Abstract:
An electric power tool has an electric drive motor whose speed is adjusted by means of an adjusting device. The adjusting device includes an adjusting element movably supported on the housing of the power tool. The adjusting element is associated with a strain measuring device that produces a measurement signal that is used to adjust the drive motor in response to a movement of the adjusting element.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on German Patent Application 10 2009 027 111.2 filed Jun. 23, 2009. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an electric power tool, in particular a hand-held power tool. 
     2. Description of the Prior Art 
     DE 103 09 057 A1 has disclosed a cordless screwdriver that has a housing containing an electric drive motor for driving a tool holder, with a battery pack that is contained in a grip housing and is provided to supply electrical energy to the drive motor. The screwdriver is actuated by means of a push button that is supported in the grip housing in a linearly movable fashion and can be adjusted with infinite variability by the user in order to drive the tool. The speed of the drive motor is adjusted via the actuation distance of the push button. 
     Usually, the position of the push button is determined by means of a variable electrical resistance that has a sliding contact along the path of the push button, with the voltage signal produced being proportional to the actuation distance of the push button. To implement the actuation distance determination, it is necessary to maintain a minimum amount of installation space in the housing of the cordless screwdriver. 
     OBJECT AND SUMMARY OF THE INVENTION 
     The object of the invention is to create a small adjusting device for regulating the motor speed in an electric power tool. The adjusting device should also be embodied in a rugged way. 
     The invention can be used in power tools that are provided with an electric drive motor. In particular, these include hand-held electric power tools such as cordless screwdrivers and drills, angle grinders, or the like. 
     The electric power tool is provided with an adjusting device for regulating the motor speed; the adjusting device includes an adjusting element movably supported on the housing of the power tool. When the adjusting element is actuated, this shifts its relative position in relation to the housing, resulting in a change in the speed of the electric drive motor. The preferably infinitely variable movement of the adjusting element is converted into an electrical signal for acting on the electric motor to produce the desired speed. 
     To obtain a signal that reflects the movement of the adjusting element in relation to the housing, the adjusting element is associated with a strain measuring device that produces a measurement signal when the adjusting element is moved. This measurement signal is preferably converted by motor electronics into an actuation signal that results in the corresponding adjustment to the speed of the drive motor. 
     The power tool according to the invention, equipped with the novel adjusting element for regulating the speed of the drive motor, is based on the principle of strain sensors or strain gauges in that the actuating movement of the adjusting element causes a component of the associated strain measuring device to stretch, which can be detected by sensor. This approach has the advantage that by contrast with embodiments from the prior art, the actuating movement of the adjusting element occurs in a virtually frictionless fashion, which is accompanied by a lower susceptibility to malfunctions and soiling. Another advantage lies in the fact that a small device can be implemented since the strain measuring device, which uses standard components, can be embodied in the form of a small unit. 
     Basically, different actuating motions of the adjusting element can be provided. These include a translatory or linear motion, a rotary or rotating motion, or a combination of rotary and translatory motion. For example, the adjusting element of the adjusting device is embodied in the form of a push button that is secured to the housing of the power tool and guided in a linearly movable fashion. The strain measuring device can detect this linear actuating motion in order to produce a corresponding signal. The strain measuring device can also detect an actuating motion in the form of a rotating motion. To implement the rotating motion, the adjusting element is either supported on the housing in a rotary fashion or is guided so that it can move in a curved path in the housing. 
     According to another suitable embodiment, the strain measuring device is affixed to the housing and the movement of the adjusting element acts on the strain measuring device, thereby generating the desired signal. Basically, however, it is also possible for the strain measuring device to be situated in or on the adjusting element, with the measurement signal being produced when the strain measuring device is acted on with the aid of a component affixed to the housing. 
     According to another advantageous embodiment, in addition to the adjusting element, the adjusting device includes a switch housing, which is affixed to the tool housing, and the adjusting element is movably guided in the switch housing. If the adjusting element is embodied in the form of a linearly movable push button, it is secured so that it is able to move in translatory fashion in guide tracks on the switch housing. The switch housing encloses an interior in which additional components can be accommodated, in particular the strain measuring device, but also optionally other components such as electronic components of the motor electronics. 
     According to an advantageous embodiment, the strain measuring device includes a flexible component that is acted on by the adjusting element or, in the case in which the flexible component is situated on the adjusting element, is acted on by a component affixed to the tool housing when the adjusting element is moved. The flexible component is embodied, for example, in the form of a bending spring that is preferably clamped in place at one end and is elastically deformed transversely to its longitudinal axis when the adjusting element is moved out of its starting position. This elastic deformation is detected with the aid of a strain sensor situated on the flexible component. The signal of the strain sensor can be supplied to the motor electronics in order to produce actuating signals that are used to regulate the speed of the electric motor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of preferred embodiments taken in conjunction with the drawings, in which: 
         FIG. 1  shows a hand-held electric power tool embodied in the form of a cordless screwdriver or drill, equipped with a push button on the housing for adjusting the motor speed; 
         FIG. 2  is a section through a motor speed adjusting device, which includes the push button, depicted in the starting position of the push button; and 
         FIG. 3  is a depiction corresponding to the one in  FIG. 2 , but with the push button in an actuated position. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Components that are the same are provided with the same reference numerals in the drawings. 
     The hand-held electric power tool  1  shown in  FIG. 1  is a cordless screwdriver or drill. The hand-held electric power tool  1  has a housing  2  that includes a motor housing  3  and a grip housing  4 . The motor housing  3  contains an electric drive motor  5  whose drive motion is transmitted via a transmission  6  to a rotary-supported tool holder  7  for holding a tool. The electric drive motor  5  is actuated by means of a push button  8  that is situated in a linearly movable fashion on the grip housing  4 . The lower region of the grip housing  4  accommodates a battery pack  9  that supplies the electric drive motor  5  with electrical energy. The speed of the electric drive motor  5  is regulated by actuating the push button  8 , which is a component of an adjusting device. The movement of the push button  8  relative to the housing  4  constitutes a measure for the speed of the electric motor  5 . 
     In detailed depictions,  FIGS. 2 and 3  each show an example of a possible embodiment variant of an adjusting device  10  that includes the push button  8  as an adjusting element and a switch housing  11  that is affixed to the tool housing. The relative movement of the push button  8  in relation to the switch housing  11  is detected by sensor and is converted by motor electronics into an actuation signal for adjusting the electric drive motor. 
     The switch housing  11  has a U-shaped cross section and on the outside, is provided with lateral legs with guide surfaces  12 ; respective sections of the push button  8  rest against these guide surfaces and are guided along them in sliding fashion. For example, the guide surfaces  12  are embodied in the form of guide grooves. The sections of the push button  8  resting against the guide surfaces  12  are embodied in the form of hook-shaped sections bent laterally in relation to the movement direction. 
     In an alternative embodiment, the guide surfaces  12  on the switch housing  11  are not embodied on the outside, but rather on the inside. In this case, the hook-shaped sections on the push button  8 , which rest against the guide surfaces  12  and are guided along them, are bent outward in order to produce the contact with the guide surfaces. 
     The switch housing  11  encloses an interior  13  in which a strain measuring device  14  is accommodated. The strain measuring device  14  includes a flexible element embodied in the form of a bending spring  15  as well as a strain sensor  16 , which is positioned on the bending spring  15  and is capable of measuring a flexion of the bending spring transverse to its longitudinal axis. One end of the bending spring  15  is clamped to the switch housing  11  with the aid of a fastening element  17 . The fastening element  17  attaches the bending spring  15  to the inner wall of a lateral leg of the switch housing  11 . The opposite end of the bending spring  15 , by contrast, is not clamped, but can elastically flex in the direction of the actuating motion of the push button  8  when placed under stress. 
     On the inside oriented toward the bending spring  15 , the push button  8  is provided with a rib  18  that contacts the elastic bending spring  15  in the region of its free end. When the push button  8  is moved in the actuating direction  19  in a linear, translatory fashion, the rib  18  of the push button presses against the free end of the bending spring  15  and moves the bending spring into the interior  13  of the switch housing  11 .  FIG. 3  shows the elastically bent bending spring  15 . 
     The interior  13  of the switch housing  11  can accommodate an electronic component  20  that is in particular a component of a set of motor electronics for adjusting the electric drive motor. Preferably, the measurement signals of the strain sensor  16  are evaluated in the electronic component  20 . 
     The flexion of the bending spring  15 , which is situated in a straight line in the switch housing  11  in the unstressed state shown in  FIG. 2  and extends orthogonal to the actuating direction  19 , is determined with the aid of the strain sensor  16 . The flexion of the bending spring  15  constitutes a measure for the translatory, linear motion of the push button  8  in the direction toward the switch housing  11 . The relationship between the translatory motion of the push button  8  and the flexion of the bending spring  15  can optionally be stored in memory in the electronic component  20 . An actuating signal for regulating the motor speed can also be produced directly from the flexion of the bending spring  15 . 
     The foregoing relates to preferred exemplary embodiments of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.