Patent Application
20110299188
The present application claims priority from Japanese applications JP 2010-90295 filed on Apr. 9, 2010 and JP 2011-25270 filed on Feb. 8, 2011, the content of which are hereby incorporated by reference into this application.
1. Field of the Invention
The present invention relates to a magnetic head slider and a magnetic disk drive. In particular, the invention relates to a magnetic head slider having a thermal actuator structure taking advantage of thermal expansion, and a magnetic disk drive employing such a magnetic head slider.
2. Description of the Related Art
For high accuracy positioning of a magnetic head slider over a track on a magnetic disk, the so-called “two-stage actuator”, including a microactuator arranged in the vicinity of the magnetic head slider, has been proposed. There are three known types of microactuators: a suspension driving type (driving a suspension which supports the magnetic head slider), a slider driving type (driving the magnetic head slider alone with respect to the suspension), and a magnetic head driving type (driving the magnetic head part alone with respect to the main body of the slider using an actuator mounted on the slider itself).
For example, JP-A-2009-170014 discloses a magnetic head driving type microactuator for a magnetic head slider proposed by the applicant of the present invention. This magnetic head slider is equipped with a head positioning thermal actuator in which a heating element is energized to cause thermal expansion of the material in the vicinity of the heating element and the track positioning of the magnetic head is carried out by use of the thermal expansion.
The magnetic head slider described in JP-A-2009-170014 includes a thin film stack part 42 stacked on the rear end of a slider substrate 41 made of Al2O3TiC. A read/write element 20 is formed in a central part of the thin film stack part 42. A heating element 21 is arranged at a lateral position several tens of microns apart from the read/write element 20. An intermediating part 25 transmits the thermal expansion in the vicinity of the heating element 21 to the read/write element 20. Since these elements are formed integrally by a thin film formation technology, the intermediating part is made of the material (generally, Al2O3) filling up the region other than the head or the heating element.
In the manufacture of the magnetic head slider having the magnetic head driving type microactuator, the heating element is formed in the same thin film formation process together with the read/write element in consideration of productivity. In general, the read element is made of a special material, the coil of the write element is made of copper, the heating element and the shield film are made of NiFe, and Al2O3 is used for the other part. Therefore, the temperature rise of the heating element, the thermal expansion and the transmission property of the thermal expansion to the head are strongly affected by physical property values (Young's modulus, thermal conductivity, specific heat, thermal expansion rate, etc.) of the Al2O3 material. Further, since the thin film stack part 42 (made principally of Al2O3) is stack up in close contact with the rear end face of the slider substrate 41 made of Al2O3TiC (generally called “AlTiC”), the properties of the head positioning thermal actuator are affected considerably by physical properties of the Al2O3TiC material in cases where the distance between the slider body (slider substrate 41) and the heating element in the thin film stack part 42 is short.
According to the simulation for the displacement response of the head to the input electric power in consideration of the physical properties of those materials constituting the magnetic head slider, the displacement of the head is 4-8 nm which is achieved by the electric power input of permissible input electric power (50-100 mW) to the heating element in the conventional heating element structure described in JP-A-2009-170014. This satisfies the displacement necessary for the high accuracy positioning required in the near future (track pitch: 50 nm). However, the time response of the head to the stepwise input power supplied to the heating element is as slow as at least approximately 0.2-0.3 ms in terms of the time constant. This corresponds only to a maximum driving frequency band of approximately 0.8 kHz at most. Thus, the properties required of the high accuracy positioning actuator cannot be satisfied unless any further improvement is made.
The time constant is mainly dominated by the responsiveness of the thermal expansion in the vicinity of the heating element and in the transmitting part (caused by the temperature rise) to the electric power input to the heating element. In order to enhance the responsiveness, it is important that the part in the vicinity of the heating element can be heated instantly and uniformly throughout the whole thickness, the cooling effect on the head slider by thermal conduction to the air flow and the disk surface (in the close vicinity of the head slider) is enhanced, and the temperature-rising region in the stationary state is restricted to the part in the vicinity of the heating element. According to the simulation, the cooling effect on the slider's air bearing surface is actually strong and the part heated by the energization and contributing to the thermal expansion is restricted to a region within approximately 10 μm of the heating element to the right or left of the heating element. Therefore, it has become clear that the thermal expansion in the vicinity of the heating element is transmitted to the head in the form of elastic strain in regions farther than approximately 10 μm from the heating element.
It is therefore an object of the present invention to provide a magnetic head slider having a thermal actuator structure capable of efficiently transmitting the thermal expansion displacement in the vicinity of the heating element to the region of the read/write element and thereby enabling high-speed displacement response.
Another object of the present invention is to provide a magnetic disk drive capable of achieving high accuracy positioning by converting high displacement efficiency of the read/write element to improvement in the response speed.
In accordance with an aspect of the present invention, there is provided a magnetic head slider flying above a rotating magnetic disk and reading and writing data from/to the magnetic disk, comprising: a read/write element for executing the reading/writing of data; at least one heating element arranged in a width direction of the magnetic head slider with respect to the read/write element for generating heat in response to energization; a thermal expansion transmitting part lying between the heating element and the read/write element for transmitting thermal expansion caused by the heating of the heating element and thereby displacing the read/write element in the width direction; and a thermal displacement restriction limiting part formed in a region covering at least part of the region of the thermal expansion transmitting part for limiting restriction on the displacement in the width direction by the thermal expansion transmitting part.
In a preferred embodiment, the magnetic head slider is made up of a slider substrate and a thin film stack part formed on a rear end face of the slider substrate. The thermal displacement restriction limiting part is configured as a low Young's modulus part which is made of a material having a Young's modulus lower than that of the thin film stack part, the thermal displacement restriction limiting part being formed in the thin film stack part facing the slider substrate so as to cover at least part of the read/write element, the thermal expansion transmitting part and the heating element in the width direction of the magnetic head slider.
Preferably, the thermal displacement restriction limiting part is a void having prescribed width, depth and thickness.
Preferably, the thermal displacement restriction limiting part is filled in with a nonconductive ceramics material and is partially formed in a region forming a air-bearing surface of the slider and in a region substantially corresponding to part of the area of a protrusion control heating element which is used for controlling protrusion of the air-bearing surface.
Preferably, the thermal displacement restriction limiting part is filled in with a nonconductive ceramics material in a region in the vicinity of a air-bearing surface of the slider and in a region substantially corresponding to part of the area of a protrusion control heating element which is used for controlling protrusion of the air-bearing surface so as not to be open to the air-bearing surface, and the void is formed to extend to a back surface of the slider opposite to the air-bearing surface so as to be open to the back surface.
Preferably, the void is filled in with a nonconductive ceramics material in a T-shape. The top of the T-shaped part forms an air bearing surface of a air-bearing surface of the slider.
Preferably, the low Young's modulus part is implemented by a low Young's modulus layer which is made of a material having a Young's modulus lower than that of the slider substrate and that of the thin film stack part.
Preferably, thermal conductivity of the material forming the low Young's modulus layer is sufficiently lower than that of Al2O3TiC as the material of the slider substrate and at most lower than or equal to that of Al2O3 as the material of the thin film stack part.
Preferably, the low Young's modulus layer extends over the whole interface between the slider substrate and the thin film stack part.
Preferably, the low Young's modulus layer is made of a polymer material.
In a preferred embodiment, the thin film stack part includes: a first thermal displacement restriction limiting part as the thermal displacement restriction limiting part containing at least part of the read/write element, the thermal expansion transmitting part and the heating element in the width direction of the magnetic head slider; and a second thermal displacement restriction limiting part arranged to extend in a direction orthogonal to the width direction of the magnetic head slider.
Preferably, the thin film stack part further includes a third thermal displacement restriction limiting part which is arranged orthogonally to the first thermal displacement restriction limiting part and the second thermal displacement restriction limiting part.
Preferably, the void covers the read/write element, the thermal expansion transmitting part and the heating element. The void is provided with one or more thin bridges each filling in the void at a position in the vicinity of the heating element.
Preferably, the void is not open to an area forming an air bearing surface or to the slider's air-bearing surface but is open to the slider's back surface. The void is provided with a thin bridge filling in the void at a position corresponding to the center of the heating element.
Preferably, the first through third thermal displacement restriction limiting parts are implemented by voids. A bridge with a small thickness as a part for filling in the void with a prescribed material is formed between the first and second thermal displacement restriction limiting parts and/or between the second and third thermal displacement restriction limiting parts.
Preferably, the magnetic head slider further comprises one or more second heating elements arranged in parallel with the heating element serving as a first heating element. When two or more second heating elements are provided, the second heating elements are connected in series and driven by the same control power as input power.
In accordance with another aspect of the present invention, there is provided a magnetic disk drive comprising: a magnetic disk on which data are recorded along tracks; a spindle motor for rotating the magnetic disk; a magnetic head slider flying above the rotating magnetic disk and reading and writing data from/to the magnetic disk (the magnetic head slider including a read/write element for executing the reading/writing of data, at least one heating element arranged in a width direction of the magnetic head slider with respect to the read/write element for generating heat in response to energization, a thermal expansion transmitting part lying between the heating element and the read/write element for transmitting thermal expansion caused by the heating of the heating element and thereby displacing the read/write element in the width direction, and a thermal displacement restriction limiting part formed in a region covering at least part of the region of the thermal expansion transmitting part for limiting restriction on the displacement in the width direction by the thermal expansion transmitting part); a heating control circuit including a phase-lead filter with a property inverse to that of the heating element and controlling and driving the heating element; a head support part for supporting the magnetic head slider; a voice coil motor for driving the head support part and thereby moving the magnetic head slider relative to the magnetic disk; a calculation circuit for calculating a position error of the read/write element with respect to the track based on data read out by the read/write element; a coarse movement control circuit for driving the voice coil motor based on the position error of the read/write element; and a fine movement control circuit for energizing the heating element based on the position error of the read/write element.
In a preferred embodiment, the magnetic head slider includes the heating element on each side of the read/write element in regard to a transverse direction. Electric power proportional to a positive value of a position error signal regarding the position error of the read/write element with respect to the track is inputted from the heating control circuit to one heating element. Electric power proportional to a negative value of the position error signal is inputted from the heating control circuit to the other heating element. The read/write element is displaced due to thermal expansion in the vicinity of the energized heating element. Feedback control is conducted so as to minimize position error caused by the displacement.
Preferably, the magnetic head slider includes first and second heating elements on one side of the read/write element in regard to a transverse direction. DC power corresponding to a center position of the positioning of the read/write element is applied by the heating control circuit to the second heating element farther from the read/write element. Electric power proportional to a positive value of a position error signal regarding the position error of the read/write element with respect to the track is inputted from the heating control circuit to the first heating element. Electric power proportional to a negative value of the position error signal is inputted from the heating control circuit to the second heating element. The read/write element is displaced due to thermal expansion in the vicinity of the first and second heating elements. Feedback control is conducted so as to minimize the position error.
By the present invention, thermal expansion can be caused efficiently to the material in the vicinity of the heating element and the thermal expansion from the heating element can be transmitted to the read/write element with high efficiency, by which a displacement of the read/write element necessary for the record track-following positioning control of the read/write element can be caused. In a preferred embodiment, the read/write element can be displaced in the direction corresponding to the track width direction (i.e. width direction of the magnetic head slider) at high speed and with high accuracy within a permissible range of temperature rise, by compensating for a delay in the time response of the heating element by use of a phase-lead compensator having a property inverse to that of the heating element. By employing the well-known proportional-integral control, etc., the positioning of the read/write element to an intended track (target track) with a high frequency control band, with high accuracy and with ease can be realized.
Referring now to the drawings, a description will be given in detail of preferred embodiments in accordance with the present invention.
The head support part 6 is supported by a post adjoining the magnetic disk 2, and a magnetic head slider 4 is fixed to the tip of the head support part 6. The magnetic head slider 4, flying over the rotating magnetic disk 2 at a position close to the disk surface by being lifted by air flow, reads and writes data from/to the magnetic disk 2. On the other hand, a voice coil motor 7 as the head actuator is provided at the rear end of the head support part 6. The voice coil motor 7 drives and pivots the head support part 6 and thereby moves the magnetic head slider 4 substantially in the radial direction of the magnetic disk 2. The magnetic head slider 4 and the voice coil motor 7 are electrically connected to a circuit board (unshown) on the back of the enclosure 10 via FPCs (Flexible Printed Circuits) 8 attached to the head support part 6.
On the gliding face 43 of the magnetic head slider 4, an air bearing surface 44 and groove areas 45 (including shallow grooves and deep grooves) are formed. The reference character “46” represents a step boundary line between the air bearing surface 44 and the groove area 45, “41b” represents the (stack part) boundary 41b between the thin film stack part 42 and the slider substrate 41, “47” represents a section line (imaginary) between a front part (shown in
Incidentally, while the read/write element 20, the heating elements 21 and the head protrusion heating element 23 are actually formed not on the gliding face 43 but inside the head slider, the elements 20, 21 and 23 are shown in
For convenience of explanation, in
The x-axis corresponds to the extending direction of each track formed on the magnetic disk 2, while the y-axis corresponds to the track width direction of the magnetic disk. In the magnetic head slider 4, a central part of the backside surface of the slider substrate 41 (on the −x side of the section line 47, unshown in
The property of the head displacement output with respect to the electric power input to the heating element is affected in the first place by the position of the heating element. Assuming that the distance from the origin to the end of the heating element 21a in the x direction is Lx, the distance in the y direction is Ly and the distance in the −z direction (depth inside the thin film stack part from the gliding face) is Lz, a simulation made it clear that the distances Lx, Ly and Lz are desired to be set approximately within the ranges Lx=3-30 μm, Ly=35-70 μm and Lz=20-40 μm. In a case where Lx=3.7 μm, Ly=40 μm and Lz=30 μm, for example, a head displacement of approximately 5 nm in the stationary state is obtained when the electric power input is 50 mW.
When the other heating element 23 is energized, the part in the vicinity of the heating element 23 thermally expands in the y direction and also in the z direction, causing the read/write element surface to protrude. This allows the magnetic head to approach the recording medium surface (magnetic disk surface) for the data reading/writing, enhancing the read/write efficiency and realizing high density recording. Incidentally, while a part of the gliding face in the vicinity of the heating element 21 (for the head positioning) also thermally expands in the z direction, the thermal expansion (protrusion) caused by the heating element 21 has to be prevented from changing the air bearing surface or affecting the flying properties of the magnetic head slider. For this reason, the position of each heating element 21 (for the head positioning thermal actuator) in the x-y plane is set not in the area of the air bearing surface 44 but in the area of the groove area 45. The level difference between the air bearing surface and the groove area is at least 10 nm or more.
Since the skirt of the gliding face's protrusion caused by the energization of the heating element 21 extends over some tens of microns, the surface at the head position is also elevated. Meanwhile, the read/write element 20 is designed to protrude its surface in the read/write operation by use of the heating element 23 and thereby reduce the clearance (distance) between the magnetic disk and the head flying over the magnetic disk. Therefore, the change in the height of the head surface caused by the head positioning heating element 21 has to be at most smaller than the protrusion of the read/write element surface (head surface) caused by the head protrusion heating element 23. Each heating element 21 is placed at least 35 μm apart from the slider's center line 48 (Ly=35-70 μm) for this reason.
As is understandable from the above explanation, a thermal actuator part (including the regions of the heating elements 21, the read/write element 20 and the thermal expansion transmitting parts 25), which is manufactured by a thin film formation process in close contact with the stack part boundary surface 41b of the slider substrate 41, is formed integrally with the slider substrate. Therefore, the displacement of the thermal actuator in the y direction was severely restricted in the conventional magnetic head slider having no void 50 (described in the JP-A-2009-170014).
In contrast, the magnetic head slider 4 in accordance with the present invention is capable of limiting (relaxing) or eliminating the restriction on the displacement in the y direction by providing the void 50 at the position close to the stack part boundary 41b, enhancing the displacement efficiency. The void 50 is a cavity in a shape like a thin rectangular parallelepiped having a prescribed depth (which can also be referred to as a “gap”).
The dimensions of the void 50 are as follows: the thickness in the x direction is desired to be approximately 2-5 μm or less (preferably, open to the gliding face) and the depth in the z direction is desired to be 40 μm or more (preferably, extending to a depth of approximately 120 μm). The length in the y direction is desired to cover at least the read/write element 20 and the heating elements 21a and 21b. In order to enhance the displacement efficiency of the read/write element, the void 50 is desired to extend to positions approximately 5 μm outside the heating elements 21 for the following reason: The region undergoing the temperature rise by each heating element 21 extends to the position approximately 5 μm outside the heating element and the thermal expansion is caused to the material in the region. To eliminate the restriction on the thermal expansion, the void length extending to the positions approximately 5 μm outside the heating elements 21 is desirable. According to a simulation, in a case where the void length in the y direction is set to reach the positions 5 μm outside the heating elements 21, the maximum displacement (or displacement efficiency) of the read/write element in the stationary state is successfully multiplied by 2.8, 4.9, 5.5 and 7.6 when the void depth in the z direction is 40 μm, 60 μm, 80 μm and 112 μm, respectively, compared to the conventional magnetic head slider having no void (described in the JP-A-2009-170014).
Incidentally, the void 50 may also be replaced with a layer formed of a material (e.g., polymer material such as resin) having a low Young's modulus (low Young's modulus layer). The void and the low Young's modulus layer may be collectively referred to as a “low Young's modulus part”, or a “thermal displacement restriction limiting part” or “thermal displacement restriction removing part” from the viewpoint of limiting or removing the restriction on the thermal displacement caused by the heating element. The void 50, capable of setting the Young's modulus of the part at zero, is functionally the most desirable. Since the thermal conductivity of air is as low as approximately 1/1000 of that of Al2O3TIC (material of the slider substrate 41) and approximately 1/50 of that of Al2O3, the void 50 has the effects of blocking the heat of the heating element 21 from flowing out to the Al2O3TiC, enhancing the temperature rise in the vicinity of the heating element, and increasing the thermal expansion efficiency with respect to the electric power input.
Meanwhile, the low Young's modulus part 5 (the void 50 or the low Young's modulus layer) reduces also the restriction on the thin film stack part in the direction toward the gliding face, and thus the low Young's modulus part 5 enhances the thermal expansion protrusion of the air-bearing surface in the vicinity of the heating element. Therefore, when the gliding face's protrusion in the vicinity of the read/write element caused by the thermal actuator should be suppressed, the low Young's modulus part 5 is desired not to be extended to the outside of the heating elements 21 but to be kept approximately within a range between the centers of the heating elements 21. A design keeping the low Young's modulus part 5 within a range between the inner ends (on the read/write element's side) of the heating elements 21 is also recommendable. Even in this case, the displacement efficiency of the read/write element can be increased considerably compared to the conventional magnetic head slider since the region of the thermal expansion transmitting parts 25 and the heating element 23 is still not restricted. The length of the low Young's modulus part 5 in the y direction in the present invention is not restricted to the example shown in
The manufacturing method of the magnetic head slider having the void 50 will be explained below referring to
First, as shown in
Subsequently, as shown in
A modification of the method of forming the void 50 is also possible. For example, the void 50 can be formed not in the thin film formation process for forming the thin film stack part 42 but after the formation of the thin film stack part including the heating elements, by creating (digging) a groove (void) from the gliding face of the magnetic head slider by executing a machining process or laser beam processing before or after the etching process is conducted to the gliding face. The machining process can be implemented by using, for example, a dicing saw which is generally used for cutting (dicing) silicon wafers, etc. In this case, the minimum value of the thickness of the rotating grinder saw is 25 μm.
It is also possible to form a low Young's modulus layer 500 by, for example, filling the groove 50′ (or void 50) formed by the laser beam processing (in a shape selected from the examples of
Several modifications are possible regarding the void 50 formed in a part of the thin film stack part 42 in the vicinity of the boundary 41b of the slider substrate 41. For example, a part of the void 50 from the gliding face 43 to some microns from the gliding face can be filled in with Al2O3, without letting the void 50 be open to the gliding face. In this case, however, the displacement efficiency of the thermal actuator deteriorates since the mechanical restriction of Al2O3 on the thermal actuator's displacement increases. For example, even when the void 50 is as deep as 112 μm (Sz=112 μm), the multiplying factor of the displacement efficiency remains no more than 2.6 if a part of the void from the gliding face to 10 μm (in the z direction) from the gliding face is replaced and filled in with Al2O3. However, the protrusion toward the gliding face caused by the thermal expansion in the vicinity of the heating element can be also suppressed by the filling with Al2O3.
Even in this case, the heat blocking effect and the limitation/removal of the slider substrate's mechanical restriction on the thermal actuator part can be expected from the aforementioned principles, by which the displacement efficiency of the thermal actuator can be enhanced. The present invention also includes this type of modifications deriving from restrictions on the technology for forming the void 50.
In spite of the deterioration in the thermal actuator's displacement efficiency, the design forming no opening of the void at the gliding face has advantages in that it requires no alteration to the conventional design conditions of the air bearing surface and it is highly compatible with the lubricant and the flying properties of the slider. Therefore, it is possible to devise some embodiments as examples forming no groove at the gliding face while successfully reducing the deterioration in the head transverse displacement efficiency of the thermal actuator as much as possible.
are added to the T-shaped shielding bridge 53 described in the above embodiment 4-2 (
The embodiment 4-3 shown in
In the embodiment 4-2 (having the T-shaped bridge in the central part) and the embodiment 4-3 (eliminating the opening of the slit (void) to the gliding face by forming shallow shielding bridges also in the groove areas), the head transverse displacement efficiency of the thermal actuator slightly deteriorates compared to the open structure and the protruding effect in the vicinity of the head (protrusion of the gliding face) caused by the heating by the heating element 21a, 21b also increases. Further, since the protrusion of the head part by the head protrusion heating element 23 is suppressed by the T-shaped bridge formed in the central part and the protruding effect tends to extend over a larger area, the peak position (where the gliding face's protrusion in the z direction is the maximum) can deviate from the part in the vicinity of the head and approach an edge of the bearing surface on the heating element's side. To prevent this deviation, the distance Ly from the center line (head) to each heating element 21a, 21b in the y direction is desired to be long (e.g., approximately 80 μm-150 μm). By this setting, the z-direction protrusion of the head part by the heating element 21 can be reduced to a fraction of that by the heating element 23. Since the change in the head flying clearance due to the gliding face's protrusion is far smaller than the protrusion of the bearing surface in the vicinity of the head, interference of the control operation of the head positioning thermal actuator with the flying height control by the heating element 23 can be prevented.
While increasing Ly has the effect of enhancing the head transverse displacement efficiency of the heating element 21, the thickness of the void 50 in the x direction may also be increased in order to further enhance the head displacement efficiency. While the thickness of the void 50 was 1 μm in all of the above examples of the head displacement efficiency, the head displacement efficiency can be multiplied by 1.5 by increasing the void thickness to 2 μm.
The magnetic head slider according to the aforementioned embodiment 1 is applied to a dual-side thermal driving microactuator in which the heating elements 21 are placed on both sides of the read/write element 20 as shown in
According to another example, the above embodiments can also be applied to a magnetic head slider having a single-side driving thermal actuator part in which the heating element 21 (e.g., 21a) is arranged on only one side of the read/write element 20. In this case, the groove 50′ or the low Young's modulus layer 500 may be formed to cover the range of the heating element 21a placed on one side. The single-side driving thermal actuator needs bias power for positioning (displacing) the read/write element at (to) the center position of the displacement. The head is precisely positioned at the intended position to the right or left of the intermediate position by decreasing or increasing the electric power (supplied to the heating element 21a) from the bias power.
In the above case where Lx=3.7 μm, the displacement efficiency multiplied 3.1 fold when an Al2O3 layer 22 μm thick was laid between the low Young's modulus layer 500 and the Al2O3TiC material (slider substrate 41) in order to relax the restriction by the slider substrate. The increase in the displacement efficiency can be understood as a result of an increase in the thermal expansion (due to enhancement of the temperature rise in the vicinity of the heating element) and the relaxation of the mechanical restriction on the thermal actuator since the Young's modulus of Al2O3 is approximately half of that of Al2O3TiC and the thermal conductivity of Al2O3 is also lower ( 1/20) than that of Al2O3TiC. This corresponds to a case where Lx=25.7 μm. Even though this example can not be regarded as adding a new material layer, this example may be considered to be within the scope of this embodiment 6 in the sense that a layer of a material having a lower Young's modulus and also having lower thermal conductivity compared to Al2O3TiC is introduced.
In short, the embodiment 6 can be regarded generally as a magnetic head slider in which a layer of a material having a Young's modulus lower than that of the slider substrate 41 (at most lower than or equal to that of Al2O3) and preferably having thermal conductivity lower than that of the slider substrate 41 (at most lower than or equal to that of Al2O3) is formed on the entire boundary 41b with the slider substrate.
While the length of the void 501 in the y direction is substantially symmetrical with respect to the x-axis in the example shown in
The magnetic head slider of the embodiment 7 is capable of further increasing the heating element's displacement efficiency by 16% to 40% compared to the embodiment 1 since the y-direction displacements of the thermal expansion transmitting part 25a, the read/write element 20 and the region 27 are less restricted compared to the embodiment 1.
In the embodiment 8, the stationary bias power, which determines the average position of the read/write element 20 in the displacement control, is applied to the heating element 21c far from the read/write element. Further, electric power for the variable displacement for the track-following control is applied to the heating element 21c as the difference from the bias power. Meanwhile, the variable displacement of the head in the −y direction is controlled by input power to the heating element 21a close to the read/write element. With this configuration, the protrusion of the gliding face of the read/write element caused by the bias power applied to the heating element 21c can be reduced remarkably while achieving high displacement efficiency of the head positioning thermal actuator.
As another modification, part or all of the L-shaped void (501, 502) of the magnetic head slider described in the embodiment 7 or 8 (
In this embodiment 9, a simulation has predicted that y-direction displacement of the read/write element amounts to 91 nm for an 50 mW input when the length of the void 501 is symmetrical with respect to the x-axis, the void 502 reaches the rear end face 42b of the thin film stack part and the position of the void 503 in the z direction is 112 μm below the gliding face. This is 18 times the displacement efficiency of the thermal actuator of the conventional magnetic head slider.
Therefore, the modification shown in
As another modification of the embodiment 9, the aforementioned structure having three voids is applicable also to a single-side driving thermal actuator having only one heating element 21 like the one shown in
In this structural example, the restriction on the head displacement is removed/relaxed while suppressing the protrusion toward the gliding face, by forming bridges (parts filled in with a material) in the void 50. In each of the above embodiments, the thermal actuator's displacement efficiency is improved by removing/relaxing the restriction on the thermal expansion displacement of the thermal actuator in the y direction. The structural examples employing a low Young's modulus material have also a heat insulation effect of preventing the heat (generated by the heating element) from leaking to the slider substrate 41 since the thermal conductivity of the low Young's modulus material is in many cases lower than that of the slider substrate (Al2O3TiC). By the promotion of the temperature rise, the thermal expansion in the vicinity of the heating element is enhanced, thereby increasing the displacement efficiency with respect to the input power. However, the heat insulation effect is valid only in the vicinity of the heating element. Therefore, the low Young's modulus part 5 formed in a region not in the vicinity of the heating element serves for the increase in the thermal actuator's displacement efficiency by reducing the mechanical restriction on the thermal expansion transmitting parts and the read/write element (and also the mechanical restriction on the region on the side opposite to the heating element).
While the embodiments 1-9 and their modifications described above have the effect of increasing the displacement efficiency of the head positioning thermal actuator, they also increases the gliding face's protrusion in the vicinity of the heating element. However, the effect of the gliding face's protrusion in the vicinity of the heating element on the flying properties of the magnetic head slider can be suppressed to a negligible level since the area in the vicinity of the heating element is not in the air bearing surface but in the shallow groove area as mentioned above. However, the gliding face's protrusion, extending over a considerably large area, can cause variations in the clearance between the head and the magnetic disk surface. Therefore, a structure capable of minimizing the protrusion toward the gliding face while also increasing the displacement efficiency of the head positioning thermal actuator is being requested.
In an example shown in
As above, the narrow bridges 60 formed in parts of the void do not substantially restrict the y-direction displacement of the thermal actuator. Meanwhile, the bridges, which are thick in the direction to the gliding face, are capable of suppressing the protrusion toward the gliding face in the vicinity of the heating elements (where the bridges are formed), by which the gliding face's protrusion in the vicinity of the head can be suppressed remarkably.
Incidentally, in order not to substantially restrict the y-direction displacement of the thermal actuator even when a plurality of bridges 60 are formed in the void 50, the width of the void (in the x direction) is desired to be greater (several μm-approximately 10 μm) compared to the embodiment 1 and the width (thickness) of the bridge (in the y direction) is desired to be small. For example, in the case where two bridges 601 and 602 of Al2O3 are formed in the void 50 as shown in
Some modifications are possible in regard to the arrangement of the bridges formed in the void shown in
In this embodiment, the embodiment 4-3 shown in
Incidentally, the embodiment of
While all the above embodiments have been designed in order to increase the displacement efficiency of the thermal actuator, the ultimate goal regarding the thermal actuator is to increase the response speed of the head positioning thermal actuator. Therefore, an embodiment directly improving the response speed of the thermal actuator will be described below.
In contrast, in the embodiment 1, the energization of the heating element 21a first causes the temperature rise in the several μm vicinity of the heating element 21a, simultaneously causing the thermal expansion in the same region. At the time point 0.1 ms after the energization, however, the temperature rise is still insufficient at positions in the thin film stack part 10 μm or more apart from the heating element 21a in the x direction, impeding the thermal expansion by the heating element 21a. At the time point 0.2-0.3 ms after the energization, the temperature of the central part of the thin film stack part rises sufficiently and the whole region in the thin film stack part around the y-position of the heating element 21a thermally expands sufficiently. Since the thermal expansion displacement of the whole region in the vicinity of the heating element 21a propagates in the −y direction at the speed of sound via the thermal expansion transmitting part 25a and displaces the read/write element in the −y direction, the transfer time (propagation time) is 10 ns or less. Therefore, the response time of the head displacement with respect to the input power to the heating element is dominated by heat propagation time in the vicinity of the heating element in the thickness direction of the thin film stack part 42 (at least approximately 0.2 ms in terms of the time constant).
Considering the above fact, the example of
By the embodiments described above, the restriction on the displacement by the thermal actuator is relaxed and the displacement efficiency is increased thanks to the low Young's modulus part formed in the thin film stack part facing the slider substrate. Further, by arranging two or more heating elements in the thickness direction of the thin film stack part and connecting the heating elements in series for the simultaneous energization as in the embodiment 11, a head positioning thermal actuator with not only the increased displacement efficiency but also the improved time responsiveness can be realized.
In the following, the positioning control of the read/write element in the magnetic disk drive equipped with the magnetic head slider configured as the above embodiments will be described.
The calculation circuit 71 calculates a position error signal PES representing the position error of the read/write element 20 with respect to the target track by obtaining the difference between the present position of the read/write element 20 (determined from servo data read out by the read/write element) and the target track (position) for the read/write element 20 (determined from a write command or a read command inputted from an external host computer). The coarse movement control circuit 72 generates a control command SV for the voice coil motor 7 (for suppressing the position error of the read/write element 20) based on the error signal PES inputted from the calculation circuit 71 and outputs the control command SV to the voice coil motor 7 via a motor driver 77.
Meanwhile, for the fine movement control circuits 74 (74a and 74b) for controlling the thermal actuator, a separation circuit 73 separates the error signal PES inputted from the calculation circuit 71 into components to be inputted to the heating elements 21a and 21b (collectively referred to as heating elements 21), respectively, in order to suppress the position error of the read/write element 20. Each fine movement control circuit 74 (74a, 74b), which is formed by a feedback control circuit, generates a control command SH for making a corresponding heater driver 76 (76a, 76b) generate electric power proportional to an error correction signal to be inputted to the corresponding heating element 21 (21a, 21b). Each time delay compensator 75 (75a, 75b) is a phase-lead filter for correcting the delay in the responsiveness of the heating element 21. Each heater driver 76 (76a, 76b) outputs electric current or electric power proportional to the control command SH to the corresponding heating element 21 (21a, 21b).
Incidentally, since the displacement of the read/write element 20 is proportional to the electric power (heating value) of the heating element 21 and the electric power of the heating element 21 is proportional to the square of the electric current of the heating element 21, the displacement control of the read/write element 20 can also be conducted by making the heater driver 76 execute the control so that the square of electric current of the heating element 21 is proportional to the position error correction signal. Since such a control method outputting electric power proportional to the control command is an already-known technique in the TFC (flying height control with the thermal actuator (heating element 23)), the control method can easily be applied to the magnetic disk drive according to this embodiment.
Here, the function and effect of the phase-lead filter, which converts the high displacement efficiency of the thermal actuator in accordance with the present invention into improvement in the displacement responsiveness, will be explained. As mentioned above, the displacement output of the thermal actuator in accordance with the present invention achieves excellently high displacement efficiency which is 3-6 times that (5 nm/50 mW) in the case where no voids, etc. are formed. However, the time constant of the thermal actuator for the stepwise power input is at least approximately 0.2 ms, which means that the control bandwidth of the thermal actuator is as narrow as 0.8 kHz. However, by using a phase-lead filter having a property inverse to that of the thermal actuator, the high displacement efficiency can be converted into a reduction in the time constant and an increase in the control bandwidth.
In the expression (1), “f0” represents the cutoff frequency where the displacement response of the thermal actuator to the input power decreases by 3 dB (to 70%). Assuming that the time constant in response to the stepwise input is τ, the relationship f0=1/(2πτ) holds. “A” represents the displacement of the read/write element in response to stationary input power, which corresponds to the displacement efficiency. Meanwhile, the transfer function 78 from the phase-lead filter's input to the heater driver's output is represented by the following expression (2):
In the expression (2), the transfer function 78 is represented as “ILF(s)” in the sense of “Inverse Lead Filter” (phase-lead filter having the inverse property) since the numerator of the transfer function 78 is identical with the denominator of the transfer function 80 (expression (1)) of the thermal actuator (transfer function having the inverse property). Another property of the ILF(s) is that it amplifies the amplitudes of frequency components higher than f0 (i.e., when f2/f0>1) and advances the phases of such frequency components. The coefficient C, representing a property of the heater driver, has a unit [mW/V] since the heater driver converts a control signal (generally processed as voltage) into electric power.
The expression (3) indicates that the cutoff frequency has increased to f2 (>f0) in the synthesized transfer property.
Here, if we assume that the cutoff frequency of the thermal actuator itself is 1 kHz and the displacement efficiency is 5 nm/10 mW (since the displacement efficiency has been increased 5 fold) for the sake of clarity, by letting f0=1 kHz, f2=5 kHz, A= 5/10 [nm/mW] and C=1 [mW/V] and substituting s=2πfi (i=(−)1/2: imaginary unit) for “s” in the expressions (1), (2) and (3), the amplitude characteristics |H(2πfi)|, |ILF(2πfi)| and |He(2πfi)| of the transfer functions 80, 78 and 81 can be drawn as shown in
In order to clarify why the responsiveness can be increased by use of the phase-lead filter by increasing the displacement efficiency, the output U(t) of the phase-lead filter and the displacement response Y(t) of the thermal actuator when stepwise input voltage of 10 V is supplied as the phase-lead filter's input u(t) in
What is important here is that the input power to the thermal actuator can not be increased without limitation since the heating temperature is limited. The maximum temperature at which physical properties of the materials of the heating elements and the parts in the vicinity of the heating elements do not change even with the repeated thermal stresses is considered to be approximately 500° C. at most. While the temperature rise is dependent on the cooling property of the heating element, the permissible input power is considered to be approximately 100 mW at most. Thus, in the case where the displacement efficiency of the thermal actuator is 5 nm/50 mW, the speeding up possible by the phase-lead filter is approximately 2-3 times at best.
Since the cutoff frequency required of the microactuator is 3 kHz or higher, the displacement efficiency is required to be at least approximately twice (5 nm/25 mW) in the case where the cutoff frequency of the thermal actuator itself is 0.8 kHz.
Incidentally, since the thermal actuator, the phase-lead filters and the heater drivers generally have high-frequency cutoff properties, the input power at the time 0 does not actually take on a finite value differently from
Next, the operation and effect of the feedback control circuit (fine movement control circuit) 74 in the control system of
In this case, the response from the track displacement (positioning target value) r to the read/write element's displacement Y and the response from the track displacement to the position error PES are given by the following expressions (5) and (6), respectively:
The amplitude characteristics and phase characteristics of Y/r and PES/r (given by the expressions (5) and (6)) when the proportional feedback gain Kp and the integral feedback gain Ki of the position signal are set at Kp=0.5 [V/nm] and Ki=2000 [V/nm·s] are shown in
In the thermal actuator, the transfer time of the displacement from the heating element to the head is 10 ns or less as mentioned above, and thus the natural frequency of the transfer system is 50 MHz or higher. Therefore, the increasing of the control band frequency (control bandwidth) is not restricted by the natural vibration of the transfer system differently from conventional voice coil actuators and suspension-driving piezoelectric microactuators. However, major problems with the thermal actuator are low time responsiveness and low displacement efficiency. Further, the protrusion of the head's gliding face caused by the thermal actuator can also cause difficulties or failures. By the present invention, the displacement efficiency can be increased while minimizing the protrusion toward the gliding face, by which the time response and the control bandwidth can be increased.
According to the preferred embodiments of the present invention, the displacement efficiency can be increased to 5 times that of the heating element itself, and even to 10 times if the embodiment 9 can be implemented. Further, a cutoff frequency of as high as 1 kHz is presumed to be possible by use of two heating elements arranged in the thickness direction of the thin film stack part. Therefore, by combining these features, the control band frequency can be increased to 5-6 kHz by use of the phase-lead filters. By applying the feedback control to this configuration, a magnetic disk drive enabling a high track density corresponding to a control band frequency of 10 kHz or higher can be realized.
Incidentally, the functional configuration of the MPU 70 is not restricted to the above example. While the control of the voice coil motor 7 and the control of the heating elements 21 are independent of each other in the embodiments, it is also possible to input the control command SH outputted by the fine movement control circuit 74 to the coarse movement control circuit 72 (via the gain model of the heating element 21) together with the error signal PES and thereby make the control of the voice coil motor 7 and the control of the heating elements 21 incoherent with each other.
Further, while the magnetic disk drive described in the above embodiments is equipped with the two-stage actuator including the voice coil motor 7 as the first stage and heating elements 21 as the second stage, the configuration of the thermal actuator is not restricted to this example. For example, the magnetic disk drive may also be equipped with a three-stage actuator further including a microactuator formed of piezoelectric elements, etc. Such a three-stage actuator can be implemented by employing the voice coil motor 7 as the first stage, the microactuator (for adjusting the position of the magnetic head slider 4A with respect to the head support part 6) as the second stage, and the heating elements 21 (thermal actuator) as the third stage.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-090295 | Apr 2010 | JP | national |
| 2011-025270 | Feb 2011 | JP | national |
