Abstract:
An actuator assembly including, a polar assembly including an inner tooth and an outer tooth, and a magnet disposed between the inner tooth and the outer tooth, the magnet operative to interact with the polar assembly to induce an electromagnetic torsion bar operative to impart a torque on a first shaft connected to the ring magnet.

Description:
BACKGROUND 
       [0001]    Power steering systems in vehicles use actuators to provide assist and sometime include capabilities such as variable effort steering and torque overlay to provide a desired response in the systems. 
         [0002]    Many actuators use a torsion bar disposed within a valve to control the valve or other input measuring device as a function of torque, and to a provide tactile feedback to the driver at the hand wheel. 
       SUMMARY 
       [0003]    The above described and other features are exemplified by the following Figures and Description in which actuators are disclosed that include an actuator assembly including, a polar assembly including an inner tooth and an outer tooth, and a magnet disposed between the inner tooth and the outer tooth, the magnet operative to interact with the polar assembly to induce an electromagnetic torsion bar operative to impart a torque on a first shaft connected to the magnet. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    Referring now to the Figures wherein like elements are numbered alike: 
           [0005]      FIG. 1  illustrates an exemplary embodiment of the magnetic torsion bar of an actuator assembly. 
           [0006]      FIGS. 2A-2D  illustrate exemplary graphs of behavior of embodiments of magnetic torsion bars. 
           [0007]      FIG. 3  illustrates an alternate exemplary embodiment of a magnetic torsion bar of an actuator assembly. 
           [0008]      FIG. 4  illustrates another alternate exemplary embodiment of a magnetic torsion bar of an actuator assembly. 
           [0009]      FIG. 5  illustrates another alternate exemplary embodiment of a magnetic torsion bar of an actuator assembly. 
           [0010]      FIG. 6  illustrates another exemplary embodiment of a magnetic torsion bar of an actuator assembly. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    Power steering systems may incorporate a torsion bar in hydraulic and electric actuators. The torsion bar typically provides a means to measure driver torque by sensing the deflection of the torsion bar. In addition, the torsion bar also provides the necessary torque coupling between the driver the rest of the steering system, thus providing a desired tactile “feel” to the user. Hydraulic and electric actuators may be magnetically actuated. For example, a magnetically actuated actuator includes a ring magnet and poles that rotate when a magnetic flux is induced. 
         [0012]      FIG. 1  illustrates the magnetic torsion bar assembly  200  of a typical electric or hydraulic steering system. The magnetic torsion bar assembly  200  includes a steering shaft  214  connected to a handwheel (not shown) and a ring magnet  209 . The ring magnet  209  includes magnetic poles  208  and  210 . Inner teeth  204  and outer teeth  206  are connected to an output shaft  212 . 
         [0013]    The magnetic portion  200  includes 15 pairs of teeth, however other embodiments may include other teeth arrangements. A pair of teeth includes an inner tooth  204  and an outer tooth  206 . The magnet  209  may be a single piece ring magnet with 15 pairs of poles. Alternatively, the magnetic pole  208  and  210  pairs may be fabricated using multiple magnets attached in a desirable pattern. Each pair of poles includes a north pole and a south pole. 
         [0014]    In operation, a driver input from a handwheel connected to the steering shaft  214  rotates the steering shaft  214 . The steering shaft  214  rotates the ring magnet  209 . Flux forces in the ring magnet  209  impart a torque on the inner teeth  204  and the outer teeth  206 . The torque rotates the inner teeth  204  and the outer teeth  206  and rotates the connected output shaft  212 . 
         [0015]    The assembly  200  include a mechanical torsion bar  218  disposed in a center cavity of the steering shaft  214 . The torsion bar is connected to the output shaft  212  and the steering shaft  214 . The torsion bar imparts a torque on the handwheel that provides a tactile response to the driver. In addition, the torsion bar ensures that the valve opening for a hydraulic power steering system is controlled as a function of the driver torque. In an electric power steering system, the deflection of the torsion bar is usually sensed with an electrical device. One disadvantage of an actuator assembly that uses a torsion bar is that the fabrication of the actuator assembly may be costly. For example, the center of the valve body is machined to align with the center of the torsion bar and a center of the magnetic actuator portion. The actuator assembly is machined or assembled to high tolerances that result in costly production procedures and waste. In certain applications, it may be desirable to have a non-linear or variable stiffness characteristic for improved steering feel and performance. The non-linear or variable stiffness characteristics are difficult to implement using a mechanical torsion bar. 
         [0016]    The assembly  200  and the additional embodiments described below may be more easily fabricated without a torsion bar  218 . If the torsion bar  218  is removed, a new method for providing the function of a torsion bar  218  is desired. The assembly  200  allows the torque response of a torsion bar  218  to be imparted by the magnetic actuator portion. Alternatively, the assembly  200  may include the torsion bar  218  to supplement its behavior with non-linear or variable stiffness characteristics. 
         [0017]    The magnetic interaction between the ring magnet  209  and the inner teeth  204  and outer teeth  206  is designed to impart a torque response similar to the mechanical torsion bar of previous systems. The torque used to replicate a mechanical torsion bar is called an electromagnetic torsion bar. 
         [0018]    By using the electromagnetic torsion bar to impart a similar torque as the mechanical torsion bar, the mechanical torsion bar may be removed from the actuator. This simplifies the fabrication of the actuator. 
         [0019]      FIG. 2A  illustrates a comparison of the torque induced by a mechanical torsion bar (Tbar) and the torque induced by an electromagnetic torsion bar. The EM torque curves represent the torque induced in rotating the magnetic actuator portion, and the Tbar torque curve represents the torque induced by a mechanical torsion bar. 
         [0020]    Typically, the range of motion of the torsion bar is limited to a maximum range of about +/−4 degrees via the use of travel stops. Thus, the use of electromagnetic torque to approximate the torque induced by a mechanical torsion bar over this range effectively allows a mechanical torsion bar to be removed or supplemented using this device. 
         [0021]      FIG. 3  illustrates an exemplary embodiment of magnetic actuator portion  400  having an electromagnetic torsion bar. The magnetic actuator portion  400  is similar in structure and operation to the magnetic actuator portion  200  described above. 
         [0022]    Determining the torque applied to upper input shaft  214  described above is desirable. The use of a separate torque sensing unit is undesirable because it adds a component to the system increasing cost and the use of space. 
         [0023]      FIG. 4  includes a magnetic sensor  407 , an outer yoke portion  402 , an inner yoke portion  411 , and a plate portion  403 . The plate portion  403  and the inner yoke portion  411  are connected to the shaft  412 . The outer yoke portion  402  and the plate portion  411  define an airgap  405 . The airgap  405  is a cavity that contains the magnetic sensor  407  that includes a magneto-sensitive element, such as, for example a Hall Effect sensor. The airgap  405  may contain a number of magneto-sensitive elements. 
         [0024]    In operation, the sensor  407  detects a relative angular displacement between the ring magnet  209  and the teeth by sensing a change in flux as the ring magnet  209  rotates. The relative angular displacement may be converted to a torque value. The use of a magneto-sensitive element allows the torque to be determined without using a separate sensing unit. An advantage of the embodiment of  FIG. 4  is that the packaging size of the actuator is reduced. 
         [0025]      FIG. 5  illustrates an embodiment of a magnetic actuator portion  500 . The magnetic actuator portion  500  is similar to the magnetic actuator portion  400  described above. The magnetic actuator portion  500  includes an outer housing portion  502 . The outer housing portion  502  remains stationary relative to the rotation of the magnetic and shaft portions of the magnetic actuator portion  500 . The outer housing portion  502  includes an upper outer housing portion  511  and a lower outer housing portion  503  that partially define an airgap  505 . The airgap provides a cavity for the position sensor  407  that operates as described above. The relative angular displacement may be converted to a torque value. 
         [0026]    In some steering systems a change in stiffness felt at the handwheel by a user is desirable as the user rotates a handwheel through a range of motion. For example, a system may be designed to have a non-linear stiffness characteristic with a stiff tactile feel when the handwheel is centered during high-speed driving, and a more compliant tactile feel when steering at higher efforts off-center, typically encountered during parking and low speed maneuvers. This is accomplished by designing the shape of the curve in  FIG. 2A  to a desired target stiffness. Alternatively, it may be desirable to have the stiffness variable with driving conditions such as vehicle speed. Variable stiffness as a function of driving conditions is complex to achieve using a mechanical torsion bar. However, the variable stiffness function may be incorporated into the magnetic actuator portion using a coil. 
         [0027]      FIG. 6  illustrates an exemplary embodiment of a magnetic actuator portion  600  that includes a coil  614 . The magnetic actuator portion  600  is similar to the magnetic actuator portion  200  (of  FIG. 1 ). The coil  614  induces a secondary field in the magnetic actuator portion  600 . 
         [0028]      FIG. 2C  illustrates a behavior of the interaction force between the pole piece and the ring magnet (illustrated above). When the coil is energized, it imparts an additional electromagnetic force between the pole piece and the ring magnet with the resulting response shown in  FIG. 2C  as a function of current. If operated with a mechanical center around the 6 degree angle the variable stiffness behavior of the system is realized between travel stops as shown in  FIG. 2D . If operated with a mechanical center around the 0 degree angle the torque overlay behavior of the system is realized between travel stops as shown in  FIG. 2B . 
         [0029]    In some embodiments, it may be desirable to apply a torque on the torsion bar, for example to automate steering for parking assist in hydraulic systems. In this case, the magnetic torque created by the coil may be designed to produce a different effect by choosing a different operating point on the magnetic torque curve as shown in  FIG. 2C . Here the mechanical center is placed to be the same as the magnetic center of the plot in  FIG. 2C  and thus realizes a variable torque overlay device between typical travel stops as shown in  FIG. 2B . 
         [0030]    The use of an electromagnetic torsion bar simplifies the application of torque overlay. The current command used to operate the actuator changes the stiffness of the actuator as felt by a user as the actuator rotates. The change in stiffness (variable stiffness) may be controlled by manipulating the cogging torque thereby varying the torque of the electromagnetic torsion bar. 
         [0031]      FIG. 6  illustrates an embodiment of a magnetic actuator portion  700  that includes the coil  614  (of  FIG. 5 ) and the outer housing  502  (of  FIG. 4 ). The magnetic actuator and torque sensing portion  700  is similar to the magnetic torsion bar and torque sensing portion  500  described above and uses the coil  614 . 
         [0032]    The technical effects and benefits of the system and methods described above allow cogging torque to effect a desired torque response of an actuator, a sensing of the torque, and the use of a torque overlay function and variable stiffness function. 
         [0033]    While the invention has been described with reference to exemplary embodiments, it will be understood by those of ordinary skill in the pertinent art that various changes may be made and equivalents may be substituted for the elements thereof without departing from the scope of the present disclosure. In addition, numerous modifications may be made to adapt the teachings of the disclosure to a particular object or situation without departing from the essential scope thereof. Therefore, it is intended that the Claims not be limited to the particular embodiments disclosed as the currently preferred best modes contemplated for carrying out the teachings herein, but that the Claims shall cover all embodiments falling within the true scope and spirit of the disclosure.