Patent Publication Number: US-6708657-B2

Title: Apparatus and method for controlling variable valve timing mechanism

Description:
FIELD OF THE INVENTION 
     The present invention relates to a control technology of a variable valve timing mechanism constituted to variably control valve timing of an intake valve and an exhaust valve by changing a rotation phase of a camshaft relative to a crankshaft using an electromagnetic brake. 
     BACKGROUND OF THE INVENTION 
     There has been known a conventional variable valve timing apparatus for an engine, for changing a rotation phase of a camshaft relative to a crankshaft by controlling a rotation delay of the camshaft relative to the crankshaft based on a friction braking by an electromagnetic brake, to variably control valve timing of intake and exhaust valves of an engine (Japanese Unexamined Patent Publication 10-153104). 
     In this variable valve timing apparatus, for example, a basic control amount of the electromagnetic brake is calculated based on a target rotation phase (target angle) and an engine rotation speed, while calculating a feedback control amount from a deviation between the target rotation phase and an actual rotation phase. Then, a final control amount (for example, duty control amount) is determined from the basic control amount and the feedback control amount, to control a current flowing in an electromagnetic coil constituting the electromagnetic brake. 
     The variable valve timing mechanism according to the above electromagnetic brake control has high response characteristics compared with a variable valve timing mechanism according to an oil pressure control. Therefore, when a change in target angle is large, an actual angle (rotation phase) is abruptly changed, resulting in a possibility that an abrupt change occurs in drivability to bring an engine stall. 
     SUMMARY OF THE INVENTION 
     The present invention has been achieved in view of the foregoing problems, and has an object of suppressing an abrupt change of valve timing, ensuring stable drivability, and preventing an occurrence of engine stall, in a variable valve timing mechanism according to an electromagnetic brake control. 
     In order to achieve the above object, the present invention is constituted to control a variable valve timing mechanism that variably controls valve timing of an intake valve or an exhaust valve by changing a rotation phase of a camshaft relative to a crankshaft due to friction braking of an electromagnetic brake, while limiting a change rate of the rotation phase. 
     According to the present invention, even when a target value of the rotation phase is largely changed, since the change rate of the rotation phase to be actually controlled is limited, an abrupt change of valve timing can be suppressed, to thereby prevent an occurrence of engine stall. 
     The other objects and features of this invention will become understood from the following description with accompanying drawings. 
    
    
     BRIEF EXPLANATION OF THE DRAWING 
     FIG. 1 is a sectional view of a variable valve timing mechanism according to an embodiment of the present invention. 
     FIG. 2 is an exploded perspective view of the variable valve timing mechanism according to the embodiment. 
     FIG. 3 is a block diagram of the variable valve timing mechanism according to the embodiment. 
     FIG. 4 is a flow chart showing a phase control according to a first embodiment. 
     FIG. 5 is a flow chart showing a phase control according to a second embodiment. 
     FIG. 6 is a table set with a coefficient “a” to be used in the second embodiment. 
    
    
     EMBODIMENT 
     An embodiment according to the invention will be explained as follows. 
     FIG. 1 is a sectional view of a variable valve timing mechanism using an electromagnetic brake in the embodiment and FIG. 2 is an exploded perspective view thereof. 
     In variable valve timing mechanism  1  shown in FIG.  1  and FIG. 2, a pulley  2  (or sprocket) is rotatably supported around an axis of an end portion  111  of a camshaft  110  rotatably supported to a cylinder head  120 . Pulley  2  is supported to camshaft  110  in a relative rotatable manner, and is rotated in synchronization with the rotation of a crankshaft of an engine. 
     On an extending line of end portion  111  of camshaft  110  is fixed a transmission member  3  with a gear being formed around an axis thereof, by a bolt  31  and the rotation of pulley  2  is transmitted to transmission member  3  through a transmission mechanism to be described later. 
     A cylindrical drum  41  with a flange is disposed on the same axis as camshaft  110 , and between drum  41  and pulley  2  is disposed a coil spring  42  for urging a rotation phase of drum  41  to retard. That is, a case member  44  is fixed to pulley  2  and an outer peripheral end of coil spring  42  is fixed to an inner peripheral surface portion of case member  44  and an inner peripheral end of coil spring  42  is fixed to an outer peripheral surface of drum  41 . 
     A gear  32  formed around the axis of transmission member  3  is in mesh with a gear  433  formed on an inner periphery of a cylindrical piston member  43  by a helical mechanism with a helical gear. 
     Engagement portions  431 ,  431  are projectingly formed on opposite two portions of an outer peripheral surface of piston member  43 , to be engaged between pawl members  21 ,  21  extending in an axial direction of camshaft  110  from a rotation center portion of pulley  2 . Piston member  43  and pulley  2  are rotated on the same phase by this engagement. 
     Engagement portions  431 ,  431  of piston member  43  are formed with male screws  432  as a center thereof being an axis of piston member  43 , respectively, to be engaged with female screws  411  formed on an inner peripheral surface of drum  41  by a screw function. 
     A drum bearing member  45  is disposed between an outer periphery of transmission member  3  and an inner periphery of drum  41 , to bear the relative rotation of them. A pawl receiving member  7   a  is disposed between drum bearing member  45  and the inner peripheral surface of drum  41 . 
     Pawl receiving member  7   a  is supported by the inner peripheral surface of drum  41  and contacts step portions  22 ,  22  formed on outer peripheral surfaces of tip end portions of pawl members  21 ,  21  to retain pawl members  21 ,  21  in a radial direction of camshaft  110 . 
     A sucked member  46  is formed with an internal spur gear  461  at a rotation center thereof and the gear  461  is engaged with a spur gear  33  formed on a tip end portion of transmission member  3 . Thereby, sucked member  46  is constituted to be slidable to transmission member  3  in an axial direction of transmission member  3  and also rotatable on the same phase as transmission member  3 . 
     A gear  413  is formed on a side surface of a flange portion  412  of drum  41  to face a gear  463  formed on one surface  462  of sucked member  46 . As a result, both of these gears are in mesh to engage drum  41  and sucked member  46  in the rotation direction. 
     A first electromagnetic solenoid  5   b  and a second electromagnetic solenoid  5   a  are positioned through a bearing member  6  so as to surround an axis line of camshaft  110 , and also to surround transmission member  3  fixed to the end portion  111  of camshaft  110 , and an outer peripheral surface of bolt  31  fixing transmission member  3 . 
     A spacer member  47  is inserted fixedly between a head portion  311  of bolt  31  and the tip end portion of transmission member  3  and, on an outer peripheral surface side of spacer member  47 , second electromagnetic solenoid  5   a  is disposed through bearing member  6 . Further, first electromagnetic solenoid  5   b  constituting an electromagnetic brake is disposed between second electromagnetic solenoid  5   a  and an outer peripheral surface of sucked member  46 . Second electromagnetic solenoid  5   a  is fixed to a case  8  by a bolt  51   a.    
     An operation of the embodiment will be explained as follows. 
     In order to change a rotation phase of camshaft  110  into an advance side, piston member  43  is moved to the axial direction of camshaft  110  by a magnetic field generated by first electromagnetic solenoid  5   b.    
     Namely, first of all, when sucked member  46  is sucked by the magnetic field generated by second electromagnetic solenoid  5   a , gear  463  of sucked member  46  and gear  413  of drum  41  are separated from each other, so that drum  41  can be relatively rotated to pulley  2 . 
     Then, drum  41  is sucked by the magnetic field generated by first electromagnetic solenoid  5   b  to be pushed against an end face of first electromagnetic solenoid  5   b , thereby performing a friction braking. Accordingly, drum  41  is subjected to a relative rotation due to a rotation delay to pulley  2  against an urging force of coil spring  42 , and piston member  43  in mesh by screw  411  and screw  432  is moved to the axial direction of camshaft  110 . Since piston member  43  and transmission member  3  are engaged by the helical mechanism, the rotation phase of transmission member  3 , as well as camshaft  110  is changed to the advance side to pulley  2  by the movement of piston member  43 . As a result, as a current value to first electromagnetic solenoid  5   b  is increased and a braking force (slide friction) against the urging force of coil spring  42  is increased, the rotation phase of camshaft  110  is changed further to the advance side of camshaft  110 . 
     As described above, since the rotation phase of camshaft  110  is changed to pulley  2  (crankshaft) depending on a rotation delay amount of drum  41  determined corresponding to the braking force by the electromagnetic brake and the braking force of the electromagnetic brake is controlled by duty-controlling a current value supplied to first electromagnetic solenoid  5   b , a change amount (advance amount) of the rotation phase can be continuously controlled by changing a duty ratio. The current value supplied to first electromagnetic solenoid  5   b  is increased in response to an increase in duty value (%) equivalent to a control amount of the electromagnetic brake. 
     FIG. 3 is a block diagram showing a control system of the variable valve timing mechanism having the above constitution. A control unit  511  incorporating therein a microcomputer for controlling the power supply to first electromagnetic solenoid  5   b  and second electromagnetic solenoid  5   a , is input with detections signals from an air flow meter  512  for detecting an engine intake air amount, a crank angle sensor  513  for detecting a crank rotation, a water temperature sensor  514  for detecting an engine cooling water temperature, an atmosphere temperature sensor  515  for detecting an atmosphere temperature, a cam sensor  516  for detecting a cam rotation and the like. 
     Control unit  511  duty-controls the power supply to first electromagnetic solenoid  5   b  to change the rotation phase of camshaft  110 . When the rotation phase reaches a target rotation phase, gear  463  of sucked member  46  and gear  413  of drum  41  are engaged with each other by cutting off the power supply to second electromagnetic solenoid  5   a , and drum  41  is fixed in a phase state at that time to pulley  2 , to cut off the power supply to first electromagnetic solenoid  5   b.    
     The duty control provided with a limiting function of the rotation phase change rate according to the invention will be described. The description will be made for the case where the variable valve timing mechanism is applied to the one that controls valve timing of an intake valve. It is assumed that a target angle is increased when the valve timing of the intake valve is controlled to an advance direction. 
     FIG. 4 shows a flowchart of a first embodiment of the duty control. 
     In FIG. 4, at S 1 , engine operating conditions an intake air amount, an engine rotation speed and the like are read in. 
     The engine rotation speed is calculated based on a detection signal from crank angle sensor  513 . 
     At S 2 , a basic target value (basic target angle) of a rotation phase is determined based on an engine load such as a basic fuel injection quantity Tp and an engine rotation speed Ne. 
     At S 3 , the number of output of detection signal Ref from cam sensor  515  in one job cycle (for example, 10 ms) of this routine is counted up. The number of output is proportional to engine rotation speed Ne. 
     At S 4 , a control direction of the valve timing is judged based on a change direction of the basic target angle. 
     When it is judged that the valve timing of the intake valve is controlled to the advance direction at S 4 , control proceeds to S 5 , wherein a limit value of a target angle change rate which is a maximum change amount of target angle is calculated as follows. 
     
       
         Limit value of target angle change rate=Advance side limit value of target angle change at advance side×Ref count value. 
       
     
     Here, the advance side limit value of target angle change rate is a fixed value set according to an advance direction control. By multiplying this limit value and Ref count value proportional to the engine rotation speed, the limit value of the target angle change rate is calculated. 
     When it is judged that the valve timing of the intake valve is controlled to a retarded condition at S 4 , control proceeds to S 6 , wherein the limit value of target angle change rate is calculated as follows. 
     
       
         Limit value of target angle change rate=Retarded side limit value of target angle change×Ref count value. 
       
     
     Here, the retarded side limit value of target angle change rate is a fixed value set according to a retard direction control, and is set to a value larger than the advance side limit value of target angle change rate. 
     Then control proceeds to S 7 , wherein it is judged whether or not deviation (absolute value) between the basic target angle set at S 2  and the target angle finally set at previous time is larger than the above calculated limit value of target angle change rate. 
     When it is judged that the deviation is larger than the limit value of target angle change rate at S 7 , control proceeds to S 8 , wherein the control direction of the valve timing is judged again in the same manner as at S 4 . 
     When it is judged that the valve timing of the intake valve is controlled to the advance direction, control proceeds to S 9 , wherein a final target angle is calculated by adding the limit value of target angle change rate for advance direction control calculated at S 5  to the target angle finally set at previous time. 
     When it is judged that the valve timing of the intake valve is controlled to the retarded direction at S 8 , control proceeds to S 10  wherein a final target angle is calculated by subtracting the limit value of target angle change rate for retarded direction control calculated at S 6  from the target angle finally set at previous time. 
     When it is judged that the deviation is equal to or less than the limit value of target angle change rate, control proceeds to S 11 , wherein a final target angle is determined as the basic target angle set at S 2 . 
     Next, control proceeds to S 12 , wherein, based on the target angle, a basic-duty value is retrieved from a basic duty map storing the basic duty value (basic control amount) which controls power supply to second electromagnetic solenoid  5   a.    
     At S 13 , a hysteresis duty value is computed from a table based on the engine rotation speed. In general, as the engine rotation speed is lower, the engine temperature is low, and also a lubricating oil supply amount is reduced, thereby resulting in an increase of viscosity resistance in the advance and retarded direction rotation of the camshaft. Therefore, the hysteresis value is set to a larger hysteresis duty value corresponding to the above condition. In the case of advance direction control, a positive hysteresis duty value is set, while in the case of retarded direction control, a negative hysterisis duty value is set. 
     At S 14 , a feedback duty value is computed by a PID operation (proportional integral and derivative). 
     At S 15 , a final duty value is calculated by adding the basic duty value, the feedback duty value, and the hysteresis duty value. At next S 16 , power supply to first electromagnetic solenoid  5   b  is controlled based on the final duty value. 
     In this way, when the change amount of target angle (basic target angle) set according to the engine operating condition is large, the change amount of target angle is limited by the limit value of target angle change rate. Therefore, an abrupt change of drivability can be restricted and an occurrence of engine stall can be prevented. 
     At the advance direction control in which an influence of drivability change caused by a change in valve timing is large and at the low engine rotation speed, the limit value of target angle change rate is further reduced to ensure the restraining function of the drivability change. At the retarded direction control in which the influence of drivability change caused by the change in valve timing is relatively small and at the high engine rotation speed, the limit value of target angle change rate is relatively increased to ensure response characteristics. 
     FIG. 5 shows a flowchart of a second embodiment for the above duty control. 
     The second embodiment is different from the first embodiment in FIG. 4 in that at S 23 , a coefficient “a” corresponding to the engine rotation speed is retrieved from a table as shown in FIG. 6, and when the limit value of target angle change rate is calculated at S 25  and S 26 , the limit value is set by multiplying the coefficient “a” and the advance side limit value of target angle change rate or the retarded side limit value of target angle change rate. The change rate of target angle can be limited as adapted better for the engine rotation speed. 
     In the case where the variable valve timing mechanism is applied to the one that controls valve timing of an exhaust valve, since a valve overlap amount is increased when the valve timing of the exhaust valve is controlled to a retarded direction, the retarded side limit value of target angle change rate may be set to a value larger than the advance side limit value of target angle change rate. 
     The entire contents of a basic Japanese Patent Application No. 2001-154370 filed May 23, 2001, a priority of which is claimed, are herein incorporated by reference.