Electric tool switch mechanism

An electric tool switch mechanism includes a trigger switch for operating to at least rotate or stop a DC motor; and a power switch connected between the DC motor and a battery power source and closed when a pressed amount of the trigger switch exceeds a predetermined threshold value. The mechanism further includes a switching device connected between the DC motor and the battery power source via the power switch; a control circuit for controlling to turn the switching device on/off using a desired on-duty when the pressed amount of the trigger switch exceeds the threshold value; and a stopping unit that is controlled by the trigger switch for forcibly turning the switching device off before the power switch is changed from on to off when the trigger switch is released.

FIELD OF THE INVENTION

The present invention relates to an electric tool switch mechanism that controls a rotational speed of an output shaft depending on a pressed amount of a trigger switch.

BACKGROUND OF THE INVENTION

A conventional electric tool switch mechanism includes a power-source contact switch connected in series with a DC motor and a battery power source, a trigger switch for determining a rotational speed of the DC motor depending on a pressed amount, a semiconductor switching device connected in series with the DC motor and the battery power source via the power-source contact switch, a second contact switch connected in parallel with the semiconductor switching device, and a control circuit for receiving a power voltage via the power-source contact switch and controlling on/off of the semiconductor switching device (see Japanese Patent Laid-open Application No. Hei 6-14576, pages 4 and 5 and FIG. 3).

In the electric tool switch mechanism, when the trigger switch is slightly pressed, the power-source contact switch is on and an operation voltage is supplied to the control circuit. In this case, the control circuit increases or decreases an on-duty of the semiconductor switching device depending on the pressed amount of a trigger switch in order to rotate the DC motor at a speed dependent on the pressed amount. When the trigger switch is pressed at its maximum, the second contact switch becomes on and bypasses the semiconductor switching device so that the power voltage is directly supplied to the DC motor. This prevents any possible loss incurred by internal resistance of the semiconductor switching device.

When the trigger switch is released to stop the rotation of the DC motor and the power-source contact switch is changed from on to off in the state where the semiconductor switching device remains powered on, an arc is generated at the power-source contact switch and the lifetime of the switch contact is reduced. Accordingly, in the conventional electric tool switch mechanism, when the pressed amount of the trigger switch is below a predetermined reference value, the on-duty of the semiconductor switching device is made 0% before the power-source contact switch is changed from on to off, so that the power-source contact switch is changed from on to off in the state where the semiconductor switching device is off.

When the electric tool switch mechanism is continuously used in the state where the trigger switch is fully pressed, the battery is over-discharged. Accordingly, a voltage detecting circuit for detecting the battery voltage is provided, and a central processing unit (CPU) in the control circuit forcibly turns the switching device off when the battery voltage detected by the voltage detecting circuit is below the predetermined reference value, in order to prevent over-discharge of the battery. However, as a user fully presses the trigger switch, the second contact switch connected in parallel with the semiconductor switching device is closed such that discharge current flows through the second contact switch, thereby over-discharging the battery.

Furthermore, in the case where the control circuit increases or decreases the on-duty of the semiconductor switching device depending on the pressed amount of a trigger switch, the on-duty of the semiconductor switching device is obtained by converting the pressed amount of the trigger switch into a voltage value, averaging it, removing noise from the voltage value, and performing operation on the voltage value. This increases an operation time in the CPU of the control circuit. Accordingly, even though the power-source contact switch is changed from on to off as the trigger switch is released, the on-duty of the semiconductor switching device may not be made 0% due to voltage measurement or operation delay. If the semiconductor switching device remains turned on when the power-source contact switch is changed from on to off, the switch contact opens in the state where current is flowing through the power-source contact switch, thereby generating an arc and reducing the lifetime of the contact.

SUMMARY OF THE INVENTION

The present invention is made in light of the aforementioned problems. It is an object of the present invention to provide an electric tool switch mechanism capable of preventing the generation of an arc when a contact opens and preventing over-discharge of a battery power source.

In accordance with the present invention, there is provided an electric tool switch mechanism including a trigger switch for operating to at least rotate or stop a DC motor; a power switch connected between the DC motor and a battery power source and closed when a pressed amount of the trigger switch exceeds a predetermined threshold value; a switching device connected between the DC motor and the battery power source via the power switch; a control circuit for controlling to turn the switching device on/off using a desired on-duty when the pressed amount of the trigger switch exceeds the threshold value; and a stopping unit controlled by the trigger switch for forcibly turning the switching device off before the power switch is changed from on to off when the trigger switch is released.

Accordingly, when the trigger switch is fully pressed, the supply of a power voltage to the DC motor can be certainly blocked by turning the switching device off because there is no switch for bypassing the switching device. For example, the supply of a power voltage to the DC motor can be certainly blocked by turning the switching device off when overcurrent is detected, thereby preventing over-discharge of the battery. In addition, the stopping unit interlocks with the trigger switch to forcibly turn the switching device off before the power switch is changed from on to off. Thus, the power switch can be changed from on to off in the state where the switching device is off, i.e., the current does not flow through the DC motor. As a result, the generation of an arc can be prevented and the lifetime of the contact can increase.

In accordance with the present invention, it may be preferable that the stopping unit includes a changeover switch that is controlled by the trigger switch, and the changeover switch is configured to change, before the power switch is changed from on to off, a connection of a control terminal of the switching device, which is for receiving a control voltage from the control circuit, from an output of the control circuit to a circuit ground.

Accordingly, since the changeover switch as the stopping unit connects the control terminal of the switching device to the ground of the circuit before the power switch is changed from on to off, the control voltage input from the control circuit to the control terminal of the switching device can be discharged in a short time so that the switching device is forcibly turned off. This allows the power switch to be changed from on to off in the state where the switching device is certainly turned off so that the generation of an arc is prevented and the lifetime of the contact can increase.

In accordance with the present invention, it may be preferable that the switch mechanism further includes a discharging path for discharging a control voltage input from the control circuit to a control terminal of the switching device, wherein the stopping unit includes a changeover switch that is controlled by the trigger switch, and the changeover switch is configured to change, before the power switch is changed from on to off, a connection of a control terminal of the switching device from an output of the control circuit to a state where an electrical circuit between a control terminal and an output of the control circuit is blocked.

Accordingly, since the changeover switch as the stopping unit blocks the electrical circuit between the control terminal of the switching device and the output of the control circuit before the power switch is changed from on to off, the control voltage input from the control circuit to the control terminal of the switching device can be discharged in a short time via the discharging path, so that the switching device is forcibly turned off. This allows the power switch to be changed from on to off in the state where the switching device is certainly turned off so that the generation of an arc is prevented and the lifetime of the contact can increase.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings so that they can be readily implemented by those skilled in the art.

First Embodiment

The first embodiment of the present invention will be described with reference toFIGS. 1 to 6.FIG. 2is a schematic diagram illustrating an electric tool having a switch device in accordance with the first embodiment of the present invention. The electric tool includes a DC motor M, a decelerator2for decelerating the rotation of the DC motor M and transmitting a rotational force to an output shaft3, and a driving circuit4for controlling the rotation of the DC motor M, all of which are accommodated in a tool body1having a handheld size.

A battery pack5having a battery E such as a rechargeable battery therein is detachably mounted to the tool body1. The output shaft3has a chuck to which a bit (i.e., a rotatable tool) such as a driver, a socket, or a drill is detachably mounted, and which can rotate the bit by rotating the DC motor M. The electric tool further includes a trigger switch6disposed at a position allowing a user's finger to reach into a gripping portion1aof the tool body1, to set a rotational speed of the DC motor M depending on a pressed amount.

FIG. 1is a circuit diagram of the driving circuit4. The driving circuit4includes a power switch SW1connected between the DC motor M and the battery E and closed when the pressed amount of the trigger switch6exceeds a predetermined threshold value, a recovery diode D1connected in parallel with the DC motor M and in a reverse direction, a switching device SW2such as a field effect transistor (FET) connected between the DC motor M and the battery E via the power switch SW1, a variable resistor VR having resistance varying with the pressed amount of the trigger switch6, a CPU7for outputting a control signal of on-duty dependent on a change in the resistance of the variable resistor VR (i.e., the pressed amount of the trigger switch6), a drive circuit8for amplifying the control signal output from the CPU7and turning the switching device SW2on/off, and a 3-terminal regulator9for dropping and smoothing a DC voltage supplied from the battery E via the power switch SW1to generate an operational voltage (e.g., DC 5V) for the CPU7and the like. Here, the CPU7and the drive circuit8include a control circuit for controlling the on and off of the switching device.

The drive circuit8includes a resistor R2having one terminal connected to a positive electrode of the battery E via the power switch SW1, and a switching device SW3consisting of an npn transistor having a collector connected to the other terminal of the resistor R2, an emitter connected to a negative terminal of the battery E, and a base connected to an output of the CPU7via a resistor R1. An output of the drive circuit8(i.e., a connection point of the resistor R2and the switching device SW3) is connected to a gate of the switching device SW2via a changeover switch10and a resistor R3. A resistor R4is connected between the gate of the switching device SW2and the negative terminal of the battery E. The changeover switch10interlocks with the trigger switch6, and has a common terminal c connected to one terminal of the resistor R3, one switching terminal a connected to the connection point between the resistor R2and the switching device SW3, and the other switching terminal b connected to the negative terminal of the battery E.

Here, when a low level signal is output from the CPU7in the state where the power switch SW1is closed and the changeover switch10is switched to the switching terminal a, the switching device SW3is turned off and the switching device SW2is turned on. Accordingly, current is supplied from the battery E to the DC motor M. Meanwhile, when a high level signal is output from the CPU7, the switching device SW3is turned on and the switching device SW2is turned off. Accordingly, current flowing through the DC motor M is blocked. In this manner, when the control signal of on-duty dependent on the resistance of the variable resistor VR is output from the CPU7, the switching device SW2is turned on/off. As a result, the DC motor M can rotate at a rotational speed dependent on the pressed amount of the trigger switch6.

As described above, the power switch SW1and the changeover switch10become on/off in response to the pressing operation of the trigger switch6, and the resistance of the variable resistor VR varies with the pressed amount of the trigger switch6. The structures of the changeover switch10and the variable resistor VR are described based onFIGS. 3 to 5.

FIG. 3is a schematic cross-sectional view illustrating the changeover switch10. The changeover switch10comprises a base11having conductor patterns12,13, and14formed on a surface thereof and having resistance of approximately zero; and a sliding member15to be slid on the base11in response to the pressing operation of the trigger switch6to connect or disconnect between the conductor pattern14and the conductor pattern12and between the conductor pattern14and the conductor pattern13.

FIG. 5shows a planar shape of the conductor patterns12,13, and14formed on the base11. The conductor patterns12,13, and14are band plates having an approximately same width. The conductor pattern12, the conductor pattern13, and the conductor pattern14are sequentially arranged from the right ofFIG. 5. The conductor pattern12acts as the switching terminal a and is electrically connected to the connection point between the resistor R2and the switching device SW3(i.e., the output of the drive circuit8). The conductor pattern13acts as the switching terminal b and is electrically connected to the negative terminal of the battery E.

The conductor pattern12is spaced apart from the conductor pattern13by an interval d1. The conductor pattern12has a narrow portion extending from one lateral end (i.e., an upper portion inFIG. 5) toward the conductor pattern13, and the conductor pattern13has a narrow portion extending from the other lateral end (i.e., a lower portion inFIG. 5) toward the conductor pattern12. The interval d1has an approximately Z shape, when viewed in plan. The conductor pattern14acts as the common terminal c and is electrically connected to one terminal of the resistor R3(i.e., an opposite side of the other switching device SW2). The conductor pattern14is spaced apart from the conductor pattern13by a greater interval d2than the interval d1and has approximately the same length as a sum of the lengths of the conductor patterns12and13.

The sliding member15is a band-shaped elastic plate having slits16respectively formed in a longitudinal direction from both ends as shown inFIG. 4. The sliding member15has a substantially H shape, when viewed in plan. The two branches of the sliding member15are bent in a same direction (diagonally downward inFIG. 3), and ends of the branches are bent in an opposite direction (diagonally upward inFIG. 3). The sliding member15has an approximately an inversed U shape, when viewed from a side and has contacts17aand17bformed at both ends. The sliding member15is disposed to cover the conductor patterns12and13and the conductor pattern14and slide on the conductor patterns12to14in an arrangement direction of the conductor patterns12to14depending on the pressed amount of the trigger switch6. The contact17bis in contact with the conductor pattern14within an entire sliding range.

Meanwhile, the variable resistor VR includes conductor patterns18,19, and21having resistance of approximately zero and a resistor pattern20having a predetermined resistance formed on the surface of the base11, and a sliding member22sliding on the base11in response to the pressing operation of the trigger switch6.

FIG. 5shows planar shapes of the conductor patterns18,19, and21and the resistor pattern20formed on the base11. The conductor patterns18,19, and21and the resistor pattern20are band plates having an approximately same width. They are arranged in the order of the conductor patterns18and19, the resistor pattern20, and the conductor pattern21from the right ofFIG. 5.

The conductor pattern18has approximately the same length as a sum of the lengths of the conductor patterns12and13. The conductor pattern18acts as the terminal e of the variable resistor VR and is electrically connected to the input of the CPU7. The conductor pattern19is spaced apart from the conductor pattern18by an interval d2. The conductor pattern19acts as the terminal g of the variable resistor VR and is electrically connected to the negative terminal of the battery E. The resistor pattern20is formed of a conductive material having a predetermined resistance and continuously from the conductor pattern19by a predetermined length.

The conductor pattern21is formed continuously from the resistor pattern20. The conductor pattern21acts as the terminal f of the variable resistor VR and is electrically connected to an output of the 3-terminal regulator9. That is, the conductor patterns19and21are electrically connected to each other via the resistor pattern20. A total length of the conductor patterns19and21and the resistor pattern20is approximately same as the length of the conductor pattern18.

Since the sliding member22is formed of the shape same as that of the sliding member15, a description thereof will be omitted. This sliding member22is disposed to cover the conductor pattern18and the continuous three patterns19,20and21, and slides on the patterns18to21depending on the pressed amount of the trigger switch6.

One contact of the sliding member22comes in contact with the conductor pattern18along the entire sliding range, and a point at which the other contact of the sliding member22is brought into contact with any one of the patterns19and20is moved in conjunction with the position of the trigger switch6as the trigger switch6is pressed. When the trigger switch6is released, the sliding member22is placed at a position indicated by a dotted line inFIG. 5such that one contact of the sliding member22is in contact with the conductor pattern18and the other contact is in contact with the conductor pattern21. In this state, the resistance of the variable resistor VR becomes approximately zero.

Operation of the switch device in accordance with the present embodiment of the present invention will be described. When the trigger switch6is released, the sliding member15is placed at a position as indicated by a solid line ofFIG. 3such that the contact17ais in contact with the conductor pattern13, and the other contact17bis in contact with the conductor pattern14. In this state, the changeover switch10is changed to the switching terminal b and the gate of the switching device SW2is connected to the negative terminal of the battery E via the resistors R3and R4. Further, the sliding member22is placed at a position as indicated by the dotted line ofFIG. 5such that one contact of the sliding member22is in contact with the conductor pattern18and the other contact is in contact with the conductor pattern21, and the resistance of the variable resistor VR becomes approximately zero.

When the trigger switch6is pressed in this state, the sliding members15and22move to the right ofFIG. 3(andFIG. 5) depending on the pressed amount of the trigger switch6. When the pressed amount of the trigger switch6exceeds a predetermined threshold value, the power switch SW1becomes on but the contact17aof the sliding member15does not reach a point i ofFIG. 5and is in contact with the conductor pattern13such that the changeover switch10remains in contact with the switching terminal b. Accordingly, even though the power voltage is supplied to the CPU7and the control signal is output from the CPU7, the control voltage is not applied to the gate of the switching device SW2and on/off of the switching device SW2does not occur. That is, the switching device SW2is forcibly turned off irrespective of the control signal from the CPU7. Further, since the other contact of the sliding member22remains in contact with the conductor pattern21, the resistance of the variable resistor VR is approximately zero.

Thereafter, when the contact17aof the sliding member15moves to the right after passing the point i ofFIG. 5as the trigger switch6is further pressed, the contact17aenters an area of the interval d1such that it is separated from the conductor pattern13and brought into contact with only the conductor pattern12. In this case, since the changeover switch10is switched to the switching terminal a and the gate of the switching device SW2is connected to the output of the drive circuit8(i.e., the control circuit) via the resistor R3, the control voltage is applied to the gate of the switching device SW2.

When the contact17aof the sliding member15passes the point i, the other contact of the sliding member22is brought into contact with the conductor pattern12, and the resistance of the variable resistor VR is approximately zero. In this case, the CPU7outputs a control signal for minimizing the on-duty of the switching device SW2, based on the resistance of the variable resistor VR. The drive circuit8turns the switching device SW2on/off in response to the control signal from the CPU7so that the voltage applied to the DC motor M becomes a minimum voltage V1and the DC motor M rotates at a minimum speed.

Thereafter, as the trigger switch6is further pressed, the other contact of the sliding member22moves from the point ii ofFIG. 5to the point iii depending on the pressed amount, and the resistance of the variable resistor VR varies from approximately zero to the maximum value depending on a position where the contact of the sliding member22is brought into contact with the resistor pattern20. At this time, the contact17aof the sliding member15slides on the conductor pattern12so that the changeover switch10remains changed to the switching terminal a.

As the resistance of the variable resistor VR varies from approximately zero to the maximum value depending on the pressed amount of the trigger switch6, the CPU7outputs a control signal of the on-duty dependent on the resistance of the variable resistor VR, the drive circuit8turns the switching device SW2on/off based on the control signal from the CPU7so that the voltage applied to the DC motor M is controlled to a predetermined voltage value between the minimum voltage V1and the maximum value Vmax. This allows the DC motor M to rotate at a desired rotational speed between a minimum speed and a maximum speed.

When the other contact of the sliding member22passes the point iii ofFIG. 5as the trigger switch6is further pressed, the resistance of the variable resistor VR becomes the maximum value while the contact moves from the point iii to the point iv corresponding to a full stroke. Accordingly, the on-duty of the switching device SW2is controlled to the maximum value and the DC motor M rotates at a maximum speed.

Meanwhile, when the trigger switch6is released, it tries to return to an off position under a restoring force of a restoring spring (not shown) and the sliding members15and22move to the left ofFIGS. 3 and 5. Until the other contact of the sliding member22reaches the point ii ofFIG. 5after passing the point iii, the resistance of the variable resistor VR varies from a maximum value to a minimum value depending on the pressed amount of the trigger switch6. Accordingly, the CPU7changes the on-duty of the switching device SW2from a maximum value to a minimum value. As a result, the rotational speed of the DC motor M returns from the maximum speed to the minimum speed.

When the other contact of the sliding member22moves to the left after passing the point ii ofFIG. 5, the resistance of the variable resistor VR becomes approximately zero. Further, the CPU7controls the on-duty of the switching device SW2to a minimum value. Thereafter, when the contact17aof the sliding member15enters the area of the interval d1, is separated from the conductor pattern12, and is brought into contact with only the pattern13, the contact of the changeover switch10is changed from the switching terminal a to the switching terminal b and the gate of the switching device SW2is connected to the negative terminal of the battery E via the resistors R3and R4. Accordingly, the driving voltage applied to the gate is discharged via the resistors R3and R4. At this time, since the switching device SW2is forcibly turned off irrespective of the control signal from the CPU7, exciting current is discharged through the DC motor M and the DC motor M stops.

Thereafter, when the pressed amount of the trigger switch6is below the predetermined threshold value, the power switch SW1becomes off. At this time, since the switching device SW2is turned off and current does not flow through the DC motor M, an arc is not generated and the lifetime of the contact of the power switch SW1is prevented from being reduced. In addition, as the power switch SW1is off, the voltage is not supplied from the 3-terminal regulator9to the CPU7and the CPU7does not operate.

As described above, in this embodiment, the stopping unit includes the changeover switch10that is controlled by the trigger switch6. The changeover switch10forcibly turns the switching device SW2off before the power switch SW1is changed from on to off when the trigger switch6is released. Specifically, the changeover switch10connects the gate (i.e., the control terminal) of the switching device SW2to the ground of the circuit (i.e., the negative terminal of the battery E) before the power switch SW1is changed from on to off.

Accordingly, the control voltage input from the drive circuit8(i.e., the control circuit) to the control terminal of the switching device SW2can be discharged in a short time so that the switching device SW2can be forcibly off. This enables the power switch SW1to be changed from on to off in the state where the switching device SW2is certainly off, thereby preventing the contact of the power switch SW1from being melt and stuck due to an arc, and increasing lifetime of the contact.

Furthermore, in this embodiment of the present invention, since there is no bypass switch connected in parallel with the switching device SW2for bypassing the switching device SW2when the trigger switch6is fully pressed, the supply of a power voltage to the DC motor M can be certainly blocked. For example, the current detecting unit is provided for detecting current flowing through the DC motor M. In the case where the current detecting unit detects overcurrent and the CPU17turns the switching device SW2off based on the detecting result, when a bypass switch that is controlled by the trigger switch6is provided, the CPU17cannot turn the bypass switch off. Accordingly, the current flowing through the DC motor M cannot be entirely blocked. However, in this embodiment, since there is no bypass switch, the CPU17can certainly block the supply of a power voltage to the DC motor by turning the switching device SW2off. Thus, the over-discharge of the battery can be prevented.

Second Embodiment

A switching device according to a second embodiment of the present invention will be described with reference toFIGS. 7A and 7B. The switching mechanism of the second embodiment is the same as that of the first embodiment in the present invention except for the changeover switch10which serves as a stopping means in the first embodiment of the present invention. Accordingly, the same elements are referred to by the same reference numerals and a description thereof will be omitted.

In the first embodiment, the changeover switch10has the common terminal c and the two switching terminals a and b. However, in the second embodiment of the present invention, the changeover switch10has only two terminals, that is, a terminal c connected to the gate of the switching device SW2via the resistor R3, and a terminal a connected to the output of the drive circuit8. In the second embodiment, the changeover switch10can connect the gate (i.e., control terminal) of the switching device SW2to the output of the drive circuit8(i.e., the control circuit) or block the electrical circuit between the gate and the output of the drive circuit8depending on the pressed amount of the trigger switch6.

A detailed description of the changeover switch10will be omitted. The changeover switch10is obtained by removing the conductor pattern13(i.e., the switching terminal b), which is electrically connected to the circuit ground, from the changeover switch10of the first embodiment as shown inFIG. 3.

When the sliding member15moves to the left ofFIG. 5and the contact17aof the sliding member15is separated from the conductor pattern12by releasing the trigger switch6after rotating the DC motor M by pressing the trigger switch6, the changeover switch10becomes off. Accordingly, the state is shifted from the state where the gate of the switching device SW2is connected the output of the drive circuit8to the state where the electrical circuit between the gate and the output of the drive circuit8is blocked. At this time, since the control voltage applied to the gate of the switching device SW2is discharged via the resistor R4of the discharging path, the switching device SW2is forcibly turned off irrespective of the control signal of the CPU7such that exciting current is discharged through the DC motor M and the DC motor M stops.

Thereafter, when the pressed amount of the trigger switch6is below a predetermined threshold value, the power switch SW1is turned off. At this time, since the switching device SW2remains in an off state and current does not flow through the DC motor M, an arc is not generated and the lifetime of the contact of the power switch SW1is prevented from being reduced. As the power switch SW1is off, the power supply from the 3-terminal regulator9to the CPU7is blocked and the CPU7does not operate.

As described above, in the second embodiment of the present invention, the changeover switch10that is controlled by the trigger switch6blocks the electrical circuit between the gate of the switching device SW2and the output of the drive circuit8before the power switch SW1is changed from on to off when the trigger switch6is released. This allows the control voltage input from the drive circuit8to the control terminal of the switching device SW2to be discharged in a short time via the discharging path so that the switching device SW2is forcibly turned off. Accordingly, the power switch SW1can be changed from on to off in the state where the switching device SW2is certainly off, so that the generation of an arc is prevented and the lifetime of the contact can increase.

In the electric tool switch mechanism in accordance with the embodiments described above, the CPU7controls the control signal of the on-duty dependent on the pressed amount of the trigger switch6to set the rotational speed of the DC motor M depending on the pressed amount of the trigger switch6. Alternatively, the CPU7may output a control signal of a predetermined on-duty when the trigger switch6is pressed above a predetermined threshold value, or the CPU7may turn the switching device SW2on/off using a predetermined on-duty to rotate the DC motor M at a constant rotational speed.

While the invention has been shown and described with respect to the embodiments, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.