Servo control apparatus and method using absolute value...

Details Servo control apparatus and method using absolute value... Servo control apparatus and method using absolute value...

1999-11-12

2003-04-08

Hudspeth, David (Department: 2651)

Dynamic magnetic information storage or retrieval

Automatic control of a recorder mechanism

Controlling the head

C360S077080

Reexamination Certificate

active

06545836

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to control systems and methods and more particularly to control systems and methods useful where a system characteristic observed for control purposes can take values of the same sign and magnitude for both positive and negative variations of a variable that is adjusted to control the system. While aspects of this invention are believed to relate very generally to many different control systems and methods, aspects of the invention find their most immediate application to the positional control of detectors used in data acquisition systems. Specific embodiments of the invention are useful in servo mechanical control systems such as high density magnetic disk drives from which data are read by precisely positioning a magnetic read element adjacent to a set of predefined data storage locations.
2. Discussion of the Related Art
The related art is illustrated with reference to several simple control systems. A common task assigned to control systems is maintaining the relative position of one object with respect to another object, where both objects may be moving in an unpredictable manner. One useful system to consider is an optical disk player, schematically illustrated in
FIG. 1
, of the type that focuses laser light from a laser
1
on a data storage surface
2
of an optical disk to read out data from the disk. Typically the laser light diverges and is focused on the data storage surface
2
by objective lens
3
. During operation, the disk may flex or vibrate so that the distance between the data storage surface
2
and the focal point of the objective lens
3
changes by enough to measurably degrade the focus of the laser light on the surface of the optical disk being read. To prevent signal variations and degradation, the disk player adjusts the position of the lens to maintain the separation between lens
3
and the data storage surface
2
near constant at the nominal “in focus” distance. Here and throughout the discussion of the background and the invention, the term nominal has its customary meaning as satisfactory or according to plan.
It is not typically practical to measure the distance between the lens
3
and the data storage system, so disk players indirectly observe this distance. For example, light reflected from the data storage surface may pass through a beam splitter
4
, be collected and refocused by lens
5
and directed to an optical detector
6
. The optical detector
6
is divided into four quadrants, as illustrated in FIG.
2
. The system is designed so that, when the distance between the lens
3
and the data storage surface
2
is equal to the nominal “in focus” distance, the light incident on the detector
6
is in focus and has an intensity distribution that varies symmetrically on the surface of the detector. Such a symmetric, “in focus” state is illustrated in FIG.
3
. Each quadrant of the detector provides a separate output voltage V
A
, V
B
, V
C
, V
D
, so the symmetric state of
FIG. 3
is associated with an effectively zero value of the observable quantity (V
A
+V
D
)−(V
B
+V
C
).
Asymmetry is introduced into the laser light used to read the data storage surface
2
so that too short of a separation between the lens
3
and the data storage surface
2
produces an asymmetric, out of focus pattern on the detector
6
like that illustrated in FIG.
2
. This asymmetry is characterized by high intensity light on quadrants B and C of detector
6
and low intensity light on quadrants A and D. Too long a separation between the lens
3
and the data storage surface
2
produces the out of focus pattern shown in
FIG. 4
, which is characterized by high intensity light on quadrants A and D of detector
6
and low intensity light on quadrants B and C. The amount of the asymmetry varies with the amount by which the separation between the lens
3
and the data storage surface
2
varies from its desired “in focus” distance. Consequently, the quantity (V
A
+V
D
)−(V
B
+V
C
) can be a useful variable to observe to control the position of the lens
3
. For example, a linear control system can be provided to adjust the lateral position of the lens
3
to provide good control of the focus of the optical disk system of
FIG. 1
using the quantity (V
A
+V
D
)−(V
B
+V
C
) as an input.
In the
FIG. 1
system, the quantity (V
A
+V
D
)−(V
B
+V
C
) provides a good observable variable for controlling the position of lens
3
(the controlled variable) and hence the focus of the system. (V
A
+V
D
)−(V
B
+V
C
) provides both a magnitude indicative of the extent of the necessary correction and the sign of the necessary correction. The sign of the necessary correction indicates whether the lens
3
is too close or too far away from the data storage surface
2
and so which lateral direction the lens
3
needs to be moved to optimize focus. (V
A
+V
D
)−(V
B
+V
C
) can be used to control the
FIG. 1
system because (1) the laser light is purposefully made asymmetric within the system and (2) the detector is made up of four independent quadrant detectors. In other words, the optical system is especially adapted to allow for the easy control of the position of the lens
3
.
While it is possible to control the system of
FIG. 1
using the asymmetric beam in association with a quadrant detector, this is not an entirely desirable situation. It is easier to design optical systems for light that varies uniformly than it is to design similar quality optical systems for asymmetrically varying beams. This is particularly true when different colors of light might be used in the optical system.
Variations of the system of
FIG. 1
that do not intentionally introduce asymmetry into the light beam could use a detector like that illustrated in
FIG. 5
, which includes an outer element
7
producing an intensity-dependent voltage output of V
T
and an inner element
9
producing an intensity-dependent voltage output of V
C
. Using such a system, the quantity V
C
/(V
C
+V
T
), schematically illustrated as a function of lens to storage surface separation S from the nominal “in focus” position S
O
, provides a measure of the focus of the system. It is possible to use this observable to adjust the focus of the system, but it is difficult. This is so because the quantity V
C
/(V
C
+V
T
) indicates a magnitude of a correction to be made, but does not provide a sign or direction for the correction. For any observed variation of the quantity V
C
/(V
C
+V
T
) from the peak, in focus value, the lens might need displacement in either a positive or a negative direction. It is thus difficult to directly control the
FIG. 1
system using only the quantity V
C
/(V
C
+V
T
).
Systems like that indicated in
FIG. 6
can be called absolute value systems, because the observed variable provides the absolute value of a correction to make, but does not indicate the sign or direction for the correction. For such absolute value systems, it is known to introduce a movement of known direction such as an oscillation to the controlled variable. This technique is known as dither or dithering. Using dither, a regular oscillation is introduced to the position of the lens
3
, which introduces a regular variation in the observed quantity V
C
/(V
C
+V
T
). By comparing the phase of the oscillations in V
C
/(V
C
+V
T
) with the phase of the oscillations in the position of the lens
3
, the control system can identify the direction of the correction to be made to the lens position. Consequently, dither allows for the mechanism illustrated in
FIGS. 5 and 6
to be used to control the
FIG. 1
optical disk player.
Dither has a variety of drawbacks. It is complex, requiring introduction of a detectable amount of motion between objects. Moreover, dither is itself a noise source, and so is generally undesirable. Consequently, it is more common to design a system to have an observable variable that provides both a magnitu

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