Abstract:
An apparatus for measuring linear velocity of a movable element relative to a stationary element includes a magnetic element fixed in relation to the stationary element. A soft-magnetic yoke is fixed in relation to the movable element to move with the movable element and is in non-contact relation with the magnetic element. A yoke pole is positioned proximate to the magnetic element and spaced therefrom by an air gap. The pole is magnetically coupled to the magnetic element so that a magnetic flux is generated in the air gap substantially orthogonal to the axis of motion. A conductive coil is coiled around a coil axis and is fixed in relation to the stationary element with the coil axis substantially parallel to the axis of movement. The coil is in non-contact relation with the yoke and resides between the magnetic element and the pole of the yoke in the magnetic flux.

Description:
BACKGROUND 
     This disclosure relates to measuring linear velocity. 
     In many cases, a need arises to measure the linear velocity of an object. It is also often desired to carry out such a measurement without mechanical contact and over a wide range of linear movement. Furthermore, the construction of the moving object of which velocity is to be measured often needs to be very robust. This imposes strict design limitations on the part of a velocity measuring device that will be mounted on the moving object. A particularly difficult case occurs when the moving object, in addition to its linear motion, also spins at a high rotational speed causing large centrifugal stresses within the object. An example application of a linear velocity sensor is in a damper, a device that exerts a damping force on a moving object proportional to a measured linear velocity of this object with an inverse sign. 
     SUMMARY 
     An apparatus for measuring linear velocity of a movable element relative to a stationary element includes a magnetic element fixed in relation to the stationary element. The magnetic element has a permanent magnet. A soft-magnetic yoke is fixed in relation to the movable element to move with the movable element relative to the stationary element. The soft-magnetic yoke is in non-contact relation with the magnetic element and has a pole positioned proximate to the magnetic element and spaced from the magnetic element by an air gap. The pole is magnetically coupled to the magnetic element so that a magnetic flux is generated in the air gap substantially orthogonal to the axis along which the movable element moves. A conductive coil is coiled around a coil axis. The conductive coil is fixed in relation to the stationary element with the coil axis substantially parallel to the axis of movement. The conductive coil is in non-contact relation with the soft-magnetic yoke and resides between the magnetic element and the pole of the soft-magnetic yoke in the magnetic flux. 
     A method includes communicating magnetic flux between a magnet fixed in relation to a stationary element and a soft-magnetic structure fixed in relation to a movable element. The method further includes generating a voltage proportional to a linear velocity of the movable element in relation to the stationary element on a conductive coil fixed in relation to the magnet and residing between the magnet and the structure. 
     An electric machine system includes a first assembly that moves in relation to a second assembly along an axis of movement. A magnet is fixed in relation to the second assembly. A coil is wound around an axis. The coil is fixed in relation to the second assembly with the axis substantially parallel to the axis of movement. A soft-magnetic structure is fixed in relation to the first assembly to move with the first assembly in relation to the second assembly. The magnet and the soft-magnetic structure cooperate to define a magnetic circuit conducting magnetic flux from the magnet through the coil substantially perpendicular to the coil axis and into the soft-magnetic structure. An electronics module is in electrical communication with the electrical coil and is fixed in relation to the second assembly. 
     The aspects above can include one or more or none of the following features. The magnetic element can include a first soft-magnetic pole element and a second soft-magnetic pole element. The permanent magnet has a pole axis extending through its north and south poles, and the pole axis can be oriented substantially parallel to axis of movement. The first soft-magnetic pole element can be magnetically coupled with the north pole of the permanent magnet, and the second soft-magnetic pole element magnetically coupled with the south pole of the magnet. The conductive coil can include a plurality of turns. The soft-magnetic yoke can include a second pole positioned proximate to the magnetic element and spaced from the magnetic element by an air gap. The second pole can be magnetically coupled to the magnetic element so that a magnetic flux is generated in the air gap substantially orthogonal to the axis of movement. A second conductive coil can be provided and coiled around the coil axis. The second conductive coil can be fixed in relation to the stationary element and in non-contact relation with the soft-magnetic yoke. The second conductive coil resides between the magnetic element and the second pole of the soft-magnetic yoke in the magnetic flux. The first conductive coil can be coupled to the second coil in series so that a voltage induced in the first and second conductive coils by movement of the yoke is additive. The first soft-magnetic pole element and the second soft-magnetic pole element can be cylindrical and substantially concentrically received within a cylindrical opening defined by the poles of the soft-magnetic yoke. Alternatively, the soft-magnetic yoke can be a solid cylinder received within the first and second soft-magnetic pole elements. The movable member can rotate about the axis of movement and the soft-magnetic yoke can be fixed in relation to the movable element to move with the movable element along the axis of movement and rotate with the movable element about the axis of movement. An electronics module can be provided that is electrically coupled to the conductive coil, and the conductive coil can be fixed in relation to the electronics module. The magnet can be fixed in relation to the electronics module. 
     The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIGS. 1A-D  are schematics depicting relative movement between a single turn of a coil and an external magnetic field. 
         FIGS. 2A-B  are schematics depicting relative movement between multiple turns of a coil and an external magnetic field. 
         FIG. 3  is a cross-sectional view of an example sensor to measure axial velocities of rotors in rotating machines. 
         FIG. 4  is a cross-sectional view of an alternative embodiment of the sensor to measure axial velocities of rotors in rotating machines. 
         FIG. 5  is a cross-sectional view illustrating an example of a rotational electric machine configured with one embodiment of the velocity sensor. 
         FIG. 6  is a cross-sectional view illustrating another example of a rotational electric machine configured with an alternative embodiment of the velocity sensor. 
         FIG. 7  is a graph showing amplitudes of the axial oscillations of the electric machine rotor resulting from the axial oscillations of a coupled rotor with a 0.01 inch amplitude. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 1A  shows a single open turn of an electrical coil exposed to an external magnetic field B. By definition, the external magnetic flux Φ 1  linked to this turn is equal to the integral of the magnetic flux density over the cross-sectional area of the turn: 
     
       
         
           
             
               
                 
                   
                     Φ 
                     1 
                   
                   = 
                   
                     
                       ∫ 
                       
                         A 
                         1 
                       
                     
                     ⁢ 
                     
                       B 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           ⅆ 
                           A 
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Assuming that the number of the flux lines crossing the area encompassed by the turn shown in  FIG. 1  represents the magnetic flux through that area, as the turn gradually translates along the z axis with respect to the magnetic field, the flux through the turn changes from six lines in  FIG. 1A , to four lines in  FIG. 1B , to two lines in  FIG. 1C , and finally to no lines in  FIG. 1D . 
     If the field geometry is such that the number of flux lines through the turn is a linear function of the turn position z with respect to the field, then the flux Φ 1  linked to the turn can be represented as a linear function of the turn position z:
 
Φ 1 =Kz.   (2)
 
     If the turn moves in the z direction with some velocity v=ż, then the flux linked to the turn will change in time and the rate of this change will be proportional to v. According to Faraday&#39;s law, the changing flux will induce a voltage on the turn terminals, also proportional to v:
 
 U=−{dot over (Φ)}   1   =−Kż=−Kv.    (3)
 
     Another approach to derive equation (3) is based on Lorenz&#39;s equation describing relativistic transformation between the electric and magnetic fields. According to Lorentz&#39;s law, a charge q moving with a velocity v in a magnetic field B experiences a force:
 
 {right arrow over (F)}   q   =q ( {right arrow over (v)}×{right arrow over (B)} ).   (4)
 
     Another way of arriving at Equation (4) is by analyzing forces acting on the charge q in the coordinate system linked to this charge. In this coordinate system, the charge is stationary and the force F q  is produced by an electric field:
 
 {right arrow over (E)}={right arrow over (v)}×{right arrow over (B)}   (5)
 
so that
 
{right arrow over (F)} q =q{right arrow over (E)}.   (6)
 
     The equation (5) establishes a relativistic relationship between electric and magnetic components of an electromagnetic field observed in two coordinate systems moving with respect to each other with a relative velocity v. 
     If an open conductive wire of an arbitrary shape moves in a magnetic field B, integrating (5) over the entire length L of the wire gives a voltage induced between the wire ends: 
     
       
         
           
             
               
                 
                   U 
                   = 
                   
                     
                       ∫ 
                       L 
                     
                     ⁢ 
                     
                       
                         ( 
                         
                           
                             v 
                             → 
                           
                           × 
                           
                             B 
                             → 
                           
                         
                         ) 
                       
                       · 
                       
                           
                       
                       ⁢ 
                       
                         
                           ⅆ 
                           
                             l 
                             → 
                           
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     If the velocity is known to be always directed in a particular direction (z axis in  FIG. 1 ), then, (7) can be rewritten as 
                     U   =       (       ∫   L     ⁢       (         e   →     z     ×     B   →       )     ·           ⁢     ⅆ     l   →           )     ⁢   v       ,           (   8   )               
where {right arrow over (e)} z  is a unity vector directed along the z axis. By comparing (3) and (8), one can see that
 
     
       
         
           
             
               
                 
                   K 
                   = 
                   
                     
                       ∫ 
                       L 
                     
                     ⁢ 
                     
                       
                         ( 
                         
                           
                             
                               e 
                               → 
                             
                             z 
                           
                           × 
                           
                             B 
                             → 
                           
                         
                         ) 
                       
                       · 
                       
                           
                       
                       ⁢ 
                       
                         
                           ⅆ 
                           
                             l 
                             → 
                           
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     Using a plurality of turns, rather than a single turn, to measure velocity increases the gain K, and if the distribution of the field B along the z axis is not uniform, improves the linearity of the system because the gain K will not be dependent on the z position of the turn with respect to the field. 
       FIG. 2  shows a similar system using a coil comprising a plurality of turns. The integral (9) in this case has to be evaluated over the entire length of the wire in the coil, or over the length of the wire where B is not zero. 
     For example, a cylindrical coil shown in  FIG. 2  is exposed to a magnetic field, which is assumed to be axisymmetric but allowed to be non-uniform in the z direction. The length of the coil is larger than the field span in the z direction. Of interest is the radial component of this field B r  because its axial component, if existed, could not contribute to the gain K since its cross product with {right arrow over (e)} z  would be zero (see equation (9). As for the tangential field component, its existence would contradict the requirement for the field to be axisymmetric. Note, however, that this approach can also be extended to non-axisymmetric systems. 
       FIG. 2A  shows the original mutual orientation of the field and the coil when the coil section shown by a thicker line from the point A 1  to the point B 1  is exposed to the field. The axial profile of the field B r (z) is shown by the shaded area. 
     In  FIG. 2B  the field is displaced from the original position by some distance Δz. The field profile corresponding to the new displaced field position is shown by a shaded area, whereas its profile corresponding to the old position is outlined by a thicker line. Now, the section of the coil exposed to the field spans from the point A 2  to the point B 2 . The sensor gain calculated using (9) will be the same for the  FIG. 2A  and  FIG. 2B  provided that the coil is wound uniformly in the z direction (with the same pitch). Indeed, the coil section A 2 -B 2  is not any different from the section A 1 -B 1 , except that it is clocked slightly differently with respect to the field, but since the field is axisymmetric this clocking would not affect the integral (9). 
       FIG. 3  depicts an example of the linear velocity sensor  10  adapted for measuring the linear velocity along a rotational axis of a rotor in an electric machine  30 , such as that shown in  FIG. 5 . The electric machine can be a motor, generator, and/or other type of electric machine. In other instances, these concepts and similar structures can be applied to measuring linear velocity of elements of other types of devices. For example, the linear velocity sensor  10  can be used to provide velocity feedback to improve the performance of active magnetic bearings, to measure velocities of sensitive elements in seismic or structural monitors, to improve the dynamic response of camera focusing or zooming mechanisms (e.g., such as that described U.S. Pat. No. 5,241,425), and/or in other applications. 
     The external magnetic field first introduced in  FIGS. 1 and 2  is generated here by a cylindrical magnet  11  with two cylindrical soft-magnetic poles  12   a  and  12   b  attached to it. A soft-magnetic yoke  13  has two poles  14   a  and  14   b  adjacent to and separated by some air gaps  15  from the surfaces of the corresponding poles  12   a  and  12   b . In certain instances, the soft-magnetic yoke  13  is mounted to or otherwise fixed in relation to the rotor (i.e., the movable element of the electric machine whose linear velocity is being measured), so that the soft-magnetic yoke  13  moves with the rotor. The magnet  11  and soft-magnetic poles  12   a  and  12   b  are mounted to or otherwise fixed in relation to a stationary element of the electric machine (e.g., the housing or other stationary element of the electric machine). The longitudinal axis  19  of the cylindrical magnet  11  and soft-magnetic poles  12   a  and  12   b  extends through the north and south poles of the magnet  11  and is substantially parallel to the rotational axis of the rotor. The magnet  11  and soft magnetic poles  12   a  and  12   b  are concentrically received within the soft-magnetic yoke  13 . The magnet  11 , poles  12   a  and  12   b , and the soft-magnetic yoke  13  form a closed magnetic circuit with some magnetic flux  16  flowing in it. This flux is directed radially (i.e., perpendicular to the longitudinal axis  19 ) in the air gaps  15  between the stationary poles  12   a  and  12   b  and the yoke poles  14   a  and  14   b . Between each stationary pole  12   a  and  12   b  and the soft-magnetic yoke poles  14   a  and  14   b  are sensing coils  17   a  and  17   b  wound around a non-conductive, generally cylindrical bobbin  18 . The longitudinal axis of the bobbin  18  coincides with the axis  19 , and the bobbin  18  is positioned proximate to the corresponding stationary poles  12   a  and  12   b.    
     Using a permanent magnet to generate the magnetic field negates the need for additional power supplies, wiring, or other electronics. Other magnets, however, may additionally or alternatively be used. 
     Two coils  17   a  and  17   b  are wound and interconnected so that the voltages induced in these coils when the rotor moves axially with velocity v would be added rather than subtracted, resulting in high output gain (i.e., higher than the single turn example described above). For example, if both coil  17   a  and coil  17   b  are wound clockwise start-to-finish as viewed from the +z direction, then the coils are connected in series with the finish of coil  17   a  connected to the finish of coil  17   b . The two starts of the coils would be the output terminals of the sensor. In other configurations, the coils may be wound and/or connected differently. 
     Alternatively, both coils  17   a  and  17   b  can be wound with a continuous wire. After the first coil is wound, the wire continues to the second coil segment and the winding continues with the winding direction being reversed. There are other methods of manufacturing the combined coil  17   a  and  17   b  without departing from the scope of the concepts herein. In certain instances, only one coil  17   a  or  17   b  could be used; however the sensor gain would be reduced by half. 
     By allowing the yoke  13  to translate and keeping the permanent magnet stationary (relative to the electric machine), the apparatus can withstand the high forces associated with, for example, high-speed rotating machinery. This is because the movable portion of the sensor (i.e., the yoke  13 ) is simple and robust. Both the magnet  11  and the coils  17 , which are mechanically the weaker parts of the device, are kept stationary. 
     Using a sensing coil configuration where the sensing coils  17  span an axial distance equal to or in excess of the linear distance traveled by the yoke  13  ensures that the sensor output is linear over a wide displacement range. 
       FIG. 4  illustrates linear velocity sensor  50  showing the yoke  53  residing within the magnet  51  and soft-magnetic poles  52   a  and  52   b . In the embodiment shown in  FIG. 4  the yoke  53  is configured as a solid cylinder of soft-magnetic material that moves inside a bore  60  of the magnet  51  and poles  52   a  and  52   b . The soft-magnetic yoke  53  has two poles  54   a  and  54   b  adjacent to and separated by some air gaps  55  from the surfaces of the corresponding poles  52   a  and  52   b . The longitudinal axis  59  of the cylindrical magnet  51  and soft-magnetic poles  52   a  and  52   b  extends through the north and south poles of the magnet  51  and is substantially parallel to the rotational axis of the rotor. The soft-magnetic yoke  53  is concentrically received within the magnet  51  and soft magnetic poles  52   a  and  52   b . The magnet  51 , poles  52   a  and  52   b , and the soft-magnetic yoke  53  form a closed magnetic circuit with some magnetic flux  56  flowing in it. This flux is directed radially (i.e., perpendicular to the longitudinal axis  59 ) in the air gaps  55  between the stationary poles  52   a  and  52   b  and the yoke poles  54   a  and  54   b . Coils  57   a  and  57   b  could be inserted in the bore, interconnected in series so that changes of the fluxes linked to the coils  57   a  and  57   b  caused by the yoke  53  movements add together for high gain output. A bobbin  58  could also be inserted within the annulus  60 , providing an additional layer of insulation between the coils  57   a  and  57   b  and the stationary soft-magnetic poles  52   a  and  52   b . Between each stationary pole  52   a  and  52   b  and the soft-magnetic yoke poles  54   a  and  54   b  are sensing coils  57   a  and  57   b  wound within a non-conductive, generally cylindrical bobbin  58 . The longitudinal axis of the bobbin  58  coincides with the axis  59 , and the bobbin  58  is positioned proximate to the corresponding stationary poles  52   a  and  52   b.    
     Furthermore, cylindrical shapes of the components specified in  FIGS. 3-4  are tailored to a rotational system and could be replaced with other shapes, depending on the configuration of the system. 
       FIG. 5  shows an example of using the velocity sensor  10  in combination with an axial electromagnetic actuator  36  and electronics  37  to damp axial oscillations of the rotor  31  in an electric rotational machine  30 . 
     The electric machine  30  shown in  FIG. 5  has a rotor  31  and a stator  33 . The rotor  31  of the electric machine  30  is supported radially without mechanical contact by means of front and rear radial Active Magnetic Bearings (AMBs)  32   a  and  32   b . The front AMB  32   a  also provides some passive axial rotor alignment using the interaction between parts of a magnetic circuit mounted on the rotating and stationary parts of the AMB  32   a , which is energized with a permanent magnet  39  within the stator of AMB  32   a . This alignment is typically needed for testing and commissioning prior to installation. When installed, the rotor  31  of the electric machine  30  will be coupled through a coupling  38  mounted on the right end of the rotor  31  to a shaft of another piece of equipment (not shown) driven by (in the case of a motor) or driving (in the case of a generator) the electric machine  30 . In this case, the coupling  38  (and the equipment coupled to the rotor  31  by the coupling) will dictate the axial position of the rotor  31 . The axial displacements of the rotor  31  in  FIG. 5  can be fairly large; however, they still have some limits beyond which the machine  30  (specifically AMBs  32   a, b ) would not operate properly. Considering these limits, non-magnetic thrust and radial backup bearings  34  are installed on the front end of the machine  30  to prevent the rotor  31  from moving beyond tolerable limits of the axial displacements. The thrust and radial backup bearings  34  also function to support the rotor  31  radially along with a rear non-magnetic radial backup bearing  35  when AMBs  32   a, b  are inactive. 
     Eliminating mechanical contact by using magnetic bearings allows the machine  30  to operate at very high rotational speeds without wear, tear, and overheating. The problem is, however, that the rotor  31  floating in space without friction is very responsive to even small axial vibrations of the equipment it is coupled to. Using an AMB to control axial movement of the rotor, however, significantly increases the cost and complexity of the machine  30 , as well as imposes much stricter requirements on the axial alignment between the rotors  31  of the electric machine  30  and the driven equipment. Thus, as described herein, employing the axial velocity sensor  10  with the axial actuator  36  provides an economical and relatively simple way of measuring and damping even small axial vibrations of the equipment. 
     To suppress possible axial vibrations of the electric machine rotor  31 , the axial damper actuator  36  and velocity sensor  10  are installed on the free (i.e., not coupled) rear end of the rotor in  FIG. 5 . A current amplifier is also provided housed within electronics module  37 , which is fixed in relation to the stationary components of the electric machine  30  (e.g., the stator  33  and housing  40  of the electric machine). Whenever there is an axial velocity of the rotor v, the axial velocity sensor  10  generates a voltage U=−Kv proportional to this velocity with the inverse sign according to Equation 3. This voltage is then input into the current amplifier, which generates a current I in the control winding of the axial damper actuator proportional to U. The latter, in its turns, exerts an axial force F d  on the rotor of the electric machine proportional to I, and, consequently, proportional to −v with some proportionality coefficient C (damping coefficient):
 
 F   d   =−Cv.  
 
F d  substantially damps axial movement of the rotor  31 .
 
       FIG. 6  shows an example of using the alternative embodiment of the velocity sensor  50  in combination with an axial electromagnetic actuator  36  and electronics  37  to damp axial oscillations of a rotor  31 ′ in an electric rotational machine  30 ′. 
       FIG. 7  shows amplitudes of the axial oscillations of the rotor  31  resulting from the axial oscillations of a coupled rotor of a driving/driven equipment with a 0.01 inch amplitude with and without the axial electromagnetic damping. 
     A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the concepts described herein. Accordingly, other embodiments are within the scope of the following claims.