Abstract:
An electric power steering system for a vehicle includes a steering wheel, a steering shaft connected to said steering wheel, and a switched reluctance motor coupled to said steering shaft. The switched reluctance motor includes a segmented stator having a plurality of stator segment assemblies that define salient stator poles and inter-pole stator slots. Each of the stator segment assemblies includes a stack of stator plates defining a stator segment core, an end cap assembly, and winding wire that is wound around the stator segment core and the end cap assembly. The rotor tends to rotate relative to the stator to a rotational position that maximizes the inductance of an energized winding. A drive circuit energizes the winding wire around the stator segment assemblies based on the rotational position of the rotor.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to electric power steering systems and, more particularly to electric power steering systems that include a switched reluctance electric motor with a segmented stator.  
         BACKGROUND OF THE INVENTION  
         [0002]    Electric power steering (EPS) systems for vehicles such as automobiles and trucks typically include a steering wheel, a motor, a controller, one or more sensors, a steering shaft, and a steering gear assembly such as a rack and pinion gear assembly or a recirculating ball steering gear assembly. The motor is coupled to the steering shaft through a worm that is connected to the motor and a worm gear that is connected to the steering shaft. The sensors typically include a torque sensor that provides a feedback signal to the controller that represents driver effort that is required to turn the steering wheel. As the driver effort increases, the motor turns the worm which, in turn, turns the worm gear that is connected to the steering shaft. The motor reduces driver effort that is required to turn the steering wheel. Other sensed parameters typically include a rotational sensor that senses shaft rotational position and that provides a feedback signal to the controller. Vehicle velocity is also typically input to the controller so that the assist provided by the EPS system varies as a function of vehicle speed.  
           [0003]    EPS systems offer improvements over conventional hydraulic assist systems by reducing overall vehicle weight and improving fuel economy. In addition, EPS systems allow for precise electronic control of the steering system. In addition to variable effort assist, the EPS system can also provide steering wheel return characteristics that may be tuned to a desired feel and/or responsiveness. The amount of tactile feedback to the driver through the steering wheel may also be electronically controlled. Specifically, the steering torque provides information to the driver regarding road conditions and vehicle maneuverability. The amount of restoring torque is a function of the chassis design and the transmissibility of rack loads back to the steering wheel. The EPS system provides active control of the transmissibility characteristics and therefore the amount of tactile feedback to the driver.  
           [0004]    Switched reluctance motors have not typically been used in EPS systems for several reasons. Reluctance motors typically include a stator that is mounted inside a motor housing and a rotor that is supported for rotation relative to the stator. Reluctance motors produce torque as a result of the rotor tending to rotate to a position that maximizes the inductance of an energized winding of the stator. As the energized winding is electrically rotated, the rotor also rotates in an attempt to maximize the inductance of the rotating energized winding of the stator. In synchronous reluctance electric motors, the windings are energized at a controlled frequency. In switched reluctance electric motors, control circuitry and/or transducers are provided for detecting the angular position of the rotor. A drive circuit energizes the stator windings as a function of the sensed rotor position.  
           [0005]    The design and operation of switched reluctance electric motors is known in the art and is discussed in Stephenson and Blake, “The Characteristics, Design and Applications of Switched Reluctance Motors and Drives”, presented at the PCIM &#39;93 Conference and Exhibition at Nuremberg, Germany, Jun. 21-24, 1993, which is hereby incorporated by reference.  
           [0006]    In switched reluctance electric motors, a rotor position transducer (“RPT”) is often used to detect the angular position of the rotor with respect to the stator. The RPT provides an angular position signal to the drive circuit that energizes the windings of the switched reluctance electric motor. The RPT typically includes a sensor board with one or more sensors and a shutter that is coupled to and rotates with the shaft of the rotor. The shutter includes a plurality of shutter teeth that pass through optical sensors as the rotor rotates.  
           [0007]    Because rotor position information is critical to proper operation of a switched reluctance electric motor, sophisticated alignment techniques are used to ensure that the sensor board of the RPT is properly positioned with respect to the housing and the stator. Misalignment of the sensor board is known to degrade the performance of the electric motor. Unfortunately, utilization of these complex alignment techniques increases the manufacturing costs for switched reluctance electric motors equipped with RPTs.  
           [0008]    The RPTs also increase the overall size of the switched reluctance electric motor, which can adversely impact motor and product packaging requirements. The costs of the RPTs and their related manufacturing costs often place switched reluctance electric motors at a competitive disadvantage in EPS system applications that are suitable for less costly induction electric motors.  
           [0009]    Another drawback with RPTs involves field servicing of the switched reluctance electric motors. Specifically, wear elements, such as the bearings, located within the enclosed rotor housing may need to be repaired or replaced. To reach the wear elements, an end shield must be removed from the housing. Because alignment of the sensor board is critical, replacement of the end shield often requires the use of complex realignment techniques. When the alignment techniques are improperly performed by the service technician, the sensor board is misaligned and the motor&#39;s performance is adversely impacted.  
           [0010]    In an effort to eliminate the RPTs and to reduce manufacturing costs and misalignment problems, “sensorless” techniques for sensing rotor position have been developed. Sensorless techniques detect the magnitude of the back-electromotive force (EMF) of an unenergized winding of the stator in the switched reluctance electric motor. The windings are commutated when the sensed EMF magnitude reaches a predetermined level. Several patents disclosing sensorless techniques for sensing rotor position in switched reluctance electric motors include U.S. Pat. No. 5,929,590 to Tang and U.S. Pat. No. 5,877,568 to Maes, et al. which are hereby incorporated by reference. Application of the sensorless techniques is limited by the relatively low back-EMFs induced in the unenergized stator windings that are associated with switched reluctance electric motors. Additional problems with the sensorless techniques are attributable to variations in the inductance and resistance of the stator windings due to assembly and tolerance variations.  
           [0011]    Conventional switched reluctance motors generally include a plurality of stator plates that are punched from a magnetically conducting material. The stator plates have a circular cross-section and are stacked together to form the stator. The stator plates define salient stator poles that project radially inward and inter-pole slots that are located between adjacent stator poles. Winding wire is wound around the stator poles. There are several methods for placing the winding wire on the stator of a switched reluctance motor. The winding wire can be initially wound and transferred onto the stator poles. Transfer winding tends to leave excess winding wire or loops around axial ends of the stator poles. Transfer winding can typically utilize approximately 60-65% of available stator slot area. Needle winding employs a needle that winds the wire directly on the stator poles. The needle, however, takes up some of the stator slot area, which reduces slot fill to approximately 50%. The positioning of winding wire on the stator poles using these methods varies from one stator pole to the next. Winding creep and other assembly variations also impact the inductance and resistance of the winding wire over time, which makes it difficult to accurately perform “sensorless” control due to the non-conformity of the salient stator poles.  
           [0012]    The design of EPS systems can be improved in several areas. Specifically, it is desirable to improve the torque density of switched reluctance electric motors that are employed by the EPS systems. By increasing the torque density, the size of the EPS systems can be reduced for a given torque output and/or the size can be maintained with an increase in torque output. Alternately, increased slot fill reduces the required battery current for a given torque and speed. EPS systems achieving higher torque density will allow designers of products equipped with EPS systems greater flexibility in product design that may lead to increased sales through product differentiation, improved performance, reduced weight, and/or improved profit margins.  
           [0013]    It would be desirable to eliminate the need for RPTs in switched reluctance electric motors that are employed by the EPS systems. It would also be desirable to assemble the stator of a switched reluctance electric motor used in the electric power steering assist systems in a highly uniform and repeatable manner to improve the performance of sensorless switched reluctance motors by reducing variations in the inductance and resistance of the stator.  
         SUMMARY OF THE INVENTION  
         [0014]    An electric power steering system for a vehicle includes a steering wheel, a steering shaft connected to said steering wheel, and a switched reluctance motor coupled to said steering shaft that reduces driver effort to turn said steering wheel. The switched reluctance motor includes a segmented stator having a plurality of stator segment assemblies. The stator segment assemblies define salient stator poles and inter-pole stator slots. Each of the stator segment assemblies includes a stack of stator plates defining a stator segment core, an end cap assembly supporting the stator segment core, and winding wire which is wound around the stator segment core and the end cap assembly. The rotor defines a plurality of rotor poles. The rotor tends to rotate relative to the stator to maximize the inductance of an energized winding. A drive circuit energizes the winding wire around the stator segment assemblies based on a rotational position of the rotor.  
           [0015]    According to other features of the invention, a worm gear is connected to the steering shaft, a worm is threadably engaged to the worm gear, and the switched reluctance motor is connected to said worm.  
           [0016]    According to other features of the invention, each stator plate has an outer rim section and a tooth-shaped pole section. A tongue and groove connection is provided for connecting the outer rim section of the stator segment cores that are associated with adjacent stator segment assemblies to define the segmented stator.  
           [0017]    As a further feature of the invention, the end cap assembly includes a pair of end caps that are secured to opposite ends of the stator segment core, and a pair of retainer plates interconnecting the end caps on opposite sides of the stator segment core. The end cap assembly defines an annular retention channel within which the winding wire is wound. The retention channel facilitates improved precision in the winding process and tends to reduce winding creep during use.  
           [0018]    The electric power steering system according to the present invention includes a switched reluctance electric machine with improved torque density. As a result, the torque output of the switched reluctance electric machine can be increased for increased steering assist and/or the dimensions of the switched reluctance electric machine can be reduced for a given torque output to reduce weight and outer dimensions of the electric power steering system. In addition, the stator segment assemblies can be manufactured with greater uniformity and with lower variations in inductance and resistance. Sensorless rotor position sensing techniques can be employed to dramatically lower the manufacturing costs of the switched reluctance machine in the electronic power system (when compared to sensed rotor position techniques) and to improve the reliability of the electric power steering system in the field.  
           [0019]    Other objects, features and advantages will be apparent from the specification, the claims and the drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1A illustrates a vehicle with an electric power steering (EPS) system;  
         [0021]    [0021]FIG. 1B is a functional block diagram and perspective view of the EPS system;  
         [0022]    [0022]FIG. 2 illustrates a segmented stator and a rotor for a switched reluctance electric motor;  
         [0023]    [0023]FIG. 3A illustrates a stator plate;  
         [0024]    [0024]FIG. 3B identifies tooth width, projection width and stator pole arc on the stator plate of FIG. 3A;  
         [0025]    [0025]FIG. 4 is a perspective view of a stator segment assembly associated with the stator;  
         [0026]    [0026]FIG. 5 illustrates a switched reluctance drive circuit and a circuit board for connecting the drive circuit to terminals of the stator segment assemblies;  
         [0027]    [0027]FIG. 6A shows the stator segment assembly with its wire windings and insulation removed to better illustrate a stack of stator plates and the end cap assembly;  
         [0028]    [0028]FIG. 6B is a plan view of the end cap assembly shown in FIG. 6A;  
         [0029]    [0029]FIG. 6C is an end view of the end cap assembly shown in FIG. 6B;  
         [0030]    [0030]FIG. 7A is similar to FIG. 6A except that an alternate end cap assembly is shown;  
         [0031]    [0031]FIG. 7B shows a plan view of the alternate end cap assembly of FIG. 7A; and  
         [0032]    [0032]FIG. 7C illustrates an end view of the alternate end cap assembly shown in FIG. 7B. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]    The following detailed description provides preferred exemplary embodiments only and is not intended to limit the scope, applicability or configuration of the present invention. Rather, the detailed description of the preferred exemplary embodiments will provide those skilled in the art with an enabling description for implementing the preferred exemplary embodiments of the present invention. It will be understood that various changes may be made in the function and arrangement of the elements without departing from the spirit and scope of the invention as set forth in the appended claims.  
         [0034]    An electric power steering (EPS) system according to the present invention includes a novel switched reluctance motor with a segmented stator. The EPS system with the segmented stator switched reluctance motor can be packaged in a smaller size for a given torque output and/or packaged at the same size with increased torque output. The novel segmented stator switched reluctance machine can be implemented using sensorless rotor position techniques while remaining cost competitive with other types of motors used in EPS systems.  
         [0035]    Referring now to FIGS. 1A and 1B, a vehicle  10  includes an electric power system steering (EPS) system  12 . The EPS system  12  includes a steering wheel  16  that is connected to an upper steering shaft  20 . A torque sensor  24  senses steering effort that is required to turn the steering wheel  16 . A worm gear  26  is connected to the upper steering shaft  20  and is threadably engaged by a worm  28 . The worm  28  is connected to a switched reluctance motor  32  that includes a segmented stator as will be described further below.  
         [0036]    A lower steering shaft  34  is connected to a steering gear assembly  36 . A universal joint (not shown) may be used between the lower steering shaft  34  and the steering gear assembly  36  if needed. In a preferred embodiment, the steering gear assembly  36  is a rack and pinion gear assembly  38 . However, skilled artisans will appreciate that the steering gear assembly  36  can be a recirculating ball gear assembly or any other suitable steering gear assembly. The rack and pinion gear assembly  38  includes a steering gear rack housing  40  and a pinion gear housing  42 . The steering gear rack housing  40  encloses a steering rack (not shown) that is connected to an inner tie rod (not shown). The pinion gear housing  42  encloses a pinion gear (not shown) that is connected at one end to the lower steering shaft  34  and whose teeth mesh with those on the steering rack. A dust boot  44  provides a flexible enclosure for the steering rack and the inner tie rods as they move laterally when the driver steers the vehicle  10 . An outer tie rod  46  is connected at one end to the inner tie rod. An opposite end of the outer tie rod  46  is connected to a steering knuckle  50 , which is connected to a rim  52  with a tire  54  mounted thereon.  
         [0037]    An EPS system controller  54  is connected to a drive circuit  56  that controls the switched reluctance motor  32 . The torque sensor  24  is connected to the EPS system controller  54 . A vehicle speed sensor  58  preferably provides a speed signal to the EPS system controller  54 . A rotational sensor  60  generates an angular position signal that is related to the angular position of the steering wheel  16 , the upper steering shaft  20 , and/or the wheel  52 .  
         [0038]    In use, the operator of the vehicle  10  turns the steering wheel  16  to turn the wheels  52  of the vehicle  10 . The torque sensor  24  senses the amount of effort that is required to turn the steering wheel  16 . The rotational sensor  60  senses the rotational position of the steering wheel  16 , the upper steering shaft  20  and/or the wheel  52 . The EPS system controller  54  factors the sensed torque, the speed of the vehicle  10 , and/or the angular orientation of the steering wheel  16 , the upper steering shaft  20  and/or the position of the wheels  52 . The EPS system controller  54  sends a control signal to the drive circuit  56  that generates a set of currents that create a magnetic field.  
         [0039]    Referring now to the drawings, the switched reluctance motor  32  is shown to include a housing  112 , a segmented stator  114  mounted in the housing  112 , and a rotor  116  supported for rotation relative to the segmented stator  114 . In accordance with the present invention, the segmented stator  114  includes a plurality of stator segment assemblies  118  that can be individually assembled and subsequently interlocked to define the segmented stator  114 . As will be detailed, each stator segment assembly  118  includes a stator segment core  120 , an end cap assembly  122 , and winding wire  124  that is wound around the stator segment core  120  and the end cap assembly  122 . The end cap assembly  122  insulates the ends of the stator segment core  120  and provides retention for additional turns of the winding wire.  
         [0040]    Referring primarily to FIGS. 2, 3A and  3 B, the stator segment core  120  is comprised of a stack of individual stator plates  126 . Each of the stator plates  126  include an arcuate outer rim section  128  and a tooth-shaped pole section  130 . An outer edge surface  132  of the outer rim section  128  is shaped for mounting to an inner wall surface  134  of the housing  112 . Each outer rim section  128  has a tongue projection  136  formed on one edge surface  138  and a groove  140  on its opposite edge surface  142 . The tongues  136  and grooves  140  may be omitted since the stator segment assemblies  118  are typically press fit in the housing  112 . Each pole section  130  of the stator plates  126  has an arcuate inner edge surface  144  and a pair of circumferentially-extending projections  146 .  
         [0041]    As previously mentioned, the stator segment core  120  is defined by a stack of the stator plates  126 . The stator plates  126  are die cut from thin sheets of magnetically conductive material. During the die cutting operation, a first pair of slits  150  are cut into the outer rim section  128  and a second pair of slits  152  are cut into the pole section  130 . Central portions between the slits  150  and  152  are deformed during the die cut operation. The slits  150  are transverse in alignment relative to the slits  152 . The stator plates  126  are subsequently stacked and press fit together. This operation results in the stator plates  126  being releasably interconnected to define the stator segment core  120 .  
         [0042]    The rotor  116  is shown to include a circular rim section  154  and a plurality of tooth-shaped pole sections  156  that project radially from the rim section  154 . A circular bore  158  is formed in the rotor  116  and may include keyways  160 . A rotor shaft (not shown) is received by the circular bore  158  of the rotor  116 . In the particular embodiment shown, the rotor  116  has eight equally-spaced rotor pole sections  156  and the segmented stator  114  has twelve equally-spaced pole sections  130 . Other rotor pole and stator pole combinations are also contemplated. In addition, each rotor pole section  156  has an arcuate outer edge surface  162  that defines an air gap  163  with respect to the arcuate inner edge surface  144  on the pole sections  130  of the stator segment core  120 .  
         [0043]    Referring to FIG. 3B, tooth width W 1 , projection width W 2 , and stator pole arc Bs are shown. As a result of segmenting the stator, the designer of the switched reluctance electric machine has greater flexibility in designing the dimensions of the stator segment assemblies. The slot opening dimension between radially inner ends of the stator teeth restricts the projection width W 2  when needle and transfer winding methods are employed. This restriction is eliminated when the segmented stator assemblies are employed because the stator teeth can be wound individually before being assembled into the stator.  
         [0044]    The tooth width W 1  determines the magnetic flux density in the stator tooth and how much area is available for winding wire in the inter-polar stator slot. The designer of the switched reluctance electric machine can select the tooth width W 1  so that it is sufficient to accommodate the maximum anticipated magnetic flux in the stator poles, but is not wider than necessary. By optimizing the tooth width W 1 , the slot area is increased, which allows additional winding wire. By increasing the current carrying capacity of the windings without causing overheating, the torque density of the switched reluctance electric machine can be improved. The design of the stator plates also depends on other factors such as the type of steel that is selected, the axial length of the stator stack, the operating speed, the overall size of the motor, and the desired magnetic flux density in the stator teeth.  
         [0045]    Referring to FIG. 4, the stator segment assembly  118  is shown fully assembled to include the stator segment core  120 , the end cap assembly  122  and the winding wire  124 . The end cap assembly  122  is preferably made from magnetically permeable material and includes a first end cap  164 A, a second end cap  164 B and a pair of elongated winding retainer sections  166 A and  166 B. The first end cap  164 A is located at one end of the stator segment core  120  and the second end cap  164 B is located at the opposite end of the stator segment core  120 . The winding retainer sections  166 A and  166 B interconnect the first and second end caps  164 A and  164 B and are located adjacent to the projections  146  near the radially inner end of the pole sections  130  of the stator segment core  120 . Preferably, the end caps  164 A and  164 B are similar in configuration. Likewise, it is preferable that the retainer sections  166 A and  166 B are similar in configuration. Snap-in connections are contemplated for connecting the opposite ends of each retainer section  166  to the end caps  164 A and  164 B. Additionally, it is contemplated that adhesives are used for bonding the end caps  164 A and  164 B to the opposite ends of the stator segment core  120 . The end caps  164 A and  164 B and the retainer sections  166  can also be molded as an integral end cap assembly  122 . Since the first end cap  164 A is similar to the second end cap  164 B, the following description of the components will use reference numerals with an “A” suffix for the first end cap  164 A and the reference numerals for similar components of the second end cap  164 B will be identical with a “B” suffix.  
         [0046]    Terminals  170  and  172  are shown in FIGS. 4 and 6A to be mounted in slots  174  and  176  (FIG. 6C) formed in an end surface  178 A of the first end cap  164 A. One end of the winding wire  124  is connected to the first terminal  170  while an opposite end of the winding wire  124  is connected to the second terminal  172 . Insulating material  177  is shown to be positioned to cover the winding wire  124  on both lateral sides of stator segment core  120 . The insulating material  177  is also positioned (but not shown) between the stator segment core  120  and the winding wire  124 .  
         [0047]    Referring to FIG. 5, a switched reluctance drive circuit  180  is shown connected via connecting wires  182 ,  184  and  186  to a printed circuit board  188 . The printed circuit board  188  is circular and has a plurality of radially outwardly projecting terminal pads  190 . Each terminal pad  190  has conductive terminal slots  192  and  194  arranged to accept installation of the terminals  170  and  172  for each stator segment assembly  118 . The drive circuit  180  operates to control energization of the winding wire  124  of the stator segment assemblies  118 . In a preferred embodiment, the switched reluctance drive circuit  180  senses rotor position using sensorless techniques that are disclosed in U.S. Pat. Nos. 5,929,590 to Tang and 5,877,568 to Maes, et al., which are hereby incorporated by reference.  
         [0048]    To more clearly illustrate the structure of the end cap assembly  122 , FIG. 6A shows the stator segment assembly  118  prior to the insulating material  177  being installed and the winding wire  124  being wound thereon. The first end cap  164 A includes an outer section  198 A and an inner section  200 A interconnected by a hub section  202 A, all defining a common face surface  204 A. The face surface  204 A abuts and is bonded to an end surface  206  of the stator segment core  120 . Similarly, the face surface  204 B of second end cap  164 B abuts and is bonded to an end surface  208  of the stator segment core  120 . When the first end cap  164 A is secured to the stator segment core  120 , its outer section  198 A extends slightly radially inward with respect to the outer edge surface of the outer rim section  128  and is parallel to the outer edge surface  132 . The hub section  202 A is aligned with pole section  130  and the inner section  200 A is aligned with and extends laterally beyond the inner edge surface  144  and the projections  146 . A similar alignment is provided when the second end cap  164 B is secured to the opposite end surface  208  of the stator segment core  120 . Moreover, the width of the hub sections  202 A and  202 B is less than or equal to the width of the pole sections  130  of the stator segment core  120 . The opposite ends of the retainer sections  166 A and  166 B are connected to the face surfaces  204 A and  204 B of the end caps  164 A and  164 B, respectively, adjacent to their inner sections  200 A and  200 B. As such, the end cap assembly  122  defines a continuous annular channel within which the winding wire  124  can be precisely installed and maintained.  
         [0049]    [0049]FIG. 6B shows the inner section  200 A of the first end cap  164 A and the inner section  200 B of the second end cap  164 B to be rectangular in shape. It is contemplated, however, that other configurations (i.e. semi-circular, square, tapered, etc.) could be used. As a further option, the retainer sections  166  could be provided as a cantilevered section that is integrally formed with the end caps  164 A and/or  164 B and adapted for connection to the inner section of the opposite end cap. To reduce the weight of the end cap assembly  122  and to simplify molding, lateral axial grooves  210  and a central axial groove  212  can be formed on the outer section of the end caps  164 A and  164 B. Likewise, a cavity  214  can also be formed to provide additional weight reduction and for simplifying the molding process.  
         [0050]    Referring now to FIGS. 7A, 7B and  7 C, an alternative cap assembly  222  is shown for connection to the stator segment core  120  and supporting the winding wire  124 . Reference numerals from FIGS. 6A, 6B and  6 C will be used where appropriate to identify similar elements. Specifically, the first end cap  224 A is generally similar to the first end cap  164 A. The alternative end cap assembly  222  includes an additional pair of retainer sections. An outer retainer section  226 A extends axially from the common face surface  204 A adjacent to the outer section  198 A for connection to the outer section  198 B of the second end cap  224 B. An outer retainer section  226 B likewise extends axially from its common face surface  204 B for connection to common face surface  204 A of first end cap  224 A. The outer retainer sections  226 A and  226 B provide additional support for the end cap assembly  122 . The outer retainer sections  226 A and  226 B fill the arcuate inner edge  230  of the outer rim section  128 . As a result, a substantially right angle projection to pole section  130  is formed. The outer retainer sections allow more precise control of the winding coil when performing precise winding and minimizes damage that may be caused by sharp edges defined by inner edge  230  and the edge surfaces  138  and  142 . The outer retainer sections  226 A and  226 B have a tapered profile to mate with the profile of inner arcuate wall surfaces  230  (FIG. 2) of the outer rim section  128 .  
         [0051]    A significant benefit of the segmented switched reluctance motor of the present invention is the ability to maximize the inductance of the switched reluctance motor. In a conventional switched reluctance motor, the spacing between adjacent stator poles or teeth is determined by the wire size and the clearance that is required by the winding method. As the stator teeth spacing decreases, the inductance generally increases. In conventional switched reluctance motors, the spacing between the adjacent stator teeth is generally limited to the greater of the wire diameter or the clearance that is required by the winding method.  
         [0052]    The switched reluctance motor according to the present invention, however, is not limited by either constraint. The spacing between adjacent stator teeth can be smaller than the wire diameter and is not limited by the winding method because the stator segment assemblies are wound before the stator is assembled. As a result, the inductance of the motor can be increased, which increases the impedance. Increasing the impedance decreases the drag load of the switched reluctance motor when an internal short circuit occurs during operation.  
         [0053]    As can be appreciated from the foregoing, the EPS system with the segmented stator, switched reluctance electric motor improves the torque density of the electric motor by allowing the stator segment assemblies to be precisely wound. Slot fill between 70-95% is achievable depending upon the diameter of the motor and the diameter of the winding wire. As a result, the torque output for the electric motor can be increased. Alternately, the outer dimensions of the electric motor can be reduced for a given torque output.  
         [0054]    The stator segment assemblies of the switched reluctance electric motor in the EPS system can be produced with a greater uniformity and with lower variations in inductance and resistance. As a result, sensorless rotor position sensing techniques can be employed, which dramatically lowers the manufacturing costs of the switched reluctance motor and improves reliability in the field. Because the manufacturing tolerances of the stator segments have been improved, less costly drive circuits can be employed and/or more accurate control can be achieved. In addition, the end cap assemblies according to the invention prevent winding creep and further help improve uniformity of the stator segment assemblies during use.  
         [0055]    Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.