Electric tool and motor control system and method thereof

Abstract

A motor control system for controlling a DC brushless motor for an electric tool is provided. The direct current brushless motor includes a rotor of an embedded magnet type and a stator having a three-phase winding. The motor control system comprises: a detection unit for detecting a phase voltage on each phase winding of a three-phase winding; the driving unit is connected between the direct-current brushless motor and the direct-current bus and comprises three bridge arms which are electrically connected with one of the three phase windings respectively, each bridge arm comprises an upper bridge arm switch and a lower bridge arm switch, and each bridge arm switch is connected with a fly-wheel diode in parallel; the control unit is electrically connected with the detection unit and the driving unit, and the control unit is configured to calculate the induced electromotive force of the winding phase in the non-working state based on the detected phase voltage; determining the position of the rotor based on the calculated induced electromotive force; and on-off of each bridge arm switch is controlled based on the position of the rotor.

Description

Electric tool and motor control system and method thereof Technical Field

[0001] The present invention generally relates to the technical field of power tools, and in particular to a motor control system and a motor control method for controlling a brushless DC motor for a power tool, and also to a power tool including the motor control system. Background Art

[0002] Power tools typically use electric motors as their power source. Brushless DC motors (BLDC) utilize electronic commutation technology instead of mechanical brushes. This allows them to retain the advantages of DC motors while also offering the simplicity, reliability, and ease of maintenance of AC motors. To achieve electronic commutation, the motor's rotor position must be determined.

[0003] In the prior art, one method for determining the rotor position of a motor is to use a position detector. This position detector, for example, a Hall effect sensor or a GMR sensor, can be placed near the rotor's magnetic poles or on a magnetic encoder. The rotor position is calculated based on the strength and direction of the magnetic field measured by the position detector. However, the use of a position sensor increases hardware cost and mechanical complexity, as well as poor system reliability.

[0004] Therefore, there has been a clear trend towards adopting position sensorless solutions recently. Current position sensorless solutions usually use RC (resistance-capacitance) filtering on the back EMF signal for detection. However, such solutions have obvious shortcomings. For example, when the motor is running at low speed, the back EMF signal is weak, making it difficult to detect the motor rotor position. To this end, the existing solution is to use pulse injection to detect the rotor position when the motor is running at low speed. Due to the need for pulse current injection, this will result in a reduction in the time range for the motor to be driven, and the output of the motor will be limited when running at low speed. In addition, when the motor is running at high speed and the load current is large, such a solution has the problem of commutation lag caused by the delay after RC filtering.

[0005] Summary of the Invention

[0006] In view of the above-mentioned problems in the prior art, the present invention aims to provide an improved control scheme, which can accurately obtain the position of the motor rotor in the full range from motor standstill to motor full speed operation without the need for a sensor for detecting the rotor position (sensorless type).

[0007] According to one aspect of the present invention, there is provided a motor control system for controlling a brushless DC motor for an electric tool, wherein the brushless DC motor comprises a rotor with an embedded magnet and a stator with a three-phase winding, and the motor control system comprises: a detection unit for detecting the phase voltage on each phase winding of the three-phase winding; a drive unit connected between the brushless DC motor and a DC bus, comprising three bridge arms, each electrically connected to one of the three phase windings, each bridge arm comprising an upper arm switch and a lower arm switch, each bridge arm switch being connected in parallel to a freewheeling diode; and a control unit, electrically connected to the detection unit and the drive unit, respectively, the control unit being configured to detect the phase voltage based on the detected phase voltage. Calculate the induced electromotive force of the winding phase in the non-working state; determine the position of the rotor based on the calculated induced electromotive force; and control the on and off of each bridge arm switch based on the position of the rotor to control the energized phase sequence of each phase winding, wherein the control unit is further configured to: judge the positive and negative of the induced electromotive force; if at the moment when the phase winding in the working state exits the working state, it is determined that the induced electromotive force is positive, then control the freewheeling diode connected in parallel with the upper bridge arm switch connected to the phase winding to be turned on; and if at the moment when the phase winding in the working state exits the working state, it is determined that the induced electromotive force is negative, then control the freewheeling diode connected in parallel with the lower bridge arm switch connected to the phase winding to be turned on.

[0008] According to another aspect of the present invention, there is provided an electric tool comprising: a brushless DC motor comprising a rotor with an embedded magnet and a stator with a three-phase winding; and a motor control system as described above, electrically connected to the brushless DC motor, for determining the induced electromotive force of the phase winding based on the phase voltage of the phase winding in a non-working state, and controlling the on and off of each bridge arm switch and the freewheeling diode based on the induced electromotive force.

[0009] According to another aspect of the present invention, a control method for a brushless DC motor for an electric tool is provided, wherein the brushless DC motor includes a rotor with an embedded magnet and a stator with a three-phase winding, and the electric tool includes a motor control system, which includes a detection unit and a drive unit, wherein the detection unit is used to detect the phase voltage on each phase winding of the three-phase winding and does not include a filter capacitor, and the drive unit includes three bridge arms, each connected to one of the three phase windings, each bridge arm includes an upper bridge arm switch and a lower bridge arm switch, and each bridge arm switch is connected in parallel with a freewheeling diode, and the control method includes: obtaining the detected phase voltage; calculating the phase voltage in the non-working state based on the phase voltage; The induced electromotive force of the winding phase in the working state is determined; the position of the rotor is determined based on the calculated induced electromotive force; and the on and off of each bridge arm switch is controlled based on the position of the rotor, so as to control the energized phase sequence of each phase winding, wherein the control method further includes: judging the positive and negative of the induced electromotive force; if at the moment when the phase winding in the working state exits the working state, it is determined that the induced electromotive force is positive, then the freewheeling diode connected in parallel with the upper bridge arm switch connected to the phase winding is controlled to be turned on; and if at the moment when the phase winding in the working state exits the working state, it is determined that the induced electromotive force is negative, then the freewheeling diode connected in parallel with the lower bridge arm switch connected to the phase winding is controlled to be turned on.

[0010] According to yet another aspect of the present invention, a machine-readable storage medium is provided, which stores executable instructions. When the instructions are executed, one or more processors are caused to perform the method described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG1 is a schematic diagram of a power tool according to an embodiment of the present invention.

[0012] FIG. 2 is a schematic structural diagram of a motor of the electric tool in FIG. 1 .

[0013] FIG3 is a circuit diagram of a motor control system according to an embodiment of the present invention.

[0014] FIG. 4 schematically illustrates a first threshold voltage and a second threshold voltage according to an embodiment of the present invention.

[0015] FIG5 schematically shows a waveform diagram of back electromotive force according to an embodiment of the present invention.

[0016] FIG6 is a flow chart of a motor control method according to an embodiment of the present invention. DETAILED DESCRIPTION

[0017] An embodiment of the present invention relates to a control scheme for a brushless DC motor, which detects the phase voltage of a phase winding, calculates the induced electromotive force of a phase winding in an inoperative state based on the phase voltage, and determines a specific rotor position, i.e., the rotor position requiring commutation, by determining the zero-crossing moment of the induced electromotive force.

[0018] The inventors of the present invention discovered that the back electromotive force generated by the rotor movement and the transformer electromotive force caused by the current change in the winding coil are consistent in the detection direction. This is because the direction of the back electromotive force is determined by the direction of the magnetic flux change of the permanent magnet, and the transformer electromotive force is caused by the position of the permanent magnet in the magnetic circuit. It can be seen that the common point between the two is that they are both caused by magnetic steel, and the motor structure using an embedded magnet type (IPM) rotor determines that the two are consistent in the detection direction. In other words, the phase relationship between the transformer electromotive force and the back electromotive force during detection is consistent. Based on this, according to an embodiment of the present invention, the rotor position detection is based on the composite signal of the back electromotive force generated by the rotor movement and the transformer electromotive force caused by the current change in the winding coil, that is, the instantaneous induced electromotive force on the phase winding is detected.

[0019] The embodiments of the present invention are particularly suitable for situations where a motor is started or runs at a low speed, because when the rotor speed is extremely low or even zero, the rotor position can still be detected by using the instantaneous induced electromotive force.

[0020] In addition, the rotor position detection scheme according to the embodiment of the present invention is also applicable when the motor is running at medium speed, high speed or full speed. Therefore, the rotor position detection scheme according to the embodiment of the present invention can be applied to the entire process of the motor from zero speed to full speed.

[0021] According to an embodiment of the present invention, the filter capacitor in the detection unit for detecting the phase voltage is removed, which ensures that the instantaneous signal of the phase voltage can be detected when the motor runs at an extremely low speed (for example, an extremely low PWM duty cycle), thereby preventing the rotor position from being lost when the motor runs at a low speed.

[0022] According to an embodiment of the present invention, the start and end of commutation freewheeling are determined by setting a suitable voltage threshold, so that the detection signal during commutation freewheeling can be treated as an invalid signal, thereby avoiding interference with commutation freewheeling.

[0023] According to an embodiment of the present invention, a control strategy is employed: in each bridge arm switch's 120° operating range, square wave control is used to keep it on during the first 60° of the switch's operation; in the second 60°, PWM control is used to alternately switch it on and off. This control strategy is also known as the ON-PWM control mode.

[0024] The specific embodiments of the present invention are described below with reference to the accompanying drawings.

[0025] FIG1 schematically illustrates a power tool 100 according to an embodiment of the present invention, which includes a brushless DC motor 10 (hereinafter referred to as motor 10) and a motor control system 20. It will be appreciated that while the power tool 100 illustrated in FIG1 may be a machine tool such as a drill or a hammer, the power tool according to an embodiment of the present invention may also be other types of machine tools, and is not limited thereto.

[0026] Motor 10 includes an embedded magnet (IPM) rotor and a stator consisting of three phase windings. According to an embodiment of the present invention, the rotor of motor 10 employs an IPM, i.e., permanent magnets are embedded within the rotor structure. The permanent magnets can be embedded in a variety of ways, and the present invention does not limit the embedding method of the permanent magnets.

[0027] FIG2 schematically illustrates an implementation of the motor 10. Referring to FIG2 , in the rotor structure, permanent magnets 111 and 112 are embedded in an iron core 113. In the stator structure, there are three coil windings 121-123, each constituting one of the three phase windings (e.g., a U-phase winding, a V-phase winding, and a W-phase winding) of the motor 10. It will be understood that FIG2 is merely exemplary and the present invention is not limited thereto. According to an embodiment of the present invention, the motor may include multiple winding coils and multiple pairs of magnetic poles, for example, 6 winding coils and 2 pairs of magnetic poles.

[0028] The motor control system 20 is electrically connected to the motor 10 and is configured to detect the rotor position of the motor 10 and control power to the motor 10 based on the rotor position. FIG3 illustrates a circuit diagram of the motor control system 20 according to an embodiment of the present invention. Referring to FIG3 , the motor control system 20 includes a detection unit 21, a drive unit 22, and a control unit 23.

[0029] The detection unit 21 is connected between the three phase windings of the motor 10 (see U, V, and W in FIG3 ) and the control unit 23. The detection unit 21 includes three detection branches, each for detecting the phase voltage on one of the three phase windings (see V_U, V_V, and V_W in FIG3 ) and transmitting the detected phase voltage to the control unit 23. No filter capacitor is included in each detection branch.

[0030] In one embodiment, the detection unit 21 includes three voltage divider circuits, namely, a first voltage divider circuit, a second voltage divider circuit, and a third voltage divider circuit, each of which is used to collect the phase voltage on a phase winding. For example, referring to Figure 3, the first voltage divider circuit is connected to the U-phase winding and includes two voltage divider units R1 and R2 connected in series, wherein one voltage divider unit R1 is connected to the connector of the U-phase winding and the other voltage divider unit R2 is connected to ground. The connection point between the one voltage divider unit R1 and the other voltage divider power supply R2 is electrically connected to a port of the control unit 23 so that the detected phase voltage V_U on the U-phase winding (i.e., the voltage on the voltage divider unit R2) is transmitted to the control unit 23.

[0031] Similarly, a second voltage divider circuit is connected to the V-phase winding and includes two voltage divider units R3 and R4 connected in series. One voltage divider unit R3 is connected to a terminal of the V-phase winding, and the other voltage divider unit R4 is connected to ground. The connection point between the first voltage divider unit R3 and the second voltage divider unit R4 is electrically connected to a port of the control unit 23 to transmit the detected phase voltage V_V on the V-phase winding (i.e., the voltage on the voltage divider unit R4) to the control unit 23.

[0032] Similarly, the third voltage divider circuit is connected to the W-phase winding and includes two voltage divider units R5 and R6 connected in series. One voltage divider unit R5 is connected to a terminal of the W-phase winding, and the other voltage divider unit R6 is connected to ground. The connection point between the first voltage divider unit R5 and the second voltage divider unit R6 is electrically connected to a port of the control unit 23 to transmit the detected phase voltage V_W on the W-phase winding (i.e., the voltage on the voltage divider unit R6) to the control unit 23.

[0033] It is worth noting that the detection unit 21 according to the embodiment of the present invention does not include a filter capacitor, that is, no capacitor for filtering is used in the detection unit 21, because the filter capacitor will cause a lag or loss of the sampling signal. This is especially important for detecting the phase voltage when the motor 10 is running at a low speed, because when the motor 10 is running at a low speed, it corresponds to a low power-on time (for example, the duty cycle of the PWM signal for controlling the bridge arm switch of the drive unit 22 is less than 4%, that is, the ON ratio of the bridge arm switch is less than 4%). In this case, the filtering function of the filter capacitor will cause the detection signal to be lost, thereby losing the rotor position. In addition, omitting the filter capacitor also plays a role in reducing costs and simplifying the structure.

[0034] It is understandable that each voltage dividing unit may include one or more resistors to achieve a suitable voltage dividing ratio, so as to transmit a voltage dividing signal of a suitable ratio to the control unit 23 .

[0035] The drive unit 22 is connected between the motor 10 and the DC bus (DC BUS). The DC bus voltage can be the power supply voltage of the DC power supply (DC). The drive unit 22 includes three bridge arms: a first bridge arm, a second bridge arm, and a third bridge arm, forming a three-phase bridge inverter circuit. Each of the three bridge arms is electrically connected to one of the three phase windings. Each bridge arm includes an upper bridge arm switch and a lower bridge arm switch, and each bridge arm switch is connected in parallel to a freewheeling diode.

[0036] Continuing to refer to Figure 3, the first bridge arm includes an upper bridge arm switch T1 and a freewheeling diode D1 connected in parallel therewith, and a lower bridge arm switch T2 and a freewheeling diode D2 connected in parallel therewith. One terminal of the upper bridge arm switch T1 and the cathode of the freewheeling diode D1 are connected to the DC bus DC BUS. One terminal of the lower bridge arm switch T2 and the anode of the freewheeling diode D2 are connected to the ground GND. The upper bridge arm switch T1 has a controlled terminal X1, which is electrically connected to a port of the control unit 23 so as to be in an on state (ON) or an off state (OFF) under a control signal from the control unit 23. The lower bridge arm switch T2 has a controlled terminal X2, which is electrically connected to a port of the control unit 23 so as to be in an on state (ON) or an off state (OFF) under a control signal from the control unit 23.

[0037] Similarly, the second bridge arm includes an upper bridge arm switch T3 and a freewheeling diode D3 connected in parallel therewith, and a lower bridge arm switch T4 and a freewheeling diode D4 connected in parallel therewith. One terminal of the upper bridge arm switch T3 and the cathode of the freewheeling diode D3 are connected to the DC bus DC BUS. One terminal of the lower bridge arm switch T4 and the anode of the freewheeling diode D4 are connected to the ground GND. The upper bridge arm switch T3 has a controlled terminal X3, which is electrically connected to a port of the control unit 23 so as to be in an on state (ON) or an off state (OFF) under a control signal from the control unit 23. The lower bridge arm switch T4 has a controlled terminal X4, which is electrically connected to a port of the control unit 23 so as to be in an on state (ON) or an off state (OFF) under a control signal from the control unit 23.

[0038] Similarly, the third bridge arm includes an upper bridge arm switch T5 and a freewheeling diode D5 connected in parallel therewith, and a lower bridge arm switch T6 and a freewheeling diode D6 connected in parallel therewith. One terminal of the upper bridge arm switch T5 and the cathode of the freewheeling diode D5 are connected to the DC bus DC BUS. One terminal of the lower bridge arm switch T6 and the anode of the freewheeling diode D6 are connected to the ground GND. The upper bridge arm switch T5 has a controlled terminal X5, which is electrically connected to a port of the control unit 23 so as to be in an on state (ON) or an off state (OFF) under a control signal from the control unit 23. The lower bridge arm switch T6 has a controlled terminal X6, which is electrically connected to a port of the control unit 23 so as to be in an on state (ON) or an off state (OFF) under a control signal from the control unit 23.

[0039] The control unit 23 is electrically connected to the detection unit 21 and the drive unit 22. The control unit 23 receives the phase voltage detection signal from the detection unit 21, determines the voltage value and polarity of the corresponding induced electromotive force based on the phase voltage detection signal, and determines the zero-crossing point of the induced electromotive force as the time when the motor 10 needs to be commutated. Based on this, the control unit 23 outputs a control signal for controlling the on and off of each bridge arm switch to the drive unit 22.

[0040] The control unit 23 may be implemented using hardware, software, or a combination of software and hardware. For example, the control unit 23 may be implemented in one or more application-specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, microcontrollers, microprocessors, electronic units designed to perform their functions, or combinations thereof. The control unit 23 may also be implemented using microcode, program code, or code segments, which may be stored in a machine-readable storage medium such as a storage component.

[0041] In one embodiment, the control unit 23 is implemented as a microcontroller (MCU).

[0042] In one embodiment, the control unit 23 is implemented as including a memory and a processor. The memory contains instructions, which, when executed by the processor, enable the processor to perform the motor control method according to the embodiment of the present invention.

[0043] In one embodiment, the control unit 23 performs analog-to-digital conversion on the phase voltage measurement signals, that is, converts the detected analog signals of the phase voltages into digital signals, so that the digital phase voltages can be used in subsequent operations of the control unit 23. For example, the control unit 23 may include a conversion module (not shown) for converting the detected analog signals of the phase voltages of each phase into digital signals.

[0044] This analog-to-digital conversion is advantageous because conventional techniques typically compare analog sampling signals with reference signals, resulting in only comparison results such as 0 or 1. The present invention, however, utilizes digital sampling signals to obtain multiple voltage measurements, which can be incorporated into calculations to yield a variety of computational results useful for motor control. Furthermore, the use of digital sampling signals improves computational accuracy.

[0045] In an embodiment of the present invention, an electrical cycle T (360°) of the brushless DC motor 10 includes six sub-cycles, each lasting T / 6. Each sub-cycle corresponds to a 60° segment. For example, the six sub-cycles of an electrical cycle T are: a 0°-60° segment, a 60°-120° segment, a 120°-180° segment, a 180°-240° segment, a 240°-300° segment, and a 300°-360° segment. The control unit 23 triggers the three phase windings with a 120° offset. In each 60° segment, two of the three phase windings are in operation, and one phase winding is inoperative. For example, in the 0°-60° segment and the 180°-240° segment, phase windings U and V are in operation, and phase winding W is inoperative. Similarly, in the 60°-120° section and the 240°-300° section, phase windings U and W are in operation, and phase winding V is in non-operation. Similarly, in the 120°-180° section and the 300°-360° section, phase windings W and V are in operation, and phase winding U is in non-operation.

[0046] The control unit 23 converts the detected phase voltage into an induced electromotive force. For example, when phase windings U and V are in operation and phase winding W is in an inoperative state, the control unit 23 converts the phase voltage on the inoperative W phase winding into the induced electromotive force of phase W using the following formula:

[0047]

[0048] Where E W is the induced electromotive force of phase W, which includes the back electromotive force generated by the rotor motion and the transformer electromotive force generated by the stator winding current change. W is the W phase voltage, U U is the U phase voltage, U V is the V-phase voltage.

[0049] When other phase windings are in a non-operating state, the conversion of their phase voltages into induced electromotive force can be achieved using a similar method and formula as above, which will not be described in detail here.

[0050] In one embodiment, the control unit 23 is configured to compare the induced electromotive force of the phase winding in the non-operating state with half the sum of the phase voltages of the two phase windings in the operating state. For example, when the phase windings U and V are in the operating state and the phase winding W is in the non-operating state, the control unit 23 compares the induced electromotive force of the W phase with half the sum of the phase voltages of the U phase and the V phase.

[0051] It is worth noting that for two phase windings in an operating state, one phase winding is connected to the DC bus, so that its phase voltage is the DC bus voltage, and the other phase winding is connected to ground, so that its phase voltage is the ground voltage (i.e., zero). In this way, the sum of the phase voltages on each phase winding of the two phase windings in an operating state is the DC bus voltage (e.g., the power supply voltage of the power supply DC). Therefore, the control unit 23 can compare the induced electromotive force of the winding phase in the non-operating state with half the DC bus voltage.

[0052] Based on the comparison results, control unit 23 determines the positive and negative signs of the induced electromotive force and its zero crossing point. When the induced electromotive force is greater than half the DC bus voltage, the induced electromotive force is determined to be positive; when the induced electromotive force is equal to half the DC bus voltage, the back electromotive force is determined to be zero; and when the back electromotive force is less than half the DC bus voltage, the induced electromotive force is determined to be negative.

[0053] For example, the control unit 23 can plot the time-varying induced electromotive force of a non-operating phase winding. During the operation of the motor 10, the induced electromotive force may change from a positive value through a zero crossing to a negative value, and from a negative value through a zero crossing to a positive value. The zero crossing of the induced electromotive force corresponds to the moment when the motor 10 switches the active phase winding, for example, when the W phase winding stops operating and the U phase winding starts operating.

[0054] In one embodiment, the control unit 23 sets a first voltage threshold and a second voltage threshold that is less than the first voltage threshold. The first voltage threshold is less than the DC bus voltage. For example, the first voltage threshold ranges from 85% to 95% of the DC bus voltage, and optionally, from 90% to 95%. The second voltage threshold is greater than the ground voltage. For example, the second voltage threshold ranges from 5% to 15% of the DC bus voltage, and optionally, from 5% to 10%. When the back electromotive force of the non-operating phase winding is greater than the first voltage threshold or less than the second voltage threshold, the control unit 23 deems the phase voltage detection signal (i.e., the detected phase voltage signal) to be an invalid signal and does not perform the above-mentioned comparison operation. In other words, when the induced electromotive force of the non-operating phase winding is between the second voltage threshold and the first voltage threshold, the control unit 23 deems the sampled signal (i.e., the detected phase voltage signal) during this period to be a valid signal and compares the induced electromotive force during this period with half the DC bus voltage.

[0055] Figure 4 illustrates a first voltage threshold TH_1 and a second voltage threshold TH_2 according to an embodiment of the present invention. As can be seen from Figure 4 , the range between the second voltage threshold TH_2 and the first voltage threshold TH_1 is smaller than the range between the ground voltage GND and the DC bus voltage. In other words, the range between the first voltage threshold TH_1 and the DC bus voltage, as well as the range between the ground voltage GND and the second voltage threshold TH_2, are considered invalid ranges in this embodiment of the present invention.

[0056] Setting such a voltage threshold is advantageous because it avoids interference from the freewheeling diode during the commutation freewheeling period. Specifically, when the freewheeling diode is in the commutation freewheeling period, the phase voltage detected is the ground voltage or the DC bus voltage, which is not the actual phase voltage, thereby generating interference from the ground voltage or the DC bus voltage. By setting the first and second voltage thresholds in this way, the low voltage detection signal and the DC bus voltage detection signal, which serve as interference signals, can be removed, thereby eliminating invalid detection signals and obtaining only valid detection signals, thereby accurately determining the rotor position.

[0057] Therefore, by properly setting the first and second threshold voltages, the start and end times of diode freewheeling can be determined. In other words, once diode freewheeling occurs, it can be detected, and once diode freewheeling ends, it can also be detected. This avoids interference caused by commutation freewheeling. Furthermore, a longer effective detection period can be retained while eliminating the ineffective detection period during commutation freewheeling.

[0058] In one embodiment, the control unit 23 is configured to execute an ON-PWM control mode. This ON-PWM control mode is executed, for example, when the motor is running at a low speed. For example, the control unit 23 executes this ON-PWM control mode during motor startup or when the motor speed is less than 2000 rpm or less than 10% of the rated speed.

[0059] In the ON-PWM control mode, the control unit 23 controls each arm switch so that, in at least one 120° operating section in which the arm is in operation, the arm switches are directly turned on (e.g., controlled by a square wave and always in the ON state) in the first 60° operating section, and are PWM-controlled to alternately turn on and off (e.g., alternately in the ON state and the OFF state) in the last 60° operating section. In one embodiment, in the ON-PWM control mode, the control unit 23 controls each arm switch so that, in each 120° operating section in which the arm is in operation, the arm switches are directly turned on in the first 60° operating section, and are PWM-controlled to alternately turn on and off in the last 60° operating section.

[0060] In one implementation of this embodiment, the control unit 23 may perform the following operations: if the induced electromotive force of the non-operating phase winding is positive when one of the two phase windings in the operating state enters the non-operating state from the operating state (for example, when the U phase exits the operating state), the freewheeling diode (i.e., the freewheeling diode connected to the DC bus) connected in parallel with the upper bridge arm switch connected to the phase winding is turned on. If the induced electromotive force of the non-operating phase winding is negative when one of the two phase windings in the operating state enters the non-operating state from the operating state (for example, when the U phase exits the operating state), the freewheeling diode (i.e., the freewheeling diode connected to the ground) connected in parallel with the lower bridge arm switch connected to the phase winding is turned on.

[0061] For example, at the moment when the W-phase winding exits the working state, if the induced electromotive force of the W-phase winding in the non-working state is positive (this is also the starting moment when the W-phase winding enters the non-working state), the freewheeling diode of the W-phase connected to the DC bus is turned on; if the induced electromotive force of the W-phase winding is negative, the freewheeling diode of the W-phase connected to the ground is turned on.

[0062] The control unit 23 can also perform the following operations: monitoring the change over time of the induced electromotive force of the phase winding in the non-operating state; if a zero crossing occurs during the process of the induced electromotive force changing from positive to negative, the zero crossing time is determined as a predetermined rotor position, i.e., a rotor position of the phase winding requiring switching to operation. If a zero crossing occurs during the process of the back electromotive force changing from negative to positive, the zero crossing time is determined as another predetermined rotor position, i.e., another rotor position of the phase winding requiring switching to operation.

[0063] This ON-PWM control mode is advantageous because when the PWM is off, interference caused by the freewheeling diode's freewheeling current (i.e., PWM off-freewheeling interference) occurs. In other words, the freewheeling current generated when the PWM is off can affect the accuracy of detection when the PWM is turned on the next time. The fundamental reason for this interference is current inertia (i.e., the current in the inductor cannot change suddenly). When the PWM is off (PWM OFF), if the freewheeling diode connected to the detection phase generates freewheeling current, it will take a certain amount of time for the current to decrease when the PWM is turned on the next time (PWM ON), so that the detected phase winding can return to a detectable state without freewheeling current.

[0064] The ON-PWM control mode according to an embodiment of the present invention can avoid the above-mentioned PWM shutdown freewheeling interference because it can control the freewheeling direction of the freewheeling diode connected to the detected phase winding when PWM is turned off, so that the freewheeling direction is consistent with the direction of the back electromotive force of the detected phase winding. As described above, if the back electromotive force of the detected phase winding is positive when a phase winding exits the working state, the freewheeling direction is controlled to the freewheeling direction in which the freewheeling diode connected to the DC bus is turned on. If the back electromotive force of the detected phase winding is negative when a working phase exits the working state, the freewheeling direction is controlled to the freewheeling direction in which the freewheeling diode connected to the ground is turned on.

[0065] Figure 5 schematically illustrates the change in induced electromotive force under the ON-PWM control mode according to an embodiment of the present invention. In Figure 5, for example, taking the case where the W-phase and V-phase windings are operating and the U-phase winding is not operating, the dotted line indicates that the induced electromotive force of the U-phase winding is zero, the area above the dotted line indicates that the induced electromotive force of the U-phase winding is positive, and the area below the dotted line indicates that the induced electromotive force of the U-phase winding is negative.

[0066] Referring to Figure 5 , the T5 (ON) segment indicates that during the first 60° of the W-phase high-arm operation, the high-arm switch T5 in operation is directly on (i.e., the high-tube is conducting). The T5 (PWM) segment indicates that during the last 60° of the W-phase high-arm operation, the high-arm switch T5 in operation is PWM-modulated (i.e., the high-tube is PWM-modulated).

[0067] Continuing to refer to Figure 5, the V6 segment represents the phase-commutation continuous flow, that is, the phase-commutation continuous flow occurs at the moment when the W phase exits the working state and the U phase enters the working state. The end time of the V6 segment represents the end time of the phase-commutation continuous flow. For example, the end time of the phase-commutation continuous flow can be determined by the above-mentioned voltage threshold. The V7 segment represents a valid detection segment. For example, the induced electromotive force of the U phase in this segment is compared with half of the DC bus voltage, and the comparison result is that the induced electromotive force is positive. The V8 segment represents the shutdown continuous flow of PWM, and the induced electromotive force of the W phase is (or close to) the linear bus voltage. The induced electromotive force at this time is compared with half of the DC bus voltage, and the comparison result is still that the induced electromotive force is positive.

[0068] 5 , the T6 (ON) section indicates that the lower bridge arm switch of the W phase enters the working state, and then the PWM modulation of T6 occurs (not shown). This process is similar to the above description of the upper bridge arm switch of the W phase in the working state.

[0069] Continuing to refer to Figure 5, before the T5 (ON) section, a situation where the induced electromotive force is negative is shown. This situation is similar to the situation where the induced electromotive force is positive described above. For example, the V1 section indicates that a bridge arm switch PWM is turned off in the working state. The V2 section indicates that a bridge arm switch PWM is turned on in the working state. The V3 section indicates PWM phase-changing continuous flow. The end moment of the V3 section indicates the end moment of phase-changing continuous flow. The V4 section indicates PWM shutdown continuous flow. The V5 section indicates a valid detection section. For example, the induced electromotive force of this section is compared with half of the DC bus voltage, and the comparison result is that the induced electromotive force is negative.

[0070] This shows that even if PWM freewheeling occurs during the detection process, the comparison result after the freewheeling is consistent with the comparison result before the freewheeling is interrupted, thus not affecting the accuracy of the zero-crossing detection result. Specifically, on the one hand, PWM freewheeling occurs before the induced electromotive force crosses zero, but this freewheeling does not affect the comparison result; on the other hand, freewheeling no longer occurs after the induced electromotive force crosses zero. Furthermore, this ON-PWM control mode can be combined with the aforementioned method of setting the threshold voltage.

[0071] FIG6 shows a motor control method 600 according to an example of the present invention. The method 600 can be executed by the control unit 23 or the motor control system 20 , and thus the above descriptions of the control unit 23 and the motor control system 20 are also applicable thereto.

[0072] 6 , in step 602 , the detected phase voltage is obtained.

[0073] In step 604 , the induced electromotive force of the winding phase in the non-operating state is calculated based on the detected phase voltage.

[0074] In step 606 , the position of the rotor is determined based on the calculated induced electromotive force.

[0075] In step 608, the on and off of each arm switch is controlled based on the position of the rotor, so as to control the energization phase sequence of each phase winding.

[0076] The method 600 also includes: determining whether the induced electromotive force is positive or negative; if the induced electromotive force is determined to be positive at the moment when the phase winding in the working state exits the working state, then controlling the freewheeling diode connected in parallel with the upper bridge arm switch connected to the phase winding to be turned on; and if the induced electromotive force is determined to be negative at the moment when the phase winding in the working state exits the working state, then controlling the freewheeling diode connected in parallel with the lower bridge arm switch connected to the phase winding to be turned on.

[0077] An embodiment of the present invention further provides a machine-readable storage medium storing executable instructions. When the instructions are executed, one or more processors are caused to perform the motor control method 600 described above.

[0078] Although some embodiments have been described above, these embodiments are given by way of example only and are not intended to limit the scope of the invention. The appended claims and their equivalents are intended to cover all modifications, substitutions and changes made within the scope and spirit of this application.

Claims

1. A motor control system for controlling a brushless DC motor for a power tool, the brushless DC motor comprising a rotor with an embedded magnet and a stator with a three-phase winding, the motor control system comprising: a detection unit, configured to detect a phase voltage on each phase winding of the three-phase winding; A drive unit connected between the brushless DC motor and the DC bus, comprising three bridge arms, each electrically connected to one of the three phase windings, each bridge arm comprising an upper bridge arm switch and a lower bridge arm switch, each bridge arm switch being connected in parallel to a freewheeling diode; as well as a control unit electrically connected to the detection unit and the drive unit, the control unit being configured to calculate an induced electromotive force of a winding phase in a non-operating state based on the detected phase voltage; The position of the rotor is determined based on the calculated induced electromotive force; and the on and off of each bridge arm switch is controlled based on the position of the rotor to control the energized phase sequence of each phase winding, Wherein, the control unit is further configured: Determining whether the induced electromotive force is positive or negative; If the induced electromotive force is determined to be positive when the phase winding in the working state exits the working state, the freewheeling diode connected in parallel to the upper arm switch connected to the phase winding is controlled to be turned on; as well as If the induced electromotive force is determined to be negative when the phase winding in the working state exits the working state, the freewheeling diode connected in parallel to the lower arm switch connected to the phase winding is controlled to be turned on.

2. The motor control system according to claim 1, wherein: The control unit is configured to perform the judgment and corresponding control when at least one of the following is satisfied: The brushless DC motor is in the starting process The speed of the brushless DC motor is less than 2000 rpm; and The rotation speed of the brushless DC motor is lower than 10% of the rated rotation speed.

3. The motor control system according to claim 1, wherein: Determining whether the induced electromotive force is positive or negative includes: comparing the induced electromotive force with half the DC bus voltage; When the induced electromotive force is greater than half of the DC bus voltage, it is determined that the induced electromotive force is positive; When the induced electromotive force is equal to half of the DC bus voltage, it is determined that the induced electromotive force is at a zero-crossing point; and When the induced electromotive force is less than half of the DC bus voltage, it is determined that the induced electromotive force is negative.

4. The motor control system according to claim 3, wherein: The control unit is further configured to: During the process of the induced electromotive force changing from positive to negative, once the induced electromotive force crosses a zero point, the rotor position at the zero point is determined as a rotor position requiring commutation; and In the process of the induced electromotive force changing from negative to positive, once the induced electromotive force crosses a zero point, the rotor position at the zero-crossing moment is determined as another rotor position requiring commutation.

5. The motor control system according to claim 1, wherein: The control unit is further configured to: The measured value of the phase voltage output by the detection unit is converted from analog to digital so as to calculate the induced electromotive force based on the digital phase voltage.

6. The motor control system according to claim 1, wherein: The control unit is further configured to: Setting a first voltage threshold and a second voltage threshold that is smaller than the first voltage threshold, wherein the first voltage threshold is lower than the DC bus voltage, and the second voltage threshold is higher than a ground voltage; Wherein, the control unit is further configured to: Determining a time period of a commutation freewheeling process of a freewheeling diode based on the first voltage threshold and the second voltage threshold; and The phase voltage detected during the phase commutation and freewheeling process is regarded as an invalid detection signal.

7. The motor control system according to claim 6, wherein: The first voltage threshold is between 85% and 95% of the DC bus voltage, and the second voltage threshold is between 5% and 10% of the DC bus voltage.

8. The motor control system according to claim 1, wherein: One electrical cycle T of the brushless DC motor comprises six sub-cycles each having a duration of T / 6, each sub-cycle corresponding to a 60° segment. The control unit is further configured to control each bridge arm switch so that: In at least one 120° operating section, each arm switch is turned on in the first 60° operating section and is PWM-controlled to be alternately turned on and off in the last 60° operating section.

9. The motor control system according to any one of claims 1 to 8, wherein: The detection unit includes three detection branches, each of which is used to detect the phase voltage on a phase winding, and each detection branch does not include any filter capacitor.

10. An electric tool comprising: A brushless DC motor including a rotor with an embedded magnet and a stator with a three-phase winding; as well as The motor control system according to any one of claims 1 to 9, electrically connected to the brushless DC motor, is used to determine the induced electromotive force of the phase winding based on the phase voltage of the phase winding in a non-operating state, and to control the on and off of each bridge arm switch and the freewheeling diode based on the induced electromotive force.

11. A method for controlling a brushless DC motor for a power tool, the brushless DC motor comprising a rotor with an embedded magnet and a stator having three-phase windings. The power tool includes a motor control system comprising a detection unit and a drive unit. The detection unit is configured to detect the phase voltage on each phase of the three-phase windings and does not include a filter capacitor. The drive unit comprises three bridge arms, each connected to one of the three phase windings. Each bridge arm comprises an upper arm switch and a lower arm switch, each bridge arm switch being connected in parallel to a freewheeling diode. The control method comprises: Obtaining the detected phase voltage; calculating an induced electromotive force of a winding phase in a non-operating state based on the phase voltage; determining a position of the rotor based on the calculated induced electromotive force; as well as Based on the position of the rotor, the on and off of each bridge arm switch is controlled to control the energized phase sequence of each phase winding. The control method further includes: Determining whether the induced electromotive force is positive or negative; If the induced electromotive force is determined to be positive when the phase winding in the working state exits the working state, the freewheeling diode connected in parallel to the upper arm switch connected to the phase winding is controlled to be turned on; and If the induced electromotive force is determined to be negative when the phase winding in the working state exits the working state, the freewheeling diode connected in parallel to the lower arm switch connected to the phase winding is controlled to be turned on.

12. A machine-readable storage medium storing executable instructions that, when executed, cause one or more processors to perform the method of claim 11.

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