Introduction - Hard Disk Drive: Mechatronics and Control - page 12

of information bits, then the rate of bits while reading differs from the actual
rate at which they were written. Thirdly, the variation in spindle speed is a
major source of disturbance in the tracking servo loop as it alters the spectrum
of disturbance related to the imperfections in the shape of the track [4]. The
variations in the track shape enters the servo loop as a disturbance, and the
spectrum of such disturbance depends on the spinning speed.
Ball-bearing spindle motors comprise a signi fi cant number of shipments in
HDD today. Demand for higher areal density and faster spindle speed are
the reasons for adopting fluid dynamic bearing (FDB) spindle motors in HDD.
Majority of drives today use FDB motors. In ball bearing motors, mechanical
contact exists between the ball and race of the bearing. It is impossible to
make the ball and race of the bearings perfect and free from defects. Any
contact with these inherent defects found in the geometry of the race ball
interfaceandthelayerofthelubricant fi lm produces lateral movements of the
spindle shaft and, therefore, of the disks. Since such movements are random in
nature and not synchronized with the rotation, they can not be modeled and
compensated exactly. These are known as non-repeatable runout or NRRO.
The NRRO is the main contributor to Track Mis-Registration (TMR), the error
between the position of the read head and the center of track when the head
positioning servomechanism tries to follow a track. The NRRO is a bottleneck
in achieving higher track density and there is an upper limit at which the ball
bearing design can no longer overcome the NRRO problem. Currently with
ball bearings, NRRO is in 0.1 µ-inch range.
Besides, the FDB spindle motors prove to be more advantageous than the
ball bearing motors in term of non-operational shock resistance. When the
motor is stationary, the FDB spindles have a larger area of surface to surface
contact compared to the ball bearing motors. So the FDB spindle can with-
stand larger non-operational shock. The dynamic motions of the spindle motor
components and disks are the main contributors to the acoustic noise in HDD.
Sound energy generated by the spinning disks, motor magnets, stator compo-
nents, and bearings is transmitted through the spindle assembly to the HDD
base casting and the top cover. In an FDB spindle, one of these sources (the
bearing) is eliminated. There exists no contact between the rotating surface
and the stationary surface of the spindle motor. Moreover, the viscosity of the
lubricant between the two surfaces is much higher than that for the lubricant
used in ball bearings. As a result of these features, the FDB spindle motors
generates less NRRO. Furthermore the damping effect of the lubricant of rel-
atively higher viscosity between the stator and the rotor of the FDB spindle
attenuates noise. As a result, the FDB spindle shows better acoustic perfor-
mance with approximately 12-15 dBA reduction of noise level compared to ball
bearing spindles. Other inherent properties of the FDB spindle include higher
damping, reduced resonances, and greater speed control.
Clamps are used to hold the disks fi rmly on the spindle shaft. Annular
rings known as spacers are placed between two disks to ensure desired gap