Patent Description:
An electric tool configured to control the rotation number of an electric motor has been known (e.g., <CIT>). The electric tool described in <CIT> includes a brushless DC motor (an electric motor), a battery voltage detector, a rotational position detector, and a control unit. The battery voltage detector is configured to detect the voltage of a battery used to drive the brushless DC motor. The rotational position detector is configured to detect the rotational position of the brushless DC motor. The control unit is configured to control a drive output to the brushless DC motor based on a signal from the rotational position detector. The control unit is configured to, when controlling the drive output to the brushless DC motor, control a conduction angle or an advance angle to the brushless DC motor such that the rotation number or the energizing current of the brushless DC motor is a target value corresponding to the battery voltage detected by the battery voltage detector. <CIT> which discloses the preamble of claim <NUM>, shows a hand-guided or stationary power tool having a drive unit with a motor that includes a rotor having a permanent magnet and a stator and having a motor control designed to trigger the motor in a first rotational speed range according to a voltage-controlled mode and to trigger the motor in a second rotational speed range following the first rotational speed range in the direction of a higher rotational speed according to a field-weakening operation.

It is an object of the present disclosure to provide an electric tool configured to increase the rotation number of an electric motor.

An electric tool according the present disclosure is defined in independent claim <NUM>. The preferred embodiments are described in claims <NUM> to <NUM>.

An embodiment of an electric tool <NUM> will now be described in detail with reference to the accompanying drawings. Note that the embodiment to be described below is only an exemplary one of various embodiments of the present disclosure and should not be construed as limiting. Also, <FIG> to be referred to in the following description of the embodiment is a schematic representation. That is to say, the ratio of the dimensions (including thicknesses) of respective constituent elements illustrated in <FIG> does not always reflect their actual dimensional ratio.

An electric tool <NUM> according to an exemplary embodiment may be used as an impact screwdriver, a drill screwdriver, or an impact wrench. The electric tool <NUM> includes an AC motor <NUM> (an electric motor) and a control unit <NUM> as shown in <FIG>. The AC motor <NUM> may be a brushless motor, for example. In particular, the AC motor <NUM> according to this embodiment is a synchronous motor and is more specifically a Permanent Magnet Synchronous Motor (PMSM). The control unit <NUM> performs control on the operation of the AC motor <NUM>.

The AC motor <NUM> includes a rotor <NUM> having a permanent magnet <NUM> and a stator <NUM> having a coil <NUM>. The rotor <NUM> includes an output shaft <NUM>. Electromagnetic interaction between the coil <NUM> and the permanent magnet <NUM> rotates the rotor <NUM> with respect to the stator <NUM>. The control unit <NUM> performs vector control for controlling a flux-weakening current (d-axis current) supplied to the AC motor <NUM> and a torque current (q-axis current) supplied to the AC motor <NUM> independently of each other. The control performed by the control unit <NUM> includes field weakening control by the vector control. In the field weakening control, the control unit <NUM> causes the flux-weakening current (d-axis current) to flow through the coil <NUM> of the AC motor <NUM>. The flux-weakening current generates, in the coil <NUM>, a magnetic flux that weakens the magnetic flux of the permanent magnet <NUM> (weakening flux). In other words, the flux-weakening current generates, in the coil <NUM>, a magnetic flux, of which the direction is opposite from the direction of the magnetic flux of the permanent magnet <NUM>. This increases the rotation number of the AC motor <NUM> (the rotation number of the output shaft <NUM>).

The control performed by the control unit <NUM> further includes regular control. The control unit <NUM> does not cause the flux-weakening current to flow through the coil <NUM> in the regular control. That is, a current that flows through the coil <NUM> in the regular control is only the torque current (q-axis current). When the torque current of the AC motor <NUM> is relatively large (e.g., the magnitude of the torque current exceeds a predetermined value), the control unit <NUM> changes its control from the field weakening control to the regular control. Thus, when the AC motor <NUM> needs a relatively large torque, the regular control provides the relatively large torque.

The electric tool <NUM> includes the AC motor <NUM>, a power supply <NUM>, a driving force transmission mechanism <NUM>, an impact mechanism <NUM>, a socket <NUM>, a trigger volume <NUM>, the control unit <NUM>, a torque measuring unit <NUM>, a bit rotation measuring unit <NUM>, and a motor rotation measuring unit <NUM> as shown in <FIG>. In addition, the electric tool <NUM> further includes a tip tool.

The impact mechanism <NUM> has an output shaft <NUM>. The output shaft <NUM> is a member to rotate with driving force transmitted from the AC motor <NUM>. The socket <NUM> is a member, which is fixed to the output shaft <NUM> and to which the tip tool is attached removably. The electric tool <NUM> is a tool for driving the tip tool with the driving force supplied from the AC motor <NUM>. The tip tool (hereinafter also referred to as a "bit") may be a screwdriver or a drill, for example. A tip tool is selected from various types of tip tools according to the intended use and attached to the socket <NUM> to have some type of machining work done. Optionally, the tip tool may be directly attached to the output shaft <NUM>.

The AC motor <NUM> is a drive source for driving the tip tool. The AC motor <NUM> includes the output shaft <NUM> for outputting rotational driving force. The power supply <NUM> is an AC power supply for supplying a current for driving the AC motor <NUM>. The power supply <NUM> includes a single or a plurality of secondary batteries. The driving force transmission mechanism <NUM> regulates the rotational driving force of the AC motor <NUM> and outputs a desired torque. The driving force transmission mechanism <NUM> includes a drive shaft <NUM> as its output member.

The drive shaft <NUM> of the driving force transmission mechanism <NUM> is connected to the impact mechanism <NUM>. The impact mechanism <NUM> transforms the rotational driving force supplied from the AC motor <NUM> via the driving force transmission mechanism <NUM> into a pulsed torque, thereby generating impacting force. The impact mechanism <NUM> includes a hammer <NUM>, an anvil <NUM>, the output shaft <NUM>, and a spring <NUM>. The hammer <NUM> is attached to the drive shaft <NUM> of the driving force transmission mechanism <NUM> via a cam mechanism. The anvil <NUM> is coupled to the hammer <NUM> and rotates along with the hammer <NUM>. The spring <NUM> biases the hammer <NUM> toward the anvil <NUM>. The anvil <NUM> is formed integrally with the output shaft <NUM>. Alternatively, the anvil <NUM> may be formed separately from the output shaft <NUM> and fixed to the output shaft <NUM>.

Unless a load (torque), of which the magnitude is greater than or equal to a predetermined value, is applied to the output shaft <NUM>, the drive shaft <NUM> and the hammer <NUM> which are coupled together via the cam mechanism turn along with each other, and in addition, the hammer <NUM> and the anvil <NUM> turn along with each other. Thus, the output shaft <NUM> formed integrally with the anvil <NUM> turns accordingly. On the other hand, if a load, of which the magnitude is greater than or equal to the predetermined value, is applied to the output shaft <NUM>, then the hammer <NUM> moves backward (i.e., moves away from the anvil <NUM>) against the spring <NUM> while being regulated by the cam mechanism. At a point in time when the hammer <NUM> is decoupled from the anvil <NUM>, the hammer <NUM> starts moving forward while turning, thus applying impacting force to the anvil <NUM> in the rotational direction and thereby turning the output shaft <NUM>.

The trigger volume <NUM> is an operating member for accepting an operating command for controlling the rotation of the AC motor <NUM>. The ON/OFF states of the AC motor <NUM> may be switched by pulling the trigger volume <NUM>. In addition, the rotational velocity of the output shaft <NUM>, i.e., the rotational velocity of the AC motor <NUM>, is adjustable by the manipulative variable indicating how deep the trigger volume <NUM> has been pulled. Specifically, the greater the manipulative variable is, the higher the rotational velocity of the AC motor <NUM> becomes. The control unit <NUM> starts or stops turning the AC motor <NUM> and controls the rotational velocity of the AC motor <NUM> according to the manipulative variable indicating how deep the trigger volume <NUM> has been pulled. In this electric tool <NUM>, the tip tool is attached to the socket <NUM>. Controlling the rotational velocity of the AC motor <NUM> by operating the trigger volume <NUM> allows the rotational velocity of the tip tool to be controlled.

The electric tool <NUM> according to this embodiment includes the socket <NUM>, thus making the tip tool replaceable depending on the intended use. However, the tip tool does not have to be replaceable. Alternatively, the electric tool <NUM> may also be designed to allow the use of only a particular type of tip tool.

The torque measuring unit <NUM> measures the operating torque of the AC motor <NUM>. The torque measuring unit <NUM> may be a magnetostriction strain sensor which may detect torsion strain, for example. The magnetostriction strain sensor makes a coil, provided in a non-rotating portion of the AC motor <NUM>, detect a variation in permeability corresponding to the strain caused by the application of a torque to the output shaft <NUM> of the AC motor <NUM> and outputs a voltage signal, of which the magnitude is proportional to the magnitude of the strain.

The bit rotation measuring unit <NUM> measures the rotational angle of the output shaft <NUM>. In this case, the rotational angle of the output shaft <NUM> is equal to the rotational angle of the tip tool (bit). As the bit rotation measuring unit <NUM>, a photoelectric encoder or a magnetic encoder may be adopted, for example.

The motor rotation measuring unit <NUM> measures the rotational angle of the AC motor <NUM>. As the motor rotation measuring unit <NUM>, a photoelectric encoder or a magnetic encoder may be adopted, for example.

The control unit <NUM> includes a computer system including one or more processors and a memory. At least some of the functions of the control unit <NUM> are performed by making the processor of the computer system execute a program stored in the memory of the computer system. The program may be stored in the memory. The program may also be downloaded via a telecommunications network such as the Internet or distributed after having been stored in a non-transitory storage medium such as a memory card.

The control performed by the control unit <NUM> includes the field weakening control and the regular control. In the field weakening control, the control unit <NUM> causes the flux-weakening current to flow from an inverter circuit section <NUM> through the coil <NUM> of the AC motor <NUM>. The control unit <NUM> does not cause the flux-weakening current to flow from the inverter circuit section <NUM> through the coil <NUM> in the regular control. When a switching condition described later are satisfied, the control performed by the control unit <NUM> is the field weakening control. The regular control is control performed such that a command value (target value) cid1 of the flux-weakening current is set to <NUM> and the flux-weakening current converges toward the command value cid1. The field weakening control is control performed such that the command value cid1 of the flux-weakening current is set to be greater than <NUM> and the flux-weakening current converges toward the command value cid1. When the command value cid1 of the flux-weakening current is greater than <NUM>, the flux-weakening current flows through the AC motor <NUM>, thereby generating the weakening flux.

As shown in <FIG>, the control unit <NUM> includes a command value generating unit <NUM>, a velocity control unit <NUM>, a current control unit <NUM>, a first coordinate transformer <NUM>, a second coordinate transformer <NUM>, a flux control unit <NUM>, an estimation unit <NUM>, and a step-out detection unit <NUM>. In addition, the electric tool <NUM> further includes the inverter circuit section <NUM> and a plurality of (e.g., two in the example illustrated in <FIG>) current sensors <NUM> and <NUM>. The control unit <NUM> is used along with the inverter circuit section <NUM> and performs feedback control to control the operation of the AC motor <NUM>.

Each of the plurality of current sensors <NUM> and <NUM> includes, for example, a Hall element current sensor or a shunt resistor element. The plurality of current sensors <NUM> and <NUM> measure an electric current supplied from the power supply <NUM> to the AC motor <NUM> via the inverter circuit section <NUM>. In this embodiment, three-phase currents (namely, a U-phase current, a V-phase current, and a W-phase current) are supplied to the AC motor <NUM>. The plurality of current sensors <NUM> and <NUM> measure currents in at least two phases. In <FIG>, the current sensor <NUM> measures the U-phase current and outputs a current measured value iu<NUM>, and the current sensor <NUM> measures the V-phase current and outputs a current measured value iv<NUM>.

The estimation unit <NUM> performs time differentiation on the rotational angle θ1, measured by the motor rotation measuring unit <NUM>, of the AC motor <NUM> to calculate an angular velocity ω1 of the AC motor <NUM> (i.e., the angular velocity of the output shaft <NUM>).

The second coordinate transformer <NUM> performs, based on the rotational angle θ1, measured by the motor rotation measuring unit <NUM>, of the AC motor <NUM>, coordinate transformation on the current measured values iu<NUM> and iv<NUM> measured by the plurality of current sensors <NUM> and <NUM>, thereby calculating current measured values id1 and iq1. That is to say, the second coordinate transformer <NUM> transforms the current measured values iu<NUM> and iv<NUM>, corresponding to currents in two phases out of the currents in three phases, into a current measured value id1 corresponding to a magnetic field component (d-axis current) and a current measured value iq1 corresponding to a torque component (q-axis current).

The command value generating unit <NUM> generates a command value cω1 of the angular velocity of the AC motor <NUM>. The command value generating unit <NUM> generates the command value cω1 according to, for example, a magnitude corresponding to the manipulative variable indicating how deep the trigger volume <NUM> (see <FIG>) has been pulled. That is to say, as the manipulative variable increases, the command value generating unit <NUM> increases the command value cω1 of the angular velocity accordingly.

The velocity control unit <NUM> generates a command value ciq1 based on the difference between the command value cω1 generated by the command value generating unit <NUM> and the angular velocity ω1 calculated by the estimation unit <NUM>. The command value ciq1 is a command value specifying the magnitude of the torque current (q-axis current) of the AC motor <NUM>. The velocity control unit <NUM> determines the command value ciq1 to reduce the difference between the command value cω1 and the angular velocity ω1.

The flux control unit <NUM> generates a command value cid1 based on the angular velocity ω1 calculated by the estimation unit <NUM>, a command value cvq1 (which will be described later) generated by the current control unit <NUM>, and the current measured value iq1 (q-axis current). The command value cid1 is a command value that specifies the magnitude of the flux-weakening current (a current component that generates a magnetic flux in a d-axis direction) of the AC motor <NUM>. When the control performed by the control unit <NUM> is the regular control, the command value cid1 generated by the flux control unit <NUM> is a command value for adjusting the flux-weakening current to <NUM>. When the control performed by the control unit <NUM> is the field weakening control, the flux control unit <NUM> determines the command value cid1 by a determination process which will be described later.

The current control unit <NUM> generates a command value cvd1 based on the difference between the command value cid1 generated by the flux control unit <NUM> and the current measured value id1 calculated by the second coordinate transformer <NUM>. The command value cvd1 is a command value that specifies the magnitude of a d-axis voltage of the AC motor <NUM>. The current control unit <NUM> determines the command value cvd1 to reduce the difference between the command value cid1 and the current measured value id1.

In addition, the current control unit <NUM> also generates the command value cvq1 based on the difference between the command value ciq1 generated by the velocity control unit <NUM> and the current measured value iq1 calculated by the second coordinate transformer <NUM>. The command value cvq1 is a command value that specifies the magnitude of a q-axis voltage of the AC motor <NUM>. The current control unit <NUM> generates the command value cvq1 to reduce the difference between the command value ciq1 and the current measured value iq1.

The first coordinate transformer <NUM> performs coordinate transformation on the command values cvd1 and cvq1 based on the rotational angle θ1, measured by the motor rotation measuring unit <NUM>, of the AC motor <NUM> to calculate command values cvu<NUM>, cvv<NUM>, and cvw<NUM>. Specifically, the first coordinate transformer <NUM> transforms the command value cvd1 for a magnetic field component (d-axis voltage) and the command value cvq1 for a torque component (q-axis voltage) into command values cvu<NUM>, cvv<NUM>, and cvw<NUM> corresponding voltages in three phases. Specifically, the command value cvu<NUM> corresponds to a U-phase voltage, the command value cvv<NUM> corresponds to a V-phase voltage, and the command value cvw<NUM> corresponds to a W-phase voltage.

The inverter circuit section <NUM> supplies voltages in three phases, corresponding to the command values cvu<NUM>, cvv<NUM>, and cvw<NUM>, respectively, to the AC motor <NUM>. The control unit <NUM> controls the power to be supplied to the AC motor <NUM> by performing Pulse Width Modulation (PWM) control on the inverter circuit section <NUM>.

The AC motor <NUM> is driven with the power (voltages in three phases) supplied from the inverter circuit section <NUM>, thus generating rotational driving force.

As a result, the control unit <NUM> controls the flux-weakening current such that the flux-weakening current flowing through the coil <NUM> of the AC motor <NUM> has a magnitude corresponding to the command value cid1 generated by the flux control unit <NUM>. Moreover, the control unit <NUM> controls the angular velocity of the AC motor <NUM> such that the angular velocity of the AC motor <NUM> is an angular velocity corresponding to the command value cω1 generated by the command value generating unit <NUM>.

The step-out detection unit <NUM> detects a step-out (loss of synchronism) of the AC motor <NUM> based on the current measured values id1 and iq1 acquired from the second coordinate transformer <NUM> and the command values cvd1 and cvq1 acquired from the current control unit <NUM>. On detecting the step-out, the step-out detection unit <NUM> transmits a stop signal cs1 to the inverter circuit section <NUM>, thus stopping the supply of power from the inverter circuit section <NUM> to the AC motor <NUM>.

Next, the determination process of the command value cid1 by the flux control unit <NUM> will be described. In this embodiment, the d-axis current when the weakening flux is generated is referred to as a negative current.

As the command value cid1 increases, the d-axis current adjusted in accordance with the command value cid1 increases. The d-axis current transitions basically at <NUM> amperes or less (see <FIG>) except for the start of the electric tool <NUM>, and when the d-axis current has a negative value, the weakening flux is generated. When the d-axis current is a negative value, and as the absolute value of the d-axis current increases, the weakening flux increases. The flux control unit <NUM> determines the command value cid1 of the d-axis current at predetermined time intervals (e.g., every several tens of microseconds) by the determination process shown in <FIG>.

When the switching condition, which is predetermined, is satisfied, the control unit <NUM> causes the flux-weakening current to flow through the coil <NUM> of the AC motor <NUM>. That is, when the switching condition is satisfied, the control performed by the control unit <NUM> is the field weakening control. The switching condition includes a condition that the AC motor <NUM> is operating within a high-velocity range. While the AC motor <NUM> is operating within the high-velocity range, schematically, the rotation number of the AC motor <NUM> is relatively high. In the present embodiment, "while the AC motor <NUM> is operating within the high-velocity range" is defined by that the rotation number of the AC motor <NUM> is greater than or equal to a predetermined rotation number R1 (see <FIG>) and the duty of the PWM control performed by the control unit <NUM> with respect to the inverter circuit section <NUM> is greater than or equal to a predetermined value. That is, the high-velocity range is an operation range within which the rotation number of the AC motor <NUM> is greater than or equal to the predetermined rotation number R1. Moreover, the high-velocity range is an operation range within which the duty (the degree of modulation) of the PWM control is greater than or equal to the predetermined value (hereinafter referred to as a "duty threshold"). The duty of the PWM control is a value obtained by dividing an ON time period in one period of the PWM signal by the length of the one period. The rotation number of the AC motor <NUM> is substantially proportional to the duty. The duty threshold is, for example, about <NUM> or <NUM>.

Moreover, the switching condition includes the condition that the value of the torque current (the value of the q-axis current) flowing through the coil <NUM> of the AC motor <NUM> is less than or equal to a predetermined current value J1 (see <FIG>). In the present embodiment, the control unit <NUM> uses the current measured value iq1 as the value of the torque current to determine whether or not the switching condition is satisfied. Note that the control unit <NUM> may use, as the value of the torque current, the command value ciq1 of the value of the torque current.

As described below, the switching condition is, as it turns out, the condition that the angular velocity ω1 is greater than or equal to a reference value, the current measured value iq1 is less than or equal to the predetermined current value J1, and the command value cvq1 is greater than or equal to a reference voltage. In this case, the reference voltage corresponds to a value obtained by converting the duty threshold into a voltage.

The control unit <NUM> uses, as a value corresponding to the rotation number of the AC motor <NUM>, the angular velocity ω1 calculated by the estimation unit <NUM>. That is, as illustrated in <FIG>, the control unit <NUM> compares the angular velocity ω1 with the reference value (step ST1) to determine whether or not the rotation number of the AC motor <NUM> is greater than or equal to the predetermined rotation number R1. The reference value is a value obtained by converting the predetermined rotation number R1 into an angular velocity. The reference value is stored in advance in, for example, memory of a microcontroller included in the control unit <NUM>. If the angular velocity ω1 is less than the reference value (step ST1: YES), the control unit <NUM> continues comparing the angular velocity ω1 with the reference value. In contrast, if the angular velocity ω1 is greater than or equal to the reference value (step ST1: NO), the control unit <NUM> removes a pulsation component of the current measured value iq1 of the q-axis current (step ST2). Specifically, the control unit <NUM> removes the pulsation component of the current measured value iq1 by a low pass filter having a cutoff frequency of several tens of hertz (e.g., <NUM>).

The control unit <NUM> then compares the current measured value iq1 with the predetermined current value J1 (see <FIG>) (step ST3). If the current measured value iq1 is greater than the predetermined current value J1 (step ST3: YES), the flux control unit <NUM> increases the command value cid1 specifying the d-axis current by a predetermined amount Δi (step ST4). That is, the flux control unit <NUM> generates the command value cid1 which specifies a d-axis current greater than the command value cid1 at a time point before the current measured value iq1 is compared with the predetermined current value J1. The predetermined amount Δi is a predetermined fixed value and is stored in advance in, for example, the memory of the microcontroller included in the control unit <NUM>.

The flux control unit <NUM> thereafter performs a limiting process of the command value cid1 (step ST5). The limiting process in this case is specifically a process in which if the command value cid1 is less than a predetermined lower limit value J2 (see <FIG>), the command value cid1 is changed to the predetermined lower limit value J2, and if the command value cid1 is greater than the predetermined upper limit value, the command value cid1 is changed to the predetermined upper limit value. The predetermined upper limit value is <NUM> amperes here. The flux control unit <NUM> outputs the command value cid1 after subjected to the limiting process.

Moreover, if the current measured value iq1 is less than or equal to the predetermined current value J1 (step ST3: NO), the flux control unit <NUM> compares the command value cvq1 specifying the q-axis voltage with the reference voltage (step ST6). The reference voltage is stored in advance in, for example, the memory of the microcontroller included in the control unit <NUM>. If the command value cvq1 is greater than or equal to the reference voltage (step ST6: NO), the flux control unit <NUM> decreases the command value cid1 specifying the d-axis current by the predetermined amount Δi (step ST7) and performs the limiting process (step ST5) to output the command value cid1. When no flux-weakening current flows through the coil <NUM> of the AC motor <NUM>, decreasing the command value cid1 in step ST7 causes a flux-weakening current to be started to flow though the coil <NUM>. That is, the control performed by the control unit <NUM> is switched from the regular control to the field weakening control. After all, the switching condition for starting the field weakening control is the condition that the angular velocity ω1 is greater than or equal to the reference value (step ST1: NO), the current measured value iq1 is less than or equal to the predetermined current value J1 (step ST3: NO), and the command value cvq1 is greater than or equal to the reference voltage (step ST6: NO).

As already described, the reference voltage corresponds to a value obtained by converting the duty threshold of the duty of the PWM control into a voltage. That is, the flux control unit <NUM> determines whether or not the duty of the PWM control is greater than or equal to the duty threshold by comparing the command value cvq1 with the reference voltage. If the command value cvq1 is greater than or equal to the reference voltage, the duty of the PWM control may be regarded to be greater than or equal to the duty threshold.

In step ST6, if the command value cvq1 is less than the reference voltage (step ST6: YES), the flux control unit <NUM> increases the count number by one (step ST8) and compares the count number with the predetermined value (here, <NUM>) (step ST9). The count number as used herein is a count number of the number times of the determination result that the command value cvq1 is less than the reference voltage is obtained. If the count number is less than or equal to <NUM> (step ST9: NO), the flux control unit <NUM> neither increases nor decreases the command value cid1, but the flux control unit <NUM> performs the limiting process (step ST5), thereby outputting the command value cid1. In contrast, if the count number is greater than the <NUM> (step ST9: YES), the count number is initialized to <NUM> (step ST10), and the flux control unit <NUM> increases the command value cid1 by the predetermined amount Δi (step ST11). Thereafter, the limiting process is performed (step ST5), and the command value cid1 is output.

In the following description, contents in steps ST3 to ST11 are summarized. If the first condition that the current measured value iq1 of the q-axis current is greater than the predetermined current value J1 is satisfied, the command value cid1 of the d-axis current is increased from a negative value toward <NUM> (step ST4). This decreases the weakening flux. In contrast, if the second condition that the current measured value iq1 of the q-axis current is less than or equal to the predetermined current value J1 and the command value cvq1 of the q-axis voltage is greater than or equal to the reference voltage is satisfied, the command value cid1 of the d-axis current is decreased from <NUM> or a negative value (step ST7). This increases the weakening flux. Meanwhile, if the third condition that the current measured value iq1 of the q-axis current is less than or equal to the predetermined current value J1 and the command value cvq1 of the q-axis voltage is less than the reference voltage is satisfied, the command value cid1 of the d-axis current is increased from a negative value toward <NUM> each time the third condition is satisfied <NUM> times (step ST11). This decreases the weakening flux. If the count number of times that the third condition is satisfied is less than or equal to <NUM>, the command value cid1 of the d-axis current is maintained.

Thus, the control unit <NUM> has a function of changing the magnitude of the flux-weakening current in the field weakening control. If the second condition (a predetermined increase condition) is satisfied in the field weakening control, the control unit <NUM> performs gradual-increase control of increasing the absolute value of the flux-weakening current over time in step ST7. In contrast, If the third condition (a predetermined decrease condition) is satisfied in the field weakening control, the control unit <NUM> performs gradual-decrease control of decreasing the absolute value of the flux-weakening current over time in steps ST8 to ST11. As used herein, "over time" includes an aspect which is not that the flux-weakening current, which is a control object, is changed in one step, and the flux-weakening current stabilizes at a current value after the change but that the flux-weakening current is changed in a plurality of steps, and the value of the flux-weakening current then stabilizes. In the present embodiment, in step ST4, ST7, or ST11, the command value cid1 of the d-axis current changes by the predetermined amount Δi, and the step ST4, ST7, or ST11 is repeated a plurality of number of times, thereby eventually stabilizing the flux-weakening current. The absolute value of the flux-weakening current gradually changes over time, thereby gradually changing the rotation number of the AC motor <NUM>. This lessens the possibility that a worker using the electric tool <NUM> feels strangeness even when the rotation number is automatically changed by the control performed by the control unit <NUM>.

Here, if the command value cvq1 is less than the reference voltage (if the second condition is satisfied) in step ST6, the control unit <NUM> performs control of increasing (gradual-increase control of) the absolute value of the flux-weakening current (d-axis current) over time in step ST7. In contrast, if the command value cvq1 is greater than or equal to the reference voltage (the second condition is no longer satisfied and the third condition is satisfied) in step ST6, the control unit <NUM> performs control of decreasing (gradual-decrease control of) the absolute value of the flux-weakening current over time in steps ST8 to ST11. As described above, if the count number of times that the third condition is satisfied is less than or equal to <NUM>, the command value cid1 of the d-axis current is maintained. Thus, the rate of change of the command value cid1 in steps ST8 to ST11 is less than the rate of change of the command value cid1 in steps ST4 and ST7. That is, the increase rate of the command value cid1 by the gradual-decrease control in the case of the third condition being continuously satisfied is less than the increase rate of the command value cid1 in the case of the first condition being continuously satisfied and than the decrease rate of the command value cid1 by the gradual-increase control in the case of the second condition being continuously satisfied. In sum, the rate of change of the flux-weakening current in the case of the gradual-decrease control is less than the rate of change of the flux-weakening current in the case of the gradual-increase control. Thus, for example, when the length of a time period during which the command value cvq1 of the q-axis voltage is less than the reference voltage is substantially equal to the length of a time period during which the command value cvq1 of the q-axis voltage is greater than or equal to the reference voltage, the flux-weakening current decreases as viewed in a time scale greater than or equal to a certain time scale. Thus, pulsation (hunting) of the flux-weakening current can be suppressed.

<FIG> shows an example of the transitions of respective parameters of the electric tool <NUM> over time when the AC motor <NUM> is controlled based on the process shown in <FIG>. In <FIG>, "battery current" refers to an output current of the power supply <NUM> of the embodiment, "battery voltage" refers to an output voltage of the power supply <NUM> of the embodiment, and "iq1" refers to the current measured value iq1 in the electric tool <NUM> of the embodiment. Moreover, in <FIG>, "id1" refers to the current measured value id1 in the electric tool <NUM> of the embodiment, and "r1" refers to the rotation number of the AC motor <NUM> of the electric tool <NUM> of the embodiment. Moreover, in <FIG>, "r2" refers to the rotation number of an AC motor of an electric tool according to a comparative example of the electric tool <NUM> of the embodiment. The electric tool according to the comparative example always maintains a state where no weakening flux flows through the AC motor. That is, in the electric tool according to the comparative example, the control performed by the control unit is always the regular control.

In this case, the electric tool <NUM> is assumed to be used as an impact screwdriver or a drill screwdriver for tightening a screw. That is, to the socket <NUM> (see <FIG>) of the electric tool <NUM>, a screwdriver is attached. A worker inserts the screwdriver into a screw at a time point before a time point T0. At the time point T0, a worker gives an operation of pulling the trigger volume <NUM> of the electric tool <NUM>, and thereby, the q-axis current (torque current) starts flowing through the AC motor <NUM>, so that the AC motor <NUM> starts rotating. Thereafter, the rotation number r1 gradually increases in accordance with the manipulative variable indicating how deep the trigger volume <NUM> has been pulled. Here, the manipulative variable indicating how deep the trigger volume <NUM> has been pulled is maximum. Thus, the rotation number r1 increases to an upper limit within an adjustable range. The electric tool of the comparative example is also assumed to be used as an impact screwdriver or a drill screwdriver in a similar manner to the electric tool <NUM> of the embodiment. Also in the electric tool of the comparative example, the rotation number r2 gradually increases in accordance with the manipulative variable indicating ow deep the trigger volume has been pulled and increases to an upper limit within an adjustable range.

At a time point T1, the rotation number r1 reaches the predetermined rotation number R1. Thereafter, at a time point T2, the switching condition described above is satisfied. Thus, in the electric tool <NUM> of the embodiment, the control performed by the control unit <NUM> is switched from the regular control to the field weakening control, so that the d-axis current (flux-weakening current) starts flowing. That is, at and after the time point T2, the current measured value id1 of the d-axis current gradually decreases from <NUM>.

As the screw is screwed into a member, the q-axis current (torque current) required to further tighten the screw may increase. As the q-axis current increases, loss in the interior resistance of the power supply <NUM> (battery) increases, and therefore, the battery voltage decreases. Thus, a reduction in the rotation number r1 due to the decrease in the battery voltage can be compensated by causing the d-axis current (flux-weakening current) to flow. That is, as compared to the case where the d-axis current is not caused to flow, the reduction in the rotation number r1 can be suppressed. In other words, the rotation number r1 of the AC motor <NUM> according to the embodiment is, while the d-axis current flows, higher than the rotation number r2 according to the comparative example.

In the electric tool <NUM> of the embodiment, at a time point T3, the magnitude of the current measured value id1 of the d-axis current is a value close to the predetermined lower limit value J2. By the limiting process (see step ST5), the current measured value id1 of the d-axis current transitions within a range not less than the predetermined lower limit value J2.

At and after a time point T4, the current measured value iq1 of the q-axis current is greater than the predetermined current value J1 (step ST3: YES). Thus, the current measured value id1 of the d-axis current increases from a negative value toward <NUM>. Moreover, around the time point T4, the screw is seated on the member. The predetermined current value J1 is set to a value less than the current measured value iq1 of the q-axis current (torque current) when the screw is seated on the member. That is, when the screw is seated on the member and the torque is relatively large, the current measured value iq1 of the q-axis current exceeds the predetermined current value J1, and in response to this, the control unit <NUM> thus decreases the absolute value of the d-axis current. In other words, the control unit <NUM> decreases the flux-weakening current. As a result, the weakening flux decreases. The decrease of the weakening flux enables the AC motor <NUM> to be driven by increased torque.

Around a time point T5, the magnitude of the flux-weakening current is <NUM>. Moreover, at a time point T6, a worker sets the manipulative variable, indicating how deep the trigger volume <NUM> has been pulled, to <NUM>, so that the AC motor <NUM> stops.

According to the embodiment described above, causing the flux-weakening current to flow through the coil <NUM> of the AC motor <NUM> can increase the rotation number of the AC motor <NUM> as compared to the case where the flux-weakening current is not caused to flow. This can shorten a time required for work such as tightening a screw by the electric tool <NUM>. In addition, the possibility that an increased rotation number may distort the shape of a hole formed in a member as a target of boring by the electric tool <NUM> used as a drill can be lessened.

Moreover, the performance characteristic of the electric tool <NUM> may vary due to variations of an induced electromotive voltage resulting from a production error of the AC motor <NUM> or variations of the voltage (the battery voltage) of the power supply <NUM>. Adjusting the magnitude of the flux-weakening current can correct the variations of the performance characteristics of the electric tool <NUM>.

Moreover, in the electric tool <NUM> of the embodiment, the weakening flux decreases when the current measured value iq1 of the q-axis (torque component) is greater than the predetermined current value J1, and the weakening flux eventually reaches <NUM>. Thus, when the torque of the AC motor <NUM> is relatively large, larger torque can be output.

That is, in a low-velocity range within which the torque of the AC motor <NUM> is relatively large and the rotation number of the AC motor <NUM> is relatively small, decreasing the weakening flux to <NUM> or to a relatively small value, the magnitude of the torque can be secured. In contrast, within a high-velocity range within which the rotation number of the AC motor <NUM> is relatively large (larger than at least the predetermined rotation number R1 (see <FIG>)), causing the flux-weakening current to flow can further increase the rotation number. That is, causing the flux-weakening current to flow enables the AC motor <NUM> to rotate at a further higher rotation number than the upper limit of the rotation number in the case of no flux-weakening current being caused to flow.

Variations of the embodiment will be enumerated below. The variations described below may be accordingly combined with each other.

The electric tool <NUM> may include an operating member configured to receive, for example, an operation of setting parameters relating to operation of the electric tool <NUM>. The operating member may include, for example, a touch panel display configured to receive an operation input and to display information relating to the operation.

The field weakening control and the regular control performed by the control unit <NUM> may be manually changeable by an operation given to an operating member different from the trigger volume <NUM>. For example, the control performed by the control unit <NUM> may be changeable by an operation given to the touch panel display.

In the AC motor <NUM>, the rotor <NUM> may include the coil <NUM>, and the permanent magnet <NUM> may include the stator <NUM>.

The electric tool <NUM> is not limited to the impact screwdriver, the drill screwdriver, or the impact wrench. Alternatively, the electric tool <NUM> may be a screwdriver or a wrench having no impact mechanism <NUM>. Alternatively, the electric tool <NUM> may be a fraise, a grinder, a cleaner, or an electric tool of a kind other than these tools.

Here, "the AC motor <NUM> operates within the high-velocity range" may be defined by that the rotation number of the AC motor <NUM> is greater than or equal to the predetermined rotation number R1 or by that the duty of the PWM control is greater than or equal to the duty threshold.

The flux control unit <NUM> may control such that the increase rate of the command value cid1 of the d-axis current when the first condition is satisfied is greater than the decrease rate of the command value cid1 of the d-axis current when the second condition is satisfied. This lessens the possibility that the d-axis current pulsates. Such a configuration can be embodied by, for example, increasing the command value cid1 of the d-axis current each time the first condition is satisfied one time and decreasing the command value cid1 of the d-axis current each time the second condition is satisfied a plurality of times.

Claim 1:
An electric tool (<NUM>), comprising:
an electric motor (<NUM>) including a permanent magnet (<NUM>) and a coil (<NUM>); and
a control unit (<NUM>) configured to perform control on operation of the electric motor (<NUM>),
the control performed by the control unit (<NUM>) includes field weakening control by which the control unit (<NUM>) causes a flux-weakening current to flow through the coil (<NUM>), the flux-weakening current being a current that generates, in the coil (<NUM>), a magnetic flux that weakens a magnetic flux of the permanent magnet (<NUM>),
wherein
the control unit (<NUM>) has a function of changing a magnitude of the flux-weakening current in the field weakening control,
wherein the electric tool (<NUM>) is characterized in that
the control unit (<NUM>) is configured to, when a predetermined increase condition is satisfied, perform gradual-increase control of increasing an absolute value of the flux-weakening current over time in the field weakening control,
wherein
the control unit (<NUM>) is configured to, when a predetermined decrease condition is satisfied, perform gradual-decrease control of decreasing the absolute value of the flux-weakening current over time in the field weakening control, the predetermined decrease condition being different from the predetermined increase condition, a rate of change of the flux-weakening current in the gradual-decrease control being less than a rate of change of the flux-weakening current in the gradual-increase control.