Abstract:
An apparatus and associated method contemplating a head gimbal assembly (HGA) and a bridge circuit including first and second microactuators attached to the HGA. Computation logic is connected to the bridge circuit and configured to independently measure an electrical output of each microactuator and to sum the electrical outputs to derive a value related to a vertically dominant modulation mode of the HGA.

Description:
RELATED APPLICATION 
       [0001]    This is a continuation application claiming benefits of the earlier filing date of U.S. patent application Ser. No. 14/607,671 entitled BIMODAL MODULATION 
     
    
     SUMMARY 
       [0002]    Various embodiments of the present technology are generally directed to the construction and use of microactuator circuitry to detect motion of an object. 
         [0003]    Some embodiments of this disclosure contemplate a head gimbal assembly (HGA) and a bridge circuit including first and second microactuators attached to the HGA. Computation logic is connected to the bridge circuit and configured to independently measure an electrical output of each microactuator and to sum the electrical outputs to derive a value related to a vertically dominant modulation mode of the HGA. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  depicts a top view of an actuator in a disc drive data storage device. 
           [0005]      FIG. 2  diagrammatically depicts a top view of the microactuator in the neutral position in the disc drive in  FIG. 1 . 
           [0006]      FIG. 3  is similar to  FIG. 2  but depicting the microactuator having rotated the head gimbal assembly (HGA). 
           [0007]      FIG. 4  diagrammatically depicts the piezoelectric effect in the PZT elements in the microactuator in  FIG. 3 . 
           [0008]      FIG. 5  is a side view of  FIG. 2  in the neutral position of the microactuator. 
           [0009]      FIG. 6  is similar to  FIG. 5  but depicting vertical-dominant mode deflection of the microactuator. 
           [0010]      FIG. 7  diagrammatically depicts the piezoelectric effect in the PZT elements in the microactuator in  FIG. 6 . 
           [0011]      FIG. 8  is a flowchart depicting steps in a method for performing in situ testing in accordance with embodiments of this technology. 
           [0012]      FIG. 9  is a schematic depiction of circuitry employed to obtain the bimodal characterization of the HGA modulation in accordance with illustrative embodiments of this technology. 
           [0013]      FIG. 10  is a bimodal frequency response function (FRF) signature of the disc drive in  FIG. 1 . 
           [0014]      FIG. 11  is an enlarged portion of the FRF signature in  FIG. 10 . 
           [0015]      FIG. 12  is an enlarged portion of the FRF signature in  FIG. 10 . 
           [0016]      FIG. 13  graphically depicts the PZT response at a constant resonant frequency in relation to varying heater power to identify head/disc contact. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    A wide variety of machines employ microactuators for precise control of moving parts. A “microactuator” for purposes of this description includes a device that mechanically deforms when subjected to an external excitation, such as a driving voltage, and that develops a sense voltage in proportion to deformation. Because of this dual functionality these devices are commonly referred to as a self-sensing actuator. For purposes of this illustrative description the microactuator can include a piezoelectric transducer (PZT), although the contemplated embodiments are not so limited. In alternative embodiments the microactuator can otherwise be constructed to include things such as a magnetorestrictive element or a piezomagnetic element and the like. 
         [0018]    For example,  FIG. 1  depicts part of a disc drive data storage device that employs a microactuator for movement control. The disc drive has a rotatable actuator  100  that is precisely moved to position a read/write head at its distal end in alignment with a data storage track formed in the surface of a rotating magnetic recording medium  102 . The read/write head, which can be a perpendicular magnetic head or a lateral magnetic head, reads and writes information by detecting and modifying the magnetic polarization of the recording layer on the surface of the storage disc  102 . The actuator  100  has a central body  106  that is journalled for rotation around an axis of rotation  108  of a bearing  110 . A voice coil  112  extends from the body  106  where it is immersed in a magnetic field from opposing magnets (only the bottom magnet  114  is depicted in  FIG. 1  for clarity sake). An arm  116  extends from the body  106  opposite the voice coil  112 . A plate  118  is attached to the distal end of the arm  116 , such as by the crimp  119  depicted in  FIG. 1 . 
         [0019]    Dual stage motion control is provided in that the voice coil  112  provides coarse position control and a microactuator  120  provides fine position control of the read/write head. The microactuator  120  includes a pair of PZT elements  122 ,  124  that selectively move a head gimbal assembly (HGA)  128  relative to the body  106 . The HGA  128  includes the plate  118 , a bulkhead  126  against which the PZTs  122 ,  124  engage, a load beam  129 , and a gimbal  130  extending from the load beam  129  that, in turn, supports the read/write head. 
         [0020]    The read/write head has a slider that is aerodynamically designed to be supported upon an air bearing created by rotation of the storage disc  102 . The surface of the slider closest to the storage disc  102  is referred to as an air bearing surface (ABS). The HGA  128  is designed to properly maintain the ABS at a desired fly height and orientation apart from the surface of the storage disc  102 . A heater  127  is provided in the read/write head to control the fly height by pole tip protrusion. The HGA  128  also includes a flexible circuit (trace)  131  that transmits electrical signals and power to the read/write head and other electronics such as the heater  127 . 
         [0021]      FIG. 2  diagrammatically depicts the PZT elements  122 ,  124  at a neutral position corresponding to a non-driven excitation state. Controlled application of a driving voltage to the PZT elements  122 ,  124  deforms them to rotate the HGA  128  relative to the body  106  as depicted in  FIG. 3 . This arrangement can be constructed by poling the PZTs across the thickness (normally done in practice) and flipping the polarities between the two PZTs in 
         [0022]      FIG. 2 . By the piezoelectric effect, applying a driving voltage that is opposite to the poled polarity of the PZT  124  causes it to shorten, whereas applying the driving voltage that is the same as the poled polarity of the PZT  122  causes it to lengthen. Shortening the PZT  124  and lengthening the PZT  122  causes the counter-clockwise rotational displacement depicted in  FIG. 3 . Reversing the polarity of the driving voltage imparts a clockwise rotational displacement in similar fashion. 
         [0023]    Driving the PZTs  122 ,  124  to achieve this rotational movement provides fine positioning control of the read/write head relative to the data storage tracks; the primary purpose of the microactuator  120  in the disc drive. The skilled artisan readily understands that the fine positioning can be achieved in alternative ways, regardless of the poled polarities of the PZTs  122 ,  124 , such as by providing individual voltages to them and perhaps phase shifting one or both of the individual driving voltages. 
         [0024]    The storage disc  102  operably rotates at high speeds, subjecting the HGA  128  to forces and exciting resonance that can significantly alter the position of the ABS. For example, these forces can distort the HGA  128  enough to create a pitch static angle that alters the flying orientation of the read/write head. Modulation-related failures are typically caused by excitation of HGA resonances. These forces and resonances can make the HGA  128  unstable for its intended purpose when it becomes unacceptably sensitive to disturbances (can be particle or contaminant interactions, for example) at the head-disc interface. Such sensitivities can cause modulation-related failures rendering the disc drive unreliable and short lived. 
         [0025]    HGA resonance stems from a wide range of excitation mechanisms and failure modes. The complex structure of the HGA results in a large number of structural modes, making it difficult to entirely design them out of the disc drive. Further, the modal response of the HGA can vary significantly from part to part. For this reason modulation failures may be experienced only on a portion of a drive population. This makes individual characterization of the modal response of each individual HGA important. Therefore, what is needed is an in situ test that compares the HGA modulation to an expected threshold to identify characteristic HGA modulation issues during the manufacturing process and thereafter. It is to those improvements and others that the embodiments of the present technology are directed. 
         [0026]    Referencing back to  FIG. 3  momentarily, recall that the counterclockwise HGA  128  rotation requires a shortening (compression) of the PZT  124  and a lengthening (tension force) on the PZT  122 . External forces acting on the HGA  128  in the offtrack dominant mode create the same result; they compress the PZT  124  and lengthen the PZT  122 .  FIG. 4  diagrammatically depicts the PZTs  122 ,  124  with their polarities swapped in relation to the longitudinal poling axis  129 . The arrows  131  facing each other represent the compressive force acting on the PZT  124  and the arrows  133  facing away from each other represent the tensile force acting on the PZT  122 . By the piezoelectric effect, the compressive force  131  acting upon the positively poled PZT  124  produces a positive sense voltage. The tensile force  133  acting upon the negatively poled PZT  122  likewise produces a positive sense voltage. For purposes of this description and meaning of the appended claims, when the two PZTs  122 ,  124  strain in the same direction at the same time, as indicated by the polarity of their sense voltages, then the mode is termed a symmetric mode. 
         [0027]      FIG. 5  is a side view of  FIG. 2  in the vertically-neutral position, and  FIG. 6  depicts external forces acting on the HGA  128  in the vertically-dominant mode. In this case both PZTs  122 ,  124  are lengthened as they are flexed downward.  FIG. 7  is similar to  FIG. 4  except that in this case both of the PZTs  122 ,  124  are subjected to tensile forces, represented by the opposing arrows  135 ,  137  facing away from each other due to the flexing. By the piezoelectric effect, the tensile force acting upon the positively poled PZT  124  produces a negative sense voltage (−V). The tensile force acting upon the negatively poled PZT  122  produces a positive sense voltage (+V). For purposes of this description and meaning of the appended claims, when the two PZTs  122 ,  124  strain in opposite directions at the same time, as indicated by the polarity of their sense voltages, then the mode is termed an asymmetric mode. 
         [0028]      FIG. 8  is a flowchart depicting steps in a method for performing an in situ HGA modulation test on a disc drive in accordance with illustrative embodiments of the present technology. The method begins in block  150  by performing in situ testing. For purposes of this description, the meaning of an “in situ” test is limited to testing that is performed using only the hardware and logic in place within the device under test; in this example within the disc drive. The in situ test specifically does not include tests that include adding extraneous data collection devices to measure data or perform a function that the disc drive is incapable of performing with the components already in place for its intended use as a data storage device. The in situ testing in block  150  can be limited to only the HGA modulation testing of this technology or it can be included as part of other testing. For example, the in situ testing  150  can be performed as part of the drive certification stage during manufacturing of the disc drive. Subsequently, the in situ testing  150  can be performed as part of a screening test performed on disc drives that are returned from commercial use for repair or refurbishing. 
         [0029]    In block  152  the disc drive&#39;s bimodal (symmetric and asymmetric modes) frequency response function (FRF) signature is obtained.  FIG. 9  is a schematic depiction of microactuator circuitry  200  for obtaining the bimodal FRF in accordance with illustrative embodiments of the present technology. The microactuator circuitry  200  generally includes a computation module  202  connected to a bridge circuit  204 . In these illustrative embodiments the bridge circuit  204  includes two separate Wheatstone bridge circuits  206 ,  208  connected in parallel to a voltage source  210 . Each of the PZTs  122 ,  124  is schematically represented as a voltage component in series with a capacitance element. The PZT  122  is included in one branch of the Wheatstone bridge  206 , and the PZT  124  is included in one branch of the Wheatstone bridge  208 . 
         [0030]    In these illustrative embodiments the PZTs  122 ,  124  are driven independently of each other by the excitation source, in this case the voltage source  210 . Driving the PZTs  122 ,  124  results in unbalanced voltages V out1 , V out2  that are each proportional to the strain in the respective individual PZT  122 ,  124 . The computation module  202  has a summing circuit  212  that independently measures the electrical output of the PZT  122  in terms of the differential voltage V a1 −V b1 . For a given excitation input V in , the voltage sensed in leg (C 2 -C 3 ) is: 
         [0000]    
       
         
           
             
               V 
               
                 b 
                  
                 
                     
                 
                  
                 1 
               
             
             = 
             
               
                 
                   C 
                   3 
                 
                 
                   
                     C 
                     2 
                   
                   + 
                   
                     C 
                     3 
                   
                 
               
                
               
                 V 
                 in 
               
             
           
         
       
     
         [0031]    Similarly, for the opposing leg (C 1 -PZT) the sensed voltage is: 
         [0000]    
       
         
           
             
               V 
               
                 a 
                  
                 
                     
                 
                  
                 1 
               
             
             = 
             
               
                 
                   
                     C 
                     p 
                   
                   
                     
                       C 
                       1 
                     
                     + 
                     
                       C 
                       p 
                     
                   
                 
                  
                 
                   V 
                   in 
                 
               
               + 
               
                 V 
                 
                   pzt 
                    
                   
                       
                   
                    
                   1 
                 
               
             
           
         
       
     
         [0032]    Therefore. the unbalanced voltage in the Wheatstone bridge circuit  206  is: 
         [0000]    
       
         
           
             
               
                 V 
                 
                   a 
                    
                   
                       
                   
                    
                   1 
                 
               
               - 
               
                 V 
                 
                   b 
                    
                   
                       
                   
                    
                   1 
                 
               
             
             = 
             
               
                 
                   ( 
                   
                     
                       
                         C 
                         p 
                       
                       
                         
                           C 
                           1 
                         
                         + 
                         
                           C 
                           p 
                         
                       
                     
                     - 
                     
                       
                         C 
                         3 
                       
                       
                         
                           C 
                           2 
                         
                         + 
                         
                           C 
                           3 
                         
                       
                     
                   
                   ) 
                 
                  
                 
                   V 
                   in 
                 
               
               + 
               
                 V 
                 
                   pzt 
                    
                   
                       
                   
                    
                   1 
                 
               
             
           
         
       
     
         [0033]    If C1=C2, C3≈Cp, then V out1 =V a1 −V b1 =V pzt1    
         [0034]    Similarly, the computation module  202  has another summing circuit  214  that independently measures the electrical output of the PZT  124  in terms of V out2 =V a2 −V b2 =V pzt2 . The asymmetric modes are calculated in terms of: 
         [0000]    
       
      
       V 
       out−asymmetric 
       =V 
       out1 
       +V 
       out2  
      
     
         [0035]    The symmetric modes are calculated in terms of: 
         [0000]    
       
      
       V 
       out−symmetric 
       =V 
       out1 
       −V 
       out2  
      
     
         [0036]    The PZTs  122 ,  124  are driven independently of each other by use of the separate Wheatstone bridges. In these embodiments the computation module  202  includes a voltage generator  211  capable of selectively altering electrical frequency of the driving voltage V in . The driving voltage is varied within a predetermined frequency range, and the resultant PZT  122 ,  124  sense voltages are simultaneously sampled at each of these frequency points. The unbalanced voltages are recorded throughout the frequency range to characterize the HGA  128  modulation by an FRF signature.  FIG. 10  graphically depicts an illustrative FRF signature  220  for the frequency range between zero to 50,000 Hz. 
         [0037]    Returning to  FIG. 8 , in block  154  the obtained FRF signature  220  can be compared to predefined thresholds to qualitatively characterize the disc drive&#39;s HGA modulation. For example,  FIG. 11  depicts an enlarged portion of the FRF  220  in  FIG. 10  at the 20 k Hz resonant frequency. The modal gain for the FRF  220  differs from that of a preexisting threshold FRF (or stored value, T) by a variance depicted by numeral  219 . For another example,  FIG. 12  is similar to  FIG. 11  except depicting the resonant frequency for the FRF  220  differs from that of the threshold T by a variance depicted by numeral  221 . The thresholds can be set by examining the modal response (FRF) of sample sets of HGAs and using statistics representative of this sample population. In these examples the modal gain values and/or resonant frequency values can be compared to expected threshold values, and if that comparison passes muster then the disc drive is deemed acceptable for commercial use. For example, the disc drive can be qualitatively approved if these values vary by no more than X%: 
         [0000]    
       
         
           
             
               
                 
                    
                   
                     
                       Gain 
                       T 
                     
                     - 
                     
                       Gain 
                       220 
                     
                   
                    
                 
                 
                   Gain 
                   220 
                 
               
               * 
               100 
             
             &lt; 
             X 
           
         
       
     
         [0038]    On the other hand, disc drives having statistically derived outlier values can be identified and heuristic rules employed to sort and correspondingly further test or rework the disc drive under test. 
         [0039]    Referring to  FIG. 8 , in block  156  the obtained FRF signature  220  can be stored in a computer memory for use as a historical baseline reference of that disc drive&#39;s HGA modulation at the time of in situ testing. The FRF signature  220  can be stored in memory either internal or external to the disc drive. 
         [0040]    In block  158  the computation module  202  tests the disc drive for proper fly height to ensure no unexpected contact will occur between the read/write head and the storage media (head/disc contact). Preferably, the computation module  202  first ascertains a resonant frequency from the FRF signature  220 , such as  222  at 20 k Hz depicted in  FIG. 10 . The computation module  202  then drives the PZTs  122 ,  124  substantially at that constant resonant frequency and varies the power to the heater  127  to deflect the read/write head toward the storage media.  FIG. 13  graphically depicts the PZT  122 ,  124  response increases and then decreases around the power setting P c  at which head/disc contact is made. Empirical testing and statistical regression can be employed to define a range  224  within which contact is expected at the selected resonant frequency. The range  224  is defined between a lower limit power (P LL ) and an upper limit power (P UL ). If P c  is within the predetermined range then the disc drive is deemed acceptable for commercial use. In addition this information can be used to filter the servo inputs driving micro-actuation to avoid these resonances during normal drive operation. 
         [0041]    It is to be understood that even though numerous characteristics of various embodiments of the present disclosure have been set forth in the foregoing description, together with details of the structure and function of various embodiments, 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 disclosure to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.