PATENT DOCUMENT

Abstract:
A control device of a control valve used for an intake air-gas system of an engine. The device includes, but is not limited to: the control valve which is an intake air throttle valve provided in the intake air-gas system provided in the intake air-gas system of the engine to control the flow rate of intake air to the engine, or an EGR valve provided in the intake air-gas system of the engine to control the flow rate of EGR gas to the engine; and a control unit which determines a target opening of the control valve in response to the operation conditions of the engine, and controls the opening of the control valve so that the opening conforms with the target opening.

Full Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a control device and a control method of a control valve which is used for an intake air (an intake air-gas) system of an engine. The device is provided the control valve which is an intake air throttle valve provided in the intake air-gas system provided in the intake air-gas system of the engine to control the flow rate of intake air to the engine, oran EGR valve provided in the intake air-gas system of the engine to control the flow rate of EGR gas to the engine; and
         a control unit which determines a target opening of the control valve used for an intake air-gas system, in response to the operation conditions of the engine, and controls the opening of the control valve used for an intake air-gas system so that the opening conforms with the target opening.       

     2. Description of the Related Art 
     As a technology reducing NOx in the exhaust gas emitted from an internal combustion engine, an exhaust gas recirculation device (hereafter abbreviated as an EGR device) is known. In the EGR device, a part of the exhaust gas in the exhaust gas passage is extracted as an EGR gas; the EGR gas is returned to an intake air passage through an EGR passage. Hence, when the EGR device is used, a mix of fresh intake air as well as a part of the exhaust gas, namely, an EGR gas is led into a combustion chamber. 
     The EGR device as described above is provided with an EGR control valve; the opening of the EGR control valve is controlled and the flow rate of the EGR gas returned to the intake air passage is controlled. 
     When the EGR control valve in the EGR device becomes out of order, the flow rate of the EGR gas returned to the intake air passage cannot be controlled. Thus, there may arise an apprehension that: the flow rate of the EGR gas becomes in surplus or shortage; or, the flow of the EGR gas is stopped. 
     Hence, various technologies in which a malfunction of the EGR control valve can be diagnosed have been proposed. 
     For instance, JP1998-122058 discloses a technology in which it is judged that the device including an EGR control valve is out of order when the device confirmed that a detected actual valve opening does not change in response to the target valve opening, while the device is detecting the actual valve opening by use of an actual valve opening detecting means. Hereby, the actual valve opening changes according to the change of the target opening after the target opening begins changing in a case where an EGR operation condition under which the target opening of the EGR control valve changes over a predetermined value holds. 
     Further, JP2007-255251 discloses an exhaust gas recirculation device as shown in  FIG. 18 , the device having: an EGR control valve  102  provided with a valve shaft  102   b ; a driving means  106  in which a reciprocating shaft  112  arranged on a line extended along the valve shaft  102   b  performs a to-and-fro movement in the axis direction; and, a control means (not shown. The exhaust gas recirculation device is configured so that the reciprocating shaft  112  of the driving means  106  opens the EGR control valve  102  by pressing an edge point of the center axis of the EGR control valve  102 , and the control means judges the occurrence of the malfunction of the EGR control valve  102  by the magnitude level of a duty ratio of the control signal oscillated toward the driving means  106  from the control means. 
     SUMMARY OF THE INVENTION 
     1. Subjects to be Solved 
     However, in each of JP1998-122058 and JP2007-255251, even in a case where the EGR control valve is out of order, the actual opening of the EGR control valve agrees with the target opening of the valve; thus, the malfunction of the valve cannot be detected under an operating condition that the target opening is not changed. 
     Above all, in a case where the target opening of the EGR control valve is 0, the EGR control valve with its own structure is provided with a function to press the valve toward the full closed direction; thus, the actual opening apparently follows the target opening so that it is difficult to detect the malfunction. 
     Further, in relation to the EGR control valve including the control valve disclosed by each technology of JP1998-122058 and JP2007-255251, when the opening is kept at a certain same level for a long duration of time, there arises a problem of loss of lube-oil (oil film breakage) in the motor bearing  101  as shown in  FIG. 18 , because of minutely small rotation perturbation of an EGR motor. Thus, the damage of the motor bearing is caused by the loss of the lube-oil, and a risk of malfunction or sticking of the EGR control valve arises. 
     Further, under the operation condition that the actual opening agrees with the target opening and the target opening is unchanged as described above, the malfunction of the control valve cannot be detected. Further, when the opening is kept at a certain same level for a long duration of time, there arises a problem of control valve sticking due to the loss of lube-oil. These problems are not limited to only the EGR control valve but also a control valve used in an air intake system of an engine, for instance, a throttle valve installed in the intake passage through which the air from the outside is supplied to the engine. 
     Consequently, in view of the problems in the conventional technologies, the present invention aims at providing a control device and a control method of a control valve which is used for an intake air-gas system of an engine, wherein: the malfunction of the control valve used for the intake air-gas system can be detected even under the operation condition that the actual opening agrees with the target opening and the target opening is unchanged; and the control valve sticking attributable to a damage of the motor bearing can be prevented, the damage being caused by a lube-oil loss due to the condition that the opening of the EGR control valve is kept at a same constant level for a certain long duration of time. 
     2. Means to Solve the Subjects 
     In order to overcome the problems as described above, the present invention discloses a control device of a control valve used for an intake air-gas system of an engine. The device includes, but is not limited to:
         the control valve which is
           an intake air throttle valve provided in the intake air-gas system provided in the intake air-gas system of the engine to control the flow rate of intake air to the engine, or   an EGR valve provided in the intake air-gas system of the engine to control the flow rate of EGR gas to the engine; and   
           a control unit which determines a target opening of the control valve in response to the operation conditions of the engine, and controls the opening of the control valve so that the opening conforms with the target opening,
           wherein the control unit is configured so that, in a case where the target opening is maintained at a same level during over a fixed duration, the target opening is changed, in time, from the target opening which is determined in response to the operation conditions of the engine and controls the opening of the control valve, in order to prevent the control valve from being out of order as well as in order to detect a failure of the control valve.   
               

     By changing the target opening in time, the opening of the control valve used for the intake air system can be prevented from being kept at a same constant level for a certain long duration of time. Hence, the sticking problem and the like of the control valve used for the intake air system can be avoided. The sticking problem and the like is attributable to the motor bearing damage caused by a lube-oil loss due to the condition that the opening of the EGR control valve is kept at a same constant level for a certain long duration of time. 
     Further, according to the above, the target opening is changed in time; thus, the technology as disclosed above can be free from a conventional problem that the malfunction cannot be detected under the operation condition that the target opening stays unchanged. Further, according to the present invention, by confirming the tracking performance of the actual opening of the control valve used for the intake air system in response to the target opening, the malfunction of the control valve used for the intake air system can be detected. 
     A preferable embodiment of the invention is the control device of the control valve used for the intake air-gas system of the engine. The control unit changes the target opening, in time, in a range of a dead zone where the flow rate of the intake air or the flow rate of the EGR gas is not influenced by the opening of the control valve used for the intake air-gas system even when the opening of the control valve is changed. 
     In the operating range of the opening of the control valve such as the EGR control valve or the throttle valve used for the intake air system, there is a dead zone in which the parameters such as the EGR gas flow rate, the EGR gas mixing ratio (i.e. EGR ratio) in the intake air, the intake-air flow rate, the oxygen excess ratio and the air excess ratio are almost unchanged even when the opening of the valve is changed. The condition of the dead zone or the existing range of the dead zone is different a control valve to a control valve and depends on the size or the structure of the valve; the range of the dead zone of a control valve is an intrinsic property of the control valve. The dead zone is usually an opening range of about 60 to 100% of the total opening range. 
     Even when the target opening is changed in time in the dead zone and the opening of the control valve used for the intake air system is changed in response to the target opening, there is little influence on the parameters such as the EGR gas flow rate, the EGR gas mixing ratio (i.e. EGR ratio) in the intake air, the intake-air flow rate, the oxygen excess ratio and the air excess ratio. Hence, the present invention can be put into practice without influencing on the engine operation condition. 
     Another preferable embodiment of the invention is the control device of the control valve used for the intake air-gas system of the engine, wherein, in temporally changing the target opening, the control unit judges that the control valve used for the intake air-gas system is out of order, in a case where a time duration in which the difference between the target opening and the actual opening of the control valve used for the intake air-gas system exceeds a predetermined allowable level continues over a predetermined allowable time duration. In this way, the malfunction of the control valve used for the intake air system can be surely detected. 
     Another preferable embodiment of the invention is the control device of the control valve used for the intake air-gas system of the engine, wherein, in changing the target opening, in time, the control unit maintains the target opening without changing the target opening, in a case where the difference between the target opening and the actual opening of the control valve used for the intake air-gas system exceeds a predetermined allowable level. 
     According to the above, it can be identified whether the cause of the malfunction is attributable to a reason that the opening of the EGR valve stays unchanged or another reason that the response to the opening command is slow, the malfunction being a condition that the difference between the actual opening and the target opening of the control valve used for the intake air system exceeds an allowable limit value. 
     Another preferable embodiment of the invention is the control device of the control valve used for the intake air-gas system of the engine, wherein the control unit forcefully fixes the target opening at a constant level in a range within the dead zone in a case where the target opening is not maintained at a same opening level over the fixed duration and the target opening is in the range within the dead zone. 
     According to the above, in a time period where it is unnecessary to change the target opening in time, the control valve used for the intake air system can be prevented from being frequently oscillated within the dead zone. In this way, the troubles such as the wear of the seal of the valve shaft and the exhaust gas leakage from the seal part can be avoided. 
     Another preferable embodiment of the invention is the control device of the control valve used for the intake air-gas system of the engine, wherein:
         the control unit holds a function representing a relationship between a parameter θ determined in response to the engine operation conditions and the target opening; and the function includes a hysteresis element.       

     Further, as a method contrivance, the present invention discloses a control method of a control valve used for an intake air-gas system of an engine, the method including, but not limited to, the steps of:
         determining a target opening of the control valve used for an intake air-gas system in response to the operation conditions of the engine, the control valve being an intake air throttle valve to control the flow rate of intake air to the engine or an EGR valve to control the flow rate of EGR gas to the engine; and   regulating the opening of the control valve so that the opening conforms with the target opening,
           wherein, in a case where the target opening is maintained at a same level over a fixed duration in time,   
           the method further includes, but not limited to, the steps of:   changing the target opening, in time, from the target opening of the control valve in response to the operation conditions of the engine; and   preventing the control valve from being out of order detecting as well as detecting a failure of the control valve.       

     A preferable embodiment of the invention is the control method of the control valve used for the intake air-gas system of the engine, wherein the target opening is changed, in time, in a range of a dead zone where the flow rate of the intake air or the flow rate of the EGR gas is not influenced by the opening of the control valve used for the intake air-gas system even when the opening of the control valve is changed. 
     Another preferable embodiment of the invention is the control method of the control valve used for the intake air-gas system of the engine, wherein, in changing the target opening, in time, it is judged that the control valve used for the intake air-gas system is out of order, in a case where a time duration in which the difference between the target opening and the actual opening of the control valve used for the intake air-gas system exceeds a predetermined allowable level continues over a predetermined allowable time duration. 
     Another preferable embodiment of the invention is the control method of the control valve used for the intake air-gas system of the engine, wherein, in changing the target opening, in time, the target opening is maintained without changing the target opening, in a case where the difference between the target opening and the actual opening of the control valve used for the intake air-gas system exceeds a predetermined allowable level. 
     Another preferable embodiment of the invention is the control method of the control valve used for the intake air-gas system of the engine, wherein the target opening is forcefully fixed at a constant level in a range within the dead zone in a case where the target opening is not maintained at a same opening level over the fixed duration and the target opening is in the range within the dead zone. 
     3. Effects of the Invention 
     According to the present invention, a control device and a control method of a control valve which is used for an intake air-gas system of an engine can be supplied, wherein: the malfunction of the control valve used for the intake air-gas system can be detected even under the operation condition that the actual opening agrees with the target opening and the target opening is unchanged; and the control valve sticking attributable to damage of the motor bearing can be prevented. The damage is caused by a lube-oil loss due to the condition that the opening of the EGR control valve is kept at a same constant level for a certain long duration of time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an outline of an EGR device to which a control device of an EGR control valve is applied; 
         FIG. 2  shows an example of the control logic by which an ECU performs the control; 
         FIG. 3  is a graph which shows an example of a characteristic of the EGR control valve; 
         FIG. 4  is a graph which shows an example of a characteristic of a throttle valve; 
         FIG. 5  shows an example of a function which determines the opening of the EGR control valve based on a parameter θ in a first mode as well as an example of a function which determines the opening of the throttle valve based on the parameter θ in the first mode; 
         FIG. 6  is a graph which shows the change of the target opening of the EGR control valve in response to an elapsed time, in a case where the target opening of the EGR control valve is changed in a dead zone; 
         FIG. 7  is a graph which shows the change of the target opening of the throttle valve in response to an elapsed time, in a case where the target opening of the EGR control valve is changed in the dead zone; 
         FIG. 8  is a flow chart which shows the control processes regarding the change of the target opening of the EGR control valve in the first mode; 
         FIG. 9  is a flow chart which shows the processes handling the judgment regarding the dead zone; 
         FIG. 10  is a flow chart which shows the processes handling the judgment regarding an abnormal condition in a case where the EGR control valve is in a sticking prevention operation mode; 
         FIG. 11  shows another example of a flow chart which shows the processes handling the judgment regarding an abnormal condition in a case where the EGR control valve is in a sticking prevention operation mode; 
         FIG. 12  is a graph which shows the change of the target opening of the EGR control valve in response to elapsed time, under a condition that the target opening of the EGR control valve is near zero; 
         FIG. 13  is a graph which shows the change of the target opening of the throttle valve, under a condition that the target opening of the EGR control valve is near zero; 
         FIG. 14  shows an example of a function which determines the opening of the EGR control valve from a parameter θ in a second mode; 
         FIG. 15  is a flow chart which shows the control processes regarding the change of the target opening of the EGR control valve in the second mode; 
         FIG. 16  is a flow chart which shows the processes regarding the judgment of a hysteresis behavior; 
         FIG. 17  is a flow chart which shows the processes in the hysteresis behavior mode; and 
         FIG. 18  shows a cross section around a conventional EGR control valve. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereafter, the present invention will be described in detail with reference to the modes or embodiments shown in the figures. However, the dimensions, materials, shape, the relative placement and so on of a component described in these modes or embodiments shall not be construed as limiting the scope of the invention thereto, unless especially specific mention is made. 
     (First Mode) 
       FIG. 1  shows an outline of an EGR device to which a control device of an EGR control valve according to a first mode of the present invention is applied. In  FIG. 1 , an engine  2  is a four stroke cycle diesel engine of four cylinders. 
     An intake air passage  8  joins the engine  2  via an intake manifold  6 . Further, the engine is connected to an exhaust gas passage  12  via an exhaust manifold  10 . 
     In the intake air passage  8 , a compressor  14   a  of a turbocharger  14  is provided. The compressor  14   a  is driven by a shaft common to the compressor  14   a  and a turbine  14   b  as described later. In the intake air passage  8 , on the downstream side of the compressor  14   a , an intercooler  16  is provided. Further, in the intake air passage  8 , on a downstream side of the intercooler  16 , a throttle valve  18  by which the flow rate of the intake air streaming through the intake air passage  8  is regulated is provided. 
     In the exhaust gas passage  12 , the turbine  14   b  of the turbocharger  14  is provided. The turbine  14   b  is driven by the exhaust gas from the engine  2 . Further, the exhaust manifold  10  is connected to an EGR passage  20  through which a part of the exhaust gas is re-circulated to the intake air side. On apart way of the EGR passage  20 , an EGR cooler  22  and an EGR control valve  24  are provided. 
     The EGR cooler  22  is provided on an exhaust manifold side of the EGR control valve  24 . Heat exchange is performed between the EGR gas and cooling water which pass through the EGR cooler  22  so that the temperature of the EGR gas is reduced. Further, the EGR control valve  24  regulates the flow rate of the EGR gas passing through the EGR passage  20 . 
     The valve opening of the EGR control valve  24  as well as the throttle valve  18  is controlled by an engine control unit (ECU)  40 . 
     The outline of the valve opening control regarding the EGR control valve  24  as well as the throttle valve  18  is now explained. Into the ECU  40 , an actual opening of the EGR control valve  24  as well as the throttle valve is inputted. Further, a detected value (a signal) detected by an intake air temperature sensor  28  fitted to the intake air passage  8  or the intake manifold  6  on the downstream side of the throttle valve  18  is inputted into the ECU  40 , via an A/D convertor  43 ; similarly, a detected value detected by an intake air pressure sensor  30  fitted to the intake air passage  8  or the intake manifold  6  on the downstream side of the throttle valve  18  is inputted into the ECU  40 , via an A/D convertor  44 . Further, a detected value detected by an air flow meter  26  fitted to the intake air passage  8  on the upstream side of the compressor  14   a  is inputted into the ECU  40 , via an A/D convertor  42 . Further, a detected value detected by an engine speed sensor  32  is inputted into the ECU  40 , via a pulse counting circuit  47   
     In the ECU  40 , based on the inputted values as described above, the target opening of the EGR control valve  24  as well as the throttle valve  18  is computed. Based on the computed result, the opening of the EGR control valve  24  is controlled via a driving circuit; and the opening of the throttle valve  18  is controlled via a driving circuit  46 . Further, In a CPU  48 , as well as the throttle valve  18  is controlled via a driving circuit, based on the inputted values as described above, the injection quantity of the fuel supplied to the engine  4  is computed; based on the computed result, the fuel injection quantity is controlled via an injector drive circuit  41 . 
       FIG. 2  shows an example of the control logic by which an ECU performs the control. In the ECU  40 , the air flow rate [kg/s], the engine speed [rpm], the intake manifold air pressure [kPa], the intake manifold air temperature [° C.] and the fuel injection quantity [mg/st] are inputted into a θ target map  51  as well as a λ 02  target map  52 ; based on the inputted values, a target θ and a target λ 02  are generated. Hereby, the θ is a value determined according to the opening of the EGR control valve  24  as well as the throttle valve  18 ; the detail will be described later. Incidentally, the λ 02  is the oxygen excess ratio. 
     Further, an estimated λ 02  is computed by a λ 02  estimating section  53  based on the data (variables) such as the air flow rate [kg/s], the engine speed [rpm], the intake manifold air pressure [kPa], the intake manifold air temperature [° C.] and the fuel injection quantity [mg/st]. 
     Further, the error between the target λ 02  and the estimated λ 02  is computed by a subtraction process  54 . And, based on the error, a PID control  55  is performed. A parameter θ is determined by the PID control  55 ; the θ is added to a target θ; and a saturation operation  57  is performed for the aggregation so that the θ is corrected. Based on the corrected θ, an opening command value for the EGR control valve  24  is determined by use of a function  58  for determining the opening of the EGR control valve  24 , the function  58  being a function with respect to the parameter θ. Further, based on the corrected θ, an opening command value for the throttle valve  18  is determined by use of a function  59  for determining the opening of the throttle valve  18 , the function  59  being a function with respect to the parameter θ. In addition, the functions  58  and  59  are memorized in the ECU  40  in advance. 
     Further, each of the EGR control valve  24  and the throttle valve  18  has a fully opened position as well as a fully closed position. In other word, the opening of each valve shows saturation behavior. Hence, when the opening of the EGR control valve  24  or the throttle valve  18  reaches the fully opened position or the fully closed position, a condition that the control error remains continues. On the other hand, the error which is inputted in the PID control  55  is kept in a non-zero condition. Therefore, the integrated value in the PID control  55  continues to increase. Thus, there arises a problem of a wind-up behavior where the control responsiveness is hindered. In order to avoid the wind-up problem, the difference between the parameter θ as the output of the PID control  55  and the corrected parameter θ as the output of the saturation operation  57  is computed by a subtraction process  60 ; based on the computed difference, namely, an error, an anti-windup compensation is performed. 
     In addition, in the operating range (as to the opening range) of each of the EGR control valve  24  and the throttle valve  18 , there is a characteristic range (hereafter called a dead zone) in which the parameters such as the EGR gas flow rate, the EGR gas mixing ratio (i.e. EGR ratio) in the intake air, the intake-air flow rate, the oxygen excess ratio and the air excess ratio are almost unchanged even when the opening of the valve is changed. 
     Based on  FIGS. 3 and 4 , the above-described dead zone is hereby explained with the oxygen excess ratio as an example of the parameters. 
       FIG. 3  is a graph which shows an example of a characteristic of the EGR control valve  24 ; and  FIG. 4  is a graph which shows an example of a characteristic of a throttle valve  18   
     In  FIG. 3 , the vertical axis denotes the oxygen excess ratio λ 02 , whereas the lateral axis denotes the valve opening [%] of the EGR control valve  24 . Further, in  FIG. 3 , the vertical axis denotes the oxygen excess ratio λ 02 , whereas the lateral axis denotes the valve opening [%] of the throttle valve  18 . 
     As shown in  FIG. 3 , in a case of the EGR control valve  24 , the oxygen excess ratio λ 02  is almost unchanged when the opening of the EGR control valve  24  is in a range of about 60 to 100%, especially, in a range of about 80 to 100%. In other words, regarding the EGR control valve  24  having the characteristic as shown in  FIG. 3 , the range of about 60 to 100% can be called the dead zone of the EGR control valve  24 . Incidentally, in the dead zone, the variables such as the EGR gas flow rate, the EGR gas mixing ratio (i.e. EGR ratio) in the intake air, the intake-air flow rate, and the air excess ratio other than the oxygen excess ratio λ 02  are also almost unchanged with respect to the change of the opening of the EGR control valve  24 , the variables being dependent on the valve opening. 
     In a similar way, regarding the throttle valve  18  having the characteristic as shown in  FIG. 4 , a range of about 70 to 100%, especially, a range of about 80 to 100% can be called the dead zone as to the opening of the throttle valve  18 . 
     The upper side of  FIG. 5  shows an example of a function which determines the opening of the EGR control valve  24  with respect to a parameter θ, whereas the lower side of  FIG. 5  shows an example of a function which determines the opening of the throttle valve  18 . The upper side and the lower side correspond to the functions  58  and  59  as shown in  FIG. 2 , respectively. 
     In the upper side drawing of  FIG. 5 , the vertical axis denotes the target opening (the command opening) as to the EGR control valve  24  and the lateral axis denotes the parameter θ; and, in the lower side drawing of  FIG. 5 , the vertical axis denotes the target opening (the command opening) as to the throttle valve  18  and the lateral axis denotes the parameter θ. 
     Hereby, θ is a variable dependent on the opening of the EGR control valve  24  as well as the throttle valve  18 . Further, when the opening of the EGR control valve  24  is 100%, the opening (0 to 100%) of the throttle valve  18  is expressed as θ to 1. In a similar way, when the opening of the throttle valve  18  is 100%, the opening (0 to 100%) of the EGR control valve  24  is expressed as 2 to 1. 
     Hence, in the upper side drawing of  FIG. 5 , the target opening of the EGR control valve  24  is 100% in response to the parameter θ in the range of θ=0 to 1; and, the target opening of the EGR control valve  24  monotonically and linearly decreases from 100% to 0% in response to the parameter θ in the range of θ=1 to 2. In the lower side drawing of  FIG. 5 , the target opening of the throttle valve  18  monotonically and linearly increases from 0% to 100% in response to the parameter θ in the range of θ=0 to 1; and, the target opening of the throttle valve  18  is 100% in response to the parameter θ in the range of θ=1 to 2. 
     Further, in the upper side drawing of  FIG. 5 , the area expressed by a symbol ‘a’ corresponds to the dead zone of the EGR control valve opening; on the other hand, the area expressed by a symbol ‘b’ is an area in which the sensitivity to the EGR control valve opening change can be acknowledged. In a similar way, in the lower side drawing of  FIG. 5 , the area expressed by a symbol ‘a’ corresponds to the dead zone of the throttle valve opening; on the other hand, the area expressed by a symbol ‘b’ is an area in which the sensitivity to the throttle valve opening change can be acknowledged. 
     Hereby, based on the control logic as shown in  FIG. 2 , a case where the parameter θ smaller than 1.0 is outputted as a command value (signal) is discussed. In this event, as is clear from  FIG. 5 , the target opening of the EGR control valve  24  is 100% in view of the conventional approach; accordingly, the opening of the EGR control valve  24  stays at an almost constant level (100%) for a long duration of time. Thus, in the conventional approach, there arises a problem of loss of lube-oil (oil film breakage) in a motor bearing as shown in  FIG. 18 ; a damage of the motor bearing due to the loss of the lube-oil; or, a risk of malfunction or sticking of the EGR control valve. Further, in the conventional approach, when the parameter θ is in the range (smaller than 1.0), the target opening is unchanged; thus, when the EGR control valve fails, the actual opening agrees with the target opening. In other words, in the conventional approach, if the actual opening is 100% in a case of the valve failure, the failure cannot be detected or acknowledged. 
     In the present invention, attention is paid to the fact that, even when the opening of the EGR control valve is changed, the above-described variable such as the oxygen excess ratio λ 02  is almost unchanged in the EGR control valve dead zone in which the opening of the valve is about 60 to 100%; to be more specific, in a case where the target opening of the EGR control valve is maintained at a same constant level for a certain prolonged duration of time, the target opening is intentionally changed in the dead zone, as shown in  FIG. 12  with the symbol c. In this way, the target opening of the EGR control valve  24  varies; thus, the malfunction such as sticking of the EGR control valve can be avoided. Hereby, the malfunction such as sticking is attributable to the failure of the motor bearing, the failure being caused by the condition in which the opening of the EGR control valve is kept at a same constant level for a certain prolonged duration of time. Thereby, the failure of the EGR control valve can be detected. 
     Changing the target opening of the EGR control valve in the dead zone as described above is feasible, when the target opening as the function with respect to the parameter θ is in the dead zone; for instance, in the upper drawing of  FIG. 5 , this change can be feasible when a condition θ&lt;θ 1  is satisfied. 
       FIG. 6  is a graph which shows the change of the target opening of the EGR control valve in response to elapsed time, in a case where the target opening of the EGR control valve is changed in a dead zone. In  FIG. 6 , the vertical axis denotes the target opening of the EGR control valve  24  and the lateral axis denotes the elapsed time. For instance, as shown in  FIG. 6 , the target opening of the EGR control valve  24  is changed in time so that the change of the target opening in time forms a wave-shaped function. In this mode of the present invention, the target opening of the EGR control valve is changed so that the graph of the change of the EGR control valve target opening in time is configured as a wave form. As a matter of course, instead of the wave form, the graph of the change may be configured as another kind of geometry such as a rectangular pulse form, so long as the target opening of the EGR control valve is changed in time. 
       FIG. 7  is a graph which shows the change of the target opening of the throttle valve in response to elapsed time, in a case where the target opening of the EGR control valve is changed in the dead zone. In  FIG. 7 , the vertical axis denotes the target opening of the throttle valve and the lateral axis denotes the elapsed time. Hereby, the target opening of the throttle valve stays in an unchanged condition. 
     In the next place, the control regarding the above-described change of the target opening of the EGR control valve is now explained in detail by use of a flow chart. 
       FIG. 8  is the flow chart which shows the control processes regarding the change of the target opening of the EGR control valve in the first mode. 
     When a series of control processes starts, it is judged whether or not a cooling water temperature is higher than a temperature T 1  in the step S 101 . The cooling water means the engine cooling water and the temperature T 1  is a prescribed temperature. When the result of the judgment in the step S 101  is negative, namely, when it is judged that the cooling water temperature is not higher than the temperature T 1 , the step S 101  is followed by the step S 108 , where the EGR is stopped so as not to perform the EGR operation; and, the control flow reaches an end. When the result of the judgment in the step S 101  is affirmative, namely, when it is judged that the cooling water temperature is higher than the temperature T 1  in the step S 101 , the step S 101  is followed by the step S 102 . 
     In the step S 102 , the judgment as to the dead zone is performed. This judgment as to the dead zone is performed according to a flow chart as shown in  FIG. 9 . By use of FIG.  8 , the dead zone judgment is explained. 
     When the control flow is started it is judged whether or not the condition θ&lt;θ 1  is satisfied in the step S 201 . Hereby, the parameter θ is a value (signal) which is ordered according to the logic described in  FIG. 2 . And, the parameter θ 1  is a coordinate of a boundary of the dead zone, the parameter corresponding to the symbol θ 1  in the upper drawing of  FIG. 5 . 
     When the judgment result in the step S 201  is affirmative, namely, the condition θ&lt;θ 1  is satisfied, the step S 201  is followed by the step S 202 , where a dead zone judgment flag is set (FLAG=ON). And, the control flow reaches an end (RETURN TO MAIN FLOW). Further, when the judgment result in the step S 201  is negative, the step S 201  is followed by the step S 203 , where a dead zone judgment flag is cleared (FLAG=OFF). And, the control flow reaches an end (RETURN TO MAIN FLOW). 
     When the dead zone judgment according to the flow chart of  FIG. 9  is finished at the step S 102  in the flow chart of  FIG. 8 , the step S 102  is followed by the step S 103 . 
     In the step S 103 , it is judged whether or not the dead zone flag is ON. 
     When the judgment result in the step S 103  is negative, namely, the dead zone flag is set at the condition FLAG=OFF, the step S 103  is followed by the step S 107 , where the opening of the EGR control valve is controlled according to the opening command for the EGR control valve which is issued the function  58  as is the case with the conventional approach, without forcefully changing the target opening of the EGR control valve. 
     When the judgment result in the step S 103  is affirmative, namely, the dead zone flag is set at the condition FLAG=ON, the step S 103  is followed by the step S 104 . 
     In the step S 104 , it is judged whether or not it is about time to take a measure to prevent the sticking of the valve. 
     As described before, when the opening of the EGR control valve is kept at a constant level for a prolonged duration of time, the problem of loss of lube-oil (oil film breakage) is caused so that the motor bearing is damaged and sticking of the EGR control valve is caused. In other words, when the target opening of the EGR control valve is not kept at a constant level for a long duration of time, the problem such as sticking can be avoided. Based on this reason, in the step S 104 , it is judged whether or not the target opening of the EGR control valve is kept at a constant level over a certain duration of time as well as whether or not the duration of time exceeds a time period of necessary maintenance to take measures to prevent sticking. To be more specific, if the target opening of the EGR control valve stays at a constant level for a prescribed duration of time, it is judged that it is time to take a measure to prevent the sticking of the valve. Incidentally, the prescribed duration of time is to be determined at every EGR control valve in consideration of the performance of the EGR control valve or the periphery devices around the engine. 
     When the judgment result in the step S 104  is negative, namely, when it is judged that it is not time to take a measure to prevent the sticking of the valve, the step S 104  is followed by the step S 107 , where the opening of the EGR control valve is controlled according to the normal control mode of the EGR control valve, namely, without forcefully changing the target opening of the EGR control valve as is the case with the conventional approach. 
     When the judgment result in the step S 104  is affirmative, namely, when it is judged that it is time to take a measure to prevent the sticking of the valve, the step S 104  is followed by the step S 106 , where the opening of the EGR control valve is controlled according to an EGR valve sticking prevention mode. And, the control flow reaches an end. In the EGR valve sticking prevention mode, as is explained by the use of  FIGS. 5 and 6 , the target opening of the EGR control valve is changed within a range in the dead zone. In this way, the target opening is controlled so as to be changed; thus, the sticking of the EGR control valve can be avoided. The sticking is attributable to the failure of the motor bearing, and the failure is caused by the condition in which the opening of the EGR control valve is kept at a same constant level for a certain prolonged duration of time. 
     Further, in the EGR valve sticking prevention mode, a malfunction of the EGR control valve can be judged on the basis of the target opening and the actual opening of the EGR control valve. The judgment of the malfunction of the EGR control valve in the EGR valve sticking prevention mode is now explained based on a flow chart of  FIG. 10 . 
       FIG. 10  is a flow chart which shows the processes handling the judgment regarding the malfunction of the EGR control valve in the EGR valve sticking prevention mode. 
     In  FIG. 10 , when the control flow is started, the step S 301  is performed. In the step S 301 , the target opening, that is, the command opening value of the EGR control valve is computed. The target opening can be obtained by use of the parameter θ and the function  58  after the computation of the parameter θ according to the processes of the logic as shown in  FIG. 2 . 
     When the step S 301  is finished, the step S 301  is followed by the step S 302 . In step S 302 , the opening command for the EGR control valve is outputted. 
     When the step S 302  is finished, the step S 302  is followed by the step S 303 . 
     In step S 303 , an EGR valve opening deviation e is computed; whereby, the deviation e means a difference between a command opening value and an actually measured opening value regarding the EGR control valve. 
     When the step S 303  is finished, the step S 303  is followed by the step S 304 . 
     In step S 304 , it is judged whether or not the absolute value |e| (i.e. abs(e)) of the EGR valve opening deviation e is greater than an allowable value. The allowable value means a least upper bound of the absolute value |e| to be allowed while the EGR control valve is used. The allowable value is a value to be determined at every EGR control valve in consideration of the performance of the EGR control valve or the periphery devices around the engine. 
     When the judgment in step S 304  is negative, namely, when the absolute value |e| is not greater than the allowable value, step S 304  is followed by step S 309 , which is described later. 
     When the judgment in step S 304  is affirmative, namely, when the absolute value |e| is greater than the allowable value, the step S 304  is followed by the step S 305 . 
     In the step S 305 , the computation according to the formula (1) below is performed.
 
 t   e   =t   e   +t   s   (1)
 
     Hereby, t e  is a cumulative time in which the absolute value |e| is greater than the allowable value; t s  is an operation period, which is a time span from the start timing to the end timing of the control flow chart in  FIG. 10 . 
     Further, the cumulative time t e  on the left side of the formula (1) is a current cumulative time; the cumulative time t e  on the right side of the formula (1) is a cumulative time at the previous timing before one period. By the computation according to the formula (1), the current cumulative time (sum) in which the absolute value |e| has exceeded the allowable value can be obtained. 
     When the step S 305  is finished, the step S 305  is followed by the step S 306 . 
     In the step S 306 , it is judged whether or not the (current) cumulative time t e  computed at the step S 305  is longer than an allowable time span. The allowable time span means an upper bound time within which the accumulation of the cumulative time where the absolute value |e| has exceeded the allowable value (error) is regarded as being allowable. In other words, the allowable time span means an upper bound value of the cumulative time t e . The allowable time span is a value to be determined at every EGR control valve in consideration of the performance of the EGR control valve or the periphery devices around the engine. 
     When the judgment in the step S 306  is affirmative, namely, when the cumulative time t e  is longer than the allowable time span, the step S 306  is followed by the step S 307 , where it is judged that the EGR valve malfunctions. In the following step S 308 , the EGR control is stopped, and the control flow reaches an end. 
     When the judgment in step S 306  is negative, namely, when the cumulative time t e  is shorter than the allowable time span, the control flow is returned to an end without any other process. 
     When the judgment in step S 304  is negative, or when the judgment in step S 306  is negative, the step S 304  or the step S 306  is followed by the step S 309 , where the cumulative time t e  is cleared (t e =0). And, the control flow reaches an end. 
     According to a series of processes shown in the flow chart of  FIG. 11  instead of the processes shown in the flow chart of  FIG. 10 , it can be judged whether or not a malfunction of the EGR control valve in the EGR valve sticking prevention mode is occurring. 
     Further,  FIG. 11  shows another example of a flow chart which shows the processes handling the judgment regarding the malfunction in a case where the EGR control valve is in the sticking prevention operation mode. 
     The steps S 401  to S 405  in the flow chart of  FIG. 11  are the same as the steps S 301  to S 305  in the flow chart of  FIG. 10 , respectively. In addition, the steps S 407  to S 410  in the flow chart of  FIG. 11  are the same as the steps S 306  to S 309 , respectively. Hence, the explanation of the steps S 401  to S 405  and the steps S 407  to S 410  in the flow chart of  FIG. 11  is omitted. 
     In  FIG. 11 , in a case where it is judged that the absolute value |e| is greater than the allowable value in the step S 404 , the step S 404  is followed by the step S 405 , where the cumulative t e  is computed. In the following step S 406 , the opening command for the EGR valve is preserved. By preserving the opening command for the EGR valve in the step S 406 , the cause of the malfunction condition that the absolute value |e|, namely, the absolute value of the difference between the command value and the actual measured-value is greater than the allowable value can be identified. In other words, the cause can be attributed to a reason that the opening of the EGR valve stays unchanged or another reason that the response to the opening command is slow. 
     In this first mode of the present invention, by use of  FIGS. 3 to 10 , a case where the opening of the EGR control valve is nearly full-opened and the target opening of the EGR control valve is in the dead zone has been explained thus far. Also, in the other case where the opening of the EGR control valve is nearly full-closed and the target opening of the EGR control valve is not in the dead zone, namely, in the case where the parameter θ is greater than the parameter θ 1 , the target opening of the EGR control valve is changed. 
     In this event, the target opening of the EGR control valve is not in the dead zone; thus, the change of the EGR gas flow rate, the EGR ratio, the intake air flow rate, the oxygen excess ratio, the air excess ratio and so on is sensitive to the change of the opening of the EGR control valve; thus, when the EGR is performed, a small opening change of the EGR control valve influences the reduction of the harmful substances in the exhaust gas. Accordingly, by making a small change to the target opening of the EGR control valve and by confirming the effect of the small change on the reduction of the harmful substances in the exhaust gas, the malfunction of the EGR control valve can be detected. 
       FIG. 12  is a graph which shows the change of the target opening of the EGR control valve in response to elapsed time, under a condition that the target opening of the EGR control valve is near zero. In  FIG. 12 , the vertical axis denotes the target opening of the EGR control valve and the lateral axis denotes the elapsed time. As shown in  FIG. 12 , the target opening of the EGR control valve is minutely changed. 
     Further,  FIG. 13  is a graph which shows the change of the target opening of the throttle valve, under a condition that the target opening of the EGR control valve is near zero. In  FIG. 13 , the vertical axis denotes the target opening of the throttle valve and the lateral axis denotes the elapsed time. As shown in  FIG. 13 , the target opening of the throttle valve stays unchanged. 
     As described above, in a case where the target opening of the EGR control valve is not in the dead zone, by minutely changing the target opening of the EGR control valve, the malfunction of the EGR control valve can be detected. Further, the sticking of the EGR control valve can be avoided. The sticking is attributable to the failure of the motor bearing, and the failure is caused by the condition in which the opening of the EGR control valve is kept at a same constant level for a certain prolonged duration of time. 
     In a case where the EGR control valve is minutely opened in a manner as described above, smoke may be generated. For all that, smoke is generally generated when the engine speed or the engine load is increased. When the engine is placed in a steady condition, the engine is not connected with the smoke generation. Further, by limiting the opening of the EGR control valve at most to the level of 4 to 8% of the full opening so as to constrain the effect of the flow rate, the problem of smoke generation can be avoided. 
     Further, in the EGR device provided with an EGR cooler as shown in  FIG. 1 , the EGR gas including smoke as well as unburned fuel is cooled when the EGR gas passes through the inside of the EGR cooler; and, the smoke is inclined to gradually become a soot deposit. Thereby, the unburned fuel plays the role of a binder of the deposit. In order to prevent the clogging of the EGR cooler, as well as, to prevent the drop in the cooling efficiency of the cooler, it is not performed to minutely change the target opening of the EGR control valve in a case where the target opening stays near 0 under a condition that the temperature of the EGR gas is low. 
     (Second Mode) 
     In a second mode of the present invention, the EGR device to which the EGR control valve is applied as well as the logic of the control thereby is the same as the EGR control valve as well as the logic of the control in the first mode. Hence,  FIGS. 1 and 2  which are used in the first mode are also used in this second mode. And, the repetition of explanation is omitted. 
     In the second mode, the function  58  in  FIG. 2  is provided with a hysteresis property. As for the function  58 , the parameter θ is the independent variable which determines the opening of the EGR control valve as the dependent variable. 
       FIG. 14  shows an example of a function which determines the opening of the EGR control valve  24  from a parameter θ in a second mode; and,  FIG. 14  corresponds to the function  58  in  FIG. 2 . 
     In  FIG. 14 , the vertical axis denotes the target opening of the EGR control valve and the lateral axis denotes the parameter θ. Further, the area expressed by a symbol ‘a’ corresponds to the dead zone of the EGR control valve opening; on the other hand, the area expressed by a symbol ‘b’ is an area in which the sensitivity to the EGR control valve opening change can be acknowledged. In this second mode, as shown in  FIG. 14 , the function  58  is provided with the hysteresis property in the range of θ from θ 2  to θ 3 . And, the parameter θ 3  is a coordinate of a boundary of the dead zone as is the case with the parameter θ 1  in the upper drawing of  FIG. 5  and the parameter θ 3  is equal to the parameter θ 1 . In addition, the parameter θ 2  is smaller than the parameter θ 3 . 
     As for the second mode, based on a flow chart as shown in  FIG. 15 , the control of the change of the EGR control valve target opening in a case where a function provided with the hysteresis property as shown in  FIG. 14  is now explained. 
       FIG. 15  is the flow chart which shows the control processes regarding the change of the target opening of the EGR control valve in the second mode. 
     When the control flow is started, in the step S 501 , it is judged whether or not a cooling water temperature is higher than a temperature T 1 . When the result of the judgment in the step S 501  is negative, namely, when it is judged that the cooling water temperature is not higher than the temperature T 1 , the step S 501  is followed by the step S 508 , where the EGR is stopped so as not to perform the EGR operation; and, the control flow reaches an end. When the result of the judgment in the step S 501  is affirmative, namely, when it is judged that the cooling water temperature is higher than the temperature T 1  in the step S 501 , the step S 501  is followed by the step S 502 . 
     In the step S 502 , a judgment as to a hysteresis behavior (a hysteresis judgment) is performed. The judgment as to the hysteresis behavior is performed according to a flow chart which is shown in  FIG. 16 . 
     By use of  FIG. 16 , the judgment as to the hysteresis behavior is explained. 
     When the control flow is started, in the step S 601 , it is judged whether or not a hysteresis judgment flag is OFF. The hysteresis judgment flag is a flag by which it is determined, in the step S 503  ( FIG. 15 ) as described later, to perform an EGR valve normal-control-mode or to perform an EGR valve sticking prevention mode. And, the hysteresis judgment flag is a value dependent on the parameter θ. 
     When the judgment result in the step S 601  is affirmative, namely, when it is judged that the hysteresis judgment flag is OFF, the step S 601  is followed by the step S 602 . 
     In the step S 602 , it is judged whether or not the parameter θ which is issued by the logic as shown in  FIG. 2  is smaller than the parameter θ 2 . When the judgment result in the step S 602  is affirmative, namely, when it is judged that θ&lt;θ 2 , the hysteresis judgment flag is changed to ON. And, the control flow reaches an end. When the judgment result in the step S 602  is negative, namely, when it is judged that θ≧θ 2 , the hysteresis judgment flag is kept at OFF. And, the control flow reaches an end. 
     Further, when the judgment result in the step S 602  is negative, namely, when the hysteresis judgment flag is ON, the step S 601  is followed by the step S 604 . 
     In the step S 604 , it is judged whether or not the parameter θ is greater than the parameter θ 3 . When the judgment result in the step S 602  is negative, namely, when it is judged that θ≦θ 3 , the hysteresis judgment flag is kept at ON without changing the flag condition. 
     According to the judgment as to the hysteresis behavior as shown in  FIG. 16 , regardless of the condition of the current hysteresis judgment flag, the hysteresis judgment flag is ON when θ&lt;θ 2 , whereas the hysteresis judgment flag is OFF when θ&gt;θ 3 . And, the control flow reaches an end. In addition, when θ 2 ≦θ≦θ 3 , the current hysteresis judgment flag is preserved, and the control flow reaches an end. 
     When the hysteresis behavior judgment by the processes as shown in  FIG. 16  are finished, the step S 502  in  FIG. 15  ends; then, the step S 502  is followed by the step S 503 . 
     In the step S 503 , it is judged whether or not the hysteresis judgment flag is ON. 
     When the judgment result is negative, namely, when the hysteresis judgment flag is OFF, the step S 503  is followed by the step S 507 , where the opening of the EGR control valve is controlled according to the opening command issued by the function  58  toward the EGR control valve without forcefully changing the target opening of the EGR control valve. 
     When the judgment result is affirmative, namely, when the hysteresis judgment flag is ON, the step S 503  is followed by the step S 504 . 
     In the step S 504 , it is judged whether or not it is time to take a measure to prevent the sticking of the valve. As for the time to take a measure to prevent sticking, the explanation is the same as that in the step S 104  of  FIG. 8 ; hence, repetition of the explanation is omitted. 
     In the step S 504 , when the judgment result is affirmative, namely, when it is judged it is time to take a measure to prevent sticking, the step S 504  is followed by the step S 506 , where the opening of the EGR control valve is controlled according to the EGR valve sticking prevention mode. And, the control flow reaches an end. In the EGR valve sticking prevention mode, as is explained by use of  FIGS. 5 and 6  in relation to the first mode, the target opening of the EGR control valve is changed within a range in the dead zone. Further, according to a series of processes shown in the flow chart of  FIG. 10 or 11 , the judgment as to whether or not a malfunction of the EGR control valve is occurring is performed. 
     In the step S 504 , when the judgment result is negative, namely, when it is judged that it is not time to take a measure to prevent sticking, the step S 504  is followed by the step S 505 , where the hysteresis behavior mode e is taken. And, the control flow reaches an end. 
     Based on  FIG. 17 , the procedure in the hysteresis behavior mode is explained.  FIG. 17  is a flow chart which shows the procedure in the hysteresis behavior mode. 
     When the control flow is started in  FIG. 17 , the step S 701  is performed. 
     In the step S 701 , the opening of the EGR control valve is fixed at 100%. And, the control flow reaches an end. In addition, in this second mode, as shown in the step S 701  of  FIG. 17 , the opening of the EGR control valve is fixed at 100% in the hysteresis behavior mode; however, if the opening in the dead zone, the opening of the EGR control valve can be fixed at a level other than 100%. 
     In other words, in the hysteresis behavior mode, the opening of the EGR control valve is maintained at a constant level in the dead zone. In a case where the hysteresis behavior mode is applied, the EGR control valve can be prevented from being frequently oscillated within the dead zone. In this way, troubles such as the wear of the seal of the valve shaft and the exhaust gas leakage from the seal part can be avoided. 
     Further, in the hysteresis behavior mode, since the opening of the EGR control valve is maintained at a constant level in the dead zone, the operation according to the hysteresis behavior mode does not influence the EGR gas flow rate, the intake air flow rate, the oxygen excess ratio, the air excess ratio and so on. 
     In the first and second modes as described above, the control of the EGR control valve has been explained; however, the device and the method as described above can be applicable to the throttle valve. 
     The present invention can be used as a control device and a control method of a control valve which is used for an intake air-gas system of an engine. A malfunction of the control valve used for the intake air-gas system can be detected even under the operation condition that the actual opening agrees with the target opening and the target opening is unchanged; and the control valve sticking attributable to a damage of the motor bearing can be prevented. The damage is caused by a lube-oil loss due to the condition that the opening of the EGR control valve is kept at a same constant level for a certain long duration of time.