WO1994017522A1

WO1994017522A1 - Rotary actuator in a magnetic recording system having square wire voice coil

Info

Publication number: WO1994017522A1
Application number: PCT/US1993/010342
Authority: WO
Inventors: Vivian H. Wang; David W. Basehore; Frank I. Morris
Original assignee: Maxtor Corporation
Priority date: 1993-01-21
Filing date: 1993-10-28
Publication date: 1994-08-04
Also published as: AU5454194A

Abstract

In a rigid disk drive having a circular disk (203) for storing digital data in a plurality of tracks, a transducer (202) for reading and writing the digital data from and to the disk (203), an actuator (201) coupled to the transducer (202) for positioning the transducer (202) from one track to another track according to a servomechanism. The actuator (201) is comprised of a stationary magnet which emanates a magnetic flux. A movable wire coil (204) is placed within this magnetic flux. When an electrical current is fed through the wire, a torque is produced which causes the actuator (201) carrying the transducer (202) to be moved. The wire comprising the coil (204) has a substantially square cross section for minimizing the resistance of the coil (204).

Description

ROTARY ACTUATOR IN A MAGNETIC RECORDING SYSTEM HAVING SQUARE WIRE VOICE COIL

FIELD OF THE INVENTION

The present invention relates to the field of magnetic recording systems. More particularly, the present invention pertains to the implementation of square wire in the voice coil of a rotary actuator.

BACKGROUND OF THE INVENTION

In the field of computer systems, digital data is typically stored onto a magnetic medium as a series of binary bits (i.e., 1 's and O's). A transducer, also known as a head, is used to write these bits onto the magnetic medium. Sometimes the same transducer is also used to read bits from the magnetic medium.

The magnetic medium usually takes the form of a circular disk. In rigid disk drive systems, a number of these disks are stacked and rotated about a spindle. A number of heads are utilized to access these disks. An actuator is implemented to position the heads in reference to fixed radial locations over the disks' surfaces. As the heads are sequentially moved radially across the spinning disks, a number of concentric rings are described. These concentric rings, onto which the binary bits are recorded, are referred to as "tracks".

Typically, the actuator consists of a voice coil motor having a permanent magnet structure and a movable bobbin or coil attached through a T plate to an armset for carrying the read-write heads. A servomechanism controls the movement of the actuator. Figure 1 is a block diagram illustrating a typical prior art actuator and servomechanism. Actuator 101 is comprised of a stationary E-frame 102, a moveable voice coil 105, and armset 106. Attached to the ends of armset 106 are a number of heads 107 for reading data from and writing data to the disks 108. Housed within E-frame 102 are two fixed magnets 103 and 104. According to the principles of electromagnetics, passing a current through a magnetic field produces a force. It is this force, induced by the current flowing in voice coil 105 through the magnetic field created by magnets 103 and 104, which acts upon actuator 101 to cause it to rotate about a pivot point. As actuator 101 is pivoted, the heads 107 are swept radially across disks 108.

A servomechanism is used to control the current supplied to voice coil 105, thereby controlling the movement of actuator 101 and heads 107. The servomechanism is comprised of a position error channel 110, controller 111 and power-current source 112. The servomechanism determines the present location of the actuator and compares this location to the desired position command input to determine the control signal for reducing the position error to zero.

When the host computer directs the servomechanism to position a head on a track which is different from the present track, the servomechanism is said to "seek". Typically, the host computer reads from and writes to the disk many times per operation. Furthermore, a block of data is frequently written onto tracks located at different parts of the disk, based on availability. Consequently, a number of seeks are often required for each operation. Since data cannot be written onto nor read from the disk during a seek, it is highly desirable for the seek to be completed as rapidly as possible. The time required to perform a seek determines the system's access time. A rigid disk's access time is a measurement of a key performance parameter. Therefore, one goal is to develop systems having ever faster access times.

One prior art method for reducing the access time is to increase the power supply voltage. By increasing the voltage, the current to the voice coil can be increased. This produces a greater torque on the actuator. A greater torque means that the actuator can be accelerated and decelerated more quickly, thereby reducing seek times. The problem, however, with this approach is that power consumption is, likewise, increased. Higher power consumption is especially detrimental for rigid disk drives destined for use in portable computer systems such as laptop and notebook applications because battery life is reduced. Another problem is that of heat dissipation. The heat produced by the increase in power must somehow be dissipated.

Another prior art method for improving access time is to reduce the moveable mass, which reduces its inertia so that it can be accelerated and decelerated more quickly. This is accomplished by reducing the dimensions of the actuator, armset, and voice coil. Furthermore, lightweight materials such as aluminum are being implemented. However, further reductions in size compromises structural integrity. Implementing more exotic materials which are stronger, yet lighter, is prohibitively expensive.

Other prior art methods for improving the access time involve increasing the gap flux density by implementing strong magnets and increasing the length of the coil wire in the gap flux density.

However, stronger magnets can become over saturated more easily, which can detrimentally impact the performance of the head. And although increasing the number of turns of the coil yields a higher torque constant, it also has the undesirable effect of increasing the overall resistance. Moreover, there are physical space limitations imposed, which determines how much wire can be utilized.

Therefore, what is needed is a mechanism which decreases the access time with minimal degradations in other rigid disk drive parameters.

SUMMARY AND OBJECTS OF THE INVENTION

In view of the problems associated with performing seeks in a rigid disk drive system, one object of the present invention is to reduce the average access time of the disk drive.

Another object of the present invention is to reduce the average access time by increasing the torque acting on the actuator.

Another object of the present invention is to increase the torque on the actuator by decreasing the resistance of the wire associated with the voice coil.

Yet another object of the present invention is to decrease the resistance of the wire used in the voice coil by implementing a square wire, rather than a circular wire.

These and other objects of the present invention are implemented in a rigid disk drive system having an actuator for positioning a read/write head in reference to a magnetic media in order to read and write data from and to certain portions of the magnetic media. The actuator holding the read/write head is swung about a pivot point by a voice coil motor. The voice coil motor is comprised of a stationary structure which houses a magnet. The magent emanates a magnetic flux. A voice coil is formed by winding a copper wire a number of turns. The copper wire used to form the voice coil has a substantially square cross-section in order to minimize the resistance of the voice coil. The voice coil is placed within the magnetic flux. When a current is fed through the voice coil, a torque is produced which causes the actuator holding the read/write head to swing about the the pivot. By minimizing the resistance in the voice coil, more current can be supplied to the voice coil with a given voltage supply. This means that larger acceleration in deceleration forces can be brought to bear upon the actuator. Consequently, this tends to reduce the average access time.

Care is taken in winding the square wire so that the wire does not become twisted in order to minimize any gaps when forming the voice coil. In the currently preferred embodiment, a copper wire having the dimensions of approximately 0.0063 inches by 0.0063 inches is utilized in forming the voice coil. A voice coil formed by such a wire has a resistance of approximately 13.5 ohms.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:

Figure 1 is a block diagram illustrating a typical prior art actuator and servomechanism.

Figure 2A is a top view showing an actuator upon which the present invention may be practiced.

Figure 2B is a side view showing an actuator upon which the present invention may be practiced.

Figure 3 is a perspective view showing another embodiment of an actuator upon which the present invention may be practiced.

Figure 4 shows a cross-sectional view of a prior art voice coil utilizing a circular wire.

Figure 5 shows a cross-sectional view of a voice coil utilizing a substantially square wire of the currently preferred embodiment of the present invention.

Figure 6 shows the forces acting on a voice coil and a magnetic flux when current is conducted through the voice coil. DETAILED DESCRIPTION

An apparatus and method for reducing the resistance of the wire used in the voice coil of a magnetic recording system to increase torque and reduce the average access time is described. In the following description, for the purposes of explanation, numerous specific details such as wire dimensions and lengths, etc., are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the present invention.

Figure 2A is a top view of an actuator 201 upon which the present invention may be practiced. Actuator 201 is used to position a number of heads 202 across the surfaces of a number of disks 203. A voice coil 204, made up the windings of a copper wire, is firmly attached to actuator 201. The voice coil 204 is placed in between two stationary magnets (not shown). The magnets emanate a magnetic flux. Voice coil 204 resides within this magnetic flux. When a current is fed to voice coil 204, the current in conjunction with the magnetic flux, produces a force which causes actuator 201 to swing about pivot point 205. Likewise, armset 206, which is attached to the body of actuator 201 , also pivots about point 205. Flexures 207, also known as gimbals, are attached to the ends of armset 206. The heads 202 are attached to the other ends of the flexures 207. Consequently, as armset 206 is pivoted, the heads 202 are radially swept across the surfaces of disks 203. The combination of the heads 202, flexures 207, armset 206, and voice coil 204 is commonly referred to as a head gimbal assembly (HGA). A number of wires run the length of armset 206 and flexures 207 which couple the heads to a flex circuit 220. During read operations, these wires conduct the electrical signal picked up by heads 202 to flex circuit 220. Flex circuit 220 is comprised of a flexible piece of plastic material with embedded conductors. Flex circuit 220 is attached to the side of actuator 201 by bracket 221 and flexes in tandem with the movement of actuator 201. The read signals are fed into a pre-amplifier and accordingly processed.

Figure 2B is a side view of actuator 201 showing the three arms of armset 206. Four flexures 207 are attached to the arms of armset 206. Two flexures are attached to the upper and lower portions of the middle arm. Four heads 202 are attached at the ends of flexures 207, one head per flexure. The four heads 202 are used to access the upper and lower surfaces of the two disks 203. Voice coil 204 is placed across from armset 206. Pivot 205 resides in between armset 206 and voice coil 204. A number of wires 230 couple the heads 202 to flex circuit 220.

A standard servomechanism is used to control the amount of current supplied to voice coil 204, thereby controlling the movement of actuator 201. In this manner, the servomechanism directs the positioning of the heads 202 for performing a seek.

Figure 3 is a perspective view showing another type of actuator 304 upon which the present invention may be utilized. Again, a number of heads 301 are used to read and write digital data from/to a platter of disks. Heads 301 are attached to flexures 302. In turn flexures 302 are attached to the ends of a plurality of arms 303. Opposite arms 303 are four tines 306 attached to the body of actuator 304. Tines 306 are used to hold voice coil 307. A crash stop 305 is used for positioning actuator 304 in its resting position.

It should be appreciated that the present invention can be utilized in the voice coils of many different actutor designs and can also be applied to linear as well as rotary actuators.

In the prior art, the wire used in a voice coil is typically circular. Figure 4 shows a cross-sectional view of a prior art voice coil 400, utilizing a circular wire 401. It can be seen that no matter how closely packed the circular wires are wound, there will always be gaps 402, in the spacing. The gaps 402 are shown as shaded areas in Figure 4.

In the currently preferred embodiment of the present invention, the voice coil is made of a square wire. Figure 5 shows a cross-sectional view of the voice coil 500 of the present invention, in which a square wire 501 is implemented. Note that gaps between the wires are minimized. The significance of this is that for a given area, a square wire can be coiled together in comparision to a circular wire implementation. In other words, the cross-sectional area occupied by a square wire is greater than the cross-sectional area occupied by a circular wire for the same total cross- sectional area. This is because the gaps associated with a coiled square wire is much less than the gaps associated with a circular wire. For example, the cross-sectional area of a wire having a diamteter of 2.0 inches is given by the equation (π) (1.0 in)² = 3.14 in². In comparison, the cross-sectional area of a 2.0 inch square wire is (2.0 in) (2.0 in) = 4.0 in². Consequently, a square wire coil has approximately _{4 0} — = 21.5%

greater cross-sectional area than a similar circular wire coil. Increasing the cross-sectional area occupied by the wire in a voice coil of a hard disk drive is advantageous because it helps reduce the average access time, as will be explained in detail below.

Figure 6 shows the forces acting on a voice coil 601 in a magnetic field when it is conducting current. According to the principles of electromagnetics, when voice coil 601 is conducting a current i in the presence of a magnetic field B (resulting from a fixed magnet), a resulting force F is produced. The force is described by the equation t = B X T. It is this force, F, which causes the actuator to pivot. For rotary actuators, this force is expressed in the form of torque. An increase in the torque acting on the actuator produces greater acceleration and decelerations of the actuator. This helps reduce the average time to perform a seek. Consequently, a greater actuator torque helps reduce the access time.

The actuator torque can be expressed as (kt i), where k_t is the torque constant, and i is the coil current. The torque constant, k_t, is equivalent to Bgl_COjir, where b_g is the flux density produced by one or more fixed magnet(s), l_COj| is the length of coil wire in the flux density, and r is the torque radius defined as the bearing-to-center-of-force distance (i.e., the distance from the pivot point to the center of the voice coil). Consequently, if the current is increased, the torque would also increase.

The current is given by Ohm's Law: V = iR or i = V/R, where V is the voltage and R is the resistance. Applied to a voice coil, V = Emf - ke(ω). Hence, decreasing the resistance of the voice coil wire increases the current.

The resistance of the wire, R, is defined by the equation R = p L/A, where p is the wire's resistivity, L is the length of the coil, and A is the coil's cross-section. It is apparent that increasing the cross-section, A, would decrease the resistance. In turn, a lower resistance leads to an increase in the current, given the same supply voltage. A greater current produces a greater torque, which means that the acceleration and deceleration forces on the actuator becomes greater. Thereby, the average access time is reduced.

In the currently preferred embodiment of the present invention, a square copper wire measuring approximately 0.0063 inches by 0.0063 inches, available from Belton Industrial Limited of Tsing Yi, Hong Kong, is utilized in the voice coil. Care is taken to ensure that the square wire is not twisted in wrapping the wire for forming the voice coil. The square wire is wrapped 291 turns, seven layers deep. The resistance of the square wire is approximately 217 ohms per 1 ,000 feet. In comparison, the resistance for a similar circular copper wire is typically 263 ohms per 1 ,000 feet. Thus, by implementing a square wire, the resistance is reduced by approximately 21.2%.

In an alternative embodiment, wire having a substantially rectangular cross-section is implemented in the voice coil. A rectangular wire also tends to minimize the coil resistance.

Thus, an apparatus and method for minimizing the resistance of a voice coil in a rigid disk drive is disclosed.

Claims

CLAIMSWhat is claimed is:

1. In a rigid disk drive having a circular disk for storing digital data in a puralilty of tracks, a transducer for reading and writing said digital data from and to said disk, an actuator coupled to said transducer for positioning said transducer from one track to another track according to a servomechanism, said actuator comprising: a stationary magnet emanating a magnetic flux; a movable wire coil residing within said magnetic flux, wherein an electrical current conducting through said wire produces a torque which moves said wire coil causing said transducer to be moved, said wire coil comprising a wire having a substantially square cross-section for minimizing resistance of said wire coil.

2. The actuator of Claim 1 , wherein said substantially square wire is coiled a plurality of times in a manner which minimizes gaps between coils.

3. The actuator of Claim 2, wherein said subtantially square wire is approximately 0.0063 inches by 0.0063 inches.

4. The actuator of Claim 3, wherein said wire coil has a resistance of less than 13.5 ohms.

5. An actuator in a magnetic storage system for positioning a head over radial locations of a magnetic media on which digital data is stored to access particular portions of said data, comprising: an actuator body; a member attached to and extending outwards from said actuator body; a flexure attached at one end to said member; a slider attached to the other end of said flexure, wherein said head is attached to said flexure; a coil of wire attached to said actuator body, wherein an electrical signal in said wire causes said actuator body to rotate about a pivot point, said wire having a square cross-section for minimizing resistance of said coil.

6. The actuator of Claim 5, wherein said wire is wound a plurality of times in a manner which minimizes gaps in forming said coil.

7. The actuator of Claim 6, wherein said wire is comprised of copper.

8. An improved actuator for positioning a transducer in reference to a magnetic media upon which digital data is stored in order to access a particular block of said data, comprising: a stationary frame for housing a magnet fixedly attached to said frame, said magnet producing a magentic flux; a moveable coil residing within said magnetic flux, wherein an electrical signal conducted through said coil is translated into radial movement of said coil and said transducer about a pivot point, wherein the improvement comprises said coil having a substantially square for minimizing resistance of said coil.

9. In a rigid disk drive having a circular disk for storing digital data in a puralilty of tracks, a transducer for accessing said digital data, an actuator coupled to said transducer for positioning said transducer from one track to another track according to a servomechanism, said actuator comprising: a stationary magnet emanating a magnetic flux; a movable wire coil residing within said magnetic flux, wherein an electrical current conducting through said wire produces a torque which moves said wire coil causing said transducer to be moved, said wire coil comprising a substantially rectangular wire for minimizing resistance of said wire coil.