Patent Publication Number: US-RE38470-E

Title: Method and device for driving at least one capacitive actuator

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of copending International Application No. PCT/DE97/02905, filed Dec. 12, 1997, which designated the United States. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a method and a device for driving at least one capacitive actuator, in particular a piezoelectrically operated fuel injection valve of an internal combustion engine. 
     Piezoelectric actuators contain a multiplicity of piezoceramic layers, and form a so-called stack, which upon the application of a voltage changes its dimensions, in particular its length s by a deviation ds, or generates an electric voltage in the event of a mechanical compression or tension. 
     Published, Non-Prosecuted German Patent Application DE 41 22 984 A1 discloses a driving device for a piezoelectric element in which the driving of the piezoelectric element is performed via corresponding electronic switches having prescribed charging and discharging times. 
     Published, Non-Prosecuted German Patent Application 196 32 872.1 A, corresponding to U.S. patent application Ser. No. 09/250,875, filed on Feb. 16, 1999, has already proposed a method for driving a capacitive actuator, in accordance with which the actuator is charged with a prescribed charging voltage until the voltage measured at the actuator during the driving operation reaches a prescribed value. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a method and a device for driving at least one capacitive actuator which overcomes the above-mentioned disadvantages of the prior art methods and devices of this general type, in which a method of voltage control during driving of at least one capacitive actuator is possible even when it is impossible to break off the charging operation during driving. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a method for driving at least one capacitive actuator, including a piezoelectrically operated fuel injection valve of an internal combustion engine, which contains: 
     charging, at a start of a driving operation, an actuator of a plurality of actuators with a prescribable charging voltage via a coil from a series circuit composed of a charging capacitor and a charge-reversing capacitor, and discharging the actuator into the charge-reversing capacitor at an end of the driving operation; 
     comparing an actuator voltage of the actuator due to the prescribable charging voltage with a prescribed desired-value voltage; 
     determining a new charging voltage for a next driving operation in dependence on a difference between the prescribed desired-value voltage and the actuator voltage; and 
     charging the charging capacitor for the next driving operation to a voltage corresponding to a difference between the new charging voltage and a voltage present across the charge-reversing capacitor. 
     With the foregoing and other objects in view there is further provided, in accordance with the invention, a device for driving at least one capacitive actuator, including: 
     a voltage source having a positive pole and a negative pole; 
     a charging capacitor disposed between the positive pole and the negative pole; 
     a control circuit controlling the voltage source; 
     a first series circuit disposed parallel to the charging capacitor and containing a charging switch connected to the positive pole for conducting current away from the positive pole and a discharging switch connected to the negative pole for conducting current toward the negative pole; 
     a ground terminal; 
     a second series circuit disposed between a connection point of the charging switch and the discharging switch and the ground terminal, the second series circuit containing a charge reversing capacitor having a recharge voltage connected to the charging switch and a coil; 
     at least one third series circuit connected in series with the second series circuit and containing an actuator having an actuator voltage and a controlled Power-MOSFET-switch; and 
     a diode disposed parallel to the at least one third series circuit and conducting from the ground terminal to the coil. 
     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 driving at least one 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 graph of an actuator voltage Up plotted against a charging voltage Uc according to the invention; 
     FIG. 2 is a diagrammatic, circuit block diagram of a device for driving a plurality of actuators; and 
     FIG. 3 is a flowchart relating to a mode of operation of the circuit according to FIG.  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention proceeds from the consideration that what is involved, as a rule, is temperature-induced variations which have a very large time constant compared with the temporal timing of successive actuator operations in an internal combustion engine, or manufacturing tolerances which do not change. There is therefore no need to carry out a control or regulation of the recharging in an actual control cycle (driving operation), it sufficing completely, instead, to determine a deviation in a driving operation and then to correct it in a subsequent driving operation. 
     In all the figures of the drawing, sub-features and integral parts that correspond to one another bear the same reference symbol in each case. Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown a diagram of the relationship between a charging voltage Uc and an actuator voltage Up, a prescribed actuator desired value Up soll  being illustrated by a dashed line. The method according to the invention is described below with the aid of this diagram and of a circuit, represented in FIG. 2, of a driving device for a plurality of capacitive actuators. 
     The driving of n fuel injection valves (not represented below) of an internal combustion engine via piezoelectric actuators P 1  to Pn is performed by a control circuit ST which is part of a microprocessor-controlled engine control unit. 
     As shown in FIG. 2, there is connected between a positive pole and a negative pole GND of a controllable voltage source SNT, preferably of a switched-mode power supply SNT, a charging capacitor C 1 . The charging capacitor C 1  can be regarded as an output capacitor of the switched-mode power supply SNT and is charged up to an output voltage Uc 1  thereof. Disposed in parallel with the charging capacitor C 1  is a series circuit composed of a charging switch X 1 , which is connected to the positive pole and passes current away from it, and a discharging switch X 2 , which is connected to the negative pole GND and passes current toward it. 
     The switches X 1  and X 2  are electronic switches, preferably thyristor switches, which pass current only in one direction, contain at least one semiconductor element and are turned on by the control circuit ST. 
     Situated between the connection point of the charging switch X 1  and the discharging switch X 2  and a frame terminal GND is a series circuit composed of a charge-reversing capacitor C 2 , a ring-around coil L, a first actuator P 1  and a first, controlled power MOSFET switch T 1 . 
     For each further actuator, a series circuit composed of an actuator P 2  to Pn and a further power MOSFET switch T 2  to Tn is connected in parallel with the series circuit composed of the first actuator P 1  and the first power MOSFET switch T 1 . 
     Disposed in parallel with the series circuits composed of the actuator P 1 -Pn and the power MOSFET switch T 1 -Tn is a diode D which passes current away from the frame terminal GND toward the ring-around coil L. Power MOSFET switches usually include inverse diodes whose function, as explained in more detail further below, are employed in operating the device according to the invention. 
     The switches X 1 , X 2  and T 1  to Tn are controlled by the control circuit ST in accordance with a program, assigned to the method according to the invention, as a function of control signals st of the engine control unit, of the actuator voltage Up and of a voltage Uc 2  present across the charge-reversing capacitor C 2  after discharging of the actuator P 1 -Pn. 
     The method according to the invention for successively driving the plurality of capacitive actuators P 1 -Pn is explained in more detail below for the actuator P 1  with the aid of the flowchart represented in FIG. 3 on the basis of the circuit shown in FIG.  2 . The individual boxes, assigned to the respective method states, are marked by reference numerals. 
     The charging voltage Uc (=Uc 1 +Uc 2 ) is prescribed an initial value A (state  0 ) in the first driving operation when the vehicle is started. The value of the charging voltage Uc can be a function of an engine temperature: Uc=f(T), since the actuator capacitance can vary by a factor 2 in the engine temperature range. 
     In the case of a first driving operation, the charge-reversing capacitor C 2  is discharged, Uc 2 =0V (state  10 ). Consequently, the output voltage of the controllable voltage source SNT is set to the voltage Uc 1 =Uc (initial value A) (state  20 ). 
     In a state  30 , in which the ring-around coil L is deenergized, all of the switches X 1 , X 2  and T 1  to Tn are turned off (of high resistance), and all the actuators P 1  to Pn are discharged. The aim is to operate the actuator P 1  in order to inject fuel into the cylinder via the assigned injection valve. Firstly, the control circuit directs the corresponding actuator P 1 , for example, (state  40 ) by turning on the power MOSFET switch T 1  assigned to it. T 1  can remain turned on (of low resistance) via a crank shaft angle KW=720° KW/Z (Z=number of cylinders), that is to say, for example, 180° KW for four-cylinder engines and 120° KW for six-cylinder engines. 
     At the start of injection, which is prescribed by the control signal st=1 (state  50 ), the charging switch X 1  is triggered by the control circuit ST (state  60 ). Consequently, the charging voltage Uc present on the series circuit composed of the capacitors C 1  and C 2  is discharged during a completely sinusoidal half oscillation via the ring-around coil L into the actuator P 1 , and the latter opens the non-illustrated injection valve. The voltage source, i.e. the switched-mode power supply SNT, remains connected to the charging capacitor C 1 , with the result that it also feeds energy into the resonant circuit. 
     After the charging process, the charging switch X 1  is automatically switched off (state  70 ), and the actuator P 1  is charged to an actuator voltage Up ist . The result in FIG. 1 is a point of intersection S of the charging voltage Uc and the actuator voltage Up ist . 
     The actual value of the actuator voltage Up ist , established at the actuator P 1  at an end of the charging cycle, is communicated to the control circuit ST which compares it with a prescribed desired value Up soll  illustrated by the dashed line in FIG. 1 (states  80  and  90 ). 
     If the actual value Up ist  is greater than the desired value Up soll  (state  80 ), a new value is determined for the charging voltage Uc: Uc new =Uc old -DU (state  110 ) is determined for the next driving operation of the actuator P 1 . A lower actuator voltage Up ist  is then set thereupon in the next driving operation. If the actual value Up ist  is, however, less than the desired value Up soll  (state  90 ), a larger, new value for the charging voltage Uc: Uc new =Uc old +DU (state  120 ) is determined for the next driving operation. If the value Up ist  is equal to the desired value Up soll  (state  100 ), the charging voltage Uc remains unchanged at the next driving operation of the actuator P 1 . As described and indicated by arrows in FIG. 1, the process of approximation to the desired value can be performed incrementally by prescribed steps DU, or according to any desired process of approximation. 
     The discharging switch X 2  is fired (state  140 ) in order to discharge the actuator P 1  at the end (removal) of the control signal st (state  130 ). The discharging circuit is closed via the inverse diode of the power MOSFET switch T 1 . The energy stored in the actuator P conducts back via the ring-around coil L into the charge-reversing capacitor C 2 ; the energy stored in it can be used for the next driving operation. 
     As soon as the actuator P 1  is discharged to the threshold voltage of the diode D connected in parallel to the “active” channel, the current still flowing is continued via the diode, thereby preventing the actuator P 1  from being charged to a negative voltage. The discharging switch X 2  is subsequently automatically switched off (state  150 ). 
     For the next driving operation of the actuator P 1 , the charging capacitor C 1  must be recharged to a voltage Uc 1 =Uc-Uc 2 , for which purpose Uc 2  is measured (state  10 ). It is thereby possible to determine Uc 1 =Uc-Uc 2  (state  20 ). The switched-mode power supply SNT is set to this value for the next driving operation of the actuator P 1 , and the charging capacitor C 1  is thereby charged to Uc 1 . The values determined in this driving operation are used to carry out the next driving operation, from state  30 . The driving operations for the other actuators P 2  to Pn correspond to the method described for the actuator P 1 .