Abstract:
In a discharge process of the solid body actuator ( 2 ), a current that discharges the solid body actuator ( 2 ) loaded with electrical energy is detected. A switching element ( 6 ) is switched from an open position to a closed position to short circuit the solid body actuator ( 2 ) for removal of electrical energy from the solid body actuator ( 2 ) through the switching element ( 6 ) depending on the current falling below a threshold of the current, wherein the magnitude of the threshold is specified.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Stage Application of International Application No. PCT/EP2009/065772 filed Nov. 24, 2009, which designates the United States of America, and claims priority to German Application No. 10 2008 061 586.2 filed Dec. 11, 2008, the contents of which are hereby incorporated by reference in their entirety. 
     TECHNICAL FIELD 
     The present application concerns a method and device for controlling a solid body actuator. 
     BACKGROUND 
     Increasingly stringent legal requirements relating to the permissible pollutant present emissions of internal combustion engines which are arranged in motor vehicles make it necessary to perform various measures by means of which the pollutant emissions are lowered. A starting point here is to lower the fuel emissions generated during the combustion process of the air/fuel mixture. In particular, the formation of soot is heavily dependent on the preparation of the air/fuel mixture in the respective cylinder of the internal combustion engine. In order to achieve very good preparation of the mixture, fuel is increasingly metered under very high pressure. In the case of diesel internal combustion engines, the fuel pressures are up to 2000 bar. For such applications, injection valves in which a solid body actuator is embodied as a piezo-actuator are becoming increasingly established. Piezo-actuators are defined by very short response times. Such injection valves are in this way suitable under certain circumstances for repeatedly metering fuel within a working cycle of a cylinder of the internal combustion engine. 
     Particularly good preparation of the mixture can be achieved if one or more pre-injections, which are also referred to as pilot injections, take place before a main injection, wherein under certain circumstances, a very small fuel quantity is to be metered for the individual pre-injection. Precise control of the injection valve is very important, in particular, for these cases. 
     An important role is assigned to the charging and discharging of the piezo-actuator in the context of the precise control of the injection valve. Rapid charging and discharging of the piezo-actuator is highly significant in particular in the case of intentional use of pilot injections. 
     DE 10 2005 040 531 A1 discloses a control device with a power source which is provided for controlling a piezo-actuator, wherein the power source can be coupled to the piezo-actuator in such a way that it can discharge the piezo-actuator, and wherein said control device has a power output stage for charging and discharging the piezo-actuator, which power output stage is electrically parallel to the power source. 
     SUMMARY 
     According to various embodiments, a method and a device for controlling a solid body actuator can be provided by means of which the solid body actuator can be rapidly discharged. 
     According to an embodiment, in a method for controlling a solid body actuator, during a discharging process of the solid body actuator,—a current is detected which discharges the solid body actuator to which electrical energy is applied, and—a switching element is switched from an open position into a closed position as a function of the current falling below a threshold which is predefined in terms of absolute value, in order to short-circuit the solid body actuator for the purpose of removing electrical energy from the solid body actuator via the switching element. 
     According to a further embodiment, the predefined threshold can be representative of a maximum average current of the switching element which is predefined in terms of absolute value. 
     According to another embodiment, a device for controlling a solid body actuator can be embodied in such a way that during a discharging process of the solid body actuator, said device detects a current which discharges the solid body actuator to which electrical energy is applied, said device switches a switching element from an open position into a closed position as a function of the current falling below a threshold which is predefined in terms of absolute value, in order to short-circuit the solid body actuator for the purpose of removing electrical energy from the solid body actuator via the switching element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments are explained below with reference to the schematic drawings, in which: 
         FIG. 1  shows a circuit diagram of a solid body actuator, a control device and a switching element, 
         FIG. 2  shows the profile of a voltage of the solid body actuator, 
         FIG. 3  shows the profile of a current of the solid body actuator, and 
         FIG. 4  shows a flowchart. 
     
    
    
     Elements with an identical design or function are characterized by the same reference symbols. 
     DETAILED DESCRIPTION 
     According to various embodiments, in a method and a corresponding device for controlling a solid body actuator, during a discharging process of the solid body actuator, a current is detected which discharges the solid body actuator to which electrical energy is applied. A switching element is switched from an opening position into a closed position as a function of the current falling below a threshold which is predefined in terms of absolute value, in order to short-circuit the solid body actuator for the purpose of removing electrical energy from the solid body actuator via the switching element. The short-circuiting enables rapid discharging of the solid body actuator. The threshold which is predefined in terms of absolute value can contribute to effective counteracting of the overheating of the switching element. 
     According to an embodiment, the predefined threshold is representative of a maximum average current of the switching element which is predefined in terms of absolute value. This makes it possible, in a particularly easy and precise way, to make a contribution to ensuring that the switching element can be operated without being damaged. In particular, thermal overloading of the switching element can be easily counteracted. 
       FIG. 1  shows a circuit diagram with a schematic illustration of a solid body actuator  2 , a control device  4  for actuating the solid body actuator  2  and a switching element  6 . 
     The solid body actuator  2  has two electrical terminals and can be embodied, for example, as a piezo-actuator. The control device  4  can be embodied, for example, as a microcontroller and comprises a processor  8 , a program memory  10  and a data memory  12 . The processor  8 , the program memory  10  and the data memory  12  are coupled to one another electrically via a system bus  14 , for example for the purpose of exchanging data. An output stage  16  is also electrically coupled to the system bus  14 , and can be controlled via the system bus  14  and is designed to apply electrical energy to the solid body actuator  2 . The output stage  16  can be embodied, for example, as a power output stage. In order to measure a current I, the output stage  16  is electrically coupled to a current detection device  18 . The control device  4  also comprises an interface  20 . 
     The control device  4  is electrically coupled to the switching element  6  via the interface  20 . The control device  4  is electrically coupled to one of the electrical terminals of the solid body actuator  2  and to the switching element  6  via the current detection device  18 . Both the switching element  6  and the other electrical terminal of the solid body actuator  2  are electrically coupled to a reference potential  22 , which can be a ground potential. 
     The switching element  6  can be embodied, for example, as a transistor. Transistors can easily be overloaded thermally through the generation of heat as a consequence of large currents. In particular, if the switching element  6  is embodied as a transistor, the maximum current I which refers to a chronological average and which flows through the switching element  6  is preferably predefined in such a way that it is adapted to the current carrying capacity of the switching element  6 . As a rule it is possible for the maximum current I which is predefined for the chronological average to be exceeded for short time intervals without leading to overheating. 
       FIGS. 2 and 3  show the profile of a voltage U plotted against the time t or the profile of the current I plotted against the time t during a charging process tc and a discharging process td. 
     In order to apply electrical energy to the solid body actuator  2 , one or more current pulses, which electrically charge the solid body actuator  2 , are predefined by the output stage  16  during the charging process tc. As a result, during the charging process tc the voltage U which is present at the solid body actuator  2  rises. In a time period between the charging process tc and the discharging process td, the voltage U which is present at the solid body actuator  2  is virtually constant. During the discharging process td, the electrical energy is conducted away to the solid body actuator  2 . The discharging process td can take place, for example, passively. In the case of a passive discharging process td, the current I which discharges the solid body actuator  2  results from the voltage U which is present across the solid body actuator  2 . 
     In order to accelerate the passive discharging process td, it is possible, for example, for both electrical terminals of the solid body actuator  2  to be coupled to the reference potential  22 . This can occur, for example, by means of the switching element  6  which can be switched from an opening position into a closed position by the control device  4 . If the switching element  6  is in the closed position, the two electrical terminals of the solid body actuator  2  are coupled to the reference potential  22 , which can also be referred to as a short-circuit of the solid body actuator  2 . The switching from the open position into the closed position of the switching element  6  takes place at a time T_Shunt. The time T_Shunt of the switching is dependent on a predefined threshold Is and occurs when the absolute value of the current I falls below the threshold Is which is predefined in terms of absolute value. 
       FIGS. 2 and 3  show the discharging process td both with and without switching of the switching element  6  from the open position into its closed position. The dashed curve in  FIG. 2  represents the voltage profile U for a case in which the switching element  6  remains in its open position and is not switched into the closed position. The current profile I which is continued by a dashed line in  FIG. 3  corresponds to this. The curve of the voltage U which is continued with the continuous line in  FIG. 2  or of the current I in  FIG. 3  represents the case for which the switching element  6  is switched into its closed position at the time T_Shunt. As is easily apparent from a comparison of the curves which are continued by dashed lines with the curves which are continued by continuous lines in  FIGS. 2 and 3 , the discharging process td is terminated earlier in the case of the short-circuiting by means of the switching element  6  than in the case of a discharging process td without the short-circuit. 
       FIG. 4  shows a flowchart with the method steps V 1  to V 4  for actuating the solid body actuator  2  by means of the control device  4 . The method steps V 1  to V 4  can be implemented, for example, in a program of the control device  4  which can be stored, for example, in the program memory  10 . 
     The program starts in a first step V 1 . During the first step V 1  variables can, for example, be initialized. 
     A second step V 2  starts as soon as the solid body actuator  2  is in the discharging process td. The current I with which the solid body actuator  2  is discharged is detected by means of the current detection device  18 . 
     In a third step V 3  it is determined when the current I falls below the threshold Is which is predefined in terms of absolute value. When the current I falls below the threshold Is which is predefined in terms of absolute value, the switching element  6  is switched from the open position into the closed position by means of the control device  4 , with the result that the solid body actuator  2  is short-circuited and from then on is discharged via the switching element  6 . 
     In an embodiment, the threshold Is is representative of a maximum average current I of the switching element  6  which is predefined in terms of absolute value. The threshold Is can, however, also be representative, for example, of a maximum average current, predefined in terms of absolute value, of another electronic component, for example of an electronic component in the output stage  16 . The switching of the switching element  6  from the open position into the closed position at the time at which the maximum average current, T_Shunt, which is predefined in terms of absolute value is undershot, permits rapid discharging of the solid body actuator  2  without the switching element  6  or other electronic components being damaged. 
     The program ends in a fourth step V 4 .