Patent Description:
At present, high-voltage brushless power tools for engineering or household use have a relatively high maintenance rate and a relatively short maintenance cycle. For example, a risk that an electronic control board of a wall grinding machine basically fails within a month or two or may fail within a week at the shortest exists.

Document <CIT> discloses a commutation control method, a device for a brushless direct current motor, and a storage medium are described. The method includes performing detection on a position of a rotor in a brushless direct current motor. The detection is further configured to be triggered by commutation of the brushless direct current motor. The method includes determining, for the brushless direct current motor, a first drive scheme corresponding to the detected position of the rotor, the first drive scheme indicates a manner in which a three-phase full-bridge circuit of the brushless direct current motor operates; updating a pulse width modulation (PWM) drive signal, the updating is performed on the basis of the first drive scheme; and using the updated PWM drive signal to control the brushless direct current motor to perform commutation.

Document <CIT> discloses a brushless DC motor system, which includes a single coil brushless DC motor and a driver for driving the single coil brushless DC motor. The brushless DC motor system has a maximum time constant Tmax. The driver comprises a control unit which is adapted for driving the brushless DC motor at a constant speed and at a variable speed by applying a PWM driving signal to the coil of the brushless DC motor with a PWM frequency larger than a ratio defined by a constant/Tmax wherein the ratio is such that a current through the coil is always bigger than a pre-defined undercurrent limit.

Document <CIT> discloses a method for controlling a single-coil BLDC motor. The method comprising: monitoring a signal indicative of a time derivative of a phase current through the coil of the BLDC motor during at least one commutation cycle; determining a first moment of the at least one commutation cycle such that at the first moment the absolute value of the time derivative of the phase current is smaller than or equal to a slope threshold; determining a rising edge of a driving signal in a commutation cycle, based on the first moment of that commutation cycle and/or based on the first moment of at least one earlier commutation cycle, and/or determining a falling edge of the driving signal in that commutation cycle based on the first moment of at least one earlier commutation cycle; driving the single-coil BLDC motor using the driving signal.

Document <CIT> discloses a controller for a BLDC motor, which includes a pulse width modulator and a control circuit. The pulse width modulator provides at least one phase control signal for a corresponding phase of the BLDC motor with a pulse width determined by a duty cycle signal. The duty cycle adjustment circuit has an input for receiving the at least one phase control signal, and an output for providing a corresponding modified phase control signal by adjusting widths of pulses of the at least one phase control signal when an average current in said corresponding phase exceeds a threshold.

Document <CIT> discloses a method and apparatus for maintaining both a DC bus voltage and a motor current within limit values, wherein, when a voltage limit condition occurs, a voltage error (i.e. DC bus voltage limit minus DC bus voltage) is used to increase an inverter output frequency until the voltage limit condition subsides, and, wherein during a current limit condition a current error signal (i.e. motor current limit minus motor current) is used to reduce inverter output frequency until the current limit condition subsides, a slew rate used to control the output frequency when neither a current nor a voltage limit condition exists, the slew rate decreased as a function of the voltage limit period durations and increases a function of the current limit period durations.

To solve the deficiencies of the existing art, an object of the present application is to provide a power tool with a long service life and high electric control stability, as defined in the independent claim attached. Preferred embodiments of the invention are defined in dependent claims attached.

The beneficial effects of the present application are described as follows: the variation of the duty cycle of the PWM signal is adjusted according to the change of the voltage of the control circuit so that excessive current or voltage spikes due to a motor commutation error can be avoided, and components in the control circuit can be prevented from being burned out, thereby ensuring the service life of the tool and the stability of the control performance.

In this application, the terms "up", "down", "left", "right", "front", and "rear" " and other directional words are described based on the orientation or positional relationship shown in the drawings, and should not be understood as limitations to the examples of this application. In addition, in this context, it also needs to be understood that when it is mentioned that an element is connected "above" or "under" another element, it can not only be directly connected "above" or "under" the other element, but can also be indirectly connected "above" or "under" the other element through an intermediate element. It should also be understood that orientation words such as upper side, lower side, left side, right side, front side, and rear side do not only represent perfect orientations, but can also be understood as lateral orientations. For example, lower side may include directly below, bottom left, bottom right, front bottom, and rear bottom.

In this application, the terms "controller", "processor", "central processor", "CPU" and "MCU" are interchangeable. Where a unit "controller", "processor", "central processing", "CPU", or "MCU" is used to perform a specific function, the specific function may be implemented by a single aforementioned unit or a plurality of the aforementioned unit.

In this application, the term "device", "module" or "unit" may be implemented in the form of hardware or software to achieve specific functions.

In this application, the terms "computing", "judging", "controlling", "determining", "recognizing" and the like refer to the operations and processes of a computer system or similar electronic computing device (e.g., controller, processor, etc.). Power tools to which the technical solutions of the present application are applicable include various high-voltage brushless power tools, such as a wall grinding machine, an electric hammer, an angle grinder, and an electric wrench. Any other power tools that use brushless motors and can perform high-power operations or that can adopt the solutions provided in the examples of the present application are within the scope of the present application.

In the examples of the present application, the wall grinding machine is used as an example for description. As shown in <FIG>, a wall grinding machine <NUM> may include a tool body <NUM>, where the tool body <NUM> includes a working head <NUM>, an operating rod <NUM>, an electric motor <NUM> mounted in the working head <NUM>, and a control device <NUM> disposed in the tool body <NUM> or independent of the tool body <NUM>. A functional piece is further provided on the working head <NUM>, such as a grinding disc.

In an example, the electric motor <NUM> is a brushless direct current (BLDC) motor. In an example, the electric motor <NUM> is a sensorless BLDC motor. In an example, the electric motor <NUM> is a sensored BLDC motor. In the present application, the BLDC motor may be an inrunner or an outrunner, and the electric motor <NUM> includes at least three-phase stator windings A, B, and C in a star connection or a triangular connection. The control device <NUM> can control the electric motor <NUM> to rotate so that the electric motor <NUM> can drive the grinding disc to perform the grinding work.

In an example, as shown in <FIG>, the control device <NUM> may include a control circuit <NUM>, where the control circuit <NUM> includes a power supply <NUM>, a driver circuit <NUM>, a parameter detection module <NUM>, and a controller <NUM>. The driver circuit <NUM> is electrically connected to the electric motor <NUM> and can drive the electric motor <NUM> to rotate. The power supply <NUM> may optionally be a battery pack. The battery pack may be composed of a group of battery cells. For example, the battery cells may be connected in series into a single power supply branch to form a 1P battery pack. The output voltage of the battery pack is changed by a specific power supply conversion module, such as a direct current-direct current (DC-DC) module, such that a power supply voltage suitable for the driver circuit <NUM>, the electric motor <NUM>, and the like is outputted to power them up. It is to be understood by those skilled in the art that the DC-DC module is a mature circuit structure and may be selected accordingly depending on the specific parameter requirements of the power tool.

In an example, the driver circuit <NUM> is electrically connected to the stator windings A, B, and C of the electric motor <NUM> and used for transmitting the current from the power supply <NUM> to the stator windings A, B, and C to drive the electric motor <NUM> to rotate. In an example, the driver circuit <NUM> includes multiple switching elements Q1, Q2, Q3, Q4, Q5, and Q6. A gate terminal of each switching element is electrically connected to the controller <NUM> and used for receiving a control signal from the controller <NUM>. A drain or source of each switching element is connected to the stator windings A, B, and C of the electric motor <NUM>. The switching elements Q1 to Q6 receive control signals from the controller <NUM> to change respective conduction states, thereby changing the current loaded to the stator windings A, B, and C of the electric motor <NUM> by the power supply <NUM>. In an example, the driver circuit <NUM> may be a three-phase bridge driver circuit including six controllable semiconductor power devices (such as field-effect transistors (FETs), bipolar junction transistors (BJTs), or insulated-gate bipolar transistors (IGBTs)). It is to be understood that the preceding switching elements may be any other types of solid-state switches, such as the IGBTs or the BJTs.

The parameter detection module <NUM> can detect an electrical parameter of the control circuit <NUM>. For example, the parameter detection module <NUM> can detect the bus voltage or bus current of the control circuit <NUM> or can detect the phase current or phase voltage of the electric motor <NUM>. In an example, the parameter detection module <NUM> may include one detection element having a function of detecting multiple parameters or may include multiple detection elements that respectively detect different electrical parameters. The parameter detection module <NUM> is electrically connected to the controller <NUM> and can transmit the electrical parameter to the controller <NUM>. Other electrical connections of the parameter detection module <NUM> in the control circuit <NUM> are not limited in this example.

To drive the electric motor <NUM> to rotate, the driver circuit <NUM> has multiple driving states. The electric motor <NUM> may have different rotational speeds or different rotational directions in different driving states. In the present application, the process is not described in detail where the controller <NUM> controls the driver circuit <NUM> to change different driving states such that the electric motor <NUM> has different rotational speeds or different rotational directions.

In an example, the driver circuit <NUM> typically has at least six driving states, and each switching of the driving state corresponds to one commutation action of the motor windings. As shown in <FIG>, when the electric motor <NUM> commutates each time, the phase voltage of a floating phase changes suddenly, and the voltage value after the sudden change is maintained for a period of time before the voltage value is eliminated. This period of time may be understood as the freewheeling time of the floating-phase winding when the electric motor commutates. It is to be understood that the higher the voltage of the motor windings, the longer the freewheeling time. However, the power of the electric motor <NUM> used in the high-voltage brushless power tool is generally relatively large, and the average inductance of the windings of the electric motor <NUM> is relatively high so that the voltage of the windings is relatively high, and the freewheeling time during commutation is also relatively long. In an example, the average inductance of the windings of the electric motor <NUM> is greater than or equal to <NUM> mH. In an example, the average inductance of the windings of the electric motor <NUM> is greater than or equal to <NUM> mH. In an example, the average inductance of the windings of the electric motor <NUM> is greater than or equal to <NUM> mH. For example, the average inductance may be <NUM> mH, <NUM> mH, <NUM> mH, <NUM> mH, <NUM> mH, or the like. In an example, the output power of the electric motor <NUM> is greater than or equal to <NUM> W, such as <NUM> W, <NUM> W, or <NUM> W.

In an example, the control circuit <NUM> further includes a filter capacitor, where the capacitance of the filter capacitor is less than or equal to <NUM>µF. For example, the capacitance of the filter capacitor could be 10µF or 15µF.

Generally, the motor control of the sensorless motor requires the accurate detection of a zero-crossing during the commutation process of the electric motor, so as to accurately estimate the rotor position of the electric motor. The zero-crossing may be understood as the moment when the floating-phase voltage is zero during the commutation process of the electric motor. If the inductance of the motor windings is relatively high and the freewheeling time of the commutation process of the electric motor is relatively long, an effective zero-crossing cannot be detected, resulting in a commutation error. The motor commutation error causes electrical spikes in the control circuit <NUM>, such as voltage spikes, current spikes, or energy spikes. The spikes may be understood as fast and short-duration electrical transients of the voltage (voltage spikes), current (current spikes), or delivered energy (energy spikes) in the control circuit. The electrical spikes in the control circuit <NUM> easily cause damage to the power components in the circuit or cause the risk of demagnetization of the electric motor.

To maintain the constant-speed stability of the electric motor, that is, to decrease the steady-state error of the electric motor running in a constant speed, the controller <NUM> can change the duty cycle of the PWM control signal to control the electric motor to operate at a constant speed. In fact, the controller <NUM> can adjust the duty cycle of the PWM signal in time according to the rotational speed information fed back by the electric motor <NUM> to implement the stable constant-speed control. In the process of the controller <NUM> adjusting the duty cycle of the PWM signal, a relatively large variation of the duty cycle may occur, resulting in problems such as more energy storage in the motor windings, relatively long freewheeling time during commutation, and the electrical spikes; or a small variation of the duty cycle may occur during regulation and control, resulting in the unobvious change of the rotational speed and increasing the steady-state error of the constant-speed control.

To solve the preceding problems, the control circuit <NUM> in this example limits the variation of the PWM duty cycle of the speed proportional integral (PI) loop.

Specifically, the variation of the PWM duty cycle is adaptively adjusted according to the voltage information of the control circuit and the rotational speed information fed back by the electric motor, so as to reduce the steady-state error of the constant-speed control of the electric motor.

Referring to <FIG>, the parameter detection module <NUM> detects the electrical parameter of the control circuit <NUM>, and then the controller <NUM> may acquire the preceding electrical parameter and adjust the variation of the duty cycle of the PWM signal according to the magnitude of the electrical parameter. In this example, the electrical parameter may be the bus voltage or bus current of the control circuit <NUM>. In this example, ΔPWM is used for denoting the variation of the duty cycle. In an example, the controller <NUM> adjusts the absolute value of the variation of the duty cycle of the PWM signal, that is, |ΔPWM| according to the magnitude of the bus voltage. For example, when the bus voltage is relatively large, the controller <NUM> reduces |ΔPWM|, that is, reduce the amount by which the PWM duty cycle increases or decreases; and when the bus voltage is relatively small, the controller <NUM> increases |ΔPWM|, that is, increase the amount by which the PWM duty cycle increases or decreases. In this example, the controller <NUM> may include a data acquisition module for acquiring the bus voltage and a data processing module for calculating the magnitude of |ΔPWM| according to the bus voltage.

In fact, when the bus voltage is relatively large, the energy stored in the motor windings is relatively high, and the freewheeling time of the floating phase during commutation is relatively long. In this case, to maintain the constant-speed stability of the electric motor, the PWM duty cycle needs to be increased or decreased. To avoid the further increase of the freewheeling time due to too much increase or decrease of the duty cycle, the controller <NUM> controls the variation of the PWM duty cycle to have a relatively low amplitude, that is, set a relatively low |ΔPWM|. Conversely, when the bus voltage is relatively small, the energy stored in the motor windings is not high, and the freewheeling time of the floating phase during commutation is relatively short. In this case, to maintain the constant-speed stability of the electric motor, the PWM duty cycle needs to be increased or decreased. Moreover, even if the PWM duty cycle is increased or decreased by a large amount, the impact on the freewheeling time is not large, and the problem that inaccurate detection of the zero-crossing leads to the commutation error does not occur. To sum up, in the present application, the variation of the PWM duty cycle is controlled according to the magnitude of the bus voltage, thereby effectively maintaining the stability of the constant-speed control of the electric motor and reducing the steady-state error of the constant-speed control. In this example, the magnitude of the phase current when the electric motor <NUM> commutates is less than <NUM> A, thereby avoiding the occurrence of the current spikes compared to the relatively large output power of the power tool.

In an example, the electrical parameter of the control circuit <NUM> has a linear or non-linear relationship with the absolute value of the variation of the PWM duty cycle. In an example, the bus voltage is inversely proportional to |ΔPWM|, that is to say, the larger the bus voltage, the smaller |ΔPWM| set by the controller <NUM>, and vice versa. For example, the bus voltage is Ud, and then Ud * |ΔPWM| = K, where K is a constant, and the value of K varies in different working conditions.

In this example, since the relative relationship between the bus voltage and |ΔPWM| is generally set unchanged in the same working condition or in the same tool, there is no need for a criterion for determining the level of the bus voltage. Based on the determination of the relative relationship between the bus voltage and |ΔPWM|, a corresponding set value of |ΔPWM| may be obtained regardless of whether the bus voltage is determined to be high or low.

In this example, the steady-state error of the electric motor running in a constant speed may be made less than or equal to <NUM>%, for example, <NUM>%, by reasonably adjusting the magnitude of |ΔPWM|.

Referring to <FIG>, the comparison of the change process of the rotational speed of the electric motor before and after the method for adjusting the PWM duty cycle protected by the examples of the present application is adopted is made. It can be seen that although a certain steady-state error of the constant-speed control of the electric motor before and after improvement exists, the steady-state error after improvement is better than the steady-state error before improvement. In this example, the steady-state error after improvement is about <NUM>%. The improvement refers to adopting the method for adjusting |ΔPWM| protected by the present application to change the PWM duty cycle to control the electric motor to operate.

Referring to <FIG>, a process of controlling a power tool may include the steps described below.

In S101, the bus voltage of a control circuit is detected.

In S102, the amplitude of the variation of the PWM duty cycle is set according to the relationship between the bus voltage and the absolute value of the variation of the PWM duty cycle.

Claim 1:
A power tool, comprising:
an electric motor (<NUM>) comprising multi-phase windings; and
a control circuit (<NUM>) for controlling the electric motor to rotate;
wherein the control circuit comprises:
a driver circuit comprising a plurality of switching elements (Q1, Q2, Q3, Q4, Q5, Q6) having a plurality of driving states to drive the electric motor to rotate;
a parameter detection module (<NUM>) for detecting an electrical parameter of the control circuit; and
a controller (<NUM>) for outputting a pulse-width modulation (PWM) control signal to control the driver circuit to change the plurality of driving states;
characterized in that
the electrical parameter comprises a bus voltage of the control circuit; and
the controller is configured:
to change an absolute value ( |ΔPWM| ) of a variation of a duty cycle of the PWM control signal according to the electrical parameter to control a steady-state error of speed of the electric motor running in a constant speed to be within a preset range;
to decrease the absolute value of said variation of the duty cycle when the bus voltage increases; and
to increase the absolute value of said variation of the duty cycle when the bus voltage decreases;
wherein an average inductance of each phase winding of said multi-phase windings is greater than or equal to <NUM> mH.