Patent Publication Number: US-2003230947-A1

Title: Fault tolerant motor actuator for steer by wire system

Description:
BACKGROUND  
       [0001] The present disclosure relates generally to electric motor actuators and, more particularly, to a fault tolerant motor actuator that may be implemented in a steer by wire system.  
       [0002] A steer by wire system is a system in which one or more steerable wheels are controlled according to an input from a device such as a steering wheel or a handwheel. Generally speaking, the angular displacement of the steering wheel inputted by an operator is detected by a sensor in the form of an electrical signal, and an electric motor is then used to actuate the steerable wheels according to this electrical signal. In addition, the handwheel typically also has a motor actuator associated therewith to provide tactile feedback to the operator. As can be appreciated, the use of electric motors in this type of environment mandates a fairly high degree of reliability associated therewith, as there is no mechanical connection between the steering wheel and the steerable wheels. Thus, it is not uncommon for these systems to provide for some type of redundancy, whether the redundancy is achieved through duplicate electric machinery or by including redundant windings within the electric motor actuators themselves.  
       [0003] However, in addition to reliability, it is also desirable to simultaneously address the problem of motor performance, especially for an application such as steer by wire. A primary concern for electric motors used in steering applications in general (especially for those motors mechanically coupled to a steering wheel) is that of torque ripple. The main sources of torque ripple include cogging torque and ripple torque, the ripple torque being a result of the harmonic contents in the line-to-line back-emf. The cogging torque is a result of the magnetic interaction between the permanent magnets of the rotor and the slotted structure of the armature in a brushless electric motor. As the leading edge of a magnet approaches an individual stator tooth, a positive torque is produced by the magnetic attraction force exerted therebetween. However, as the magnet leading edge passes and the trailing edge approaches, a negative torque is produced. The instantaneous value of the cogging torque varies with rotor position and alternates at a frequency that is proportional to the motor speed and the number of slots. The amplitude of the cogging torque is affected by certain design parameters such as slot opening/slot pitch ratio, magnet strength and air gap length.  
       [0004] Existing approaches to improving torque performance include the use of skewed, arc-shaped magnets that increases the complexity and costs of the manufacturing process. Furthermore, motors with relatively high number of slots (e.g., 27-slot/6-pole, 24-slot/6-pole) also increase the manufacturing and winding costs. Accordingly, it is desirable to be able to implement a motor actuator for a system (such as a steer by wire system) that provides both fault tolerance and acceptable torque performance, but that is also relatively simple in design and inexpensive to manufacture.  
       SUMMARY  
       [0005] The above discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by a fault tolerant electric motor for steering actuation. In an exemplary embodiment, the motor includes a stator assembly having a first group of stator windings and a second group of stator windings, thereby forming a redundant pair of stator windings. The first and second groups of stator windings are located within opposite hemispheres of the stator assembly. A rotor assembly is rotatingly disposed within the stator assembly, and has a plurality of magnets disposed around the periphery of a rotor core. Each of the plurality of magnets is arranged into a pair of segments, one of which is shifted from the other with respect to an axis of rotation of the rotor assembly.  
       [0006] In a preferred embodiment, the stator assembly further includes a plurality of stator teeth, each having a pair of grooves formed within inward facing ends thereof. Each of the segments of the plurality of magnets further includes a substantially flat shaped inner surface and a substantially circular outer surface. In addition, each segment has a width of about 76.5 mechanical degrees with respect to the axis of rotation, and one of the pair of segments is shifted from the other of the pair of segments by about 15 mechanical degrees with respect to the axis of rotation.  
       [0007] In another aspect, an actuator for a steering system includes an electric motor having a stator assembly and a rotor assembly rotatingly disposed within the stator assembly. The stator assembly has a first group of stator windings and a second group of stator windings, thereby forming a redundant pair of stator windings. The first and the second group of stator windings are located within opposite hemispheres of said stator assembly. The stator assembly further includes a plurality of stator teeth each having a pair of slots formed within inward facing ends thereof. The rotor assembly has a plurality of magnets disposed around the periphery of a rotor core, wherein each of the plurality of magnets is arranged into a pair of segments, one of the pair of segments being shifted from the other of the pair of segments with respect to an axis of rotation of the rotor assembly.  
       [0008] In another aspect, a steer-by-wire system for a vehicle includes a master control unit responsive to a steering wheel position signal from a steering wheel unit, and a road wheel unit responsive to a road wheel command signal generated by the master control unit for steering the vehicle. The steering wheel unit further includes a motor actuator for providing tactile feedback to an operator of the vehicle. The motor actuator has a stator assembly with a first group of stator windings and a second group of stator windings, thereby forming a redundant pair of stator windings. The first and second groups of stator windings are located within opposite hemispheres of the stator assembly. A rotor assembly is rotatingly disposed within the stator assembly, the rotor assembly having a plurality of magnets disposed around the periphery of a rotor core. Each of the plurality of magnets is arranged into a pair of segments, one of the pair of segments being shifted from the other of the pair of segments with respect to an axis of rotation of the rotor assembly.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0009] Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures:  
     [0010]FIG. 1 is a cross-sectional view of a brushless electric motor in accordance with an embodiment of the invention;  
     [0011]FIG. 2 is a side view of the rotor assembly of the motor shown in FIG. 1;  
     [0012]FIG. 3 is a perspective view of the rotor assembly shown in FIG. 2;  
     [0013]FIG. 4 illustrates the cogging torque performance of the motor configuration of FIG. 1;  
     [0014]FIG. 5 illustrates the line-to-line back-emf performance of the motor configuration of FIG. 1; and  
     [0015]FIG. 6 is a system block diagram illustrating an exemplary steer-by-wire system that may employ the brushless electric motor of FIG. 1. 
    
    
     DETAILED DESCRIPTION  
     [0016] Disclosed herein is a brushless electric motor, which may be used as an actuator in a steering system such as a steer by wire system. It should be appreciated however, that the specific application of the motor is not necessarily limited to a steer by wire system or even to a steering system in general. Rather, it is contemplated the following motor embodiment(s) may be implemented as an actuator in any application where redundancy, torque performance and cost are of concern.  
     [0017] Referring initially to FIG. 1, there is shown a cross-sectional view of a brushless electric motor  100  in accordance with an embodiment of the invention. The motor  100  includes a rotor assembly  102  rotatingly disposed within a stator assembly  104 . The stator assembly  104  features a plurality of salient stator teeth  106 , defining a plurality of corresponding slots  108  therebetween. As can be seen from the embodiment depicted, the stator assembly  104  has a total of six slots  108 . The stator assembly further includes a first set of stator windings  110  and a second set of stator windings  112 , disposed within opposite hemispheres of the stator  104 , as indicated by the dashed line  114 . Thus configured, the motor  100  is provided with a redundant pair of stator windings.  
     [0018] Within each hemisphere, individual phase windings  116  are wound around each of the stator teeth  106 . In the example illustrated, there are a total of three phase windings  116  wound around the three corresponding stator teeth of the first hemisphere, the three windings labeled as A 1 -A 1 ′, B 1 -B 1 ′, and C 1 -C 1 ′. Similarly, there are three phase windings  116  wound around the stator teeth of the second hemisphere, labeled A 2 -A 2 ′, B 2 -B 2 ′, and C 2 -C 2 ′. This concentrated winding arrangement allows for a relatively inexpensive manufacturing process, in addition to a redundant set of windings. Effectively, two motors reside within the stator assembly. When configured as a single motor, the first and second sets of stator windings  110 ,  112  are connected in parallel. Alternatively, each set may be connected to separate power supplies in a fully redundant arrangement. Moreover, since each set of stator windings is within a separate hemisphere, there exists complete decoupling therebetween. Because each slot  108  simultaneously houses two separate phase windings, appropriate electrical isolation is disposed therebetween. It will also be seen in FIG. 1 that the stator teeth  106  each include a pair of “dummy slots” or grooves  118  formed in the inward ends thereof. As will be described in further detail, the grooves  118  are used to reduce the amplitude of the cogging torque, while increasing the frequency of the cogging.  
     [0019] The rotor assembly  102  includes a shaft  120  protruding from a core  122  that is preferably made from a plurality of lamina of iron, steel, or other magnetic material. In addition, there are four “bread-loaf” rotor magnets  124  disposed around the circumference of the core, thereby forming a four-pole motor. As seen in the cross-sectional view of FIG. 1, the term “bread-loaf” is used to describe the general shape of the rotor magnets  124 , in that they have a flat inner surface  126  and a circular outer surface  128 .  
     [0020] As a result of the formation of the grooves  118  within the stator teeth  106 , a cogging component of 36 pulses per revolution (in addition to the 12 pulses per revolution caused by the stator teeth  106  without the grooves) is introduced into the motor. In order to cancel these cogging components, each magnet  124  is segmented into two pieces, which are shifted by about 15 mechanical degrees from one another with respect to the axis of rotation of the rotor assembly  102 . This segmentation and shifting of the segments is shown in further detail in FIGS. 2 and 3. As can be seen, each of the magnets  124  is each divided into a pair of segments, designated  124   a  and  124   b . In this configuration, the individual magnet segments (having edges perpendicular to the axis of the motor shaft  120 ) are easier to manufacture than a skewed arc magnet.  
     [0021] Various simulations were run with the above-described rotor assembly configuration in order to optimize the design. While the magnet width cannot be optimized for both cogging and harmonics, the cogging amplitude is minimized by introducing dummy-slotted teeth and segmented magnets. FIG. 4 is a graph illustrating the cogging torque performance of the segmented and shifted rotor magnets  124 , as compared with a design utilizing single-piece magnets. As is seen in the graph, there is significantly less cogging torque ripple with the segmented/shifted rotor magnet configuration. Particularly, the shifting of the magnet segments by 15 mechanical degrees causes the canceling of both 12 and 36 pulse per revolution cogging components.  
     [0022] In order to improve the harmonic performance, the magnet width of each segment was selected to be about 76.5 mechanical degrees, a width wherein both the 5 th  and 7 th  harmonic components in the motor-induced voltage are at a minimum and are about equal to one another. In addition, the shifting also reduces the amplitude of the 5 th  and 7 th  harmonic components by almost 75%. For the non-segmented configuration, the peak-to-peak cogging torque is about 20 milli-Newton meters (mN·m), whereas for the segmented configuration, the peak-to-peak cogging torque is less than 1.0 mN·m. FIG. 5 illustrates the line-to-line back-emf for designs with and without magnet segmentation and shifting. As is shown, the resultant harmonic content of both 5 th  and 7 th  harmonics is around 0.5% of the fundamental frequency with the magnet segmenting and shifting. This is an improvement over the back-emf waveform without segmenting and shifting, wherein the harmonic content is about 2-3%.  
     [0023] Finally, FIG. 6 is a system block diagram illustrating an exemplary steer-by-wire system  200  in which the above described motor  100  may be used as an actuator. A steering wheel unit  202  detects the position and movement of a steering wheel (not shown) and sends a steering wheel position signal  204  to a master control unit  206 . The master control unit  206  combines the information of the steering wheel position signal  204 , a feedback torque sensor signal  208 , with a vehicle speed signal  210  from a vehicle speed sensor  212  and tie-rod force signals  214 ,  216  from a road wheel unit  218 . Using these input signals, the master control unit  206  produces road wheel command signals  220 ,  222  (one for a left and right road wheel respectively) that are sent to the road wheel unit  218 . In addition, a steering wheel torque command signal  224  is sent from the master control unit  206  to the steering wheel unit  202 .  
     [0024] Thus, in one aspect, the motor  100  may be included as an actuator within the steering wheel unit  202  to provide tactile feedback to an operator of the vehicle. In another aspect, the motor  100  may also be used in the road wheel unit  218  to produce the steering angle on the steerable wheels.  
     [0025] The above described motor design provides a robust, cost effective, reliable solution for applications such as steering actuators. In one aspect, a 6-slot, 4-pole device allows for a simpler stator winding process, wherein redundant windings are disposed in opposing hemispheres of the stator assembly. Thereby, the redundant pair of windings are also decoupled from another. In a further aspect, the motor torque ripple performance is enhanced through the grooves formed within the stator teeth, as well as by the magnet width of the “bread-loaf” rotor magnet configuration. By configuring the magnets in a segmented, shifted arrangement, the harmonic components in the line-to-line back emf and cogging torque are also minimized.  
     [0026] While the invention has been described with reference to a preferred embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.