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EX800M
3.4
Rotary Acceleration Feed Forward (RAFF)™
Rotary Acceleration Feed Forward (RAFF) helps to overcome the effects of rotational vibration (RV)
on a hard drive by generating an additional control effort to counter the RV disturbances, thereby
keeping the drive head(s) within the safe operating region during reading and writing operations.
Figure 3-1. Dual Linear Sensor Rotational Acceleration Feed Forward (RAFF)
Position
Reference
The RAFF implementation has three major components: RV sensing, RV control effort feed-
forwarding and adaptation to environmental conditions.
RV sensing in the RAFF implementation is accomplished by using two relatively inexpensive
linear accelerometers placed on the printed circuit board assembly (PCBA). The sensor locations
are optimized for separation distance and PCB mounting conditions. Since the difference signal
from two similar linear accelerometers placed in a parallel orientation and separated by some
distance is indicative of RV, the signals are subtracted from each other to generate a Differential
Sensor Signal (DSS).
RV control effort feed-forwarding is achieved by digitizing the DSS, then, sending it to the
microprocessor of the drive. Using a control algorithm, the microprocessor generates a control
effort signal based on the DSS. This feed forward control effort is in addition to the conventional
servo control approach in hard drive operations.
Adaptation to environmental conditions is crucial to the successful deployment of RAFF. The
WDT design intelligently applies RAFF selectively and adapts to with individual drive parameters
to maintain maximum performance in the hard drive.
3.5
Perpendicular Magnetic Recording (PMR)
In perpendicular magnetic recording (PMR), the magnetization of each data bit is aligned vertically to
the spinning disk, rather than longitudinally as has been the case in hard drive technology for decades.
In longitudinal recording, as the bits become smaller and closer together, they experience an increasing
demagnetizing field, much like two bar magnets that are placed end-to-end repel one another. A
property of the media called coercivity must be increased to counteract the demagnetization to keep the
bits stable under thermal fluctuations; otherwise data corruption may occur over time. Higher media
coercivity has pushed the recording head write field to the limit of known materials.
In perpendicular recording, the adjacent bits attract instead of repel (as with bar magnets placed side by
side,) creating more thermally stable bits. In addition, the media contains a magnetically soft
underlayer (SUL) beneath the recording layer. This SUL allows a larger effective write field, thus higher
coercivity media, enabling further increases in density. Lastly, because of the vertical orientation of the
2679-771339-A05
Rotational
Vibration
Disturbance
Position
Position
Error
Feedback
Controller
Microprocessor
RELEASED 8/13/13
Drive
Mechanical
Coupling
Motor Power
Driver
Product Features
Position
Voice Coil Motor and
Actuator Arm
19
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