usually adopt methods such as rotor pre-positioning together with forced commutation or initial position identification together with forced commutation to achieve. However, the starting load of power tools (such as electric drills) changes greatly. In the case of heavy loads, the reliability of forced commutation is low, often resulting in startup failures. Specifically, in the case of heavy-load startup, the existing forced commutation startup scheme commutes according to the set time without rotor position information, and cannot achieve 120° precise commutation control, it is difficult to output sufficient torque, the startup reliability is low, the startup effect is poor, and it will cause the startup to reverse or even fail. In addition, after rotor pre-positioning or initial position identification in the existing technology, a certain number of forced commutations are still required, gradually increasing the duty cycle of the drive pulse and reducing the commutation interval, so that the motor can reach a certain speed to detect the zero crossing of the back electromotive force and then switch to back electromotive force zero crossing commutation. However, the driving time of such an approach is too long to meet the fast startup requirements of some power tools.

[0039] To this end, the present disclosure proposes an improved starting method for a brushless DC motor. After determining the sector where the initial position of the rotor is located, by continuously applying acceleration pulses in a determined acceleration direction, the rotor is quickly accelerated to a point where the zero-crossing of the subsequent effective back electromotive force can be detected, skipping the low-speed forced commutation stage and directly using zero-crossing commutation, thereby achieving a fast and reliable start of the motor.

[0040] Figure 4 FIG. shows a schematic flowchart of a method for starting a brushless DC motor according to an embodiment of the present disclosure. This method is applicable to the starting stage of a brushless DC motor. More specifically, this method is applicable to quickly entering zero-crossing commutation of the back electromotive force of a brushless DC motor.

[0041] In step S410, a plurality of voltage pulses are injected in the directions corresponding to the central angles of a plurality of sectors, and a plurality of current response values are obtained. Subsequently, in step S420, based on the current response values, the sector where the initial rotor angle is located when the brushless DC motor starts is determined. Due to the saturation salient pole effect in the embedded motor, the initial position of the rotor can be identified based on pulse injection, thereby identifying which sector the rotor is in. As is well known, the 360° of one full rotation of the rotor can be divided into six sectors, and the angles of sectors I to VI correspond to 0° to 60°, 60° to 120°, 120° to 180°, 180° to 240°, 240° to 300°, and 300° to 360° respectively. Therefore, injecting a plurality of voltage pulses in the directions corresponding to the central angles of a plurality of sectors can correspond to inputting voltage pulses in six directions of 30°, 90°, 150°, 210°, 270°, and 330°, and obtaining the current response values in each direction. Subsequently, based on the current response values in these six directions, the initial rotor angle when the brushless DC motor starts can be determined.

[0042] After determining the sector where the initial rotor angle is located when starting, in step S430, according to the sector where the initial rotor angle is located, the continuous acceleration direction for applying the rotor acceleration pulse is determined, and in step S440, the rotor acceleration pulse is applied in this continuous acceleration direction until the zero-crossing of the back electromotive force of the non-conducting phase corresponding to the continuous acceleration direction is detected. Subsequently, in step S450, commutation start can be performed based on the detected zero-crossing of the back electromotive force.

[0043] Here, a direction can be selected as the continuous acceleration direction according to the angle corresponding to the sector where the initial rotor angle is located. Different from the prior art, after selecting the continuous acceleration direction, instead of performing commutation acceleration every 60°, the voltage vector in this continuous acceleration direction is used to continuously accelerate until the zero-crossing of the back electromotive force of the non-conducting phase corresponding to the continuous acceleration direction is detected.

[0044] In addition, it can be understood that in the illustrated embodiment, the brushless DC motor includes a three-phase winding. The continuous acceleration direction is to conduct two of the three-phase windings, and the third phase is the non-conducting phase. For example, when using the BA voltage vector for acceleration, the zero-crossing point of the back electromotive force of phase C is directly detected. In other words, in the motor starting scheme of the present disclosure, rotor acceleration pulses will be continuously applied in this continuous acceleration direction without operating to change the direction of the rotor acceleration pulse based on commutation, and the zero-crossing point of the back electromotive force of the non-conducting phase corresponding to this continuous acceleration direction is directly detected.

[0045] After determining the initial rotor position, the motor starting algorithm of the present disclosure directly drives the motor with a large duty cycle and conducts vectors separated by 120° to accelerate the motor within one sector to a sufficient speed to detect the next zero-crossing point, and immediately switches to zero-crossing commutation. Specifically, according to the current load, the duty cycle of the rotor acceleration pulse is set so that the rotor accelerates to be able to detect the effective zero-crossing point of the back electromotive force of the non-conducting phase. Thus, by providing a sufficiently large acceleration pulse duty cycle under the current load, the zero-crossing point detection of the back electromotive force of the non-conducting phase is directly achieved. This method is particularly applicable to application scenarios with large load changes or requiring rapid startup, such as the startup scenario of an electric drill.

[0046] In one embodiment, in order to achieve direct zero-crossing detection in the continuous acceleration direction, it is necessary to further determine the initial rotor angle (for example, in units of 30°), and thus select the continuous acceleration direction. Here, determining the sector where the initial rotor angle is located at the start of the brushless DC motor based on the current response value may include: determining whether the initial rotor angle is before or after the central angle of the sector where it is located. That is, on the basis of determining the sector where the initial rotor angle is located, it can be further determined whether the initial rotor angle is before or after the central angle of the sector where it is located. For example, in sector I (0° to 60°), the central angle is 30°. When the rotor rotates in the CCW direction, that is, when the motor rotates forward, 0° to 30° lags behind the central angle and is after the central angle of the sector where it is located; while 30° to 60° is ahead of the central angle and is before the central angle of the sector where it is located. On this basis, determining the continuous acceleration direction for applying the rotor acceleration pulse according to the sector where the initial rotor angle is located may include: when the initial rotor angle is before the central angle of the sector where it is located, determining the direction 180° ahead of the central angle of the sector where it is located as the continuous acceleration direction; and when the initial rotor angle is after the central angle of the sector where it is located, determining the direction 120° ahead of the central angle of the sector where it is located as the continuous acceleration direction.

[0047] When the initial rotor angle is before the central angle of the sector it is in, in order to have sufficient acceleration time to detect the zero-crossing of the back electromotive force of the non-conducting phase, a direction that is 60° ahead of the conventional acceleration direction which is 120° ahead of the central angle of the sector can be selected as the continuous acceleration direction as described above. In a preferred embodiment, in order to ensure the steady acceleration of the motor rotor, before applying the acceleration pulse in the continuous acceleration direction, auxiliary acceleration can also be performed in the conventional acceleration direction that is 120° ahead of the central angle of the sector for a period of time. At this time, the starting method of the present disclosure further includes: when the initial rotor angle is before the central angle of the sector it is in, determining the direction that is 120° ahead of the central angle of the sector as the auxiliary acceleration direction; and before applying the rotor acceleration pulse in the continuous acceleration direction, applying the rotor acceleration pulse in the auxiliary acceleration direction. Thus, since the torque applied in the auxiliary acceleration direction at the initial angle is greater than the torque in the continuous acceleration direction, it is possible to pre-accelerate for a period of time in the auxiliary acceleration direction before switching to the continuous acceleration direction for acceleration. The duration of applying the rotor acceleration pulse in the auxiliary acceleration direction can be determined according to the current load. Similarly, the duty cycle of applying the rotor acceleration pulse in the auxiliary acceleration direction can be determined according to the current load.

[0048] As described in the previous steps S410 and S420, the initial position of the rotor can be identified based on pulse injection (IPD) using the salient pole effect of the embedded motor, thereby identifying which sector the rotor is in. Here, phase A is taken as an example for illustration. Fig. 6 shows the self-inductance change curve of phase A winding considering the combined influence of the cross-axis and direct-axis magnetic fluxes and the stator winding magnetomotive force. Specifically shown in the figure is the change in the inductance L aa of phase A winding considering the combined influence of the cross-axis and direct-axis magnetic fluxes and the stator winding magnetomotive force. The cross-axis is the q-axis, the direct-axis is the d-axis, the position of the d-axis corresponds to the angle of the rotor position θ, and the position of the q-axis is 90° ahead of the angle of the rotor position θ. At this time, the inductance L aa of phase A winding can be specifically expressed as:

[0049] L aa =L s0 +L s1 cos2θ+L s2 cosθ

[0050] In the formula, L s0 represents the self-inductance of phase A winding, L s1 is the amplitude of the second harmonic generated by the cross-axis and direct-axis magnetic fluxes (the generation of L s1 is mainly related to the salient pole structure, and L s1 <0), L s2 is the amplitude of the fundamental wave component generated by the winding magnetomotive force.

[0051] It can be seen from this that the phase winding inductance is jointly determined by the cross-axis and direct-axis flux linkages and the winding magnetomotive force. When the winding magnetomotive force is in the same direction as the d-axis, the inductance is the smallest. When the angle between the winding magnetomotive force and the d-axis is 90° or 270°, the inductance value is the largest.

[0052] Since the brushless DC motor is in a stationary state at startup and the θ value is constant (i.e., the position of the d-axis remains unchanged), the relative position between the rotor position and the voltage application direction can be determined by changing the stator voltage application direction and observing the response.

[0053] Specifically, when voltages of equal amplitude are applied from different directions, different stator winding magnetomotive forces will be generated, affecting the saturation degree of the magnetic circuit and thus the inductance value. At this time, the saturated salient pole effect can be summarized as follows: when the angle between the stator magnetomotive force and the d-axis is less than 90°, the inductance decreases; when the angle is greater than 90°, the inductance increases. The change in the inductance value will affect the change in the response current. Therefore, by applying pulses in different diagonal directions, the position of the d-axis (i.e., the initial θ value) can be determined based on the magnitude of the response current.

[0054] Specifically, the rising speed of the winding current is inversely proportional to the inductance. For the winding opposite to the N pole of the rotor, the inductance is the smallest and the current rises the fastest. Therefore, the winding inductance can be detected by injecting pulse voltages of a certain width into each phase and detecting the current magnitude. To prevent the rotor position from changing during the detection process, each phase T is turned on in diagonal order on At time, the ADC (analog-to-digital converter) is triggered at the end of the pulse to collect the bus current, and then T is turned off off At time to dissipate the winding current. Six current sampling values are obtained, and the conduction vector corresponding to the maximum value among them is found to determine the initial rotor position.

[0055] To this end, the multi-angle voltage pulse injection corresponds to the voltage pulse injection in six directions, and six current response values are obtained accordingly. Correspondingly, determining the sector where the initial rotor angle is located when the brushless DC motor starts based on the current response values may include: determining the sector where the initial rotor angle is located based on the angle corresponding to the maximum current response value among the six current response values.

[0056] Furthermore, in the present disclosure, the position of the initial rotor angle in the sector can be further determined based on the analysis of the six response values. Figure 6A -B gives an example of the six current response values obtained when the rotor is in different positions.

[0057] In Figure 6A In the example, as shown in the upper left part of the figure, the rotor is in a position facing the axis of the A-phase winding. That is, θ = 0. The inductances of each phase winding are as shown in Figure 6A In the upper right part. At this time, as shown in Figure 6AAs shown in the lower part, if pulses with a conduction time of T are sequentially injected in the AB, BA, CA, AC, BC, and CB directions on and with the same amplitude (it is necessary to wait for the turn-off time T after the injection of the previous pulse off before injecting the next pulse), among the six current responses obtained accordingly, the two current values corresponding to the conduction vectors AB and AC that are symmetric about the left and right of the A-phase winding are simultaneously the largest, and the current values corresponding to the BC and CB conduction vectors perpendicular to the A-phase winding are simultaneously the smallest.

[0058] And Figure 6B in the example of, as shown in the upper left part of the figure, the rotor is at the midpoint position between the A-phase and B-phase windings. That is, θ = 11π / 6. The inductance of each phase winding is as Figure 6B shown in the upper right part. At this time, as Figure 6B shown in the lower part, if pulses with a conduction time of T are sequentially injected in the AB, BA, CA, AC, BC, and CB directions on and with the same amplitude, among the six current responses obtained accordingly, the current value corresponding to the conduction vector AB consistent with the d-axis direction is the largest, the current value corresponding to the conduction vector BA opposite to the d-axis direction is the second largest, and the differences between the current values of the vector AB and its adjacent vectors AC and CB on the left and right are both greater than the set threshold.

[0059] As above Figure 6A shows the current response when the rotor is at the sector boundary (the figure shows entering sector I from sector VI), Figure 6B and shows the situation when the rotor is at the central angle of the sector (the figure shows that the rotor angle is at the midpoint of sector VI). When the rotor position is outside the six sector switching angles, there must be a maximum value among the six current responses. At this time, the sector where the rotor is located can be determined according to the angle corresponding to this maximum value. And when the rotor position is outside the six sector switching angles and not at the sector center, it can be determined whether the rotor is currently before or after the central angle of the sector where it is located according to the relative magnitudes of the current response values of the two injection angles adjacent to the angle corresponding to the maximum value.

[0060] From the above Figure 6A and Figure 6B examples, it can be seen that when the d-axis position of the rotor is closer to a certain conduction vector, the current value of the corresponding vector is larger, and the current values of other vectors decrease. For example, when it is detected that the current value corresponding to the vector AB is the largest, there are still three situations for the actual rotor position: 1) facing the AB vector; 2) within the area 30° to the right of the AB vector; 3) within the area 30° to the left of the AB vector. Figure 7Examples are given where the rotor d-axis is to the right of the AB vector, directly facing the AB vector, and to the left of the AB vector. When the motor rotates forward, the rotor rotates from 300° to 0° within sector VI. The central angle of the sector corresponds to the position of the AB vector. The left side of the figure shows the case where the rotor d-axis is within 30° to the right of the AB vector, that is, after the middle angle of sector VI; the middle shows the case where the rotor d-axis is directly facing the AB vector, that is, at the middle angle of sector VI; the right side shows the case where the rotor d-axis is within 30° to the left of the AB vector, that is, before the middle angle of sector VI.

[0061] When as Figure 7 shown on the left, when the rotor d-axis is within 30° to the right of the AB vector, the current response value corresponding to the AB vector is the largest, and among the two vectors CB and AC adjacent to the AB vector, the current response value corresponding to the CB vector is greater than the current response value corresponding to the AC vector. At this time, the difference between the current response value corresponding to the CB vector and the maximum current response value is less than a predetermined threshold.

[0062] When as Figure 7 shown in the middle, when the rotor d-axis is directly facing the AB vector, the current response value corresponding to the AB vector is the largest, and among the two vectors CB and AC adjacent to the AB vector, the current response value corresponding to the CB vector is equal to the current response value corresponding to the AC vector.

[0063] When as Figure 7 shown on the right, when the rotor d-axis is within 30° to the left of the AB vector, the current response value corresponding to the AB vector is the largest, and among the two vectors CB and AC adjacent to the AB vector, the current response value corresponding to the AC vector is greater than the current response value corresponding to the CB vector. At this time, the difference between the current response value corresponding to the AC vector and the maximum current response value is less than a predetermined threshold.

[0064] Therefore, determining the sector where the initial rotor angle is located when the brushless DC motor starts can also include: when the difference between the maximum current response value and the current response value of the previous angle is less than a predetermined threshold, determining that the initial rotor angle is before the central angle of the sector where it is located; when the difference between the maximum current response value and the current response value of the subsequent angle is less than a predetermined threshold, determining that the initial rotor angle is after the central angle of the sector where it is located.

[0065] Figure 8 Fig. shows a schematic flowchart of a motor starting process based on IPD detection according to an embodiment of the present disclosure. As shown in the figure, after receiving a motor start command, 6 vector pulses can be injected in sequence. As described above, a conduction time of T can be injected in sequence in the directions of AB, BA, CA, AC, BC, and CB onPulses with the same amplitude. It should be understood that the pulses can be injected sequentially in these six directions in any order. And the current values at the end of each pulse can be collected as the current response values of the six injected pulses respectively.

[0066] If there is a case where the current value is less than the minimum threshold, a phase loss fault is considered, and the motor can enter the fault state and report an error. If the current values are all greater than the minimum threshold, find the vector corresponding to the maximum current, such as Figure 8 The AB vector in the example. At this time, the sector where the rotor is located can be determined. For example, when the current response value corresponding to the AB vector is the largest, it is determined that the rotor is located in sector VI.

[0067] Subsequently, determine whether the rotor is on the left or right side of the current sector, that is, after or before the middle angle. If the difference between the current response value corresponding to the AB vector and the current response value corresponding to the adjacent vector on the left (i.e., the AC vector) is less than the predetermined threshold, it is considered that the rotor position is as Figure 7 Described on the right, the d-axis of the rotor is within 30° to the left of the AB vector. That is, at this time, the d-axis of the rotor is located in sector VI at a position closer to the switching angle of 0° for leaving the sector. At this time, since the interval between the d-axis of the rotor and the direction of the BC vector (the BC vector is separated from the AB vector by 120°) is less than 120°, the direction of the BC vector can be first selected as the auxiliary acceleration direction, and acceleration is performed in the direction of the BC vector. After accelerating for a period of time (for example, the acceleration time is determined according to the current load), then switch to the BA vector direction separated from the AB vector by 180° as the continuous acceleration direction to continuously accelerate the rotor, and immediately detect the zero-crossing commutation of the C-phase back electromotive force, thereby entering the zero-crossing commutation, and the startup ends

[0068] And if the difference between the current response value corresponding to the AB vector and the current response value corresponding to the adjacent vector on the right (i.e., the AC vector) is less than the predetermined threshold, it is considered that the rotor position is as Figure 7 Described on the left, the d-axis of the rotor is within 30° to the right of the AB vector. That is, at this time, the d-axis of the rotor is located in sector VI at a position closer to the switching angle of 300° for entering the sector. At this time, since the interval between the d-axis of the rotor and the direction of the BC vector (the BC vector is separated from the AB vector by 120°) is greater than 120°, the direction of the BC vector can be directly selected as the continuous acceleration direction to continuously accelerate the rotor, and immediately detect the zero-crossing commutation of the A-phase back electromotive force, thereby entering the zero-crossing commutation, and the startup ends.

[0069] Figure 9The figure shows the terminal voltage waveform during motor startup according to an embodiment of the present disclosure. The blue, green, and yellow waveforms in the figure may respectively correspond to the terminal voltages of phase C, phase A, and phase B. As shown in the figure, an IPD detection pulse injection is performed at -80 μs, and then the position of the rotor can be determined based on six current response values, for example, within 30° to the right of the AB vector shown on the left side. At this time, an acceleration pulse can be directly applied in the BC vector direction, and the zero-crossing point of the back electromotive force of phase A can be directly detected. As shown by the vertical line in the figure, the zero-crossing point is detected and commutation is performed at position x1, and conventional zero-crossing commutation-based PWM modulation is performed at the vertical dotted line position of x2. In the figure, the time required to switch from IPD pulse injection to zero-crossing commutation does not exceed 10 ms, thereby achieving rapid startup of the motor. Figure 7 within 30° to the right of the AB vector shown on the left side. At this time, an acceleration pulse can be directly applied in the BC vector direction, and the zero-crossing point of the back electromotive force of phase A can be directly detected. As shown by the vertical line in the figure, the zero-crossing point is detected and commutation is performed at position x1, and conventional zero-crossing commutation-based PWM modulation is performed at the vertical dotted line position of x2. In the figure, the time required to switch from IPD pulse injection to zero-crossing commutation does not exceed 10 ms, thereby achieving rapid startup of the motor.

[0070] In addition, the present disclosure can also be implemented as a brushless DC motor control system. Figure 10 The figure shows a schematic diagram of the composition of a brushless DC motor according to an embodiment of the present disclosure. As shown in the figure, in addition to the motor part, the brushless DC motor further includes a power supply, a microcontroller (MCU), a driver, an inverter, and a bus current acquisition device. Specifically, the pulses sent by the microcontroller are converted into voltage pulses by the driver and act on the inverter to complete multiple voltage pulse injections in directions corresponding to the central angles of multiple sectors. For example, voltage pulse injections are performed in the directions of the central angles of six sectors respectively. The bus current acquisition device obtains multiple current response values, for example, six current response values. The microcontroller determines the sector where the initial rotor angle is located when the brushless DC motor starts up based on the current response values, and determines the continuous acceleration direction for applying the rotor acceleration pulse according to the sector where the initial rotor angle is located. The pulses sent by the microcontroller are converted by the driver into rotor acceleration pulses applied in the continuous acceleration direction and act on the inverter until the zero-crossing point of the back electromotive force of the non-conducting phase corresponding to the continuous acceleration direction is detected, and commutation startup is performed based on the detected zero-crossing point of the back electromotive force.

[0071] More specifically, the microcontroller can determine the PWM duty cycle of the conducting vector according to the current load, and immediately detect the zero-crossing point of the back electromotive force immediately after applying the conducting vector in the continuous acceleration direction, skip the low-speed forced commutation stage, and immediately switch to zero-crossing commutation operation to ensure no reverse rotation and large torque output during startup.

[0072] In one embodiment, the present disclosure can also be implemented as a power tool, including a brushless DC motor started by the method described above. The starting load of the brushless DC motor is determined by the torque required by the power tool, that is, the duty cycle of the rotor acceleration pulse applied in the continuous acceleration direction is determined according to the torque required by the power tool. In some cases, it is also necessary to determine the duration and duty cycle of the rotor acceleration pulse applied in the auxiliary acceleration direction according to the torque required by the power tool. This power tool can especially be a power tool that needs to start quickly, such as a drill.

[0073] As described above in conjunction with the drawings, a brushless DC motor and its starting method, as well as a power tool equipped with a brushless DC motor, are described. The brushless DC motor starting scheme of the present disclosure uses an initial position recognition algorithm. Based on the maximum current values corresponding to 6 injection pulses to judge the sector, and according to the relationship between the current values adjacent to the maximum value on the left and right and the maximum value (whether the difference is within the threshold range), further judge the position of the rotor in the left or right half area of the sector. According to the different rotor positions detected above, set the corresponding starting conduction vector and a sufficiently large PWM duty cycle to accelerate the rotor sufficiently to be able to detect the next valid back electromotive force zero crossing, skip the low-speed forced commutation stage, and immediately switch to zero-crossing commutation, so that the motor can output sufficient torque and start reliably.

[0074] This method avoids the problems of weak load-carrying capacity and easy reverse rotation in the prior art starting methods. Further, during starting, it is possible to judge whether there is a phase loss fault according to the IPD detection result, and since the zero crossing of the back electromotive force of the non-conducting phase can be detected without multiple forced commutation drags during starting, and then zero-crossing commutation is used, the starting speed can be increased and the torque output ability can be improved.

[0075] The various embodiments of the present disclosure have been described above. The above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and variations are obvious to those of ordinary skill in the art in the technical field without departing from the scope and spirit of the described embodiments. The selection of the terms used herein is intended to best explain the principles of the embodiments, the practical application, or the improvement of the technology in the market, or to enable other ordinary skill in the art in the technical field to understand the embodiments disclosed herein.

Claims

1. A method for starting a brushless DC motor, comprising: Injecting a plurality of voltage pulses in directions corresponding to the central angles of a plurality of sectors, and obtaining a plurality of current response values; Determining the sector in which the initial rotor angle is located when the brushless DC motor starts based on the current response values; Determining the continuous acceleration direction for applying the rotor acceleration pulse according to the sector in which the initial rotor angle is located; Applying a rotor acceleration pulse in the continuous acceleration direction until the zero-crossing point of the back electromotive force of the non-conducting phase corresponding to the continuous acceleration direction is detected; And Based on the detected zero-crossing point of the back electromotive force, performing commutation start.

2. The method according to claim 1, wherein The determining the sector in which the initial rotor angle is located when the brushless DC motor starts based on the current response values includes: Determining whether the initial rotor angle is before or after the central angle of the sector in which it is located, and The determining the continuous acceleration direction for applying the rotor acceleration pulse according to the sector in which the initial rotor angle is located includes: In the case where the initial rotor angle is before the central angle of the sector in which it is located, determining the direction 180° ahead of the central angle of the sector as the continuous acceleration direction; and In the case where the initial rotor angle is after the central angle of the sector in which it is located, determining the direction 120° ahead of the central angle of the sector as the continuous acceleration direction.

3. The method according to claim 2, wherein, The determining the sector in which the initial rotor angle is located when the brushless DC motor starts based on the current response values includes: Based on the angle corresponding to the maximum current response value among six current response values, determining the sector in which the initial rotor angle is located; In the case where the difference between the maximum current response value and the current response value of the previous angle is less than a predetermined threshold, determining that the initial rotor angle is before the central angle of the sector in which it is located; and In the case where the difference between the current response value and the current response value of the subsequent angle is less than a predetermined threshold, determining that the initial rotor angle is after the central angle of the sector in which it is located.

4. The method according to claim 2, further comprising: In the case where the initial rotor angle is before the central angle of the sector in which it is located, determining the direction 120° ahead of the central angle of the sector as the auxiliary acceleration direction; and Before applying the rotor acceleration pulse in the continuous acceleration direction, applying the rotor acceleration pulse in the auxiliary acceleration direction.

5. The method according to claim 4, wherein Before applying the rotor acceleration pulse in the continuous acceleration direction, applying the rotor acceleration pulse in the auxiliary acceleration direction includes: According to the current load, determining the duration and duty cycle of applying the rotor acceleration pulse in the auxiliary acceleration direction.

6. The method according to claim 1, wherein, Applying a rotor acceleration pulse in the continuous acceleration direction until the zero-crossing point of the back electromotive force of the non-conducting phase corresponding to the continuous acceleration direction is detected includes: According to the current load, setting the duty cycle of the rotor acceleration pulse so that the rotor accelerates to be able to detect the effective zero-crossing point of the back electromotive force of the non-conducting phase.

7. The method according to claim 1, wherein Applying a rotor acceleration pulse in the continuous acceleration direction until the zero-crossing point of the back electromotive force of the non-conducting phase corresponding to the continuous acceleration direction is detected includes: A rotor acceleration pulse is continuously applied in the continuous acceleration direction without performing an operation of changing the addition direction of the rotor acceleration pulse based on commutation, and a back electromotive force zero crossing of the non-conducting phase corresponding to the continuous acceleration direction is directly detected.

8. The method according to claim 1, wherein The brushless DC motor includes a three-phase winding, and the continuous acceleration direction is to conduct two of the three-phase windings, where the third phase is the non-conducting phase.

9. A brushless DC motor control system, comprising: a motor; a power supply; a microcontroller; a driver; an inverter; and a bus current acquisition device, wherein, the pulse sent by the microcontroller is converted by the driver into a voltage pulse acting on the inverter to complete the injection of a plurality of voltage pulses in a direction corresponding to the central angle of a plurality of sectors, the bus current acquisition device obtains a plurality of current response values; the microcontroller determines the sector where the initial rotor angle is located when the brushless DC motor starts according to the current response values, and determines the continuous acceleration direction for applying the rotor acceleration pulse according to the sector where the initial rotor angle is located; and the pulse sent by the microcontroller is converted by the driver into a rotor acceleration pulse applied in the continuous acceleration direction and acts on the inverter until a back electromotive force zero crossing of the non-conducting phase corresponding to the continuous acceleration direction is detected, and commutation startup is performed based on the detected back electromotive force zero crossing.

10. A power tool, comprising a brushless DC motor started by using the method according to any one of claims 1-8, wherein the starting load of the brushless DC motor is determined by the torque required by the power tool.

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