Abstract:
A PZT limiter in a microactuator type dual stage actuated (DSA) suspension limits travel of the PZT, particularly of the cantilevered end of the PZT and particularly during non-operational shock. The PZT limiter thus limits the stresses including bending placed on the PZT during the shock event and thus helps to prevent cracking of the PZT. Additionally, by limiting displacement of the PZT, the limiter improves the mechanic and electrical performance of the suspension during operation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Patent Application No. 62/035,290 filed Aug. 8, 2014. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of suspensions for disk drives. More particularly, this invention relates to the field of a PZT travel limiter for micro dual stage actuated (DSA) suspensions. 
     2. Description of Related Art 
     Disk drive assemblies typically include a voice coil motor (VCM) which mover the suspension in order to position the read/write head on the spinning magnetic disk medium. Dual stage actuated (DSA) suspensions are suspensions that, in addition to the VCM, include at least one actuator mounted somewhere on the suspension itself in order to effect fine positional movements of the read/write head, also called the head slider. Milli-actuators are broadly classified as actuators that move the entire front end of the suspension: spring, load beam, flexure and slider. Micro-actuators are broadly classified as actuators that move only the slider, moving it relative to the load beam, or moving the read-write element only. The actuators are usually piezoelectric devices, though other types of actuators have been proposed and used. 
     Without admitting that  FIG. 1  is “prior art” within the legal meaning of that term,  FIG. 1  is a bottom plan view of the area around flexure  20  in a micro DSA suspension  10  according to a prior design by the assignee of the present application. Such a design is generally shown in U.S. Pat. No. 8,879,210 issued to Hahn, which is assigned to the present applicant. In the suspension  10  of  FIG. 1 , a flexure  20  is mounted to what will be called the bottom side of load beam  14 . Load beam or beam portion  14  extends in a generally horizontal direction and supports flexure  20  and read/write head  38  which is mounted to a gimbaled portion  30  of the flexure. Flexure  20  includes a gimbal  24  which supports read/write head  38  in a gimbaled arrangement so that the read/write head pitches and rolls freely in response to surface irregularities in a surface of the spinning data disk as the data disk magnetic media platter surface travels underneath the read/write head. In addition to gimbal  24  which is typically made of a stainless steel support layer, flexure  20  also includes a flexible circuit  34  which includes an insulating layer such as polyimide and a signal conducting layer such as copper or a copper alloy such as copper/beryllium. A bent finger  26  acts as a tongue limiter to limit travel of read/write head  38  during inertial shock events in order to prevent damage to the suspension. Lifter tab  18  is located at the distal end of suspension  20 . The “distal” end of a suspension or load beam is the end that is opposite the proximal end, i.e., the “distal” end is the cantilevered end. The “proximal” end of a suspension or load beam is the end that is supported, i.e., the end that is mounted to an actuator arm. 
     Two piezoelectric actuators  40 , sometimes referred to as microactuators, are located on laterally opposite sides of the suspension. Actuators  40  act in push/pull fashion with one actuator typically contracting while the other actuator expands, or vice versa, in order to effect fine movements of read/write head  38  and thereby to position read/write head  38  precisely over the data track desired. For simplicity, the term “PZT” may be used herein as shorthand to refer to the piezoelectric actuators, it being understood that not all piezoelectric actuators comprise lead zirconate titanate (PZT) material. A first and distal end  42  of PZT  40  is affixed to a relatively movable portion of the flexure, in this case specifically to PZT bonding pads or actuator attachment locations  32  which are part of gimbaled portion  30 . A second and proximal end  44  of PZT  40  is affixed to a relatively fixed portion of flexure  20  that substantially does not move relative to load beam  10 . A small drop of conductive adhesive  46  such as conductive epoxy carries the PZT driving voltage to the metallized top surface of PZT  40  which defines the PZT&#39;s top electrode. When PZT  40  expands or contracts in its longitudinal direction in response to a driving voltage, it pushes or pulls on gimbaled portion  30  to rotate that gimbaled portion and thus move read/write head  38 . A servo feedback loop keeps read/write head  38  properly positioned over the desired data track on the data disk. 
       FIG. 2  is a top perspective view of the suspension  10  of  FIG. 1 . Tongue limiter  26  extends through aperture  16  in load beam  14 , and typically includes T-arms  27  which may or may not be bent as shown. 
     SUMMARY OF THE INVENTION 
     The inventors have discovered that, due to the large distance between welds that hold flexure  20  to load beam  14  in the suspension of  FIG. 1  and in similar suspensions, PZTs  40  experience a large displacement under shock conditions. 
       FIG. 3  is a side elevation view of the suspension  10  of  FIG. 1  under such shock conditions. Suspension  10  includes base plate  12  and spring region  13  which are not shown in the prior figures. As can be seen in the figure, shock loading produces a very large PZT displacement D. The large PZT displacement and bending stresses created within the PZT thereby can cause PZTs  40 , which are relatively fragile, to crack. The large displacement can also make for poor micro DSA suspension mechanical and electrical performance during an operational shock event, due to both off-track and/or off-height problems created by the mechanical displacement, as well as to electrical disruptions created in the data track servo feedback loop caused by voltages that are induced by mechanical strains within the PZTs. The problem arises in DSA suspension designs in which the PZTs are not rigidly affixed at both their ends to the load beam or to any portion of the load beam proximal to the load beam such as the base plate. 
     In order to mitigate and/or avoid the various problems associated with PZT displacement in prior suspension designs, according to the invention a PZT displacement limiter is added to the suspension in order to limit the Z-direction displacement or travel of each PZT. In the illustrative embodiment, the PZT limiters are formed integrally with, and bent from, the same stainless steel layer or other metal support layer from which the flexure is formed, in much the same way that the head slider limiter or tongue limiter is formed. 
     The PZT limiters can engage corresponding portions of the load beam to limit the vertical displacement of the PZTs, such as by projecting through an aperture(s) in the load beam and engaging the other side thereof, in much the same way that a conventional tongue limiter engages the load beam through an aperture therein to limit the Z-direction displacement of the tongue and hence the read/write head that is attached to the tongue. The maximum travel distance is thus predefined by a gap distance between interacting components. 
     In an exemplary embodiment, the vertical travel of the PZT is constrained by a limiter that takes the form of a finger that extends from the PZT mounting region on the gimbaled portion of the flexure. The finger is bent so that it extends from the PZT mounting region which is on the bottom side of the load beam, through an aperture in the load beam and to the top side of the load beam, and abuts against the load beam top surface when the predefined maximum travel distance has been reached. The distal end of the PZT, which his mounted to the gimbaled region, can travel up and down relatively freely within a predefined limited travel distance, until the bent finger abuts up against the load beam&#39;s top surface which acts as a stop to prevent further vertical movement of the finger. The two PZTs each have their own associated limiter, in addition to the standard tongue limiter which limits travel of the read/write head. By limiting the vertical travel of the frame PZTs in response to shock, the PZT limiters help to prevent cracking and other damage to the PZTs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a bottom plan view of the area around the flexure in a micro DSA suspension according to a prior design. 
         FIG. 2  is a top perspective view of the suspension of  FIG. 1 . 
         FIG. 3  is a side elevation view of the suspension of  FIG. 1  under shock condition. 
         FIG. 4  is a bottom perspective view of a flexure according to an illustrative embodiment of the invention. 
         FIG. 5  is a top perspective view of a suspension according to an illustrative embodiment of the invention including the flexure of  FIG. 4 . 
         FIG. 6  is a side elevation view of the suspension of  FIG. 5 . 
         FIG. 7  is an exploded view of the suspension of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 4  is a bottom perspective view of a flexure  120  according to an illustrative embodiment of the invention. Flexure  120  includes a stainless steel support layer  22  from which gimbal  24  is integrally formed, a flexible circuit  34 , and a tongue limiter  126  formed from the stainless steel layer. Flexure  120  also includes two PZT limiters  150  according to the invention. Each PZT limiter  150  comprises a finger that extends from PZT bonding pad or actuator attachment location  32  in a proximal direction, and having two bends  152 . Because PZT bonding pads  32  are part of the gimbaled region  130  which defines a region  123  which is movable relative to load beam  114 , the end of the PZT that is mounted to PZT bonding pad  32  will be relatively unconstrained in the vertical direction. In other words, in the absence of PZT limiters  150 , the PZTs would be effectively cantilevered with their cantilevered ends travelling relatively freely in the vertical direction in response to vertical shock. In contrast, the ends of the PZTs that are bonded to flexure portion  121  that does not move relative to the load, are not free to travel vertically. 
     The PZT limiters  150  limit the Z-displacement of the two PZTs  40  on a gimbal-mounted DSA suspension  10 , and in particular limit the Z-displacement of distal ends  42 . Preferably tongue limiter  126  and the two PZT limiters  150  are all formed at the same time, including being bent at the same time, thus resulting in no additional manufacturing steps and no significant additional cost. 
       FIG. 5  is a top perspective view of a suspension  110  according to an illustrative embodiment of the invention including both a load beam  114  and the flexure  120  of  FIG. 4 . Limiter fingers  150  and  126  extend through aperture  116  in load beam  114 , from the bottom side of load beam  114  to its top side, with limiter fingers  150  extending to positions over their respectively associated PZTs  40 . The limiters  150  are located adjacent the distal ends of their respective PZTs  40 , and are located closer to those distal ends of the PZTs than to the proximal ends of the PZTs. When the suspension experiences a shock having a large enough vertical component, the ends of bent fingers  150  come into contact with, and abut up against, the top surface of load beam  114 . The top surface of load beam  114  therefore acts as a stop  117  to limit any further travel of fingers  150  and thus to prevent further deflection of the cantilevered distal ends  42  of PZTs  40 . More generally, a portion of the gimbaled region of the flexure engages a corresponding portion of the load beam to limit vertical displacement of the PZTs  40 , or at least to limit movement of an otherwise relatively unconstrained end of PZT&#39;s  40 . 
     Preferably the limiters  126  and  150  are all separate, allowing the greatest freedom of movement to gimbaled region  130 , and without any material connecting the limiters in order to minimize the mass and weight of the limiters. 
       FIG. 6  is a side elevation view of the suspension of  FIG. 5 , showing PZT limiters  150  extending from the bottom side of load beam  114  up through aperture  116  and to the top side of the load beam, the fingers  150  being limited in their vertical travel distance by stop(s)  117 . The separation between finger  150  and stop surface  117  at a quiescent condition defines a gap, the gap defining the maximum vertical movement of the PZTs. 
       FIG. 7  is an exploded view of the suspension of  FIG. 5 . Also visible in this figure is vibration damper  60  which is conventional. 
     Other structures and manufacturing methods are possible, as long as a stop is provided that limits the travel distance of the PZTs, and/or otherwise limits the stresses that will be placed on the PZTs. For example, instead of the limiter being a bent finger or tab that is formed integrally with the flexure and interacts with a corresponding aperture and/or other stop surface formed from the load beam, the limiter could be a bent finger or tab that is formed integrally with the load beam and interacts with a corresponding aperture and/or other stop surfaced formed from the aperture, similar to the arrangement show in U.S. Pat. No. 7,751,149 issued to Mei. The limiter could be a feature such as a finger that extends from the load beam, and is bent so that it extends underneath the PZT and directly limits the movement of the PZT by abutting against it. The limiter could be flexible to absorb shock, or could comprise multiple-step limiters with one or more of the limiters absorbing shock, such as disclosed in U.S. Pat. No. 7,719,797 to Mei. The limiter could include an offset portion to facilitate bending of the limiter after joining the flexure to the load beam, as disclosed in U.S. Pat. No. 7,551,401 to Ciurea et al. All of those patents are assigned to the present applicant, and all are incorporated by reference as if set forth fully herein for their teachings of limiter structures. Furthermore, the two PZT limiters and the tongue limiter could extend through the same aperture in the load beam, or they could extend through separate apertures. All of the limiter structures and travel-limiting techniques disclosed herein, and all of the limiter structures and travel-limiting techniques disclosed in the references which are incorporated by reference herein, constitute various means for limiting travel of at least the cantilevered ends of the PZTs. 
     It will be understood that the terms “generally,” “approximately,” “about,” “substantially,” and/or “coplanar” as used within the specification and the claims herein allow for a certain amount of variation from any exact dimensions, measurements, and arrangements, and that those terms should be understood within the context of the description and operation of the invention as disclosed herein. 
     It will further be understood that terms such as “top,” “bottom,” “above,” and “below” as used within the specification and the claims herein are terms of convenience that denote the spatial relationships of parts relative to each other rather than to any specific spatial or gravitational orientation. Thus, the terms are intended to encompass an assembly of component parts regardless of whether the assembly is oriented in the particular orientation shown in the drawings and described in the specification, upside down from that orientation, or any other rotational variation.