BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates in general to a variable valve system of an internal combustion engine, which is able to vary a lift degree (or work angle) of engine valves (viz., intake and/or exhaust valves) in accordance with an operation condition of the engine. More specifically, the present invention relates to such variable valve system and a method of assembling the same. 
   2. Description of the Related Art 
   Hitherto, in the field of variable valve systems of an internal combustion engine, various types have been proposed and put into practical use. One of the systems is shown in U.S. Pat. No. 6,615,777. 
   The variable valve system of this US patent generally comprises a valve lift varying mechanism that, by rotating a control shaft, varies a lift degree (or work angle) of engine valves (viz., intake and/or exhaust valves) and an actuating mechanism that drives or rotates the control shaft in accordance with an operation condition of the engine. 
   The actuating mechanism comprises an electric motor, an output shaft driven by the electric motor, a screw nut having an inner thread meshed with an outer thread formed on the output shaft, a link member having a forked end pivotally connected to diametrically opposed portions of the screw nut through bearing pins and a lever member having one end pivotally connected to the other end of the link member and the other end rotatably connected to the control shaft through a pin. The control shaft has adjusting cams integrally connected thereto. 
   When, upon energization of the electric motor, the output shaft is rotated about its axis, the screw nut is moved axially forward or rearward along the output shaft pivotally moving both the link member and the lever member. With this, the control shaft is turned about its axis to a desired angular position. 
   SUMMARY OF THE INVENTION 
   However, due to its inherent construction, the actuating mechanism disclosed by the above-mentioned US patent fails to show an exact position control of the screw nut relative to the output shaft. More specifically, because of the nature of the meshed engagement between the screw nut and the output shaft, the maximally moved position of the screw nut relative to the output shaft is not exactly defined or controlled, which tends to induce a poor positioning of the control shaft at the maximally turned angular position. Of course, such poor positioning of the control shaft has an undesired influence on the controllability of the variable valve system. 
   It is therefore an object of the present invention to provide a variable valve system of an internal combustion engine, which is free of the above-mentioned drawback. 
   That is, an object of the present invention is provide a variable valve system of an internal combustion engine, which can exhibit a satisfied controllability of the system throughout all ranges of the angular position of the control shaft. 
   In accordance with a first aspect of the present invention, there is provided a variable valve system of an internal combustion engine for varying an operation manner of an engine valve by controlling an angular position of a control shaft, which comprises a stopper mechanism that determines an angular range in which the control shaft is permitted to rotate about its axis; an actuating mechanism that actuates the control shaft to rotate about its axis; and a position matching device that is practically assembled only when the actuating mechanism is being assembled, the position matching device when assembled restricting operation of the actuating mechanism in such a manner as to match a maximally operated position of the actuating mechanism with a maximally operated angular position of the control shaft. 
   In accordance with a second aspect of the present invention, there is provided a variable valve system of an internal combustion engine for varying an operation manner of an engine valve by controlling an angular position of a control shaft. The variable valve system comprises a stopper mechanism that determines an angular range in which the control shaft is permitted to rotate about its axis; and an actuating mechanism that actuates the control shaft to rotate about its axis, the actuating mechanism comprising an externally threaded shaft that is turned about its axis in accordance with the operation condition of the engine; an internally threaded nut member operatively engaged with the threaded shaft so that turning of the threaded shaft induces an axial movement of the nut member along the threaded shaft, the nut member being contactable with the position matching device when the latter is assembled; a transmission mechanism provided between the control shaft and the nut member to convert the axial movement of the nut member to a rotary motion of the control shaft; a housing that houses therein the threaded shaft, the threaded nut member and the transmission mechanism; and a position matching device that is practically assembled only when the actuating mechanism is being assembled, the position matching device when assembled restricting operation of the actuating mechanism in such a manner as to match a maximally operated position of the actuating mechanism with a maximally operated angular position of the control shaft. 
   In accordance with a third aspect of the present invention, there is provided a method of assembling a variable valve system of an internal combustion engine, the variable valve system varying an operation manner of an engine valve by controlling an angular position of a control shaft and comprising a stopper mechanism that determines an angular range in which the control shaft is permitted to rotate and an actuating mechanism that actuates the control shaft to rotate about its axis. The method comprises placing a positioning bolt at a given position of a way along which an element of the actuating mechanism moves, the given position being a position where the element contacts the positioning bolt when the stopper mechanism causes the control shaft to take a maximally operated position; causing the element of the actuating mechanism to contact with the positioning bolt; connecting the actuating mechanism and the control shaft; removing the positioning bolt from the given position; placing a close bolt to the given position in place of the positioning bolt. 
   Other objects and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partially sectional plan view of a variable valve system of the present invention; 
       FIG. 2  is a front view of a stopper mechanism employed in the variable valve system of the present invention; 
       FIG. 3  is a sectional view of an essential portion of the variable valve system of the invention, showing a connection between a control shaft and an actuating mechanism; 
       FIG. 4  is a sectional view of the actuating mechanism in a condition to induce the minimum lift of engine valves; 
       FIG. 5  is a view similar to  FIG. 4 , but showing a condition to induce the maximum lift of intake valves of the engine; 
       FIG. 6  is a sectional view of a portion of the variable valve system where a positioning bolt is operatively engaged with a positioning opening; 
       FIG. 7  is a view similar to  FIG. 6 , but showing a condition wherein a close bolt is engaged with the positioning opening in place of the positioning bolt; 
       FIG. 8  is a perspective view of the variable valve system of the present invention; 
       FIGS. 9A and 9B  are views taken from the direction of the arrow “A” of  FIG. 8 , in which  FIG. 9A  shows a valve closing operation under the lowest lift of the intake valves, and  FIG. 9B  shows a valve opening operation under the lowest lift of the intake valves; 
       FIGS. 10A and 10B  are views similar to  FIGS. 9A and 9B , but in which  FIG. 10A  shows a valve closing operation under the highest lift of the intake valves, and  FIG. 10B  shows a valve opening operation under the highest lift of the intake valves; 
       FIG. 11  is a graph showing a valve lift characteristic of each intake valve, which is provided by the variable valve system of the present invention; 
       FIG. 12  is a front perspective view of a guide cap used in the present invention; 
       FIG. 13  is a back perspective view of the guide cap; and 
       FIGS. 14 to 18  are views for explaining the steps for properly connecting a lever member of the actuating mechanism to the control shaft. 
   


   DETAILED DESCRIPTION OF THE INVENTION 
   In the following, the present invention will be described in detail with reference to the accompanying drawings. 
   For ease of understanding, various directional terms, such as, right, left, upper, lower, rightward and the like are used in the following description. However, such terms are to be understood with respect to only a drawing or drawings on which corresponding part or portion is shown. 
   Before describing the detail of the invention, an outlined construction of a variable valve system  100  of the invention will be described with reference to  FIGS. 8 ,  9 A,  9 B,  10 A and  10 B. 
   As will be understood from  FIG. 8 , variable valve system  100  shown in the drawing is designed for multi-cylinder internal combustion engines of a type that has two intake valves  2  and  2  for each cylinder. 
   That is, variable valve system  100  is constructed to control operation of paired intake valves  2  and  2  (viz., engine valves) for each cylinder of the engine. Intake valves  2  and  2  are slidably guided by a cylinder head  1  (see  FIG. 9A ) through valve guides (not shown). Each intake valve  2  has a valve spring  3  for being biased in a closing direction, and has a valve lifter  16  mounted on a stem thereof. 
   As will be described in detail hereinafter, variable valve system  100  generally comprises a valve lift mechanism  4  that induces an open/close condition of intake valves  2  and  2 , a valve lift degree varying mechanism  5  that is incorporated with valve lift mechanism  4  to vary a lift degree (or work angle) of intake valves  2  and  2  and an actuating mechanism  6  that actuates the valve lift degree varying mechanism  5  (more specifically, a control shaft  32  of this mechanism  5 ) in accordance with an operation condition of the engine. 
   It is to be noted that the work angle of engine valve  2  is an event corresponding to a period or span in terms of crank angle, that elapses from a time when the valve  2  is just opened to a time when the valve  2  is just closed in each operation cycle of the engine. 
   As is seen from  FIG. 8 , valve lift mechanism  4  comprises a hollow drive shaft  13  that is rotatably held on an upper portion of cylinder head  1  through bearings  14  (see  FIG. 9A ), a drive cam  15  for each cylinder, that is fixed, through a press-fitting or the like, to hollow drive shaft  13  to rotate therewith, two swing cams  17  and  17  for each cylinder, that are integrally mounted on a cylindrical camshaft  20  rotatably disposed on hollow drive shaft  13  and operatively contact with valve lifters  16  and  16  of intake valves  2  and  2  to induce an open/close operation of intake valves  2  and  2  and a power transmitting mechanism “PTM” that is arranged between drive cam  15  and each of swing cams  17  and  17  to transmit a torque of drive cam  15  to swing cams  17  and  17 . Actually, due to an after-mentioned linkage construction of power transmitting mechanism “PTM”, the rotary motion of drive cam  15  is converted to a swing motion of swing cams  17  and  17 . 
   Hollow drive shaft  13  extends along an axis of the engine. Although not shown in the drawings, hollow drive shaft  13  has one end to which a torque is applied from a crankshaft of the engine through a sprocket fixed to the end of drive shaft  13  and a timing chain that is put around the sprocket and the crankshaft. That is, drive shaft  13  is driven or rotated by the crankshaft of the engine. Usually, an operation phase varying mechanism (not shown) is arranged between the crankshaft and drive shaft  13  for varying or controlling an operation phase of drive shaft  13  relative to the crankshaft of the engine. 
   As is seen from  FIG. 9A , each of bearings  14  comprises a main bracket  14   a  that is mounted on cylinder head  1  to rotatably support drive shaft  13 , a sub-bracket  14   b  that is mounted on main bracket  14   a  to rotatably support an after-mentioned control shaft  32  and a pair of connecting bolts  14   c  and  14   c  that pass through both sub-bracket  14   b  and main bracket  14   a  to tightly connect these brackets  14   b  and  14   a  to cylinder head  1 . 
   Drive cam  15  is a circular disc that has a center axis “Y” displaced or eccentric from a center axis “X” of drive shaft  13 . More specifically, the circular disc  15  has at an eccentric portion thereof a circular opening through which drive shaft  13  passes. For the integral rotation of drive cam  15  with drive shaft  13 , drive shaft  13  is secured to the circular opening of the drive cam  15  through press-fitting or the like. 
   The two swing cams  17  and  17  are substantially the same in construction and have a generally triangular cross section. These two swing cams  17  and  17  are integrally mounted on axially opposed end portions of cylindrical camshaft  20  that is swingably disposed about hollow drive shaft  13 . 
   As shown in  FIG. 9A , each swing cam  17  has a cam nose portion  21  and a cam surface  22  at its lower side. 
   As is seen from this drawing, cam surface  22  of each swing cam  17  includes a base round part that extends around the cylindrical outer surface of camshaft  20 , a lump part that extends from the base round part toward cam nose portion  21  and a lift part that extends from the lump part to a maximum lift point defined at the leading end of cam nose portion  21 . That is, under operation, these parts of cam surface  22  slidably contact an upper surface of the corresponding valve lifter  16  thereby to induce the open/close operation of the corresponding intake valve  2  in accordance with a swing movement of swing arms  17  and  17 . 
   As is understood from  FIG. 8 , power transmitting mechanism “PTM” comprises a rocker arm  23  that is pivotally disposed about control shaft  32  positioned above drive shaft  13 , a link arm  24  that pivotally connects one wing part  23   a  (see  FIG. 9A ) of rocker arm  23  to drive cam  15 , and a link rod  25  that pivotally connects the other wing part  23   b  of rocker arm  23  to one of swing cams  17  and  17 . 
   As is seen from  FIGS. 8 and 9A , rocker arm  23  has at its middle part a cylindrical bore (no numeral) in which an after-mentioned control cam  33  is rotatably disposed. 
   As shown in  FIG. 9A , wing part  23   b  of rocker arm  23  is pivotally connected to one end of link rod  25  through a pivot pin  27 . The other wing part  23   a  of rocker arm  23  is pivotally connected to a radially projected arm portion  24   b  of link arm  24  through a pivot pin  26 . 
   The two wing parts  23   a  and  23   b  of rocker arm  23  extend radially outward from axially opposed end portions of the bored middle part of rocker arm  23 . 
   As is understood from  FIG. 9A , link arm  24  comprises an annular base portion  24   a  that rotatably receives therein the above-mentioned drive cam  15  and the above-mentioned radially projected arm portion  24   b  that is pivotally connected to wing part  23   a  of rocker arm  23  through pivot pin  26 . 
   As is best seen from  FIGS. 8 and 9A , link rod  25  is a curved channel member that has an upper end  25   a  pivotally connected to wing part  23   b  of rocker arm  23  through pivot pin  27  and a lower end  25   b  pivotally connected to swing cam  17  through a pivot pin  28 . 
   Although not shown in the drawings, pivot pins  26 ,  27  and  28  are equipped at one ends with respective snap rings for holding link arm  24  and link rod  25  at their properly set positions. 
   In the following, valve lift degree varying mechanism  5  will be described in detail with reference to the drawings. 
   As is seen from  FIG. 8 , valve lift degree varying mechanism  5  comprises control shaft  32  that extends in parallel with the above-mentioned drive shaft  13  and is rotatably held by bearings  14  (see  FIG. 9A ), and a control cam  33  for each cylinder, which is secured to control shaft  32  to rotate therewith. As has been mentioned hereinabove, control cam  33  is rotatably disposed in the cylindrical bore provided in the middle part of rocker arm  23 . That is, control cam  33  serves as a swinging fulcrum of rocker arm  23 . 
   As is described hereinabove and seen from  FIGS. 1 ,  2  and  9 A, control shaft  32  is rotatably held between main-bracket  14   a  and sub-bracket  14   b  of each bearing  14  that is tightly mounted on cylinder head  1 . 
   As is seen from  FIGS. 1 and 3 , control shaft  32  is integrally formed, at an end portion thereof near actuating mechanism  6 , with a trapezoidal flange  32   a  that has at radially projected two portions respective threaded bores  32   b . It is however to be noted that these bores  32   b  are not positioned at diametrically opposite portions of control shaft  32 . That is, the bores  32   b  are provided at asymmetric positions with respect to an axis of control shaft  32 . 
   As is best seen from  FIG. 3 , at a left end of control shaft  32 , there is integrally formed an annular projection  32   c  that projects leftward from a center portion of the trapezoidal flange  32   a.    
   As is seen from  FIGS. 1 and 2 , for controlling or restricting a rotation range of control shaft  32 , there is provided a stopper mechanism  29 . 
   Stopper mechanism  29  comprises first and second stopper pins  29   a  and  29   b  that are projected from sub-bracket  14   b  of bearing  14 , and a stopper arm  29   c  that is fixed to control shaft  32 . As is seen from these drawings, upon rotation of control shaft  32 , stopper arm  29   c  is brought into contact with first or second stopper pin  29   a  or  29   b  thereby to restrict the rotation range of control shaft  32 . 
   As is seen from  FIG. 9A , control cam  33  is a circular disc that has a center axis “P 2 ” displaced or eccentric from a center axis “P 1 ” of control shaft  32 . More specifically, the circular disc  33  has at an eccentric portion thereof a circular opening through which control shaft  32  passes. For the integral rotation of control cam  33  with control shaft  32 , control shaft  32  is secured to the circular opening of control cam  33  through press-fitting or the like. 
   In the following, actuating mechanism  6  will be described with reference to  FIGS. 1 to 5  and  8 . It is to be noted that actuating mechanism  6  shown in  FIG. 8  has some parts removed for clarifying the arrangement of essential elements of the mechanism  6 . 
   As is seen from  FIG. 1 , actuating mechanism  6  generally comprises a cylindrical housing  35  that is mounted on one end of cylinder head  1  in a manner to extend perpendicular to control shaft  32  and thus perpendicular to drive shaft  13 , an electric motor  36  that is connected to one axial end of cylindrical housing  35 , and a ball-screw type transmission mechanism  37  that is installed in cylindrical housing  35  for transmitting a torque of electric motor  36  to control shaft  36  while reducing the rotation speed. 
   As is seen from  FIGS. 4 and 5 , cylindrical housing  35  is constructed of an aluminum alloy or the like and includes generally an elongate lower bore  35   a  that extends axially along the housing  35  and an upper bore  35   b  that extends upward from a middle portion of elongate lower bore  35   a . That is, these two bores  35   a  and  35   b  are merged to constitute a so-called part housing room. As shown, in elongate lower bore  35   a , there is arranged the above-mentioned ball-screw type transmission mechanism  37 , and into upper bore  35   b , there is projected trapezoidal flange  32   a  of control shaft  32 . 
   Although not shown in  FIG. 1 , the two bores  35   a  and  35   b  have respective openings that are covered by respective covers through sealing members. As shown in  FIG. 4 , elongate lower bore  35   a  has a left end  35   c  opened and a right end closed by a wall  35   d.    
   As is seen from  FIGS. 4 and 8 , electric motor  36  is of a DC type which comprises a cylindrical casing  38  that has an opened base end  38   a  tightly connected to the opened end  35   c  (see  FIG. 4 ) of elongate lower bore  35   a . Electric motor  36  has an output shaft  36   a  rotatably held by a retainer  39  tightly received in the opened left end  35   c  (see  FIG. 4 ). For sealing output shaft  36   a , there is provided a mechanical seal between retainer  39  and output shaft  36   a.    
   As is seen from  FIG. 8 , electric motor  36  is controlled by a control unit  40 . That is, control unit  40  outputs an instruction signal to electric motor  36  by processing various information signals fed thereto. These information signals are, for example, signals from a crank angle sensor  41 , an air flow meter  42 , an engine cooling water temperature sensor  43  and a rotation angle sensor  44  for control shaft  32 . By processing these information signals, control unit  40  derives a current operation condition of the engine and outputs an instruction signal to electric motor  36  in accordance with the derived operation condition of the engine. 
   Referring back to  FIG. 4 , ball-screw type transmission mechanism  37  generally comprises a ball-screw shaft  45  that extends axially in elongate lower bore  35   a  to be coaxially connected to output shaft  36   a  of electric motor  36 , a ball-nut  46  that is disposed about ball-screw shaft  45  to operatively engage with the same, a lever member  47  that is secured to the above-mentioned trapezoidal flange  32   a  of control shaft  32  by means of connecting bolts  58 , and a channel shaped link member  48  that pivotally connects lever member  47  and ball-nut  46 . Lever member  47  and link member  48  thus constitute a transmission mechanism. 
   Ball-screw shaft  45  is formed with a threaded outer surface  49  except axially opposite end portions  45   a  and  45   b  thereof. As shown in  FIG. 4 , opposite end portions  45   a  and  45   b  of ball-screw shaft  45  are rotatably held by left and right ball bearings  50  and  51  which are tightly held in elongate lower bore  35   a.    
   Left end portion  45   a  of ball-screw shaft  45  has a hexagonal head  45   a ′ that is axially movably received in a hexagonal socket  52  that is fixed to a leading end of output shaft  36   a  of electric motor  36 . Thus, output shaft  36   a  and ball-screw shaft  45  can rotate together like a single unit while being permitted to move axially relative to each other. 
   As is seen from  FIGS. 4 ,  5  and  8 , ball-nut  46  is meshed with ball-screw shaft  45  so that rotation of ball-screw shaft  45  about its axis induces a forward or rearward movement of ball-nut  46  along ball-screw shaft  45 . That is, ball-nut  46  is a cylindrical member that has a bore whose inner surface is formed with a spiral thread  53  that is meshed with a spiral thread  49  formed on the outer surface of ball-screw shaft  45 . 
   A plurality of fine balls  54  are operatively received in spiral thread  53  of ball-nut  46  for achieving a smoothed movement of ball-nut  46  along ball-screw shaft  45 . Two deflectors (no numerals) are provided by spiral thread  53  of ball-nut  46  to produce an endless screw passage of the threads in and along which fine balls  54  run endlessly under movement of ball-nut  46  along ball-screw shaft  45 . 
   Thus, in operation, rotation of ball-screw shaft  45  about its axis is converted to the axial movement of ball-nut  46  through fine balls  54 . 
   As is seen from  FIG. 4 , ball-nut  46  is formed with a round projection  55  to which lower ends of the above-mentioned link member  48  are pivotally connected through a pivot pin  57 . As shown in  FIG. 8 , at axially opposite sides of round projection  55 , there are provided curved cuts  56  for permitting a swing movement of rounded lower ends of the channel-shaped link member  48 . 
   For achieving a proper positioning of ball-nut  46  at the time when transmission mechanism  37  is being assembled in housing  35 , there is provided a position matching device. 
   That is, by using this position matching device, the leftmost position of ball-nut  46  in  FIG. 4  relative to ball-screw shaft  45  is assured. In other words, the most-clockwise position of control shaft  32  in the same drawing, that induces a minimum lift degree of intake valves  2 , is assured. 
   That is, as is seen from  FIGS. 3 ,  4  and  6 , the position matching device comprises a threaded positioning opening  65  provided in a wall of housing  35  and an elongate positioning bolt  66  detachably fixed to positioning opening  65 . As is seen from  FIG. 4 , positioning bolt  66  is positioned just before left ball bearing  50 . 
   As is seen from  FIG. 6 , when positioning bolt  66  is properly fixed to positioning opening  65 , a rod portion  66   a  of positioning bolt  66  serves as a stopper for stopping excessive axial movement of ball-nut  46 . That is, in such case, left end surface  46   a  of ball-nut  46  is brought into abutment with rod portion  66   a  of positioning bolt  66 , as shown. 
   It is now to be noted that the position of ball-nut  46  determined by positioning bolt  66  corresponds to an angular position of control shaft  32  determined by the above-mentioned stopper mechanism  29  (see  FIG. 1 ). That is, in such case, stopper arm  29   c  of control shaft  32  is in abutment with first stopper pin  29   a , and as will be described in detail hereinafter, intake valves  2  of the engine are subjected to a minimum lift operation. 
   When the assembling work of transmission mechanism  37  is finished, positioning bolt  66  is removed and in place of it, a close bolt  67  is fitted to positioning opening  65 , as is seen from  FIG. 7 . 
   As is seen from  FIG. 4 , between ball-nut  46  and right ball bearing  51 , there is compressed through respective retainers  69   a  and  69   b  a coil spring  68  that is disposed about ball-screw shaft  45 . Thus, coil spring  68  functions to bias ball-nut  46  in such a direction as to induce the minimum lift operation of intake valves  2 . 
   As is seen from  FIG. 4 , lever member  47  is trapezoidal in shape and comprises a base portion  47   a  that is secured to trapezoidal flange  32   a  of control shaft  32  by means of two bolts  58  and a radially projected portion  47   b  that extends radially outward from base portion  47   a.    
   As is seen from  FIG. 3 , the two bolts  58  are engaged with the above-mentioned asymmetrically arranged threaded bores  32   b  of the flange  32   a  of control shaft  32 . For receiving bolts  58 , base portion  47   a  of lever member  47  are formed with two arcuate openings  47   c . Although not well shown in the drawings, each arcuate opening  47   c  is shaped to extend around the axis of control shaft  32 . 
   As is seen from  FIGS. 1 and 3 , lever member  47  is formed at its back surface with an annular recess  47   d  into which the above-mentioned annular projection  32   c  of control shaft  32  is snugly received. 
   As is seen from  FIG. 4 , radially projected portion  47   b  of lever member  47  is formed with an opening (no numeral) through which an after-mentioned pin  59  passes for pivotally connecting the link member  48  to the radially projected portion  47   b.    
   Link member  48  having a generally U-shaped cross section is produced by pressing a flat metal plate. That is, link member  48  comprises two parallel wall portions and a bridge portion that extends between the two parallel wall portions. 
   As is seen from  FIGS. 1 and 3 , particularly  FIG. 3 , the above-mentioned rotation angle sensor  44  is arranged to face the lever member  47 . 
   Rotation angle sensor  44  comprises a cylindrical metal member  60  that is coaxially connected to the leading end of control shaft  32  passing through an opening  47   e  of lever member  47 . A round plastic holder  61  is secured to the leading end of cylindrical metal member  60 . For this fixing, an integral molding technique is used. As shown, round plastic holder  61  has the same diameter as the metal member  60 . Round plastic holder  61  is formed with a diametrically extending groove (no numeral) in which a circular permanent magnet  62  is snugly and tightly received. As shown, the magnet  62  is received in the center part of the groove and the depth of the groove is greater than the thickness of magnet  62 . Although not shown in the drawings, the magnet  62  has at its diametrically opposed portions flat edges that intimately abut against inner surfaces of the two walls that define therebetween the groove. With this, undesired radial leakage of magnetic force from the magnet  62  is suppressed or at least minimized. It is to be noted that the magnet  62  is positioned away from heads of bolts  58  by a sufficient distance. 
   As is best seen from  FIG. 3 , rotation angle sensor  44  further comprises a plastic circular casing  63  that is fixed to the housing  35 . For this fixing, a stepped portion of circular casing  63  is snugly received in a circular opening  35   e  of housing  35 . Circular casing  63  is formed at an inner side thereof with a cylindrical recess  63   a  that receives therein round plastic holder  61  keeping an annular clearance therebetween. A Hall-element  64  is embedded in circular casing  63  in a manner to surround the round plastic holder  61 . 
   As is seen from  FIG. 8 , information signal produced by rotation angle sensor  44  is processed by control unit  40 . By detecting the N-pole and S-pole from the magnet  62 , the rotation angle sensor  44  senses a rotation angle (or angular position) of control shaft  32 , that is fed to control unit  40 . 
   It is to be noted that, as is understood from  FIGS. 3 ,  14  and  15 , to opening  35   e  of housing  35 , there is detachably fitted a guide cap  70  for keeping the connection between lever member  47  and link member  48  at the time when various parts of the actuating mechanism  6  are being assembled in housing  35 . 
   The detailed construction of guide cap  70  is shown in  FIGS. 12 and 13 . Guide cap  70  is made of a molded plastic and comprises a cylindrical cap proper  70   a  that is to be fitted in the opening  35   e  of housing  35 , a flange portion  70   b  that is integrally formed on one axial end of the cap proper  70   a , and a curved recess  70   c  provided at a bottom portion of cap proper  70   a . For the purpose that will be described hereinafter, the curved recess  70   c  has such a shape as to correspond to a unit that includes lever member  47  and link member  48 . 
   Furthermore, guide cap  70  has a cylindrical recess  70   d  into which the above-mentioned round plastic holder  61  is to be inserted. As shown, the cylindrical recess  70   d  is exposed to a generally middle portion of the curved recess  70   c . Furthermore, guide cap  70  has at both sides of cylindrical recess  70   d  a pair of openings  70   e  into which the heads of the above-mentioned bolts  58  are to be roughly inserted. These openings  70   e  are larger than heads of bolts  58 . As shown, these paired openings  70   e  are exposed to longitudinally opposed portions of the curved recess  70   d.    
   In the following, steps for connecting lever member  47  to control shaft  32  and assembling rotation angle sensor  44  after assemblage of actuating mechanism  6  to cylinder head  1  will be described in detail with reference to  FIGS. 6 and 14  to  18 . 
   As is seen from  FIG. 14 , prior to connecting housing  35  of actuating mechanism  6  to cylinder head  1  by means of four connecting bolts  71 , the above-mentioned positioning bolt  66  is deeply engaged with positioning opening  65  of housing  35  and ball-screw shaft  45  is turned in a direction about its axis to cause ball-nut  46  to take the leftmost position in  FIG. 4  contacting with rod portion  66   a  of positioning bolt  66 , as is understood from  FIG. 6 . 
   Then, guide cap  70  is fitted into opening  35   e  of housing  35  assuring a positioning therebetween. As is seen from  FIG. 15  that shows a back view of housing  35 , upon fitting of guide cap  70  into opening  35   e , a given part of guide cap  70  pushes ball-nut  46  to turn about the shaft  45  in a direction to near electric motor  36 , so that the curved recess  70   c  of guide cap  70  neatly receive both lever member  47  and link member  48  keeping the leftmost position of ball-nut  46  that is in contact with rod portion  66   a  of positioning bolt  66 . 
   Thus, lever member  47  and link member  48  are suppressed from making a free movement as well as inclination toward this side in  FIG. 15 . Then, with this condition kept, base portion  47   a  of lever member  47  is brought into contact with flange  32   a  of control shaft  32  while inserting the annular projection  32   c  of control shaft  32  into annular recess  47   d  of base portion  47   a , as will be understood from  FIG. 3 . With this, coaxial arrangement between control shaft  32  and lever member  47  is assured. 
   Then, as is seen from  FIG. 16 , after passing through openings  70   e  of guide cap  70  and arcuate openings  47   c  of lever member  47 , the two bolts  58  are engaged with threaded bores  32   b  of flange  32   a  of control shaft  32 . During this time, by using the arcuate shape of the openings  47   c , base portion  47   a  of lever member  47  is somewhat turned relative to flange  32   a  of control shaft  32  to assure a proper positioning between lever member  47  and flange  32   a  of control shaft  32 . Then, the two bolts  58  are strongly turned against threaded bores  32   b  of flange  32   a . With this, lever member  47  is properly fixed to the flange  32   a  of control shaft  32 , in such a manner as is shown in  FIG. 3 . 
   Then, as is seen from  FIG. 17 , guide cap  70  is detached from opening  35   e  of housing  35 . Thus, under this condition, cylindrical metal member  60 , round plastic holder  61 , permanent magnet  62  and heads of the bolts  58  are exposed to the outside through the opening  35   e , as is seen from this drawing. 
   Then, as is seen from  FIG. 18 , plastic circular casing  63  is fitted to circular opening  35   e  of housing  35  and secured to the same by means of two bolts  72 . Thereafter, positioning bolt  66  is removed from positioning opening  65 , and as is seen from  FIG. 7 , close bolt  67  is fixed to positioning opening  65  to close the same. Then, a terminal (not shown) provided on circular casing  63  is connected to control unit  40  by a suitable harness. 
   In the following, operation of variable valve system  100  actuated by actuating mechanism  6  will be briefly described with reference to the drawings, particularly  FIGS. 4 ,  5 ,  8 ,  9 A,  9 B,  10 A and  10 B. 
   For ease of understanding, the description on the operation will be commenced with respect to a condition wherein the associated engine starts to run at a lower speed, such as a speed at idling. 
   In such case, as is seen from  FIG. 5 , electric motor  36  is actuated in accordance with an instruction signal outputted from control unit  40 . As is seen from  FIG. 4 , upon this, a torque produced by electric motor  36  is transmitted to ball-screw shaft  45  to rotate the same. With this, ball-nut  46  is moved axially leftward along ball-screw shaft  45  allowing fine balls  54  to run in and along a passage that is defined by and between spiral thread  53  of ball-nut  46  and spiral thread  49  of ball-screw shaft  45 . That is, ball-nut  46  is moved toward electric motor  36  and finally to the leftmost position that has been determined by positioning bolt  66 . 
   During the leftward movement of ball-nut  46  on ball-screw shaft  45 , lever member  47  and thus control shaft  32  are turned clockwise in  FIG. 4 . (It is to be noted that the clockwise turning of control shaft  32  in  FIG. 4  induces a counterclockwise turning of the same in  FIGS. 9A to 10B ). 
   Upon this, as is seen from  FIGS. 9A and 9B , control cam  33  secured to control shaft  32  is turned counterclockwise about the axis “P 1 ” of control shaft  32  moving the thickest cam part thereof upward away from drive shaft  13 , and finally control cam  33  takes the angular position as shown in these drawings. In other words, in this case, the entire construction of rocker arm  23  takes a relatively high position. Thus, under this condition, as is seen from  FIG. 9A , the uppermost position that can be taken by pivot pin  27  provided between left wing part  23   b  of rocker arm  23  and upper end  25   a  of link rod  25  is a first position that is remote from drive shaft  13 . This means that as is seen from  FIGS. 9A and 9B , under operation of the variable valve system  100 , link rod  25  and thus swing cam  17  are forced to operate at a position remote from valve lifter  16 . 
   Accordingly, when, due to rotation of drive shaft  13 , drive cam  15  is rotated in annular base portion  24   a  of link arm  24 , rocker arm  23  is forced to swing reciprocating link rod  25  and swing cam  17  at such a position remote from valve lifter  16 . That is, as is understood from  FIG. 9B  and the graph of  FIG. 11 , under this condition, the valve lift shows the smallest degree “L 1 ” inducing a retarded open timing of intake valves  2  and  2  thereby minimizing the over wrap period with the associated exhaust valves. Thus, improved fuel consumption and stable running of the engine are obtained under such lower speed condition of the engine. In  FIG. 11 , reference “BDC” indicates a bottom dead center and reference “TDC” indicates a top dead center. 
   While, when the engine is subjected to a high speed operation, control unit  40  (see  FIG. 8 ) controls electric motor  36  to run in a reversed direction. As is seen from  FIG. 5 , upon this, ball-nut  46  is moved rightward on and along ball-screw shaft  45 . That is, ball-nut  46  is moved away from electric motor  36  allowing fine balls  54  to run in and along the passage defined by and between spiral thread  53  of ball-nut  46  and spiral thread  49  of ball-screw shaft  45 . 
   Accordingly, as is seen from  FIG. 5 , lever member  47  and thus control shaft  32  are turned counterclockwise in the drawing. (It is to be noted that the counterclockwise turning of control shaft  32  in  FIG. 5  induces a clockwise turning of the same in  FIGS. 9A to 10B ). 
   Upon this, as is seen from  FIGS. 9A ,  10 A and  10 B, control cam  33  is turned clockwise about the axis “P 1 ” of control shaft  32  moving the thickest cam part thereof downward toward drive shaft  13 , and finally control cam  33  takes the angular position as shown in  FIGS. 10A and 10B . In other words, in this case, the entire construction of rocker arm  23  takes a relatively low position. Thus, under this condition, as is seen from  FIG. 10A , the uppermost position that can be taken by pivot pin  27  is a second position that is near drive shaft  13  as compared with the above-mentioned first position. This means that as is seen from  FIGS. 10A and 10B , under operation of variable valve system  100 , link rod  25  and thus swing cam  17  are forced to operate at a position near valve lifter  16 . 
   Accordingly, when, due to rotation of drive shaft  13 , drive cam  15  is rotated in annular base portion  24   a  of link arm  24 , rocker arm  23  is forced to swing reciprocating link rod  25  and swing cam  17  at such a position near valve lifter  16 . That is, as is seen from  FIG. 10B  and the graph of  FIG. 11 , under this condition, the valve lift shows the largest degree “L 2 ”. As is seen from the graph of  FIG. 11 , the close timing of each intake valve  2  is retarded in accordance with an advancement of the open timing. That is, the work angle is increased. Thus, intake air charging efficiency is increased and thus sufficient engine power is obtained in such high speed condition. 
   As is described hereinabove, in accordance with the present invention, due to employment of the position matching device ( 65 ,  66 ) that includes threaded positioning opening  65  of housing  35  and positioning bolt  66  detachably connectable to the opening  65 , the most-moved position (viz., the leftmost position in  FIG. 4 ) of ball-nut  46  can be previously set, which matches with the most-tuned angular position of control shaft  32  determined by the stopper mechanism  29 . Accordingly, even after positioning bolt  66  is removed from the opening  65 , the most-moved position of ball-nut  46  is assuredly provided by the stopper mechanism  29 . This means that as is understood from  FIG. 4  under such most-moved positioning of ball-nut  46 , there is left a certain clearance between ball bearing  50  and ball-nut  46  and thus the most-turned angular position of control shaft  32  is not affected by the operation range of ball-nut  46 . 
   That is, as is seen from  FIG. 1 , under the most-turned angular position of control shaft  32  wherein stopper arm  29   c  contacts first stopper pin  29   a , intake valves  2  are forced to show their smallest lift degree “L 1 ” as is shown by  FIGS. 9A and 9B . In this case, as is seen from  FIG. 4 , ball-nut  46  stays near ball bearing  50  without contacting the same. Thus, the angular operation range of control shaft  32  is not affected by the most-moved position of ball-nut  46 , but affected by only the stopper mechanism  29 . 
   Accordingly, control shaft  32  can have a higher positioning accuracy at the most-turned angular position, which brings about a higher valve lift controllability of intake valves  2 . 
   As is mentioned hereinabove, due to practical usage of guide cap  70 , lever member  47  and link member  48  can be stably held keeping the relative positioning therebetween at the time of assembling the actuating mechanism  6 . Thus, the work for connecting the lever member  47  to control shaft  32  is readily and precisely carried out. Furthermore, the practical usage of guide cap  70  facilitates the convey of actuating mechanism  6  to a desired position, and facilitates the preparation for connecting the actuating mechanism  6  to an associated engine. 
   After connecting the lever member  47  to control shaft  32 , guide cap  70  is removed from opening  35   e  of housing  35  and plastic circular casing  63  is fitted to opening  35   e  to close the same. Casing  63  thus has a function to close opening  34   e  as well as a function to hold Hall-element  64 , which means reduction in number of parts used and thus reduction in cost. 
   Furthermore, due to usage of guide cap  70 , the work for turning bolts  58  to fix lever member  47  to flange  32   a  of control shaft  32  is readily made. Actually, openings  70   e  of guide cap  70  that accommodate heads of bolts  58  serve as a guide means for bolts  58 . Presence of openings  70   e  facilitates the work for detaching guide cap  70  from opening  35   e  of housing  35 . 
   Due to usage of close bolt  67  fitted to positioning opening  65 , the interior of housing  35  is protected from dust and the like. 
   Due to usage of coil spring  68  that biases ball-nut  46  in an axial direction, backlash of ball-nut  46  is suppressed or at least minimized. Furthermore, due to presence of such coil spring  68 , direct contact of ball-nut  46  against the other ball bearing  51  is avoided. 
   For example, the arrangement of electric motor  32  may change in accordance with the layout of engine room. Furthermore, in place of electric motor  32 , a hydraulic motor or the like may be used. 
   For connecting round plastic holder  61  to the leading end of cylindrical metal member  60  (see  FIG. 1 ), a threaded coupling or the like may be used. The diameter of the holder  61  may change in accordance with size of permanent magnet  62 . The holder  61  may be made of a hard rubber, aluminum or the like as long as it exhibits a non-magnetic characteristic. 
   In place of ball-screw shaft  45  and ball-nut  46  that employ a plurality of fine balls  54 , a normal bolt-nut arrangement may be used. 
   Although the foregoing description is directed to the system for controlling intake valves  2  of the engine, the present invention is applicable to exhaust valves and both intake and exhaust valves. 
   The entire contents of Japanese Patent Application 2004-177783 filed Jun. 16, 2004 are incorporated herein by reference. 
   Although the invention has been described above with reference to the embodiment of the invention, the invention is not limited to such embodiment as described above. Various modifications and variations of such embodiment may be carried out by those skilled in the art, in light of the above description.
A variable valve system of an internal combustion engine varies an operation manner of an engine valve by controlling an angular position of a control shaft. The system comprises a stopper mechanism that determines an angular range in which the control shaft is permitted to rotate about its axis; an actuating mechanism that actuates the control shaft to rotate about its axis; and a position matching device that is practically assembly only when the actuating mechanism is being assembled. The position matching device, when assembled, restricts operation of the actuating mechanism in such a manner as to match a maximally operated position of the actuating mechanism with a maximally operated angular position of the control shaft.