Abstract:
A current source contains a first switching element which is provided with a control input and is embodied and arranged in such a way that an output flow on an output side of a current source can be adjusted according to a control signal at the control input. The current source also contains a reference resistance that is electrically coupled to a first switching element in such a way that a potential difference above the reference resistance represents the output flow. The adjustment signal of a regulator unit depends on the voltage difference above the reference resistance, is the control signal of the first switching element, and contains a time function element which limits a first value of the output flow to a maximum duration and then reduces the value of the output flow.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a current source and a control device. These current sources are particularly used in a control device, which is particularly provided for the use for one or several piezo actuators. 
     Increasingly strict legal regulations regarding the permitted pollutant emissions of combustion engines, which are arranged in vehicles, make it necessary to undertake diverse measures, by which the pollutant emissions are reduced. A starting point here is to reduce the pollutant emissions generated during the combustion process of the air/fuel mixture. The formation of carbon-particulate matter is especially heavily dependent on the conditioning of the air/fuel mixture in the respective cylinder of the combustion engine. So as to achieve a very good mixture preparation, fuel is increasingly added under a very high pressure. In the case of diesel combustion engines, the fuel pressures are up to 2000 bar. For these uses, injection valves are increasingly used with a piezo actuator as actuator. Piezo actuators are characterized by very short reaction times. Injection valves of this type are thus possibly suitable to metering fuel several times within a work cycle of a cylinder of the combustion engine. 
     A particularly good mixture preparation can be achieved if one or more pre-injections take place prior to a main injection, which is also referred to as pilot injection, wherein a very small amount of fuel mass is to be metered for the individual pre-injection. A precise piloting of the injection valve is particularly important in these cases. 
     In connection with the precise piloting of the injection valve, an important role is accorded to the charging and discharging of the piezo actuator. For this purpose, a performance end step is regularly provided, which however cannot discharge the piezo actuator completely during the discharge process or cannot perform this with the demanded and available durations. For the complete discharging, a switching element is provided in this regard which can assume this object, but which is thereby subject to thermal loads. The switching element is part of a current sink, which can also be called current source. 
     BRIEF SUMMARY OF THE INVENTION 
     It is the object of the invention to create a current source and a control device which is simple and reliable. 
     The object is achieved by the characteristics of the independent claims. Advantageous embodiments of the invention are characterized in the dependent claims. 
     The invention is characterized by a current source with a first switching element, which has a control input and is formed and arranged in such a manner that an output current can be adjusted at the output side of the current source as a function of a control signal at its control input. A reference resistor is electrically coupled to the first switching element in such a manner that a voltage drop over the reference resistor is representative of the output current. A regulator unit is provided, the input signal of which depends on the voltage drop over the reference resistor. The input signal of the regulator unit is the control signal of the first switching element. It comprises a timing relay which limits a first amount of the output current to a maximum duration and afterwards reduces the amount of the output current. With a suitable setting of the maximum duration, a high amount of the output current can thus be adjusted for the maximum duration, without thermally damaging the first switching element. The first switching element is further protected better against the thermal destruction in the case of a short circuit at the output of the current source on a supply potential by the reduction of the amount of the output current occurring after the maximum duration. In particular, a very high protection against thermal destruction can thus be ensured by a suitable reduction of the output current, that is, without a high wiring effort, and thereby, a very economical current source is possible. The current source is thus particularly short-circuit proof with regard to its output. 
     The invention is achieved by the use of such a current source with regard to the control device. According to an advantageous embodiment of the current source, the maximum duration depends on an integral of the output current. Thus, a thermal overload of the first switching element can be reliably avoided even with an interim switching off of the output current. 
     According to a further advantageous embodiment of the current source, the first amount of the output current can be adjusted again after a given relaxation duration. This has the advantage that the first amount can quickly be adjusted again with a suitable choice of the relaxation duration and that a thermal damage of the first switching element can be avoided at the same time in this manner. 
     According to a further advantageous embodiment of the current source, it is embodied in such a manner that the relaxation duration starts with the omission of an external source which drives the output current. Such an external source can for example be a piezo actuator or also a voltage source, such as for example a wiring system supply. Thus, a targeted and short-circuit proof discharge of the piezo actuator is possible in a particularly reliable manner. 
     According to a further advantageous embodiment of the current source, the timing relay comprises a RC member, which is formed by a resistor and a capacitor arranged electrically in series. The RC member is arranged electrically parallel to the reference resistor. A control input of a timing relay switching element is electrically coupled to a measuring tap point of the RC member, which lies electrically between the resistor and the capacitor of the RC member. The timing relay switching element is arranged in such a manner that it influences the input signal of the regulator unit as a function of its control signal. The timing relay is formed particularly simply in this manner. 
     According to a further advantageous embodiment of current source, the timing relay comprises a resistor and an auxiliary timing relay switching element, which are arranged electrically in series. The resistor can be connected in an electrically parallel manner to the reference resistor by means of the auxiliary timing relay switching element. A control input of a further timing relay switching element is electrically coupled to a measuring tap point, which lies electrically between the resistor and the auxiliary time relay switching element. The further timing relay switching element is arranged in such a manner that it influences the input signal of the regulator unit as a function of its control signal. A simple wired realization is thus possible. 
     In this connection, it is particularly advantageous if a control output of a control unit is electrically coupled to a control input of the auxiliary timing relay switching element. This control input can thus be used simply, and possibly other influential magnitudes can be considered during adjustment of the maximum duration or possibly the relaxation duration. 
     Exemplary embodiments of the invention are explained below with reference to the schematic drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  shows an injection valve with a control device including a current source, 
         FIG. 2  shows a more detailed illustration of the control device with the current source, and 
         FIG. 3  shows a further embodiment of the current source. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     Elements having the same construction or function are designated with the same reference numerals in all figures. 
     An injection valve ( FIG. 1 ) has an injector housing  1  with a recess, into which is inserted a piezo actuator PAKT 1 , that is, a piezo actuator, which is coupled to a transducer  6 . The transducer  6  is arranged in a leakage space  8 . A switching valve  10 , which is preferably formed as a servo valve, is arranged in such a manner that it de-energizes a leakage fluid depending on its switching position, which is preferably fuel in this embodiment. The switching valve is coupled to the piezo actuator PAKT  1  via the transducer  6 , and is driven thereby, that is, the switching position of the switching valve  10  is adjusted by means of the piezo actuator PAKT  1 . The piezo actuator PAKT  1  can possibly also act on the switching valve  10  without the interconnection of the transducer  6 . The switching valve  10  is arranged in a valve plate  12 . It comprises a valve member, the position of which can be adjusted by means of the piezo actuator PAKT  1  and which is in abutment with the valve plate in a switching position and prevents the de-energization of fuel into the leakage space in this manner. In a further switching position, it is distanced from a wall of the valve plate  12  and enables the de-energization of the fuel into the leakage space  8  in this manner. The piezo actuator comprises a stack of 60 piezo elements. The stack  60  of piezo elements includes for example 200 piezo elements, which are layered on one another. The stack  60  of the piezo elements is preferably surrounded by a tubular spring, which clamps the stack  60  of the piezo elements between the transducer  6  and a closure element. 
     The injection valve further comprises a needle guide body  14  and a nozzle body  16 . The valve plate  12 , the needle guide body  14  and the nozzle body  16  form a nozzle component assembly, which is secured to the injector housing  1  by means of a nozzle clamping nut  18 . 
     The needle guide body  14  has a recess which is continued as a recess of the nozzle body  16  in the nozzle body  16 , and in which is arranged a nozzle needle  24 . The nozzle needle  24  is guided in the needle guiding body  14 . A nozzle spring  26  clamps the nozzle needle  24  into a closure position, in which it prevents a fuel flow through an injection aperture  28 . 
     A control space  30  is formed at the axial end of the nozzle needle  24 , which faces the valve plate  12 , which space is hydraulically coupled to a high pressure bore  32  by means of an inlet throttle  31 . If the switching valve  10  is in its closure position, the control space  30  is hydraulically decoupled from the leakage space  8 . This results in the pressure in the control space  30  essentially adapting to the pressure in the high pressure bore  32  after the closure of the switching valve  10 . The high pressure bore  32  is hydraulically coupled to a high pressure fuel store during the use of the injection valve in a combustion engine and is supplied with fuel under a pressure of for example up to 2000 bar in this manner. 
     Due to the fluid pressure in the control space  30 , a force is applied to a front surface of the nozzle needle  24  in the closure direction of the nozzle needle  24  via the control space  30 . The nozzle needle  24  further comprises a shoulder, which is axially spaced from its front surface, which is charged with fluid flowing through the high pressure bore  32  in such a manner that a force acting in an opening manner acts on the nozzle needle  24 , that is, opposite to the closure direction. In its closure position, the nozzle needle  24  prevents a fuel flow through the injection nozzle  28 . If the nozzle needle  24  moves into the control space  30  starting from its closure position, it releases the fuel flow through the injection nozzle  28 , in particular in its opened position, in which it is in abutment with the region of the wall of the control space  30 , which is formed by the valve plate  12 . 
     Whether the nozzle needle  24  is in its opened or closed position depends on whether the force which is generated at the shoulder of the nozzle needle  24  by the pressure of the fluid prevailing there, is larger or smaller than the force which is generated by the nozzle spring  26  and the pressure acting on the front surface of the nozzle needle  24 . 
     If the switching valve  10  is in its opened position, fluid flows from the control space  30  through the switching valve  10  into the leakage space  8 . With a suitable dimensioning of the inlet throttle, the pressure in the control space  30  drops then, which finally leads to a movement of the nozzle needle into its opened position. The pressure of the fluid in the leakage space  8  is clearly smaller than the pressure of the fluid in the high pressure bore. 
     A control device  38  is assigned to the injection valve. The control device  38  is embodied so as to generate an input signal SG for the piezo actuator PAKT 1  of the injection valve. The control device  38  is preferably further embodied so as to generate input signals for further piezo actuators PAKT 2 - 4 , which are assigned to further injection valves. 
     The input signal SG is preferably a current signal, which is preferably pulse-height modulated. Starting with a start of a charging process, a given number of pulses, as for example 20, is preferably generated with a given temporal duration and period, until the charging process is finished. The electric energy to be supplied to the piezo actuator PAKT  1  during the charging process is adjusted via the height of the respective pulse. The energy to be supplied to the piezo actuator PAKT  1  during a charging process LV is determined as a function of operating parameters. The energy supplied to the piezo actuator PAKT  1  influences its axial lift and thereby also the course of the pressure in the control space  30 . 
     The control device  40  is further embodied so as to carry out a discharge process of the piezo actuator PAKT  1 . A given number of discharge pulses is preferably generated for this, as e.g. 20, with a given temporal duration and period. The electric energy which is removed from the piezo actuator PAKT  1  during the discharging process is adjusted via the height of the respective discharge pulses. The energy removed from the actuator influences its axial lift reduction. 
     One part of the control device  38  is depicted according to  FIG. 2 . The control device  38  comprises a voltage amplifier  42 , which is also called DC/DC transducer, and which is electrically coupled to a wiring system  40 , which is embodied so as to supply the voltage amplifier  42  with a given voltage, and thus forms a voltage source. The wiring system comprises a vehicle battery for example. 
     The voltage amplifier  42  is electrically coupled to a performance end stage  46 . A capacitor  44  is preferably interconnected, and in such a manner that in the interim electrical energy can be stored in the capacitor  44  during the discharge process of the respective piezo actuators PAKT  1  to PAKT  4  and can be used for future charging processes. 
     The performance end stage  46  of the control device  38  is electrically coupled to the piezo actuators PAKT  1  to PAKT  4 , which are embodied separately from the control device  38 , that is, in the injection valves. One performance end stage  46  is preferably assigned to several piezo actuators PAKT  1  to PAKT  4  due to cost reasons. The choice of the respective piezo actuator PAKT  1  to PAKT  4  to be charged or discharged preferably takes place via choice switching elements TSEL  1  to TSEL  4 . 
     During a discharge process, which is controlled by the performance end stage  46 , a residual charge remains in the respective piezo actuator PAKT 1  to PAKT 4  after the given number of discharge pulses. If this residual charge is to be removed from the respective piezo actuator PAKT 1  to PAKT 4 , a current source  48  of the control device  38  is activated, which is provided for this purpose. 
     The current source  48  comprises a regulator unit  52  and a first switching element T 1 . The first switching element T 1  is embodied and arranged in such a manner that an output current I_A can be adjusted at the output side of the current source  48  depending on a control signal at a control input  54  of the first switching element T 1 . The output current I_A adjusts itself in the illustrated current direction. It represents a discharge current for the respective piezo actuator PAKT 1  to PAKT 4 . During the operation, the regulator unit  52  generates an input signal at its output, which is the control signal of the first switching element T 1 . 
     The current source  48  further comprises a reference resistor R_S, which is electrically coupled to the first switching element T 1  in such a manner that a voltage drop over the reference resistor R_S is representative of the output current I_A. A voltage divider comprising resistors R 1  and R 2  is arranged in an electrically parallel manner to the reference resistor R_S. A control input  56  of a second switching element T 2  is electrically coupled to a measuring tap point, which electrically lies between the resistors R 1  and R 2 . The control input of the second switching element T 2  is thus loaded with a control signal, which is preferably a voltage U 1 . 
     The second switching element T 2  influences an input signal of the regulator unit  52 , which acts as a control signal on the control input  54  of the first switching element T 1 . The second switching element is preferably a bipolar transistor in an emitter circuit with a coupling against the current which takes place via a resistor R 3 . An output  57  of the regulator unit is electrically coupled to the input of the first switching element T 1 , and to a resistor R 7 , which is again connected to a supply potential U_V. The resistor R 7  thus acts as a series resistor. The switching element T 2  thus generates the input signal for the control input  54  of the first switching element in dependence on the voltage drop over the reference resistor R_S. In particular, it drives a current through the resistor R 7 , the amount of which depends on the voltage drop at the reference resistor R_S. The amount can be adjusted by the corresponding dimensioning of the voltage divider, which is formed by the resistors R 1  and R 2 , the resistor R 3 , and also the resistor R 7  and by the supply potential U_V. 
     The current flow through the resistor R 7  then leads to a voltage drop over the resistor R 7  and thus adjusts a voltage U 2  at the control input  54  of the first transistor, which preferably forms the control signal at the control input  54  of the first transistor. A desired first amount of the output current I_A of the current source  48  can thus be adjusted by a suitable dimensioning of the resistors R 1 , R 2 , R 3 , R 7  and the reference resistor R_S, further the second switching element T 2  and the first switching element T 1  and the supply potential U_V. 
     The first amount of the output current can for example be between 2 and 5 A. The first amount of the output current I_A leads to a high heat input into the first switching element T 1 . The first switching element T 1  is dimensioned with regard to its heat storage capacity and its heat removal in such a manner, that it can accommodate the first amount of the output current I_A only for a limited duration without the danger of a thermal damage. It is thereby dimensioned in such a manner that a discharge of the residual charge in the respective piezo actuators PAKT 1  to PAKT 4  after the completion of the given number of the discharge pulses, which are controlled by the performance end stage  46 , can discharge by means of the first amount of the output current I_A via the current source  48 . 
     A third switching element T 3  is provided for switching the output current I_A on and off, which is operated in such a manner that the control signal of the first switching element can be brought to a switching-off value as a function of a control signal at its control input  64 , by the first switching element not allowing a flow of the output current I_A. The third switching element T 3  is therefore preferably electrically coupled to the resistor R 7  at its output side, and can couple this electrically to the reference potential GND in dependence on its switching position. This results in that, in the switching position of the third switching element, in which the resistor R 7  is electrically coupled to the reference potential GND via the third switching element T 3 , the voltage U 2  takes up a lower voltage level in the vicinity of the reference potential GND, and thus blocks the first switching element T 1 . However, if the third switching element T 3  is in its switching position, in which it does not couple the resistor R 7  to a reference potential GND, the voltage U 2  can be adjusted by the regulator unit  52 . The third switching element T 3  is preferably electrically coupled to an output of the control unit of the current source or the control device with its control input  64 . The control signal preferably applies a voltage U 4  to the control input  64 . 
     The regulator unit  52  further comprises a timing relay  58 . The timing relay  58  is formed to limit the first amount of the output current I_A to a maximum duration and afterwards to reduce the amount of the output current I_A. The timing relay comprises a RC member, which is an electrical series connection of a resistor R 5  and a capacitor C 1 . The RC member is arranged in an electrically parallel manner with the reference resistor R_S. The timing relay  58  further comprises a timing relay switching element T_ZG, which has a control input  62 , which is electrically coupled to the RC member between the resistor R 5  and the capacitor C 1 . A voltage U 3  is preferably applied as input signal to the control input  62  of the timing relay switching element T_ZG. The timing relay switching element T_ZG is electrically coupled to the output  57  of the regulator unit  52  on the output side and thus influences the input signal of the regulator unit and thus the control signal of the first switching element T 1 . The timing relay switching element T_ZG is preferably a bipolar transistor, which is arranged in an emitter circuit and which is coupled against a current by means of the resistor R 4 . It thus drives a current through the resistor R 7 , which depends on the voltage U 3  at its control input  62 . 
     If the third switching element T 3 , starting from its switching position, in which it couples the resistor R 7  to the reference potential GND, is controlled into the switching position, in which it decouples the resistor R 7  from the reference potential, the first amount of the output current I_A will first adjust itself, namely controlled by means of the second switching element T 2 , and leads to the corresponding voltage drop over the reference resistor R_S, and thus also over the RC member connected in parallel. This results in the charging of the capacitor C 1 , until the voltage U 3  at the control input of the timing relay switching element T_ZG leads to an interconnection of the timing relay switching element. 
     The current through the resistor R 7  is thus also influenced depending on the voltage U 3  at the control input  62  of the timing relay switching element T_ZG, and thereby leads to a reduction of the amount of the output current I_A with a suitable dimensioning of the capacitor C 1  and the resistor R 5  or also the resistor R 4  and R 7  and the reference resistor R_S. 
     A maximum duration can thus be adjusted by suitable dimensioning, while the output current can adopt the first amount. By the integrating behavior of the RC member, an interim switching off of the output current or an interim reduction of the output current can be considered. The maximum duration is thereby suitably provided in such a manner that a thermal damage of the first switching element T 1  can be prevented by the heat entry which is introduced by the first amount of the output current I_A. 
     By the dimensioning of the component of the timing relay  58 , that is, the resistors R 4 , R 5 , the capacitor C 1  and the timing relay switching element T_ZG, the amount of the reduction of the output current I_A can then be further adjusted after the expiration of the maximum duration, and thus a suitable lower second quantity of the output current of for example 100 to 200 mA can be adjusted, by means of which a complete discharge from a fully loaded state of the respective piezo actuators PAKT 1  to PAKT 4  is possible for example in the case of a failure of the end stage, or the thermal destruction of the first switching element T 1  can also be avoided reliably during a short circuit of the output of the current source  48  to the wiring system  40 . 
     But the reduction of the output current I_A caused by the timing relay  58  then leads again to a reduction of the voltage drop over the reference resistor R_S. 
     The RC member, the resistor R 4  and the reference resistor R_S are dimensioned in such a manner that, even with an interconnected timing relay switching element T_ZG, the voltage U 3  stays at a value at which the timing relay switching element T_ZG remains interconnected, as long as a current is driven as output current of the current source through an external source. The external source can for example be the respective piezo actuator PAKT 1  to PAKT 4 , or also the voltage source  40 . 
     A relaxation duration then only starts with the omission of the external source, which can also be caused by taking up the switching position of the third switching element T 3 , into which it couples the resistor R 7  with the reference potential GND. 
     In this manner, the relaxation duration can be adjusted in a suitable manner, after which the timing relay switching element T_ZG blocks again and thus the first amount of the output current can be adjusted again. By a suitable choice of the relaxation duration, the first amount of the output current I_A can on the one hand be adjusted again as fast as possible, but it can also be ensured on the other hand that the first switching element T 1  is not thermally overloaded. 
     A resistor R 6  is provided as protective circuit. 
     In a second embodiment of the current source  48  (see  FIG. 3 ), the timing relay  58  is distinguished in that the resistor R 5  is connected in series with an auxiliary timing relay switching element T_HZG instead of the RC member, namely in such a manner that the resistor R 5  can be electrically connected in parallel to the reference resistor R_S in a switching position of the auxiliary timing relay switching element T_HZG, namely through coupling with the reference potential, and is electrically decoupled from the reference potential GND in a further switching position of the auxiliary timing relay switching element T_HZG. In this case, a control input  66  of the auxiliary timing relay switching element T_HZG is preferably electrically coupled to a further output of the control unit, which is also called timer output, and the reduction of the first amount can thus be adjusted at the control input  66  of the auxiliary timing relay switching element T_HZG via corresponding control signals. For this, the control unit  53  is then preferably suitably designed for sensing the output current I_A or a magnitude representing it, and possibly for integrating and thus determining the corresponding maximum duration or relaxation duration. The control signal at the control input  66  of the auxiliary timing relay switching element T_HZG is preferably a voltage U 5 . A resistor R 8  is furthermore preferably provided. 
     Of course, the timing relay can alternatively also be embodied in such a manner that the maximum duration is not dependent on the integral of the output current and is for example only related to the switching-on of the output current I_A. The same also applies for the relaxation duration.