WO2001026098A1

WO2001026098A1 - Hard disk drive with high bandwith

Info

Publication number: WO2001026098A1
Application number: PCT/GB2000/003761
Authority: WO
Inventors: John Stewart Heath
Original assignee: John Stewart Heath
Priority date: 1999-10-06
Filing date: 2000-10-02
Publication date: 2001-04-12
Also published as: WO2001026098B1

Abstract

A disk drive has a pivoted actuator arm including a recording head, a central housing for a pivot bearing assembly, and a servo motor coil. The central housing has a thickness approximately the same as the coil. A wide thin plate which may be of stainless steel is attached to the coil and passes between the poles of the servo motor magnet. The plate is also attached to a flat area of the housing. A second similar plate may be attached to the other side of the coil to form a sandwich. A tapered ring exerts a compressive force acting between the housing and the pivot bearing assembly, attaching the two by friction. A ring within the pivot bearing assembly supports the compressive force and compensates for differential thermal expansion. The lowest mechanical resonant frequency of such an arm may be twice that of the prior art enabling substantial improvements in recording density and performance.

Description

DESCRIPTION

HARD DISK DRIVE WITH HIGH BANDWIDTH

TECHNICAL FIELD This invention relates to data recording disk drives, and more particularly to the construction of arms for rotary actuators used to move the recording heads across the disk surfaces and to follow the recorded data tracks.

BACKGROUND ART

Disk drives are information storage devices with rotating disks having concentric data tracks containing the information, and a rotary actuator used to move recording heads to the desired tracks and to maintain them centred over the tracks to perform record and read-back operations. One factor which determines the density of the storage of information storage in disk drives is the accuracy with which the recording heads can follow the extremely narrow data tracks on the disk. The heads have to be well centred on a track at every instant in order to ensure that data is correctly recorded or read-back. A servo controller drives a servo motor to keep the heads centred on track against disturbances such as spindle wobble, shock, vibration, and pivot bearing hysteresis, and its success in overcoming these disturbances is set by the gain of the servo. Servo gain can be defined as the restoring force applied by the servo motor to the actuator arm expressed as a ratio of the tracking error, and the gain which may be applied is limited by mechanical resonance of the actuator arm. Resonance introduces a delay into the servo loop, for which the servo controller is unable to compensate without a limitation of gain. As a consequence the frequency range, or bandwidth, over which the servo is able to correct disturbances is limited to about 25% of the lowest resonant frequency of the actuator structure, This limitation on servo bandwidth defines the accuracy with which the heads follow the tracks and also the speed of response in moving to a new track. Thus it is an important object in mechanical design of disk drive actuator arms to have the system resonant frequency of the arm (the lowest of many such resonances) as high in frequency as possible.

The normal practice for construction of actuator arms is that there is a pivot bearing in a central housing or body, and to one side of the housing head arms project which carry the recording heads. On the side of the housing opposite to the heads the actuator arm has a servo motor coil which moves within a fixed magnetic field. The coil is wound of insulated wire and is supported on either side by two yoke beams which project from the central housing. There are two aspects of the construction of the actuator arm which may dominate its lowest resonant frequency. Firstly the rigidity or stiffness of the structure of the actuator arm in relation to the mass of its extremities determines a mode of vibration in which the arm bends about the pivot rather as a butterfly flaps its wings. The frequency of this " butterfly mode" may be increased by increased stiffness (rigidity) of the arm in relation to its mass. Secondly the stiffness in a radial direction of the pivot in relation to the total mass of the actuator arm determines a mode of vibration in which the arm moves as a rigid body to either side of its rest position. The frequency of this "rigid body mode" may be increased by reducing the mass of the actuator arm. These two modes of vibration often have similar frequencies and are manifested as a single or system mode of vibration at a frequency near to the lowest of the two individual frequencies. The housing, head arms, and yoke, have proportions which are carefully computed to maximise the frequency of the butterfly mode, but an opposing constraint on the design is that the coil has to fit closely within the magnet gap in order to exert a high turning moment as this strongly determines the time to move the heads. This constraint results in coil structures which are the least rigid part of the arm structure for three reasons. Firstly the coil has little rigidity of itself, being formed of many turns of wire bonded together through resin coverings. Secondly the strength of the yoke beams is limited because, due to the presence between them of the coil, they are relatively narrow at the point of attachment to the housing. Thirdly any stiffening structure introduced into the central space within the coil can only be weakly connected to the rest of the actuator arm because the coil intervenes. US 5,790,348 for example teaches a coil with the central space of the coil almost filled with plastic material, but a very limited structural connection between this filling and the actuator arm. This connection extends as far as to connect with the material in the central space link but without entering the magnet gap. Extending such a structural member further, sandwiched against a flat servo motor coil and within the servo motor magnet gap has not been practised with flat wound coils in the prior art because of the high priority placed on efficient use of the magnet. In disk drives with linear actuators and servo motor coils which are wound on cylindrical formers, and in disk drives with rotary actuators and coils wound on rectangular formers and which are not flat in the plane of rotation, such a sandwich is known. An example is UK 1,342,495 (US 3,849,800). Such three dimensional coils belong to the era prior to the adoption of rare earth servo motor magnets, and have for many years been superseded by flat coils. Because actuator arms with flat coils exhibit weakness as described, it is common practice to fall back on damping to reduce the amplitude of the resonance by absorbing energy. US 5,790,348 is an example, but a more common approach is to attach the coil to the yoke through a thick layer of soft adhesive, as for example in US 5,623,759. Then the softness of this adhesive layer is a further factor reducing the stiffness of attachment of the coil. Both the data published in US 5,790,348, and measurements of the transmission characteristic of examples of actuator arms similar to US 5,623,759, show that the degree of damping achieved by these means is not great. The reason for this is that in these prior arts damping material is only subject to a small proportion of the total maximum strain (relative distortion) that occurs during a vibration cycle.

BRIEF SUMMARY OF THE INVENTION Consequently it is an object of this invention to provide a disk drive with increased bandwidth attained through an actuator arm in which the lowest frequency of vibrational resonance of the arm is increased. This necessitates increasing the frequencies of both the butterfly mode and the rigid body modes of vibration. The butterfly mode of vibration is increased by stiffening the coil structure of the arm with a thin wide plate. The rigid body mode of vibration is increased by reducing the mass of the arm through means which are enabled by the use of the same wide plate to attach the coil. It is well known that the bending stiffness of a rectangular section beam is proportional to its width in the plane of bending raised to the third power (Roark & Young 1975 McGraw Hill, pp 64 & 97). It has been found that a wide thin plate may be added to the actuator arm structure entering into the magnet gap which, benefiting from the third power relationship, has stiffness greater to that of the coil and yoke, but because of its thinness does not significantly effect the motor efficiency. The material for such a plate requires to have high rigidity yet be non-magnetic and to have poor electrical conductivity. Stainless steel and carbon fibre composite have been demonstrated as suitable, and ceramics or cermets would be if produced in very thin plates. Most common metals such as aluminium are ruled out because of higher conductivity or magnetic properties. Low mass is desirable but not essential. It has been found that when such a plate is added to an actuator which is otherwise according to the prior art, then the yoke arms are redundant and may be removed, so reducing the mass and inertia of the arm and reducing seek time of the disk drive.

The benefit of this invention may be applied as a palliative to existing disk drive actuator arms in order to improve performance without great change in the design and manufacturing process. This is exemplified in the first two embodiments. In the third embodiment the head arms are made of composite material, reducing the inertia of the actuator arm and seek time, and are extended into the magnet gap to form the plates which support the servo motor coil. In the fourth embodiment two plates are used and the mass of the housing is eliminated. In the fifth embodiment improvements are described which have been found to gain the maximum advantage from the invention when applied to an industrial and commercial situation.

The invention can be regarded as a disk drive including a base, a disk mounted on a spindle, a servo control system, and a pivoted actuator arm mounted on a pivot bearing assembly. The actuator arm has a housing, and a recording head, and the actuator arm has a servo motor coil wound of insulated wire which is flat in the plane of rotation of the actuator arm. The servo motor coil moves within the gap of a servo motor magnet, and is attached to the housing by a plate parallel to the plane of said servo motor coil which passes into the gap of said servo motor magnet. The plate is attached to the housing over an area where the housing has thickness substantially the same as the thickness of the servo motor coil, and the area has substantially the same width as the servo motor coil. The invention can also be regarded as a disk drive having a base, a disk mounted on a spindle, and a pivoted actuator arm mounted on a pivot bearing assembly. The actuator arm has a housing, head arms with recording heads, and a servo motor coil which is flat in the plane of rotation of the actuator arm and passes within the gap of the servo motor magnet. The actuator arm also has two plates extending within the magnet gap, the plates being structurally connected to the actuator arm, and the coil being sandwiched between the plates and attached to them. This invention may further be regarded as a disk drive including a base, a disk mounted on a spindle, a servo motor magnet system, and an actuator arm mounted on a pivot bearing assembly. The actuator arm has a housing, head arms carrying recording heads, and a servo motor coil which is flat in the plane of rotation of said actuator arm and which passes within the gap of a servo motor magnet. The housing has a member with a flat surface co-planar to the plane of rotatioa The actuator arm has a plate parallel to the plane of the servo motor coil, and which passes within the gap of the servo motor magnet. The plate is attached to the co-planar surface of the coil and to the housing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 A perspective view of a disk drive according to the first embodiment.

FIGURE 2 A section through a servo motor coil and magnet similar to FIGURE 1.

FIGURE 3 A perspective view of the actuator arm according to the second embodiment.

FIGURE 4 A perspective view of a variation of the second embodiment.

FIGURE 5 Transmission characteristic of an arm according to the second embodiment.

FIGURE 6 A perspective view of a disk drive according to the third embodiment. FIGURE 7 A longitudinal section through the actuator arm of FIGURE 6.

FIGURE 8 A perspective view of a disk drive according to the fourth embodiment.

FIGURE 9 A longitudinal section through the actuator arm of FIGURE 8.

FIGURE 10 A perspective view of a disk drive according to the fifth embodiment.

FIGURE 11 A perspective view of the actuator arm of FIGURE 10. FIGURE 12 A longitudinal section through the actuator arm of FIGURE 10.

FIGURE 13 A cut-away view of the pivot bearing assembly of FIGURE 12.

DETAILED DESCRIPTION OF THE INVENTION

A specific embodiment of the invention is described here with reference to the accompanying drawings. FIGURE 1 shows a hard disk drive comprising a base 1 with a motor driven spindle with rotational axis 4 carrying disks 3. In this figure the upper of two disks 3 and the base 1 are cut away for clarity. An actuator arm has a pivot bearing assembly comprising a pivot bearing 5 mounted within a housing 11. Pivot bearing 5 has a pivotal axis 6 and is mounted by its shaft to base 1. The actuator arm has head arms 12 with head mount blocks 7 from which recording heads 13 are loaded by load beams 14 against disks 3. Also attached to housing 11 is a yoke comprising two protruding beams 16 which are in a common radial plane and form a fork. Housing 11, arms 12, and yoke 16, are formed as a single part in aluminium alloy. Within the yoke 16 is mounted a servo motor coil 15 of trapezoidal shape and rectangular cross section which is substantially flat in the plane of rotatioa Coil 15 is attached to yoke 16 with a layer of adhesive 20. Coil 15 is energised by electronic servo circuits (not shown) which control the pivotal position of the actuator arm and thus the tracking of heads 13 on disks 3. Pivotal motion is over a range of about 32 degrees and throughout this angular movement coil 15 is entirely between the flat surfaces of opposing lower pole 8 and upper pole 9 (shown chain dotted in FIGURE 1) of a servo motor magnet mounted from base 1. The description to this point has been of a state of the art disk drive.

This embodiment of the invention comprises plate 17 added to the actuator arm. Plate 17 is cut from a sheet of a composite material approximately 0.25 mm thick and is bonded at 24 in FIGURE 2 to flat and coplanar surfaces of coil 15 and yoke 16. Plate 17 is also bonded to the central arm 18 of head arms 12. Thus plate 17 enters into the gap between magnet poles 8 and 9 and plate 17 also strongly couples coil 15 rotationally to head arms 12. Plate 17 passes through slots 19 in housing 11 where it is bonded to housing 11 and is also shaped (reference 22) to press radially inward against pivot bearing assembly 5. Plate 17 is cut from commercially available composite material in sheet form in which woven carbon fibres are impregnated and cured in epoxy resia Approximately 60% of the volume of the composite is carbon fibre. The carbon fibres are highly resistant to change of length but very flexible, consequently the modulus of stiffness of this composite material varies with orientation. To gain maximum stiffness advantage from the properties of the material, plate 17 is cut so that both the warp and weft fibres run at about 45 degrees to the arm centreline. Plate 17 is split (reference 23) along the centre line of the arm 18 and shaped so that it may be assembled by being curled and sprung apart to pass through the slots 19 of housing 11. Pivot bearing 5 is inserted after assembly of the plate 17 to housing 11. Plate 17 is substantially wider in the central section near the pivot 5 than is the case with state of the art actuator arms since being thin it can penetrate between disks 3 without contact.

The advantage in stiffness of this invention over the state of the art may be appreciated from the following table comparing the resistance to bending ( which is the product of Young's modulus of the material (E) and 2nd moment of area (I) ) of the cross section illustrated in FIGURE 2, this being the most critically weak section of the state of the art arm.

State of the art arm Plate 17 of this invention

Thickness T mm 1.5 0.25

Width W mm 5 25

Number of beams N 2 1

Second moment of area I mm⁴ 31.25 325.5

= NxTxW³ / 12

Material Aluminium Carbon fibre composite

Young's Modulus E GPa 71 70

Resistance to bending Ex I 2220 22800

This example shows that the stiffness added by the invention at this section can be ten times greater than the stiffness of the state of the art actuator arm. Readers familiar with structural mechanics will be able to further appreciate that the beams 16 of the yoke of the state of the art actuator arm gain little benefit from being widely separated since no part of the structure resists shear strain between the two sides of what would otherwise constitute an I-beam. Plate 17 however provides the required shear strength, further increasing the bending resistance above the tabulated values. The vibrational characteristics of such an actuator arm have been found to depend on the adhesive used to bond plate 17 to the other parts. If the adhesive is rigid such as is the case with a cyano-acrylate then the frequency of the structure is maximised. If however the material is softer, as may be the case with partially crosslinked epoxy resin, then the resonant frequency is lower but the amplitude of the resonance can be greatly reduced by through damping or energy dissipation in the bond. It has also been found that the vibrational characteristics of the arm are determined by both the choice of adhesive and the thickness of the adhesive.

A second specific embodiment of the invention is illustrated in FIGURE 3. The same components of a state of the art disk drive as described previously for the first embodiment are present with the same reference numerals. A plate 30 is bonded to the flat and co-planar surfaces of coil 15 and yoke 16. Plate 30 is about 0.25 mm thick and made of Carbon fibre/Epoxy composite material as described previously. Plate 30 is shaped to cover the majority of the flat surfaces of coil 15 and yoke 16, but the outer corners of plate 30 may be removed with a small advantage in inertia similar to reference 32 of figure 1. It may be appreciated that in comparison with the previous embodiment, plate 30 due to its smaller extent of adhesion is less well rotationally coupled to head arms 12, and can contribute less extra rigidity to the actuator arm at the pivot, however despite this, the arrangement has been found to be very effective both in increasing the resonant frequency and in damping the vibrational resonances of the coil 15 and yoke 16. FIGURE 5 shows as a dotted line the mechanical transmission characteristic measured for a state of the art arm. (The transmission characteristic relates the amplitude of vibration of the head mounting block (7) to the current in coil 15, measured as a function of frequency.) This can be compared with the transmission characteristic for the same arm with plate 30 bonded to it, shown as a solid line in FIGURE 5. The benefit gained by the addition of plate 30 is indicated by the reduction in amplitude of the resonant peaks which increases the servo gain margin as indicated in FIGURE 5. The adhesive used to bond plate 30 for these tests (partially crosslinked epoxy resin) was selected to give the greatest reduction in amplitude over a wide range of temperature. It may be recalled from the background section of this document that state of the art disk drives have soft adhesive used to bond coil 15 to yoke 16 specifically to achieve damping. The reason for the greatly enhanced damping found with this invention in comparison with the state of the art may be understood by considering the strain applied to the damping material during a vibration cycle. The mode of the vibration (which is in the plane of the parts) is a sideways deflection of the outer limb of coil 15 and consequent bending of yoke 16. In the state of the art disk drive such deflection induces strain in the adhesive 20 which is much less than the deflection of the outer parts. In comparison, plate 30 of this invention, having far greater stiffness than the coil and yoke, induces strain in the adhesive bond 24 to plate 30 which is comparable in magnitude to the motion of the outer limb of coil 15. Thus the energy dissipated in the adhesive is much greater. This embodiment provides the advantages of the invention with the minimum of change to the existing parts and may thus be applied to products quickly and with little investment.

Slight variations of this embodiment providing a further improvement in performance through reduced inertia are illustrated in FIGURE 4. The yoke arms 16 of FIGURE 3 are reduced in length (reference 39) and lightening holes 42 are introduced into plate 30. It has been found that either or both of these variations when applied to an actuator, do not effect the reduced amplitude of the transmission characteristic which remains substantially as illustrated as a solid line in FIGURE 5.

A third embodiment of the invention is illustrated in FIGURES 6 and 7. A hard disk drive comprises a base 1 with a motor driven spindle with rotational axis 4 carrying a disk 3. Base 1 also carries a servo motor magnet having a lower pole 38 and an upper pole 39 with a gap between their flat surfaces. An actuator arm is mounted on a pivot bearing assembly 29. Pivot bearing assembly 29 has a collar 33 and a pivotal axis 31 and is mounted on base 1. The actuator arm comprises two head arms 34 and 35 which are bonded to collar 33 of pivot bearing assembly 29. Head arms 34 and 35 each carry at one end recording heads 13 loaded by load beams 14 against disk 3. Arms 34 and 35 extend either side of pivot bearing 30, being wide near to pivot 29, tapering toward each end, and having stiffening flanges 37 along the outer edges extending from head mount blocks 36 back past pivot 29. Arms 34 and 35 are made of a composite material approximately 0.25 mm thick and may be manufactured by preparing a composite of carbon fibre fabric impregnated in soft partially cured resia This composite is then fully cured while being pressed in a mould having the form of the arm. A servo motor coil 36 of trapezoidal form with a rectangular cross section and flat in the plane of rotation of the actuator arm, is located at the end of the actuator arm opposite to heads 13. The actuator arm may pivot through an angular range of about 32 degrees, through which angle coil 36 is substantially within the gap between lower pole 38 and upper pole 39 of the servo motor magnet. Arms 34 and 35 extend within the magnet gap where they have the form of thin plates with width comparable to the width of servo motor coil 36. Coil 36 is sandwiched between them and bonded across the majority of the proximate surfaces. It has been found that the corners 40 of arms 34 and 35 may be cut away to reduce inertia, without the resonant frequency of the structure being significantly reduced. It may be appreciated that in this embodiment the plates bonded to servo motor coil 36 are extremely well coupled rotationally to the actuator arm, being integral with its principal parts. Consequently an actuator arm according to this embodiment has a higher resonant frequency and greater structural damping than the previous embodiments. The embodiment also has lower inertia than state of the art actuator arms because carbon fibre is substituted for the more usual aluminium which has an inferior ratio of rigidity to mass.

A fourth embodiment of the invention is illustrated in FIGURES 8 and 9. A hard disk drive comprises a base 1 with a motor driven spindle with rotational axis 4 carrying a disk 3. Base 1 also carries a servo motor magnet having a lower pole 48 and an upper pole 49 with a gap between their flat surfaces. An actuator arm is mounted on a pivot bearing assembly 50. The actuator arm comprises two head support arms 44 and 45 which are made of aluminium and are integral with a collar 43 mounted on pivot bearing assembly 50. Head arms 44 and 45 each carry at one end mount blocks 51 with recording heads 13 loaded by load beams 14 against disk 3. The actuator arm further comprises two plates 54 and 55 which may be made of a composite material of carbon fibre fabric impregnated with a resin such as epoxy. Plates 54 and 55 are rigidly attached to head arms 44 and 45 and to collar 43. A servo motor coil 46 of trapezoidal form with a rectangular cross section and flat in the plane of rotation of the actuator arm, is located at the end of the actuator arm opposite to heads 13 where it is sandwiched between plates 54 and 55. At any operational position of the actuator arm coil 46 and the adjacent parts of plates 54 and 55 are substantially within the gap between lower pole 38 and upper pole 39 of the servo motor magnet. The shape of plates 54 and 55 in plan view has a strong influence on the resonant frequency and inertia of the actuator arm. The best compromise, which may be determined exactly by mathematical analysis, is found when the plate is relatively wide near to the pivot and tapers toward each end. Variations from the ideal shape however may be required by practical considerations such as assembly or the need to minimise obstruction of air flow over the disk. Such variation may include departure from symmetry about the centre line. Plates 54 and 55 are preferably flat or nearly so because testing has shown that significant departure from flatness reduces stiffness due to buckling distortion of the outer parts of the plates, and the reduced stiffiiess is manifested as reduced resonant frequency. However it has been found that the extent of such a reduction of resonant frequency can be reduced if the plates are connected together at one or more places where vibrational buckling occurs.

It may be appreciated that this embodiment in comparison with the third embodiment has extra rotational inertia due to the mass of the head support arms 44 and 45.

Consequently an actuator arm according to this embodiment has a lower resonant frequency and is slower to move. The aluminium head support arms 44 and 45 however provide a practical advantage in that the recording heads may be attached to them by the ball swaging method which has many advantages in disk drive manufacture. The avoidance of the need to develop a new head attachment process can justify the loss in performance due to inertia. A fifth embodiment of the invention incorporating improvements resulting from engineering development and prototyping is described here with reference to the accompanying drawings. This embodiment has been submitted FIGURE 10 shows a disk drive comprising a base 61 with a motor driven spindle, a rigid recording disk 63 with rotational axis 62, and a rotary actuator. The rotary actuator has an actuator arm 65 pivotally mounted on base 61 through pivot bearing assembly 66, and a servo motor magnet assembly mounted on base 61 and having a lower pole 67 and an upper pole 68 (shown chain dotted). Supporting parts of the servo motor magnet are omitted from figure 10 for clarity. Actuator arm 65 pivots about pivotal axis 64 such that parts of actuator arm 65 rotate within a magnetic field existing in a gap 69 between facing flat surfaces of upper pole 68 and lower pole 67.

FIGURE 1 1 shows the rotary actuator, other parts being omitted for clarity. The actuator arm has a central hub or housing 70 in the form of a flat collar, and two head arms 71. Housing 70 and head arms 71 are formed as a single part in aluminium. Head arms 71 carry mount blocks 72 with recording heads 73 loaded by load beams 74 against the recording disk. Actuator arm 65 also has a servo motor coil 75 wound from insulated wire with a trapeziodal form in plan view and which is flat in the plane of rotatioa Coil 75 is located opposite head arms 71 to balance the mass about pivotal axis 64, and coil 75 is attached to housing 70 by two thin stainless steel plates 76. Plates 76 have a maximum width where they are attached to the housing. This maximum width is comparable with the width of the widest part of servo motor coil 75, and is much greater then the diameter of pivot bearing assembly 66. (Width being the dimension in the direction orthogonal to both axis 64 and to the long axis of actuator arm 65). Thus plates 76 are bonded to the opposing flat sides of servo motor coil 75 and to opposing flat and C-shaped areas 77 of housing 70 which extend to either side of pivot bearing assembly 66. The thickness of housing 70 over at least the C-shaped area 77 to which plates 76 are attached, is approximately the same as the thickness of coil 75. Thus housing 70 and coil 75 are sandwiched by plates 76 which are substantially flat. The width of housing 70 is approximately the same as the width of the plates 76, allowing the area 77 over which they are attached to extend across the entire width of plates 76, this being substantially the maximum width of the coil. At operational positions of actuator arm 65 , the straight and radially directed sides 92 of servo motor coil 75 and the adjacent parts of plates 76 are substantially within the gap 69 (shown in figure 12) between upper pole 68 and lower pole 67 of the servo motor magnet. During operation of the disk drive, electrical current is supplied by the servo controller to coil 75 which interacts with the magnetic field within gap 69 to cause the coil to move, and rotate the actuator arm, Thus the position of the recording heads is controlled. Plates 76 are about 0.1 mm thick (exaggerated in figure 11) and made of non-magnetic stainless steel so are not attracted to the magnet poles. Stainless steel has high electrical resistivity which reduces the undesirable effects of electrical currents which are induced in the plates by operation of the servo motor.

Pivot bearing assembly 66 is shown in section in FIGURE 12 and in a cut-away view in FIGURE 13, and comprises two ball bearings 80 each bonded to a shaft 81 and to an outer tube 82. During manufacture of the pivot bearing assembly 66, a force is applied to the bearings 80 during bonding to outer tube 82 and to shaft 81 so that each bearing applies an axial force acting on the other bearing, shaft 81 being in tension and outer tube 82 in compressioa By this means, which is known as axial preload, clearance is removed from both of bearings 80. A compensating ring 84, made of the same material as the housing (aluminium) is located within outer tube 82 and between bearings 80 contacts and supports the inner surface of outer tube 82. The outer profile of compensating ring 84 is relieved so that contact with outer tube is restricted to a narrow band 85 central to the length of outer tube 82.

Pivot bearing assembly 66 is housed within housing 70 and is firmly attached to housing 70 by a compression ring 86 pressed axially into a radial space between them.

Compression ring 86 has a cylindrical inner surface which is supported by outer tube 82, and an outer surface 87 having a conical taper with an included angle which should be less than three degrees, in order to be self locking. Compression ring 86 is made of aluminium and is cut through at reference 88 to form a C-shape so that it may be more easily be compressed in diameter. To receive compression ring 88, housing 70 has a tapered bore with a matching taper angle and with two internal notches 89. To assemble the pivot bearing assembly to the actuator arm, pivot bearing assembly 66 and housing member 70 are supported in the correct relative position, then compression ring 86 is pressed in an axial direction into the annular space between them. This compresses ring 86 by a wedging action, fixing housing 70 to the outer tube 82 of pivot bearing assembly 66 by friction.

The invention as described so far provides attachment of the coil through plates which are comparable in width to the coil and are attached across this full width to a housing. The theoretical justification for this emphasis on width and particularly the width of the connection to the housing, follows from the theory of bending of beams, where maximum stress and strain occurs at points most remote from the bending neutral axis, i.e. at the extremes of width; and at the section of greatest bending moment, Le. at the housing. (Formulas for Stress and Straia 5th Ed. Roark and Young. McGraw-Hill, Art 7.1, p92) The theoretical predictions have been found to be justified when the invention is reduced to practice. In respect of the width of the attachment between the housing and the structure (plates in this case) supporting the coil, this invention may be distinguished from prior art actuators, where the width of the housing is, for economy of mass, not much greater than the diameter of the pivot bearing assembly, that is much less than the width of the coil at its distal corners. Despite the increased width of the housing, its greatly reduced thickness according to the invention results in housing having a mass that is less than that of housings of the prior art. This combination of structural properties gives simultaneously both an increase in the "butterfly" mode of vibration of the arm (through increased stiffness), and an increase in the "rigid body" mode of vibration (through reduced mass). A large increase in system mode vibration frequency to 10 kHz has been measured for disk drives with 95 mm disks, compared with prior art actuators employing neither plates 16 nor a wide flat housing where system mode vibration frequency in the range 4 kHz to 5 kHz is commoa

The radically reduced thickness of housing 70 of this invention compared with the thickness or length of prior art housings, requires an alternative means of attachment of the pivot bearing assembly to the actuator arm. This means of attachment is provided by the compression ring 86. Its principle advantage is that it provides intimate structural connection between the pivot bearing assembly and the housing of the actuator arm through almost the entire surface of the tapered bore in housing 70. Thus the stiffness of the pivot bearing assembly, upon which the frequency of the rigid body mode of actuator arm vibration depends, is not reduced by the means of attachment to the actuator arm. This contrasts strongly with the prior art where structural connection between the pivot bearing assembly and the actuator arm or some intermediate member is often localised to a few lines of contact with consequent reduced stiffness and reduced frequency of vibratioa The compression ring has two other attributes needed for its role in coupling the actuator arm to the pivot bearing assembly, namely low mass, and freedom from contaminating adhesives.

The construction of pivot bearing assembly 66 as described above is particularly suited to the means of attachment of the pivot bearing assembly 66 to housing 70 by compression of a ring. This may be appreciated from the following discussion. It is necessary that the disk drive should operate over a wide temperature range. The bearing preload, which affects several aspects of servo response, must remain fairly constant as temperature changes. For this to be so it is necessary for shaft 81 and the outer tube 82 of the pivot bearing assembly to be made of the same material, and preferably made of the same material as the bearings, namely steel. But housing 70 is of aluminium, so these parts which differ greatly in their intrinsic thermal expansion must be attached in such a way that twisting movement or movement in the direction of the pivotal axis of the head mount blocks 72 with respect to pivotal axis 64 is minimised despite temperature change. This is a well known problem in disk drive actuator desiga The way that the invention as described above solves this problem is by ensuring that thermal distortion is symmetrical about the central plane of the bearing and therefore causes no relative motion of mount blocks 72. This end is achieved through compensating ring 84 which supports outer tube 70 against the inward thrust of tapered ring 86. Outer tube 70 being much thinner than compensating ring 84 has less hoop stiffiiess also, and thus the expansion of the composite structure of the steel outer tube 70 and compensating ring 84 is dominated by ring 84, and the aluminium housing 70 to which tube 70 is attached. Thus outer tube 70 expands naturally as steel at both its ends while at a central section it is constrained to expand as aluminium. Being symmetrical, such distortion causes no twisting or tilting of housing 70 relative to pivotal axis 4 and consequently no undesirable motion of mount blocks 72. It will be appreciated that because of the support given by compensating ring 84, outer tube 70 may be made thinner than it otherwise would without the possibility of being crushed under the inward compressive thrust of tapered ring 86. There is a net reduction of the mass supported by the pivot bearing. Some features are now described which although not essential to the operation or understanding of the invention, have been found to provide the best function. Notches 89 in housing member 70 provide extra resilience to the structure surrounding the pivot bearing. Such notches are optional to the operation of the invention but can further reduce contention between the expansion of steel and aluminium parts. A circular plastic gaiter 90 shaped to fit against outer tube 82 and against housing member 70 encloses the protruding part of tapered ring 86. The purpose of gaiter 90 is to trap any wear particles generated during assembly by the insertion of ring 86, preventing the particles from interfering with operation of the disk drive.

Plates 76 can be cut away in the corners furthest from housing 70 (reference 90) as this can result in a small increase in resonant frequency. Lightening holes 91 in plates 76 can have a similar effect, but it may be understood that for best performance such cut-aways and lightening holes must be carefully proportioned otherwise the resonant frequency of the actuator arm may be reduced if the stiffness of the structure is unduly effected.

An advantageous method of bonding plates 76 to housing 70 and to servo motor coil 75 is for plates 76 to be pre-coated with a fusible adhesive such as the plastic material polyamide on the inner sides. With the parts in their correct relative positions, they can be bonded together by pressing the assembly first between hot platens to fuse the polyamide, and then between cold platens to quickly solidify the adhesive. This "hot melt adhesive" process has the advantage of speed compared with the assembly processes which have to be used to attach the coils to state of the art disk drives. It will be clear to those skilled in the art that the advantages which have been referred are attained by the present invention and that the spirit of the invention as described and claimed may be adapted in many ways.

INDUSTRIAL APPLICABILITY The invention described in each of the embodiments applicable directly to industry. This is evidenced by the feet that the embodiments are improvements to the head actuator mechanism used in all hard disk drives produced today. A prototype of the fifth embodiment has been evaluated by a major disk drive manufacturer and found to be entirely practical as well as exhibiting the various advantages described above. The resonant frequency of the actuator arm was raised from around 5000 Hz in the manufacturer's original form to 9600 Hz with the improvements of the fifth embodiment of this inventioa

Claims

1. A disk drive including a base, at least one disk mounted on a spindle, a servo control system, and an actuator arm mounted on and including a pivot bearing assembly, said actuator arm having head arms with recording heads, and said actuator arm having a servo motor coil which is flat in the plane of rotation of said actuator arm and which moves within the gap of a servo motor magnet, said actuator arm having a plate parallel to the plane of said servo motor coil, said plate extending within the gap of said servo motor magnet, and said plate being attached to said servo motor coil and to another part of said actuator arm.

2. A disk drive as in claim 1 in which said plate has a section with width comparable with or greater than the width of said servo motor coil.

3. A disk drive as in claim 1 in which said plate is bonded to a substantial proportion of the area of a flat side of said servo motor coil.

4. A disk drive as in claim 1 in which said plate has lightening holes.

5. A disk drive as in claim 1 in which there are two of said plates.

6. A disk drive as in claim 1 in which said plate is made of stainless steel.

7. A disk drive as in claim 1 in which said plate is made of carbon fibre composite.

8. A disk drive including a base, a disk mounted on a spindle, a servo motor magnet, and an actuator arm mounted on a pivot bearing assembly, said actuator arm having a housing, head arms with recording heads, and a servo motor coil of insulated wire which is flat in the plane of rotation of said actuator arm and passes within the gap of said servo motor magnet, said actuator arm having two plates extending within said magnet gap, said plates hieing structurally connected to said actuator arm, and said coil being sandwiched between said plates and attached to said plates.

9. A disk drive as in claim 8 in which said coil is bonded to said plates

10. A disk drive as in claim 8 in which said pivot bearing assembly has a collar, said plates being structurally integral with members bonded to said collar.

11. A disk drive as in claim 8 in which said plates are integral extensions of with said head arms.

12. A disk drive as in claim 8 in which said head arms are made of carbon fibre composite material.

13. A disk drive as in claim 8 in which a corner of said plate is cut away.

14. A disk drive including a base, a disk mounted on a spindle, a servo motor magnet system, and an actuator arm mounted on a pivot bearing assembly, said actuator arm having a housing, head arms with recording heads, and a servo motor coil wound from insulating wire which is flat in the plane of rotation of said actuator arm and passes within the gap of a servo motor magnet, said actuator arm having two substantially flat plates parallel to the plane of said servo motor coil, and extending within said the gap of said servo motor magnet, said housing having opposing surfaces which are separated by a distance substantially equal to the thickness of said coil, said plates being attached to said opposing surfaces of said housing and to opposing sides of said coil, said plates remaining substantially flat.

15. A disk drive as in claim 14 in which said plates have width comparable with or greater than the width of said servo motor coil.

16. A disk drive as in claim 14 in which said plates are bonded to said servo motor coil and to said housing by fusion of a hot melt adhesive.

17. A disk drive as in claim 14 in which said member of said housing with opposing flat surfaces is a collar.

18. A disk drive as in claim 14 in which said pivot bearing includes two ball bearings, an outer tube, and a compensating ring located between said bearings, said compensating ring supporting said outer tube, and said compensating ring having a thermal expansion coefficient to match said housing .

19. A disk drive as in claim 14 in which said actuator arm is attached to said pivot bearing assembly by frictional forces exerted by a ring having a conical tapered surface, said ring being compressed between said pivot bearing assembly and a bore in said housing.

20. A disk drive as in claim 19 in which a gaiter is in contact with said pivot bearing assembly and with said housing member, such that particles generated by the compression of said tapered surface into said housing member are trapped within said gaiter.