Patent Publication Number: US-2018043521-A1

Title: Safety method and handheld power tool

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a control method for a hand-held power tool including a rotating tool, in particular a hammer drill or an electric screwdriver. 
     U.S. Pat. No. 7,552,781 discloses a hammer drill including an anti-kickback system. A rotational rate sensor determines, based on a displacement of a vibrating mass, a rotational speed of the electric screwdriver about a working axis. A safety function is activated based on the determined rotational speed. The safety function reduces the torque on the tool. 
     The triggering of the safety function is to take place in a reliable manner. In this case, both a triggering in the absence of potential danger to the user as well as a non-triggering in the presence of potential danger is to be avoided. 
     SUMMARY OF THE INVENTION 
     The hand-held power tool according to the present invention includes a tool holder for holding a tool on a working axis and a motor for rotationally driving the tool holder about the working axis. The motor is situated in a power-tool housing and a handle is fastened on the power-tool housing for guiding the hand-held power tool during operation. A rotary motion sensor detects a rotary motion of the power-tool housing about the working axis. A monitor ascertains a holding force based on an amplitude of the rotary motion in a frequency range between 0.4 Hz and 4 Hz. A safety device reduces a torque output to the tool holder when the rotary motion exceeds a limiting value. The limiting value is established as a function of the holding force. 
     The safety device adapts its triggering behavior to an ascertained holding force of the user. The influence of the holding force on the mean motion of the hand-held power tool is not to be significantly differentiated from other influences on the mean motion. In particular, the various applications and associated different typical motions of one and the same hand-held power tool make it difficult to identify the holding force. According to the present invention, it has been found that, in a narrow frequency range, the rotary motion about the working axis is significant for the holding force. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following description explains the present invention on the basis of exemplary specific embodiments and figures. 
         FIG. 1  shows a hammer drill; and 
         FIG. 2  shows a block diagram of a controller of the hammer drill. 
     
    
    
     Identical or functionally identical elements are indicated by identical reference numerals in the figures, unless indicated otherwise. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a hammer drill  1  as an exemplary embodiment of a hand-held power tool. Hammer drill  1  includes a tool holder  2 , into which a shaft end  3  of a tool, e.g., of drill bit  4 , may be inserted. A motor  5 , which drives a hammer mechanism  6  and an output shaft  7 , forms one primary drive of hammer drill  1 . A battery pack  8  or a mains power line supplies motor  5  with current. A user may guide hammer drill  1  with the aid of a handle  9  and may start hammer drill  1  with the aid of a main button  10 . During operation, hammer drill  1  continuously rotates drill bit  4  about a working axis  11  and may thereby hammer drill bit  4  into a substrate in direction of impact  12  along working axis  11 . 
     Hammer drill  1  includes a safety device  13 , which protects the user against an excessive repercussive torque of drill bit  4 . Hammer drill  1  exerts a repercussive torque onto the user, which results as a reaction to the torque transmitted by drill bit  4  onto the workpiece. Provided the substrate yields during drilling, the repercussive torque is uniform and low. In the event that a drill bit is jammed in the workpiece, a high repercussive torque results due to the abruptly braked rotating assemblies. The user is no longer able to sufficiently counteract this repercussive torque, and entire hammer drill  1 , including handles  9 , therefore begins to rotate about the rotational axis of drill bit  4 . Safety device  13  monitors a rotary motion of handle  9  relative to working axis  11  and reduces the torque output to tool holder  2  if it is expected that the instantaneous rotary motion will result in a rotation of entire hammer drill  1  by a critical angle. The critical angle is, for example, 60 degrees. The reduction in the torque takes place, for example, by stopping motor  5  with the aid of a brake  14 . 
     Safety device  13  detects the rotary motion of handle  9  with the aid of a rotary motion sensor  15 . Rotary motion sensor  15  is, for example, a gyro sensor, which directly determines the angular velocity about working axis  11 . The gyro sensor includes, e.g., an oscillatingly suspended chip. The Coriolis force associated with the rotary motion influences the oscillation frequency of the chip. On the basis of the oscillation frequency, rotary motion sensor  15  ascertains the angular velocity triggering the Coriolis force. Rotary motion sensor  15  may be situated in the vicinity of working axis  11  or offset with respect to working axis  11  in power-tool housing  16  or handle  9 . 
     Safety device  13  evaluates the angular velocity and ascertains whether a user-endangering situation is present. One exemplary simple evaluation of safety device  13  is based on a comparator  17 , which compares whether the angular velocity exceeds a limiting value for the angular velocity. In this case, safety device  13  triggers a suitable protective measure. For example, safety device  13  activates brake  14  of motor  5 . Motor  5  is preferably braked to a standstill. 
     Another evaluation ascertains, for example, an instantaneous torsion angle of power-tool housing  16  with respect to a preceding point in time. An evaluation unit  18  integrates the angular velocity starting at the point in time. If the torsion angle exceeds a limiting value for the torsion angle, safety device  13  triggers the suitable protective measure. 
     Another evaluation unit  18  combines the instantaneous angular velocity and the instantaneous torsion angle. For example, a future torsion angle is estimated. The future torsion angle is the sum of the instantaneous torsion angle and the product of the instantaneous angular velocity and a fixed period of time of, e.g., 10 ms. The future torsion angle is compared with a limiting value for the future torsion angle and, if necessary, safety device  13  is triggered. Instead of ascertaining the future torsion angle, a table including pairs of limiting values for the angular velocity and the instantaneous torsion angle may be utilized. Safety device  13  is triggered when both limiting values of one pair are exceeded. 
     Safety device  13  ascertains the user behavior during on-going operation, also outside of a potentially critical situation. A monitor  19  ascertains the mean holding force which the user applies against a rotation of hammer drill  1  about working axis  11 . The holding force of the user is primarily dependent on the user&#39;s physical strength, but also on the user&#39;s attentiveness, activity, the spatial orientation of hammer drill  1 , etc. Drill bit  4 , which rotates at a largely constant rotational speed and is acted upon by a largely constant number of strikes, generates vibrations in hammer drill  1  during drilling. The amplitude of the vibrations is dependent on the substrate, the tool, the contact pressure and the holding force of the user. Although a multitude of unknown variables affect the amplitude of the vibrations, there appears to be a dependence of the amplitude dominated by the holding force in a narrow frequency range about 2 Hz. Monitor  19  utilizes this dependence in order to ascertain a measure for the holding force. Monitor  19  contains a bandpass filter  20  having a mid-band frequency between 0.4 Hz and 4 Hz, to which measuring signal  21  of rotary motion sensor  15  is supplied. The amplitude of output signal  22  of bandpass filter  20  is detected as a measure for the holding force. For example, output signal  22  may be rectified in a rectifier  23 . The rectified signal may be supplied to a discriminator  24  and may be assigned, for example, to one of three categories “weak holding force,” “mean holding force,” and “strong holding force.” 
     Safety device  13  triggers brake  14  as a function of the holding force. A user having a firm grip may probably also slow down a rapidly rotating hammer drill  1  before a critical angle is reached, as compared to a user having a less firm grip. Safety device  13  changes the limiting value of comparator  17  as a function of the ascertained holding force. For example, the limiting value for the “strong holding force” is set to be greater than for the “weak holding force.” Preferably, the limiting value increases constantly or incrementally as the holding force increases. 
     Brake  14  may be, for example, a mechanically acting brake, which clamps drill bit  4 . Preferably, the drive train is decoupled in this case from motor  5  with the aid of a slipping clutch or an electrically activated clutch. According to one preferred embodiment, brake  14  is to be implemented together with motor  5 . Motor  5  is switched into a generator mode and the generated electrical power is introduced into an ohmic resistor. Alternatively, a current may be supplied into motor  5 , in particular in the case of a reluctance motor, in such a phase-controlled way that the electromechanical force counteracts the rotary motion of motor  5 . The torque may also be reduced with the aid of an electrically controlled clutch.