Abstract:
An active magnetic wheel bearing including at least two electromagnetic units arranged to support a wheel hub flange of a vehicle within a bearing outer ring. The active magnetic wheel bearing requires little change to surrounding known structure of the wheel hub flange unit. In addition a method of control and operation of the magnetic wheel bearing is presented.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to an active magnetic bearing, in particular an active magnetic bearing for a vehicular wheel application and a method of controlling an active magnetic wheel bearing. 
       BACKGROUND 
       [0002]    Magnetic bearings are known, for example in U.S. Pat. No. 5,300,843 and U.S. Pat. No. 4,920,290. Magnetic bearings operate and support loads by using electromagnetic levitation, for instance, by using electromagnetic forces to levitate a rotating shaft in three dimensional space. A current flow is supplied to electromagnets distributed around an inner circumferential space of the bearing, generating magnetic fields that support shafts or other rotating objects, and are maintained in position by actively controlling the electromagnets, leaving no contact between the bearing and the rotating object or mass. 
         [0003]    Wheel bearing applications are also known in the art. For example, prior art wheel bearings include an inner ring, outer ring and rolling elements between the rings, integrally assembled on a wheel hub and mounted to a vehicle using a suspension member or knuckle arrangement known in the art. Wheel bearings can also be made as a separate unitary assembly that is assembled onto an outer diameter of a wheel hub, and fixed onto the assembly by a variety of methods known in the art, including a press fit. 
       SUMMARY 
       [0004]    According to aspects illustrated herein, there is provided a magnetic bearing which includes; an axis; a wheel hub flange arranged to connect to a wheel, including a flange with a radial face directed toward a wheel and a cylindrical hub extending axially from a flange and arranged to connect to a vehicle wheel shaft; an electromagnetic modification unit fixedly assembled onto outer an cylindrical surface of the cylindrical hub, including a first axial end having a hollow cylindrical shape, a second axial end having a hollow cylindrical shape and an integrally formed axial position disc extending radially outward from a second axial end and having first and second radial faces; an outer ring axially aligned with the electromagnetic modification unit to form a gap there between, including a first axial end axially aligned with first axial end of electromagnetic modification unit, second axial end axially aligned with second axial end of electromagnetic modification unit and wheel knuckle mounting feature arranged to connect to a wheel knuckle; first electromagnetic unit fixedly assembled at first axial end of outer ring and arranged to magnetically levitate first axial end of the electromagnetic modification unit in radial space; and a second electromagnetic unit fixedly assembled at the second axial end of the outer ring, radially aligned with the axial position disc of electromagnetic modification unit  6  and arranged to magnetically levitate second axial end  22  of the electromagnetic modification unit in axial space. The magnetic bearing is capable of providing controllable radial, axial and moment load support to meet desired applications requirements. 
         [0005]    According to aspects illustrated herein, there is provided a method of operating a magnetic wheel bearing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Various embodiments are disclosed, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, in which: 
           [0007]      FIG. 1  is a perspective view of a cylindrical coordinate system demonstrating spatial terminology used in the present application; 
           [0008]      FIG. 2  is a cross sectional view of an active magnetic wheel bearing according to one example embodiment; 
           [0009]      FIG. 3  is a schematic view of a control system for an active magnetic wheel bearing according to one example embodiment; 
           [0010]      FIG. 4  is a a schematic view of a control system for an active magnetic wheel bearing according to a second example embodiment; 
           [0011]      FIG. 5  is a schematic view of a control system for an active magnetic wheel bearing according to a third example embodiment; 
           [0012]      FIG. 6  a schematic view of a control system for an active magnetic wheel bearing according to a fourth example embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    At the outset, it should be appreciated that like drawing numbers on different drawing views identify identical, or functionally similar, structural elements of the disclosure. It is to be understood that the disclosure as claimed is not limited to the disclosed aspects. 
         [0014]    Furthermore, it is understood that this disclosure is not limited to the particular methodology, materials and modifications described and as such may, of course, vary. It is also understood that the terminology used herein is for the purpose of describing particular aspects only, and is not intended to limit the scope of the present disclosure. 
         [0015]    Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs. It should be understood that any methods, devices or materials similar or equivalent to those described herein can be used in the practice or testing of the disclosure. 
         [0016]      FIG. 1  is a perspective view of cylindrical coordinate system  10  demonstrating spatial terminology used in the present application. The present application is at least partially described within the context of a cylindrical coordinate system. System  10  includes longitudinal axis  11 , used as the reference for the directional and spatial terms that follow. Axial direction AD is parallel to axis  11 . Radial direction RD is orthogonal to axis  11 . Circumferential direction CD is defined by an endpoint of radius R (orthogonal to axis  11 ) rotated about axis  11 . 
         [0017]    To clarify the spatial terminology, objects  12 ,  13 , and  14  are used. An axial surface, such as surface  15  of object  12 , is formed by a plane co-planar with axis  11 . Axis  11  passes through planar surface  15 ; however any planar surface co-planar with axis  11  is an axial surface. A radial surface, such as surface  16  of object  13 , is formed by a plane orthogonal to axis  11  and co-planar with a radius, for example, radius  17 . Radius  17  passes through planar surface  16 ; however any planar surface co-planar with radius  17  is a radial surface. Surface  18  of object  14  forms a circumferential, or cylindrical, surface. For example, circumference  19  passes through surface  18 . As a further example, axial movement is parallel to axis  11 , radial movement is orthogonal to axis  11 , and circumferential movement is parallel to circumference  19 . Rotational movement is with respect to axis  11 . The adverbs “axially,” “radially,” and “circumferentially” refer to orientations parallel to axis  11 , radius  17 , and circumference  19 , respectively. For example, an axially disposed surface or edge extends in direction AD, a radially disposed surface or edge extends in direction R, and a circumferentially disposed surface or edge extends in direction CD. 
         [0018]      FIG. 2  is a cross sectional view of active magnetic wheel bearing  1  according to one example embodiment. Magnetic bearing  1  includes; axis AR; wheel hub flange  2  arranged to connect to a wheel (not shown), including flange  3  with radial face  4  directed toward a wheel (not shown) and cylindrical hub  5  extending axially from flange  3  and arranged to connect to a vehicle wheel shaft (not shown); electromagnetic modification unit  6  fixedly assembled onto outer cylindrical surface  20  of the cylindrical hub  5 , including first axial end  21  having a hollow cylindrical shape, second axial end  22  having a hollow cylindrical shape and integrally formed axial position disc  23  extending radially outward from second axial end  22  and having first and second radial faces  30 , 31 ; outer ring  10  axially aligned with electromagnetic modification unit  6  to form a gap therebetween, including first axial end  41  axially aligned with first axial end  21  of electromagnetic modification unit  6 , second axial end  42  axially aligned with second axial end  22  of electromagnetic modification unit  6  and wheel knuckle mounting feature  43  arranged to connect to a wheel knuckle (not shown); first electromagnetic unit  50  fixedly assembled at first axial end  41  of outer ring  10  and arranged to magnetically levitate first axial end  21  of electromagnetic modification unit  6  in radial space; and second electromagnetic unit  51  fixedly assembled at second axial end  42  of outer ring  10 , radially aligned with axial position disc  23  of electromagnetic modification unit  6  and arranged to magnetically levitate second axial end  22  of electromagnetic modification unit  6  in axial space. Magnetic bearing  1  is capable of providing controllable radial, axial and moment load support to meet desired applications requirements. For clarity electromagnetic modification unit  6  is also termed a rotor in magnetic bearing  1 , in particular in the control diagrams of  FIGS. 3-6 . 
         [0019]    In addition, according to the example embodiment emergency support element  55  is positioned axially between electromagnetic units  50 ,  51  and radially positioned outer ring  10  and between electromagnetic modification unit  6 , such that if either or both electromagnetic units  50 ,  51  fail or do not operate properly, cylindrical hub  5  will remain supported and axially and radially aligned within outer ring  10 . Emergency support element  55  may be a plain bearing, roller bearing or any other support element known in the art. Seals  60 ,  61  can also be used at opposite axial ends of outer ring  10 . In the example embodiment, seals  60 ,  61  are pressed on inner cylindrical surface  70  of outer ring  10  and outer cylindrical surface  72  of modification unit  6 . Alternatively seals can be pressed on mating components to outer ring  10  and unit  6 , for example electromagnetic unit  51  at second axial end  42  and spacer  80  at first axial end  41 . 
         [0020]    In order to properly locate unit  6  and associated wheel hub flange  2  in a desired reference position, at least one radial position sensor  90  and one axial position sensor  91 . In the example embodiment of  FIG. 2 , first radial position sensor  90  is fixedly assembled at first axial end  21  and arranged to sense radial position, y, of electromagnetic modification unit  6  and first axial position sensor  91  is fixedly assembled at second axial end  22  of electromagnetic modification unit  6  to sense axial position, x, of electromagnetic modification unit  6 . To improve radial and axial position monitoring, it will be understood by one skilled in the art that multiple radial and axial position sensors  90 ,  91  may be used and circumferentially distributed around electromagnetic modification unit  6 . 
         [0021]    Electromagnetic modification unit  6  is needed to avoid changes to the form of wheel hub flange  2  and to provide a proper magnetic field support for electromagnetic units  50 , 51 . It will be understood by one skilled in the art that wheel hub flange  2  could be modified to include all the features of modification unit  6 , dispensing of the need for unit  6 . 
         [0022]    A control system for magnetic wheel bearing  1  will now be described. The objective of the control system for bearing  1  is to maintain electromagnetic modification unit  6  and associated wheel hub flange  2  in a desired reference position with respect to axial and radial space (x, y) by means of producing a control signal. The control should be robust enough to quickly respond despite disturbances and noise in the system.  FIG. 3  shows a general block diagram for the control strategy and structure for magnetic wheel bearing  1  for one of the control axes, y, according to one example embodiment. The desired reference position is referenced as Uref and the actual sensed position is referenced as Us in the Figure. A change in displacement, y, as sensed by radial position sensor  90 , with respect to the reference position will produce a response from proportional integral derivative (PID) controller  100 , which will control the voltage through radial coils  110  of magnetic bearing  1 , and consequently the current flow. PID controllers are known in the art and, generally calculate an error value as the difference between a measured process variable and a desired set point. The controller attempts to compensate the error by adjusting the process through use of a manipulated variable, in this case a voltage, which depends on the position of the shaft and the current through the coils of the magnetic bearing . In the case of axial position control, coils  111  would be engaged. This current will generate a change in the magnetic fluxes and therefore in the magnitude of magnetic bearing  1  forces, as a result unit  6  will be positioned to the reference coordinate. 
         [0023]    Voltage amplifier  101  feeds magnetic bearing  1  with an appropriate voltage value depending on the PID control laws. PID  100  responds accordingly to the signal generated by the difference between the reference position with respect to the displacements measured by sensors  90 ,  91 . The control has to be able to respond automatically to disturbances or external forces like, weight, bumps, etc. Also, it has to compensate for all the noises generated from the electronic devices. 
         [0024]    Table 1 shows the inputs and outputs to the control system for magnetic bearing  1  control system. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 INPUTS 
                 OUTPUTS 
               
               
                   
                   
               
             
             
               
                   
                 Forces acting on the rotor: 
                 Displacements on: 
               
               
                   
                 1. Forces generated from the AMB (F m ) 
                 1. Sensors 
               
               
                   
                 2. External forces (F d  and F g ) 
                 2. Voltage 
               
               
                   
                 2.1 Weight 
                   
               
               
                   
                 2.2 External forces (impact factor) 
               
               
                   
                   
               
             
          
         
       
     
         [0025]    Table 2 shows one example of the control considerations for a particular magnetic wheel bearing application. 
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Specifications 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Maximum positional stiffness = 60 kN/mm 
               
               
                   
                 Airgap = 600 μm 
               
               
                   
                 Voltage = 300 V 
               
               
                   
                 Current = 100 A 
               
               
                   
                 Current stiffness = 1.02 kN/Amp 
               
               
                   
                 0.5 g force 
               
               
                   
               
             
          
         
       
     
         [0026]    An example embodiment of operation and the control states of magnetic bearing  1  will now be disclosed. In the first control state, the vehicle and magnetic bearing  1  are turned off and without power. In the second control state, the vehicle is on, but, static. During this state magnetic bearing  1  must position wheel hub flange  1  and rotor or modification unit  6  in the initial design coordinates. In the third control state, the vehicle is moving at certain speed. Wheel hub flange  2  and rotor  6  rotating and magnetic bearing  1  must keep the flange  2  and rotor  6  levitated with a constant (target) airgap, a 1 , a 2 , a 3  (See  FIG. 2 ). In the fourth control state, the vehicle is moving and suddenly is under an impact condition. The control must be able to respond relatively instantaneously (estimated response time up to 20 ms) in order to keep wheel hub flange  2  and rotor  6  levitating and without any contact with electromagnetic units  50 ,  51 . In a fifth control state, wheel hub flange  2  is rotating but electromagnetic units  50 ,  51  fail or stop operating properly due to a loss of supply power. There must be a secondary power source that can relatively instantaneously supply power in case the primary source fails. 
         [0027]    To better understand control system  200 , and further define operation, as shown in  FIG. 4 , control system  200  can be subdivided into three subsystems: mechanical, electrical and control. 
         [0028]    In the mechanical system, rotor  6  may be modeled by the following equation of motion, in this case for one axis: 
         [0000]      mγ+ωGγ= F   m   +F   g   +F   d 
 
         [0000]    where,
 
m=mass
 
ω=speed
 
G=gyroscopic force
 
P m =mafnetic force
 
F g =gravity force=mg
 
P α —disturbance forces
 
The magnetic force is dependent on the current through the coils and the airgaps a 1 , a 2 , a 3 . This relationship is non-linear, however, a typical linearized implementation of such force is:
 
         [0000]    
       
      
       P=K 
       i 
       i+K 
       d 
       γ 
      
     
         [0000]    Where K i  and K d  are the current and position gradients in the desired operating point. 
         [0029]    For the electrical system the voltage is dependent on the change of the position of rotor  6 . This relation between position, current and voltage can be expressed as follows: 
         [0000]    
       
         
           
             
                 
             
              
             
               v 
               = 
               
                 Rt 
                 + 
                 
                   L 
                    
                   
                     
                        
                       t 
                     
                     
                        
                       t 
                     
                   
                 
                 + 
                 
                   ? 
                 
               
             
           
         
       
       
         
           
             
               ? 
             
              
             
               indicates text missing or illegible when filed 
             
           
         
       
     
         [0000]    where 
         [0030]    ν=voltage 
         [0031]    R=reluctance 
         [0032]    L=inductance 
         [0033]    K ν =coefficient reflecting the change in voltage with respect to the magnetic field 
         [0034]    For the control system, magnetic bearing  1  is controlled by the implementation of PID controller  100  (baseline controller) as shown in the equation below. Depending on the results, the use of filters or optimal control can be used. 
         [0000]    
       
         
           
             
                 
             
              
             
               
                 ? 
               
               = 
               
                 
                   ? 
                 
                 + 
                 
                   Nt 
                   s 
                 
                 + 
                 
                   
                     K 
                     d 
                   
                    
                   s 
                 
               
             
           
         
       
       
         
           
             
               ? 
             
              
             
               indicates text missing or illegible when filed 
             
           
         
       
     
         [0000]    Where u(s) is the output voltage and g(s) is the error. 
         [0035]    In an alternative embodiment of control and operation of magnetic wheel bearing  1 , as shown in  FIG. 5 , a more precise Adaptive Backstepping Controller (ABC)  105  might be used to optimize centerline position x, control effort, and response time. Direct sensing might be used for position, current, and a wheel speed sensor as inputs to the controller. Vehicle speed (v) input may be used to achieve desired dynamic bearing stiffness. 
         [0036]    A further example embodiment is shown in  FIG. 6 , where ABC controller is further optimized by using established estimators with Lyapunov functions  107 . Using such schema, current and voltage could be estimated as inputs to the Adaptive Observer Back steeping Controller (AOBC)  106 . In such an arrangement error estimations and deviation from desired control parameter tracking could be delegated to estimator  107 . 
         [0037]    The diagram shown in  FIG. 4  incorporates the three subsystems for illustrative purposes. It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.