Method and device for operating an actuator with a capacitive element

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.

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.

DETAILED DESCRIPTION

InFIG. 1, an internal combustion bears in its entirety the reference character10. It encompasses a combustion chamber12into which fresh air is introduced through an inlet valve14and an intake duct16. The hot combustion gases are discharged from combustion chamber12through an outlet valve18and an exhaust duct20.

Fuel is introduced directly into combustion chamber12through an injector22that is activated by a control unit24and receives fuel under high pressure from a fuel system26. Injector22encompasses a valve needle (not depicted inFIG. 1) that is actuated by a piezoactuator28. The fuel/air mixture present in combustion chamber12after an injection is ignited by a spark plug30(note in this context that the use of injector22is not confined to gasoline internal combustion engines, but that it may also be used in diesel internal combustion engines).

As is evident fromFIG. 2, piezoactuator28encompasses a multi-layer piezo positioner32whose 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 resistance34is connected in parallel with multi-layer piezo positioner32. Multi-layer piezo positioner32and ohmic resistance34thus constitute an RC element.

Piezoactuator28may be connected, for example via a hydraulic coupler (not depicted), to the valve needle of injector22, and may influence the position of the valve needle depending on the voltage present at multi-layer piezo positioner32. 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 positioner32and ohmic resistance34are, via their one terminal, on the one hand grounded (reference character36) and on the other hand connected to an evaluation block38that is part of open- and closed-loop control unit24and is discussed in greater detail below. At their other terminal, multi-layer piezo positioner32and ohmic resistance34are on the one hand again connected to evaluation block38and on the other hand connected to an output stage switch40. As once again discussed in detail below, the manner of connection of evaluation block38makes it possible to sense, by means thereof, the voltage drop occurring through RC element32,34.

Output stage switch40is activated by a control block42that receives and processes different input signals, also including signals from evaluation block38. Multi-layer piezo positioner32and ohmic resistor34can be connected via output stage switch40to an energy source44. Additionally disposed between output stage switch40and energy source44is a monitoring device46whose exact function will once again be discussed in detail below.

Control block42additionally activates a further output stage switch48that may ground (reference character50) the other terminal of capacitative element32and of ohmic resistance34. Piezoactuator28is connected to open- and closed-loop control unit24via a line52and a connector54.

During normal operation of internal combustion engine10, injector22with multi-layer piezo positioner32works as follows: When fuel is to be injected by injector22into combustion chamber12of internal combustion engine10, first output stage switch40is closed by control block42, and second output stage switch48is opened. Multi-layer piezo positioner32is thus connected to energy source44. The voltage now present at capacitative element32causes an elongation of the capacitative element which, as already indicated above, causes valve needle of injector22to lift off from a corresponding valve seat and open a passage for fuel from fuel source26into combustion chamber12.

When the injection of fuel into combustion chamber12is to be terminated, output stage switch48is closed by control block42(output stage switch40having been opened again immediately after the end of the charging operation). Both terminals of multi-layer piezo positioner32are thus grounded (reference characters36and50), so that multi-layer piezo positioner32discharges again and becomes correspondingly shorter. As a result, the valve needle of injector22once again comes into contact against the corresponding valve seat so that communication between fuel system26and combustion chamber12is again interrupted.

Reliable operation of capacitative element32may 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 cable52or a loose connector54, multi-layer piezo positioner32is no longer connected to open- and closed-loop control device24and thus may no longer be activated. If the connection between multi-layer piezo positioner32and open- and closed-loop control device24is interrupted while multi-layer piezo positioner32is charged, i.e. while an injection of fuel into combustion chamber12of internal combustion engine10is occurring, then without corresponding countermeasures, that injection may not be terminated. This may result in severe damage to internal combustion engine10.

To prevent this, ohmic resistance34is connected in parallel with multi-layer piezo positioner32. This resistance is dimensioned so that the time constant resulting from multi-layer piezo positioner32and ohmic resistance34(which constitute an RC element) is so great that no significant discharge of capacitative element32occurs within the usual activation time period that is necessary and usual for a fault-free injection of fuel into combustion chamber12. On the other hand, the time constant is set so that multi-layer piezo positioner32is sufficiently discharged within the maximum time available before injector22must definitely be closed in order not to damage internal combustion engine10. When appropriately dimensioned, ohmic resistor34therefore acts as a so-called “bleeder resistance.”

If a break in line52or a detachment of connector54occurs while injector22is open, multi-layer piezo positioner32is therefore discharged through ohmic resistance34, and injector22is thus closed again. Ohmic resistance34is therefore an important safety device of injector22. The knowledge that this safety device is functional may thus enhance the overall operating reliability of internal combustion engine10. The functionality of ohmic resistance34is determined, during a coasting mode of the internal combustion engine, during startup and during shutdown of open- and closed-loop control device24, as follows (seeFIG. 3):

The exemplary method depicted inFIG. 3begins with a Start block56. After this, in block58multi-layer piezo positioner32is charged to a defined voltage U. Simultaneously, a time counter t is set to zero. The subsequent query in block60checks whether the value of time counter t is greater than or equal to a time threshold t1. If that is not the case, the time counter is then incremented in62, and the query in block60is made again. If time counter t is greater than or equal to time threshold t1, the voltage U1at that time t1is then measured in block64.

The next step66queries whether the content of time counter t is greater than or equal to a second time threshold t2. If that is not the case, the time counter is then incremented in block68. If it is the case, the value U2of the voltage at time t2is then ascertained in block70.

The voltage in the RC element constituted by multi-layer piezo positioner32and ohmic resistance34decreases over time according to an exponential function, the exponential function being determined substantially by a time constant. By measuring voltage U1at time t1and voltage U2at time t2, the time constant may be determined and, if the capacitance of capacitative element32is known, therefore the value R of ohmic resistance34. This calculation of the value R is performed in block72.

Block74then queries whether the value R is greater than a limit value G. If the response to the query in block74if No, this indicates that ohmic resistance34is working correctly (block76). If, however, a solder joint with which ohmic resistance34is connected to multi-layer piezo positioner32is defective, for example, the value R of ohmic resistance34rises sharply and exceeds limit value G. In this case the response to the query in74is Yes, and that logical signal is further processed in block78in a manner depicted below in detail. The checking of the functionality of ohmic resistance34ends in an End block80.

FIG. 4depicts the processing in processing block78in detail. That processing contains substantially a combination of the logical Yes result of query block74with the logical results of the diagnosis of the functionality of capacitative element32by way of monitoring block46(seeFIG. 2). Block82queries whether multi-layer piezo positioner32is or is not functional. If a defect is present, a bit B2=1 is set at the output of block82. If no defect is present, bit B2=0 is set at the output of block82. Analogously, a bit B1=1 is set at the output of query74if the value R of ohmic resistance34is greater than the limit value G, i.e. if there is a defect in ohmic resistance34. The same bit B1is set to zero when ohmic resistance34is working in fault-free fashion.

The respective outputs of queries74and82are fed into three logical AND blocks84,86, and88. The output of query block74is inverted in block90before being fed into block84, and the output of query block82is inverted in block92before being fed into block86. The two AND blocks84and86are connected on the output side to an OR element94whose output is again connected to an OR element96. The output of AND block88leads directly to an OR element98.

OR elements96and98ensure that the exemplary method described inFIG. 4is performed for all the cylinders1through i of internal combustion engine10. The output of OR element96leads to an alarm block100, and the output of OR element98to a second alarm block102.

If both bits B1and B2are equal to zero (capacitative element32and ohmic resistance34are each working correctly), a bit =0 is also present at the respective outputs of AND blocks84,86, and88, so that ultimately neither alarm block100nor alarm block102is activated. If, however, bit B1=1 (ohmic resistance34is defective), and bit B2=0 (capacitative element32is working correctly), this results in a bit=1 at the output of AND block86, so that ultimately alarm block100is activated.

The same also applies to the case in which bit B1=0 (ohmic resistance34is working correctly), but bit B2=1 (capacitative element32is defective). In this case a logical value of 1 is present at the output of AND block84, once again ultimately resulting, via OR element94, in the activation of alarm block100. Lastly, if bit B1=1 (ohmic resistance34is defective) and bit B2=1 (capacitative element32is defective), this then results in a bit=1 at the output of AND block88, which ultimately causes the activation of alarm block102.

Alarm block100causes 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 engine10is reduced. Upon activation of alarm block102, on the other hand, the affected cylinder is shut down, fuel pressure is reduced and, if applicable, the entire internal combustion engine10is shut down. The exemplary method depicted inFIG. 4thus permits a graduated reaction, depending on whether only piezo positioner32or only ohmic resistance34, or both piezo positioner32and ohmic resistance34simultaneously, are defective.