Patent Description:
As a drive source of an electric tool for driving an end tool, an electric motor using AC power is widely used, as disclosed in <CIT>. In the case of the electric motor using the AC power, compared to an electric tool using a DC motor with a battery, an output is high, and further, a long time operation is possible. On the other hand, when using the AC power, it is required to connect a power supply cord to a socket. Accordingly, available locations may be limited, compared to a portable electric tool with a battery. When the electric tool using the AC power is used in a location with no socket, an extension cord <NUM> wound around a cord reel <NUM> is often used to supply power from a socket <NUM> to an electric tool <NUM>, as shown in <FIG>. In this case, the cord reel <NUM> is placed within a range where a power supply cord <NUM> extending from the electric tool <NUM> can be connected. When observing an actual using site, the extension cord <NUM> having a length of approximately <NUM> or more is often used.

<CIT> relates to an electric power tool which includes a brushless motor having a plurality of stator windings and configured to rotate in accordance with voltages applied to the plurality of stator windings, an induced voltage being generated in accordance with a rotation of the brushless motor, a rectifier circuit configured to rectify an AC voltage, a smoothing capacitor configured to smooth the AC voltage rectified by the rectifier circuit to a pulsation voltage having a maximum value larger than the induced voltage and a minimum value smaller than the induced voltage and an inverter circuit configured to perform switching operations to output the pulsation voltage to the plurality of stator windings by rotation.

When the power supply cord <NUM> is connected to the socket <NUM> through the long extension cord <NUM> as in the related art, in addition to the factors of instability of commercial power supply voltage supplied to the socket <NUM>, the influence of the voltage drop due to the extension cord <NUM> cannot be ignored. Accordingly, there is a problem that the torque or rotation number of the motor of the electric tool <NUM> becomes lower than a rated output. For example, in a case where the commercial power supply of 230V is used and the extension cord <NUM> of <NUM> having a cross-section area of <NUM><NUM> and an AC conductor resistance of <NUM>Ω/km is used, the voltage drop of approximately <NUM> V is caused when current is <NUM> A. Accordingly, the voltage inputted to the electric tool <NUM> is dropped more than <NUM>%, compared to the rated <NUM> V. As the length of the power supply cord used becomes longer, the voltage drop due to the extension cord <NUM> is further increased. Further, the output variation of the electric tool <NUM> is increased in a circumstance where the voltage of the commercial power supply itself is unstable. Even in a configuration that the battery is not directly connected to the electric tool but connected to the electric tool through a cord, the electric tool is affected by the voltage drop due to the cord.

Aspects of the present invention has been made in consideration of the above situations and an object thereof is to provide an electric tool that is capable of maintaining the rated output even when the voltage variation in the power inputted to the electric tool occurs to some extent.

Another object is to provide an electric tool that can be operated in the rated output even when the voltage drop due to the usage of an extension cord occurs, by controlling the duty ratio of a motor to a value less than <NUM>% when the rated power is inputted.

Another object of the present invention is to provide an electric tool that is capable of realizing the motor control that is strong to the input voltage variation of the power without an increase in manufacturing cost.

According to the invention, the problem is solved by the subject matter outlined in independent claim <NUM>. Advantageous further developments of the invention are set forth in the dependent claims.

According to the present invention, it is possible to realize an electric tool where a decrease in a torque or rotation number of a motor can be suppressed and a stable operation can be performed even when the power supply is unstable and thus the voltage variation occurs or even when the voltage drop occurs due to the usage of a cord reel or a power cable, etc.
The foregoing and other objects and features of the present invention will be apparent from the detailed description below and accompanying drawings.

Hereinafter, an illustrative embodiment of the present invention will be described with reference to the drawings. In the following drawings, an impact wrench is used as an example of an electric tool, the same components are denoted by the same reference numerals and a duplicated description thereof is omitted. Further, as used herein, an upper-lower direction, a left-right direction and a front-rear direction are described as directions shown in the drawings.

As shown in <FIG>, an impact wrench <NUM> includes a housing <NUM>, a motor <NUM>, a deceleration mechanism <NUM>, a hammer <NUM>, an anvil <NUM>, a light <NUM>, a control unit <NUM> and a power supply cord <NUM>. An end tool (not shown) is mounted to the anvil <NUM> that is an output shaft. <FIG> shows a mounting part <NUM> to which a hexagon socket as an end tool can be mounted. However, instead of the mounting part <NUM>, a mounting hole and a mounting mechanism may be provided, to which a driver bit with a hexagonal cross-section or other end tools can be mounted in an one-touch method. The housing of the impact wrench <NUM> in a broad sense is configured by the resin housing <NUM> and a metal hammer case <NUM>. The hammer case <NUM> is covered by a resin cover <NUM>. The deceleration mechanism <NUM> and a striking mechanism are accommodated in the interior of the hammer case <NUM>. A leading end of the anvil <NUM> is exposed to the outside from a through hole on a leading end side of the hammer case <NUM>. The housing <NUM> is configured by three parts including a body part 2a, a handle part 2b and a board accommodating part 2c. The body part 2a has a substantially cylindrical shape. The handle part 2b is formed so as to extend in a substantially vertical direction from the body part 2a. The board accommodating part 2c is formed at a lower end portion (on a side apart from the body part 2a) of the handle part 2b. The motor <NUM>, the deceleration mechanism <NUM>, the hammer <NUM> and the anvil <NUM> are arranged in series in a rotation axis direction. Rotation of the motor <NUM> is transmitted to the end tool through the deceleration mechanism <NUM> and the striking mechanism having the hammer <NUM> and the anvil <NUM>. In the present embodiment, an impact wrench including a power transmission mechanism is illustratively described as an example of the electric tool. The power transmission mechanism is composed of the deceleration mechanism <NUM> and the striking mechanism. However, the power transmission mechanism is not limited thereto, and the electric tool may use other power transmission mechanism or other end tool.

The handle part 2b is provided with a trigger <NUM>. The trigger <NUM> is connected to a trigger switch <NUM> accommodated in the handle part 2b to set a rotation speed of the motor <NUM>. A forward/reverse switching switch <NUM> (forward/reverse switching lever <NUM>) is provided immediately above the trigger <NUM>. The forward/reverse switching switch <NUM> is a connection portion of the handle part 2b and the body part 2a and switches the rotation direction of the motor <NUM>. The light <NUM> is an LED (Light Emitting Diode). As a light button (not shown) or the trigger <NUM> is depressed, the light <NUM> is turned on. The light <NUM> illuminates the end tool and its surroundings. In this way, a worker can perform a work by using a bright light of the light <NUM> even in a dark place.

The control unit <NUM> is accommodated in the board accommodating part 2c and controls the rotation of the motor <NUM>. The power supply cord <NUM> is extended at a lower side of the board accommodating part 2c and supplies power from the outside. An operation panel <NUM> is provided at an upper side of the board accommodating part 2c and sets the maximum rotation number of the motor in a plurality of levels. A worker can set the rotation number of the motor in three levels of low, middle and high levels by pressing a button (not shown) on the operation panel <NUM>. The control unit <NUM> includes a control circuit board <NUM> and a power supply circuit board <NUM>, which are mainly accommodated in the board accommodating part 2c. The control unit <NUM> controls the rotation speed of the motor <NUM> by adjusting the amount of power supplied to the motor <NUM> in accordance with an operation amount of the trigger <NUM>. A diode bridge (not shown) is mounted on the power supply circuit board <NUM>. The diode bridge rectifies and converts the commercial power supply into DC current (e.g., converts AC <NUM> V into DC <NUM> V).

The motor <NUM> is a brushless DC motor. The motor <NUM> includes an output shaft <NUM> extending in a longitudinal direction, a rotor <NUM> fixed to the output shaft <NUM> and having a plurality of magnets 32a, and a stator <NUM> disposed so as to surround the rotor <NUM> and having a plurality of coils (stator coils) <NUM>. The motor <NUM> of the present embodiment is a <NUM>-phase <NUM>-pole <NUM>-slot motor. However, the number of the poles and the number of the slots are not limited thereto. The brushless DC motor having other number of poles and slots may be used. A cooling fan <NUM> is provided at a front side of the motor <NUM> of the output shaft <NUM> and between the motor <NUM> and the deceleration mechanism <NUM>. The cooling fan <NUM> receives air from an intake port 23a (see <FIG>) by rotating in synchronization with the motor <NUM>. The cooling fan <NUM> allows the air to pass through each part of the motor <NUM>, thereby cooling each part. Then, the cooling fan <NUM> allows the air to be discharged to the outside from an exhaust port 23b (see <FIG>) to be described later. A circuit board <NUM> is provided at a rear side in an axial direction of the motor <NUM>. The circuit board <NUM> is provided for mounting an inverter in a direction substantially perpendicular to an axial direction of the output shaft <NUM>. The circuit board <NUM> is a substantially circular double-sided board. The contour of the circuit board <NUM> is substantially the same as the contour of the motor <NUM>. A switching element <NUM> made of a semiconductor such as a FET (Field Effect Transistor) or a position detection element (not shown) such as a hall IC is mounted on the circuit board.

The deceleration mechanism <NUM> is configured by a planetary gear mechanism having a plurality of gears. The deceleration mechanism <NUM> allows the rotation of the output shaft <NUM> to be decelerated at a predetermined reduction ratio and to be transmitted to a spindle <NUM>. Here, the spindle <NUM> and the hammer <NUM> are connected to each other by a cam mechanism. The cam mechanism is configured by a V-shaped spindle cam groove formed in an outer peripheral surface of the spindle <NUM>, a hammer cam groove formed in an inner peripheral surface of the hammer <NUM>, and a ball <NUM> engaged with these cam grooves. The hammer <NUM> is rotated by the spindle <NUM> and provided at its front end with a colliding portion <NUM>. The colliding portion <NUM> is projected axially forward in a convex shape. The anvil <NUM> is provided at its rear end with a collided portion <NUM>. The collided portion <NUM> is extended radially in a concave shape. As the hammer <NUM> is rotated, the colliding portion <NUM> collides with the collided portion <NUM> in a rotation direction. The colliding portion <NUM> and the collided portion <NUM> are symmetrically formed at two locations on a rotation plane of the hammer <NUM> and the anvil <NUM>, which are opposed to each other. The hammer <NUM> is always urged forward by a spring <NUM>. During the stop of the hammer, the engagement of the ball <NUM> and the cam grooves allows the hammer <NUM> to be positioned at a position that is separated from an end surface of the collided portion <NUM> of the anvil <NUM> by a gap.

When the spindle <NUM> is rotationally driven, the rotation of the spindle <NUM> is transmitted to the hammer <NUM> through the cam mechanism. -Until the hammer <NUM> is rotated a half turn, the colliding portion <NUM> of the hammer <NUM> is engaged with the collided portion <NUM> of the anvil <NUM> to rotate the anvil <NUM>. When the relative rotation of the spindle <NUM> and the hammer <NUM> occurs by an engagement reaction force at that time, the hammer <NUM> starts to retreat toward the motor <NUM> while compressing the spring <NUM> along the spindle cam groove of the cam mechanism. Further, the retreat movement of the hammer <NUM> allows the colliding portion <NUM> of the hammer <NUM> to go beyond the collided portion <NUM> of the anvil <NUM> and thus the engagement between the hammer <NUM> and the anvil <NUM> is released. In this case, the hammer <NUM> is moved by an urging force of the spring <NUM> while being rapidly accelerated in a rotation direction and forward by the action of the cam mechanism and the elastic energy accumulated in the spring <NUM>, in addition to the rotational force of the spindle <NUM>. The colliding portion <NUM> of the hammer <NUM> is re-engaged with the collided portion <NUM> of the anvil <NUM>, thereby starting to rotate integrally therewith. At this time, a strong rotary striking force is applied to the anvil <NUM>. Accordingly, the strong rotary striking force is transmitted to an end tool (not shown) that is mounted to the mounting part <NUM> at a leading end of the anvil <NUM>. Thereafter, the same operation is repeated, so that the rotary striking force is intermittently and repeatedly transmitted to a bolt or the like from the end tool. In this way, the bolt or the like is tightened.

<FIG> is a side view showing an appearance of the impact wrench <NUM> according to the illustrative embodiment of the present invention. The housing <NUM> is configured by two split housings that can be provided at the left and right in the longitudinal direction. The left and right housings <NUM> are fixed to each other by a plurality of screws <NUM>. A plurality of intake ports 23a for taking the outside air is formed at a rear end surface of the body part 2a. A plurality of exhaust ports 23b for discharging the outside air taken into the housing <NUM> is formed at both left and right sides of the body part 2a and in the vicinity of the peripheries of the cooling fan <NUM> (see <FIG>).

<FIG> is a rear view of the impact wrench <NUM> according to the illustrative embodiment of the present invention. The intake ports 23a provided at the back side of the body part 2a are provided at the left and right housings <NUM>, respectively. The intake ports 23a can be relatively large openings so as to take a sufficient amount of outside air. The shape of the body part 2a of the housing <NUM> is a cylindrical shape whose inner wall substantially conforms to an outer diameter of the motor <NUM>. Below the body part 2a, the handle part 2b having a diameter smaller than that of the body part 2a is arranged so as to extend downward.

Next, a configuration and operation of a drive control system of the motor <NUM> will be described with reference to <FIG> is a block diagram showing the configuration of the drive control system of the motor. In the present embodiment, the motor <NUM> is configured by a three-phase brushless DC motor. The motor <NUM> is a so-called inner rotor type and includes the rotor <NUM>, three position detection elements <NUM> and the stator <NUM>. The rotor <NUM> is configured by embedding the magnet 32a (permanent magnet) having a pair of N-pole and S-pole therein. The position detection elements <NUM> are arranged at an angle of every <NUM>° so as to detect the rotation position of the rotor <NUM>. The stator <NUM> is composed of star-connected three-phase windings U, V, W. The three-phase windings U, V, W are controlled at the current energization interval of <NUM>° electrical angle on the basis of position detection signals from the position detection elements <NUM>.

An inverter circuit <NUM> that is mounted on the circuit board <NUM> is mainly configured by six FETs (hereinafter, simply referred to as "transistor") Q1 to Q6 and a flywheel diode (not shown). The six FETs Q1 to Q6 are connected in three-phase bridge form. As an input power supply to the inverter circuit <NUM>, the present embodiment uses direct current supplied from a rectifier circuit <NUM> where an AC power supply <NUM> is employed as a diode bridge. Typically, a smoothing capacitor or the like is used in the rectifier circuit <NUM> so as to reduce the ripple included in the output wave. However, in the present embodiment, the output of the rectifier circuit <NUM> is directly supplied to the inverter circuit <NUM> without interposing a capacitor. Of course, the smoothing capacitor may be used. Each gate of the six transistors Q1 to Q6 connected in the bridge type is connected to a control signal output circuit <NUM>. Further, a source or drain of the six transistors Q1 to Q6 is connected to the star-connected armature windings U, V, W. Thereby, the six transistors Q1 to Q6 perform a switching operation by a switching element driving signal that is outputted from the control signal output circuit <NUM>. The six transistors Q1 to Q6 supply power to the armature windings U, V, W by using DC voltage applied to the inverter circuit <NUM> as the three-phase (U-phase, V-phase, W-phase) AC voltages Vu, Vv, Vw.

A control circuit (controller) is mounted on the control circuit board <NUM>. The control circuit is configured by an operation unit <NUM>, a current detection circuit <NUM>, a voltage detection circuit <NUM>, an applied voltage setting circuit <NUM>, a rotation direction setting circuit <NUM>, a rotor position detection circuit <NUM>, and the control signal output circuit <NUM>, etc. The operation unit <NUM> includes a CPU 84a for outputting a drive signal based on a processing program and data, a ROM 84b for storing a control data or program corresponding to a flowchart (to be described later), a RAM 84c for temporarily storing data, and a timer 84d, etc. For example, the operation unit <NUM> can be realized by using a microcomputer where the above parts are incorporated. The current detection circuit <NUM> is a current detection means for detecting the current inputted to the inverter circuit <NUM> by measuring the voltage across a shunt resistor <NUM>. The detected current is inputted to the operation unit <NUM>. In this way, the operation unit <NUM> can monitor the value of current flowing through the motor <NUM>. In the present embodiment, the shunt resistor <NUM> is provided between the rectifier circuit <NUM> and the inverter circuit <NUM>, thereby detecting the value of current flowing through a semiconductor switching element. However, the shunt resistor <NUM> may be provided between the inverter circuit <NUM> and the motor <NUM>, thereby directly detecting the current flowing through the motor <NUM>.

The applied voltage setting circuit <NUM> inputs a predetermined voltage to the operation unit <NUM>, in response to a movement stroke of the trigger <NUM>. The operation unit <NUM> sets the voltage applied to the motor <NUM>, that is, a duty ratio of PWM signal, in accordance with the predetermined voltage. The rotation direction setting circuit <NUM> is a circuit for setting the rotation direction of the motor <NUM> by detecting a forward rotation operation or a reverse rotation operation by the forward/reverse switching lever <NUM> (forward/reverse switching switch <NUM>) of the motor <NUM>. The rotor position detection circuit <NUM> is a circuit for detecting a positional relationship between the rotor <NUM> and the armature windings U, V, W of the stator <NUM> based on output signals of the three position detection elements <NUM>. The control signal output circuit <NUM> supplies PWM signal to the transistors Q1 to Q6 based on the output from the operation unit <NUM>. The power supplied to each of the armature windings U, V, W is adjusted by the control of a pulse width of the PWM signal, so that the rotation number of the motor <NUM> in the rotation direction set can be controlled. Although not shown in <FIG>, an output signal of a rotation setting switch of the motor <NUM> is inputted to the operation unit <NUM> and adapted to switch a torque value (or the rotation number of the motor). The operation unit <NUM> serves as a rotational speed setting unit for setting the maximum rotation number of the motor <NUM> in accordance with the output signal. This refers to the following control method. Specifically, in a case where an initial duty ratio D0 is set to <NUM>%, the duty ratio of the motor <NUM> is controlled in <NUM> to <NUM>% during the rotation number of "high level," the duty ratio of the motor <NUM> is controlled in <NUM> to <NUM>% during the rotation number of "middle level," and the duty ratio of the motor <NUM> is controlled in <NUM> to <NUM>% during the rotation number of "low level. " Further, although not shown in <FIG>, a lighting circuit is connected to the operation unit <NUM> to control the lighting of the light <NUM> by an LED or the like for illuminating the vicinity of the end tool.

Next, a rotation control procedure of the motor <NUM> according to the illustrative embodiment of the present invention will be described with reference to <FIG> and <FIG>. The most essential feature of the present embodiment is that a high-output motor capable of achieving higher rotation number and torque than the rated output required is used as the motor <NUM>, and in a case where the power supply voltage is a rated value, even at the maximum output setting of the electric tool, an operation is carried out in a state of suppressing the output of the high-output motor. <FIG> are diagrams for explaining a relationship between a duty ratio (reference duty ratio), and no-load input voltage V0 and load voltage V1. The duty ratio is set and used for the rotation control of the motor <NUM>. The operation unit <NUM> respectively measures the no-load input voltage V0 immediately before the pulling of the trigger <NUM> (or, from the pulling of the trigger <NUM> to the start of the motor <NUM>) and the load voltage V1 after the start of the motor <NUM>, by using the voltage detection circuit <NUM>. <FIG> is a diagram showing a relationship between the no-load input voltage V0 and the initial duty ratio D0. Here, "the duty ratio" refers to a ratio of a period of a pulse wave and a pulse width in a PWM (Pulse Width Modulation) control. The PWM control performs modulation by changing the duty ratio of a pulse wave supplied to the motor <NUM>. Normally, the duty ratio D can be represented by D = τ/T (here, τ: pulse width, T: period). In a general electric tool, when the electric tool is operated at the maximum output by using the rated AC power supply, the motor <NUM> is driven in the duty ratio of <NUM>%, i.e., the motor <NUM> is operated without a pulse modulation. Therefore, the type of the motor <NUM> to be used is selected to suit the rated power supply and the required rated output. However, in the present embodiment, the motor <NUM> to be used has a characteristic that the motor is operated in the reference duty ratio D of about <NUM>% during the rated voltage. This means that the motor becomes overspeed and cannot withstand thermally when the motor <NUM> is driven in the duty ratio D of <NUM>% at the rated voltage. In order to achieve such a motor characteristic, it is sufficient that the windings of the coil of the motor <NUM> are reduced and a thick winding is used to allow a large current. Therefore, it does not lead to the large size of the motor <NUM> itself and cost increase.

In the present embodiment, in the impact wrench <NUM> of rated voltage alternating current <NUM> V (<NUM>), as shown in <FIG>, the duty ratio (initial duty ratio) D0 in the initial setting is set to <NUM>% when the measured no-load input voltage V0 is equal to or greater than <NUM> V. The duty ratio (initial duty ratio) D0 in the initial setting is set to <NUM>% when the no-load input voltage V0 is less than <NUM> V. In this way, the motor <NUM> is started. Large current flows through the motor <NUM> when the motor <NUM> is started. Accordingly, the voltage drop occurs when the extension cord <NUM> shown in <FIG> is used or when the power supply voltage is not stable. In the present embodiment, the power supply voltage after the start of the motor <NUM> (load voltage V1) is measured and the duty ratio D1 during the rotation is adjusted in accordance with the load voltage V1. <FIG> shows an example thereof. When the load voltage V1 is equal to or greater than <NUM> V, the duty ratio D1 is still <NUM>%. When the load voltage V1 is equal to or greater than <NUM> V but less than <NUM> V, the duty ratio D1 becomes <NUM>%. When the load voltage V1 is less than <NUM> V, the duty ratio D1 becomes <NUM>%. In order to change the duty ratios D0, D1 in accordance with the no-load input voltage V0 and the load voltage V1 in this way, the combinations thereof are previously stored as parameters in the ROM 84b of the operation unit <NUM> and the motor <NUM> is controlled by using the parameters, so that it is possible to supplement the output reduction of the impact wrench <NUM> due to the variation of the AC voltage. Here, <NUM> V may be set based on a predetermined range including the rated voltage <NUM> V. In the present embodiment, it is determined whether or not the load voltage is equal to or greater than <NUM> V based on the range of ±<NUM> V from the rated voltage <NUM> V. However, the value of the reference voltage may be set as appropriate.

<FIG> is a flowchart for explaining a rotation control procedure of the motor according to the illustrative embodiment of the present invention. This flowchart is started when the power supply cord <NUM> of the impact wrench <NUM> is connected to the socket <NUM> or the cord reel <NUM> and the microcomputer included in the operation unit <NUM> is thus activated. A computer program that is previously stored in the ROM 84b is executed by the CPU 84a, so that a series of steps shown in <FIG> can be realized through software. First, as an initial setting after the power supply is turned on, the operation unit <NUM> sets the values of the initial duty ratio D0 and the load duty ratio D1 to <NUM>% (Step <NUM>). Subsequently, the operation unit <NUM> detects the no-load input voltage V0 from the output of the voltage detection circuit <NUM> (Step <NUM>). Subsequently, the operation unit <NUM> determines whether or not the no-load input voltage V0 measured is equal to or greater than <NUM> V (Step <NUM>). Here, when it is determined that the no-load input voltage V0 is equal to or greater than <NUM> V, the operation unit <NUM> sets the duty ratio D0 in the initial setting to <NUM>% by using the values stored in the table of <FIG> (Step <NUM>). When it is determined that the no-load input voltage V0 is less than <NUM> V, the operation unit <NUM> sets the duty ratio D0 in the initial setting to <NUM>% (Step <NUM>). Subsequently, the operation unit <NUM> detects whether or not the trigger <NUM> is pulled by a worker (Step <NUM>). When it is detected that the trigger <NUM> is not pulled, the procedure returns to Step <NUM>. When it is detected in Step <NUM> that the trigger <NUM> is pulled, the motor <NUM> starts to rotate (Step <NUM>). In the trigger switch <NUM> of the present embodiment, a variable switch is used and the rotation number of the motor <NUM> is changed in proportional to the pulled amount of the trigger. Accordingly, the duty ratio D0 set has a value (maximum value) when the trigger <NUM> is pulled to the maximum. When the pulled amount of the trigger <NUM> is small, for example, when the trigger <NUM> is pulled about a half, the operation unit <NUM> sets the duty ratio in accordance with the pulled state of the trigger. For example, the operation unit <NUM> adjusts the duty ratio within the range of <NUM> to <NUM>%. Meanwhile, in the case of the electric tool where the trigger <NUM> is not a variable capacitance switch but an ON/OFF switch, the electric tool is driven in the duty ratio D0 set when the trigger switch is turned on.

Next, once the motor <NUM> is activated, the operation unit <NUM> detects the load voltage V1 at that time from the output of the voltage detection circuit <NUM> (Step <NUM>). Subsequently, the operation unit <NUM> determines whether or not the detected load voltage V1 is equal to or greater than <NUM> V (Step <NUM>). When it is determined that the load voltage V1 is equal to or greater than <NUM> V, the operation unit <NUM> resets the duty ratio D1 during the rotation to <NUM>% and the procedure proceeds to Step <NUM> (Step <NUM>). When it is determined in Step <NUM> that the load voltage V1 is less than <NUM> V, the operation unit <NUM> determines whether or not the load voltage V1 is equal to or greater than <NUM> V (Step <NUM>). When it is determined that the load voltage V1 is equal to or greater than <NUM> V, the operation unit <NUM> resets the duty ratio D1 during the rotation to <NUM>% and the procedure proceeds to Step <NUM> (Step <NUM>). When it is determined in Step <NUM> that the load voltage V1 is less than <NUM> V, the operation unit <NUM> resets the duty ratio D1 during the rotation to <NUM>% and the procedure proceeds to Step <NUM> (Step <NUM>). The operation unit <NUM> switches the duty ratio D1 to the reset value and resumes the rotation control of the motor (Step <NUM>). When the trigger switch <NUM> is still turned on, the procedure returns to Step <NUM> (Step <NUM>). When the trigger switch <NUM> is turned off, the motor is stopped and the procedure returns to Step <NUM> (Step <NUM>).

By the above procedures, the operation unit <NUM> performs the rotation control of the motor <NUM> in the range of <NUM> to <NUM>% of the maximum value of the duty ratio. Even when the AC power supply supplied to the impact wrench <NUM> drops below the rated value by performing the control in this way, the output is compensated by increasing the duty ratio of the motor <NUM> to be controlled and therefore the rated output of the impact wrench <NUM> can be secured. In the present embodiment, the maximum value of the duty ratio is varied in the range of <NUM> to <NUM>%. However, in the case where the range is controlled in the range of <NUM> to <NUM>%, it is possible to maintain the rated output even when the voltage to be supplied drops to about 200V. Further, instead of driving the motor <NUM> while adjusting the duty ratio D1 determined by using the voltage V1 measured immediately after the pulling of the trigger <NUM> until the trigger <NUM> is returned, the operation unit <NUM> may be configured so that the procedure does not return to Step <NUM> in Step <NUM> but is in a standby state. In this way, the duty ratio D1 is not be switched while the electric tool is operated.

Next, a rotation control procedure of a motor according to a second embodiment of the present invention will be described with reference to the flowchart of <FIG>. The basic control procedure in the second embodiment is the same as in the first embodiment. However, the measuring procedure for the no-load input voltage V0 in the second embodiment is different from the first embodiment. That is, instead of periodically measuring the no-load input voltage V0 when the power supply voltage is connected, the no-load input voltage V0 is measured for a short time immediately after the pulling of the trigger and before the rotation start of the motor <NUM>. Here, a state of the motor <NUM> before the start refers to a state before current flows through the motor <NUM> and a state of the motor <NUM> after the start refers to a state where current flows through the motor <NUM>. First, the power supply cord <NUM> of the impact wrench <NUM> is connected to the socket <NUM> or the cord reel <NUM>, so that the microcomputer included in the operation unit <NUM> is started. Further, as an initial setting, the values of the initial duty ratio D0 and the load duty ratio D1 are set to <NUM>% (Step <NUM>). Further, the value of the counter n for the measurement of the number of times of sampling is cleared to zero. Subsequently, the operation unit <NUM> determines whether or not the trigger switch <NUM> is turned on. When it is determined that the trigger switch is turned on, the procedure proceeds to Step <NUM>. When it is determined that the trigger switch is not turned on, the procedure is in a standby state (Step <NUM>). In Step <NUM>, it is determined whether or not a sampling time T1 has elapsed after the turn on of the trigger switch <NUM> or from the time previously measured. The procedure is in a standby state until the sampling time T1 has elapsed (Step <NUM>). When the sampling time T1 has elapsed, the operation unit <NUM> measures the no-load input voltage V0 and adds the value of the counter n (Step <NUM>). Subsequently, the operation unit <NUM> determines whether or not the number n of times of sampling reaches a predetermined value. When it is determined that the number n of times of sampling does not reach the predetermined value, the procedure returns to Step <NUM> (Step <NUM>). When the number n of times of sampling is equal to N in Step <NUM>, the operation unit <NUM> calculates an average value of the no-load input voltage V0 that is measured by N times (Step <NUM>). As such, in the second embodiment, the no-load input voltage V0 is measured by a predetermined number of times of sampling and then the average voltage is calculated. In this way, the influence of the voltage variation or noise, etc., can be effectively prevented, so that the measurement accuracy can be improved. In addition, a method of using a peak voltage (highest value) may be used as a method of calculating the no-load input voltage V0. A method of calculating an average value from the data excluding the highest and lowest values may be used or other calculation methods may be used.

Next, the operation unit <NUM> determines whether or not the measured no-load input voltage V0 is equal to or greater than <NUM> V (Step <NUM>). When it is determined that the no-load input voltage V0 is equal to or greater than <NUM> V, the operation unit <NUM> sets the duty ratio D0 in the initial setting to <NUM>% by using the values stored in the table of <FIG> (Step <NUM>). When it is determined that the no-load input voltage V0 is less than <NUM> V, the operation unit <NUM> sets the duty ratio D0 in the initial setting to <NUM>% (Step <NUM>). Subsequent procedures are the same as a step group <NUM> (Steps <NUM> to <NUM>) shown in <FIG> and a duplicated description thereof is thus omitted. According to the second embodiment, the no-load input voltage V0 is measured immediately after the pulling of the trigger and before the start of the motor, so that it is possible to accurately carry out the voltage measurement. Further, the motor <NUM> can be started in a suitable duty ratio and excessive voltage can be prevented from being applied to the motor <NUM>.

Next, a rotation control procedure of a motor according to a third embodiment of the present invention will be described with reference to the flowchart of <FIG>. As shown by a step group <NUM>, the control before the start of the motor in the third embodiment is the same as in the second embodiment. However, in Step <NUM>' of the third embodiment, an operation of assigning <NUM>(Ω) to Z is executed, as an impedance value Z0 for computing the voltage drop of the extension cord. This value may be previously stored in the ROM 84b of the operation unit <NUM>. The other steps are the same as the steps <NUM> to <NUM> in the second embodiment and a duplicated description thereof is thus omitted. When the average value of the no-load input voltage V0 sampled by the number of N times after the pulling of the trigger is calculated by the step group <NUM>, the duty ratio D0 (%) is computed by using the formula (<NUM>) (Step <NUM>). <MAT> (Here, D0=<NUM> when the computed value of the duty ratio D0 exceeds <NUM>).

Since the duty ratio is calculated by the equation in this way, it is possible to finely respond to the voltage variation. Accordingly, the motor <NUM> can be started in an optimal duty ratio D0, compared to the first and second embodiments.

Next, the rotation of the motor <NUM> is started as a worker pulls the trigger <NUM> (Step <NUM>). Subsequently, the operation unit <NUM> detects the load voltage V1 at that time from the output of the voltage detection circuit <NUM> when the motor <NUM> is activated. However, in order to perform the detection at timing when a predetermined time T2 has elapsed from the start of the motor, the operation unit <NUM> is in a standby state until the predetermined time T2 has elapsed (Step <NUM>). For example, the predetermined time T2 may be about <NUM>. When the predetermined time T2 has elapsed from the start of the motor <NUM>, the operation unit <NUM> detects the load voltage V1 at that time from the output of the voltage detection circuit <NUM> and simultaneously detects the current Iin at that time from the output of the current detection circuit <NUM> (Step <NUM>). Subsequently, the operation unit <NUM> calculates an expected tool applied voltage V2 applied to the electric tool by using the formula (<NUM>) (Step <NUM>).

Subsequently, the operation unit <NUM> calculates the duty ratio D1 by using the calculated V2 by the formula (<NUM>) (Step <NUM>).

The operation unit <NUM> switches the duty ratio to the duty ratio D1 obtained by the calculation to continuously perform the rotation control of the motor (Step <NUM>). The operation unit <NUM> causes the procedure to return to Step <NUM> when the trigger switch <NUM> is still turned on (Step <NUM>). The operation unit <NUM> stops the motor and then causes the procedure to return to Step <NUM>' when the trigger switch <NUM> is turned off (Step <NUM>).

According to the third embodiment, the no-load input voltage V0 is measured immediately after the pulling of the trigger and before the start of the motor, so that it is possible to accurately carry out the voltage measurement. Accordingly, the motor <NUM> can be started in a suitable duty ratio. Furthermore, immediately after the start of the motor <NUM>, the expected tool applied voltage V2 is calculated in consideration of the voltage drop situation of the power supply cord. Then, the optimal duty ratio D1 is set on the basis of the expected tool applied voltage V2. The motor <NUM> can be optimally controlled and operated at constant output even in a poor environment where the voltage drop is large.

In Step <NUM>, a PID control may be executed by using a transfer function G. In this case, the proportional gain Kp is equal to V2/V1 and the transfer function G is calculated from the following formula (<NUM>).

Here, Tis(sec): Integration time
Tds(sec): Derivative time.

Here, the integration time and the derivative time are parameters and set to suit the actual operation of the motor. Then, D1 is calculated from D1 = G × D0 and the motor control is performed.

Next, a rotation control procedure of a motor according to a fourth embodiment of the present invention will be described with reference to the flowchart of <FIG>. The control for the steps (step group <NUM>) after the start of the motor <NUM> in the fourth embodiment is the same as in the third embodiment. However, the control before the start of the motor <NUM> in the fourth embodiment is different from the control in the third embodiment. First, the power supply cord <NUM> of the impact wrench <NUM> is connected to the socket <NUM> or the cord reel <NUM>, so that the microcomputer included in the operation unit <NUM> is started. Further, as an initial setting, the values of the initial duty ratio D0 and the load duty ratio D1 are set to <NUM>%. Simultaneously, as an impedance value Z0 for computing the voltage drop of the extension cord, <NUM>(Ω) is assigned to Z (Step <NUM>). The initial values of these parameters may be previously stored in the ROM 84b of the operation unit <NUM>. Subsequently, in order to measure the no-load input voltage V0 at timing when a predetermined time T1 has elapsed from the start of the microcomputer, the operation unit <NUM> is in a standby state until the predetermined time T1 has elapsed (Step <NUM>). Subsequently, the operation unit <NUM> detects the no-load input voltage V0 at that time from the output of the voltage detection circuit <NUM> when the predetermined time T1 has elapsed (Step <NUM>). Then, the operation unit <NUM> calculates the duty ratio D0 in the initial setting from the formula (<NUM>) (Step <NUM>).

When the calculation is completed, the operation unit <NUM> detects whether or not the trigger switch <NUM> is turned on. When it is detected that the trigger switch <NUM> is turned on, the procedure proceeds to step group <NUM>. When it is detected that the trigger switch <NUM> is still turned off, the procedure returns to Step <NUM>. Control procedures in the step group <NUM> are the same as in the third embodiment shown in <FIG> and a duplicated description thereof is thus omitted.

As such, in the fourth embodiment, the measuring procedure for the no-load input voltage V0 is simplified compared to the third embodiment. Further, in the control after the start of the motor <NUM>, the duty ratio D1 is finely controlled. Accordingly, the electric tool such as the impact wrench can be stably used in a stable state even in an environment where the power supply voltage is unstable and the voltage variation is large.

Hereinabove, the present invention has been described with reference to the illustrative embodiments.

In the above-described illustrative embodiments, an electric tool to which a commercial power supply is supplied from the outside by a power supply cord has been illustratively described. However, the present teachings can be similarly applied to an electric tool to which direct current is supplied from the outside or a battery. Further, in the electric tool of the above-described illustrative embodiments, the impact wrench has been illustratively described. However, the present invention is not limited to the impact wrench but can be similarly applied to an impact driver, a driver drill, an electric circular saw, a hammer drill or any other electric tool using a motor as a power source.

This application claims priority from <CIT>.

Claim 1:
An electric tool (<NUM>) comprising:
a motor (<NUM>) configured to be driven by a PWM control of a semiconductor switching element (Q1-Q6) by using a power supply (<NUM>), the power supply (<NUM>) being an AC power supply (<NUM>) supplied from the outside through a power supply cord (<NUM>);
a power transmission mechanism (<NUM>, <NUM>, <NUM>) configured to transmit a rotation of the motor (<NUM>) to an end tool so as to drive the end tool; and
a controller (<NUM>) configured to control a rotation of the motor (<NUM>) by a duty ratio of a PWM signal of the PWM control,
characterized in that the electric tool (<NUM>) further comprises an extension cord (<NUM>) configured to connect between the power supply and the power supply cord (<NUM>) and that the controller (<NUM>) is configured to set the duty ratio
to a reference duty ratio less than <NUM>%, when a voltage of the power supply (<NUM>) applied to the electric tool (<NUM>) is lower than a rated voltage of the electric tool (<NUM>) and within a predetermined range including a rated voltage of the electric tool (<NUM>), and
the controller (<NUM>) is configured to drive the motor (<NUM>) by setting the duty ratio to be higher than the reference duty ratio, when the voltage of the power supply (<NUM>) applied to the electric tool (<NUM>) is lower than the predetermined range including the rated voltage by a voltage drop which occurs due to the extension cord (<NUM>) connected between the power supply (<NUM>) and the power supply cord (<NUM>).