Motor braking using selectively connectable resistors

Abstract

An apparatus and method for braking a motor using selectively connected resistors. The method includes: controlling a power switching network to drive a motor of a power tool using a motor controller in response to actuation by a user input; and determining variable tool characteristics using the motor controller. The method further includes determining, using the motor controller, that the user input has stopped actuation. The method also includes controlling the power switching network to brake the motor using the motor controller when the variable tool characteristics meet a tool characteristic threshold, and controlling a braking circuit to brake the motor using the motor controller when the variable tool characteristics do not meet the tool characteristic threshold. The braking circuit includes one or more resistive loads and is selectively connected to motor terminals of the motor.

Description

[0001] Cross-references to related applications

[0002] This application claims priority to U.S. Provisional Patent Application No. 62 / 859,274, filed June 10, 2019, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The embodiments described herein relate to a motor braking circuit for braking the motor of a power tool. Background Technology

[0004] Some power tools include braking controls to stop the motor after a trigger is released. To meet certain industry standards, the motor may need to stop completely within a specified time period (e.g., a predetermined time). Some power tools include large, costly braking resistors used to absorb the energy generated by the motor during braking. Summary of the Invention

[0005] When the braking resistor absorbs excess current from the motor, the large resistor used for braking generates excessive heat, which concentrates in one location. The resistors and components used for heat dissipation from the system increase the manufacturing cost of the power tool. Therefore, at least some of the embodiments described herein provide improved techniques for braking motors that reduce the cost of power tools, improve thermal management, and provide additional space-saving layout options for the power tool.

[0006] Some embodiments provide a power tool including: a power source, a motor, a power switching network connected between the power source and the motor, and a user input configured to be actuated to drive the motor. The power tool also includes a braking circuit including one or more resistive loads and configured to be selectively coupled to motor terminals of the motor, and a motor controller connected to the power switching network and the braking circuit. The motor controller is configured to control the power switching network to drive the motor in response to actuation of the user input and to determine variable tool characteristics. The motor controller is also configured to determine whether the variable tool characteristics meet a tool characteristic threshold when the user input stops actuating. The motor controller is further configured to control the power switching network to brake the motor when the variable tool characteristics meet the tool characteristic threshold, and to control the braking circuit to brake the motor when the variable tool characteristics do not meet the tool characteristic threshold.

[0007] The motor can be a three-phase motor with three motor terminals. One or more resistive loads in the braking circuit can include three resistive loads, each connected to one of the three motor terminals.

[0008] The braking circuit may also include one or more brake switches, and the motor controller controls the one or more brake switches to selectively connect one or more resistive loads to the motor terminals. The one or more brake switches may include field-effect transistors (FETs) controlled by the motor controller. The motor controller may perform pulse-width modulation (PWM) control on the one or more brake switches to brake the motor. The braking circuit is configured to selectively connect the motor terminals to each other, selectively connect the motor terminals to ground, or selectively connect the motor terminals to a power supply terminal.

[0009] The variable tool characteristic can be the system impedance, and when the system impedance is higher than the system impedance threshold, the variable tool characteristic satisfies the tool characteristic threshold.

[0010] The variable tool characteristic can be the motor current, and when the motor current is below the regenerative braking threshold, the variable tool characteristic satisfies the tool characteristic threshold.

[0011] The motor controller can also be configured to determine when the motor current drops below a regenerative braking threshold while the motor is being braked using a braking circuit, and in response to determining that the motor current has dropped below the regenerative braking threshold, switch from controlling the braking circuit to braking the motor to controlling the power switching network to brake the motor.

[0012] The motor controller can also be configured to use a power switch network to perform regenerative braking to redirect braking current back to the power source.

[0013] Power tools may include a direct-connect power interface, thereby providing a connection between a power source and a power switching network without the need for an on / off switch controlled by a trigger on the power tool. In some cases, a connection between a power source and a power switching network is provided without a mechanical on / off switch and / or without an electrical solid-state switching device.

[0014] Some embodiments provide a method for braking a motor of a power tool, comprising: controlling a power switching network to drive the motor of the power tool using a motor controller in response to actuation by a user input; and determining variable tool characteristics using the motor controller. The method further includes determining, using the motor controller, that the user input has stopped actuation and, in response to the user input stopping actuation, determining whether the variable tool characteristics meet a tool characteristic threshold. The method also includes controlling the power switching network to brake the motor using the motor controller when the variable tool characteristics meet the tool characteristic threshold, and controlling a braking circuit to brake the motor using the motor controller when the variable tool characteristics do not meet the tool characteristic threshold. The braking circuit, having one or more resistive loads, is selectively connected to the motor terminals of the motor.

[0015] Selectively connecting the braking circuit to the motor terminals may also include using a motor controller to control one or more brake switches to connect one or more resistive loads to the motor terminals. The method may also include using the motor controller to perform PWM control of one or more brake switches to brake the motor using the braking circuit.

[0016] This method may include using a power switch network to perform regenerative braking to redirect braking current to the power source when braking a motor using a power switch network.

[0017] Other aspects of the invention will become apparent from consideration of the detailed description and accompanying drawings. Attached Figure Description

[0018] Figure 1 A side view of a power tool according to some embodiments is shown.

[0019] Figure 2 The following are illustrated according to some embodiments. Figure 1 A block diagram of power tools.

[0020] Figure 3A and Figure 3B The following are illustrated according to some embodiments. Figure 1 A block diagram of the power switch network and braking circuit of a power tool.

[0021] Figure 4 Braking according to some implementation methods Figure 1 A flowchart of a method for developing a motor for an electric tool.

[0022] Figure 5 Braking according to some implementation methods Figure 1 A flowchart of a method for developing a motor for an electric tool.

[0023] Before explaining any embodiments of the invention in detail, it should be understood that the application of the invention is not limited to the construction details and component arrangements set forth in the following description or shown in the drawings. The invention can have other embodiments and can be practiced or implemented in different ways. Furthermore, it should be understood that the wording and terminology used herein are for descriptive purposes and should not be considered limiting. The terms “comprising,” “including,” and “having,” and variations thereof, as used herein, are intended to cover the items listed thereafter and their equivalents and additional items. The terms “installation,” “connection,” “support,” and “linkage,” and variations thereof, are used broadly and cover direct and indirect installation, connection, support, and linking. Moreover, “connection” and “linkage” are not limited to physical or mechanical connections or links and may include direct or indirect electrical connections or links.

[0024] It should be noted that the present invention can be implemented using multiple hardware and software-based devices and multiple different structural components. Furthermore, as described in the following paragraphs, the specific configurations shown in the accompanying drawings are intended to illustrate embodiments of the invention, and other alternative configurations are also possible. Unless otherwise stated, the terms "processor," "central processing unit," and "CPU" are used interchangeably. When the terms "processor," "central processing unit," or "CPU" are used to identify a unit performing a specific function, it should be understood that, unless otherwise stated, these functions can be performed by a single processor or multiple processors arranged in any form (including parallel processors, serial processors, cascaded processors, or cloud processing / cloud computing configurations). Detailed Implementation

[0025] Figure 1 A power tool 100 incorporating a brushless direct current (DC) motor is shown. In brushless motor power tools such as power tool 100, switching elements are selectively enabled and disabled by control signals from a controller to selectively apply power from a power source (e.g., a battery pack) to drive the brushless motor. Power tool 100 is a brushless hammer drill having a housing 102, which includes a handle portion 104 and a motor housing portion 106. Power tool 100 also includes an output driver 107 (shown as a chuck), a torque setting dial 108, a forward / reverse selector 110, a trigger 112, a battery interface 114, and a lamp 116. Although Figure 1 A hammer drill is shown, but in some embodiments, the motor described herein is incorporated into other types of power tools, including drills, impact drives, impact wrenches, angle grinders, circular saws, reciprocating saws, wire trimmers, leaf blowers, vacuum cleaners, etc.

[0026] Figure 2A simplified block diagram of a brushless power tool 100 is shown, which includes a power supply 122, a power switch network 124, a motor 126, Hall sensors 128, a motor controller 130, a user input 132, a braking circuit 134, and other components 136 (battery pack, fuel gauge, work light (LED), current / voltage sensor, etc.). The power supply 122 provides DC power to the various components of the power tool 100 and may be a power tool battery pack, which is rechargeable and uses, for example, lithium-ion battery technology. In some cases, the power supply 122 may receive AC power (e.g., 120V / 60Hz) from a tool plug connected to a standard wall socket, and then filter, regulate, and rectify the received power to output DC power. Each Hall sensor 128 outputs motor feedback information, such as an indication (e.g., a pulse) as the rotor's magnet rotates past the surface of the Hall sensor 128. Based on the motor feedback information from the Hall sensors 128, the motor controller 130 can determine the rotor's position, speed, and acceleration. Motor controller 130 also receives user control from user input 132, such as by pressing trigger 112 or moving forward / reverse selector 110. In response to motor feedback information and user control, motor controller 130 transmits control signals to power switching network 124 to drive motor 126, as per [reference needed]. Figure 3A and Figure 3B To explain in more detail. In some embodiments, the power tool 100 may be a sensorless power tool, which does not include the Hall sensor 128 or other position sensors for detecting the position of the rotor. Instead, the position of the rotor may be detected based on the inductance of the motor 126 or the back electromotive force generated in the motor 126. Although not shown, the motor controller 130 and other components of the power tool 100 are electrically connected to the power supply 122 so that the power supply 122 provides power to them.

[0027] Figure 3A and Figure 3BA circuit diagram of a power switching network 124 and a braking circuit 134 is shown. The power switching network 124 includes multiple high-side power switching elements 140 (e.g., field-effect transistors [FETs]) and multiple low-side power switching elements 144 (e.g., FETs). The braking circuit 134 includes multiple braking resistors 148 (148a, 148b, 148c, respectively) and multiple braking switching elements 152 (152a, 152b, 152c, respectively), such as braking field-effect transistors (FETs). A motor controller 130 provides control signals to control the high-side FETs 140 and the low-side FETs 144 to drive the motor based on motor feedback information and user control, as described above. For example, in response to detecting a pull of trigger 112 and an input from the forward / reverse selector 110, motor controller 130 provides a control signal to selectively enable and disable FETs 140 and 144 (e.g., sequentially, in pairs), thereby causing power from power supply 122 to be selectively applied to the stator coils of motor 126 to cause rotation of the rotor. More specifically, to drive motor 126, motor controller 130 enables a first high-side FET 140 and a first low-side FET 144 pair for a first time period (e.g., by providing a voltage at the gate terminals of the FETs). In response to determining that the rotor of motor 126 has rotated based on a pulse from Hall sensor 128, motor controller 130 disables the first FET pair and enables the second high-side FET 140 and the second low-side FET 144. In response to determining that the rotor of motor 126 has rotated based on a pulse from Hall sensor 128, motor controller 130 disables the second FET pair and enables the third high-side FET 140 and the third low-side FET 144. In response to a further pulse received from Hall sensor 128 indicating that the rotor of motor 126 has rotated, motor controller 130 disables the third FET pair and re-enables the first high-side FET 140 and the third low-side FET 144. This cyclical activation of the high-side FET 140 and low-side FET 144 pairs is repeated sequentially to drive motor 126. Furthermore, in some embodiments, the control signal includes a pulse width modulation (PWM) signal with a duty cycle proportional to the trigger pull of trigger 112 to control the speed or torque of motor 126.

[0028] To stop motor 126, motor controller 130 short-circuit low-side FET 144 (i.e., enables low-side FET 144 and disables high-side FET 140) to allow back electromotive force to flow through the motor coils of motor 126. The back electromotive force provides braking force on the rotor magnets. For power tools 100 where a faster stop of motor 126 is desired (e.g., saw, grinder, etc.), additional resistance is used to brake motor 126. Figure 3A and Figure 3BAs shown, the motor controller 130 controls the brake switch element 152 to close, thereby connecting multiple brake resistors 148 to the current path of the motor 126. The multiple brake resistors 148 absorb excess current and stop the motor 126 more quickly (compared to the power tool 100 without brake circuit 134).

[0029] exist Figure 3A and Figure 3B In the example shown, braking circuit 134 includes three resistors 148, each for a terminal (U, V, and W) of motor 126. Braking resistors 148 are selectively connected to corresponding motor terminals by controlling braking switching element 152. Motor controller 130 provides control signals to control braking switching element 152, thereby braking the motor. For example, in response to detecting the release of trigger 112, motor controller 130 provides control signals to selectively enable braking switching element 152, causing residual power in motor 126, or at least a majority thereof, to be absorbed by braking resistors 148. In braking circuit 134, excess current from motor 126 is directed to ground or the negative terminal of power supply 122, rather than returning to motor 126. Additionally, braking circuit 134 is separate from power switching elements 140, 144 to brake motor 126 without the use of power switching elements 140, 144.

[0030] In some embodiments, the braking circuit 134 short-circuits the motor terminals. For example, the brake switch 152 connects a braking resistor 148 between the motor terminals such that when the brake switch 152 is activated, the current in the motor terminals is absorbed by the braking resistor 148.

[0031] In some embodiments, the motor controller 130 may implement PWM control of the brake switching element 152 to brake the motor 126. The motor controller 130 selectively activates the brake switching element 152 to direct current from the motor coil to the braking resistor 148.

[0032] use Figure 3A and Figure 3B One advantage of providing multiple resistors 148 is that it avoids the use of large, costly resistors. Additionally, heat from the resistors 148 dissipates over a larger area within the power tool 100, resulting in a lower heat density caused by braking.

[0033] exist Figure 3AIn the example shown, power tool 100 includes a direct-connect power interface 156. That is, it provides a connection between power supply 122 and power switching network 124 without including a mechanical on / off switch controlled by trigger 112 of power tool 100. Typically, power tools may include mechanical switches, such as relays or solid-state driven switches coupled in the current path between power supply 122 and power switching network 124. The mechanical switch is used to enable or disable power from power supply 122 to power switching network 124 and is mechanically controlled by trigger 112. As an alternative to a mechanical switch mechanically controlled by trigger 112, power tool 100 may not include a mechanical switch, but may instead include, for example, a FET 158 controlled by motor controller 130 to selectively connect or disconnect power supply 122 to or from power switching network 124 (e.g., based on trigger pull). In some embodiments, the direct-connect power interface 156 does not include a mechanical switch or an electro-solid-state switching device (i.e., FET 158) (e.g., ...). Figure 3B (As shown), this allows power to be supplied directly from the battery pack to the power switching network 124. Specifically, in some embodiments, a direct-connect power interface 156 without mechanical or electrical switches is advantageous for providing the braking circuit 134. When the FET 158 is removed to form the direct-connect power interface 156, the braking switching element 152 is used to brake the motor while not connecting a low impedance to the power supply 122. The function of the mechanical relay or FET 158 can therefore be performed by the braking switching element 152.

[0034] Figure 4 This is a flowchart of an example method 400 for braking the motor 126 of a power tool 100. In the example shown, method 400 includes detecting the pulling of a trigger (in block 404) using a motor controller 130. When the trigger 112 is pulled, the motor controller 130 receives an input from a user input 132 indicating that the trigger 112 has been pulled. For example, a trigger sensor (e.g., a push-button switch, Hall sensor, potentiometer, or force sensor) may detect that the trigger is pressed and output a signal to the motor controller 130 indicating a pulling state (e.g., pulled or not pulled). In some embodiments, the motor controller 130 may also receive an input indicating the distance the trigger has been pulled, which indicates a desired speed for variable speed control of the motor.

[0035] Method 400 includes using a motor controller 130 to control a power switching network 124 to drive a motor 126 (in block 408). The power switching network 124 includes a plurality of high-side power switching elements 140 and a plurality of low-side power switching elements 144. The motor controller 130 provides control signals to control the high-side FETs 140 and the low-side FETs 144, thereby driving the motor 126 based on motor feedback information and user control, as described above.

[0036] Method 400 also includes determining the system impedance using motor controller 130 (in block 412). Motor controller 130 can detect the impedance of motor 126 and / or power switching network 124. In one example, the motor impedance is measured during manufacturing via simulation and / or experimentation and stored in the memory of motor controller 130. Motor controller 130 determines the system impedance (including battery and / or power supply impedance) by communicating with power supply 122 (e.g., the battery pack controller of power supply 122) to retrieve the power supply impedance or by measuring the impedance of power supply 122 using voltage and / or current sensors. The system impedance is then determined based on the power supply impedance and motor impedance. Generally, motor braking is more effective when the system impedance is higher. However, at low speeds or low torque operation, the system (e.g., motor 126 and / or power switching network 124) may not have sufficient impedance to stop the motor within a specified stopping time. Therefore, in some embodiments, motor controller 130 continuously determines and tracks the system impedance.

[0037] Method 400 includes using motor controller 130 to determine whether trigger 112 has been released (in block 416). When trigger 112 is released, motor controller 130 receives an input from the trigger sensor of user input 132 indicating that trigger 112 has been released. Motor controller 130 determines the trigger release state based on the input from user input 132. When trigger 112 has not been released, method 400 continues to drive the motor.

[0038] When trigger 112 is released, method 400 includes using motor controller 130 to determine whether the system impedance is greater than or equal to an impedance threshold (in block 420). When an input from user input 132 indicates that the trigger has been released, motor controller 130 determines whether the system impedance is sufficient to brake motor 126. In some embodiments, motor controller 130 may determine the system impedance after trigger 112 has been released.

[0039] When the system impedance is not greater than or equal to an impedance threshold, method 400 includes using motor controller 130 to control braking circuit 134 to brake motor 126 (in block 424). For example, motor controller 130 may enable brake switching element 152 to connect braking resistor 148 to motor terminals (U, V, and W). Braking resistor 148 draws current from motor 126 and stops motor for a specified stop time. The resistance value of braking resistor 148 is selected based on the specified stop time. For example, the shorter the stop time, the larger the resistance value of the selected braking resistor 148. In some embodiments, motor controller 130 uses braking circuit 134 to brake motor 126 even when the system impedance is sufficient to brake motor 126 (e.g., blocks 420 and 424 are bypassed). When braking circuit 134 is used to brake motor 126, motor current flows through braking resistor 148 and to ground or to the negative terminal of power supply 122. Alternatively, the braking circuit 134 forms a closed circuit with the motor terminals (U, V, and W) so that motor current flows through the braking resistor 148.

[0040] When the system impedance is greater than or equal to an impedance threshold, method 400 includes using motor controller 130 to control power switching network 124 to brake motor 126 (in block 428). Motor controller 130 selectively enables, for example, low-side FET 144 to direct current from motor 126 to ground or negative terminal of power supply 122. In some embodiments, motor controller 130 may direct motor current to power supply 122, for example, through the freewheeling diode of high-side FET 140, to charge power supply 122 using braking current. Method 400 is repeated to determine the next trigger pull of power tool 100.

[0041] Figure 5 This is a flowchart of an example method 450 for braking the motor 126 of a power tool 100. In the example shown, method 450 includes detecting the pulling of a trigger using a motor controller 130 (in block 454). As described above, a trigger sensor of user input 132 can provide an indication of trigger pulling to the motor controller 130. Method 450 includes controlling a power switching network 124 using the motor controller 130 to drive the motor 126 (in block 458). The motor controller 130 provides control signals to control the high-side FET 140 and the low-side FET 144 to drive the motor 126 based on motor feedback information and user control, as described above.

[0042] Method 450 also includes using motor controller 130 to determine the motor current (in block 462). Motor controller 130 can use a current sensor to detect the amount of current flowing through motor 126 to determine the torque of motor 126. During regenerative braking, the current generated by the rotor magnets in motor 126 is directed to power source 122, for example, to charge power source 122. However, excessive current flowing to power source 122 may damage power source 122 or other electrical components of power tool 100. Therefore, in some embodiments, motor controller 130 continuously determines and tracks the motor current of motor torque for regenerative braking.

[0043] Method 450 includes using motor controller 130 to determine whether trigger 112 has been released (in block 466). Motor controller 130 determines the trigger release state based on input from the trigger sensor from user input 132. When trigger 112 has not been released, method 450 continues to drive motor 126.

[0044] When trigger 112 is released, method 450 includes using motor controller 130 to determine whether the motor current is greater than or equal to a regenerative current threshold (in block 470). When an input from user input 132 indicates that trigger 112 has been released, motor controller 130 determines whether the motor current or motor torque is greater than or equal to the regenerative current threshold (e.g., using a comparison operation). The regenerative current threshold can be selected as a current that can be safely used to charge power supply 122 during motor braking of motor 126. In some embodiments, motor controller 130 can determine the motor current or torque after trigger 112 has been released.

[0045] When the motor current exceeds the regenerative current threshold, method 450 includes using motor controller 130 to control braking circuit 134 to brake motor 126 (in block 474). For example, motor controller 130 may enable braking switch element 152 to connect braking resistor 148 to motor terminals (U, V, and W). Braking resistor 148 draws current from motor 126 and stops motor 126 for a predetermined stopping time. In some embodiments, motor controller 130 uses braking circuit 134 to brake motor 126 even when motor current is below the regenerative current threshold. When braking circuit 134 is used to brake motor 126, motor current flows through braking resistor 148 and to ground or to the negative terminal of power supply 122. Alternatively, braking circuit 134 forms a closed circuit with motor terminals (U, V, and W) such that motor current flows through braking resistor 148.

[0046] When the motor current is not greater than the regenerative current threshold, method 450 includes using motor controller 130 to control power switching network 124 to perform regenerative braking of motor 126 (in block 478). Motor controller 130 selectively enables, for example, one of the low-side FETs 144 to allow freewheeling current (induced by the still-rotating rotor magnet) to flow through the enabled low-side FET 144 (e.g., 144a), the coupling winding of the motor coil phase, and the freewheeling diode of the coupled adjacent low-side FET 144 (e.g., 144b). The freewheeling current is partially stored as potential energy (i.e., voltage) within the motor coil winding coupled between the adjacent low-side FETs 144. Motor controller 130 then disables the previously enabled low-side FET 144, which directs the stored potential energy as current through high-side FET 140 to power supply 122 to charge power supply 122. Method 450 is repeated, in which motor controller 130 returns to block 454 to determine the next trigger pull of power tool 100.

[0047] In some implementations, when the motor current is higher than the regenerative current threshold in block 470, the motor controller 130 temporarily brakes the motor in block 474, and then returns to block 470 to determine whether the motor current has decreased below the regenerative current threshold. The motor controller 130 continuously cycles between blocks 470 and 474 until the motor current decreases below the regenerative current threshold, and then the motor controller proceeds to block 478.

[0048] The above design allows for the use of resistors with a size one-third (1 / 3) smaller than those used in previous designs (relying on a single large braking resistor). Furthermore, the braking transistor (FET) used in the braking circuit 134 can be smaller than a braking FET used with a single large braking resistor because the braking current is distributed across multiple current paths rather than concentrated in a single braking FET and resistor pair. Additionally, extra redundancy can be achieved, for example, allowing other braking resistors 148 in the operating path to be available for the braking motor 126 in the event of a failure in one of the braking switching elements 152.

[0049] Although Figure 3A and Figure 3B The braking circuit 134 shown includes three braking current paths (i.e., through three braking FETs 152 and three associated braking resistors 148, one pair for each motor phase), but in some embodiments, the braking circuit 134 includes only two braking current paths along only two of the three motor phases (i.e., through two braking FETs 152 and associated braking resistors 148).

[0050] In some implementations... Figure 4 Block 420 and Figure 5Block 470 can be more generally described as a block in which the motor controller 130 determines, based on variable tooling characteristics, whether to activate enhanced braking using the motor braking circuit 134. Figure 4 In the case of block 420, the variable tool characteristic is the motor impedance, and the motor controller 130 determines to initiate enhanced braking in response to the motor impedance being greater than an impedance threshold. Figure 5 In the case of block 470, the variable tool characteristic is the motor current, and the motor controller 130 determines to initiate enhanced braking in response to the motor current being greater than the regenerative current threshold.

[0051] Therefore, the various embodiments described herein provide motor braking circuits for power tool motors.

Claims

1. An electric tool, comprising: power supply; motor; A power switch network connected between the power source and the motor; User input, which is configured to be actuated to drive the motor; A braking circuit, which includes one or more resistive loads and is configured to be selectively connected to the motor terminals of the motor; as well as A motor controller, which is connected to the power switch network and the braking circuit, is configured to: In response to the actuation of the user input, the power switching network is controlled to drive the motor. Determine the variable tool properties. It is determined that the user input has been stopped. In response to the user input being stopped, determine whether the variable tool characteristic meets the tool characteristic threshold. When the variable tool characteristic meets the tool characteristic threshold, the power switching network is controlled to brake the motor, and When the variable tool characteristic does not meet the tool characteristic threshold, the braking circuit is controlled to brake the motor; The variable tool characteristic is either system impedance or motor current.

2. The power tool according to claim 1, wherein, The motor is a three-phase motor including three motor terminals, and the one or more resistive loads of the braking circuit include three resistive loads.

3. The power tool according to claim 1, wherein, The braking circuit also includes one or more braking switches, wherein the motor controller controls the one or more braking switches to selectively connect one or more braking resistors to the motor terminals.

4. The power tool according to claim 3, wherein: The one or more brake switches include field-effect transistors (FETs) controlled by the motor controller; and The motor controller performs pulse width modulation control of the one or more brake switches to brake the motor using the brake circuit.

5. The power tool according to claim 1, wherein, The braking circuit is configured to selectively connect the motor terminals to one of the following groups: each other, ground, and the power supply terminal.

6. The power tool according to claim 1, wherein, Controlling the power switching network to brake the motor includes using the power switching network to perform regenerative braking to redirect braking current back to the power source.

7. The power tool according to claim 1, wherein, The motor controller is also configured to: When the variable tool characteristic is the motor current, when the motor is braked using the braking circuit, it is determined that the motor current drops below the regenerative braking threshold. as well as In response to determining that the motor current has dropped below the regenerative braking threshold, the control of the braking circuit to brake the motor is switched to the control of the power switching network to brake the motor.

8. The power tool according to claim 7, wherein, Controlling the power switching network to brake the motor includes using the power switching network to perform regenerative braking to redirect braking current back to the power source.

9. The power tool of claim 1 further includes a direct-connect power interface to provide connection between the power supply and the power switching network in the absence of an on / off switch controlled by the user input.

10. A method for braking a motor of an electric tool, comprising: In response to actuation by user input, the motor controller of the power tool controls the power switching network to drive the motor of the power tool; The variable tool characteristics are determined using the motor controller; The motor controller is used to determine that the user input has been stopped from actuating; In response to the user input being stopped, the motor controller is used to determine whether the variable tool characteristic meets the tool characteristic threshold. When the variable tool characteristic meets the tool characteristic threshold, the motor controller is used to control the power switch network to brake the motor; as well as When the variable tool characteristic does not meet the tool characteristic threshold, the motor controller is used to control the braking circuit to brake the motor, wherein the braking circuit having one or more resistive loads is selectively connected to the motor terminals of the motor. The variable tool characteristic is either system impedance or motor current.

11. The method according to claim 10, wherein, The motor is a three-phase motor including three motor terminals, and the one or more resistive loads of the braking circuit include three resistive loads.

12. The method according to claim 10, wherein, Selectively connecting the braking circuit to the motor terminals further includes: The motor controller is used to control one or more brake switches to connect one or more resistive loads to the motor terminals.

13. The method of claim 12, further comprising: The motor controller is used to perform pulse width modulation control of one or more brake switches to brake the motor using the brake circuit.

14. The method of claim 10, further comprising: When the motor is braked using the power switch network, regenerative braking is performed using the power switch network to redirect the braking current back to the power source.

15. The method according to claim 10, wherein, Also includes: When the variable tool characteristic is the motor current, when the motor is braked using the braking circuit, the motor controller determines that the motor current has decreased to below the regenerative braking threshold. as well as In response to determining that the motor current has dropped below the regenerative braking threshold, the control of the braking circuit to brake the motor is switched to the control of the power switching network to brake the motor.

16. The method of claim 15, further comprising: When the motor is braked using the power switch network, regenerative braking is performed using the power switch network to redirect the braking current back to the power source.

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