Abstract:
The invention relates to a hand-guided electric tool having a motor and a pulse width modulator for generating a pulse width modulated signal for operating the motor. A unit is provided for reducing the EMC interferences emitted by the electric tool.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a 35 USC 371 application of PCT/EP2008/063855 filed on Oct. 15, 2008. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a hand-held power tool. 
     2. Description of the Prior Art 
     Hand-held power tools such as drills, cordless screwdrivers, jigsaws, angle grinders, or electric hedge trimmers that are powered by batteries, rechargeable batteries, or a power cord are generally known. Power tools of this kind have electric motors, which, according to the prior art, are operated with a pulse width modulated voltage or, in the case of corded tools, by means of phase-angle control. As schematically depicted in  FIG. 1 , a pulse width modulated voltage periodically alternates between a high and low voltage value. When the high voltage value is present, the motor is switched on. When the low voltage is present, the motor is switched off. The ratio of the on-time T on  during a period of pulse width modulated voltage to the total time T PWM  of a period of pulse width modulated voltage is referred to as the mark/space ratio and determines the effective electrical output supplied by the motor. 
     One problem when using the pulse width modulation method is the production and emission of harmonics. Because of the periodic switching back and forth between a high and low voltage level and the resulting current changes, harmonics are produced whose frequency is an uneven multiple of the modulation frequency f PWM  of the pulse width modulation ( FIG. 2 ). These harmonics are emitted in the form of EMC interference. Excessively powerful EMC interference can negatively affect other electrical devices such as communication systems. 
     OBJECT AND SUMMARY OF THE INVENTION 
     The object of the present invention is to disclose a device that reduces the amplitudes of the EMC interference emitted by a hand-held power tool. 
     The object underlying the invention is attained by means of a power tool with the defining characteristics according to the invention. 
     In one embodiment of the invention, a hand-held power tool has a motor and a pulse width modulator for producing a pulse width modulated signal for operating the motor. According to the invention, the emitted EMC interference is reduced by using filter elements such as capacitors, chokes, and combinations thereof. 
     In another embodiment, the emitted interference is reduced by flattening or smoothing the edges of the pulse width modulated signal, which reduces the share of high-frequency signal components, i.e. harmonics. 
     In another embodiment of the invention, the carrier frequency of the pulse width modulated signal can be modulated using a noise signal or pseudorandom signal. 
     The pulse width modulated signal with a random-modulated carrier frequency advantageously has no discrete spectral lines with multiples of the carrier frequency of the pulse width modulation. Instead, each of these spectral lines is spread out over a frequency band. This distributes the total power of each high-frequency signal component over a frequency interval and as a result, the amplitudes of the individual signal maxima decrease. 
     In another embodiment, a clock pulse produced by a clock-pulse generator is modulated by a clock-pulse modulator using a noise signal or pseudorandom signal; the modulated clock signal is supplied to a pulse width modulator, which produces a pulse width modulated signal with a noise-modulated carrier frequency. The noise signal or pseudorandom signal can be produced by an analog noise generator. The noise signal or pseudorandom signal can also be produced as a digital pseudorandom number and converted into an analog pseudorandom signal by a smoothing element. 
     In a preferred embodiment of the invention, a microcontroller is provided as the pulse width modulator. 
     In another preferred embodiment, the modulation of the carrier frequency of the pulse width modulated signal is carried out digitally by a microcontroller using a noise signal or pseudorandom signal. 
     In this embodiment of the invention, the noise modulation of the carrier frequency of the pulse width modulated signal can be advantageously implemented entirely at the software level. As a result, no additional hardware components are required, incurring no increase in costs or in the assembly complexity required to manufacture the power tool. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, parts that are the same or function in the same manner have been provided with the same reference numerals. The invention is explained in greater detail below in conjunction with the accompanying drawings, in which: 
         FIG. 1  schematically depicts a pulse width modulated voltage signal with a constant carrier frequency; 
         FIG. 2  schematically depicts an emitted spectrum of a pulse width modulated signal with a constant carrier frequency; 
         FIG. 3  schematically depicts a pulse width modulated voltage signal with a noise-modulated carrier frequency; 
         FIG. 4  schematically depicts an emitted spectrum of a pulse width modulated signal with a noise-modulated carrier frequency; 
         FIG. 5  schematically depicts a hand-held power tool; 
         FIG. 6  schematically depicts a device for producing a pulse width modulated signal with a noise-modulated carrier frequency in a power tool; 
         FIG. 7  schematically depicts another device for producing a pulse width modulated signal with a noise-modulated carrier frequency in a power tool; and 
         FIG. 8  schematically depicts another device for producing a pulse width modulated signal with a noise-modulated carrier frequency in a power tool. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a schematic depiction of the variation in time of a pulse width modulated voltage signal. The pulse width modulated voltage signal periodically alternates between a high and low voltage value. If a motor of a power tool  100  (shown in  FIG. 5 , for example) is operated using a pulse width modulated voltage signal, then the voltage differences produce a chronological variation of the current flowing through the motor, but the inductance of the motor smoothes out this variance. Changes in the amperage produce a change in the torque and therefore the speed of the motor, but the inertia of the motor delays these changes. The two voltage levels alternate with each other at a carrier frequency f PWM . The mark/space ratio between the on-time T on  and the total period duration T PWM  influences the average output supplied by the motor of the power tool  100 . With a sufficiently high carrier frequency f PWM , a motor speed occurs that is virtually constant over time and is dependent on the mark/space ratio. 
     Because of the fixed carrier frequency f PWM , the spectrum of the pulse width modulated voltage signal in  FIG. 1  has a number of discrete spectral lines at uneven multiples of the modulation frequency f PWM . This spectrum is schematically depicted in  FIG. 2 . The high-frequency signal components of the spectrum of the pulse width modulated signal and the resulting motor current are emitted in the form of EMC interference. The amplitudes of the individual discrete spectral lines in this case can exceed current or future permissible limit values. 
     One possibility for reducing EMC interference is to use filter elements such as capacitors, chokes, and combinations thereof. The use of additional components, however, increases the size of the power tool  100  and the assembly complexity required for its manufacture, thus incurring additional costs. 
     Another possibility for reducing the interference emitted by the pulse width modulation is to flatten or smooth the edges of the pulse width modulated signal. This reduces the share of high-frequency signal components, i.e. harmonics. A flattening of the signal edges can, for example, be achieved by the fact that the driver module that produces the pulse width modulated signal is intentionally embodied to be slower, for example through the insertion of a series resistance. However, doing so also protracts the switching time of a power switch situated after the driver module, thereby increasing switching losses. The power consumption of a power tool  100  increases as a result. This can negatively affect the running time of a power tool  100  powered by a battery or rechargeable battery. 
     Another possibility for reducing the emitted EMC interference is to reduce the amplitudes of the harmonics of the pulse width modulated signal by using a noise signal or pseudorandom signal to modulate the carrier frequency of the pulse width modulated signal. This is schematically depicted in  FIG. 3 .  FIG. 3  shows the variation in time of a pulse width modulated voltage signal with a noise-modulated carrier frequency. The voltage alternates between a high and low voltage level over time. The period b 1 , b 2  of a cycle composed of a high and low voltage signal is subjected to a chronologically random modulation, which is depicted in exaggerated fashion in  FIG. 3 . The mark/space ratio of the on-time a 1 , a 2  to the total duration of a cycle b 1 , b 2 , however, is constant over time.  FIG. 4  schematically depicts the harmonic spectrum of such a pulse width modulated voltage signal with a noise-modulated carrier frequency. Instead of discrete spectral lines, the spectrum has amplitudes, which are expanded over finite frequency intervals by uneven multiples of the carrier frequency f PWM  and whose height is reduced in comparison to the amplitudes of the spectrum in  FIG. 2 . A motor of a power tool  100  operated with a pulse width modulated voltage signal with a noise-modulated carrier frequency as shown in  FIG. 3  therefore emits only reduced-amplitude EMC interference. 
       FIG. 5  is a schematic view of a hand-held power tool  100 . 
       FIG. 6  shows a part of a first embodiment of a power tool  100  according to the invention. The power tool  100  has a clock-pulse generator  110  that produces a constant clock pulse  111 . An analog noise generator  150  emits an analog random signal  151 . A clock-pulse modulator  152  modulates the constant clock pulse  111  using the analog random signal  151  to produce a modulated clock signal  153 . In a preferred embodiment, the clock-pulse generator  110  and clock-pulse modulator  152  are combined to form a single unit. The modulated clock signal  153  is supplied to a pulse width modulator  154 , which uses it to produce a pulse width modulated set point voltage with a noise-modulated carrier frequency  106 . For example, the pulse width modulator  154  can be implemented in the form of a microcontroller. A voltage source  101  emits a constant voltage  102 . The voltage source  101  can, for example, be a rechargeable battery built into the power tool  100  or a battery that is inserted into the power tool  100 . A power switch  103  uses the constant voltage  102  and the pulse width modulated set point voltage with a noise-modulated carrier frequency  106  to produce a voltage  104  that has the same pulse width modulation with a noise-modulated carrier frequency as the pulse width modulated set point voltage with a noise-modulated carrier frequency  106 . The pulse width modulated voltage with a noise-modulated carrier frequency  104  is supplied to a motor  105  of the power tool  100 . The rotation speed of the motor  105  is determined by the mark/space ratio of the pulse width modulation produced by the pulse width modulator  154 . Components required for this, e.g. switches and set point transmitters, are not shown in  FIG. 6  for the sake of clarity. The power switch  103  can be a semiconductor element such as a MOSFET. 
       FIG. 7  shows a part of a second embodiment of a power tool  100  according to the invention. The power tool  100  has a clock-pulse generator  110  that produces a constant clock pulse  111 . A digital pseudorandom number generator  160  generates a digital pseudorandom number  161 . The digital pseudorandom number generator  160  can, for example, be a microcontroller, which uses a suitable algorithm to generate a digital pseudorandom number  161  and serially transmits it bitwise via a port pin. A smoothing element  162  converts the digital pseudorandom number  161  into an analog pseudorandom signal  163 . The smoothing element  162  can, for example, be an RC low pass. A clock-pulse modulator  152  uses the analog pseudorandom signal  163  to modulate the constant clock pulse  111 , transforming it into a modulated clock signal  153 . In a preferred embodiment, the clock-pulse generator  110  and clock-pulse modulator  152  are combined to form a single unit. The modulated clock signal  153  is supplied to a pulse width modulator  154 , which produces a pulse width modulated set point voltage with a noise-modulated carrier frequency  106 . A voltage source  101  integrated into the power tool  100  emits a constant voltage  102 . A power switch  103  uses the constant voltage  102  and the pulse width modulated set point voltage with a noise-modulated carrier frequency  106  to produce a pulse width modulated voltage with a noise-modulated carrier frequency  104 , which drives a motor  105  of the power tool  100 . 
       FIG. 8  shows a part of another embodiment of a power tool  100  according to the invention. The power tool  100  has a clock-pulse generator  110  that produces a constant clock pulse  111 . A digital pseudorandom number generator  160  generates a digital pseudorandom number  161 . A pulse width modulator  170  uses the constant clock pulse  111  and the digital pseudorandom number  161  in a method according to the invention to produce a pulse width modulated set point voltage with a noise-modulated carrier frequency  106 . The pulse width modulator  170  can, for example, be a microcontroller. 
     The pulse width modulator  170  has a counter  171 , a defined fold-back value  172 , and a defined overflow value  173 . The counter  171 , the fold-back value  172 , and the overflow value  173  can, for example, be embodied in the form of a memory register of the microcontroller. The pulse width modulator  170  increases the value of the counter  171  by the number  1  with each clock cycle of the constant clock pulse  111 . If the value of the counter  171  is less than the fold-back value  172 , then the pulse width modulator  170  emits a high voltage level as a set point voltage (a noise-modulated carrier frequency  106 ). If the value of the counter  171  is greater than or equal to the fold-back value  172 , then the pulse width modulator  170  emits a low voltage level as a set point voltage (a noise-modulated carrier frequency  106 ). If the value of the counter  171  is less than the overflow value  173 , then the pulse width modulator  170  waits for the next clock cycle of the constant clock pulse  111  in order to then repeat the above-described process, starting from the increase of the counter  171 . 
     If the value of the counter  171  is equal to the overflow value  173 , then the value of the counter  171  is reset to a starting value, for example the value 0. Otherwise, the fold-back value  172  and the overflow value  173  for the subsequent clock cycle of the pulse width modulated set point voltage are modulated with a noise-modulated carrier frequency  106 . The overflow value  173  determines the period length of the carrier frequency of the pulse width modulated set point voltage with a noise-modulated carrier frequency  106 . The ratio of the fold-back value  172  to the overflow value  173  yields the mark/space ratio of the pulse width modulated set point voltage with a noise-modulated carrier frequency  106  and should vary as little as possible between the individual clock cycles of the carrier frequency of the pulse width modulated set point voltage with a noise-modulated carrier frequency  106 . Ideally, the fold-back value  172  and the overflow value  173  are therefore multiplied by the digital pseudorandom number  161 . In an alternative embodiment of the power tool  100  according to the invention, the digital random number  161  is added to the fold-back value  172  and the overflow value  173 . This embodiment has the advantage that it is less computationally demanding for the pulse width modulator  170  to execute an addition than to execute a multiplication. If the value of the digital random number  161  is small compared to the fold-back value  172 , then the resulting variation of the mark/space ratio of the pulse width modulated set point voltage with a noise-modulated carrier frequency  106  is negligibly low and averages out over the course of time. In a particularly preferred embodiment, the pulse width modulator  170  has an additional register for storing a constant fold-back value and an additional register for storing a constant overflow value. The new fold-back value  172  and the new overflow value  173  are calculated in each new period of the carrier frequency of the pulse width modulated set point voltage with a noise-modulated carrier frequency  106 , based on the constant fold-back value and overflow value stored in the additional registers. This prevents the overflow value  173  and the fold-back value  172  from deviating too far from their initial values over time. 
     A voltage source  101  integrated into the power tool  100  emits a constant voltage  102 . A power switch  103  uses the constant voltage  102  and the pulse width modulated set point voltage with a noise-modulated carrier frequency  106  to produce a pulse width modulated voltage with a noise-modulated carrier frequency  104 , which drives a motor  105  of the power tool  100 . 
     The foregoing relates to the preferred exemplary embodiments of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.