Abstract:
A hammer drill comprising a body ( 2 ) containing an electric motor ( 4 ) for driving an output ( 6 ) of the drill is disclosed. A vibration transducer ( 12 ) senses vibrations generated by the motor and produces a vibration signal dependent upon the sensed vibrations. An electronic module ( 10 ) includes a controller ( 10   a ) for controlling the rotational speed of the motor, and a signal processor ( 10   b ) for receiving the vibration signal from the vibration transducer, determining the rotational speed of the motor based on the vibration signal, and providing an output signal to the controller to cause the controller to control the rotational speed of the motor.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to portable power tools, and relates particularly to a portable power tool having means for detecting and controlling the speed of rotation of the motor of the portable power tool. 
       BACKGROUND OF THE INVENTION 
       [0002]    Power tools such as hammer drills are well known in the art and are generally provided with an electric motor driving a spindle for receiving the shank of a tool or bit such as a drill bit or a chisel bit. Such hammer drills comprise an impact mechanism that converts the rotational drive from the motor to a reciprocating drive causing a piston to reciprocate within the spindle. The piston reciprocatingly drives a ram by means of a closed air cushion located between the piston and the ram, and the impacts from the ram are then transmitted to the tool or bit of the hammer. The rotational movement of the motor and the reciprocating piston cause vibrations having various superimposed frequencies that are transmitted through and from the power tool. 
         [0003]    Furthermore, it is also well known in the art that the cutting speed of the tool bit depends, inter alia, on the diameter of the tool bit, the appropriate rotational speed of the motor and the generated hammer frequency for a particular work material. Accordingly, prior art power tool are known to include motor speed control knobs which can be manually adjusted by an operator to set the speed of motor and/or hammer frequency to the recommended speed for a given tool bit diameter and/or work material. When the power tool is in use, the resistance of the work material to the cutting action of the tool bit varies, which can cause unpredictable variations of the rotational speed of the motor and therefore affect the efficiency of the power tool. Also, overheating of the motor during operation can occur if the tool encounters excessive resistance. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    It is an object of the present invention to provide a portable power tool having means for automatically determining and controlling the rotational speed of the motor during operation. 
         [0005]    According to the present invention there is provided a power tool comprising a housing, an electric motor within the housing for driving an output of the tool, a vibration transducer for sensing vibrations generated by the motor and producing a vibration signal dependent upon the sensed vibrations, a controller for controlling the rotational speed of the motor, and a signal processor for receiving the vibration signal from the vibration transducer, determining the rotational speed of the motor based on the vibration signal, and providing an output signal to the controller to cause the controller to control the rotational speed of the motor. 
         [0006]    By providing a signal processor for receiving the vibration signal from the vibration transducer, determining the rotational speed of the motor based on the vibration signal, and providing an output signal to the controller to cause the controller to control the rotational speed of the motor, this provides the advantage of enabling the rotational speed of the motor to be kept relatively constant, irrespective of the resistance caused by the work material during operation. This maximises, for example, the cutting efficiency of the power tool, and enables the motor to be protected from overheating. 
         [0007]    The controller and the signal processor may be integrated within a single electronic module. This provides the advantage of saving space within the housing of the power tool and reducing the complexity and cost of manufacture. 
         [0008]    The signal processor may be adapted to enhance and/or isolate a component of the vibration signal caused by the rotation of the motor. This provides the advantage of minimizing false readings by improving the selectivity and quality of the vibration signal of interest, e.g. the signal caused by radial vibration of the motor. 
         [0009]    The signal processor may be adapted to produce a frequency spectrum of the sensed vibration signal and select at least one frequency component according to amplitude and/or frequency. This provides the advantage of facilitating the selection process to find the frequency component that can be used to determine the rotational speed of the motor, for example by simply (i) selecting a frequency component with the highest amplitude and/or (ii) selecting a frequency component within a specific frequency range known from the manually selected speed settings of the power tool. 
         [0010]    The signal processor may be adapted to provide an output signal determined according to the difference between the determined rotational speed and a preselected target rotational speed of the motor. 
         [0011]    The vibration transducer may be mounted on the body adjacent to the motor. 
         [0012]    The vibration transducer may be adapted to detect radial vibrations caused by the motor. 
         [0013]    The vibration transducer may include at least one piezo-electric sensor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    An embodiment of the present invention will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawings in which: 
           [0015]      FIG. 1  shows a cross-sectional side view of a hammer drill embodying the present invention; 
           [0016]      FIG. 2  shows a graph of a typical vibration signal received from the vibration transducer of  FIG. 1 ; 
           [0017]      FIG. 3  shows a graph of the Fourier Transform (amplitude vs. frequency) of the vibration signal shown in  FIG. 2 ; and 
           [0018]      FIG. 4  shows flow process charts of the main-routine executed by the controller and the sub-routines soft-start and motor-control executed within the main-routine. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    Referring to  FIG. 1 , a hammer drill comprises a body  2  in which is mounted a motor  4 . The motor  4  rotatingly drives a chuck  6 , for receiving a drill bit (not shown), via a gearbox  8 . The rotational speed of the motor  4  is controlled by an electronic module  10  comprising a controller  10   a  and a signal processor  10   b,  the function of which will be described in greater detail below. 
         [0020]    A vibration transducer  12  is mounted on the body  2  near the motor  4  but not on the axis of rotation  14  of the spindle  16  of the motor  4 . The vibration transducer  12  can be any type of sensor, for example, a piezo-electric sensor, but must be capable of detecting vibrations over a range of frequencies. The vibration transducer  12  measures the amplitude of the vibration caused by the motor  4  in a radial direction from the axis of rotation  14  of the spindle  16 . 
         [0021]      FIG. 2  shows a graph of a typical vibration signal  20  produced by the vibration transducer  12  when the hammer drill is operating, the graph showing amplitude versus time. The vibration signal  20  generally represents vibrations from many sources from within the hammer drill. For example, vibration is generated by the rotation of the rotor of the motor  4  due to imperfectly rotationally symmetrical alignment of the rotor with motor rotational axis  14 . Other vibrations may be caused by the gears  8  and rotation of the spindle  6  or by the reciprocating drive of the impact mechanism. The vibration signal  20  is then fed into the signal processor  10   b  of the electronic module  10 . 
         [0022]      FIG. 3  shows a graph of a frequency spectrum (amplitude vs. frequency) provided by the signal processor  10   b  by applying a Fourier Transform algorithm to the vibration signal  20  of  FIG. 2  in order to isolate the various frequencies of the vibration signal  20 . For example, the vibration caused by the imperfectly symmetrical rotation of the armature of the motor  4  causes a spike  22  (frequency component) as shown in the graph of  FIG. 3 , i.e. it generates a vibration of relatively large amplitude at a particular frequency. The resulting signal at the particular frequency of the spike  22  is then filtered to enhance and/or isolate the component of the vibration signal  20  or, at least, enhance the major component of the vibration signal  20  caused by the motor  4 . 
         [0023]    The frequency of the vibration caused by the motor  4  is directly proportional to the rotational speed of the motor  4 . As such, determining the frequency will enable the rotational speed of the motor  4  to be calculated. If, for example, the rotational speed of the motor  4  increases, the frequency of the vibration increases. Similarly, if the rotational speed of the motor  4  decreases, the frequency of the vibration decreases. Thus, by measuring the frequency component of the rotational movement of the motor  4 , the signal processor  10   b  can determine the rotational speed of the motor  4  and provide an output signal, based on the difference between the determined rotational speed and a pre-selected target speed, for the controller  10   a  in order to automatically adjust the rotational speed of the motor  4 . 
         [0024]      FIG. 4  shows a flow process chart of the main-routine executed by the electronic module  10  during operation of the power tool. A detailed description of the main routine and its sub-routines (i) soft-start and (ii) motor-control is given below: 
         [0025]    Within the main-routine, the operator first ensures that power is provided by plugging in the power tool at step S 10  and manually switches on the power tool by pressing the switch-on button at step S 20 . The controller  10   a  will then set the maximum rotational speed of the motor  4  according to the speed dial setting at step S 30  and start the soft-start sub-routine at step S 40  to protect the motor from damage by gradually increasing the motor speed until reaching the target rotational speed of the motor  4 . The rotational speed of the motor  4  is then maintained by the motor-control subroutine at step S 50  by constantly monitoring and adjusting the rotational speed of the motor  4  until the operator manually switches off the power tool at step S 60 . 
         [0026]    Within the soft-start sub-routine of step S 40 , the firing angle of a triac (not shown) provided within or controlled by the controller  10   a  is increased at step S 110  and a bandwidth filter in the processor  10   b  is adjusted automatically at step S 120 . The vibration transducer  12  measures the vibration of the motor  4  at step S 130  and provides the vibration signal  20  to the signal processor  10   b,  where the vibration signal  20  is, for example, filtered using an adjustable bandwidth filter at step S 140 . A frequency spectrum of the vibration signal  20  is generated by means of a Fast Fourier Transformation at step S 150  and the most recent frequency spike caused by the rotational movement of the motor  4  is selected at step S 160  according to, for example, the amplitude, in order to determine the instantaneous rotational speed of the motor  4  which is then compared at step S 170  to a target rotational speed of the motor  4 . As long as the current rotational speed is smaller than the target rotational speed of the motor  4 , the soft-start routine returns to step S 110  and is repeated using an increased Triac firing angle with each iteration. When the target rotational speed of the motor  4  is reached, the soft-start routine is terminated at step S 180  and the motor-control routine of step S 50  is started at step S 210 . 
         [0027]    Within the motor-control routine, the rotational speed of the motor  4  is adjusted according to the speed dial setting at step S 210  and the bandwidth filter coefficient is adjusted automatically, if necessary, at step S 220 . The vibration transducer  12  measures the vibration of the motor  4  at step S 230  and provides the vibration signal  20  to the signal processor  10   b,  where the vibration signal  20  is, for example, filtered using an adjustable bandwidth filter at step S 240 . A frequency spectrum of the vibration signal  20  is generated at step S 250  by means of a Fast Fourier Transformation and the most recent frequency spike caused by the rotational movement of the motor  4  is selected at step S 260  according to, for example, the amplitude, in order to determine the instantaneous rotational speed of the motor  4  which is then compared to the target rotational speed of the motor  4 . The rotational speed of the motor  4  is then adjusted, if necessary, at step S 270  and the motor-control routine is repeated until the operator manually switches off the power tool. 
         [0028]    It will be appreciated by persons skilled in the art that the above embodiment has been described by way of example only and not in any limitative sense, and that various alterations and modifications are possible without departing from the scope of the invention as defined by the appended claims.