Measuring and testing – Rotor unbalance
Reexamination Certificate
1998-03-31
2001-02-20
Kwok, Helen C. (Department: 2856)
Measuring and testing
Rotor unbalance
C360S099080
Reexamination Certificate
active
06189371
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for adjusting the rotation balance of a disk, and in particular to a method and apparatus for aligning the center of rotation of a disk with the axis of rotation of a spindle during disk drive assembly.
BACKGROUND OF THE INVENTION
Typically, a disk drive has a circular magnetic disk for storing data. The disk is mounted on a spindle which is rotated about a longitudinal axis or axis of rotation. Data is read from and written to the surface of the disk while the disk is rotating. To rotate the disk, a spindle motor transmits a driving force to rotate the spindle. A mechanism then positions a magnetic head over a desired area of the disk surface to read data already recorded on the disk or to write data to the disk. To position the head accurately, it is very important that the center of the disk be precisely aligned with the spindle to prevent the disk from being offset during rotation. Therefore, a process is needed to align the center of the disk with the axis of rotation of the spindle. In other words, a process is needed to keep the offset of the disk within an allowable error (tolerance), and fix the disk to the spindle during manufacturing.
The operation of adjusting the offset of a rotating disk to be within an allowable error is generally referred to as the adjustment of rotation balance. Rotation balance has two components: static balance and dynamic balance. Static balancing refers to decreasing the vibration component generated by a translational force during rotation. Dynamic balancing refers to decreasing the vibration component generated by torque during rotation. Generally, if the distance between a plurality of masses fixed to an axis of rotation or spindle is short, the effect of the dynamic imbalance is negligible relative to the effect of the static imbalance. However, as the distance increases, the effect of the dynamic imbalance increases; and the dynamic imbalance can no longer be ignored.
In a hard disk drive, when a small number of disks are fixed to the spindle, the length of the spindle is small; therefore, the dynamic balance may be ignored. However, as the number of disks increases, both the dynamic and static balance need to be adjusted. Preferably, the amount of static and dynamic imbalance is 0. However, in practice, it is very difficult to completely eliminate all imbalance. Therefore, a process is needed to adjust the static and dynamic balance during disk drive manufacture so that the amount of imbalance falls below a predetermined threshold value to prevent problems when the disk drives are used. The adjustment of the static and dynamic balance will be described below.
Adjusting Static Balance
Initially the adjustment of the static balance will be described while referring to
FIGS. 1
,
2
a
,
2
b
and
2
c
.
FIG. 1
is a diagram of the force resulting from disk offset. A magnetic disk drive
200
has a disk
204
, e.g. a magnetic disk, fixed to a spindle
202
. Because the outer diameter of the spindle
202
is smaller than the inner diameter of a central hole in the disk, a gap as large as several hundred microns forms between the disk and the spindle. Accordingly, when the disk
204
is fixed to the spindle
202
, the center of the spindle of the disk
0
is frequently offset, by amount e, from the center of gravity G of the disk, therefore the disk is frequently imbalanced. If the disk is imbalanced, a translational force P is generated when the disk
204
rotates.
FIGS. 2
a
,
2
b
and
2
c
show the relationship between the periodic motion of the disk and the direction of the translational force resulting from the offset. The translational force P, with a continuously changing direction, is exerted between the disk
204
and the spindle
202
. As the amount of vibration from the translational force P increases, errors are more likely to occur when reading or writing data.
Referring back to
FIG. 1
, a disk
204
of mass m rotates at angular velocity &ohgr; while offset by dimension e relative to the center O of the spindle
202
of a fixed disk drive
200
. The translational force P has a vertical component F urging the disk drive
200
downward. The magnitude of the vertical force F is determined by the following equation (1):
F=me&ohgr;
2 sin(&ohgr;
t
) (1)
For example, if a 2.5 inch aluminum disk rotates at 3,600 revolutions per minute (RPM) with a offset of 0.1 mm, the maximum value of the magnitude of force F is about 10 g. Equation (1) shows that the magnitude of force F is directly proportional to both the rotation frequency and the number of disks. If either the rotation frequency or the number of disks increases, then the magnitude of force F will increase. For example, if the 2.5 inch aluminum disk above is rotated at 4,800 RPM, the magnitude of force F is 1.8 times larger than at 3,600 RPM. The static balance adjustment means decreasing the amount of vibration caused by the translational force during disk rotation.
Adjusting the Dynamic Balance
Next, the adjustment of the dynamic balance will be described while referring to
FIGS. 3
a
and
3
b
. FIG.
3
a shows a plurality of disks
204
fixed to a spindle
202
.
FIG. 3
b
is a dynamic model of the plurality of disks of
FIG. 3
a
. In
FIG. 3
b
, according to rigid body kinematics, the rotation imbalance of a distributed mass system can be equivalently represented by two concentrated weights
201
,
203
on two arbitrarily selected planes S
1
, S
2
on the axis
202
. The two concentrated weights
201
,
203
have mass m
1
and m
2
, respectively, and have corresponding vectors r
1
and r
2
. The planes S
1
, S
2
are spaced apart at a distance
1
along the spindle
202
. During rotation, a vibration component is generated by the torque based on the equivalent amount of imbalance m
1
·r
1
, m
2
·r
2
between the two planes. Adjusting the dynamic balance decreases the amount of vibration from the torque.
To satisfy the increased demand for high speed and large storage capacity, disk drives have more disks and rotate faster. Consequently, the problem of efficiently reducing the imbalance has become more significant. Therefore, the adjustment of static balance and dynamic balance has become increasingly important in disk drive manufacture.
Using a conventional technique, imbalance is adjusted by: rotating a disk, measuring the position and amount of imbalance, and attaching a counterweight with a controlled mass to a side opposite a stopped position. The counterweight must be attached while the disk is stopped. Furthermore, the balance adjusting operation must be repeated many times for accurate adjustment. Because the disk is repeatedly rotated and stopped, the method has the disadvantage of taking a long time.
Another method has been proposed which uses a screw as a counterweight. However, this method not only had the disadvantages discussed above, but also had the disadvantage of being difficult to make a fine balance adjustment since the tapped hole has a predetermined position.
In addition, Japanese Published Unexamined Patent Application No. 60-187966 discloses a method of rotating the disk at a frequency higher than a critical speed while the disk is offset and fixing the disk to the axis when the rotation is brought close to 0. However, rotating the disk above the critical speed puts a very large load on the spindle motor and may reduce the life of the disk drive. Therefore, this method is not always practical.
Recently, Japanese Published Unexamined Patent Application No. 3-69060 has disclosed a method using an offset compensator to overcome the disadvantages discussed above. The offset compensator has several components. A clamp, such as a magnet, is capable of moving radially across a disk. A spindle motor part guide is formed to enable the spindle motor to move radially with the clamp. A device measures the amount of offset between the center of a concentric circular or spiral track on the disk and the center of rotation of the spindle motor part using a push-pull an
International Business Machines - Corporation
Knight G. Marlin
Kwok Helen C.
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