Rotary actuator system with zero skew angle variation

A transducer head is located on a head arm. The head arm has a roller surface which contacts a fixed surface at a point of contact. The roller surface is attached to the fixed surface such that the roller surface may roll along the fixed surface without slipping. The roller surface and the fixed surface are such that a line between the point of contact and a disk center is always perpendicular to a line between the disk center and the transducer head for all positions as the roller surface rolls along the fixed surface.

TECHNICAL FIELD 
1. Background of the Invention 
This invention relates to a data storage disk drive and more particularly 
to a rotary actuator for a read/write head. 
2. Description of the Prior Art 
In disk drive technology it is becoming increasingly important that the 
relative angle between the read/write or transducer head and the data 
tracks change very little, if at all, as the head moves across the surface 
of the disk. The relative angle between the head and the tracks is also 
known as skew. Magnetic recording disks can now achieve very large track 
densities and even a small change in the head skew can interfere with 
proper reading and writing of data on the disk. In optical disk drive 
systems, array heads are now being used. These heads focus two or more 
laser spots onto a single track at the same time. Any large variation in 
the skew of the head will make it impossible to align both laser spots on 
the same track. 
Currently, head actuators consist of three types: linear, rotary, and 
combinations of both. Linear actuators consist of a sliding arm to which 
the head is attached. The arm is aligned along a radius of the disk and 
the head moves along the radius as the arm slides in and out without any 
variation in the skew. The problem with linear actuators is that they 
require a number of roller bearings to hold the arm. These bearings add 
unwanted inertia and frictional force to the system. 
The use of linear actuators in optical systems causes additional problems. 
Generally, in an optical system a lens and a mirror are located at the 
head of the actuator arm and the rest of the optical system is located at 
a fixed location. The lens and mirror move relative to the rest of the 
fixed optical system which can make proper alignment of the system very 
difficult. One way to solve this alignment problem is to locate all of the 
optical system at the head. However, the relatively large amount of mass 
at the end of the actuator arm greatly reduces its speed, and hence, 
increases the access time of the disk drive system. U.S. Pat. No. 
4,161,004 issued July 10, 1979 to Dalziel, et al. illustrates a typical 
linear actuator system. 
Rotary actuators pivot about a point or roll about a surface. The advantage 
of a rotary actuator is that it is much lighter and faster than the linear 
actuator. Rotary actuators are especially applicable to optical disk drive 
systems because the lens and mirror can be located at the head while the 
rest of the optical system can be positioned on the arm near the pivot 
point. Since the optical system parts are now fixed relative to one 
another, the alignment is not a problem. Also, the addition to the inertia 
of the arm is very small when the bulk of the optical system is located 
near the pivot point. Thus, the access time can still be quite fast. 
The major problem with the rotary actuator is that the skew angle between 
the head and the data tracks can vary by as much as 10 to 15 degrees as 
the head moves across the disk. Optimizing the position of the head and 
the dimensions of the head arm can reduce the skew angle variance, 
however, the variance may still be too large for high density magnetic or 
optical disk drive systems. 
U.S. Pat. No. 4,751,597 issued June 14, 1988 to Anderson; U.S. Pat. No. 
4,200,894 issued Apr. 29, 1980 to Kaseta, et al.; U.S. Pat. No. 3,500,363 
issued Mar. 10, 1970 to Shill; and U.S. Pat. No. 4,794,586 issued Dec. 27, 
1988 to Korth all illustrate typical rotary actuator systems. 
U.S. Pat. No. 4,556,924 issued Dec. 3, 1985 to Quist, Jr., et al. and U.S. 
Pat. No. 4,775,907 issued Oct. 4, 1988 to Shtipelman disclose rotary 
actuators which have heads which compensate for the skew angle variance. 
However, both of these devices require a relatively large number of 
additional parts which correspondingly slows the access time. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a rotary actuator system 
comprises a data storage disk, a fixed surface, an arm to which a head is 
connected and means to attach the arm to the fixed surface. The arm has a 
roller surface which contacts the fixed surface at a point of contact. The 
attachment means allows the roller surface of the arm to roll without 
sliding along the fixed surface. The fixed surface and the roller surface 
are such that a line between the point of contact and the disk center is 
always perpendicular to a line between the disk center and the read/write 
head for all positions as the roller surface rolls along the fixed 
surface. The result is that a zero skew angle is established between the 
data tracks on the disk and the head which does not vary as the actuator 
moves the head across the data tracks. 
For a fuller understanding of the nature and advantages of the present 
invention reference should be made t the following detailed description 
taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a first embodiment of the rotary actuator system of the 
present invention. The system has a data storage disk 10 which has an 
outer data track 12 and an inner data track 14. Disk 10 may be either a 
magnetic or optical storage disk. Disk 10 rotates about a fixed position 
spindle axis 16 located at the center of disk 10. A fixed member 20 is 
shown located a distance beyond the outer edge of disk 10. Fixed member 20 
may also be located within the outer edge of disk 10. Member 20 has a 
fixed surface 22 which is a cylindrical arc centered about axis 16. The 
cylindrical arc of surface 22 is concentric with the disk 10. 
An actuator arm 30 has a head section 32 which contains a transducer head 
34 attached to a first end. Head 34 is positioned over disk 10 and may be 
either a magnetic or optical transducer head. Head section 32 is attached 
to a roller member 36. Roller member 36 has a roller surface 38 which lies 
in a plane perpendicular to disk 10. The longitudinal axis of the head 
section 32 is perpendicular to roller surface 38. Roller surface 38 abuts 
fixed surface 22 along a point of contact 40 which is perpendicular to 
disk 10. (Point of contact 40 is actually a line of contact which appears 
as a point when seen from a top view. Thus, for simplicity, all lines of 
contact will be referred to as points of contact.) An attachment means, 
which will be shown in more detail below, attaches roller member 36 to 
fixed member 20 such that roller surface 38 rolls along fixed surface 22 
without slipping. Head 34 is positioned along arm 30 such that a line 42 
from the active portion of head 34 to axis 16 is perpendicular to a line 
44 from axis 16 to line of contact 40. In other words, a radius of disk 10 
through the point of contact 40 is always perpendicular to a radius of 
disk 10 through the head 34. 
The operation of the present invention may now be understood. As head 34 
moves inward or outward from disk 10, the roller surface 38 rolls along 
the fixed surface 22. The line of contact 40 will correspondingly shift 
along the arc of fixed surface 22. For any position, the line 42 (between 
head 34 and axis 16) will always be perpendicular to line 44 (the line 
between axis 16 and line of contact 40). The result is that head 34 will 
always have the same skew angle relative to axis 16 and to the data 
tracks. 
Another way to visualize this is to imagine disk 10 moving relative to head 
34 instead of the other way around. Suppose head 34 and its arm 30 are 
fixed. Now suppose that disk 10 is enlarged such that its outer 
circumference lies along surface 22. If disk 10 is allowed to roll along 
the surface 38, it can be seen that the center of the disk 10 will always 
lie along line 42. If disk 10 rolls to the left, the center axis moves to 
a point 50. If disk 10 rolls to the right, the center axis moves to a 
point 52. In either case, the skew angle between the head 34 and the data 
tracks does not change. From the perspective of the head 34 the center of 
the disk moves along a straight line relative to it. 
Of course, in actuality the head 34 moves and the disk axis 16 remains 
fixed. From the perspective of the fixed axis 16, the head 34 will appear 
to move along an arc. However, for any position of head 34, the lines 42 
and 44 remain perpendicular and the skew angle does not change. 
In addition to plane surface 38 and circular arc 22, there are a number of 
other complementary surface pairs which may be used in the present 
invention. Again, it is best to visualize the head a being fixed and the 
disk rolling relative to the head. As long as the center of the disk moves 
along a straight line relative to the fixed head, then the surfaces are 
complementary and may be used in the present invention. 
If the curve of the head arm surface is described by: 
EQU Y=f(x) 
then the mating surface is characterized by: 
##EQU1## 
FIG. 2 shows a complementary pair of surfaces (a logarithmic spiral 60 and 
a tilted plane 62). The spiral is described by: 
##EQU2## 
As the spiral 60 rolls along plane 62, a center point 64 will always move 
along a straight line along or parallel to the x axis. To adapt these 
surfaces to the present invention, the spiral surface 60 is substituted 
for surface 22 such that point 64 is located at axis 16. The tilted plane 
62 is then substituted for surface 38 and the position of head 34 is 
adjusted such that lines 42 and 44 are perpendicular. 
FIG. 3 shows another complementary pair of surfaces. A square wheel 70 
rolls along a catenary surface 72. 
##EQU3## 
As the square wheel 70 rolls along surface 72, a point 74 moves along the x 
axis. To adapt these surfaces to the present invention, the flat surface 
wheel 70 is substituted for surface 22 with point 74 located at axis 16. 
The catenary surface 72 is then substituted for surface 38 and the 
position of head 34 is adjusted such that lines 42 and 44 are 
perpendicular. 
FIG. 4 shows two congruent parabolic curves which may be used in the 
present invention. A parabolic fixed surface 76 is defined as: 
##EQU4## 
A parabolic wheel is defined as: 
##EQU5## 
Wheel 78 has a center point 79. The wheel 78 may be substituted for surface 
22 with point 79 located at axis 16. The surface 76 may be substituted for 
surface 38 and the position of head 34 adjusted such that lines 42 and 44 
are perpendicular. 
Additional complementary surfaces are shown in the article "Rockers and 
Rollers" by Gerson B. Robison, Mathematics Magazine, January 1960, pp. 
139-144. 
FIG. 5 graphically shows a way to approximate complementary surfaces. A 
wheel 80 with its center initially at a point 84 at the origin moves along 
the X axis. The periphery of wheel 80 is initially in contact with a 
surface 82 at a point 90. A plurality of points 92-100 are located on the 
periphery of wheel 80 and the line segments between each pair of points 
define the outer surface of the wheel 80. A plurality of lines 102-112 
represent the radii between point 84 and points 90-100, respectively. A 
plurality of points 120-128 represent the positions of points 92-100, 
respectively on the Y axis if wheel 80 were rotated about point 84. A 
point 130 on surface 82 lies along a line horizontally projected from 
point 120. The line segment between points 90-92 is rotated about point 90 
and the intersection of this rotation and the horizontal line from point 
120 is point 130. To define a point 132 on surface 82 which corresponds to 
point 94 of wheel 80, a line is projected horizontally from point 122. A 
line segment equal to the distance between the points 92-94 is rotated 
about point 130. The intersection is the point 132. The procedure is 
repeated to find points 134, 136 and 138 which correspond to points 96, 98 
and 100, respectively. 
As wheel 80 rolls along surface 82, the point 84 will move along the X 
axis. The wheel 80 ma be substituted for surface 22 with point 84 located 
at axis 16. The surface 82 may be substituted for surface 38 and the 
position of head 34 adjusted such that lines 42 and 44 are perpendicular. 
Additional complementary surfaces may be derived in the same manner. 
It has been shown that there are a number of complementary surfaces which 
may be used in the present invention. A further improvement of the 
actuator system of FIG. 1 would be to move arm 34 such that its 
longitudinal axis is closer to the center of rotation. 
FIG. 6 shows a rotary actuator system of the present invention with an arm 
having a longitudinal axis close to the center of rotation. A data storage 
disk 200 rotates about a spindle 202. Spindle 202 is fixed relative to a 
disk drive body 204 and has a central spindle axis 206. Disk 200 may be a 
magnetic or optical disk. Disk 200 has an outer data track 208, a center 
data track 210 and an inner data track 212. 
A fixed member 220 is attached to body 204 by a pair of posts 222. Member 
220 has a curved fixed surface 226 which is perpendicular to the plane of 
disk 200. An arm 230 has a roller surface 232 which is in contact with 
fixed surface 226 of member 220. Arm 230 is attached to member 220 by a 
plurality of bands 240. Bands 240 allow roller surface 232 to rotate along 
fixed surface 226 without slipping. 
Arm 230 has a voice coil motor or counter weight section 242. Arm 230 has a 
center line segment 244 which extends from the initial point of contact 
246 between surfaces 226 and 232 to a head 248. A line 250 from point 246 
to axis 206 is perpendicular to a line 252 from axis 206 to the active 
portion of a transducer or read/write head 248. Head 248 may be either a 
magnetic or optical head. As arm 230 rotates clockwise, head 248 moves to 
a position 260 and a point 262 becomes a point of contact between surface 
226 and surface 232. A line 264 between point 262 and axis 206 is 
perpendicular to a line 266 between axis 206 and head 248. As head 248 
moves to a position 270, a point 272 becomes a line of contact between 
surfaces 226 and 232. A line 274 between point 272 and axis 206 is 
perpendicular to a line 276 between axis 206 and head 248. The skew angle 
of head 248 does not vary as it moves across disk 200. 
In this embodiment the roller surface 232 is a cylindrical arc having a 
radius approximately equal to twice the length of line segment 244. Fixed 
surface 226 is a complementary curved surface similar to wheel 60 in FIG. 
2. The exact shape of surface 226 is derived by fitting a smooth curve 
through the points using the method shown in FIG. 5. However, it has been 
found that a cylindrical arc having the same radius as the cylindrical arc 
of surface 232 (a radius approximately equal to twice the length of line 
segment 244) is a close approximation to the ideal curve. This 
approximation does introduce a skew angle variance of less than 0.05 
degrees. 
FIGS. 7, 8 and 9 show a perspective, top and side view, respectively of the 
bands 240 joining member 220 and arm 230. The shapes of member 220 and arm 
230 have been exaggerated to more clearly illustrate the operation of 
bands 240. Member 220 has an offset ledge 300 which is offset a distance 
form the edge of surface 226. Arm 230 has a similar ledge 302 which is 
offset a distance from the edge of surface 232. Bands 240 are wrapped in a 
figure eight pattern around ledges 300 and 302 and are secured to a pair 
of flat surfaces 310 and 312 behind surfaces 300 and 302, respectively. 
The offset ledges 300 and 302 insure that bands 240 always have a 
directional force holding the arm 230 to member 220. If the bands 240 were 
attached directly to surfaces 226 and 232, then at the point of contact, 
the bands 240 would be tangential to surfaces 226 and 232 and there would 
be no force in a direction perpendicular to the surfaces which would hold 
the surfaces together. 
While the preferred embodiments of the present invention have been 
illustrated in detail, it should be apparent that modifications and 
adaptations to those embodiments may occur to one skilled in the art 
without departing from the scope of the present invention as set forth in 
the following claims.