Abstract:
A power tool has a power head  11;  a motor  10  configured to drive the power head; a handle  13  at which a switch  30  and a trigger  31  are installed; a force sensor  33  installed at the handle and electrically connected to the switch and configured to output voltage in response to force acted thereon; and a control unit configured to control the motor in response to the output voltage from the force sensor. A method for operating a power tool is also disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This non-provisional patent application claims priority under 35 U.S.C. §119(a) from Patent Application No. 200810216255.6 filed in The People&#39;s Republic of China on Sep. 19, 2008. 
       FIELD OF THE INVENTION 
       [0002]    This invention relates to power tools and in particular, to a power tool having force sensing components. 
       BACKGROUND OF THE INVENTION 
       [0003]    Drills are a typical kind of power tool widely used in industry or home. As shown in  FIG. 4 , a known power tool  10  in the form of a drill usually comprises a power head  11  in the form of a chuck for holding drills etc, a motor  12  for driving the power head  11 , a handle  13 , a battery  14  for providing a source of power to the motor  12  and other electric components, a switch  30  a installed at the handle  13 , a trigger  31  for activating the switch and an enclosure  15 . When a user holds the handle  13  and turns on the switch  30  by pushing the trigger  31 , the motor  12  is activated to drive the power head  11 . However, often a user is not ready to use the drill at the moment of holding the handle of the drill. As such, the user especially an inexperienced user may be scared when the drill works before he/she is ready. Furthermore, output of the motor is not being used efficiently, which results in waste of energy 
       SUMMARY OF THE INVENTION 
       [0004]    Hence there is a desire for an improved power tool. 
         [0005]    Accordingly, in one aspect thereof, the present invention provides a power tool comprising: a power head; a motor configured to drive the power head; a handle accommodating a switch and a trigger; a force sensor fitted to the handle and electrically connected to the switch, the force sensor being configured to output a signal in response to force acting thereon; and a control unit configured to control the motor in response to the signal from the force sensor. 
         [0006]    Preferably, the power tool further comprises an enclosure enclosing the motor and forming the handle, wherein the enclosure defines an opening for the force sensor to facilitate the force sensor sensing a force applied by a user. 
         [0007]    Preferably, the opening is covered by a cover made of soft material. 
         [0008]    Preferably, the soft material comprises rubber. 
         [0009]    Preferably, the control unit comprises a comparison device configured to compare the value of the signal from the force sensor with preset values stored therein, and a signal amplifier configured to output a corresponding value of voltage/current to the motor in response to the result of the comparison. 
         [0010]    Preferably, the signal from the force sensor is a voltage signal. 
         [0011]    Preferably, the control unit is installed in the switch. 
         [0012]    Preferably, the switch is activated when the trigger is depressed. 
         [0013]    According to a second aspect, the present invention provides a method of operating a power tool which comprises a motor and a handle accommodating a switch, a trigger and a force sensor, the method comprising: holding the handle of the power tool and operating the trigger to activate the switch and bring the power tool into a state ready for working; and applying a force to the force sensor by pushing on a portion of the handle to start the motor. 
         [0014]    Preferably, the method includes increasing the force acting on the force sensor by pushing harder, to increase the power output from the motor. 
         [0015]    Preferably, the method includes decreasing the force acting on the force sensor by pushing less on the handle, to reduce the power output from the motor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    A preferred embodiment of the invention will now be described, by way of example only, with reference to figures of the accompanying drawings. In the figures, identical structures, elements or parts that appear in more than one figure are generally labelled with a same reference numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are generally chosen for convenience and clarity of presentation and are not necessarily shown to scale. The figures are listed below. 
           [0017]      FIG. 1  illustrates a power tool, in the form of a drill, in accordance with a first embodiment of the present invention; 
           [0018]      FIG. 2  is an enlarged view of a portion of the power tool of  FIG. 1 ; 
           [0019]      FIG. 3  is a schematic block diagram of an electronic circuit of the power tool of  FIG. 1 ; and 
           [0020]      FIG. 4  illustrates a known power tool in the form of a drill. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    The power tool  10  of  FIG. 1  is a drill having a power head  11 , a motor  12 , a power source  14 , and an enclosure  15  for enclosing the motor  12  and which forms a handle  13 . The handle  13  has installed therein a power switch  30  and a trigger  31  connected to the switch  30 . The switch  30  has a locating slot  32 . A pin  35  of a force sensor  33  is inserted in the locating slot  32  and is electrically connected to the switch  30 . A heat sink (not shown) may be installed in the handle  13  contacting with the force sensor  33  and/or switch  30  for facilitating heat dissipation there from. The power source  14  is adapted to provide electric power to the motor  12 , the switch  30 , the force sensor  33  and other electric components. The power source  14  may be a battery accommodated within a housing attached to one end of the handle  13 . 
         [0022]    An enlarged view of the switch  30  and surrounding components is shown in  FIG. 2 , which is an enlargement of a portion of the power tool of  FIG. 1 . The enclosure  15  may be made of a plastics material and is preferably injection molded. The enclosure  15  defines an opening  51  corresponding to the force sensor  33 . The force sensor  33  comprises a sensing surface  34  and the pin  35 . The sensing surface  34  may extend outwardly through the opening  51 . A cover  16  which may be attached to the enclosure  15 , covers the opening  151  and the force sensor  33 . Preferably, the cover  16  closely contacts the sensing surface  34  of the force sensor  33 . The cover  16  may be made of soft material for example, rubber, such that force acting on the cover  16  is transferred to the sensing surface  34  of the force sensor  33 . The cover  16  may absorb shock when the power tool  10  works. The pin  35  of the force sensor  33  is inserted into the locating slot  32  of the power switch  30  to thereby retain the force sensor  33  to the switch  30 . When a force F acts on the outer surface of the cover  16 , the force F is transferred to the sensing surface  34 . Consequently, the force sensor  33  will generate a voltage signal corresponding to the magnitude of the applied force F. 
         [0023]    Referring to  FIG. 3 , the power switch  30  comprises a control unit  40  which comprises a comparison device  42  and a signal amplifier  44 . A group of voltages V 1 , V 2  . . . V n  are preset in the comparison device. The increasing force applied to the cover  16  may be presented as F 0 , F 0 *(1+y), F 0 *(1+2y), . . . F 0 *(1+ny), the corresponding voltage signal generated by the force sensor is represented as V 0 , V 0 *(1+x), V 0 *(1+2x), . . . V 0 *(1+nx), wherein F 0  presents an initial holding force acting on the handle  13  when the user holds the handle of the power tool, V 0  presents an initial voltage value, y presents increased percentage of the force being applied to the cover  16 , and x presents increased percentage of the value of the voltage signal generated by the force sensor. Usually, different users will apply different holding forces. However, the difference is not great and the holding force is usually in a common range. Therefore, the voltage V 1  may be preset to be much greater than the usual maximum initial voltage V 0  corresponding to the maximum holding force in the range. 
         [0024]    In operation, a user holds the handle  13  of the power tool  10  and depresses the trigger  31  to activate the switch  30 . The user applies to the cover  16  a holding force F 0 . Consequently, the force sensor  33  outputs a voltage V 0  which is transferred to the comparison device  42 . Since the voltage V 0  is much less than the preset voltage V 1 , the control unit  40  of the power switch  30  keeps the motor turned off. When the user is ready to use the power tool, the user pushes the cover  16  in a predetermined direction and therefore applies an increased force F 0 *(1+y), F 0 *(1+2y), . . . F 0 *(1+ny) on the cover  16 . Accordingly, the force sensor  33  outputs a signal with an increased voltage V 0 *(1+x), V 0 *(1+2x), . . . V 0 *(1+nx) to the comparison device  42  which compares the increased voltage with the preset voltage V 1 , V 2  . . . V n . When the voltage V 0 *(1+x) is greater than the corresponding preset voltage V 1 , the signal amplifier  44  outputs corresponding voltage/current to start the motor  12  which drives the power head  11 , in response to signals from the comparator  42 . When the voltage V 0 *(1+nx) is greater than the corresponding preset voltage value Vn, the signal amplifier  44  outputs correspondingly increased voltage/current to the motor  12  which results in the motor  12  outputting correspondingly increased power. Understandably, when the user reduces the force applied to the cover  16 , the signal amplifier  44  outputs correspondingly decreased voltage/current to the motor  12  which results in the motor  12  outputting correspondingly reduced power. When the decreased voltage output from the force sensor  33  is less than the preset voltage V 1 , the control unit  40  will switch the motor off. Thus the motor is controlled by the force that the user applies to that portion of the power tool corresponding to the force sensor. 
         [0025]    In the preferred embodiment of the present invention, the motor of the power tool does not work at the moment the user holds the handle of the power tool. Instead the motor does not start until the user consciously increases the force acting on the handle, to thereby avoid scaring and/or damaging the user and wasting of energy. 
         [0026]    In the present invention, the power tool may be cordless drills, drills, scissors, screwdrivers etc. Optionally, the control unit can control the rate of increase in speed or power out of the motor in response to an increase or decrease in the force applied to the power tool. 
         [0027]    In the description and claims of the present application, each of the verbs “comprise”, “include”, “contain” and “have”, and variations thereof, are used in an inclusive sense, to specify the presence of the stated item but not to exclude the presence of additional items. 
         [0028]    Although the invention is described with reference to one or more preferred embodiments, it should be appreciated by those skilled in the art that various modifications are possible. Therefore, the scope of the invention is to be determined by reference to the claims that follow. 
         [0029]    For example, the output from the force sensor is described as a voltage signal, but other types of signals, such as a current signal or a digital signal may be used and it is the value of the output signal which the comparison device compares with predetermined or stored values in order to control the operation of the motor.