Rotary Acceleration Feed Forward (Raff); Perpendicular Magnetic Recording (Pmr); Figure 3-1 Dual Linear Sensor Rotational Acceleration Feed Forward (Raff) - Western Digital WD1002FBYS Technical Reference Manual

WD RE3 XL333M
3.3

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)
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.
3.4

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.
2679-701179-A01 RELEASED 11/13/08 (WD CONFIDENTIAL)
Product Features
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