CN115967331A - Electric tool and control method thereof - Google Patents

Abstract

The invention discloses an electric tool and a control method thereof, comprising the following steps: the device comprises a shell, an alternating current power supply input device, a motor and a drive circuit, wherein the drive circuit comprises a plurality of electronic switch current detection modules and a control module, and the phase current value of the motor is acquired in real time through the current detection modules at periodic time intervals; when the acquired phase current value exceeds a preset current threshold value, turning off the electronic switch which is in a conducting state in the remaining time of the current time interval, and turning on the electronic switch which is controlled to be turned on currently by the control module when the current driving signal period is finished; the period of the driving signal varies randomly within a second preset period range. By adopting the technical scheme, the Zhou Xianliu gradual control method applicable to the alternating current electric tool can be provided, so that the electric tool can effectively inhibit large current under a heavy-load working condition and improve the use hand feeling of a user.

Description

Electric tool and control method thereof

technical field

The invention relates to an electric tool, in particular to a control method suitable for an AC electric tool.

Background technique

Under heavy load conditions, AC power tools, especially for high-voltage brushless tools, are prone to overcurrent due to the large enough power supply capacity of the power grid, which not only easily damages electronic components, but also significantly affects the power tools to a large extent. use feel.

Contents of the invention

In order to solve the deficiencies of the prior art, the present invention provides a current limiting control method suitable for AC electric tools, which can effectively suppress the large current under heavy load conditions without affecting the use feel of the electric tool.

In order to achieve the above object, the present invention adopts the following technical solutions:

An electric tool, comprising: a casing; an AC power input device, used to connect the power required for the operation of the electric tool; a motor, arranged in the casing; a drive circuit, the drive circuit includes a plurality of electronic switch; a current detection module, used to obtain the phase current value of the motor; a control module, electrically connected to the drive circuit, and the control module outputs a drive signal to control the drive circuit to run the motor; the control The module is also configured to: obtain the phase current value of the motor in real time through the current detection module in a periodic time interval; when the obtained phase current value exceeds a preset current threshold, in the current time interval Turn off the electronic switch that is in the conduction state within the remaining time, and turn on the electronic switch currently controlled by the control module to conduct when the previous drive signal cycle ends; the periodic time interval The duration of each time interval in is the same as the current period corresponding to the driving signal; the period of the driving signal varies randomly within the range of the second preset period.

Further, the random change within the range of the second preset period follows the law of normal distribution.

Further, it also includes: a rectification module, configured to be electrically connected to the AC power input device, and convert the AC power into DC power for use by the electric tool; a power supply circuit, electrically connected to the rectification module, configured to be at least the The control module supplies power; the capacitor circuit is electrically connected between the rectification module and the drive circuit.

Further, the current detection module includes a plurality of current detection resistors.

Further, the motor is set as a brushless DC motor.

Further, the brushless DC motor is controlled by the driving signal.

Further, the capacitive circuit includes at least one electrolytic capacitor.

A control method for an electric tool, the electric tool includes a casing; an AC power input device, used to connect the power required for the operation of the electric tool; a motor, arranged in the casing; a drive circuit, the The drive circuit includes a plurality of electronic switches; a current detection module, used to obtain the phase current value of the motor; a control module, electrically connected to the drive circuit;

The control method includes: the control module outputs a drive signal to control the drive circuit to run the motor, and limits the current of the motor at periodic time intervals; Obtain the phase current value of the motor in real time through the current detection module within the time interval; if the phase current value exceeds the preset current threshold, turn off and be in the conduction state during the remaining time of the current time interval the electronic switch, and then at the end of the previous drive signal cycle, turn on the electronic switch currently controlled by the control module; the duration of each time interval in the periodic time interval is the same as the The current period corresponding to the driving signal is the same; the period of the driving signal changes randomly within the second preset period range.

Further, the random change within the range of the second preset period follows the law of normal distribution.

The electric tool and its control method disclosed in the present invention effectively limit the large current of the electric tool under heavy load conditions, and at the same time set the period of the driving signal within a preset range to meet the random change of the normal distribution law, The EMI characteristics of the drive circuit are reduced through the frequency shaking strategy, which greatly optimizes the feel of the electric tool and improves the reliability and life of the electric tool.

Description of drawings

Fig. 1 is a perspective view of an electric tool as an embodiment;

Fig. 2 is the circuit block diagram of the circuit system as a kind of embodiment;

Fig. 3 is a block circuit diagram of a rectifier module as an embodiment;

Fig. 4 is a block circuit diagram of a current detection module as an embodiment;

Fig. 5 is a block circuit diagram of a current detection module as another embodiment;

Fig. 6 is a waveform diagram of a motor control signal as an embodiment;

FIG. 7 is a waveform diagram of a motor current limiting control method as an embodiment;

FIG. 8 is a flow chart of a motor current limiting control method as an embodiment;

Fig. 9 is a frequency continuously changing waveform diagram of a PWM signal as an embodiment;

Fig. 10 is a frequency continuously changing waveform diagram of a PWM signal as an embodiment;

Fig. 11 is a flowchart of a control method of a motor as an embodiment;

Fig. 12 is a waveform diagram of a PWM signal and a current-limiting period as another embodiment;

Fig. 13 is a flow chart of a motor control method as another embodiment;

Fig. 14 is a waveform diagram of a PWM signal and a current-limiting period as yet another embodiment;

Fig. 15 is a flow chart of a motor control method as yet another embodiment.

Detailed ways

The present invention will be specifically introduced below in conjunction with the accompanying drawings and specific embodiments.

The electric tool of the present invention may be a hand-held electric tool, a garden tool, or a garden vehicle such as a vehicle-type lawnmower, which is not limited herein. The power tools of the present invention include but are not limited to AC power tools such as sanders, drills, impact drivers, tapping machines, fastener drivers, etc., as long as these power tools can adopt the essence of the technical solutions disclosed below, they can fall into this category protection scope of the invention. In addition, it should be noted that, for the convenience of description, only some structures related to the present invention are shown in the drawings but not all structures.

Referring to FIG. 1 , an electric tool 10 is exemplarily shown, which is an angle grinder. The electric tool 10 mainly includes: a housing 11 , a motor 13 , functional components 14 , an AC power input device 15 and a circuit system 12 inside the housing 11 .

The electric machine 13 includes stator windings and a rotor. In some embodiments, the motor 13 is a three-phase brushless motor comprising a rotor with permanent magnets and electronically commutated three-phase stator windings U, V, W. In some embodiments, the three-phase stator windings U, V, W are connected in a star connection. In other embodiments, the three-phase stator windings U, V, and W are connected in an angled manner. However, it must be understood that other types of brushless motors are also within the scope of this disclosure. Brushless motors may include less or more than three phases.

The functional part 14 is used to realize the functions of the electric tool 10 . The functional part 14 is driven and operated by the motor 13 . The functional elements are different for different power tools. For the angle grinder, the functional part 14 is an angle grinder, which is used for grinding or cutting.

The AC power input device 15 is used to access the power required for the electric tool 10 to work. As one of the specific implementation manners, the power supply in this embodiment may optionally be set as an AC power supply. Specifically, the AC power input device 15 includes an AC plug for connecting to 120V or 220V AC mains.

Referring to the circuit system 12 of an embodiment of the electric tool 10 shown in FIG. .

The rectifier module 21 constitutes a DC unit of the electric tool 10 . The rectification module 21 is configured to receive the AC power from the AC power input device 15 and output the DC bus voltage, that is, to convert the AC power input by the AC power input device 15 into a pulsating DC output. The rectification module 21 is electrically connected to the AC power input device 15 . As a specific embodiment, as shown in FIG. 3 , the rectification module 21 includes a rectification bridge composed of four diodes D1, D2, D3, and D4. direction of the pulsating DC output.

The capacitor circuit 22 is connected in parallel to the DC bus of the electric tool 10 , that is, connected in parallel between the positive and negative poles of the DC unit in the circuit system 12 . As one specific implementation manner, the capacitor circuit 22 is optionally connected in parallel between the rectification module 21 and the drive circuit 24 . Specifically, the capacitor circuit 22 includes an electrolytic capacitor C. The capacitor circuit 22 is electrically connected to the rectification module 21. The pulsating direct current output by the rectification module 21 is filtered by the electrolytic capacitor C and converted into a smooth direct current output to reduce harmonic interference in the pulsating direct current. Preferably, the ratio of the capacitance of the electrolytic capacitor C to the rated power of the motor 13 is greater than 20 μF/KW and less than 80 μF/KW. This saves space and ensures that there are no capacitive elements with large physical dimensions in the hardware circuit.

The power supply circuit 23 is used to at least supply power to the control module 25 . As a specific embodiment, the power supply circuit 23 is electrically connected to the rectification module 21 , and converts the electric energy rectified by the rectification module 21 into a power supply voltage output suitable for the control module 25 . For example, in order to supply power to the control module 25, the power supply circuit 23 drops the voltage from the AC power input device 15 and rectified by the rectifier module 21 to 15V to supply power to the control module 25.

The drive circuit 24 is electrically connected to the rectifier module 21 for driving the motor 13'. The input terminal of the drive circuit 24 receives the voltage from the rectification module 21, and under the drive of the drive signal output by the control module 25, the voltage is distributed to each phase winding on the stator of the motor 13 in a certain logical relationship, so that the motor 13 starts and generates Continuous torque. Specifically, the drive circuit 24 includes a plurality of electronic switches. In some embodiments, the electronic switch includes a field effect transistor (FET), in other embodiments the electronic switch includes an insulated gate bipolar transistor (IG-BT), or the like. In some embodiments, the driving circuit 24 is a three-phase bridge circuit. The drive circuit 24 includes three electronic switches Q1 , Q3 , Q5 arranged as high-side switches and three electronic switches Q2 , Q4 , Q6 arranged as low-side switches.

Three electronic switches Q1 , Q3 , Q5 as high-side switches are respectively arranged between the power supply line of the rectifier module 21 and the coils of each phase of the motor 13 . Three electronic switches Q2, Q4 and Q6 as low-side switches are respectively arranged between the coils of each phase of the motor 22 and the ground wire.

Each gate terminal UH, UL, VH, VL, WH, WL of the six electronic switches Q1-Q6 is electrically connected to the control module 25, and each drain or source of the electronic switch is connected to the stator winding of the motor 13. The electronic switches Q1-Q6 change the on or off state at a certain frequency according to the drive signal output by the control module 25, thereby changing the power state of the rectifier circuit 21 loaded on the winding of the motor 13.

The drive circuit 24 is a circuit for rotationally driving the motor 13 by switching the energization state of each phase winding of the motor 13 and controlling the respective energization currents of each phase winding. The order and timing of the conduction of each phase winding depends on the position of the rotor. In order to make the motor 13 rotate, the driving circuit 24 has multiple driving states. In one driving state, the stator winding of the motor 13 will generate a magnetic field. The control module 25 outputs control signals based on different rotor positions to control the driving circuit 24 to switch the driving state so that The magnetic field generated by the stator winding rotates to drive the rotor to rotate, thereby realizing the driving of the motor 13 .

The rotation speed detection module 26 is used for acquiring at least one of the measured rotation speed of the motor 13 and the position of the rotor. In some embodiments, the rotation speed detection module 26 includes a sensor that can directly detect the speed and position of the motor 13, such as a Hall sensor. In some other embodiments, the rotation speed detection module 26 is configured to estimate the rotor position of the motor 13 at least according to the phase voltage of the motor 13 and the current value of the stator winding.

The current detection module 27 is used to collect the current of the motor 13 , which may be the bus current of the motor 13 or the phase current of each phase winding of the motor 13 . As a specific embodiment, the current detection module 27 detects the phase current of each phase winding of the motor 13, and the bus current of the motor 13 can be obtained by calculating the detected three-phase current values. In some embodiments, the current detection module 27 Hall current sensors are included to directly detect the phase currents of the respective phase windings of the motor 13 . As another specific embodiment, as shown in FIG. 4 , current detection resistors R1, R2 and R3 are connected in series between the drive circuit 24 and each phase winding of the motor 13, and the current detection module 27 detects the voltage across the detection resistor. The phase current or bus current of each phase winding can be calculated. Specifically, the current detection module 27 respectively detects the voltages at both ends of the current detection resistors R1 , R2 and R3 to calculate the phase currents of the three-phase stator windings U, V and W. As another specific embodiment, as shown in FIG. 5 , the current detection module 27 is used to detect the internal resistance of the electronic switch in the conducting state in the drive circuit 24, based on the internal resistance of the electronic switch in the conducting state and its The voltage values at both ends are calculated to obtain the current passing through the electronic switch, and the current of the electronic switch is the phase current corresponding to the winding of the motor 13 . Specifically, the current detection module 27 respectively detects the voltages at both ends of the three drive switches Q1, Q3, and Q5 of the high-side switch to calculate the corresponding phase currents of the three-phase stator windings U, V, and W. In this way, the electric tool can detect the phase current corresponding to the winding of the motor 22 without adding hardware, which saves costs.

The control module 25 is at least electrically connected to the power supply circuit 23 , the driving circuit 24 and the current detection module 27 for controlling the operation of the driving circuit 24 . In some embodiments, the control module 25 adopts a dedicated control chip (for example, MCU, micro control unit, Microcontroller Unit).

In some specific implementations, as shown in FIG. 6 and FIG. 7 , the control module 25 outputs a driving signal with a cycle of T ₁ and a frequency of f ₁ for controlling the conduction states of a plurality of electronic switches Q1-Q6 to drive the motor 13. Preferably, the motor 13 is set as a three-phase brushless DC motor, and the driving signal is optionally set as a PWM signal. At the same time, the control module 25 limits the current of the motor 13 within the periodic time interval T2. Those skilled in the art can understand that the periodic time interval here can be understood as the current limiting period of the motor. T2 is defined as the current limiting period of the motor 13 .

As a specific embodiment, as shown in FIG. 7 , the current limiting period _T2 is set to be the same as the period _T1 of the PWM signal. The control module 25 is configured to obtain the phase current value of the motor 13 in real time through the current detection module 27 in the current period of the current limiting period _T2 , and compare the phase current value with the preset current threshold, if the phase current value exceeds If the current threshold is preset, then the electronic switch is turned off during the remainder of the current period _T2 of the current limiting period, thereby disconnecting the current flowing to the motor 13, and the electronic switch is turned on when the period _T1 of the previous PWM signal ends, The current to the motor 13 is restored. Specifically, the setting of the preset current threshold can be set separately according to the type selection of the motor in the actual application and the actual application scenario, and the setting method of the preset current preset is not limited in the present invention. It should also be noted here that the electronic switch that is turned off in the above technical solution is specifically the electronic switch that is currently in the on state, and the electronic switch that is turned on is the electronic switch that is turned on under the control of the current driving signal.

Under heavy load conditions, the motor 13 may over-current due to the large power supply capacity of the grid voltage, thereby damaging the components in the electric tool and reducing the service life of the electric tool. Referring to Fig. 7, the motor 13 has an overcurrent phenomenon under heavy load conditions, which is shown at point b in the figure. Using the above-mentioned current limiting technical solution, when the phase current value I _phase exceeds the preset current threshold I _ref , the control module 25 immediately turns off the electronic switch so that the current flowing through the motor 13 does not increase any more, as shown by point a in the figure. At the end of the current cycle of the PWM signal, that is, as shown by point c in the figure, the control module 25 turns on the electronic switch again, so as to resume the current flowing to the motor 13 . What needs to be explained here is that the current detection module is mainly set in the present invention to detect the phase current of the motor 13, and is used to realize the cycle-by-cycle current limiting of the motor. Those skilled in the art can understand that it can also be used to detect the bus current value of the motor by detecting the current value of the motor. Realize the cycle-by-cycle current limiting of the motor, which will not be described in detail here.

A control method for a current-limiting period of the motor 13 in the electric tool 10 will be specifically described below in conjunction with FIG. 8 , and the method includes the following steps:

S101. Obtain the phase current value of the motor.

S102, judging whether the phase current value of the motor exceeds a preset current threshold, if yes, execute step S103; if not, execute step S104.

S103, turning off the electronic switch that is currently turned on.

S104, judging whether the current current limiting cycle is over, if yes, execute step S105; if not, execute step S101.

S105, turn on the electronic switch controlled by the current driving signal, and return to step S101.

In some other specific embodiments, when the grid voltage increases or decreases relatively substantially, the grid voltage will fluctuate, thereby affecting the user experience. In view of this, the present invention proposes to effectively compensate the grid voltage while effectively limiting the current of the motor. As shown in FIG. 9 , specifically, the control module 25 controls the period _T1 of the PWM signal used to drive the motor 13 to continuously change within the first preset period range, and simultaneously sets the current limiting period T2 of the motor 13 and the period _T2 of the PWM signal. Period _T1 is the same. Wherein, the first preset period range is optionally set to [0.5T ₀ , 2T ₀ ], and T ₀ is an initial period of the PWM signal. Specifically, when the grid voltage is lower than or equal to the current back electromotive force of the motor 13, the period _T1 of the PWM signal remains unchanged; otherwise, when the grid voltage is higher than the current back electromotive force of the motor 13, the period _T1 of the PWM signal is between Continuous change within the first preset range. Specifically, as shown in FIG. 9 and FIG. 10, the period _T1 of the driving signal is set to be continuously changed within the preset period range, which can be obtained by the following formula:

Among them, f ₀ is the initial frequency corresponding to the initial period T ₀ of the PWM signal, f ₁ is the frequency corresponding to the period T ₁ of the PWM signal, and θ is the phase of the current grid voltage.

A control method for cycle-by-cycle current limiting of the motor 13 in the electric tool 10 will be described in detail below in conjunction with FIG. 11 , the method includes the following steps:

S201. Obtain the phase current value of the motor.

S202, judging whether the phase current value of the motor exceeds the preset current threshold, if yes, execute step S203; if not, execute step S204.

S203, turning off the electronic switch that is currently turned on.

S204, judging whether the current current limiting cycle is over, if yes, execute step S205; if not, execute step S206.

S205, turning on the electronic switch that is controlled to be turned on by the current driving signal.

S206. Obtain the grid voltage and the back electromotive force of the motor.

S207 , judging whether the current back electromotive force of the motor exceeds the grid voltage, if yes, execute step S208 , if not, execute step S201 .

S208, reset the period T1 of the driving signal.

S209, reset the current limiting period T2, and return to step S201.

In the above-mentioned embodiments, the present invention discloses a control method for cycle-by-cycle current limiting of electric tools. The current limiting cycle is always the same as the cycle of the PWM signal. When the detected phase current of the motor is greater than the preset current threshold, the electronic switch is turned off. At the end of the previous cycle, the electronic switch is turned on again, and at the same time, the phase current of the motor is continuously detected in real time. On the other hand, setting the period of the PWM signal to continuously change within the first preset period range with the fluctuation of the grid voltage can effectively compensate for the fluctuation of the grid voltage, and improve the user's feeling of use and the service life of the electric tool.

In some other specific embodiments, the control module 25 controls the period T1 of the PWM signal used to drive the motor ₁₃ to change randomly within the second preset period range, and sets the current limiting period _T2 of the motor 13 and the period of the PWM signal T1 is always the same. Referring to FIG. 12 , specifically, the period _T1 of the PWM signal varies randomly within the second preset period range, which can be set to conform to the law of normal distribution after the initial period _T0 of the PWM signal is superimposed with white noise. Specifically, the initial period _T0 of the PWM signal used to drive the motor 13 is set to 100us, and the period _T1 of the PWM signal after superimposing white noise changes randomly within the second preset period range [98us, 102us], and the change satisfies The law of normal distribution. It should be noted here that the second preset range set by the present invention can be set by those skilled in the art according to the actual application scene of the electric tool.

Another control method for cycle-by-cycle current limiting of the motor 13 in the electric tool 10 will be specifically described below in conjunction with FIG. 13 , the method includes the following steps:

S301. Obtain the phase current value of the motor.

S302, judging whether the phase current value of the motor exceeds the preset current threshold, if yes, execute step S303; if not, execute step S304.

S303, turning off the electronic switch that is currently turned on.

S304, judging whether the current current limiting cycle is over, if yes, execute step S305; if not, execute step S301.

S305, reset the period T1 of the driving signal.

S305. Set the current limiting period T2 according to the reset period T1 of the driving signal.

S305, turn on the electronic switch controlled by the current driving signal, and return to step S301.

In the above-mentioned embodiments, the present invention discloses another control method for cycle-by-cycle current limiting of electric tools. The current limiting cycle is always the same as the cycle of the PWM signal. When the detected phase current of the motor is greater than the preset current threshold, the electronic switch is turned off. When the previous period of the PWM signal ends, the electronic switch is turned on again, and at the same time, the phase current of the motor continues to be detected in real time. On the other hand, the period of the PWM signal is set to change randomly within the range of the second preset period, and the above random change satisfies the law of normal distribution. In this embodiment, the period of the PWM signal is set within a preset range to meet the random change of the normal distribution law, and the EMI of the drive circuit is reduced through the frequency shaking strategy, so as to improve the reliability of the electric tool.

In some other specific embodiments, the control module 25 outputs a PWM signal with an initial period of T ₀ to control the driving circuit 24 to drive the motor 13 to operate. At the same time, the control module 25 also obtains the phase current value of the motor 13 in real time through the current detection module 27 at periodic time intervals, and compares the obtained real-time phase current value with the preset current range. Referring to FIG. 14 , specifically, the preset current range includes a first preset current threshold I _ref1 and a second preset current threshold I ref2 , and the first preset current threshold I _ref1 is set as an upper limit value, and the second preset current threshold I _ref1 is set as an upper limit value, and the second preset current threshold Let the current threshold I _ref2 be set as the lower limit value. When the phase current value of the motor 13 obtained by the control module 25 is higher than the first preset current threshold I _ref1 , the control module 25 immediately turns off the electronic switch currently in the conduction state; once the phase current value of the motor 13 obtained by the control module 25 When the current value is lower than the second preset current threshold I _ref2 , the control module 25 controls to turn on the electronic switch turned on by the current driving signal. In the specific setting process, the difference between the first preset current threshold I _ref1 and the second preset current threshold I _ref2 is inversely proportional to the inductance of the motor and proportional to the back electromotive force during normal operation of the motor. Those skilled in the art can The preset current range is reasonably designed according to the motor type selection and the actual application scenario.

Another control method of cycle-by-cycle current limiting of the motor 13 in the electric tool 10 will be described in detail below in conjunction with FIG. 15 , the method includes the following steps:

S401. Obtain the phase current value of the motor.

S402, judging whether the phase current value of the motor is higher than the first preset current threshold, if yes, execute step S403; if not, execute step S404.

S403, turning off the electronic switch that is currently turned on.

S404, judging whether the motor phase current value is lower than the second preset current threshold, if yes, execute step S405, if not, execute step S401.

S405, turning on the electronic switch that is controlled to be turned on by the current driving signal. Return to step S401.

In the above-mentioned embodiments, the present invention discloses another control method for cycle-by-cycle current limiting of an electric tool. The phase current of the motor is obtained through the current detection module, and once it is detected that the phase current exceeds the first preset current threshold, the electronic switch is turned off; Once the phase current value of the motor is lower than the second preset current threshold, the electronic switch is turned on to resume the current flowing to the motor. The technical solutions in the above embodiments can simply and effectively suppress the large current that occurs during the operation of the motor without affecting the feel of the electric tool.

Note that the above are only preferred embodiments of the present invention and applied technical principles. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described herein, and that various obvious changes, readjustments and substitutions can be made by those skilled in the art without departing from the protection scope of the present invention. Therefore, although the present invention has been described in detail through the above embodiments, the present invention is not limited to the above embodiments, and can also include more other equivalent embodiments without departing from the concept of the present invention, and the present invention The scope is determined by the scope of the appended claims.

Claims (9)

1. An electric tool, comprising: case; An AC power input device, used to connect the power required for the electric tool to work; a motor arranged in the housing; a drive circuit comprising a plurality of electronic switches; a current detection module, configured to obtain the phase current value of the motor; a control module electrically connected to the drive circuit, the control module outputs a drive signal to control the drive circuit to run the motor; The control module is further configured to: Obtaining the phase current value of the motor in real time through the current detection module in a periodic time interval; When the obtained phase current value exceeds the preset current threshold, turn off the electronic switch that is in the conduction state during the remaining time of the current time interval, and turn on the electronic switch when the previous cycle of the driving signal ends. The control module currently controls the electronic switch that is turned on; It is characterized in that, The duration of each time interval in the periodic time intervals is the same as the current period corresponding to the driving signal; The period of the driving signal varies randomly within the range of the second preset period. 2. The power tool according to claim 1, wherein: The random variation within the range of the second preset period follows the law of normal distribution. 3. The power tool according to claim 1, wherein: Also includes: A rectifier module, configured to be electrically connected to the AC power input device, and convert the AC power into DC power for use by the electric tool; a power supply circuit electrically connected to the rectifier module and configured to at least supply power to the control module; The capacitor circuit is electrically connected between the rectification module and the driving circuit. 4. The power tool according to claim 1, wherein: The current detection module includes a plurality of current detection resistors. 5. The power tool according to claim 1, wherein: The motor is set as a brushless DC motor. 6. The power tool according to claim 5, wherein: The brushless DC motor is controlled by the drive signal. 7. The power tool according to claim 1, wherein: The capacitive circuit includes at least one electrolytic capacitor. 8. A control method for an electric tool, the electric tool comprising a casing; an AC power input device for connecting to the power required for the operation of the electric tool; a motor arranged in the casing; a drive circuit, The drive circuit includes a plurality of electronic switches; a current detection module, used to obtain the phase current value of the motor; a control module, electrically connected to the drive circuit; The control methods include: The control module outputs a drive signal to control the drive circuit to operate the motor, and to limit the current of the motor at periodic time intervals; The control module obtains the phase current value of the motor in real time through the current detection module within the periodic time interval; if the phase current value exceeds the preset current threshold, during the current time interval Turning off the electronic switch that is in the conduction state within the remaining time, and then turning on the electronic switch that the control module is currently controlling to conduct at the end of the previous cycle of the driving signal; It is characterized in that, The duration of each time interval in the periodic time intervals is the same as the current period corresponding to the driving signal; The period of the driving signal varies randomly within the range of the second preset period. 9. The control method according to claim 8, characterized in that, The random variation within the range of the second preset period follows the law of normal distribution.

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