Abstract:
An electronic variable valve apparatus may be stably controlled regardless of variations of engine condition with minimized usage of map tables by employing a sliding surface calculation for controlling the electronic variable valve apparatus in order to achieve a target cam angle.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims priority to and the benefit of Korean Patent Application No. 10-2007-0121456 filed in the Korean Intellectual Property Office on Nov. 27, 2007, the entire contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   (a) Field of the Invention 
   The present invention relates to an internal combustion engine. More particularly, the present invention relates to a method and apparatus for controlling an electronic variable valve apparatus of an internal combustion engine. 
   (b) Description of the Related Art 
   Performance of an internal combustion engine, especially a gasoline engine, substantially depends on how efficiently an air can be drawn into a combustion chamber. 
   For better intake efficiency, a variable valve apparatus for varying valve timing is employed such that optimal amount of air can be drawn into the combustion chamber for various engine speeds. 
   A hydraulic variable valve apparatus that is typically employed has a drawback in that, when the engine speed is low or engine oil is at a low temperature, a torque for operating the apparatus is increased. In addition, such a hydraulic variable valve apparatus does not usually provide sufficient variation of cam angle. 
   In order to solve such drawbacks, an electronic variable valve apparatus is widely studied as an alternative for such a hydraulic variable valve apparatus. 
   The electronic variable valve apparatus shows many merits. For example, it shows higher response speed that a conventional hydraulic variable valve apparatus. An oil pump of an engine may be downsized since the electronic variable valve apparatus does not require a hydraulic pressure. The electronic variable valve apparatus can be properly operated even if the engine speed is low or the engine oil is at a low temperature, which means that a load for starting the engine may be reduced. Furthermore, an exhaust gas may be reduced when the engine is at a low temperature. 
   In addition the electronic variable valve apparatus may be operated at a wider range of an angle, such that the merits of varying the valve timing may be maximized. 
   The electronic variable valve apparatus are typically driven by an electronic clutch or a motor. 
   The scheme employing the electronic clutch costs less but it is harder to control. The scheme employing the motor cost more but it is easier to control. 
   An example of the electronic variable valve apparatus can be found in Japanese Patent Laid-Open Publication No. 2002-276310. 
   In order to control an angle of a camshaft according to the conventional scheme, an engine control unit calculates an angular difference Δθ between a reference angle depending on an engine state and a current angle detected by a cam angle sensor, and determines whether the angular difference Δθ is above a predetermined error value. 
   When the angular difference Δθ is less than the predetermined error value, the current control is maintained, and a clutch release coil and a brake control coil is not applied with a current. 
   When the angular difference Δθ is above the predetermined error value, it is determined whether the angular difference Δθ is positive or negative. If the angular difference Δθ is positive, a current is applied to the clutch release coil and the brake control coil so as to perform an advance control. If the angular difference Δθ is negative, a current is applied to the clutch release coil and the brake control coil so as to perform a retardation control. 
   According to the above scheme where a current to be applied to the clutch release coil and the brake control coil is on/off controlled in order to control an angle of a camshaft, calibration maps should be provided depending on control responsiveness, angular error, and engine states. 
   According to such a scheme, huge amount of experimentation is required in order to prepare sufficiently precise calibration maps, which causes the cost for newly designing a vehicle to increase very high. 
   The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in an effort to provide a method and apparatus for controlling an electronic variable valve apparatus having advantages of a stable control with minimized usage of map tables by employing a sliding surface calculation. 
   An exemplary embodiment of the present invention provides an apparatus for controlling an electronic variable valve apparatus that includes: a cam angle sensor that detects a cam angle; a crank angle sensor that detects a crank angle; and a controller that determines a target cam angle and controls the electronic variable valve apparatus in order to achieve the target cam angle, based on a sliding surface calculation. 
   The controller may include: a synchronization unit that obtains a current cam angle by synchronizing a cam angle signal and a crank angle signal; a comparator that obtains a deviation between the target cam angle and the current cam angle; a control unit that outputs a control signal for adjusting the cam angle for advance and retardation based on the deviation received from the comparator; and an actuation unit that controls the electronic variable valve apparatus according to the control signal from the control unit. 
   An exemplary embodiment of the present invention provides a method of controlling a clutch type electronic variable valve apparatus that includes: setting a reference cam angle depending on an engine operation state; detecting a current cam angle, an engine speed, and an engine oil temperature; calculating a sliding surface; calculating a deviation of the current cam angle from the reference cam angle; determining whether the calculated deviation is above a reference value; calculating an estimated current for maintaining the sliding surface when the calculated deviation is above the reference value; calculating a application current using the calculated sliding surface and the estimated current; and adjusting the current cam angle by operating the electronic variable valve apparatus by a driving duty ratio that is converted from the application current. 
   According to an exemplary embodiment of the present invention, a clutch type electronic variable valve apparatus may be stably controlled regardless of variations of engine condition such as an engine oil temperature. 
   The above features and advantages of the present invention will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated in and form a part of this specification, and the following Detailed Description of the Invention, which together serve to explain by way of example the principles of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other features of the present invention will now be described in detail with reference to certain exemplary embodiments thereof illustrated the accompanying drawings which are given hereinbelow by way of illustration only, and thus are not limitative of the present invention, and wherein: 
       FIG. 1  is a block diagram for an apparatus for controlling an electronic variable valve apparatus of an internal combustion engine according to an exemplary embodiment of the present invention; 
       FIG. 2  is a block diagram that shows a detailed configuration of an apparatus for controlling an electronic variable valve apparatus of an internal combustion engine according to an exemplary embodiment of the present invention; 
       FIG. 3  is an exploded view of an electronic variable valve apparatus of an internal combustion engine according to an exemplary embodiment of the present invention; 
       FIG. 4  is a flowchart for a method for controlling an electronic variable valve apparatus of an internal combustion engine according to an exemplary embodiment of the present invention; 
       FIG. 5  and  FIG. 6  show graphs obtained by applying the scheme of controlling an electronic variable valve apparatus according to an exemplary embodiment of the present invention in conditions of different engine oil temperatures. 
   

   It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment. 
   In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing. 
   &lt;Description of Reference Numerals Indicating Primary Elements in the Drawings&gt;
           100 : cam angle sensor  200 : crank angle sensor     300 : controller  400 : electronic variable valve apparatus       

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   Hereinafter reference will now be made in detail to various embodiments of the present invention, examples of which are illustrated in the accompanying drawings and described below. While the invention will be described in conjunction with exemplary embodiments, it will be understood that present description is not intended to limit the invention to those exemplary embodiments. On the contrary, the invention is intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims. 
     FIG. 1  is a block diagram for an apparatus for controlling an electronic variable valve apparatus of an internal combustion engine according to an exemplary embodiment of the present invention. 
   As shown in  FIG. 1 , an apparatus for controlling an electronic variable valve apparatus of an internal combustion engine according to an exemplary embodiment of the present invention includes a cam angle sensor  100 , a crank angle sensor  200 , a controller  300 , and an electronic variable valve apparatus  400 . 
   The cam angle sensor  100  detects an angular position of a camshaft (hereinafter called a cam angle) of an internal combustion engine, and provides information for the detected angular position of the camshaft to the controller  300 . 
   The crank angle sensor  200  detects an angular position of a crankshaft (hereinafter called a crank angle) of the internal combustion engine, and provides information for the detected crank angle to the controller  300 . 
   Based on the cam angle received from the cam angle sensor  100  and the crank angle received from the crank angle sensor  200 , the controller  300  determines a target cam angle, and controls the electronic variable valve apparatus  400  by a method involving a sliding surface calculation such that the cam angle may become the target cam angle. 
   The electronic variable valve apparatus  400  receives a control signal from the controller  300 , and controls the cam angle of the camshaft according to the control signal received from the controller  300 . 
     FIG. 2  is a block diagram that shows a detailed configuration of an apparatus for controlling an electronic variable valve apparatus of an internal combustion engine according to an exemplary embodiment of the present invention. 
   As shown in  FIG. 2 , the controller  300  includes a synchronization unit  310 , a comparator  320 , a control unit  330 , and an actuation unit  340 . 
   The synchronization unit  310  obtains a current cam angle by synchronizing a square wave signal for the cam angle received from the cam angle sensor  100  and a square wave signal for the crank angle received from the crank angle sensor  200 . 
   The comparator  320  compares the current cam angle received from the synchronization unit  310  and a target cam angle depending on a driving condition, and outputs the comparison result. 
   The control unit  330  receives the comparison result form the comparator  320 , and outputs a control signal for adjusting the cam angle of the camshaft  600  for advance and retardation. 
   The actuation unit  340  controls an operation of the electronic variable valve apparatus  400  of an electronic clutch type according to the control signal received from the control unit  330  such that a cam angle of the camshaft  600  may become a target angle depending on the current engine state. 
   Spring torque may be regarded as disturbance element for the control system and thus element  500  is added to simulate the spring torque in this model. 
     FIG. 3  is an exploded view of an electronic variable valve apparatus of an internal combustion engine according to an exemplary embodiment of the present invention. 
   As shown in  FIG. 3 , the electronic variable valve apparatus  400  includes an electronic clutch  410 , a damper disk  420 , an advance plate  430 , an exterior shaft  440 , an interior shaft  450 , and a chain sprocket  460 . 
   The electronic clutch  410  is mounted at a chain cover of an engine, and may be magnetized so as to make a contact with the damper disk  420  and generate a frictional force when a control signal is applied. 
   The interior shaft  450  is mounted at an end of the camshaft. The advance plate  430  is engaged with exterior circumference of the interior shaft  450  by helical gears. The exterior shaft  440  is engaged with an exterior circumference of the advance plate  430  by helical gears. Thus, a spline shaft unit is formed. 
   A chain sprocket  460  is placed behind the interior shaft  450  and the exterior shaft  440  of the spline shaft unit on the camshaft, and enables power delivery from a sprocket of a crankshaft by a timing chain. 
   The damper disk  420  is placed in front of the interior shaft  450  of the spline shaft unit. A rear side of the damper disk  420  is supported by the advance plate  430 , and a torsion coil spring is placed between the rear side of the damper disk  420  and the exterior shaft  440 . A friction surface is formed at a front side of the damper disk  420  such that a frictional force is generated by contacting the electronic clutch  410 . 
   Such an electronic variable valve apparatus  400  may be expressed as a second order differential equation of the following Equation 1. 
   
     
       
         
           
             
               
                 
                   
                     Jd 
                     × 
                     
                       
                         ⅆ 
                         
                           θ 
                           2 
                         
                       
                       
                         
                           ⅆ 
                           2 
                         
                         ⁢ 
                         t 
                       
                     
                   
                   + 
                   
                     Dd 
                     × 
                     
                       
                         ⅆ 
                         θ 
                       
                       
                         ⅆ 
                         t 
                       
                     
                   
                   + 
                   
                     ( 
                     
                       
                         Kn 
                         × 
                         θ 
                       
                       + 
                       T 
                     
                     ) 
                   
                 
                 = 
                 
                   μ 
                   × 
                   r 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   d 
                   × 
                   kl 
                   × 
                   I 
                 
               
             
             
               
                 ( 
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
                 ) 
               
             
           
         
       
     
   
   Here, T denotes a spring torque, Jd denotes a momentum inertia, θ denotes a cam angle, Dd denotes a viscosity coefficient, Kn denotes a spring constant, μ denotes a frictional coefficient of the clutch, rd denotes an effective radius of the clutch, Kl denotes an attractive force of the clutch, and I denotes an applied current. 
   The above Equation 1 may be changed to the following Equation 2. 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         ⅆ 
                         
                           θ 
                           2 
                         
                       
                       
                         
                           ⅆ 
                           2 
                         
                         ⁢ 
                         t 
                       
                     
                     + 
                     
                       a 
                       ⁢ 
                       
                         
                           ⅆ 
                           θ 
                         
                         
                           ⅆ 
                           t 
                         
                       
                     
                     + 
                     
                       b 
                       × 
                       θ 
                     
                     + 
                     c 
                   
                   = 
                   
                     d 
                     × 
                     I 
                   
                 
                 , 
                 where 
                 , 
                 
                   
 
                 
                 ⁢ 
                 
                   a 
                   = 
                   
                     Dd 
                     Jd 
                   
                 
                 , 
                 
                   b 
                   = 
                   
                     Kn 
                     Jd 
                   
                 
                 , 
                 
                   c 
                   = 
                   
                     T 
                     Jd 
                   
                 
                 , 
                 
                   d 
                   = 
                   
                     
                       
                         μ 
                         × 
                         r 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         d 
                         × 
                         kl 
                       
                       Jd 
                     
                     . 
                   
                 
               
             
             
               
                 ( 
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
                 ) 
               
             
           
         
       
     
   
   Next, the first step to derive the controller is to decide the expression of error. Therefore, in the Equation 2, an estimated error {tilde over (θ)} is defined as {tilde over (θ)}=θ−θd. The next step is to define a sliding surface S and thereby sliding surface S is defined as S=({tilde over (θ)}′+λ{tilde over (θ)}). S′ can be expressed as the following Equation 3.
 
 S′=dI−aθ′−bθ−c+ 2{tilde over (θ)}″  (Equation 3)
 
   From the Equation 3, an estimated current Î for maintaining the sliding surface may be obtained as the following Equation 4. 
   
     
       
         
           
             
               
                 
                   I 
                   ^ 
                 
                 = 
                 
                   
                     
                       
                         θ 
                         ″ 
                       
                       ⁢ 
                       d 
                     
                     + 
                     
                       a 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         θ 
                         ′ 
                       
                     
                     - 
                     
                       λ 
                       ⁢ 
                       
                         θ 
                         ~ 
                       
                     
                     + 
                     
                       b 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       θ 
                     
                     + 
                     c 
                   
                   d 
                 
               
             
             
               
                 ( 
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   4 
                 
                 ) 
               
             
           
         
       
     
   
   From the Equation 4, an application current Ieq for a nonlinear control is obtained as the following Equation 5.
 
 Ieq−Î+K sgn ( S )  (Equation 5)
 
   Here, K is a control constant. 
   An operation of adjusting a cam angle according to an engine state is described hereinafter. 
   When the engine is running, the chain sprocket  460  is driven by the engine through a timing chain, and accordingly the camshaft connected thereto rotates. 
   The spline shaft unit having the advance plate  430 , the exterior shaft  440 , and the interior shaft  450  that are engaged with each other by helical gears also rotates with the rotation of the chain sprocket  460 . In addition, the damper disk  420  placed in front of the interior shaft  450  also rotates in the same way. 
   At this time, the controller  300  receives a cam angle signal from the cam angle sensor  100  and a crank angle signal from the crank angle sensor  200 , and determines a target cam angle according to a current engine state. Then, the controller  300  outputs a control signal for adjusting the cam angle to the electronic variable valve apparatus  400 . 
   Then, the electronic clutch  410  in the electronic variable valve apparatus  400  is magnetized and moves to the damper disk  420  in the rotational axis so as to make a contact therewith, such that a frictional torque is generated by the friction surface of the damper disk  420 . 
   Therefore, the damper disk  420  receives a force shown in an arrow. 
   Therefore, the interior shaft  450  engaged with the chain sprocket  460  varies an angle of the chain sprocket  460  that is connected with the camshaft. 
   Therefore, the cam angle is varied by the change of the angle of the chain sprocket  460 . 
   While such an operation is performed, the synchronization unit  310  in the controller  300  obtains a current cam angle by comparing signals from the cam angle sensor  100  and the crank angle sensor  200 . 
   The obtained current cam angle is compared with the target cam angle at the comparator  320 , and the comparison result is provided to the control unit  330 . 
   Depending on the comparison result, the control unit  330  varies a level of the current applied to the electronic clutch  410  of the electronic variable valve apparatus  400  through the actuation unit  340  until the current cam angle becomes the target cam angle. 
     FIG. 4  is a flowchart for a method for controlling an electronic variable valve apparatus of an internal combustion engine according to an exemplary embodiment of the present invention. 
   Firstly at step S 101 , the controller  300  calculates a reference cam angle θr depending on an engine operation state. 
   Then, at step S 102 , the controller  300  obtains a current cam angle θ and an engine speed based on signals from the cam angle sensor  100  and the crank angle sensor  200 . 
   In addition, the controller  300  may obtain an oil temperature of engine oil, and calculates a change rate of the cam angle. 
   A control responsiveness of the clutch type electronic variable valve apparatus depends on the condition of the engine speed and engine oil temperature. Therefore, at step S 103 , the controller  300  calculates a compensation current value corresponding to the engine speed and engine oil temperature. 
   Subsequently at step S 104 , the controller  300  sets the sliding surface S with the current cam angle θ, by calculating the estimated error and its derivative. 
   Then, at step S 105 , the controller  300  calculates a deviation of the cam angle as a difference between the current cam angle θ and the reference cam angle θr, and determines whether the deviation is above a reference value, i.e. a minimal permissible deviation. 
   When the cam angle deviation is less than the reference value, the process returns to the step of s 102 . When the cam angle deviation is above the reference value, the controller calculates the estimated current Î for maintaining the sliding surface at step S 106 . 
   Then at step S 107 , the controller  300  calculates the application current Ieq by the Equation 5 using the calculated sliding surface S and the estimated current Î. 
   Then, the controller  300  converts the application current Ieq to a driving duty ratio at step S 108 , and operates the electronic variable valve apparatus  400  by outputting the duty ratio at step S 109  such that the cam angle may become the target cam angle. 
     FIG. 5  shows a graph obtained by applying the scheme of controlling an electronic variable valve apparatus according to an exemplary embodiment of the present invention in a condition that an engine oil temperature is 60° C. and the engine speed 2,000 RPM. 
   As shown in  FIG. 5 , it is experimentally confirmed that the cam angle precisely follows the reference cam angle. 
     FIG. 6  shows a graph obtained by applying the scheme of controlling an electronic variable valve apparatus according to an exemplary embodiment of the present invention in a condition that an engine oil temperature is 0° C. and the engine speed 2,000 RPM. 
   As shown in  FIG. 6 , it is experimentally confirmed that the cam angle precisely follows the reference cam angle without tuning a specific parameter even if engine condition is changed. 
   While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.