Abstract:
A control method for an electro-mechanical camshaft phase shifting devices in general, and a control method for an electro-mechanic camshaft phase shifting device with a self-locking mechanism in particular. The control method takes advantage of a cam shaft reaction torque in conjunction with a frictional self-locking feature of an electro-mechanical camshaft phase shifting device to simplify the control structure and to reduce the actuating torque required for the associated electric machine, consequently reducing the size of electric machine.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims priority to U.S. Provisional Application Ser. No. 61/247,229, filed Sep. 30, 2009, the entire disclosure of which is incorporated by reference herein. 
     
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0002]    The present application is related to U.S. patent application Ser. No. 12/441,841 filed on Mar. 18, 2009 as a U.S. National Stage of PCT/US2007/078755 filed on Sep. 18, 2007 and published as WO 2008/036650 A1. 
         [0003]    The present application is related to U.S. patent application Ser. No. 12/517,920 filed on Jun. 5, 2009 as a U.S. National Stage of PCT/US2007/024822 filed on Dec. 4, 2007 and published as WO 2008/070066 A1. 
         [0004]    The present application is related to U.S. Provisional Patent Application Ser. No. 60/978,568 filed on Oct. 9, 2007. 
         [0005]    The present application is related to U.S. Provisional Patent Application Ser. No. 61/121,694 filed on Dec. 11, 2008. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0006]    Not Applicable. 
       BACKGROUND OF THE INVENTION 
       [0007]    The present invention is related generally to a camshaft adjustment mechanism for use in an internal combustion engine, and in particular, to a control structure utilizing cam reaction torque to control an electro-mechanical camshaft phase shifting device. 
         [0008]    Camshaft phase shifting devices are used more often in gasoline engines to vary valve timing for benefits of improving fuel economy and exhaust gas quality. There are many types of cam shaft phase shifting devices. Hydraulic cam phase shifting devices are commonly seen current applications. The major challenges for these hydraulic cam phasers include obtaining required slew rate in slow-speed operation, maintaining accurate cam shaft angular position, and extending the range of operating temperature. To reduce high pollutant emissions, it is highly desirable to adjust cam phase angle before or during engine startup. This requires the cam-shaft phase shifting device to be controlled prior to or during engine startup. These difficulties can be overcome by electro-mechanical cam-shaft phase shifting devices. 
         [0009]    In International Patent Cooperation Treaty Application Ser. No. PCT/US2007/078755, an electro-mechanical camshaft phase shifting device (eCPS) is disclosed. The device includes a three-shaft gear unit and an electric machine. The three shaft gear unit, comprising an input shaft, an output shaft and a control shaft, features a frictional self-locking mechanism. The output shaft is locked to the input shaft unless torque is applied to the control shaft. Upon receiving command from the engine ECU, the electric machine, connected to the control shaft, can be operated in three modes to achieve desired performance objectives. The three operating modes include the neutral mode in which the electric machine exerts no torque on the control shaft, the motoring mode in which the electric machine exerts a driving torque on the control shaft, and the generating mode in which the electric machine exerts braking torque on the control shaft. 
         [0010]    Similarly, in International Patent Cooperation Treaty Application Ser. No. PCT/US2007/024822, a control structure for a electro-mechanical camshaft phase shifting device is disclosed. The control structure uses both feed forward and feed back loops to generate control signals for the electric machine, and thus provides a concrete means for an eCPS to realize the three different operating modes. 
       BRIEF SUMMARY OF THE INVENTION 
       [0011]    Briefly stated, the present disclosure provides a control method for an electromechanical camshaft phase shifting device in general, and a control method for an electro-mechanic camshaft phase shifting device with a self-locking mechanism in particular. The control method takes advantage of cam shaft reaction torque in conjunction with the frictional self-locking feature of the eCPS to simplify the control structure and to reduce the actuating torque required for the electric machine, consequently reducing the size of electric machine. 
         [0012]    The camshaft phase shifting device of the present disclosure includes a coaxially arranged three-shaft gear system, having an input shaft, an output shaft and a control shaft for adjusting the phase angle between the input and output shafts. The input shaft is coupled with the engine crank shaft, the output shaft is coupled with the cam shaft, and the control shaft is coupled with the rotator of an electric machine. The method of control is developed from a so-called torque-time based control structure. The dynamic response of the system, and thus the desired phase angle of the cam shaft, is controlled and maintained by a controller that produces a torque command with a constant amplitude and variable width based on a signal or signals it receives. The signal or signals received includes a cam shaft phase angle error signal, defined as the deviation of cam phase shift angle from a reference value. The torque command (a voltage signal for example) is then converted by an electric machine into an electro-magnetic torque exerted on the control shaft of the camshaft phase shifting device. The length in time during which the torque is applied is determined by the pulse width of the torque command. 
         [0013]    In one embodiment of the present disclosure, the torque command can be a signed constant whose amplitude is changeable based on the cam shaft speed in either a continuous or stepwise fashion. 
         [0014]    In one embodiment of the present disclosure, the torque command may be smaller than the amplitude of a camshaft reaction torque reflected on the control shaft. 
         [0015]    In one embodiment of the present disclosure, the controller includes an on-and-off switch to turn off the torque command for energy savings when a self-locking mechanism is determined to be active. 
         [0016]    The foregoing features and advantages set forth in the present disclosure, as well as presently preferred embodiments, will become more apparent from the reading of the following description in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0017]    In the accompanying drawings which form part of the specification: 
           [0018]      FIG. 1  schematically illustrates a control structure of the present disclosure for controlling an electro-mechanical cam phase shifting device; 
           [0019]      FIG. 2  illustrates the interconnections between an input shaft, an output shaft, a control shaft, and a three-coaxial shaft gearing system of the present disclosure; 
           [0020]      FIG. 3  illustrates a sectional view of an electro-mechanical camshaft phase shifting device with the three-coaxial shaft gearing system; 
           [0021]      FIG. 4  illustrates a plot of the torque, phase angle shifting speed, and shifting angle of the output shaft with respect to the input shaft; and 
           [0022]      FIG. 5  schematically illustrates an alternate control structure of the present disclosure for controlling an electro-mechanical cam phase shifting device. 
       
    
    
       [0023]    Corresponding reference numerals indicate corresponding parts throughout the several figures of the drawings. It is to be understood that the drawings are for illustrating the concepts set forth in the present disclosure and are not to scale. 
         [0024]    Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. 
       DETAILED DESCRIPTION 
       [0025]    The following detailed description illustrates the invention by way of example and not by way of limitation. The description enables one skilled in the art to make and use the present disclosure, and describes several embodiments, adaptations, variations, alternatives, and uses of the present disclosure, including what is presently believed to be the best mode of carrying out the present disclosure. 
         [0026]    Turning to the Figures, and to  FIG. 1  in particular, a control structure for controlling the electro-mechanical cam phase shifting device is shown. The system shown in  FIG. 1  is comprised of an engine  10 , an engine control unit (ECU)  20 , a phase shifting device  30  and a controller  40 . The phase shifting device  30  includes a three-shaft gearing system, having three co-axially arranged rotatable shafts as depicted in  FIGS. 2 and 3 . The input shaft  16  to the phase shifting device  30  is connected through a sprocket  18  and a chain drive (not shown) to the engine crank shaft. The output shaft  14  of the phase shifting device  30  is connected to the engine cam shaft  12 . A control shaft  34  of the phase shifting device  30  is coupled to the rotor  31  of an electric machine  32 . 
         [0027]    The phase shifting device  30  has a built-in frictional self-locking mechanism, which enables the output shaft  14  to lock up with the input shaft  16  and therefore to transmit torque between the two shafts with a 1:1 speed ratio if no torque is applied to the control shaft  34 . Under this condition, there will be no phase shift between input shaft  16  and output shaft  14 . Frictional locking between the input shaft  16  and the output shaft  14  can only be unlocked by applying adequate torque to the control shaft  34 . 
         [0028]    During operation, the required torque to unlock the input shaft  16  from the output shaft  14  is generated by the electric machine  32  coupled to the control shaft  34  in response to a torque command received by the electric machine from the controller  40 . When the phase shifting device  30  is unlocked, there may be a slight difference between the speed of the input shaft  16  and the output shaft  14 . This allows the cam shaft  12  connected to the output shaft  14  to shift in angular position with respect to the input shaft  16 . 
         [0029]    The cyclical nature of the reactive torque to the cam shaft from valve springs in the engine  10  can be utilized in conjunction with the resistive nature of frictional torque from the self-locking mechanism to reduce the actuation torque required to be generated on the control shaft  34  by the electric machine  32 . 
         [0030]    Turning to  FIG. 4 , it will be seen that T C  denotes the cam shaft reaction torque, T E  denotes the effective electric machine actuation torque, and T R  denotes the effective resistant torque. The phrase “effective” means the torque values are converted from their origins and are seen or measured on the cam shaft. The maximum frictional resistant torque can be reasonably expressed as T R—max =qT C  where q&gt;1, for the gear train to have a self-locking feature. Assume that the change in reaction torque T C  follows a square wave as shown in  FIG. 4 , and the actuation direction is the positive direction for torque and speed. To take the advantage of frictional resistant torque in reducing actuation torque, set 
         [0000]      T E &lt;T C   
         [0000]    and chose q such that 
         [0000]      T E &gt;(q−1)T C .
 
         [0031]    Thus, when reaction torque T C  is aligned with actuation torque T E , we have 
         [0000]    ti  T   E   +T   C   &gt;qT   C   =T   R—max   
         [0032]    T E +T C  will overcome T R—max  to unlock the gear train and accelerate the output shaft  14  with respect to the input shaft  16 . Accordingly, the output shaft  14  starts to shift the phase angle in a positive direction. When reaction torque T C  changes direction, it works against actuation torque T E . Since 
         [0000]      T E &lt;T C &lt;T C +T R—max , 
         [0000]    T C  +T R—max  takes over T E  and slows output shaft  14  down with respect to input shaft  16  until it reaches the same speed as the input shaft  16 . During deceleration, the output shaft  14  continues to phase with respect to the input shaft  16  in the positive direction at a decreasing rate until the phase difference becomes zero. At this moment the resistant torque T R  reverses direction and assists T E  to maintain the balance between the actuation torque T E  and the reaction torque T C , that is 
         [0000]        T   E   +T   R   =T   C . 
         [0033]    The output shaft  14  does not change phase with respect to the input shaft  16  until the reaction torque T C  becomes positive again during the next cycle.  FIG. 4  illustrates the torque, phase angle shifting speed, and shifting angle of the output shaft  14  with respect to the input shaft  16 . Three regimes are identified for each cycle of reaction torque T C  during actuation. They are respectively referred to as the acceleration regime, the deceleration regime, and the dwell regime. The phase angle shift per cycle varies with the amplitude of actuation torque T E , and the cumulative phase angle shifted during the actuation is a function of both the amplitude and duration of the actuation torque T E . This forms the basis for torque-time based control structure. 
         [0034]    In real applications, the variation of reaction torque T C  does not follow an ideal square wave form, and the transitions between the dwell and acceleration regimes and between the acceleration and deceleration regimes may not coincide with the zero-crossing point of the reaction torque T C . However, this does not alter the torque-time based control structure. 
         [0035]    To implement the torque-time based control structure of the present disclosure, the controller  40  generates a torque command, which can be a voltage signal, based on information it receives from the engine ECU  20  and the cam shaft angle sensors. The received information includes, but is not limited to, a cam shaft phase shift angle set point (reference), and an actual cam shaft phase shift angle measured and/or computed from angular position sensor signals. The actual cam shaft phase shift angle is compared to the reference value to generate a differential (error) signal. The differential or error signal is then fed to a compensator to generate a torque command with an amplitude restricted not to exceed a chosen value for T E . This value can be lower than the maximum reaction torque T C  but has to be higher than the differential between the maximum frictional torque and the maximum reaction torque. In applications, the amplitude of chosen actuation torque T E  may be adjusted to suite for engine speed or other conditions. The duration of the actuation torque command is controlled by a timing logic in the controller  40 , and is based on error signal or signals. 
         [0036]    The torque command generated by the controller  40  is in turn used to command the electric machine for controlling and adjusting the cam shaft phase angle to decrease the error signal or signals sent to the controller  40 . In doing so, the desired cam shaft phase shift is achieved. 
         [0037]    Optionally, the torque-time based controller  40  may further include a PID compensator  42 , as shown in  FIG. 5 . The compensator can be primarily a proportional-and-derivative controller (PD). In addition, as is further shown in  FIG. 5 , the controller  40  may further include a feed forward branch (or a processor)  44  for processing and computing an anticipated torque disturbance. The resulting signal is fed forward to, and combined with, the output signal of the PID controller, forming the base for the torque command signal controlling the operation of the electric machine  32 . 
         [0038]    Since, as described above, the phase shifting device  30  features a self-locking mechanism, it is possible to turn the controller  40  and the electric machine  32  off for energy savings when the actual cam phase shift angle is in a close proximity to the desired value (reference value or set point). This is done, for example, by sending a signal from the controller  40  to the electric machine  32  commanding a zero torque output. 
         [0039]    It is also possible to move the derivative portion of the PID compensator  42  to a feed back path to reduce the effects of impulse (sudden change) in reference input. 
         [0040]    Those of ordinary skill in the art will recognize that the control system of the present disclosure may be implemented with other types of compensators using alternative control laws, such as model predictive controller (MPC), to replace the PID compensator  42 . 
         [0041]    The current invention may include other embodiments that can be derived from the current torque-time based control structure. 
         [0042]    As various changes could be made in the above constructions without departing from the scope of the disclosure, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.