Abstract:
The active vibration suppression of a slider suspension arm is achieved by including as part of the suspension arm two bimorph piezoelectric elements. A first bimorph piezoelectric element is attached to the top of the suspension arm and acts as a sensor. A second bimorph piezoelectric element is attached to the bottom of the suspension arm and acts as an actuator. The actuator is controlled as a function of a voltage measured by the sensor so that the actuator damps periodic vibrations occurring in the suspension arm.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority from provisional application Ser. No. 60/101,948 filed on Sep. 25, 1998, for “Active Vibration Suppression of Glide Head Suspension Arm” by Wei H. Yao, Ramesh Sundaram, and David S. Kuo. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to slider suspension arms, and more particularly to a slider suspension arm having a bimorph sensor and bimorph actuator used to suppress vibration in the suspension arm. 
     As the density of data recorded on magnetic discs continues to increase, the flying height of magnetic transducers with respect to the disc must be reduced to accurately read and write information on the disc. As a result, the magnetic recording disc must accommodate the lower fly height of the transducer and the slider supporting it, meaning that the disc surface must be extremely smooth and uniform. In order to certify that a magnetic disc is adequately smooth for use in a disc drive system, glide height tests are performed on the disc. 
     In addition to the general requirement of reduced fly height, magnetoresistive (MR) heads are extremely sensitive to small physical defects in the surface of the disc, such as undulations on the disc surface and microscopic debris on the disc. When the MR head strikes a defect, there is a momentary frictional heating of the MR element, known as a thermal asperity. This heating effect increases the resistance of the MR head, which causes data errors and loss of information in reading the disc. Thus, it is important to ensure the surface of any disc is relatively free of defects which may adversely affect the ability of the MR head to function. 
     Thus, one of the final steps in manufacturing a disc is to perform a glide height test. In conducting a glide height test, a single disc is placed on a spin stand and the disc is spun at extremely high speeds, often approaching over 10,000 revolutions per minute (rpm). A glide head suspended on a suspension arm is moved across the surface of a disc as the disc is spun. A typical glide head often comprises a piezoelectric transducer mounted on an air bearing slider. During the glide height test, the glide head “flies” over a disc surface at a predetermined height above the disc surface, known as the glide height. If contact occurs between the glide head and a disk asperity or a defect, the glide head is forced to vibrate and deform. 
     The slider deformation results in corresponding deformation of the piezoelectric transducer, and creates a potential difference between the electrodes of the piezoelectric element. When the contact occurs, many vibration modes of the piezoelectric element and slider are excited simultaneously, and each mode generates a voltage at its specific frequency. Signals generated by the piezoelectric element are fed to a pre-amplifier and a band pass filter. A digital data acquisition system on the glide tester then processes the filtered data, which can then be used to determine whether the disc passes or fails the glide height test. Should the disc fail a glide height test, it is possible to use a burnishing head to attempt to smooth out surface asperities. 
     One problem in performing a glide height test is the potential for the suspension arm to experience vibrations during the glide height test. Vibrations occur in the suspension arm during glide tests due to a variety of causes. First, the windage created by the disc as the disc is spun is very strong and can create vibration in the suspension arm. Vibration can also occur as the suspension arm is moved across the surface of the disc by an actuator motor. Finally, should a minor defect be encountered on the surface of the disc, such a defect may also cause the suspension arm to vibrate. If the suspension arm begins to vibrate during a glide test, the results of the glide test are much less actuate. Vibration in the suspension arm results in an uneven fly height of the glide head. Uneven fly height in turn results in the glide head missing some defects, or over-detecting minor defects which may not have an effect on the ultimate functioning ability of the disc. 
     In addition to glide head suspension assemblies, the same problem occurs in other slider suspension assemblies. Vibrations in the suspension arm which carries a magnetoresistive (MR) head will similarly result in an uneven fly height of the MR head over the surface of a disc, which in turn adversely affects the ability of the MR head to read data from the disc and write data to the disc. Vibrations may also occur in suspension arms which carry burnishing heads. Vibrations in a burnishing head assembly result in an uneven fly height of the burnishing head, which adversely affects the ability of the burnishing head to accurately burnish a disc asperity. 
     Thus, there is a need in the art for a slider suspension arm which can counteract the effects of vibration occurring in the suspension arm as the slider is flown over a rotating disc. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is an improved slider suspension assembly which acts to control and suppress any vibration which may occur in the suspension arm as the slider is moved over the surface of a rotating disc. Included as part of the suspension arm are two bimorph piezoelectric elements. A first piezoelectric bimorph element is attached to the top of the suspension, and a second piezoelectric bimorph element is attached to the bottom of the suspension. One of the piezoelectric bimorph elements acts as an actuator while the other element acts as a sensor. 
     As the suspension arm vibrates, a first piezoelectric bimorph element acts as a sensor. The vibration in the suspension arm causes the first piezoelectric bimorph element to likewise vibrate. As the first piezoelectric bimorph element is deformed due to the vibration, it generates a voltage. The voltage indicates the bending vibration frequency and amplitude of the suspension arm. Because the bending vibration of the suspension arm is repetitive, the other piezoelectric bimorph element acting as an actuator can be used to actively damp the vibration by providing a voltage signal to the second element which is 180 degrees out of phase with the sensed signal. Once the vibration of the suspension arm is damped, a precise fly height of the slider can be achieved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top view of a glide height test system for supporting a glide head over the surface of a disc. 
     FIG. 2 is a top view of a glide head suspension assembly. 
     FIG. 3 is a cross-sectional view the glide head suspension assembly of FIG. 2 taken along line  3 — 3 . 
     FIG. 4 is a diagram of a cantilevered bimorph piezoelectric bending motor configured for parallel operation. 
     FIG. 5 is a diagram of a cantilevered bimorph piezoelectric bending motor configured for series operation. 
     FIG. 6 is a block diagram indicating the process of suppressing a vibration in a glide head suspension arm. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a top view of a disc test system  10  supporting a glide head  24  over the surface of a disc  30 . The test system  10  includes an actuator motor  12  arranged to rotate an actuator arm  16  around an axis  14  on a support spindle  15 . The suspension arm  18  is connected to the actuator arm  16  at a mounting block  20 . A gimbal  22  is connected to an end of the suspension arm  18 , and carries a slider or glide head  24 . The disc  30  rotates around its axis  32  so that windage is encountered by the glide head  24  to keep it aloft a small distance (the glide height) above the surface of the disc  30 . 
     When conducting a glide height test, the disc  30  is rotated so that the glide head  24  flies over the surface of the disc  30 . When any part of the glide head  24  contacts a protrusion or other irregularity in the surface of the disc  30 , sensors on the glide head  24  indicate this fact. To ensure the glide head  24  detects all asperities or defects on the disc surface, it is important that the glide head  24  maintain a uniform fly height. 
     Vibration in the suspension arm  18  results in an uneven fly height of the glide head  24  over the surface of the disc  30 . The suspension arm  18  has been known to experience vibration due to the strong wind created by the disc  30  as the disc  30  is spun at extremely high speeds. In addition, the actuator motor  12  may cause vibration in suspension arm  18  as the motor  12  moves the actuator arm  16  across the surface of the disc  30 . Finally, the suspension arm  18  may vibrate as a result of the glide head  24  encountering a protrusion or irregularity on the surface of the disc  30 . 
     FIG. 2 is a side view of a glide head suspension assembly  32  which controls and suppresses any vibration which may occur in the suspension arm  18  during a glide height test. FIG. 2 more clearly shows the suspension arm  18 , the gimbal  22 , and the glide head  24 . The gimbal  22  comprises a tongue portion  34  and an aperture  36 . The suspension arm  18  has a pre-load tip  38  located at its distal end and a bimorph actuator  40  located on its surface. 
     The suspension arm  18  provides a support structure for the glide head suspension assembly  32  and carries the glide head  24 . The suspension arm  18  is formed to apply a pre-load force against the glide head  24  at the pre-load tip  38 . This pre-loading serves to bias the glide head  24  toward the surface of a disc. The gimbal  22  is attached under the suspension arm  18 , and the glide head  24  is attached to the gimbal  22 . In an exemplary embodiment, the gimbal  22  is shaped with an arcuate end portion at a proximal end  22   a , and arm portions  22   b ,  22   c  extending toward its distal end, forming the aperture  36  between the arm portions  22   b ,  22   c . The slider  24  is attached to the tongue portion  34  at the distal end of the gimbal  22 . The gimbal  22  is designed to impart flexibility to the glide head  24  and allow the glide head  24  to follow the surface of the disc more closely than if the glide head  24  were mounted directly on the suspension arm  18 , as is well known in the art. 
     The bimorph actuator  40  is located on the top surface of the suspension arm  18 . The bimorph actuator  40  is controlled to dampen any vibration occurring in the suspension arm  18 . A similarly shaped sensor  42  is located on the bottom surface of the suspension arm  18  (shown in FIG. 3) which is used to sense the vibration occurring in the suspension arm  18 . In an exemplary embodiment, the actuator  40  is triangularly shaped to fit the shape of the suspension arm  18  and is affixed to the suspension arm  18  using an epoxy or suitable adhesive. 
     FIG. 3 is a cross section of the glide head suspension assembly  32  taken along line  3 — 3  of FIG.  2 . FIG. 3 shows both the actuator  40  and a bimorph sensor  42 . The bimorph sensor  42  is located on the bottom of suspension arm  18  between the gimbal  22  and the suspension arm  18 . Much like the bimorph actuator  40 , the bimorph sensor  42  is affixed to the gimbal  22  and suspension arm  18  using a suitable epoxy or adhesive. In one embodiment, the bimorph actuator  40  and sensor  42  each comprise a bimetal strip with two sheets of piezoelectric material of opposite polarity adhered together to form a bending element. 
     A piezoelectric material is used in the bimorph sensor  42  and bimorph actuator  40  due to the useful properties exhibited by piezoelectric materials. Piezoelectric materials generate an electrical response to a mechanical stimulus. Thus, as the piezoelectric material is deformed, due to vibrations, the material generates a voltage representative of the strain of deformation. Correspondingly, applying a voltage to the piezoelectric material causes it to deform. 
     Because the bimorph sensor  42  is affixed to the suspension arm  18 , any vibration in the suspension arm  18  also causes the bimorph sensor  42  to vibrate. When the piezoelectric material of the bimorph sensor  42  vibrates, it generates a voltage. This voltage can be measured and used to determine certain information relating to the vibration, such as its frequency and amplitude. The bimorph actuator  40  can then be used to dampen the vibration by applying a corresponding voltage to the actuator  40 . The suspension arm  18  is fabricated from a relatively flexible material. A voltage can be applied to the actuator  40  to cause it to deform, and as the actuator  40  deforms, so does the suspension arm  18 . As a result, the actuator  40  can be used to hold the glide head at a desired uniform fly height by damping vibrations in the suspension arm  18 . 
     FIGS. 4 and 5 show in greater detail suitable bimorph configurations for use in the present invention and illustrate how the piezoelectric material can be configured to achieve a desired deformation based on an applied voltage. Shown in FIG. 4 is a cantilevered bimorph piezoelectric strip  50  configured for parallel operation. The bimorph piezoelectric strip  50  is restrained at one end by connection to a solid object  52 . The bimorph piezoelectric strip  50  includes a bottom electrode  54 , a first piezoelectric element  56 , a shared electrode  58 , a second piezoelectric element  60 , and a top electrode  62 . In the “parallel” configuration depicted in FIG. 4, the piezoelectric elements  56  and  60  are poled in the direction of arrows  70  and  72 . A first voltage is applied at the terminal  64  to the bottom electrode  54 , and at the terminal  68  to the top electrode  62 . A second voltage is applied at the terminal  66  to the shared electrode  58 . 
     In the “parallel” configuration, one piezoelectric element  60  will contract, and the other piezoelectric element  56  will expand, in response to the first and second voltages applied at the terminals  64 ,  66 , and  68 . The result is a bending motion (shown in dashed lines) of the bimorph piezoelectric strip  50 , since one end of the motor is restrained by its connection to a solid object  52 . The amount of bending of the strip  50  is precisely controlled by the voltages applied to the terminals  64 ,  66 , and  68 . Applying opposite voltages to the terminals  64 ,  66 , and  68  causes similar bending in the opposite direction. 
     FIG. 5 is a diagram illustrating a cantilevered bimorph piezoelectric strip  50  configured for series operation. Just as in FIG. 4, bimorph piezoelectric strip  50  includes bottom an electrode  54 , a first piezoelectric element  56 , a shared electrode  58 , a second piezoelectric element  70 , and a top electrode  62 . The piezoelectric element  60  is poled in the direction of arrow  74  and the piezoelectric element  56  is poled in the opposite direction, shown by arrow  76 . A first voltage is applied at the terminal  68  to one piezoelectric element  60 , and a second voltage is applied at the terminal  64  to the other piezoelectric element  56 . As a result, bimorph piezoelectric strip  50  bends as indicated in dashed lines, since one end of the strip is restrained by the connection to solid object  52 . Applying opposite voltages to terminals  64  and  68  causes similar bending in the opposite direction. 
     The “series” configuration is the simplest and most economical, since it requires only two connections to the outside surfaces of piezoelectric elements  56  and  60 . However, the “series” configuration yields less deflection per volt of applied potential than the “parallel” configuration shown in FIG.  4 . The “parallel” configuration is more complex, requiring three electrical connections, the additional connection being made to shared electrode  58 . Either the parallel configuration of FIG. 4 or the series configuration of FIG. 5 are acceptable for use in the glide head suspension assembly  32 . 
     One suitable piezoelectric material for use in the present invention is a polyvinylideneflouride (PVDF) film. When using a PVDF film, the amount of tip deflection and the force developed are given by the following formulas: 
     
       
         Δ x =¾ d   31 (l 2   /t   2 ) V   
       
     
     
       
           F ={fraction (3/2)} Y w d   31 ( t/l ) V   
       
     
     In the above formula, Δx is the displacement at a DC voltage, F is the generated force, d 31  is the piezoelectric coefficient in the length direction l, w, t, and l, are the width, thickness, and length, respectively, of the PVDF film, V is the applied voltage, and Y is Young&#39;s Modulus of the PVDF film. PVDF film has a Young&#39;s Modulus of Y=4×10 9  giga pascals (GPa) and a piezoelectric coefficient of d 31 =23×10 −12 . For a piece of PVDF film having a length, thickness, and width of 2×10 −3 , 9×10 −6 , and 2×10 −3  meters respectively, applying 120 volts results in a deflection Δx of 57 micrometers (μm) and a generated force of 0.07 grams force. 
     A voltage of between 0 and 120 volts can thus be applied to the actuator  40  to cause it to deflect a desired amount (up to 57 μm in one embodiment) and generate a desired force. If a higher force is desired, a multi-layer construction having more piezoelectric layers is possible. The resulting output force is proportionally increased by the number of piezoelectric layers added. 
     FIG. 6 is a block diagram explaining the control structure of the glide head suspension assembly. The control structure comprises a sensor  80 , am actuator control system  82 , and an actuator  84 . The sensor  80  provides input to the actuator control system  82 , and the actuator control system  82  sends a signal to the actuator  86  to control the actuator  86  based on the sensor&#39;s  80  input. 
     The sensor  80  comprises a layer of bimorph material located on the suspension arm  18 . The layer can be affixed to either the top of the suspension arm  18 , or the bottom. As explained above, since the bimorph material is affixed to the suspension arm  18 , as the arm vibrates, the bimorph material also vibrates. As the bimorph material is deformed by the vibrations, the bimorph material generates a voltage. In an exemplary embodiment, the sensor  80  may be realized by the structure associated with the bimorph sensor  42  shown in FIG.  3 . 
     Vibrations in the suspension arm  18  occur at a low frequency, between about 1 kHz to about 10 kHz, and low frequency vibrations typically have a high amplitude. The voltage from the sensor  80  is representative of the deformation occurring in the suspension arm  18 , making it possible to use the voltage to determine both the frequency and the amplitude of the vibration. Once the voltage is measured from the sensor  80 , it must be analyzed to determine whether there is a low frequency portion indicating a periodic vibration occurring in the suspension arm  18 . 
     The actuator control system  82  receives the voltage signal from the sensor  80  and operates to detect repetitive characteristics in the voltage signal to determine whether the vibration is periodic. If the voltage characteristic indicates a periodic vibration, the actuator  84  can be used to damp the vibration. There are several options well known in the art for determining whether the voltage signal indicates a repetitive vibration. For instance, a phase locked loop (PLL) can be used, as can Fourier transform signal processing. 
     If a repetitive vibration is found, a voltage can be input from the actuator control system  82  to the actuator  84  to dampen that vibration. The actuator control system  82  may contain control circuitry which filters, inverts, delays or in some manner transforms the signal from the sensor  80  to control the actuator  84  in such a way as to cause the actuator  84  to damp the vibration in the suspension arm  18 . For example, one method of damping the vibration is to simply invert the signal received from the sensor  80  and input it to the actuator  84  using an inverter  83  as part of the actuator control system  82 . The inverter  83  sends a periodic signal identical to that sensed by the sensor  80 , except out of phase by 180 degrees, to the actuator  84 . It is also possible to simply delay the signal by 180 degrees, and then input it to the actuator  84 . 
     The actuator  84  is also located on the suspension arm, typically placed on an opposite side from the sensor  80 . In an exemplary embodiment, the actuator  84  may be realized by the structure associated with the bimorph actuator  40  shown in FIG.  3 . The signal sent to the actuator  84  generates a force in the actuator  84  and causes the piezoelectric material to undergo a deformation opposite the deformations experienced by the sensor  80 . As a result, the net deformation experienced by the suspension arm is zero, and the vibrations occurring in the suspension arm are damped. 
     Using this method to damp vibrations in the suspension arm is extremely effective. In an exemplary embodiment, it is possible to collect the information relating to the vibration and generate an appropriate signal to control the actuator to damp the vibrations completely within  10  periods of sensing the vibration. Further, the sensor  80  can be used monitor the effectiveness of the actuator  84 . If the actuator  84  is not damping the vibration, sensor  80  will continue to sense the vibration and thus continue to send a signal to the actuator control system  82 . The actuator control system  82  can then signal actuator  84  to damp the continuing vibration. Thus, the fly height of the glide head can be kept more uniform throughout the fly height testing process. 
     Though the invention has been discussed in terms of being used on a glide height test suspension assembly, the invention is not so limited. The present invention can be used to control and suppress vibrations occurring in a variety of slider suspension assemblies. Specifically, similar vibration problems occur in MR head suspension assemblies and burnishing head suspension assemblies, and the present invention is suitable for use in both applications. Similarly, though the actuator and sensor of the present invention have been described herein as bimorph configurations, it should be understood that other actuator and sensor configurations that are well known in the art may also be utilized to implement the suspension arm vibration damping system of the present invention. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.