A DUAL MAGNETIC FLUX PATH LINEAR VOICE COIL MOTOR

Abstract: A dual flux path linear voice coil motor (VCM) for use as a disk file linear actuator has a magnetically permeable central core (10) which is separated by a non-magnetic spacer (44) into two sections (40, 42) spaced apart along a longitudinal axis parallel to the VCM's linear access direction. The central core has recessed parallel flat faces (46, 47) equally spaced on opposite sides of the longitudinal axis which form pockets in the central core. Permanent magnets (48, 49) are embedded in the pockets of the central core and span the space between the central core sections. The VCM generates an essentially constant force on the coil for a constant current, regardless of coil stroke position or direction of movement.

Description: This invention relates to a dual magnetic flux path linear voice coil motor (VCM) for positioning read/write transducer heads to appropriate data tracks in a data recording disk file. A typical disk file linear VCM comprises a coil movable through the magnetic field of a permanent magnet stator. The VCM includes a fixed magnetically permeable central core and a magnetically permeable outer core surrounding and spaced from the central core. Permanent magnets are attached to the outer core and spaced from the central core to define a working gap through which magnetic flux passes. A movable coil is supported within the working gap and is connected to a carriage which supports the read/write heads. The application of current of proper magnitude and polarity to the coil produces a magnetic field which reacts with the magnetic field in the working gap to apply force to the coil and carriage to move the attached read/write heads radially relative to the disks. In disk files which have a relatively high density of data tracks on the disks, a servo control system is typically used to position the heads to the desired data tracks in the minimum time and to maintain the heads precisely over the center lines of the desired tracks during read or write operations. In order to determine the value of the control current to be applied to the coil of the VCM, it is necessary to know the relationship between the current applied to the VCM and the acceleration of the coil and carriage. This relationship is given by the acceleration factor (K f /M) which defines the acceleration of the coil and carriage per unit of input current. The term K f is the force factor which relates force applied to the coil to the input current, and M is the mass of the movable portion of the VCM (i.e. the coil, carriage, and attached head/arm assemblies). The value of the force factor K f typically varies with the linear stroke position of the coil in the gap of the VCM. In addition, the profile of K f with position is different for different directions of movement. Conventional servo control systems cannot compensate for any variation in K f with position, but rather assume a constant K f regardless of the actual K f . For this reason, it is desirable to design the VCM in order to make K f constant with position, as well as to make the K f profile the same regardless of the direction of movement. Certain disk file linear VCMs have a dual magnetic flux path through which the coil moves. In such disk files the variation of K f with position and direction is accentuated because at certain times the coil is subjected to the magnetic flux from both magnetic flux paths. US-A- 4439699 and US-A 4661729 describe a dual magnetic flux path VCM which has a gap in the outer return path to provide a symmetrical permanent magnetic flux distribution in the VCM. US-A 4652779 relates to improvements in the support structures for energised moving coils in rotary actuators. There is disclosure in US-A 652779 of division of a centre pole of the actuator assembly into two parts to achieve the prevention of reactance flux (due to coil inductance) from travelling through the assembly. JP-A 6295957 relates to a linear motor with a movable yoke moving integrally with a movable coil to facilitate the bypass of magnetic flux generated by electrical conduction to the movable coil. The linear motor has plate-formed permanent magnets on internal opposing surfaces of its stationary yoke. The invention seeks to provide a dual flux path linear VCM which generates an essentially constant force per unit of input current regardless of the position of the coil along the gap or the direction of movement of the coil. A dual magnetic flux path linear voice coil motor (VCM) for transducer head positioning in a disk file comprising a magnetically permeable central core having a central longitudinal axis and substantially two parallel faces on opposite sides of the central longitudinal axis; a magnetically permeable outer core generally surrounding the central core and having parallel inner walls, each of the inner walls being generally parallel to a corresponding face of the central core and having a generally flat permanent magnet attached thereto, and being spaced from the central core so as to define a gap; an electrically conductive coil surrounding the central core and located within the gap; and means for so supporting the coil within the gap as to allow linear movement of the coil through the gap along a path generally parallel to the longitudinal axis of the central core; the motor is characterised, according to the invention, by the central core comprising two generally like sections spaced apart along the longitudinal axis; and by each of the central core faces having attached thereto a generally flat permanent magnet having a magnetic polarity opposing the magnetic polarity of the magnet on the inner wall which it faces. The preferred VCM has a magnetic stator which comprises a rectangularly shaped central core divided into like sections and separated by a non-magnetic spacer, a surrounding outer core, and permanent magnets attached to both the central core and the outer core and spaced apart so as to define the working gap for the coil. The magnetically permeable central core has rectangularly shaped pockets for receipt of flat permanent magnets which span the non-magnetic spacer. Flat permanent magnets are attached to the corresponding inner walls of the outer core. A shorted copper turn surrounds the central core and is located between the central core and the coil. The arrangement of the magnets on the central core and the outer core, together with the non-magnetic spacer which separates the central core into two sections, provides a VCM with a flat K f profile which is essentially constant in both directions of coil movement. How the invention can be carried out will now be described by way of example, with reference to the accompanying drawings, in which:- Fig. 1 is an exploded view illustrating the component parts of a VCM embodying the invention; Fig. 2 is a cut-away perspective view of the central core of the VCM of Fig. 1, illustrating the non-magnetic insert and the embedded magnets; Fig. 3 is a longitudinal sectional view illustrating the dual magnetic flux paths of the VCM; Fig. 4 is a graphical comparison of force factor (K f ) as a function of coil position for a VCM without a non-magnetic insert and a VCM embodying the invention; and Fig. 5 is a graphical comparison of coil current rise time for a VCM with a conventional core and a VCM embodying the invention. As shown in Fig. 1, the major components of a VCM embodying the invention, are a central core 10 with embedded permanent magnets (not shown in Fig. 1); an outer core having two sections 12, 14 which generally surround the central core 10; permanent magnets, such as permanent magnet 16, on each of the outer core sections; and a movable coil 18. The outer core sections 12, 14 have generally flat inner walls to which the permanent magnets are attached, such as permanent magnet 16 attached to the inner wall of section 14. A like permanent magnet 17 (Fig. 3) is attached to the inner wall of outer core section 12. The outer core permanent magnets are generally parallel to and spaced apart from corresponding parallel flat faces of central core 10 so as to define a working gap through which the coil 18 is free to move. Attached to the coil 18 is a carriage (not shown) which has means for mounting the rigid arms (not shown) which support the read/write heads (not shown). The carriage is attached to the coil 18 by means of struts, such as struts 22, 24. As is well known in the art, high precision wheeled bearings attached to the carriage ride on guide rails, thereby permitting linear movement of coil 18 along longitudinal axis 38. When current is applied to coil 18 the current interacts with the magnetic flux in the working gap and creates a force in a direction parallel to the central longitudinal axis 38 of the central core 10, which is the radial direction of access of the read/write heads relative to the disks. The read/write heads are thus moved radially inwardly or outwardly with respect to the data recording disks, depending on the direction of current in the coil. The coil 18 is supported for free movement within the gap by means of the bearing assemblies and their associated guide rails. Referring now to Fig. 2, the construction of the central core with a non-magnetic insert and embedded magnets is illustrated. The central core 10 is formed of two generally like sections 40, 42 of magnetically permeable material, such as electrical iron (99.7% Fe). The sections 40, 42 are spaced apart along the longitudinal axis 38, which represents the access directions of the VCM, by a non-magnetic spacer 44. In the preferred embodiment the spacer 44 is made of a nickel-copper alloy, such as Monel (66.5% Ni, 31.5% Cu), and is attached to the sections 40, 42 by brazing so that sections 40, 42 and insert 44 provide a unitary structure for the central core 10. The central core 10 has two generally parallel flat faces 46, 47 (Fig. 3) on opposite sides of longitudinal axis 38 which form part of rectangularly-shaped cut-outs defining pockets. Located within each of the pockets is a permanent flat rectangularly shaped magnet, such as magnet 48 located adjacent to face 46, and a like magnet 49 located adjacent to face 47 (Fig. 3). A rectangularly shaped copper sleeve 50 surrounds the central core 10 and embedded magnets 48, 49. The copper sleeve 50 acts as a shorted turn and improves the response time of the VCM to the application of current by reducing the reluctance in the magnetic circuit. The dual magnetic flux paths of the VCM are illustrated in Fig. 3, which is a sectional view along axis 38, the path of movement of coil 18. The outer core sections 12, 14 include spacers 60, 62 and 64, 66 respectively, which are also formed of magnetically permeable material. The spacers can also be integrally formed as part of core sections 12, 14. Each of the magnets 48, 49 in the central core and their opposing magnets 16, 17 respectively, in the outer core have their magnetic polarities opposed to one another so that a magnetic field passes generally perpendicularly through a working gap 51 defined by the spaced-apart magnets. Thus as shown in Fig. 3 there are two magnetic flux paths, each flux path being located essentially in one-half of the stroke position of the coil 18 through the working gap 51. The central core 10 with non-magnetic insert 44 and embedded magnets 48, 49 substantially improves the performance of the dual flux path VCM, as illustrated in Fig. 4. The dashed lines of curves A, B represent the force generated on the coil 18 as a function of stroke position for radially inward and outward directions, respectively, for a constant current in a VCM identical to that represented in Figs. 1 to 3, but without the non-magnetic insert in the central core. The force curves A and B are nonlinear, especially near the ends of the stroke. In addition, curves A and B are not coincident, so that the VCM will have different response characteristics, depending on the direction of movement of the coil 18. The solid lines of curves C and D in Fig. 4 represent similar force curves but with the VCM embodying the invention and thus having a central core with a non-magnetic insert. The result is that the force on coil 18 is substantially more linear with stroke position and is essentially the same in both directions of movement. The non-magnetic insert 44 must have a finite width to completely separate and space apart the two central core sections 40, 42. Where the central core sections 40, 42 are not completely separated, e.g. if only a partial groove is located in the central core, the benefits shown in Fig. 4 are not obtainable. The improved coil current rise time of a VCM embodying the invention is illustrated in Fig. 5. Curve A represents the coil current rise time for a VCM similar to that shown in Fig. 1, but with a central core having no embedded magnets and no non-magnetic insert. Curve B represents the current rise time for the VCM as described herein.

Claims: A dual magnetic flux path linear voice coil motor (VCM) for transducer head positioning in a disk file comprising: a magnetically permeable central core (10) having a central longitudinal axis, two generally like sections (40, 42) spaced apart along the longitudinal axis and substantially two parallel faces on opposite sides of the central longitudinal axis; a magnetically permeable outer core (12, 14) generally surrounding the central core and having parallel inner walls, each of the inner walls being generally parallel to a corresponding face of the central core and having a generally flat permanent magnet (16, 17) attached thereto, and being spaced from the central core (51) so as to define a gap, each of the central core faces having attached thereto a generally flat permanent magnet (48, 49) having a magnetic polarity opposing the magnetic polarity of the magnet on the inner wall which it faces; an electrically conductive coil (18) surrounding the central core and located within the gap; and means (22, 24) for so supporting the coil within the gap as to allow linear movement of the coil through the gap along a path generally parallel to the longitudinal axis of the central core. A VCM as claimed in claim 1, further comprising a magnetically impermeable spacer (44) physically interconnecting the two sections of the central core. A VCM as claimed in claim 1 or claim 2, wherein the central core magnets are located within pockets recessed into the faces of the central core. A VCM as claimed in any preceding claim, further comprising a shorted electrically conductive turn (50) surrounding the central core and located between it and the coil. A VCM as claimed in any preceding claim, further comprising a carriage supporting at least one head and attached to the coil.

Inventors: MAWLA, KHALID ABDUL
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1989
Main classification: H02K41
IPC codes: H02K33, H02K41