Patent Publication Number: US-2012045976-A1

Title: Handheld electric machine tool

Description:
FIELD OF THE INVENTION 
     The present invention relates to a handheld electric machine tool including a housing with a grip area, a tool area for a tool that is drivable in a linear and/or rotary oscillating manner, an operating part on the housing for activation of the tool and/or the electric machine tool by the user, a drive unit disposed in the housing for producing a working motion of the tool, an electronic unit disposed in the housing for acting upon the drive unit with the required machining output consisting of at least control and/or regulating signals, an operating voltage unit for supplying an electrical DC voltage to the electronic unit, the drive unit including at least one excitation actuator having a volume of excitation-active material, which excitation actuator when in operation is electrically supplied by the operating voltage unit and is controlled or regulated by the electronic unit. 
     BACKGROUND INFORMATION 
     Handheld electric machine tools are characterized by being portable and by being held and guided by a user by hand when in operation. They may be cordlessly operated by battery packs or may be operated with mains current. In particular, they generally consist of only one housing which is completely held by the user. 
     European Patent No. EP 1 598 171 B1 describes a mechanical configuration of a welding head of a portable welding gun in which an ultrasound actuator acts upon the welding head with mechanical power. 
     SUMMARY 
     The present invention relates to a handheld electric machine tool including a housing with a grip area, a tool area for a tool that is drivable in a linear and/or rotary oscillating manner, an operating part on the housing for activation of the tool and/or the electric machine tool by the user, a drive unit disposed in the housing for producing a working motion of the tool, an electronic unit disposed in the housing for acting upon the drive unit with the required machining output consisting of at least control and/or regulating signals, an operating voltage unit for supplying an electrical DC voltage to the electronic unit, the drive unit including at least one excitation actuator having a volume of excitation-active material, which excitation actuator when in operation is electrically supplied by the operating voltage unit and is controlled or regulated by the electronic unit. 
     The electronic unit is configured to operate the at least one excitation actuator in a resonant frequency. 
     If the excitation actuator is operated with its resonant frequency, it is possible, with a sufficiently high Q factor of the oscillating system, for a high mechanical output power to be delivered corresponding to an electrical input power. The excitation actuator may be an ultrasound excitation generator, especially a piezo actuator in the style of a Langevin oscillator. The piezo actuator has excitation-active material as the piezoelectric material. Typically, the Q factor of the undamped oscillating system lies at values above 100, typically above 500. The resonance system of the excitation actuator, which has the resonant frequency, includes the Langevin oscillator with piezoelectrically active material, and components coupled to the oscillator, especially components that amplify the ultrasound and/or transmit it to a machining site. Such components are known, for example, as boosters or sonotrodes. This makes possible a reduction in overall size and makes it possible to provide a compact device. That advantageously produces a compact, high-performance electric machine tool which is handy at the same time. 
     It is also possible for a plurality of excitation actuators, for example of the same or also of differing resonant frequency, to be provided as drive components. Alternatively, it is also possible for one or more further drive components, such as an electric motor, to be provided. The various drive components may be operated as alternatives or in combination. If the at least one excitation actuator is operated in resonance, the power yield is particularly high, so that, for a given output power of the electric machine tool, the construction may be particularly compact, which is also conducive to comfortable handling of the handheld electric machine tool. The proposed electric machine tool is a one-piece implement with which it is possible to dispense with troublesome connection cables between separate housing parts. The electric machine tool may be operable cordlessly with non-rechargeable or rechargeable batteries or—in addition or alternatively—may be operable by mains power via a mains cable. The tool may be an interchangeable tool detachably connected to the excitation actuator or it may be fixedly connected to the excitation actuator. The connection may, for example, be integral or non-positive. The electric machine tool is especially a machining tool with which objects or surfaces may be machined or modified, such as, for example, drills, hammer drills, cutting tools, grinding machines, milling machines, saws, welding devices and the like. 
     In accordance with an advantageous development of the present invention, the electronic unit may include a regulating unit with frequency matching for adjustment of the resonant frequency of the at least one excitation actuator. Advantageously, during operation of the electric machine tool the resonant frequency may be continuously adapted if, for example, the resonant frequency of the excitation actuator changes due to temperature change, changing of the tool coupled to the excitation actuator or upon loading of the tool. In that manner, an optimum power yield is always made possible in operation. Advantageously, the electronic unit may include a phase-regulating chain with which the resonant frequency may be excited with high accuracy. In that manner, a phase shift between the electrical current and electrical voltage supplied to the piezoelectrically active material to excite the ultrasonic oscillations may be set and maintained at a fixed value, especially  0 ° phase difference between current and voltage signal, thereby enabling an optimum power yield to be achieved. 
     In accordance with an advantageous development of the present invention, the volume of the piezoelectrically active material may be at least 0.2 cm 3 , preferably 0.5 cm 3 , especially at least 1 cm 3 . Advantageously, it is possible for a sufficient ultrasound power to be achieved with a small overall size of the excitation actuator. 
     In accordance with an advantageous development of the present invention, the at least one excitation actuator may have a power density of at least 5 Watt/cm 3 , preferably at least 20 Watt/cm 3 , based on the volume of the piezo-electrically active material of the at least one excitation actuator. A correspondingly high power density is advantageous for a handheld compact electric machine tool having the smallest possible dimensions and low production costs. 
     In accordance with an advantageous development, the at least one excitation actuator may have, at the tip of the tool, an oscillation amplitude of at least 3 μm, preferably at least 8 μm, especially at least 12 μm. A correspondingly high oscillation amplitude is advantageous for good power transfer to the workpiece and hence for a high rate of work progress by the electric machine tool. 
     In accordance with an advantageous development of the present invention, on the input side of the electronic unit an electrical power for acting upon the at least one excitation actuator may be at least 20 Watt. Advantageously, it is thereby possible to ensure sufficient power for an electric machine tool. Customary power outputs in the do-it-yourself sector are, for small cutting systems, approximately from 20 Watt to 250 Watt, preferably from 50 Watt to 150 Watt. For higher-powered applications, for example drilling, power outputs starting at 50 Watt up to 1000 Watt, preferably from 200 Watt to 500 Watt, are required. In the professional trade sector, the power requirement for small systems is approximately from 50 to 400 Watt, preferably from 100 to 250 Watt. In the case of large systems, power outputs of from 200 Watt to 2000 Watt, preferably from 400 Watt to 1000 Watt, are employed. It is nevertheless possible to produce an electric machine tool with handy dimensions which not only is capable of being gripped or held by the hand of the user but also affords a sufficiently high power output for machining purposes. 
     In accordance with an advantageous development of the present invention, a maximum electric excitation field strength of the at least one excitation actuator may be in the range below 300 V/mm (based on the thickness, especially disc thickness, of the piezoelectrically active material), preferably in the range from 50 V/mm to 220 V/mm. At a disc thickness of the excitation actuator of typically from 1 mm to 10 mm, preferably from 2 mm to 6 mm, and especially of around 5 mm, the electrical voltages are below 1000 Volt. That advantageously makes it possible for the excitation actuator to be used in the handheld electric machine tool with sufficient mechanical output power while having advantageously small dimensions. 
     In accordance with an advantageous development of the present invention, an electrical output voltage of the operating voltage unit when supplied by electrochemical storage devices may be within from 3 Volt to 100 Volt DC, preferably in the range from 3.5 V to 40 V, and especially may be 36 Volt, 24 Volt, 18 Volt, 14.4 Volt, 12 Volt, 10.6 Volt, 7.2 Volt and 3.6 Volt. It is advantageously possible to use non-rechargeable battery packs or rechargeable battery packs that are small and light enough to still afford easy handling of the electric machine tool at high power output. 
     In accordance with an advantageous development of the present invention, a DC voltage component of the electrical output voltage of the operating voltage unit when supplied with mains voltage may be within from 0.5 U mains  (effective value of mains voltage) to 2 U mains . Preferably, for example with the use of a bridge rectifier with smoothing capacitor, 1.4 U mains . In a further embodiment, the mains voltage may be transformed using an input-side transformer to a voltage suitable for the operating voltage unit. 
     In accordance with an advantageous development of the present invention, the operating frequency of the at least one excitation actuator may be in the range of from 10 kHz to 1000 kHz, preferably from 30 kHz to 50 kHz, and especially from 35 kHz to 45 kHz, more especially around 40 kHz. With increasing frequency, the overall size of the components decreases and the mechanical load on the oscillating system increases, producing in the selected frequency range advantageous proportions with high output power and favorable weight of the electric machine tool. 
     In accordance with an advantageous development of the present invention, the operating voltage unit may include an electrochemical storage device, preferably a rechargeable electrochemical storage device. The operating voltage unit takes up only very little space, which is advantageous in terms of the compactness and weight of the electric machine tool. Advantageous systems are those based on, for example, lithium ions (Li ions) or also nickel-metal hydride (NiMeH), nickel-cadmium (NiCd) or also lead and the like. These may be fixedly integrated in the housing and recharged via a charging connection. Alternatively, the operating voltage unit may be in the form of an exchangeable system, with replaceable electrochemical storage devices which may also be rechargeable externally if appropriate and which may be plugged into a holder provided for the purpose in or on the housing. Depending on the power output required, the rated voltage of the operating voltage unit may be, for example, from 3 Volt DC to 48 Volt DC, for example 12 Volt DC. 
     In accordance with an advantageous development of the present invention, the operating voltage unit may include an AC/DC transformer unit. In that case, a mains connection may also be provided for the electric machine tool, and rectification and smoothing of the mains voltage may take place in the operating voltage unit. Although the conditioning of the mains voltage requires more space than an energy storage device, the further space-saving and compact construction in a single housing still makes simplified operation and handling of the electric machine tool possible. 
     In accordance with an advantageous development of the present invention, the electronic unit may be concentrated on a printed circuit board. That allows a particularly space-saving arrangement in the housing. The electronic activation system of the excitation actuator is particularly compact. 
     In accordance with an advantageous development of the present invention, for signal filtering and for inductive compensation of the at least one excitation actuator at least one inductance may be provided in a power circuit of the electronic unit acting upon the at least one excitation actuator with electrical power. It is possible to achieve a space-saving layout of the power inductances in a single coil core. The signal filtering and inductive compensation of the piezo actuator, which is beneficial in the case of excitation actuators, may be provided directly by a specifically adjusted stray inductance of a transmission transformer that is required in any case, or may be afforded by an inductance wound on the same coil core. An additional coil core with a further inductance in the power circuit may thereby be omitted. 
     In accordance with an advantageous development of the present invention, at least drive unit, electronic unit and operating voltage unit may be distributed in the housing in such a manner that a center of gravity lies in the region of the grip part. The user is able to handle the electric machine tool safely and conveniently. Safety and ease of use are enhanced. 
     In accordance with an advantageous development of the present invention, the drive unit may include, in addition to the at least one excitation actuator, at least one further drive component. Advantageously, a motion produced by the at least one excitation actuator may be superimposed on the working motion of a tool driven by the at least one further drive component, thereby enabling work progress to be considerably improved and making the machining easier. 
     In accordance with an advantageous development of the present invention, the at least one excitation actuator may form a main energy consumer of the electric machine tool, for which preferably at least 50% of the electrical input power may be provided. In an advantageous development, at least 75%, preferably at least 80%, of the electrical input power may be provided for the excitation actuator. The rate of work progress of the electric machine tool when using ultrasound is especially high, and therefore a further energy consumer, especially a further drive component, such as a drill, chisel, cutter or the like, may be smaller. That means that the drive and associated electronic components and the energy supply may also be smaller, which in turn allows enhanced ease of use and improved handling of the handheld electric machine tool. 
     In accordance with an advantageous development of the present invention, one or more operating indicators for an activated state of the at least one excitation actuator may be provided. The indicators may be optical and/or acoustic and/or haptic. The operating safety of the electric machine tool is increased, since it is clearly evident when the excitation actuator is activated and capable of delivering mechanical power. 
     In accordance with an advantageous development of the present invention, the drive unit which imparts a working motion to the tool may impart superimposed oscillations to the tool. The drive unit may have as a further drive component, for example, an electric drive motor which is housed in the housing of the electric machine tool. The motor shaft is normally coupled via a gear unit to a tool shaft which is the carrier of the tool and executes the working motion. The tool is usually to be fastened to the tool shaft in an interchangeable manner. 
     The electric machine tool may, for example, be used for chip-generating machining of workpieces, where, to reduce the chip size, the excitation actuator, which is able to produce superimposed oscillations in the tool, is advantageously disposed in the electric machine tool. Those superimposed oscillations are superimposed on the working motion of the tool. 
     According to the type of electric machine tool and depending on the tool used and the material of the workpiece to be machined, the superimposed oscillations, which emanate not from the drive motor but from the excitation actuator, may be generated with a frequency that results in a significant reduction in the chip size. Since smaller chips also have a smaller heat capacity, the chips are able to cool down in a shorter time, thereby reducing the fire risk. Furthermore, the smaller chips per se lead to a reduced risk of injury, since their momentum is lower. 
     The frequency of the superimposed oscillations is expediently in the ultrasound range and may thus be, for example, at least 20 kHz. That comparatively high frequency has, on the one hand, the advantage that oscillations in that order of magnitude are no longer audible to humans, and therefore no noise nuisance occurs. On the other hand, it has been found that oscillations at and above that order of magnitude are particularly effective in significantly reducing the size of the chips produced in the machining of a workpiece. 
     It may be expedient to generate superimposed oscillations that are in considerably greater orders of magnitude. In principle, oscillations up to and including the megahertz range come into consideration. In addition, it is also possible to generate superimposed oscillations of lower frequency. 
     Owing to the superimposition on the working motion of the tool, on the one hand, and owing to the generally distinctly higher frequency, the generation of the superimposed oscillations has no effect on the working motion and hence on the result of the workpiece machining operation. In addition, the superimposed oscillations are usually of only a very small amplitude, so that the machining of the workpiece is not impaired. 
     The advantageous generation of superimposed oscillations in the tool may be used both in rotary and in translational or in a mixture of rotary and translational working motions of the tool. In accordance with an advantageous embodiment, the electric machine tool is in the form of a grinding device, for example an angle grinder, having as the tool a grinding wheel supported on a tool shaft, the motion of the tool being exclusively a rotary motion in that case. There also come into consideration, however, translational motions, for example in the case of hacksaws which execute an oscillatory stroke movement. 
     The superimposed oscillations may, in accordance with an advantageous embodiment, be excited orthogonally to the plane of motion of the tool in which the working motion takes place. For example, in the case of grinding wheels, the superimposed oscillations may be applied in the direction of the tool shaft carrying the grinding wheel. In the case of a translational working motion, on the other hand, the superimposed oscillation takes place perpendicularly to the translational motion. 
     In accordance with a further advantageous embodiment, it is also possible, however, for the superimposed oscillations to excite the tool in the plane of motion. In the case of a grinding wheel, this means that the grinding wheel is excited perpendicularly to the tool shaft, so that the vector of the excitation lies in the plane of motion of the grinding wheel. 
     It may furthermore be advantageous to cause the superimposed oscillations emanating from the excitation actuator to act upon a bearing of the tool, in which case the oscillations also propagate via the bearing to the tool. In the case of a plurality of bearings, this is preferably done via the bearing that is near the tool in order to avoid loading of the gear unit and the drive motor by the superimposed oscillations. 
     As excitation actuator, it is possible to use active actuators of various configurations capable of being excited by supply of energy to generate oscillations. In accordance with an advantageous embodiment, it may be provided that the excitation actuator is in the form of a Langevin oscillator, with piezo elements clamped therein, which changes its dimensions as a result of application of a voltage. As a result of being acted upon by an appropriately high-frequency voltage, the piezo element is able to expand and contract in the desired frequency of the superimposed oscillations, the excitation actuator being coupled to a component in the force transmission chain between drive unit or drive motor and tool so that the oscillations of the excitation actuator may propagate into the tool. As already described previously, the excitation is preferably effected by way of a bearing of the tool shaft carrying the tool. In accordance with an advantageous embodiment, it is provided that the excitation actuator is in the form of a magneto-restrictive excitation actuator, which is especially suitable for generating ultrasonic oscillations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages will be apparent from the following description of the figures. Exemplary embodiments of the present invention are illustrated in the figures. The figures and the description contain numerous features in combination. The person skilled in the art will advantageously also consider the features individually and combine them to make sensible further combinations. 
         FIG. 1  shows an exemplary embodiment of a handheld electric machine tool configured as a cutting tool. 
         FIG. 2  shows a further exemplary embodiment of a handheld electric machine tool configured as a drill. 
         FIG. 3   a ,  3   b  show an outline sketch of an activation assembly with an AC voltage power supply by mains current or with a DC voltage power supply by a battery pack ( FIG. 3   a ) and an advantageous clocking for reducing the overall size of a filter unit ( FIG. 3   b ). 
         FIG. 4  shows a progression of an ultrasound amplitude along a sonotrode. 
         FIG. 5  shows an impedance characteristic for detecting a resonant frequency of an excitation actuator. 
         FIG. 6  shows an equivalent circuit diagram of an ideal transformer. 
         FIG. 7  is a sectional view of an electric machine tool in the form of an angle grinder. 
         FIG. 8  is a detailed view of the grinding wheel of the angle grinder of  FIG. 7 , disposed on a tool shaft, the tool shaft being received in bearings and the bearing near the tool being acted upon with high-frequency oscillations transversely to the shaft axis by an excitation actuator. 
         FIG. 9  shows the grinding wheel of  FIG. 8  with bearing and excitation actuator in plan view. 
         FIG. 10  shows a further exemplary embodiment, in which the excitation actuator acts upon the tool shaft carrying the grinding wheel with high-frequency oscillations in the axial longitudinal direction. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     In the Figures, components that are identical or of the same kind are numbered with identical reference numerals. 
     To explain the present invention,  FIGS. 1 and 2  show different examples of handheld electric machine tools  10 . FIG.  1  shows a cutting tool with elongate housing shape;  FIG. 2  shows a drill with T-form housing shape. 
     Handheld electric machine tool  10  includes a housing  20  with a grip area  40 . A user holds electric machine tool  10  at the grip area  40  and is able to guide electric machine tool  10 . Grip area  40  may, where appropriate, be decoupled from other areas of the housing by a damping element, not shown. Electric machine tool  10  further includes a tool area  50  for a tool  60  which is drivable in a linear and/or oscillating manner, for example a cutter ( FIG. 1 ) or a drill ( FIG. 2 ) or another tool corresponding to another type of device. 
     An operating part  30  on the housing is used for activation of tool  60  and/or electric machine tool  10  by the user. Operating part  30  may, for example, be a switch or a controller or may also include a plurality of operating elements, one of which may be provided, for example, for switching on electric machine tool  10  and one of which may be provided for switching on and/or controlling tool  60 . 
     Arranged in housing  20  there is a drive unit  80  which, in the examples shown in  FIG. 1  and  FIG. 2 , includes only one drive component which is formed by an excitation actuator  100 . The latter may be in the form of a piezo-excited Langevin oscillator (also called a piezo actuator) which includes a volume of piezoelectrically active material  102 , for example piezo-ceramic discs which are pressed together and which undergo a change in length when acted upon by electrical voltage. When high-frequency electrical voltage is applied, in a conventional manner ultrasound is generated which is passed via a coupling element  106  to a tool  60 . Coupling element  106  may be a conventional sonotrode. The length and shape and also the material of coupling element  106  determine a resonant frequency of excitation actuator  100 . Tool  60  may also have an influence on the resonant frequency. In the embodiment variants in  FIG. 1  and  FIG. 2 , excitation actuator  100  is configured in such a way that Langevin oscillator and coupling element  106  are combined in a unit, and the total length thereof approximately corresponds to half the wavelength λ/2 of the ultrasonic oscillation. Other embodiment variants may provide that excitation actuator  100  is composed of a plurality of components of length λ/2. These may be: oscillation generators, known as converters, specifically, for example, a Langevin oscillator, amplitude transformation pieces  104 , known as boosters, where applicable lengthening pieces, and coupling element  106  known as a sonotrode. 
     An electronic unit  200  arranged in housing  20  serves to apply at least control and/or regulating signals to drive unit  80  and to supply voltage to excitation actuator  100 . An operating voltage unit  90 , in the form of a non-rechargeable or rechargeable battery pack with non-rechargeable or rechargeable batteries  92  here, serves to provide an electrical DC voltage for electronic unit  90  which converts the operating voltage into a high-frequency voltage signal with which excitation actuator  100  is excited into oscillation in the desired manner. 
     Electronic unit  200  is configured to operate the at least one excitation actuator  100  in a resonant frequency f_res. Electronic unit  200  includes a regulating unit  224  for re adjustment of the resonant frequency f_res of excitation actuator  100 . Regulating unit  224  may include a phase regulating chain capable of exciting excitation actuator  100  into its resonant frequency, with a phase shift between incoming current and incoming voltage being set to 0°. Preferably, resonant frequency f_res is regulated accordingly if the resonant frequency changes owing to heating or changing load at the tool. Alternatively, frequency re-adjustment may also be carried out by regulating to a maximum of the current fed into excitation actuator  100 . 
     If excitation actuator  100  is a piezo actuator, the volume of piezoelectrically active material  102 , for example stacked piezoelectric discs, is advantageously at least 0.2 cm 3 , preferably 0.5 cm 3 , especially at least 1 cm 3 . Excitation actuator  100  may have a power density of at least 5 Watt/cm 3 , preferably at least 20 Watt/cm 3 , based on the volume of piezoelectrically active material  102  of excitation actuator  100 . The power density makes use possible in a handheld electric machine tool  10  with sufficient power delivery of tool  60 . 
     Activation of tool  60  by activation actuator  30  may be indicated by a signal element  122  ( FIG. 2 ). 
     In  FIG. 1 , electronic unit  200  is integrated in a particularly space-saving manner on a single printed circuit board  210 . In  FIG. 2 , the electronic unit is divided between two printed circuit boards  212 ,  214 , one being disposed in the main part and one being disposed in the grip part of T shaped housing  20 , which grip part juts out at right angles to the main part. Advantageously, drive unit  80 , electronic unit  200  and operating voltage unit  90  are distributed in housing  20  in such a way that a center of gravity lies in the region of grip part  40 . 
       FIG. 3   a  shows an outline sketch of an activation of excitation actuator  100 , for example in the form of piezo actuator  100 , with an AC voltage power supply from a mains supply network or with a DC voltage power supply with a battery pack. 
     When electronic unit  200  has a mains power supply, for example 220 Volt AC, a component assembly  94  is provided that rectifies and smoothes the AC voltage. Electronic unit  200  includes a power generating unit  222  into which the DC voltage is fed and which is coupled to excitation actuator  100  via a suitable filter unit  226 . A regulating unit  224  provides the regulating signals for excitation actuator  100 . The operating frequency of excitation actuator  100  is in the range of from 10 kHz to 1000 kHz, preferably from 30 kHz to 50 kHz, and especially from 35 kHz to 45 kHz, more especially around 40 kHz. 
     If power is supplied by operating voltage unit  90  using non-rechargeable or rechargeable batteries  92 , it is possible to reduce the space required, since it is possible to omit component assembly  94  for rectifying and smoothing. The electrical output voltage of operating voltage unit  90  is preferably below 100 Volt, and is approximately 36 Volt or 10.8 Volt. 
     The maximum electric excitation field strength of the at least one excitation actuator is preferably in the range below 300 V/mm (based on the thickness, especially disc thickness, of the piezoelectrically active material), preferably in the range of from 50 V/mm to 220 V/mm. At a disc thickness of excitation actuator 100 of typically from 1 mm to 10 mm, preferably 2 mm to 6 mm, and especially of around 5 mm, the electrical voltages are below 1000 Volt. 
     In one embodiment variant, power generating unit  222  may be implemented by 4 MOSFET semiconductors in a conventional full bridge topology. In a further variant, the generation of the operating signal may also be effected by a conventional half bridge topology with, for example, a mid point capacitor for filtering the DC component. 
       FIG. 3   b  illustrates one possibility for making the overall size of filter unit  226  as small as possible. For that purpose, power unit  222  may be driven by regulating unit  224  in such a manner that, by sine-triangle modulation for example, it generates instead of simple square-wave signals a square-wave voltage that is more similar to a sine. Depending on the level of the clocking, that is, the number of individual pulses that together reproduce a sine, the content of undesirable harmonics may be distinctly reduced, which results in a smaller design of filter unit  226 . For this, the number of square-wave pulses per cycle duration of the sinusoidal signal is greater than 6, preferably in the range of from 6 to 100, especially in the range of from 10 to 26. In one embodiment variant, the number and width of the square-wave pulses of regulating unit  224  may also be varied during operation, for example with changes in load. 
       FIG. 4  shows a progression of an ultrasound amplitude along an excitation actuator  100  in the form of a piezo actuator. Coupling element  106  is in the form of a sonotrode. The region of excitation actuator  100  adjoining piezoelectric material  102  is referred to together with piezo discs  102  as a converter. Piezoelectric material  102  is excited by the supplied high-frequency AC voltage into oscillations which are transmitted into coupling element  102  via the converter. In the case of a three-stage structure of excitation actuator  100  such as that shown in  FIG. 4 , excitation actuator  100  additionally consists of a booster  104  for amplitude matching. Along the length M of excitation actuator  100  the amplitude Amp of the excited oscillation increases on average. Variations in the resonant frequency f_res of the oscillating system of excitation actuator  100  (where applicable with attached tool) during operation are preferably compensated for, for example using a phase regulating chain already described above with which the phase shift between the electrical voltage fed into excitation actuator  100  for excitation thereof and the electrical current fed in is regulated to zero (phase zero regulation), or using a maximum regulation of the electrical current fed into excitation actuator  100 . 
       FIG. 5  shows an impedance characteristic of an excitation actuator implemented by a piezo actuator with the resonant frequencies f_res and f_res 2 . Curve A shows the variation of the impedance Imp as a function of the frequency f, which passes through an impedance minimum at resonant frequency f_res and through an impedance maximum at f_res 2 . The frequency f_res is referred to as series resonance, and f_res 2  as parallel resonance. 
     Curve B shows the variation of the phase shift between current and voltage, which has a zero crossing at the resonant frequency and changes from −90° below the resonant frequency f_res to +90° above the resonant frequency f_res. On passing through the parallel resonance f_res 2 , the phase shift changes from +90° below the resonant frequency to −90° above the resonant frequency. 
     For signal filtering and for inductive compensation of the at least one excitation actuator  100 , at least one inductance may be provided in a power circuit of the electronic unit, which circuit acts upon the at least one excitation actuator  100  with electrical power. It is possible to obtain a space-saving layout of the power inductances together with the transmission transformer in a single coil core. The signal filtering and inductive compensation of the piezo actuator, which is beneficial in the case of excitation actuators  100 , may be provided directly by a specifically adjusted stray inductance of a transmission transformer that is required in any case, or may be afforded by an inductance wound on the same coil core. An additional coil core with a further inductance in the power circuit may thereby be omitted. 
     To illustrate this,  FIG. 6  shows an equivalent circuit diagram with an ideal transformer. The inductance M is used for the actual transfer from primary side to secondary side. The stray inductances occur since it is never possible for the windings to be ideally coupled. L 1  and L 2  form the part of the magnetic field that cannot be “captured” by the secondary coil. L 1  and L 2  are to be regarded in electrical terms as being like an air-core coil. 
     Electric machine tool  10  shown as an angle grinder in  FIG. 7  includes a housing  20  consisting of a motor housing  22  and a grip housing  24 , a damping element  26  being disposed between motor housing  22  and grip housing  24 . Electric machine tool  10  is held at grip housing  24  which forms grip area  40 . Motor housing  22  houses a drive unit  80  with a drive component in the form of an electric drive motor  82  which is coupled to and drives a tool shaft  64  via a gear unit  62 . Tool shaft  64  is the carrier for a tool  60 , in the form of a grinding wheel, which is fastened interchangeably to tool shaft  64 . 
     In  FIG. 8 , tool shaft  64  and tool  60  fastened thereto in the form of a grinding wheel are shown in a detail view. Tool shaft  64 , which has longitudinal axis L, is rotatably supported in bearings  70  and  72  spaced apart from each other in housing  20 . Situated on tool shaft  64  at the opposite end face from the grinding wheel, there is a beveled wheel  74  via which tool shaft  64  is driven by electrical drive motor  82 . 
     To reduce the size of the chips produced during machining of a workpiece with tool  60  in the form of a grinding wheel, tool  60  in the form of a grinding wheel is set into high-frequency oscillation in addition to its rotary working motion. This involves superimposed oscillations which are superimposed on the working motion of tool  60  in the form of a grinding wheel. Those superimposed oscillations are generated with the aid of excitation actuator  100  which is also disposed in housing  10  of handheld electric machine tool  10  as a further drive component of drive unit  80  and which directly or indirectly excites tool  60  in the form of a grinding wheel into the superimposed oscillations. In the exemplary embodiment shown in  FIG. 8 , excitation actuator  100  acts upon tool-side bearing  70  of tool shaft  64  and generates superimposed oscillations that are directed orthogonally to longitudinal axis L of tool shaft  64 . Those superimposed oscillations directed orthogonally to longitudinal axis L are also transmitted via tool shaft  64  to tool  60  in the form of a grinding wheel which similarly executes superimposed oscillations orthogonally to longitudinal axis L and thus in its plane of motion. 
     It is also possible for excitation actuator  100  to be positioned at a different location, for example at bearing  72  remote from the tool or directly at a position on tool shaft  64  or on tool  60  in the form of a grinding wheel in order for tool  60  to be acted upon directly by superimposed oscillations. 
     Various active actuators may be used as excitation actuator  100 . Preference is given to the use of actuators that generate high-frequency oscillations in the ultrasound range, especially in a frequency range of at least 20 kHz, but with frequencies in higher orders of magnitude coming into consideration, especially up to and including the megahertz range, or also smaller frequencies. 
     By way of example, there is used as excitation actuator  100  a piezo element whose length changes as a result of application of an electrical voltage. Since piezo elements respond very rapidly to voltage changes, by applying a high-frequency voltage it is possible to produce a correspondingly rapid change in length in the excitation actuator, which exerts an effect on tool  60  which by way of example is in the form of a grinding wheel here. 
     Excitation actuator  100  may also be in the form of a magneto-resistive actuator in which the electrical resistance is changed by application of an external magnetic field. 
     In the exemplary embodiment shown in  FIGS. 8 and 9 , the superimposed oscillations are generated in the direction of arrow  110 , orthogonally to longitudinal axis L of tool shaft  64  and tool  60  in the form of a grinding wheel. In the exemplary embodiment shown in  FIG. 10 , on the other hand, excitation with the superimposed oscillations takes place in accordance with arrow direction  110 , in the direction of longitudinal axis L of tool shaft  64  and tool  60  and thus perpendicularly to the plane of motion of tool  60  in the form of a grinding wheel. Excitation actuator  100 , by which the superimposed oscillations are generated, acts either directly upon tool shaft  64  or one or both bearings  70  and  72  or directly upon tool  60  with the superimposed oscillations in the axial direction.