Dynamic information storage or retrieval – Storage or retrieval by simultaneous application of diverse... – Magnetic field and light beam
Reexamination Certificate
1999-12-14
2003-05-13
Neyzari, Ali (Department: 2655)
Dynamic information storage or retrieval
Storage or retrieval by simultaneous application of diverse...
Magnetic field and light beam
C369S300000, C369S112240, C369S126000
Reexamination Certificate
active
06563767
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a head actuator supporting a magnetic head for recording information on and/or reproducing information from a disc-shaped recording medium and for causing movement of the head in the direction along the radius of the disc-shaped recording medium for positioning the head in a target position. This invention also relates to a recording and/or reproducing apparatus incorporating this head actuator.
In a magnetic disc device or an optical disc device for recording and/or reproducing information signals for a disc-shaped recording medium, attempts are now being made to reduce the size as well as to increase the recording density thereof. For example, a hard disc device may be utilized, in which the track density and the recording density of the recording medium of the disc are raised to increase the recording capacity.
In this hard disc drive of increased recording capacity, the head needs to be precisely positioned on a target track of the disc to which the head is to be accessed.
In general, if a moving object with an inertial moment J is performing a sinusoidal movement with an amplitude A and a frequency f, the amplitude of the angular acceleration A·sin(2&pgr;ft), determined from a second order differential of the displacement with respect to time, is equal to A·(2&pgr;f)
2
, and the amplitude of the inertial force determined by the product of the angular acceleration and the inertial moment is equal to J·A·(2&pgr;f)
2
. Therefore, the driving force T, necessary for causing movement of a moving object with an inertial moment J, is given by
T∝inertial moment (J)
T∝amplitude (A)
T∝square of frequency of motion (f).
Thus, if the driving force is constant, the amount of oscillation of the moving object (amplitude) is decreased in inverse proportion to the frequency of motion.
By these mechanical properties, if, in a system in which an object having an inertial moment is kept in motion and its position is controlled, the positioning error is increased roughly in proportion to the square of the frequency, thus deteriorating the control performance.
For enlarging the servo control range of the head positioning system, it is necessary to raise the servo loop gain so that the head/track relative offset will be within the allowable residual servo error value. However, in a single-stage actuator, it has been shown that, for the following reason, the actuator driving power needs to be proportionate to the fourth power of the frequency.
That is, the relationship between the driving force T and the driving current i is given by the equation:
Kt·i=T=J·A
·(2
&pgr;f
)
2
(1)
where Kt is a torque constant.
On the other hand, since the driving power P is proportionate to the square of the driving current i,
P∝i
2
=((
J/Kt
)·
A
·(2
&pgr;f
)
2
)
2
(2)
so that it can be rewritten in a proportional form to
P∝i
2
∝f
4
(3)
That is, the driving power P is proportionate to the fourth power of the frequency of motion.
Therefore, if the rotational speed (rpm) of the disc is doubled, with the head to track misregistration (track misregistration TMR) being fixed, track-related oscillations on the disc are shifted to a doubled value as a whole towards the high side. Thus, the driving power of the actuator is increased by a power of 4 from that for the original disc rotational speed, that is to a 16-fold value.
It is noted that the smaller the actuator size, the smaller the inertial moment J, such that the torque constant to inertial moment ratio Kt/J is increased. Thus, by dividing the actuator into two steps, namely the coarse movement step and the fine movement step, and by reducing the inertial moment to as small a value as is possible for the fine movement driving mechanism, the Kt/J ratio can be correspondingly increased. For example, if the single-stage actuator in current use is used as a coarse movement driving mechanism, and Kt/J of the fine driving movement mechanism is e.g., 36 times that of the coarse movement driving mechanism, the maximum frequency and the driving power that can be controlled are as follows:
That is, as for the controllable band, from equation (1) above,
(
Kt/J
)·
i=A
−(2
&pgr;f
)
2
.
Therefore, if the driving current i and the amplitude A are constant,
f
2
∝Kt/J
, so that
f
∝{square root over ((
Kt/J
))}
and hence the maximum frequency f (fine) that can be controlled by the fine movement driving mechanism is
f
(fine)=
f
(coarse)·{square root over (36)}
=f
(coarse)·6.
Therefore, the frequency f (fine) is six times the maximum frequency f (coarse) that can be controlled by the coarse movement driving mechanism.
As for the driving power, since
P
∝((
J/Kt
)·
A
·(2
&pgr;f
)
2
)
2
from equation (2), if the amplitude A and the frequency f are constant,
P
∝(
J/Kt
)
2
.
Therefore, the driving power P (fine) required for the fine movement driving mechanism is
P
(fine)=
P
(coarse)·(1/36)
2
=P
(coarse)·(1/1296)
or 1/1296 of the driving power P (coarse) required for the rough movement driving mechanism.
Meanwhile, the most difficult problem in improving the track follow mode is that the angular velocity of a rotary actuator in performing track following is extremely small. It has been known that the frictional force generated between the bearing and a ball shaft for an extremely small angular velocity is such that the displacement curve representing the force of rolling friction generally describes a hysteresis loop for an extremely small width of displacement of the order of the rollout angle. The hysteresis is susceptible to irregular changes dependent upon the temperature or humidity, such that, due to non-linearity of the frictional force, prediction of displacement is extremely difficult. This in turn renders positioning control difficult. Thus, in a conventional single-stage actuator, non-linearities produced by the bearing tend to frustrate attempts towards increasing the track density.
In order to cause a magnetic head of a hard disc device to follow a high rotational speed rpm and high track density associated with a conventional voice coil motor (VCM), a double-stage micro-actuator system in a variety of systems may be utilized, such as a system combining a VCM for rough movement and a piezo element for fine movement (PZT), a piggy back system employing a VCM along with rough movement/fine movement, or a system for driving a slider by a piezo element for fine movement (PZT) or an electrostatic actuator.
The micro-actuator for double stage servo is roughly classified into a type mounted on the base of a suspension for driving the head in its entirety, a type for driving a slider and a type for driving a head element.
In the two-stage servo micro-actuator, used for positioning the head to high accuracy, the following problem is met in connection with these respective types.
That is, in the type mounted on the base of the suspension for driving the entire head, it is difficult to enhance the servo bandwidth due to the mass and vibration characteristics of the suspension.
In the slider driving type, the servo range, limited by the acceleration (driving force), is narrower than that in the head element due to the mass weight of the slider.
In the head element driving type, the production process is complex and varied because of integration of the actuator formation and the head element manufacturing steps.
Therefore, insofar as the structure is concerned, the driven movable part of the actuator smaller in size and weight than the slider and can be actuated by a smaller force. In addition, the manufacturing process of the slider driving type micro-actuator is not as complex as the head element driving type and is suited to a batch process.
On the other hand, in an optical disc device, a double-stage actuator for tracking has been used. It is noted that the optical pickup is appreciably larger in size than the magnetic pickup. Since the optical pickup is not of the floating type, as in th
Neyzari Ali
Ryan Matthew K.
Sony Corporation
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