Abstract:
An apparatus and associated method is provided for suspending a test object in a gravitational field from a support member, exciting the test object by noncontactingly engaging it with a predetermined waveform force, and characterizing the test object qualitatively in relation to an observed modal frequency response of the test object to the excitation.

Description:
BACKGROUND 
       [0001]    Market demands have caused successive recent generations of data storage devices to be continually smaller but more capable. That is, consumers today want and get ever-greater storage capacity and processing speed in a smaller package. This dichotomy has been and will continue to be met by designers who factor in higher bit-areal storage densities, faster data transfer speeds, tighter transducer flying heights, and more robust mechanical components. 
         [0002]    All these design factors make data storage devices more susceptible to vibration. Vibrations once conveniently ignored for being negligible must now be effectively managed to prevent perturbances that can render position-control systems ineffective due to track misregistration errors and servo tracking errors. For instance, the smaller and stiffer mechanical components in miniaturized assemblies have relatively higher natural frequencies, and as such are more sensitive to external excitation. Testing of such components requires more scrutiny of the modal frequency response in order to successfully design away from resonant frequencies that create such perturbances. 
         [0003]    There are generally two categories of previously attempted solutions for measuring a data storage device component&#39;s modal frequency response. In the first, an impact hammer is used to excite the test object. In the second, a mechanical shaker device is used to excite the test object. The former disadvantageously does not provide a continuous and stable periodic excitation. The latter disadvantageously distorts the modal frequency response due to the mass loading associated with the requirement of attaching the shaker to the test object, and the fact that the excitation forces are transmitted through the attachment link. What is lacking in the art is an apparatus and method that provides free-state excitation for a modal frequency response analysis. It is to that improvement in the art that the claimed embodiments are directed. 
       SUMMARY 
       [0004]    Claimed embodiments are generally directed to free-state modal frequency response testing of components in a data storage device. 
         [0005]    In some embodiments an apparatus and associated method is provided for suspending a test object in a gravitational field from a support member, exciting the test object by noncontactingly engaging it with a predetermined waveform force, and characterizing the test object qualitatively in relation to an observed modal frequency response of the test object to the excitation. 
         [0006]    These and various other features and advantages which characterize the claimed embodiments will become apparent upon reading the following detailed description and upon reviewing the associated drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a partially exploded isometric view of a data storage device that is well suited for use in practicing the embodiments of the claimed invention. 
           [0008]      FIG. 2  is a diagrammatic depiction of a test apparatus that is constructed in accordance with embodiments of the claimed invention. 
           [0009]      FIG. 3  is a diagrammatic depiction of a test apparatus that is constructed in accordance with alternative embodiments of the claimed invention. 
           [0010]      FIG. 4  shows test results comparing modal frequency responses obtained by shaker testing of related art attempted solutions and by acoustic wave force testing of the claimed embodiments. 
           [0011]      FIG. 5  shows test results comparing modal frequency responses obtained by shaker testing of related art attempted solutions and by magnetic wave force testing of the claimed embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Referring to the drawings as a whole, and for now in particular to  FIG. 1  which is an isometric depiction of a data storage device  100  that is well suited for use in carrying out the claimed embodiments. A base  102  and a cover  104  with a sealing member interposed therebetween provide a sealed enclosure for a number of components. These components include a spindle motor  108  that has one or more data storage mediums (sometimes referred to as “discs”)  110  affixed thereto in rotation. 
         [0013]    Adjacent the disc  110  is an actuator assembly  112  that is pivotally supported by a cartridge bearing  114 . The actuator assembly  112  includes an eblock having a cantilevered actuator arm  116  supporting a load arm  118  that, in turn, supports a read/write transducer (or “head”)  120  in a data transfer relationship with the adjacent disc  110 . 
         [0014]    A recording surface of the disc  110  is divided into a plurality of tracks  122  over which the head  120  is moved. The tracks  122  can have head position control information written to embedded servo sectors. Between the embedded servo sectors are data sectors for storing user data. The head  120  stores input data to the tracks  122  and retrieves output data from the tracks  122 . The output data can be previously stored user data or it can be servo data used to position-control the head  120  relative to a desired track  122 . 
         [0015]    The actuator assembly  112  is positionally controlled by a voice coil motor (VCM)  124  that includes an actuator coil  126  immersed in a magnetic field generated by a magnet assembly  128 . A pair of steel plates  130  (pole pieces) mounted above and below the actuator coil  126  provides a magnetically permeable flux path for a magnetic circuit of the VCM  124 . During operation of the data storage device  100  current is passed through the actuator coil  126  forming an electromagnetic field, which interacts with the magnetic circuit of the VCM  124 , causing the actuator  112  to move the head  120  radially across the disc  110 . 
         [0016]    To provide the requisite electrical conduction paths between the head  120  and data storage device control circuitry, head wires of the head  120  are affixed to a flex circuit  132 . The flex circuit  132  is routed at one end from the load arms  118  along the actuator arms  116 , and is secured to a flex connector at the other end. The flex connector supports the flex circuit  132  where it passes through the base  102  and into electrical communication with a printed circuit board assembly (“PCBA”), mounted to the underside of the base  102 . A preamplifier/driver (preamp) conditions read/write signals passed between the control circuitry and the head  120 . 
         [0017]      FIG. 2  is a diagrammatic depiction of an apparatus for testing the modal frequency response of the cover  104  ( FIG. 1 ) in accordance with embodiments of the present invention. The apparatus includes a fixture  134  that suspends the cover  104  from one end thereof. The cover  104  is thereby free-hanging from the fixture  134 , subject only to a gravitational field. A solenoid  136  is energized by electrically powering a coil  138  having a predetermined number (N) of coil turns. When energized, the solenoid  136  produces a magnetic wave force  140  that excites the cover  104  with a predetermined excitation force. 
         [0018]    A motion sensing measurement device, such as a laser Doppler vibrometer (“LDV”)  142 , detects the resonance response of the cover  104  to the excitation force. The output signal from the LDV, a vibration signature signal, is analyzed by a signal analyzer  144  to provide results in a useful format. In these illustrative embodiments the signal analyzer  144  performs a Fourier transformation on the vibration signature signal from the LDV  142  to produce a mechanical bode plot  146 , showing the resonance and phase relationship of the cover  104  in response to the excitation. Measured values can be compared to a predetermined threshold constructed across all frequencies of interest in order to qualitatively characterize a cover  104  under test. A modal frequency response can be performed to qualify the cover  104  in terms of a comparison of the resonant frequency modes in comparison to predetermined frequency ranges that are desirably avoided. 
         [0019]    A function generator  148  produces a desired excitation force  140  from the solenoid  136  by varying an excitation signal  150  in terms of an input frequency of an excitation voltage. The magnetic wave force  140 , F(x,t), that is generated by the solenoid  136  is a function of time (t) and position (x) as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     F 
                      
                     
                       ( 
                       
                         x 
                         , 
                         t 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       - 
                       
                         1 
                         2 
                       
                     
                      
                     
                       φ 
                       2 
                     
                      
                     
                       
                          
                       
                       
                          
                         x 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where φ is the magnetic flux generated by the solenoid and           is the reluctance between the solenoid and the cover  104  under test. Reluctance           is determined by the distance between the solenoid  136  and the cover  104 . It is inversely proportional to the solenoid cross sectional area, A, and the permeability of the air, μ 0 , within the solenoid  136 : 
         [0000]    
       
         
           
             
               
                 
                   
                     = 
                     
                       
                         
                           x 
                           0 
                         
                         - 
                         x 
                       
                       
                         
                           μ 
                           0 
                         
                          
                         A 
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     
                       
                         
                            
                         
                          
                       
                       
                          
                         x 
                       
                     
                     = 
                     
                       
                         - 
                         1 
                       
                       
                         
                           μ 
                           0 
                         
                          
                         A 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0020]    By magnetic circuit analogy, magnetic motif force is: 
         [0000]    
       
         
           
             
               
                 
                   
                     Ni 
                     = 
                     
                       φ 
                        
                       
                           
                       
                        
                     
                   
                    
                   
                     
 
                   
                    
                   
                     φ 
                     = 
                     
                       
                         Ni 
                       
                       = 
                       
                         
                           Ni 
                            
                           
                               
                           
                            
                           
                             μ 
                             0 
                           
                            
                           A 
                         
                         
                           
                             x 
                             0 
                           
                           - 
                           x 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where N in the number of coil turns and i is the input current into the solenoid  136 . By substituting equations (2) and (3) into (1) yields: 
         [0000]    
       
         
           
             
               
                 
                   
                     F 
                      
                     
                       ( 
                       
                         x 
                         , 
                         t 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       1 
                       2 
                     
                      
                     
                       
                         
                           N 
                           2 
                         
                          
                         
                           μ 
                           0 
                         
                          
                         
                           Ai 
                           2 
                         
                       
                       
                         
                           ( 
                           
                             
                               x 
                               0 
                             
                             - 
                             x 
                           
                           ) 
                         
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0021]    V is the input voltage to the solenoid. V 0  is the amplitude of the voltage and ω is the input angular frequency of the voltage: 
         [0000]      V=V 0  sin ωt   (5) 
         [0022]    By analyzing the electric circuit of the solenoid  136 , the following relationship is defined: 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     = 
                     
                       
                         L 
                          
                         
                           
                              
                             i 
                           
                           
                              
                             t 
                           
                         
                       
                       + 
                       iR 
                     
                   
                    
                   
                     
 
                   
                    
                   
                     
                       
                          
                         i 
                       
                       
                          
                         t 
                       
                     
                     = 
                     
                       
                         
                           
                             V 
                             0 
                           
                           L 
                         
                          
                         sin 
                          
                         
                             
                         
                          
                         ω 
                          
                         
                             
                         
                          
                         t 
                       
                       - 
                       
                         
                           R 
                           L 
                         
                          
                         i 
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0023]    By solving the first order differential equation (6): 
         [0000]    
       
         
           
             
               
                 
                   i 
                   = 
                   
                     
                       [ 
                       
                         
                            
                           
                             
                               - 
                               
                                 R 
                                 L 
                               
                             
                              
                             t 
                           
                         
                         + 
                         
                           
                             
                               V 
                               0 
                             
                             
                               L 
                                
                               
                                 ( 
                                 
                                   1 
                                   + 
                                   
                                     ω 
                                     2 
                                   
                                 
                                 ) 
                               
                             
                           
                            
                           sin 
                            
                           
                               
                           
                            
                           ω 
                            
                           
                               
                           
                            
                           t 
                         
                         - 
                         
                           
                             
                               V 
                               0 
                             
                             L 
                           
                            
                           
                             ω 
                             
                               ( 
                               
                                 1 
                                 + 
                                 
                                   ω 
                                   2 
                                 
                               
                               ) 
                             
                           
                            
                           cos 
                            
                           
                               
                           
                            
                           ω 
                            
                           
                               
                           
                            
                           t 
                         
                       
                       ] 
                     
                     2 
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
         [0024]    Substituting equation (7) into (4) yields a relationship used by the function generator  148  to vary the excitation signal  150  in terms of an input frequency of an excitation voltage to produce a desired excitation force: 
         [0000]    
       
         
           
             
               
                 
                   
                     F 
                      
                     
                       ( 
                       
                         x 
                         , 
                         t 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           
                             N 
                             2 
                           
                            
                           
                             μ 
                             0 
                           
                            
                           A 
                         
                         
                           2 
                            
                           
                             
                               ( 
                               
                                 
                                   x 
                                   0 
                                 
                                 - 
                                 x 
                               
                               ) 
                             
                             2 
                           
                         
                       
                        
                       
                         [ 
                         
                           
                              
                             
                               
                                 - 
                                 
                                   R 
                                   L 
                                 
                               
                                
                               t 
                             
                           
                           + 
                           
                             
                               
                                 V 
                                 0 
                               
                               
                                 L 
                                  
                                 
                                   ( 
                                   
                                     1 
                                     + 
                                     
                                       ω 
                                       2 
                                     
                                   
                                   ) 
                                 
                               
                             
                              
                             sin 
                              
                             
                                 
                             
                              
                             ω 
                              
                             
                                 
                             
                              
                             t 
                           
                           - 
                           
                             
                               
                                 V 
                                 0 
                               
                               L 
                             
                              
                             
                               ω 
                               
                                 ( 
                                 
                                   1 
                                   + 
                                   
                                     ω 
                                     2 
                                   
                                 
                                 ) 
                               
                             
                              
                             cos 
                              
                             
                                 
                             
                              
                             ω 
                              
                             
                                 
                             
                              
                             t 
                           
                         
                         ] 
                       
                     
                     2 
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
         [0025]    Turning now to  FIG. 3  which is a diagrammatic depiction of an apparatus for testing the modal frequency response of the cover  104  ( FIG. 1 ) in accordance with equivalent alternative embodiments of the present invention. As before, the fixture  134  suspends the cover  104  from one end thereof and subjects it to a gravitational field. However, here a cone speaker  152  is energized by an excitation signal  154  to produce an acoustic wave force  156  that excites the cover  104  with a predetermined excitation force. 
         [0026]    Also as above, the LDV  142  detects the resonance response of the cover  104  to the excitation force. The vibration signature signal from the LDV  142  is analyzed by the signal analyzer  144  to produce the mechanical bode plot  146 . 
         [0027]    A function generator  158  produces a desired excitation force  156  from the cone speaker  152  by varying the excitation signal  154  in terms of a sound pressure level, P(r), that is proportional to the excitation voltage. The transient pressure, p f (r,t), can be expressed as follows: 
         [0000]        p   f ( r,t )= P ( r ) cos[2π ft +φ( r )]  (1) 
         [0000]    where f is the acoustic frequency, and φ(r) is the phase state of the acoustic wave at radius r. A change in frequency f will excite instantaneous sound pressure, which causes energy density change in terms of: 
         [0000]        e   Δf ( r,t )= p (Δ f,r,t ) 2   /ρc   2   =P ( r ) 2  cos 2 [2πΔ ft +Δφ( r )]/ρ c   2    
         [0000]    where ρ is the density of air and c is the speed of sound. The acoustic force F(r,t) at an object of area dS with drag coefficient d r (r) is calculated based on the sound energy density change in Equation (2): 
         [0000]        F   Δf ( r,t )= d   r ( r ) dS×e   Δf ( r,t ) 
         [0000]    or, in terms of the sound pressure, P(r): 
         [0000]        F   Δf ( r,t )= d   r ( r ) dS×P ( r ) 2  cos 2 [2πΔ ft +Δφ( r )]/ρ c   2    
         [0028]    Generally, the embodiments described contemplate a modal frequency response tester wherein a fixture operably suspends a test object in a gravitational field. The tester also possesses a means for qualitatively characterizing the test object in relation to observing its modal frequency response to a free-state waveform excitation force. 
         [0029]    For purposes of this description and meaning of the appended claims, the phrase “means for qualitatively characterizing” expressly means the structural aspects of the embodiments disclosed herein and the structural equivalents thereof. For example, without limitation, the meaning of “means for qualitatively characterizing” expressly does not include previously attempted solutions that employ a mechanical contacting engagement with the test object to deliver the excitation.  FIG. 4 , for example, are test results obtained during experimentation comparing the results of shaker testing to that of acoustic testing with the apparatus depicted in  FIG. 3 . The skilled artisan will note the amount of distortion in the shaker test modal frequency response that is effectively eliminated by the free-state excitation of the claimed embodiments.  FIG. 5  likewise shows the elimination of distortion in the modal frequency response by using the magnetic testing apparatus of  FIG. 2  in comparison to shaker testing. 
         [0030]    It is to be understood that even though numerous characteristics and advantages of various embodiments of the present invention have been set forth in the foregoing description, together with details of the structure and function of various embodiments of the invention, this detailed description is illustrative only, and changes may be made in detail, especially in matters of structure and arrangements of parts within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. For example, the particular elements may vary in type or arrangement without departing from the spirit and scope of the present invention. 
         [0031]    In addition, although the embodiments described herein are directed to a data storage device, it will be appreciated by those skilled in the art that the claimed subject matter is not so limited and various other applications can utilize the present embodiments without departing from the spirit and scope of the claimed invention.