Abstract:
The present invention provides an external rotor stepper motor consisting of four individual, independently controlled, co-planar stepper motors optimized for vehicle direct wheel drive applications. The innovative design eliminates the need for bifilar coil winding, each of the motors&#39; coils being single conductor, each coil being wound in the same direction throughout the entire motor. Each of the four independent motors is simultaneously energized. Because adjacent, outside coils of adjacent motors are simultaneously energized at the same polarity, there is substantially no energy wasting cancellation of magnetic flux. Carefully chosen radially spacing of the four independent, co-planar motors also eliminates substantially all flux interaction between the motors. Because similar coils of the four motors are all connected in parallel, the net inductance presented driver circuits is minimized. This yields fast rise time pulses and improved efficiency through minimized reactance of the motor.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of stepper motors and, more particularly, to a multi-phase stepper motor having an outer rotor adapted for directly driving a vehicle wheel. 
     BACKGROUND OF THE INVENTION 
     In response to environmental and geopolitical problems, it has become of some urgency to make available motor vehicles consuming relatively less fossil fuel than current fossil-fuel vehicles. While the ultimate target remains “zero-emissions” vehicles that utilize virtually no fossil fuel, realization of this goal appears far-reaching because of lack of progress in development of batteries capable of propelling a vehicle for times, distances, and speeds the driving public has come to expect. 
     It is now believed that in the interim, hybrid vehicles utilizing a combination of fossil-fueled engines and electric drive means may well be the answer, falling short of “zero-emissions,” but nevertheless greatly increasing the distance one can travel on a gallon of fossil fuel. One such concept includes providing an electric motor for each wheel, the motors being driven by a generator, which, in turn, is driven by a fossil-fuel engine. In such an instance, the engine can be run solely for the benefit of the generator and therefore, can be small and run at a constant, most efficient speed, greatly reducing fossil fuel requirements. To reduce the fossil fuel requirements further, it is necessary that the wheel motors be as efficient as possible. 
     Two types of rotary electrical motors have been heretofore considered—axial permanent magnet motors and radial permanent magnet motors. Numerous examples of both types appear in the prior art. Typically, axial permanent magnet motors feature a stator disk, or drum, with a central opening and electrical conductor windings wound through the central opening and across the outer peripheral edge of the stator disk. The stator disk typically is fixed in place. A rotor is usually mounted on a shaft near the stator disk. The rotor is provided with permanent magnets extending radially from its center. 
     In operation, a polyphase alternating electrical current passed through the windings of the stator disk creates a magnetic flux, to which the permanent magnets of the rotor respond, thereby turning the rotor, the shaft to which the rotor is fixed, and the vehicle wheel. The portion of the windings overlying the outer peripheral edge of the stator disk does not accomplish useful work. The magnetic field generated by the windings on the outer peripheral edge of the stator is not coupled with any of the permanent magnets in the rotor and is therefore wasted. 
     In general, in radial permanent magnet motors, the stator is annularly shaped and is concentrically disposed around a generally cylindrically shaped rotor. The stator is provided with electrically conductive windings wound about and in between teeth, which extend radially inwardly from the stator toward the rotor. Portions of the windings (“end-turn wire”) extend around the outer periphery of the stator. The rotor is provided with permanent magnets of alternating polarity disposed around the periphery of the rotor. The permanent magnets of the rotor react to a magnetic field created by current through the stator windings to cause the rotor to turn. The rotor generally is connected to a shaft, which turns with the rotor and accomplishes work. In the radial motor, the end-turn wires of the stator are not useful in creating the magnetic field that couples with the rotor permanent magnets. 
     Thus, there is a need for an efficient, electrically powered motor-wheel for vehicles and there is a need for more efficient motor components for use in such a vehicle wheel. 
     Stepper motors have been utilized since the 1940s to provide readily controllable, angular motion for a wide variety of applications. Heretofore, stepper motor designs were incapable of providing the necessary torque and electrical efficiency necessary for direct-drive vehicle wheel applications. In addition, stepper motors have required complex control and driver systems, which have also limited their application to direct-drive wheel motor applications. The inventive motor and controller designs, however, overcome these torque, efficiency, and control limitations of the prior art and provide a simple, low-cost, high-efficiency, high torque motor which is easily controlled and driven. The inventive motor design is optimized for direct-drive vehicle applications. 
     DISCUSSION OF THE RELATED ART 
     Several attempts at designing electric, direct-drive wheel motors for vehicles have been made. 
     U.S. Pat. No. 5,584,902 for SELF-PROPELLED WHEEL FOR WHEELED VEHICLES, issued Apr. 20, 1999 to Chahee P. Cho teaches one such motor. CHO, however, teaches an axial, permanent magnet motor having a stator drum fixed to an axle and a wheel hub and rim rotatably mounted to the axle. Permanent magnets are fixed on the wheel and oppose an outer lateral face of the stator drum. Excitation of the stator drum causes the permanent magnets, and thereby the wheel, to rotate. The CHO motor, while having a similar function to the stepper motor of the invention, has a totally different rotor (i.e., wheel) and stator structure, and is not a stepper motor. In addition, neither a sensor system (i.e., concentric rings of sensors) nor a detented parking brake arrangement are shown or suggested in CHO. 
     Another motor similar to the CHO motor is taught in U.S. Pat. No. 5,509,492 for DRIVE INCLUDING AN EXTERNAL ROTOR MOTOR, issued Apr. 23, 1996 to Bernd Pfannschmidt. The PFANNSCHMIDT motor, while serving a similar function to the inventive motor is still not a stepper motor. Like CHO, PFANNSCHMIDT neither teaches nor suggests either a sensor system (i.e., concentric rings of sensors) or a detented parking brake arrangement. 
     None of the prior art, individually or taken in combination, is seen to anticipate or suggest the stepper motor of the present invention. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided an external rotor stepper motor consisting of four individual, independently controlled, co-planar stepper motors optimized for vehicle direct wheel drive applications. The innovative design eliminates the need for bifilar coil winding, each of the motor coils being wound with a single conductor, each coil also being wound in the same direction throughout the entire motor. The space between pole pieces of the motor allows the individual coils to be prewound and installed on the motor pole pieces during motor assembly. Each of the four independent motors is simultaneously energized. 
     Because adjacent, outside coils of adjacent motors are simultaneously energized at the same polarity, there is substantially no energy wasting cancellation of magnetic flux. Carefully chosen radial spacing of the four independent, co-planar motors also eliminates substantially all flux interaction between the motors. Because similar coils of the four motors are all connected in parallel, the net inductance presented to driver circuits is minimized. This yields fast rise time pulses and improved efficiency through minimized reactance of the motor. The physical dimensions of the motor have been chosen to provide a motor suitable for directly driving a vehicle wheel. 
     It is therefore an object of the invention to provide an external rotor stepper motor for direct drive of a vehicle wheel. 
     It is another object of the invention to provide an external rotor stepper motor having multiple, individual, independent coplanar stepper motors. 
     It is a further object of the invention to provide an external rotor stepper motor in which all coils are wound in the same direction for manufacturing economy. 
     It is an additional object of the invention to provide an external rotor stepper motor having single conductor (i.e., non-bifilar) coil windings. 
     It is another object of the invention to provide an external rotor stepper motor having coils that may be pre-fabricated and slid onto the stator during assembly. 
     It is a still further object of the invention to provide an external rotor stepper motor having concentric rings of sensors disposed in the external rotor to control drive to the stepper motor. 
     It is another object of the invention to provide an external rotor stepper motor having concentric rings of sensors disposed in the external rotor to allow smooth starts of the stepper motor by selecting an optimum initial energization for the stepper motor. 
     It is yet another object of the invention to provide an external rotor stepper motor having a plunger and detent to mechanically hold the rotor of the stepper motor when the motor is deenergized. 
     It is a still further object of the invention to provide an external rotor stepper motor having a controller using lead angle modulation to maximize the torque of the stepper motor as a function of motor speed. 
     It is another object of the invention to provide an external rotor stepper motor controller that recaptures kinetic energy from the stepper motor during braking and coasting of the motor. 
     It is an additional object of the invention to provide an external rotor stepper motor that may be stacked into multi-motor units for increased power output. 
     It is a further object of the invention to provide an external rotor stepper motor wherein a stack of the motors may be used to provide a central power plant for a motor vehicle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A complete understanding of the present invention may be obtained by reference to the accompanying drawings, when taken in conjunction with the detailed description thereof and in which: 
     FIG. 1 is an axially oriented, cross-sectional, schematic view of a typical conventional  30 , external rotor stepper of the prior art; 
     FIG. 2 a  is an axially oriented, cross-sectional, schematic view of the four-section stepper motor of the invention; 
     FIG. 2 b  is an axially oriented, cross-sectional, schematic view of the four-section stepper motor of FIG. 2 a  showing detents in the perimeter of the rotor; 
     FIG. 3 a  is a schematic, cross-sectional view of one implementation of a magnetic sensor pickup; 
     FIG. 3 b  is a schematic, cross-sectional view of one implementation of a magnetic sensor pickup of FIG. 3 a  shown in relation to the rotor of the motor of FIG. 2 a;    
     FIG. 3 c  is a schematic, cross-sectional view of an alternate implementation of a magnetic sensor pickup; 
     FIG. 3 d  is a schematic, cross-sectional view of an alternate implementation of a magnetic sensor pickup of FIG. 3 c  shown in relation to the rotor of the motor of FIG. 2 a;    
     FIG. 4 is schematic view showing a detented parking brake suitable for use with the motor of FIG. 2 b;    
     FIG. 5 is an electrical schematic diagram showing a possible driver configuration for use with the motor of FIG. 2 a;    
     FIG. 6 is a schematic, cross-sectional view of a motor vehicle equipped with the composite, external rotor stepper motor of the invention; and 
     FIG. 7 is a schematic block diagram of the vehicle of FIG.  6 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention relates to a stepper motor having an external rotor adapted for direct wheel drive in traction and other similar applications. Unlike polyphase stepper motors of the prior art, the motor of the present invention uses four independent, coplanar  30  stepper motors dispersed radially. All stator pole pieces of each independent motor are adjacent one another. In other words, all pole pieces of motor “A” are grouped, all pole pieces of motor “B” are also grouped, etc. 
     Referring first to FIG. 1, there is shown an axially oriented, cross-sectional view of a typical conventional  30 , external rotor stepper motor of the prior art, generally at reference number  100 . Stepper motor  100  has a stator  102  having 12 pole pieces  104 , each pole piece  104  having three teeth  106  disposed on an outward surface thereof. An external rotor  108  is arranged concentrically around the stator  102 , the rotor  108  having teeth  110  on an inner surface thereof, adapted for interaction with teeth  106  on the pole pieces  104  of the stator  102 . Coils  112  are wound around a central region  114  of the pole pieces  104 . Coils  112 , which appear in cross section in FIG. 1, are typically wound in a bifilar fashion to allow finer rotational control of the rotor  108  (e.g., microstepping). Pole pieces  104  containing respective ones of coils  112  are labeled A 1 , A′ 1 , B 1 , B′ 1 , c 1 , C′ 1 , A 2 , A′ 2 , B 2 , B′ 2 , c 2 , C′ 2 , respectively. A, B, and C refer to the three phases of motor  100 . It will be noted that phase A is divided into A 1  and A 2  portions, each portion having two pole pieces  104  diametrically opposed to one another. Likewise, phases B and C are also split and diametrically opposed. 
     It will be noted that pole pieces  104  as substantially equidistantly distributed around the perimeter of the stator  102 . This is the conventional approach to stepper motor design. Unlike the stepper motor of the invention, stepper motor  100  is a single stepper motor having three phase components. The inventive stepper motor disclosed hereinbelow deviates significantly from this prior art arrangement and consists of four individual, self-contained, coplanar, 30 stepper motors. 
     In operation, prior art motor  100  is rotated by sequentially energizing coils  112  as is well known to those of skill in the stepper motor arts. 
     Referring now to FIG. 2 a , there is shown an axial sectional view of the stepper motor of the invention, generally at reference no.  200 . The four discrete motors  202 ,  204 ,  206 ,  208 , labeled A, B, C, and D, respectively, each have three pole pieces  210  in the stator  212 . The chosen shape of pole pieces is stubby” (i.e., short and fat) to minimize instability caused by their vibration. Longer pole pieces  210  of the prior art were more prone to such vibrations because of their length. Pole pieces  210  are cantilevered from their attachment to the body of stator  212 . 
     In the embodiment chosen for purposes of disclosure, each pole piece  210  has four teeth  232 . The three pole pieces  210  for each motor  202 ,  204 ,  206 ,  208  are spaced apart approximately 26° from one another. Individual pole pieces  210  within each of the motors  202 ,  204 ,  206 ,  208  are separated from each other by approximately 14°. This means that the radial gap between pole pieces  210  within each motor  202 ,  204 ,  206 ,  208  is slightly wider than the approximately 12° width of each pole piece  210  itself. This arrangement provides a short flux path and keeps most of the flux generated by each coil  226  contained substantially within the particular pole piece  210  upon which that coil  226  is wound. 
     In addition, the wide spaces separating pole pieces  210  facilitate the installation of the coils  226  during manufacturing. These large gaps between pole pieces allow the coils  226  to be wound externally and then slipped over the pole pieces  210 . In prior art motors having smaller spaces between pole pieces, it was necessary to wind the motor coils in situ, a more costly process. In situ winding makes factory rework more difficult and probably precludes field servicing of the motor  200 , if or when necessary. 
     The sets of three pole pieces  210  forming the stator  212  portion of each of the four motors  202 ,  204 ,  206 ,  208  are not disposed equidistantly around the circumference of the stator  212  but rather have a gap of approximately 25° between the outer, adjacent pole pieces  210  of any two adjacent motors  202 ,  204 ,  206 ,  208 . This gap in spacing ensures that there is substantially no interaction of flux (i.e., primarily residual flux) in the rotor  214  generated by each of two adjacent motors  202 ,  204 ,  206 ,  208 . In other words, the magnetic flux generated by each of the four motors  202 ,  204 ,  206 ,  208  is substantially self-contained. It is estimated that a lamination stack thickness of approximately two inches for the stator  212  and the rotor  214  will produce motors  200  having sufficient torque that four motors  200  (i.e., one on each vehicle wheel) is sufficient to adequately power a passenger vehicle such as a car. Of course, other motor configurations. (e.g., greater lamination height) could be provided for lower or higher torque applications. 
     The external rotor  214  is disposed concentrically with the stator  212 . It is anticipated that a gap of approximately 0.005 inch will be provided between the stator  212  and the rotor  214 . In the embodiment chosen for purposes of disclosure, the rotor  214  contains 99 teeth  230 . It will be recognized that the inventive principles described herein are not limited to the particular embodiment chosen for purposes of disclosure but that the principles may readily be expanded to other configurations. Therefore, the invention is not considered limited to a rotor having 99 teeth  230 . 
     In the embodiment chosen for purposes of disclosure, the rotor  214  has an inner diameter of approximately 12 inches and an outer diameter of approximately IS inches. The choice of the number of teeth  230  allows the necessary offsets of teeth  230  to enable the stepper motor  200  to rotate. For example, by choosing  99  teeth  230 , when the teeth  232  of the pole pieces  210  designated A 2 , B 2 , C 2  and D 2  are directly aligned with corresponding teeth  230  in the rotor  214 , the teeth  230  of the rotor  214  are misaligned with the teeth  232  of the pole pieces  210  designated A 1 , B 1 , C 1 , and D 1  by two-thirds of a tooth pitch. In other words, only one-third the width of teeth  230  of the rotor  214  is aligned with corresponding the teeth  232  of the stator  212 . Likewise, there is a one-third of a tooth pitch misalignment between the pole pieces&#39; teeth  230  of the rotor  214  and the corresponding teeth  232  of the pole pieces  210  designated A 3 , B 3 , C 3 , and D 3 . In this case, only one-third the width of the teeth  230  of the rotor  214  overlaps their corresponding teeth  232  of the stator  212 . 
     It is desirable for reasons that will be described hereinbelow that the number of teeth  230  on the rotor  214  be an integral multiple of the number of pole pieces  210  in each motor  202 ,  204 ,  206 ,  208 . It will be recognized that other numbers of teeth  230  could be chosen in designs adapted for other operating circumstances or environments. 
     Sensors or sensor actuators  224  are disposed in three concentric rings  218 ,  220 ,  222  in the approximately 1.5 inch space  216  (calculated as 0.5×(15−12)) between the inner and outer diameters of the rotor  214 . Each concentric ring  218 ,  220 ,  222  of the sensors  224  typically consists of 18 sensors uniformly disposed therearound. The sensors  224  in each concentric ring  218 ,  220 ,  222  are, therefore, disposed radially approximately 20° from one another but are offset approximately 6.67° from corresponding sensors in other concentric rings  218 ,  220 ,  222 . 
     Because of the nature of stepper motors when stopped, the rotor  214  and the stator  212  will be locked (i.e., detented) in one of a number of discrete positions relative to one another. The number of sensors  224  is chosen such that at any of these detented positions, at least one of the sensors  224  in one of the concentric rings  218 ,  220 ,  222  will be activated. The operation of the sensors  224  in cooperation with a stepper motor controller will be described in detail hereinbelow. One ring  218 ,  220 ,  222  of the sensors  224  is provided for each pole piece  210  of each motor  202 ,  204 ,  206 ,  208 . In alternate embodiments, the inventive stepper motor could be formed from individual stepper motors having structures other than the 3Ø structure and other numbers of bands of sensors could be utilized. Likewise, it is possible to implement the inventive stepper motor with fewer than three bands of sensors. Consequently, the invention is not considered limited to the number of sensors or the number of bands of sensors chosen for purposes of disclosure. 
     Refer now also to FIGS. 3 a - 3   d . While it is likely that the rotor  214  will carry a sensor actuator (not specifically identified), for example, a hole serving as an actuating part of a see-through sensor system, etc., the term sensor will be used throughout whether an actual sensor or an actuator for a sensor is placed on the rotor  214 . 
     Typically, in a vehicle environment, a sensor system relatively immune to dirt, temperature variation, and vibration is required. One sensor system deemed suitable for use in the vehicle wheel application is a magnetic sensor system. A series of raised bumps, dimples, or other projections (identified as “sensors”  224 ,  224 ′) on rotor  214  could interact with a stationary pick-up coil  240  adjacent the rotor  214 . 
     There are sensor systems well known to those skilled in the art wherein a pickup coil  240  is supplied with a high frequency AC signal (not shown)—a 1 KHz signal, for example. Pickup coil  240 , could be wound on the center leg of a steel or ferrite yoke  242 ,  242 ′. While ferrite might be chosen because of its high magnetic permeability, non-existent eddy current losses, ease of manufacturing, and low cost, it is believed that at the extremely low temperatures possibly encountered in a vehicle wheel environment, ferrite could potentially become too brittle and fail in service. Rather, traditional E-cores formed from laminations may be preferred. M-27 Silicon steel laminations having a thickness in the range of 0.004 inch are known and appear suitable for constructing yoke  242 . It will be recognized that other materials, yoke shapes, and lamination thicknesses could be chosen to satisfy a particular operating circumstance or environment. 
     It is desirable to have sensors  224 ,  224 ′ pass as closely as possible to the core  240 . In the embodiment of FIGS. 3 a  and  3   b , typical air gaps  243  and  244  may be approximately 0.020 inch. As the raised bumps (i.e., sensors  224 ) on the rotor  214  pass the pick-up coil  240 , the magnetic interaction with the pick-up coil  240  is readily detected. FIG. 3 b  shows a schematic representation of three assemblies of yoke  242  and coil  240  of FIG. 3 a  interacting with sensors  224  in concentric bands  218 ,  220 ,  222 . 
     There is a high potential for vibration in the vehicle wheel environment, which could result in possible variation in the relative positions of the sensor actuators  224  on rotor  214  and pickup-up coil  240 . Consequently, an alternate embodiment (FIGS. 3 c ,  3   d ) uses sensor actuators  224 ′ consisting of rectangular bars of laminated silicon steel or other high-permeability material cemented or otherwise affixed to the face of the rotor  214 . Laminations  256  minimize eddy currents in sensor actuators  224 ′. It will be recognized that other material and/or other shapes of laminated or non-laminated material could also be used to implement sensor actuators  224 ′. 
     Other suitable sensor systems meeting these requirements are also well known to those of skill in the art and any suitable sensor may be used to produce a sensed output as will be described in detail hereinbelow. 
     Each pole piece  210  has a coil  226  wound thereupon. Unlike stepper motors of the prior art, the coils  226  are wound from a single conductor (i.e., are not bifilar windings typically formed by winding two parallel conductors), and wound in the same direction on each pole piece  210 . By altering control electronics, it is possible to use bifilar-wound coils as well but single layer coils are considered preferable. By using single layer coils, larger diameter (i.e., smaller AWG number) wire may be used. This lowers the DC resistance of coils  226 , which also improves the overall efficiency of motor  200 . 
     Windings  226  may be wound externally to stepper motor  200  and the pre-wound coils then slipped over pole pieces  210  during motor assembly. The windings may be secured to pole pieces  210  using any suitable means. Coils  226  may be retained on pole pieces  210  in any suitable manner such as a detented retention arrangement (not shown), a clip or other fastener (not shown), or by a suitable adhesive. A removable retention arrangement such as a detent system is preferred to facilitate rework of motors during manufacturing or to allow easy field replacement of one or more coils  226 . This approach appears to provide a low manufacturing cost for windings  210 . 
     Adjacent ones of the coils  210  will always be energized at the same polarity and all energized coils are designed for connection in parallel to present the minimum load inductance to the driver circuits. This results in minimized current rise times, which allows the stepper motor  200  to respond more quickly to energization signals. Heating of the coils  210  is also thereby minimized, again contributing to a high efficiency motor design. As already discussed, the physical separation of pole pieces  210  in individual motors  202  and the separation of the motors  202  from one another contains flux and minimizes eddy currents. This also contributes to motor efficiencies by substantially eliminating minor losses caused by heating any eddy currents. 
     Referring now also to FIG. 5, there is shown an electrical schematic diagram of one possible drive configuration for motor  200 . Each coil  226  associated with pole pieces  210  designated A 1  B 1  C 1  D 1  are connected in parallel in a drive group  248 . Likewise, coils  226  associated with pole pieces  210  designated A 2 , B 2 , C 2 , and D 2  are parallel-connected in drive group  250 . Finally, coils  226  associated with pole pieces  210  designated A 3 , B 3 , C 3 , and D 3  are parallel-connected in drive group  252 . Each drive group  248 ,  250 ,  252  contains four transistors  246  which act as switching elements to power their respective sets of coils  226 . Power for energizing coils  226  through transistors  246  comes from a power source  254  designated +V. +V  254  is typically supplied in vehicular applications from the battery  308  (FIG.  7 ). 
     The use of a variable voltage power supply to the motor  200  is highly desirable. Voltage may be controlled responsive to rotational speed of the motor  200 , increasing voltage being applied as the rotational speed increases. This allows a lower voltage to be applied when the motor  200  is at rest thereby helping to eliminate bolting (i.e., a jerky, lurching) starts as the motor  200  is started. In effect, the motor  200  may be supplied by a substantially constant current, regardless of its rotational speed. Controller  304  (FIG. 7) may be designed to provide such a variable voltage power supply. 
     Refer now to FIGS. 2 b  and  4 . A series of 99 V-shaped detents  228  are spaced around the perimeter of the rotor  214 . These are adapted to receive a mating plunger  230  disposed externally to the stator  212 , actuatable after the stepper motor  200  has been deenergized. It will be recognized that the plunger  230  may be affixed to any stationary portion of a vehicle proximate the rotor  214 . It will also be recognized that more than one plunger  230  may be used to further secure the rotor  214  when the motor  200  is in an energized state. The interaction of one or more of the plungers  230  with respective ones of the detents  228  provides a mechanical “parking” brake function. In the anticipated application as a direct-drive wheel motor for a vehicle, the activation of the detent  228 /plunger  230  system would keep a vehicle from rolling once the stepper motors  200  were deenergized. Plunger  230 /detent  228  combinations could be implemented in a variety of ways. One simple way would be to have plungers  230  biased by a spring  236  towards detents  228 . A solenoid  238  or similar coil could counteract the biasing force of the spring  236  and hold plungers in a withdrawn position when the motor  200  was rotating. In alternate embodiments, detents  228  could be formed in a sprocket, not shown, axially aligned with the rotor  214  rather than in the periphery of the rotor  214 . 
     The four independent motors  202 ,  204 ,  206 ,  208  are designed for simultaneous operation. The coils  210  are, for purposes of operational description, designated A 1 , B 2 , C 3 , etc., coil A 1  being associated with the first pole piece  210  of the motor A (reference no.  202 ), etc. The anticipated energization sequence of the coils  226  (for forward motion) is: 
     
       
         
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 A1, A2   B1, B2   C1, C2   D1, D2 
               
             
          
           
               
                   
                 A2, A3   B2, B3   C2, C3   D2, D3 
               
             
          
           
               
                   
                 A3, A1   B3, B1   C3, C1   D3, D1 
               
             
          
           
               
                   
                 A1, A2   B1, B2   C1, C2   D1, D2 
               
             
          
           
               
                   
                 A2, A3   B2, B3   C2, C3   D2, D3 
               
             
          
           
               
                   
                 A3, A1   B3, B1   C3, C1   D3, D1, etc. 
               
               
                   
                   
               
             
          
         
       
     
     It will be recognized that reversing the energization sequence will allow the rotor  214  to rotate in the opposite direction. 
     Coils A 1 , B 1 , C 1 , and D 1  are connected in parallel. Likewise, coils A 2 , B 2 , C 2 , and D 2  as well as A 3 , B 3 , C 3 , and D 3  are also connected in parallel. 
     The placement of sensors  224  allows a motor control system (described in detail hereinbelow) to ascertain an exact position of the rotor  214  relative to the stator  212  under any condition. The exact location of the rotor  214  is particularly important when the motor is at rest. The motor controller, having ascertained the exact position of the rotor  214  relative to the stator  212 , can provide an initial energization that prevents a sudden jerk, which could occur if an initial energization was arbitrary regardless of the rotor  214 /stator  212  positional relationship. 
     In operation, coil switching is accomplished responsive to signals from the sensors  224 . As the motor  200  rotates, successive ones of the sensors are “made.” As each successive sensor is made, the currently energized coils are deenergized and the next series of coils are energized. 
     The particular embodiment of the stepper motor  200  chosen for purposes of disclosure is designed according to the following assumption and suppositions. First, it is assumed that the wheel outer diameter D is approximately 15 inches. This provides a perimeter according to S=·×D of approximately 47.12 inches or approximately 3.9 feet per motor revolution. Because a mile equals approximately 5,280 feet, the stepper motor  200  must revolve approximately 1,354 revolutions/mile. At a speed, for example, of 70 miles per hour (mph), the stepper motor  200  must rotate approximately 94,769 revolutions/hour. Because the rotor  214  is equipped with 99 teeth and is a 30 motor, this means that the motor  200  must step at the rate of approximately 8,213 steps/second, a rate within the capability of the stepper motor  200  and conventional controller technology. 
     Referring now to FIGS. 6 and 7, a side, schematic view and a functional block diagram are shown of a simple wheeled land vehicle  400 —a car, for example. Four wheels  402  are disposed at the four corners of the vehicle  400 , each of the wheels  402  equipped with a stepper motor  200  as described hereinabove. Each motor  200  is connected to a controller  404  by interconnections  406 . Each interconnection  406  may include a power connection, sensor connections, activation lines for brake devices including the detents  228  (FIG. 4) and plungers  230  (FIG.  4 ), and any other power or communications interconnections needed. 
     Power to energize the motors  200  flows outward over interconnection  406  and generated power from the motors  200  flows inward when the vehicle  400  is in a braking or regenerating mode. A battery  408  is connected to the controller  404  and provides power for the motors  200 . It will be understood that the battery  408  must be charged, either by an on-board or an external power source (not shown). No power source or charging controller is shown, as the actual source of power for charging the battery  408  forms no part of the present invention. It will be recognized that the controller  404  includes the necessary componentry (not shown) to manage charging of the battery  408 . 
     Controller  404  is adapted to receive sensor input from the sensors  224  (FIG. 2) disposed in each of the motors  200 . In addition, the controller  402  is connected to the accelerator  410 , brake pedal  412 , steering mechanism  414 , and direction controller  416 . 
     When vehicle  400  is stopped and the vehicle is “turned off,” the controller  402  actuates the plungers  230  (FIG. 2) allowing them to enter the detents  228  (FIG.  2 ), thereby providing a parking brake function for the wheels  402 . It will be recognized that this parking brake function may be implemented on any combination of wheels  402 , not necessarily on all four wheels  402  chosen for purposes of disclosure. 
     Control of the stepper motor is particularly easy because of the simplified energization scheme described hereinabove. Of course, the energization sequence is merely reversed to cause the vehicle  400  to travel in reverse. A simple direction control  416  in the cab of the vehicle  400  may be used to shift the vehicle  400  between a forward and reverse direction. 
     Refer now also to FIG.  2 . The arrangement of sensors  224  in concentric rings  218 ,  200 , and  222  guarantees that at least one sensor  224  in one of the concentric rings  218 ,  220 ,  224  will be active (i.e., “made”) when the rotor  214  of the motor  200  is at rest. This allows the controller  402  to select an appropriate initial energization for the motor  200  when the vehicle  400  is at rest. That is, if the motor  200  were to be randomly energized, the tendency would be for the vehicle  400  to lurch as the rotor  214  and the stator  212  attempted to align themselves in accordance with the energization. By selecting an optimum energization, the controller  402  ensures that starts of the vehicle  400  will be as smooth as possible. 
     Because each wheel  302  of the vehicle  300  may be controlled completely independently, several possibilities for vehicle are provided. First, by coupling the controller  304  to steering wheel  414 , the speed of motors  200  driving the front wheels  302  may be varied so as to facilitate steering vehicle  300 . Because sensors  224  associated with each wheel motor  200  provide wheel speed data to the controller  304 , each motor may be controlled independently responsive to a steering signal from steering wheel  414 . It will be recognized that other similar sophisticated vehicle control techniques could readily be implemented because of the totally independent wheel motor control. 
     Braking the vehicle  33  may also be accomplished electrically. When breaking is required as indicated by pressure on brake pedal  312 , the controller  304  retards the signal from the sensors  224 , thereby retarding the motors  200  and slowing the vehicle  300 . A fringe benefit of this arrangement is that the kinetic energy of the vehicle  300  is recaptured as the motors  200  enter a regenerative mode (i.e., act as generators instead of motors). The generated energy from the motors  200  during braking is directed to the battery  308  by the controller  304 . It is anticipated that vehicle  300  will also include conventional frictional brakes (not shown) as are well know to those skilled in the vehicle arts. These frictional brakes could be directly actuated by brake pedal  312  through a conventional hydraulic system (not shown), or, in alternate embodiments, could be electrically actuated by the controller  304  if regenerative braking was inadequate for decelerating the vehicle  300  at a sufficient rate. The frictional braking system forms no part of the present invention. 
     It will also be recognized that signals from the sensors  224  associated with individual wheels  202  could be used to indicate slippage during deceleration (braking). The controller  304  could readily take action thereby forming an anti-lock braking system (ABS) either independently, or in cooperation with a frictional braking system. 
     As is well known in the stepper motor arts, closed loop control generally results in superior motor control. This is true because each succeeding energization is directly controlled by positional information from one or more sensors  224 . Controlling the motor(s)  200  in the vehicle  400  is no exception. In addition, a technique known as lead angle modulation is used to optimize the torque of the motors  200  at any operating speed. By using inputs from one or more sensors  224 , the controller  402  may readily determine the rotational velocity of the rotor  214 . As speed increases, the switching point for applying the energy to the next set of coils in the sequence may be varied, thereby altering the lead angle at which energy is applied to a particular combination of motor coils  226 . Lead angle control may be accomplished electronically by the controller  402  by delaying a sensor signal depending upon the rotational velocity of the rotor  214 . In addition, the lead angle may be varied depending upon whether the vehicle  400  is accelerating or decelerating. 
     While the preferred embodiment of the present invention is directed to individual wheel motors for a vehicle, it will be recognized that the inventive motors could also be axially stacked. This creates several possibilities. First, one or more motors  200  could be used on individual wheels of a vehicle for applications requiring torque. Second, multiple motors  200  could be stacked face to face to form a central power plant for a vehicle or for another application normally powered by a conventional internal combustion engine. The stacked motor could be located within a vehicle and its output coupled to the vehicle&#39;s wheel through a conventional drive train. 
     Since other combinations, modifications, and changes varied to fit particular operating requirements and environments will be apparent to those skilled in the art, the invention is not considered limited to the chosen preferred embodiments for purposes of this disclosure, but covers all changes and modifications which do not constitute departures from the true spirit and scope of this invention. 
     Having thus described the invention, what is desired to be protected by Letters Patent is presented in the subsequently appended claims.