Abstract:
A drive circuit for controlling at least one device having first and second electromagnetically operable actuators, the drive circuit having first and second terminals for connection to a voltage supply and comprising a first controllable switch in connection with one side of the actuators and the first terminal. Each actuator has an associated second controllable switch in connection with the other side of the respective actuator, whereby opening and closing of the first and second switches serves to control selection and actuation of the actuators. The circuit further comprises a sensor arrangement for providing independent sensing of the current flowing through each of the actuators, each actuator having an associated diode in connection with the respective second controllable switch to ensure current flows through the sensor arrangement when the first controllable switch is opened and the second controllable switch is closed.

Description:
TECHNICAL FIELD 
     The invention relates to a drive circuit for a device having two electromagnetically operable actuators. In particular, the invention relates to a drive circuit for a dual-valve fuel injector, forming part of the fuel system of a vehicle internal combustion engine, the fuel injector having two electromagnetically operable actuators, one for each of the two valves. 
     BACKGROUND OF THE INVENTION 
     In conventional fuel injectors there is a single actuator for a spill control valve controlled by means of a drive circuit. A drive circuit for controlling a four cylinder internal combustion engine having four fuel injectors is described in PCT application WO96/27198. The actuator for each fuel injector valve has a winding through which a current is passed to actuate the valve. A first controllable switch (commonly referred to as the high side switch) is connected in series between one end of the winding forming part of each actuator and a first terminal of the drive circuit connected to the positive terminal of a DC supply. The high side switch is therefore connected to one end of each of the four actuators. Additionally, a diode is connected in the path between the DC supply and the high side switch. 
     The other end of each winding is connected in series to a second controllable switch (commonly referred to the low side switch), which in turn is connected to the negative terminal of the DC supply via a resistor. The current flowing through each winding can be determined by means of this resistor. In addition, the anode connection of a second diode, one associated with each winding, is in connection with the end of the winding connected to the respective low side switch. The cathode connection of each second diode is in connection with the negative terminal of the DC supply via a capacitor. Thus, depending on whether the low side switch of the associated winding is switched on or off, the winding of each valve connects to the negative terminal of the battery either via a capacitor or by means of the resistor. 
     When it is required to actuate one of the valves, the current in the associated winding is allowed to rise to a high value and is then allowed to fall to a lower value after which it is maintained for a period at a mean level by subjecting the high side switch to a modulated pulse, thereby serving to regulate the current flow through the windings, until such time as the current is turned off to de-actuate the valve. 
     In more recently developed internal combustion engines, dual-valve fuel injectors are to be employed, each fuel injector having a spill valve and a nozzle control valve requiring independent control by an associated actuator. Thus, there is a requirement for a drive circuit for controlling an assembly of fuel injectors each having two actuators. For this purpose it would be possible to employ the drive circuit for a conventional single-actuator fuel injector assembly, as hereinbefore described. However, in order to do so, a duplicate of each circuit component is required for the additional actuator in each fuel injector. This requires a drive circuit of considerable complexity. In particular, four wire connections are needed to each fuel injector from the DC supply, two for each winding of each actuator. This is particularly difficult to accommodate within a single fuel injector body. 
     It is an object of the present invention to provide a drive circuit for a dual-actuator fuel injector having reduced complexity. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a drive circuit for controlling at least one device having first and second electromagnetically operable actuators, the drive circuit having first and second terminals for connection to voltage supply means and comprising a first controllable switch in connection with one side of the actuators and the first terminal, each actuator having an associated second controllable switch in connection with the other side of the respective actuator, whereby opening and closing of the first and second switches serves to control selection and actuation of the actuators, the drive circuit further comprising sensing means for providing independent sensing of the current flowing through each of the actuators, each actuator having an associated diode in connection with the respective second controllable switch to ensure current flows through the sensing means when the first controllable switch is opened and the second controllable switch is closed. 
     The drive circuit provides the advantage that dual-actuator device can be powered with a more simplified circuit than is possible using a conventional means. In particular, the drive circuit limits the numbers of connection required to the actuators. This is particularly important when the drive circuit is used to implement control of valve actuators in a dual-valve fuel injector system. 
     Furthermore, the drive circuit enables independent sensing of the current in each actuator. Therefore current glitch detection may be performed independently for each actuator during the period in which current flows through the sensing means. Current glitch detection is important in fuel injector control as it provides a means of determining when valve closure occurs. 
     The sensing means may be provided by two resistors, one resistor being associated with a different one of the two actuators, each resistor being connected on one side to the second controllable switch associated with the respective actuator. 
     The drive circuit may be controlled by an electronic control unit, providing switching pulses to the first and second controllable switches. The switching control pulses include current glitch detection pulses to initiate switching of the first and second controllable switches when current sensing is required. 
     The drive circuit may further comprise a blocking diode connected between each actuator and the respective second controllable switch to prevent any unintentional short-circuit current flow through the respective actuator. Preferably, the blocking diodes are located within the electronic control unit (ECU). 
     In a preferred embodiment of the invention, the drive circuit controls a plurality of devices arranged in two banks, the drive circuit comprising separate first controllable switches for each of the two banks, the second controllable switches being arranged such that each second controllable switch is common to actuators of devices in both banks. 
     The drive circuit may be powered by a DC supply voltage connected to the first and second terminals. 
     Alternatively, the drive circuit may further comprise a third controllable switch to enable mixed-voltage operation, whereby selection of a higher voltage supply provides an initial higher current to an actuator to initiate actuation thereof and selection of a lower voltage supply provides a subsequent lower current to the actuator sufficient to hold the actuator in a desired position within the device, selection being effected by switching of the third controllable switch. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example only, with reference to the following figures in which; 
     FIG. 1 is a first embodiment of the drive circuit of the present invention for controlling six fuel injectors, each injector having two actuators; 
     FIGS.  2 ( a ) and  2 ( d ) show current waveforms through the windings of the actuators in one of the dual-valve fuel injectors shown in FIG. 1; 
     FIGS.  2 ( c ) and  2 ( b ) shows control pulses which may be employed to control the drive circuit shown in FIG. 1; 
     FIG. 3 is a modification of the drive circuit shown in FIG. 1, having improved fault tolerance; and 
     FIG. 4 is a further modification of the drive circuits shown in FIGS. 1 and 3 for mixed-voltage operation. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows an assembly of six dual-valve fuel injectors  10 A- 10 F, arranged in two banks,  10 A- 10 C and  10 D- 10 F. Each fuel injector has two actuators, one for controlling each of its two valves. The actuators are denoted by references  12 A, 12 B,  14 A, 14 B,  16 A, 16 B,  18 A, 18 B,  20 A, 20 B and  22 A, 22 B. One of the valves of each fuel injector is a spill valve, corresponding to actuators denoted with a reference sign ‘A’, and the other valve is a nozzle control valve, corresponding to actuators denoted with a reference sign ‘B’. Each actuator comprises a winding through which a current may be passed to actuate the associated valve. 
     The actuators  12 A- 22 A and  12 B- 22 B are controlled by means of two high side switches  24 A, 24 B and six low side switches  26 A- 26 F. Preferably, the switches are field effect transistors (FETs). The low side switches  26 A-F are arranged such that each one controls two actuators of the same type i.e. each low side switch  26 A- 26 F either controls two actuators for spill valves or two actuators for nozzle control valves. The low side switches  26 A- 26 F and the high side switches  24 A, 24 B are under the control of an electronic control unit (ECU) (not shown) which would be familiar to a person skilled in the art and will not be described in further detail. Preferably, the high and low side switches may be located within the ECU. 
     The low side switches are arranged in two banks,  26 A- 26 C and  26 D- 26 F, each bank having an associated resistor  28 A, 28 B. Each bank of fuel injectors  10 A-C and  10 D-F is connected to a different one of the high side switches  24 A, 24 B. The actuator connections to the high side switches  24 A, 24 B are such that each of the two actuators sharing a common low side switch is connected to a different one of the high side switches  24 A, 24 B. For example, the actuators  12 A and  18 A are connected, on one side, to the low side switch  26 A whereas the other side of actuator  12 A is connected to the high side switch  24 A, the other side of actuator  18 A being connected to the high side switch  24 B. In addition, the sides of each actuator  12 A and  18 A, sharing the common low side switch  26 A, are also connected to the anode of a diode  30 A, the cathode of which is connected to a DC supply V 1 . Diodes  30 A- 30 F connected in this way are included for each of the actuator pairs sharing a common low side switch  26 A- 26 F. 
     The high side switches  24 A, 24 B are connected on one side to a DC supply V 1 , typically a 50V supply, and on the other side to the cathode of an associated diode,  32 A and  32 B respectively, the anode of each diode  32 A, 32 B being connected to ground. 
     The arrangement is such that independent current sensing, by means of the resistors  28 A, 28 B, is possible for each of the actuators  12 A- 22 A,  12 B- 22 B. Independent current sensing through each of the actuators is desirable as it enables current glitch detection to be performed, as will be described hereinafter. Furthermore, it is necessary that the fuel injectors  10 A- 10 B can be operated sequentially without overlapping. 
     With the high side switch  24 A closed, actuation of each of the actuators  12 A, 14 A,  16 A,  12 B, 14 B, 16 B of the first bank of fuel injectors  10 A- 10 C is achieved by closing the appropriate low side switch  26 A- 26 F. Actuator selection and current holding in a selected actuator is performed by each low-side switch with the corresponding high side switch turned on. Recirculation occurs through the high side switch  24 A, and the corresponding diode  30 A- 30 F, when the corresponding low side switch is turned off. 
     The drive circuit will now be described in detail with reference to fuel injector  10 A, comprising actuators  12 A and  12 B. FIG.  2 ( a ) shows the current waveform, I S , through the winding of actuator  12 A controlling the spill valve and FIG.  2 ( d ) shows the current waveform, I B , through the winding of actuator  12 B controlling the nozzle valve. The continuous lines shown in FIGS.  2 ( a ) and  2 ( d ) represent the current which is sensed by the sensing resistor,  28 A or  28 B, whereas the dashed line portions  70 , 72  represent the actual current flowing through the actuator winding. 
     With reference to FIG.  2 ( a ), at time t1, the current, I S , through actuator  12 A increases at a high rate, during which period both the high and low side switches  24 A, 26 A are closed. When the peak current value, I SMAX , is reached the high side switch  24 A is opened and the current decays at a slower rate through the resistor  28 A, providing the low side switch  26 A remains closed. If the high side switch  24 A is closed again, the current, I S , will start to increase. If the low side switch  26 A is opened, and with the high side switch closed, recirculation occurs through the high side switch  24 A and diode  30 A, causing the current through the winding of the actuator to decay. Thus, by opening and closing the low side switch  26 A, the current, I S , through the actuator  12 A can be held about a mean holding value, as shown in FIG.  2 ( a ), with the current decaying when the low side switch  26 A is open and the current increasing when the low side switch  26 A is closed. Pulsing the low side switch  26 A in this way thereby serves to regulate the current through the winding of the actuator  12 A. The opening and closing of the low side switch  26 A will be referred to as the “chopping mode”. 
     In the chopping mode, the actuator  12 A is supplied with the mean holding current such that it actuates upon the spill valve to hold it in the desired position within the fuel injector  10 A. When it is required to de-actuate the spill valve, both the high side switch  24 A and the low side switch  26 A are opened to allow rapid decay of the current, I S , to zero. 
     The operation above has been described with reference to control of the actuator  12 A by means of switches  24 A and  26 A. Simultaneously with this, actuator  12 B for controlling the nozzle control valve of the fuel injector  10 A is also controlled by high side switch  24 B and low side switch  26 D. FIG.  2 ( d ) shows the current waveform, I N , in the actuator  12 B controlling the nozzle control valve of fuel injector  10 A. The current increase is initiated at time t 3  by closing the low side switch  26 D (the high side switch  24 A already being closed). As described previously for actuator  12 A, the current is increased to a maximum value at which time the low side switch  26 D is opened so that the current recirculates back through the high side switch  24 A and diode  30 D. Following this, the current can be maintained at a mean holding value, using the chopping mode as described above, by opening and closing the low side switch  26 D. 
     It is an important feature of fuel injector operation to be able to determine precisely when valve closure has occurred. This is done by means of current glitch detection i.e. detecting the glitch in the current through the actuator winding at the instant valve closure occurs. In order to perform current glitch detection, it is essential to be able to monitor the current through the sensing resistor  28 A during current recirculation. In the drive circuit shown in FIG. 1, recirculation occurs through the closed high side switch  24 A and diode  30 A when the low side switch  26 A is open, and it is not therefore possible to sense the current, I S , during this recirculation time as the current, I S , is not then passing through the sensing resistor  28 A. It is therefore necessary to employ a control scheme whereby the high side switch  24 A is opened and the low side switch  26 A is closed, for a fixed or predetermined period, to enable current sensing in the resistor  28 A. 
     This is done by means of glitch window pulse, GW 1 , as shown in FIG.  2 ( b ) which is used to open and close the high and low side switches  24 A, 26 A. The glitch window pulse GW 1  is provided by the ECU as for the other switch control pulses. By opening the high side switch  24 A and closing the low side switch  26 A when the current, I S , through actuator  12 A reaches the maximum value, recirculation is forced through the sensing resistor  28 A, thereby enabling the current to be sensed. During this recirculation period, current glitch detection can therefore be performed as the current is flowing through the sensing resistor  28 A, the current decaying at a low rate. 
     As for actuator  12 A control, glitch window GW 2  can be used to open the high side switch  24 A and to close the low side switch  26 D to enable the recirculating current associated with actuator  12 B to be sensed by means of resistor  28 B. Thus, current glitch detection can be performed to determine when closure of the nozzle control valve occurs, at which time the chopping mode is initiated to provide the mean holding current required to hold the nozzle valve in the required position. 
     An advantage of the drive circuit of the present invention is that, because the low side switch  26 D is not on the same low side switch bank as switch  26 A, the current through actuator  12 B can be sensed independently from the current through corresponding actuator  12 A as the recirculating current in each case is sensed by a different resistor  28 A, 28 B. It will be appreciated that it is important to ensure that the current waveforms applied to the two actuators of a common fuel injector (e.g. actuators  12 A and  12 B) are chosen such that glitch windows can be introduced at the desired time on one of the actuators without substantially affecting the operation of the other. 
     In addition to the glitch window pulses, GW 1  and GW 2 , an additional pulse, T RMAX , may be used, as shown in FIG.  2 B. The pulse, T RMAX , is initiated immediately before glitch window pulse GW 2  is applied and serves to maintain the high side switch  24 A in the closed position and the low side switch  26 A in the closed position for a fractionally longer period than when in regular chopping mode, such that the current I S  increases above the value of I SMAX1  to the slightly higher value I SMAX2 . In this way, the decay of current I S  is prevented from decaying to too low a value during glitch window pulse GW 2 . 
     Control of each of the actuators of the five other fuel injectors,  10 B- 10 F, can be implemented in the same way as described above for actuators  12 A and  12 B of fuel injector  10 A. It will be appreciated that for any particular actuator, a relatively fast current decay occurs in recirculation mode when the corresponding low side switch and the corresponding high side switch are both open. If, at this time, the other low side switch associated with the same high side switch remains closed, the current decay in the actuator corresponding to this other low side switch will decay slowly. 
     The drive circuit enables independent current sensing in each of the actuators of a dual-valve fuel injector, while requiring only marginally more complex sensing circuitry than is required for a single-valve fuel injector. It is possible to interchange the high side switches  24 A, 24 B with the low side switches  26 A- 26 F such that the current sensing resistor  28 A, 28 B are located in connection with the high side switches  24 A, 24 A. However, this does have the undesirable effect of there being a high, common voltage across the resistors  28 A, 28 B. 
     FIG. 3 shows a modification of the control circuit shown in FIG. 1 which has an improved level of fault tolerance. In the circuit shown in FIG. 3, series blocking diodes  40 A, 40 B,  42 A, 42 B,  44 A, 44 B,  46 A, 46 B, 48 A, 48 B,  50 A, 50 B are included in the path between the actuators  12 A, 12 B- 22 A- 22 B and their respective low side switches  26 A- 26 F. In the event that a short circuit to ground occurs in one of the fuel injectors  10 A- 10 B, the diodes  40 A, 40 B- 50 A, 50 B ensure that no current can flow back through the shorted connection. Thus, the three fuel injectors of the other fuel injector bank,  10 - 10 C or  10 D- 10 F, can still be kept running, providing a so-called“limp home capability”. The diodes also provide the advantage that any small currents which may undesirably flow through a unpowered actuator during the time when a powered actuator is in recirculation mode, are blocked. 
     Preferably, the diodes  40 A, 40 B- 50 A, 50 B should be arranged close to or inside the ECU providing the drive circuit control pulses to protect against the fuel injector return wires shorting to ground. Locating the diodes  40 A, 40 B- 50 A, 50 B within the ECU does, however, require six additional connectors on the ECU. Alternatively, the diodes may be located with the ECU connector cover, although may give rise to manufacturing difficulties. 
     The actuators of the fuel injectors  10 A- 10 F shown in FIGS. 1 and 3 are powered by means of a DC supply, V 1 , typically supplying 50V. Conventionally, however, in fuel injector assemblies comprising a number of single-valve fuel injectors, a high voltage regulated supply can be used to power the actuators by means of a battery voltage, such as a 12V car battery, in combination with a booster converter and by storing energy in a tank capacitor which is subsequently used to power the actuators. Such a scheme may also be used in conjunction with the present invention. However, the drive circuit of the present invention requires more power than an assembly of single-valve fuel injectors due to the increased number of actuators in the assembly. This requires the use of a booster converter of increased size. 
     Alternatively, with reference to FIG. 4, mixed-voltage operation may be employed whereby a battery voltage, such as a 12V car battery voltage, is used to supply the chopping mode current and a higher voltage is used to generate the fast current rise e.g. after time t 1  in FIG.  2 ( a ). As in FIGS. 1 and 3, the control circuit shown in FIG. 4 comprises two high side switches  24 A and  24 B connected to the cathode of a blocking diode  32 A and  32 B respectively, the anode of each blocking diode being connected to ground. High side switch  24 B is connected to a further switch  60 , referred to as a high boost switch, connecting the high side switch  24 B to a high voltage supply, V 2 , typically 50V. The high boost switch  60  is connected to ground via a tank capacitor  62 . A battery  66 , such as a 12V car battery, also supplies power to the high side switches  24 A, 24 B (i.e. in place of voltage supply V 1  in FIGS. 1 and 3) through a diode  64  to prevent current flow back to the battery  66 . 
     When the high boost switch  60  is closed, the high supply voltage supplies the current required for the fast current increase upon e.g. after time t1 in FIG.  2 ( a ). When the switch  60  is opened, the battery voltage  66  is available, via the diode  64 , to provide the chopping mode holding current. Thus, by controlling the opening and closing of the high boost switch  60 , either a higher supply voltage (V 2 ) or a lower voltage (battery  66 ) can be supplied to the selected actuator. 
     For a selected fuel injector, a relatively fast current decay occurs in both actuators when both the corresponding low side switches and the corresponding high side switch are open. The inductive energy stored in the actuators is returned to the high voltage supply V 2  via the“parasitic” diode between the source and the drain of the high boost switch  60 . If one of the low side switches associated with the same high side switch remains closed, the current will decay more slowly in the actuator and the current in the other actuator of the fuel injector will decay at the faster rate, returning its energy to the high voltage supply V 3 . 
     It will be appreciated that although mixed-voltage operation, provided by DC supply V 3  and battery  66 , is shown in FIG. 4 in combination with the use of series blocking diodes  40 A, 40 B- 50 A, 50 B, mixed voltage operation may also be employed in embodiments of the invention which do not include these diodes. 
     The fuel injector assemblies shown in FIGS. 1,  3  and  4  may be employed in an internal combustion engine having six cylinders, one fuel injector  10 A- 10 F being associated with a different one of the six cylinders and each fuel injector having two valves. It will be appreciated, however, that the control circuit may be adapted to control an increased or reduced number of dual-valve fuel injectors.