Abstract:
A method and a device for transmitting an elastic deflection of a piezoelectric element to an actuator by using a direct voltage source, generated from a supply voltage, for charging or discharging the piezoelectric element. An actuating motion of the actuator is modified as a function of the control voltage of the piezoelectric element, the voltage gradient being simultaneously adjusted to the control voltage.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a method and a device for charging and discharging a piezoelectric element where electric charge carriers are transported back and forth between a direct voltage source and the piezoelectric element in order to transmit an elastic deflection of the piezoelectric element to an actuator.  
         BACKGROUND INFORMATION  
         [0002]    Conventionally, piezoelectric elements have the characteristic of contracting or expanding as a function of a direct voltage applied to them or of a direct voltage established across them. The practical implementation of actuators using piezoelectric elements may be suitable when the actuator has to perform quick and/or frequent movements. Among other things, the piezoelectric element may be used as an actuator in fuel injectors for internal combustion engines. For certain applications it may be necessary that different degrees of expansion or, if needed, varying degrees of expansion be induceable in the piezoelectric element as precisely as possible; for example, when the piezoelectric element is used as an actuator in a fuel injection system. Through direct or indirect transmission to a control valve, different degrees of expansion of the piezoelectric element correspond to the displacement of an actuator, for example, a nozzle needle. The displacement of the nozzle needle results in the opening of injection orifices. The duration of the opening of the injection orifices corresponds to a desired injected quantity as a function of a free cross section of the orifices and an applied pressure. The control valve which controls the movement of the nozzle needle need not be triggered directly, but may be triggered via a hydraulic coupler starting at the piezoelectric element.  
           [0003]    The piezoelectric element, together with the hydraulic coupler, the adjoining control valve and the nozzle needle, forms a complex spring-mass system. No excessive oscillations are to be induced in the spring-mass system by the triggering, because this would affect the desired injected quantity. Excitation of oscillations of the piezoelectric element may thus not be arbitrarily short.  
           [0004]    Assuming a predefined voltage level, there is a lower time threshold at which a trigger duration, composed of a charging operation and a holding operation, may not be shortened any further without causing oscillations of the spring-mass system.  
           [0005]    A conventional operating mode using a specific trigger cycle may make a non-oscillating deflection of piezoelectric elements possible via the modification of the trigger duration; however, the lower time threshold as a limit remains. In the case of a technical requirement for arbitrarily small injected quantities under an extremely high rail pressure in fuel injection systems, non-oscillating triggering is no longer possible using the conventional methods.  
         SUMMARY  
         [0006]    According to an example method according to the present invention, the level of an actuating motion of the actuator may be modified by charging the piezoelectric actuator to a variable voltage. The charging and discharging periods of the actuator remain unchanged in order to safely prevent the above-mentioned oscillations, even when the control voltage varies. This new second operating mode may be used for implementing minute quantities, for example, in fuel injection systems having a high rail pressure, since minute actuating motions of the actuator may be performed in a simple manner without causing oscillations of the spring-mass system.  
           [0007]    According to the present invention, a control unit may be provided which selects, prior to the start of the control cycle, whether triggering takes place in the first operating mode or in the second operating mode. All data necessary for a suitable selection between the operating modes may be detected and parameterized in the control unit according to the present invention. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0008]    [0008]FIG. 1 shows a schematic illustration of an example embodiment of a fuel injection system having a direct triggering of an actuator.  
         [0009]    [0009]FIG. 2 shows a schematic illustration of an example embodiment of a fuel injection system having an indirect triggering of the actuator.  
         [0010]    [0010]FIG. 3 shows a schematic illustration of an example embodiment of a fuel injection system.  
         [0011]    [0011]FIG. 4 shows a diagram illustrating the control cycle in the limit range of a lower time threshold with the formation of a voltage level U 1  and unchanged charging and discharging periods (second operating mode).  
         [0012]    [0012]FIG. 5 shows a diagram illustrating a control cycle in areas greater than or equal to the lower time threshold at a predefined constant direct voltage level U 1  (first operating mode).  
         [0013]    [0013]FIG. 6 shows a schematic illustration of an example embodiment of a device according to the present invention in a block diagram format. 
     
    
     DETAILED DESCRIPTION  
       [0014]    Control valve  42 , which moves an actuator  14 , is used for transmission of a deflection of a piezoelectric element  10  to actuator  14 . This direct transmission mode is depicted in FIG. 1. The transmission mode according to the present invention is depicted in FIG. 2 and is explained in detail in the following. For the transmission of the deflection of piezoelectric element  10  to control valve  42 , a hydraulic coupler  28  is interposed. Subsequently, the actuating motion of hydraulic coupler  28  is transmitted to actuator  14  via control valve  42 . A nozzle needle  44 , which opens injection orifices  64  for a predetermined time period, is moved by the deflection of actuator  14  in both transmission modes.  
         [0015]    [0015]FIG. 3 shows a schematic illustration of a fuel injection system. A supply voltage  12  feeds a direct voltage source  13  which in turn supplies a charging and discharging unit  40 . Data from fuel injection systems, which is used for triggering a trigger module  46 , may be parameterized for controlling and regulating in a microcontroller  34  of a control unit. A first operating mode  16  and a second operating mode  32 , which are explained in detail later, are formed for triggering trigger module  46 . In both operating modes  16  and  32 , charging and discharging unit  40  may be activated via trigger module  46 . Electric charge carriers of direct voltage source  13  are transmitted to and from piezoelectric element  10 . The transmission takes place within a control cycle  20  (FIGS. 4 and 5) which may be formed by a charging operation  22 , a holding operation  24 , and a discharging operation  26 . In both operating modes  16  and  32 , charging operation  22  and discharging operation  32  may be implemented in an identical time period. During charging operation  22 , and generally during holding operation  24 , piezoelectric element  10  is mechanically deflected. The deflection is transmitted to hydraulic coupler  28  via a piston  50 . Then the transmission takes place from hydraulic coupler  28  to piston  52  and subsequently to control valve  42 . Different responses of control valve  42  and consequently of actuator  14 , or of nozzle needle  44 , take place as a function of selected operating mode  16  or  32 .  
         [0016]    The following explanation of the example embodiment is based on a constant rail pressure of a rail chamber  60 . The explanation is also based on piezoelectric element  10 , piston  50 , hydraulic coupler  28 , and piston  52 , which together with the moving discrete masses inside the control valve up to nozzle needle  44  form the complex spring-mass system.  
         [0017]    As described above, both operating modes  16  and  32  operate according to control cycle  20  on which the following description of operating modes  16  and  32  is based.  
         [0018]    As shown in FIG. 3, a valve element  43  rests in a first seat  66  and seals a return line  54  in second operating mode  16  before the triggering of piezoelectric element  10 . Due to the pressure of rail chamber  60 , nozzle needle  44  is kept in its closed state.  
         [0019]    As shown in FIG. 4, charging operation  22  of piezoelectric element  10  takes place via a variable voltage gradient  18  until a voltage level U 1  is reached. Holding operation  24  follows on the same voltage level U 1 .  
         [0020]    Charging operation  22  and holding operation  24  are implemented observing a lower time threshold  30 . Lower time threshold  30  is the sum of the time periods of charging operation  22  and holding operation  24 , during which the spring-mass system has just not yet started to oscillate.  
         [0021]    As shown in FIG. 3, starting from the deflection of piezoelectric element  10 , hydraulic coupler  28  is mechanically deflected via piston  50  and piston  52 .  
         [0022]    After being deflected from first seat  66  by piston  52  inside control valve  42 , valve element  43  is displaced into a middle position  68  between seat  66  and seat  70  without sealing rail bypass  62 . Return line  54  is simultaneously opened. Depressurization via return line  54  takes place via an intake throttle  58 , a discharge throttle  56 , and rail bypass  62 . Intake throttle  58  has a smaller cross section than discharge throttle  56 . The rail pressure of rail chamber  60  acts on the back of discharge throttle  56  via opened rail bypass  62  and on the front of discharge throttle  56  via intake throttle  58 . The pressure in the area of nozzle needle  44  drops only to the point where nozzle needle  44  opens in a partial lift or at least opens more slowly than it would at a lower pressure, and injection orifices  64  are opened.  
         [0023]    The spring-mass system does not oscillate since, for implementation of a partial lift, voltage gradient  18  has been changed within charging operation  22  of control cycle  20  observing lower time threshold  30 .  
         [0024]    As shown in FIG. 4, voltage level U 1  drops back during discharging operation  26  and the deflection of piezoelectric element  10  goes back to zero.  
         [0025]    As shown in FIG. 3, hydraulic coupler  28  and pistons  52  and  50  yield to the rail pressure of rail chamber  60  via rail bypass  62  following valve element  43 . Valve element  43  seals return line  54  and is replaced into first seat  66 . Nozzle needle  44  is simultaneously closed again by the restored rail pressure of rail chamber  60  inside of control valve  42 .  
         [0026]    As shown in FIG. 3, a valve element  43  rests in the first seat and seals return line  54  in first operating mode  32 , conventionally, before the triggering of piezoelectric element  10 . Due to the pressure of rail chamber  60 , nozzle needle  44  is kept in its closed state.  
         [0027]    As shown in FIG. 5, charging operation  22  of piezoelectric element  10  takes place until predetermined voltage level U 2  is reached. Holding operation  24  follows on the same voltage level U 2 .  
         [0028]    Charging operation  22  and holding operation  24  are implemented observing lower time threshold  30 .  
         [0029]    As shown in FIG. 3, hydraulic coupler  28  is mechanically deflected, due to the deflection of piezoelectric element  10 , via piston  50  and piston  52 .  
         [0030]    After deflection from first seat  66  via piston  52  inside of control valve  42 , valve element  43  is displaced into second seat  70  so that rail bypass  62  is sealed. Return line  54  is simultaneously opened. Depressurization via return line  54  takes place via intake throttle  58  and discharge throttle  56 . Intake throttle  58  having a smaller cross section than discharge throttle  56  creates a pressure drop on nozzle needle  44  in a time period that is predefinable via the throttle cross sections. Since in this operating mode  32  the rail pressure of the rail chamber does not act upon the back of discharge throttle  56  via opened rail bypass  62 , the pressure on nozzle needle  44  drops to the point that nozzle needle  44  opens in a full lift and injection orifices  64  are opened completely.  
         [0031]    The spring-mass system does not oscillate, because in order to implement the full lift, according to FIG. 5, charging operation  22  and holding operation  24  within control cycle  20  has been performed on a predetermined voltage level U 2  observing lower time threshold  30 . Subsequently, voltage level U 2  drops back during discharging operation  26  and the deflection of piezoelectric element  10  moves back to zero.  
         [0032]    As shown in FIG. 3, hydraulic coupler  28  and pistons  52  and  50  yield to the rail pressure of rail chamber  60  following valve element  43  via rail bypass  62 . Valve element  43  leaves second seat  70  and seals return line  54 , being re-placed into first seat  66 . Nozzle needle  44  is simultaneously closed again by the restored rail pressure of rail chamber  60  inside of control valve  42 .  
         [0033]    The device for charging and discharging a piezoelectric element  10 , shown in FIG. 6 and described in detail in the following, has charging and discharging unit  40 . Direct voltage source  13 , situated between supply voltage  12  and charging and discharging unit  40 , is assigned to charging and discharging unit  40 . Direct voltage source  13  together with charging and discharging unit  40  and microcontroller  34  are parts of the control unit.  
         [0034]    A piezoelectric element  10 , connected to a hydraulic coupler  28 , is assigned to charging and discharging unit  40  in a conventional manner. Piston  50  on the input side of hydraulic coupler  28  and piston  52  on the output side of hydraulic coupler  28  are situated between hydraulic coupler  28  and piezoelectric element  10  (FIG. 3). Control valve  42  is assigned to piston  52  on the output side of hydraulic coupler  28 . Control valve  42  has an actuator  14 , which may be, for example, a nozzle valve  44 . The device also includes microcontroller  34  and trigger module  46  and is directly assigned to charging and discharging unit  40 . Microcontroller  34  has a controller  36  with which, in the second operating mode, an actuating motion of actuator  14  is modifiable by variation of the control voltage applied, the voltage gradient also being changed simultaneously. Within controller  36 , microcontroller  34  has the first operating mode, using which an actuating motion of actuator  14  may be determined as a function of a holding operation  24  over time. Operating modes  16  and  32  in microcontroller  34  may be selected as a function of the recorded and parameterized data of microcontroller  34 . This makes it possible to determine which operating mode  16  or  32  is subsequently assigned to trigger module  46 . In accordance with microcontroller  34  and controller  36 , the sequence of operating modes  16  or  32  may be combined freely and are transmittable to charging and discharging unit  40  via trigger module  46 .