Hardware Reference
In-Depth Information
1. The ratio of EM (electro-magnetic) power to the volume of motor must
be high. Hard disk drive is a compact electromechanical system and,
therefore, very limited space is available for mounting any component
including the spindle motor. But, on the other hand, the motor must have
the capability of driving the disks to rotate at the required speed, and
must generate torque high enough to drive that load. This requirement
is crucial especially for small form factor HDD's.
2. The HDD spindle motor operates at high speed for long time. Higher
spindle speed is always in demand in order to reduce the latency in
reading/writing of data. The industry have experienced this trend of
continuously increasing trend of spindle speed over past years, and some
products have already reached rotational speed of 15,000 rpm or higher.
Moreover, an HDD is required to operate continuously for years. Many
motors cannot stand such high speed operation for such long time due to
the limitations in the structures and components used in those motors.
3. Accurate speed is another important requirement for HDD spindle mo-
tor. Any variation in the spindle speed causes jitter between the read-
back information and the read clock. With increasing linear density of
recording, small jitter in synchronization can cause erroneous detection
of binary bits. Therefore, the accuracy of the rotational speed must be
maintained and the spindle speed must be controlled robustly.
4. Reliability of the motor must be good. The HDD is a key component for
mass data storage in PCs and other computing systems, and it may oper-
ate continuously for years. Although the HDD may spend the most of its
running time in idle mode, i.e., no reading or writing of data being per-
formed, the spindle motor operates continuously while other components
remain inactive.
5. Leakage magnetic
fi
eld must be low. HDD utilizes the magnetic
fi
eld to
read/write data onto the media. Hence, the process of data access is very
sensitive to external magnetic
fi
eld, which includes the leakage magnetic
fi
eld from the spindle motor. This issue will be especially crucial when
the industry will adopt perpendicular recording technology.
6. Runout is the horizonatal and vertical movement of the spindle and disks
with respect to the center of rotation, and it may be contributed by many
factors such as misalignment of stator and rotor, defects in bearing, mis-
alignment between spindle and disks etc. Runout must be kept low as it
in
fl
uences severely the recording density in HDDs (see section 4.3.7).
Therefore, reducing the runout, especially the non-repeatable runout
(NRRO), of the spindle motor is very important for the HDD products.
This requirement has propelled the development of special bearings like
the
fl
uiddynamicbearing,whicharenowusedwidelyintheHDDprod-
ucts. The issue of runout is elaborated again in section 4.3.7.
