Abstract:
A method of controlling exhaust gas recirculation by means of an EGR valve ( 55 ). An electromagnetic actuator ( 41 ) is associated with a housing ( 47 ) to transmit movement of an actuator output ( 75 ) into reciprocating movement of the EGR valve in response to changes in an electrical input signal ( 95 ), and the method comprises generating ( 93 ) a compensator gain value to modify the electrical input signal ( 95 ). The improved method provides a valve position sensor ( 79 ) and generates a position signal ( 97 ) representing instantaneous valve position. The next step is storing ( 105 ) a first relationship (DC THLD )of the electrical input signal ( 95 ) required to change the instantaneous valve position, then, during ongoing operation, generating ( 105 ) a then-current, second relationship (DC) of the electrical input signal ( 95 ) required to change the instantaneous valve position. Next is comparing ( 105 ) the second relationship (DC) to the first (DC THLD ) and generating ( 111 ) a corresponding difference factor, then using ( 111 ) that difference factor to modify ( 93 ) the compensator gain value correspondingly. This method enables the system to adapt to changes in system friction.

Description:
BACKGROUND OF THE DISCLOSURE  
         [0001]    The present invention relates to an exhaust gas recirculation (EGR) system for controlling the flow of exhaust gas from an exhaust gas manifold to an intake manifold of an internal combustion engine, and more particularly, to an improved method for controlling such an EGR system.  
           [0002]    Although the use of the present invention is not limited to any particular type or configuration of engine, its use is especially advantageous in connection with a heavy duty diesel engine, and the invention will be described in connection therewith. Furthermore, although the present invention may be utilized advantageously in connection with the control of various engine elements, such as electromagnetically-operated engine poppet valves on camless engines, and control rods for VGT (variable geometry turbine) systems, the invention is especially advantageous when utilized in connection with an EGR system, and will be described in connection therewith.  
           [0003]    EGR systems are utilized in automotive vehicles (i.e., including both passenger cars and trucks) in order to help reduce engine emissions, and are desirable especially on heavy duty diesel engines. Such EGR systems typically utilize an EGR poppet valve that is disposed between the engine exhaust manifold and the engine intake manifold. The EGR poppet valve is operable, when in an open position, to permit recirculation of exhaust gas from the exhaust manifold back into the intake manifold. As is well know to those skilled in the art, such recirculation of exhaust gasses is helpful in reducing various engine emissions. As is also well known to those skilled in the art, when the engine is operating under relatively heavy torque loads (such as while accelerating or shifting gears at low speeds), the EGR valve will typically be closed, or nearly closed, whereas, when the engine is operating under relatively lighter torque loads (such as at steady state engine speed, in a higher gear ratio), the EGR valve will typically be fully open, or almost fully open.  
           [0004]    An electromagnetic actuator is preferably employed for moving the EGR poppet valve between its open and closed positions, because the recirculation of exhaust gasses is appropriate and helpful only at certain times during the operation of the engine, in accordance with the previous discussion, and it is desirable to be able to change the position of the EGR poppet valve very quickly to adjust to varying vehicle and engine operating conditions. EGR valves of the type with which the present invention may be utilized are illustrated and described in U.S. Pat. Nos. 5,937,835 and 6,102,016, both of which are assigned to the assignee of the present invention and are incorporated herein by reference.  
           [0005]    Electrically actuated EGR valve systems preferably utilize software-implemented control logic, such that the EGR poppet valve is operating under closed loop control when the EGR poppet valve is being moved from a closed position to an open position, and when it is being moved from an open position to a closed position. As used herein, the term “closed loop” in regard to the control of the EGR poppet valve will be understood to mean that the control logic is constantly “reading” the position of the valve, and utilizing the resulting position signal as part of the feedback to the control logic. The closed loop control logic controls electric current to an electric motor which serves as the actuator to move the EGR poppet valve, and control the opening/closing position thereof. In such systems, the control logic typically generates pulse width modulated (PWM) signals to power the actuator motor, and modulate the movement of the EGR poppet valve, moving it from one position to another.  
           [0006]    As is also well know to those skilled in the art of position control using DC motors, it is not sufficient, when designing the control logic for an engine component such as an EGR poppet valve, to merely establish a baseline relationship of EGR poppet valve position as a function of control current, and thereafter assume that the position-versus-current relationship will remain constant (i.e., equal to the baseline relationship). For example, it is now well know to those skilled in the art of controlling electrically actuated devices to adjust the gain compensation within the control circuit as a function of the ambient temperature of the device being controlled. In the course of developing the commercial embodiment of an EGR poppet valve system of the type to which the present invention relates, the assignee of the present invention has taken into account the typical, well known system variables (e.g., fluctuations in system voltage, ambient temperature, etc.), and has built into the EGR system control logic the appropriate compensation for variations in such factors. However, it has been observed by the assignee of the present invention that there have still been aspects of the overall EGR system performance, on the developmental systems, which have not been fully acceptable.  
           [0007]    As a result of the development of the present invention, it has been observed by the inventor of the present invention that the performance of the EGR system can change substantially, over a relatively short period of time, especially when the EGR system is operating under conditions such that the EGR poppet valve remains open during a major portion of a given time period. It has now been determined that at least one likely cause of such changes in the performance of the EGR system relates to the system “friction”, and especially, the static friction (i.e., the friction when the system is not moving) which must be overcome to achieve initial movement of the poppet valve. The friction being referred to hereinabove would include that in the gear train or drive train between the electromagnetic actuator (motor) and the poppet valve, as well as that associated with the engagement of the poppet valve stem and the bore in which the stem reciprocates. In some EGR systems, there may also be seals, or other elements which provide a frictional “drag” which resists movement of the poppet valve.  
           [0008]    Unfortunately, it has now been determined that, not only does the system have to overcome the static friction in order to begin to move the EGR poppet valve, but also, the total amount of the static friction which must be overcome can change substantially. It is now believed that a major cause of the changing static friction is the exhaust gas soot, and the various other contaminants from the EGR gas (all of which are hereinafter, for simplicity, collectively referred to as “soot”), which build up at various locations, such as on the valve stem. If the EGR poppet valve remains open for an extended period of time, such as an hour, there may be enough of a build-up of soot to change the static friction of the system by 20 or 30 percent, or more, thus requiring substantially more electric power than usual to overcome the friction and achieve initial movement of the EGR poppet valve.  
           [0009]    However, as a further complication in attempting to compensate for the build-up of soot, and the resulting increase in the static coefficient of friction (“COF”), it is also known that during operation of the vehicle engine, the built-up soot can get burned-off, thus decreasing the static COF. In other words, the static COF goes up and down, as a function of the driving cycle. Furthermore, when the static COF is relatively high (because of soot as explained previously) and the difference between the static COF and the dynamic COF (i.e., when the system is moving) becomes fairly large, controlling accurately the movement of the EGR valve becomes even more difficult, as there is a tendency for the valve to “overshoot” its commanded position. This is true because a relatively higher current is needed to overcome the static friction, and get the valve moving, but then the current to the motor is excessive, once the valve begins to move, in view of the much lower dynamic COF. The overshoot problem typically means that it takes a longer time to get the EGR valve to the desired position, which may result in more exhaust gas being released than was intended. Also, in the event of overshoot of the EGR valve position, there can be unintentional engagements with mechanical stops which comprise part of the system, causing excessive wear and reducing the durability of the EGR assembly.  
         BRIEF SUMMARY OF THE INVENTION  
         [0010]    Accordingly, it is an object of the present invention to provide an improved control member system, and an improved method for controlling such a system, which achieves a greater consistency and predictability in the operating performance of the system.  
           [0011]    It is a more specific object of the present invention to provide such an improved method of controlling an EGR valve system which substantially eliminates one of the major sources of variation in overall system performance.  
           [0012]    It is another object of the present invention to provide an improved method of controlling such a system, which accomplishes the above-stated objects by compensating for variations in system friction over a period of time.  
           [0013]    The above and other objects of the invention are accomplished by the provision of an improved method of controlling the movement of an assembly in an internal combustion engine. The assembly includes a control member moveable between a closed position, blocking communication from a first engine gas passage to a second engine gas passage, and an open position. The assembly further includes housing means, the control member being disposed within the housing means for reciprocable movement therein. An electromagnetic actuator operably associated with the housing means has an actuator output. A drive train is operable to transmit movement of the actuator output into reciprocating movement of the control member in response to changes in an electrical input signal, the method of controlling the movement comprising the steps of generating a compensator gain value to modify the electrical input signal.  
           [0014]    The improved method of controlling the movement is characterized by providing a position sensor operable to sense a position of the control member and generate a position signal representing instantaneous control member position. The next step is storing a first relationship of the electrical input signal required to change the instantaneous control member position. During ongoing operation of the internal combustion engine, the next step is generating a then-current, second relationship of the electrical input signal required to change the instantaneous control member position. Next, the method compares the second relationship to the first relationship and generates a corresponding difference factor, and uses that factor to modify the compensator gain value correspondingly. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a schematic view of a diesel engine including an exhaust gas recirculation (EGR) system of the type with which the control method of the present invention may be utilized.  
         [0016]    [0016]FIG. 2 is a transverse cross section of the exhaust gas recirculation valve and control system, shown schematically in FIG. 1.  
         [0017]    [0017]FIG. 3 is a simplified logic control diagram of the type which would be utilized to control the EGR valve and control system shown in FIGS. 1 and 2.  
         [0018]    [0018]FIG. 4 is a “state” flow chart, illustrating the various states (conditions) of the control system of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]    Referring now to the drawings, which are not intended to limit the invention, FIG. 1 is a schematic of a vehicle internal combustion engine, and more specifically, of a heavy duty diesel engine. As is shown schematically in FIG. 1, the diesel engine includes an engine block  11 , an intake manifold  13 , and an exhaust manifold  15 . Disposed forwardly of the engine block  11  is an engine radiator  17 , by means of which engine coolant flowing through the engine block  11  may be cooled. As is well know to those skilled in the art, the radiator  17  would typically be connected to the engine block  11  by means of a pair of hoses or conduits  19  and  21 .  
         [0020]    Associated with the exhaust manifold  15  is an EGR valve assembly, generally designated  23 . The assembly  23  includes an EGR valve portion  25 , an EGR valve actuator portion  27 , and an actuator electronic control portion  29 . Associated with the engine block  11  is an EGR cooler  31 , the function of which is to cool the relatively hot exhaust gasses which are communicated from the EGR valve assembly  23  to the intake manifold  13 . In order to accomplish this cooling of the exhaust gasses, the EGR valve portion  25  is connected by means of a duct or pipe  33  to the cooler  31 , and exhaust gasses passing through the cooler  31  then flow through a duct or pipe  35  to the intake manifold  13 , the details of which are not essential to the present invention and which, therefore, will not be described further herein.  
         [0021]    The vehicle includes a battery  37  which is connected by means of a pair of electrical leads  39  to the actuator electronic control portion  29 , thus providing the electrical power for an electric motor  41 , which comprises part of the EGR valve actuator portion  27 . It should be understood that the present invention is not limited to any particular type or configuration of electric motor, for reasons which will become apparent subsequently, and within the scope of the present invention, various other forms of an electromagnetic actuator could be utilized. The vehicle is also provided with a fairly conventional engine control module (ECM), generally designated  43 .  
         [0022]    The ECM  43  receives input from the electronic control portion  29  (such as a signal representative of instantaneous EGR valve position), and provides appropriate command signals to the electronic control portion  29  (such as a PWM signal representative of the desired EGR valve position) by means of a data link  45 . Although FIG. 1 schematically illustrates the electronic control portion  29  and the ECM  43  as separate components/sub-systems, it should be apparent to those skilled in the vehicle electronic control art that the portion  29  would likely be included within the ECM  43 . Hereinafter, the command signal from the ECM  43  is also referred to by the designation “45”. The data link  45  is also used to send/receive information for diagnostic purposes, for example, to comply with various OBD (on-board-diagnostics) regulations.  
         [0023]    Referring now primarily to FIG. 2, the EGR valve assembly  23  is shown in some detail. The assembly  23  includes a manifold mounting portion  47 , a heat transfer (cooling) portion  49 , and the valve actuator portion  27 . The manifold mounting portion  47  defines a flow passage  51 , and at the upstream end thereof, the portion  47  and the flow passage  51  are connected to the exhaust manifold  15  (shown schematically in FIG. 2). At the downstream end of the flow passage  51  the manifold mounting portion  47  is connected to the duct  33 , such that the exhaust gases may eventually flow to the intake manifold  13 .  
         [0024]    The manifold mounting portion  47  also defines a bore  53  within which an EGR valve, generally designated  55 , is reciprocally supported for axial movement therein. The EGR valve  55  includes a valve stem  57  that is integrally formed with a poppet valve portion  59 , and an input stem portion  61  that is coupled to the valve stem  57  by any suitable coupling means, such that the input stem portion  61  and the valve stem  57  have common axial movement. It should be understood, however, that the configuration of the EGR valve  55  as just described is not an essential feature of the invention, and various other poppet valve configurations could be utilized within the scope of the present invention. The manifold mounting portion  47  further includes a valve seat  63  against which the poppet valve portion  49  seats or engages when the EGR valve  55  is closed. It should be noted that in FIG. 2, the EGR valve  55  is shown in an open position. As is well known to those skilled in the EGR valve art, a typical EGR valve doesn&#39;t have just one “open” position, but instead, has a range of open positions, depending upon the then-current operating conditions of the engine.  
         [0025]    The EGR valve actuator portion  27  includes, by way of example only, an actuator housing  65  to which is attached a housing cover  67 . Attached to the exterior of the housing cover  67  is the casing of the electric motor  41 . Although the particular construction and specification of the electric motor  41  are not essential features of the present invention, the motor  41  is preferably of the relatively high speed, continuously rotating type, and is preferably one with a high torque-to-inertia ratio, such as a permanent magnet DC commutator motor. As is described in greater detail below, control logic controls the functioning of the electric motor  41  by means of a pair of electrical connections  71  and  73  (not shown in the schematic of FIG. 1).  
         [0026]    The electric motor  41  of the EGR valve actuator portion  27  provides a low torque, high speed rotary output at a motor output shaft  75  which drives a gear train, generally designated  77 . The gear train  77  translates the relatively low torque, high speed rotary output of the motor  41  into a relatively high torque, low speed rotary output which is then converted by means of a linkage, not shown herein, into axial movement of the input stem portion  61 , and of the EGR valve  55 . However, it should be apparent to those skilled in the art that the use of the present invention is not limited to any particular configuration of EGR valve gear train or actuator, etc.  
         [0027]    Attached to the actuator housing  65  is a sensor assembly, generally designated  79 , the function of which is to sense, either directly or indirectly, the axial position of the EGR valve  55 . The sensor assembly  79  converts the sensed position into an appropriate electrical signal that is transmitted as an input to the control logic in the ECM  43  (the logic to be described hereinafter), which controls the functioning of the electric motor  41 . In the preferred embodiment, the sensor assembly  79  is a resistive position sensor of the type typically used in the vehicle industry for throttle position measurements.  
         [0028]    Referring now primarily to FIG. 3, the basic control logic utilized to provide the electrical input signal to the electric motor  41  will be described briefly. It should be understood that the control logic could take various forms, and what is illustrated and described in FIG. 3 is by way of example only.  
         [0029]    In FIG. 3, a position command signal  81  is communicated to a pre-filter device  83 , the output of the device  83  comprising a filtered command signal  85 . The pre-filter device  83  functions in the manner of a low-pass filter, and provides a second degree of freedom which can be used to alter the dynamic time response of the system. The device  83  is intended to remove certain undesirable high frequency components of the position command signal  81 , and especially those which are near the natural frequency of the EGR valve assembly  23 . The signal  85  is communicated to a summing junction  87 , the other input to which is an inverted position feedback signal  89 , such that the output of the summing junction  87  comprises an error signal  91 . As used herein, it will be understood that the term “error” refers to an error in the position of the EGR valve  55 , i.e., the difference between the commanded position and the actual position.  
         [0030]    The error signal  91  is communicated to a control device  93  which, by way of example only, may include the control logic (compensator and “state” machine) and an amplifier circuit. The output of the control device  93  comprises a command signal (referred to hereinafter in the appended claims as an “electrical input signal”)  95  which is the actual command signal transmitted from the electronic control portion  29  to the electrical connections  71  and  73  of the electric motor  41 . Typically, the command signal  95  would comprise a PWM (pulse width modulated) signal, as is well know to those skilled in the art. The command signal  95  is transmitted to the electric motor  41  which then, in response to the command signal  95 , positions the EGR valve  55 , in the manner described previously.  
         [0031]    In the control logic of FIG. 3, the “output” from the element labeled “41” (the electric motor) is a valve position signal  97 , which is the output signal from the sensor assembly  79 , and represents actual instantaneous valve position, i.e., the actual linear position of the poppet valve portion  59  relative to the valve seat  63 . The position signal  97  is fed back to an inverting amplifier  99 , which merely inverts the polarity of the position signal  97  to generate the inverted position feedback signal  89 , in preparation for transmitting the signal  89  to the summing junction  87 .  
         [0032]    As was mentioned in the BACKGROUND OF THE DISCLOSURE, it is well known to adjust the gain (i.e., the gain of the compensator of the control device  93 ) in accordance with variations in system parameters, such as system voltage and ambient temperature. However, in accordance with an important aspect of the present invention, the gain of the compensator of the control device  93  is also varied as a function of changes in a parameter to be referred to hereinafter as the system “friction number”, or friction index, which comprises an arbitrary value, having no units. The friction number is representative of the instantaneous level of friction in the entire EGR valve system, i.e., all of the friction in the system which will ultimately affect the movement of the EGR valve  55 . Those skilled in the art will understand that the friction number is not to be confused with the co-efficient of friction (COF) associated with any particular pair of engaging surfaces.  
         [0033]    For purposes of the subsequent description of the invention, the focus will be on the situation in which the EGR valve  55  is moved from a closed position to a particular, commanded (desired) open position, although it will be understood by those skilled in the art that the present invention would also be applied, and in the same manner, in connection with moving the EGR valve  55  from a particular open position to either the closed position, or to a new, commanded (desired) open position which is less open than the starting position.  
         [0034]    Referring now primarily to FIG. 4, there is shown a flow diagram of the system control algorithm, generally designated  101 , which comprises an important aspect of the present invention. In the algorithm  101  (also referred to as a “state machine”), there are six states representative of different operating modes for the EGR valve assembly  23 . The six states of the system include an OFF state  103 , a CALIBRATE state  105 , a NORMAL state  107 , a WAIT state  109 , a STICKING state  111  and a LIMIT CYCLE state  113 .  
         [0035]    In the OFF state  103 , the entire system is off because the engine is not operating and the vehicle ignition and electrical system are off. The system exits the OFF state  103  whenever the vehicle ignition switch is turned “ON”, and proceeds to the CALIBRATE state  105 .  
         [0036]    In the CALIBRATE state  105 , the current (or duty cycle) of the command signal  95 , designated “DC” in FIG. 4, which is required to change the instantaneous position of the EGR valve  55  is compared to a known threshold value, designated “DC THLD ” in FIG. 4. When that comparison is completed, the algorithm exits the CALIBRATE state  105 . If the command signal  95  (DC) is greater than the threshold value (DC THLD ), the system proceeds to the STICKING state  111 . If the command signal  95  is less than the threshold value, the system proceeds to the NORMAL state  107 .  
         [0037]    In the NORMAL state  107 , there is a continuous monitoring of the error signal  91  (see FIG. 3), designated in FIG. 4 as “E”, in the general sense, but also designated at some places in FIG. 4 as “E N ”, to indicate the instantaneous value of the error at a particular sample time. If the error signal  91  (E N ) is equal to or greater than a threshold value of error, designated “E THLD ” in FIG. 4, then the algorithm exits the NORMAL state  107  and goes to the WAIT state  109 . In the WAIT state  109 , if EN is greater than the threshold value E THLD  for a time period “t” which is greater than a threshold time period, designated T THLD  in FIG. 4, then the algorithm exits the WAIT state  109  and goes to the STICKING state  111 . Alternatively, if at any time the instantaneous error signal EN is less than the threshold value E THLD , the algorithm exits the WAIT state  109  and returns to the NORMAL state  107 .  
         [0038]    While the algorithm is in the NORMAL state  107 , if the time derivative dP/dt of the desired position command signal (signal  81  in FIG. 3), but designated “P” in FIG. 4, is approximately zero, and the time derivative of the error signal dE/dt is greater than a predetermined derivative error threshold DE THLD  for the error signal, the algorithm exists the NORMAL state  107  and goes to the LIMIT CYCLE state  113 . In other words, if the EGR valve  55  is moving when no change in the desired position “P” is being commanded, then the algorithm proceeds to the LIMIT CYCLE state  113 . In the LIMIT CYCLE state  113 , the compensator gain in the amplifier device  93  is reduced in an attempt to prevent (or eliminate) oscillation of the EGR valve  55 . Typically, but not necessarily, this reduction in gain would be accomplished by using a look-up table of the type well known to those skilled in the art, to select a value for the gain, based upon the then-current value for dE/dt, the time derivative of the error signal  91 .  
         [0039]    While the algorithm  101  is in the Limit Cycle state  113 , if the above-described condition (the time derivative dP/dt of the signal  81  being approximately zero and the time derivative dE/dt of the error signal  91  being greater than the error threshold E THLD ) ceases to be true, then the algorithm exits the LIMIT CYCLE state  113  and returns to the NORMAL state  107 .  
         [0040]    In accordance with an important aspect of the invention, the STICKING state  111  is that condition of the EGR valve system  23  in which the valve was commanded to move toward a particular open condition, but the fact that the error signal EN was greater than the threshold value E THLD  (and for a time “t” greater than the threshold time value T THLD ) indicates that the command signal  95  was insufficient, in view of the then-current level of friction in the system to achieve the desired position “P” (signal  81  in FIG. 3) of the EGR valve  55 .  
         [0041]    When the algorithm is in the STICKING state  111 , an instantaneous friction number is calculated (with the current or duty cycle DC required to change the position of the EGR valve  55  being representative of the instantaneous friction number). Then, in the STICKING STATE  111 , the instantaneous friction number is compared to a reference friction number to generate a difference factor, threshold value DC THLD  being representative of the reference friction number. The difference factor is then used to modify the compensator gain in the control device  93 . For example, if the reference friction number were “10” and, after some period of operation of the engine, the friction number calculated while the algorithm  101  is in the STICKING STATE  111  would have a value of “13”, that would indicate a thirty percent increase in the friction number, and the difference factor would be 1.30, indicating that the compensator gain would have to be decreased by about thirty percent (divided by a factor of about 1.3) in order to compensate for the increased level of friction in the system.  
         [0042]    In actual practice of the invention, it would again be typical to provide a look-up table and, using the example above, the current value of the friction number ( 13 ) would be found in the look-up table to find the corresponding value for the compensator gain in the control device  93 . In other words, the change to be made in the compensator gain may not be in a linear relationship with the changes in the friction number. In addition, the desired control of the EGR valve  55  may require other changes in the algorithm  101 , and for example, the change in the friction number may also be used to select different coefficients for use in the pre-filter device  83 , in order to reduce any overshoot of the position of the EGR valve  55 .  
         [0043]    The invention has been described in great detail in the foregoing specification, and it is believed that various alterations and modifications of the invention will become apparent to those skilled in the art from a reading and understanding of the specification. It is intended that all such alterations and modifications are included in the invention, insofar as they come within the scope of the appended claims.