JP4148124B2 - Capacitive load element drive device - Google Patents

Description

  The present invention relates to a drive device for a capacitive load element composed of a piezoelectric element or the like.

  For example, the use of a piezo-driven injector as a fuel injection device for a vehicle diesel engine has been studied. The piezo stack constituting the injector has a characteristic of extending by charging and contracting by discharging, and the nozzle needle moves to an open position or a closed position by expansion and contraction of the piezo stack, and accordingly, fuel injection of the injector is performed. Charging and discharging of the piezo stack are performed according to a fuel injection start command and a stop command, and charging and discharging are performed by a so-called multi-switching charging and discharging method.

  A multi-switching charging method will be briefly described. At the time of charging, first, a charging switch provided between the power supply unit and the piezo stack is turned on, and then the charging switch is turned off when the charging current of the piezo stack reaches a predetermined current. At this time, the on-time of the first charging switch is stored. When the current of the piezo stack decreases to 0 A, the charge switch is turned on for the on time stored at the first charge. Thereafter, each time the current drops to 0 A, the discharge switch is repeatedly turned on / off based on the first on-time. According to this charging method, the charging energy per hour is constant, and the extension amount of the piezo stack can be appropriately managed.

  On the other hand, at the time of discharge, the discharge switch is turned on as the discharge starts, and the electric energy stored in the piezo stack is discharged to the energy storage coil connected in series to the piezo stack. Thereafter, when the discharge current of the piezo stack reaches a predetermined cut-off current, the discharge switch is turned off, and the energy of the energy storage coil is recovered by the power supply unit via a diode or the like. Thereafter, the discharging operation is repeatedly executed.

  Further, in the discharge device of the capacitive load element disclosed in Patent Document 1, the interruption current for turning off the discharge switch at the time of discharge is set to a time in order to suppress the fluctuation of the discharge time by making the discharge energy of the capacitive load element (piezo stack) constant. A configuration that raises it along with it is adopted.

However, the existing discharge methods have the following problems. That is, if the cut-off current is increased with the time after the start of discharge as in Patent Document 1, it is possible to perform discharge control that substantially follows the capacity change of the piezo stack, but in reality the capacity change cannot be predicted. In many cases, the desired effect may not always be obtained. Therefore, the discharge time may fluctuate unexpectedly. Therefore, further improvement is desired to drive the piezo stack with high accuracy.
JP 2003-92438 A

  The main object of the present invention is to provide a drive device for a capacitive load element that can accurately drive the capacitive load element by optimizing the discharge characteristics.

  According to the first aspect of the present invention, when the switch means is turned on during discharging of the capacitive load element, the capacitive load element and the energy recovery element are energized, and the energy once stored in the capacitive load element is stored in the energy recovery element. To be recovered. Thereafter, when the discharge current of the capacitive load element reaches the cutoff threshold, the switch means is turned off and the switch means is turned on again after the discharge current is lowered, and this discharge operation is repeatedly performed until the discharge of the capacitive load element is completed. . Through this repeated discharge operation, all the energy of the capacitive load element is released. In the present invention, in particular, the load voltage of the capacitive load element is detected, and the cutoff threshold is variably set according to the detected load voltage. In other words, when the capacitance of the capacitive load element changes due to temperature fluctuation or the like as the discharge progresses, the load voltage of the capacitive load element changes accordingly, and the time until discharge completion (discharge time) varies. Accordingly, since the cutoff threshold value is variably set according to the load voltage, the discharge time can be made constant regardless of the capacity change. Therefore, the discharge characteristics can be optimized, and the capacitive load element can be driven with high accuracy.

  In the first aspect of the present invention, as described in the second aspect, the cutoff threshold value is preferably negative feedback controlled in accordance with the load voltage. That is, when the load voltage is high, the cutoff threshold value is decreased, and when the load voltage is low, the cutoff threshold value is increased. By realizing such negative feedback control, it is possible to optimize the cutoff threshold, and to reliably suppress fluctuations in the discharge time.

  According to the third aspect of the present invention, in the device that charges the capacitive load element so that the amount of energy stored in the capacitive load element is constant, the load voltage increases when the capacitance of the capacitive load element is small. When the capacitance of the element is large, the load voltage is lowered. In such a case, in particular, the discharge characteristics can be favorably maintained by variably setting the cutoff threshold as described above.

  In the invention according to claim 4, the switch means is turned on or off in synchronization with a clock signal output at a constant period within the discharge period of the capacitive load element. That is, the on / off interval of the switch means is defined by the clock signal. In this case, it is possible to intentionally delay discharge without increasing the energy recovery capability of the energy recovery element (for example, the inductance of the energy storage coil) or lowering the cutoff threshold. .

  According to the fifth aspect of the present invention, when the switch means is turned on during discharging of the capacitive load element, the capacitive load element and the energy recovery element are energized, and the energy once stored in the capacitive load element is stored in the energy recovery element. To be recovered. After the switch means is turned on, when the discharge current of the capacitive load element reaches the cutoff threshold, the switch means is turned off, and then the switch means is turned on again in synchronization with the clock signal. Repeated until completion. Through this repeated discharge operation, all the energy of the capacitive load element is released. In this case, since the on / off interval of the switch means is defined by the clock signal, it is possible to increase the energy recovery capability of the energy recovery element (for example, the inductance of the energy storage coil) or lower the cutoff threshold. Even if it is not performed, it is possible to intentionally delay the discharge. Therefore, the capacitive load element can be driven with high accuracy.

  In the inventions of the fourth and fifth aspects, as described in the sixth aspect, it is preferable that the on-operation of the switch means is permitted only when the clock signal becomes either the H level or the L level. Thereby, for example, even if the discharge current is sufficiently reduced after the switch means is turned off, the switch means is not turned on unless the clock signal is at the active level of the switch means. In this case, unlike the configuration in which the switch means is simply turned on / off according to the discharge current level, it is possible to have an operation waiting time of the switch means. Therefore, it is possible to reduce the number of times of switching of the switch means, and it is possible to reduce circuit loss.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS An embodiment of the invention will be described below with reference to the drawings. The present embodiment embodies the present invention as a common rail fuel injection system for a diesel engine for vehicles, and particularly uses a piezo drive injector as a fuel injection valve of the fuel injection system. First, the configuration of the fuel injection system will be described with reference to FIG.

  In FIG. 3, a high-pressure supply pump 11 is connected to the common rail 10. Fuel is pumped from the fuel tank 12 as the high-pressure supply pump 11 is driven, and high-pressure fuel corresponding to the injection pressure is continuously stored in the common rail 10. . The common rail 10 is connected to an injector 13 provided corresponding to each cylinder of the engine, and the high pressure fuel in the common rail 10 is injected and supplied to each cylinder as the injector 13 is driven (in FIG. 3). Only one injector 13 is shown).

  The ECU 20 generates an injection signal TQ based on the engine operating state and the like each time, and outputs the injection signal TQ to the injector drive circuit 21. The injector drive circuit 21 controls the timing of the start and end of injection by the injector 13 based on the injection signal TQ. In this case, the injector 13 is configured to perform an injection operation by expansion and contraction of the piezo stack. When the piezo stack is charged and discharged based on the injection signal TQ, the piezo stack expands and contracts, and fuel injection starts and ends. Is done.

  The fuel supplied from the common rail 10 to each injector 13 is used not only as fuel injected into each cylinder but also as control hydraulic pressure of the injector 13. The fuel is supplied from the injector 13 to the fuel tank 12 via the low-pressure drain line 14. It is designed to be refluxed. A pressure sensor 15 for detecting the fuel pressure is attached to the common rail 10. The ECU 20 controls the opening degree of the adjustment valve 16 based on the output of the pressure sensor 15 to adjust the amount of fuel pumped to the common rail 10. As a result, the fuel pressure in the common rail 10 is maintained at an appropriate pressure.

  The structure of the injector 13 will be described with reference to FIG. The injector 13 has a rod-like body, and is attached so that the lower side in the drawing penetrates the combustion chamber wall of the engine and protrudes into the combustion chamber. The injector 13 is roughly divided and includes a nozzle unit 31, a back pressure control unit 32, and a piezo drive unit 33.

  In the nozzle portion 31, the needle 34 has a large-diameter portion 34 a slidably supported in the nozzle holder 35, and a tip cone portion 34 b of the needle 34 is attached to an annular sheet portion 36 formed at the tip portion of the nozzle holder 35. Is configured to be seated or separated. High pressure fuel is introduced into the fuel chamber 37 formed between the inner wall of the nozzle holder 35 and the needle 34 through the high pressure passage 38 from the common rail 10 described above, and the fuel introduced into the fuel chamber 37 is fed into the needle. 34 is ejected from the nozzle hole 39 as the seat 34 is separated. Since the step surface 34 c of the needle 34 is pushed by the fuel pressure in the fuel chamber 37, an upward force (separating direction) is always applied to the needle 34.

  A back pressure chamber 41 is formed above the needle 34, and high pressure fuel is supplied to the back pressure chamber 41 from the high pressure passage 38 through the in-orifice 42. The high pressure fuel supplied to the back pressure chamber 41 acts on the upper surface of the needle, and urges the needle 34 downward (sitting direction) together with the spring 43. The pressure in the back pressure chamber 41 (back pressure) is controlled by a piezo drive unit 33 described later.

  The back pressure chamber 41 communicates with the valve chamber 45 via the out orifice 44. This valve chamber 45 has a ceiling surface 45a formed in an upward conical shape, and communicates with the low pressure chamber 46 at the uppermost portion of the ceiling surface 45a. The low pressure chamber 46 communicates with the drain line 14 described above via the low pressure passage 47. The bottom surface 45b of the valve chamber 45 is a flat surface, and a high-pressure control passage 48 branched from the high-pressure passage 38 is opened on the bottom surface 45b. A ball valve 50 whose lower surface is cut horizontally is accommodated in the valve chamber 45. The ball valve 50 is a valve body that can move up and down. When the ball valve 50 is lifted, the spherical surface is seated on the ceiling surface 45a of the valve chamber 45 so that the communication between the valve chamber 45 and the low pressure chamber 46 is cut off. By sitting on the bottom surface 45 b of the chamber 45, the communication between the valve chamber 45 and the high-pressure control passage 48 is blocked.

  As shown in the figure, when the ball valve 50 is raised and the communication between the valve chamber 45 and the low pressure chamber 46 is interrupted, the back pressure chamber 41 is maintained in a high pressure state. Sit down. On the contrary, when the ball valve 50 is lowered and the communication between the valve chamber 45 and the high pressure control passage 48 is cut off, the back pressure chamber 41 is connected to the drain line 14 via the valve chamber 45, the low pressure chamber 46, and the low pressure passage 47. As a result, the pressure of the back pressure chamber 41 is lowered, and the needle 34 is separated.

  The ball valve 50 is lowered or raised by the expansion and contraction of the piezo stack 51 in the piezo drive unit 33. That is, a displacement expansion chamber 52 is formed above the low pressure chamber 46, and a large diameter piston 53 and a small diameter piston 54 are accommodated separately in the displacement expansion chamber 52. On the upper side of the large-diameter piston 53, a piezo stack 51 formed by laminating a large number of piezo elements as capacitive load elements is disposed. The large-diameter piston 53 is pressed against the piezo stack 51 by a spring 55 disposed below the large-diameter piston 53 and is displaced in the vertical direction by the same amount as the expansion and contraction of the piezo stack 51. The lower end portion of the small diameter piston 54 is in contact with the ball valve 50. The displacement expansion chamber 52 is filled with fuel. When the piezo stack 51 is extended, the upper large-diameter piston 53 is lowered, and when the fuel in the displacement expansion chamber 52 is pressurized, the pressurizing force causes the lower small diameter. The piston 54 is pushed downward. At this time, since the small-diameter piston 54 has a smaller diameter than the large-diameter piston 53, the extension amount of the piezo stack 51 is expanded and transmitted to the small-diameter piston 54.

  At the start of injection, first, the piezo stack 51 expands with the charging by the injector drive circuit 21. Then, the large-diameter piston 53 and the small-diameter piston 54 are lowered, the ball valve 50 is pushed down, and the back pressure in the back pressure chamber 41 is reduced. As a result, the needle 34 is separated and fuel injection is started. Further, when the injection is stopped, the piezo stack 51 contracts with the discharge by the injector drive circuit 21. Then, the large-diameter piston 53 and the small-diameter piston 54 are raised, and the push-down of the ball valve 50 is released. Since high pressure fuel is acting on the ball valve 50 from the high pressure control passage 48, the ball valve 50 is raised and the communication between the valve chamber 45 and the low pressure chamber 46 is blocked. Then, the back pressure in the back pressure chamber 41 rises, the needle 34 is seated, and fuel injection is stopped.

  Next, the electrical configuration of the injector drive circuit 21 will be described with reference to FIG. In the injector drive circuit 21, the DC power supply unit 60 includes a DC / DC converter 62 that generates a DC voltage of several tens to several hundreds V from the in-vehicle battery 61, and a buffer capacitor 63 that is connected in parallel to the DC / DC converter 62. Is provided. The buffer capacitor 63 has a relatively large capacitance and has a configuration that maintains a constant voltage even when the piezo stack 51 is charged.

  The buffer capacitor 63 is connected in parallel with a charge switch 65 that is turned on / off when the piezo stack 51 is charged and a discharge switch 66 that is turned on / off when the piezo stack 51 is discharged. In parallel, diodes 67 and 68 are connected. One end of the energy storage coil 69 is connected to a connection point between the charge switch 65 and the discharge switch 66, and the other end of the energy storage coil 69 is connected to each piezo stack 51. Each piezo stack 51 is connected to a parallel circuit including a selection switch 70 and a diode 71 for selecting the piezo stack 51 to be driven each time.

  The charge switch 65, the discharge switch 66, and the selection switch 70 are switched between an on state and an off state by a charge / discharge control unit (not shown), and are configured by a semiconductor switching element such as a MOSFET or a mechanical relay switch. Has been. In the present embodiment, the discharge switch 66 corresponds to “switch means”, and the energy storage coil 69 corresponds to “energy recovery element”.

  The charging / discharging operation of the piezo stack 51 by the injector driving circuit 21 having the above configuration will be described. First, when the injection signal TQ is turned on by the ECU 20, the charging switch 65 is turned on, and charging is started for the piezo stack 51 in which the selection switch 70 is turned on. For this charging, a known charging method known as a multi-switching method is used. Specifically, the charging current of the piezo stack 51 is detected, and when the charging current reaches a predetermined current (for example, 25 A), the charging switch 65 is turned off. At this time, the first on-time of the charging switch 65 is stored. When the current of the piezo stack 51 decreases to 0 A, the charging switch 65 is turned on for the on time stored at the time of the first charging. Thereafter, each time the current decreases to 0 A, the discharge switch 65 is repeatedly turned on / off based on the first on-time. According to this charging method, the charging energy per time is constant. For this reason, even if the capacity of the piezo stack 51 changes due to temperature fluctuation or the like, a certain amount of electrical energy can be stored in the piezo stack 51.

  When the injection signal TQ is turned off by the ECU 20, the piezo stack 51 is discharged by the on / off operation of the discharge switch 66. The discharge of the piezo stack 51 includes the characteristic configuration of the present invention and will be described in detail below. FIG. 1 shows a circuit configuration relating to the discharge of the piezo stack 51. The discharge switch control circuit 80 shown in FIG. 1 forms part of the charge / discharge control unit, and the discharge switch 66 is turned on or off by the discharge control signal Toff output from the discharge switch control circuit 80. ing. In the present embodiment, the discharge switch control circuit 80 corresponds to “discharge control means” and “threshold variable setting means”.

  In FIG. 1, voltage dividing resistors 75 and 76 are connected in parallel to the piezo stack 51, and a voltage of the piezo stack 51 (hereinafter referred to as a piezo voltage Vpzt) is detected by the voltage dividing resistors 75 and 76. In the discharge switch control circuit 80, the piezo voltage Vpzt is input to the non-inverting input terminal of the operational amplifier 81. The output terminal of the operational amplifier 81 is connected to the base of the transistor 82, and the inverting input terminal of the operational amplifier 81 is connected to the emitter of the transistor 82. Further, voltage dividing resistors 83, 84, 85 are connected in series to the constant voltage source Vcc, and the collector of the transistor 82 is connected to the connection point of the voltage dividing resistors 83, 84, and the voltage dividing resistors 84, 85 are connected to each other. The non-inverting input terminal of the comparator 87 is connected to the connection point.

  A resistor 86 is connected between the emitter of the transistor 82 and the ground, and the resistor 86 is also connected to the inverting input terminal of the operational amplifier 81, whereby the voltage of the resistor 86 is controlled to be Vpzt. Therefore, the current flowing through the resistor 86 is defined by Vpzt and the resistor 86, and flows from the constant voltage source Vcc via the resistor 83 and the transistor 82. As a result, the voltage drop of the resistor 83 increases due to the increase of Vpzt, and the voltage at the connection point of the voltage dividing resistors 84 and 85 decreases. That is, as Vpzt increases, the voltage at the same connection point (the reference voltage of the comparator 87) decreases.

  The injector drive circuit 21 is configured to detect the discharge current Idis of the piezo stack 51 by the current detection resistor 77, and the comparator 87 has a detection value (voltage conversion value) of the discharge current Idis and a cutoff threshold value. The reference voltage is compared. In this case, if the Idis detection value is smaller than the reference voltage, the comparator output becomes H level, and if the Idis detection value is larger than the reference voltage, the comparator output becomes L level. The comparator 87 is a hysteresis comparator. When the output is at the H level, the voltage at the connection point of the voltage dividing resistors 84 and 85 is set as the high side reference voltage VH1, and when the output is at the L level, the low side reference voltage is set. 0V is set as VL1.

  The AND circuit 88 receives the output of the comparator 87, the output of the discharge period signal output circuit 90, and the output of the clock generation circuit 91. The AND circuit 88 outputs the logical product of each input as the discharge control signal Toff. The discharge period signal output circuit 90 outputs a discharge period signal that becomes H level for a predetermined time after the injection signal TQ output from the ECU 20 becomes L level. The clock generation circuit 91 is a circuit that generates and outputs a clock signal having a constant period within the discharge period. An example of the circuit configuration is shown in FIG.

  The configuration and operation of the clock generation circuit 91 shown in FIG. 2 will be described with reference to the time chart of FIG. Here, FIG. 6 is a time chart for explaining the discharge operation of the piezo stack 51. In particular, (a) to (e) show the clock generation procedure.

  In FIG. 6, when the injection signal TQ becomes L level at the timing t1, the discharge period signal output circuit 90 accordingly sets the discharge period signal to H level for a certain time. After t1, since the inverted signal of the discharge period signal becomes L level, the transistor 103 is turned off in the configuration of FIG. Therefore, a constant current flows from the constant current source 101 to the capacitor 102, and the capacitor voltage gradually increases. At this time, the output of the comparator 104 is at L level, and the transistor 105 is off, so that the clock signal is at H level.

  The reference voltage of the comparator 104 is the high-side reference voltage VH2 if the comparator output is L level, and the reference voltage is the low-side reference voltage VL2 if the comparator output is H level. When the capacitor voltage reaches the reference voltage VH2 at timing t3, the output of the comparator 104 becomes H level, and both the transistors 105 and 106 are turned on. Therefore, the clock signal is inverted to L level. At this time, the reference voltage of the comparator 104 is switched from the high-side VH2 to the low-side VL2. When the output of the comparator 104 becomes H level, the transistor 107 is turned on, so that a constant current flows to the ground via the transistor 107, and the rise of the capacitor voltage is stopped. In addition, since the capacitor 102 is discharged through the current mirror circuit unit 108, the capacitor voltage decreases.

  Thereafter, when the capacitor voltage reaches the reference voltage VL2 at timing t5, the output of the comparator 104 becomes L level again, and the transistors 105 to 107 are turned off. Then, the clock signal is raised to the H level. Thereafter, during the discharge period up to timing t6 when the discharge period signal becomes L level, the capacitor voltage is repeatedly increased and decreased as described above, and a clock signal having a constant period is output accordingly.

  Next, the discharge operation of the piezo stack 51 will be described using the time chart of FIG. Here, the description will be made based on (f) to (i) of FIG. 6 together with the clock signal waveform described above.

  At timing t1, both the discharge period signal and the clock signal are at the H level, and the Idis detection value is equal to or lower than the reference voltage VH1, so that the output of the comparator 87 is at the H level. Therefore, the discharge control signal Toff becomes H level, and the discharge switch 66 is turned on. Thereby, the discharge of the piezo stack 51 is started, and the discharge current Idis starts to rise. That is, the electrical energy stored in the piezo stack 51 is recovered by the energy storage coil 69, so that the piezo stack 51 is discharged.

  Thereafter, when the Idis detection value reaches the reference voltage VH1 at timing t2, the output of the comparator 87 becomes L level, so that the discharge control signal Toff becomes L level, and the discharge switch 66 is turned off. When the discharge switch 66 is turned off, the electrical energy transferred from the piezo stack 51 to the energy storage coil 69 so far flows through the diode 67 and is transferred to the buffer capacitor 63. Therefore, the discharge current Idis decreases. The reference voltage is switched from VH1 to VL1 (0V) when the output of the comparator 87 becomes L level.

  When the Idis detection value reaches the reference voltage VL1 at timing t4 as the discharge current Idis decreases, the output of the comparator 87 becomes H level. Thereafter, after the timing t5 when the clock signal becomes H level, the discharge control signal Toff becomes H level, and the discharge switch 66 is turned on. Thereafter, similarly to the above, the discharge control signal Toff is generated according to the change in the Idis detection value and the clock signal, and the discharge switch 66 is turned on / off. As described above, the discharge switch 66 is turned on / off once per unit clock of the clock signal.

  Then, after the discharge is repeated, when the piezo voltage Vpzt is reduced to 0V, the discharge of the piezo stack 51 is completed.

  In FIG. 6, as the discharge of the piezo stack 51 proceeds, the piezo voltage Vpzt gradually decreases, and the reference voltage VH1 of the comparator 87 is variably set as shown in the figure as the Vpzt decreases. That is, negative feedback control is performed such that when the piezo voltage Vpzt is high, the reference voltage VH1 of the comparator 87 is lowered, and when the piezo voltage Vpzt is low, the reference voltage VH1 is raised.

  In the present embodiment, since charging control is performed so that the amount of charging energy is constant, the piezo voltage Vpzt increases when the piezo capacity is small, and conversely, when the piezo capacity is large, the piezo voltage Vpzt decreases. Even in such a case, the reference voltage VH1 of the comparator 87 is variably set as described above. For example, when the piezo capacitance is small and the piezo voltage Vpzt is high, the discharge current Idis is kept low and the discharge is delayed. . Further, when the piezo capacity is large and the piezo voltage Vpzt is low, the discharge current Idis becomes high and the discharge is accelerated. It is also possible to cope with the case where the piezo capacity fluctuates due to temperature change or the like. Thereby, the discharge time is constant regardless of the capacity fluctuation of the piezo stack 51, and the discharge characteristics can be optimized. Further, the discharge energy can be made constant, and the amount of piezo displacement becomes constant even when the piezo capacity is changed. The change in the discharge energy is preferably within 20%, and more preferably within 5%.

  FIG. 7 is a diagram for explaining the effect of discharge control in the present embodiment. FIG. 7A shows data of the present embodiment in which the reference voltage VH1 is variably set, and FIG. 7B shows that the reference voltage VH1 is constant. The prior art data are shown respectively. In FIG. 7, the discharge energy (50 μs discharge energy) for 50 μs from the start of discharge and the time required for releasing 90% of the energy accumulated in the piezo stack 51 (90% discharge time) are shown in FIG. It shows corresponding to.

  In the data shown in (b), both the 50 μs discharge energy and the 90% discharge time change depending on the piezo capacity. The larger the piezo capacity, the larger the 50 μs discharge energy and the shorter the 90% discharge time. I understand that there is. This means that the discharge characteristics of the piezo stack 51 are not constant when, for example, the piezo capacity changes due to temperature fluctuations. On the other hand, in the data shown in (a), it can be seen that both the 50 μs discharge energy and the 90% discharge time are substantially constant regardless of the piezoelectric capacity. This means that the discharge characteristics of the piezo stack 51 can be made constant even when the piezo capacity changes due to temperature fluctuation, for example.

  According to the embodiment described above in detail, the following excellent effects can be obtained.

  When discharging the piezo stack 51, the reference voltage VH1 of the comparator 87 is variably set according to the piezo voltage Vpzt and the discharge switch 66 is turned on / off, so that the discharge time can be made constant. Accordingly, the discharge characteristics can be optimized and the piezo stack 51 can be driven with high accuracy. As a result, the control accuracy of the fuel injection amount is improved.

  Since only a negative feedback control unit in the discharge switch control circuit 80 can correct the piezo capacitance variation, a low-cost configuration can be realized as compared with correction control using a microcomputer or the like.

  In order to turn on or off the discharge switch 66 in synchronization with a clock signal output at a constant period within the discharge period of the piezo stack 51, a configuration in which the discharge switch 66 is simply turned on / off according to the discharge current Idis (simply current) Unlike the configuration in which the discharge switch 66 is sandwiched, an operation waiting time (for example, t4 to t5 in FIG. 6) of the discharge switch 66 can be provided. Therefore, it is possible to reduce the number of switching times of the discharge switch 66, and it is possible to reduce circuit loss. Further, it is possible to intentionally delay the discharge without increasing the inductance of the energy storage coil 69 or decreasing the reference voltage VH1.

  In addition, this invention is not limited to the content of description of the said embodiment, For example, you may implement as follows.

  The amount of negative feedback in the discharge switch control circuit 80 may be changed in accordance with the piezo characteristic so as to have an appropriate temperature characteristic.

  In the above embodiment, the discharge switch control circuit 80 is configured to input the discharge period signal that is at the H level for a certain period of time after the injection signal TQ is turned off to the AND circuit 88, but this discharge period signal is not used. Good. That is, the AND circuit 88 outputs the discharge control signal Toff by the logical product of the output of the comparator 87 and the clock signal. Similar effects can be obtained even with this configuration.

  The clock signal output means is not limited to the clock generation circuit 91 shown in FIG. 2, but may have other configurations. In short, what is necessary is just to output a clock signal with a fixed period within the discharge period after the injection signal TQ is turned off.

  In the above embodiment, the reference voltage VH1 of the comparator 87 is variably set according to the piezo voltage Vpzt when the piezo stack 51 is discharged, and the on / off of the discharge switch 66 is synchronized with the clock signal. It can also be realized without requiring signal synchronization. Even with such a configuration, it is possible to optimize the discharge characteristics.

  The discharge control means and the variable threshold value setting means are preferably realized by a hardware circuit as described above from the viewpoint of cost, but may be realized by a microcomputer. For example, the load voltage (piezo voltage Vpzt) may be detected by a microcomputer, and the cutoff threshold value (reference voltage VH1) may be changed according to a command from the microcomputer in accordance with the load voltage.

  It is also possible to apply the piezo element (piezo stack) as a capacitive load element to a drive device other than the fuel injection injector and to embody the present invention.

It is a circuit diagram which shows the structure regarding the discharge of a piezo stack. It is a circuit diagram which shows the structure of a clock generation circuit. It is the schematic which shows the structure of a fuel-injection system. It is sectional drawing which shows the structure of an injector. It is an electrical block diagram of an injector drive circuit. It is a time chart which shows the discharge operation of a piezo stack. It is a figure for demonstrating the effect of embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 20 ... ECU, 21 ... Injector drive circuit, 51 ... Piezo stack, 66 ... Discharge switch, 69 ... Energy storage coil, 75, 76 ... Voltage dividing resistance as load voltage detection means, 77 ... Current detection as discharge current detection means Resistance, 80 ... discharge switch control circuit, 91 ... clock generation circuit.

Claims (6)

In a capacitive load element driving apparatus comprising a capacitive load element having temperature capacity characteristics and controlling charging and discharging of the capacitive load element during element drive,
An energy recovery element connected in series to the capacitive load element and recovering energy once stored in the capacitive load element;
Switch means provided in the middle of an energization path for energizing the capacitive load element and the energy recovery element, and for switching the energization path;
A discharge current detecting means for detecting a discharge current from the capacitive load element;
When discharging the capacitive load element, when the discharge current detected by the discharge current detection means reaches the cutoff threshold after the switch means is turned on, the switch means is turned off and the switch means is turned on again after the discharge current is reduced. Discharge control means for repeatedly performing this discharge operation until the discharge of the capacitive load element; and
Load voltage detecting means for detecting a load voltage of the capacitive load element;
Threshold variable setting means for variably setting the cutoff threshold according to the detected load voltage;
A drive device for a capacitive load element, comprising:   2. The drive device for a capacitive load element according to claim 1, wherein the threshold value variable setting means performs negative feedback control of the cutoff threshold value according to the load voltage.   The drive device for a capacitive load element according to claim 1, further comprising charge control means for charging the capacitive load element so that the amount of energy stored in the capacitive load element is constant.   The capacitive load element according to any one of claims 1 to 3, wherein the discharge control means turns on or off the switch means in synchronization with a clock signal output at a constant period within a discharge period of the capacitive load element. Drive device. In a capacitive load element drive device comprising a capacitive load element having temperature capacity characteristics and controlling charging and discharging of the capacitive load element during element drive,
An energy recovery element connected in series to the capacitive load element and recovering energy once stored in the capacitive load element;
Switch means provided in the middle of an energization path for energizing the capacitive load element and the energy recovery element, and for switching the energization path;
A discharge current detecting means for detecting a discharge current from the capacitive load element;
Clock signal output means for outputting a clock signal generated at a constant period;
When discharging the capacitive load element, when the discharge current detected by the discharge current detecting means reaches the cutoff threshold after the switch means is turned on, the switch means is turned off and then the switch means is synchronized with the clock signal. A discharge control means which is turned on again and repeatedly performs this discharge operation until the discharge of the capacitive load element is completed;
A drive device for a capacitive load element, comprising:   The clock signal is a binary signal that becomes an H level or an L level at a constant period, and the discharge control means allows the switch means to be turned on only when the clock signal becomes an H level or an L level. Item 6. The capacitive load element drive device according to Item 4 or 5.

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