Abstract:
A method and a device for charging a capacitive actuator are described. The capacitive actuator, in particular for a fuel injection valve of an internal combustion engine, is charged or discharged with different charging and discharging times. In order to shorten the charging time, the capacitance of the recharging capacitor which is dimensioned for a maximum charging time is reduced at a predefined time during the charging process. Two exemplary embodiments of a device for carrying out the method are explained in more detail.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of copending International Application PCT/DE00/02216, filed Jul. 6, 2000, which designated the United States. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a method for charging a capacitive actuator, in particular for a fuel injection valve of an internal combustion engine. The invention also relates to a device for carrying out the method. 
     One of the advantages when actuating fuel injection valves of an internal combustion engine by piezo actuators instead of solenoids is the short switching time of the actuators, which leads to steep needle edges and low degrees of variation of the injected quantities of fuel. From the point of view of combustion technology, charging times that are as short as possible are to be aimed at. 
     In order to achieve a more gentle combustion profile, the quantity of fuel is divided into a pre-injection quantity and main injection quantity, which permits slower combustion and thus makes it possible to reduce the combustion noise. The actuators have previously been actuated with a constant charging and discharging time (a duration of the transfer of charge from a power source to the actuator, or vice versa), which must be very short (for example 100 μs) so that a predefined pre-injection fuel quantity can still be injected even in the highest load range or rotational speed range of the internal combustion engine. 
     The charging process takes place, for example, as a ringing process which includes the charging from one charge source (of a series connection of a charging capacitor and of a recharging capacitor) via a recharging coil to the actuator. An inductance of the recharging coil determining, together with capacitances of the recharging capacitors and of the actuator, the time constant for the charging and discharging processes (the charging and discharging time). Such a device is known from German Patent DE 196 52 801. 
     German Patent DE 195 29 667 C2 discloses a configuration for the actuation of two piezoelectric actuators in which the frequency of the oscillating circuits in which the piezoelectric actuators are disposed can be changed in order to compensate for temperature effects and aging effects. 
     Published, Non-Prosecuted German Patent Application DE 197 14 607 A1 describes a method for incrementally charging and discharging a piezoelectric element. The recharging process is switched over to a specific point in time after the start of charging from a charging path with a resistor and a capacitor to a charging path with a coil and a further capacitor. The discharging process takes place in reverse order. 
     However, the short charging times lead to high noise emissions in frequency ranges which are unpleasant for human ears. This is felt to be very troublesome, for example in a motor vehicle, if the combustion noises are low when the internal combustion engine is idling. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a method and a device for charging a capacitive actuator which overcome the above-mentioned disadvantages of the prior art devices and methods of this general type, which makes possible a significant reduction in the noise emissions of the actuator. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a method for charging a capacitive actuator from a charge source through a series circuit formed of a recharging capacitor and a recharging coil, and for discharging the actuator into the recharging capacitor having a much smaller capacitance than the charge source. The method includes the steps of dimensioning the recharging capacitor to have a maximum capacitance for a predefined maximum charging time; and reducing the capacitance of the recharging capacitor to a predefined value at a specific point in time after a start of a charging process for achieving a shorter charging time. 
     The charging times and the discharging times of the actuator can be varied, in particular in a low-load and idling range of the internal combustion engine, by various measures during the charging process, for example in a range between 100 μs and 200 μs. 
     The method according to the invention consists in the fact that the overall capacitance of the recharging capacitors via which the actuator is charged, that is to say in this case the capacitance of at least two recharging capacitors which are connected in parallel and which make possible, for example, a maximum charging time of 200 μs, is reduced at a specific point in time during a charging process by switching off at least one of the parallel recharging capacitors, as a result of which the charging time is shortened. 
     The following applies to the selection of optimum charging times. The duration of the charging time limits the minimum period of fuel injection. This is critical in particular at high injection pressures because the injected quantity of fuel rises with the fuel pressure that is proportional to the load, given an identical period of injection. In order to achieve a specific injection quantity, in particular a low pre-injection quantity, ever shorter injection periods are therefore necessary as the fuel pressure increases. 
     On the other hand, in the case of a main injection, the injection quantities are load-dependent and/or pressure-dependent. Given a low load, small injection quantities are required, but given a high load large injection quantities with a high fuel pressure are required. The correlation between the fuel quantity and fuel pressure permits the use of relatively long charging times for the main injection, even in the high load range. 
     Within certain limits, for example between 100 μs and 200 μs, different charging times of a capacitive actuator have no influence on the injection profile which is relevant for a combustion process, with the exception of delay effects (delays of the start and end of injection) which can be compensated by shifting the timing of the actuation signals. 
     In accordance with an added mode of the invention, there are the steps of reaching the maximum capacitance of the recharging capacitor using a parallel connection of at least two recharging capacitors; and disconnecting at least one of the two recharging capacitors from the charge source at the specific point in time after the start of the charging process. 
     In accordance with an additional mode of the invention, there is the step of using the actuator in a fuel injection valve of an internal combustion engine. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a device for charging a capacitive actuator. The device contains a charge source to be connected to a power source, and a first series circuit disposed between the charge source and the capacitive actuator. The first series circuit has a first charge switch, a first blocking diode connected to the first charge switch, a first recharging capacitor connected to the first blocking diode, and a recharging coil connected to the first recharging capacitor. A reference potential terminal is provided. A discharge switch connects a connecting point of the first blocking diode and the first recharging capacitor to the reference potential terminal. At least one second series circuit is provided and contains a second charge switch, a second blocking diode connected to the second charge switch, and a second recharging capacitor connected to the second blocking diode. The second series circuit is connected in parallel with a third series circuit composed of the first charge switch, the first blocking diode and the first recharging capacitor. A control circuit is connected to and controls the discharge switch, the first charge switch and the second charge switch. A third diode is provided for conducting current in a direction of the discharge switch and is connected between the first and second recharging capacitors. A fourth diode is provided for conducting the current and is disposed between the first recharging capacitor and the discharge switch. The first charge switch and the second charge switch are switched on simultaneously, by the control circuit, to charge the capacitive actuator, and one of the first charge switch and the second charge switch is switched off at a specific point in time for removing the capacitive effect of one of the first and second recharging capacitors. 
     In accordance with an additional feature of the invention, if the discharge switch is conductive, the capacitive actuator is discharged through the first recharging capacitor and through the second recharging capacitor. 
     In accordance with a further feature of the invention, the first charge switch, the second charge switch and the discharge switch are MOSFET switches. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a device for charging a capacitive actuator. The device includes a charge source to be connected to a power source, and a first series circuit disposed between the charge source and the capacitive actuator. The first series circuit has a first charge switch, a first blocking diode connected to the first charging switch and conducts way from the first charge switch, a first recharging capacitor connected to the first blocking diode, and a recharging coil connected to the first recharging capacitor. A reference potential terminal is provided. A second blocking diode is connected to a connection point of the first blocking diode and the first recharging capacitor and conducts current toward the reference potential terminal. A third blocking diode is connected in series with the second blocking diode and has a current conducting direction equivalent to that of the second blocking diode. A discharge switch is connected to the third blocking diode and couples the connecting point of the first blocking diode and of the first recharging capacitor to the reference potential terminal through the second blocking diode and the third blocking diode. A second series circuit is provided and is formed of a second recharging capacitor, a second charge switch connected to the second recharging capacitor, and a fourth blocking diode connected to the second charge switch. The second series circuit is connected between the reference potential terminal and a connecting point of the first recharging capacitor and the recharging coil. The fourth blocking diode conducts current in a direction from the reference potential terminal to the second recharging capacitor. The fourth blocking diode has a cathode connected to the connecting point of the second and third blocking diodes. A control circuit is connected to and controls the discharge switch, the first charge switch and the second charge switch. The first charge switch and the second charge switch are switched on simultaneously, by the control circuit, to charge the capacitive actuator, and one of the first charge switch and the second charge switch is switched off at a specific point in time for removing the capacitive effect of one of the first and second recharging capacitors. 
     In accordance with another feature of the invention, if the discharge switch is conductive, the capacitive actuator is discharged through the first recharging capacitor, and through the second recharging capacitor and the second charge switch or the fourth blocking diode. 
     In accordance with a concomitant feature of the invention, the second charge switch is operated inversely with respect to the charge switch, that is to say the second charge switch is switched on when the discharge switch is switched off, and vice versa. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a method and a device for charging a capacitive actuator, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block circuit diagram of a device according to the prior art; 
     FIG. 2 is a block circuit diagram of a first exemplary embodiment of the device according to the invention; 
     FIG. 3 is a graph of charging and discharging times of the exemplary embodiment shown in FIG. 2; 
     FIG. 4 is a block circuit diagram of a second exemplary embodiment of the device according to the invention; and 
     FIG. 5 is a graph of the charging and discharging times of the exemplary embodiment shown in FIG.  4 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown a basic circuit of a known device for charging and discharging a capacitive actuator P. The basic circuit is composed of a series circuit that is connected to a ground reference potential at both ends and is composed of a charge source which can be charged from a power source V, a charging capacitor C 1 , a charge switch T 1 , a blocking diode D 1 , a recharging capacitor C 2 , a recharging coil L and one or more actuators P, P′ which are connected in parallel, and a selection switch S, S′ connected in series with each of the actuators P, P′. A terminal of the recharging capacitor C 2  which leads to the charge switch T 1  can be connected to the ground reference potential via a discharge switch T 2  which is in series with a further blocking diode D 2 . The two switches T 1  and T 2  are controlled by a control circuit or switch ST. S The capacitance of the charge capacitor C 1  is significantly higher than that of the recharging capacitor C 2 : C 1  &gt;&gt;C 2 . 
     When the terms charging, discharging or selection switches are used, switches are preferably to be understood which are is switched on or off, for example thyristors, or MOSFETs (with a diode in a series connection) which automatically become non-conductive again if the current flowing then drops to zero. 
     The charging of the actuator P takes place by closing (switched on) the charge switch T 1 . Here, the charge moves backward and forward with a current I in the form of a half sinusoidal oscillation of the charge source (the charging capacitor C 1 ) via the recharging capacitor C 2  and the recharging coil L to the actuator P. During the charging time, the actuator voltage U rises to a specific value, and the actuator P opens the fuel injection valve. 
     If the current I drops to zero, the charge switch T 1  is opened again (switched off), and the actuator voltage U is maintained until the discharge process starts when the discharge switch T 2  is closed (switched on). The charge then moves backward and forward from the actuator P into the recharging capacitor C 2  via the recharging coil L. The actuator voltage U drops to zero again, the current I drops to zero and the fuel injection valve is closed by the actuator P. The discharge switch T 2  must be opened again (switched off) before the next charging process. An injection process is thus terminated. Recharging into the charging capacitor C 1  is prevented by the blocking diode D 1 . 
     FIG. 2 shows a circuit of a first exemplary embodiment according to the invention, which differs from the known circuit according to FIG. 1 in that connected in parallel with a first series circuit composed of a charge switch T 1   a , a blocking diode D 1   a  and a recharging capacitor C 2   a  is a second series circuit of the same type. The second series circuit is composed of a further charge switch T 1   b , a further blocking diode D 1   b  and a further recharging capacitor C 2   b  The terminals of the two recharging capacitors C 2   a  and C 2   b  which face the charging switches T 1   a  and T 1   b  are connected to one another by a diode D 2   b  which conducts current from the recharging capacitor C 2   b  to the recharging capacitor C 2   a . Further series circuits of this type that are connected in parallel can be provided, which is indicated by dotted arrows. 
     The mode of operation of the circuit is explained below with reference to the diagram in FIG. 3 showing a current profile I in the actuator P and the switched settings of the charge switches T 1   a  and T 1   b  as well as the discharge switch T 2 . 
     The two recharging capacitors C 2   a  and C 2   b  are dimensioned in such a way that the actuator P, (or P′) is charged from a parallel connection of the two capacitors C 2   a  and C 2   b  with a desired, maximum charging time of, for example, 200 μs. 
     For this purpose, at a point in time T 0  (FIG.  3 ), both charge switches T 1   a  and T 1   b  are switched on simultaneously, as a result of which the actuator P is charged from the capacitors C 1 , C 2   a  and C 2   b  via the recharging coil L, and a sinusoidal current I begins to flow through the actuator P, which has been selected by the selection switch S. A voltage at both of the recharging capacitors C 2   a  and C 2   b  drops uniformly. If both charge switches T 1   a  and T 1   b  (shown by dashed lines) remain switched on until the current I (dashed curve) drops to zero at the point in time t 3 , the charging time is t 3 −t 0 =200 μs. 
     According to the invention, in order to achieve a shorter charging time, the charge switch T 1   a , for example, is prematurely opened at the point in time t 1 , i.e. switched off. As a result, the current continues to flow only from the series circuit of the two capacitors C 1  and C 2   b , as a result of which the current I (unbroken curve) already drops to zero at the point in time t 2 , at which point in time the second charge switch is also switched off. As a result of this measure, the charging time only then has the duration t 2 −t 0 . The end of the charging time which starts at the point in time t 0  can be varied in this way between &lt;t 1  and t 3 , as a result of which charging times of &lt;100 μs up to the selected maximum, here 200 μs can be selected. At the end of the charging process (t 2 ), there is still a voltage of, for N example, +80 V at the first recharging capacitor C 2   a , which has not been entirely discharged, while the voltage at the second recharging capacitor C 2   b  can be −50 V, for example. 
     During the discharging of the actuator P, starting for example at the point in time t 4 , both charge switches T 2   a  and T 2   b  are already switched off, the discharge switch T 2  is switched on. As a result, the actuator P is discharged via the recharging coil L into both recharging capacitors C 2   a  and C 2   b  which are now connected in parallel by the diodes D 2   a  and D 2   b . The second recharging capacitor C 2   b  is charged until it reaches the voltage (+80 V) of the first recharging capacitor C 2   a . Both recharging capacitors are then uniformly charged further until the actuator P is discharged. In this way, each discharging time corresponds to the respective preceding charging time. In the selected example, the discharging time (charging time t 0  to t 2 ) therefore already ends at the point in time t 5  (unbroken curve), instead of at the point in time t 6  (dashed curve). 
     The respective selection switch, S or S′, must be switched on, at least from the start (t 0 ) of the charging time up to the end of the discharging time (t 5  or t 6 ). 
     FIG. 4 shows the circuit of a second exemplary embodiment according to the invention, which differs from the known circuit according to FIG. 1 in that connected in series with the second blocking diode D 2  is a third blocking diode D 3  with the same current conducting direction, in that a series circuit composed of a second recharging capacitor C 2   b , a further charge switch T 3  and a fourth blocking diode D 4  is connected to reference potential from the connecting point of the recharging capacitor C 2   a  and the recharging coil L. The anode of the fourth blocking diode D 4  conducting current in the direction from the reference potential to the second recharging capacitor C 2   b , and in that the cathode of the fourth blocking diode D 4  is connected to the connecting point of the second and third blocking diodes D 2 , D 3 . C 1  &gt;&gt;C 2   a , C 2   b  also applies here. The two recharging capacitors C 2   a  and C 2   b  are also dimensioned in the exemplary embodiment in such a way that the charging of the actuator P (or P′) takes place from a parallel connection of the two capacitors C 2   a  and C 2   b  with a desired, maximum charging time of, for example, 200 μs. 
     For this purpose, at the point in time t 0  (FIG.  5 ), both charge switches T 1  and T 3  are switched on simultaneously, as a result of which the actuator P is charged from the capacitors C 1 , C 2   a  and C 2   b  via the recharging coil L, and a sinusoidal current I begins to flow through the actuator P, which has been selected by the selection switch S. 
     The voltage at both recharging capacitors C 2   a  and C 2   b  drops uniformly. If both charge switches T 1  and T 3  remain switched on until the current I (dashed curve) drops to zero at the point in time t 3 , the charging time is thus t 3 −t 0 =200 μs. 
     In order to achieve a shorter charging time, the charge switch T 1  is prematurely opened at the point in time t 1 , i.e. switched off. As a result, the current continues to flow only from the recharging capacitor C 2   b  via the recharging coil L to the actuator P, and from the actuator P via the selection switch, the blocking diode D 4  and the further charge switch T 3  back into the recharging capacitor C 2   b , as it were as a “freewheeling current” in order to discharge C 2   b  and L, until the current drops to zero at the point in time t 2  (unbroken curve from t 1  to t 2  in FIG.  5 ). During this time the further charge switch T 3  must be switched on. 
     As a result, in the exemplary embodiment also, the charging time continues to have only the duration t 2 −t 0 . The end of the charging time which starts at the point in time t 0  can in this way be varied between &lt;t 1  and t 3 , as a result of which charging times of &lt;100 μs up to the selected maximum, here 200 μs, can be selected. 
     At the end of the charging process (t 2 ), there is still, as in the first exemplary embodiment, a voltage of, for example, +80 V at the first recharging capacitor C 2   a  which was not entirely discharged, while the voltage at the second recharging capacitor C 2   b  can be, for example, −50 V. 
     During the discharging of the actuator P, starting at the point in time t 4  (charge switch T 1  is switched off), the discharge switch T 2  is switched on. If the further charge switch T 3  is still switched on at this point in time, the actuator P is discharged, as already described in the first exemplary embodiment, via the recharging coil L into both recharging capacitors C 2   a  and C 2   b  which are now connected in parallel by the diode D 2 , the second recharging capacitor C 2   b  being charged until it reaches the voltage (+80 V) of the first recharging capacitor C 2   a . Both recharging capacitors are then uniformly charged further until the actuator P is discharged. In this way, any discharging time corresponds again to the respectively preceding charging time. In the selected example (charging time t 0  to t 2 ), the discharging time therefore already ends at the point in time t 5  (unbroken curve), instead of at the point in time t 6  (dashed curve). 
     During the discharging of the actuator P, starting at the point in time t 4  (FIG.  5 ), in which the charge switch T 1  is switched off, the discharge switch T 2  is switched on. Here, the charge switch T 3  is either still actively conducting or, if it is embodied as a MOSFET, conducts current in the direction of the discharge switch T 2  (illustrated by dashed lines in FIG. 5) through the arbitrarily inverse diode. 
     As a result, the actuator P is discharged via the recharging coil L into both recharging capacitors C 2   a  and C 2   b  which are connected in parallel, the second recharging capacitor C 2   b  being charged again until it reaches the voltage (+80 V) of the first recharging capacitor C 2   a . Both recharging capacitors are then uniformly charged further until the actuator P is discharged. In this way, any discharging time corresponds to the respectively preceding charging time. In the selected example (charging time t 0  to t 2 ), the discharging time therefore already ends at the point in time t 5  (unbroken curve), instead of at the point in time t 6  (charging time t 0  to t 3 , shown by the dashed curve). The respective selection switch S or S′ must be switched on at least from the start (t 0 ) of the charging time up to the end of the discharging time (t 5  or t 6 ). 
     In the second exemplary embodiment with a shortened charging time (charge switch T 1  is switched off before the further charge switch T 3 ), the fuel injection quantity can be minimized by operating the further charge switch T 3  and the discharge switch T 2  inversely. T 3  is switched on when T 2  is switched off, and vice versa, as a result of which the discharging time follows the charging time immediately. In the event of T 1  and T 3  being synchronously switched on at the point in time to and switched off at the point in time T 3 , an inverse operation of T 2  and T 3  is to be avoided. If, in fact, T 1  and T 3  are switched off simultaneously and T 2  is switched on, T 1  and T 2  are switched on owing to brief overlaps and the charging capacitor C 1  and the power source V are thus short-circuited.