Dynamic magnetic information storage or retrieval – Automatic control of a recorder mechanism
Reexamination Certificate
1998-09-29
2002-01-29
Hudspeth, David (Department: 2753)
Dynamic magnetic information storage or retrieval
Automatic control of a recorder mechanism
C360S075000, C318S432000, C318S433000
Reexamination Certificate
active
06342984
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to hard disk drives. More particularly, the present invention relates to a disk drive including DC to DC voltage converter for increasing voltage to its spindle motor and VCM.
2. Description of the Prior Art
In hard disk drives, data is stored on magnetic media disks in concentric data tracks, which are divided into groups of data sectors. Servo information including track number, sector number, and tracking information is recorded in radially continuous narrow wedges between the groups of data sectors. The disk drive includes an actuator assembly having a voice coil motor (VCM), an actuator arm extending from the VCM, and a transducer head disposed at the end of the actuator arm. One way of improving disk drive performance is to reduce data access time, including the time required to move the transducer head of the actuator assembly from a current data track to a selected target data track.
The disk drive performs a seek operation to move the transducer head from a present data track to a target data track. The disk drive includes a servo system employed to seek to the target data track and thereafter follow the target data track on the disk. The servo system controls the VCM so that the VCM swings the actuator and its attached transducer head to access the target data track. Therefore, if the torque developed (T
d
) by the VCM could be increased, a corresponding reduction in data access time could be achieved. T
d
is given by the following Equations I:
T
d
=Kt·I
coil
Equations I
Kt=Ke(Kt in Nm/A; Ke in V/rad/sec)
Bemf=Ke·Vel
motor
(Vel
motor
in rad/sec)
V
applied
=V
source
−Bemf=I
coil
·R
coil
I
coil
=(V
source
−Ke·Vel
motor
)/R
coil
Where:
T
d
is the torque developed by the VCM;
Kt is the Torque constant of the VCM;
I
coil
is VCM coil current;
Ke is the Voltage constant of the VCM i.e. the Bemf factor;
Bemf is the back emf of the VCM;
Vel
motor
is the velocity of the VCM;
V
applied
is the voltage applied to the VCM coil;
V
source
is the voltage of the VCM power source supplying power to the VCM; and
R
coil
is the total resistance of the VCM coil from the VCM power source to ground.
As can be shown by Equations I above, T
d
can be increased by increasing Kt and/or I
coil
. Unfortunately, increasing I
coil
causes a corresponding heating of the VCM coil, proportional to the square of the current, which can result in heat related breakdown of VCM components, including the VCM coil itself. In particular, seek operations require the highest current to be supplied to the VCM in order to achieve competitive access times. Conversely, increasing Kt equates to an equal increase in Ke which increases the Bemf of the VCM. Therefore, the Bemf approaches V
source
as Kt is increased. Consequently, if V
source
cannot be increased, less voltage V
applied
is available and I
coil
is reduced as Kt is increased thereby limiting the performance of the VCM. The V
source
supplied to the VCM in disk drives, however, is fixed because, for compatibility with host systems, a standard power supply having a fixed voltage is used to supply power to the VCM. This fixed voltage power supply to the VCM therefore limits how much effect the VCM can have on reducing access times.
For reasons stated above and for other reasons presented in greater detail in the detailed description of the present specification, there is a desire to reduce data access time during seek operations in disk drives. In particular, there is a need to improve VCM performance by increasing the torque constant of the VCM to achieve a corresponding decrease in data access time in seek operations in disk drives. It would also be desirable to increase the torque constant of the VCM to permit a reduction in VCM coil current to reduce I
2
R power losses in the system while still maintaining a given torque in the VCM.
In disk drives, disks are typically stacked on a spindle assembly. The spindle assembly is mechanically coupled to a spindle motor which rotates the disks at a high spin-rate. A spindle motor driver typically includes power field effect transistors (FETs) to drive the spindle motor. A microprocessor is typically employed to ascertain when to apply a run signal, a coast signal, or a brake signal to the spindle motor driver to control the operation of the spindle motor.
The torque developed (T
d
) by the spindle motor is given by the following Equations II:
T
d
=Kt·I
m
Equations II
Kt=Ke(Kt in Nm/A; Ke in V/rad/sec)
Bemf=Ke·&ohgr;
V
applied
=V
source
−Bemf=I
m
·(R
m
+R
fet
)
I
m
=(V
source
−Ke·&ohgr;)/(R
m
+R
fet
)
Where:
T
d
is the torque developed by spindle motor;
Kt is the Torque constant of spindle motor;
I
m
is spindle motor current;
Ke is the Voltage constant of the spindle motor i.e. the Bemf factor;
Bemf is the back emf of the spindle motor;
&ohgr; is the rotational velocity of the spindle motor;
V
applied
is the voltage applied to the spindle motor;
V
source
is the voltage of the spindle motor source supplying power to the spindle motor;
R
m
is the total resistance of the spindle motor and wire connections between the spindle motor and the power FETs; and
R
fet
is the resistance in the power FETS that are turned on for controlling current
I
m
flowing through the spindle motor.
Power dissipation in the windings of the spindle motor are given by the following Equation III:
P=I
m
2
·R
m
Equation III
Power dissipation in the power FETs are given by the following Equation IV:
P=I
m
2
·R
fet
Equation IV
It is known to increase the RPM of the spindle motor to reduce rotational latency and increase disk transfer rate in the disk drive. However, as disk drives employ spindle motors operating at higher RPMs, the increased drag at higher speeds causes an additional drag torque to be applied to the disk assembly. This drag torque increases at approximately the square of the spin-rate increase of the spindle motor. As can be shown by Equations II-IV above, the increased T
d
required to off-set the additional drag torque resulting from higher speed spindle motors, can be obtained by increasing Kt and/or I
m
. Unfortunately, increasing I
m
causes a corresponding heating of the windings of the spindle motor as shown by the above power dissipation Equation III and also a corresponding heating of the power FETs used to drive the spindle motor as shown by the above Equation IV. The additional heat generated by the disk drive can cause heat related breakdown of components in the disk drive, including the spindle motor windings and the power FETs themselves. In addition, power supplies in high performance computer systems which employ disk drives having spindle motors operating at high RPMs, typically limit the peak and average current output from the computer system's power supply circuitry to reduce the cost of the computer system.
The T
d
can be increased by increasing Kt, but increasing Kt equates to an equal increase in Ke which increases the Bemf of the spindle motor. Therefore, the Bemf approaches V
source
as Kt is increased. Consequently, if V
source
cannot be increased, less voltage V
applied
is available and I
m
is reduced as Kt is increased thereby limiting the T
d
of the spindle motor. The V
source
supplied to the spindle motor in disk drives, however, is fixed because, for compatibility with host systems, a standard power supply having a fixed voltage is used to supply power to the spindle motor.
For reasons stated above and for other reasons presented in greater detail in the detailed description of the present specification, there is a desire to reduce the spindle motor current (I
m
) disk drives, especially in high performance disk drives operating at 10,000 RPMs and higher. It would be desirable to increase the torque constant of the spindle motor to permit a reduction in I
m
to reduce I
2
R power losses in the disk drive wh
Hussein Hakam D.
Oettinger Eric G.
Plutowski Eugene F.
Davidson Dan I.
Hudspeth David
Shara Milad G
Western Digital Technologies, INC
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