Abstract:
A positioner includes a capacitative element with which an ohmic resistance is connected in parallel. The value of the ohmic resistance is sensed at specific points in time. To enhance operating reliability during operation of the positioner, correct functioning of the ohmic resistance is monitored, and a fault signal is outputted upon detection of a malfunction.

Description:
RELATED APPLICATIONS 
   This application is a 371 of PCT/DE 03/02172 filed on Jun. 30, 2003. 
   FIELD OF THE INVENTION 
   The present invention relates to a method for operating an actuator having a capacitative element, an ohmic resistance being connected in parallel with the capacitative element and the value of the ohmic resistance being sensed at specific points in time. 
   BACKGROUND INFORMATION 
   A method is referred to in Published Patent Application No. DE 199 58 406 A1, which describes a piezoactuator that is used, for example, in a fuel injector. The piezoactuator behaves similarly to a capacitative element in electrical terms, and is therefore itself often referred to as a capacitative element. The capacitative element is longer or shorter depending on its charge state. The change in length of the capacitative element is transferred to a valve element of the fuel injector. 
   In the event of an interruption in activation of the capacitative element, or a malfunction of one of the components, it may happen that the capacitative positioner remains continuously in one specific position because it can no longer be discharged. In a context of use in a fuel injector, the result of this may be, for example, that the latter remains in the open position for a long period of time, and fuel is continuously injected into the combustion chamber of the internal combustion engine. This may result in severe damage to the internal combustion engine. 
   The ohmic resistance is provided to prevent such a situation. It enables discharging of the capacitative element even when the actual control line is interrupted, e.g. due to a cable break or a contact fault. The value of the ohmic resistance is dimensioned such that the time constant resulting from the capacitative element and the ohmic resistance is so great that no significant discharge of the capacitative element occurs within the usual activation time period that is usual for fault-free injection. On the other hand, the time constant is set so that the capacitative element is sufficiently discharged within the maximum time available before the valve must definitely be closed in order not to damage the internal combustion engine. 
   German Published Patent Application No. 199 58 406 proposes to sense the value of the ohmic resistance at specific points in time, and to draw conclusions therefrom as to the nature and/or the temperature of the capacitative element. The temperature dependence of the capacitative element may thereby be corrected. 
   SUMMARY OF THE INVENTION 
   The present invention may enhance operating reliability when a capacitative element is used. 
   This may be achieved, in the context of a method of the kind cited initially, in that correct functioning of the ohmic resistance is monitored, and a fault signal is outputted upon detection of a malfunction. 
   An exemplary method according to the present invention may monitor the functionality of the ohmic resistance representing a safety device in order to detect states in which that safety device can no longer perform the function assigned to it. This in turn may make it possible, for example, to seek out in timely fashion a maintenance facility that can repair the safety device, i.e. the ohmic resistance, and thus restore the operating reliability of, for example, an internal combustion engine. 
   In a first exemplary embodiment, the value of the ohmic resistance may be sensed and compared with a limit value. A corresponding exemplary method for sensing the value of the ohmic resistance is described in German Published Patent Application No. 199 58 406. Sensing the value of the ohmic resistance and comparing the sensed value with a limit value may be simple and reliable capability for checking the functionality of the ohmic resistance. This is because if the ohmic resistance loses contact with the capacitative element, e.g. as a result of a poor solder joint, the value of the ohmic resistance rises sharply. This may be unequivocally detected by way of the claimed comparison with a limit value. It may also be possible to monitor whether the value of the ohmic resistance is within a tolerance band. 
   In another exemplary embodiment, the value of the ohmic resistance may be sensed during a startup phase of a control unit with which the capacitative element is activated, and/or during a shutdown phase of the control unit when the latter is being switched off. The above-claimed sensing of the value of the ohmic resistance may not be possible in every case during normal operation of the capacitative element. This is because in order to sense the value of the ohmic resistance (in accordance with the exemplary method indicated in German Published Patent Application No. 199 58 406), it may be necessary to charge the capacitative element to a certain voltage and to sense the discharge curve through the ohmic resistance. A sufficiently high voltage may be important in this context, since the error may become too great at excessively low voltages. This may not be achievable during normal operation of the capacitative element. 
   Prior to actual operation of the capacitative element, however, there exist a startup phase of the control unit that activates the capacitative element. During this startup phase of the control unit, for example, a self-test may be executed and certain initial values are set. The same also applies to the shutdown phase of the control unit, which may be necessary for controlled shutoff of the capacitative element and of the device in which the capacitative element is being used. During these phases, the capacitative element is not yet being used as intended, so that charging and discharging for test purposes produce no interference here. 
   The capacitative element may be used in an injector of an internal combustion engine, and the value of the ohmic resistance may be sensed during a coasting mode of the internal combustion engine. No fuel is usually injected into the internal combustion engine while the internal combustion engine is in coasting mode. It may therefore be appropriate to use this operating state in order to sense the value of the ohmic resistance. 
   In an exemplary embodiment of a method according to the present invention, correct functioning of the capacitative element may be monitored. A corresponding exemplary method therefore is described in European Patent Application No. 1 138 905. With this exemplary method according to the present invention, therefore, on the one hand correct functioning of the capacitative element may be monitored, i.e. a determination may be made as to whether activation from the control unit to the capacitative element is OK (cable break, loose connector, etc.); and on the other hand, correct functioning of the ohmic resistance, i.e. the safety device of the capacitative element, may be monitored. A high level of safety may thus be achieved with this exemplary embodiment of the method according to the present invention. 
   According to an exemplary embodiment, a first fault signal may be outputted when it is determined that the ohmic resistance is working correctly and the capacitative element not correctly, or when it is determined that the capacitative element is working correctly and the ohmic resistance not correctly. The user of the capacitative element may be, in this fashion, given concrete information regarding that specific malfunction. He or she may thus react accordingly, i.e. seek out a maintenance facility. 
   In this context, the capacitative element may be used in an injector of an internal combustion engine, and the first fault signal may cause a reduction in the maximum permitted torque of the internal combustion engine. The internal combustion engine is thus shifted into a “safety mode” in which it may continue to be operated, but only in such a manner that no permanent damage to the internal combustion engine occurs. 
   In this context a second fault signal may be outputted when it is determined that on the one hand the ohmic resistance and on the other hand the capacitative element are not working correctly. The result is to create a graduated fault message that informs the user not only of the existence of a malfunction, but also about the nature and severity of the malfunction. The user may thus react to the reported malfunctions in particularly effective and specific fashion. It is understood in this context that the second fault signal indicates a more serious malfunction than the first fault signal. This is because if on the one hand the ohmic resistance and on the other hand the capacitative element are not working correctly, this means that the risk of damage to the apparatus being operated with the capacitative element may be particularly high. 
   If the capacitative element is used in an injector of an internal combustion engine, the second fault signal should cause the affected cylinder to be shut off, the fuel pressure to be reduced, and/or the internal combustion engine to be shut down. These actions may reduce the risk of permanent damage to the internal combustion engine, or may entirely rule out such a risk. 
   It may also be desired if the first and/or the second fault signal result(s) in an input into a fault memory and/or the triggering of a specific alarm signal. This may facilitate diagnosis at the maintenance facility and appropriate reaction by the user. 
   The present invention also relates to a computer program to carry out the aforesaid exemplary method when it is executed on a computer. In this context, the computer program may be stored on a memory, in particular on a flash memory. 
   The present invention further relates to an open- and/or closed-loop control unit for operating an internal combustion engine. In this context, such an open- and/or closed-loop control unit may encompass a memory on which a computer program of the aforesaid kind is stored. 
   Also the subject matter of the present invention may include an internal combustion engine having a combustion chamber, having at least one injector that encompasses a capacitative element as actuator and that encompasses an ohmic resistance connected in parallel with the latter. To enhance the operating reliability of the internal combustion engine, it is proposed that it encompass an open- and/or closed-loop control device of the aforesaid kind. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  schematically depicts an internal combustion engine having an injector that encompasses a piezoactuator. 
       FIG. 2  shows a detail of the piezoactuator of  FIG. 1  and a control unit for activating it. 
       FIG. 3  is a flow chart of an exemplary method operating the piezoactuator of  FIG. 1 . 
       FIG. 4  is a flow chart of a further exemplary method for operating the piezoactuator of  FIG. 1 . 
   

   DETAILED DESCRIPTION 
   In  FIG. 1 , an internal combustion bears in its entirety the reference character  10 . It encompasses a combustion chamber  12  into which fresh air is introduced through an inlet valve  14  and an intake duct  16 . The hot combustion gases are discharged from combustion chamber  12  through an outlet valve  18  and an exhaust duct  20 . 
   Fuel is introduced directly into combustion chamber  12  through an injector  22  that is activated by a control unit  24  and receives fuel under high pressure from a fuel system  26 . Injector  22  encompasses a valve needle (not depicted in  FIG. 1 ) that is actuated by a piezoactuator  28 . The fuel/air mixture present in combustion chamber  12  after an injection is ignited by a spark plug  30  (note in this context that the use of injector  22  is not confined to gasoline internal combustion engines, but that it may also be used in diesel internal combustion engines). 
   As is evident from  FIG. 2 , piezoactuator  28  encompasses a multi-layer piezo positioner  32  whose length depends on its electrical charge state. Since a multi-layer piezo positioner of this kind has electrical properties similar to those of a capacitative element, it may also itself be referred to as a capacitative element. An ohmic resistance  34  is connected in parallel with multi-layer piezo positioner  32 . Multi-layer piezo positioner  32  and ohmic resistance  34  thus constitute an RC element. 
   Piezoactuator  28  may be connected, for example via a hydraulic coupler (not depicted), to the valve needle of injector  22 , and may influence the position of the valve needle depending on the voltage present at multi-layer piezo positioner  32 . In an exemplified embodiment that is not depicted, the piezoactuator actuates a hydraulic control valve that causes a motion of the valve needle by way of a pressure change in a control chamber. 
   Multi-layer piezo positioner  32  and ohmic resistance  34  are, via their one terminal, on the one hand grounded (reference character  36 ) and on the other hand connected to an evaluation block  38  that is part of open- and closed-loop control unit  24  and is discussed in greater detail below. At their other terminal, multi-layer piezo positioner  32  and ohmic resistance  34  are on the one hand again connected to evaluation block  38  and on the other hand connected to an output stage switch  40 . As once again discussed in detail below, the manner of connection of evaluation block  38  makes it possible to sense, by means thereof, the voltage drop occurring through RC element  32 ,  34 . 
   Output stage switch  40  is activated by a control block  42  that receives and processes different input signals, also including signals from evaluation block  38 . Multi-layer piezo positioner  32  and ohmic resistor  34  can be connected via output stage switch  40  to an energy source  44 . Additionally disposed between output stage switch  40  and energy source  44  is a monitoring device  46  whose exact function will once again be discussed in detail below. 
   Control block  42  additionally activates a further output stage switch  48  that may ground (reference character  50 ) the other terminal of capacitative element  32  and of ohmic resistance  34 . Piezoactuator  28  is connected to open- and closed-loop control unit  24  via a line  52  and a connector  54 . 
   During normal operation of internal combustion engine  10 , injector  22  with multi-layer piezo positioner  32  works as follows: When fuel is to be injected by injector  22  into combustion chamber  12  of internal combustion engine  10 , first output stage switch  40  is closed by control block  42 , and second output stage switch  48  is opened. Multi-layer piezo positioner  32  is thus connected to energy source  44 . The voltage now present at capacitative element  32  causes an elongation of the capacitative element which, as already indicated above, causes valve needle of injector  22  to lift off from a corresponding valve seat and open a passage for fuel from fuel source  26  into combustion chamber  12 . 
   When the injection of fuel into combustion chamber  12  is to be terminated, output stage switch  48  is closed by control block  42  (output stage switch  40  having been opened again immediately after the end of the charging operation). Both terminals of multi-layer piezo positioner  32  are thus grounded (reference characters  36  and  50 ), so that multi-layer piezo positioner  32  discharges again and becomes correspondingly shorter. As a result, the valve needle of injector  22  once again comes into contact against the corresponding valve seat so that communication between fuel system  26  and combustion chamber  12  is again interrupted. 
   Reliable operation of capacitative element  32  may be important for the overall operating reliability of the internal combustion engine. Without corresponding countermeasures, it may happen that, for example in the event of a break in cable  52  or a loose connector  54 , multi-layer piezo positioner  32  is no longer connected to open- and closed-loop control device  24  and thus may no longer be activated. If the connection between multi-layer piezo positioner  32  and open- and closed-loop control device  24  is interrupted while multi-layer piezo positioner  32  is charged, i.e. while an injection of fuel into combustion chamber  12  of internal combustion engine  10  is occurring, then without corresponding countermeasures, that injection may not be terminated. This may result in severe damage to internal combustion engine  10 . 
   To prevent this, ohmic resistance  34  is connected in parallel with multi-layer piezo positioner  32 . This resistance is dimensioned so that the time constant resulting from multi-layer piezo positioner  32  and ohmic resistance  34  (which constitute an RC element) is so great that no significant discharge of capacitative element  32  occurs within the usual activation time period that is necessary and usual for a fault-free injection of fuel into combustion chamber  12 . On the other hand, the time constant is set so that multi-layer piezo positioner  32  is sufficiently discharged within the maximum time available before injector  22  must definitely be closed in order not to damage internal combustion engine  10 . When appropriately dimensioned, ohmic resistor  34  therefore acts as a so-called “bleeder resistance.” 
   If a break in line  52  or a detachment of connector  54  occurs while injector  22  is open, multi-layer piezo positioner  32  is therefore discharged through ohmic resistance  34 , and injector  22  is thus closed again. Ohmic resistance  34  is therefore an important safety device of injector  22 . The knowledge that this safety device is functional may thus enhance the overall operating reliability of internal combustion engine  10 . The functionality of ohmic resistance  34  is determined, during a coasting mode of the internal combustion engine, during startup and during shutdown of open- and closed-loop control device  24 , as follows (see  FIG. 3 ): 
   The exemplary method depicted in  FIG. 3  begins with a Start block  56 . After this, in block  58  multi-layer piezo positioner  32  is charged to a defined voltage U. Simultaneously, a time counter t is set to zero. The subsequent query in block  60  checks whether the value of time counter t is greater than or equal to a time threshold t 1 . If that is not the case, the time counter is then incremented in  62 , and the query in block  60  is made again. If time counter t is greater than or equal to time threshold t 1 , the voltage U 1  at that time t 1  is then measured in block  64 . 
   The next step  66  queries whether the content of time counter t is greater than or equal to a second time threshold t 2 . If that is not the case, the time counter is then incremented in block  68 . If it is the case, the value U 2  of the voltage at time t 2  is then ascertained in block  70 . 
   The voltage in the RC element constituted by multi-layer piezo positioner  32  and ohmic resistance  34  decreases over time according to an exponential function, the exponential function being determined substantially by a time constant. By measuring voltage U 1  at time t 1  and voltage U 2  at time t 2 , the time constant may be determined and, if the capacitance of capacitative element  32  is known, therefore the value R of ohmic resistance  34 . This calculation of the value R is performed in block  72 . 
   Block  74  then queries whether the value R is greater than a limit value G. If the response to the query in block  74  if No, this indicates that ohmic resistance  34  is working correctly (block  76 ). If, however, a solder joint with which ohmic resistance  34  is connected to multi-layer piezo positioner  32  is defective, for example, the value R of ohmic resistance  34  rises sharply and exceeds limit value G. In this case the response to the query in  74  is Yes, and that logical signal is further processed in block  78  in a manner depicted below in detail. The checking of the functionality of ohmic resistance  34  ends in an End block  80 . 
     FIG. 4  depicts the processing in processing block  78  in detail. That processing contains substantially a combination of the logical Yes result of query block  74  with the logical results of the diagnosis of the functionality of capacitative element  32  by way of monitoring block  46  (see  FIG. 2 ). Block  82  queries whether multi-layer piezo positioner  32  is or is not functional. If a defect is present, a bit B 2 =1 is set at the output of block  82 . If no defect is present, bit B 2 =0 is set at the output of block  82 . Analogously, a bit B 1 =1 is set at the output of query  74  if the value R of ohmic resistance  34  is greater than the limit value G, i.e. if there is a defect in ohmic resistance  34 . The same bit B 1  is set to zero when ohmic resistance  34  is working in fault-free fashion. 
   The respective outputs of queries  74  and  82  are fed into three logical AND blocks  84 ,  86 , and  88 . The output of query block  74  is inverted in block  90  before being fed into block  84 , and the output of query block  82  is inverted in block  92  before being fed into block  86 . The two AND blocks  84  and  86  are connected on the output side to an OR element  94  whose output is again connected to an OR element  96 . The output of AND block  88  leads directly to an OR element  98 . 
   OR elements  96  and  98  ensure that the exemplary method described in  FIG. 4  is performed for all the cylinders  1  through i of internal combustion engine  10 . The output of OR element  96  leads to an alarm block  100 , and the output of OR element  98  to a second alarm block  102 . 
   If both bits B 1  and B 2  are equal to zero (capacitative element  32  and ohmic resistance  34  are each working correctly), a bit =0 is also present at the respective outputs of AND blocks  84 ,  86 , and  88 , so that ultimately neither alarm block  100  nor alarm block  102  is activated. If, however, bit B 1 =1 (ohmic resistance  34  is defective), and bit B 2 =0 (capacitative element  32  is working correctly), this results in a bit=1 at the output of AND block  86 , so that ultimately alarm block  100  is activated. 
   The same also applies to the case in which bit B 1 =0 (ohmic resistance  34  is working correctly), but bit B 2 =1 (capacitative element  32  is defective). In this case a logical value of 1 is present at the output of AND block  84 , once again ultimately resulting, via OR element  94 , in the activation of alarm block  100 . Lastly, if bit B 1 =1 (ohmic resistance  34  is defective) and bit B 2 =1 (capacitative element  32  is defective), this then results in a bit=1 at the output of AND block  88 , which ultimately causes the activation of alarm block  102 . 
   Alarm block  100  causes an input into a fault memory and the illumination of a warning light. In addition, the maximum torque that may be generated by internal combustion engine  10  is reduced. Upon activation of alarm block  102 , on the other hand, the affected cylinder is shut down, fuel pressure is reduced and, if applicable, the entire internal combustion engine  10  is shut down. The exemplary method depicted in  FIG. 4  thus permits a graduated reaction, depending on whether only piezo positioner  32  or only ohmic resistance  34 , or both piezo positioner  32  and ohmic resistance  34  simultaneously, are defective.