Abstract:
A hub cap assembly couplable to an axle of a vehicle and operable to obtain data indicative of wheel velocity includes a housing, a hub cap rotatably coupled to said housing, and a speed sensor coupled to said hub cap. The assembly may be inserted into or otherwise coupled to an axle of a vehicle.

Description:
RELATED APPLICATION DATA 
       [0001]    This application claims priority of U.S. Provisional Application No. 60/744,649 filed on Apr. 11, 2006, which is incorporated herein by reference in its entirety. 
     
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to speed sensors and, more particularly, to a device and method for measuring a true speed of a wheel. 
       BACKGROUND OF THE INVENTION 
       [0003]    Modern brake control systems monitor the rotation of one or more wheels and modify the braking force applied to the wheel or brakes as necessary for proper braking. Such brake control systems are in widespread use in commercial and military aircraft, as well as in ground transportation vehicles. In use, these systems provide improved braking control, antiskid and reduced stopping distances. 
         [0004]    One of the central elements of a brake control system is the wheel speed sensor or transducer, which provides data regarding the instantaneous wheel speed of the various wheels of the vehicle. An accurate measure of wheel speed is an important first step in detecting and then controlling braking conditions such as wheel skid. Known wheel speed transducers convert the rotational speed of the associated wheel to electrical signals, and these signals then are employed by brake control circuitry, such as antiskid control circuitry and/or automatic brake control circuitry, to control the braking activity of the vehicle. 
         [0005]    Conventional systems for monitoring wheel speed have employed analog and/or digital sensors, wherein the wheel speed sensor produces a signal having a frequency that is proportional to the rotational speed of the wheel. The signal is transmitted from the sensor at each wheel to respective sensing and control circuits, which control the braking pressure to prevent skidding of the respective wheels. Wheel speed transducers typically include one part which is fixed to the axle or aircraft frame, while another part is connected to and rotates with the associated wheel. 
         [0006]    Further, conventional methods of deriving wheel speed utilize the axle as a reference point. That is, the sensor is located in or on the axle and the speed is translated from a hubcap to the sensor via a coupling, such as a blade-clip interface, for example. When a misalignment between the axle and hubcap is present, however, an error or oscillation is observed in the sensor signal, which results in inaccuracies in measured speed. Signal errors in this sense are due in part to an elliptical path of the coupling induced by the misalignment. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention enables measurement of true wheel speed, which is obtained by direct measurement of rotational velocity at the hubcap. The invention comprises a hub cap assembly that includes a hub cap rotatably coupled to a hub cap housing. The entire assembly may be inserted in or otherwise attached to an axle of a vehicle, without the need for precise alignment with the axle. Since the measurement sensors are within the housing, the measured speed is true to the hub cap, which eliminates problems associated with the axle centerline being offset from the hub cap centerline. Further, the coupling, which can be a source of dynamic vibration and a point of wear, can be eliminated, thereby reducing costs. Not only is the measured speed determined with increased accuracy, the components used to manufacture the system need not be high precision components, thereby reducing manufacturing costs. 
         [0008]    According to one aspect of the invention, there is provided a hub cap assembly couplable to an axle of a vehicle, said assembly operable to obtain data indicative of wheel velocity. The hub cap assembly includes a housing, a hub cap rotatably coupled to said housing, and a speed sensor coupled to said hub cap and said housing. 
         [0009]    According to another aspect of the invention, there is provided a method for measuring wheel speed using a hub cap assembly comprising a housing, a speed sensor and a hub cap, the method including using the sensor to measure a rotational velocity of the hub cap relative to the hub cap assembly. 
         [0010]    According to another aspect of the invention, there is provided an aircraft wheel speed monitoring system for detecting a speed of an aircraft wheel. The system includes a hubcap rotatable with the aircraft wheel, and a sensor operatively coupled to the hubcap. The sensor includes at least two channels 90 degrees out of phase with respect to each other. 
         [0011]    According to another aspect of the invention, there is provided an aircraft wheel speed monitoring system for detecting a speed of an aircraft wheel. The system includes a hub cap rotatable with the aircraft wheel, and a resolver operatively coupled to said hub cap. The resolver is operable to provide data indicative of an angular position of said wheel. 
         [0012]    To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The forgoing and other embodiments of the invention are hereinafter discussed with reference to the drawings. 
           [0014]      FIG. 1  is a schematic diagram illustrating an exemplary relationship between a velocity reference point and a centerline of a hubcap. 
           [0015]      FIG. 2  is a schematic diagram illustrating an exemplary relationship between a velocity reference point and a hubcap centerline in accordance with the invention. 
           [0016]      FIG. 3  is a cross-sectional view of an exemplary aircraft brake. 
           [0017]      FIG. 4  is an axial view of the actuator modules of the brake of  FIG. 3 . 
           [0018]      FIG. 5  is a schematic diagram of an exemplary aircraft brake control system. 
           [0019]      FIG. 6  is a schematic diagram of an exemplary axle and hubcap including a wheel speed sensor in accordance with the invention. 
           [0020]      FIG. 7  is a schematic diagram illustrating an exemplary orbit of an effector through a detection range of a detector in accordance with the invention. 
           [0021]      FIG. 8  is a graph illustrating signals produced from an exemplary wheel speed sensor in accordance with the invention. 
           [0022]      FIGS. 9A-9C  are perspective views of an exemplary axle and hubcap including a wheel speed sensor in accordance with the invention. 
           [0023]      FIGS. 10A-10B  are cross-sectional views of an exemplary axle and hubcap in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    Conventionally, wheel speed is referenced with respect to the axle, wherein a sensor is mounted in or on the axle and the speed of the hubcap is translated from the hubcap to the sensor via a coupling (e.g., a clip is fixed to and rotates with the hubcap, and a blade, which interfaces with the clip, is normally part of the sensor and rotates with the clip causing the sensor to produce a speed signal). For various reasons, e.g., manufacturing tolerances and/or coupling techniques, misalignment occurs between the hubcap centerline and the axle centerline. This misalignment causes the orbit of the clip about the axle centerline to be offset, thereby creating an oscillation or “accordion effect” in the velocity signal, as indicated in the graph  10  of  FIG. 1 . 
         [0025]    For example,  FIG. 1  illustrates a circle  12  having a first point  14  (i.e., the center point of the circle) and a second point  16  offset from the first point  14 . The circle  12  can represent an orbit of the clip, the first point  14  can represent a centerline of the hubcap, and the second point  16  can represent a centerline of the axle. As the hubcap rotates, the clip rotates with the hubcap such that a center of the clip orbit coincides with the hubcap centerline  14  (i.e., the centerline  14  and center of the clip orbit are the same point). However, since the axle centerline  16  does not coincide with the hubcap centerline  14 , the clip orbit  12  is skewed with respect to the axle centerline  16 . This causes the sensor to measure multiple velocity components (e.g., angular and orbiting), which creates an oscillation in the velocity signal graph  10 . This is illustrated in  FIG. 1  by the arrows  18 , each of which have different lengths or magnitudes from the axle centerline  16  (the velocity reference point for this example) to the clip orbit  12 . 
         [0026]    The present invention enables the determination of true wheel speed by eliminating harmonics associated with a hubcap having multiple components of velocity (e.g., angular velocity and orbiting velocity). More specifically, wheel speed is referenced from the hubcap and/or hubcap centerline, and not from the axle or axle centerline. In other words, relative motion is translated out of the hub cap rather than through a coupling back to the axle. Thus, if misalignments are present between the hubcap and the axle, the wheel speed sensor does not record these as oscillations (the sensor effectively is immune to such misalignments). This arrangement assures that the only component of velocity that is sensed is the true angular velocity of the hubcap, without harmonics associated with the orbiting velocity. 
         [0027]    For example, and with reference to  FIG. 2 , the circle  12 , first point  14  (hubcap centerline) and second point  16  (axle centerline) correspond to the above-described points. However, velocity is directly measured at the hubcap (e.g., velocity is referenced from the hubcap centerline  14 ) as opposed to measuring velocity from the axle (e.g., velocity being referenced from the axle centerline  16 ). In this scenario, the sensor&#39;s pickup (also referred to as a detector) is held in a constant position relative to an orbit of the sensor&#39;s effector, resulting in elimination of harmonics (misalignment errors) associated with offsets between the axle centerline  16  and the hubcap centerline  14 . The graph  20  of  FIG. 2  illustrates a feedback signal free from harmonics, as do the arrows  22 , which all have the same length or magnitude from the hubcap centerline  14  (the velocity reference point) to the clip orbit  12 . 
         [0028]    As used herein, an effector is defined as a device used to produce a desired result or event. Examples of an effector include slits in an optical disk, a magnetic object, electrical windings, etc. 
         [0029]    The invention will now be described in more detail with respect to several embodiments. Because the invention was conceived and developed for use in aircraft braking systems, it will be herein described chiefly in this context. However, the principles of the invention in their broader aspects can be adapted to other types of braking systems, such as in train brake systems or earth moving vehicle brake systems, for example. 
         [0030]    Referring now to  FIG. 3 , an exemplary wheel and brake is generally indicated at  30 . The assembly  30  generally comprises a brake  31  and an aircraft wheel  32 , which is coupled to a hubcap  33 . The wheel  32  and/or hubcap  33  are supported for rotation by bearings  34   a  and  34   b  on an aircraft axle  35 . The aircraft axle  35  forms a wheel mount and is attached to the end of an aircraft landing gear strut (not shown) or a truck attached to the end of a landing gear strut. 
         [0031]    The brake  31  includes a brake head or housing  50  which is attached by bolt fasteners  51  to a torque tube  52 , which in turn is attached by bolt fasteners  53  to a torque take-out flange on the aircraft axle  35 . The torque tube  52  is surrounded by stationary brake elements and rotary brake elements that are interleaved. The stationary and rotary brake elements are in the form of stator disks  45  and rotor disks  46 , and the interleaved arrangement thereof is commonly referred to as a brake disk stack, the same being designated by reference numeral  47 . The stator disks  45  are splined to the torque tube  52  and the rotor disks  46  are splined to the wheel  32  interiorly of the wheel&#39;s rim. As is conventional, the splined connection may be effected by a plurality of spline or drive keys that are spaced around the circumference of the rim/torque tube to permit axial movement of the rotor/stator disks while being held to the wheel/torque tube against relative rotation. 
         [0032]    The disk stack  47  is located between a back pressure member  61  and the brake head  50 . The back pressure member  61  is formed by a radial flange at the outer end of the torque tube  52 . The radial flange  61  carries thereon a plurality of circumferentially spaced torque pucks  63  engaged with the last brake disk  64  at the rear end of the disk stack  47 . The torque pucks  63  may be attached in a known manner to the radial flange  61  by several torque pucks which have the stems thereof loosely fitted in holes in the radial flange to permit some swiveling movement thereof. The torque pucks  63  in the illustrated embodiment secure the last brake disk  64  against rotation relative to the torque tube  52 . In a modified arrangement, the radial flange could be configured to engage directly the disk pack, and still other arrangements could be used. 
         [0033]    Pressure is applied to the front end of the disk stack  47  by one or more disk engaging members which in the illustrated embodiment are the inboard ends of one or more actuator rams  55 . The actuator rams  55  are included in respective electric actuator modules  56  mounted to the brake head  50  by removable bolt fasteners  57  or other suitable means enabling quick and easy attachment and detachment of the actuator modules to and from the brake head. As shown in  FIG. 4 , a plurality of the electric actuator modules  56  are mounted in a circular arrangement around the rotational axis of the wheel, preferably with the actuator rams circumferentially equally spaced apart. The electric actuator modules  56  each have extending therefrom a cable  59  (only two shown) for effecting electrical connection to a brake controller  44 , as described below with respect to  FIG. 5 . It is noted that while the brake is described with respect to an electric braking system and electric actuator means, the brake may be implemented using other actuator means (e.g., hydraulic, pneumatic, etc.). 
         [0034]    Further, and as described in more detail with respect to  FIG. 6 , a wheel speed sensor (not shown in  FIG. 3 ) includes a first part coupled to and rotatable with the hubcap  33 , and a second part coupled to a support structure, independent of the aircraft axle  35 . The first part rotates with the hubcap such that the first part moves about the hubcap centerline in a circular orbit. Additionally, the second part is positionally fixed to the support structure. The wheel speed sensor can be an encoder, an AC or DC tach, a resolver, a Hall effect sensor, or any other device that can be used to measure a change in relative position with respect to time. 
         [0035]    Referring to  FIG. 5 , an exemplary schematic diagram of an aircraft brake control system  60  is shown. The system  60  includes a brake pedal  62  located in the aircraft, wherein the brake pedal  62  generates a signal proportional to an amount of pedal deflection or desired braking force. The signal generated by the brake pedal  62  is provided to a brake controller  64 , which also receives data relating to the wheels and brakes  30  (e.g., wheel speed, brake torque, brake temperature, etc.). The brake controller  64  can include a microprocessor  64   a , read only memory (ROM)  64   b , random access memory (RAM)  64   c , and input/output module  64   d , each of which are communicatively coupled via a system bus  64   e  or the like. A braking program can reside in ROM  64   b  and can be executed by the microprocessor  64   a  so as to implement a braking function. 
         [0036]    The brake controller  64  is operatively coupled to a brake actuator  66 , such as one or more hydraulic valves, electric motors, or the like, which in turn drive respective actuator rams  55 . Based on a braking command from the pedal  62  and data relating to the wheel and brakes, the braking controller  64  provides a signal to the brake actuator  66  so as to implement the braking command while preventing wheel skid (antiskid control), control the deceleration of the wheel, or other speed related logic functions of brake control. 
         [0037]    Referring now to  FIG. 6 , there is shown a simplified side view of an exemplary wheel assembly  70  for an aircraft, wherein the brake assembly and associated components have been omitted for clarity. The wheel assembly  70  includes a hub cap assembly  71  that interfaces with the aircraft axle  35 , as well as the wheel  32  via fasteners  32   a . The hub cap assembly  71  comprises both rotating  71   a  and non-rotating  71   b  portions, as well as a wheel speed sensor  72 . The wheel speed sensor  72  includes a first part  72   a  coupled to the rotating portion  71   a , and a second part  72   b  coupled to the non-rotating portion  71   b  of the hub cap assembly  71 . The wheel speed sensor  72  will be described in more detail below. 
         [0038]    The non-rotating portion  71   b  includes an axle interface receiver  74  that fits within the aircraft axle  35 . The axle interface receiver  74 , for example, may be cylindrical in shape and dimensioned so as to fit within the inner diameter D 1  of the aircraft axle  35 . Preferably, the axle interface receiver  74  is dimensioned so as to provide a non-interference fit with the aircraft axle  35 . 
         [0039]    A proximal end  74   a  of the axle interface receiver  74  includes a connector  76 , such as an electrical or optical connector, which is couplable to the brake controller  64 . The connector  76  provides a means for supplying signals from the hub cap assembly  71  to the brake controller  64 , as described in more detail below. A distal end  74   b  of the axle interface receiver  74  includes a bearing  78  or the like, which couples the non-rotating portion  71   b  of the hub cap assembly with the rotating portion  71   a  of the hub cap assembly. The second portion  72   b  of the wheel speed sensor  72 , which may be an optical sensor, for example, also is attached to the distal end  74   b  of the axle interface receiver  74 . 
         [0040]    The axle interface receiver  74  also includes one or more anti-rotation mechanisms  80 , such as a spring clip, o-ring, or the like. The anti-rotation mechanism  80  interfaces with an outer surface  82  of the axle interface receiver  74  and an inner surface  84  of the aircraft axle  35 . The anti-rotation mechanism  80  need only provide a force sufficient to inhibit rotation from forces transferred to the non-rotating portion  71   b  via the bearing  78  (e.g., from bearing drag). As will be appreciated, these transmitted forces typically are small in magnitude, and thus the opposing force created by the anti-rotation mechanism  80  need only overcome these forces. 
         [0041]    For example, the deformation of a rubber o-ring between the outer surface  82  of the axle interface receiver  74  and the inner surface  84  of the aircraft axle  35  typically is sufficient to prevent rotation of the axle interface receiver  74 . The o-ring may interface with a slot formed in the axle interface receiver (e.g., slot  84 ) and/or in the aircraft axle  35 . Alternatively, an anti-rotation mechanism embodied as a spring clip may interface directly with the inner and outer surface of the aircraft axle  35  and axle interface receiver  74 , respectively, or the clip may interface with slots, tabs or the like formed in the axle and/or axle interface receiver  74 . As was noted above, the axle interface receiver  74  and components attached thereto are non-rotating parts, i.e., they remain in a relatively fixed position with respect to the aircraft axle  35  without substantial movement during wheel rotation. 
         [0042]    The rotating portion  71   a  of the hub cap assembly  71  includes the hub cap  33  and hub cap axle  84 , wherein the hub cap  33  and hub cap axle  84  rotate about a common centerline (i.e., the hub cap centerline  14 ). As will be appreciated, the hub cap axle  84  and hub cap  33  may be integrally formed as a one-piece unit, or they may be separate pieces that are fastened to one another. Also included in the rotating portion  71   a  is the first part  72   a  of the wheel speed sensor  72 , which may be a disk comprising slits formed therein. The first part  72   a  is coupled to and rotates with the hub cap axle  84  about the hub cap centerline  14 . 
         [0043]    In the exemplary embodiments illustrated herein, the wheel speed sensor  72  comprises an optical encoder, wherein the first part  72   a  includes a disk having an effector  88  that comprises a plurality of slits formed in the disk, and the second part  72   b  includes a detector  90  that comprises one or more optical pickups. One or more leads (not shown) electrically or optically couple the detector  90  to the connector  76 , thereby providing data to the brake controller  64 . As will be appreciated, the wheel speed sensor  72  may be embodied in other forms without departing from the scope of the invention. 
         [0044]    With further reference to  FIG. 7 , a relative position of an effector  88  with respect to the hubcap  33  and/or the hubcap centerline  14  is fixed (i.e., the hubcap and effector rotate about the common centerline  14 , wherein at least a portion of the first part  72   a , such as the effector  88 , exhibits a circular orbit about the centerline  14 ). Further, the wheel speed sensor  72  includes a second part  72   b  having a detector  90 , wherein the second part  72   b  is attached to the distal end  74   b  of the axle interface receiver  74  such that a relative position of the detector  90  with respect to the hubcap centerline  14  is fixed. 
         [0045]    The detector  90  has a detection range such that as the effector  88  passes through the detection range, data relating to wheel speed is obtained. Moreover, the detector  90  is positioned such that the detection range corresponds to a radius of the effector orbit. Preferably, the effector orbit follows a trajectory that is substantially tangential to a predefined trajectory between an upper detection limit and a lower detection limit of the detector  90 , wherein the upper detection limit, lower detection limit and predefined trajectory correspond to different orbits around the hubcap centerline  14 . For example, the detector  90  of the second part  72   b  can detect the effector  88  provided the effector  88  passes between an upper detection limit  92   a  and a lower detection limit  92   b . The upper detection limit  92   a  can correspond to a first orbit having a radius R 1  about the centerline  14 , and the lower detection limit  92   b  can correspond to a second orbit having a radius R 2  about the centerline  14 , wherein R 1 &gt;R 2 . Then, the effector  88  can follow or be tangential to an orbit  92   c , wherein the orbit  92   c  has a radius R 3  and a center point located at the centerline  14 , and wherein R 1 &gt;R 3 &gt;R 2 . 
         [0046]    Alternatively, the speed sensor may be a resolver, and the hub cap axle  84  may be directly or indirectly coupled to an input shaft of the resolver. As is known, a resolver can provide absolute position data within one revolution of the resolver input shaft. Thus, the angular orientation of the hub cap axle  84  (and thus the aircraft wheel) can be determined even at zero speed. Further, axle rotational velocity (and thus wheel velocity) can be determined from the change in angular axle position with respect to time. Other examples of a speed sensor include an AC or DC tach, a hall effect sensor, a quadrature sensor, a pulse tach, or the like. 
         [0047]    The wheel speed sensor  72  can be embodied as a sensor that comprises multiple channels, as shown in  FIG. 8 . When multiple channels are employed, it is preferable that the channels are out of phase from one another by ninety degrees, wherein each channel provides a predetermined number of pulses per revolution. In a two channel system, for example, the rising and falling edges of the respective channels can be used to double the pulses per revolution from the sensor  72 . For example, the rising  94   a  and falling  94   b  edges of a first channel (A and −A) can be used to derive a new pulse train  94   c  having double (2×) the pulses per revolution. Similarly, the rising  94   d  and falling  94   e  edges of a second channel (B and −B) can be used to derive a new pulse train  94   f  having double the pulses per revolution. Further, the two channels  94   c ,  94   f  can be doubled again as described herein to achieve four times (4×) the original channel frequency  94   g.    
         [0048]    During normal operation, the 4× channel can be used to provide enhanced performance of the brake control. Should a channel fail, the system can operate using the 2× channel provided by the single channel (A or B). This approach enables high resolution speed feedback in a small package, while providing redundancy at a lower resolution. 
         [0049]      FIGS. 9A and 9B  are perspective views of the hub cap assembly  71 , wherein  FIG. 9A  illustrates a front portion of the hubcap  33 , while  FIG. 9B  illustrates the axle receiver interface  74  and a rear portion of the hubcap  33 .  FIG. 9C  is a perspective view of axle receiver interface  74  without the hubcap  33 . As can be seen in  FIGS. 9B and 9C , the axle receiver interface  74  includes the aforementioned anti-rotation mechanism  80  that interfaces with the aircraft axle  35  so as to inhibit rotation of the axle interface receiver  74  with respect to the aircraft axle  35 . The anti-rotation mechanism will be described in more detail with respect to  FIG. 10B . Also shown in  FIGS. 9B and 9C  is wiring slot  74   c  formed in the axle interface receiver  74 . The wiring slot  74   c  facilitates routing of electrical or optical leads from the sensor  72  to the connector  76  and helps visually in assembling while reducing weight. Also shown in  FIGS. 9A-9C  is a flange  95  formed on the distal end  74   b  of the axle interface receiver  74 . The flange  95  can provide a convenient location for attaching components, such as portions of the speed sensor  72 , for example. 
         [0050]    With further reference to  FIG. 10A , a cross sectional view of the hub cap assembly  71  is shown coupled to the aircraft axle  35 . Due to manufacturing tolerances, a centerline  16  of the aircraft axle  35  may be offset from a centerline  14  of the hubcap  33  (the offset is exaggerated for sake of clarity). However, because the first part  72   a  and second part  72   b  of the speed sensor  72  are part of the hub cap assembly  71  as described herein, the hubcap velocity is effectively measured from the point of view of the hubcap centerline  14 , and not the aircraft axle  35  or axle centerline  16 . Thus, the measured velocity is the true speed of the hubcap  33  (and thus the wheel attached to the hubcap). In the exemplary embodiment of  FIG. 10A , the first part  72   a  is secured to the hub cap axle  84  via a sleeve  84   a , which may be pressed on the rotating hub cap axle  84 , for example. 
         [0051]    Referring now to  FIG. 10B , the anti-rotation mechanism  80  is shown in more detail. In the exemplary embodiment of  FIG. 10B , the anti-rotation mechanism  80  comprises a spring clip  96  coupled to the axle interface receiver  74 , and an interface  98 , such as a slot, recess or the like formed in the aircraft axle  35 . As the axle interface receiver  74  is inserted into the aircraft axle  35 , the spring clip  96  deforms or compresses as the axle interface receiver  74  is initially inserted into the aircraft axle  35 , and then expands to engage the interface  98  of the aircraft axle  35 , thereby inhibiting rotation of the axle interface receiver  74  with respect to the aircraft axle  35 . 
         [0052]    The anti-rotation mechanism  80  may be embodied in other forms, and the spring clip retention mechanism is merely exemplary. For example, and as described above, the anti-rotation mechanism  80  can be an O-ring that engages an outer wall of the axle interface receiver and an inner wall of the aircraft axle  35 . The anti-rotation mechanism  80  can be any device that will fix the axle interface receiver  74  against rotation relative to the aircraft axle  35 . 
         [0053]    Accordingly, a wheel speed measuring system has been disclosed that provides a measurement of true wheel speed, without errors associated with orbiting velocities (from misalignment). The system can be used to provide improved braking control and enhanced anti-skid control, thereby improving braking performance. Moreover, since the hubcap and/or hubcap centerline are used as a reference point for speed measurement, the interface between the axle and the wheel assembly need not be precise, thereby reducing machining costs as well as eliminating weight, wear and cost associated with the conventional coupling, for example. 
         [0054]    Although the invention has been shown and described with respect to a certain preferred embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.