Abstract:
A method is provided for sensing the position of a camshaft in an internal combustion engine having a camshaft phaser for controllably varying the phase relationship between a crankshaft of the internal combustion engine and the camshaft, the camshaft phaser being actuated by an electric motor and having a gear reduction mechanism with a predetermined gear reduction ratio and rotational position means for determining the rotational position of the electric motor. The method includes generating a rotational position signal indicative of the rotational position of the electric motor by using the rotational position means to determine the rotational position of the electric motor and calculating the position of the camshaft based on the rotational position signal and the gear reduction ratio of the gear reduction mechanism.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application claims the benefit of U.S. provisional patent application Ser. No. 61/378,048 filed Aug. 30, 2010, the disclosure of which is hereby incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD OF INVENTION 
       [0002]    The present invention relates to sensing the position of a camshaft in an internal combustion engine, and more particularly to sensing the position of a camshaft in an internal combustion engine which includes an electric variable cam phaser (eVCP). 
       BACKGROUND OF INVENTION 
       [0003]    Camshaft phasers (“cam phasers”) for varying the timing of combustion valves in internal combustion engines are well known. A first element, known generally as a sprocket element, is driven by a chain, belt, or gearing from an engine&#39;s crankshaft. A second element, known generally as a camshaft plate, is mounted to the end of an engine&#39;s camshaft. A common type of camshaft phaser used by motor vehicle manufactures is known as a vane-type cam phaser. U.S. Pat. No. 7,421,989 shows a typical vane-type cam phaser which generally comprises a plurality of outwardly-extending vanes on a rotor interspersed with a plurality of inwardly-extending lobes on a stator, forming alternating advance and retard chambers between the vanes and lobes. Engine oil is supplied via a multiport oil control valve, in accordance with an engine control module, to either the advance or retard chambers, to change the angular position of the rotor relative to the stator, as required to meet current or anticipated engine operating conditions. 
         [0004]    Knowing the rotational position of the camshaft can be useful, for example, for combustion control and diagnostic functions. In vane-type cam phasers, camshaft position sensing is typically accomplished by using a target wheel rotating with the camshaft which induces a signal on one or more sensors positioned next to the target wheel. The target wheel is disk shaped, and the edge thereof is varied along its periphery in some fashion, for example, by cutting a series of slots along the periphery of the wheel in a predetermined pattern. At least one sensor is used to detect the slots as they pass by the sensor. This type of camshaft rotational position sensing may require one complete revolution in order to synchronize. In other words, it may require one complete revolution in order to sense the pattern of slots to establish the position of the camshaft. Knowing the rotational position of the camshaft more quickly when the internal combustion engine is started or stopped may be desirable. 
         [0005]    While vane-type cam phasers are effective and relatively inexpensive, they do suffer from drawbacks. First, at low engine speeds, oil pressure tends to be low, and sometimes unacceptable. Therefore, the response of a vane-type cam phaser may be slow at low engine speeds. Second, at low environmental temperatures, and especially at engine start-up, engine oil displays a relatively high viscosity and is more difficult to pump, therefore making it more difficult to quickly supply engine oil to the vane-type cam phaser. Third, using engine oil to drive the vane-type cam phaser is parasitic on the engine oil system and can lead to requirement of a larger oil pump. Fourth, for fast actuation, a larger engine oil pump may be necessary, resulting in additional fuel consumption by the engine. Lastly, the total amount of phase authority provided by vane-type cam phasers is limited by the amount of space between adjacent vanes and lobes. A greater amount of phase authority may be desired than is capable of being provided between adjacent vanes and lobes. For at least these reasons, the automotive industry is developing electrically driven cam phasers. 
         [0006]    One type of electrically driven cam phaser being developed is shown in U.S. patent application Ser. No. 12/536,575; U.S. patent application Ser. No. 12/844,918; U.S. patent application Ser. No. 12/825,806; U.S. patent application Ser. No. 12/848,599; U.S. patent application Ser. No. 12/965,057; U.S. patent application Ser. No. 13/102,138; U.S. patent application Ser. No. 13/112,199; U.S. patent application Ser. No. 13/155,685; and U.S. patent application Ser. No. 13/184,975; which are commonly owned by Applicant and incorporated herein by reference in their entirety. The electrically driven cam phaser is an electric variable cam phaser (eVCP) which comprises a flat harmonic drive unit having a circular spline and a dynamic spline linked by a common flexspline within the circular and dynamic splines, and a single wave generator disposed within the flexspline. The circular spline is connectable to either of an engine camshaft or an engine crankshaft driven rotationally and fixed to a housing, the dynamic spline being connectable to the other thereof. The wave generator is driven selectively by an electric motor to cause the dynamic spline to rotate past the circular spline, thereby changing the phase relationship between the crankshaft and the camshaft. The electric motor may be a brushless DC motor. Brushless DC motors have three or more separate coils and replace the commutator and brushes, which are present in conventional electric motors, with an electronic circuit. Typically, three Hall Effect sensors are used to detect the position of a rotor of the motor. The circuit alternately switches the power on and off in the coils based on input from the Hall Effect sensor inputs, in turn creating forces in each coil that make the motor spin. The Hall Effect sensors are capable of detecting rotor position reliably even at zero RPM as long as the engine controller is still powered. 
         [0007]    What is needed is a way to determine the rotational position of a camshaft in an internal combustion engine equipped with an eVCP without the need for additional components. What is also needed is a way to determine the rotational position of a camshaft in an internal combustion equipped with an eVCP even at zero RPM. 
       SUMMARY OF THE INVENTION 
       [0008]    Briefly described, a method is provided for sensing the position of a camshaft in an internal combustion engine having a camshaft phaser for controllably varying the phase relationship between a crankshaft of the internal combustion engine and the camshaft where the camshaft phaser is actuated by an electric motor and includes a gear reduction mechanism with a predetermined gear reduction ratio and rotational position means for determining the rotational position of the electric motor. The method includes generating a rotational position signal indicative of the rotational position of the electric motor by using the rotational position means to determine the rotational position of the electric motor. The method also includes calculating the position of the camshaft based on the rotational position signal and the gear reduction ratio of the gear reduction mechanism. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0009]    This invention will be further described with reference to the accompanying drawings in which: 
           [0010]      FIG. 1  is an exploded isometric view of an eVCP in accordance with the present invention; 
           [0011]      FIG. 2  is an axial cross-section of an eVCP in accordance with the present invention; 
           [0012]      FIG. 3  is a radial cross-section through line  3 - 3  of  FIG. 2 ; 
           [0013]      FIG. 4  is an exploded isometric partial cut-away view of an eVCP in accordance with the present invention; 
           [0014]      FIG. 5  is an isometric view of an eVCP in accordance with the present invention; 
           [0015]      FIG. 6  is a radial cross-section as in  FIG. 3  now shown in the maximum advance valve timing position; 
           [0016]      FIG. 7  is a radial cross-section as in  FIG. 3 , now shown in the maximum retard valve timing position; and 
           [0017]      FIG. 8  is a plot showing the voltage of each Hall Effect sensor and the voltage of each phase of an electric motor used to actuate an eVCP. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0018]    Referring to  FIGS. 1 and 2 , an eVCP  10  in accordance with the present invention comprises a flat harmonic gear drive unit  12 ; a rotational actuator  14  that is preferably a DC electric motor, operationally connected to harmonic gear drive unit  12 ; an input sprocket  16  operationally connected to harmonic gear drive unit  12  and drivable by a crankshaft (not shown) of engine  18 ; an output hub  20  attached to harmonic gear drive unit  12  and mountable to an end of an engine camshaft  22 ; and a bias spring  24  operationally disposed between output hub  20  and input sprocket  16 . Electric motor  14  may be a brushless three-phase radial-flux DC motor. 
         [0019]    Harmonic gear drive unit  12  comprises an outer first spline  28  which may be either a circular spline or a dynamic spline as described below; an outer second spline  30  which is the opposite (dynamic or circular) of first spline  28  and is coaxially positioned adjacent first spline  28 ; a flexspline  32  disposed radially inwards of both first and second splines  28 ,  30  and having outwardly-extending gear teeth disposed for engaging inwardly-extending gear teeth on both first and second splines  28 ,  30 ; and a wave generator  36  disposed radially inwards of and engaging flexspline  32 . 
         [0020]    Flexspline  32  is a non-rigid ring with external teeth on a slightly smaller pitch diameter than the circular spline. It is fitted over and elastically deflected by wave generator  36 . 
         [0021]    The circular spline is a rigid ring with internal teeth engaging the teeth of flexspline  32  across the major axis of wave generator  36 . The circular spline serves as the input member. 
         [0022]    The dynamic spline is a rigid ring having internal teeth of the same number as flexspline  32 . It rotates together with flexspline  32  and serves as the output member. Either the dynamic spline  28  or the circular spline  30  may be identified by a chamfered corner  34  at its outside diameter to distinguish one spline from the other. 
         [0023]    As is disclosed in the prior art, wave generator  36  is an assembly of an elliptical steel disc (not shown) supporting an elliptical bearing (not shown), the combination defining a wave generator plug. A flexible bearing retainer (not shown) surrounds the elliptical bearing and engages flexspline  32 . Rotation of the wave generator plug causes a rotational wave to be generated in flexspline  32  (actually two waves 180° apart, corresponding to opposite ends of the major ellipse axis of the disc). 
         [0024]    During assembly of harmonic gear drive unit  12 , flexspline teeth engage both circular spline teeth and dynamic spline teeth along and near the major elliptical axis of the wave generator  36 . The dynamic spline  28  has the same number of teeth as the flexspline  32 , so rotation of the wave generator  36  causes no net rotation per revolution therebetween. However, the circular spline  30  has slightly fewer gear teeth than does the dynamic spline  28 , and therefore the circular spline  30  rotates past the dynamic spline  28  during rotation of the wave generator plug, defining a gear ratio therebetween (for example, a gear ratio of 50:1 would mean that 1 rotation of the circular spline past the dynamic spline corresponds to 50 rotations of the wave generator). Harmonic gear drive unit  12  is thus a high-ratio gear transmission; that is, the angular phase relationship between first spline  28  and second spline  30  changes by 2% for every revolution of wave generator  36 . 
         [0025]    Of course, as will be obvious to those skilled in the art, the circular spline  30  may instead have slightly more teeth than the dynamic spline  28  has, in which case the rotational relationships described below are reversed. 
         [0026]    Still referring to  FIGS. 1 and 2 , input sprocket  16  is fixed to a generally cup-shaped sprocket housing  40  that is fastened by bolts  42  to first spline  28  in order to prevent relative rotation therebetween. Coupling adaptor  44  is mounted to wave generator  36  and extends through sprocket housing  40 , being supported by bearing  46  mounted in sprocket housing  40 . Coupling adapter  44  may be made of two separate pieces that are joined together as shown in  FIG. 2 . Coupling  48 , mounted to the motor shaft of electric motor  14  and pinned thereto by pin  50 , engages coupling adaptor  44 , permitting wave generator  36  to be rotationally driven by electric motor  14 , as may be desired to alter the phase relationship between first spline  28  and second spline  30 . 
         [0027]    Output hub  20  is fastened to second spline  30  by bolts  52  and may be secured to engine camshaft  22  by central through-bolt  54  extending through output hub axial bore  56  in output hub  20 , and capturing stepped thrust washer  58  and filter  60  recessed in output hub  20 . In an eVCP, it is necessary to limit radial run-out between the input hub and output hub. In the prior art, this has been done by providing multiple roller bearings to maintain concentricity between the input and output hubs. Referring to  FIG. 2 , radial run-out is limited by a single journal bearing interface  38  between sprocket housing  40  (input hub) and output hub  20 , thereby reducing the overall axial length of eVCP  10  and its cost to manufacture. Output hub  20  is retained within sprocket housing  40  by snap ring  62  disposed in an annular groove  64  formed in sprocket housing  40 . 
         [0028]    Back plate  66 , which is integrally formed with input sprocket  16 , captures bias spring  24  against output hub  20 . Inner spring tang  67  is engaged by output hub  20 , and outer spring tang  68  is attached to back plate  66  by pin  69 . In the event of an electric motor malfunction, bias spring  24  is biased to back-drive harmonic gear drive unit  12  without help from electric motor  14  to a rotational position of second spline  30  wherein engine  18  will start or run, which position may be at one of the extreme ends of the range of authority or intermediate of the phaser&#39;s extreme ends of its rotational range of authority. For example, the rotational range of travel to which bias spring  24  biases harmonic gear drive unit  12  may be limited to something short of the end stop position of the phaser&#39;s range of authority. Such an arrangement would be useful for engines requiring an intermediate park position for idle or restart. 
         [0029]    The nominal diameter of output hub  20  is D; the nominal axial length of first journal bearing  70  is L; and the nominal axial length of the oil groove  72  formed in either output hub  20  (shown) and/or in sprocket housing  40  (not shown) for supplying oil to first journal bearing  70  is W. In addition to journal bearing clearance, the length L of the journal bearing in relation to output hub diameter D controls how much output hub  20  can tip within sprocket housing  40 . The width of oil groove  72  in relation to journal bearing length L controls how much bearing contact area is available to carry the radial load. Experimentation has shown that a currently preferred range of the ratio L/D may be between about 0.25 and about 0.40, and that a currently preferred range of the ratio W/L may be between about 0.15 and about 0.70. 
         [0030]    Oil provided by engine  18  is supplied to oil groove  72  by one or more oil passages  74  that extend radially from output hub axial bore  56  of output hub  20  to oil groove  72 . Filter  60  filters contaminants from the incoming oil before entering oil passages  74 . Filter  60  also filters contaminants from the incoming oil before being supplied to harmonic gear drive unit  12  and bearing  46 . Filter  60  is a band-type filter that may be a screen or mesh and may be made from any number of different materials that are known in the art of oil filtering. 
         [0031]    Extension portion  82  of output hub  20  receives bushing  78  in a press fit manner. In this way, output hub  20  is fixed to bushing  78 . Input sprocket axial bore  76  interfaces in a sliding fit manner with bushing  78  to form second journal bearing  84 . This provides support for the radial drive load placed on input sprocket  16  and prevents the radial drive load from tipping first journal bearing  70  which could cause binding and wear issues for first journal bearing  70 . Bushing  78  includes radial flange  80  which serves to axially retain back plate  66 /input sprocket  16 . Alternatively, but not shown, bushing  78  may be eliminated and input sprocket axial bore  76  could interface in a sliding fit manner with extension portion  82  of output hub  20  to form second journal bearing  84  and thereby provide the support for the radial drive load placed on input sprocket  16 . In this alternative, back plate  66 /input sprocket  16  may be axially retained by a snap ring (not shown) received in a groove (not shown) of extension portion  82 . 
         [0032]    In order to transmit torque from input sprocket  16 /back plate  66  to sprocket housing  40  and referring to  FIGS. 1 ,  2 , and  5 , a sleeve gear type joint is used in which back plate  66  includes external splines  86  which slidingly fit with internal splines  88  included within sprocket housing  40 . The sliding fit nature of the splines  86 ,  88  eliminates or significantly reduces the radial tolerance stack issue between first journal bearing  70  and second journal bearing  84  because the two journal bearings  70 ,  84  operate independently and do not transfer load from one to the other. If this tolerance stack issue were not resolved, manufacture of the two journal bearings would be prohibitive in mass production because of component size and concentricity tolerances that would need to be maintained. The sleeve gear arrangement also eliminates then need for a bolted flange arrangement to rotationally fix back plate  66  to sprocket housing  40  which minimizes size and mass. Additionally, splines  86 ,  88  lend themselves to fabrication methods where they can be net formed onto back plate  66  and into sprocket housing  40  respectively. Splines  86 ,  88  may be made, for example, by powder metal process or by standard gear cutting methods. 
         [0033]    Now referring to  FIGS. 3 and 4 , eVCP  10  is provided with a means for limiting the phase authority of eVCP  10 . Sprocket housing  40  is provided with first and second arcuate input stop members  90 ,  92  which extend axially away from first surface  94  (also shown in  FIG. 2 ) of sprocket housing  40 , the first and second lengths of which are defined by the arcuate or angular distances α 1 , α 2  respectively. First surface  94  is the bottom of the longitudinal bore which receives output hub  20  within sprocket housing  40 . First arcuate input stop member  90  includes first advance stop surface  96  and first retard stop surface  98  which define the ends of first arcuate input stop member  90 . Similarly, second arcuate input stop member  92  includes second advance stop surface  100  and second retard stop surface  102  which define the ends of second arcuate input stop member  92 . First arcuate input opening  104  is defined between first advance stop surface  96  of first arcuate input stop member  90  and second retard stop surface  102  of second arcuate input stop member  92 . First arcuate input opening  104  has a third length defined by the arcuate or angular distance α 3 . Similarly, second arcuate input opening  106  is defined between first retard stop surface  98  of first arcuate input stop member  90  and second advance stop surface  100  of second arcuate input stop member  92 . Second arcuate input opening  106  has a fourth length defined by the arcuate or angular distance α 4 . 
         [0034]    Now referring to  FIGS. 1 ,  3 , and  4 , output hub  20  includes corresponding features which interact with first and second arcuate input stop members  90 ,  92  and first and second arcuate input openings  104 ,  106  to limit the phase authority of eVCP  10 . Output hub  20  is provided with first and second arcuate output stop members  108 ,  110  which extend axially away from second surface  112  (also shown in  FIG. 2 ) of output hub  20 , the fifth and sixth lengths of which are defined by the arcuate or angular distances α 3 ′, α 4 ′ respectively. Second surface  112  is the end of output hub  20  which faces toward first surface  94 . First arcuate output stop member  108  includes third advance stop surface  96 ′ and fourth retard stop surface  102 ′ which define the ends of first arcuate output stop member  108 . Similarly, second arcuate output stop member  110  includes fourth advance stop surface  100 ′ and third retard stop surface  98 ′ which define the ends of second arcuate output stop member  110 . First arcuate output opening  114  is defined between fourth retard stop surface  102 ′ of first arcuate output stop member  108  and fourth advance stop surface  100 ′ of second arcuate output stop member  110 . First arcuate output opening  114  has a seventh length defined by the arcuate or angular distance α 2 ′. Similarly, second arcuate output opening  116  is defined between third retard stop surface  98 ′ of second arcuate output stop member  110  and third advance stop surface  96 ′ of first arcuate output stop member  108 . Second arcuate output opening  116  has an eighth length defined by the arcuate or angular distance α 1 ′. 
         [0035]    In order to establish the phase authority of eVCP  10 , first and second arcuate input stop members  90 ,  92  are axially and radially received within second and first arcuate output openings  116 ,  114  respectively. Similarly, first and second arcuate output stop members  108 ,  110  are axially and radially received within first and second arcuate input openings  104 ,  106  respectively. The arcuate stop members and each corresponding arcuate opening within which the arcuate stop member is received are sized such that the angular distance of each angular opening minus the angular distance of the corresponding arcuate stop member is equal to the phase authority of eVCP  10 . For example, angular distance α 1 ′ minus angular distance ca equals the phase authority of eVCP  10 . Stated another way, if the phase authority for eVCP  10  is 50 degrees, then angular distance α 1 ′ (in degrees) minus angular distance α 1  (in degrees) equals 50 degrees. 
         [0036]    Angular distances α 1 , α 2  of first and second arcuate input stop members  90 ,  92  are preferably equal and first and second arcuate input stop members  90 ,  92  are preferably angularly spaced in a symmetric manner. Similarly, angular distance α 3 ′, α 4 ′ of first and second arcuate output stop members  108 ,  110  are preferably equal and first and second arcuate output stop members  108 ,  110  are preferably angularly spaced in a symmetric manner. As can now be seen, distinct eVCPs can be provided for different engine application requiring different amounts of phase authority simply by redesigning the input stop members and the output stop members to achieve the desired phase authority. 
         [0037]    Angular distances α 3 , α 4  of first and second arcuate input openings  104 ,  106  are preferably equal and first and second arcuate input openings  104 ,  106  are preferably angularly spaced in a symmetric manner. Similarly, angular distance α 1 ′, α 2 ′ of first and second arcuate output openings  114 ,  116  are preferably equal and first and second arcuate output openings  114 ,  116  are preferably angularly spaced in a symmetric manner. 
         [0038]    In operation, when eVCP  10  is commanded to provide maximum valve timing advance, electric motor  14  will actuate harmonic gear drive unit  12  to rotate output hub  20  with respect to sprocket housing  40  until first and third advance stop surfaces  96 ,  96 ′ are in contact with each other ( FIG. 6 ). At the same time, second and fourth advance stop surfaces  100 ,  100 ′ are in contact with each other. Similarly, when eVCP  10  is commanded to provide maximum valve timing retard, electric motor  14  will actuate harmonic gear drive unit  12  to rotate output hub  20  with respect to sprocket housing  40  until second and fourth retard surfaces  102 ,  102 ′ are in contact with each other ( FIG. 7 ). At the same time, first and third retard surfaces  98 ,  98 ′ are in contact with each other. 
         [0039]    Now referring to  FIGS. 1 and 2 , electric motor  14 , which is preferably a three-phase brushless DC motor, includes motor housing  118  which may be bolted to engine  18  in order to prevent relative rotation therebetween. Motor housing  118  includes rotor  120  therewithin which is rotatable relative to motor housing  118 . Motor housing  118  also includes stator  122  therewithin which is fixed to motor housing  118  to prevent relative rotation therebetween. Rotor  120  includes multi-pole ring magnet  124  surrounding the perimeter thereof. In this example, multi-pole ring magnet  124  includes five pole pairs where each pole is preferably equal in angular length, that is, about 36°. Stator  122  includes three electrical windings  126   a ,  126   b ,  126   c ; each winding establishing a phase of electric motor  14 . Electrical windings  126   a ,  126   b ,  126   c  are preferably spaced equiangularly. 
         [0040]    Electric motor  14  includes a rotation position means including three Hall Effect sensors  130   a ,  130   b ,  130   c  that are used to detect the rotational position of rotor  120 . Hall Effect sensors  130   a ,  130   b ,  130   c  generate a rotational position signal indicative of the rotational position of rotor  120 . The rotational position of rotor  120  may also be referred to as the rotational position of electric motor  14 . One Hall Effect sensor is disposed between each of the three electrical windings  126   a ,  126   b ,  126   c  in stator  122 . Hall Effect sensors  130   a ,  130   b ,  130   c  are preferably spaced equiangularly. Each Hall Effect sensors  130   a ,  130   b ,  130   c  sends the rotational position signal to engine control module (ECM)  132  which alternately switches the power on and off to electrical windings  126   a ,  126   b ,  126   c  of the three phases based on input from the Hall Effect sensors  130   a ,  130   b ,  130   c , in turn creating forces in each electrical winding  126   a ,  126   b ,  126   c  that make rotor  120  rotate about its central axis. Hall Effect sensors  130   a ,  130   b ,  130   c  are capable of detecting the position of rotor  120  even at zero RPM as long as ECM  132  is still powered. Since rotor  120  of electric motor  14  is connected to engine camshaft  22  through harmonic gear drive unit  12 , the position of rotor  120 , θ actuator , correlates to the position of engine camshaft  22 , θ camshaft , based on the position of the crankshaft, θ sprocket , according to equations A and B below. Therefore, Hall Effect sensors  130   a ,  130   b ,  130   c  can also be used to detect the position of engine camshaft  22  even at zero RPM as long as ECM  132  is still powered. Using Hall Effect sensors  130   a ,  130   b ,  130   c  to determine the position of engine camshaft  22  eliminates the need for a separate sensor for determining the position of engine camshaft  22 . 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       θ 
                       camshaft 
                     
                     - 
                     
                       θ 
                       sprocket 
                     
                   
                   = 
                   
                     
                       Phase 
                        
                       
                           
                       
                        
                       Angle 
                     
                     2 
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   A 
                 
               
             
             
               
                 
                   
                     
                       θ 
                       camshaft 
                     
                     - 
                     
                       θ 
                       sprocket 
                     
                   
                   = 
                   
                     
                       1 
                       
                         Gear 
                          
                         
                             
                         
                          
                         Ratio 
                       
                     
                     × 
                     
                       ( 
                       
                         
                           θ 
                           sprocket 
                         
                         - 
                         
                           θ 
                           actuator 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   B 
                 
               
             
           
         
       
     
         [0041]    When engine  18  is turned off by the operator of the motor vehicle, power may continue to be supplied to ECM  132  until after engine  18  comes to a complete stop. Since power is still supplied to ECM  132  when engine  18  comes to a complete stop, Hall Effect sensors  130   a ,  130   b ,  130   c  can determine the rotational position of engine camshaft  22  even when engine  18  is no longer rotating. The position of engine camshaft  22  can then be stored in non-volatile random access memory  134  of ECM  132 . Because eVCP  10  uses a high gear ratio harmonic drive unit, the rotational position of engine camshaft  22  will not change due to residual cam torque generated by open intake or exhaust valves pushing against engine camshaft  22 . After the position of engine camshaft  22  is stored in the non-volatile random access memory  134  of ECM  132 , power to ECM  132  may be discontinued. In this way, when engine  18  is commanded to be started by the operator of the motor vehicle, the rotational position of engine camshaft  22  can be immediately recalled from the ECM  132  without the need to synchronize the rotational position of engine camshaft  22  with the crankshaft of engine  18 . 
         [0042]    Diagnostics may be performed upon start up of engine  18  in order to determine if engine camshaft  22  is not in the position it should be in based on the position of engine camshaft  22  at the time engine  18  was stopped. The rotation position of engine camshaft  22  may change, for example, due to the motor vehicle being pushed or being parked on a hill. If the diagnostics indicate that engine camshaft  22  is not in the position it should be, Hall Effect sensors  130   a ,  130   b ,  130   c  can be used to synchronize the position of engine camshaft  22  with the crankshaft of engine  18  in a conventional manner. For example, a  58 X crankshaft sensor (not shown) which produces a pulse every six degrees of crankshaft rotation with two missing pulses every 360 degrees of crankshaft rotation as an index can be used to determine the crankshaft position, θ sprocket . This requires a maximum of one engine rotation (one-half camshaft rotation) to reach the two missing pulses in order to obtain the absolute crankshaft position. ECM  132  commands eVCP  10  to full advance such that first and third advance stop surfaces  96 ,  96 ′ are in contact with each other ( FIG. 6 ) and second and fourth advance stop surfaces  100 ,  100 ′ are in contact with each other. With the crankshaft position known and eVCP  10  fully advanced, the absolute position of engine camshaft  22  is now known. Equations A and B above may now be used to determine the phase angle based on crank position and camshaft position. This same process may be used to synchronize the position of engine camshaft  22  if ECM  132  is not equipped with non-volatile random access memory  134  and power has been shut off to ECM  132  in an engine shutdown event. 
         [0043]    This same process may be used to determine the camshaft position. 
         [0044]      FIG. 8  shows a plot of the voltage of electrical winding  126   a ,  126   b ,  126   c  of electric motor  14  (left ordinate axis) over time as well as the voltage of each Hall Effect sensors  130   a ,  130   b ,  130   c  (right ordinate axis). The voltage of electrical windings  126   a ,  126   b ,  126   c  are represented by traces  136   a ,  136   b ,  136   c  respectively while the voltage of Hall Effect sensors  130   a ,  130   b ,  130   c  are represented by traces  138   a ,  138   b ,  138   c  respectively. Each vertical section of traces  138   a ,  138   b ,  138   c  represents a transition from one pole to an adjacent pole of multi-pole ring magnet  124  passing a corresponding Hall Effect sensor. As can be seen, traces  136   a ,  136   b ,  136   c ,  138   a ,  138   b , and  138   c  produce a pattern that can be used to always determine the relative actuator position. 
         [0045]    Stopping the engine  18  may also be desirable in order to conserve fuel when the motor vehicle is not in motion and engine  18  would otherwise be idling, for example, when stopped at a stop sign or traffic light. When engine  18  is stopped under these circumstances, power continues to be supplied to the ECM  132 . Therefore, eVCP  10  can be used, even when engine  18  is stopped, to position engine camshaft  22  to a rotational position that benefits restart of engine  18  when motion of the motor vehicle is commanded. Of course, eVCP  10  can also be used prior to engine  18  being stopped to position engine camshaft  22  to a rotation position that benefits restart of engine  18  when motion of the motor vehicle is commanded. One such rotational position may be what is referred to in the art as decompression mode. In decompression mode, engine camshaft  22  is phased relative to the crankshaft of engine  18  such that minimal pressure is generated in the combustion chamber of engine  18  such that minimal torque is required to overcome the pressure inside the combustion chambers as engine  18  is restarted. 
         [0046]    In order to restart engine  18  that has been stopped for the purpose of conserving fuel when there is no motion of the motor vehicle, one proposal has been made to use fuel and spark to instantly produce driving power. In this technique, high pressure fuel is injected directly into the combustion chamber of engine  18 . The high pressure charge of fuel is then ignited to create torque to restart engine  18 . This technique eliminates the need for use of the conventional starter which may delay restart of engine  18 . This technique is aided by stopping engine  18  in a specific optimized position. Knowing the position of engine camshaft  22  as the internal combustion engine approaches zero RPMs can aid in stopping engine  18  in the specific optimized position. In one example, using the position of engine camshaft  22  in conjunction with ECM  132  controlling throttle and spark, engine  18  can be stopped at the specific optimized position. 
         [0047]    While stator  122  has been described as having three electrical windings, each establishing a phase, it should now be understood that each phase may include more than one winding. When the stator includes more than one winding for each phase, the windings may be arranged in an alternating pattern such that adjacent windings are of different phases. 
         [0048]    While the embodiment described herein describes input sprocket  16  as being smaller in diameter than sprocket housing  40  and disposed axially behind sprocket housing  40 , it should now be understood that the input sprocket may be radially surrounding the sprocket housing and axially aligned therewith. In this example, the back plate may be press fit into the sprocket housing rather than having a sleeve gear type joint. 
         [0049]    While the embodiment described herein includes first and second input stop members, it should now be understood that more or fewer arcuate input stop members may be included. Similarly, more or fewer arcuate output stop members may be included. 
         [0050]    While the embodiment described herein describes angular distances α 1 , α 2  of first and second arcuate input stop members  90 ,  92  as equal and first and second arcuate input stop members  90 ,  92  are angularly spaced in a symmetric manner, it should now be understood that the first and second arcuate input stop members may be have unequal lengths and may also be spaced asymmetrically. This will result in the first and second arcuate output members being unequal in length and being spaced asymmetrically. 
         [0051]    The embodiment described herein describes harmonic gear drive unit  12  as comprising outer first spline  28  which may be either a circular spline or a dynamic spline which serves as the input member; an outer second spline  30  which is the opposite (dynamic or circular) of first spline  28  and which serves as the output member and is coaxially positioned adjacent first spline  28 ; a flexspline  32  disposed radially inwards of both first and second splines  28 ,  30  and having outwardly-extending gear teeth disposed for engaging inwardly-extending gear teeth on both first and second splines  28 ,  30 ; and a wave generator  36  disposed radially inwards of and engaging flexspline  32 . As described, harmonic gear drive unit  12  is a flat plate or pancake type harmonic gear drive unit as referred to in the art. However, it should now be understood that other types of harmonic gear drive units may be used in accordance with the present invention. For example, a cup type harmonic gear drive unit may be used. The cup type harmonic gear drive unit comprises a circular spline which serves as the input member; a flexspline which serves as the output member and which is disposed radially inwards of the circular spline and having outwardly-extending gear teeth disposed for engaging inwardly-extending gear teeth on the circular spline; and a wave generator disposed radially inwards of and engaging the flexspline. 
         [0052]    While the embodiment of eVCP  10  described herein includes harmonic gear drive unit  12  driven by electric motor  14 , it should be understood that harmonic gear drive unit  12  may be replaced with any number of gear drive units or gear reduction units commonly known for transmitting torque from a driving member to a driven member. 
         [0053]    While this invention has been described in terms of preferred embodiments thereof, it is not intended to be so limited, but rather only to the extent set forth in the claims that follow.