Patent Publication Number: US-7721691-B2

Title: Variable valve mechanism for internal combustion engine

Description:
TECHNICAL FIELD 
   The present invention relates to a variable valve mechanism that allows a maximum lift amount of a valve to be varied using a variable mechanism. More particularly, the present invention relates to a variable valve mechanism that has a control shaft operating a variable mechanism connected via a worm gear mechanism to an actuator rotatably driving the control shaft. 
   BACKGROUND ART 
   A known variable valve mechanism, as that disclosed, for example, in Japanese Patent Laid-open No. 2000-234507, varies a maximum lift amount and open/close timing of a valve according to an engine operating condition. The variable valve mechanism disclosed in Japanese Patent Laid-open No. 2000-234507 includes a variable mechanism and an actuator. The variable mechanism varies the maximum lift amount and open/close timing of a valve according to an angular position of a control shaft. The actuator controls the angular position of the control shaft. The actuator is connected to the side of a worm gear of a worm gear mechanism. The control shaft is connected to the side of a worm wheel of the worm gear mechanism. Accordingly, rotation of the actuator is inputted to the control shaft with rotational speed thereof being reduced by the worm gear mechanism. 
   The known variable valve mechanism cited above includes a restriction mechanism that restricts maximum angular positions of the control shaft in forward and backward rotation. The restriction mechanism includes a restriction pin that rotates integrally with the worm wheel and a restriction member fixed to an accommodation cover of the worm gear mechanism. The restriction pin abuts on the restriction member, so that the worm wheel is prevented from further rotating. The maximum angular position of the control shaft is thereby restricted. In addition, an elastic body is integrally fixed to the restriction member to absorb impact that would otherwise be received when the restriction pin contacts the restriction member. 
   Including the above-mentioned document, the applicant is aware of the following documents as a related art of the present invention. 
   Japanese Patent Laid-open No. 2000-234507 
   Japanese Patent Laid-open No. 2002-349215 
   DISCLOSURE OF THE INVENTION 
   In the aforementioned known variable valve mechanism, however, the actuator can be rotated to exceed a limit amount because of a system failure or the like. In such cases, though the angular position of the worm wheel is directly restricted by the restriction mechanism, the maximum angular position of the worm gear can only be indirectly restricted by the worm wheel. Consequently, a screw-in action of the worm gear has causes the worm gear to be in excessive mesh with the worm wheel, resulting at times in a locked-up or damaged worm gear mechanism. 
   The present invention addresses these problems discussed above and it is an object of the present invention to provide, for a variable valve mechanism that has a control shaft operating a variable mechanism connected via a worm gear mechanism to an actuator rotatably driving the control shaft, a structure that can prevent the worm gear mechanism from being locked up or damaged by an excessive rotation of the actuator and the variable valve mechanism from being damaged by an excessive rotation of the control shaft. 
   In accomplishing the above object, according to a first aspect of the present invention, there is provided a variable valve mechanism for an internal combustion engine comprising: a variable mechanism for varying a maximum lift amount of a valve according to an angular position of a control shaft; and an actuator connected to the control shaft via a worm gear mechanism, the variable valve mechanism varying the maximum lift amount of the valve by rotatably driving the control shaft via the worm gear mechanism using the actuator; wherein the worm gear mechanism includes a worm gear connected to the actuator and a worm wheel connected to the control shaft; and wherein the worm wheel has teeth that are in mesh with the worm gear over a predetermined angular range including a required rotational range of the control shaft and is formed to be brought out of mesh with the worm gear outside the predetermined angular range. 
   According to the first aspect of the present invention, the worm wheel has the teeth that are in mesh with the worm gear over a predetermined angular range thereof. If the actuator rotates to exceed a limit amount as a result of a system failure or the like, a point of contact between the worm wheel and the worm gear exceeds the predetermined angular range, which brings the worm wheel and the worm gear out of mesh with each other. This shuts off an input of rotation from the worm gear to the worm wheel. A locked-up or damaged worm gear mechanism as a result of screw-in action of the worm gear or a damaged variable valve mechanism as a result of excessive rotation of the control shaft can be prevented. 
   According to a second aspect of the present invention, there is provided the variable valve mechanism as described in the first aspect, wherein the required rotational range of the control shaft includes an angular range of the control shaft ranging from an angular position that corresponds to a minimum setting value of the maximum lift amount of the valve to an angular position that corresponds to a maximum setting value thereof; and wherein the predetermined angular range is set such that, when the control shaft rotates in a small lift direction to exceed the angular position associated with the minimum setting value, the worm wheel and the worm gear are brought out of mesh with each other before the maximum lift amount of the valve reaches a minimum limit value required for achieving a marginal amount of intake air that allows an optimum operating condition of the internal combustion engine to be maintained. 
   According to the second aspect of the present invention, even if the control shaft rotates in the small lift direction to exceed the angular position associated with the minimum setting value of the maximum lift amount of the valve as a result of the actuator&#39;s rotating to exceed the limit amount because of a system failure or the like, the worm wheel and the worm gear are brought out of mesh with each other before the maximum lift amount of the valve reaches the minimum limit value. This prevents the control shaft from rotating any further. Accordingly, the maximum lift amount of the valve can be prevented from becoming smaller than the minimum limit value, thus achieving the marginal amount of intake air that allows an optimum operating condition of the internal combustion engine to be maintained. 
   According to a third aspect of the present invention, there is provided the variable valve mechanism as described in the first aspect, wherein the required rotational range of the control shaft includes an angular range of the control shaft ranging from an angular position that corresponds to a minimum setting value of the maximum lift amount of the valve to an angular position that corresponds to a maximum setting value thereof; and wherein the predetermined angular range is set such that, when the control shaft rotates in a large lift direction to exceed the angular position associated with the maximum setting value, the worm wheel and the worm gear are brought out of mesh with each other before the maximum lift amount of the valve reaches a maximum limit value that can prevent a collision between the valve and a piston. 
   According to the third aspect of the present invention, even if the control shaft rotates in the large lift direction to exceed the angular position that corresponds to the maximum setting value of the maximum lift amount of the valve as a result of the actuator&#39;s rotating to exceed the limit amount because of a system failure or the like, the worm wheel and the worm gear are brought out of mesh with each other before the maximum lift amount of the valve reaches the maximum limit value. This prevents the control shaft from rotating any further. Accordingly, the maximum lift amount of the valve can be prevented from becoming larger than the maximum limit value, thus avoiding a collision between the valve and the piston. 
   According to a fourth aspect of the present invention, there is provided the variable valve mechanism as described in the first aspect, wherein the variable mechanism includes: a rocking member that rocks about an axis disposed in parallel with a camshaft; a rocking cam surface formed on the rocking member, the rocking cam surface coming in contact with a valve support member supporting the valve to press the valve in a lift direction; a slide surface formed on the rocking member so as to oppose to a cam; an intermediate member sandwiched between the cam and the slide surface; and an operative coupling mechanism that varies a position of the intermediate member on the slide surface through operative coupling with rotation of the control shaft, wherein the predetermined angular range is set such that the worm wheel and the worm gear are brought out of mesh with each other before the position of the intermediate member on the slide surface reaches an extreme end of the slide surface when the control shaft rotates to exceed the required rotational range. 
   According to the fourth aspect of the present invention, even if the control shaft rotates to exceed the required rotational range as a result of the actuator&#39;s rotating to exceed the limit amount because of a system failure or the like, the worm wheel and the worm gear are brought out of mesh with each other before the position of the intermediate member on the slide surface reaches the extreme end of the slide surface. This prevents the control shaft from rotating any further. This prevents the intermediate member from exceeding the extreme end of the slide surface so that the intermediate member may not fall off the space between the cam and the slide surface. 
   According to a fifth aspect of the present invention, there is provided the variable valve mechanism as described in any one of the first through fourth aspects, further including: an urge means for urging the worm wheel toward a side, in which teeth of the worm wheel are engaged with the worm gear, if the worm wheel and the worm gear are brought out of mesh with each other as a result of an excessive rotation of the worm wheel. 
   According to the fifth aspect of the present invention, the worm wheel has teeth thereof engaged with the worm gear even if the worm wheel and the worm gear are brought out of mesh with each other. The worm wheel can therefore be brought into mesh with the worm gear by turning the worm gear in a backward direction. This allows the control shaft to be rotated again via the worm gear mechanism, making it possible to resume the operation of the variable valve mechanism quickly. 
   According to a sixth aspect of the present invention, there is provided the variable valve mechanism as described in the fifth aspect, wherein the urge means includes: a first spring that urges the worm wheel in the small lift direction with a spring force according to an amount of rotation of the worm wheel in the large lift direction; and a second spring that urges the worm wheel in the large lift direction with a spring force according to an amount of rotation of the worm wheel in the small lift direction. 
   According to the sixth aspect of the present invention, by using a spring as a means for urging the worm wheel, the worm wheel can be urged with an urging force according to the amount of rotation of the worm wheel in the direction opposite to the direction of rotation. This prevents an excessive force from acting between the worm wheel and the worm gear while the two are in mesh with each other. The worm wheel and the worm gear can also be reliably brought into mesh with each other, should the two are brought out of mesh with each other. 
   According to a seventh aspect of the present invention, there is provided the variable valve mechanism as described in any one of the first through sixth aspects, further including: an angular position sensor for producing an output of a signal in response to an angular position of the control shaft; a control means for controlling the actuator such that the angular position of the control shaft is made to coincide with a target angular position based on the signal of the angular position sensor; a switch a signal of which is changed before and after a predetermined reference angular position when the control shaft rotates; and a correction means for correcting the signal of the angular position sensor based on deviation between a signal to be outputted from the angular position sensor when the control shaft is at the reference angular position and a signal actually outputted from the angular position sensor when the signal of the switch changes. 
   According to the seventh aspect of the present invention, a signal correction is made with reference to a change in the switch signal when the actuator is to be controlled based on the signal of the angular position sensor. This can prevent the angular position of the control shaft from being deviated due to deviation in the signal of the angular position sensor. Consequently, the control shaft can be prevented from being rotated in excess of the required rotational range as affected by deviation in the signal of the angular position sensor due to a voltage drop or the like. 
   According to an eighth aspect of the present invention, there is provided the variable valve mechanism as described in any one of the first through sixth aspects, further including: an angular position sensor for producing an output of a signal in response to an angular position of the control shaft; a control means for controlling the actuator such that the angular position of the control shaft is made to coincide with a target angular position based on the signal of the angular position sensor; and a correction means for correcting the signal of the angular position sensor based on the relationship between the magnitude of a power supplied to the actuator and the signal of the angular position sensor. 
   According to the eighth aspect of the present invention, a signal correction is made with reference to the magnitude of the power supplied to the actuator when the actuator is to be controlled based on the signal of the angular position sensor. This can prevent the angular position of the control shaft from being deviated due to deviation in the signal. Consequently, the control shaft can be prevented from being rotated in excess of the required rotational range as affected by deviation in the signal of the angular position sensor due to a voltage drop or the like. 
   In accomplishing the above object, according to a ninth aspect of the present invention, there is provided a drive system including a worm gear mechanism reducing a rotational speed of an actuator and a drive shaft outputting rotation with a reduced speed, wherein the worm gear mechanism includes a worm gear connected to the actuator and a worm wheel connected to the drive shaft; and wherein the worm wheel has teeth formed thereon only for a predetermined angular range including a required rotational range of the drive shaft, with which the worm gear meshes, and the worm wheel is brought out of mesh with the worm gear over any ranges outside the predetermined angular range. 
   According to the ninth aspect of the present invention, the worm wheel has teeth formed thereon only for a predetermined angular range. If the actuator rotates to exceed the limit amount as a result of a system failure or the like, the point of contact between the worm wheel and the worm gear exceeds the predetermined angular range, thus bringing the worm wheel and the worm gear out of mesh with each other. This shuts off an input of rotation from the worm gear to the worm wheel. A locked-up or damaged worm gear mechanism as a result of screw-in action of the worm gear or a damaged element being driven as a result of excessive rotation of the drive shaft can be prevented. It is to be noted that the drive system according to the ninth aspect of the present invention is applicable to not only the variable valve mechanism for the internal combustion engine, but also any mechanism or system to be driven having a limited angular range of an input shaft (drive shaft). 
   According to a tenth aspect of the present invention, there is provided the drive system as described in the ninth aspect, further including: an urge means for urging the worm wheel toward a side, in which the teeth of the worm wheel are engaged with the worm gear when the worm wheel and the worm gear are brought out of mesh with each other as a result of an excessive rotation of the worm wheel. 
   According to the tenth aspect of the present invention, the teeth of the worm wheel remain engaged with the worm gear even when the worm wheel and the worm gear are brought out of mesh with each other. By rotating the worm gear in the opposite direction, therefore, the worm wheel and the worm gear can be once again brought in mesh with each other. This allows the control shaft to be rotated again via the worm gear mechanism, making it possible to resume the operation of the element to be driven quickly. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a perspective view for illustrating a general structure of a variable valve mechanism according to a first embodiment of the present invention. 
       FIG. 2  is a view showing the variable valve mechanism as viewed from the direction of arrow A in  FIG. 1 . 
       FIGS. 3A and 3B  are views showing lift operations of the variable valve mechanism,  FIG. 3A  showing a condition of the variable valve mechanism, in which a valve is closed and  FIG. 3B  showing a condition of the variable valve mechanism, in which the valve is open. 
       FIGS. 4A and 4B  are views showing operations for changing a maximum lift amount of the variable valve mechanism,  FIG. 4A  showing a condition of a large lift and  FIG. 4B  showing a condition of a small lift. 
       FIG. 5  is a view showing a worm gear mechanism as viewed from the direction of arrow B in  FIG. 1 . 
       FIG. 6A  is a view showing a condition, in which a control shaft rotates in a large lift direction to exceed a correct operating range in the arrangement shown in  FIG. 5 . 
       FIG. 6B  is a view showing a condition, in which the control shaft rotates in a small lift direction to exceed the correct operating range in the arrangement shown in  FIG. 5 . 
       FIG. 7A  is a view showing a worm gear mechanism according to a second embodiment of the present invention as viewed from the direction of arrow B of  FIG. 1 . 
       FIG. 7B  is a view showing a condition, in which the control shaft rotates in the small lift direction to exceed the correct operating range in the arrangement shown in  FIG. 7A . 
       FIG. 8  is a diagram showing changes in a signal from a reference switch and changes in a signal from a lift sensor relative to an angular position of a worm wheel. 
       FIG. 9  is a diagram showing changes in the magnitude of a supply current fed to a motor and changes in the signal from the lift sensor relative to the angular position of the worm wheel. 
   

   BEST MODE FOR CARRYING OUT THE INVENTION 
   First Embodiment 
   A first embodiment of the present invention will be described below with reference to  FIGS. 1 through 7B . 
   General Structure of the Variable Valve Mechanism According to this Embodiment 
     FIG. 1  is a perspective view showing a general structure of a variable valve mechanism according to the first embodiment of the present invention. Referring to  FIG. 1 , a variable valve mechanism  100  according to the embodiment of the present invention is interposed between a camshaft  120  and intake valves  104 . The variable valve mechanism  100  operatively couples a rotational movement of a cam  122  to a vertical movement of the intake valves  104 . The variable valve mechanism  100  includes a control shaft  132  that is disposed in parallel with the camshaft  120 . Varying an angular position of the control shaft  132  allows an operative coupling condition to be changed between the rotational movement of the cam  122  and the vertical movement of the intake valves  104 , which, in turn, varies an acting angle and a maximum lift amount of the intake valves  104 . 
   An internal combustion engine has the variable valve mechanism  100  for each cylinder though they are omitted in  FIG. 1 . For example, for an inline four-cylinder engine, four variable valve mechanisms  100  are disposed in series with the camshaft  120 . Only one control shaft  132  is disposed in parallel with the camshaft  120  and the variable valve mechanism  100  of each cylinder shares this control shaft  132 . Accordingly, the variable valve mechanisms  100  for all four cylinders are simultaneously controlled by controlling the angular position of this single control shaft  132 , so that the acting angles and the maximum lift amounts of all intake valves  104  can be varied simultaneously. 
   The control shaft  132  is rotatably driven by a motor  10  that serves as an actuator. A worm wheel  30  is secured to an end portion of the control shaft  132 . A worm gear  20  fixed to an output shaft  12  of the motor  10  is in mesh with the worm wheel  30 . The worm wheel  30  and the worm gear  20  constitute a gear mechanism (worm gear mechanism). Rotation of the motor  10  is inputted to the worm wheel  30  via the worm gear  20 . This varies the angular position of the control shaft  132 , which simultaneously achieves changing of the acting angles and the maximum lift amounts of all intake valves  102 . The variable valve mechanism  100  according to the embodiment of the present invention is characterized in arrangements of the worm wheel  30 , which will be detailed later. 
   Rotation of the motor  10  is controlled by an ECU (Electronic Control Unit)  60  that provides an overall control of the internal combustion engine. The ECU  60  controls a rotational movement of the motor  10  by using a signal outputted from a lift sensor  50  as a reference signal. The lift sensor  50  is an angular position sensor mounted on an end of the control shaft  132 . The lift sensor  50  produces an output of a signal according to the angular position of the control shaft  132 . 
   Detailed Arrangement of Variable Valve Mechanism 
   The arrangement of the variable valve mechanism  100  will be described in detail below. 
     FIG. 2  is a view showing the variable valve mechanism  100  as viewed from the direction of arrow A that extends in parallel with an axis of the control shaft  132  in  FIG. 1 . As shown in  FIG. 2 , the intake valve  104  is supported by a rocker arm  110  in the variable valve mechanism  100 . A variable mechanism  130  is interposed between the cam  122  and the rocker arm  110 . The variable mechanism  130  operatively couples a rocking movement of the rocker arm  110  to a rotational movement of the cam  122 . The variable mechanism  130  is capable of continuously changing an operative coupling condition between the rotational movement of the cam  122  and the rocking movement of the rocker arm  110 . The variable valve mechanism  100  is adapted to variably control the variable mechanism  130  so as to change a rocking movement and rocking timing of the rocker arm  110 , thereby continuously changing valve opening characteristics of the intake valve  104  including the maximum lift amount, acting angle, and valve timing. 
   The variable mechanism  130  includes the aforementioned control shaft  132 . A control arm  162  is secured to the control shaft  132 . The control arm  162  protrudes in a radial direction of the control shaft  132 . An arcuate link arm  164  is fitted to the protrusion. The link arm  164  has a proximal end portion rotatably connected to the control arm  162  by a pin  166 . The pin  166  is eccentric from a center of the control shaft  132 , serving as a fulcrum of rocking motion of the link arm  164 . 
   In addition, a rocking cam arm  150  is rockably supported on the control shaft  132 . The rocking cam arm  150  is disposed in pair so as to sandwich the control arm  162 . The internal combustion engine according to the embodiment of the present invention includes two intake valves  104  for each cylinder, though they are omitted in  FIG. 2 . Accordingly, the variable valve mechanism  100  is arranged so as to drive two intake valves  104 . The rocking cam arm  150  is disposed in association with each of the intake valves  102 . 
   The rocking cam arm  150  is disposed such that a leading end thereof is oriented toward an upstream side in the direction of rotation of the cam  122 . In accordance with the embodiment of the present invention, the camshaft  120  rotates in a clockwise direction as shown by an arrow in  FIG. 2 . The rocking cam arm  150  includes a slide surface  156  formed on a side thereof opposing the cam  122 . The slide surface  156  contacts a second roller  174  to be described later. The slide surface  156  is curved mildly toward the side of the cam  122  and is formed such that the distance from a center of the cam  122  becomes greater at farther distances from a center of the control shaft  132  as the center of rocking. 
   A rocking cam surface  152  ( 152   a ,  152   b ) is formed on a side of the rocking cam arm  150  opposite the slide surface  156 . The rocking cam surface  152  includes a non-acting face  152   a  and an acting face  152   b . The non-acting face  52   a  is a peripheral surface of a cam base circle and formed with a constant distance from the center of the control shaft  132 . The acting face  152   b  is formed on a leading end side of the rocking cam arm  150  so as to be connected and continued smoothly into the non-acting face  152   a . The acting face  152   b  is formed such that the distance from the center of the control shaft  132  (i.e., a cam height) becomes greater toward the leading end of the rocking cam arm  150 . When the non-acting face  152   a  is not differentiated from the acting face  152   b  in this specification, the face will be simply referred to as the rocking cam surface  152 . 
   A first roller  172  and the second roller  174  are disposed between the slide surface  156  of the rocking cam arm  150  and a surface of the cam  122 . Both the first roller  172  and the second roller  174  are rotatably supported on a coupling shaft  176  secured to a leading end portion of the aforementioned link arm  164 . The second roller  174  is provided for each of the rocking cam arms  150 . The first roller  172  is disposed between the pair of second rollers  174 . The first roller  172  is in contact with the cam  122 , while the second roller  174  is in contact with the slide surface  156  of the corresponding rocking cam arm  150 . The link arm  164  can pivots about the pin  166 . Accordingly, the first and second rollers  172 ,  174  can rock along the slide surface  156  and the surface of the cam  122 , respectively, while keeping a predetermined distance from the pin  166 . In accordance with the embodiment of the present invention, the control arm  162  and the link arm  164  constitute an operative coupling mechanism that varies the position of the second roller  174  on the slide surface  156  through operative coupling with rotation of the control shaft  132 . 
   The rocking cam arm  150  includes a spring seat  158  formed therein. A lost motion spring  168  having a distal end fixed to a stationary portion of the internal combustion engine is hooked onto the spring seat  158 . The lost motion spring  168  according to the embodiment of the present invention is a compression spring. An urging force from the lost motion spring  168  acts as an urging force pressing the slide surface  156  up against the second roller  174 . The urging force also acts as an urging force pressing the first roller  172  coaxially integrated with the second roller  174  up against the cam  122 . As a result, the first roller  172  and the second roller  174  are positioned correctly by being sandwiched from both sides between the slide surface  156  and the cam  122 . 
   The above-referenced rocker arm  110  is disposed downward of the rocking cam arm  150 . The rocker arm  110  includes a rocker roller  112  disposed so as to oppose the rocking cam surface  152 . The rocker roller  112  is rotatably mounted at a middle portion of the rocker arm  110 . The rocker arm  110  has a first end, to which a valve shaft  102  that supports the intake valve  104  is mounted. The rocker arm  110  also has a second end supported rotatably by a hydraulic lash adjuster  106 . The valve shaft  102  is urged in a closing direction, i.e., a direction of pushing up the rocker arm  110  by a valve spring not shown. Further, the rocker roller  112  is pressed against the rocking cam surface  152  of the rocking cam arm  150  by this urging force and the hydraulic lash adjuster  106 . 
   Operation of Variable Valve Mechanism 
   (1) Lift Operation of the Variable Valve Mechanism 
   The lift operation of the variable valve mechanism  100  will be described with reference to  FIGS. 3A and 3B .  FIG. 3A  is a view showing a condition of the variable valve mechanism, in which the intake valve  104  is closed in a process of lift operation.  FIG. 3B  is a view showing a condition of the variable valve mechanism, in which the intake valve  104  is fully open in the process of lift operation. 
   In the variable valve mechanism  100 , the rotational movement of the cam  122  is first inputted to the first roller  172  that is in contact therewith. The first roller  172 , together with the second roller  174  coaxially integrated therewith, rocks about the pin  166 . This rocking movement is inputted to the slide surface  156  of the rocking cam arm  150  that supports the second roller  174 . The slide surface  156  is pressed up against the second roller  174  at all times by the urging force of the lost motion spring. Accordingly, the rocking cam arm  150  rocks about the control shaft  132  according to the rotation of the cam  122  transmitted thereto via the second roller  174 . 
   More specifically, when the camshaft  120  rotates from the condition shown in  FIG. 3A , a point of contact of the first roller  172  on the cam  122  approaches a vertex portion of the cam  122  as shown in  FIG. 3B . The first roller  172  is then relatively pressed downward by the cam  122  and the slide surface  156  of the rocking cam arm  150  is pressed down by the second roller  174  integrated with the first roller  172 . As a result, the rocking cam arm  150  is rotated in the clockwise direction in  FIG. 3B  about the control shaft  132 . 
   Rotation of the rocking cam arms  150  shifts a position of contact of the rocker roller  112  on the rocking cam surface  152  from the non-acting face  152   a  to the acting face  152   b . This presses down the rocker arm  110  according to the distance of the acting face  152   b  from the center of the control shaft  132 , causing the rocker arm  110  to rock in the clockwise direction about the point of support by the hydraulic lash adjuster  106 . As a result, the intake valve  104  is pressed down by the rocker arm  110  and opened. Referring to  FIG. 3B , a rotation amount of the rocking cam arm  150  becomes the greatest when the position of contact of the first roller  172  on the cam  122  reaches the vertex portion of the cam  122 . At the same time, the lift amount of the intake valve  104  becomes the greatest in this case. 
   As the camshaft  120  further rotates, the position of contact of the first roller  172  on the cam surface  124  moves past the vertex portion of the cam  122 . Then, the rocking cam arm  150  is rotated this time in a counterclockwise direction in  FIG. 3B  about the control shaft  132  by the urging force of the lost motion spring and the valve spring. Rotation of the rocking cam arm  150  in the counterclockwise direction moves the position of contact of the rocker roller  112  on the rocking cam surface  152  toward the side of the non-acting face  152   a . As a result, the lift amount of the intake valve  104  decreases. When the position of contact of the rocker roller  112  on the rocking cam surface  152  eventually changes from the acting face  152   b  to the non-acting face  152   a  as shown in  FIG. 3A , the lift amount of the intake valve  104  becomes zero. Specifically, the intake valve  104  is closed. 
   (2) Lift Amount Change Operation of Variable Valve Mechanism 
   The lift amount change operation of the variable valve mechanism  100  will be described with reference to  FIGS. 4A and 4B .  FIG. 4A  is a view showing a condition of the variable valve mechanism  100  with the maximum lift amount, in which the variable valve mechanism  100  operates so as to give the intake valve  104  (omitted in  FIG. 4A ) a large lift.  FIG. 4B  is a view showing a condition of the variable valve mechanism  100  with the maximum lift amount, in which the variable valve mechanism  100  operates so as to give the intake valve  104  a small lift. 
   When the maximum lift amount is changed from the lift amount shown in  FIG. 4A  to that shown in  FIG. 4B , the control shaft  132  is rotatably driven, in the condition shown in  FIG. 4A , to rotate in a direction opposite to the rotating direction of the camshaft  120  (i.e., the counterclockwise direction in  FIG. 4A ). The control arm  162  is thereby rotated to an angular position shown in  FIG. 4B . As the control arm  162  is rotated, the second roller  174  moves along the slide surface  156  in a direction away from the control shaft  132 . At the same time, the first roller  172  moves along the cam  122  toward the upstream side in the direction of rotation of the cam  122 . The control arm  162  and the link arm  164  constitute an operative coupling mechanism that varies the position of the second roller  174  on the slide surface  156  through operative coupling with rotation of the control shaft  132 . 
   The second roller  174  moves in the direction away from the control shaft  132 . This results in a longer distance between a rocking center of the rocking cam arm  150  and a position of contact P 2  of the second roller  174  on the slide surface  156  and thus a reduced rocking angular range of the rocking cam arm  150 . This is because the rocking angular range of the rocking cam arm  150  is inversely proportional to the distance between the rocking center and the position of contact P 2  that is an input point of vibration. The reduction in the rocking angular range of the rocking cam arm  150  results in a final position of contact P 3 , to which the rocker roller  112  can reach, being moved on the acting face  152   b  toward the side of the non-acting face  152   a . The maximum lift amount of the intake valve  104  is thereby reduced. An angle during which the rocker roller  112  remains disposed on the acting face  152   b , serves as the acting angle of the intake valve  104 . Movement of the final position of contact P 3  to the side of the non-acting face  152   a  results in a reduced acting angle of the intake valve  104 . 
   When the maximum lift amount is changed from the lift amount shown in  FIG. 4B  to that shown in  FIG. 4A , on the other hand, the control shaft  132  is rotatably driven, in the condition shown in  FIG. 4B , to rotate in the same direction as the rotating direction of the camshaft  120  (i.e., the clockwise direction in  FIG. 4B ). The control arm  162  is thereby rotated to an angular position shown in  FIG. 4A . The second roller  174  then moves in a direction approaching the control shaft  132 . As a result, the distance between the rocking center of the rocking cam arm  150  and the position of contact P 2  of the second roller  174  on the slide surface  156  is shortened, thus increasing the rocking angular range of the rocking cam arm  150 . The increase in the rocking angular range of the rocking cam arm  150  results in the final position of contact P 3 , to which the rocker roller  112  can reach, moving toward the side of a leading end of the acting face  152   b . The maximum lift amount and the acting angle of the intake valve  104  are then increased. 
   Detailed Arrangement of Worm Gear Mechanism 
   The gear mechanism (worm gear mechanism) that transmits a driving force of the motor  10  to the control shaft  132  will be described in detail below. 
     FIG. 5  is a view showing the worm gear mechanism as viewed from the direction of arrow B (the direction opposite to that viewed in  FIGS. 2 through 4B ) that extends in parallel with the axis of the control shaft  132  in  FIG. 1 . As described earlier, the worm gear mechanism includes the worm gear  20  fixed to the output shaft  12  of the motor and the worm wheel  30  fixed to the control shaft  132 . In  FIG. 5 , the more the control shaft  132  rotates in the clockwise direction, the less largely the maximum lift amount of the intake valve  104  is changed. Further, the more the control shaft  132  rotates in the counterclockwise direction, the more largely the maximum lift amount of the intake valve  104  is changed. In the following, rotation of the control shaft  132  in the clockwise direction is referred to as rotation in a small lift direction and rotation thereof in the counterclockwise direction is referred to as rotation in a large lift direction. 
   The worm wheel  30  according to the embodiment of the present invention is formed into a sector shape, and not a circular shape which is commonly found. Accordingly, the worm wheel  30  has teeth  32  formed only on a limited angular range θ WHEEL  thereof. The worm wheel  30  meshes with screw threads  22  of the worm gear  20  only within this limited angular range θ WHEEL  thereof. To state it another way, the worm wheel  30  is out of mesh with the worm gear  20  over any ranges outside this limited angular range θ WHEEL . 
   The above-referenced angular range θ WHEEL  includes a required rotational range θ A  of the control shaft  132 , that is, an angular range of the control shaft  132  from an angular position that corresponds to a minimum setting value of the maximum lift amount of the intake valve  104  to an angular position that corresponds to a maximum setting value thereof. Rotation of the worm wheel  30  in the small lift direction causes a contact point (a contact point on a line extended in an orthogonal direction relative to an axis of the worm gear  20  and connecting the center of the worm wheel  30  with the shortest center distance) P WORM  between the worm wheel  30  and the worm gear  20  to reach a small lift side boundary B MIN  of the required rotational range θ A . At this time, the maximum lift amount of the intake valve  104  becomes the minimum setting value as shown in  FIG. 4B . On the other hand, rotation of the worm wheel  30  in the large lift direction causes the aforementioned contact point to reach a large lift side boundary B MAX  of the required rotational range θ A . At this time, the maximum lift amount of the intake valve  104  becomes the maximum setting value as shown in  FIG. 4A . 
   The aforementioned angular range θ WHEEL  also includes adjustment margins θ B1 , θ B2  set on corresponding ends on the outside on both sides of the required rotational range θ A . These adjustment margins θ B1 , θ B2  are set to eliminate any discrepancies between a design value and an actual value of the required rotational range θ A  that occur as a result of dimensional errors in each element. The margin values are calculated based on tolerances of each element. An angular range of these adjustment margins θ B1 , θ B2  added to the required rotational range θ A  represents a correct operating range of the control shaft  132 . The ECU  60  controls rotation of the motor  10  such that the control shaft  132  rotates through this correct operating range. 
   The angular range θ WHEEL  further includes allowance ranges θ C1 , θ C2  set outside the adjustment margins θ B1 , θ B2 . These allowance ranges θ C1 , θ C2  represent an angular range until the worm wheel  30  no longer rotates after the contact point P WORM  falls outside the correct operating range of the control shaft  132 . When the control shaft  132  rotates in the large lift direction to exceed the correct operating range, the contact point P WORM  enters the allowance range θ C1 . As the contact point P WORM  eventually exceeds the allowance range θ C1 , the worm wheel  30  is out of mesh with the worm gear  20 , thus causing the worm gear  20  to rotate idly. When, on the other hand, the control shaft  132  rotates in the small lift direction to exceed the correct operating range, the contact point P WORM  enters the allowance range θ C2 . As the contact point P WORM  eventually exceeds the allowance range θ C2 , the worm wheel  30  is out of mesh with the worm gear  20 , thus causing the worm gear  20  to rotate idly. 
   Each of the aforementioned allowance ranges θ C1 , θ C2  is set in consideration of, for example, deviation of a signal from the lift sensor  50 . The ECU  60  determines the angular position of the control shaft  132  based on the signal from the lift sensor  50 . Accordingly, if there is any deviation in the signal from the lift sensor  50 , the following event could occur. Specifically, when the control shaft  132  is made to rotate to an angular position associated with the minimum setting value or the maximum setting value of the maximum lift amount, the control shaft  132  may be rotated to exceed slightly the above-referenced correct operating range. If the allowance ranges θ C1 , θ C2  are set excessively largely, however, the variable valve mechanism  100  could be damaged by an excessively rotated control shaft  132 , should the motor  10  rotate erratically because of a system failure or the like. In the worm wheel  30  according to the embodiment of the present invention, therefore, the allowance ranges θ C1 , θ C2  are set as detailed below. 
   The allowance range θ C1  on the large lift side is set based on a maximum limit value of the maximum lift amount of the intake valve  104 . The larger the maximum lift amount of the intake valve  104 , the smaller a clearance between the intake valve  104  and a piston (not shown) when the valve is open. The maximum limit value refers to a limit value of the maximum lift amount, at which collision between the intake valve  104  and the piston can be avoided. The allowance range θ C1  is set such that the worm wheel  30  and the worm gear  20  are out of mesh with each other before the maximum lift amount reaches the above-referenced maximum limit value when the control shaft  132  rotates in the large lift direction to exceed the correct operating range. 
   The allowance range θ C2  on the small lift side is set based on a minimum limit value of the maximum lift amount of the intake valve  104 . The smaller the maximum lift amount of the intake valve  104 , the more the amount of air drawn into a combustion chamber is decreased. The minimum limit value refers to a limit value of the maximum lift amount required to achieve a marginal amount of intake air that allows an optimum operating condition of the internal combustion engine to be maintained. The allowance range θ C2  is set such that the worm wheel  30  and the worm gear  20  are out of mesh with each other before the maximum lift amount reaches the above-referenced minimum limit value when the control shaft  132  rotates in the small lift direction to exceed the correct operating range. 
   Each of the allowance ranges θ C1 , θ C2  is set in consideration also of the position of the second roller  174  on the slide surface  156 . When the control shaft  132  rotates in the large lift direction, the second roller  174  moves on the slide surface  156  toward the leading end position thereof. When the control shaft  132  rotates in the large lift direction, the second roller  174  moves on the slide surface  156  toward the trailing end position thereof. If the second roller  174  exceeds an extreme end of the slide surface  156  as a result of the control shaft  132  rotating excessively, the first roller  172  and the second rollers  174  fall out of a space between the cam  122  and the rocking cam arms  150 . Accordingly, each of the allowance ranges θ C1 , θ C2  is set such that the worm wheel  30  and the worm gear  20  are out of mesh with each other before the position of the second roller  174  on the slide surface  156  reaches the extreme end of the slide surface  156  when the control shaft  132  rotates to exceed the correct operating range. 
   The gear mechanism according to the embodiment of the present invention includes a shock absorber  40  for restricting rotation of the worm wheel  30  in the small lift direction. The shock absorber  40  is disposed in the small lift direction relative to the worm wheel  30  within a plane of rotation of the worm wheel  30 . The shock absorber  40  is fixed to a stationary portion that the internal combustion engine includes. As shown by a dotted line in  FIG. 5 , when the worm wheel  30  rotates in the small lift direction, the worm wheel  30  is adapted to abut against a head portion of the shock absorber  40  before and after the contact point P WORM  enters the allowance range θ C2  on the small lift side. 
   Operation and Effects of Gear Mechanism 
   The operation and effects of the gear mechanism having the arrangements as described heretofore will be described with reference to  FIGS. 6A and 6B . 
   A case will first be described, in which the control shaft  132  rotates in the large lift direction to exceed the correct operating range because of erratic rotation of the motor  10  as a result of a system failure or the like. As described earlier, the teeth  32  of the worm wheel  30  are formed only in the limited angular range θ WHEEL . Moreover, the allowance range θ C1  on the large lift side included in the limited angular range θ WHEEL  is set such that the worm wheel  30  and the worm gear  20  are out of mesh with each other before the maximum lift amount of the intake valve  104  reaches the maximum limit value. Accordingly, if the control shaft  132  rotates in the large lift direction to exceed the correct operating range, the worm wheel  30  and the worm gear  20  are out of mesh with each other, as shown in  FIG. 6A , before the maximum lift amount of the intake valve  104  reaches the maximum limit value, thereby preventing the control shaft  132  from rotating any further. This prevents the maximum lift amount of the intake valve  104  from exceeding and becoming larger than the maximum limit value, thus avoiding a collision between the intake valve  104  and the piston. 
   A case will next be described, in which the control shaft  132  rotates in the small lift direction to exceed the correct operating range. As described earlier, the allowance range θ C2  on the small lift side included in the angular range θ WHEEL , over which the teeth  32  of the worm wheel  30  are formed, is set such that the worm wheel  30  and the worm gear  20  are out of mesh with each other before the maximum lift amount of the intake valve  104  reaches the minimum limit value. Accordingly, if the control shaft  132  rotates in the small lift direction to exceed the correct operating range, the worm wheel  30  and the worm gear  20  are out of mesh with each other, as shown in  FIG. 6B , before the maximum lift amount of the intake valve  104  reaches the minimum limit value, thereby preventing the control shaft  132  from rotating any further. This prevents the maximum lift amount of the intake valve  104  from exceeding and becoming smaller than the minimum limit value. The marginal amount of intake air that allows the operating condition of the internal combustion engine to be maintained optimally can thereby be achieved. 
   The arrangement, in which the worm wheel  30  and the worm gear  20  are out of mesh with each other as described above, may indeed prevent the control shaft  132  from malfunctioning as a result of an excessive rotation thereof. It is, however, not possible to control the angular position of the control shaft  132  if the worm wheel  30  and the worm gear  20  are left out of mesh with each other. To resume operation of the variable valve mechanism  100  by letting the mechanism  100  recover from the failed state, it is necessary to allow the worm wheel  30  to be in mesh with the worm gear  20  once again so that rotation of the control shaft  132  can be controlled. In this respect, the gear mechanism according to the embodiment of the present invention allows the worm wheel  30  to be easily brought back into a state of being in mesh with the worm gear  20  as described below. 
   Torque produced from the reaction force of the lost motion spring and valve spring acts on the control shaft  132  at all times. This torque acts in a direction of closing the intake valve  104 ; specifically, in a direction of rotating the control shaft  132  in the small lift direction. The more the control shaft  132  is positioned at an angular position on the side of the large lift, the greater the magnitude of the torque. Accordingly, in the condition in which the worm wheel  30  and the worm gear  20  are out of mesh with each other because of an excessive rotation of the control shaft  132  in the large lift direction, a torque Ta in the small lift direction acts on the worm wheel  30 . The torque Ta acts to press the worm wheel  30  up against the worm gear  20  at all times as shown in  FIG. 6A , so that a condition is maintained, in which the teeth  32  of the worm wheel  30  are engaged with the screw threads  22  of the worm gear  20 . Accordingly, rotating the motor  10  in the small lift direction allows the screw threads  22  of the worm gear  20  to pull the teeth  32  of the worm wheel  30 , thus making the worm wheel  30  be in mesh with the worm gear  20  again. In the embodiment of the present invention, the lost motion spring and the valve spring correspond to a “first spring” according to an aspect of the present invention. 
   When the control shaft  132  rotates in the small lift direction, on the other hand, the torque produced from the reaction force of the lost motion spring and valve spring decreases. In the meantime, the worm wheel  30  abuts against the shock absorber  40 , which causes torque generated from a reaction force of the shock absorber  40  to act on the control shaft  132 . The magnitude of the torque generated from the shock absorber  40  is directly proportional to a compressed amount of a spring  42 . The magnitude of the torque becomes greater with the control shaft  132  located at angular positions more on the small lift side. Accordingly, in the condition in which the worm wheel  30  and the worm gear  20  are out of mesh with each other because of an excessive rotation of the control shaft  132  in the small lift direction, a torque Tb in the large lift direction acts on the worm wheel  30 . The torque Tb acts to press the worm wheel  30  up against the worm gear  20  at all times as shown in  FIG. 6B , so that a condition is maintained, in which the teeth  32  of the worm wheel  30  are engaged with the screw threads  22  of the worm gear  20 . Accordingly, rotating the motor  10  in the large lift direction allows the screw threads  22  of the worm gear  20  to pull the teeth  32  of the worm wheel  30 , thus making the worm wheel  30  be in mesh with the worm gear  20  again. In the embodiment of the present invention, the spring  42  of the shock absorber  40  corresponds to a “second spring” according to another aspect of the present invention. 
   As described in the foregoing, according to the gear mechanism in accordance with the embodiment of the present invention, the worm wheel  30  can be urged in a direction opposite to the direction of rotation of the worm wheel  30  according to the amount of rotation thereof by using the reaction force of the lost motion spring or valve spring during rotation of the control shaft  132  in the large lift direction and using that of the spring  42  during rotation of the control shaft  132  in the small lift direction. This prevents an excessive force from acting on the worm wheel  30  and the worm gear  20  when the worm wheel  30  and the worm gear  20  are in mesh with each other. Further, the worm wheel  30  and the worm gear  20  can be reliably brought back into mesh with other when the two are out of mesh with each other. 
   Second Embodiment 
   A second embodiment of the present invention will be described below with reference to  FIGS. 7A ,  7 B, and  8 . 
   A variable valve mechanism according to the second embodiment of the present invention is characterized in that an arrangement for correcting deviation of a signal from a lift sensor  50  is newly added to the basic structure of the arrangements according to the first embodiment of the present invention. In each of  FIGS. 7A ,  7 B, and  8 , like reference numerals refer to like parts and duplicate descriptions will be omitted or simplified. 
     FIG. 7A  is a view showing a worm gear mechanism according to the second embodiment of the present invention as viewed from the direction of arrow B of  FIG. 1 .  FIG. 7A  corresponds to  FIG. 5  according to the first embodiment of the present invention. Referring to  FIG. 7A , a shock absorber  40  includes a lever  44  newly added thereto. The lever  44  is fixed to the shock absorber  40 . When a worm wheel  30  abuts on the shock absorber  40 , therefore, the lever  44  is displaced integrally with the shock absorber  40  according to the amount of rotation of the worm wheel  30 . 
   A reference switch  62  is disposed along a trajectory of movement of the lever  44 . The reference switch  62  is connected to an ECU  60 . A signal from the reference switch  62  is outputted to the ECU  60  at all times. The signal from the reference switch  62  turns on from an off state according to abutment of the lever  44 . In  FIG. 7A , when the worm wheel  30  rotates in the small lift direction, the worm wheel  30  abuts on the shock absorber  40 . The angular position of the worm wheel  30  at this time is referred to as “A.” When the worm wheel  30  is located at the angular position A and the shock absorber  40  and the lever  44  are located at a position indicated by a dotted line, the signal of the reference switch  62  is off. As the worm wheel  30  further rotates in the small lift direction to reach the angular position of “B” in  FIG. 7A  and the lever  44  then contacts the reference switch  62  as shown by a solid line in  FIG. 7A , the signal of the reference switch  62  turns on. The angular position B is set so as to be achieved when a control shaft  132  is in the correct operating range. 
     FIG. 7B  is a view showing a condition, in which the worm wheel  30  further rotates in the small lift direction, so that the worm wheel  30  and a worm gear  20  are out of mesh with each other. In this condition, the angular position of the worm wheel  30  is “C” in  FIG. 7B . The angular position “B” of the worm wheel  30  shown in  FIG. 7B  corresponds to position B in  FIG. 7A . With the worm wheel  30  at the angular position B, the shock absorber  40  and the lever  44  are located at positions indicated by a dotted line in  FIG. 7B . While the worm wheel  30  rotates from angular position B to angular position C, the reference switch  62  is kept pressed by the lever  44 . The signal from the reference switch  62  is therefore kept on. 
     FIG. 8  is a diagram showing changes in the signal from the reference switch  62  and changes in the signal from the lift sensor  50  relative to the angular position of a worm wheel  30 . The angular position of the worm wheel  30  has one-to-one correspondence with a maximum lift amount of an intake valve  104 . As described above, the signal of the reference switch  62  is off when the angular position of the worm wheel  30  is located at a position on the large lift side relative to position B. Further, the signal is on when the angular position of the worm wheel  30  is located at a position on the small lift side relative to position B. The signal of the lift sensor  50 , on the other hand, changes in direct proportion to the angular position of the worm wheel  30 . The signal becomes greater as the angular position is more on the large lift side. The ECU  60  determines the current angular position of the worm wheel  30  (angular position of the control shaft  132 ) using the signal from the lift sensor  50 . The ECU  60  then controls rotation of a motor  10  such that the angular position of the worm wheel  30  coincides with a target angular position as determined from an operating condition of an internal combustion engine and the like. 
   The reference switch  62  is a simple structure having its signal turned on or off. If the reference switch  62  is properly installed, there is no likelihood that deviation occurs in the signal relative to the angular position of the worm wheel  30 . With the lift sensor  50 , however, a voltage drop or other effect can produce deviation in the signal. For example, referring to  FIG. 8 , an actual signal indicated by a solid line may be deviated toward the small lift side with reference to a design signal shown by a dash-double-dot line. If there is deviation in the signal from the lift sensor  50  as shown above, an error results in the maximum lift amount of the intake valve  104  controlled based on this signal. 
   The ECU  60  therefore corrects the signal of the lift sensor  50  with reference to the signal of the reference switch  62 . More specifically, the signal of the lift sensor  50  is measured at a time that the signal of the reference switch  62  is turned on from the off state. Any deviation of the thus measured signal from the design signal (“deviation in signal” shown in  FIG. 8 ) is set as a correction signal for correcting the signal of the lift sensor  50 . This makes the signal of the lift sensor  50  that has undergone the correction coincide with the design signal. It therefore becomes possible to control the angular position of the control shaft  132  based on accurate position information. 
   As described in the foregoing, according to the second embodiment of the present invention, the signal of the lift sensor  50  is corrected with reference to the change in the signal of the reference switch  62  when rotation of the motor  10  is controlled based on the signal of the lift sensor  50 . Deviation in the angular position of the control shaft  132  as affected by deviation in the signal of the lift sensor  50  can therefore be prevented. Accordingly, should there be deviation produced in the signal of the lift sensor  50  as affected by a voltage drop or the like, an error can be prevented from occurring in controlling the maximum lift amount of the intake valve  104 , which would otherwise be caused as a result of the deviation. In addition, it is also possible to prevent the control shaft  132  from rotating to exceed the correct operating range, which would otherwise be caused as a result of the deviation in the signal of the lift sensor  50 . 
   Third Embodiment 
   A third embodiment of the present invention will be described below with reference to  FIG. 9 . 
   A variable valve mechanism according to the third embodiment of the present invention is characterized in that the mechanism allows deviation of a signal of a lift sensor  50  to be corrected without adding any new arrangement to the structure of the arrangements according to the first embodiment of the present invention. 
     FIG. 9  is a diagram showing changes in the magnitude of a supply current fed to a motor  10  and changes in the signal from the lift sensor  50  relative to the angular position of a worm wheel  30 . Angular positions A, B, and C indicated on the abscissa of  FIG. 9  correspond, respectively, to the angular positions A, B, and C of the worm wheel  30  exemplified in the second embodiment of the present invention. Referring to  FIG. 9 , the motor supply current decreases gradually as the angular position of the worm wheel  30  changes toward the small lift side. This is because of the following reason. Specifically, with reaction forces of a lost motion spring and a valve spring decreasing, a driving force required of the motor  10  decreases. If the angular position of the worm wheel  30  is on the small lift side more than the angular position A, however, the motor supply current gradually increases. This is the reason why with the worm wheel  30  at an angular position more on the small lift side than the angular position A, the reaction force of a shock absorber  40  acts on the worm wheel  30 . 
   The motor supply current is directly proportional to the driving force required of the motor  10 . The driving force required of the motor  10  is defined by the angular position of a control shaft  10 . Assuming that the relationship between the motor supply current and the driving force, and that between the required driving force and the angular position of the control shaft  10 , are constant, it may be safe to assume that the relationship between the motor supply current and the angular position of the control shaft  10  (angular position of the worm wheel  30 ) is constant. According to the third embodiment of the present invention, the motor supply current is used as a reference signal for correcting deviation of the signal from the lift sensor  50 . A method of correcting deviation of the signal from the lift sensor  50  according to the third embodiment of the present invention will be described below. 
   The motor supply current becomes the minimum value when the worm wheel is located at the angular position A, that is, when the worm wheel  30  abuts on the shock absorber  40 . As the worm wheel  30  rotates further in the small lift direction to be located at the angular position B, the motor supply current increases by ΔA than the minimum value. An ECU  60  uses this difference in current ΔA for correcting the signal of the lift sensor  50 . More specifically, the signal of the lift sensor  50  when the motor supply current increases from the minimum value to the difference in current ΔA is measured and the signal of the lift sensor  50  is corrected by using, as a correction signal, the deviation between the measured signal and the design signal (“deviation in signal” shown in  FIG. 9 ). This makes the signal of the lift sensor  56  after the correction coincide with the design signal. The angular position of the control shaft  132  can thereby be controlled based on accurate position information. 
   As described heretofore, according to the third embodiment of the present invention, the signal of the lift sensor  50  is corrected with reference to changes in the motor supply current when rotation of the motor  10  is controlled based on the signal of the lift sensor  50 . As in the second embodiment of the present invention, therefore, deviation in the angular position of the control shaft  132  as affected by deviation in the signal of the lift sensor  50  can be prevented. Moreover, the third embodiment of the present invention offers the advantage of achieving the same effect as that derived from the second embodiment of the present invention without including any new reference switches in the mechanism. 
   MISCELLANEOUS 
   The present invention has been described with reference to specific embodiments that are to be considered as only illustrative and not restrictive, and the present invention is not to be limited to the details given herein, but can be implemented in various manners without departing from the spirit thereof. For instance, the variable valve mechanism according to the present invention may also be applied to an exhaust valve, in addition to the intake valve, in which the present invention is embodied.