Patent Publication Number: US-2023139444-A1

Title: Hand-held machine tool

Description:
The invention relates to a hand-held machine tool comprising a drive device by means of which a tool that can be brought into operative connection with the machine tool can be actuated, with a control device for actuating the drive device and at least one capacitive sensor element operatively connected to the control device being provided. 
     BACKGROUND 
     Hand-held machine tools designed with a capacitive sensor are known from practice. By means of the capacitive sensor, for example, a power switch can be enabled and a drive device can then be actuated via a switch that can be actuated by a user. A so-called dead man function can be provided by means of the capacitive sensor. The capacitive sensor is arranged in a grip region of the machine tool, for example, and can detect a user&#39;s hand located in the grip region. 
     SUMMARY OF THE INVENTION 
     However, the determination of the presence of a user&#39;s hand is subject to interference, which can lead to an incorrect detection. For example, the drive device can have a negative impact on the functionality of a sensor arranged in the vicinity of the drive device and possibly lead to an incorrect detection by the capacitive sensor. Furthermore, when using a capacitive sensor, it may be the case that contact in an undesired region of the sensor leads to a detection. 
     It is an object of the present invention to provide a hand-held machine tool with which the risk of an incorrect detection can be reduced, and in particular a spatially limited detection is possible. 
     A hand-held machine tool is thus provided which comprises a drive device by means of which a tool that can be brought into operative connection with the machine tool can be actuated, with a control device for actuating the drive device and at least one capacitive sensor element operatively connected to the control device being provided. 
     According to the invention, it is proposed that at least one element of the machine tool that is arranged in the region of the capacitive sensor element is grounded. 
     The at least one grounded element makes it possible, in a structurally simple manner, to reduce or in particular completely exclude any influence on the at least one capacitive sensor element, for example by the drive device. The risk of an incorrect detection by the at least one capacitive sensor element can thus be reduced or avoided by grounding the at least one element. 
     Furthermore, with the solution according to the invention, regions of the capacitive sensor element can be shielded in a simple manner in such a way that an actuation, for example by a hand, in the shielded region of the capacitive sensor element does not lead to a detection. In this way, a targeted detection in a desired region can be achieved in a structurally simple manner, with preferably only an interaction with an unshielded region of the capacitive sensor element, for example in an unshielded detection region, leading to a detection. 
     A single grounded element or a plurality of grounded elements can be provided which is or are arranged in particular in the vicinity of at least one capacitive sensor element. 
     The at least one grounded element is at least partially arranged between a capacitive sensor element and a possible source of interference, such as the drive device. In this way, interfering influences can be particularly effectively reduced. 
     In order to ground the at least one element, it can be provided that it is connected to a negative pole of a storage battery of the hand-held machine tool. As an alternative to this, it can also be provided that, in the case of a mains-operated hand-held machine tool, the at least one element is connected to a grounding line of the mains line. If the mains line does not have a grounding line, it can be provided that the hand-held machine tool is designed with a DC/AC converter and the at least one element is connected to a negative pole of the DC circuit for grounding. 
     The presence of a user&#39;s hand in a detection region of the sensor element, for example in the grip region or in a holding region, can be determined by means of the sensor element and the control device. The presence of a hand can be detected, for example, by the fact that, when charged with a constant voltage, the sensor element can absorb a greater electrical charge due to the hand. It can be provided, for example, that the sensor element is alternately charged and discharged and the presence of a hand is assumed when a defined threshold value for the determined electrical charge is exceeded. 
     The hand-held machine tool can relate to all kinds of hand-held machine tools, such as grinding machines, for example angle grinders, sawing machines, such as reciprocating saws, jigsaws, circular saws or chain saws, drills, chisel hammers or the like. 
     In an advantageous embodiment of a hand-held machine tool according to the invention, the at least one element encompasses or surrounds the sensor element at least partially, in particular completely. As a result, the sensor element is particularly effectively shielded from interfering influences and the risk of an incorrect detection is reduced. 
     The at least one grounded element encompasses the sensor element in the extension direction of the sensor element preferably only partially, in particular starting from a region connected to the control device as far as a region provided for detection; i.e. a detection region. In this way, in addition to reducing the influence of interfering influences, a detection which can be attributed to an interaction for example of a user&#39;s hand with an undesired region of the sensor element can be avoided. In this way, it can preferably be achieved that detection is only possible in the desired detection region of the sensor element. 
     In a structurally simple embodiment according to the invention, the at least one grounded element is a line which at least partially encompasses the sensor element in particular in a spiral shape, and thereby leads to shielding of the sensor element in the encompassed region. 
     In a structurally simple implementation of a sensor element, said element is designed as a line, the line extending in particular from the control device, in particular a control element of the control device, until it adjoins a detection region which is arranged in a holding region or grip region of the machine tool, for example. The line is preferably made of copper. 
     In a structurally simple embodiment of a hand-held machine tool according to the invention, the at least one element extends, starting from the control element, in particular almost completely in the region of the sensor element as far as a detection region of the sensor element. 
     In an inexpensive embodiment according to the invention, the at least one element is a part of the machine tool that in particular comprises a metal material, in particular a part of a housing of the machine tool. 
     In an advantageous embodiment of a hand-held machine tool according to the invention, the sensor element is at least partially arranged in an element that can be releasably connected to a housing of the machine tool, in particular an additional grip, a protective device such as a protective hood, or the like. In particular, a region of the sensor element that is not arranged in the connectable element is preferably shielded. 
     The control device can be designed with a control element, the at least one sensor element being operatively connected to the control element. The control element is connected to a main control unit of the control device in particular via lines and can thereby be arranged at a distance from the main control unit. 
     The at least one sensor element and the control device, in particular the control element, are preferably part of a capacitively measuring sensor. The control element is preferably designed as a printed circuit board and has a control unit and a signal generator, the signal transmitter being designed in particular to charge the sensor elements, which are designed for example as lines, with a defined voltage. The control unit is preferably designed to detect changes in charge in the at least one sensor element, whereby, for example, the presence of a hand is detected if the determined charge is greater than a defined threshold value. Such changes in the determined charge occur, for example, when a user&#39;s hand is in a defined region of the machine tool, since this increases the overall capacitance. The control element then sends in particular a signal to the control device, which then preferably enables the operation of the machine tool in response to a user request. To determine the actual electrical charge of a sensor element, a capacitor having a small capacitance compared to a capacitance of the sensor element can be provided, which capacitor is fully charged with the electrical charge of the sensor element and then discharged, and the number of processes is counted until the relevant sensor element is completely discharged. If a defined threshold value for the number of these processes is exceeded, this can be interpreted as the presence of a hand. 
     In an advantageous embodiment of a machine tool according to the invention, a power switch that is operatively connected to the control device and can be actuated by a user can be provided, the control device being designed to transfer the power switch between an inactive state and an active state, with a user actuation of the power switch in the active state of the power switch leading to an actuation of the drive device. By means of the control device, the power switch can in particular thus be actuated in such a way that a user actuation of an actuating element that is operatively connected to the power switch leads to the coupling of the drive device with current, for example from a storage battery or from a mains supply, and thus to the actuation of the drive device. In a simple manner, the situation can be achieved whereby the machine tool can only be actuated when a hand located for example in the grip region can be detected by means of the control device in cooperation with the at least one capacitive sensor element and/or the control element. It can also be provided that the power switch in the active state is transferred to the inactive state if the control device in cooperation with the at least one capacitive sensor element and/or the control element no longer detects a hand in the grip region. A so-called dead man function can thus be implemented in a simple manner. In the active state, the power switch is designed to guide a motor current to the drive device, as a result of which the tool can be actuated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages can be found in the following description of the drawings. An embodiment of the present invention is shown in the drawings. The drawings, the description and the claims contain numerous features in combination. A person skilled in the art will expediently also consider the features individually and combine them to form meaningful further combinations. 
       In the drawings: 
         FIG.  1    is a greatly simplified longitudinal sectional view of a hand-held machine tool designed as an angle grinder, with a tool being arranged on a driven shaft of the angle grinder; 
         FIG.  2    is a longitudinal sectional view of the angle grinder according to  FIG.  1   ; 
         FIG.  3    is a further longitudinal sectional view of the angle grinder according to  FIG.  1    and  FIG.  2   ; 
         FIG.  4    is a side view of the angle grinder according to  FIGS.  1  to  3   ; 
         FIG.  5    is a cross-sectional view of the angle grinder according to  FIGS.  1  to  4    along the line A-A in  FIG.  4   ; 
         FIG.  6    is a simplified cross-sectional view of the angle grinder according to  FIGS.  1  to  5    along the line A-A in  FIG.  5   ; 
         FIG.  7    is a simplified three-dimensional view of the angle grinder according to  FIGS.  1  to  6    without part of a housing, with an additional grip being shown; 
         FIG.  8    is a schematic view of an operating principle of a sensor element; and 
         FIG.  9    is a simplified three-dimensional view of a section of the angle grinder according to  FIGS.  1  to  8   , in which a shielded sensor element can be seen in more detail; 
         FIG.  10    is an exemplary sequence of a method for actuating the angle grinder according to  FIGS.  1  to  9   . 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    to  FIG.  7    and  FIG.  9    show a hand-held machine tool  1  according to the invention which is designed as an angle grinder in the illustration shown. According to an alternative embodiment, the machine tool  1  can also be designed as a drill, a hammer drill, a saw, a chisel hammer or the like. 
     The machine tool  1 , which is designed as an angle grinder in the drawings, has a housing  2  and a tool  3  which is designed as a cutting disk or grinding wheel, for example, and can be releasably connected to a driven shaft  7  of the machine tool  1 . The tool  3  can be actuated by a drive or a drive device  4  which is in particular designed as an electric motor and can be supplied with current in particular by means of a storage battery  5  that can be connected to the machine tool  1 . The storage battery  5 , like the housing  2 , is not shown in all of the drawings. 
     According to an alternative embodiment, the machine tool  1  can also be supplied with electric current from a network by means of a power cable. 
     The drive device  4  for actuating the tool  3 , in this case in a rotating movement, is arranged in an interior of the housing  2  along with a gear mechanism  6 . The drive  4 , which is designed for example as an electric motor, the gear mechanism  6  and the driven shaft  7  to which the tool  3  is operatively connected are arranged relative to one another and are interconnected in such a way that a torque generated by the electric motor  4  can be transmitted to the gear mechanism  6  and finally to the driven shaft  7 . A freely rotating end of the driven shaft  7  which in this case projects downward from the housing  2  is connected to the tool, which is designed here as a cutting disk  3 , for example via a clamping device (not shown in more detail). The torque of the driven shaft  7  is thus transmitted directly to the cutting disk  3 . 
     A protective device designed as a protective hood  12  is provided and can preferably be releasably connected to the housing  2  of the machine tool  1  in a conventionally known manner and can be provided to protect a user when the machine tool  1  is in operation. 
     The housing  2  has at least one holding region  22  or grip region which is provided for holding and guiding the machine tool  1  by one or both hands of a user. A switch  20  which can be actuated by a user when the machine tool  1  is held in the grip region  22  is associated with the holding region  22 . 
     As an alternative or in addition to this, the machine tool  1  can have a further grip device  26  (see, for example  FIG.  7   ) which, for example in a head region  24  of the machine tool  1  that faces the tool  3 , can be releasably brought into operative connection with the housing  2  of the machine tool  1  and provides a different working position for the user, for example.  FIG.  7    shows an additional grip  26  of this type, which can be releasably connected to the housing  2  in the region of a connection point  28 . 
     The machine tool  1  also has a control device  8  which is designed in this case with main control electronics  9  and a control element  10 . The control element  10  is preferably designed as a printed circuit board and is electrically and electronically connected to the main control electronics  9 , which in the present case are arranged in the region of the storage battery  5 . 
     A power switch  21  which can be user-actuated by means of the switch  20  is in turn connected to the main control electronics  9 . The power switch  21  is designed for example as a MOSFET and is provided for enabling a current from the storage battery  5  to the electric motor  4 . 
     The machine tool  1  is designed in the present case with a so-called dead man function. For this purpose, the power switch  21  can be transferred by the control device  8  between an active state, in which a user actuation of the switch  20  leads to an actuation of the electric motor  4 , and an inactive state, in which a user actuation of the switch  20  does not lead to an actuation of the electric motor. To transfer the power switch  21  between the active state and the inactive state, the control element  10  is designed in the manner explained in more detail below. 
     In the present case, the control element  10  is operatively connected to a plurality of sensor elements  30 ,  31 ,  32 ,  33 ,  34 ,  35 , four of which can be seen in  FIG.  1    (sensor elements  34  and  35  being shown in  FIG.  7    for example). The sensor elements  30 ,  31 ,  32 ,  33 ,  34 ,  35  are in this case each designed with a line or designed as a line. The sensor elements  30 ,  31 ,  32 ,  33 ,  34 ,  35 , which in the present case interact with one another only via the control element  10 , are of substantially comparable design here, but extend from the control element  10  into different regions of the machine tool  1 . 
     The control element  10  has in particular a control unit and a signal generator, the signal transmitter being designed in particular to charge the sensor elements  30 ,  31 ,  32 ,  33 ,  34 ,  35  designed with lines with a defined voltage. The control unit is preferably designed to detect changes in charge in the at least one sensor element, whereby, for example, the presence of a hand is detected if the determined charge is greater than a defined threshold value. 
     A vertical direction H, a longitudinal direction L and a transverse direction Q are used in the following for a more detailed description of the machine tool  1 . In the present embodiment, the longitudinal direction L relates to a direction between the storage battery  5  and the head region  2 . The vertical direction H in this case substantially corresponds to an extension direction of the driven shaft  7  and the transverse direction is substantially perpendicular to the longitudinal direction L and the vertical direction H and substantially represents the extension direction of the grip device  26 . 
     A first sensor element  30  extends from the control element  10  initially upward substantially in the vertical direction H and, from a region near the surface, forward in the longitudinal direction L toward the head region  24 . The first sensor element  30 , in its region extending substantially in the longitudinal direction L within the housing  2 , is arranged in a region close to the surface of the housing  2  and extends substantially in the holding region  22 . The first sensor element  30  extends in the longitudinal direction L substantially into a region in which the electric motor  4  is arranged. 
     A second sensor element  31  extends from the control element  10  initially downward substantially in the vertical direction H and, from a region near the surface, forward in the longitudinal direction L toward the head region  24 . The second sensor element  31 , in its region extending substantially in the longitudinal direction L within the housing  2 , is again arranged in a region close to the surface of the housing  2  and extends substantially in the holding region  22 . The second sensor element  31  extends in the longitudinal direction L substantially into a region in which the electric motor  4  is arranged. 
     A third sensor element  32  extends from the control element  10  initially upward substantially in the vertical direction H and, from a region near the surface, to the rear in the longitudinal direction L toward the base region  25 . The third sensor element  32 , in its region extending substantially in the longitudinal direction L within the housing  2 , is arranged in a region close to the surface of the housing  2  and extends substantially in the holding region  22 . The third sensor element  32  extends in the longitudinal direction L substantially into a region in which the storage battery  5  is arranged. The third sensor element  32  can also, as can be seen in  FIGS.  3  and  4   , extend into a base region  25  of the machine tool  1  in which the storage battery  5  is arranged. 
     A fourth sensor element  32  extends from the control element  10  initially downward substantially in the vertical direction H and, from a region near the surface, to the rear in the longitudinal direction L toward the base region  25 . The fourth sensor element  33 , in its region extending substantially in the longitudinal direction L within the housing  2 , is arranged in a region close to the surface of the housing  2  and extends substantially in the holding region  22 . The fourth sensor element  32  extends in the longitudinal direction L substantially into a region in which the storage battery  5  is arranged. The fourth sensor element  33  can also, as can be seen in  FIGS.  3  and  4   , extend into the base region  25  of the machine tool  1 . 
     The first sensor element  30  and the third sensor element  32  are thus arranged in a region of the housing  2  that is upper with respect to the vertical direction H and the second sensor element  31  and the fourth sensor element  33  are arranged in a region of the housing  2  that is lower with respect to the vertical direction H. 
       FIG.  7    shows a fifth sensor element  34  and a sixth sensor element  35  in a greatly simplified manner, these likewise being connected to the control element  10  in a manner comparable to the sensor elements  30 ,  31 ,  32 ,  33 . The fifth sensor element  34  extends from the control element  10  to the grip device  26  and in particular within the grip device  26  further into a grip region  27  of the grip device  26 . It can be provided here that the fifth sensor element  34  is designed in two parts, with a first part  37  which extends inside the housing  2  and a second part  38  which extends inside the grip device  26 . A connection device  28  for connecting the grip device  26  to the housing  2  is designed such that, when the grip device  26  is connected to the housing  2 , the first part  37  of the fifth sensor element  34  is operatively connected to the second part  38  of the fifth sensor element  34 . 
     The sixth sensor element  35 , which can also be seen in  FIG.  7   , extends from the control element  10  to the protective device  12 , with it being possible to select a path of the sixth sensor element  35  within the housing  2  in principle as desired, depending on the existing installation space conditions. The sixth sensor element  35  can preferably be used to determine whether the protective device  12  is connected to the housing  2 , and in particular whether the protective device  12  is connected to the housing  2  in the intended manner, that is to say correctly. 
     In an alternative embodiment, it can also be provided that simply any desired combination of the sensor elements  30 ,  31 ,  32 ,  33 ,  34 ,  35  is provided. As an alternative to this, further sensor elements can also be provided, with it also being possible to arrange sensor elements for example in a region of the housing  2  which is lateral with respect to the transverse direction Q. 
     The sensor elements  30 ,  31 ,  32 ,  33 ,  34 ,  35  are presently all designed as capacitive sensor elements, it being possible to determine a change in charge in the relevant sensor element  30 ,  31 ,  32 ,  33 ,  34 ,  35  in conjunction with the control element  10 . To determine a change in charge, it can be provided that the relevant sensor element  30 ,  31 ,  32 ,  33 ,  34 ,  35  is charged by the control device  8  or the control element  10  up to a defined voltage and the relevant sensor element  30 ,  31 ,  32 ,  33 ,  34 ,  35  is then discharged. It can be provided in this case, for example, that a capacitor which has a small capacitance comparable to the typical charging of a sensor element  30 ,  31 ,  32 ,  33 ,  34 ,  35  is associated with the control element  10  and it is determined how often this capacitor can be charged by the relevant sensor element  30 ,  31 ,  32 ,  33 ,  34 ,  35  until the relevant sensor element  30 ,  31 ,  32 ,  33 ,  34 ,  35  is completely discharged. 
     The charge that the relevant sensor element  30 ,  31 ,  32 ,  33 ,  34 ,  35  can absorb is substantially constant. A human hand has a capacitance which is added to the capacitance of the sensor element  30 ,  31 ,  32 ,  33 ,  34 ,  35  when the human hand comes within close range of the relevant sensor element  30 ,  31 ,  32 ,  33 ,  34 ,  35 . Since the relevant sensor element  30 ,  31 ,  32 ,  33 ,  34 ,  35  is charged to a constant voltage, the charge absorbed by the sensor element  30 ,  31 ,  32 ,  33 ,  34 ,  35  increases for example in the presence of a hand so that, when the relevant sensor element  30 ,  31 ,  32 ,  33 ,  34 ,  35  is discharged, a greater amount of charge is present than in a state without the human hand. 
     In particular, the sensor elements  34  and  35  can be designed such that the respectively associated defined threshold value is exceeded when the grip device  26  or the protective device  12  is arranged in the desired manner on the housing  2  and the relevant defined threshold value is not exceeded if no grip device  26  or no protective device  12  is mounted or is not mounted on the housing  2  in the desired manner, for example. 
     A defined threshold value for the determined electrical charge is associated with each sensor element  30 ,  31 ,  32 ,  33 ,  34 ,  35 , with the exceeding of the relevant threshold value being interpreted as the presence of a hand or the presence of the grip device  26  or the protective device  12 . 
     It can be provided that the control element  10  transfers the power switch  21  from the inactive state to the active state if at least one determined electrical charge of a sensor element  30 ,  31 ,  32 ,  33 ,  34 ,  35  or multiple determined electrical charges of different sensor elements  30 ,  31 ,  32 ,  33 ,  34 ,  35  exceeds the respectively associated threshold value. Thus, a prerequisite for transferring the power switch  21  from the inactive state to the active state can be that the charge determined by the fifth sensor element  34  exceeds the corresponding threshold value and/or the charge determined by the sixth sensor element  36  exceeds the corresponding threshold value, thus requiring the presence of the grip device  26  and/or the protective device  12  to activate the power switch  21 . 
     An actual transfer of the power switch  21  from the inactive state to the active state only occurs, for example, if, in addition to the above-mentioned prerequisite, a predefined condition is present at the sensor elements  30 ,  31 ,  32 ,  33  and, for example, the determined electrical charge of one or more of these sensor elements  30 ,  31 ,  32 ,  33  exceeds the respectively associated defined threshold value. 
     It can be determined, for example, whether the machine tool  1  is being held by a user in the desired manner. This can be determined by the fact that the determined charges of defined sensor elements which are arranged in corresponding regions of the machine tool  1  exceed the respectively associated threshold value and/or the determined charges of other defined sensor elements which are arranged in corresponding regions of the machine tool  1  do not exceed the respectively associated threshold value. It can thus be determined, for example, whether the machine tool is being held in the holding region  22  and in the grip region  27  and the power switch  21  is transferred to the active state only in this case, for example. 
     As an alternative or in addition to this, it can be provided that the power switch  21  is not transferred to the active state when a determined charge of a defined sensor element which is arranged for example in a specific region, for example in a region located close to the tool  3 , exceeds the associated threshold value. 
     In addition, by positioning the sensor elements  30 ,  31 ,  32 ,  33 ,  34 ,  35  accordingly, different grip positions of a user can be determined depending on the determined charges of the respective sensor elements  30 ,  31 ,  32 ,  33 ,  34 ,  35 , and the drive device  4  can be operated in different operating modes depending on the determined grip position. This can be provided, for example, in the case of a machine tool  1  designed as an angle grinder when using a tool  3  designed as a cutting disk or as a grinding wheel, in which the machine tool  1  is held differently and the use of the different tools  3  have different optimal speeds. 
     As an alternative or in addition to this, it can also be provided that the power switch  21  is only transferred from the inactive state to the active state if the electrical charge determined by a sensor element  30 ,  31 ,  32 ,  33 ,  34 ,  35  is less than the relevant defined threshold value. This can be provided in particular to prevent a user&#39;s hand from being in a defined region of the machine tool  1 , for example. A defined region can be a region in which a user must not have their hand to operate the machine tool  1 , for example. As an alternative or in addition to this, the defined region can be a danger region which is located in particular in the vicinity of the tool  3 . In addition to the embodiment shown, the corresponding sensor elements  30 ,  31 ,  32 ,  33 ,  34 ,  35  can be arranged in the desired regions of the machine tool  1 . 
     It can also be provided that the power switch  21  is transferred from the active state back to the inactive state if the electrical charge determined by a sensor element  30 ,  31 ,  32 ,  33 ,  34 ,  35  falls below the respectively defined threshold value, since this is due, for example, to a removal of a hand from the holding region  22  or the grip region  27  or to a removal of the protective device  12  and/or the grip device  12  and thus an undesirable operating state. 
     The fifth sensor element  34  can be provided to determine only the presence of the grip device  26 . As an alternative or in addition to this, it can also be provided that the presence of a user&#39;s hand in the grip region  27  can also be determined by means of the fifth sensor element  34 . 
     In order to reliably prevent, for example, a hand that is located in a region differing from the intended detection region, for example in a region outside the grip region  27 , from leading to undesired exceeding of the associated defined threshold value, it can be provided that the region of the fifth sensor element  35  remote from the detection region is shielded and, for example, the presence of a user&#39;s hand in this region does not lead to the associated defined threshold value of the fifth sensor element  34  being exceeded. 
     A section of the hand-held machine tool  1  or the angle grinder can be seen in greater detail in  FIG.  9   , in which the head region  24  of the angle grinder  1 , which is at least partially, in particular completely designed with a metal housing  70  (not shown in  FIG.  9    but see for example  FIG.  7   ). A rear part  71  of the housing  2  can be seen which has a plurality of connection points  72 ,  73  for connecting the rear part  71  of the housing  2  to the metal housing  70 , two of which can be seen here. 
     In order to protect the fifth sensor element  34  in regions not intended for detection, for example in the region extending in the rear housing  71 , from interference, undesired detections or incorrect detections for example due the drive device  4 , the part of the fifth sensor element  34  extending in the region of the rear housing  71  is presently enclosed by a further element  75 . The further element  75  is designed in this case as a line  75 , which presently encompasses or encloses the line of the fifth sensor element  34  in a spiral shape. 
     In the present case, the line  75  is connected to a negative pole of the storage battery  5  for grounding. As an alternative to this, it can also be provided that the line  75  is connected to a further sensor input. 
     In the case of mains-operated machine tools, the line can be connected to a neutral, grounded conductor for grounding. If an AC-DC converter is provided, the line  75  can also be connected to the negative pole of the DC circuit for grounding. 
     The fifth sensor element  34 , which is made as thin as possible to reduce parasitic capacitance effects, is connected in the region of the connection point  72  to an electrically conductive element  77  which, when the rear housing part  71  is connected to the metal housing  70 , is conductively connected to a further region of the fifth sensor element  34  which extends in particular into the grip region  27  of the grip device  26 . 
     By means of the line  75 , which in particular almost completely encompasses the fifth sensor element  34  in the rear housing part  71  from the control element  10  to the connection point  72 , undesired detections by the fifth sensor element  34  are reliably prevented in the region of the rear housing part  71 . By arranging the line  75  accordingly, only a selected region of the sensor element  34  or also multiple regions of the sensor element  34  can also be shielded to the desired extent and the risk of an undesired detection in this region or regions can be reliably prevented. Regions of the sensor element  34  intended for detection can thus be determined very precisely. 
     In order to also reliably shield the region of the fifth sensor element  34  extending in the metal housing  70  and to prevent an undesired detection in this region, the metal housing  70  is grounded in the present case and connected to the negative pole of the storage battery  5  analogously to the line  75 . For this purpose, a metal element  80  or metal contact is presently provided in the region of the connection point  73  and is connected to the negative pole of the storage battery  5  via a connecting element  81  in the region of the rear housing part  71 . This can reliably prevent contact with the metal housing  70  leading to a detection by the fifth sensor element  34 . 
     Accordingly, the sensor elements  30 ,  31 ,  32 ,  33 ,  34 ,  35  can be shielded in selected regions to prevent an undesired detection in these regions, it being possible to provide one or more further grounded elements, for example a grounded line or a grounded housing part, for this purpose. 
     There is greater interference for the sensor elements  30 ,  31 ,  32 ,  33 ,  34 ,  35  in a region  61  of the machine tool  1  in which the electric motor  4  is arranged, for example, than in a region  60  further away from the electric motor  4  (see for example  FIG.  3   ). In order to prevent the relevant defined threshold value of the relevant sensor element  30 ,  31 ,  32 ,  33 ,  34 ,  35  from being exceeded due to such interference, it can be provided that the defined threshold values of the individual sensor elements  30 ,  31 ,  32 ,  33 ,  34 ,  35  differ from one another, with in particular those defined threshold values of sensor elements  30 ,  31 ,  34 ,  35  arranged in the region of the drive device  4  being greater than the defined threshold values in sensor elements  32 ,  33  remote from the drive device  4 . 
     For example, it can be provided that the relevant defined threshold value is composed of the sum of a contact value that is substantially identical for all sensor elements  30 ,  31 ,  32 ,  33 ,  34 ,  35  and a basic signal value that is dependent on the respective ambient conditions. The basic signal value is in particular higher the greater the interference in the region of the relevant sensor element  30 ,  31 ,  32 ,  33 ,  34 ,  35 . 
     It can be provided here that in particular the defined threshold values of the sensor elements  30 ,  31 ,  34 ,  35  which are arranged in the regions close to the drive device  4  are set or calibrated on the basis of the defined threshold values of the sensor elements  32 ,  33  which are arranged in regions remote from the drive device  4 . The setting or calibration of the defined threshold values of the sensor elements  32 ,  33  can take place at defined time intervals or continuously during operation of the machine tool  1  on the basis of one or more of the defined threshold values of one or more of the sensor elements  30 ,  31 ,  34 ,  35 . 
       FIG.  8    shows, by way of example, a section of an end region  41 , remote from the control element  10 , of a sensor element  40  which is fundamentally comparable to the sensor elements  30 ,  31 ,  32 ,  33 ,  34 ,  35  and can be used as an alternative or in addition to one or more of the sensor elements  30 ,  31 ,  32 ,  33 ,  34 ,  35 . The end region  41  of the sensor element  40  in this case has two planar elements  42 ,  43 , with a first element  42  being arranged at a distance from and so as to be displaceable relative to the second element  43 . 
     The first element  42  represents a base plate, for example, which is electrically and electronically connected to a line  44  of the sensor element  40 , which line in turn is provided for coupling to the control element  10 . The first element  42  and the second element  43  can have a planar design, as shown in  FIG.  8   . In an alternative embodiment, it can also be provided that the elements  42  and  43  are designed differently and for example are curved so that the sensor element  40  can be arranged in a simple manner, for example also in the grip region  27 . The sensor element  40  can partially or almost completely encompass the grip region  27  circumferentially, for example, so that a defined grip strength in the grip region  27  can be determined by means of the sensor element  40 . 
     In the present case, the first element  42  and the second element  43  are interconnected via a spring device  45  which forces the second element  43  into a position spaced apart from the first element  42 . 
     As an alternative or in addition to this, an in particular elastically deformable material can be arranged between the first element  42  and the second element  43 , by means of which the elements  42 ,  43  are in particular interconnected. 
     The first element  42  and the second element  43  are designed in particular with a metal material. If a distance between the first element  42  and the second element  43  is changed, for example by means of an actuating element which can be actuated by a user and which is preferably arranged in the holding region  22  and/or the grip region  27  of the machine tool  1 , a current electrical charge of the sensor element  40  is changed accordingly. This is due to the fact that the second element  43  together with the first element  42  represents a capacitor of which the electrical charge varies on the basis of a distance between the elements  42 ,  43 , and increases as the distance decreases. The functional principle here corresponds to the procedure described in more detail above for determining a change in charge. 
     Analogously to the above explanations, a defined threshold value is provided for the electrical charge, the exceeding of which can be interpreted as the user actuating the sensor element  40 . By means of the sensor element  40 , depending on the selection of the spring constant of the spring device  45  or the selection of the elastically deformable material, a minimum force can easily be specified with which the actuating element connected to the second element  43  can be actuated in order to exceed the defined threshold value and to transfer the power switch  21  in the manner described above to the active state. An unintentional transfer of the power switch  21  from the inactive state to the active state can be reliably prevented in this way. 
     As can be seen in particular from  FIGS.  2  to  5   , the machine tool  1  has a main channel  50  which extends in the longitudinal direction L of the machine tool  1  and which extends substantially from a base region  51  in which the storage battery  5  and the main control electronics  9  are located to the head region  24  of the machine tool  1 . Cooling air is guided through the main channel  50  and sucked in through ventilation openings in the base region  51  of the machine tool  1  by means of a fan, and is guided through the main channel  50  in the longitudinal direction L, in particular for cooling the electric motor  5 . 
     The respective sensor elements  30 ,  31 ,  32 ,  33 ,  34 ,  35  are in this case each almost completely arranged in a region separated from the main channel  50 . The regions are preferably designed as separate channels  51 ,  52 ,  53 ,  54 , with each sensor element  30 ,  31 ,  32 ,  33  being associated with a separate channel  51 ,  52 ,  53 ,  54 . A single sensor element  30 ,  31 ,  32 ,  33  is arranged in each channel  51 ,  52 ,  53 ,  54  in this case, although it can also be provided that two or more sensor elements are at least partially arranged in one channel. Besides the sensor elements  30 ,  31 ,  32 ,  33 , a separate channel can also be associated with the sensor elements  34 ,  35  in a comparable manner. 
     Each channel  51 ,  52 ,  53 ,  54  extends here substantially from the control element  10  to an end region of the relevant sensor element  30 ,  31 ,  32 ,  33  that is remote from the control element  10 , the channels  51 ,  52 ,  53 ,  54  presently being substantially completely separated from the main channel  50 . 
     The sensor elements  30 ,  31 ,  32 ,  33  are separated from the main channel  50  by the channels  51 ,  52 ,  53 ,  54  in such a way that the sensor elements  30 ,  31 ,  32 ,  33  are reliably protected from interfering influences present in the main channel  50 , for example in the form of moisture and/or dirt, for example conductive grinding dust. Such interfering influences could interfere with the functionality of the sensor elements  30 ,  31 ,  32 ,  33  and possibly lead to an incorrect detection. This is reliably prevented by the arrangement of the sensor elements  30 ,  31 ,  32 ,  33  in the respective channels  51 ,  52 ,  53 ,  54  separately from the main channel  50 . 
     In addition to the sensor elements  30 ,  31 ,  32 ,  33 , the control element  10  is also presently separated from the main channel  50  and arranged in a separate region or chamber  56 . Thus, in a manner comparable to the sensor elements  30 ,  31 ,  32 ,  33 , the control element  10  is protected from conditions present in the main channel  50  which could interfere with the functionality of the control element  10 . 
     As a result of these measures, the risk of an incorrect detection by one or more of the sensor elements  30 ,  31 ,  32 ,  33  due to conditions present in the main channel  50  is greatly reduced in a simple manner. 
     In order to form the channels  51 ,  52 ,  53 ,  54  in a structurally simple manner, the housing  2  is presently designed in two parts in such a way that preferably each channel  51 ,  52 ,  53 ,  54  and the chamber  56  are jointly formed by at least two parts of the housing  2 . This makes it easy to assemble the machine tool  1 . 
       FIG.  10    shows an example of an embodiment of a method for operating the machine tool  1 . 
     The method begins at the start S, in particular when a user actuates the switch  20 , for example. In a first step S 1 , the power switch  21  is set to the inactive state. The power switch  21  is preferably preset to the inactive state. 
     In a second step S 2 , the electrical charges of the respective sensor elements  30 ,  31 ,  32 ,  33 ,  34 ,  35 ,  40  are determined and these are compared in step S 3  with the respectively associated defined threshold values. In step S 4 , it is checked whether a predefined condition exists between the determined electrical charges and the respective threshold values of the sensor elements  30 ,  31 ,  32 ,  33 ,  34 ,  35 ,  40 , as described in more detail above by way of example. 
     If the query in step S 4  is negative and the predefined condition is not present, the power switch  21  is transferred to the inactive state or left in the inactive state in step S 5  and the method is continued with step S 2 . 
     If the query result in step S 4  is positive, the power switch  21  is transferred to the active state or left in the active state in step S 6 , so that a user actuation of the switch  20  leads to an activation of the drive device  4 . The method is then continued with step S 2 . 
     In step E, the method is ended in particular when there is no longer any user actuation.