Abstract:
A velocity sensor including a damper assembly having a central axis, a magnet extending parallel to the central axis, the magnet having a magnetic axis radially oriented with the central axis, and a coil extending parallel to the central axis and radially oriented with the central axis, wherein movement of the damper assembly with respect to the magnet induces a voltage in the coil.

Description:
This application claims priority from U.S. Provisional Patent App. No. 60,527,604 filed Dec. 5, 2003, the contents from which is incorporated herein by reference. 
    
    
     BACKGROUND 
     The present invention is directed to a velocity sensing system and, more particularly, to a velocity sensing system for a damper or the like. 
     A typical damper assembly, such as the one shown in  FIG. 1 , includes a damper body  12  having a piston (not shown) slidably disposed within the damper body  12 . The damper assembly includes a piston rod  14  fixed to the piston and extending outwardly from the damper body  12 . The damper assembly typically is associated with a spring and is mounted between a wheel assembly and frame or body of a vehicle, such as an automobile or a truck. The piston is located in a fluid-filled cavity (not shown) of the damper body  12 . When a load, shock or vibrational force displaces the associated wheel assembly relative to the vehicle body, the force drives the piston into, or out of, the damper body  12 ; the movement of the piston through the relatively viscous fluid within the cavity dampens the movement of the wheel in a well-known manner. 
     The damper assembly also may include a dust tube  16  coaxially received over the damper body  12 . The dust tube  16  provides mechanical protection to the damper body  12  and reduces the introduction of dust and other contaminants into the damper body  12 . When the damper body  12  is moved relative to the piston and piston rod  14 , the damper body  12  moves along its central axis relative to the dust tube  16 . 
     It is often desired to track the state of the damper assemblies of a vehicle so that an on-board control unit may account for the state of such damper assemblies in controlling the damping characteristics of the damper assemblies, as well as during control of braking systems, steering systems and the like. In particular, the control of dampers in real time damping systems requires measurement of the instantaneous relative damper velocity (i.e., the velocity of the piston relative to the damper body  12  or the velocity of the damper body  12  relative to the dust tube  16 ) as a control variable. 
       FIG. 1  illustrates a first system, generally designated  10 , for determining the velocity of the damper body  12  relative to the dust tube  16 . The system  10  includes a concentrated magnet  19  (such as a ring magnet) mounted to the top of the damper body  12 . The dust tube  16  includes a coil  20  located on and distributed along its inner surface, with the coil  20  being coaxial with the dust tube  16  and damper body  12 . Movement of the damper body  12  relative to the dust tube  16  causes a voltage to be induced in the coil  20 , which may then be sensed to determine the damper velocity. 
     The theoretical or idealized flux of the system  10  is shown as line  22  in  FIG. 1 . The flux  22  exits the magnet  19  in a radial direction and extends across a radial gap  24  to the dust tube  16 . The flux  22  extends up the dust tube  16 , radially across the top  26  of the dust tube  16  and axially along the piston rod  14  to return to the magnet  19  to close the flux loop  22 . 
     The system  10  of  FIG. 1  may provide adequate data when the stroke of the damper is relatively small (i.e., less than two times the diameter of the damper body  12 ). However, when the stroke of the damper is relative large (i.e., more than two times or four times the diameter of the damper body  12 ) the performance of the system  10  of  FIG. 1  may be unacceptable. In particular, as the distance of the magnet  19  from the top  26  of the dust tube  16  (through which the flux  22  passes) increases, the size of the flux path  22  increases and flux loss increases accordingly, which degrades the performance of the system  10 . Furthermore, the flux  22  of the system  10  of  FIG. 1  is sensitive to disruption caused by the radial flux produced by magnetorheological (“MR”) fluid type dampers, which typically include solenoids located inside the damper body  12 . 
       FIGS. 2A and 2B  illustrate an alternative system, generally designated  30 , for determining damper velocity. The system  30  includes two diametrically opposed magnets  32 ,  34  positioned adjacent to and extending axially along the length of the damper. Each magnet  32 ,  34  has a polarity such that the long, flat face  36  thereof facing the damper body has a north or south polarity and the opposite face  38  has an opposite polarity. The magnets  32 ,  34  are mounted such that the faces thereof facing the damper body  12  have opposite polarity. Each magnet  32 ,  34  is mounted on a flux carrier or flux collector  40  which is part of, or coupled to, an external dust tube (not shown). The system  30  of  FIGS. 2A and 2B  differs from the system in  FIG. 1  in that the magnets  32 ,  34  are located on the dust tube instead of on the damper body  12  and the magnets  32 ,  34  are distributed or extend along the entire length of the damper travel rather than being concentrated at the end of the damper tube  12 . Conversely, the coil  20  of the system of  FIG. 1  that was distributed along the entire length of travel is now concentrated at the top of the dust tube as two separate coils  44 ,  46 . 
     The idealized flux of the system  30  of  FIGS. 2A and 2B  is shown therein as flux path  48 . The flux travels from the north pole or face of one of the magnets  34  and across the damper body  12  to the south pole of the other magnet  32  in a generally radial or circumferential direction. The flux  48  then travels axially along the flux collector  40  to the top  50  of the dust tube. The flux  48  then travels generally radially or circumferentially to the other flux collector  40  and returns to the north pole of the magnet  34  to close the flux loop  48 . The sense coils  44 , 46  may be located on the flux collectors  40  (see  FIG. 2A ) or at the top  50  of the dust cover (see  FIG. 2B ) to sense the voltage generated by movement of the damper body  12 . 
     The system  30  of  FIGS. 2A and 2B  can provide a reduced-quality output signal because the flux  48  is required to be carried axially to the top  50  of the dust tube to pass through the coils  44 ,  46 . This requires a relatively long flux path  48  (especially during extended travel or long strokes of the damper body  12 ) that reduces the signal strength due to flux leakage. 
     Accordingly, there is a need for a velocity sensing system wherein flux leakage is reduced and signal strength is increased. 
     SUMMARY 
     A first embodiment of the present invention provides a velocity sensing system including a damper assembly having a central axis, a magnet extending parallel with the central axis, the magnet having a magnetic axis radially oriented with the central axis, and a coil extending parallel with the central axis and radially oriented with the central axis, wherein movement of the damper assembly with respect to the magnet induces a voltage in the coil. 
     A second embodiment of the present invention provides a velocity sensor including a damper assembly extending along a central axis, a magnet extending parallel with the central axis, the magnet having a magnetic axis radially oriented with the central axis, and a coil extending parallel with the central axis and radially oriented with the central axis, wherein the damper, magnet and coil form a three-dimensional flux path such that the amount of flux depends upon the position of the damper with respect to the magnet and coil. 
     A third embodiment of the present invention provides a method for sensing velocity including the steps of providing a damper assembly having a central axis, aligning a magnet and a coil with the central axis such that the magnet and coil extend parallel with the central axis, and moving the damper assembly relative to the magnet and coil to induce a voltage in the coil. 
     Other embodiments, objects and advantages of the present invention will be apparent from the following description, the accompanying drawings and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front elevational view, shown partially in section, of a first prior art velocity sensing system; 
         FIG. 2A  is a front perspective view of a second prior art velocity sensing system in a jounce position; 
         FIG. 2B  is a front perspective view of the system of  FIG. 2A  in a rebound position; 
         FIG. 3A  is front perspective view of the velocity sensing system of the present invention; 
         FIG. 3B  is a top plan view of a section of the system of  FIG. 3A  taken along line  3 B- 3 B of  FIG. 3A ; 
         FIG. 4A  is an alternative embodiment of a magnet/pole/coil assembly of the system of  FIG. 3B ; 
         FIG. 4B  is an alternative embodiment of a magnet/pole/coil assembly of the system of  FIG. 3B ; 
         FIG. 4C  is an alternative embodiment of a magnet/pole/coil assembly of the system of  FIG. 3B ; 
         FIG. 4D  is an alternative embodiment of a magnet/pole/coil assembly of the system of  FIG. 3B ; 
         FIG. 4E  is an alternative embodiment of a magnet/pole/coil assembly of the system of  FIG. 3B ; 
         FIG. 4F  is an alternative embodiment of a magnet/pole/coil assembly of the system of  FIG. 3B ; 
         FIG. 4G  is an alternative embodiment of a magnet/pole/coil assembly of the system of  FIG. 3B ; 
         FIG. 4H  is an alternative embodiment of a magnet/pole/coil assembly of the system of  FIG. 3B ; 
         FIG. 4I  is an alternative embodiment of a magnet/pole/coil assembly of the system of  FIG. 3B ; 
         FIG. 4J  is an alternative embodiment of a magnet/pole/coil assembly of the system of  FIG. 3B ; 
         FIG. 4K  is an alternative embodiment of a magnet/pole/coil assembly of the system of  FIG. 3B ; 
         FIG. 4L  is an alternative embodiment of a magnet/pole/coil assembly of the system of  FIG. 3B ; 
         FIG. 4M  is an alternative embodiment of a magnet/pole/coil assembly of the system of  FIG. 3B ; 
         FIG. 4N  is an alternative embodiment of a magnet/pole/coil assembly of the system of  FIG. 3B ; 
         FIG. 4O  is an alternative embodiment of a magnet/pole/coil assembly of the system of  FIG. 3B ; and 
         FIG. 5  is a graph of flux linkage per turn as a function of axial position. 
     
    
    
     DETAILED DESCRIPTION 
     The system of the present invention, generally designated  60 , is a self-powered integrated relative velocity sensor with a distributed magnet and coil that may be used with real time damping systems. The system  60  is suitable for use with dampers with a wide range of strokes and is insensitive to common mode magnetic signals as produced by, for example, MR dampers. The output of the sensing system  60  is insensitive to the temperature of the coil (i.e., resistance) since voltage (and not current) is the measurement variable. 
     As shown in  FIGS. 3A and 3B , the system  60  includes a soft iron pole  62 , which is part of, or mounted on, the inside of a dust tube (not shown) of a damper. The soft iron pole  62  is oriented generally axially relative to the dust tube and damper body  12  and extends generally the entire length of the dust tube. The system  60  further includes a coil  64  that is coupled to the soft iron pole  62 . The coil  64  is wrapped around a magnet  66  that is coupled to the coil  64  and/or soft iron pole  62 . Both the magnet  66  and the coil  64  extend generally the entire axial length of the soft iron pole  62  and dust tube. The magnet  66  has a polarity such that the long, flat face  68  facing the damper body  12  has a north or south polarity and the opposite face  70  (i.e., facing the soft iron pole  62 ) has an opposite polarity. The soft iron pole  62  provides structural support to the coil  64  and magnet  66  and also acts as a flux carrier. 
     Thus, the system  60  uses a magnet  66  and a coil  64  mounted on a soft magnetic iron pole or flux carrier  62  as part of an external dust tube. This differs from the systems  10 ,  30  of  FIGS. 1 and 2  in that a magnet  66  is located on the dust tube and is distributed along the entire length of the damper travel, instead of the concentrated magnet at the end of the damper body  12 , as in the system of  FIG. 1 , or the two distributed magnets on the dust tube as in the system of FIG.  2 . Furthermore, the coils  20 ,  44 ,  46  of the systems of  FIGS. 1 and 2  that were distributed along the entire length of travel or concentrated at the top of the dust tube are now wrapped around the magnet  66  that extends along the entire length of damper travel. 
     The idealized flux path  80  of the system  60  is shown in  FIG. 3B . The flux path  80  extends in a clockwise direction from the north face  70  of magnet  66 , through the soft iron pole  62 , across a gap to the damper body  12 , circumferentially along the damper body  12  and finally across another gap to the south face  68  of the magnet  66  to complete the closed loop  80 . Thus, the flux path  80  may be generally square or circular in top view. However, the flux is three-dimensional and may extend generally axially along the entire length of the soft iron pole  62 , coil  64  and magnet  66 . Therefore, the idealized flux may be visualized as a “tube” which may be generally square or generally circular in cross section extending along the soft iron pole  62 , magnet  66  and coil  64 . 
     The flux path  80  is relatively small, especially as compared to the systems  10 , 30  of  FIGS. 1 and 2 . Therefore, the flux leakage is significantly reduced and signal strength is significantly increased. 
     When the damper body  12  is moved relative to the dust tube, the damper body  12  moves axially relative to the soft iron pole  62 , coil  64  and magnet  66 . Thus, movement of the damper tube  12  relative to the soft iron pole  62 , coil  64  and magnet  66  creates a voltage in the coil  64  that is directly proportional to the relative velocity. When the damper body  12  of the system  60  is moved downwardly, the total flux is reduced. In other words, the axial overlap between the damper tube  12  and the soft iron pole  62 , coil  64  and magnet  66  is reduced, and the length of the idealized three-dimensional flux “tube” is reduced. 
     Movement of the damper tube  12  when there is reduced overlap produces a lower coil flux linkage as compared to when there is greater overlap and thus higher coil flux linkage. Ideally, the relation between flux linkage and damper position is a linear relationship, starting with a minimum value when there is no overlap between the damper tube  12  and magnet, and rising uniformly to a maximum value when there is complete overlap between the damper body  12  and the magnet  66 . Thus, as can be seen in Eq. 1 discussed below, the linear relationship ensures that the induced voltage is directly proportional to the relative velocity. 
     Because the voltage induced in the coil  64  by movement of the damper body  12  has a directly proportional, or nearly directly proportional, relationship with the velocity of the damper body  12  relative to the dust tube, this represents a significant improvement over the systems  10 ,  30  of  FIGS. 1 ,  2 A and  2 B. In particular, the systems  10 ,  30  may have exponential or other non-linear relationships between induced voltage and velocity that cause difficulty in determining the velocity of the damper tube. Furthermore, as will be discussed in greater detail below, the system or sensor  60  of the present invention has greater sensitivity than those of  FIGS. 1 ,  2 A and  2 B. 
       FIGS. 3A and 3B  illustrate a sensor system  60  with a radially oriented magnet  66  and coil  64 . By a “radially oriented magnet” it is meant that the magnetic axis of the magnet  66  (which extends between the poles of the magnet) is radially oriented with respect to the central axis of the dust tube/damper body  12 . By a “radially oriented coil” it is meant that the central axis of the coil  64  is radially oriented with respect to the central axis of the dust tube/damper body  12 . 
       FIGS. 4A through 4O  illustrate variations upon the general concept shown in  FIG. 3B . A design with dual radially oriented magnets  66  and dual radially oriented coils  64  is shown in  FIG. 4A .  FIG. 4B  illustrates a design with a single radially oriented circumferential coil  64 . The term “circumferential coil” means that the central axis of the coil  64  is oriented with the circumferential direction relative to the central axis of the dust tube/damper body. Thus, for example, the coil  64  of  FIG. 4B  may be wound about the soft iron pole  62  such that the coil  64  is wrapped around the long flat faces of the soft iron pole  62  facing the damper body  12  and on the opposite side of the soft iron pole  62 . The curved soft magnetic iron cores shown in these figures can also be straight with the magnets mounted perpendicular to the core. 
     A variety of single radially oriented magnet  66 /pole  62 /coil  64  designs are shown in  FIGS. 4C through 4G .  FIG. 4D  illustrates the configuration shown in  FIG. 3B . Some of these configurations have controlled leakage paths that are useful for magnets requiring high load lines (e.g., Alnico magnets). A number of radially oriented circumferential coil designs are shown in  FIGS. 4H through 4K . The configuration shown in  FIG. 4K , has a controlled leakage path that is useful for magnets requiring high load lines. The configuration shown in  FIG. 4L  may be preferred due to its symmetrical shape and the coil shielding/protection provided by the dual radial iron poles. Finally there are a number of configurations possible with a single radial magnet  66  and dual soft iron poles, as shown in  FIGS. 4L through 4O . Many additional configurations are possible by distributing two or more of these sensors around the circumference of the dust tube. These sensors can be connected together electrically to boost the sensor output and/or to suppress output variations due to non-uniform air gap between the damper body  12  and the dust tube resulting from vibrations or manufacturing tolerances or both. 
     The voltage induced in the coil  64  due to the motion of the damper body  12  relative to the sensor (i.e., soft magnetic iron pole  62 , magnet  66  and coil  64 ) is given by the following equation: 
     
       
         
           
             
               
                 
                   
                     V 
                     coil 
                   
                   = 
                   
                     
                       
                         ⅆ 
                         λ 
                       
                       
                         ⅆ 
                         t 
                       
                     
                     = 
                     
                       
                         
                           ∂ 
                           λ 
                         
                         
                           ∂ 
                           z 
                         
                       
                       · 
                       
                         
                           ⅆ 
                           z 
                         
                         
                           ⅆ 
                           t 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
     It can be shown that the derivative of the flux linkage (λ) with respect to the axial position (z) is a constant and therefore it follows that: 
     
       
         
           
             
               
                 
                   
                     
                       ∂ 
                       λ 
                     
                     
                       ∂ 
                       z 
                     
                   
                   = 
                   
                     k 
                     E 
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   ) 
                 
               
             
           
         
       
     
     The second term in (Eq. 1) is the relative velocity between the stationary sensor assembly and the moving damper body; that is: 
     
       
         
           
             
               
                 
                   
                     
                       ⅆ 
                       z 
                     
                     
                       ⅆ 
                       t 
                     
                   
                   = 
                   Velocity 
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ) 
                 
               
             
           
         
       
     
     Hence the sensor&#39;s induced voltage is directly proportional to the relative damper velocity as follows: 
     
       
         
           
             
               
                 
                   
                     V 
                     coil 
                   
                   = 
                   
                     
                       
                         ⅆ 
                         λ 
                       
                       
                         ⅆ 
                         t 
                       
                     
                     = 
                     
                       
                         
                           
                             ∂ 
                             λ 
                           
                           
                             ∂ 
                             z 
                           
                         
                         · 
                         
                           
                             ⅆ 
                             z 
                           
                           
                             ⅆ 
                             t 
                           
                         
                       
                       = 
                       
                         
                           k 
                           E 
                         
                         · 
                         Velocity 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                     ⁢ 
                     4 
                   
                   ) 
                 
               
             
           
         
       
     
     Thus, a voltage sensor may monitor the voltage induced in the coil and a control unit may convert the induced voltage signal into a relative damper velocity signal by application of Eq. 4. 
     As long as eddy current effects are minimal and the coil current is close to zero the sensor output will show little delay in the velocity signal. The coil voltage can also be integrated to produce a signal proportional to the position of the damper as follows: 
     
       
         
           
             
               
                 
                   
                     z 
                     ⁡ 
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       z 
                       ⁡ 
                       
                         ( 
                         
                           t 
                           reset 
                         
                         ) 
                       
                     
                     + 
                     
                       ∫ 
                       
                         
                           
                             V 
                             coil 
                           
                           
                             K 
                             E 
                           
                         
                         ⁢ 
                         
                           ⅆ 
                           t 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                     ⁢ 
                     5 
                   
                   ) 
                 
               
             
           
         
       
     
     The initial position of the damper body  12 , z(t reset ), can be obtained by a Hall effect sensor or equivalent magnetic sensor that is affixed to the damper body  12  in any number of ways known to those skilled in the art. The Hall effect sensor can be fixed at an arbitrary location (i.e., at the center of the stroke of the piston) such that the integration step of Equation 5 can be utilized during each stroke to accurately track the position of the piston. The integration of the coil voltage as described by Eq. 5 from the time a reset signal is generated, t reset , to the current time, t, can be accomplished by a simple analog circuit as known to those skilled in the art or by various other means. 
     The flux linkage per turn as a function of axial position for the prior art system  30  and the system  60  of the present invention are shown in  FIG. 5 . Line  160  corresponds to the system  60  of the present invention and line  130  corresponds to the prior art system  30  of  FIG. 2 . It can be seen that the upper line  160  displays a linear or nearly linear relationship between flux (and flux linkage) and position as is desired for a velocity sensor. Furthermore, the slope of the line (corresponding to the constant K E ) for the system  60  of  FIG. 3  is 980 mv/(m/s), which is almost three times the slope of the line for the system  30  of  FIG. 2 . 
     Thus, the output of the system  60  of  FIG. 3  is significantly greater than that of the system  30  of  FIGS. 2A and 2B  while using significantly less magnet material and soft magnetic iron structures. Thus, the system  60  of the present invention provides significantly higher output at less cost. Furthermore, the high level of output of the system  60  of the present invention may eliminate the need for amplification or on-board sensor electronics. Higher outputs can also be achieved through additional turns, wider magnets and flux collectors, better magnets, and by distributing a number of these sensors along the circumference of the dust tube, as described above. 
     Finally, because the system and sensor  60  of the present invention includes a flux path that is oriented in a radial plane of the dust tube/damper body  12 , the system  60  is less prone to disturbance by the components of a MR damper. In particular, operation of the piston in a MR damper may cause magnetic flux lines to be formed in a radial plane of the dust tube/damper body  12  that radiate outwardly from the MR piston that is referred to here as a “common mode field” with respect to the sensor. However, because the flux path of the system of the present invention is also oriented in a radial plane, the magnetic flux cause by a MR piston does not have a net effect upon the flux of the sensor since the coil is sensitive only to differential radial fields, and therefore the common mode component causes little or no disturbance. In contrast, for example, the system of  FIG. 1  is sensitive to the common mode field (i.e., a field that is constant in sign or in radial direction at a given z position) while rejecting a differential mode field (i.e., a field that varies in sign or in radial direction at a given z position) and therefore would be sensitive to any fields produced by an MR damper. Thus, the system  60  of the present invention is generally insensitive to common mode magnetic signals. 
     Although the invention is shown and described with respect to certain embodiments, equivalents and modifications will occur to those skilled in the art upon reading and understanding the specification. The present invention includes all such equivalents and modifications and is limited only by the scope of the claims.