Head suspension having reduced torsional vibration

A transducer suspension for a drive system has a beam which is bendable in a vertical direction for holding the transducer adjacent to the media, but is torsionally stiff for quicker access times and reduced noise and errors. This inherently contradictory performance occurs by joining a laterally elongated brace which is torsionally stiff about its elongated dimension across a pair of laterally disposed strips to cause the strips to flex in unison, allowing bending but reducing torsional motion in the suspension about an axis which is substantially perpendicular to the torsionally stiff axis of the brace. The torsionally stiff brace may take the form of a tube having a rectangular, triangular or circular cross-section, and may be formed of material cut from the suspension to define the hinge strips. The brace may have reduced longitudinal dimensions at the connections to the strips to further filter torsional versus bending vibrations. A plurality of braces joined to hinge portions may also occur in a single suspension for increased effect.

TECHNICAL FIELD
 The present invention relates to suspensions for transducers that are held
 in close proximity to relatively moving media, particularly for load beams
 holding disk drive heads.
 BACKGROUND OF THE INVENTION
 Information storage drive systems often operate by holding an
 electromagnetic transducer or head next to a relatively moving magnetic
 media, such as a disk or tape. The media typically moves in a single
 direction, so that a stationary transducer can read or write bits of data
 on a single track that passes next to the transducer, while the employment
 of a transducer that can move in a direction transverse to the media
 motion allows access to multiple tracks. Such moveable transducers are
 commonly attached near distal ends of suspensions in order to facilitate
 access to various tracks. It is important that a suspension accurately
 position the transducer along as well as across a recording track in order
 to eliminate errors and reduce noise.
 In order to provide such accurate positioning, a suspension load beam is
 typically stiff in both lengthwise ("longitudinal") and sideways
 ("lateral") directions, which are both oriented substantially parallel to
 the plane of the media. Lateral and longitudinal stiffness affords rapid
 access of a head to various tracks of the media, reduces settling time and
 increases the ability of the head to follow a single track, all essential
 to increased drive performance. Stiffness in a given direction of a beam
 generally correlates with high resonant vibration frequencies of the beam
 in vibration modes corresponding to those directions, so a greater
 stiffness or flexibility can be translated into, respectively, a higher or
 lower resonant vibration frequency. In other words, high resonant
 vibration frequencies generally reduce positioning error, as perturbations
 such as may be induced by transducer movement between tracks do not result
 in large position errors and are quickly diminished, allowing the
 transducer to accurately read or write.
 On the other hand, flexibility of a beam in a direction toward and away
 from the media, which is termed the "vertical" or "perpendicular"
 direction in the present invention, is generally desirable to allow the
 transducer to conform to variations in media height or positioning and to
 provide a spring force for holding the transducer next to the media, which
 may include controlling fly height. This bending is typically achieved by
 creating a spring or hinge portion near a base or mounting end of the
 beam. Flexibility in the vertical direction, however, may allow
 undesirable low-frequency resonant vibrations to occur in a torsional
 mode, since torsional vibration can occur when laterally spaced portions
 of the beam bend in opposite vertical directions. A torque on the beam is
 generally induced with any lateral acceleration, which occurs for instance
 when the head shifts between tracks, since the center of mass of the head
 is not aligned with the torsional axis of the beam or gimbal. There is a
 fundamental conflict between the need to allow vertical bending of the
 beam and the need to reduce the amplitude of torsional vibrations, since
 the torsional vibrations are simply vertical bending in which one side of
 the beam is out of phase with the other. These torsional vibrations can
 cause significant off-track motion leading to noise and errors in reading
 and/or writing data, and impeding any reduction in transducer access time
 between tracks.
 An object of the present invention is to provide means for preferentially
 increasing torsional stiffness of a transducer suspension without reducing
 the vertical bending flexibility.
 SUMMARY OF THE INVENTION
 The above object is achieved by providing, in at least one place along the
 length of a drive system arm or beam, a laterally extensive region of
 vertical flexibility, commonly termed a "hinge" region, with a laterally
 extending brace joined to a longitudinally limited portion of the hinge.
 Although the purpose of the brace is to mitigate twisting of the beam
 about a longitudinal beam axis, the brace itself is stiff against twisting
 about a lateral axis. This lateral torsional stiffness of the brace
 prevents the vertically flexible region attached to the brace from
 simultaneously bending upward on one side of the beam and downward on the
 other, thereby increasing longitudinal torsional stiffness. At the same
 time, overall bending of the beam is decreased only along the small
 portion of the length joined to the laterally extending brace, allowing
 the beam to provide a relatively soft spring force holding the transducer
 adjacent the media.
 The reason for the success of this invention can be understood by comparing
 the bending and twisting motions that are possible with a generally
 planar, flexible yet resilient structure, such as a sheet of metal.
 Bending of such a structure along a given line in the plane does not cause
 bending along a perpendicular line in the plane but instead tends to
 prevent such perpendicular bending. In contrast to bending motion,
 however, torsional motion about a given line in the plane causes torsional
 motion about a second perpendicular line in the plane. Reducing torsional
 motion about an axis can thus be accomplished by reducing torsional motion
 about a perpendicular axis. A brace that is torsionally stiff about a
 first direction but limited in length in a second, perpendicular direction
 can therefore limit torsional flexibility about the second direction
 without substantially limiting bending along the second direction.
 Several types of torsionally stiff braces may be employed to raise the
 torsional frequency of a suspension beam. Torsional stiffness is caused by
 resistance to motion about a torsional axis, and thus benefits from a
 structure having shear strength that is distanced to the axis, the
 distance providing a moment arm that leverages the strength. In general, a
 tube-like structure offers high torsional stiffness per unit weight, since
 a tube has a high shear strength at a distance from its axis but no mass
 at the axis. Such a tube, extending laterally across a beam, may be joined
 to a hinge for reducing torsional vibrations in a perpendicular,
 longitudinal direction. The tube may be cylindrical, offering a high
 torsional stiffness and a minimal reduction in bendable length along a
 joined area. Alternatively, a tube may have rectangular or triangular
 cross-sections which afford greater attachment area, providing more
 assuredly reduced torsional vibration at a cost of greater non-bendable
 length. On the other hand, for a multilayer beam having a hinged area
 formed by a cutout in a layer or layers, a laterally-extending,
 longitudinally-limited brace may be formed across the hinge by folding the
 cutout layers and/or leaving the multilayers. The brace should be formed
 of a material having a matching thermal expansion coefficient as the hinge
 in order to avoid temperature induced changes in the spring-like hinge,
 such matching being inherent when a layer is folded back on itself to form
 the brace.
 The brace, the hinge and the entire beam may alternatively be formed of
 materials micromachined with semiconductor processing techniques, such as
 etched silicon with embedded or deposited conductive leads for connecting
 the transducer with drive electronics. Materials such as silicon (Si),
 silicon carbide (SiC), silicon dioxide (SiO.sub.2), alumina (Al.sub.2
 O.sub.3) or other strong, workable substances may be used in this case. A
 stainless steel or other metal suspension beam may also be etched to form
 hinge strips or deposited with additional layers for a laminated brace,
 for example. Alternatively, a laminated brace including a plurality of
 rolled or pressed metal layers may be formed by metal working such as
 cutting, stamping, bonding and/or welding, such as laser spot welding.
 A plurality of such laterally extending braces adjoined by vertically
 flexible regions can be formed along a suspension beam to increase the
 vertical flexibility, with the provision that such segmentation should not
 be periodic in order to avoid creating resonant structures. Also, due to
 the improved performance provided by the torsionally stiff brace, the beam
 can be tailored to further reduce torsional vibrations by increasing its
 torsional stiffness in areas not selected for vertical bending,
 improvements which would not have been noticeable otherwise in relation to
 hinge induced vibrations.

DESCRIPTION OF THE INVENTION
 Referring now to FIG. 1, a suspension 30 for a hard disk drive transducer
 35 includes a "load beam" 33 holding the transducer near a distal end, and
 a baseplate 37 for attaching the beam to a rotary actuator arm. The beam
 33 is designed to generally pivot about an actuator axis which is parallel
 to the Z direction shown and substantially normal to a major surface of a
 disk, not shown, over which the transducer 35 glides or slides, in order
 to access various tracks on the disk. The beam 33 is elongated along an X
 direction and also extends substantially along a lateral or Y direction,
 the X and Y extensions of the beam resulting in stiffness in those
 directions and thereby providing accurate positioning of the transducer 35
 on the disk surface in the X and Y directions. The beam 33 is thinnest in
 the Z direction, to allow bending toward and away from the disk and to
 provide a relatively soft and accurately controlled spring force for
 holding the transducer 35 adjacent to the disk. The beam 33 may be made of
 stainless steel having a thickness of a few mils, or other resilient
 materials, and has a tapered width that is generally greater closer to the
 base plate 37 and lesser adjacent to the transducer 35. A pair of flanges
 40 and 42 are formed by rolling edges of the tapered portion of the beam
 33, the flanges stiffening that portion against bending in the Z
 direction, which also reduces torsional (twisting) motion of that portion
 about the X direction, since such torsional motion involves bending of
 laterally offset parts of the beam in opposite Z directions.
 A hinge region 44 is formed by the thin, unflanged portion of the beam 33
 close to the baseplate 37, which has been cut out at opening 46 to further
 decrease resistance to bending in the Z direction, while maintaining a
 pair of laterally spaced strips 48 and 50, which maintain X and Y
 direction stiffness. The sum of the lateral or Y dimensions of the strips
 48 and 50 is termed the "active width" of the hinge 44. The hinge 44 may
 provide a force holding the transducer adjacent to the disk, which in disk
 drive industry jargon is termed a "load" or "preload" and is measured in
 units of "grams," where a "gram" is the force produced by a 1 gram mass in
 standard gravity. Since the beam 33 is most flexible in the hinge region
 44, this region provides the primary contribution to both bending and
 torsional motion. A brace 53 spans the opening 46 and is joined near a
 middle of strips 48 and 50 so as to leave a substantial portion of the
 strips free on either side of the brace. The brace 53 extends much further
 in the Y direction than in either the X or Z direction, and has been
 designed to be torsionally stiff (against twisting) about the Y direction.
 The brace 53 in this embodiment is hollow for reduced mass and has a
 rectangular cross-section in an X-Z plane. Since the brace 53 severely
 restricts motion about its Y axis, the strips 48 and 50 are similarly
 constrained, essentially preventing each strip from sloping oppositely in
 the X-Z plane from the other strip and thereby minimizing torsional motion
 of the beam 33. Due to the limited extent of the brace 53 in the X
 direction, however, needed Z direction bending of the hinge is not
 significantly constrained. Moreover, a reduction in Z direction bending
 can be compensated for by increasing the length of the strips 48 and 50 in
 the X direction to achieve a desired spring constant.
 In order to better illustrate the surprising results of the present
 invention, FIG. 2 shows a pair of flexible, resilient strips 55 and 57
 which extend primarily along the X direction between a pair of rigid bars
 59 and 60, the strips and bars representing a simplified hinge region of
 the prior art. In an unperturbed state, the strips 55 and 57 would be
 parallel to each other, as would the bars 59 and 60. The strips 55 and 57,
 which are free to bend in the Z direction, are shown twisting about the X
 direction due to a torque on bar 60 relative to bar 59. Note that this
 torsional motion about the X direction also causes a twisting of strips 55
 and 57 about the Y direction in parts of the strip far from the bars 59
 and 60. Thus a portion 62 of strip 55 disposed about midway between bars
 59 and 60 is seen to slope upward while a corresponding portion 64 of
 strip 57 is seen to slope downward. Preventing such torsional motion of
 the portions 62 and 64 about the Y direction would therefore prevent the
 torsional motion about the X direction depicted in FIG. 2. The present
 invention is directed to preventing this X direction torsion with a brace
 extending along and torsionally stiff about the Y direction, provided that
 the brace does not extend so far in the X direction so as to prevent
 bending of the strips in the Z direction.
 Application of some engineering formulas can also be used to understand the
 current invention. The twisted strips 55 and 57 of FIG. 2 must each act
 like a beam with one end fixed and one end guided so as to have equal
 slopes at their ends, as required by rigid bar 60. A governing equation
 for this condition is:
 y=PL.sup.3 /12EI,
 where y is the vertical deflection, P is the applied load, L is the beam
 length, E is the elastic modulus, and I is the area moment of inertia of
 the beam cross-section. In addition, the twist of each leg is given by:
EQU .theta.=TL/KG,
 where .theta. is the angular deflection, T is the applied torque, L is the
 beam length, G is the shear modulus, and K is the stiffness constant for a
 rectangular section. For the geometry shown in FIG. 2, the torsional
 stiffness of the hinge is given by:
EQU S=T/.theta.=2KG/L+2A.sup.2 12EI/L.sup.3,
 where S is the torsional stiffness and A is the arm from load beam rotation
 to the center of the leg.
 In considering the torsional stiffness of the hinge, note that most
 variables are linearly related to S, while A and L are nonlinearly
 related. If A becomes larger, meaning that the width of the hinge is
 increased, then the stiffness increases as a square of the width. The most
 dramatic effect on torsional stiffness, however, is due to changing the
 hinge length L, since the stiffness is essentially a cubic function of the
 length. The bending spring constant also increases at a higher than linear
 rate, however, the bending spring constant is a substantially linear
 function of hinge length if the stiff load beam portion is significantly
 longer than the hinge portion. A temptation arises to increase torsional
 stiffness by simply decreasing hinge length. But as hinge length
 decreases, either material thickness or active width must decrease in
 order to provide a given suspension stiffness, or "spring rate".
 Decreasing material thickness affects load beam design, manufacturing
 processes, suspension robustness, and increases stress on the hinge.
 Decreasing active width similarly increases stress levels of the hinge.
 For a given spring rate and gram load, the following hinge equations are
 applicable:
EQU .sigma.=mc/I; c=h/2; and I=bh.sup.3 /12;
 where .sigma. is the stress, m is the moment (force/length of load beam), b
 is the active width, h is the material thickness, I is the area moment of
 inertia, and c is the length from a neutral axis to an outermost fiber.
 Solving the above equations yields:
EQU .sigma.=m (h/2)/(bh.sup.3 /12)=6m/bh.sup.2.
 In other words, decreasing thickness h increases stress .sigma. at a
 squared rate. Thus, attempting to increase torsional stiffness by
 decreasing the length of the hinge while maintaining a desired spring
 constant also increases stress levels, which may lead to failure of the
 hinge. On the other hand, the addition of a torsionally stiff brace such
 as element 53 in FIG. 1 may divide the length L roughly in half and
 thereby increase the torsional stiffness in each half dramatically, while
 the bending stiffness is decreased only by the reduction in length
 occupied by the brace.
 FIG. 3 illustrates an alternate embodiment in which a suspension 68 for a
 transducer 70 is formed of a continuous sheet of metal such as steel with
 a generally triangular shape, with a hinge region 66 adjacent to a
 baseplate 69. A generally triangular cutout 67 may optionally be formed
 for added flexibility of the hinge region 66. A pair of torsionally stiff
 braces 71 and 72 are formed of a generally cylindrical tube which may be
 made of steel or other strong materials and which has been cut to a length
 matching the hinge. The braces 71 and 72 have been crimped slightly to
 broaden the interfaces between the braces and the hinge 66 where the
 braces and hinge are joined by gluing, welding or other bonding
 techniques. An adhesive used for bonding braces 71 and 72 to hinge 66 may
 additionally serve as damping material to reduce vibrations. A generally
 cylindrical and hollow tube such as brace 71 or 72 is believed to offer a
 high torsional stiffness and a low reduction in overall bending length of
 the hinge 66. The pair of braces 71 and 72 joined to hinge areas each act
 as vibrational filters for the suspension, allowing low frequency bending
 vibrations while reducing low frequency torsional vibrations.
 FIG. 4 focuses on a hinge region 80 of another embodiment of the present
 invention, which is formed by making a pair of somewhat U-shaped cuts in a
 resilient planar sheet 82, the cuts tracing a generally rectangular
 outline and separated by small uncut regions 84 and 86. The flaps 88 and
 90 of the sheet 82 within the U-shaped cuts are then bent upward for
 increased rigidity against Z direction bending, forming a torsionally
 stiff brace 92 spanning most of the hinge region 80.
 FIG. 5 shows an embodiment in which the flaps 88 and 90 of the sheet 82
 within the U-shaped cuts are folded together to form a generally
 triangular, torsionally stiff brace 95 that separates the thin strips 94
 and 96 which act as hinges. The flaps 88 and 90 are bonded together along
 a ridge 98 by gluing, welding, soldering or other known bonding methods.
 Laser welding along ridge 98 is a preferred bonding method. Although the
 brace 92 in this example does not join all of the bendable material of
 flaps 94 and 96, the brace offers a rigid boundary to those flaps that
 discourages torsional motion about an axis perpendicular to the length of
 the brace.
 In FIG. 6, another embodiment of a torsionally stiff brace 100 spanning a
 pair of flexible hinge strips 102 and 104 is formed, like the embodiment
 depicted in FIG. 4, by making a pair of somewhat U-shaped cuts in a
 resilient planar sheet 106, the cuts tracing a generally rectangular
 outline and separated by small uncut regions, although in this example the
 cuts differ slightly in longitudinal extent. A pair of flaps 110 and 112
 defined by the cuts are then folded over an uncut region between the cuts
 and on top of each other to form the trilayer brace 100. A number of laser
 welding spots 115 join the flaps 110 and 112 together, which can
 alternatively be overlapped to form a tube shaped brace, similar to that
 shown in FIG. 3. As before, the brace is designed to preferentially reduce
 torsional motion about an axis of the planar sheet 106 transverse to the
 elongated brace, particularly for the situation in which the sheet is part
 of a suspension for a drive system transducer.
 FIGS. 7A and 7B show a hinge region 118 of a laminated, wireless suspension
 120 for a drive system which has a mostly planar beam 125 extending
 between a mounting area 127 and a head platform 124. This suspension 120
 is designed to work with an ultralight contact head, not shown, which is
 conductively bonded to the head platform 124, while the mounting area 127
 attaches to a rotary actuator via an adapter, also not shown. The
 suspension 120 is formed essentially of a pair of stainless steel layers
 sandwiched together with epoxy and patterned to obtain desired mechanical
 and electrical properties.
 The disk-facing layer of steel, which is preferably gold-coated to enhance
 conductivity, is shown in FIG. 7B as fashioned into a pair of conductors
 121 and 122 that are patterned adjacent to the head platform 124 to allow
 gimbal movement of the head or slider, not shown, which is to be attached
 to the platform. The conductors 121 and 122 are patterned in the hinge
 area 118 to cover and therefore add to the stiffness of a brace 128,
 except for a small electrically insulative gap between the conductors. An
 island 129 of the conductive layer remains disposed between the conductors
 121 and 122 in the middle of the beam 125 in order to reduce the area and
 therefore the capacitance between the conductors and the steel stiffener
 layer without significantly reducing the strength of the beam or
 increasing the inductance of the circuit formed by the conductors.
 The side of the suspension 120 designed to face away from its associated
 disk is shown in FIG. 7A to have portions of the stiffener layer between
 the beam 125 and the mounting area 127 cut away by sawing or etching, thus
 forming a pair of thin, longitudinal strips 130 and 133, the strips being
 flexible due to their limited width and extended length, so that the
 strips have a low bending stiffness. Extending laterally between and
 connecting the strips 130 and 133 is a torsionally stiff brace 128, which
 is stiff, in part, due to its longitudinal as well as its lateral extent,
 much as the entire hinge region would be stiff if nothing had been cut
 out. The brace 128 is connected to the hinge strips 130 and 133 by a pair
 of bridges 137 and 138, which are longitudinally reduced in order to limit
 their constraint of bending by the hinge strips. Spaces between the brace
 128 and hinge strips 130 and 133 are made as thin as possible while
 maintaining operational clearance between the strips 130 and 133 and the
 brace 128, in order to minimize the lateral extent of bridges 137 and 138,
 so that the torsional stiffness of brace 128 transfers efficiently into a
 torsional stiffness of the strips. Having the brace 128 extend
 longitudinally much further than the bridges 137 and 138 provides the
 strips 130 and 133 with the torsional stiffness of the brace while
 minimizing the bending stiffness imparted from the brace to the strips.
 Tiny ribbons of conductors 121 and 122 also extend between beam 125 and
 mounting area 127, negligibly affecting the spring constant of the hinge
 region 118. A slight bend 135 in the beam 125 may be provided for assuring
 a parallel relationship between the beam and the disk.
 FIG. 8 shows a suspension 150 for a transducer 152 which may read and/or
 write on a disk or tape, the suspension defined by a generally tapered
 outline between a mounting end 155 and a transducer end 157. The
 suspension 150 has a pair of laterally spaced, longitudinally extending
 strips 160 and 161, which are connected by a number of laterally
 extending, longitudinally limited, torsionally stiff braces 166. The
 strips 160 and 161 are thus divided into a number of segments bordered by
 at least one of the braces 166. Each brace 166 holds adjoining strip 160
 and 161 segments from sloping upward at one strip and downward at another,
 and thus imparts a torsional stiffens to the tapered suspension 150. At
 the same time, the braces 166 allow the strips 160 and 161 to bend up or
 down in unison at each segment, thereby providing flexibility for the
 transducer to bend in a direction perpendicular to the strips and braces.
 The segmented suspension 150 may be conveniently produced by semiconductor
 processing techniques of masking and etching, for instance, of a silicon
 wafer upon which a silicon carbide layer has been formed, the silicon
 patterned to form the braces 166 and the silicon carbide forming the
 strips 160 and 161. A plurality of conductive leads may be deposited on a
 media facing side of the suspension, and connected to the transducer in a
 contiguous or gimbaled manner.
 FIG. 9 gives an overview of a torsionally stiff, vertically flexible
 suspension 30 of FIG. 1 as employed within a hard disk drive 180. The
 suspension is attached at its mounting end to an member 185 of a rotary
 actuator 188, so that the head 35 can access various tracks of a disk
 surface 189 as shown by arrow 190, while the member pivots about axis 192.
 The disk surface 189 is spinning rapidly in a direction of arrow 195,
 demanding flexibility of the arm 33 in the direction toward and away from
 the disk surface 189 in order to accommodate any asperities or waviness of
 the disk surface without damage to the head, which may be within a few
 microinches of that surface. This vertical flexibility is afforded by
 hinge region 44, while torsionally stiff brace 53 prevents unwanted
 torsional motion of the suspension. Although not shown, similar
 torsionally stiff and vertically flexible suspensions may be employed in
 disk drives having linear acuators or in servo controlled tape drives.