Abstract:
The present device provides a fuel injector including a two-input, self-triggering monostable driver circuit for an electromechanical valve is disclosed. Upon receipt of an initiation signal, the driver circuit generates a predetermined current profile in a coil. A current sensing mechanism senses the current flowing through the coil, enabling a current threshold generator to detect a preset peak current threshold state. Upon detection of the preset peak current threshold state, the current threshold generator establishes a new hold current threshold state in the coil. A rapid decay generator forces a rapid transition from peak current to hold current within the coil. The hold current is maintained within preset hysteretic limits until the valve cycle is terminated by removing the initiation signal from the inputs.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/105,242 filed Oct. 22, 1998, which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to electromechanical valve control circuitry in general and particularly to driving circuitry for controlling single-coil fuel injectors. More particularly, this invention relates to a driver circuit and method for driving single-coil fuel injectors with a two-pin input connector. 
     BACKGROUND OF THE INVENTION 
     Minimizing heat generation is important for fuel injectors that inject high vapor pressure fuels such as, for example, Liquid Propane Gas (LPG). Excessive heat generation within LPG fuel injectors often results in vapor lock because LPG is highly volatile and can boil at relatively low temperatures. Vapor lock is an undesirable condition wherein fuel changes from a normally liquid state to a gaseous state within the fuel ring or injector body, obstructing the flow of liquid fuel and adversely affecting fuel metering and engine performance. 
     Dual-coil fuel injectors are known to reduce heat generation. However, dual-coil injectors require three connections to the Engine Control Unit (ECU), a common center tap and two control connections, in contrast, only two connections to the ECU are required for single-coil injectors. The dual-coil third connection results in increased cost and complexity owing to the need to precisely align the electrical leads from the coils through sealing o-rings to connector terminals during the assembly process. In addition, the dual-coil third connection necessitates additional wire and connector hardware in the injector harness, resulting in increased cost and complexity over single-coil designs. 
     Single-coil fuel injectors solve many of the manufacturing and hardware cost problems described above. However, prior single-coil injector driver circuits have been known to dissipate excessive energy in the form of heat within the injector coil, making them unsuitable for LPG applications due to the risk of vapor lock. Thus, a need exists for a highly efficient, self-triggered monostable single-coil fuel injector driver circuit suitable for use with LPG systems that delivers the performance normally associated with peak and hold drivers while actually being interfaced to an ECU that only provides a saturated transistor driver. Further, a need exists for a single-coil fuel injector driver circuit capable of being mounted within a single-coil LPG injector housing without causing excessive heat build up within the injector. 
     SUMMARY OF THE INVENTION 
     The present invention provides a fuel metering device actuated by an electromagnetic assembly. The electromagnetic assembly includes a coil and armature. A housing cinctures the fuel metering device. An electrical connector is disposed on the housing, the electrical connector includes two pins that are exposed to the exterior of the fuel injector. A self-triggering driver circuit is disposed within the housing. The driver circuit has two inputs, each operatively connected to one of the connector pins. The driver circuit is configured to generate a predetermined current profile in the coil upon an initiation signal. The initiation signal is created by generating an electrical potential between the connector pins. In a preferred embodiment, the fuel injector housing includes an over-molded member. In a preferred embodiment, the fuel injector is a liquid propane fuel injector. 
     In a preferred embodiment, the self-triggering driver circuit has a current source that generates a current in a electromechanical valve coil. A current sensor senses the current flowing through the coil. A current threshold generator, having a peak current threshold state, corresponding to a peak current value generated by the current source, and a hold current threshold state, corresponding to a hold current value generated by the current source, transitions from the peak current threshold state to the hold current threshold state when the current sensor senses a predetermined peak current flowing through the coil. A current regulator regulates the output of the current source according to the state of the current threshold generator. A rapid decay generator is activated upon transition of the current threshold generator from the peak current threshold state to the hold current threshold state. The rapid decay generator causes rapid current decay through the coil from the peak current value to the hold current value, thus minimizing the energy dissipated in the form of heat within the coil. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate presently preferred embodiments of the invention, and, together with the general description given above and the detailed description given below, serve to explain features of the invention. 
     FIG. 1 is a functional block diagram of a electromechanical valve driver circuit embodying the principles of the present invention. 
     FIG. 2 illustrates a preferred embodiment of an electromechanical valve driver circuit according to the present invention. 
     FIG. 3 is an oscilloscope trace illustrating a output current waveform according to the present invention. 
     FIG. 4 is an oscilloscope trace illustrating a build-to-peak and peak-to-hold transition waveform according to the present invention. 
     FIG. 5 is an oscilloscope trace illustrating a hold regulation and hold-to-off transition waveform according to the present invention. 
     FIG. 6 illustrates a preferred packaging scheme for an embodiment of the invention enclosed within a fuel injector housing having two input pins mounted within an electrical connector. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a functional block diagram illustrating the operation of the electromechanical valve driver circuit according to a preferred embodiment. The self-triggering driver circuit is preferably housed within a fuel injector having a connector with only two pins, as described below in reference to FIG. 6. A current source  152  feeding electromechanical valve coil  154  is controlled by a current regulator  168 . A current threshold generator  192  establishes a peak current threshold for current regulator  168 . Upon reaching a preset peak current threshold, the current threshold generator  192  establishes a hold current threshold while a rapid decay generator  132  simultaneously forces the current in the coil  154  to rapidly decay from the peak threshold value to the hold threshold value. The current regulator  168  maintains the current in the injector at the hold threshold value, within preset hysteretic limits, for the duration of the electromechanical valve cycle. 
     FIG. 2 illustrates a preferred embodiment of the electromechanical valve driver circuit diagrammed in FIG. 1 in a Liquid Propane Gas (LPG) fuel injector application. Battery voltage Vcc, preferably about 12 volts, is provided to a first circuit input  370  located on a first end of regulator resistor  78 , which is preferably 1K. The second end of resistor  78  is connected to the cathode of zener regulator  80 , which is preferably 5.1 volts and 1 watt. The anode of zener regulator  80  is connected to a second circuit input  380  located on floating ground terminal ECU. The anode of electrolytic capacitor  82 , which is preferably 1 uF, is connected to the cathode of zener regulator  80 . The cathode of capacitor  82  is connected to ECU. 
     The main current string has current source transistor  52 , which is a preferably PNP type, having emitter  52   e  connected to battery voltage Vcc and collector  52   c  connected to a first terminal of the injector coil  54  and to the cathode of recirculation diode  44 . The anode of recirculation diode  44  is connected to ECU. The drain  56   d  of current sink transistor  56 , which is preferably a MOSFET, is connected to a second terminal of the injector coil  54  and to the cathode of zener diode  42 . The source  56   s  of transistor  56  is connected to a first end of sense resistor  58 , which is preferably 0.5 ohm, 1%, and to the anode of zener diode  42 . As will be appreciated by one skilled in the art, the drain and source connections of transistor  56  may be reversed. The second end of sense resistor  58  is connected to ECU. The gate  56 g of transistor  56  is connected to a first end of bias resistor  60 , which is preferably 27K, and to a first end of bias resistor  62 , which is preferably 10K. The second end of resistor  60  is connected to ECU. The second end of resistor  62  is connected to Vcc. 
     The base  52   b  of transistor  52  is connected to a first end of bias resistor  64 , which is preferably 680 ohms, and to a first end of bias resistor  66 , which is preferably 100 ohms, 3 watts. The second end of resistor  64  is connected to Vcc. The second end of resistor  66  is connected to the drain  68   d  of current regulator transistor  68 , which is preferably a JFET. The source  68   s  of transistor  68  is connected to ECU. The gate  68   g  of transistor  68  is connected to a first end of resistor  70 , which is preferably 1K. The second end of resistor  70  is connected to the output of threshold regulating comparator  72 . 
     The non-inverting input  72 + of comparator  72  is connected to a first end of resistor  74 , which is preferably 4.75K, 1%, and to a first end of resistor  76 , which is preferably 9.09K, 1%. The second end of resistor  74  is connected to ECU. The second end of resistor  76  is connected to the cathode of zener regulator  80 . A first end of positive feedback resistor  88 , which is preferably 43.2K, is connected to non-inverting input  72 + and the second end of resistor  88  is connected to the output of comparator  72 . A first end of resistor  90  is connected to the output of comparator  72  and the second end of resistor  90  is connected to the cathode of zener regulator  80 . The inverting input  72 − of comparator  72  is connected to a first end of capacitor  84 , which is preferably 10 nF. The second end of capacitor  84  is connected to ECU. 
     The output of comparator  92  is connected to a first end of resistor  94 , which is preferably 1K, 1%, and a first end of capacitor  30 , which is preferably 22 nF. The second end of resistor  94  is connected to non-inverting input  72 + of comparator  72 . The non-inverting input  92 + of comparator  92  is connected to a first end of resistor  96 , which is preferably 16K, and a first end of resistor  98 , which is preferably 4.7K. The second end of resistor  96  is connected to the non-inverting input  72 + of comparator  72 . The second end of resistor  98  is connected to the output of comparator  92 . The inverting input  92 − of comparator  92  is connected to the first end of capacitor  84 . 
     The second end of capacitor  30  is connected to the non-inverting input  32 + of comparator  32  and to a first end of resistor  34 , which is preferably 22K. The second end of resistor  34  is connected to ECU. The cathode of zener regulator  80  is connected to a first end of resistor  36 , which is preferably 33K, and to the cathode of diode  38 . The second end of resistor  36  is connected to the non-inverting input  32 + of comparator  32 . The anode of diode  38  is connected to the output of comparator  32 . A first end of resistor  40 , which is preferably 1K, is connected to the first end of capacitor  84 . The second end of resistor  40  is connected to the first end of sense resistor  58 . 
     Referring now to FIG. 3, the circuit described above generates substantially current profile  200  through injector coil  54  for the duration of injector driver pulse  100 . The circuit is activated by an initiation signal that is created by placing a potential across the first and second circuit inputs,  370  and  380 , respectively. The initiation signal may be created, for example, when injector driver pulse  100 , generated within an engine control unit, causes terminal ECU  380  to become effectively grounded. Grounding of terminal ECU may be accomplished by any desired method, for example by saturation of an open collector switch within the engine control unit. 
     Upon effectively grounding terminal ECU, a positive voltage is applied to the gate  56   g  of current sink MOSFET transistor  56  by the voltage divider formed by resistors  60  and  62 , forcing MOSFET  56  to conduct. Current supplied by current source PNP transistor  52  builds on the L/R time constant in the injector coil  54  and passes through current sense resistor  58 . The voltage across current sense resistor  58  grows linearly in relation to the current flowing through the injector coil  54 , in accordance with Ohm&#39;s Law. 
     A current regulator, which preferably includes JFET transistor  68  and comparator  72 , supplies base current to PNP transistor  52  whenever the threshold voltage present at non-inverting input  72 + of comparator  72  exceeds the voltage across sense resistor  58 . During the current incline period  210  in FIG. 3, the threshold voltage present at non-inverting input  72 + of comparator  72  is established primarily by the voltage divider formed by resistors  74  and  76 . 
     A current threshold generator, which preferably includes comparator  92 , resistor  94 , and voltage divider formed by resistors  74  and  76 , shunts the first end of 1K resistor  94  to ground when the voltage across the sense resistor  58  reaches a level corresponding to the desired peak current  220  in FIG.  3  through injector coil  54 . This effectively places resistor  94  in parallel with resistor  74 , and establishes a new lower threshold voltage at the non-inverting input  72 + of comparator  72 , corresponding to the hold current level  240  in FIG.  3 . As a consequence, the voltage at the inverting input  72 − of comparator  72  will exceed the voltage at the non-inverting input  72 +, forcing the gate  68   g  of JFET  68  to substantially ECU (ground) potential and removing the base current from current source PNP transistor  52 , thereby making it non-conducting. 
     Simultaneously, a rapid decay generator, which preferably includes capacitor  30 , comparator  32 , zener diode  42 , and MOSFET  56 , triggers a one-shot timing pulse. The one-shot causes the voltage at inverting input  32 − to exceed the voltage at non-inverting input  32 +, and forces MOSFET  56  into a non-conducting state and isolates injector coil  54 . Zener diode  42  clamps the injector coil/MOSFET drain junction at approximately  38  volts. Recirculation diode  44  clamps the PNP collector  52   c  to within about 0.7 volts of ECU ground, thereby protecting PNP transistor  56  by ensuring that the Vce breakdown voltage is not exceeded. 
     The large zener voltage across the injector coil induces a large rate of change in the coil current. The rate of change of current through the injector coil  54  is governed by the equation di/dt=−V/L, where i is the current through the coil; di/dt is the instantaneous rate of change of the current through the coil; V is the voltage across the coil; and L is a constant representing the inductance of the coil. It can readily be seen from the above equation that increasing the voltage, V, will increase the rate of current decay, di/dt, through the injector coil  54 . FIGS. 3 and 4 illustrate the rapid change  230  from the peak current threshold  220  to the hold current threshold  240 . 
     Hysteresis in the hold current threshold state is governed by a dither control, which preferably includes positive feedback resistor  88  and comparator  72 , together forming a Schmitt trigger. For the remainder of the injector pulse, dithering around the hold threshold current under control of the sense resistor  58  and recirculation diode  44  regulates the injector coil current. At the termination of the injection pulse, the saturated switch driver of the ECU removes terminal ECU from ground. This results in both PNP transistor  52  and MOSFET transistor  56  becoming non-conducting, causing the injector coil current to decay rapidly to zero  250  via zener diode  42  and recirculation diode  44 . 
     FIG. 6 illustrates a preferred embodiment wherein the driver circuit  300  is mounted within a over-mold housing  310  of a fuel injector  320 . The driver circuit  300  may be mounted to the injector  320  on fasteners  330 . Over-mold housing  310  provides a hermetic seal against moisture and includes electrical connector  340  having connector pins  350  and  360  electrically connected to circuit inputs  370  and  380 , respectively. Electrical connector  340  provides access for a wiring harness connector. 
     While the present invention has been disclosed with reference to certain preferred embodiments, numerous modifications, alterations, and changes to the described embodiments are possible without departing from the sphere and scope of the present invention, as defined in the appended claims. Accordingly, it is intended that the present invention not be limited to the described embodiments, but have the full scope defined by the language of the following claims, and equivalents thereof