Abstract:
Disclosed are a method and a device for more rapidly switching inductive fuel injection valves. According to the invention, the magnetic retaining forces generated by remanence in a bistable valve comprising an opening and closing coil or by eddy currents in a standard valve comprising an opening coil and a closing spring are eliminated with the aid of a negative current that flows through the coil in a direction running counter to the direction of the operating current. Additionally, the magnetic yoke and armature that are used are made of materials having different conductivities in order to be able to close the valve even more quickly.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a device for switching inductive fuel injection valves. 
     Tighter statutory emission standards and the obligation to achieve increasingly efficient utilization of fuel have been critical factors over the last several years in advancing the introduction of high-pressure direct injection systems for diesel and gasoline engines, since by this means the quality of the fuel mixture generation is significantly improved. 
     Features of said systems are very high fuel injection pressures of up to 2000 bar and more (diesel) and in excess of 100 bar (gasoline), as well as the metering of the fuel in a plurality of partial injections per injection cycle. 
     As a result of this adaptation of the fuel metering to the dynamics of the combustion cycle, a host of functional improvements can be achieved:
         in the gasoline engine: greater efficiency, lower raw emissions;   in the diesel engine: fewer engine noises (knocking), reduction in soot particles, less NOx generation, better cold start performance.       

     In many diesel engines fuel is still injected at periodic intervals even during the exhaust stroke in order for instance to achieve the regeneration of a particle filter in the exhaust system by burning off the soot particles. 
     The multiplicity of said functions that are possible using modern direct-injection systems has subsequently resulted in a massive tightening of the requirements in terms of the precision and dynamics of the injection valves. Thus, for example, valve switching times of 100 to 500 μs are now required in order to be able to inject even minimum fuel quantities down to a few μg with high precision and high timing accuracy at the high system pressures. 
     This has finally enabled piezoelectric technology to make the breakthrough, since this technology permits a much faster and more precise valve actuation compared to traditional solenoid technology. It has meanwhile become standard for diesel engines in passenger cars. 
     Since the piezoelectric ceramic used here reacts spontaneously to a change in control voltage with a change in the volume of the injected fuel quantity, a very fast, almost delay-free actuation of the injection valves is possible. In contrast thereto, in the case of the conventional solenoid valve a current flow must first be built up in the inductance-susceptible exciter winding, which current flow can then actuate the valve, though only after reaching a specific current value. 
     Admittedly, however, the advantages of piezoelectric technology for high-pressure injection valves are associated with considerable costs, so that there is an urgent need to continue using solenoid injection valves as well for less demanding high-pressure direct-injection systems. 
     A typical example of this are large-volume, slow-running diesel truck engines, such as, say, 6-cylinder engines with a cylinder volume of 9 liters and maximum operating speeds of about 1800 rpm. In addition to the low speed, the requirements in terms of minimum injection quantities are also reduced owing to the large engine displacement. The number of injection pulses per injection cycle is also lower, since e.g. a pre-injection to reduce the typical diesel “rattling” due to the already very high running noise of the truck engine can be dispensed with. 
     Studies have meanwhile shown that solenoid injection valves, while suitable in principle for such applications, still require some further developments. Thus, in order for standard solenoid valves which have a coil (winding) for magnetically opening and a spring for closing the valve to be made suitable for use in direct-injection systems, the closing delay must be reduced. 
     The main obstacle during the closing of a standard solenoid valve of this kind are the eddy currents in the magnetic material of the valve which decay only slowly after the actuation current has been turned off and prevent a fast closing of the valve. This behavior defines the minimum valve opening time and consequently increases the smallest possible fuel injection quantity. 
     In the case of bistable injection valves having two windings and fixing of the valve in the respective end position by means of remanence forces, a reduction is required both in the turn-on time for opening the valve and in the turn-off time for closing the valve. 
       FIG. 1  shows a schematic of a known circuit arrangement for operating a coil of a fuel injection valve using the PWM (Pulse Width Modulation) mode of operation. There, one terminal of the coil L 1  is connected by means of a first switching transistor T 1  to the positive pole V+ of a supply voltage source V and the other terminal is connected by means of a second switching transistor T 2  to reference potential GND. The source terminal of the first switching transistor T 1  is connected to one terminal of the coil L 1 , and its drain terminal to the positive pole V+. The source terminal of the second switching transistor T 2  is connected to reference potential GND and its drain terminal to the other terminal of the coil L 1 . In addition, a freewheeling diode D 1  is arranged to conduct current from reference potential GND to one terminal of the coil L 1  and a recuperation diode D 2  is arranged to conduct current from the other terminal of the coil L 1  to the positive pole V+ of the supply voltage source. 
     The circuit according to  FIG. 1  operates as follows: prior to the start of a turn-on operation let both switching transistors T 1 , T 2  be non-conducting. At turn-on start (opening signal EO, rising edge) both switching transistors T 1 , T 2  are switched to the current-conducting state. This causes the supply voltage V, where V=48V for example, to be applied to the coil inductance. A current flows through the coil L 1 , which current quickly increases. 
     Upon reaching a predefined upper current setpoint value at which the valve opens, switching transistor T 1  is switched to non-conducting by means of the PWM unit PWM and the coil current now flows through the coil L 1  via the freewheeling diode D 1  and switching transistor T 2 , slowly decreasing in the process. If the current now reaches a lower predefined setpoint value, switching transistor T 1  is again switched to conducting, whereupon the coil current increases once again. 
     By repeated switching of switching transistor T 1  between the conducting and non-conducting state the coil current can thus be held at an approximately constant value during the turn-on time of the valve. At the end of the turn-on time (falling edge of the opening signal EO) both switching transistors T 1  and T 2  (in the case of a standard valve with closing spring) are switched to non-conducting simultaneously, whereupon the coil L 1  discharges via the freewheeling diode D 1  and the recuperation diode D 2  into the supply voltage source V and the valve closes. 
       FIG. 2  shows, as described above, in the upper track the voltage profile and in the lower track the current profile in the opening coil L 1  during the opening time of a standard fuel injection valve. 
       FIG. 3  shows the principle of a bistable fuel injection valve. The valve needle  1  is movably mounted in a housing  4  and is shown in the “OPEN” position. It butts against the left-hand magnetic yoke  2 . The left-hand magnetic yoke  2  encloses the opening coil A-B (rectangles A and B with beveled edge). The left-hand magnetic yoke has been magnetized by means of a preceding actuation current in the opening coil A-B so that it now, when the current decays, holds the valve needle  1  in the “OPEN” position. 
     In this position the path is free for the highly pressurized fuel to pass from the inlet a (in the direction of the arrow) to the outlets b and c and on to the valve nozzles (not shown), which are thereby opened. In the following description the term “fuel” can also refer to a “hydraulic medium”, in which case instead of a fuel circuit a hydraulic circuit can be provided by means of which a fuel injection valve is controlled by means of hydraulic pressure transmission. 
     In order to close the valve an actuation current is now conducted through the closing coil C-D such that the valve needle  1  moves to the right-hand magnetic yoke  3 . After the closing current is switched off, the valve needle  1  is held in the “CLOSED” position by the magnetization of the right-hand magnetic yoke  3 . 
     This causes the path from the inlet a to the outlets b and c to be closed. At the same time the outlets b and c are connected to the return lines r which are implemented as circular lines and reduce the fuel pressure between the outlets b, c and the valve nozzles (not shown), as a result of which the valve is closed. 
     Since a bistable valve has two coils, namely an opening and a closing coil, the circuit arrangement according to  FIG. 1  has to be provided twice per valve: once for driving the opening coil A-B (L 1  in  FIG. 1 ) and once for driving the closing coil C-D. 
     DE 100 18 175 A1 discloses a circuit arrangement for operating a lift armature actuator for a charge cycle valve, wherein at the end of the actuation cycle a current is sent through the coil in the opposite direction to the actuation current in order to initiate a faster changeover of the switching state. 
     Methods of this kind are also known for example from DE 199 21 938 A1, DE 195 26 681 A1 and DE 40 16 816 A1. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the invention is to provide an improved device for faster switching of inductive fuel injection valves which
         reduces the opening and closing delay in the case of bistable valves, and   reduces the closing delay in the case of standard solenoid valves (with closing spring).       

     This object is achieved according to the invention by a device according to the features of claim  1  or  6 . 
     Advantageous developments of the invention may be derived from the dependent claims. 
     As is well-known, the valve switching times are reduced in the case of a bistable valve when the magnetic holding forces generated during the activation of a coil are eliminated by selective quenching of the remanence of the other coil, and in the case of a standard valve (with closing spring) when the magnetic holding forces—induced by the decaying eddy currents—are eliminated during the deactivation of the coil. 
     In both cases it is necessary for this purpose to impress a negative current pulse into the respective coil, whereby the current level and time characteristic of said current pulse must correspond as exactly as possible to the magnetic requirements of the valve. 
     Exemplary embodiments according to the invention are explained in more detail below with reference to a schematic drawing, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1 : is a schematic of a known circuit arrangement for PWM operation of an inductive fuel injection valve, 
         FIG. 2 : shows the voltage and current profiles during PWM operation of the fuel injection valve according to  FIG. 1 , 
         FIG. 3 : shows a detail view of a bistable fuel injection valve, 
         FIG. 4 : shows an inventive circuit arrangement for PWM operation of an inductive fuel injection valve, 
         FIG. 5   a : shows voltage and current profile at the current mirror of the inventive circuit arrangement, 
         FIG. 5   b : shows the time characteristic of operating current and negative current during the opening and closing of a bistable valve, 
         FIG. 6 : shows a control device for the negative current in the case of a bistable fuel injection valve, 
         FIG. 7 : shows a control device for the negative current in the case of a standard injection valve with opening coil and closing spring, 
         FIG. 8 : shows an inventive circuit arrangement for operation of a plurality of valve coils, 
         FIG. 9 : shows the time characteristic of the valve switching movements, without ( 9   a ) and with demagnetization current ( 9   b ), 
         FIG. 10 : shows a further circuit arrangement, 
         FIG. 11 : shows a control unit for the circuit arrangement according to  FIG. 10 , 
         FIG. 12 : shows the signal shapes in said control unit, 
         FIG. 13 : shows a control unit for the circuit arrangement according to  FIG. 10 , 
         FIG. 14 : is a schematic representation of a standard solenoid injection valve, and 
         FIG. 15 : shows the generation of transitory, opposite field directions. 
     
    
    
     DESCRIPTION OF THE INVENTION 
       FIG. 4  shows an inventive circuit arrangement for PWM operation of a coil, for example the opening coil L 1  of an inductive fuel injection valve. The circuit part (T 1 , T 2 , D 1 , D 2 ) used for controlling the valve operating current has already been explained in the description relating to  FIG. 1 . 
     As described there, one terminal of the coil L 1 , for example the opening coil of the valve, is connected by means of the first switching transistor T 1  to the positive pole V+ of the supply voltage source V and the other terminal is connected by means of the second switching transistor T 2  to reference potential GND. The source terminal of the first switching transistor T 1  is connected to one terminal of the coil L 1 , and its drain terminal to the positive pole V+. The source terminal of the second switching transistor T 2  is connected to reference potential GND, and its drain terminal to the other terminal of the coil L 1 . 
     The freewheeling diode D 1  is arranged to conduct current from reference potential GND to one terminal of the coil L 1  and the recuperation diode D 2  is arranged to conduct current from the other terminal of the coil L 1  to the positive pole V+ of the supply voltage source. 
     In addition, the circuit has been extended by five transistors T 3  to T 7 , five resistors R 1  to R 5 , one capacitor C 1  and one diode D 3 , as well as by the integration of the onboard voltage source Vbat present in the vehicle. 
     The third transistor T 3  is connected in parallel with the freewheeling diode D 1 : its source terminal is connected to reference potential GND, and its drain terminal to the connecting point of freewheeling diode D 1  and one terminal of the coil L 1 . Said transistor serves in the current-conducting state to connect the terminal of the coil L 1  connected to the first switching transistor T 1  to reference potential GND. 
     The transistors T 4  to T 6  together with the resistors R 2  to R 4  form a complementary Darlington current mirror which supplies a negative current. Said current mirror T 4 -T 6  is connected via a first resistor R 1  to the positive pole V+ of the supply voltage V. The source terminal of the fourth transistor T 4  is connected to the other terminal of the coil L 1 , while the source terminal of the sixth transistor T 6  is connected via the series circuit of the seventh transistor T 7  and the fifth resistor R 5  to reference potential GND. The gate terminals of the third transistor T 3  and the seventh transistor T 7  are connected to one another and to the output of a control device, which is shown in  FIGS. 6 and 7 , for the purpose of generating a negative current control signal NSC for the negative current. 
     Connected into the circuit between the terminal of the first resistor R 1  connected to the current mirror T 4 -T 6  and reference potential GND is a capacitor C 1  which is charged up by the vehicle onboard voltage source Vbat via a protection diode D 3  and supplies the current mirror T 4 -T 6  with energy, said current mirror being controlled by the seventh transistor T 7  which is connected as a current source. 
     As long as the control signal NSC has low level (0V) at the gate terminal of the third transistor T 3 , said transistor T 3  and also the seventh transistor T 7  are switched to the non-conducting state, with the result that no current flows at the output of the current mirror formed by the source terminal of the fourth transistors T 4  either. The circuit is inactive; no current flows through the coil L 1  in the negative direction (in the direction from transistor T 4  to transistor T 3 ). 
     If the control signal NSC jumps to high level (e.g. +5V), the third transistor T 3  is switched to conducting and connects one terminal of the coil L 1  to reference potential GND. Simultaneously, a current begins to flow through the seventh transistor T 7 , the magnitude of said current being determined by the value of the fifth resistor R 5  and the base voltage (+5V) of the seventh transistor T 7  minus its base-emitter voltage (5V−0.7V≈4.3V). 
     Furthermore, said current also flows through the sixth transistor T 6  and the third resistor R 3 , at which transistors it generates a voltage drop. According to the principle of operation of a current mirror comprising emitter resistors (for negative current feedback), the same voltage drop will develop between the base terminal of the fifth transistor T 5  and the second resistor R 2 . If the value of resistor R 2  is now chosen to be substantially less than the value of R 3 , a correspondingly higher current through R 3  is required for that purpose:
 
 I   R2   /I   R3   =R 3/ R 2
 
     The fifth transistor T 5  together with the fourth transistor T 4  forms a complementary Darlington transistor. Accordingly, the major portion of the current I R2  flowing through the second resistor R 2  will flow through the fourth transistor T 4 . 
     No current flow is necessary for static control of the fourth transistor T 4 , which is embodied as a MOS FET; instead, a gate-source voltage corresponding to the drain current and the control characteristic must be set. If the value of the fourth resistor R 4  is selected such that I D(T4) =I R2  (drain current through T 4 =current through the second resistor R 2 ) the condition applies:
 
 U   GS(T4)   /R 4= I   R3 ,
 
where U GS(T4) =gate-source voltage of the fourth transistor T 4  and I R3 =current through the third resistor R 3 , then approximately identical currents flow through the two transistors T 5  and T 6 . This improves the accuracy of the current transmission ratio I R2 /I R3  in the current mirror to such an extent that even large transmissions of, for example, &gt;1000:1 can be represented stably and reproducibly. In the illustrated example, an output current of 2 A through transistor T 4  is controlled by means of a control current of, for example, 2 mA through transistor T 7 . The current mirror is supplied from the capacitor C 1 .
 
     At the beginning of a negative current pulse initiated by the signal NSC, capacitor C 1  is charged up by means of the first resistor R 1  to the potential of the supply voltage V+ (e.g. +48V). In this case a current through the opening or closing coil in the opposite direction to the direction of the actuation current is defined as the negative current. 
     The value of R 1  is chosen here as high enough so that its current flow is substantially less than the negative current flowing through the second resistor R 2  and the fourth transistor T 4 . The value of R 1  must nonetheless be small enough to permit a charging-up of the capacitor C 1  to the potential V+ in the intervals between two successive negative current pulses. 
     Capacitor C 1  is now discharged by the (negative) current flowing through the second resistor R 2  and the fourth transistor T 4  through the coil L 1  and the third transistor T 3  and its voltage becomes less than the vehicle onboard voltage Vbat. This causes the protection diode D 3  to become conducting and capacitor C 1  to be clamped to the vehicle onboard voltage Vbat. What is achieved thereby is that at the beginning of a negative current pulse the high supply voltage V+ enables a fast current buildup in the coil L 1  and subsequently is low enough so as not to allow any unnecessary power dissipation to occur in the fourth transistor T 4 . 
       FIG. 5   a  shows the voltage and current profiles at the current mirror T 4 -T 6 , the upper track showing the voltage U C1  at the capacitor C 1 . As the negative current pulse I L1  grows, the voltage U C1  drops until it is clamped at approx. 11.3V. Following termination of the negative current pulse the voltage U C1  increases once again to V+. The lower track shows the negative current pulse I L1 . The setpoint value of 2 A is reached already after 38 μs. 
     In the case of bistable valves it has been shown that the duration of the negative current pulse should be set to the time period that the current in the other coil needs to reach its operating value. This enables the control signal NSC to be obtained in a simple manner. All that is required is a flip-flop which can be set at the start of the valve activation and reset in turn when the operating current is reached for the first time. 
       FIG. 6  shows a circuit of such a control device in the case of a bistable valve for the negative current through one coil, for example the opening coil L 1 , by means of the closing signal of the other coil, for example the closing coil. 
     Said circuit consists solely of a flip-flop IC 1 A. The flip-flop IC 1 A (terminal CLK) is set by means of the rising edge e.g. of the closing signal ES for the closing coil (not shown), such that the flip-flop&#39;s output Q, at which the signal NSC appears, assumes high level. 
     At this point in time the output of the PWM unit PWM (see  FIGS. 2 and 4 ) connected to terminal CLR-Not of the flip-flop IC 1 A receives high level. If the current through the closing coil reaches its operating value, said output switches to low level and consequently also clears the flip-flop IC 1 A, with the result that the latter&#39;s output signal NSC at the output Q returns to low level. Thus, the signal NSC supplied to the base terminal of the transistors T 3  and T 7  of the circuit for the opening coil L 1  has high level for as long as the current through the closing coil needs until it reaches its operating value for the first time. 
     For a bistable valve, a circuit according to  FIG. 4  and  FIG. 6  is required both for the opening and for the closing coil in order to generate the negative current. It is important to note that the appropriate PWM unit for opening the valve controls the negative current pulse in the closing coil of the valve and the appropriate PWM unit for closing the valve controls the negative current pulse in the opening coil of the valve. The time characteristic of operating current and negative current for opening and closing a bistable valve is represented schematically in  FIG. 5   b.    
     For a standard valve with opening coil and closing spring, the negative current of the single coil L 1  must be controlled at the end of the opening signal EO, as shown in  FIG. 7 . 
     In the case of the control unit according to  FIG. 7 , the negative current serves to quench the eddy currents which still continue to flow in the magnetic circuit of the standard valve after the turning-off and decaying of the current in the opening coil. Toward that end, a negative current should be conducted through the opening coil L 1  immediately after termination of the valve activation (falling edge of the actuation (opening) signal EO. For that purpose the circuit according to  FIG. 7  includes a timing element (monoflop IC 2 ) for determining the duration of the negative current pulse through the coil L 1 , which timing element is triggered by means of a falling edge of the signal EO inverted by means of an inverter IC 4 . 
     Only one circuit according to  FIG. 4  and  FIG. 7  is required in each case for a standard valve. 
     In a further advantageous embodiment of the circuit according to  FIG. 4 , diode D 1  can be omitted, in which case the substrate diode of transistor T 3  takes over its function, i.e. freewheeling. 
     The advantages of the inventive circuit according to  FIG. 4  are as follows:
         a time-variable supply voltage is produced, as a result of which the power dissipation in the current source can be kept low;   the Darlington current mirror is supplied from a capacitor which is initially charged up to the potential of the supply voltage V+ in order to achieve a rapid current increase in the coil inductance.       

     For bistable valves having two actuation windings, the negative current is controlled by means of a signal from the drive electronics which controls the current profile in the opposite coil in each case. 
     For standard valves with closing spring, the negative current is controlled by means of the falling edge of the actuation (opening) signal. 
     In the further course of the negative current the capacitor voltage is clamped to the vehicle onboard voltage Vbat. 
     In a further advantageous exemplary embodiment, the energy required for the demagnetization can also be applied in an accelerated manner. This is beneficial when the fastest possible start of the valve movement is required. For this purpose the negative current is specified not by means of a predefined, largely constant value for a specific time period, as  FIG. 5   a  shows, but as an approximately triangular current pulse with predefined maximum value ( FIG. 9   b ). 
     The speed of the current rise is therein determined by the inductance of the coil and the supply voltage V. The peak value of the current is also higher than in the case of the first embodiment variant, since the demagnetization energy is produced in a shorter time. 
     In  FIG. 9  the valve switching times without ( FIG. 9   a ) and with demagnetization current ( FIG. 9   b ) are compared with one another. In the figure
         the top track: shows the demagnetization current,   the middle track: shows the valve movement, and   the bottom track: shows the control signal (falling edge).       

     A circuit diagram for a circuit arrangement of this kind is shown in  FIG. 10 . The circuit essentially corresponds to the embodiment according to  FIG. 4 , except that resistor R 1 , capacitor C 1 , diode D 3 , and the connection to the vehicle onboard voltage source Vbat are omitted. Also, the resistors R 2  and R 3  are connected directly to the positive pole V+ of the supply voltage and a resistor R 7  is inserted between the source terminal of transistor T 3  and the ground terminal GND. 
     In addition, the current source T 4 -T 6  is now configured for a substantially higher constant current—for example 8 A—by the choice of the value ratio of the resistors R 2  and R 3 . 
     When the negative current control signal NSC is activated by means of the closing signal, the transistor T 3  assigned to the opening coil is switched—as described with reference to FIG.  4 —to the conducting state, and simultaneously the current source T 4  to T 6  by means of transistor T 7 . According to the inductance of the coil L 1  (opening coil), the current through it will now rise over time ( FIG. 9   b , top track). Said current can be observed as the negative current sense voltage NSS at the resistor R 7 . Once said voltage NSS has reached a predefined value, the negative current control signal NSC is switched to 0V, thereby terminating the current flow. 
     The valve switching time determined in a measured exemplary embodiment of the circuit according to  FIG. 10  is shortened for example from 620 μs (without demagnetization current,  FIG. 9   a ) to 504 μs (with demagnetization current,  FIG. 9   b ). The current source T 4 - 6  also possesses a protection function, since the current from T 6  will be limited in the event of a shorting of the right-hand terminal of the coil L 1  to reference potential. 
     The valve coils are located in the injection valve (not shown) on the engine block of the internal combustion engine outside the electronic control device, and a shorting of the feed lines to vehicle ground is a common fault. This must not, however, result in damage to the electronics. 
     The negative current sense voltage NSS is evaluated and the negative current control signal NSC is controlled by means of a suitable control unit, which is described in  FIG. 11 . 
     The control unit according to  FIG. 11  implemented for a bistable injection valve contains a monoflop IC 2 , a flip-flop IC 1 A, a comparator Comp 1 , and an AND element IC 3 A having three inputs. The closing signal ES is connected to the trigger input Ck of the monoflop IC 2 , to an input of the AND element IC 3 A and to the reset input CLR-Not of the flip-flop IC 1 A. 
     The signal NSS (negative current sense) tapped at the resistor R 7  in  FIG. 10  is connected to the non-inverting input of the comparator Comp 1 , to the inverting input of which a reference voltage Vref is supplied. The output of the comparator Comp 1  is connected to the trigger input CLK of the flip-flop IC 1 A. 
     The output Q of the monoflop IC 2  is connected to a second input of the AND element, whose third input is connected to the inverting output Q-Not of the flip-flop IC 1 A. 
     The signal NSC (negative current control) appears at the output of the AND element IC 3 A, and a signal NSD (negative current diagnosis) appears at the non-inverting output Q of the flip-flop IC 1 A. 
     The control signal already described in  FIG. 6 , the closing signal ES for example, controls the turning-on of the negative current for the opening coil L 1  in this case also. However, the negative current is now turned off when a predefined current value is reached, though this current value must be smaller than the setpoint value of the current of the current source T 4 - 6 . 
     The signal profiles of the control unit shown in  FIG. 11  are presented in  FIG. 12 . At the beginning let the closing signal ES have low level. This level is also present at the reset input CLR-Not of the flip-flop IC 1 A, with the result that a negative current diagnosis signal NSD with low level is present at its non-inverting output Q. Corresponding thereto, the inverting output Q-Not of flip-flop IC 1 A has high level. 
     The rising edge of the control signal ES clocks the monoflop IC 2 , whose output Q now assumes high level for the duration of the monoflop time. The AND element IC 3 A combines the signals ES, Q of IC 2  and Q-Not von IC 1 A. Since all these signals now have high level, the signal NSC at the output of AND element IC 3 A likewise assumes high level by means of the rising edge of the control signal ES. The negative current begins to increase. 
     As a result the transistors T 3  and T 4  ( FIGS. 9   b  and  10 ) become conductive, so that a current starts to flow through the coil L 1  ( FIG. 10 ). Said current also flows through resistor R 7 , a corresponding voltage drop, negative current sense signal NSS, being produced. Comparator Comp 1  now compares this voltage NSS with the reference voltage Vref. 
     If NSS&lt;Vref, then the output of the comparator Comp 1  has low level. If the value of NSS exceeds the value of Vref, the output of the comparator Comp 1  jumps to high level and sets the downstream flip-flop IC 1 A. The latter&#39;s inverting output Q-Not jumps to low level and switches the signal NSC to low level via the AND element IC 3 A, thereby causing the negative current in the opening coil L 1  to be turned off. Similarly, the signal NSD at the non-inverting output Q jumps to high level. 
     A potential malfunction can be detected by observation of the instant in time at which said voltage jump occurs or of whether it occurs. The type of fault can also be detected. If there is a shorting to reference potential in one of the feed lines of the coils, no current will flow through resistor R 7  and the signal NSD remains at low level. This also applies in the case of a line break. 
     It is therefore sufficient to interrogate the signal NSD  3  immediately before the opening signal EO or closing signal ES is turned on. 
     The time constant of the monoflop IC 2  is chosen such that the desired value of the negative current is reliably reached, yet a thermal overloading of the power transistor T 4  of the current source is avoided in the event of shorting to reference potential. 
     If the signal NSS (negative current sense) has not exceeded the value of Vref before the time constant has expired, the downstream flip-flop IC 1 A will not be triggered. The signal NSD at the non-inverting output Q remains at low level. The output Q of the monoflop IC 2  goes to low level again and blocks the AND element IC 3 A, with the result that the latter&#39;s output signal NSC goes to low level. 
     In the case of a bistable valve, a circuit according to  FIG. 10  and  FIG. 11  is required again in each case for the opening coil and for the closing coil. 
     For a standard valve with closing spring, the control unit of which is shown in  FIG. 13 , the control unit according to  FIG. 11  is supplemented to the extent that the opening signal EO, before being supplied to the monoflop IC 2 , the AND element IC 3 A and the flip-flop IC 1 A, is inverted by means of an inverter IC 4 , with the result that the monoflop IC 2  is triggered only by the falling edge of the signal EO. 
     As shown in  FIG. 8  for a circuit arrangement according to  FIG. 4 , in a further advantageous embodiment according to the invention, the circuit arrangement according to  FIG. 4  or  FIG. 10  can be expanded for the purpose of actuating a plurality of valves, i.e. all (for example four or six) fuel injection valves of an internal combustion engine without the need to increase the number of circuits proportionally. This can be achieved by the addition of additional diodes D 7  to D 10  in series with the drain terminal of the third transistor T 3 , additional diodes D 4   a  to D 6   a  and D 4   b  to D 6   b  in series with the source terminal of the transistor T 4 , and/or a further transistor T 3   b  or a further current mirror T 4   b -T 7   b , R 2   b -R 5   b.    
     For this purpose, however, an additional selection circuit (not shown) is required which selects the current path desired in each case by suitable control of T 3 , T 3   b , T 7 , T 7   b.    
     The main obstacle during closing are, as already explained, the eddy currents in the magnetic material of the valve, which decay slowly after the actuation current is turned off and prevent fast closing of the valve. For this reason steel with low electric conductance is generally used. 
     In order to reduce the closing delay in the case of standard solenoid valves even further, according to the invention, in addition to the use of a negative current pulse, use is also made of the different decay times of eddy currents in magnetic materials having different electric conductances. 
       FIG. 14  shows a schematic representation of a standard solenoid injection valve with coil S 4  and closing spring S 3 . The coil S 4  is enclosed by the magnetic yoke S 5 . The valve needle S 7  and the armature S 6  connected thereto is pressed against a valve seat (not shown) by the closing spring S 3  and thereby closes the valve opening (not shown). When the coil S 4  is excited, the armature S 6  is attracted against the force of the closing spring S 3  and the valve thereby opened. 
     For that purpose, contrary to the above-described rule, according to the invention a material having the highest possible conductance is chosen for the armature S 6  in order to allow the eddy currents to decay as slowly as possible in the armature. The magnetic yoke S 5 , on the other hand, consists as in the prior art of material having low electric conductance. 
     In this way it is possible, during the closing of the valve through application of a negative current pulse to the coil S 4  to temporarily achieve a field reversal in the magnetic yoke S 5  while the original exciter field in the armature S 6  has not yet completely decayed. 
     This temporarily results in a repulsive force between magnetic yoke S 5  and magnetic armature S 6  in the gap between magnetic yoke and magnetic armature, which significantly accelerates the commencement of the closing movement and the closing cycle of the valve. 
       FIG. 14  shows the unbroken field lines  14   a  (on the left) with the valve open and the dashed field lines  14   b  (on the right) in the closing cycle during the temporarily induced field reversal. 
       FIG. 15  shows in schematic form the generation of temporary opposite field directions between magnetic yoke S 5  and armature S 6 . 
     The bottom diagram shows the time characteristic of the negative current pulse applied to the coil during the closing cycle of the injection valve. 
     The field strengths or holding forces generated due to eddy currents are shown in the top diagram. The respective value of the eddy current is assigned a magnetic field strength and hence a holding force. 
     The top curve  15   a  shows the profile of the field strength effective in the armature S 6 —which consists of material having the highest possible electric conductance—while the bottom curve  15   b  shows the profile of the field strength effective in the magnetic yoke S 5 —which is made of material having low electric conductance. 
     Also shown is the line  15   c , which represents the holding force of the closing spring S 3 . 
     At the instant in which the field strength influenced by the negative current pulse—curve  15   b —becomes negative and so reverses its direction, the repulsive force between magnetic yoke S 5  and armature S 6  begins to take effect. This force is at its greatest at the point marked by a double arrow. 
     The combination of negative current pulse at the end of the exciter current and suitable choice of the magnetic material properties therefore produces overall a substantial reduction in the turn-off delay in the case of standard solenoid valves.