Abstract:
There is proposed a control unit for an internal combustion engine, which comprises a boost circuit, a switching element, a current detecting resistor and a controller and is designed to be actuated such that the boost circuit is used to boost a power source voltage to create a boosted voltage and the controller is used to control the switching element so as to enable the boosted voltage to flow to the injector solenoid coil. This control unit is designed such that, when the boost circuit goes out of order, the injector solenoid coil is excited by making use of the power source voltage without using the boosted voltage and without creating a peak current to thereby generate a first holding current required for opening the injector and a second holding current required for retaining the opened state of the injector, the second holding current being lower in intensity than the first holding current.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a control unit for an internal combustion engine of automobiles. In particular, the present invention relates to a control unit for an internal combustion engine, which is equipped with means which is capable of driving an injector of automobile in such a manner that even if the boost circuit in the injector-driving circuit of the control unit goes out of order, the injector is enabled to be stably driven while leaving the circuit structure as it is. 
   2. Description of the Related Art 
   With respect to the injector-driving circuit for automobiles, the one described in JP Published Patent Application No. 2003-27994 A has been conventionally known, wherein a valve-opening current control circuit is actuated to transmit an electric current needed for opening the valve to the injector and then a switching element is turned OFF to enable the electric current to moderately fall, after which the switching element is brought into a state of Half-ON by making use of a steep fall control circuit of downstream side and then the electric current is switched to holding current. 
   Further, JP Published Patent Application No. 2004-124890 A discloses a fuel-feeding apparatus for an engine, wherein the supply of electric current to a solenoid is performed at the time when the theoretical product between a valve-opening signal and a holding signal is materialized, and if the time required for enabling the valve-opening current to reach a predetermined current level counted from the start of the fuel injection pulse is shorter than a predetermined time period, a fuel injection system is judged as being abnormal. 
     FIG. 1  illustrates a circuit diagram of the driving circuit (for one cylinder) of solenoid coil for the injector in the conventional control unit for an internal combustion engine. This circuit consists of two sections, i.e. one of which being a boost circuit which is constituted by an injector controller  5 , a boost coil  1 , a diode  2 , a switching MOSFET  3 , an electrolytic capacitor  4  and a current detecting resistor  6 ; and the other being an injector driving circuit which is constituted by an injector controller  5 , a peak current MOSFET  7 , a holding current MOSFET  8 , a downstream side MOSFET  11 , a reverse current-preventing diode  9 , a flywheel diode  12 , an injector solenoid coil  10  and a current-detecting resistor  13 . 
   When the driving signals shown at the second, third and fourth rows of  FIG. 2  are applied to the peak current MOSFET  7 , the holding current MOSFET  8 , the downstream side MOSFET  11 , respectively, the intensity of current flowing to the injector solenoid coil  10  is monitored by the current-detecting resistor  13  shown in  FIG. 1  and then the current intensity thus monitored is fed back to the injector controller  5 , thereby enabling an injector current  10 -A shown at the first row of  FIG. 2  to flow to the injector solenoid coil  10 . 
   In the case of this circuit configuration, there is a problem that if the electrolytic capacitor  4  of boost circuit goes out of order due to the GND short-circuit, the electric current from the holding current MOSFET  8  is caused to backflow to the peak current MOSFET  7 , thereby making it impossible to enable electric current to flow to the injector solenoid coil  10  and hence making it impossible to actuate the injector. 
   Further, when it is judged that the boost circuit has gone out of order, the driving signals shown at the second, third and fourth rows of  FIG. 3  are applied to the peak current MOSFET  7 , the holding current MOSFET  8 , the downstream side MOSFET  11 , respectively, thereby making it possible to create a current waveform shown at the first row of  FIG. 3 . However, there is a problem that in the case of the waveform shown in  FIG. 3 , since it is constant in current waveform, a maximum current needed for opening the injector is caused to flow all over the entire current conducting region, resulting in an increased heating of the driving circuit, thereby greatly restricting the upper limit of the engine speed. 
   Furthermore, in the case of this circuit configuration, there is also a problem that although it may be possible, with the addition of Zener diode, to perform a steep fall of electric current, it will lead to a great heat build-up due to the provision of Zener diode, thereby necessitating further restriction of the upper limit of the engine speed. 
   SUMMARY OF THE INVENTION 
   The present invention has been made to overcome the problems mentioned above and, therefore, according to the present invention, there is provided a control unit for an internal combustion engine, which comprises a boost circuit, a switching element, a current detecting resistor and a controller and is designed to be actuated such that the boost circuit is used to boost a power source voltage to create a boosted voltage and the controller is used to control the switching element so as to enable the boosted voltage to flow to the injector solenoid coil, wherein the control unit is characterized in that, when the boost circuit goes out of order, the injector solenoid coil is excited by making use of the power source voltage without using the boosted voltage and without creating a peak current to thereby generate a first holding current required for opening the injector and a second holding current required for retaining the opened state of the injector, the second holding current being lower in intensity than the first holding current. 
   The above-described control unit for an internal combustion engine according to the present invention may be constructed such that it is provided with a current channel for enabling the boost circuit to communicate, via a switching element for peak current and a reverse current-preventing diode, with the injector solenoid coil, and with a current channel, which is disposed parallel with the first-mentioned current channel, for enabling a power source to communicate, via a switching element for holding current and the reverse current-preventing diode, with the injector solenoid coil, thereby creating a current channel for enabling the power source voltage to be directly transmitted to the injector solenoid coil. 
   The above-described control unit for an internal combustion engine according to the present invention may be constructed such that a downstream side switching element and the current detecting resistor are successively disposed on the downstream side of the injector solenoid coil and, additionally, a flywheel diode is located between the downstream side of the current detecting resistor and the upstream side of the injector solenoid coil, wherein the electric current flowing into the injector solenoid coil is monitored as the electric current falls by making use of the flywheel diode and the downstream side switching element is shut down at a threshold level of the electric current immediately before the fall of the electric current becomes zero. 
   According to another aspect of the present invention, there is further provided a control unit for an internal combustion engine, which comprises a boost circuit, a switching element, a current detecting resistor and a controller and is designed to be actuated such that the boost circuit is used to boost a power source voltage to create a boosted voltage and the controller is used to control the switching element so as to enable the boosted voltage to flow to the injector solenoid coil, wherein the control unit is characterized in that, when the boost circuit goes out of order, the injector solenoid coil is excited by making use of the power source voltage without using the boosted voltage and without creating a peak current to thereby generate a pre-charge current for promoting the opening of the injector, a first holding current required for opening the injector and a second holding current required for retaining the opened state of the injector, the second holding current being lower in intensity than the first holding current. 
   In the last-mentioned control unit for an internal combustion engine according to the present invention may be constructed such that it is provided with a current channel for enabling the boost circuit to communicate, via a switching element for peak current and a reverse current-preventing diode, with the injector solenoid coil, and with a current channel, which is disposed parallel with the first-mentioned current channel, for enabling a power source to communicate, via a switching element for holding current and the reverse current-preventing diode, with the injector solenoid coil, thereby creating a current channel for enabling the power source voltage to be directly transmitted to the injector solenoid coil. 
   Further, the last-mentioned control unit for an internal combustion engine according to the present invention may be constructed such that a downstream side switching element and the current detecting resistor are successively disposed on the downstream side of the injector solenoid coil and, additionally, a flywheel diode is located between the downstream side of the current detecting resistor and the upstream side of the injector solenoid coil, wherein the electric current flowing into the injector solenoid coil is monitored as the electric current falls by making use of the flywheel diode and the downstream side switching element is shut down at a threshold level of the electric current immediately before the fall of the electric current becomes zero. 
   Alternatively, the control unit for an internal combustion engine according to the present invention may be constructed such that when a waiting time for the fall of electric current is prolonged, the downstream side switching element is shut down timely before the fall of electric current overlaps with the rise of electric current flowing into the injector solenoid coil of a counter cylinder. 
   According to the control unit for an internal combustion engine which is designed to actuate the injector as set forth in claim  1  or  2 , when the boost circuit goes out of order, the injector solenoid coil is excited by making use of the power source voltage without using the boosted voltage, thereby enabling to generate a first holding current required for opening the injector and a second holding current which is lower in intensity than the first holding current and required for retaining the opened state of the injector. As a result, it is now possible to secure the fail-safe function that the injector can be actuated even when the boost circuit goes out of order. Additionally, since the second holding current for retaining the opened state of the injector is made lower in intensity than the first holding current, it is now possible to inhibit the build-up of heat in the driving circuit as compared with the case where the injector current is kept constant in intensity. 
   Further, according to the invention set forth in claim  3  or  6 , since it is designed such that when the boost circuit goes out of order, the electric current flowing into the injector solenoid coil is monitored as the injector-driving current falls and then the electric current thus monitored is fed back to the drive controller so as to enable the downstream side switching element to shut down at a threshold level of the electric current immediately before the fall of the electric current becomes zero. As a result, it is now possible to prevent regenerative current from flowing toward the boost circuit and hence to prevent the further deterioration of the damage of the boost circuit. 
   Further, according to the invention set forth in claim  4  or  5 , due to the provision of the fail-safe function that the injector can be actuated even when the boost circuit goes out of order, due to the suppression of heat build-up in the driving circuit through the employment of the second holding current of lower intensity than that of the first holding current for retaining the opened state of the injector, and due to the employment of the pre-charge current, it is now possible to improve the responding properties for opening the injector without necessitating the employment of the boost circuit, thereby making it possible to achieve the high-precision control of the injector. 
   Further, according to the invention set forth in claim  7 , it is possible to prevent the mutual intervention among the cylinders of the internal combustion engine, thus making it possible to realize the stable control of the engine. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit diagram of the driving circuit (for one cylinder) of solenoid coil for the injector in the conventional control unit for an internal combustion engine; 
       FIG. 2  is a diagram illustrating the input voltage waveform and the injector current waveform in the driving circuit of solenoid coil for the injector in the conventional control unit for an internal combustion engine; 
       FIG. 3  is a diagram illustrating the input voltage waveform and the injector current waveform to be employed as the boost circuit has gone out of order in the driving circuit of solenoid coil for the injector in the conventional control unit for an internal combustion engine; 
       FIG. 4  is a circuit diagram of the injector driving circuit (for one cylinder) according to Embodiment 1 of the present invention; 
       FIG. 5  is a diagram illustrating the input voltage waveform and the injector current waveform (during the normal operation) in the injector driving circuit (for one cylinder) according to Embodiment 1 of the present invention; 
       FIG. 6  is a diagram illustrating the input voltage waveform and the injector current waveform during the failure of the boost circuit in the injector driving circuit (for one cylinder) according to Embodiment 1 of the present invention; 
       FIG. 7  is a circuit diagram of the injector driving circuit (for one cylinder) according to Embodiment 2 of the present invention; 
       FIG. 8  is a diagram illustrating the input voltage waveform and the injector current waveform (during the normal operation) in the injector driving circuit (for one cylinder) according to Embodiment 2 of the present invention; 
       FIG. 9  is a diagram illustrating the input voltage waveform and the injector current waveform during the failure of the boost circuit in the injector driving circuit (for one cylinder) according to Embodiment 2 of the present invention; 
       FIG. 10  is a circuit diagram of the injector driving circuit (for two cylinders) according to Embodiment 3 of the present invention; 
       FIG. 11  is a diagram illustrating the input voltage waveform and the injector current waveform of a first cylinder (during the normal operation) in the injector driving circuit according to Embodiment 3 of the present invention; 
       FIG. 12  is a diagram illustrating the input voltage waveform and the injector current waveform of a second cylinder (during the normal operation) in the injector driving circuit (for one cylinder) according to Embodiment 3 of the present invention; 
       FIG. 13  is a diagram illustrating the input voltage waveform and the injector current waveform of a first cylinder during the failure of the boost circuit in the injector driving circuit (for one cylinder) according to Embodiment 3 of the present invention; and 
       FIG. 14  is a diagram illustrating the input voltage waveform and the injector current waveform of a second cylinder during the failure of the boost circuit in the injector driving circuit (for one cylinder) according to Embodiment 3 of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Next, specific embodiments representing preferred embodiments for carrying out the present invention will be explained in detail with reference to drawings. 
   Embodiment 1 
     FIG. 4  shows a circuit diagram of Embodiment 1 of the present invention. In this circuit, a boost circuit is constituted by an injector controller  14 , a boost coil  15 , a diode  16 , a switching MOSFET  17 , an electrolytic capacitor  18  and a current detecting resistor  20 . This boost circuit is designed to boost a battery voltage VB which is supplied from an external component. A high voltage VH that has been boosted by the boost coil  15  is applied to the upstream side of a peak current MOSFET  21  and the downstream side of this MOSFET  21  is connected with the anode side of a reverse current-preventing diode  24 . The cathode side of the reverse current-preventing diode  24  is connected with an injector solenoid coil  25 . 
   To the upstream side of a holding current MOSFET  22  is applied a battery voltage VB which is supplied from an external component and the downstream side of the holding current MOSFET  22  is connected with the anode side of a reverse current-preventing diode  23 . The cathode side of the reverse current-preventing diode  23  is connected with the injector solenoid coil  25 . The downstream side of the injector solenoid coil  25  is connected with the upstream side of the downstream side MOSFET  26  and the downstream side of this MOSFET  26  is connected with a current-detecting resistor  19 . The anode side of a flywheel diode  27  is connected with GND and the cathode side thereof is connected with the cathode sides of the reverse current-preventing diodes  23  and  24 . 
   When the driving signals shown at the second, third and fourth rows of  FIG. 5  are applied to the peak current MOSFET  21 , the holding current MOSFET  22 , the downstream side MOSFET  26 , respectively, the intensity of current flowing to the injector solenoid coil  25  is monitored by the current-detecting resistor  19  and then the current intensity thus monitored is fed back to the injector controller  14  to control the electric current. As a result, an electric current constituted by a peak current for opening the injector, a first holding current which is needed for keeping the open state of the injector, and a second holding current needed for suppressing the heat build-up is permitted flow to the injector solenoid coil  25 . 
   When an abnormal intensity of the electric current flowing to the injector solenoid coil  25  is detected by the injector controller  14  through the monitoring by means of the current-detecting resistor  19 , or when the abnormality of boosted voltage that has been detected by the injector controller  14  is not amended even after the elapse of a predetermined period of time, the boost circuit is judged as gone out of order. In this case, since a peak current is to be formed by making use of the boost circuit at the peak current rise period  29  shown in  FIG. 5 , it is no longer possible to create the current waveform shown at the first row of  FIG. 5  due to this failure of the boost circuit. 
   Therefore, the driving signals to be applied to the MOSFETs  21 ,  22  and  26  are changed to the waveforms shown at the second, the third and the fourth rows of  FIG. 6 , respectively, thereby changing it to the injector current shown at the first row of  FIG. 6 . In this case, in different to the conventional circuit, since the power source to be supplied to the peak current MOSFET  21  and to the holding current MOSFET  22  is divided and the reverse current-preventing diodes  23  and  24  are additionally incorporated, it is possible, even if the boost circuit has gone out of order, to supply electric current from the power source to the holding current MOSFET  22 , thereby making it possible to create the current waveform shown at the first row of  FIG. 6 . As a result, it is possible to flow electric current to the injector solenoid coil  25  and hence to actuate the injector. 
   In the case of the current waveform shown in  FIG. 6 , which is consisted of two steps with no peak current, it is possible to avoid not only the increase of heat build-up of the driving circuit but also great restriction on the upper limit of the engine speed, that may result from a constant current waveform shown in  FIG. 3 . 
   Further, when both of the peak current MOSFET  21  and the holding current MOSFET  22  are turned OFF and the downstream side MOSFET  26  is turned ON, the energy of electric current flowing to the injector solenoid coil  25  is enabled to be consumed by the flywheel diode  27 , so that the boost circuit is no longer required to be used. 
   Furthermore, since it is designed such that the electric current flowing into the injector solenoid coil  25  is monitored as the electric current falls and then the downstream side MOSFET  26  is turned OFF at a threshold level immediately before the fall of the electric current becomes zero, it is now possible to enable a Zener diode  28  connected to a location between the drain-gate regions of the MOSFET  26  to consume the electric energy of the injector solenoid coil. 
   As described above, it is now possible to avoid the heat build-up of the circuit that might have been caused to develop as the boost circuit goes out of order in the prior art as shown in  FIG. 3 . 
   Embodiment 2 
     FIG. 7  shows a circuit diagram of Embodiment 2 of the present invention. In this circuit, a boost circuit is constituted by an injector controller  49 , a boost coil  36 , a diode  37 , a switching MOSFET  38 , an electrolytic capacitor  39  and a current detecting resistor  41 . This boost circuit is designed to boost a battery voltage VB to be supplied from an external component. A high voltage VH that has been boosted by the boost coil  36  is applied to the source side of a peak current MOSFET  42  and the drain side of this MOSFET  42  is connected with the anode side of a reverse current-preventing diode  45 . The cathode side of the reverse current-preventing diode  45  is connected with an injector solenoid coil  47 . 
   To the drain side of a holding current MOSFET  43  is applied a battery voltage VB which is supplied from an external component and the source side of the holding current MOSFET  43  is connected with the anode side of a reverse current-preventing diode  44 . The cathode side of the reverse current-preventing diode  44  is connected with the injector solenoid coil  47 . The downstream side of the injector solenoid coil  47  is connected with the drain side of a downstream side MOSFET  48  and the source side of this MOSFET  48  is connected with a current-detecting resistor  35 . The anode side of a flywheel diode  40  is connected with GND and the cathode side thereof is connected with the cathode sides of the reverse current-preventing diodes  44  and  45 . The cathode side of regenerating diode  46  is connected with the source side of the peak current MOSFET  42  and the anode thereof is connected with the drain side of the downstream side MOSFET  48 . 
   When the driving signals shown at the second, third and fourth rows of  FIG. 8  are applied to the peak current MOSFET  42 , the holding current MOSFET  43 , the downstream side MOSFET  48 , respectively, the intensity of current flowing to the injector solenoid coil  47  is monitored by the current-detecting resistor  35  and then the current intensity thus monitored is fed back to the injector controller  49  to control the electric current. As a result, an electric current at the first row of  FIG. 8  is permitted to flow to the injector solenoid coil  47 . 
   When an abnormal intensity of the electric current flowing to the injector solenoid coil  47  is detected by the injector controller  49  through the monitoring by means of the current-detecting resistor  35 , or when the abnormality of boosted voltage that has been detected by the injector controller  49  is not amended even after the elapse of a predetermined period of time, the boost circuit is judged as gone out of order. In this case, since the current waveform shown at the first row of  FIG. 8  is formed by making use of the boost circuit at the periods  52 ,  53 ,  55  and  57  shown in  FIG. 8 , it is no longer possible to create the current waveform shown in  FIG. 8  due to this failure of the boost circuit. 
   Therefore, the driving signals to be applied to the MOSFETs  42 ,  43  and  48  are changed to the waveforms shown at the second, the third and the fourth rows of  FIG. 9 , respectively. In this case, in different to the conventional circuit, since the power source to be supplied to the peak current MOSFET  42  and to the holding current MOSFET  43  is divided and the reverse current-preventing diodes  44  and  45  are additionally incorporated, it is possible, even if the boost circuit has gone out of order, to supply electric current from the power source to the holding current MOSFET  43 , thereby making it possible to create the current waveform shown at the first row of  FIG. 9 . As a result, it is possible to flow electric current to the injector solenoid coil  47  and hence to actuate the injector. 
   Further, when both of the peak current MOSFET  42  and the holding current MOSFET  43  are turned OFF and the downstream side MOSFET  48  is turned ON, the energy of electric current flowing to the injector solenoid coil  47  is enabled to be consumed by the flywheel diode  40 , so that the boost circuit is no longer required to be used and regenerative current is not required to be transmitted to the boost circuit side. 
   Especially, during the holding current-falling period  64  and on the occasion where the circuit is out of order, it is possible to enable electric current to fall down to zero without necessitating the monitoring of current if an ordinary boost circuit is employed. However, if the boost circuit is not employed, since the downstream side MOSFET  48  is turned ON to enable the flywheel diode to consume the energy of electric current, it is required to continue the monitoring of electric current and the downstream side MOSFET  48  is turned OFF immediately before the intensity of electric current becomes zero. By doing so, it is now possible to enable the waveform of the injector current shown in the first row in  FIG. 9  to change in terms of the time and the intensity of electric current of each time period. As a result, it is now possible to accelerate the injector-opening response, to realize a high-precision control and to suppress the heat build-up. 
   Embodiment 3 
     FIG. 10  shows a circuit diagram of Embodiment 3 of the present invention. In this circuit, a boost circuit is constituted by an injector controller  70 , a boost coil  65 , a diode  66 , a switching MOSFET  67 , an electrolytic capacitor  68  and a current detecting resistor  69 . This boost circuit is designed to boost a battery voltage VB which is supplied from an external component. A high voltage VH that has been boosted by the boost coil  15  is applied to the source side of a peak current MOSFET  72  and the drain side of this MOSFET  72  is connected with the anode side of a reverse current-preventing diode  74 . The cathode side of the reverse current-preventing diode  74  is connected in parallel with injector solenoid coils  78  and  79 . 
   To a holding current MOSFET  71  is applied a battery voltage VB which is supplied from an external component and the source side of the holding current MOSFET  71  is connected with the anode side of a reverse current-preventing diode  73 . The cathode side of the reverse current-preventing diode  73  is connected in parallel with the injector solenoid coils  78  and  79 . 
   The downstream sides of the injector solenoid coils  78  and  79  are connected with the drain side of the downstream side MOSFETs  80  and  81 , respectively, and the source sides of these MOSFETs  80  and  81  are connected with a current-detecting resistor  82 . The anode side of a flywheel diode  75  is connected with GND and the cathode side thereof is connected with the cathode sides of the reverse current-preventing diodes  73  and  74 . Further, the cathode sides of regenerating diodes  76  and  77  are connected with the source side of the peak current MOSFET  72  and the anode thereof is connected with the drain sides of the downstream side MOSFETs  80  and  81 . 
   When the driving signals shown at the second, third and fourth rows of  FIG. 11  are applied to the peak current MOSFET  72 , the holding current MOSFET  71 , the downstream side MOSFET  80 , respectively, and, successively, the driving signals shown at the second, third and fourth rows of  FIG. 12  are applied to the peak current MOSFET  72 , the holding current MOSFET  71 , the downstream side MOSFET  80 , respectively, the current intensities  78 -A and  79 -A of the injector solenoid coils  78  and  79  are monitored by the current-detecting resistor  82  and then the current intensities thus monitored are fed back to the injector controller  70  to control the electric current. As a result, the electric current shown at the first row in  FIG. 11  is permitted flow to the injector solenoid coil  78  and then the electric current shown at the first row in  FIG. 12  is permitted flow to the injector solenoid coil  79 . 
   When an abnormal intensity of the electric current flowing to the injector solenoid coils  78  and  79  is detected by the injector controller  70  through the monitoring by means of the current-detecting resistor  82 , or when the abnormality of boosted voltage that has been detected by the injector controller  70  is not amended even after the elapse of a predetermined period of time, the boost circuit is judged as gone out of order. 
   Since the current waveform shown at the first row of  FIG. 11  is formed by making use of the boost circuit at the periods  85 ,  86 ,  88  and  90  shown in  FIG. 11  in the case where the boost circuit has been gone out of order, it is no longer possible to create the current waveform of  FIG. 11  due to this failure of the boost circuit. Therefore, the driving signals to be applied to the MOSFETs  72 ,  71  and  80  are changed to the waveforms shown at the second, the third and the fourth rows of  FIG. 13 , respectively. In this case, in different to the conventional circuit, since the power source to be supplied to the peak current MOSFET  72  and to the holding current MOSFET  71  is divided and the reverse current-preventing diodes  73  and  74  are additionally incorporated, it is possible, even if the boost circuit has gone out of order, to supply electric current from the power source to the holding current MOSFET  71 , thereby making it possible to create the current waveform shown at the first row of  FIG. 13 . As a result, it is possible to flow electric current to the injector solenoid coil  78  and hence to actuate the injector. 
   Further, since the current waveform shown at the first row of  FIG. 12  is formed by making use of the boost circuit at the periods  93 ,  94 ,  96  and  98  shown in  FIG. 12  in the case where the boost circuit has been gone out of order, it is no longer possible to create the current waveform of  FIG. 12  due to this failure of the boost circuit. Therefore, the driving signals to be applied to the MOSFETs  72 ,  71  and  81  are changed to the waveforms shown at the second, the third and the fourth rows of  FIG. 14 , respectively. In this case, since electric current is enabled to be supplied from the power source to the holding current MOSFET  71  even if the boost circuit has gone out of order, it is possible to create the current waveform shown at the first row of  FIG. 14 . As a result, it is possible to flow electric current to the injector solenoid coil  79  and hence to actuate the injector. 
   Further, when both of the peak current MOSFET  72  and the holding current MOSFET  71  are turned OFF and either the downstream side MOSFET  80  or the downstream side MOSFET  81  is turned ON, the energy of electric current flowing to the injector solenoid coil  78  or  79  is enabled to be consumed by the flywheel diode  75 , so that the boost circuit is no longer required to be used. Additionally, during the periods  103  and  105  shown in  FIG. 13  as well as during the periods  110  and  112  shown in  FIG. 14 , the electric current flowing to the injector solenoid coil  78  or  79  is monitored, thereby enabling the downstream side MOSFET  80  or  81  to turn OFF at a threshold level immediately before the driving current falls to zero. In this manner, the regenerative current can be prevented from flowing toward the boost circuit. 
   However, since it is necessary to wait until a moment immediately before the falling of the driving current becomes zero, there is an increasing possibility that the falling to of the driving current may overlap with the rise of the counter cylinder. Therefore, in a situation where the preceding injector current transmitted at first is likely to overlap with the succeeding injector current transmitted subsequently, the priority should be given to the transmission of the succeeding injector current, so that even if the fall of driving current does not reach a threshold value at that point, the downstream side MOSFET  80  or  81  is actuated to turn OFF. By doing so, the aforementioned overlapping can be prevented. Further, since it is designed to generate a precharge current at the period  99  of  FIG. 13  and the period  106  of  FIG. 14  to thereby excite the injector, it is possible to promote the opening of the injector and to improve the minimum injection quantity. 
   The present invention is applicable not only to the injector solenoid coil of automobiles but also to any demand for fail-safe in every kinds of actuators which are designed to be actuated through the control of electric current by making use of a boost circuit.