Abstract:
A method is disclosed for controlling a valve having a spring with a spring force, an actuator with an actuator force opposing the spring force, and a pin that is actuated by the actuator, a seal element that can be coupled with the pin, and a seal seat, the valve being closed when the seal element sits against the seal seat. The method includes determining an expected natural opening time when the seal element is lifted off the seal seat based on a pressure difference in front and behind the valve, impressing a specified current on the actuator at a point in time within a specified interval prior to the natural opening time, the seal element sitting against the seal seat at the point in time such that the pin contacts the seal element and the actuator force matches the spring force, which is reduced by a specified value.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Stage application of International Application No. PCT/EP2012/058122 filed May 3, 2012, which designates the United States of America, and claims priority to DE Application No. 10 2011 075 271.4 filed May 4, 2011, the contents of which are hereby incorporated by reference in their entirety. 
     TECHNICAL FIELD 
     The invention relates to a method and a device for controlling a valve, e.g., a valve used in a high-pressure pump in an accumulator-type injection system of an internal combustion engine. 
     BACKGROUND 
     Certain internal combustion engines include an accumulator-type injection system having a high-pressure pump for delivering fluid. Such valves are subjected to intense stresses, in particular if they are subjected to continuous loading such as is the case for example in high-pressure pumps. Since high-pressure pumps are subjected to pressures of for example 2000 bar and greater, high demands are placed on the valves in pumps of said type. Noises can occur both during the closing and during the opening of said valves. 
     SUMMARY 
     One embodiment provides a method for controlling a valve which has a spring with a spring force, an actuator with an actuator force that can counteract the spring force, a pin that can be actuated by means of the actuator, a sealing element that can be coupled to the pin, and a sealing seat, such that the valve is closed when the sealing element bears against the sealing seat, comprising: determining an expected natural opening time at which the sealing element lifts from the sealing seat owing to a pressure difference upstream and downstream of the valve, and applying an electrical current with a predefined value to the actuator at a time, within a predefined interval before the natural opening time, at which the sealing element bears against the sealing seat, such that the pin makes contact with the sealing element and the actuator force corresponds to the spring force reduced by a predefined value. 
     In a further embodiment, in a first operating mode, the actuator has the current applied to it with a predefined non-constant profile proceeding from a starting value of the current, at which the pin is in a position in which it permits the closure of the valve, measurement values of the current are determined temporally in succession, a reference value of the current assumes the present measurement value of the current when the profile of the measurement values of the current deviates by a predefined magnitude from the profile of the current applied to the actuator, wherein the reference value of the current is representative of the actuator force corresponding to the spring force reduced by a predefined value, and wherein in a second operating mode, the predefined value of the current that is applied to the actuator at the time within the interval before the natural opening time corresponds to the reference value. 
     In a further embodiment, the spring is designed for opening the valve, and wherein in the first operating mode, the applied current decreases linearly proceeding from the starting value of the current, and at the starting value of the current, the actuator force is greater than the spring force. 
     In a further embodiment, the applied current is set as a function of a pulse width modulation. 
     In a further embodiment, the measurement values of the current are determined as a function of a voltage across a shunt resistor. 
     In a further embodiment, the reference value of the current is determined as a function of a temperature of a fluid in the valve or in a predefined region of the valve. 
     In a further embodiment, the valve is arranged in an injection system of an internal combustion engine, and the reference value of the current is determined as a function of characteristic values of the internal combustion engine. 
     In a further embodiment, the reference value of the current is stored in an operating-point-dependent characteristic map. 
     In a further embodiment, in the second operating mode, measurement values of the current are determined as a function of a voltage across a shunt resistor. 
     In a further embodiment, the natural opening time is determined as a function of a temperature of a fluid in the valve or in a predefined region of the valve and/or as a function of the pressure within the valve. 
     In a further embodiment, the valve is arranged in an injection system of an internal combustion engine, and the natural opening time is determined as a function of characteristic values of the internal combustion engine. 
     In a further embodiment, the natural opening time is stored in an operating-point-dependent characteristic map. 
     In a further embodiment, in the second operating mode, after the natural opening time, an electrical current is applied to the actuator such that the pin moves into a position in which it does not permit the closure of the valve. 
     Another embodiment provides a device for controlling a valve which has a spring with a spring force, an actuator with an actuator force that can counteract the spring force, a pin that can be actuated by means of the actuator, a sealing element that can be coupled to the pin, and a sealing seat, such that the valve is closed when the sealing element bears against the sealing seat, wherein the device is designed for: determining an expected natural opening time at which the sealing element lifts from the sealing seat owing to a pressure difference upstream and downstream of the valve, and applying an electrical current with a predefined value to the actuator at a time, within a predefined interval before the natural opening time, at which the sealing element bears against the sealing seat, such that the pin makes contact with the sealing element and the actuator force corresponds to the spring force reduced by a predefined value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Example embodiments of the invention are explained below with reference to the drawings, in which: 
         FIG. 1  shows a schematic view of a pump with a valve in a longitudinal section, 
         FIG. 2  shows a schematic view of the valve in a longitudinal section, 
         FIG. 3  shows a schematic view of the valve in three operating states, and 
         FIG. 4  shows a schematic view of current profiles during the control of the valve. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiment of the present invention provide a method and a device for controlling a valve, which method and device permit precise and quiet operation of the valve. 
     In one embodiment, the valve has a spring with a spring force. The valve furthermore has an actuator with an actuator force that can counteract the spring force. The valve has a pin that can be actuated by means of the actuator. The valve also has a sealing element that can be coupled to the pin. The valve has a sealing seat, such that the valve is closed when the sealing element bears against the sealing seat. 
     An expected natural opening time, at which the sealing element lifts from the sealing seat owing to a pressure difference upstream and downstream of the valve, is determined. An electrical current with a predefined value is applied to the actuator at a time within a predefined interval before the natural opening time. At said time, the sealing element bears against the sealing seat. The electrical current is applied such that the pin makes contact with the sealing element and the actuator force corresponds to the spring force reduced by a predefined value. 
     This has the advantage that the valve can be opened slowly such that the noise generated by the valve can be kept low, and reliable and sufficiently fast opening of the valve can nevertheless be achieved. Furthermore, wear of the valve can be kept low. 
     The value by which the spring force is reduced is in particular dependent on the pressure, or the force on the sealing element resulting from the pressure, at the natural opening time. 
     The interval in which the predefined current value is applied to the actuator is predefined as being very short, such that the time at which the predefined current value is applied to the actuator immediately precedes the natural opening time. At the time at which the predefined current value is applied to the actuator, the sealing element is still just bearing against the sealing seat, and in the subsequent step, at the natural opening time, the sealing element lifts from the sealing seat. 
     In further embodiments, in a first operating mode, the actuator has a current applied to it with a predefined non-constant profile proceeding from a starting value of the current, at which the pin is in a position in which it permits the closure of the valve. Measurement values of the current are determined temporally in succession, and a reference value of the current assumes the present measurement value of the current when the profile of the measurement values of the current deviates by a predefined magnitude from the profile of the current applied to the actuator. The reference value of the current is representative of the actuator force corresponding to the spring force reduced by the predefined value. In a second operating mode, the predefined value of the current that is applied to the actuator at the time within the predefined interval before the natural opening time corresponds to the reference value. Thus, the value of the current that is applied at the time before the natural opening time can be determined in the first operating mode. 
     In further embodiments, the reference value of the current and/or the natural opening time are/is dependent on a temperature of a fluid in the valve or in a predefined region of the valve. This has the advantage that reference values of the current and/or the natural opening time can be determined under different, in particular temperature-dependent operating conditions of the valve. 
     In further embodiments, the valve is arranged in an injection valve of an internal combustion engine, and the reference value of the current and/or the natural opening time are determined as a function of characteristic values of the internal combustion engine. This has the advantage that reference values of the current and/or the natural opening time can be determined under different operating conditions of the internal combustion engine. 
     In further embodiments, the reference value of the current and/or the natural opening time are stored in an operating-point-dependent characteristic map. This has the advantage that the reference values of the current and/or the natural opening time can be recorded, in particular during the course of the first operating mode, for different operating conditions of the valve and/or of the internal combustion engine, and can be used, during the second operating mode, for the respectively present operating state of the valve and/or of the internal combustion engine. 
     In particular, in the second operating mode, after the natural opening time, an electrical current is applied to the actuator such that the pin moves into a position in which it does not permit the closure of the valve. 
       FIG. 1  shows a pump  10  having a pump housing  12 . The pump  10  is in particular in the form of a high-pressure pump, preferably a radial piston pump. A pump piston  14  is movably mounted in the pump housing  12 . A pressure chamber  16  is situated in the pump housing  12  at one end of the pump piston  14 . To enable the pressure chamber  16  to be filled with fluid, said pressure chamber has an inflow line  18  in which there is preferably arranged a valve  20  in the form of an inlet valve. The valve  20  in the form of an inlet valve is preferably in the form of a digitally switchable valve. The valve  20  facilitates the filling of the pressure chamber  16  and, during the filling process, prevents a return flow of the fluid out of the inflow line  18 . The pressure chamber  16  also has an outflow line  22  in which there is arranged a further valve  24  in the form of an outlet valve. Fluid can thereby be discharged from the pressure chamber  16 . 
     The pump  10  also has a drive shaft  26  which is operatively connected to an eccentric ring  28  and which can be rotated clockwise in a direction of rotation D. Instead of the eccentric ring  28 , use may also be made of a camshaft. The pump  10  may alternatively also be designed as a crank-drive pump. 
       FIG. 2  shows the valve  20  with a valve housing  29  which has a recess  30 . A spring  32 , a pin  34  and a sealing element  36  are arranged in the recess  30 . The spring  32 , by virtue of its being supported on a wall of the recess  30 , preloads the sealing element  36  via the pin  34 . The pin  34  has a first cylindrical part  34   a  and a second cylindrical part  34   b , wherein the first part  34   a  has a greater diameter than the second part  34   b.    
     Also situated in the recess  30  is a sealing seat  38  which is arranged fixedly with respect to the valve housing  29  and which has passage recesses  40 . Fluid can flow via the passage recesses  40  when the sealing element  36  is not bearing against the sealing seat  38 . 
     The valve  20  also has an actuator  42  which is in particular in the form of a magnet coil. The first part  34   a  of the pin  34  is arranged within the actuator  42  and can be actuated by the actuator  42 . 
     The mode of operation of the pump  10  and of the valve  20  will be described below: 
     By means of a rotational movement of the drive shaft  26  in a direction of rotation D, the pump piston  14  is moved by means of the eccentric ring  28  toward the drive shaft  26  until said pump piston reaches a bottom dead center. Here, the valve  20  opens owing to a spring force F_ 1  of the spring  32  and the pressure difference upstream and downstream of the valve  20 . The sealing element  36  lifts from the sealing seat  38  (FIG.  3 ). Said time at which the sealing element  36  lifts from the sealing seat  38  is referred to as natural opening time NP ( FIG. 4 ). 
     The pressure chamber  16  is now filled with fluid. By means of a further rotational movement of the drive shaft  26  in the direction of rotation D, the pump piston  14  is moved away from the drive shaft  26  by the eccentric ring  28 , and in the process compresses the fluid situated in the pressure chamber  16 . At a predefined time, the valve  20  is closed by virtue of a current being applied to the actuator  42 , whereby an actuator force F_ 2 , which counteracts the spring force F_ 1 , can act on the pin  34 . Owing to the movement of the pin  34  in the direction of the actuator force F_ 2  and the prevailing pressure conditions upstream and downstream of the valve  20 , the sealing element  36  can abut against the sealing seat  38 , and a fluid flow through the passage recesses  40  is prevented. The fluid that is compressed in the pressure chamber  16  can now, in its entirety, be discharged out of the pump  10  via the further valve  24  in the form of an outlet valve. The pump piston  14  has now reached a top dead center TP (camshaft signal NS,  FIG. 4 ). 
     If the pump  10  is a high-pressure fuel pump of an injection system of an internal combustion engine, the highly pressurized fuel may pass to a fluid accumulator in the form of a high-pressure fuel accumulator, the so-called common rail. 
     Both during the opening and during the closing of the valve  20 , mechanically and hydraulically induced noises can be generated at the valve  20 . The noises generated during the opening of the valve  20  will be described below on the basis of  FIG. 3 . During the opening of the valve, in a first step, the sealing element  36  abuts against the valve housing  29  ( FIG. 3A ), whereby a first noise can be generated. After making contact with the valve housing  29 , the sealing element  36  can move in the direction of the pin  34  again, while the pin  34  itself is moved toward the sealing element  36  by the spring force F_ 1  of the spring  32 . A further noise can be generated if the sealing element  36  and the part  34   a  of the pin  34  impact against one another ( FIG. 3B ). The pin  34  is subsequently moved toward the sealing seat  38  by the spring force F_ 1  of the spring  32 . A further noise can be generated if the part  34   b  of the pin  34  impacts against the sealing seat ( FIG. 3C ). 
     The control of the valve  20  will be presented in detail below for a valve which is open in a deenergized state ( FIG. 4 ). It is self-evident that this may be applied correspondingly to a valve which is closed in a deenergized state. 
     The natural opening time NP of the valve  20  is predefined as a function of the top dead center TP. The natural opening time NP is, in embodiments, dependent on further influential variables, in particular the temperature and the pressure in the valve or in regions at the valve. Proceeding from top dead center TP, the valve is closed until the natural opening time NP. That is to say, the sealing element  36  lies against and is in contact with the sealing seat  38 . 
     The current that is applied to the actuator is set, for example increased proceeding from the time of top dead center, such that, at a time within a predefined time interval before the natural opening time, the force F_ 2  has built up to such an extent that the actuator force F_ 2  substantially corresponds to the spring force F_ 1 . At the time within the predefined interval, the actuator force F_ 2  is smaller, to a minimal extent, than the spring force F_ 1 . The actuator force F_ 2  corresponds to a spring force F_ 1  reduced by a predefined value F_ 3 . The predefined value F_ 3  is for example dependent on the pressure conditions upstream and downstream of the sealing element  36 . In embodiments, at the time within the interval before the natural opening time NP, the actuator force F_ 2  is equal to the spring force F_ 1 . The predefined interval before the natural opening time is selected to be infinitesimally small. 
     Correspondingly, the current is applied to the actuator  42  such that, immediately before the natural opening time NP, the actuator force F_ 2  is smaller, to a minimal extent, than the spring force F_ 1 . As a result, at the natural opening time NP and immediately before the natural opening time NP of the valve  20 , the pin  34  is in a central position between a maximum position of the pin  34  in which the pin  34 , in particular the part  34   b , is spaced apart from the sealing seat to a maximum extent, and a maximum position in which the pin  34 , in particular the part  34   b , bears against the sealing seat. 
     In the central position, that end of the pin  34  which faces away from the spring makes contact with the sealing element  36  but exerts no force or virtually no force on the sealing element  36 . If, after the natural opening time, the sealing element  36  moves away from the sealing seat  38  in the direction of the valve housing  29  owing to the pressure conditions, the pin follows said sealing element owing to the spring force F_ 1 . Owing to the current applied to the actuator, the impetus to which the pin is subjected owing to the spring force  34  is weakened. Furthermore, owing to the movement of the pin  34 , induction is effected in the actuator  42 , which changes the measurement values I_AV of the current. 
     Owing to the fact that the pin moves only from the central position to the end position, the distance covered by the pin is reduced. This shortens the required time until the pin arrives in the right-hand position. In this way, a low speed can be used for the movement of the pin. This reduces the impact energy (cf.  FIGS. 3A to 3C ). In particular, the reduced impact impetus of the pin against the end stop  38  results in quiet opening of the valve with little wear. 
     During operation, the gradient of the trend of the measurement values I_AV can be determined. A predefined change in the gradient is representative of the natural opening time NP. As shown in  FIG. 4 , the trend of the measurement values I_AV has a bend at the natural opening time NP of the valve  20 . By determining the predefined change in the gradient of the trend of the measurement values I_AV, the natural opening time can be readjusted during operation, in particular during the second operating mode. A shift of the natural opening time NP may result for example from a temperature change within the valve or from a change in the pressure of the fluid in the valve. 
     The high-resolution illustration of the profile of the current in  FIG. 4  also shows fluctuations of the current caused by a pulse width modulation. The profile of the current should however basically be regarded as linear, and the linear profile of the current merely has the relatively very small fluctuations of the pulse width modulation superposed thereon. For an opening of the valve  20 , in a first operating mode, a current with a predefined profile is applied to the actuator  42 . The first operating mode may also be referred to as detection mode. The value for the current that is applied to the actuator at the time before the natural opening time NP is detected in the first operating mode. 
     The applied current decreases proceeding from a starting value. At the starting value of the current, the actuator force F_ 2  is greater than the spring force F_ 1 . The pin  34  is thus pushed in the direction of the spring  32 , counter to the spring force F_ 1 . In this state, the valve  20  can be closed. If the valve  20  is arranged in the pump  10 , the applied current assumes the starting value when the pump piston  14  reaches top dead center TP. 
     Subsequently, measurement values I_AV of the current are determined temporally in succession. A present value of a current change with respect to time is determined as a function of two successive measurement values I_AV of the current. The current is reduced linearly until the present value of the current change with respect to time deviates from a preceding value of the current change with respect to time by a predefined magnitude, that is to say the deviation between the present value of the current change with respect to time and the preceding value of the current change with respect to time exceeds a predefined threshold value. The present value of the current change with respect to time assumes, for example, a positive value, whereas the preceding value of the current change with respect to time has a negative value. The change in the value of the current change with respect to time relative to the preceding values of the current change with respect to time is caused by an induction of an opposing current in the actuator  42  owing to a movement of the pin  34  in the direction of the spring force F_ 1 . A reference value I_REF of the current now assumes the present measurement value I_AV of the current. The reference value I_REF of the current is representative of the actuator force F_ 2  being equal to the spring force F_ 1  reduced by the predefined value F_ 3 . 
     The applied current is preferably set as a function of a pulse width modulation. In particular, it is advantageous for times for the determination of the measurement values I_AV of the current to be dependent on the times of the maxima of the pulse-width-modulated current. 
     The frequency of the pulse width modulation may be adapted to the desired time for the determination of the measurement values I_AV of the current. In this way, it is possible in particular for the spacing of the maxima of the pulse-width-modulated current to be adapted to the desired measurement points for the determination of the measurement values I_AV of the current. 
     It is particularly advantageous for the measurement values I_AV of the current to be determined by means of a voltage measurement across a shunt resistor. 
     The first operating mode, referred to as detection mode, may in particular be implemented as a function of ambient conditions such as the temperature of the valve  20  or a temperature of a fluid in the valve  20  or as a function of characteristic values of the internal combustion engine, for example the rotational speed or operating time of the internal combustion engine. In exemplary embodiments, the detection mode is implemented every time the internal combustion engine is started. In embodiments, the reference values I_REF of the current determined for the different operating states of the valve  20  or of the internal combustion engine, and the natural opening time NP, are recorded by being stored in an operating-point-dependent characteristic map. In this way, the reference values I_REF of the current and the natural opening time NP can be recorded, during the course of the first operating mode, for different operating conditions of the valve  20  and/or of the internal combustion engine. 
     In a second operating mode, also referred to as working mode, the current with the predefined value is applied to the actuator  42  at the time that lies in the predefined interval before the natural opening time NP. In particular, the time is dependent on the camshaft signal NS, and is dependent in particular on the time of top dead center TP. The predefined value corresponds to the reference value I_REF determined in the first operating mode. The measurement values I_AV of the current increase linearly ( FIG. 4 ) proceeding from the starting value at the time of top dead center TP. If the valve  20  is arranged in the pump  10 , then the applied current assumes the final value of the current when the pump piston  14  is close to bottom dead center. 
     The final value of the current is representative of the pin  34  being in a position in which it does not permit the closure of the valve  20 . In other words, this means that the spring force F_ 1  is only slightly greater than the actuator force F_ 2 . The pin  34  can come into contact with the sealing element  36  softly, and thus fix the position of the sealing element  36  with respect to the sealing seat  38 , whereby the valve  20  can be held open in an effective manner. As a result of the slow movement of the pin  34 , the noise generated by the valve  20  can be kept very low, and reliable and sufficiently fast opening of the valve  20  is nevertheless possible. As a result of the slow movement of the pin  34 , it is furthermore possible for the wear of the valve  20  to be kept low.