Abstract:
A current source includes a first switching element configured to adjust an output current according to a control signal received at a control input. The current source also includes a reference resistance, which is electrically coupled to the first switching element such that the output current flows through the reference resistance. The latter includes first and second individual reference resistors, connected in series, and a diode connected in parallel to the first reference resistor. The first reference resistor has a higher impedance than the second reference resistor. A controller receives a predetermined reference potential and an actuating signal that constitutes the control signal of the first switching element. A second switching element supplies the controller with an actual value consisting of the voltage across the first and second individual reference resistors, when the element adopts a first switching position and consisting of the voltage drop across the second individual reference resistor, when the element adopts a second switching position.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a current source, a control device and a method for operating said control device, especially for use for a piezo actuator. 
     Ever more stringent legal requirements in relation to permissible pollutant emissions of internal combustion engines which are disposed in motor vehicles make it necessary to take various measures through which the pollutant emissions are reduced. One approach in this case is to reduce the pollutant emissions created during the process of air/fuel mixture combustion. The formation of soot in particular is heavily dependent on the preparation of the air/fuel mixture in the respective cylinder of the internal combustion engine. In order to obtain a very good mixture preparation, fuel is increasingly dispensed at very high pressure. In the case of diesel internal combustion engines the fuel pressures amount to up to 2000 bar. Injection valves with a piezo actuator as an actuator are increasingly being used for such applications. The outstanding feature of piezo actuators is their very short response time. These types of injection valve are thus suitable if necessary for dispensing fuel several times within an operating cycle of a cylinder of the internal combustion engine. 
     An especially good mixture preparation can be achieved if one or more pre-injections, also known as pilot injections, are performed before a main injection, with the mass of fuel to be dispensed if necessary for each pilot injection being very small. A precise activation of the injection valve is very important, especially in these cases. 
     The charging and discharging of the piezo actuator assumes an important role in conjunction with the precise activation of the injection valve. For this purpose a power output stage is regularly provided, which cannot however be completely discharged during the discharging process of the piezo actuator. For complete discharging a circuit element is then provided in this connection which can assume this task but in doing so however is subject to thermal stresses. 
     SUMMARY OF THE INVENTION 
     The object of the invention is, in accordance with a first aspect, to create a simple current source by means of which two different output currents can be set. In accordance with a further aspect the object of the invention is to create a control device and a method for operating said control device by means of which two different output currents are able to be set in a simple manner. 
     The object is achieved by the features of the independent claims. Advantageous embodiments of the invention are identified in the subclaims. 
     The outstanding feature of the invention in accordance with a first aspect is a current source with a first circuit element, which has a control input and is embodied so that, depending on a control signal at its control input, an output current can be set on the output side of the current source. It also has a reference resistance, which is electrically coupled to the first circuit element so that the output current flows through the reference resistance. The reference resistance features first and second individual reference resistances arranged in series. Furthermore a diode is connected in parallel to the first reference resistance and is connected in a conductive direction corresponding to the stipulated current direction through the reference resistance. The first individual reference resistance is of higher impedance than the second individual reference resistance. The current source also includes a controller to which a predetermined reference potential is supplied as a setpoint value and of which the corrective signal is the control signal of the first circuit element. The current source also has a second circuit element which is embodied and arranged to apply to the controller a first potential difference as a setpoint value which is representative for a voltage drop across the first and second individual reference resistance in a first switching position and with a second potential difference as a required value which is representative for a voltage drop across the second individual reference resistance in a second switching position. This has the advantage of only one reference potential being needed to realize two different output currents of the current source. This is accompanied by a lower outlay in terms of circuitry, especially by comparison with providing a corresponding circuit arrangement for implementing two different reference potentials. 
     The other outstanding feature of the reference resistance is that it has different impedance values depending on the voltage drop across the first and second reference resistance. This is implemented particularly simply by connecting the diode in parallel to the first individual reference resistance. The fact that the first individual reference resistance is of higher impedance than the second individual reference resistance thus allows clearly different output currents to be set at the current source and this can be done highly accurately in each case, even in respect of an offset of the corrective signal of the controller. In addition the first and second individual reference resistances can each be embodied with the small tolerance suitable for the respective output current. 
     In accordance with an advantageous embodiment of the first aspect of the invention the controller and the second circuit element are arranged in an integrated circuit and the reference resistance and the first circuit element are arranged externally to the integrated circuit. The advantage of this is that few inputs or outputs of the integrated circuit are needed to realize the current source. On the other hand the integrated circuit can also be employed independently for the output current desired for the respective application and can be adapted by suitable individual reference resistances and a correspondingly suitable first circuit element to the respective application. The current source can thus be produced at low cost in overall terms. 
     In accordance with a further advantageous embodiment of the first aspect of the invention a first control parameter path couples the controller electrically on its output side to a first node point which is arranged electrically between the first circuit element and the reference resistance. The first control path element has a predeterminable impedance. Thus the behavior of the controller can be set in a simple manner relative to a first output current which is assigned to the first switching position of the second circuit element. 
     In accordance with a further advantageous embodiment of the first aspect of the invention a second control parameter path is provided which couples the controller electrically on its output side to a second node point which is arranged electrically between the first and second individual reference resistance. The second control parameter path has a predeterminable impedance. In this way the control behavior can be set in a simple manner relative to a second output current which is assigned to the second switching position of the second circuit element. 
     In accordance with a further advantageous embodiment of the first aspect of invention the predeterminable impedance in the first and/or second control parameter path is embodied so that a proportional and integral control behavior is embodied. This has proved to be especially favorable. 
     In accordance with a further advantageous embodiment of the first aspect the predetermined impedance of the second control parameter path is around at least one order of magnitude smaller than the predeterminable impedance of the first control parameter path. This has the advantage that the control behavior in respect of the setting of the first or of the second output current can be made practically independently. 
     In accordance with a second aspect the outstanding feature of the invention is a control device with the current source which is provided for controlling a piezo actuator, with the current source being able to be coupled with the piezo actuator so that it can discharge the piezo actuator. 
     The control device has a power output stage for charging and discharging the piezo actuator, which is arranged electrically in parallel to the current source. 
     The advantages of the first aspect of the invention and its advantageous embodiment correspond to those of the second aspect of the invention. 
     In accordance with of a third aspect a method for operating the control device is provided in which the power output stage is activated for discharging the piezo actuator and subsequently the second circuit element is set to its second switching position for further discharging of the power output stage, which sets the first output current. If there is an error in the power output stage the second circuit element is set to its first switching setting for discharging the piezo actuator, which sets the second output current. This has the advantage of enabling the output current, and in fact the second output current, to be selected suitably high for almost complete residual discharging of the piezo actuator, without damaging the first circuit element thermally. Furthermore the output current, and indeed the first output current can be set suitably low, in order not to thermally overload the first circuit element and simultaneously be able to completely discharge the piezo actuator, if discharging supported by the output stage is not possible or is rendered more difficult, especially also to establish a secure state in the event of specific error states or defects of the output stage. Thus good protection can be given to persons wishing to replace the controller. 
     Exemplary embodiments of the invention are explained below with reference to schematic diagrams. The figures show: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  an injection valve with a control device and 
         FIG. 2  a more detailed diagram of parts of the control device in accordance with  FIG. 1 . 
       Elements with identical construction or which function in the same way are identified by the same reference symbols in all figures. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An injection valve ( FIG. 1 ) has a injector housing  1  with a recess into which a piezo actuator PAKT 1 , is fitted, which is coupled to a transmitter  6 . The transmitter  6  is arranged in a leakage space  8 . A switching valve  10 , which is preferably embodied as a servo valve, is arranged so that it controls a leakage fluid, which is preferably the fuel, as a function of its switching position. The switching valve is coupled via the transmitter to the piezo actuator PAKT 1  and is driven by it, meaning that the switching position of the switching valve  10  is set by means of the piezo actuator PAKT 1 . If necessary the piezo actuator PAKT 1  can also act on the switching valve  10  without the intervention of the transmitter  6 . The switching valve  10  is arranged in a valve plate  12 . It comprises a valve element, the position of which can be set by means of the piezo actuator PAKT 1  and which, in one switch position, is in contact with the valve plate and thus prevents the fuel being driven back into the leakage space. In a further switching position it is at a distance from a wall of the valve plate  12  and thus enables the fuel to be driven back into the leakage space  8 . The piezo actuator comprises a stack of piezo elements. The stack of piezo elements includes for example around 200 piezo elements layered one above the other. The stack of piezo elements is preferably surrounded by a coil spring which tensions the stack of piezo elements between the transmitter  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 module which is attached by means of a nozzle clamping nut  18  to the Injector housing  1 . 
     The needle value body  14  has a recess which is continued as the recess of the nozzle body  16  in the nozzle body  16  and in which an injector needle  24  is arranged. The injector needle  24  is guided in the needle guide body  14 . A nozzle spring  26  holds the injector needle  24  in a closed position by preventing fuel from flowing through an injection bore  28 . 
     At the axial end of the injector needle  24 , which faces towards the valve plate  12 , is embodied a control chamber  30  which is hydraulically coupled via an inlet flap  31  to a high-pressure bore  32 . If the switching valve  10  is in its closed position, the control chamber  30  is hydraulically decoupled from the leakage space  8 . The result of this is that after the switching valve closes  10  the pressure in the control chamber  30  essentially equalizes the pressure in the high-pressure bore  32 . When used in the injection valve in an internal combustion engine the high-pressure bore  32  is hydraulically coupled to a high-pressure accumulator and is thus supplied with fuel at a pressure of for example up to 2000 bar. 
     As a result of the fluid pressure in the control chamber  30  on a face of the injector needle  24 , a force is exerted via the control chamber  30  in the closing direction of the injector needle  24 . The injector needle  24  also features, spaced axially from its front face, a step to which fluid which is flowing through the high-pressure bore  32  is applied such that a force acts to open the injector needle  24  i.e. against the closure direction. In its closed position the injector needle  24  suppresses a flow of fuel through the injection nozzle  28 . If the injector needle  24 , starting from its closed position, moves into the control chamber  30 , it releases the flow of fuel through the injection nozzle  28 , especially in its open position, by coming into contact with the area of the wall of the control chamber  30  which is embodied by the valve plate  12 . 
     Whether the injector needle  24  is in its open position or in its closed position depends on whether the force which is generated at the step of the injector needle  24  by the pressure of the fluid bus obtaining there is greater than or less than the force generated by the nozzle spring  26  and the pressure acting on the front face of the injector needle  24 . 
     If the switching valve  10  in its open position, fluid flows from the control chamber  30  through the switching valve  10  into the leakage space  8 . With suitable dimensioning of the inlet flap the pressure in the control chamber  30  then drops, which finally leads to a movement of the injector needle into its open position. The pressure of the fluid in the leakage space  8  is far lower 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 for creating a corrective signal SG for the piezo actuator PAKT 1  of the injection valve. The control device  38  is preferably further embodied for creating a corrective signal for further piezo actuators PAKT 2 - 4 , which are assigned to further injection valves. 
     The corrective signal SG is preferably a current signal, which is preferably pulse-height-modulated. Beginning from a start of a charging process LV, a predetermined number of pulses, so for example 20, are generated, with a predetermined duration and period, until the charging process is completed. 
     The height of the respective pulse sets the electrical energy to be supplied during the charging process to the piezo actuator PAKT 1 . 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 stroke and thus also the progress of the pressure in the control chamber  30 . 
     The control device  40  is further embodied to execute a discharging process of the piezo actuator PAKT 1 . Preferably a predetermined number of discharge pulses is created for this purpose, 20 for example, with a predetermined duration and period. The height of the respective pulse sets the electrical energy to be removed during the discharging process from the piezo actuator PAKT 1 . The energy removed from the actuator influences its axial stroke reduction. 
     A part of the control device  38  is shown with reference to  FIG. 2 . The control device  38  comprises a voltage amplifier, also called a DC/DC converter which is coupled electrically to a vehicle electrical system  40  which is embodied to supply the DC/DC converter  42  with a predetermined voltage and thus forms a voltage source. The vehicle electrical system includes a vehicle battery for example. 
     The DC/DC converter  42  is coupled electrically to a power output stage  46 . Preferably a capacitor  44  is connected between them and is connected so that electrical energy in the discharge process of the respective piezo actuator PAKT 1  to PAKT 4  can be buffered in the capacitor  44  and used for future charging processes. 
     The power output stage  46  of the control device  38  is electrically coupled to piezo actuators PAKT 1  to PAKT 4 , which are embodied separately from the control device  38  and are embodied in the injection valve. Preferably one power output stage  46  is assigned to a number of piezo actuators PAKT 1  to PAKT 4  for cost reasons. The respective piezo actuators PAKT 1  to PAKT 4  to be charged or discharged are preferably selected using selection elements TSEL 1  to TSEL 4 . 
     In a discharging process which is controlled by the power output stage  46  a residual charge remains in the respective piezo actuator PAKT 1  to PAKT 4  after the predetermined number of discharge pulses. If this residual charge is also to be removed from the respective piezo actuator PAKT 1  to PAKT 4  a current source  48  which is provided for this purpose is activated by the control device  38 . 
     The current source  48  comprises an application-specific integrated circuit, also known as an ASIC. Embodied in the ASIC  50  is a controller  52  which preferably includes an operational amplifier. The controller  52  is coupled electrically-conductively on the output side to a control input  54  of a first circuit element T 1 . During operation the controller  52  creates a corrective signal at its output which is a control signal of the first circuit element T 1 . 
     The first circuit element T 1  is embodied and arranged so that, depending on the control signal, an output current I_A can be set on the output side of the current source. The output current I_A represents a discharge current for the respective piezo actuator PAKT 1  to PAKT 4  in the current direction indicated. 
     The current source  48  further includes a reference resistance R_S, which is coupled electrically to the first circuit element T 1  so that the output current I_A flows through the reference resistance R_S. The reference resistance R_S has a first and second individual reference resistance R 5 , R 6  arranged in series and a diode D 1  which is connected in parallel to the first individual reference resistance R 5  and is connected in the conducting direction corresponding to the stipulated direction through the reference resistance R_S. The first individual reference resistance R 5  is of higher impedance than the second individual reference resistance R 6 . Depending on input currents I_A to be set, the ratio between the first and the second individual reference resistance R 5 , R 6  amounts to around 50 for example. 
     Further embodied in the second ASIC  50  is the second circuit element T 2 , which is embodied and arranged so that a first potential difference U 1  is applied as an actual value to the control  52  which is representative of a voltage drop across the first and second individual reference resistances R 5  and R 6 , in a first switching position, and a second potential difference U 2  is applied as an actual value, which is representative of voltage drop across the second reference resistance R 6 , in a second switching position of the second circuit element. A reference potential U_REF can be applied to the controller  52  at one of its other inputs. 
     The current source  48  also features preferably a first control parameter path  56 , through which a first node point  58  is electrically coupled to the controller  52  on its output side. The first node point  58  is arranged electrically between the first circuit element T 1  and the reference resistance R_S. The first controller path element  46  has a predeterminable impedance. 
     Furthermore a second control parameter path  60  is preferably provided which electrically couples the controller  52  on its output side to a second node point  62 . The second node point  62  is arranged electrically between the first and second reference resistance R 5 , R 6 . The second control parameter path  60  likewise has a predeterminable impedance. 
     It has proved to be especially advantageous for the first control parameter path  56  to have a first impedance, which is preferably embodied as resistance R 1  in a series circuit with a capacitor C 1 . Furthermore the first control parameter path  56  advantageously has a second impedance, which is preferably embodied as resistor R 3 . 
     Likewise the second control parameter path  60  preferably has a third impedance, which is preferably embodied as a series circuit comprising a resistor R 2  and a capacitor C 2 . Preferably a fourth impedance, which is preferably embodied as resistor R 4 , is provided in the second control parameter path  60 . Control parameters are set by the first and second impedances which are effective in the first switching position of the second circuit element T 2  for setting the output current I_A, which is designated as the first output current for this switching position. 
     Control parameters can be set by the third and fourth impedances which are relevant to the setting of the output current I_A in the second switching position of the second circuit element T 2 . For this case the output current I_A will be designated as the second output current. 
     By suitable selection of the first and second or of the third and fourth impedance any given control behavior, for example a P, PI or PID control behavior, can be set for setting the output current I_A. It has proved especially suitable to embody the first and second impedance or the third and fourth impedance in order to produce a proportional-integral control behavior (PI). For this purpose the first and second impedances or the third and fourth impedances are then embodied in accordance with the circuit arrangement shown in  FIG. 2  with the resistors R 2 , R 3 , R 4  and the capacitors C 1  and C 2 . 
     It has further proved to be very advantageous to embody the control parameters which are defined by the impedance values of the first and second impedances for the first control parameter path or by the third and fourth impedances for the second control parameter path  60  to clearly differ and to preferably embody them so that the control parameters assigned to the second control parameter path  60  are preferably at least one order of magnitude smaller than those assigned to the first control parameter path  56 . The third and fourth impedances are thus preferably smaller than the first and second impedances. This allows a setting behavior for setting the first or second output current to be set practically independently. 
     In the event of an error, for example for an error in the power output stage  46 , the current source is suitable for taking away the entire electrical energy stored in the respective piezo actuator PAKT 1  to PAKT 4 , which for example can lie between 70 and 100 mJ. For this purpose the reference-potential U_REF is supplied to the controller  52  as the setpoint value and the second circuit element T 2  is put into the first switching position, in which the controller  52  is supplied with the first potential difference U 1  as the setpoint value. The corrective signal of the controller thus depends in this case on the difference between the reference potential U_REF and the first potential difference U 1 . The reference potential U_REF is suitably embodied for interaction with the reference resistance R_S, and here especially with the first individual reference resistance R 5 , so that the first output current assumes the desired value and in this area the diode D 1  is not yet operated in the conducting direction, i.e. the voltage drop at the first individual reference resistance R 5  is still smaller than the conducting voltage of the diode D 1 . The second individual reference resistance R 6  is embodied with a much lower impedance than the first individual reference resistance R 5 . The result of this is that in the first switching position of the second circuit element T 2  only a very small voltage drops across the second individual reference resistor R 6 . The dimensioning of the first individual reference resistance R 5 , of the resistance R 3  and of the reference potential difference UREF influence the first output current. 
     Thus the desired low amount of first output current can be set in the first switching position of the second circuit element and thus for example for an error of the power output stage of the respective piezo actuator PAKT 1  to PAKT 4  completely discharged by the current source  48 , without placing too much thermal stress on the first circuit element T 1 . 
     Preferably the control parameters for the first switching position of the second circuit element are set so that a relatively hard switching of the output current I_A is also possible. 
     In a normal mode of the control device, for discharging the respective piezo actuator PAKT 1  to PAKT 4  the power output stage  46  is first activated in accordance with the procedure already outlined above. Subsequently, to remove the remaining residual discharge in the respective piezo actuator PAKT 1  to PAKT 4  the reference potential U_REF is supplied to the controller  52  as the setpoint value. The second circuit element is put into its second switching position by the second potential difference U 2  being supplied as the actual value to the controller  52  on its input side. The result of this is that on the output side of the controller  52  the corrective signal is created so that the first switching element T 1  sets the output current I_A to the second output current. The first switching element T 1  is preferably embodied as a self-blocking N-MOSFET transistor and is operated in the pinch-off region. Thus in this area, with the control signal at the control input of the first circuit element T 1  remaining the same, a practically constant output current I_A is guaranteed, regardless of the potential difference dropping to a reference potential. However, by feeding back the first potential difference U 1  or in the second switching position of the second circuit element the second potential difference U 2  at the controller  52  interference influences such as a temperature-dependent switching characteristic of the first circuit element T 1  or production variations of the first circuit element T 1  can however be compensated for. It has proved especially advantageous in this case to set the control parameters which are effective in the second switching position of the second circuit element T 2  so that the output current I_A does not change too suddenly, i.e. is not switched too hard. The specific most suitable characteristics of the control parameters essentially depend on the respective characteristic of the respective piezo actuator PAKT 1  to PAKT 4 . 
     The second output current typically has a much higher amount than the first output current. The second output current can for example mount to around 5 A, whereas the first output current in can amount to around 100 mA. The first individual reference resistance R 5  is embodied with such a high impedance that for the second output current it is bridged by the diode D 1  and the voltage drop across resistor R 5  is almost constant regardless of deviations of the second output current. 
     By suitable dimensioning of the first and second individual reference resistances R 5  and R 6  and of the resistance R 4  and of the reference voltage U_REF, in the second switching position of the second circuit element T 2  it can be ensured that the diode D 1  is driven in the conductive range and the resistance characteristic in this state of the reference resistance R_S is definitively determined by the second individual reference resistance R 6  which is embodied with suitably low impedance. The especially very high second output current can be set very precisely with the same reference potential U_REF as the first output current. 
     The total electrical energy to be removed from the respective piezo actuator PAKT 1  to PAKT 4  by the power output stage  46  and the current source  48  amounts for example to between 70 and 100 mJ. Typically only around the last tenth of the electrical energy is removed in normal operation by the current source from the respective piezo actuator  1 . 
     Because of the high second output current, which amounts to 5 A for example, the residual discharging can be undertaken by a very short appropriate control of the first switching element T 1  and is thus accompanied by a high thermal stress for the first switching element unit, which only lasts for a very short time however and thus, given suitable thermal capacity of the first circuit, does not lead to any thermal destruction of the first switching element. By contrast the removal in the event of an error of the entire electrical energy stored in the respective piezo actuator PAKT 1  to PAKT 4  would involve very high thermal stress for the first switching element, if this were to occur in the second switching position of the second element, i.e. with the second output current could thus possibly lead to its thermal destruction or would on the other hand require the first switching element T 1  to be dimensioned very generously, resulting in high space requirements and thereby higher costs. The option of setting the second output current makes a compact dimensioning of the first switching element T 1  possible.