Abstract:
A combustion-powered driving tool for driving nails or other fasteners in which the starting characteristics of a motor in the tool are improved by varying the amount of voltage applied to the motor when starting the motor and during normal operations so that the motor is driven to reach the rotational speed required in normal operations quickly. Therefore, the combustion-powered driving tool does not require the use of an expensive low-inertia motor, but can use a relatively inexpensive core-type motor or the like with inferior starting characteristics, while improving the work efficiency and user-friendliness of the combustion-powered driving tool.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a combustion-powered tool, and more particularly to a combustion-powered, fastener-driving tool for driving nails or other fasteners. In such a fastener-driving tool, liquefied gas contained in a gas tank is injected into a combustion chamber, where the liquefied gas is mixed with air and ignited. The power generated from this combustion drives a piston, which in turn drives the nail or other fastener into a workpiece.  
         [0003]     2. Description of the Related Art  
         [0004]     Combustion-powered tools of the type described above are disclosed in U.S. Pat. Nos. 4,483,474; 4,403,722; 4,522,162 and 5,592,580. A typical combustion-powered tool primarily includes a housing, handle, trigger switch, head cap, combustion chamber frame, push lever, cylinder, piston, driver blade, motor, fan, gas tank, spark plug, exhaust check valve, magazine, and tail cover. The head cap seals one end of the housing. The handle is fixed to the housing and includes a trigger switch, as well as a built-in battery. The combustion chamber frame is disposed inside the housing and is capable of moving in the lengthwise direction thereof. A spring urges the combustion chamber frame in a direction away from the head cap, but the frame is capable of opposing the urging force of the spring to contact the head cap with an end nearest the same.  
         [0005]     The push lever is movably disposed on the opposite end of the housing from the head cap and is coupled with the combustion chamber frame. The cylinder is fixed to the housing at a position enabling the cylinder to be in fluid communication with the combustion chamber frame for guiding the movement of the frame. Exhaust holes are formed in the cylinder. The piston is capable of sliding in a reciprocating motion in the cylinder. When the end of the combustion chamber frame contacts the head cap, a combustion chamber is formed by the head cap, the combustion chamber frame, the cylinder, and the end of the cylinder nearest the head cap. The driver blade extends from the side of the piston opposite the combustion chamber to the other end of the housing. The motor is supported on the head cap. The fan is positioned in the combustion chamber and fixed to the motor. When driven by the motor, the fan accelerates combustion by creating a turbulent flow with respect to combusted gas, non-combusted gas, and air in the combustion chamber. The fan also introduces outside air into the housing when the combustion chamber frame separates from the head cap to clear gas out of the combustion chamber frame and functions to cool the peripheral sides of the cylinder. The gas tank is accommodated in the housing and contains a liquefied flammable gas that can be injected into the combustion chamber via a channel formed in the head cap. The spark plug is exposed in the combustion chamber for igniting the mixture of flammable gas and air. The exhaust check valve selectively covers the exhaust holes.  
         [0006]     The magazine is disposed on the end of the housing opposite the head cap and accommodates nails or other fasteners. The tail cover is provided between the magazine and the push lever for supplying a fastener from the magazine to a position aligned with the driver blade.  
         [0007]     In order to hermetically seal the combustion chamber when the combustion chamber frame contacts the head cap, a sealing member (seal ring) is provided one on a prescribed surface of the head cap that contacts the top part of the combustion chamber frame and one on the edge of the cylinder on the head cap side that contacts the bottom of the combustion chamber frame.  
         [0008]     When the push lever is pressed against a workpiece, the combustion chamber is formed; liquefied gas from the gas tank mounted in the housing is injected into the combustion chamber; and the fan mixes air with the flammable gas. If the trigger switch is operated at this time, the spark plug ignites the gas-air mixture, causing explosive combustion. This combustion drives the piston and, consequently, the driver blade, to drive a nail into a workpiece, such as wood. The combustion chamber frame is maintained in contact with the head cap for a prescribed time after the explosive combustion. After exhausting the gas, the exhaust check valve is closed to seal the combustion chamber, and a thermal vacuum is obtained on the combustion chamber side when a drop in temperature causes the pressure in the combustion chamber to drop. As a result, the piston rises due to the pressure differential above and below the piston.  
         [0009]     The above-described conventional combustion-powered driving tool is involved with the following drawbacks.  
         [0010]     (1) Pressing the push lever against the workpiece switches on a head switch (or push switch). The head switch (or push switch) actuates the motor, which drives the fan to rotate. When the user operates the trigger switch, the spark plug fires, igniting the gas-air mixture. However, if the trigger switch is operated in a relatively short time period after the motor and the fan begin to rotate, the motor and fan have not yet reached a rotational speed capable of producing a sufficient driving force. In such cases, a low driving force is produced.  
         [0011]     There have been proposals for overcoming this problem that include use of an expensive low-inertia motor, or “coreless” motor, and methods for regulating the interval from the point that the head switch (or push switch) is turned on until the gas-air mixture is ignited. However, these methods are expensive and greatly reduce work efficiency and user-friendliness.  
         [0012]     (2) A secondary battery is used as a power source for driving the motor and igniting the sparkplug. An extra battery needs to be provided when the tool is used continuously or used frequently over a long period of time.  
         [0013]     (3) When the tool is used under a low temperature circumstance, the liquefied gas injected into the combustion chamber is not sufficiently vaporized and thus cannot be mixed with air. In such a condition, explosive combustion does not occur even if the trigger switch is turned ON. Re-triggering the switch does not cause the explosive combustion to occur. The tool has to be separated from the workpiece and is again pressed against the workpiece to inject the liquefied gas into the combustion chamber. If this procedure is taken, explosive combustion may be taken place when the trigger switch is again turned ON. However, repeated injection of the liquefied gas consumes the gas in vain and the duration of time the tool is continuously usable with the loaded gas tank is shortened.  
         [0014]     (4) Because a high voltage is applied to an electric circuit accommodated in the tool and a large current is flowing therein when the tool is operating, a voltage caused by noises is induced on the wiring of the tool, which prevents the tool from operating normally.  
       SUMMARY OF THE INVENTION  
       [0015]     In view of the foregoing, it is an object of the present invention to provide a combustion-powered tool that is cheaper and more efficient and user-friendly than the combustion-powered tool of the prior art.  
         [0016]     It is another object of the present invention to provide a combustion-powered tool that can be used for a long period of time without replacing a gas tank.  
         [0017]     In order to achieve the above and other objects, there is provided according to one aspect of the invention a combustion-powered driving tool for driving fasteners into a workpiece, that includes a housing, a head section, a motor, a battery, a motor drive controlling section, a cylinder, a piston, a combustion chamber frame, a fan, and a sparkplug. The head section seals one end of the housing and has a flammable gas channel formed therein. The motor drive controlling section is supplied with the operating voltage of the battery and controls a voltage applied to the motor. The piston is slidably movably disposed inside the cylinder. The combustion chamber frame moves to contact and separate from the head section and forms a combustion chamber together with the head section, the cylinder, and the piston when the combustion chamber frame is in contact with the head section. The fan is rotatably disposed in the combustion chamber and driven to rotate by the motor. The sparkplug is exposed in the combustion chamber for igniting a mixture of air and flammable gas in the combustion chamber. The flammable gas is supplied into the combustion chamber via the flammable gas channel. Explosive combustion caused by firing of the sparkplug moves the piston and a fastener is driven into the workpiece in accordance with the movement of the piston. The motor drive control section applies a first voltage to the motor when the combustion chamber is formed by the combustion chamber frame moving toward and brought into contact with the head section, and a second voltage to the motor, wherein the first voltage is greater than the second voltage.  
         [0018]     The motor drive controlling section may include an up converter that steps up the operating voltage of the battery and outputs a stepped up voltage. The motor drive controlling section applies the stepped up voltage to the motor as the first voltage. In this case, the motor drive controlling section may apply the operating voltage of the battery to the motor as the second voltage.  
         [0019]     Alternatively, the motor drive controlling section may include a down converter that steps down the operating voltage of the battery and outputs a stepped down voltage. The motor drive controlling section applies the stepped down voltage to the motor as the second voltage. In this case, the motor drive controlling section may apply the operating voltage of the battery to the motor as the first voltage.  
         [0020]     It is preferred that with the first voltage applied to the motor, the motor reach to the steady rotational speed within 130 ms.  
         [0021]     The motor drive control section may apply a third voltage to the motor after the explosive combustion is taken place, wherein the second voltage is greater than the third voltage.  
         [0022]     According to another aspect of the invention, there is provided a combustion-powered driving tool for driving fasteners into a workpiece, that includes a housing, a head section, a motor, a battery, a power source section, a motor drive controlling section, a first switch, a second switch, a cylinder, a piston, a combustion chamber frame, a fan, a sparkplug, and a controller. The head section seals one end of the housing and has a flammable gas channel formed therein. The power source section is supplied with the operating voltage of the battery and generates a reference voltage. The motor drive controlling section is supplied with the operating voltage of the battery and the reference voltage from the power source section and drives the motor based on the operating voltage and the reference voltage. The first switch detects whether the tool is pressed against the workpiece and outputs a first signal indicative of a detected condition. The second switch instructs driving the fastener into the workpiece and outputs a second signal indicative of an instruction to drive the fastener. The combustion chamber frame moves to contact and separate from the head section and forms a combustion chamber together with the head section, the cylinder, and the piston when the combustion chamber frame is in contact with the head section. The fan is rotatably disposed in the combustion chamber and driven to rotate by the motor. The sparkplug is exposed in the combustion chamber for igniting a mixture of air and flammable gas in the combustion chamber. The flammable gas is supplied into the combustion chamber via the flammable gas channel, wherein explosive combustion caused by firing of the sparkplug moves the piston toward the second end of the housing and a fastener is driven into the workpiece in accordance with the movement of the piston. The controller controls the power source section so as not to generate the reference voltage in order to reduce power consumption when at least one of the first signal and the second signal indicates that the tool is left unused for a prescribed period of time. The controller may further control the power source section so as not to generate the reference voltage in order to reduce power consumption when at least one of the first signal and the second signal indicates that at least one of the first switch and the second switch malfunctions.  
         [0023]     According to still another aspect of the invention, there is provided a combustion-powered driving tool for driving fasteners into a workpiece, that includes a housing, a head section for sealing one end of the housing and having a flammable gas channel formed therein, a motor, a battery for supplying an operating voltage, a power source section that is supplied with the operating voltage of the battery, a motor drive controlling section that is supplied with the operating voltage of the battery and drives the motor, a first switch that detects whether the tool is pressed against the workpiece and outputs a first signal indicative of a detected condition, a second switch that instructs driving the fastener into the workpiece and outputs a second signal indicative of an instruction to drive the fastener, a cylinder, a piston slidably movably disposed inside the cylinder, a combustion chamber frame that moves to contact and separate from the head section and that forms a combustion chamber together with the head section, the cylinder, and the piston when the combustion chamber frame is in contact with the head section, a fan rotatably disposed in the combustion chamber and driven to rotate by the motor, a sparkplug exposed in the combustion chamber for igniting a mixture of air and flammable gas in the combustion chamber, the flammable gas being supplied into the combustion chamber via the flammable gas channel, wherein explosive combustion caused by firing of the sparkplug moves the piston toward the second end of the housing and a fastener is driven into the workpiece in accordance with the movement of the piston, and a controller that actuates the sparkplug to ignite the mixture of air and flammable gas in the combustion chamber in response to the second signal and regardless of the first signal.  
         [0024]     According to further aspect of the invention, there is provided a combustion-powered driving tool for driving fasteners into a workpiece, that includes a housing, a head section for sealing one end of the housing and having a flammable gas channel formed therein, a motor, a battery for supplying an operating voltage, a power source section that is supplied with the operating voltage of the battery and generates a reference voltage, a motor drive controlling section that is supplied with the operating voltage of the battery and the reference voltage from the power source section and that drives the motor based on the operating voltage and the reference voltage, a first switch that detects whether the tool is pressed against the workpiece and outputs a first signal indicative of a pressed condition of the tool, a second switch that instructs driving the fastener into the workpiece and outputs a second signal indicative of an instruction to drive the fastener, a third switch that connects the battery and the power source section when turned ON, a cylinder, a piston slidably movably disposed inside the cylinder, a combustion chamber frame that moves to contact and separate from the head section and that forms a combustion chamber together with the head section, the cylinder, and the piston when the combustion chamber frame is in contact with the head section, a fan rotatably disposed in the combustion chamber and driven to rotate by the motor, a sparkplug exposed in the combustion chamber for igniting a mixture of air and flammable gas in the combustion chamber, the flammable gas being supplied into the combustion chamber via the flammable gas channel, wherein explosive combustion caused by firing of the sparkplug moves the piston toward the second end of the housing and a fastener is driven into the workpiece in accordance with the movement of the piston, and a controller that is supplied with the reference voltage from the power source section when the third switch is ON, wherein the controller is rendered inoperative when neither the first signal nor the second signal is output even if the third switch is ON.  
         [0025]     According to yet another aspect of the invention, there is provided a combustion-powered driving tool for driving fasteners into a workpiece, that includes a housing, a head section for sealing one end the housing and having a flammable gas channel formed therein, a motor, a battery for supplying an operating voltage, a power source section that is supplied with the operating voltage of the battery and generates a reference voltage, a motor drive controlling section that is supplied with the operating voltage of the battery and the reference voltage from the power source section and that drives the motor based on the operating voltage and the reference voltage, a first switch that detects whether the tool is pressed against the workpiece and outputs a first signal indicative of a pressed condition of the tool, a second switch that instructs driving the fastener into the workpiece and outputs a second signal indicative of an instruction to drive the fastener, a cylinder, a piston slidably movably disposed inside the cylinder, a combustion chamber frame that moves to contact and separate from the head section and that forms a combustion chamber together with the head section, the cylinder, and the piston when the combustion chamber frame is in contact with the head section, a fan rotatably disposed in the combustion chamber and driven to rotate by the motor, a sparkplug exposed in the combustion chamber for igniting a mixture of air and flammable gas in the combustion chamber, the flammable gas being supplied into the combustion chamber via the flammable gas channel, wherein explosive combustion caused by firing of the sparkplug moves the piston toward the second end of the housing and a fastener is driven into the workpiece in accordance with the movement of the piston, and a controller that generates a start signal instructing to drive a fastener into the workpiece. The fastener is driven into the workpiece when both the second signal and the start signal are generated. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]     The particular features and advantages of the invention as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which:  
         [0027]      FIG. 1  is a cross-sectional view showing a combustion-powered driving tool according to a first embodiment of the invention;  
         [0028]      FIG. 2  is a block circuit diagram showing a control circuit for controlling the voltage applied to the motor according to the first embodiment of the invention;  
         [0029]      FIG. 3A  is a time chart showing the changes in voltage applied to the motor when using the circuit in  FIG. 2 ;  
         [0030]      FIG. 3B  is a time chart of the rotational speed of the motor when using the circuit in  FIG. 2 ;  
         [0031]      FIG. 4  is a cross-sectional view showing a combustion-powered driving tool according to a second embodiment of the invention;  
         [0032]      FIG. 5  is a side view showing the combustion-powered driving tool shown in  FIG. 4 ;  
         [0033]      FIG. 6A  is a part of a circuit diagram showing a control circuit for controlling the voltage applied to the motor according to the second embodiment of the invention;  
         [0034]      FIG. 6B  is a remaining part of the circuit diagram showing the control circuit according to the second embodiment of the invention, wherein combining the circuit diagrams in  FIGS. 6A and 6B  in relevant portions provides an entire circuit diagram;  
         [0035]      FIG. 7  is a time chart showing the changes in voltage applied to the motor and the rotational speed of the motor when using the circuit in  FIGS. 6A and 6B ;  
         [0036]      FIG. 8  is a flowchart showing control of a head switch when using the circuit in  FIGS. 6A and 6B ; and  
         [0037]      FIG. 9  is a flowchart showing control of a trigger switch when using the circuit in  FIGS. 6A and 6B . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0038]     Next, a first embodiment will be described while referring to  FIGS. 1, 2  and  3 A- 3 B, wherein the combustion-powered tool of the present invention is applied to a combustion-powered, fastener-driving tool. In the following description, it is assumed that the tool is held in an orientation in which nails are fired toward a downward direction.  
         [0039]     A combustion-powered, fastener-driving tool  1  has a housing  2  that forms an outer framework. The housing  2  includes a main housing section  2 A and a tank chamber  2 B provided alongside the main housing section  2 A in the lengthwise direction. An intake hole (not shown) is formed in the top of the main housing section  2 A, while an exhaust hole (not shown) is formed in the bottom of the same.  
         [0040]     A head cover  4  is mounted on the top of the main housing section  2 A. A gas tank  5  containing flammable gas is removably accommodated in the tank chamber  2 B. A handle  7  extends outward from the tank chamber  2 B. The handle  7  is provided with a trigger switch  6  and a built-in battery  30  (see  FIG. 2 ) having a nominal voltage of 7.2 V, for example. Disposed below the main housing section  2 A and the tank chamber  2 B are a magazine  8  loaded with nails (not shown) and a tail cover  9  for guiding the nails in the magazine  8  to a prescribed position.  
         [0041]     A push lever  10  is movably supported on the bottom end of the main housing section  2 A with respect to the position of the nail set by the tail cover  9 . A coupling unit  12  fixed to a combustion chamber frame  11  described later is joined to the push lever  10 . When the tip of the push lever  10  contacts a workpiece W and the entire housing  2  is pushed in a direction toward the workpiece W, the upper portion of the push lever  10  can recede into the main housing section  2 A.  
         [0042]     A head cap  13  is fixed in the top end of the main housing section  2 A. A motor  3  is supported in the head cap  13  by a spring  3 A. A fan  14  is fixed to a rotational shaft of the motor  3 . A spark plug  15  that fires when the trigger switch  6  is operated is also retained in the head cap  13 . A head switch  16  (see  FIG. 2 ) is provided in the main housing section  2 A for detecting that the combustion chamber frame  11  is at the top end of a stroke when the entire tool is pressed against the workpiece W. When the push lever  10  rises to a prescribed position, the head switch  16  is switched on, activating the motor  3 , which in turn begins rotating the fan  14 . The fan  14  is configured from a hub and six vanes equally spaced apart around the hub and extending outwardly from the hub. The fan  14  rotates at a rate of approximately 12,000 rpm.  
         [0043]     A fuel injection channel  17  is formed in the side of the head cap  13  nearest the tank chamber  2 B. An end of the fuel injection channel  17  penetrating the bottom surface of the head cap  13  forms an injection nozzle  18 , while the other end forms a connector for connecting to the gas tank  5 . A first sealing member  19  formed of an O-ring is mounted on the head cap  13  for forming a seal between the head cap  13  and the combustion chamber frame  11  when the top of the combustion chamber frame  11  is placed in contact with the head cap  13 .  
         [0044]     The combustion chamber frame  11  disposed in the main housing section  2 A is capable of reciprocating movement in the lengthwise direction of the main housing section  2 A and is capable of contacting the bottom surface of the head cap  13 . As described above, the coupling unit  12  is joined with the push lever  10  and fixed to the bottom end of the combustion chamber frame  11 . Accordingly, the combustion chamber frame  11  moves along with the movement of the push lever  10 . A cylinder  20  is fixed to the main housing section  2 A for guiding movement of the combustion chamber frame  11  by contacting the inner wall of the same. A compressed coil spring  22  is interposed between the bottom surface of the cylinder  20  and the coupling unit  12  for urging the combustion chamber frame  11  away from the head cap  13 . Exhaust holes  21  are formed near the bottom of the cylinder  20  and are in fluid communication with the exhaust hole in the main housing section  2 A described above. A check valve (not shown) is provided on the outer side of the exhaust holes  21  for selectively blocking the same. A bumper  23  is also provided in the bottom of the cylinder  20 . A second sealing member  24  formed of an O-ring is mounted on the top of the cylinder  20  for forming a seal between the inner wall near the bottom of the combustion chamber frame  11  and the outer wall near the top of the cylinder  20  when the combustion chamber frame  11  contacts the head cap  13 .  
         [0045]     A piston  25  capable of reciprocating movement while sliding against the inner wall of the cylinder  20  is provided inside the cylinder  20 . When the top end of the combustion chamber frame  11  contacts the head cap  13 , a combustion chamber  26  is formed by the head cap  13 , the combustion chamber frame  11 , the end of the cylinder  20  nearest the head cap, the piston  25 , and the first and second sealing members  19  and  24 . When the combustion chamber frame  11  separates from the head cap  13 , a first channel S 1  in fluid communication with the outside air forms between the head cap  13  and the top end of the combustion chamber frame  11 , and a second channel S 2  in communication with the first channel S 1  forms between the bottom end of the combustion chamber frame  11  and the top end of the cylinder  20 . The second channel S 2  allows combustion gas and fresh air to pass outside the cylinder  20  and to be discharged through the exhaust hole in the main housing section  2 A.  
         [0046]     A plurality of ribs  27  is provided on the section of the combustion chamber frame  11  forming the combustion chamber  26 , extending in the axial direction of the combustion chamber frame  11  and protruding radially inwardly. In cooperation with the rotation of the fan  14 , the ribs  27  promote the mixture of air and flammable gas in the combustion chamber  26  through agitation. The intake hole described above that is formed in the top of the main housing section  2 A supplies air into the combustion chamber  26 , while combustion gas in the combustion chamber  26  is discharged through the exhaust holes  21  and the exhaust hole formed in the bottom of the main housing section  2 A.  
         [0047]     A driver blade  28  extends from the side of the piston  25  opposite the combustion chamber  26  to the end of the main housing section  2 A. The driver blade  28  is capable of impacting a nail in the tail cover  9  along the same axis as the nail. When propelled downward, the piston  25  collides with the bumper  23  and stops.  
         [0048]     The fan  14 , spark plug  15 , and injection nozzle  18  are all disposed in or exposed in the combustion chamber  26 . The fan  14  achieves three functions. First, before the spark plug  15  fires, rotation of the fan  14  mixes air and flammable gas in the combustion chamber  26  by agitation when the combustion chamber frame  11  is contacting the head cap  13 . Second, when the spark plug  15  fires, rotation of the fan  14  generates a turbulent flow that promotes combustion. Third, when the combustion chamber frame  11  separates from the head cap  13  after driving the nail, the first and second channels S 1  and S 2  are formed and the fan  14  functions to clear combustion gas from the combustion chamber  26  and to cool the cylinder  20 .  
         [0049]      FIG. 2  shows a control circuit incorporated in the tool  1  shown in  FIG. 1 . The control circuit controls the operating voltage of the motor  3  to drive the fan  14 . When the head switch  16  is closed, a first timer  31  and a second timer  32  operate for a prescribed interval and energize associated exciting coils  33   a  and  34   a  of relay switches  33  and  34 , respectively. While the exciting coils  33   a  and  34   a  are energized, the relay switches  33  and  34  close contacts  33   b  and  34   b . When the relay contact  33   b  is closed, a voltage converter  39  increases the voltage of the battery  30  (7.2 V) to 12 V and applies the 12 V to the motor  3  via the contact  34   b  and a diode  40 . After the time measured by the second timer  32  has elapsed, the contact  34   b  is opened and the voltage of 7.2 V from the battery  30  is applied to the motor  3  via the contact  33   b  and a diode  41 . Here, the voltage converter  39  is configured of a step-up transformer  35 , a switching transistor  36  that repeatedly turns on and off in a prescribed cycle, a diode  37 , and a capacitor  38 . Thus, the voltage converter  39  operates as an up converter.  
         [0050]      FIG. 3A  is a timing chart showing the voltage applied to the motor  3  and  FIG. 3B  is a timing chart showing the rotational speed of the motor  3 . The solid line indicates the applied voltage and rotational speed according to the first embodiment of the invention, while the dotted line indicates the rotational speed when the voltage applied to the motor  3  is not controlled and only the nominal voltage of 7.2 V from the battery  30  is applied.  
         [0051]     As is clear from  FIGS. 3A and 3B , the rotational speed of the motor  3  indicated by the solid line rises quickly when the voltage applied to the motor  3  is increased from the nominal voltage of 7.2 V to 12 V, reaching the prescribed rotational speed (12,000 rpm in the first embodiment) in less than 130 ms. When only the nominal voltage is used, the motor  3  does not reach the prescribed rotational speed even after 300 ms has elapsed, as indicated by the dotted line.  
         [0052]     Therefore, the time measured by the second timer  32  is set less than or equal to 130 ms from the moment the head switch  16  is closed, while the time measured by the first timer  31  begins from the moment the head switch  16  is closed and ends at the moment when a prescribed time has elapsed after the head switch  16  is opened. More specifically, the time to be measured by the first timer  31  is set to a length that allows the combustion chamber  26  to be opened after driving the nail and fresh air to be introduced into the combustion chamber  26 .  
         [0053]     From the perspective of energy conservation, the circuit in  FIG. 2  is problematic in that the step-up transformer  35  consumes a large amount of power. However, it was found that the rotational speed of the motor  3  can still be increased quickly by applying the nominal voltage of 7.2 V from the battery  30  when exciting the motor  3  and stepping down the battery voltage to 6 V, for example, during normal operations. In this case, the number of turns in the coil of the motor  3  or the like can be set so that the rotational speed of the motor  3  reaches 12,000 rpm during normal operations with an applied voltage of 6 V. In this connection, the number of turns in the coil of the motor  3  reaches 12,000 rpm during normal operations with an applied voltage of 6 V. In this connection, the number of turns in the coil of the motor  3  according to the first embodiment described above has also been set to achieve a rotational speed of 12,000 rpm with an applied voltage of 7.2 V.  
         [0054]     In contrast to the conventional combustion-powered fastener-driving tool employing a fan with four vanes, the tool  1  according to the first embodiment of the invention employs the fan  14  having six vanes. With this increase in the number of vanes, scavenging time can be shortened as compared with the conventional tool. With the same scavenging time, the voltage applied to the motor  3  can be decreased, so that power can be conserved.  
         [0055]     A combustion-powered driving tool having the construction described above enables the motor that drives the fan to start rapidly so that the fan can quickly reach the rotational speed for normal operations. Accordingly, the flammable gas and air can be reliably mixed through agitation to ensure that operations are reliable and simple, thereby improving work efficiency and user-friendliness. Since it is not necessary to use an expensive low-inertia motor, the present invention can provide an inexpensive combustion-powered driving tool.  
         [0056]     The combustion-powered driving tool described above makes it possible to conserve energy, thereby increasing the life of the battery. Also, the tool makes it possible to achieve rapid driving, thereby improving user-friendliness.  
         [0057]     A combustion-powered, fastener-driving tool according to a second embodiment of the invention will be described while referring to  FIGS. 4 through 9  where like components and parts as appeared in  FIG. 1  are designated by like reference numerals and duplicate description thereof is omitted. In  FIG. 4 , reference numerals  251  and  201  designate a trigger switch and a push switch that function similar to the trigger switch  6  and the head switch  16  of  FIGS. 1 and 2 , respectively.  
         [0058]     In the vicinity of the trigger switch  251  and above the magazine  8 , a main switch  101  is disposed. When the main switch  101  is closed or turned ON, the voltage across the battery  30  is applied to a control circuit  51  shown in  FIGS. 6A and 6B  and the tool  1  is placed in a usable condition. On the other hand, when the main switch  101  is opened or turned OFF, the control circuit  51  is not powered. Therefore, by turning the main switch  101  OFF, it is possible to block dissipation of energy of the battery  30  when the tool  1  is not used.  
         [0059]     The push switch  201  is provided in the lower part of the housing  2 . Similar to the head switch of the first embodiment, the push switch  201  detects that the combustion chamber frame  11  is at the top end of a stroke when the tool  1  is pressed against the workpiece W.  
         [0060]      FIGS. 6A and 6B  show a circuit diagram of the control circuit  51  according to the second embodiment of the invention. It should be noted that  FIG. 6A  shows a part of the control circuit  51  and  FIG. 6B  shows a remaining part thereof. Combining the two diagrams in relevant portions provides the entire circuit diagram. The control circuit  51  is configured from a power source section  100 , a battery voltage detecting section  150 , a push switch section  200 , a trigger switch section  250 , a microcomputer  300 , an oscillator  310 , a charging circuit section  400 , an ignition circuit section  450 , a motor drive controlling section  500 , and a display section  600 .  
         [0061]     The power source section  100  includes a main switch  101 , a regulator  115  for generating a drive voltage of the microcomputer  300  and reference voltages, an FET  109 , transistors  102 ,  108 ,  114 , a diode  112 , capacitors  105 ,  113 ,  116 ,  118 , and resistors  103 ,  104 ,  106 ,  107 ,  110 ,  111 .  
         [0062]     The voltage of the battery  30  (7.2 V) is applied to the regulator  115  through the diode  112  and the regulator  115  generates a voltage (3.3 V) for operating the control circuit  51 . The regulator  115  has a terminal R 1  for controlling the output from the regulator  115 . The power source section  100  further includes a self-holding circuit  130  for holding an output stop signal from the P 14  terminal of the microcomputer  300 . The output stop signal is for stopping the voltage output from the regulator  115 . The output stop signal is held by the self-holding circuit  130  even after the microcomputer  300  is not powered. To stop the voltage output from the regulator  115 , the microcomputer  300  outputs a HIGH signal from its P 14  terminal, causing the FET  109  to turn ON which in turn causes the transistor  114  to turn OFF and the transistors  102  and  108  to turn ON. Thus, the output stop signal is transmitted to the regulator  115 . When the voltage output from the regulator  115  is stopped, the output stop signal, which has been supplied from the P 14  terminal of the microcomputer  300 , is no longer supplied therefrom. However, due to the self-holding circuit  130 , the transistor  108  is held ON in the absence of the output stop signal. This condition continues as far as the battery  30  is not removed or the main switch  101  is not turned OFF. Hence, the control circuit  51  is placed in a low power consumption mode in which the voltage is not output from the regulator  115 . Under the low power consumption mode, the tool is not capable of being operated. The low power consumption mode can be canceled by turning OFF the main switch  101  and then turning ON the main switch  101  again.  
         [0063]     Generation of the output stop signal from the microcomputer  300  can prevent the battery  30  from being consumed in vain when the tool  1  is left unused for a long period of time while switching ON the main switch  101 . The same is true when the tool  1  is rested with the push lever  10  held in a pressed condition and the push switch  201  switched to ON, and when the contact point of the push switch  201  is melted and normally held ON.  
         [0064]     A reset IC  117  is connected to the P 6  terminal of the microcomputer  300  and outputs a reset signal thereto when the battery  30  is loaded and the main switch  101  is turned ON or when the output voltage from the regulator  115  is out of a set range.  
         [0065]     The battery voltage detecting section  150  includes a voltage detection stop circuit  151 , a pair of voltage division resistors  158  and  159 , and a capacitor  160 . The voltage detection stop circuit  151  is configured of FETs  155 ,  157 , and resistors  153 ,  154 ,  156 . When the power source section  100  is placed in the low power consumption mode and when no voltage is output from the regulator  115 , both the FETs  155  and  157  are rendered OFF, thereby disabling the battery voltage detecting section  150 . Hence, the voltage division resistors  158  and  159  do not consume power in vain. The resistors  158  and  159  divides the voltage across the battery  30  and the voltage developed across the resistor  159  is applied to the P 8  terminal of the microcomputer  300 .  
         [0066]     The push switch section  200  includes a push switch  201 , resistors  202 ,  203 , diodes  204 ,  205  and a capacitor  206 . When the tool  1  is pressed against the workpiece W and the push switch  201  is turned ON, a LOW signal is applied to the P 20  terminal of the microcomputer  300 . The push switch  201  and the trigger switch  251  are provided in positions apart from the substrate of the control circuit  51  and these switches are connected to the relevant positions using cables.  
         [0067]     Here, a problem arises such that the cables pick up noises produced at the time of ignition, resulting in a voltage induced on the cables, which causes the ground side of the push switch  201  to be positive in polarity. The diodes  204 ,  205  are provided so that the induced voltage is applied thereto. Thus, an unduly high voltage can be prevented from being applied to the microcomputer  300 .  
         [0068]     The trigger switch section  250  includes resistors  252 ,  253 , diodes  254 ,  255  and a capacitor  256 , and operates in a similar fashion to the push switch section  200 .  
         [0069]     The microcomputer  300  has a reset input port  301 , an output port  302 , a central processing unit (CPU)  303 , a RAM  304 , a ROM  305 , an analog-to-digital (A/D) converter  306 , an output port  307 , a timer  308 , and an input port  309 . The microcomputer  300  controls rotation of the motor  3  and operation of the ignition circuit  450 . An oscillator  310  disposed outside the microcomputer  300  is connected to the timer  308 . While the second embodiment uses the microcomputer  300 , a digital circuit may be employed in lieu of the microcomputer  300  to achieve the same job imposed on the microcomputer  300 .  
         [0070]     The charging circuit section  400  is provided for charging an ignition capacitor  401  and includes the ignition capacitor  401 , a transformer  403 , diodes  402 ,  404 ,  406 , transistors  408 ,  411 , an FET  405 , and resistors  403 ,  407 - 410 ,  412 ,  413 . Charging the capacitor  401  is commenced when the trigger switch  251  is turned ON. An ON signal issued from the trigger switch  251  is transmitted via two paths to the charging circuit  400 . The first path includes a route A indicated in  FIG. 6B  wherein the ON signal is applied to the base of the transistor  411  to render the latter ON and is thus transmitted to the collector of the transistor  408 . On the other hand, the ON signal transmitted via the second path is applied to the P 19  terminal of the microcomputer  300 . Upon receipt of the ON signal, the microcomputer  300  outputs a LOW signal intermittently from the P 11  terminal to the base of a transistor  408 , thereby ON/OFF switching the transistor  408 . The ON signal transmitted via the two paths causes the FET  405  to perform ON/OFF switching. As a result, a high voltage is developed across the secondary side of the transformer  403 , and the ignition capacitor  401  is charged thereby.  
         [0071]     As described above, the charging circuit  400  does not start charging the ignition capacitor  401  if the trigger switch  250  is held OFF. This is true even if a voltage developed by a noise is applied to the P 19  terminal of the microcomputer  300  and a charge signal is output from the microcomputer  300  instructing to charge the ignition capacitor  401 .  
         [0072]     The ignition circuit  450  includes an ignition plug  15 , a thyristor  457 , a transistor  453 , a diode  458 , and resistors  451 ,  452 ,  454 ,  456 . A LOW signal is output from the P 9  terminal of the microcomputer  300  as an ignition signal, which signal renders the transistor  453  ON. A gate signal is applied to the gate of the thyristor  457  to render the latter ON. When the thyristor  457  is turned ON, electric charges retained in the ignition capacitor  401  are discharged. As a result, the voltage across the secondary side of the transformer  459  is boosted up to about 15,000 V, causing the ignition plug  15  to ignite. The microcomputer  300  operates to apply the ON signal to the gate of the thyristor  457  for 10 milliseconds after the ignition circuit is rendered operative.  
         [0073]     The motor driving controlling section  500  includes a first-stage driving circuit  510  used when starting up the motor  3 , a second-stage driving circuit  540  used when the motor  3  rotates at a steady condition, and a third-stage driving circuit  570  used at the time of scavenging. The motor driving controlling section  500  operates when the tool  1  is pressed against the workpiece W and the push switch  201  is turned ON.  
         [0074]     The first-stage driving circuit  510  includes transistors  514  through  516  and resistors  511  through  513 . When the push switch  201  is turned ON, the microcomputer  300  outputs a LOW signal from the P 10  terminal, which renders the transistor  514  OFF and the transistors  515  and  516  ON. As a result, the motor  3  is applied with the battery voltage (7.2 V).  
         [0075]     The second-stage driving circuit  540  and the third-stage driving circuit  570  operate in a similar fashion. However, these driving circuits output different voltages to be applied to the motor  3  depending on the base voltages of the transistors  550 ,  580 . Specifically, the second-stage driving circuit  540  outputs 6 V and the third-stage driving circuit  570  outputs 5 V.  
         [0076]      FIG. 7  is a time chart showing changes in voltage applied to the motor  3  and the rotational speed of the motor  3  in accordance with the second embodiment of the invention. When the tool  1  is pressed against the workpiece W, flammable gas is injected into the combustion chamber  26  from the gas tank  5  and the push switch  201  is turned ON. Air and flammable gas are mixed through agitation. Early start of agitation ensures explosive combustion and the nail driving operation can be performed without fail. After driving a nail into the workpiece W, the tool  1  is separated from the workpiece W. The motor  3  continues rotating even after the tool  1  is separated from the workpiece W for the purpose of scavenging exhaust gas and cooling the cylinder  20 .  
         [0077]     As shown in  FIG. 7 , the voltage applied to the motor  3  changes in three steps. Specifically, the voltage applied to the motor  3  at the time of start-up (hereinafter referred to as “first-stage voltage”) is the highest, the voltage applied to the motor  3  during a steady condition (hereinafter referred to as “second-stage voltage”) is the second highest, and the voltage applied to the motor  3  at the time of scavenging (hereinafter referred to as “third-stage voltage”) is the lowest. The relationship among the first-stage, second-stage and third-stage voltages is not limited to that shown in  FIG. 7  but can be such a relationship that the first-stage voltage is equal to or greater than the second-stage voltage, and the second-stage voltage is equal to or greater than the third-stage voltage. However, the first-stage, second-stage and third-stage voltages must not be equal to each other. By establishing the above-described relationship, air and flammable gas can be quickly mixed through agitation at the time of start-up of the motor  3 . After air and flammable gas are well mixed, the motor  3  is driven at a constant top speed to achieve a steady condition. At the time of scavenging, the motor  3  is driven at a possible minimum speed to make the exhaust gas scavenge and the cylinder  20  cool down. With the control of the motor  3  as described above, explosive force can be sufficiently strong and the dissipation of the battery  30  can be reduced.  
         [0078]     It should be noted that driving the fan  14  is performed irrespective of ON/OFF of the trigger switch  250  but performed depending solely on the operation of the push switch  201 . Similarly, charging and igniting operations are performed irrespective of ON/OFF of the push switch  201  but performed depending solely on the operation of the trigger switch  250 . As such, even if flammable gas injected into the combustion chamber  26  is not sufficiently vaporized and mixed with air due to circumferential temperature and/or inner pressure of the gas tank  5 , ignition to the flammable gas can be achieved by triggering the trigger switch  251  several times while pressing the tool  1  against the workpiece W.  
         [0079]     Referring back to  FIG. 6B , the display section  600  includes an LED  601  and resistors  602  and  603 . When the battery  3  is loaded into the tool  1  and the main switch  101  is turned ON, HIGH and LOW signals are cyclically generated from the P 16  terminal of the microcomputer  300  and a LOW signal is generated from the P 15  terminal of the microcomputer  300 . Thus, the LED  601  flickers with green light to thereby indicate the operator that the tool  1  is in a usable condition. When the tool  1  is pressed against the workpiece W and the motor  3  is driven, the microcomputer  300  generates a LOW signal from the P 15  terminal and a HIGH signal from the P 16  terminal. Then, the LED  601  is continuously lit with green light to thereby indicate the operator that the nail driving operation can be started. When the battery voltage is not at a nominal level and the control circuit  51  is not in the low power consumption mode, the microcomputer  300  generates a HIGH signal from the P 15  terminal and a LOW signal from the P 16  terminal. Then the LED  601  is lit with red light to thereby alert the operator that the battery  30  needs charging.  
         [0080]     Operation of the control circuit  51  will be described while referring to the flowcharts shown in  FIGS. 8 and 9 .  FIG. 8  is a flowchart relating to the push switch  210  and  FIG. 9  to the trigger switch  251 .  
         [0081]     Referring first to the flowchart of  FIG. 8 , prior to executing initial settings in step (hereinafter abbreviated to “S”)  100 , the battery  30  is loaded into the tool  1  (S 001 ) and the main switch  101  is turned to ON (S 002 ). Then, it is judged whether both the push switch  201  and the trigger switch  251  are OFF (S 003 ). The purpose for confirming that these two switches are OFF is to see if the switches malfunction. Should either the push switch  201  or the trigger switch  251  be ON at the initial stage of the operation, the contact point of the switch may, for example, be defective. The tool  1  does not operate if both switches are OFF. Specifically, although the main switch  101  is ON and the microcomputer  300  is supplied with power, the microcomputer  300  is rendered inoperative when either the push switch  201  or the trigger switch  251  is ON even if the main switch  101  is ON.  
         [0082]     Next, initial settings are executed (S 100 ). After the initial settings are executed, it is judged whether the tool  1  is currently being used (S 102 ). In this embodiment, the tool  1  is determined to be a non-use condition if the duration of time the push switch  201  is continuously OFF continues more than 60 minutes. The purpose for investigating the non-use condition of the tool  1  is to prevent the battery  30  from being unnecessarily dissipated. If the tool  1  is left unused for a long period of time, dissipation of the battery  30  is to be stopped.  
         [0083]     When it is judged that the push switch  201  is being OFF for more than 60 minutes (S 102 :YES), the power source section  100  is switched to the low power consumption mode (S 134 ). In the low power consumption mode, the microcomputer  300  stops its operation. Cancellation of the low power consumption mode can be implemented by turning OFF the main switch  101  to reset the self-holding circuit  130  and then turning the main switch  101  ON again. When the low power consumption mode is canceled, the tool  1  is placed in a usable condition. After the lower power consumption mode is set (S 134 ), the routine waits until the main switch  101  is turned OFF (S 136 ). If the main switch  101  is turned OFF (S 136 :YES), the routine returns to S 002 .  
         [0084]     If the push switch  201  is not being OFF for more than 60 minutes (S 102 :NO), then it is judged whether the push switch  201  is turned ON (S 104 ). When the push switch  201  is turned ON (S 104 :YES), the motor drive controlling section  500  is energized to drive the fan  14 . The rotation of the fan  14  mixes air and flammable gas injected into the combustion chamber  26  through agitation. In this embodiment, when the push switch  201  is turned ON, all of the motor driving circuits  510 ,  540  and  570  are driven (S 106 , S 108 , S 110 ). The voltage applied to the motor  3  in this situation is equal to the voltage across the battery  30 , i.e., 7.2V.  
         [0085]     Next, it is judged whether 100 milliseconds have been expired from the timing when the push switch  201  is turned ON (S  112 ). The time of 100 milliseconds is considered to be sufficient duration for the motor  3  to reach to a steady rotational speed. If 100 milliseconds have been expired (S 112 :YES), then the first-stage driving circuit  510  is turned OFF. As a result, the voltage applied to the motor  3  is decreased to 6V. The motor  3  continues rotating at the steady rotational speed.  
         [0086]     Next, it is judged that the tool  1  is separated from the workpiece W by detecting that the push switch  201  is turned OFF (S 116 ). If separation of the tool  1  from the workpiece W is detected (S 116 :YES), then it is judged whether or not 2 seconds have been expired from the time when the tool  1  is separated from the workpiece W (S 120 ). When 2 seconds have been expired (S 120 :YES), then the second-stage driving circuit  540  is turned OFF (S 124 ). As a result, the voltage applied to the motor  3  is decreased to 5V and the rotational speed of the motor  3  is decreased.  
         [0087]     By preserving 2 second waiting time in S 120 , the change in the rotational speed of the motor  3  can be prevented even if the push switch  201  is momentarily turned OFF during this period due to reaction of the tool  1 . Thus, generation of beats caused by the change in the rotational speed of the motor  3  can be prevented. The waiting time in S 120  is not limited to 2 seconds but different duration of time may be set.  
         [0088]     If the push switch  201  is turned ON during the 2 seconds waiting time (S 122 :YES), then the routine proceeds via S 116  to S 118  where it is judged whether or not the ON state of the push switch  201  continues for more than 60 seconds. The purpose for the 60 seconds continuous ON time detection of the push switch  201  in S 118  is to prevent an unintentional driving of the motor  3  and dissipation of the battery  30  resulting from the motor driving. The motor  3  is unintentionally driven if the push lever  10  is held in a pressed condition for some reasons. Further, if the push switch  201  is continuously ON for more than 60 seconds, the wired circuit may be short-circuited or the push switch  201  is defective. Accordingly, if the push switch  201  is continuously ON for more than 60 seconds (S 118 :YES), then the low power consumption mode is set (S 134 ). On the other hand, if the push switch  201  is not continuously ON for more than 60 seconds (S 118 :NO), then the routine returns to S 116 . It is not intended to limit the duration of time for the continuous ON time detection of the push switch  201  in S 118  to 60 minutes but different duration of time can be set.  
         [0089]     After the second-stage driving circuit  540  is turned OFF (S 124 ), it is judged whether or not 7 seconds have been expired from the time when the push switch  201  is turned OFF (S 126 ). When the push switch  201  is turned OFF, that is, when the tool  1  is separated from the workpiece W, the combustion chamber  26  is in fluid communication with atmosphere. The motor  3  is forcibly driven for 7 second after the push switch  201  is turned OFF to scavenge the exhaust gas and cool the cylinder  20 .  
         [0090]     If the push switch  201  is turned ON before expiration of 7 seconds (S 130 ), it is determined that the nail driving operation is again performed. Accordingly, the second-stage driving circuit.  540  is again turned ON to apply 6V to the motor  3 . When 7 seconds have been expired from the time when the push switch  201  is turned OFF (S 126 :YES), then the third-stage driving circuit  570  is turned OFF (S 128 ) to thereby stop driving the motor  3 , whereupon the routine returns to S 102 .  
         [0091]     Referring next to the flowchart of  FIG. 9 , the battery  30  is loaded into the tool  1  (S 001 ) and the main switch  101  is turned ON (S 002 ). Next, it is judged whether both the push switch  201  and the trigger switch  251  are OFF (S 003 ). If both of the push switch  201  and the trigger switch  251  are OFF (S 003 :YES), then initial settings are performed (S 200 ).  
         [0092]     After the initial settings are performed, it is judged whether or not the trigger switch  251  is continuously OFF for more than 60 minutes (S 202 ). If the judgement in S 202  is affirmative (S 202 :YES), the tool  1  is determined to be in a non-use condition. Therefore, the power source section  100  is set to the low power consumption mode (S 226 ). When the main switch  101  is turned OFF (S 228 :YES), the routine returns to S 002 .  
         [0093]     When judgement in S 202  indicates that the tool  1  is in use condition (S 202 :NO), then it is judged whether the operator triggers the trigger switch  251 . If the trigger switch  251  is continuously ON for 20 milliseconds (S 204 :YES), it is determined that the trigger switch  251  is triggered. Chattering caused by vibration of the tool  1  may turn the trigger switch  251  ON. However, generally, the ON duration of the trigger switch  251  does not last 20 milliseconds, therefore, S 204  can detect only when the operator triggers the trigger switch  251 .  
         [0094]     When it is detected that the operator triggers the trigger switch  251 , the voltage V across the battery  30  is detected (S 206 ). Depending on the detected battery voltage V, a charge time T for charging the ignition capacitor  401  is determined (S 208 ). The charge time T is set to longer if the battery voltage V is lowered. Then, the charging circuit section  400  is turned ON to start charging the ignition capacitor  401  for duration of time T set in S 208 .  
         [0095]     When the charge time T has been expired (S 212 :YES), then the charging circuit section  400  is turned OFF (S 214 ). After charging the ignition capacitor  401  is complete, the ignition circuit section  450  is turned ON for 10 milliseconds (S 216 , S 218 ) to ignite the mixture of flammable gas and air with the spark of the ignition plug  15 . After the ignition is performed, the ignition circuit section  450  is turned OFF (S 220 ).  
         [0096]     Next, it is judged whether the trigger switch  251  is turned OFF. In order to exclude influence of chattering, whether the trigger switch  251  is continuously OFF for 10 milliseconds is detected (S 222 ). When the trigger switch  251  is OFF (S 222 :YES), then the routine returns to S 202 . On the other hand, when the trigger switch  251  is ON (S 222 :NO), it is judged whether or not the trigger switch  251  is continuously ON for more than 60 seconds (S 224 ). If the judgement in S 224  is affirmative, it is assumed that the trigger switch  251  is defective for some reasons. Accordingly, the power source section  100  is set to the low power consumption mode (S 226 ) to stop the operation of the microcomputer  300 . After the low power consumption mode is set, the routine returns to S 002  if the main switch  101  is turned OFF (S 228 :YES).  
         [0097]     Two pieces of programs corresponding to the flowcharts in  FIGS. 8 and 9  are run separately on the same time base. It should be noted that when the trigger switch is turned ON while the push switch is OFF, the liquefied gas is not injected into the combustion chamber. Accordingly, the fastener is prevented from accidentally driven into the workpiece even if the ignition is taken place.  
         [0098]     Although the present invention has been described with respect to specific embodiments, it will be appreciated by one skilled in the art that a variety of changes and modifications may be made without departing from the scope of the invention. For example, certain features may be used independently of others and equivalents may be substituted all within the scope of the invention.