Abstract:
In a method for operating a power tool with an energy accumulator, in particular a rechargeable energy accumulator, which supplies power to an electric drive motor, a clock frequency is generated by an electronic unit, with which a gate of a MOSFET—which supplies operating voltage to the drive motor—is switched on with each cycle, and a switching-off of the MOSFET is carried out within one cycle using different signals, as a function of operating parameters.

Description:
CROSS-REFERENCE TO A RELATED APPLIACTION 
   The invention described and claimed hereinbelow is also described in German Patent Application DE 102005020377.9 filed on May 2, 2005. This German Patent Application, whose subject matter is incorporated here by reference, provides the basis for a claim of priority of invention under 35 U.S.C. 119(a)-(d). 
   BACKGROUND OF THE INVENTION 
   The present invention is directed to a method for operating a power tool. 
   When rechargeable energy accumulators are used with power tools, total discharge of the energy accumulator must be avoided, since this can cause the energy accumulator to become irreversibly damaged, or, at the least, its service life can be greatly reduced. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the present invention to provide a method for operating a power tool, which is a further improvement of the existing methods. 
   The method according to the present invention for operating a power tool with an energy accumulator is suited for use, in particular, with power tools with lithium ion batteries. 
   The energy accumulator is successfully protected, and good regulating properties for the power tool are attained. A clock frequency is generated by an electronic unit, with which a gate of a MOSFET, which supplies operating voltage to the drive motor, is switched on with each cycle, it being possible to switch off the MOSFET within one cycle using different signals, as a function of operating parameters. When the MOSFET is switched off, the first result is not that the drive motor is shut off, but rather that a pulse width of the drive motor is reduced. The motor current and, therefore, the torque remain high or even at a maximum level, and only the rotational speed decreases. 
   Preferably, when a specified threshold value of an operating parameter of the power tool is reached, a pulse width of triggering signals of the drive motor is reduced. The power tool merely reduces the pulse width, but does not shut off completely. 
   In a favorable embodiment, when a maximum permissible threshold of an electric current from the energy accumulator is reached, the pulse width is reduced. 
   In a further favorable embodiment, when a maximum permissible power loss of the energy accumulator and/or the drive motor is reached, the pulse width is reduced. Resetting the gates does not result in the power tool being switched off. Instead, it only results in a reduction in the rotational speed. Advantageously, the power loss is determined by determining a signal in proportion to ohmic losses in the energy accumulator and/or the drive motor, and a power loss that is above permissible losses is integrated. To determine the power loss, the current from the energy accumulator can be determined based on a current-proportional signal and converted to a signal that is proportional to the square of the current, a difference between the current value and the current threshold can be calculated, and the difference can be integrated with respect to time. The power loss that is above the permissible losses is therefore integrated. 
   In a favorable embodiment, when a maximum permissible temperature of the energy accumulator is reached, a measured current signal is modified such that the drive motor is gently turned off. By intentionally and specifically adulterating the current signals, an uncomfortable, abrupt shut-off can be prevented. 
   In a favorable embodiment, when a permissible minimum electrical voltage of the energy accumulator is fallen below, the gate is shut off. Preferably, the drive motor is not shut off completely until the energy accumulator is discharged. 
   In a favorable embodiment, drilling-site illumination of the power tool is not switched off until the drive motor is shut off completely. The power tool operator can continue working nearly interference-free even when threshold values are exceeded or fallen below, while the energy accumulator is reliably protected against harmful total discharge. 
   It is advantageously provided that the pulse width of the gate voltage is adjusted in an on-off switch of the power tool using a potentiometer. Using a logarithmic curve of the pulse width as a function of a displacement path of the on-off switch, the rotational speed of the power tool can be adjusted very precisely between 0 and 100%. 
   Further embodiments, aspects and advantages of the present invention also result independently of their wording in the claims, without limitation to generality, from an exemplary embodiment of the present invention presented below with reference to the drawing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic diagram of a preferred power tool in accordance with the present invention; 
       FIG. 2  shows a sequence of functions of a preferred method in accordance with the present invention; and 
       FIG. 3  shows an example of an overall schematic diagram of a preferred electronic unit in accordance with the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a schematic diagram of a preferred power tool. An electronic unit  15  controls gate impulses of a MOSFET  17  acting as a switch, which supplies operating voltage from an energy accumulator  12  to a switch  11  and, therefore, a drive motor  10 .  FIG. 3  shows, as an example, an overall schematic diagram of electronic unit  15 . The interplay of individual functional blocks of the method according to the present invention is depicted in  FIG. 2 . 
   Switch  11  switches the direction of rotation of drive motor  10  between right and left rotation as desired. In a free-wheeling phase, the motor current flows through a free-wheeling diode  18 . Gate X 4  of MOSFET  17  can be applied to switch  11 . A measuring resistor  19  is connected between source X 2  of MOSFET  17  and negative terminal X 1  of energy accumulator  12 , as shown in  FIG. 3 , with which an electrical current flow from energy accumulator  12  is measured. 
   A temperature of energy accumulator  12  is determined using a sensor  16  and transmitted to temperature input X 8 . A capacitor  14  is located parallel to energy accumulator  12 . An on-off switch  13  turns the supply voltage to drive motor  10  and, via input X 5 , to electronic unit  15  on or off. 
   These functional blocks protect the energy accumulator, which is preferably designed as a lithium ion battery, and provide the power tool with good regulating properties. 
   The electronic unit  15  generates, in voltage block  20 , a clock frequency of a few kHz, preferably between 7 and 10 kHz, and particularly preferably 8 kHz. At the beginning of each cycle, a gate voltage in gate voltage block  21  is applied to input S, which therefore turns on MOSFET  17 . MOSFET  17  can be turned off via resetting at input R during a cycle using various signals. 
   The electric current from energy accumulator  12  flows through measuring resistor  19  and generates a current-proportional signal, which is amplified. When a maximum value is reached, in particular a maximum permissible discharge current of energy accumulator  12 , e.g., approx. 30 A when a lithium ion battery is used, the gate voltage in gate voltage block  21  is reset. As a result, the power tool merely reduces its pulse width, but does not shut off completely. The motor current and, therefore, the torque remain at a maximum level, and only the rotational speed decreases. 
   Current-proportional signal I is determined in current measurement block  23  and subsequently converted in integration block  24  to a signal I eff , which is proportional to the square of the current. A limit I grenz  is subtracted from this new signal I eff . This difference I eff −I grenz  is integrated with respect to time. Signal I eff  is proportional to the ohmic losses (I 2 ·R) in energy accumulator  12  and drive motor  10 . That is, the power loss above the permissible losses is integrated. When this integral reaches a limit, the gate voltage in gate voltage block  21  is reset. This does not cause the power tool to shut off, either. It only reduces the rotational speed. 
   Voltage U bat  of energy accumulator  12  is monitored continually in functional block  25 , to protect lithium ion batteries against total discharge. Lithium ion batteries are highly susceptible to total discharge. If the actual value falls below limit U batt,min , which is adjusted as a function of load (I·R compensation), electronic unit  15  turns off gate X 4 , i.e., the gate voltage is rest in gate voltage block  21 . To perform I·R compensation, functional block  25  receives a signal from current measurement block  23 . This initially results in a reduced rotational speed. If energy accumulator  12  is discharged, the power tool shuts off completely. In this case, drilling-site illumination  28  in functional block  26  is switched off. 
   When energy accumulator  12  is designed as a lithium ion battery, as is known, e.g., with nickel-based cells, it must be protected against excessive temperatures. The temperature in energy accumulator  12  is determined (functional block  27 ) using a temperature resistor  16 —preferably a NTC resistor—which is preferably integrated in energy accumulator  12 . If a specified limit is exceeded, electronic unit  15  adulterates the current signals in functional block  24  such that the power tool is shut off gently and not abruptly. 
   Finally, the pulse width of the gate voltage can be adjusted using a potentiometer in on-off switch  13  (pulse-width block  22 ). Using a logarithmic dependence between a displacement path of the potentiometer and the pulse width, the rotational speed of the power tool can be adjusted very precisely between 0 and 100%. 
   Particularly preferably, the power tool is a cordless screwdriver with a lithium ion battery. 
   It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of constructions differing from the types described above. 
   While the invention has been illustrated and described as embodied in a method for operating a power tool, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. 
   Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention. 
   What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.