Abstract:
A method of charging or discharging a piezoelectric element, in which electric charge carriers are transported back and forth between a supply voltage of a direct voltage source and a piezoelectric element in order to transmit an elastic deflection of the piezoelectric element to an actuator. An actuating movement of the actuator is modified as a function of the level of a voltage being applied to the piezoelectric element, a reduction of the level of the applied voltage being compensated for by at least one additional transmission of electric charge carriers during a holding phase of the piezoelectric element.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to Application No. 101 55 391.9, filed in the Federal Republic of Germany on Nov. 10, 2001, which is expressly incorporated herein in its entirety by reference thereto. 
     FIELD OF THE INVENTION 
     The present invention relates to a method of charging and discharging a piezoelectric element. 
     BACKGROUND INFORMATION 
     Piezoelectric elements have a 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 is suitable in particular when the appropriate actuator has to perform quick and/or frequent movements. Among other things, the piezoelectric element is used in fuel injectors for internal combustion engines. For certain applications it is necessary that the piezoelectric element be able to be brought to different expansions or if needed to varying expansions 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 expansions of the piezoelectric element correspond to the displacement of an actuator, like a nozzle needle for example. 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 fuel quantity as a function of a free cross-section of the orifices and an applied pressure. The transmission of the expansion of the piezoelectric element to the control valve is differentiated into two basic transmission modes. In the first, direct, transmission mode, the nozzle needle is moved directly by the piezoelectric element via a hydraulic coupler. In the second transmission mode, the movement of the nozzle needle is controlled by a control valve which is triggered by the piezoelectric element via a hydraulic coupler. The second, the indirect transmission mode, corresponds to the main industrial application and is the basis for further explanations. The hydraulic coupler has essentially two characteristics; first, the reinforcement of the stroke of the piezoelectric element, and second, the decoupling of the control valve from a static thermal expansion of the piezoelectric element. The hydraulic coupler transmits the elastic deflection of the piezoelectric element to the control valve within a control cycle. In order to function accurately, the coupler must be sufficiently filled with a fluid. In each control cycle, which includes a charging operation, a holding operation, and a discharging operation, a portion of the fluid being present in the coupler is pressed out via leak gaps. 
     In particular during the charging and holding operation, where the piezoelectric element is charged to a certain voltage, the hydraulic coupler undergoes a certain deflection and moves a valve element of the control valve from a first seat to a second seat. To ensure an accurate opening of the nozzle needle, the valve element of the control valve, as a rule, must be in contact with the second seat and must seal against a high pressure applied in a rail chamber. If this is not the case, as a rule, an unintentional deflection of the actuator occurs due to the pressure changes of a control space above the actuator and thus resulting in an imprecise injected fuel quantity. Due to existing leakage losses at the leak gaps of the hydraulic coupler, a decrease of coupler pressure results during the holding operation. The level of a setpoint direct voltage of the piezoelectric element, applied within the charging operation, is reduced in response to the decrease in the coupler pressure. If the coupler pressure drops to a certain level, then the valve element of the control valve can no longer be held in the second seat and leakages occur in the second seat of the control valve. Thus, after a short time, leakages in the sealing area of the second seat occur, in particular at high pressures in the rail chamber. This results in a pressure change inside the control space. This pressure change results in an unintended actuating movement of the actuator and thus to an imprecise injected fuel quantity. The decrease in the voltage applied to the piezoelectric element during the holding phase indicates a leakproof condition of the control valve on the second seat and thus a correct function of the injector. 
     SUMMARY 
     The method according to the present invention provides an actuating movement of the actuator that is changed as a function of the level of the voltage applied to the piezoelectric element, a decrease in the level of the voltage applied during a holding phase being compensated for by at least one additional transmission of electric charge carriers from the direct voltage source to the piezoelectric element. Thereby the pressure in the coupler is elevated again to a level where a leakproof condition in the second seat may be ensured. 
     In an exemplary embodiment of the present invention, after a reduction in the setpoint direct voltage of the piezoelectric element within the holding operation of the control cycle, a retransmission of electric charge carriers from the direct voltage source to the piezoelectric element takes place at a predefined point in time. This method results in the compensation for the pressure drop in the coupler due to retransmission of electric charge carriers and in the prevention of the movement of the actuator at a wrong instant, and thus in a reliable control and an accurate regulation of the fuel quantity being injected. 
     The present invention is explained below in greater detail using an exemplary embodiment with reference to the drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of the method for a fuel injector. 
     FIG. 2 is a diagram of the voltage characteristic of a control cycle as a function of time. 
     FIG. 3 is a diagram of the injected fuel quantity within a control cycle as a function of time. 
     FIG. 4 is a diagram of the voltage characteristic of a control cycle as a function of time. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates a method according to the present invention. A supply voltage  14  feeds a direct voltage source  16  which supplies a charging and discharging unit  62 . Electric charge carriers of direct voltage source  16  are transmitted to and from piezoelectric element  10 . The transmission occurs within a control cycle  20 , which includes a charging operation  22 , a holding operation  24 , and a discharging operation  26  (FIG.  2 ). During charging operation  22  and essentially during holding operation  24 , piezoelectric element  10  is mechanically deflected. The mechanical deflection occurs by applying a predefined direct voltage  12  and reaches the predefined deflection at the end of charging operation  22  at a setpoint direct voltage  68 . 
     The respective charging operation  22  is illustrated in FIG. 2 in a voltage/time diagram. 
     The maximum deflection of piezoelectric element  10  is transmitted to a hydraulic coupler  28  via a piston  36 , as illustrated in FIG.  1 . The transmission occurs from hydraulic coupler  28  to a piston  38  and subsequently to a control valve  32 . Between piston  36  and hydraulic coupler  28  and piston  38 , part of the fluid present in hydraulic coupler  28  is pressed out of the hydraulic coupler via leak gaps in each control cycle  20 . 
     A valve element  60  of control valve  32  is situated initially in a first seat  56 . After triggering of piezoelectric element  10  and transmission of the mechanical deflection of piezoelectric element  10  via piston  36 , hydraulic coupler  28 , and piston  38 , valve element  60  is displaced from first seat  56  to a second seat  58 . This closes a rail bypass  52  of a rail chamber  50 , which is under high rail pressure. In the further example embodiments, it may be assumed that the rail pressure in rail chamber  50  is kept constant. After valve element  60  has reached second seat  58  in control valve  32 , holding operation  24  starts within control cycle  20 . 
     According to FIG. 1, liquid fuel already present inside control valve  32  and in a control space  64  flows to control space  64  via an inlet throttle  44  during holding operation  24 . A portion of the fuel from control space  64  reaches return line  40  via an outlet throttle  42 , resulting in a drop of rail chamber dependent pressure in control space  64 , and actuator  18  opens. The opening of actuator  18  is triggered by the pressure-dependent deflection of a nozzle needle  34 , whereby injection orifices  54  are opened. 
     FIG. 3 illustrates a charging operation  22  and subsequent holding operation  24  on the basis of a diagram which illustrates injected fuel quantity  70  as a function of time. It is recognizable that—after a hydraulic delay within the charging operation, caused by the inertia of hydraulic coupler  28 , as well as of hydraulically operated nozzle needle  34 —after completion of charging operation  22  a fuel quantity  70  is injected, in particular during holding operation  24 . 
     If the coupler pressure drops, because of the leakage effects between piston  36  and piston  38 , then setpoint direct voltage  68  of piezoelectric element  10  drops during holding operation  24 , according to FIG.  2 . Due to the dropping pressure in the coupler, valve element  60  is no longer securely held in second seat  58 . Additional leakages in the sealing area between valve element  60  and rail bypass  52  occur initially. 
     FIG. 2 illustrates, on the basis of the characteristic curve, the direct voltage drop of setpoint direct voltage  68  to lower voltage limit  30  within holding operation  24 . 
     FIG. 3 illustrates that injected fuel quantity  70  may remain unaffected by the drop of setpoint direct voltage  68  within holding operation  24  and may even further increase, since the drop in setpoint direct voltage  68  to lower voltage limit  30  may not yet result in a displacement of valve element  60  from second seat  58 . 
     A further drop in the coupler pressure during holding operation  24  to below lower voltage limit  30  may result in displacement of valve element  60  from the second seat in the direction of the first seat and opening of rail bypass  52 . Due to the opening of rail bypass  52 , the rail pressure of rail chamber  50  acts via opened rail bypass  52  on the back of outlet throttle  42  and via inlet throttle  44  back to inlet throttle  44  on the front of outlet throttle  42 . Through this procedure, the pressure rises in control space  64  compared to the situation of a closed rail bypass  52 . The pressure rise in control space  64  results in an unintended closing operation of actuator  18 , in particular nozzle needle  34 . Therefore a smaller cross-section of injection orifices  54  is opened and desired injected fuel quantity  70  is not achieved. 
     This procedure is not illustrated in FIGS. 1 through 4, since according to the present invention, as shown in FIG. 2, a compensation operation  66  is added when a lower voltage limit  30  is reached. Piezoelectric element  10 , being isolated from the voltage supply during holding operation  24 , is triggered again after the drop of setpoint direct voltage  68  to lower voltage limit  30  and a new transmission of electric charge carriers of direct voltage source  16  to piezoelectric element  10  takes place. Compensation operation  66  may hereby ensure that valve element  60  is held further on second seat  58  of control valve  32 . 
     It is further possible, according to the present invention, to repeat compensation operation  66  several times during holding operation  24 . 
     The method is not limited to returning the voltage from lower voltage limit  30  to setpoint direct voltage  68  within compensation operation  66 . There is the possibility, while leaving the process steps described so far (charging operation  22  and holding operation  24 ) unchanged, to raise the reduced setpoint direct voltage  68  to a voltage level  72  which lies above setpoint direct voltage  68 . This possible process step is illustrated in FIG. 4 on the basis of the characteristic curve within compensation operation  66 . 
     As illustrated in FIG.  1  and FIG. 4, compensation operation  66  may be followed by an additional holding operation  24  in which a voltage drop is observed again as a function of setpoint direct voltage  68  according to FIG. 2 or voltage level  72  according to FIG.  4 . 
     As illustrated in FIG. 3, injected fuel quantity  70  also remains constant during holding operation  24  following compensation operation  66 . 
     In practice, discharging operation  26  follows within one of the above mentioned holding operations  24  before reaching lower voltage limit  30 . Due to the retransmission of electric charge carriers from the piezoelectric element to charging/discharging unit  62 , the elastic deflection of piezoelectric element  10  is canceled. The rail pressure of rail chamber  50  acts on valve element  60 . Starting from piezoelectric element  10  via pistons  36  and  38 , the pressure of hydraulic coupler  28 , causing the deflection of valve element  60 , is canceled. Valve element  60  leaves second seat  58  and closes return line  40  and is displaced back onto first seat  56 . Due to the restored rail pressure of rail chamber  50  inside of control valve  32  and control space  64 , actuator  18 , in particular nozzle needle  34 , are again completely closed. Injected fuel quantity  70  drops back to zero, as FIG. 3 illustrates on the basis of the characteristic curve. 
     It has been assumed in the previous explanations that compensation (re-loading) takes place when voltage limit  30  is reached. It is however also within the framework of the present invention if compensation starts at a preselectable point in time, independently of the actual voltage across piezoelectric element  10 . For example, compensation may be initiated after a preselectable time period has elapsed after the start of holding operation  24 . It is also possible to repeat the compensation automatically in preselectable time intervals within the holding operation.