Electrostatic track following using patterned media

A transducing head positioning system for use with patterned media. The patterned media comprises a plurality of data tracks and grooves. The grooved tracks can be used in connection with electrodes on a slider to create an electrostatic motor for microactuation of the slider.

BACKGROUND OF THE INVENTION

The present invention relates to a disc drive system. In particular, the present invention relates to a head positioning system capable of accommodating ever higher areal density of computer discs.

Disc drive systems are well known in the art and comprise several discs, each disc having concentric data tracks for storing data. The discs are mounted on a spindle motor, which causes the discs to spin. As the discs are spinning, a slider suspended from an actuator arm “flies” a small distance above the disc surface. The slider carries a transducing head for reading from or writing to a data track on the disc.

In addition to the actuator arm, the slider suspension comprises a bearing about which the actuator arm pivots. A large scale actuator motor, such as a voice coil motor (VCM), is used to move the actuator arm over the surface of the disc. When actuated by the VCM, the slider can be moved from an inner diameter to an outer diameter of the disc along an arc until the slider is positioned above a desired data track on the disc. Called tracking, this method of positioning the slider above the desired track on the disc allows the transducing head on the slider to either read from or write data to a selected track on the disc.

The areal recording density of the disc is typically given in tracks per inch (TPI). There is constant pressure to increase the areal density of discs, and thus increase the number of tracks per inch on the disc. As the tracks per inch increase, the accuracy of the system used to position the transducing head above the desired track on the disc must increase in proportion. This requires increasing the bandwidth of the servo system used to position the actuator arm.

There are many sources of error which reduce the track positioning accuracy of current slider suspension systems. The actuator arm is designed to be flexible to improve the ability of the slider to more closely follow the surface of the disc. However, this flexibility can result in the occurrence of unwanted resonances in the suspension as the suspension is moved across the disc surface during tracking. These unwanted resonances in the suspension reduce the ability to accurately control the slider positioning system at the required frequency. In addition, forces acting at the VCM, the bearing, and the actuator arm may all introduce potential error into the final tracking ability of the slider by adding to the resonance experienced in the actuator arm.

In an attempt to manage the amount of resonance in the suspension, secondary microactuators have been placed between the suspension and the slider. Moving the slider directly by using a form of microactuator has reduced, but has not eliminated the effect of suspension resonances. In particular, as the actuation force is applied to the slider by the microactuator, an equal and opposite reaction force is applied to the suspension, which in turn can create other resonance disturbances in the suspension. Control systems have been developed which attempt to compensate for the resonance and vibration experienced by the slider. However, such attempts reduce, but do not eliminate the effect of suspension resonances.

Microactuators and control systems have improved the tracking accuracy of sliders to where areal densities of up to 200,000 TPI may be possible. However, current goals are for discs having areal densities of as high as 500,000 to 1,000,000 TPI. At such areal densities, current slider positioning methods become inadequate.

Thus, there is a need in the art for an improved head positioning system which is capable of accommodating discs with ever higher areal densities.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to a head positioning system capable of positioning a slider over a medium having up to 500,000 to 1,000,000 TPI. The head positioning system comprises a slider positioned over a patterned media. The patterned media comprises a disc having a plurality of tracks and grooves. Located on the slider's medium opposing surface are a plurality of electrodes. The electrodes can be selectively activated so that together with the tracks on the disc, the electrodes operate as an electrostatic motor. Using the electrostatic motor, the slider can be finely positioned at a desired track on the disc.

DETAILED DESCRIPTION

FIG. 1is a perspective view of a disc drive actuation system10for positioning a slider12over a selected data track14of a magnetic storage medium16, such as a disc. The actuation system10includes a voice coil motor (VCM)18arranged to rotate a slider suspension20about an axis22. The slider suspension20includes a load beam24connected to an actuator arm26at a slider mounting block. A flexure28is connected to the end of the load beam24, and carries the slider12. The slider12carries a magneto-resistive (MR) element (not shown) for reading and/or writing data on the concentric tracks14of the disc16.

The disc16rotates around an axis30, which causes the slider12to “fly” a small distance above the surface of the disc16. To position the slider12at a desired track14on the disc16, the VCM18actuates the slider suspension20about the axis22so that the suspension20is moved in an arc across the surface of the disc16. This arc shaped movement allows the slider12to be moved from an inner diameter to an outer diameter of the disc16so that the slider can be positioned above the desired track14on the disc16. A variety of sources of positioning error are introduced during this process and while the disc drive10is capable of operation at up to 200,000 TPI, it is inadequate for TPI's in excess of that number.

The present invention improves upon the disc drive illustrated inFIG. 1to allow for TPI of up to 500,000 to 1,000,000 TPI.FIG. 2is a greatly enlarged side view of a slider40positioned above a patterned medium42, such as a disc. The slider40comprises a slider substrate44and a disc opposing surface46. On the disc opposing surface46are a plurality of phase1electrodes48, phase2electrodes50, and phase3electrodes52.

The patterned media42comprises a disc having a plurality of tracks54, labeled A-E, and grooves56. It is anticipated that such a patterned media, in the form of tracks54and grooves56, will be necessary in order to increase the bit density possible before the onset of the superparamagnetic limit, which currently sets a lower size limit on the magnetic particle size of the storage media.

The tracks54on the disc42can be used in conjunction with the phase1,2, and3electrodes48-52to form an electrostatic motor58for microactuation of the slider40. An application of a voltage to a phase1,2, or3electrode48-52, causes an electrostatic attraction between the activated electrode48-52and the closest track54on the disc42over which the activated electrode48-52is located. By controlling the application of voltage to selected phase1,2, or3electrodes48-52, it is possible to finely position the slider40above a selected location on the disc42.

When moving the slider40using the electrostatic motor58, an electrostatic force is generated between the disc42and the slider40. As a result, unlike using a microactuator between the slider and the suspension to move the slider, there is no reactive force acting on the suspension when using the electrostatic motor. This reduces the negative effects of suspension resonances on the tracking ability of the slider40.

One method of operating the electrostatic motor58is to apply a voltage to only one set of either the phase1, phase2, or phase3electrodes48-52at any given time. The set of electrodes48-52is activated by applying an electrical potential to the selected electrodes48-52while the other electrodes48-52remain at ground potential, as does the disc42. More specifically, activating the phase1electrodes48will cause the phase1electrodes48to center over the closest data track54on the disc42, or tracks A and E as shown in FIG.2. Activating the phase2electrodes52and deactivating the phase1electrodes50will cause the slider40to move ⅓ of a track56to the left (as viewed inFIG. 2) so that the phase2electrode50is centered over track B. Similarly, activating the phase3electrodes52and deactivating the phase1electrodes48will cause the slider40to move ⅓ of a track54to the right, so that the phase3electrode52is centered over data track D. Thus, by properly sequencing the application of voltage to phase1, phase2, and phase3electrodes48-52, it is possible to move the slider40any desired distance across the disc surface42.

A control system can be used to control the tracking of the slider40by controlling the application of voltage to the electrodes48-52. Such a control system can be used to cause the slider40to lock onto a desired track54by selectively activating the electrodes48-52. In doing so, the control system causes the electrostatic motor58to act similar to a stepper motor. As a result, the control system allows for automatic track following without a closed loop servo feedback system.

For instance, a positioning resolution of ⅙ of a track can be obtained by first activating the phase1electrodes48, centering the phase1electrodes over tracks A and E. Next, both phase1electrodes48and phase3electrodes52are activated, causing the slider to move slightly so that the phase1electrodes48are no longer centered over tracks A and E, but are shifting slightly to the right (as viewed in FIG.2). Finally, activating the phase3electrodes52but not the phase1electrodes48results in the slider moving to the right (as viewed inFIG. 2) until the phase3electrode52is centered over track D. As a result of this sequence of activation, the slider is moved ⅓ track to the right. Thus, continuos analog positioning can be obtained by adjusting the voltages applied to the electrodes48-52in a continuous analog manner, rather than by simple digital switching.

The patterned media42may have various ratios of tracks54to grooves56. For instance, as is illustrated inFIG. 2, the width of the tracks54may be equal to the width of the grooves56. Other ratios may be suitable for use with the present invention as well, including a track width to groove width ratio of 60/40. Similarly, the spacing of the electrodes48-52on the slider40may vary based on the track54spacing on the disc42. InFIG. 2, the spacing of the electrodes48-52on the slider40is 4/3 of the track54to track54spacing on the disc42.

Regardless of the spacing of the tracks54, it is desirable that the width of the electrodes48-52on the slider40be about equal to the width of the tracks54on the disc42. If the electrodes48-52are wider than the tracks54, the ability of the electrodes48-52to follow the tracks54is diminished. If the electrodes48-52are narrower than the tracks54, the electrodes48-52may wander the width of the tracks54, reducing the performance and accuracy of the tracking ability of the slider40.

The maximum force available for moving or holding the slider40with respect to the tracks54on the disc42depends on several factors, including: the number and length of active electrodes48-52; the gap between the slider40and the disc42; and the voltage applied to the active electrodes48-52. The relationship of these factors is given in the below equation:Flat=N⁢ɛo⁢L2⁢d⁢V2
Where Flatis the lateral force in newtons, N is the number of active electrodes, d is the gap between the disc and slider electrodes in meters, ε0, is the dielectric constant of the gap (8.854×10−12F/m for air or a vacuum), L is the length of the active electrodes in meters, and V is the voltage between the active slider electrodes and the disc.

When an active electrode48-52is positioned directly above a track54, the alignment force is essentially zero. Once the slider starts moving off a track, the lateral force begins to increase. The maximum lateral force occurs when the active electrode is misaligned from a track by about the fly height, or gap. The lateral force stays essentially at the maximum until the active electrode48-52moves further away from a track54than the fly height.

As is shown in the above equation, the lateral positioning force varies with the square of the voltage between the slider electrodes48-52and the disc42. The amount of voltage that can be applied to the slider electrodes48-52is limited by a variety of factors. At a gap of 0.1 microinches (about 2.5 nanometers), the voltage limit is not due to the electrical breakdown strength of air, which actually increases for air at standard pressure for a spacing of less than about 7 micrometers. Rather, at gaps of less than about 1 nanometer, electron tunneling currents are likely to be the limiting factor.

At gaps of 2.5 nanometers, the voltage limit will likely be due to field emission. The electric field at which significant field emission occurs varies greatly depending on the material used for the electrodes48-52. The table below gives the magnetic field strength at which significant field emission occurs for a variety of materials:

Using the above values for maximum electric field strength, the maximum possible voltage for a gap of 2.5 nanometers would be 35 volts for Tungsten, 9.25 volts for Lanthanum hexaboride, 1.25 volts for the Beryllia particle metal/insulator system, and 0.175 for the p-doped diamond material.

Another limiting factor for electrode voltage is the normal attraction force between the disc and the slider electrodes. The normal force is given in the below equation:Fn=N⁢ɛo⁢Lw2⁢d2⁢V2
where w is the width of the slider electrodes and all the variables are the same as those previously defined above.

The normal force (Fn) is equal to the lateral force (Flat) multiplied by w/d. Thus, for slider electrodes having a width of 25 nanometers and a gap of 2.5 nanometers, the normal force is 10 times larger than the lateral positioning force. Typically, the total pre-load force supported by the air bearing in current slider designs is about 2.5 grams. As such, the electrostatic attraction force needs to be limited to about 1 gram unless a drastic redesign of the air bearing is performed because of the risk that the normal force will overcome the pre-load force such that the slider crashes into the disc.

In addition to the amount of voltage applied to the electrodes, the lateral positioning force available for moving or holding the slider40depends on the length of the electrodes on the slider. The patterned storage medium is typically a disc having concentric data tracks on its surface. The curvature of the concentric data tracks on the disc limits the length of the electrodes on the slider. This is because the radius of the data tracks at the inner diameter of the disc will be smaller than the radius of the data tracks at the outer diameter of the disc. As a result, the depth of curvature of the data tracks at the inner diameter is not equal to the depth of curvature of the data tracks at the outer diameter of the disc. Thus, the electrodes must be of a length that allows them to “fit” the depth of curvature of the data tracks at both the inner diameter, as well as at the outer diameter.

To illustrate this,FIG. 3shows a simplified diagrammatic view of a slider positioned at various locations on a disc. Shown inFIG. 3are several data tracks60having varying depths of curvature corresponding to data tracks on a disc at the inner diameter moving toward the outer diameter. In addition, a slider62having two electrodes64is shown. At a first position66, the slider62is positioned closest to the inner diameter. When so positioned, the electrodes64on the slider62do not match the data tracks60. Similarly, at a third position70, corresponding to an outer diameter of the disc, the electrodes64on the slider62likewise do not match the data tracks60. The only time the electrodes64are exactly aligned with the data tracks60is when the slider62is located at a second position68, located somewhere between the inner and outer diameter of the disc.

The longer the electrodes64, the more difficult it is to ensure that the electrodes64properly align with the data tracks60at both the inner diameter and the outer diameter such that the electrodes64can efficiently be used to position the slider62. As such, it is desired to have electrodes64with a length that will minimize the misalignment of the electrodes64and data tracks60.

FIG. 4is an illustration of curvature depth of a circle. For a circle defined by r2=x2+y2, the curvature depth d is a function of the radius r and the distance y from the radius. This relationship is given by the following equation:
d=r−√{square root over ((r2−y2))}

FIG. 5is a table80illustrating the amount of track curvature, and the difference in that curvature with respect to a radius of 28 millimeters, for various radii from 16 to 44 mm. Each cell of the table80contains two numbers. The upper number is the curvature, and the lower number is the change in curvature between that radius and the 28 millimeter radius. The far left column of the table80represents the orthogonal offsets from the radius line of between 0.000 and 0.100 mm. This offset in the y axis corresponds to the length of the electrodes on the slider.

As illustrated inFIG. 3above, as the change in curvature with respect to the 28 mm radius increases, the misalignment between the slider electrodes and the data tracks will increase. Thus, the maximum tolerable y value is limited by the change in curvature with respect to the 28 mm radius. Because the curvature is symmetric about the radius line, the electrode length can be twice the maximum tolerable y value in the table80.

The misalignment typically occurs at the ends of the slider electrodes. For the 500,000 TPI example illustrated above, if the misalignment between the ends of the electrodes and the data tracks exceeds 1 microinch, that portion of the electrode will be attracted more strongly to the adjacent track than the correct track, and the net positioning force will be reduced. However, the curvature mismatch at the ends of the slider electrodes will cause an offset in the opposite direction at the center, so the actual amount of curvature mismatch before the ends of the slider electrodes provide a negative contribution to the positioning force will be greater than 1 microinch but less than 2 microinches.

In light of this effect, the numbers in the table illustrated inFIG. 5can assist in determining suitable lengths of electrodes. The numbers in area82indicate a positive contribution to positioning force for the entire length of the electrodes. The numbers in area84indicate a possible negative contribution from part of the length of the electrodes. Finally, the numbers in area86indicate a definite negative contribution from part of the electrode length. Thus, as can be determined from table80inFIG. 5, for a minimum track radius of 20 millimeters and a maximus radius of 44 millimeters, with the slider electrodes centered about the radius line, the length of the slider electrodes for maximum force is between 0.12 millimeters and 0.16 millimeters (2×0.060 and 2×0.080 mm).

To help maintain proper alignment of the slider electrodes to the tracks on the disc as the slider moves from the inner diameter to the outer diameter of the disc, it is necessary to utilize a suspension mechanism which maintains a fixed, rather than a rotating orientation of the slider to the disc. Currently, the slider is moved over the disc using a VCM with a central pivot bearing, such as that illustrated inFIG. 1above. Using a VCM results in an arc shaped path as the slider moves over the disc, rather than a linear shaped path. One method of obtaining the required linear path of the slider over the disc is to utilize linear actuation mechanisms.

One suitable linear actuation mechanism involves adding compliant springs between the slider and the slider suspension, similar to springs used in connection with other head level microactuators. A VCM or other primary positioning mechanism can still be used for coarse positioning and seeking to within some relatively small distance of the desired track. Once the primary positioning mechanism has positioned the slider to within about a half of a track of the desired position, the electrostatic motor can be actuated by activating the proper slider electrodes. The compliant springs will allow the slider to lock in and follow the desired track. Using the electrostatic motor allows for fine positioning, with no additional servo feedback required. In addition, if initial positioning error results in the slider being off by one or more tracks, the electrostatic positioning can also be used to step to the desired track.

A second method of linear actuation, though similar to the first, reduces the mass that must be moved by the electrostatic actuation. Reducing the moving mass greatly reduces the seek time of the electrostatic motor. Placing the compliant springs between slider and the slider suspension, as in method1, requires moving the entire slider. In contrast, the second method involves placing compliant springs between the head and the slider. To do so, the compliant springs may be fabricated during the slider and transducing head manufacturing process. When the compliant springs are located between the head and the slider, only the head (and a thin substrate and overcoat encapsulating the head) must be actuated using the electrostatic motor. As a result, the moving mass is greatly reduced, and the electrostatic positioning acceleration is increased by 10 to 100 times.

A third method of linear actuation requires eliminating the primary positioning system entirely. In its place, only the electrostatic actuation is used for both track following and the coarse positioning used during seeking. To provide the required alignment of the slider electrodes to the disc as the slider sweeps across the disc surface, a folded flexure suspension mechanism may be used. The folded flexure suspension mechanism will increase the mass that must be moved by the electrostatic motor. As a result, seek times will be longer. Due to the longer seek times, this method may be better suited to low cost applications where speed is not the most important parameter.

In addition to the length of the electrodes, the number of the slider electrodes located on the slider will have an effect on the performance of the electrostatic motor.FIG. 6is a simplified bottom plan view of a slider illustrating the medium opposing surface of a slider90. The slider90has a trailing edge92and an air bearing surface (ABS)94. Located on the ABS are the electrodes, indicated generally by96. The number of slider electrodes96is determined by the TPI and the width of the trailing edge air bearing surface94.

A slider may have an ABS width of about 110 micrometers. The entire width of a slider may be 1 millimeter, in which case the ABS amounts to about one tenth of the total slider width. Based on a slider having these parameters, the number of electrodes which can be located on the ABS is approximately 541: (100 μm)(500 tracks/microinch)/(25.4 μm/microinch)(¼ active electrodes per data track)=541.

If the entire width of the trailing edge of the slider, rather than just the width of the ABS, was used for the electrodes, the number of electrodes could be increased by about 9 times. However, any such attempt to place electrodes on the entire width of the slider would require a drastic redesign of the air bearing.

Tables 2a-2c and 3a-3c below provide a comparison of the results of modeling the performance of electrostatic slider positioning systems having: 1) electrodes across the trailing edge ABS; and 2) having electrodes across the entire trailing edge. Each table shows the lateral positioning force, Flat, and normal attraction force, Fn, as well as the resulting lateral accelerations possible at selected voltages.

Tables 2a-2c illustrate the performance of an electrostatic slider positioning system with electrodes located across the trailing edge ABS. The data in Tables 2a-2c is based on a slider having 541 active slider electrodes covering a width of about 110 microns, which is typical of the width of a trailing edge airbearing surface. In addition, the length of the electrodes was about 1.20×10−4meters, while the gap between the disc and the slider electrodes was about 2.50×10−9meters.

Table 2a provides the acceleration in g's for a Femco Si slider having amass of 6×10−7kilograms. Table 2b provides the acceleration in g's for a Pico AlTiC slider having a mass of 1.6×10−6kilograms. Looking at Tables 2a and 2b, a 1 gram normal force (Fn) imposes a 3 volt limit on the electrode voltage, which generates 1.0 mN of positioning force (Flat). As shown in Table 2a, moving the entire slider would provide an acceleration of 176 g's for a 0.6 mg Femco Silicon slider. As shown in Table 2b, an acceleration of 66 g's can be achieved for a 1.6 mg AlTic Pico slider.

An acceleration of 176 g's is near the maximum anticipated acceleration for seeking in high performance drives. In comparison, an acceleration pf 66 g's is rather modest performance which would have limited applications in high performance drives. If a simple air bearing design change allowed operation of the electrodes at 5 volts, the resulting normal attractive force of 2.9 grams, and a lateral force of about 219 mN, the Femco Silicon slider would have an acceleration of 488 g's and the AlTiC Pico slider would have an acceleration of about 183 g's. Both such performances would be suitable for high performance drives.

Table 2c shows the accelerations in g's for four head level actuators, such as by using compliant springs between the slider and head as described above. Table 2c shows the results of modeling four actuators each having a different moving mass. As can be seen in Table 2c, the head level actuators achieve high seek acceleration even at one volt electrode potential.

For a 30 microgram moving mass and a one volt potential, the resulting acceleration is 390 g's. This acceleration allows for a single track switch (for a disc having 500,000 TPI) in about 11 microseconds, neglecting settling time. With springs that allow one micrometer of motion, up to 20 tracks could be covered in 50 microseconds. With springs that allow five micrometers of travel, up to 100 tracks could be covered in 110 microseconds. At two volts, these times are reduced by a factor of two.

Tables 3a-3c illustrate the results of modeling the performance of an electrostatic slider positioning system having electrodes located across the entire trailing edge of the slider. Shown in Tables 3a-3c are the lateral positioning force (Flat) and normal attraction force (Fn), and the resulting lateral acceleration possible for various cases of moving mass at voltages from 0.5 to 5 volts. The data in Tables 3a-3c is based on a slider having 4920 active slider electrodes covering the entire width of the slider, or about 1000 microns. The length of the electrodes was about 1.20×10−4meters, while the gap between the disc and the slider electrodes was about 2.50×10−9meters.

The system illustrated in Tables 3a-3c requires a substantially redesigned trailing edge airbearing that also covers the entire width of the slider at the trailing edge. This embodiment, used with a primary positioning means for coarse track seeking and compliant springs between the slider and the suspension for electrostatic microactuation, provides higher performance than the embodiment illustrated in Tables 2a-2c.

Table 3a provides the acceleration in g's for a Femco Si slider having a mass of 6×10−7kilograms. Table 3b provides the acceleration in g's for a Pico AlTic Slider having a mass of 1.6×10−6kilograms. The tables illustrate that acceptable performance for high performance drives is obtained at as low as 1.5 volts for the heavier Pico AlTiC slider and 1.0 volts for the Femco silicon slider.

As can be seen by comparing Tables 3a and 3b with Tables 2a and 2b, the wider trailing edge air bearing associated with Tables 3a and 3b allows greater normal force, possibly up to ten times greater, than the normal forces provided by the similar sliders illustrated in Tables 2a-2b. In addition, as shown in Tables 3a and 3b, at a voltage of three volts the acceleration of the Femco Silicon slider is 1600 g's and the acceleration of the Pico AlTiC slider is 600 g's. Both of these accelerations are significantly greater than the accelerations of 176 g's and 66 g's achieved by the Femco Silicon slider of Table 2a and the Pico AlTiC slider of Table 2b at the same voltage.

Table 3c below illustrates that for a lower performance, low cost drive, the primary positioning means can be entirely replaced by a simple folded flexure suspension that allows the slider to sweep the entire inner diameter to outer diameter range of the disc using only the electrostatic stepping action of the slider electrodes and the disc tracks. Though larger mass, the need for a primary positioning means is eliminated.