Abstract:
A very small fixed current is passed through the coil of a solenoid during use thereof when the solenoid is in its “off” state of cyclic PWM command, the fixed current being less than the current required to actuate the solenoid to the “on” position. While the current is flowing through the coil, the voltage across the coil is measured. The resistance of the coil is calculated according to Ohm&#39;s Law. When the actual resistance is known, the duty cycle imposed on the solenoid may be altered to compensate for the resistance deviation from nominal. This allows, for example, fuel injectors for internal combustion engines to deliver the correct amount of fuel under all operating conditions.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to solenoid actuators; more particularly, to means for detecting changes in resistance of solenoid windings; and most particularly, to apparatus and method for continuously determining the resistance of a solenoid winding during use and for correcting for changes in such resistance.  
       BACKGROUND OF THE INVENTION  
       [0002]     Solenoid actuators are well known. In a typical solenoid actuator, an electromagnet includes a wound wire coil surrounding primary and secondary pole pieces. When a current is passed through the windings, an axial magnetic field is generated. A ferromagnetic armature movably disposed in the axial field is urged axially of the windings. The strength of the field, and thus of the actuator, is dependent upon the current at any given voltage. As is well known in the art, the current is inversely proportional to the resistance of the windings. Resistance of the coil can vary with the temperature of the coil. Resistance of the coil can also diminish progressively with length of use.  
         [0003]     It is known to use solenoids to actuate fuel injectors for, among others, internal combustion engines and hydrocarbon reformers. Such a fuel injector typically is controlled in known fashion by pulse width modulation (PWM) control of the solenoid actuator; that is, the injector is fully open for a desired fraction of the time of a full injection cycle. Because the resistance and thus the action of a solenoid varies with temperature, it is further known to apply a correction factor to the PWM control, based on coil temperature. For example, if the nominal resistance of a coil is 12 ohms (Ω), but due to ambient temperature and run time the coil resistance is actually 10.5 Ω, then fuel flow through the injector would be greater than desired. At the lesser resistance, current flow is increased and this in turn decreases the opening time and increases the closing time of the fuel injector, effectively increasing total flow in each cycle. Such errors in flow are of special concern for fuel injector operation in open loop during cold engine starts since accurate fuel dispensing is essential to reducing cold-start engine exhaust emissions.  
         [0004]     Currently, some known engine management systems (EMS) predict or infer coil temperature from other operating parameters, such as coil duty cycle, voltage, ambient temperature, operation time, and engine temperature, and then alter the pulse bandwidth to compensate for inferred resistance variation from nominal. These approaches share a common shortcoming in relying on inference and not measuring the actual resistance of a solenoid&#39;s coil.  
         [0005]     What is needed is a means for measuring directly the resistance of a solenoid coil during use thereof, and for adjusting the energizing of the solenoid to compensate for coil resistance deviations from nominal.  
         [0006]     It is a principal object of the present invention to provide improved control of a duty cycle of a solenoid.  
         [0007]     It is a further object of the invention to improve fuel efficiency of an internal combustion engine.  
         [0008]     It is a still further object of the invention to reduce unburned hydrocarbon emissions in engine exhaust.  
       SUMMARY OF THE INVENTION  
       [0009]     Briefly described, a very small fixed current is passed through the windings of a solenoid during use thereof when the solenoid is in its “off” state of PWM command. The fixed current is lower than the current required to actuate the solenoid to the “on” position. While the small fixed current is flowing through the solenoid, the voltage across the coil is measured. Based on Ohm&#39;s law, the resistance of the coil is equal to the measured voltage divided by the fixed current (R=V/I). Because the current is being controlled and is constant, the actual resistance is directly proportional to the voltage measured. When the actual resistance is known, the duty cycle imposed on the solenoid may be altered to compensate for the resistance deviation from nominal. This allows, for example, fuel injectors for internal combustion engines to deliver the correct amount of fuel under all operating conditions, including variations in temperature and changing conditions due to length of use. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0011]      FIG. 1  is an elevational cross-sectional view of a prior art solenoid-actuated fuel injector; and  
         [0012]      FIG. 2  is a schematic diagram of an electronic control circuit for determining the actual resistance of a solenoid coil during a duty cycle thereof. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]     Referring first to  FIG. 1  of the drawings in detail, numeral  10  generally indicates a representative solenoid actuated fuel injector, the performance of which can benefit from control circuitry in accordance with the invention. Injector  10  is substantially as disclosed in U.S. Pat. No. 6,264,112, the relevant disclosure of which is incorporated herein by reference.  
         [0014]     Injector  10  includes a continuous endoskeletal injector tube  12  which is centered on a central axis  14  and encloses a continuous passage  15  through the injector from an inlet end  16  of the tube to an outlet end  18 . Preferably, the tube  12  has no openings except at the inlet and outlet ends and defines a continuous imperforate passage in which fuel is conducted and kept separate from all the components of the injector that are mounted externally of the tube. These include a separately formed coil assembly including a solenoid coil  22  extending around and closely adjacent to the tube but isolated thereby from the fuel in the tube. A magnetic coil body  24  surrounds the coil  22  and has upper and lower ends  26 ,  28  fixed to the outer surface of the tube.  
         [0015]     A support element  30  is formed as a tubular member that slides over the tube and engages the body  24  surrounding an upper portion thereof. The support element includes a slot  32  for receiving a retainer clip, not shown, that holds the injector inlet end within a cup, not shown, of an associated fuel rail. The support element also provides a backup surface  34  at one end for constraining a seal ring  36  of the conventional O-ring type. A push-on seal retainer  38  is frictionally or otherwise retained on the inlet end  16  of the injector tube  12  to form with the other parts an annular groove in which the seal ring  36  is retained. A split spacer ring  46  extends around the lower end of the body  24  and engages an annular O-ring seal  48  which is retained, in part, by an expanded diameter portion  50  at the lower end of the injector tube  12 .  
         [0016]     Within the injector tube  12 , an inlet fuel filter  52  is provided at the inlet end of the tube. A tubular magnetic pole is fixed within the tube in engagement with its interior surface. The pole extends from adjacent the upper end  26  of the body  24  to a position within the axial extent of the coil  22 . An injection valve  56  is reciprocable within the tube  12  and includes a ball end  58  connected with a hollow armature  60  that slides within the tube. A biasing spring  62  engages the armature and an adjusting sleeve  64  fixed within the magnetic pole  54  to urge the injection valve downward toward a closed position.  
         [0017]     Within the expanded diameter portion  50  of the tube  12 , a valve seat  66  and a lower valve guide  68  are retained by crimped over portions of the tube outlet end  18 .  
         [0018]     The lower valve guide  68  is a disc positioned between the valve seat and a flange-like surface formed by the expanded diameter tube portion  50  to guide the ball end  58  of the injection valve. Lower valve guide  68  includes openings  70  to allow fuel flow through the guide  68  to a conical surface  72  of the valve seat against which the ball end  58  seats in the valve closed position. A central discharge opening  74  of the valve seat  66  connects the conical surface  72  with a circular recess  76  in which a multi-hole spray director  78  is press fitted or otherwise retained. An outer seal ring  80  is captured in a groove of the valve seat and prevents fuel from leaking around the valve seat and bypassing the discharge opening  74 .  
         [0019]     In operation, energizing of the coil  22  draws the armature  60  upward into engagement with the end of the magnetic pole  54 , moving the ball end  58  of valve  56  upward away from the conical surface  72  of the valve seat  66 . Fuel is then allowed to flow through the tube  12  and valve seat  66  and out through the director  78  directly or indirectly into an associated combustion chamber of an associated internal combustion engine  99 . Upon de-energization of the coil  22 , the magnetic field collapses and spring  62  seats the valve  56  on conical surface  72 , cutting off further fuel injection flow.  
         [0020]     Referring to  FIG. 2 , an injector resistance measurement circuit  100  in accordance with the invention is applicable to measurement of electrical resistance in solenoid coils generally and, for example, coil  22  in fuel injector  10  during operation thereof. Circuit  100  outputs a voltage value  112  corresponding to the resistance of coil  22  by controlling the current through the injector coil and measuring the voltage across the injector coil while the injector is normally commanded “off”. Controlling the current through the injector makes the denominator of the resistance calculation R=V/I a “constant value”. Thus, with a constant current, the resistance of the injector is directly proportional to the voltage measured across it. Note that the current imposed for resistance measurement during the coil “off” state, for example, about 50 mA, must be much less than the current required for energizing the solenoid to the “on” state, such that the solenoid armature does not move during resistance measurement.  
         [0021]     The injector is turned on and off by an injector driver Q 2 . The injector driver may be external to the current measurement circuit. The design of circuit  100  allows the circuit to be applied to existing injector driver systems.  
         [0022]     The injector is “on” when the injector logic signal  104  is high, and the injector is “off” when the injector logic signal is low. Injector driver Q 2  acts as a switch to complete the circuit to apply the full power available to the injector. Current through the injector is controlled by section  106  of the circuit that consists of the following components: operational amplifier U 1 C, resistors R 5 , R 6 , R 7 , capacitors C 1 , C 5 , and Q 1 . Preferably, Q 1  and Q 2  are logic level driven power Field Effect Transistors (FETs). R 6  acts as a “current sense” resistor with a value of about 50 ohms. The voltage measured across R 6  is directly proportional to the current through it, and is the feedback signal to the current control circuit. R 6  is part of a series circuit that includes the injector, Q 1 , and R 6 , so the current through R 6  is the same as the current through the injector (the current through each component in a series circuit is the same). Q 1  acts as a current control device to the series circuit (Injector, Q 1 , and R 6 ). Q 1  is controlled by operational amplifier U 1 C. U 1 C is configured to control the current through R 6  such that the voltage across R 6  results in a voltage at the negative input of U 1 C equal to the voltage at the positive input of U 1 C. The output of U 1 C will change in order to keep the voltages at the two inputs of U 1 C the same. The voltage at the positive input of U 1 C is set to a constant value of about 2.5V with a voltage divider circuit made by R 5  and R 7 . R 7  is adjustable for precise setting of the current control circuit. R 5  and R 7  divide the voltage provided from a regulated 5V supply (which is described later) to keep it at a constant level as the injector supply voltage varies.  
         [0023]     If the voltage across R 6  is controlled to a constant 2.5 volts, then the current through R 6  is being controlled to a constant 50 mA.  
         [0024]     C 1  is in the current control schematic to keep high frequency noise on the inputs from affecting the output. (C 1  is not critical to the circuit operation.) C 5  acts as a decoupling capacitor for the voltage at the low side of the injector, which helps keep the voltage from “ringing” or oscillating as the current through the injector is effectively being controlled. When injector driver Q 2  is turned on to complete the injector drive circuit, R 6  is effectively shorted out, forcing the voltage across R 6  to be near zero. When this happens, the output of U 1 C turns fully on to try to compensate for the low reading on its negative input. This turns Q 1  on fully which completes the injector driver circuit to turn the injector on as intended.  
         [0025]     A “snubber circuit”  108  comprising diodes D 1  and D 2  is necessary to provide a ethod to quickly dissipate the stored energy in injector coil  22  when injector driver  102  is turned off. When the injector driver circuit is “opened” by turning off driver  102 , the injector coil  22  becomes a current source. The current sourced from the injector causes the voltage at the drain of Q 1  to become potentially very large. Zener Diode D 2  conducts reverse current when the voltage at its cathode reaches the zener voltage relative to its anode. When this current flows, snubber circuit  108  is considered “active”. The voltage at the drain of Q 1  is thus limited to the sum of the zener voltage of D 2 , the forward voltage drop of D 1 , and the supply voltage “IGN”, which may be, for example, 75V+0.65V+13.5V, or 89.15 Volts. This high voltage allows a fast dissipation of the energy stored in the injector, which allows it to turn off very quickly.  
         [0026]     In injector voltage measurement circuit  110 , operational amplifier U 1 B is configured as a differential amplifier (along with resistors R 1 , R 2 , R 3 , and R 4 ) to measure the voltage across the injector coil  22 . Resistors R 1 , R 2 , R 3 , and R 4  are selected to provide a fixed gain of around 2 to the amplifier circuit. Preferably, R 3  has a power rating above 0.125 watt, to dissipate more power when the snubber circuit is active. The output of U 1 B is about two times the voltage across the injector coil. The output of U 1 B cannot exceed the supply voltage, so when the injector is “on”, the output is nearly the same as the supply voltage. Zener diode D 3  serves to protect the negative input of U 1 B during the time when the injector turns off, and snubber circuit  108  is active. Zener diode D 3  limits the voltage to the zener voltage of the diode during the transient.  
         [0027]     The injector voltage output  112  from U 1 B is two times the voltage across the injector coil while the current through the injector is 50 mA. The output value at this output in volts is thus effectively one tenth of the resistance value R of injector coil  22  in ohms.  
         [0028]     An enable measurement circuit  114  is used to prevent readings when the injector coil is “on”. Circuit  114  comprises operational amplifier U 1 A configured as a comparator, with potentiometer resistor R 8  used to set the voltage being compared to. The output of U 1 A is high only when the voltage across R 6  is higher than the voltage at the negative input of U 1 A, which is set by the voltage divider circuit of potentiometer resistor R 8 . The voltage across R 6  is low when the injector coil is “on”, and thus, the enable measurement circuit output is low.  
         [0029]     C 2  is a decoupling capacitor for the supply voltage to the operational amplifier UlA. Preferably, U 1 A, U 1 B, and U 1 C are components of a single quad package high-speed precision operational amplifier integrated circuit.  
         [0030]     Note that the injector voltage output signal  112  from U 1 B is not necessarily representative of the injector resistance when the enable measurement signal is high. Further processing of the “injector voltage” and “enable measurement” signals is needed to prevent reading the injector resistance while the injector&#39;s stored energy is being dissipated. The injector voltage output reading  112  at the output of U 1 B can safely be considered representative of the injector resistance about 2 milliseconds after the enable measurement signal goes high.  
         [0031]     A voltage regulator section  116  is needed to provide a constant voltage source to be used for reference voltages in the current control, and the enable measurement portions of the circuit. Voltage regulator VR 1 , along with capacitors C 3  and C 4 , make up the voltage regulator circuit. The input is the injector supply voltage “IGN”, and the output is 5 Volts (as long as the injector supply voltage is in its normal operating range of between 8 and 16 Volts).  
         [0032]     Referring still to the example of a solenoid coil in a fuel injector, pulse-width controlled operation of injector  10  and control circuit  100  is controlled by a convention programmable controller or a computer (not shown) which may be an Engine Control Module (ECM) for an internal combustion engine  99  ( FIG. 1 ). The computer is provided with a desired fuel flow aim and is programmed with a conventional look-up table of base pulse width vs. coil resistance, preferably expressed as negative and positive pulse width trim factors. Such a table is readily determined in known fashion without undue experimentation.  
         [0033]     In operation, the computer monitors voltage  112 , and thus coil resistance, during “off” portions of the injector cycle. A base pulse width for the fuel injector is calculated, based on a nominal coil resistance and plurality of computer input parameters, for example, engine air flow, the injector gain rate, system voltage, and manifold absolute pressure. As coil resistance changes, the computer refers to the table to shorten or lengthen the pulse width to maintain the desired actual flow from the fuel injector. In the example discussed above, wherein the actual resistance is 10.5 Ω rather than the nominal resistance of 12 Ω, the value in the look-up table would be negative, reducing the pulse width of the device to compensate for the higher anticipated flow. Conversely, the look-up value would be positive if the coil resistance becomes higher than the nominal 12 Ω.  
         [0034]     While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.